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SECONDARY
A new insight into the melting behaviour and
performance of fused MgCl2/KCl refining fluxes
Fluxes containing 25% wt MgCl2 are shown to be as effective at removing alkali metals and inclusions as more
costly fluxes containing 40% wt MgCl2, due to the occurrence of a third eutectic point in the MgCl2–KCl system
close to this composition. Trace additions of fluoride rare earth salts also enhance performance as does injection
through a rotary of granulated fused flux injector (RFI).
By John Courtenay & Michael Bryant
agnesium
chloridepotassium chloride (MgCl2KCl) refining fluxes have
become widely adopted as an
environmentally-acceptable means of
removing alkali metals and oxides from
molten aluminium. These fluxes were
initially based on the binary system
MgCl2–KCl which exhibits two low
melting point eutectics; one at about 60%
mole magnesium chloride and one at
about 40% mole magnesium chloride.
Recently, products containing 25%
magnesium chloride have been found to
exhibit equal removal efficiencies and
satisfactory melting behaviour.
The aim of the current work was to
reconcile the practical results with the
phase equilibriums.
The findings indicate the existence of a
third low melting point eutectic at about
30% mole magnesium chloride as well as
a significant performance improvement
by a small addition of fluoride salt.
In 1995 Beland et al(1) described the role
of the liquid magnesium chloride
intermediate as the rate controlling
species in removal of alkalis by chlorine
fluxing. Many Alcan sites replaced the use
of chlorine gas with the alternative
cleaning process of solid flux injection
using a mixture of fused magnesium
chloride and potassium chloride injected
by means of a newly developed rotary
flux injection technique known as RFI(2).
M
The positions of the two commonly
referred eutectics correspond more
closely to commercially used materials
with the first occurring at 61% or 55.5%
mole and the second at 42% or 36.5%
mole. Interestingly a third eutectic is
shown occurring at 31.5% mole (35%
wt) which is also confirmed by thermal
analysis work carried out by Seifert and
Eubach.
It seems clear from all this work that a
third eutectic definitely exists but there
are still differing views as to the exact
nature of the MgCl2/KCl system.
Injection of fused MgCl2 flux was
found to be completely equivalent to
chlorine injection in terms of both alkali
metal removal and cleanliness and with
the advantage of a reduction in emissions
of around 90%.
MgCl2 – KCl SYSTEM
The classic binary magnesium chloridepotassium chloride phase system, after
Ivanov(3), shows two eutectics occurring,
the first at 65% weight (59.5% mole)
magnesium chloride and the second at
38% wt (32.5% mole) magnesium
chloride.
Based on this phase diagram it has been
normal practice to choose a composition
corresponding closely to one of the two
eutectics namely 60/40 MgCl2/KCl (by
weight %) or 40/60 MgCl2/KCl (weight %).
Both compositions are proven to be
effective at removing alkalis and
inclusions but little is known about the
relative effectiveness. It is obviously also
important to know whether there are
other eutectics and compounds in the
system that might be of practical use.
A detailed study into the phase
equilibrium shows a number of different
views as to the exact nature of the MgCl2
/KCl diagram. Research by Dr Ditze
predicts a system with three eutectics
and two compounds, but with the
absence of a peritectic and eutectoid as
shown in Fig 1.
WORK BY MQP
Initially it was generally believed that
flux efficiency was related to the MgCl2
content. However by the time MQP
started its own development programme
in 2001, production data was available
from use of 65% and 40% MgCl 2
containing fluxes, indicating there to be
no significant difference in terms of
alkali and inclusion removal efficiency.
Since the main component of raw
material cost in these fluxes is the MgCl2
an extensive programme of laboratory
and casthouse testing was carried out to
compare the performances of MgCl2/
KCl fluxes in the range 40% wt MgCl2
and 60% wt MgCl2, as well as fluxes in a
nominal composition range of 25% to
30% MgCl2 which exhibited single
៉
melting point behaviour.
770°C
Clean reference
៌
714°C
៌
615°C
Liquid
Refinal 40%
Refinal 25%
60% MgCl2
60% Admix
426°
38.9 Mass%
431°C
KMgCl2
56.1 Mass%
KCl + Liquid
K2MgCl2
MQP.qxp
485°C
Control
MgCl2
+ Liquid
Lower production bound
467°C
428°C 55.5%
60%, 40% and 25% fused
MgCl2
31.5% 36.5%
KCl
M41
MoI%MgCI2
MgCl2
1 MgCl2 - KCl phase system (% mole) after Grjotheim
K, Holm J L, Roetnes M, Acta Chem Scand 26 (1972),
p3802, with additions by Ditze
2 The Prefil test plots of metal flow rates for metal
treated with various flux compositions and fused or
mechanically mixed
The authors are with MQP Ltd, 6 Hall Croft Way, Knowle, Solihull, B93 9EW, UK
Tel & Fax +44 (0)1564 200443 e-mail [email protected]
24 ALUMINIUM INTERNATIONAL TODAY MARCH/APRIL 2008 www.aluminiumtoday.com
60% unfused MgCl2 and
unreated control
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SECONDARY
LABORATORY TESTING
Initially, attempts were made to measure
sodium removal in small-scale tests but
results
were
unreliable.
Instead
measurement of inclusion removal was
used and it was found that Prefil tests
carried out by N-Tec Ltd gave
reproducible results. The Prefil test
measures the flow-rate of molten metal
through a micro filter at constant
temperature and pressure. Very clean
metal flows quickly giving a steep straight
line in the test plot and metal cleanliness
is proportional to the slope.
As can be seen in Fig 2, a 60% MgCl2
unfused mechanical mixture 60/40
MgCl2/KCl (60% Admix) gave no
improvement over the untreated control.
Fused fluxes containing 60% MgCl2, 40%
MgCl2 and 25% MgCl2 all showed a
significant and similar improvement over
the control.
This agrees with work of De Young(4)
which shows no difference in sodium
removal for fluxes containing either 20%
or 60% MgCl2.
CASTHOUSE DATA
In a first series of tests at a smelter casting
5xxx alloy, comparisons were made
between a 40% MgCl2 fused product and
a 60-65% MgCl2 fluxed product applied
manually in two identical 50t furnaces.
The results showed that, on average,
the 40% MgCl2 product performed
slightly better than the 60-65% product
with 89% sodium removal from an
average starting level of 39ppm sodium
compared with 82% sodium removal for
the higher MgCl2 containing flux at an
average starting level of 35ppm sodium.
In a separate series of tests at another
smelter, results were obtained adding
40% MgCl2 and 25% MgCl2 fused
products via an RFI rotary flux injection
unit. The addition rate was 20kg of
material in a 23 minute stirring cycle.
The average sodium removal for the
first test series with 40% MgCl2 was 76%
whilst the results for the second series
with 25% MgCl2 flux gave a 77%
removal. In each case a series of sodium
samples were taken every 5 minutes.
It was concluded that both the 40%
MgCl2 and 25% MgCl2 compositions,
based on this limited series of tests, were
equivalent in alkali removal performance.
As an outcome both the 40% product
and the 25% product were evaluated on
an extended basis and then adopted into
full scale production, with continuous
monitoring, over a period of 12 months.
The results obtained over this period
confirmed that the two compositions
gave equivalent performances which led
to the important conclusion that the
Prefil laboratory test technique can
reasonably predict actual performance in
the casthouse in terms of both alkali and
inclusion removal. It follows that the
same technique can be employed to
benchmark product performance and to
TG/mg
Peak 120.5°C
DTA/uV
exc
Peak 158.5°C
Masseänderung -0.205mg
Masseänderung -0.050mg
(1.2)
Peak 436.8°C
Temperature/°C
3 Differential thermal analysis trace to 825ºC for experimental product corresponding to the
third eutectic
develop new improved formulations
which the next section of this paper is
concerned with.
Flux Type
FORMULATIONS WITH ACTIVE ADDITIVES
Refinal 100 (40%wt MgCl2)
425
427
Refinal 250 (25%xt MgCl2)
Refinal 252XF (25%wt MgCl2 plus fluoride) 421
According to Bilodeau(5), the rate
controlling step in the alkali removal
process, when application is by injection
through RFI, is not furnace mixing, but
transfer across the liquid salt droplet/
molten aluminium interface. He
concluded that the rate-controlling step
was the salt-melt interfacial area, with the
reaction with MgCl2 having little effect.
Use of the RFI clearly improves the
reaction kinetic by breaking up the salt
droplets in the reaction zone which
reduces the droplet size and increases the
interfacial surface area available.
It is known Ref(6 - 7) that certain active
additives, particularly alkaline earth metal
fluorides have a strong effect in reducing
interfacial tension between liquid salt
droplets and molten aluminium alloys
leading to a reduction in droplet size and
improved transfer across the salt droplet –
melt interface.
MQP therefore undertook research
into the effect of fluorides in MgCl2
based fused fluxes on inclusion removal
rates. It was found in laboratory tests that
a very small amount of fluoride additive
gave a significant improvement in
performance. The most successful
additive was then selected for inclusion in
batches of 25% MgCl2-based fused fluxdesignated Refinal 252XF.
Prefil comparison tests were carried out
and these showed greater removal
efficiency of inclusion using Refinal
252XF than either the standard Refinal
100 (40% MgCl2) or the standard Refinal
250 (25% MgCl2).
A field test was then carried out at a
smelter casthouse applying 35kg of
Refinal 252XF in 40 minutes through an
RFI on an 85t furnace casting 1xxx alloy.
The kinetic index, an indicator of sodium
removal, was 54% better than data for
standard Refinal 250 which corresponds
well with the improvement seen in the
laboratory Prefil tests.
Refinal 252XF has now been
MP (°C)
Table 1 Melting point tests on Refinal fluxes
successfully used for over 12 months at a
remelt operation and, in addition to
consistent
sodium
removal.
Environmental
monitoring
has
confirmed no detectable fluoride
emissions in the furnace stack gases.
MELTING BEHAVIOUR
To better understand the melting
behaviour of the standard and fluoride
containing fluxes, differential thermal
analysis and differential scanning
calorimetry (DSC) have been carried out
at Freiberg University and Henkel
Laboratories respectively.
All samples were tested in the range of
380-825°C. No solid KCl, which could
have been expected for Refinal 250 and
252XF according to the binary diagram,
were detected and all the products
exhibited a sharp thermal effect within
the range of 408-435ºC (table 1).
Refinal 250 and 252XF however
showed a slightly diffused melting zone
between 410ºC and 435ºC whereas
Refinal 100 showed a single melting
point.
Recent work has produced a product
using industrial raw materials that
corresponds exactly to the third eutectic.
The experimental product exhibits a
sharp melting point at 436ºC, as shown in
the differential thermal analysis (DTA)
trace (Fig 3).
DISCUSSION
Whilst Refinal 100 coincides with the
second eutectic in the system formed at
36.5% mole (41%wt), the Refinal 250 and
Refinal 252XF compositions with 25% wt
MgCl2 appear to be slightly hypo-eutectic
with respect to the third eutectic
occurring at 31.5% mole (38.9% wt).
Practical measurements on the other hand
using differential thermal analysis (DTA) ៉
26 ALUMINIUM INTERNATIONAL TODAY MARCH/APRIL 2008 www.aluminiumtoday.com
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SECONDARY
and DSC showed no evidence of any KCl
solids indicating eutectic melting
behaviour. However if the composition
was still slightly hypo-eutectic then, as
solid flux is introduced into the melt,
approximately 90-95% would melt at
423°C with any remaining KCl solid
melting by the time the liquidus
temperature of 630°C was reached and
therefore this would have no practical
effect on performance in the casthouse.
Regarding the question as to which is
the active ingredient in fused MgCl2/KCl
fluxes, it must be concluded from
available thermodynamic data that it is the
MgCl2 containing phase which is active. If
this is correct then an explanation is
required as to why the alkali removal
performance is observed to be
independent of the MgCl2 content in the
fused salt flux.
Ditze has proposed that the
concentration of MgCl2 in the molten salt
droplet has little influence on the kinetics
because the rate of salt addition applied is
ten times that required to satisfy the
requirement for stoichometric reaction. Of
far greater importance are the kinetic
factors.
Bilodeau considered the factors
influencing the alkali removal kinetic and
concluded that the reaction with MgCl2
was not rate limiting. Rather the kinetics of
alkali removal in the reaction zone are
controlled by the salt-melt interfacial area.
He noted that the reaction kinetic was
strongly increased by reducing the
bubble/droplet diameter. This can be
achieved by applying shear forces to the
droplets as in the case of addition via the
RFI rotary flux injector where the droplets
are effectively chopped up by the rotor.
Hinze demonstrated that the splitting of
a droplet occurs when the shear forces are
sufficient to overcome the interfacial
tension and internal viscous forces.
Thus the reaction kinetic can be
increased by increasing the shear force
applied or reducing the interfacial tension.
The addition of fluorides to aluminium
melts has been shown(7) to cause a marked
decrease in interfacial tension and
therefore this can be expected to increase
the reaction kinetic as observed in the tests
with the application of Refinal 252 XF.
CONCLUSIONS
Ɂ A third eutectic has been confirmed at
31.5 mole % MgCl2 in the MgCl2/KCl
phase diagram.
Ɂ Differential thermal analysis of Refinal
100 with 40wt% MgCl2 confirms that it is
a eutectic composition corresponding to
the second eutectic in the MgCl2/KCl
system.
Ɂ Differential thermal analysis of Refinal
250 and Refinal 252XF with 25% wt
MgCl2 confirm that they exhibit low
melting point eutectic behaviour with
melting occurring over a narrow range
from 409°C-424°C and no detectable
KCl solids up to 825ºC.
Ɂ The Refinal 250 and Refinal 252 XF
compositions, although appearing to be
slightly hypo-eutectic with respect to the
theoretical equilibrium diagram for the
MgCl2/KCl phase system, correspond
closely to the third eutectic probably as a
result of reagents of greater purity being
used to derive the published phase
diagram compared with the less pure
reagents applied industrially.
Ɂ Laboratory and field test data confirm
that Refinal 100 with 40 wt % MgCl2 and
Refinal 250 with 25% wt % MgCl2
provide equal performance for alkali and
inclusion removal as compositions
corresponding to the first eutectic at 65
wt% MgCl2.
Ɂ The active component in fused MgCl 2
KCl fluxes is MgCl2, existing separately
or in the compounds in the system.
Ɂ The amount of MgCl
2 applied in
practice is currently an order of
magnitude higher than that required for
stoichiometric removal of Na, Ca and
non-metallic inclusions. This is a
consequence of the need to achieve an
adequate distribution of a relatively small
amount of flux in the aluminium melt.
Thus the concentration of MgCl2 in the
individual salt droplets, providing that it
is always greater than that required to
satisfy the reaction, has little influence on
the reaction kinetic.
Ɂ Laboratory and initial field test data
indicate that a minute addition of an
active fluoride compound to a 25 wt%
MgCl2 flux material leads to a significant
increase in the kinetic index and an
improvement in inclusion and alkali
removal performance over and above that
of the other eutectic compositions tested.
Ɂ The mechanism by which the increase
in reaction kinetic is achieved is
considered to be due to the effect of
minute quantities of fluorine on the salt
droplet interfacial tension leading to a
decrease in stable droplet diameter and
improved transfer across the salt droplet –
melt interface.
ACKNOWLEDGMENTS
The authors would like to thank Dr Andre Ditze at
TU Clausthal, Prof Voigt at TU Freiberg and Mr
Matthias Rohmann of Rheinkalk HDW for their
assistance in undertaking this project and for
giving permission to publish their work.
REFERENCES
1 G Beland, C Dupuis and J-P Martin, ‘Improving fluxing
of Aluminium Alloys Light Metals, 1995, 1189-1195
2 G Beland et al, ‘Rotary Flux Injection: Chlorine-Free
Technique for Furnace Preparation’ Light Metal, 1998,
843-847
3 A I Ivanov, Sbornik Statei Obshchei Khim, Akad Nauk
S S S R ,1, 1953, 758
4 D H DeYoung, ‘Salt Fluxes for Alkali and Alkaline Earth
removal from Molten Aluminium, 7th Australian Asian
Pacific Conference Aluminium Casthouse Technology,
2003,
5 J F Bilodeau, C Lakroni and Y Kocaefe, ‘Modelling of
Rotary Injection Process for Molten Aluminium
Processing’ Light Metals, 2001, 1009-1015
6 K J Freisen, T A Utigard, C Dupuis and J-P Martin,
‘Coalescence behaviour of Aluminium Droplets under a
Molten Flux Cover’ Light Metals, 1997, 857-864
7 A Silney and T A Utigard, ‘Interfacial Tension between
Aluminium, Aluminium Alloys and Chloride – Fluoride
melts ‘ Light Metals, 1997, 871 – 878
Rio Tinto Alcan AP-Xe smelting offers reduced energy
t the TMS conference in New
Orleans, USA last month Rio
Tinto Alcan announced that it has
begun development of the next
generation of its AP Technology series.
To be called AP-Xe, this new
technology could potentially lead to a step
fall in energy consumption of up to 20%
along with reduced environmental impact
and improved full economic costs.
AP-Xe will be developed in phases,
including a potentially workable 'drained
cathode' that, taken together with other
technologies under development, could
result in the lowering of unit energy
consumption in smelting by up to 20%.
The drained cathode concept enables
the anode – cathode distance (ACD) to be
A
reduced thus lowering the resistance of
the cell. In conventional high current
operations the magnetic fields cause
'waves' to form in the liquid aluminium
which can contact the anode and short
circuit the cell if the ACD is too small.
The drained cathode uses a coating on the
cathode surface, for example a titanium
diboride carbon composite, which enables
the pool of liquid metal to be maintained
at only a few millimetres thick and thus
enable the ACD to be minimised.
This technology is designed to be
retrofitted to previous AP series cells.
While the maximum energy consumption
savings are expected from greenfield
applications, significant savings could also
be achieved in retrofitted cells.
Drained cathode cells are already under
test on an industrial scale and the
immediate next phase will be the
progressive start-up of a 10-cell AP30 test
section at Rio Tinto Alcan’s site in StJean-de-Maurienne, France. If successful,
an industrial scale-up of this pot-line
technology could begin within five
years.With approximately 7500 AP
Technology cells in operation around the
world today, representing some 6Mt of
installed capacity, AP Technology is one
of the global leaders. Facilities reaching
close to 1Mt are under construction with,
in addition, several million tonnes in
various stages of evaluation, either
internal to Rio Tinto Alcan or with
potential partners or licensees.
28 ALUMINIUM INTERNATIONAL TODAY MARCH/APRIL 2008 www.aluminiumtoday.com