Mechanism of Magnesium Oxide
Chlorination by Hydrogen Chloride in a Molten Salt
Martin Lamy
Department of Mining and Metallurgical Engineering
McGill University, Montréal
November 2001
A thesis submitted to the Faculty of Graduate Studies and
Research in partial fulfilment of the requirements
for the degree of Master of Engineering
© Martin Lamy, 2001
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Abstract
The reaction of HCI gas with MgO partieles suspended in a molten salt containing
magnesium chloride was studied. Chlorination experiments were carried out in a batch
stirred tank reactor.
The determination of the MgCb concentration as a function of
chlorination time indicated that the chlorination of MgO to MgCI 2 occured through an
intermediate step.
The intermediate species was assumed to be MgOHGI forming
according to the reaction :
MgqS) + HG/(g)
-7
MgOHG/(d)
Alkalimetric titration and thermal decomposition by injection of dry nitrogen were used to
confirm the existence of MgOHCI in the molten salt during chlorination.
Infrared
spectrometry was also tentatively used to identify MgOHCI but, without success due to
the hygroscopie nature of the material.
Conventional mass transfer theory was used to develop a model for the reaction of solid
MgO particles with HGI dissolved in the molten salt to form MgOHGI.
The model
assumed shrinking particle behaviour for the MgO and was confirmed by the data from
the chlorination experiments.
Moreover, experiments conducted at different HGI flow rates showed that the reaction
was controlled by mass transfer of HGI dissolved in the molten salt across the liquid film
surraunding the MgO particle to the surface of the particle. The following rate law was
obtained:
the constant Kn was found to vary between 0.04 and 0.08
S-1
when the gas flow rate
was varied fram 2.5 to 7.5 SLPM at constant temperature and agitation speed of 500 oC
and 500 RPM, respectively.
Résumé
Ce projet de recherche porte sur l'étude de la réaction entre le HCI gazeux et le MgO
solide en suspension dans un sel fondu contenant du MgCb.
Des expériences de
cinétique de chloruration ont été effectuées dans un réacteur agité. La détermination de
la variation de la concentration du MgCI 2 durant la chloruration a permis d'établir que la
réaction de MgO avec HCI pour former du MgCI 2 s'effectuait par l'entremise d'une étape
intermédiaire. MgOHCI a été considéré comme l'espèce intermédiaire formé selon la
réaction:
Mgqs) + HC/(g)
~
MgOHC/(d)
La technique de titrage alkalimétrique ainsi que la décomposition par injection d'azote
sec ont été utilisées pour confirmer l'existence de MgOHCI dans le sel fondu durant la
chloruration. Cependant, l'analyse des échantillons par spectrométrie infrarouge n'a pas
permis de montrer la présence de MgOHCI à cause de la nature hygroscopique du
matériel.
Un modèle cinétique a été établi pour la réaction entre le MgO et le HCI dissous dans le
sel fondu pour former MgOHCI.
Le modèle assume que la vitesse de réaction des
particules de MgO est proportionnel à leur surface et que la réduction du diamètre est
uniforme durant la réaction (Le. "shrinking particle").
Une analyse par régression
linéaire effectuée en utilisant les données cinétiques a permis de valider le modèle.
De plus, des expériences effectuées à différents débits de HCI ont démontré que la
réaction était contrôlée par le transfert du HCI dissous dans le sel fondu à travers le film
liquide à la surface des particules de MgO à la surface elle-même. La relation suivante
a été obtenue pour l'évolution de la concentration de MgO dans le temps:
La constante Kn variait de 0.04 à 0.08
S-1
lorsque le débit de HCI était augmenté de 2.5
à 7.5 SLPM à 550°C et 500 RPM.
Il
Acknowledgements
1 wish to express my greatest gratitude and appreciation to
my supervisor,
Dr. Ralph L. Harris. for his guidance, encouragement and financial support throughout
the course of this research.
1 also want to thank Noranda Inc. Technology Centre for allowing me to perform my
experiments in their laboratory. A special thanks goes to Eduardo Delrincon who helped
me to solve various mechanical problems.
The friendly support of Craig Taylor, my
industrial supervisor, and Kirsten Wallace was really appreciated.
Finally, 1 want to mention that this work would never have been possible without the
support of my fiancée, Valérie. 1could never express through written words the thanks 1
owe to her.
III
Nomenclature
Symbol
aBUbble
HGI
a Bulk
HGI
a Surface
HGI
AbTDt
Ap
C
CM9GI2
C~9G12
C M90
C~9O
C~9O
C~goeq
C~goeq
C~9OHGI
CM9(OHlz
C~9(OH)2
Co
0
kL
Kn
kp
ks
kSL
kR
Description
HCI activity at the bubble/molten salt
interface
HCI activity in the bulk of the molten salt
units
sJ. units
mole/m 3
mole/m 3
HCI activity at the surface of a MgO particie
mole/m 3
Total bubbles surface area
m2
m2
Surface area of a MgO particle
mole/m 3
Concentration of the solute at the disk
surface
Concentration of MgCb
wt%
mole/m 3
Concentration of MgCI 2 at time "t"
wt%
mole/m 3
Concentration of MgO
ppm
mole/m 3
Initial MgO concentration
ppm
mole/m 3
MgO concentration measured by
alkalimetric titration
Alkalinity after MgOHCI decomposition
ppm
mole/m 3
ppm
mole/m 3
Alkalinity before MgOHCI decomposition
ppm
mole/m 3
MgOHCI concentration measured by
alkalimetric titration
Concentration of Mg(OH)2
ppm
mole/m 3
mg/I
kg/m 3
Solubility of Mg(OHh in water
mg/I
kg/m 3
mole/m 3
2
m /s
mis
Concentration of solute in the Iiquid phase
Diffusivity
Gas-liquid mass transfer coefficient
Specific reaction rate constant
Surface area constant
M2/ppm 2/3
System constant
ppm
1/s
mole/m 3
Liquid-solid mass transfer coefficient
mis
mis
msample
Reaction rate constant
Alkalinity at the MgOHCI end point in
alkalimetric titration
Alkalinity at time "t" during alkalimetric
titration
Total alkalinity of sample in alkalimetric
titration
Mass of solid sam pie
M HGI
Molarity of HCI solution
lEP
mMgoeq
t
mMgOeq
0
mMgOeq
m4/mole 2/3
9
kg
9
kg
9
kg
9
kg
mole/I
IV
Nomenclature continued
Symbol
MM90
n Mgo
nMgOHC/
bas
Description
Molecular weight of MgO
s.i. units
kg/mole
units
g/mole
Number of moles of MgO
mole
Number of moles of MgOHGI
mole
Number of moles of "basic" oxygen
mole
tot
n0
Total number of moles of oxygen
mole
np
Number of MgO particles in the melt
unitless
Mass transfer rate of HGI across the liquid
film surrounding a HGI bubble
Mass transfer rate of HGI across the liquid
film surrounding a MgO particle
Mass transfer rate of HGI at the surface of a
MgO particle due to chemical reaction
Number of moles of MgO reacted from a
single particle
Total number of moles of MgO reacted
mole/s
Radius of a MgO particle at a given time
during chlorination
Initial radius of a MgO particle
m
m
Mg(OH)2 supersaturation level
unitless
n0
• l>fb
NHc/
• l>fp
NHc/
• Surface
NHcl
N Mgo
Tot
NMgO
rp
ri
p
SM9(OHh
v
VHC/
Vm
'U
Pm
PMgO
ü)
QL
QSL
OR
Specifie reaction rate
Volume of HGI used in titration
Melt volume
Kinematic viscosity
Melt density
Density of MgO
Angular velocity
Resistance to mass transfer in the gasliquid interface
Resistance to mass transfer in the liquidsolid interface
Resistance to mass transfer due to
chemical reaction at the surface of a MgO
particle
mole/s
mole/s
moles
moles
mole/s
ml
m3
m3
m2/s
kg/m 3
kg/m 3
radIs
s/m 3
s/m 3
s/m 3
Indices:
aq : aqueous, s : solid, 9 : gas, d : dissolved species
V
Table of Contents
1
Introduction
1
2
Literature Review
3
2.1 Electrolytic Production of Magnesium
3
2.1.1 The Magnola Process
5
2.1.1.1 Feed preparation
6
2.1.1.2 Leaching and purification
6
2.1.1.3 Brine dehydration
7
2.1.1.4 Melt chlorination
8
2.1.1.5 Electrolysis
8
2.2 Effect of oxides on cel! performance
10
2.3 Solubility of MgCI 2 hydrolysis product in molten salts
12
2.4 Techniques for the determination of MgCI2 hydrolysis products
15
2.4.1 Volumetrie determination of hydrogen
15
2.4.2 Alkalimetric Speciation
17
2.4.3 Carbothermal Reduction
21
2.5 Chlorination of magnesium oxide by hydrogen chloride in molten salts
23
3
Proposed Reaction Mechanism for MgO chlorination
29
4
Experimental
36
4.1 Laboratory Scale Chlorination Reactor
36
4.1.1 Fumace and Reaction Vesse!.
37
4.1.2 Rotary Gas lnjector
38
4.1.3 Gas Handling System
40
4.2 Chlorination Experiments
.41
4.2.1 Melting
41
4.2.2 Pre-chlorination
41
4.2.3 Degassing
42
4.3 Determination of reaction kinetics
43
4.4 Quantitative decomposition of MgOHCI
.45
4.5 Analytical Techniques
.46
VI
4.5.1
5
Determination of Total Alkalinity
4.5.2 Determination of MgCb
.48
4.5.3 Alkalimetric Speciation
.48
4.5.4 Infrared Spectrometry
51
Experimental Results
53
5.1 Chlorination Experiments
53
5.1.1 ObseNation of melt surface
53
5.1.2 Oxide Speciation During MgO Chlorination
54
5.1 .2.1 Magnesium chloride analysis
54
5.1.2.2 Alkalimetric Titration
55
5.1.2.3 Quantitative Decomposition of MgOHCI
56
5.1.2.4 Infrared Spectrometry
57
5.2 Verification of the kinetic model for the Chlorination of MgO to MgOHCI.
5.2.1
6
.46
Linear regression analysis
60
60
5.2.2 Effect of HCI flow rate on reaction kinetics
62
Interpretation of Results
65
6.1 ObseNation of melt surface
65
6.2 Oxide Speciation During MgO Chlorination
65
6.2.1
Magnesium Chloride Analysis
65
6.2.2 Alkalimetric Titration
66
6.2.3 Quantitative Decompositon of MgOHCI
67
6.2.4 Infrared Spectrometry
68
6.3 Verification of the Kinetic Model for the Chlorination of MgO to MgOHCI
6.3.1
Linear Regression Analysis
69
69
7
Conclusion
72
8
Recommendations for Future Work
74
References
78
VII
List of Figures
Figure 1 : Magnola process flowsheet
5
Figure 2 : Schematic illustration of a version of the Alcan MPC [1]
9
Figure 3 : Solubility of MgO in the NaCI-MgCb system [24]
14
Figure 4 : MgO saturation mole fraction in the MgCI 2-MgF2 system at 840°C [25]
14
Figure 5 : Apparatus for the determination of hydrogen in dehydrated carnallite [26]
16
Figure 6 : Schematic alkalimetric speciation curve (adapted from [29])
19
Figure 7 : Sample HCI injection curve [11]
20
Figure 8 : Typical speciation curves
21
Figure 9 : Change in content of hydrolysis products in molten carnallite during
chlorination with HCI [33]
24
Figure 10 : Chlorination and dissolution rates of MgO in molten carnallite as
a function of rotation speed [34]
25
Figure 11 : Chlorination rate as a function of argon concentration in
HCI-Argon mixture at 500°C and 500 RPM [34]
26
Figure 12 : Typical curve of C~:o as function of time [5]
28
Figure 13 : Schematic of the HCI activity profile for the system "MgO-HCI-Molten Salt"
during the reaction of MgO with HCI to form MgOHCI
31
Figure 14 : Laboratory scale chlorination reactor schematic
36
Figure 15 : Top view of the baffled graphite crucible
37
Figure 16 : Schematic of the baffled graphite crucible and rotary gas injector [42]
38
Figure 17 : Detailed schematic of impeller (Ieft) and shaft (right)
39
Figure 18 : HCI solution injection curve during determination of total alkalinity
0
.47
Figure 19 : Sample Alkalimetric Speciation
50
Figure 20 : Schematic of a mull sample [46]
51
Figure 21 : Infrared spectrum of Fluorolube [46]
52
Figure 22 : Aspect of melt surface during chlorination
53
Figure 23 : Variation of MgO concentration as a function of time for
a typical chlorination experiment
54
VIII
Figure 24: Comparison between experimental and calculated MgCI 2 concentration
assuming direct chlorination of MgO to MgCI 2
55
Figure 25 : MgO and MgOHCI concentration as a function of time
56
Figure 26 : MgOHCI decomposition by dry nitrogen injection
56
Figure 27 : IR spectrum of a dehydrated MgCI 2·6H 20 sam pie containing
70-80
wt % MgOHCI
58
Figure 28 : IR spectrum of an oxide free melt..
58
Figure 29 : IR spectrum of a sample containing ",. 1 wt % MgOHCI
59
Figure 30 : CM90 as a function of time
61
Figure 31 : C~:o as a function of time
61
CM90 as a function of time
62
Figure 33 : Plot of residual values of C~:o as a function of time
62
Figure 34 : Effect of HCI flow rate on chlorination kinetics
63
Figure 35 : Effect of varying HCI flow rate on the value of Kn
64
Figure 32 : Plot of residual values of
Figure 36 : Variation of the degree of Mg(OHh saturation calculated for the data in
Figure 25
67
List of Tables
Table 1 : Energy consumption in electrolytic magnesium cells [8]
10
Table 2: Equilibrium concentration of MgOHCI in MgCI 2 KCI-NaCI melts
(KCI:NaCI
= 0.454 by weight) at 500°C
Table 3 : Solubility of MgO in selected molten chloride systems
12
13
Appendices
Appendix
1-
List of suppliers for consumables
75
Appendix Il - Particle size distribution of MgO powder
76
Appendix III - Sample Calculations
77
IX
1 Introduction
The preparation of anhydrous magnesium chloride cell feed is one the central steps in
the electrolytic production of magnesium. Moreover, the oxide content of the anhydrous
cell feed must be low in order to ensure a high current efficiency. Magnola Métallurgie
Inc. (Danville, Qc) currently uses a continuous stirred tank reactor into which hydrated
magnesium chloride prills are contacted with a molten salt under hydrogen chloride gas
atmosphere to dehydrate the prills. As the prills dissolve in the molten salts, any oxide
materials react with HCI to form magnesium chloride, leaving a magnesium chloride rich
molten salt with a low oxide content.
Initially, magnesium oxide was believed to be the only stable oxide in the melt and it was
assumed that the chlorination of magnesium oxide to magnesium chloride proceeded in
one step. However, the literature indicates that magnesium hydroxychloride is stable
when dissolved in the molten salt and that the reaction of magnesium oxide with
hydrogen
chloride
gas
might
proceed
through
the
formation
of
magnesium
hydroxychloride as an intermediate species.
Thus, there was a need to develop a fundamental understanding of the chlorination of
magnesium oxide with hydrogen ch/oride gas in a molten salt.
Experiments were
conducted with two objectives :
1. to prove the existence of MgOHCI as an intermediate species during chlorination of
MgO with HCI in a molten salt having a composition suitable for the production of
magnesium by electrolysis;
2. to develop a model and to measure the kinetics of MgOHCI formation during MgO
chlorination.
This thesis begins with Chapter Two, which is a Iiterature survey. The chapter begins
with an introduction to the electrolytic production of magnesium. Then, the Magnola
1
process is presented in some detail. The effect of oxide on cell performance is also
discussed. The last three sections present information required to develop a model for
the chlorination of magnesium oxide with hydrogen chloride gas : i)
hydrolysis product in molten salts, ii)
solubility of MgCI 2
techniques for the determination of MgCI 2
hydrolysis products, and iii) review of literature on chlorination of magnesium oxide by
hydrogen chloride in molten salts.
ln Chapter Three, a reaction mechanism for the chlorination of magnesium oxide is
proposed. A rate law is derived based upon mass transfer equations and simplifying
assumptions.
Chapter Four describes the apparatus for the experiments, experimental procedures and
analytical techniques. The experimental test program is designed to : i) confirm the
hypothesis presented in Chapter Three as to the mechanism of reaction between MgO
and HCI to form MgOHCI, and ii) measure the kinetics of reaction.
Chapter Five presents the experimental results.
Visual observations made during
chlorination experiments are presented. Then, the results of oxide speciation (Le. ratio
of MgO/MgOHCI) during chlorination obtained from i) MgCI 2 analysis, ii) alkalimetric
titration, iii) quantitative decomposition of MgOHCI, and iv) infrared spectrometry are
presented. Results from the linear regression analysis of the data are presented.
Chapter Six present an interpretation of the results of the testwork.
From the
experimental results, a rate law was obtained.
Conclusions from this research are presented in Chapter Seven. Recommendations for
future work are presented in Chapter 8.
2
2 Literature Review
2.1
Electrolytic Production of Magnesium
Electrolysis of magnesium chloride dissolved in a molten salt mixture accounts for 75 %
of the total magnesium production [1].
The remaining magnesium is produced from
metallothermic reduction of magnesium oxide 1 by ferro silicon.
The metallothermic
reduction is used commercially in the Pidgeon [2,3] and the Magnetherm 2 [4] processes.
Modern electrolytic processes are fundamentally similar and consist of two main steps :
1. Preparation of anhydrous MgCI 2 cell feed
2. Electrolysis
Anhydrous magnesium chloride can be produced by the carbo-chlorination of
magnesium oxide in the presence of carbon and impure magnesium chloride at a
temperature of 1000°C. This process is used by Norsk Hydro at its Porsgrunn (Norway)
plant. The main reactions taking place are:
MgO+ C + C/2 -7 2MgC/2 + CO
Eqn.1
MgO+CO+C/2 -7 MgC/2 +C02
Eqn.2
ln an other process developed by Norsk Hydro [5] and in operation at its Becancour (Qc,
Canada) plant, a purified magnesium chloride brine is concentrated to 50-55 % and
dehydrated in a series fluid bed dryer in the presence of air to a composition
corresponding to MgCI 2 ·2H 20. The final dehydration of the prills is accomplished in a
multiple tray dehydration tower in the presence of pure HCI.
1
Feed materials include magnesite(MgC02 ), dolomite (MgCOs·CaCOs), magnesia (MgO) and dolime
(MgO·CaO).
2
Pechiney ceased its operation late 2001
3
Magnesium chloride rich anhydrous feed can also be obtained from the dehydration of
carnallite (MgCI 2 ,KCI·6H 20) [6]. This technology was developed by the Russians.
involves dehydrating carnallite to 0.3-0.5 wt% water by heating it
furnace.
It
in an electrical
After dehydration, the carnallite contains 3 wt% MgO. The dehydrated
carnallite, mixed with ground petroleum, is continuously fed into the fusion chamber
where melting of the carnallite is accompanied by a partial hydrolysis of magnesium
chloride. The melt temperature in the fusion chamber is held at about 500°C. The melt
passing from the fusion chamber to the chlorination chamber contains magnesium oxide
and carnallite hydroxychlorides. A chlorine-air mixture is fed under pressure into the
lower zone of each chlorination chambers through tuyeres. The flow of chlorine stirs the
melt and reacts with magnesium oxide and carnallite hydroxychlorides to produce
magnesium chloride. The chlorinated carnallite contains 0.5-1.0% magnesium oxide.
Lower oxide content is achieved by gravity separation in a settling furnace and by
collecting the overflowing molten salt.
Theoretically, it would be possible to recover magnesium metal by electrolysis of pure
magnesium chloride.
However, this is impractical due to the high melting point and
volatility and low electrical conductivity of pure MgCb.
Moreover, the solubility of
magnesium metal in pure MgCI 2 is high enough to cause a significant decrease in
current efficiency. Therefore, the electrolysis is performed in a molten salt containing
MgCI 2 along with various additives, the most common being NaCI, CaCI 2 , KCI and CaF2 •
The electrolyte composition is adjusted such that its density is higher than the density of
liquid magnesium.
Therefore, the molten magnesium is collected at the top of the
electrolyte. Each magnesium producer operates its own type of electrolysis cel! due to
the proprietary nature of the industry. The I.G. Farben monopolar cell [7] which uses an
internai diaphragm to prevent
magnesium and chlorine recombination is used by
several producers. Diaphragm less cells are aise used by Norsk-Hydre [1] and VAMI 3
[6].
3
Vsesooyzny Alyuminiev
Magniyevy Institut (Ali-Union Aluminium and Magnesium Institute),
Leningrad, U.S.S.R
4
Ali the technologies that have been presented up to now have been proven to be
capable of producing magnesium with profitability. However, some of these processes
require massive capital investment whereas other require intensive maintenance.
Métallurgie Magnola Inc. (Danville, Qc) is exploiting a novel process for the primary
production of magnesium [8]. The process is innovative in the preparation method for
anhydrous magnesium chloride and uses high efficiency multipolar electrolysis cells.
The present research being related to the preparation of anhydrous feed in the Magnola
process, the next section describes the process in more detail.
2.1.1 The Magnola Process
Figure 1 shows a simplified version of the magnola process flowsheet.
Figure 1 : Magnola process flowsheet
:~~
...
=:.
~-:::-.K=_="'t---"I!ICll1Q­
+r-----+-----:lr---:a.
WCJIIlG
5
The process can be divided as follows :
1. Feed preparation
2. Leaching and purification
3. Brine dehydration
4. Melt chlorination
5. Electrolysis and casting
2.1.1.1 Feed preparation
The raw material for the Magnola process is serpentine (3MgO·2Si02·2H 20) derived
from the tailings of asbestos mining.
Typical composition of the raw material is
40% MgO, 38% Si02, 5% Fe compounds (Fe203 and Fe(OH)2), 13% H20 and varying
amounts of CaO and A1 203. The feed has a homogenous mineralogy, chemistry and
particle size distribution.
Therefore, the feed only needs to be passed through
a
scalping screen to remove lumps of material.
The screened feed is then slurried in recycle water and fresh make-up water. The slurry
is passed through a magnetic separator.
The benefits of magnetic separation are :
improved Mg/Fe separation, lower MgO requirement for neutralization and better
filterability of the leach cake.
2.1.1.2 Leaching and purification
The demagnetized fraction of the feed slurry is then fed to the leach circuit where it is
leached with a combination of hydrogen chloride gas and hydrochloric acid. Nagamori
et al. [9] have analysed the thermodynamic and technico-economic aspects of the HCIleach of serpentine.
The excess hydrochloric acid in the final stage is neutralized with MgO and the
unleached residue removed from the impure brine solution by filtration. The leach cake
is sent to a residue disposai pond.
6
The impure brine is treated by ch/orine injection under agitation to oxidize such
impurities as ferrous chloride, manganous chloride and nickelous chloride.
MgO and
50% caustic is added to precipitate the impurities in the form of hydroxides and to
neutralize the solution.
The precipitated impurities are then removed by filtration.
Further removal of nickel, boron and manganese is pertormed in an ion exchange
column.
The purified brine containing 350 9 MgCb/L is pumped to storage.
This
correspond to a magnesium extraction yield from the feed of about 80%.
2.1.1.3 Brine dehydration
A fluid bed drier is used to dehydrate the purified brine to produce free flowing granules
of hydrated magnesium chloride (MgCb·xH 2 0). The exact number of hydration is not
made public by Magnola. However, it is known that equilibrium dehydration magnesium
chloride in air occurs through the following series of steps [10] :
MgCI 2 ·6H2 0 -7 MgCI 2 ·4H 2 0 + 2H 2 0
Eqn.3
MgCb·4H 2 0 -7 MgCb·2H 20 + 2H 2 0
Eqn.4
MgCI 2 ·4H 20 -7 MgOHCI + HCI + 2H 20
Eqn.5
MgCI 2 ·2H2 0 -7 MgCb·H 20 + H2 0
Eqn.6
185-230°C
MgCI 2 ·2H 20 -7 MgOHCI + HCI + H2 0
Eqn. 7
Above 230°C
MgCI 2 ·H 20 -7 MgCb + H2 0
Eqn.8
MgCI 2 ·H 20 -7 MgOHCI + HCI
Eqn.9
MgOHCI -7 MgO + HCI
Eqn.10
95-115°C
135-180°C
According to Laroche et al. [11], MgOHCI is the predominant oxide species in the prills.
It is reported that the oxide level in the prills varies between 1 to 2 wt% expressed as
MgO. The concentration of impurities in the prills is below 0.7 wt% and mainly consists
of sodium and calcium chloride.
7
2.1.1.4 Melt chlorination
The partially dehydrated MgCI 2 prills are transferred to the "Super-Chlorinator", which is
an electrically heated (A.C.) solid-liquid-gas reactor. In this reactor, the prills are
contacted with a molten salt into which hydrogen chloride is injected. A sufficient HCI
partial pressure is maintained in the freeboard of the reactor to allow water of hydration
to flash off as steam while minimizing reaction of water with MgCI 2 dissolving into the
molten salt.
Ficara [5] studied the thermodynamic aspects of the dehydration of
magnesium chloride having different degrees of hydration dissolving in molten salts of
the ternary system MgCb-CaCb-NaCI.
The oxides that are initially present in the prills enter the melt and are chlorinated.
Depending on whether the oxide material is MgO or MgOHCI, the overall chlorination
reaction will be :
Eqn. 11
or
MgOHC/+HC/ ~ MgC/2 +H2 0
Eqn. 12
Sufficient residence time in the reactor produces a molten salt having an oxide content
low enough for electrolysis.
2.1.1.5 Electrolysis
The oxide free magnesium rich molten salt is fed to the Alcan Multi Polar Cell (MPC)
(Sivilotti [12,13]). The cell electrolyte is reported to contain MgCI 2 , NaCI, CaCI 2 and
MgF2 • Figure 2 shows a schematic of a version of the Alcan MPC.
The cell is divided in two compartments :
1. the metal compartment, into which the molten anhydrous magnesium chloride
containing feed is charged and from which liquid magnesium metal is tapped ;
2. the electrolysis compartment, where the electrolysis of MgCI 2 occurs between the
graphite bipolar electrodes.
8
Figure 2: Sehematie illustration of a version of the Alean MPC [1]
2
1
(1) Steel cathode, (2) graphite anode, (3) graphite bipolar electrodes, (4) liquid
magnesium, (5) submarine
Thermal regulation of the cell is achieved by a heat exchanger (not shown in the
schematic) which is installed in the metal compartment. The melt level is maintained
constant during feeding, tapping and electrolysis by varying the pressure in the
submarine.
The Alcan MPC is a high power-efficiency cell.
Table 1 compares the energy
consumption in various electrolytic magnesium cells.
The life of the cell is determined by the graphite consumption and by the accumulation of
sludge at the bottom of the cell.
Graphite consumption results from the reaction of
carbon with oxygen from air, moisture or oxide contaminants in the feed. Air ingress in
the cell is expected to be low owing to sealing arrangements. Therefore, the amount of
residual oxide in the feed determines the life of the cell.
9
Table 1 : Energy consumption in electrolytic magnesium cells [8]
Company
Cell
kW.h/kg of Mg
Technology
Dow Chemical 4
Dow
18.5
MagCorp
Russian Diaphragmless
13-15
Dead Sea Mag
Russian Diaphragmless
13-15
Porsgrun
I.G. Farben (Old)
12-13
Bécancour
I.G. Farben (New)
12-13
Alcan - MPC
10-12
Norsk Hydro
Magnola
Theoretical minimum
2.2
7.0
Effect of oxides on cell performance
The main impurities present in electrolytes for magnesium production are Fe, Si, AI, Mn,
Ni, Cr, B, sulfates, moisture, MgO and MgOHCI. These impurities are introduced in the
electrolyte with the feed or from reaction between the electrolyte and the cell
components, and affect both the cathodic and anodic process. A general discussion of
the effect of the listed impurities on electrolysis can be found in Strelets [6]. Among
these impurities, MgO and MgOHCI significantly affect the electrolysis process. These
oxides are introduced with the feed or from reaction between MgCb with moisture or air
in the atmosphere above the electrolyte.
MgO is known to form a passivation layer on the steel cathodes of the magnesium
electrolytic cells. Strelets [14] et al. showed that in the electrolytic production of Mg from
fused MgCiQ-KCI electrolyte MgO was deposited at the cathode as a black film
containing some iron. This caused passivation of the cathode surface and a decrease in
current efficiency. The loss of current efficiency was explained by the fact that the MgO
4
This process is no longer in use. The process used MgCI 2·2H 20 as the feed for the electrolysis cell
which resulted in low current efficiency
10
film made the cathode less wettable by metallic magnesium. Therefore, fine magnesium
droplets which were not easily recoverable from the electrolyte were formed on the
cathode. Zhemchnzhina et al. [15] observed that finely crystallized MgO was present on
the surface of the cathode when current efficiency was low.
Komura et al. [16] showed that liquid magnesium dispersion in MgCI 2-KCI melts was
stabilized by the presence of MgO. The stabilisation occurs through the formation of a
MgO coating on the surface of the magnesium droplets. Therefore, magnesium droplets
are prevented from coalescing and the fine magnesium droplets do not gain enough
buoyancy to be collected at the top of the electrolyte. Moreover, MgO (3.6 g/cm 3) being
denser than the electrolyte (typically <2 g/cm 3), the magnesium droplets can become
heavy enough to settle at bottom of the cell.
MgO also affects the anodic process by reacting with chlorine and carbon at the anode.
Haarberg et al. [17,18] studied the anodic behavior of CaO and MgO in melts of their
respective pure chlorides. The reaction mechanisms were found to involve a positively
charged complex species that migrated and dissociated at the anode.
The overall
chlorination of MgO at the anode is given by the following overail reaction :
2MgO+ 2C/2 + C ~ 2MgC/2 + CO2
Eqn. 13
Eqn. 13 causes the carbon from the anode to be consumed, leading to an increase in
the anode to cathode distance. Consequently, the ohmic resistance between the anode
and the cathode is increased which in turn increases the specific power consumption.
MgOHCI is thought to be present in molten chloride in the form of MgOH+ ions. The
hydroxyl ion can be reduced at the cathode according to the reaction :
Eqn. 14
11
The MgO thus formed will precipitate at the cathode, possibly forming a passivation
layer.
2.3 Solubility of MgCI2 hydrolysis product in molten salts
MgOHCI and MgO are the two hydrolysis products expected in melts containing MgCI2.
According to Ivanov [19], magnesium hydroxychloride dissociates in the electrolyte into
MgOH+ and
cr
ions.
Other states of the hydroxychloride are improbable, for the
following reasons :
1. Melts of KCI and NaCI without MgCb do not undergo hydralysis.
process
cr
+
H20~OH-
Therefore, the
+ HCI cannot lead to formation of a substantial amount of
hydroxyl ions. Consequently, the hydraxyl group is strongly bonded to magnesium
and does not form an independent anion
2. Mixed complexes such as [MgOHCI 2L [MgOHCb] 2- must be very unstable, owing to
counterpolarization of chlorine by protons of the hydroxyl group and to the sharp
difference in the nature of the chlorine-magnesium and oxygen-magnesium bonds.
Savinkova [20] measured the equilibrium concentration of MgOHCI in MgCI 2-KCI-NaCI
melts at various pH 20/pHCI ratios and MgCI2 concentrations. Selected results fram this
work are shown in Table 2.
Table 2 : Equilibrium concentration of MgOHCI in MgCI2KCI-NaCI melts
(KCI:NaCI 0.454 by weight) at 500°C
=
PHCI, atm
PH20, atm
MgCI 2, mole %
MgOHCI, mole %
MgCI2 activity
0.113
0.427
32.40
1.56
0.0130
0.148
0.324
39.88
1.96
0.0293
0.275
0.325
53.03
2.51
0.1001
Adapted from [20]
12
MgOHCI is also known to decompose when present in excess to saturation. Savinkova
[21] studied the crystallisation of MgO from MgOHCI in hydrolyzed MgCI2-KCI melts. In
the experiment, preliminary dehydrated carnallite (1.7 wt% H20, 7 wt% MgOHCI) and
pure KCI were melted together and the melt was maintained at constant temperature.
MgOHCI concentration was measured over time. The MgOHCI concentration decreased
as a result of thermal decomposition followed by crystallisation of MgO. Typical
equilibrium concentration of MgOHCI was approximately 1.5 wt% for a melt containing
35.8 wt% MgCI2.
The solubility of MgO in molten chlorides is negligible compared to the solubility of
MgOHCI. Table 3 shows the solubility of MgO in selected molten chloride systems.
Table 3 : Solubility of MgO in selected molten chloride systems
Temparature,
MgO solubility,
oC
XMgO
NaCI (pure)
850
0.51x10-(
Inyushkina [22]
KCI (pure)
820
1.99x10-(
Inyushkina [22]
0.5 NaCI, 0.5 KCI
727
2.0x10-0
Combes [23]
0.06 MgCb, 0.12 CaCb, 0.82 NaCI
850
2.2x10-b
Boghosian [24]
Composition, mole fraction
Reference
Boghossian et al. [24] measured the solibility of MgO in the system NaCI-MgCI2. The
solubility of MgO was found to increase with MgCI2 concentration, as shown in Figure 3.
13
Figure 3 : Solubility of MgO in the NaCI-MgCI2 system [24]
0,004
1
C
a
0.003
a
j
\t
C
c
'08
0.OQ2
(0) NaCI-MgCI2• T .. 730°C;
(0) NaCI-MgCI2• T 830 oc;
=
8
0
0
0,001
0
C
C
0,000'
0.0
rdJdJc
0.2
0.4
0.6
0.8
1.0
)('MgC12
The increase in MgO solubility with the MgCI 2
concentration was explained by the
formation of an oxide complex according to the following reaction :
Eqn.15
Mediaas et al. [25] measured the solubility of MgO in the system MgCb-MgF2 . Figure 4
shows the saturation mole fraction of MgO as a function of MgF2 mole fraction.
Figure 4 : MgO saturation mole fraction in the MgCI2-MgF2 system at 840°C [25]
0.012 ,....,......-......,.....................P"""T"".............,..'T""""...........'"'T1i"""""'.........,..,
0.01
0.004
o.ooz
OI.....l-.&...I.....t...J.-'-'.....t....Jl..L.oI-I-.........L..L~.........~oI-I-......
o
0.1
0.4
0.5
14
From Figure 4, the MgO saturation mole fraction remained constant at approximately
0.0065 up to XMgF2 equal to 0.05 and increased linearly for XMgF2 greater than 0.05.
According to Sharma [26]
and Strelets [6], industrial electrolyte for the production of
magnesium does not require a concentration of fluoride higher than 1.5 wt% on a MgF2
basiss. Thus, the increase in solubility due to fluoride addition should be negligible for
industrial electrolyte.
2.4 Techniques for the determination of MgCI2 hydrolysis products
2.4.1 Volumetrie determination of hydrogen
This method is also referred to as the magnesium method. It is based on the fact that
MgOHCI or other hydroxyl containing compounds react with incandescent magnesium to
give off hydrogen.
MgOHCI and its decomposition product, HCI, react with Mg
as
follows:
2MgOHCI(s) + Mg(s)
-7
2MgO(s) + MgCI 2(s) + H2(g)
2HCI(g) + Mg(s)
-7
MgCI 2 + H2(g)
Eqn.16
Eqn.17
Vilnyanskii et al. [27] devised a method in which the volume of hydrogen produced when
a sample of dehydrated carnallite was contacted with incandescent magnesium was
measured. The experimental apparatus for the determination of hydrogen in dehydrated
carnallite is shown in Figure 5.
The first step consisted of charging magnesium shavings into the reaction tube. Then,
the furnace was turned on and hydrogen was blown through the apparatus in order to
displace air from the system.
The hydrogen blow was also needed to saturate the
magnesium with hydrogen. This was necessary because the solubility of hydrogen in
magnesium is high.
Data from Bakke et al. [28] indicate that the solubility of hydrogen
in magnesium varies between 25 to 30 cm 3 H2/100 9 Mg.
Hydrogen saturation was
continued for approximately 2.5 hours. After reaching a temperature of about 600°C the
5
Most metal fluorides (MF x) undergo an exchange reaction with MgCI 2
(x-1)MF x + MgCI 2 ~ (x-1)MCl x + MgF2
15
level of fluid (75% H2 S04 ) in the buret was set at zero and valves b) and f) were closed.
The carnallite sampie was then poured from the test tube into the reaction tube. The
volume of gas evolved was read on the buret and converted to hydrogen content in the
carnallite. The analysis itself, without considering the saturation period, took about 3
minutes. A relative errer of 4 % was obtained when replicates of dehydrated carnallite
samples were analysed.
Figure 5: Apparatus for the determination of hydrogen in dehydrated
f
9
a
d
carnallite [26]
a) Hydrogen supply, b) Inlet valve, c) Test tube, d) Reaction tube with Mg shavings
e) Furnace, f) Purge Valve, g) Buret
The obvious drawback of this method is that it is sensitive to moisture.
Magnesium
would also react with moisture from the sample according to the following reaction :
Eqn.18
16
Thus, moisture contamination would lead to an overestimation of the hydrogen content
of the sam pie. This was exemplified in the article by the fact that the greater than 3 mm
fraction of a carnallite sample gave a 1.0% hydrogen concentration while for the less
than 0.6 mm fraction from the same sample, the analysed Hydrogen was 0.4%.
Vilnyanskii [29] developed another volumetrie method of determining the hydrogen
content of dehydrated carnallite. In this other method, aluminum was used instead of
magnesium. The solubility of hydrogen in solid aluminum is about 0.25 cm 3/100 g AI
which is much lower than in solid magnesium. Therefore, it was possible to reduce the
saturation period from 2.5 hours to 40 minutes.
2.4.2 Alkalimetric Speciation
Savinkova et al. [30] first proposed a method to overcome the problem of moisture
contamination encountered with the volumetrie determination of hydrogen in dehydrated
carnallite. The method was developed on the basis that when MgOHCI is dissolved in
water, it forms Mg(OH)2 according to the following reaction :
2MgOHCI(s) ~ Mg(OHh(s) + MgCI2(aq)
Eqn.19
Eqn. 19 assumes that the MgOHCI concentration is such that the concentration of
Mg(OH)2 exceeds its solubility limit.
Since MgOHCI was a dissolved species in
carnallite, its dissolution in water would lead to the precipitation of very fine Mg(OH)2
partieles.
The specifie surface of the fine Mg(OHh precipitates would significantly
exceed the specifie surface of MgO which is practically insoluble in carnallite.
Thus,
when the carnallite containing MgO and MgOHCI was dissolved in water and titrated
with an acid, there was two stages of acid consumption. The first stage was associated
with the Mg(OH)2 from the dissociation of MgOHCI that rapidly reacted with the acid.
The second stage consisted in the slow reaction of MgO with the acid. The MgO and
Mg(OHh partieles were assumed to react with the acid following a shrinking partiele
modal.
17
Although it was not clearly stated, it is important to understand that the second stage of
acid consumption consisted in the reaction of the acid with Mg(OH)2 from the hydrolysis
of the MgO particle. Fruhwirth et al. [31] studied the dissolution and hydration kinetics of
MgO in water. It was shown that MgO hydrolysed in water as follows :
Eqn.20
The hydrolysis of MgO (Eqn. 20) is a slow reaction. Thus, the rate Iimiting step in the
reaction of MgO with the acid was not the reaction with Mg(OH)2, but rather the
hydrolysis of MgO. This becomes clearer when one considers the technique that was
used.
The technique first consisted in determining the total alkalinity of the carnallite sample
expressed as MgO equivalent (MgOeq ). Then a mass of carnallite equivalent to 0.1 9 of
MgOeq was dissolved in water.
The sample was
titrated with 0.1 N H2S04 in the
presence of methyl red indicator6 at a rate that provided a constant slightly red colour.
Initially, the rate of acid addition was high due to the fast reaction of Mg(OH)2 from
MgOHCI. Then, the rate of acid addition was adjusted such that the pH would remain
constant, close to pH 5. The amount of acid which was added to the sam pie was
recorded as a function of time and converted to an equivalent content of reacted
magnesium oxide.
The amount of MgOeq left unreacted was then computed and
(MgO eq) 1/3 was plotted as a function of time.
Figure 6 shows a schematic alkalimetric
speciation curve.
6
Transition range (pH) : 4.2 (red) ta 6.3 (yellow)
18
Figure 6 : Schematic alkalimetric speciation curve (adapted from [29])
-
~
T""
C)
c
c
'co
E
~
i
oC)
:lE
o
th
th
ta
:lE
time
Figure 6 shows that 3 distinct zones were observed on the titration curves, The initial
slope (1) corresponded to the reaction of Mg(OH)2 from MgOHCI. The second part of
the curve (2) was explained by the fact that the MgO was present as polydisperse
particles. Finally, the last part of the curve represented the hydrolysis of MgO. The
content of MgO in the sampie was obtained by extrapolating the second segment (3) of
the curve to the vertical axis of the graph. The validity of the method was checked
against the magnesium method.
The relative error between the two methods was
reported to be below 3% for MgO and below 15 % for MgOHCI, both results being
reported as MgOeq •
Laroche et al. [11] improved the alkalimetric speciation method by monitoring the pH
with an indicator electrode and an automatic titrator. In their version of the method,
0.05 N HCI was used as the titrant. The sample was first analysed for its total alkalinity
expressed in wt % MgOeq according to the following reaction :
Eqn.21
Part of the same sample was weighed to contain approximately 7 mg MgOeq • The
sample was dissolved in 100 cm 3 of water and homogenised. At the beginning of the
19
titration, the HCI solution was rapidly injected into the solution to reduce the pH to 5.00
without overshooting by manual control of the injection rate of the HCI solution. Once a
pH of 5.00 was reached, the injection rate of HCI solution was automatically adjusted by
a control unit such that the pH would remain constant at 5.00.
The titration was
continued while maintaining a pH of 5.00 for more than ten minutes. A curve of injected
HCI as a function of time was then obtained as shown in Figure 7.
Figure 7: Sample Hel injection curve [11]
9.0.....--------------·~n~~-
.-~
.. .,.4PkW
e6.0+-7F-------------------~!
E
~
-
l
Ü
:a:
t
li
~
!
13.0 .~.--.- . . -.~- .....----.-------.'"--...-.j
;
aO+---,
- - r - - - - - - , r - - - -__--....,..,_
omm
~
m
_.~
~'.
i1
4
W
Tne(lrirt
From the HCI injection curve, the mass of MgOeq remaining at any time was computed.
The cubic root of the mass of MgOeq remaining was plotted against time.
Figure 8
shows typical speciation curves.
20
Figure 8 : Typical speciation curves
......._._...._._.._...._---_.._--_._----.-_.._-------...__.._ _. _ - - - j
1
2.00 . , - - - - - .
~
!
"iD
E 1.50 -I+------.:::~::__-------------------..........,
ti
•
li
~
l1.00
+ + - - \ - - - - - - - - -.-
-
-.----.- .
!
f
!
1
~
!
---.....J
11gb MgOHCIIMgO (ratio -il 000)
=
. .:::.,--=-=-=-=-=:::::::J
..::.~~:==:~~::.:;;Wet;;.AI;.:Oll;lde;.:ge::M~I'lI;;:tId=...
:drD;;;;IylIt;
.....;.;;;...;;;;;:._==_::.;.;;••_:.:.-.:=:;.::.;,:
...::.;,:
...
0.50
l1li90 zone (fer 1i!ll1i1Ki èllamplfl)
0.00 + L - - - - - r - - - - r - - - - - - r - - - - - - r - - - - - - , . - - - - - - - - J
o
5
10
15
20
25
30
Tine (min)
Figure 8 provides a proof of concept. The fiat curve represents the result of the titration
of a MgO doped electrolyte. Since the sam pie contained only MgO, a straight line was
expected. The other curve represents the result obtained for a sam pie of molten salt in
which wet argon was slowly injected. Injection of wet argon was believed to lead mainly
to the formation MgOHCI. Hence, two distinct reactions zones were expected as shown
in Figure 8. The MgO content of the sampie was obtained by extrapolating the tangent
to the MgO reaction curve to time zero. The method was intended to be semiquantitative.
2.4.3 Carbothermal Reduction
Vinstad [32] used this technique for the determination of MgO and MgOHCI in
hydrolysed
MgCI 2 and MgCI 2-NaCI melts. It was assumed that the only hydrolysis .
products were MgO and MgOHCI.
Thus, the total oxygen content and the base
21
equivalent oxygen content were connected to the content of MgO and MgOHCI as
follows:
tot
no
bas
no
=
=
n MgO
n MgO
+ nMgOHCI
1
+-
2
nMgOHCI
Eqn.22
Eqn.23
Solving Eqn 22 and 23 simultaneously gave the MgO and MgOHCI content of the
sam pie.
The total oxygen content (n~t )of the sample was determined using a LECO TC-436
oxygen and nitragen determinator. The analysis began with the fusion of a sample kept
in a tin capsule in a graphite crucible. The oxygen in the sample reacted with the
graphite from the crucible to form CO. The CO was oxidised to CO2 in a CuO tower.
The amount of CO2 formed was determined by infrared absorption spectrametry. Since
this method determined ail the oxygen present in the system, it made the method
sensitive to moisture. Water absorbed by the salt sample could react directly with the
graphite to form CO or react with MgCI 2 to form MgO or MgOHCI. Consequently, it was
not possible to distinguish between contributions fram absorbed water and from oxides
already present in the sampie. Therefore, the following precautions had to be taken in
order to minimise water absorption:
1. samples were kept and handled in a diffusion tight glove box with low water content;
2. equipment used for sample preparation were heated at 150°C and transferred in the
glove box while hot;
3. pieces of material of more than 20 mg were analysed in order to minimise the
surface of the sample.
22
The base equivalent oxygen content (no baS) was determined by iodometric titration. First,
the sample was dissolved in a known amount of HCI according to the following reaction :
Eqn.24
Excess HCI was then reacted with KI03 an KI according to the following reaction :
Eqn.25
Is-
was titrated with sodium thiosulphate in presence of soluble starch as an indicator,
the titration reaction being :
Eqn.26
Substituting the value of n~t and
ngas in Eqn. 22 and 23 respectively, and solving
simultaneously gave the number of moles of MgO and MgOHCI present in the sample.
2.5
Chlorination of magnesium oxide by hydrogen chloride in molten salts
Lukmanova et al. [33], studied the chlorination of molten carnallite containing MgO and
hydroxyl compounds assumed to be MgOHCI.
It was observed that the hydrogen
content of the melt increased as the chlorination proceeded.
To explain this
observation, it was proposed that an intermediate species containing an hydroxyl group
formed during the chlorination of MgO.
Since MgOHCI was known to be stable in
molten carnallite, it was suggested that the chlorination of suspended particles of MgO in
molten carnallite by HCI proceeded in two steps :
MgO + HGI -7 MgOH+ + GI-
Eqn.27
Eqn.28
23
The presence of MgOHCI as an intermediate reaction product was also proven by
decomposing it by passing nitrogen through the melt. The decomposition reaction was
the following :
MgOHG/(d)
~
Eqn.29
Mgqs) + HG/(g)
Eqn. 29 indicates that MgOHCI will exist in the melt under sufficient partial pressure of
HCI. The activity of HCI in a freshly formed inert gas bubble being zero, HCI would
diffuse to the gas-liquid interface where it would finally diffuse inside the bubble. The
HCI is then carried out of the melt with the inert gas bubble.
Eqn. 30 shows the overail
reaction for HCI elimination from the melt.
Eqn.30
Savinkova et al. [34] also studied the chlorination of hydrolyzed carnallite.
Molten
carnallite containing 1.2% MgOHCI and 5.6% MgO was chlorinated with HCI. Figure 9
shows the change in content of hydrolysis products in molten carnallite during
chlorination with HGI.
Figure 9 : Change in content of hydrolysis products in molten carnallite during
chlorination with HCI [33]
6
j
~
.i
M
..
(3
::J:
0
Cl
:E
...0
0
Cl
:E
80
f2f)
160
Time, min
1- MgO
2- MgOHCI in MgO equivalent
24
Figure 9 shows that MgO concentration decreased rapidly fram 5.6 to 0.6% while
MgOHCI concentration dropped sharply during the first ten minutes and remained
constant at 0.5% up to a time of 60 minutes. This behaviour indicated that MgOHCI was
forming from the reaction of MgO with HCI.
The rotating disk method was used by Stupina et al. [35] to study the reaction
mechanism of the chlorination of magnesium oxide by hydrogen chloride in molten
carnallite. This method is useful to study diffusion controlled process. Under laminar
flow over the disk surface, the specific rate of a reaction that is controlled by diffusion
processes is given by the Levich [36] equation :
Eqn.31
Figure 10 shows the dependence of the rate of chlorination and dissolution on the
rotation speed of the disk.
Figure 10 : Chlorination and dissolution rates of MgO in molten carnallite as a
function of rotation speed [34]
A
A
1
7
8
6
'1
5
6
~
S
J
2
--
1
°5
1- Chlorination, T = 550 oC
2- Chlorination, T = 800 oC
3- Dissolution, T = 750 oC
li
'1
•
8
9
J
e.-
"
J
Z
10 11 B 1
A) Chlorination or dissolution rate, (g/cm 2 ·sec).1 06
B) Square raot of angular velocity, sec- 1/2
25
The linear dependence between the chlorination or dissolution rate and the square root
of the angular velocity indicated that both processes were controlled by diffusion The
following chlorination mechanism was proposed :
1. Dissolution of HGI in the melt
2. Transfer of HGI by convention and diffusion to the reaction front
3. Dissolution of MgO
4. Interaction of dissolved MgO and HGI
5. Removal of the reaction products into the melt volume
Even though it was not stated in the article, the relatively large difference between the
rates of dissolution and chlorination of MgO indicated that the reaction front was more
likely to be at the surface of the disk. In the same work, the influence of dilution of
hydrogen chloride with argon on the rate of chlorination was studied as shown in
Figure 11.
Figure 11 : Chlorination rate as a function of argon concentration in Hel-Argon
mixture at SOO°C and SOO RPM [34]
A
5
J
2
--...&....._-..1-_---10._ _""'"--_.:;..
o
20
40
EO
60
A) Ghlorination rate, (g/cm 2·sec).10 6
8) Argon content in the HGI-Argon mixture, Vol. %
26
Lowering of the chlorination rate with decreasing content of hydrogen chio ride in the gas
lead to the conclusion that the chlorination of MgO was controlled by the diffusion of HGI
in the molten salt.
ln US Patent 5565080 [37], the chlorination of MgO with HGI in a molten salt containing
magnesium chloride for the production of an anhydrous feed for magnesium electrolysis
is described. The chlorination was studied in a stirred tank reactor and was found that
chlorination proceeded at an average rate of 6.42 mol MgO/h in a melt containing initially
1.82 wt% MgO.
The most recent research on the kinetics and mechanism of the chlorination of MgO by
HGI was performed by Ficara [5]. In this research, the chlorination of suspended MgO
particles (d avg
=
5 JlI11) in a ternary molten salt containing MgGI 2-NaGI-GaCb
was
studied in an agitated batch reactor. A rate law was derived based upon the following
assumption :
1. MgO was insoluble in the molten salt
2. Reaction occurred at the particle surface:
Mgqs) + 2HC/(d)
-7
MgC/2 (d) + H2 0
3. Mass transport of HGI from the bulk of the liquid to the MgO particle was
rate Iimiting
Assuming a shrinking particle particle model, the following relationship was obtained for
the concentration of MgO as a function of time :
Eqn.32
Kinetic experiments were performed in order to validate the proposed mode!. According
to Eqn. 32, a linear relationship was expected between C~:o and time f, if the proposed
model was valid. Figure 12 shows a typical curve of C~:o a a function time.
27
Figure 12 : Typical curve of c~go as function of time [5]
1.8...-----------------------,
1.6
1.4
.•.... " .•. " .. " •.. ,.,., .• ,.,
~
..... 1.2
...
S2
••
....•••••••• , ••.•••••• , , .• , ,
,
.
, .. ,
.
••
··,·,·····'·····fi·'····,·····,··,·······,·········
,
• , .. ,
.
,
.
'
t
,
. . . . . . . . . ,
,
,
.. •
~, il'
~
, . ,
.
•
0.6
. . . . . . • . . . . . . . . . . . . . . . . . . • . • " ,., '• . , ••.. , .•. , •. " ..
0.4
•• ,.,., ••. , ...•..•..• " ......••..•...••••...•.•..•.
•
•••••
0.2 +---+--+--+-I-+--t--+--i-+--+--+--+-+-+--+--t-'
240
210
180
90
120
150
so
30
o
T1me (min)
From Figure 12,
it is seen the linearity was verified for the first 200 minutes of
chlorination. The concentration remained almost constant for the next 35 minutes of
chlorination.
The detection limit of the analytical method used for the determination of
the MgO concentration was reported to be 0.056 wt%. Therefore, only the data from the
first 200 minutes of chlorination were considered in the regression analysis. Thus, their
was some uncertainty regarding the shape of the curve in the low MgO concentration
region. The effect of MgCb concentration, temperature, agitation and gas flow rate on
the value of km, which represented the conversion rate per unit area of particle surface,
was also investigated. It was observed that temperature and MgCI 2 concentration had
little effect on km whereas increased agitation and gas flowrate lead to higher value of
km, From these observations, it was concluded that the system was controlled by mass
transfer of HCI dissolved in the molten salt to the MgO particle.
28
3
Proposed Reaction Mechanism for MgO chlorination
The present study concerns the reaction of HCI gas with MgO that is suspended in a
molten salt. The HCI gas is introduced into the molten salt via a submerged nozzle thus
giving rise to bubbles of HCI that pass upwards through the salt. Solid MgO is weil
wetted by molten salt with the result that MgO particles are dispersed in the salt. MgO is
sparingly soluble in most alkali chlorides (Section 2.3) with the result that reaction of
MgO with HCI must occur at the particle surface.
The solubility of HGI in the pure molten chlorides comprising the salt used in this study is
lower than 50 ppm [38,39,40]. Therefore, the solubility of HCI is the molten salt mixture
is expected to be low. The form of dissolved HGI is not known, so for the present
purposes, it was assumed that the HCI was dissolved as a molecule that is transported
through the salt by diffusion and convenction.
The literature suggests that the transformation of solid MgO to MgGI2 dissolved in the
molten salt may proceeed through an intermediate species, namely MgOHGI, which is
soluble in the molten salt. The reaction between MgO and HCI being :
Mgqs) + HC/(d)
~
MgOHC/(d)
Eqn.33
The MgOHCI then reacts with HCI to form MgGI2 which is dissolved in the molten salt.
This second reaction is not to be considered in the present study.
Eqn. 33 represents the overail reaction mechanism for the chlorination of MgO with HCI
to form MgOHCI. However, this reaction must occur through a series of individual mass
transfer steps. The Nernst film theory [41 cited in 42] is useful in the modelling of mass
transfer between different phases.
In this theory, it is postulated that the entire
resistance to mass transfer is in a thin, stagnant region of the liquid phase at the
interface, called a film.
In the present model, the MgO particles are assumed to be
29
spherical and surrounded by a liquid film of thickness 8
rp
•
On the other hand, modelling
of liquid-gas mass transfer is best achieved using the two-film theory of Whitman [43
cited in 41]. According to this model, a stagnant gas film exists on one side of the HCI
bubble/molten salt interface and a liquid film exists on the other side with the controlling
factors being molecular diffusion through each of the films.
ln order for a HCI molecule to reach the MgO particle surface and react there, it passes
through the following steps :
81.
Transport in the bulk gas phase of the bubble to its surface;
82.
Transport across agas side boundary layer at the molten saltlbubble interface;
83.
Dissolution
84.
Transport across a liquid side boundary layer at the molten salt bubble/ interface;
85.
Transport in the bulk molten salt;
86.
Transport across a second liquid side boundary layer at the liquid/MgO particle
interface;
87.
Chemical reaction at the surface of the MgO particle
MgO(s) + HCI(g) ~ MgOHCI(ads,s)
88.
Desorption and dissolution of MgOHCI
MgOHCI(adS,S) ~ MgOHCI(d)
Chlorination is performed with pure HCI, therefore no concentration gradient exists in the
gas phase and, consequently, resistance in the gas phase was absent. Thus, HCI
bubbles were assumed to be spherical and surrounded by a Iiquid film of thickness 8rb
and 81 was assumed not to be rate limiting. Also, 83 was assumed not to be rate
Iimiting because the process was carried out at high temperature where molecular
diffusion is favoured. The fact that the molten salt was weil agitated makes it possible to
eliminate 85 as being the possible rate limiting step. For the same reason and because
MgOHCI is soluble in the molten salt, 88 was assumed not to be rate limiting. Hence, no
char formed on the surface of the MgO particles and the solid shrank during the course
of the reaction and ultimately disappeared.
30
From the arguments above, S4, S6 and S7 were left as being the possible rate limiting
step. Figure 13 shows a schematic of the HCI activity profile for the system "MgO-HCIMolten Salt" during the reaction of MgO with HCI to form MgOHCI.
Figure 13 : 5chematic of the HCI activity profile for the system "MgO-HCI-Molten
Salt" during the reaction of MgO with Hel to form MgOHCI
a Bubble
HGI
Bulk
a HGI
MgO(s) + HCI(g) -7 MgOHCI(ads,S)
MgOHCI(adS,S)
-7
MgOHCI(d)
Surface
aHGI
The following expressions are obtained for the rate of mass transfer of HCI for S4 and
86:
• orb
Bubble
N HGI = k L AP (aHGI
BUlk)
aHGI
Eqn.34
sUrface)
aHGI
Eqn.35
-
• orp
Bulk
N HGI = k SL AP (aHGI
-
Assuming that the reaction between MgO and HCI is first order with respect to the
activity of HGI at the partiele surface, the following expression was obtained for the mass
transfer rate of HCI due to chemical reaction :
• Surface
N HGI
-
k R AP aSurface
HGI
Eqn.36
31
Eqns. 34 to 36 state that the mass transfer rate of HGI across a given interface is
proportional to an activity gradient multiplied by a proportionality constant and the
surface area of the interface. However, if the praportionality constant is rearranged to
the denominator then the mass transfer rate of HGI can be expressed as a function of a
gradient and a resistance to mass transfer. For example, the mass transfer rate of HGI
acrass the Iiquid film surrounding the HGI bubble can be expressed as :
• orb
=
N HGI
(aBUbble _ aBulk )
HGI
HGI
=
(aBUbble _ aBUlk )
HGI
HGI
1
Eqn.37
QL
kLA b
At steady state, the rate of each individual steps is the same and from the stoichiometry
of Eqn. 33 the following equality is obtained :
• orb
NHGI
• orp
•
Surface
= NHGI = NHGI
=
dNM9Q
dt
Eqn.38
Mass transfer resistances can be combined to give an expression for the mass transfer
rate of HGI fram the HGI bubble to the surface of the MgO particle.
dNMgQ
_
Bubble _ a sUrface)
(aHGI
HGI
dt
Eqn.39
The activity of HGI at the bubble/liquid interface is unity since HGI is a pure gas. It can
be assumed that the activity of HGI at the surface of the particle will be much lower than
unity. Therefore, the numerator in Eqn. 39 can be set equal to 1. Substitution of the
respective expressions for the mass transfer resistances in Eqn. 39 gives :
dNMgQ
dt
_
1
Eqn.40
32
The previously derived equation takes into account the mass transfer of HCI from a
single bubble to a single particle. However, MgO will be present in the molten salt as
discrete particles. Thus, by multiplying both sides of Eqn. 40 by the number of MgO
particles present in the melt the following expression is obtained :
Eqn.41
Since MgO is measured in term of concentration, it is convenient to rearrange Eqn. 41
as follows:
Eqn.42
dt
Using the density of the melt and the molecular weight of MgO, Eqn. 42 can be
expressed in term of the concentration of MgO in ppm by :
dCMgo
dt
_
Eqn.43
Eqn. 43 can be further simplified by expressing the following constant for a melt
containing a given amount of MgO particles :
6
k = _1x1_0_--:nP,-M_M--=g_o
5
PmVm
Eqn.44
33
Substitution of ks in Eqn. 44 gives :
dCMgo
_
dt
Eqn.45
The total surface area of HCI bubbles will be a constant for a given temperature, gas
flow rate and impeller rotation speed. However, no direct measurement of HCI bubble
sizes were available for the melt studied, and consequently, the bubbles total surface
area will be left as a constant in Eqn. 45.
ln Eqn. 45, the surface area of the MgO particles is a function of the radius of the
particles.
As MgO is converted to MgOHCI, the particles shrink and the surface
available for reaction diminishes. The surface area of the MgO particles must be
expressed in term of MgO concentration? for integration of Eqn. 45. Assuming a
spherical particle shape, the surface area of a single MgO particle is given by :
Eqn.46
Assuming a monodisperse powder, the initial number of particles is obtained from the
initial MgO concentration using the following expression:
Eqn.47
By rearranging Eqn. 47 an expression for the radius of the MgO particles as a function of
concentration is obtained :
Eqn.48
7 It should be noted that this is not a concentration per so, but rather a mass density since MgO is not
dissolved. However, it was believed that such nomenclature would be confusing.
34
Combining Eqn. 46 and 48 gives the surface area of a single particle as a function of
MgO concentration:
c7g0 J2/3
(c (r{f
Eqn.49
Ap = 41t
MgO
Regrouping the constants in Eqn. 49 gives :
Eqn.5D
Where k p =
41t(r~ )
i
(CM90 )
Eqn.51
2/3
Substitution of these expressions in Eqn. 45 gives the differential form of the rate law :
dCMgo _
Eqn.52
dt
Separating the variables and integrating using the initial condition CMgolt=o
= C~90
gives
the following expression for the concentration of MgO as a function of time :
1 ( CMgO - Ci)
1 (1
1)(MgO
C )1/3
1 (k A
-1(
MgO +1(
k+j(
rot
s
L
b
P
SL
-( CiM90) 1/3
)J = t
Eqn.53
R
From Eqn. 53, the linear dependence between MgO concentration and time for the three
possible rate controlling-steps are:
t
84.
Transport of HGI across the liquid film surrounding HGI bubble : CM90
S6.
Transport of HCI from the bulk molten salt across the liquid film surrounding a
MgO particle HCI to the surface of the MgO particle : C~:o
S7.
oc
0<:
t ;
Reaction between MgO and HCI at the surface of the MgO particle : C~:o
oc
t.
35
4
4.1
Experimental
Laboratory Scale Chlorination Reactor
Ali testwork was carried out in a laboratory scale chlorination reactor. The apparatus,
which will be referred hereafter as the chlorinator, is a batch reactor. A general layout
schematic of the installation is shown in Fig. 14. It has three main components:
1. fumace and reaction vessel
2. rotary agitation system
3. gas handling system.
Figure 14 : Laboratory scale chlorination reactor schematic
Gas Line
Rotary Union
Hollow shaft
Melt Thermocouple
f
__ ArgOn
Purge
Insulation
c::::::;==~JI=-
Glowbar
Furnace
Impeller
Furnace
Thermocouple
Graphite
Crucible
The chlorinator components are described in the following sections.
36
4.1.1 Furnace and Reaction Vessel
Ail the experiments required
a charge of solid chlorides and fluorides to form the
synthetic electrolyte to be melted prior to the chlorination experiment. Melting was
achieved in a specially designed graphite crucible. The crucible, containing the charge,
was placed
in a "glow bar" type resistance fumace (GNA Industrial Fumace lnc).
Heating was achieved by 12 silicon carbide glow bars (1 Squared R - 5.20Q).
The
temperature was controlled via a stainless steel sheathed type K thermocouple (Omega)
located in the heating zone of the fumace and a Partlow MIC 2000 On/Off control 1er. In
addition to the fumace temperature, the temperature of the melt was monitored using a
stainless steel sheathed type K thermocouple (Omega) protected by an alumina tube
(Vesuvius) immersed in the melt. The thermocouple assembly was connected ta a Cole
Parmer 89500-10 display.
Fiberfrax Duraboard was used to cover the top of the
fumace. lnsulation was completed by a 1" thick Fiberfrax Durablanket. A removable lid
was also made from Fiberfrax Duraboard in order to have easy access to the melt.
The molten material was contained inside a baffled cylindrical graphite crucible (Speer 890S). Graphite was used because it is one of the few materials that resists HGI attack
in chloride melts. The crucible, including the baffles, was machined from a single block
of graphite. Figure 15 shows a photograph of the baffled graphite crucible.
Figure 15 : Top view of the baffled graphite crucible
37
The graphite crucible was contained in Morganite EX 552 silicon carbide crucible.
Magnesia refractory was rammed in the space left between the two crucibles. The use of
an outer crucible was necessary to protect the graphite from oxidation and because the
molten salt slightly seeped through the graphite.
4.1.2 Rotary Gas Injector
Agitation and dispersion of the gases in the molten salt was achieved using a graphite
(Speer - 890S) rotary gas injector [42]. Figure 16 shows a general schematic of the
baffled graphite crucible and rotary gas injector.
Figure 16 : Schematic of the baffled graphite crucible and rotary gas injector [42]
3
/
~--2
.1
The rotary gas injector (1), including impeller (2) and shaft (3) was located at a
distance (C) from the bottom of the crucible. The impeller had diameter (0) and
width (W). Aiso shown in Figure 16 is the height (h) that the molten salt was above the
upper part of the impeller and total bath height (H). For a 7.5 kg charge of salt and a
38
distance between the lower part of the impeller to the bottom of the crucible of 5 cm
(C
= 5 cm), the value of "h" and "H" were respectively 18 and 27 cm. The position of the
assembly corresponding to a 5 cm distance was initially determined with the empty
crucible in place. Figure 17 shows a detailed schematic of the impeller and shaft.
Figure 17 : Detailed schematic of impeller (Ieft) and shaft (right)
s
~'-Sb
Both, impeller and shaft were made fram graphite (Speer-890S). The impeller was 3.0"
(7.6 cm) in diameter and 1.5" (3.8 cm) thick. Gas was delivered via 9 holes of 0.125"
(3 mm) in diameter, located at 0.5" (1.3 cm) from the bottom of the impeller.
The
impeller was attached to a 1.5" (3.8 cm) diameter shaft, 3D" (76 cm) long, with a
threaded end of 8 turns per inch of a length of 1" (2.54 cm)and a 5/16" (8 mm) diameter
central bore.
The graphite shaft was joined to a hollow steel shaft using a steel coupling. The steel
shaft was mounted vertically on two self centering pillow block bearings spaced 22 cm
39
apart. The steel coupling was machined from a steel cylinder. One end was threaded to
connect to the steel shaft. The other end was a cylindrical sleeve with an internai a-ring
seal. The graphite shaft was inserted inside the sleeve, forming agas tight connection.
A clamp secured the graphite shaft to the coupling and insured concentricity of the rotary
injector. Gas was injected through the rotary injector via the steel shaft using a rotary
union (Oeublin model 1205-000-003)
The steel shaft was driven by a 1 hp electric motor (Pacifie Scientific) connected via a
pulley and belt assembly. The motor rotation speed was adjusted using a Ratiotrol
speed controler (Incom International Inc). The entire driving mechanism and the rotary
injector assembly were built ante an stand that could be adjusted for height. An electric
winch raised or lowered the impeller in the crucible.
4.1.3 Gas Handling System
Experiments required the injection of HGI and nitrogen separately in the molten salt.
Three separate 316 stainless steel 3/8" 00 (9.5 mm) gas lines were connected to a tee
union. A third gas line was connected to the cross union and directed to a flow meter.
Since only one gas was needed at a time, only one flow meter was necessary to
measure the flow rate through the rotary injector. A Gilmont (model GF-5541-230)
flowmeter with glass bail was used to measure the flow rate. Precise control of the gas
flow rate was achieved using a needle valve mounted at the bottom of the flowmeter. A
calibration table was generated for the two gases using the GF-4000 Ver. 1.0. software
from Gilmont.
The interior of the furnace was purged with argon to protect the salt from excessive
moisture absorption and oxidation during melting. The argon was delivered in the free
board of the crucible through a 1/8" ID (3 mm) dense alumina tube (Vesuvius). The flow
rate to the purge was controlled using a Gilmont (model GF-5541-230) flowmeter with
glass bail.
40
4.2
Chlorination Experiments
4.2.1 Melting
The first step of a chlorination experiment consisted of preparing 7.5 kg of solid salt with
a composition of
~
35 wt% MgCI 2 , 2 wt% KCI, 1.5 wt% CaF2 and balance NaCI/CaCI2
with mass ratio of 2.2. This composition was chosen based on the information that
DOW [44] used these concentration of additives in their electrolytic cell. It was decided
arbitrarily to use a concentration of 35 wt% MgCI 2 • The salt could not be charged ail at
once in the crucible due to its low bulk density. It was important to blend the salt to
obtain an homogeneous charge in order to ensure efficient melting since the liquidus for
the present salt composition was approximately 450 oC as compared to 801 oC, the
melting point of NaCI alone. Once the salt was blended, approximately 2/3 of the salt
was charged in the crucible.
Once the first portion of salt was charged in the fumace, the top of the fumace was
c10sed with the Duraboard lid and the argon purge was installed in the freeboard. The
fumace temperature was set at 550 oC and the argon flow rate was set at approximately
2.5 SLPM s. Once the salt in the crucible was completely molten (after approximately
1.5 hours), the balance of the charge was carefully added using a scoop. After a total of
approximately 2-3 hours, the charge was completely molten and the bath thermocouple
was inserted in the melt.
4.2.2 Pre-chlorination
Chlorination experiments required the initial molten salt to be free of any oxide. Even
though the free board of the crucible was purged with argon during melting of the salts,
the initial charge contained some moisture and, consequently, the melt contained a
relatively high concentration of oxides. For this reason it was necessary to initially
chlorinate (Le. pre-chlorinate) the melt with HCI to convert these oxides to chlorides.
Once the temperature of the melt had reached 550°C, the electrolyte was ready to be
41
chlorinated. As a first step, the lid was removed from the furnace and the rotary gas
injector was lowered at a distance of 111 above the top of the melt. The rotary injector
was allowed to sit above the melt in the furnace for at least 2 minutes to drive off any
moisture that could result in splashing of the molten salt. Then, HCI was injected into
the electrolyte through the rotary gas injector at a constant flow rate of 5 SLPM. The
rotary injector was further lowered in the electrolyte and the distance between the lower
part of the impeller and the bottom of the graphite crucible was adjusted to 211 (5 cm).
The agitation was started and set at 500 RPM.
4.2.3 Degassing
HCI is believed to have a limited solubility in the molten salt since the solubility of HCI in
alkali chlorides entering the composition of the present molten salt is lower than 50 ppm
at their melting temperature [36,37,38]. The presence of HCI in the molten salt would
interfere in the determination of total alkalinity since HCI is an acid.
Therefore, any
residual HCI needed to be removed from the melt prior to verifying the oxide
concentration. Thus, after 60 minutes of pre-chlorination, HCI was turned off and argon
was injected in the melt to remove HCI. The argon flow rate during degassing was set
at 2 SLPM and the rotation speed at 200 RPM. It was not necessary to remove the
impeller from the melt before switching from Hel to argon since the operating
temperature was weil above the liquidus temperature making the impeller unlikely to
plug.
After 10 minutes, the argon was turned off and the impeller was stopped and raised
above the surface of the melt. A sampie of approximately 20 9 was drawn from the melt
using a pin tube attached to a rubber bulb. The upper portion of the sample (Le. near
the bulb) was allowed to solidify before the extremity of the pin tube was removed from
the melt. This was done in order to avoid the risk of sucking some molten salt in the
bulb. Then, the pin tube was quickly removed from the melt, turned upside down and
gently f1icked in order to evacuate any entrapped gas.
The sampie was analyzed to
solidify and cool before it was analyzed for its total alkalinity (Section 4.5.1). If the total
8
Standard liter per minute at 70 oF, 1 atm
42
alkalinity was lower than 10 ppm MgO eq , the melt was ready to perform the chlorination
experiment. If not, HGI injection was continued for an extra 20 minutes, after what the
melt was degassed and its total alkalinity verified again.
4.3
Determination of reaction kinetics
The first step was to adjust the HGI flow rate to the desired value for the experiment.
This was done with the impeller raised in the freeboard of the crucible. The HGI valve
was fully opened and the flow rate was adjusted to the desired value using the needle
valve on the flow meter. Once the flow rate was properly set, the HGI valve was e1osed.
This needed to be done to obtain reliable data in the early stage of chlorination (0-10
minutes).
The determination of reaction kinetics was performed by the integral method of analysis
[45] in which the MgO concentration of the molten salt was measured as a function of
chlorination time. A melt containing approximately 1 wt% of MgO was prepared by
adding MgO powder having an average partiele size of 2 J.lIT1 to the molten salt. The
particle size distribution of the MgO powder is shown in Appendix 1.
Gold MgO did not
mix easily with the electrolyte because it tended to agglomerate at the surface of the
molten salt. For this reason the rotary gas injector was used to achieve a good mixing.
Prior to MgO addition, the rotary gas injector with argon flowing through it at 1 SLPM
was lowered in the melt. Then, approximately 70 9 of dry and decarbonated MgO were
added using a large pyrex funnel to avoid material spill or loss to the ventilation system.
Once ail the MgO was added, the melt was agitated at 200 RPM for 5 minutes. Such a
low rotation speed was chosen in order to avoid foaming. Preliminary experiments had
shown that the presence of MgO in the molten salt made it prone to foam excessively to
the extent of overflowing the crucible.
At the end of the 5 minutes mixing period,
agitation speed was adjusted to 500 RPM in view of the kinetic test and then stopped
immediately.
One sam pie was immediately taken in order to obtain the initial MgO
concentration of the melt.
43
As mentioned before, special care was taken in order to have decarbonated and
moisture free MgO powder. This needed to be done because the MgO that was used in
these experiments was indicated by the supplier to contain moisture. Weight loss at
1000 oC was reported to be 10 wt%. Also, magnesium carbonate can form by reaction
of MgO with CO2 from air. Any moisture in the MgO lead to the formation of MgOHCI in
the molten salt when the MgO was added at the beginning of the test. This situation
needed to be avoided since ail the present interpretation of the data was based on the
assumption that MgO was the only oxide species in the melt prior to HCI injection. For
these reasons, MgO was heated in an electric resistance furnace at 550 oC for at least 3
hours prior using it in a chlorination experiment. This was deemed sufficient to remove
ail the absorbed water and to decompose any MgC03 that may have formed since its
decomposition temperature is about 350 oC.
After that the MgO addition was completed, the rotary injector was lowered in the melt
as shown in Figure 3. The agitation was started. HCI was turned on and the stop watch
was started. The electrolyte was sampled over time as follows : sample each minute for
the first 10 minutes and then at 15, 20, 30, 45, 60, 75, 90 min. Samples were removed
from the pin tubes and stored in tightly sealed plastic bottles.
The test was terminated by stopping agitation and raising the rotary gas injector into the
freeboard. Then HCI was turned off and the gas line was purged with argon to prevent
corrosion in the event of moisture ingress into the systemduring the downtime. Finally,
the electrolyte was ladled out of the graphite crucible using a steel cup and poured into
an alumina crucible where it was allowed to solidify. The solid block of salt was
discarded.
44
4.4
Quantitative decomposition of MgOHCI
ln Section 2.5, it was mentioned that MgOHGI dissolved in a molten salt was
decomposed by bubbling nitrogen through the melt.
The decomposition and HGI
removal reactions were as follows :
MgOHG/(d)
--7
Mgqs) + HG/(g)
Eqn.54
Eqn.55
If the inert gas was injected for a sufficiently long period of time, ail the MgOHCI would
eventually be decomposed to MgO.
Since the alkalinity of the melt is measured as
MgO eq , and that MgOHGI corresponds to 0.5 MgO eq on a molar basis, an increase in
alkalinity would be measured as decomposition proceeds.
Thus, a single experiment was designed to quantitatively decompose MgOHCI in the
molten salt. It consisted of preparing a melt containing MgOHGI by chlorinating a molten
salt containing 1 wt% MgO. The HGI flow rate was adjusted to 5 SLPM and agitator
speed was set at 500 RPM. The kinetic model developed in the present work predicted
that under these conditions, in a melt containing initially 1 wt% MgO, ail the MgO would
be converted to MgOHCI in about 15 minutes. Therefore, HGI injection was stopped
after 25 minutes. At this time, any alkalinity in the melt was assumed to be entirely due
to the presence MgOHCI. The impeller was raised into the freeboard and the melt was
sampled to determine its initial alkalinity.
Then, dry nitrogen was injected through the impeller at a flow rate of 0.5 SLPM. The
impeller was slowly lowered in the melt and agitation speed was set at 175 RPM.
The
use of dry nitrogen was required because any moisture present in the gas would cause
the melt to hydrolyze. Also, slow agitation speed and low gas flow rate were used in
order to avoid foaming. Foaming was observed in previous experiments where argon
was injected in melts containing MgO and/or MgOHGI under vigorous agitation. The
melt was sampled over time using pin tubes and the melt alkalinity was determined.
45
Assuming that the only oxide species present initially was MgOHCI, the expected total
alkalinity of the melt when ail MgOHCI had decomposed was given by Eqn. 56.
f
i
Eqn.56
CMgOeq = 2 CMgOeq
The factor 2 in Eqn. 56 arises from the stoichiometry of the titration reactions of MgO
and MgOHCI, MgO requiring twice as much HCI as compared to MgOHCI on a molar
basis, for an equivalent amount of MgO.
4.5
Analytical Techniques
4.5.1 Determination of Total Alkalinity
ln the present work, a technique that takes benefit of an automatic tiration system was
used. The technique involved the precise determination of the pH of a blank sample of
water. The deionized water that was used throughout this work was found to have a
blank pH of 5.00. This pH was selected as the end point value for the titration. Any
alkalinity in excess of pH 5 was assumed to result from the contribution of MgO or
MgOHCI present in the sampie.
Thus, samples from the chlorination experiments were analyzed by titrating with 0.05N
aqueous HCI solution. The samples were first broken in small pieces. Approximately
1.5 9 of salt was precisely weighed in a polypropylene beaker using a Sartorius Model
A200S analytical balance. The sample was dissolved in 100 ml of deionized water.
Titration was carried out using a METROHM automatic titrator. The system comprised a
microsyringe feeding unit Model 665 Dosimat.
Data acquisition was performed via a
Model 686 Titroprocessor processing unit. A Model 674 sampie changer along with a
Model 664 control unit allowed series analyses to be performed automatically. The HCI
solution feeding to the sampie was executed in 3 steps as shown in Figure 18.
ln the first range (start feed), the feeding rate was increased continuously up to the
maximum feed rate determined by the operator.
Then, the HCI solution was
continuously fed (continuous feeding) to the sample at the maximum feed rate. Once
the pH had reached a value of 5.00, the HCI solution was injected as discrete pulse
46
(pulse range). The next pulse was only injected if the measured drift was less than the
set "drift" in mV/s. The total volume of Hel solution was recorded and converted to
MgO eq using the following equation :
CMgQ
= 0.0806 VHC1
M HC1
Eqn.57
msample
Bq
Figure 18: Hel solution injection curve during determination of total alkalinity [46]
U/rrN
UCE~
u,
VI-
1
.1
1
1
1.
.,.
.1
1
1
1
1
1
1
1
1
1:gt
aI.-
ota
g.s
i!"a
tOI -1
-
- s"CI
If
&or!
VI_
u
47
4.5.2 Determination of MgCI2
Samples from the chlarination experiments were analyzed for MgCI 2 concentration by
titrating with 0.5N aqueous NaOH solution. The titration reaction was :
MgC/ 2(aq) + 2NaOH(aq)
--7
Mg(OH)2(S) + 2NaC/(aq)
Eqn.58
The samples were first broken in small pieces but not crushed to a fine powder in arder
to minimize moisture pickup. Approximately 3 9 of salt were weighed precisely in a
polypropylene beaker. The sampie was dissolved in 100 ml of deionized water. The
titration was performed using the METROHM automatic titration described in Section
4.5.1
The analysis was done by step wise addition of NaOH solution to the sampie.
The titration was stopped automatically when the pH of the solution had reached a
value of 12.5. The equivalence point for this titration was located around pH 11.2. The
exact equivalence point was determined by the processing unit using a procedure based
on the Fortuin method and adapted by METROHM.
4.5.3 Alkalimetric Speciation
This technique was previously described in Section 2.4.1. The analysis was performed
using the METROHM automatic titrator.
The processing unit was connected to a
computer via a RS232C data transfer interface. Titration progress was monitored on the
computer screen via a labView V graphical interface. This allowed the operator to easily
adjust the injection rate. The volume of HCI solution injected as a function of time was
recorded and the data analyzed to obtain the MgOHCI and MgO concentration of the
sample.
Samples were prepared by grinding in an agate mortar to obtain a fine powder. Part of
the
sample was then analyzed for its total alkalinity.
corresponding to approximately 7 mg MgOeq of alkalinity
A mass of the sample
was weighed
in a
48
polypropylene beaker using a precision balance. The sample was dissolved in 100 ml
of deionized water by gently agitating the beaker for less than 10 seconds.
The titration needed to be started as soon as possible to minimize hydrolysis of MgO.
The titrant was an aqueous -0.05 M HGI solution. The titration procedure included a
30 seconds homogenization period prior to HGI injection. The time interval between
water addition and the beginning of the titration was always less than 40 seconds. Data
acquisition and HGI injection were started at the same time. At the beginning of the
titration, HGI was added rapidly a rate of approximately 5 mUmin to reduce the solution
pH to 5 without overshooting. This was achieved through operator experience. Once a
pH of 5 was reached, the HGI injection rate was reduced to a value that kept the pH
constant for at least 10 minutes.
The volume of HGI injected as a function of time was converted to remaining alkalinity
using Eqn. 59.
Eqn.59
The factor9 in front of the second term of Eqn. 59 arises from the stoichiometry of
Eqn.21.
Then, the remaining alkalinity was plotted as a function of time to give the alkalimetric
titration curve. Figure 19 shows a typical alkalimetric titration curve. The end point for
MgOHGI titration was determined by extrapolating the tangent to the MgO reaction curve
to time zero according to Figure 8. From the respective stoechiometry of the reactions
of MgO and MgOHGI with HGI during the alkalimetric titration, the conversion factor 1D
9
10
1L . 40,3 9 MgO .1000 mg =20.15 mg·L
1000 mL 2 moles HCI
9
mole
2 mole HCI/ mole MgO x 76.8 g/mole MgOHCI
1 mole HCI/ mole MgOHCI
40.3 g/mole MgO
= 3.81 9 MgOHCI
9 MgOeq
49
between the mass MgO eq consumed in the first step of the titration and MgOHCI was
found to correspond to 3.81 9 MgOHC/
9 MgOeq
Figure 19 : Sample Alkalimetric Speciation
2,0..---------------------;
~1'5~
o
or::n
mMgOeq
::::E
U)
CIl
~
1,0
c
~
:;;:
r::n
c
c
0,5
,~---
~ mMgOeq
EP
'co
E
Q)
a:
_
0,0
-!-----,-----r----.,....-----,.----i
10
5
°
15
20
25
Time, min
Thus, fram Figure 19, MgOHCI and MgO concentrations of the initial sampie were given
by:
C•
- 381 x
MgOHCI -
o
EP
mMgOeq - mMgoeq
•
Eqn.60
msample
· _
CMgO -
EP
mMgoeq
Eqn.61
msample
50
4.5.4 Infrared Spectrometry
Selected samples from the chlorination kinetics experiments were analyzed using
infrared (IR) spectrometry. The objective was to verify if the hydroxyl group of MgOHCI
could be identified using IR spectrometry. The samples were prepared using the mulling
technique described below. IR spectra acquisition was performed using a Bruker IFS66
spectrometer. The system comprised a DTGS detector equipped with a KBr window.
This type of window enable to cover the mid-infrared range from 4000 to 400 cm- 1•
Data treatment was performed using OPUS V.2.2 software.
Smith [47] presented a
detailed review of the application of infrared spectrometry to the analysis of inorganic
materials.
The mulling technique is an easy and fast qualitative technique that has been used in
quantitative analysis in the pasto The technique consists of mixing the powdered material
to analyze with a few drops of oil (the mulling agent). The mixture is ground further in an
agate mortar to disperse the solid in the oil. The sample/oil slurry is then smeared on a
KBr window, and a second KBr window is squeezed against the first to form a thin film
sandwich. Figure 20 shows a schematic of a mull sample.
Figure 20: Schematic of a mull sample [46]
IR Beam
/KBrWindow
51
ln this experiment, Fluorolube was used as the mulling agent. Fluorolube is a mixture of
chlorofluorocarbons, long chain alkanes where the CH bonds have been replaced by
C-F and C-CI bonds. Thus, the absorption peaks of Fluorolube are located in the low
wavenumber region of the IR spectrum. The choice of Flurolube was then obvious since
hydroxyl absorption peaks are expected around 3800 cm- 1• The infrared spectrum of
Fluorolube is shown in Figure 21. Note the absence of absorption above 1500 cm- 1•
Figure 21 : Infrared spectrum of Fluorolube [46]
80
10
60
20
10
b
4000
~~.""''-'''-~
3500
3000
'.
'
••
,
2500
••
1
•
1 ••
2000
...-.,.......- ... -
1500
·...f -l··....-.. ....1..~
1000
llaYel\U!lber' kit-tl
As a first step, approximately 0.17 9 of sample was weighed exactly on an analytical
balance. The sample was placed in an agate mortar and 3 drops of Fluorolube were
added. The mixture was ground for 2 minutes in order to obtain a good dispersion of the
solid in the oil.
A small amount of the solid/oil mixture was smeared at the center of a
25 mm x 4 mm KBr disk (Spectratech). Then, a 0.025 mm plastic spacer was placed on
the KBr disk and a second identical KBr disk was firmly squeezed against the first. The
sam pie was installed on a sample holder which was mounted in the spectrometer to
perform the spectrum acquisition.
52
5
5.1
Experimental Results
Chlorination Experiments
5.1.1 Observation of melt surface
It was observed during a chlorination experiment that the surface of the melt changed
from a uniformly agitated surface to a foamy surface after a certain time. Figures 22 a)
and b) show a picture of the melt surface after 3 and 10 minutes of HCI injection during a
chlorination experiment.
Figure 22 : Aspect of melt surface during chlorination
Temperature
= 550°C, HCI flowrate = 5 SLPM,
a)
After 3 minutes of chlorination
b)
After 10 minutes of chlorination
Agitator speed
= 500 RPM
Legend : 1 - Crucible wall, 2 - Agitated and gassed electrolyte, 3 - Agitator shaft
53
5.1.2 Oxide Speciation During MgO Chlorination
5.1.2.1 Magnesium chloride analysis
The analysis of the variation in magnesium chloride concentration in the melt during
chlorination allowed to determine if MgO formed an intermediate species prior to forming
MgCI 2 • From the stoichiometry of Eqn. 11, which is the direct chlorination of MgO to
MgCI 2 , the expected increase in MgCI 2 concentration, if there was no intermediate
reaction, is related to the decrease in MgO concentration by the following equation :
Eqn.62
Therefore, the expected MgCb concentration at any time if no intermediate reaction
occurred was given by :
Eqn.63
The change in MgO concentration in Eqn. 63 was computed from the data of the kinetic
tests. Figure 23 shows the variation of MgO concentration (reported as MgO eq ) as a
function of time for a typical chlorination experiment.
Figure 23 : Variation of MgO concentration as a function of time for a typical
chlorination experiment
12000
10000
E
8000
!1
6000
Q.
Q.
0
4000
2000
Tlme, mIn
Temperature
= 550°C,
HCI flowrate
= 2.5 SLPM,
Agitator speed
= 500
RPM
54
Figures 24 compares the experimental MgCb concentration with the calculated value
using Eqn. 11 with the data from Figure 23.
Figure 24 : Comparison between experimental and calculated MgCI2 concentration
assuming direct chlorination 11 of MgO to MgCI2
32,1
31,9
31,7
:::R
0
~
~
31,5
ü
Cl
::E
ü
31,3
31,1
30,9
0
2
4
6
8
10
Time, min
--+- Calculated tram Eqn.62 using data tram data in Figure 23
Il
Experimental
5.1.2.2 Alkalimetric Titration
Alkalimetric titration was used to determine the amount of MgO and MgOHCI in the
electrolyte as chlorination proceeded.
Figure 25 shows the evolution of MgO and
MgOHCI concentration as a function of time for the kinetic test presented in Figure 23.
11
95.3 ppm MgC/2 x
40.3 ppm MgO
100 = 2.36x1 0- 4 wt%MgC/2
1000000
ppm MgO
55
Figure 25 : MgO and MgOHCI concentration as a function of time
12000
CI'
i
c0
10000
--+-MgO
_MgOHCI
8000
~
...cl'!!
CIl
u
6000
c
0
u
CIl
"C
4000
'>Ci
0
2000
0
0
10
20
30
40
50
60
Time, min
5.1.2.3 Quantitative Decomposition of MgOHCI
Figure 26 shows the change in alkalinity of a melt initially containing MgOHCI when dry
nitrogen was injected in the bath.
Figure 26 : MgOHCI decomposition by dry nitrogen injection
2500,......-------------....,
2000
--+- Decomposition of MgOHCI
E 1500
_
2:
J
Dry nitrogen injection in an
oxide free melt
Cl
:a: 1000
500
o ~=-===;:::::II::::=!::;:::===:~--J
o
100
50
150
200
Time, min
56
The initial amount of MgOHCI was produced
by adding 1 wt% of MgO particles to the
molten salt and chlorinating for 25 minutes under a HCI flow rate of 5 SLPM and agitator
speed of 500 RPM.
A blank run was also performed in which dry nitrogen was injected
in an oxide free melt to obtain a baseline value of the alkalinity of the melt under the
actual experimental conditions.
5.1.2.4 Infrared Spectrometry
Many attempts were made in order to identify MgOHCI by infrared spectrometry.
Selected IR spectra are presented in this section.Figure 27 shows the IR spectra of a
sample of MgOHCI 12 (70-80 wt% MgOHCI, bal. MgCI 2 ) obtained by dehydrating
MgCb·6H 20. Figure 28 shows the spectra of a melt containing no oxide. This spectrum
corresponded to the background signal. Selected samples from the chlorination
experiments were analyzed with IR spectrometry. The spectrum of a sample containing
approximately 1 wt% MgOHCI concentration, determined using the alkalimetric titration
techniques, is shown in Figure 29.
12
The material and the ehemieal analysis were supplied by Noranda Teehnology Centre Ine.
57
Figure 27: IR spectrum of a dehydrated MgC12-6H20 sample containing
70-80 wt % Mg OHCI
...
Ill)
~-+---r--'--...,....---r-..--r----r--r----r---r----r------.----r----r-.....I-
4CDI:AlJ36œ32lll300021ED2BOO2260DIO
Figure 28 : IR spectrum of an oxide free melt
_ _~_ _.L.-_---L_ _.-l--....
~~--'-_.l...-....L----L_.J..---L.---I_..L-_--IL...-_--L.
Ill)
rd
a
uS
~-+--....-r---r---r-......---r---.--.------'r-----r---"'---..-----.----.-.....J-
'!OOO:AlJ 36œ3250 3000 27&1 2BOO 22502000
58
Figure 29: IR spectrum of a sample containing
z
1 wt % MgOHCI
~~--r-...---'---.-"""--....---'...---.----..----....----r---"'---r----....-...I-
1!II03?&O:BI)32IiD3Ol102i'Sl2II022Sl2lJlO
59
5.2
Verification of the kinetic model for the Chlorination of MgO to MgOHCI
5.2.1 Linear regression analysis
From the MgCI 2 analysis during the early stage of chlorination, it was assumed that MgO
chlorination proceeded exclusively by the formation of MgOHCI, following a shrinking
particle model, during the early stage of chlorination.
Therefore, the kinetic data
(MgO eq) from the chlorination experiments were converted to actual MgO concentration
(CMgO). In a melt containing initially only suspended MgO partieles, if the only reaction
occurring is the formation of MgOHCI from MgO, the decrease in actual MgO
concentration is related to the decrease in the total alkalinity (MgO eq) by the following
equation:
Eqn.64
Therefore, actual MgO concentration at any time was given by :
Eqn.65
Raw kinetic data and actual MgO concentrations, calculated from Eqn. 65, are
presented in Appendix Il for different HCI flow rates.
Graphs of
C MgO
= t(t) and
C~:o = t(t), as shown in Figure 30 and 31, were plotted and analyzed for the strength of
the correlation between the two variables. The value of
C MgO
at any given time was
computed from the total basicity ( CMgOeq) assuming that the only reaction taking place in
the first ten minutes of chlorination was Eqn. 33.
60
Figure 30 : CM9Q as a function of time
10000
9000
8000
r
7000
E
c.
c.
j
2
= 0,9663
0.9337 < r < 0.9957
6000
5000
4000
3000
2000
1000
0
0
2
4
6
8
10
12
Tlme, min
Figure 31
C~:o as a function of time
25,-------------------..,
20
l\l
E 15
Q,
Q,
~ = 0,9914
51
ô
r
Q.
10
0.9829 < Irl < 0.9989
5
O+----r-----r----..----r----r------l
4
o
2
6
8
10
12
lime, min
61
Figure 32 and 33 show the corresponding plot of residual values 13 for Figure 30 and 31.
Figure 32 : Plot of residual values of CMgo as a function of time
1000,.......-----------,
800
cl
ë5
600
400
200
iii
:::J
'C
~
O+---"u----.--------,--------,----Jij'-----j
-200
-400
-600
-800
-1000
• + ...
+
-1...-
o
---'
8
2
10
Figure 33 : Plot of residual values of C~;o as a function of time
0,6
,•
0,4
J0
iii
0,2
+
+
...
.. •
0
:::J
'C
'gj -0,2
a:
..
•
•
-0,4
-0,6
0
2
.1N2 ",IN3
+IN1
•
...
..
....
•
•
'Il
..
•
...
+
4
• ••
I~
...
6
8
10
Tlme, min
5.2.2 Effect of Hel flow rate on reaction kinetics
The regression analysis indicated that the rate controlling-step was the mass transfer
from the liquid to the surface of the solid or by surface reaction of the HGI with the MgO
since a plot of C~;o versus time provided a better correlation (to be further dicussed).
13
Residual
= (Experimental Value - Regression Value)
62
Therefore, the effect of HGI flow rate on reaction kinetics was explored to determine if
the reaction was controlled by mass transfer fram the liquid to the surface of the solid
MgO particle or by the reaction of HGI with MgO at the surface of a solid MgO partiele.
Figure 34 shows the effect varying HGI flowrate on the reaction kinetics when C~:o
versus time was plotted.
Figure 34 : Effect of Hel flow rate on chlorination kinetics
22
---r--------------,
20
HGI flow rate (SLPM): • 2.5
~ 18
E
• 7.5
... 5.0
a.
Co
12-
16
~
rf 14
12
10 -!---..,.---.,----,-----,-----,--1
o
2
6
4
8
10
12
Time,min
According to Eqn. 53, the slope of the curve shown in Figure 34 must be divided by the
corresponding initial MgO concentration to the power 1/3 to to be a measure of the
magnitude of the liquid-solid mass transfer coefficient (kSL )' This arises because the
slope in Figure 34 is equal to
kk(_1_+_1_J.
k
k
s p
SL
Therefore, it is proportional to the
R
praduct kskpwhich is in turn proportional to (C~gO)1I3. Since the initial concentration of
MgO varies slightly for each test, this needs to be taken into account. The resulting
constant is labelled Kn• In Figure 34, the last data point on each curve corresponds to
the last sample taken before MgCI 2 began to form. Therefore, extrapolation of the curve
to zero concentration gives the time for complete reaction of MgO.
63
Figure 35 shows the effect of varying the HGI flow rate on the value of Kn•
Figure 35 : Effect of varying Hel flow rate on the value of Kn
--------=------,
0,100 .........
0,080
0,060
'(1)
ë
~ 0,040
0,020
0,000 +---.,...--r----r----,..-----l
246
8
10
°
Hel flow rate, SLPM
64
6 Interpretation of Results
6.1
Observation of melt surface
Observation of the melt surface provided some indications that the physical properties of
the melt changed over time during a chlorination experiment.
At the beginning of the
chlorination, the surface of the melt was a uniformly agitated surface and free of any
foam.
After 10 minutes of chlorination, the surface changed to a completely foamy
surface. This was taken as an indication that the physical properties of the molten salt
changed over time. The reaction between MgO and HCI to form MgOHCI in this period
of time could have gradually changed the system from a solid-liquid-gas system to a
liquid-gas system since MgO is almost insoluble in the molten salt whereas MgOHCI is
soluble. Also, this was an indication that water was evolved from the melt only after a
certain period of time leading to the formation of foam at the surface of the melt.
Unfortunately, no data on the foaming behaviour of electrolyte gassed with HCI and
H2 0 were found in the literature.
6.2
Oxide 5peciation During MgO Chlorination
6.2.1 Magnesium Chloride Analysis
The change in MgCI 2 concentration of a melt during chlorination was shown in
Figure 24.
The error bars on the experimental measurements represents the 95%
confidence interval obtained from three replicated samples taken from the same melt.
The results indicated that MgCI 2 concentration remained constant for the first ten
minutes of chlorination. Moreover, the expected increase in MgCI 2 concentration in the
case of direct chlorination of MgO to MgCI 2 is shown to be statistically different from the
experimental MgCb concentration. From these results, it can be concluded that MgO is
forming an hydroxyl containing compound, most likely to be MgOHCI during the first step
of the chlorination mechanism.
65
6.2.2 Alkalimetric Titration
The results of alkalimetric titrations peformed on selected samples from the chlorination
experiments were presented in Figure 25.
The MgO concentration was found to
decrease continuously as a function of time whereas the MgOHCI concentration initially
increased and reached a maximum of 10700 ppm MgOHCI at 15 minutes. This was in
agreement with the results of the MgCI 2 analysis since the MgCb concentration
remained constant and, therefore, no significant reaction of MgOHCI with HCI occured
during this period of time.
It was pointed out in Section 2.4.2, that there is no way of determining the accuracy of
this method and the results from the alkalimentric titration are used qualitatively. One of
the major concerns with this method was that it was developed on the basis that
Mg(OH)2 particles formed in the water upon dissolution of a sample were relatively sm ail
compared to MgO particles. These assumption would need to be verified a priori before
performing the analysis, which was impossible. The size of the Mg(OHh particles will be
dependent on the degree of "supersaturation,,14 of Mg(OHh with respect to its solubility
in water (o:z7 mg/L). The degree of supersaturation is defined as follows :
Eqn.66
Since the alkalimetric titration is performed with an amount of sampie corresponding to a
constant alkalinity of 7 mg MgO eq , the Mg(OHh supersaturation will change as a function
of MgO/MgOHCI oxide ratio in the sample. Figure 36 shows the variation of the degree
of Mg(OH)2 supersaturation calculated for the data in Figure 25.
The term supersaturation is used here to express the extent to which the actual concentration of
Mg(OHh exceeds its solubility Iimit
14
66
Figure 36 : Variation of the degree of Mg(OHh saturation calculated for the data in
Figure 25
12,...--------------,
20
40
60
Tlme, min
Figure 36 clearly shows that the Mg(OH)2 supersaturation is not constant. Therefore,
the nucleation and growth of Mg(OH)2 particles will vary as a function of the oxide ratio
in the sample.
At low supersaturation, growth will be favoured whereas at high
supersaturation, nucleation will be favoured.
6.2.3 Quantitative Decompositon of MgOHCI
Figure 26 showed the decomposition of MgOHCI by dry argon injection. The initial
alkalinity of the melt was 914±44 ppm MgOeq and was assumed to be exclusively in the
form of MgOHCI. The uncertainty represents the 95% confidence interval of 3 replicated
measurements. Argon was injected in the melt for three hours in order to completely
decompose MgOHCI. The final alkalinity of the melt corresponded to 21 00±1 05 ppm
MgOeq . The uncertainty was arbitrarily taken to be 5% based on experience with melts
containing suspended MgO. One has to remember that when MgOHCI decomposes,
the system changes trom an homogenous system, in which MgOHCI is dissolved, to a
suspension of MgO particles.
67
Substituting C~goeq= 914 ppm in Eqn. 56, the total theoretical alkalinity when ail MgOHCI
had decomposed was C~goeq = 1828±88 ppm. The experimental value of C~goeq is not
included in this interval. This can be explained by the fact that, even though precautions
were taken to make the fumace protected from air by top purging with argon, there was
still some air or moisture ingress in the fumace. This resulted in an increase in alkalinity
of the melt over time. This phenomena was demonstrated by injecting dry nitrogen in an
oxide free melt for a long period of time. The total alkalinity of the melt increased slowly
up to a concentration of 153 ppm MgO eq • Then, if the oxide contribution from air or
moisture contamination is substracted from the experimental value, a corrected
experimental value of C~goeq= 1842 is obtained. This last value lies within the error limit
of the theoretical value. Therefore, it can be concluded that, within the limit of error of
the experimental method, the melt initially contained only MgOHCI. Thus, when a melt
containing initially 1 wt% of suspended MgO particles is chlorinated with HCI at 5 SLPM
and 500 RPM for 25 minutes, MgO is completely converted to MgOHCI.
6.2.4 Infrared Spectrometry
It was possible to identify the hydroxyl group in a sampie of MgOHCI obtained by
dehydration of MgCI 2 ·6H2 0 by analysing it with infrared spectrometry. This was shown
in Figure 27, where there is an absorption peak characteristic of hydroxyl group at
3600 cm- 1• The MgOHCI sample was believed to be less sensitive to moisture pickup
because of its low chloride salt content « 30 wt% MgCI 2 ).
It is interesting to note in
Figure 27, as weil as in Figures 28 and 29, that the part of the spectrum located below
1500 cm- 1 corresponded to the spectrum of Fluorolub.
However, it was not possible to identity hydroxyl group in sample trom the chlorination
experiments using infrared spectrometry. It can be explained by moisture absorption by
the sample during crushing. This effect of moisture was obseNed in Figure 28, which
showed the spectrum of a sample from an oxide free melt. Therefore, no characteristic
hydroxyl absorption peak (3600 cm- 1) was expected for this sample [48,49]. However,
68
significant absorption took place between 3125 and 3625 cm-1, this region being
characteristics of the water spectrum. Moreover, melt samples were analysed for their
water content using the Karl-Fisher [50] method and it was foundthat simple crushing of
the salt in air lead to 1 wt% of moisture pickup. Therefore, the water contamination was
higher than the actual concentration of hydroxychloride. For this reason, the spectrum
(Figure 29) of a sample expected to contain approximately 1 wt% of MgOHCI does not
show any hydroxyl absorption peak.
6.3
Verification of the Kinetic Model for the Chlorination of MgO to MgOHCI
6.3.1 Linear Regression Analysis
The fact that MgCI 2 concentration remained constant during the early stage of
chlorination was deemed to be sufficient to assume that MgOHCI was produced
according to the following reaction :
Mgqs) + HG/(g)
--7
MgOHG/(d)
Eqn.67
Therefore, the data were analysed in order to determine the rate controlling-step of the
chlorination mechanism.
According to Eqn. 53, there would be a Iinear relationship between GM90 or G~:o and
time. Figure 30 showed the variation of GM90 as a function of time fitted with a linear
curve. The coefficient of determination (~) is 0.9663 with a 95 % confidence interval
[0.9337, 0.9957] which indicates a good correlation between the two variables. Figure
31 showed the variation of G~:o as a function of time fitted with a Iinear curve. In this
case, the coefficient of determination (~) is 0.9914 with a 95 % confidence interval
[0.9829, 0.9989]. Therefore, there is a good correlation for both relationships. However,
the larger confidence interval for the Iinear regression of G~:o as a function of time
made the value of the correlation of determination statistically different. Moreover, a
plot of the residuals of GM90 as a function of time (Figure 32) showed trends in the
69
dispersion of the data. On the other hand, the residuals of C~:o as a function of time
(Figure 33) shows more randomly distributed data points, which is an indication of the
existence of a Iinear relation between C~:o and time.
The Iinear relation between C~:o and time indicated that the rate controlling-step was
one of the following :
1.
Transport of HCI from the bulk molten salt across the Iiquid film surrounding a
MgO particle to the surface of the particle ;
2.
Reaction between MgO and HCI at the surface of a MgO partiele.
According to Doraiswamy [51 l, mass transfer to a solid particle is affected by
hydrodynamic factors. On the other hand, chemical reaction is significantly affected by
temperature but not by hydrodynamic factors. Therefore, it was decided to carry out
chlorination experiments at different HCI flow rates to determine the rate limiting-step.
Figure 35 showed the effect of HCI flow rate on chlorination kinetics. The Using the data
in Figure 34, the normalized slope (K n) was obtained for each of the gas flow rate.
Normalisation consisted in dividing the slope of the curve by the corresponding initial
MgO concentration (C~go) to the power 1/3. This was necessary in order for the slope
to be a measure of the magnitude of the liquid-solid mass transfer coefficient (kSL )' This
complication arises because the product of k s and k p in Eqn. 53 is proportional to
(C~gO)1/3.
Figure 35 shows the effect of varying HCI flow rate on Kn. The error bars
represent the 95 % confidence interval on the slopes of the curves in Figure 34. The
graph shows that increasing the HCI flow rate increased the value of Kn • This is an
indication that increased HCI flow rate increases mass transfer of HCI to the solid MgO
particles.
Therefore, it can be concluded that the rate limiting-step in the reaction
between MgO and HCI to produce MgOHCI is the transport of HCI from the bulk molten
salt across the liquid film surrounding a MgO particle to the surface of the partiele.
70
The reaction was described by the following equation :
(c
MgO
)1/3
= (Ci
MgO
)1/3 _
K (Ci
n
MgO
)1/3
t
Eqn.68
or
Eqn.69
The magnitude of Kn in Eqn. 69 was an indication of the rate of mass transfer of HGI to
the MgO particles when the chlorination conditions (Le. impeller speed, gas flow rate,
temperature) were changed for a constant volume of molten salt.
71
7 Conclusion
The chlorination of MgO partieles with HCI gas in a molten salt was studied in a batch
agitated reactor. It was shown that the MgCI 2 concentration of the molten salt remained
constant during the first ten minutes of chlorination when the reaction was performed at
550°C and 500 RPM with a HCI flow rate of 2.5 SLPM. From this observation, it was
proposed that MgOHCI formed as an intermediate species during the chlorination of
MgO to MgCb. Different techniques were used to identify and quantify the concentration
of MgOHCI samples from chlorination experiments. The alkalimetric titration technique
showed that the MgOHCI concentration increased to a maximum during the first 15
minutes and then decreased as chlorination progressed.
In a single experiment
MgOHCI, was quantitatively decomposed by injecting dry nitrogen into the molten salt.
These results indicated that the chlorination of MgO to MgCI 2 proceeded initially by the
formation of MgOHCI according to the following reaction :
Mgqs) + HC/(g) -7 Mg( OH)C/(d)
However, infrared spectrometry was proven not to be a valid technique to differentiate
between MgO and MgOHCI in solid salt samples owing to the hygroscopie nature of this
material.
Conventional mass transfer theory was used to develop a model for the reaction of solid
MgO partieles with HCI dissolved in a molten salt.
The model assumed shrinking
particle behaviour of the MgO. Linear regression analysis was performed on the data
from the chlorination experiments. It was established that the reaction of MgO with HCI
to form MgOHCI followed a shrinking particle model. Chlorination experiments carried
out at different HCI flow rates while keeping the temperature and the rotation speed
constant showed that the reaction was controlled by mass transfer of HCI dissolved in
the molten salt across the liquid film surrounding the MgO particle to the surface of the
particle.
72
The following rate law was obtained :
Kn, which is a measure of the mass transfer in the system, was found to vary between
0.04 and 0.08 when the gas flow rate was varied from 2.5 to 7.5 SLPM at 550°C and
500 RPM.
73
8
Recommendations for Future Work
The work performed in this research showed that there is no satisfactory analytical
technique to determine the concentration of MgO and MgOHCI in a molten salt.
Therefore, there is a need to develop an easy and reliable technique for the
determination of these two oxide specie. Future work could look at improving the actual
alkalimetric titration technique or at developing a new technique.
The kinetics of chlorination of MgOHCI with HCI to form MgCb could also be measured.
The challenge in this system would be to determine the location of the reaction front
since MgOHCI is soluble in the molten salt.
Also, MgOHCI might become unstable in the electrolyte has the temperature increases.
This would need to be investigated because it could change the reaction mechanism for
the chlorination of MgO to MgCI 2 to a single step reaction.
74
Appendix 1- List of suppliers for consumables
Item
Description
Supplier
MgCb
>99%, Anhydrous
Alfa Aesar
CaCI 2
>99%, Anhydrous
EM Science
NaCI
>99%
EM Science
KCI
>99%+
EM Science
CaF2
>99.95 %
Alfa Aesar
MgO
>99%, -325 mesh powder
Sigma-Aldrich
HCI
Anhydrous (Cylinder)
Air Liquide
Ar
>99.997%, Refrigerated Liquid
Air Liquide
N2
>99.995% Extra Dry (Cylinder)
Praxair
CsCI
99.9%
Aldrich
Fluorolube
Oil
Spectratech
75
Appendix Il - Particle size distribution of MgO powder
Volume (%)
10 ,-r--+--+--t-+-+++t----t-+----+i--tt:::::=:;:=::::io==--r-T-ï1 00
o
o
o
o
o
o
o
o
10
OI-k---i~~~W$:lSW~W~~~t::EJi:::J;;:"1S1"
~J-.~~-.-J0
.1
100.0
Particle Diameter (IJm.)
76
Appendix III - Sample Calculations
Test ID : IN1 - Conditions: 550°C, 500 RPM and 2.5 SLPM HCI
Sampie ID
Time, min MgO eq , ppm MgCI 2 , wt%
IN1.1
IN1.3
IN1.4
IN1.5
IN1.6
IN1.7
IN1.8
IN1.9
IN1.10
IN1.11
IN1.12
IN1.13
IN1.14
IN1.15
IN1.16
IN1.17
IN1.18
0
1
2
3
4
5
6
7
8
9,25
10
20
30
40
50
60
70
9001
8608
8105
7715
6837
6563
6339
6022
5783
5523
5253
2600
1189
529
193
Fouled
82
At 1 minute: MgO remaining
= 9001
31,2
31,2
31,3
31,2
31,3
31,3
31,5
31,2
31,4
31,2
31,2
MgO
remaining,
ppm
9001
8215
7209
6429
4673
4125
3677
3043
2565
2045
1505
- 2 x (9001-8608)
(MgO remaining
ppm 1/3
= 8215 ppm
)1/;J,
20,8
20,2
19,3
18,6
16,7
16,0
15,4
14,5
13,7
12,7
11,5
(Eqn.65)
Linear regression analysis :
25
- r - - - - - - - - - - - - - -.. . .
20
;:l
E
8::
15
~
l
10
Y =-0,8596x + 20,905
R2 = 0,9914
~
5
o -t----;--.......,-----r--,...----...,---\
o
2
4
6
8
10
12
Time, min
= 0.8596 ppm /8 = 0.041 8-1
K
n
20.905ppm
77
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