Materials Transactions, Vol. 53, No. 3 (2012) pp. 477 to 482
© 2012 The Japan Institute of Metals
Use of Chlorine to Remove Magnesium from Molten Aluminum
Estéfano Aparecido Vieira1, Jose Roberto de Oliveira1, Gianni Ferreira Alves1,
Denise Crocce Romano Espinosa2 and Jorge Alberto Soares Tenório2
Federal Institute of Espírito Santo ­ Department of Metallurgical and Materials Engineering,
Av. Vitória, 1729, Vitória-ES, 29040-780 Brazil
2
Polytechnic School ­ University of São Paulo ­ Department of Metallurgical and Materials Engineering,
Av. Prof. Mello Moraes 2463, São Paulo ­ SP, 05508-030 Brazil
1
Removal of Mg from aluminum scraps, known as demagging, has been widely applied in the aluminum industry. This work discusses
bubble-formation theories and magnesium kinetic removal from aluminum scraps using chlorine and inert gas fluxing. The interfacial area of the
bubbles and residence time were estimated using a mathematical model. To inject gaseous chlorine, three types of nozzles were used with
varying internal diameter. In addition, a porous plug, as well as varying input chlorine flow and concentration were used. The use of lower
chlorine concentration improves efficiency because the interfacial tension is reduced therefore, more and smaller bubbles are formed. The model
proposed herein is consistent with the experimental data. [doi:10.2320/matertrans.M2011256]
(Received August 19, 2011; Accepted December 13, 2011; Published February 25, 2012)
Keywords: aluminum, refining, reactions rates
1.
Introduction
Magnesium is introduced as an alloy element to produce
aluminum cans; however, most aluminum alloys do not
contain Mg as an alloying element. Consequently removal of
Mg may be necessary if different types of scraps are mixed
during the aluminum recycling process. The removal of
contaminants is frequently necessary in the secondary
aluminum industry.1­3) Chlorine may also be helpful in
removing alkaline metals such as Na, K, and Li. Another use
of Cl2(g) in the Al foundry is related to the removal of
hydrogen and also to improving the removal of solid particles
such as TiB2, Al2O3, MgO and Al4C3.4,5)
Several techniques have been developed to remove the Mg
from molten Al. The use of Cl2(g) flow, electrolysis and
reactive powders are the main alternatives to decrease Mg
concentrations in Al baths.2,6,7) Muñoz-Arroyo et al.8) used
rich silica based compounds such as Ca(Si7Al2)18·6H2O,
KAlSi3O8 and SiO2. In such cases the goal is to form
MgAl2O4.
The chlorine injection technique is essentially performed
introducing the gas inside the molten Al through a graphite or
stainless steel nozzle. Usually a mixture of gases is applied.
Under such conditions nitrogen or argon are the bases of the
mixture and chlorine is the active gas. Nitrogen typically is
employed more commonly because of its lower cost. An
improvement to this process is to use a rotor. The main
advantage of this device is that it produces a dispersion of
small bubbles (less than 5 mm) inside the bath.
Table 1 shows the main reactions occurring in the process
of Mg removal by the chlorine introduction. Gibbs free
energy equations are also presented. Data for the reactions
involving moisture and oxygen are also exhibited. These
compounds may also be found under industrial conditions.
Through these calculi, all the reactions investigated are
feasible, since at, for example, at 790°C; all Gibbs free
energies are less than zero. Furthermore, it is possible to
conclude that the Mg removal by chlorine is thermodynamically feasible.
The first reaction to occur when chlorine is purged in
molten aluminum is reaction (1); next, aluminum chloride
reacts with dissolved magnesium following reaction (5). No
gas emissions are observed when the aluminum chloride
bubbles have enough time to react completely with
magnesium. The reaction kinetics will depend on parameters
such as temperature, magnesium concentration, metal stirring
and contact area. According to Lagowski12) the reaction
between pure chlorine and dissolved magnesium has a
maximum efficiency at around 710°C.
Fu et al.1) observed the same behavior described by
Lagowski, and also stated that below 710°C emissions of
chlorine and aluminum chloride occur, but above this
temperature only aluminum chloride emissions were observed. According to these authors this behavior is explained
because of the highly negative Gibbs free energy for the
aluminum chloride and also because of the high amount of
liquid aluminum in direct contact with the chlorine gas.
Magnesium chloride melts at 710°C, hence, below of this
temperature, this compound forms a solid phase on the
bubbles that restrain Cl2(g) reacts with Al(l) so this could
explain the percentage increase of aluminum chloride
formation.
2.
Objective
In the present work, the aim was to study the kinetics of
Mg depletion from Al can scraps using Cl2(g) (gaseous
chlorine) and Ar(g) (argon). Thus, a kinetic model is proposed
to explain the experimental data obtained.
3.
Methodology
All tests were performed by melting 1.75 « 0.2 kg of an
Al­1.5 mass% Mg alloy in graphite crucibles. The charged
graphite crucible was placed inside a hot chamber of a
laboratory electric furnace. The furnace was set at 790°C,
because under this condition the magnesium chloride is a
liquid phase. The gas purging was carried out through an
478
E. A. Vieira, J. Roberto de Oliveira, G. F. Alves, D. C. R. Espinosa and J. A. S. Tenório
Table 1 Standard free Gibbs energy as a function of temperature of the mainly reactions that occur in the Mg removal from the
aluminum.9­11)
Reaction
Al(l) + 3/2Cl2(g) ¼ AlCl3(g)
#
¦G° (J·mol¹1)
1
¹586,872 + 10.45T log T + 29.47T
Mg(l) ¼ Mg
2
¹14,538 ¹ 1.254T
®
Mg(l) + Cl2(g) ¼ MgCl2(l)
3
¹618,013 ¹ 56.76T log T + 304.18T
6.27
Min 6.27
Mg + Cl2(g) ¼ MgCl2(l)
4
¹603,475 ¹ 56.76T log T + 305.43T
2/3AlCl3(g) + Mg ¼ MgCl2(l) + 2/3Al(l)
5
¹212,227 ¹ 63.75T log T + 285.79T
®
2Al(l) + 3/2O2(g) ¼ Al2O3(s)
6
¹1,696,077 ¹ 15.68T log T + 385.48T
16.72
Mg + 1/2O2(g) ¼ MgO(s)
3H2O(g) + 2Al(l) ¼ Al2O3(s) + 6H
7
8
¹593,025 ¹ 1.00T log T + 113.49T
¹1,229,940 ¹ 16.64T log T ¹ 186.08T
6.27
®
3Mg + Al2O3(s) ¼ 3MgO(s) + 2Al(l)
9
¹82,998 + 12.67T log T ¹ 45.02T
18.81
Mg(g) ¼ Mg(l)
10
¹129,455 + 95.05T
1.67
Tests performed.
Gas
Nozzle
Flow (mL·s¹1)
Designation
®
®
®
no purged
Ar
Ar
1.8* (mm)
1.8* (mm)
1.00
2.50
Ar1810
Ar1825
Ar
1.8* (mm)
4.17
Ar1842
Cl2
0.4* (mm)
2.50
Cl20425
Cl2
1.8* (mm)
2.50
Cl21825
Cl2
4.7* (mm)
2.50
Cl24725
Cl2
Plug (**80 µm)
1.00
Cl2P10
Cl2
Plug (**80 µm)
2.50
Cl2P25
Cl2
Cl2 + N2
Plug (**80 µm)
Plug (**80 µm)
4.17
2.50/2.00
Cl2P42
Cl2N2P2520
*internal diameter
**pore diameter
Fig. 1
Results and Discussion
As aforementioned before, the liquid Al­Mg alloy was
kept at 790°C to verify the loss of Mg under such conditions.
Figure 2 presents the Mg concentration as a function of
elapsed time. In this case Mg removal follows the reaction 9
1.54
percentage of Mg, (wt%)
alumina duct, which was immersed 115 mm beneath the
molten aluminum surface. The gas flows were measured at
room temperature but the devices corrected the gas flows to
the Standard Condition for Temperature and Pressure (STP).
Two types of nozzles were applied: (i) a graphite porous
plug with external diameter of 15 mm and mean pore
diameter of 80 µm. (ii) an alumina nozzle with three different
internal diameters: 4.7, 1.8 and 0.4 mm. The samples from
molten aluminum were extracted after 0, 1800, 3600, 5400,
7200, 9000 and 10800 s. Table 2 presents the tests performed
and their particular designation. The magnesium concentrations were measured using atomic absorption (AA)
spectrophotometry in accordance with the ASTM standard
procedure.13)
Another four tests were performed. In the first one, no
purging gas was used, and the bath was just laid resting
inside the hot furnace atmosphere. The others one was
executed through the purging of argon. Figure 1 shows the
sketch of the laboratory apparatus.
Experimental apparatus.
1.2
1.53
1.0
1.52
0.8
1.51
0.6
1.50
1.49
0.4
1.48
0.2
1.47
1.46
mass of Mg, m/g
Table 2
4.
«kJ
12.54
0.0
0 1 2 3 4 5 6 7 8 9 10 11
3
time x 10 , t /s
Fig. 2
Test performed without gas injection.
of Table 1. In such conditions the oxide layer of aluminum
formed on the surface of the liquid reacts with magnesium.
It is proposed that the rate of Mg depletion is given by the
following equation:
dc
¼ kðc ce Þn
dt
ð1Þ
Where c is Mg concentration in the liquid aluminum and may
be expressed in mol·kg¹1, ce is the Mg concentration at
equilibrium and k is the proportionality constant which is
Use of Chlorine to Remove Magnesium from Molten Aluminum
dc
¼ kc
dt
ð2Þ
and
c ¼ ci e
kt
ð3Þ
2
Using eq. (3) and data from Fig. 2, a R equal to 0.987 was
determined. Although this model matches very well with
experimental results, another model can be proposed. In this
case, one can consider Higbie’s model14) where the reaction
is controlled by the thickness of the boundary layer.
dc
Aμ
¼ melt hc ðc ce Þ
dt
MT
ð4Þ
Where A is the reaction Area (m2), MT is total mass of alloy in
(kg), μmelt is density of the melt (kg·m¹3) and hc is the mass
transfer coefficient of dissolved Mg (m·s¹1) and may be
expressed by:
1=2
D
hc ¼ 2
ð5Þ
³t
Where D is the magnesium diffusion coefficient in the liquid
aluminum (m2·s¹1) and t is the time of exposure (s) so:
dc
¼ k0 t1=2 ðc ce Þ
ð6Þ
dt
Zt
ZC
dc
¼ k0 t1=2
dt
ð7Þ
Ci c
0
ð8Þ
lnðcÞ ¼ 2k0 t1=2 þ lnðci Þ
Using these equations and the experimental results and
also considering ce ¹ c, the obtained R2 is equal to 0.996.
Consequently, one can state that both models may describe
the magnesium loss under such conditions. However, this
second model matches better with experimental results than
the first one.
The magnesium concentration remained nearly unchanged
during the entire test, so very little magnesium oxidation
occurred and also an insignificant amount of magnesium was
lost due to evaporation. Thermodynamic evaluations demonstrated that the amount of Mg vapor that may be present in
the bubbles is in the magnitude of 0.0003% of the total Mg
spent in the process. According to Tenório & Espinosa15)
under these conditions MgO·Al2O3 is the oxide appearing
on the molten surface. Magnesium removal throughout the
oxidation process was very slow; consequently, the oxide
layer was able to protect the molten aluminum from further
magnesium oxidation. Only 5.2% of the initial magnesium
was lost after 3 h.
Figure 3 presents the results obtained when argon was
purged in the aluminum bath. The use of argon increased the
rate of magnesium removal compared to the previous test.
However, eq. (3) did not match these experimental results, so
eq. (4) was tried. Therefore, the opening of the aluminum
oxide on the surface caused by bubbles was considered
1.7
percentage of Mg, (wt%)
dependent on interfacial area. Lastly dc/dt may be expressed
in mol·s¹1, kg·s¹1 or mass%·s¹1 and a choice may be made
adjusting the corresponding k. According to thermo dynamic
calculi, ce is very low compared with concentrations obtained
in this work. Consequently if n is equal to 1, then one can
write:
479
1.6
1.5
1.4
1.3
1.2
1.1
1.0
0.9
no purged
-1
1.0 mL.s
-1
2.5 mL.s
-1
4.2 mL.s
0 1 2 3 4 5 6 7 8 9 10 11
-3
time x 10 , t /s
Fig. 3 Test performed with injection of argon 1.0, 2.5 and 4.2 mL·s¹1.
during the initial stages of the argon injection. A period of
1800 s was considered to stabilize the bath surface. This time
is necessary to stabilize the opened area on the bath surface
for oxidation to take place. Hence the initial time for the
model was 1800 s.
According to Fig. 4 the removal of magnesium by surface
oxidation can be described by eq. (6). The same equation
also describes the removal of magnesium using inert gas.
Table 3 presents the values of k and it was assumed n = 1.
As seen in Table 3, eq. (6) matches well with experimental
results. In Fig. 3 one can notice that the removal of Mg was
far superior when argon was purged than when no gas was
purged.
This result shows that the bubbles appearing on the surface
creates a discontinuity in the oxide layer, and consequently
the oxidation process occurs in these regions. Additionally,
Fig. 5 shows that k varies as the flow increases; such
behavior can be attributed to the increase on the oxidation
interface on the surface of the molten bath as the flow
increases.
Experiments Cl20425, Cl21825, Cl24725 and Cl2P25 were
carried out to understand the kinetics of Mg removal as a
function of bubble size and chlorine injection. Figure 6(a)
shows the results obtained for the test Cl20425. These
experimental data agree with the mass balance, and no
emissions of Cl2(g) or Al2Cl6(g) are expected. For this test the
experimental data are close to the stoichiometric curve
throughout the entire test. Figure 6(b) exhibits the results
from the test in which the 1.8 mm nozzle was used, and one
can observe that the experimental data follow the stoichiometric curve until the Mg concentration is around
0.12 mass%.
At this point, the kinetics of Mg removal changes, and this
point is identified as the critical Mg concentration and is also
characterized by the beginning of the Al chloride Al2Cl6(g)
emissions. For the 4.7 mm nozzle, Fig. 6(c) Al2Cl6(g)
emissions appeared at Mg concentration around 0.80 mass%.
This fact indicates that the bubbles are greater than previous
ones.
On the other hand, according to Fig. 6(d) the critical Mg
concentration is around 1.0 mass% when the porous plug was
used. Consequently, the greatest part of the Al chloride did
not react.
480
E. A. Vieira, J. Roberto de Oliveira, G. F. Alves, D. C. R. Espinosa and J. A. S. Tenório
(a)
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
no purged
-1
1.0 mL.s
-1
2.5 mL.s
-1
4.2 mL.s
0.0
-0.1
(b)
0.5
ln (c)
ln (c)
0.6
-0.2
20
40
no purged
-1
1.0 mL.s
-1
2.5 mL.s
-1
4.2 mL.s
0.1
R2=0.9932
0.0
-0.1
60
80
1/2
time, t /s
100
120
-0.2
-0.2 -0.1 0.0
0.1 0.2 0.3
2k t + ln(c i)
0.4
0.5
Fig. 4 Experimental curves of magnesium concentration as a function of time (a) relationship between experimental data and (b)
adjustment using Higbie’s Model.
R2
Test
¹2kB © 104
(s¹1/2)
R2
No gas
6.34
1.00
Cl2P25
440.4
0.97
Ar1810
53.05
0.98
Cl2P42
24.7
1.00
Ar1825
63.94
0.99
Cl24725
180.6
1.00
Ar1842
87.10
0.98
®
®
®
-1/2
¹2kB © 104
(s¹1/2)
3
constant x 10 , -k/s
Table 3 Kinetic parameter obtained using the diffusion model.
Test
4
3
5
2
1
0
0
1
2
3
-1
flow, Q/mL.s
4
5
Fig. 5 k as a function of argon flow using a nozzle with 1.8 mm internal
diameter.
(a)
(b)
1.6
o Cl20425
1.4
percentage of Mg, (wt%)
percentage of Mg, (wt%)
1.6
stoichiometric
1.2
1.0
0.8
0.6
0.4
0.2
o Cl21825
1.4
stoichiometric
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0.0
0
2
4
6
8
10
0
12
2
-3
o Cl24725
1.4
stoichiometric
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
-3
time x 10 , t /s
Fig. 6
8
10
12
10
12
time x 10 , t /s
(d)
1.6
percentage of Mg, (wt%)
percentage of Mg, (wt%)
1.6
6
-3
time x 10 , t /s
(c)
4
o Cl2P25
1.4
stoichiometric
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
10
12
-3
time x 10 , t /s
Influence of orifice diameter on the kinetics of Mg concentration as a function of time using flow of 2.5 mL·s¹1 of Cl2(g).
Use of Chlorine to Remove Magnesium from Molten Aluminum
The rate of Mg removal from Al is directly related to the
bubble size. The prediction of the size of bubbles formed
when a gas is injected inside a liquid was made by the
Reynolds (Reo), Weber (Weo) and Froude (Fro) orifice
numbers.16) Table 4 presents the calculi of bubbles diameter
for the tests performed. An expression to define the bubble
size can be derived as a function of the Reo, Weo and Fro
numbers.
The diameters of the bubbles were estimated through the
expression:16)
db ¼ 3:8ð·d0 =μ ‘ gÞ1=3
ð9Þ
In this case db is the bubble diameter, do is the orifice
diameter, · is the interfacial tension, do is the nozzle external
diameter, μl is the Al density, and g is the gravity
acceleration. The calculi for tests Cl20425, Cl21825 and
Cl24725 showed that the calculated bubble diameter increases
as much as the nozzle internal diameter increases.
The increase in bubble size explains the behaviors
observed in Fig. 6 in which the critical Mg concentration
increases following the higher diameter of the bubbles. In this
respect, the increase of bubble size causes an increase in the
Al chloride emission or a premature deviation from the
stoichiometric behavior. In the case of the porous plug, there
is a critical injection velocity Vc where the bubbles start to
coalesce. The critical velocity can be calculated through the
expression (10).17)
Vc ¼ ½ð2³·Þ=ð0:6db μ ‘ Þ1=2
ð10Þ
The critical flux or velocity is a function of the pore
diameter, shape and size of the porous plug, depth of the
Table 4
Experimental and calculated parameters.
V (m·s¹1)
db (mm)
Vc (m·s¹1)
Cl20425
Cl21825
27.95
8.0
®
2.30
13.0
®
Cl24725
0.46
20.0
®
Cl2P10
0.25
10.5
0.49
Cl2P25
0.62
12.0
0.46
Cl2P42
1.07
13.2
0.44
Cl2N2P2520
1.10
14.6
0.42
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
nozzle inside the liquid, fluid dynamic characteristics of the
liquid and gas. Koide et al.17) proposed the expression (11)
to predict the mean bubble diameter when a porous plug is
used:
2 30:16
V2
6
7
6
7
¾2 gdp
1=3 6
7
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
db ¼ 1:65ðdp ·=gμ ‘ Þ 6 s
ð11Þ
ffi 7
2
4
dp V μ ‘ 5
¾2 ·
Where ¾ is the void fraction in the porous plug, dp is the
pore diameter and V is the velocity of the gas. The calculi for
the test Cl2P25 are present in Table 4. The data showed that
in this test, the velocity imposed on the system was superior
to the critical velocity. Thus, the bubbles coalesced, which
explains the behavior observed in Fig. 6(d).
Figure 7 presents the results of the second set of trials
carried out using the porous plug. The Cl2 fluxes employed
were 1.0, 2.5 and 4.2 mL·s¹1 of Cl2. When 1.0 mL·s¹1 of Cl2
was purged, the results were better than at 4.2 mL·s¹1. As
shown in Table 4, in the first case the critical velocity was not
reached, so a large amount of small bubbles was formed. As a
result the stoichiometric behavior was reached, and therefore
no Al chloride was emitted.
On the other hand, when 4.2 mL·s¹1 of Cl2 was injected,
the emissions started from the beginning of the experiment.
Each time the critical Mg is reached, Mg removal may occur
by both the reaction with the Al chloride and by the oxidation
on the surface of liquid Al. Since the bubbles that rise to the
liquid surface generate an opening in the protective oxide
layer.
In the test Cl2N2P2520, a mixture of Cl2 and nitrogen
was used. The results of this experiment are plotted in Fig. 7.
The Mg was removed and low Al chloride emissions
occurred. However, in the test Cl2P25, the same efficiency
was not observed. The decrease in the Cl2 concentration in
the purged gas resulted in the decrease in the critical Mg
concentration.
This effect is in close agreement with the results of
Fu et al.;1) although in their experiments the initial Mg
concentration was around 0.1 mass% and flow constant.
Thus, in the test Cl2N2P2520 low Al chloride emissions
happened even though the flux was above the critical
(a)
(b)
-3
-1
stoichiometric 2.5 x 10 mL.s
-3
-1
stoichiometric 4.2 x 10 mL.s
Cl2P42
Cl2P25
0
1
2
3
4
5
6
-3
7
time x 10 , t /s
8
9 10 11
percentage of Mg, (wt%)
percentage of Mg, (wt%)
Test
481
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-3
stoichiometric 1.0 x 10 mL.s
-1
-3
stoichiometric mix(2.5Cl 2 + 2.0Ar) x 10 mL.s
-1
Cl2P10
Cl2N2P2520
0
1
2
3
4
5
6
7
8
9 10 11
-3
time x 10 , t /s
Fig. 7 Concentration of Mg as a function of time using a pore plug. Mixture with argon and low rate of injection improves the process
performance.
482
E. A. Vieira, J. Roberto de Oliveira, G. F. Alves, D. C. R. Espinosa and J. A. S. Tenório
velocity. Table 4 shows the calculated data for this test. One
possible reason for this higher Mg removal is because of the
stirring increase caused by the mixture of Cl2(g) and N2(g)
decreasing the diffusion path for Al2Cl6(g) in the boundary
layer.
Finally, the experiments show that the Mg removal rate can
follow two distinct kinds of behaviors, the first being
stoichiometric. In this case, the Al chloride has enough time
to react before reaching the surface, and the total Cl2 flow
determine the velocity of Mg removal. In this case, the simple
expression (12) describes this behavior:
c ¼ ci Q
PMMg 100t
22:4MT
dc dcðAl2 Cl6ðgÞ Þ dcðO2ðgÞ Þ
¼
þ
dt
dt
dt
ð13Þ
The Mg removal is established by the sum of two states.
The transitory state is expressed by dcðAl2 Cl6ðgÞ Þ refers to the
amount of Mg reacted with Al chloride. The permanent state
dcðO2ðgÞ Þ is the parcel of Mg that reacts with oxygen on the
surface. Consequently, the eq. (14) describes the Mg removal
kinetics:
dc
¼ þk1 t1=2 c þ k2 t1=2 c
dt
dc
¼ þðk1 þ k2 Þt1=2 c
dt
ð14Þ
ð15Þ
Where k1 and k2 are constants.
Thus, eq. (15) describes the Mg removal kinetics. The
calculi to obtain the reaction rate constants (k1 + k2) were
performed for tests Cl24725, CL2P25 and CL2P42 and results
are shown in Table 4 and Fig. 8.
This agreement indicates that the diffusion of Mg in the
boundary layer is the rate controlling step. Under industrial
conditions, the critical Mg concentration is a very important
parameter to be determined. The time to reach the critical
concentration is close to the time to start the chloride
emission.1) The results showed that the critical Mg concentration and the time to reach this concentration are influenced
by the system adopted to inject Cl2(g), by the gas flux, and
also by the Cl2(g) concentration in the gas. The developed
equations showed that the Mg removal kinetic is controlled
by the Mg transport in the boundary layer either for the
oxidation parcel or by the reaction with Al chloride.
Cl2P42 - 4.2 x 10 mL.s
0.3
Cl24725 - 2.5 x 10 mL.s
0.0
Cl2P25 - 2.5 x 10 mL.s
-3
-1
-3
-1
-3
-1
ln (c)
-0.3
-0.6
-0.9
R2=0.9937
-1.2
-1.5
-1.5 -1.2 -0.9 -0.6 -0.3 0.0 0.3 0.6
2k t + ln(ci)
ð12Þ
Where, ci is the initial Mg content in mass%, Q is the total
Cl2 flux in L, MT is the Al mass, t is the time, and PMMg is the
Mg molecular weight. In the second case, the bubbles reach
the surface before all reactions have finished.
The bubble holding time in the liquid is not enough to
ensure the reaction between the Al chloride and the Mg.
Under such conditions, the removal of Mg is caused by two
separate phenomena. The first one refers to the Mg removed
by the reaction with the Al chloride (Table 1 and reaction 5),
and the second refers to the oxidation on the surface
(Table 1 and reaction 9). In both cases, the controlling step
of the process is the Mg diffusion in the boundary layer.
Furthermore, the overall rate of Mg removal is given by the
eq. (13):
0.6
Fig. 8 Relationship between experimental results for chlorine injection and
adjustment curve of Higbie’s Model.
5.
Conclusions
(1) The mg removal from liquid aluminum on the tests
when no gas is purged is controlled by diffusion of Mg in the
bath.
(2) Argon purging increases the Mg removal because the
bubbles break the protective layer.
(3) Chlorine injection increases the removal of Mg. The
controlling step is the diffusion on the boundary layer to form
Al2Cl6(g). When the bubbles reach the surface, the oxidation
of Mg by the atmosphere also occurs.
(4) The use of a mixture of inert gas and chlorine enhances
Mg removal. This effect may be explained by the decrease in
the boundary layer caused by the bath stirring.
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