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Methanol as Anti-solvent for Recycling of Caustic Soda Solution Used in Aluminum products manufacturing.

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Dev. Chem. Eng. Mineral Process. 13(1/2), pp. 14 7-157, 2005.
Methanol as Anti-solvent for Recycling of
Caustic Soda Solution Used in Aluminum
Products Manufacturing
K. Fukui'*, K. Maeda', T. Yanagisawa', K. Honda3
and M. Yoshida3
' Dept of Mechanical and System Engineering, University of Hyogo,
Shosha 2167, Himeji 671-2201, Japan
Shanghai Asia Aluminum Products Co. Ltd., Qinpu County,
Shanghai 201 713, China
Takuma Plant Co. Ltd., Kinrakuji 2-2-33, Amagasaki 660-0806, Japan
The recycling of caustic soda solution in aluminum processing is proposed in order to
decrease the energy consumption and environmental impact. Methanol as an
anti-solvent in the caustic soda solution was found to be the most effective solvent for
reducing gibbsite solubility. The gibbsite solubility in caustic soda was significantly
reduced when methanol was added by more than 40 wt%. It was found that the
gibbsite solubility with 55 w t% methanol was about a halfof that without methanol.
Methanol has low latent heat, and can be economically separated from the caustic
soda solution. This recycling leads to an increase in the dissolution rate of alumina
and/or aluminum, at atmospheric pressure, with a more concentrated caustic soda
solution (up to 500 g/L NaOH). A more concentrated caustic soda solution can be
recycled more economically, for example, that used in the die recovery process in
aluminum forming plants. Optimization by the controlled addition or mixing of
methanol in the caustic solution easily controls the growth rate and nucleation rate of
gibbsite clystals in the crystallizer Synergy of these advantages of the present
recycling method may contribute to more cost effective operations, and minimization
of environmental impactsfrom the disposal of spent caustic soda solution.
* Author for correspondence
K.Fukui, K.Maeda. I: Yanagisawa, K.Honda and M.Yoshida
Large amounts of concentrated caustic soda (NaOH) solution are commonly used in
alumina refining and for aluminum forming worldwide. All alumina is produced by
processing bauxite, using the so-called Bayer process. Whereas gibbsite (aluminum
trihydroxide, Al(OH)3)is the dominant mineral component in bauxites, other bauxites
contain boehmite and diaspore in monohydrate form. Generally, the bauxites may
contain all three aluminum hydroxide phases, and it is difficult to determine the
optimal condition for the extraction process. In the commercial Bayer process,
gibbsite is dissolved in a caustic soda solution containing 130-190 g/L NaOH at about
41 3K. Boehmite requires elevated temperatures above 473K, or approximately 343K
in 350 g/L NaOH solution. However, diaspore is dissolved at the higher temperature
of 523K and a pressure of 3.5 MPa in 260-400 g/LNaOH solution.
The aluminum processing industry produces many forms of the final aluminum
products, e.g. plate and pipe, often by extrusion using dies through to etching
processes for surface finishing. The old extrusion dies are reused after dissolving the
aluminum pieces adhered in and on the dies. The dye cleaning process requires a
concentrated caustic solution of 400 g/L NaOH to maintain a high dissolution rate,
depending upon the optimal formation rate of aluminum products. In the surface
etching process, a caustic solution of 100-200 g/L NaOH is used for surface finishing.
Therefore, the concentration of caustic soda solution for dissolving alumina or
aluminum depends upon the type of aluminum manufacturing processes.
From an environmental point of view, it has become even more important to
develop energy-conserving and economical techniques in the recycling of caustic
solutions. However, all of the processes mentioned above require individual recycling
processes because of the variety of caustic soda concentrations used in the extraction
or dissolution processes. Sodium aluminate (NaA102) solutions formed in these
processes can be recovered from the recycled soda solution by precipitating gibbsite
crystals from the sodium aluminate solutions. The optimal recycling process for
caustic soda requires a hgher efficiency of separation of gibbsite from the sodium
aluminate liquor in order to improve the cost performance of the total process. If we
can reduce the amount of aluminum dissolved in the recycled-caustic solution, then
the production rate of gibbsite crystals in the precipitation process would increase,
and the recycled caustic solution with less aluminate soda would have more capacity
to dissolve alumina or aluminum at higher dissolution rates. The most important point
for developing the recycling process is that the gibbsite precipitation has to be carried
out in a caustic solution of less than 200 g/L NaOH with hydrolysis, according to the
level of the gibbsite solubility.
The resulting caustic aluminate solution is cooled to become supersaturated, and
the caustic aluminate is converted to gibbsite crystals in the crystallizer. The caustic
solution is recycled, after condensing the caustic solution by either evaporating off
water or adding new caustic soda. During these processes, the gibbsite crystallizer is
operated at caustic soda concentrations of between 120-200 g/L NaOH and a
temperature of about 313-333K. Gibbsite is effectively crystallized in a solution of
120-170 g/L NaOH because the solubility of gibbsite in a caustic solution decreases
sharply at caustic soda concentrations lower than 200 g/L NaOH, as discussed later.
Methanol as Anti-solventfor Recycling of Caustic Soda Solution
This means that the caustic solution to be recycled has to be concentrated by
evaporation if the caustic aluminate solution generated in the dissolution process is
higher than 200 g/L NaOH. Such a recycling system for caustic soda solutions
consumes a large amount of evaporation energy of water. Another problem is that the
very low growth rate of gibbsite crystals (1-5 pm/hr) requires a long residence time in
a large-volume crystallizer. The crystallizer has to be stable in operation with fines for
gibbsite seeding, because secondary nucleation is insufficient in the crystallizer.
We have conducted the present study to crystallize and remove the gibbsite
dissolved in the caustic soda solution as completely as possible, by dramatically
reducing the gibbsite solubility with anti-solvent. The method proposed here can be
applied to the development of a new process for caustic solution recycling. This new
process will consume less energy and will improve control of the gibbsite crystallizer
in the alumina and aluminum manufacturing fields.
Solubility of Gibbsite in Caustic Soda Solution and Conditions of
Gibbsite Dissolution and Crystallization
This investigation into the solubility curve of gibbsite essentially contributes to the
improvement of recycling processes for caustic soda solutions. The solubility of
gibbsite can sluft at elevated temperatures and pressures. However, in order to achieve
the high temperature and pressure, the initial cost of the aluminum process increases
considerably. The solubility of gibbsite-caustic soda solution is described in the
following chemical reaction:
NaA102(L)+ 2 H 2 0 e Al(OH),(S) 3. +NaOH(L)
The gibbsite crystals are generally dissociated in a caustic solution, and gibbsite
cannot exist in a liquid phase. However, the caustic aluminate dissolved in the caustic
solution can crystallize out gibbsite crystals by adding water (hydrolysis). Gibbsite
solubility in a caustic solution at normal pressure has been reported by Misra and
Wlute [I], and Miki [2]. The results are shown in Figure 1 where the total content of
aluminum dissolved in the caustic solution (g/L Al) is plotted against the total content
of caustic soda in the solution (g/L NaOH). The phase diagram contains three regions:
(i) the gibbsite precipitation region of less than 300 g/L NaOH; (ii) the high solubility
region of 300-420 g/L NaOH even at low temperature; and (iii) the aluminate soda
crystallization region above 420 g/L NaOH. Such properties of the phase diagram
hndamentally provide useful information for designing the recycling process of
caustic soda solution. Gibbsite solubility decreases sharply in the range of 200-300
g/L NaOH, but more gradually at less than 100 g/L NaOH. The crystallization of
gibbsite from caustic aluminate solution should be performed effectively at about
120-170 g/L NaOH and low temperature. Therefore, the refining processes for
gibbsite-type bauxites operate most economically in the range 130-190 g/L NaOH.
The caustic soda solutions in the extraction processes from boehmite and diaspore
K. Fukui, K. Maeda, T Yanagisawa,K. Honda and M.Yoshida
require a more concentrated solution above 200 g/L NaOH. The solution of 400 g/L
NaOH is used for refreshing the die in the aluminum forming process. The recycling
processes of these caustic solutions require dilution with water in order to effectively
crystallize gibbsite. To completely recycle the caustic solutions, they have to be
condensed by evaporating off some water to form the concentrated caustic solution
necessary for the dissolution or extraction processes.
An alternative method is to change the gibbsite solubility using appropriate
additives. The additive materials in a caustic soda solution are limited because they
must be mutually soluble with an aqueous solution. Thus, most organic solvents are
invalid since they will only give immiscible solutions. Inorganic salts are also not
effective because it is difficult to separate the salts from the caustic solution.
Consequently, we have to look for an anti-solvent that can effectively dissolve in the
caustic soda solution.
The gibbsite phase diagram shows that the recycling efficiency of caustic soda
solutions, in the Bayer process and aluminum manufacturing, depends to a limited
extent on the gibbsite solubility difference between the gibbsite dissolving and
crystallizing processes. To increase the efficiency of the recycling process, we focused
on reducing the gibbsite solubility by adding organic and inorganic materials or
solvents. We conducted solubility experiments for about twenty additives in the
caustic soda solution. The selected organic solvents were expected to work as
anti-solvents for gibbsite, and the selected salts were expected to work as co-salts for
gibbsite. Results showed that the organic solvents with the exception of methanol
made the caustic soda solution immiscible, and they could not be used as anti-solvents.
Furthermore, inorganic salts could not work well as co-salts, and showed the dilution
effect only. Methanol was found to be the most adaptable solvent to the caustic
solution recycling process from the viewpoint of energy and economic efftciencies.
Methanol may be useful for increasing the gibbsite precipitation rate, and controlling
the morphology of gibbsite crystals [3, 41. However, it is not known whether
methanol added in abundance to the caustic solution, which causes a remarkable
reduction in the gibbsite solubility, is attractive enough to justify rebuilding of the
caustic solution recycling process.
This alternative was investigated by measuring the solubility of gibbsite in caustic
soda solution at different methanol concentrations and different temperatures.
According to the chemical equilibrium in Equation (l), the equilibrium constant (Kg)
is given by:
Kg = [ N u O H ] [ H , 0 ] 2
The equilibrium constant is a function of the temperature and methanol content.
Therefore, we proposed the use of the complete van’t Hoff model [ 5 ] to present the
solubility of gibbsite for a methanol anti-solvent system:
Kg = exp 12.8 -7C2 +1800%T]
Methanol as Anti-solventfor Recycling of Caustic Soda Solution
T-kOH [s/L]
Figure 1. Solubility of gibbsite and caustic soda aluminate in a caustic
soda solution.
MeOH hw(x M i i a st a1
-MeOH hUlX cdlc'd
MeOH XlM%cskd
T-NaOH [gIL] (MeOH free basis)
Figure 2. Solubility of gibbsite in a caustic soda solution with methanol.
K . Fukui, K. Maeda, 1: Yanagisawa,K. Honda and M.Yoshida
where C is the methanol weight fraction (in caustic soda and caustic aluminate-free
basis); R is the gas constant; and T is the absolute temperature; T-A1 is calculated
from the caustic aluminate concentration [NaAlO,]; T-NaOH is calculated from the
sum of caustic aluminate [NaA102] and free caustic soda maOH]. The gibbsite
solubility with and without methanol at 303K is shown in Figure 2; note that the
concentrations of A1 and NaOH are expressed on a methanol-free basis. The results
show the dramatic effect of anti-solvent methanol upon gibbsite solubility, when
methanol is present above 50 wt% in the caustic soda solution.
Recycling Process of Caustic Soda Solution
Figure 3 shows the flowchart of the normal recycling process for caustic soda
solutions. The aluminate-soda solutions generated during alumina extraction and
aluminum dissolution processes are recovered by gibbsite precipitation, and the
resulting soda solution is concentrated by evaporation. The evaporation process
consumes a large amount of thermal energy if the extraction or dissolution process
requires a caustic soda solution more concentrated than 200 g/L NaOH. This process
needs a large quantity of seeding because secondary nucleation alone in the
crystallizer is insufficient for controlling the crystallizations [ 6 ] . Figure 4 shows the
new recycling process using anti-solvent methanol. In this process, it is necessary to
separate methanol from the caustic solution by using a distillation unit. The methanol
and caustic solutions are recycled individually to the previous processes for extraction
and precipitation.
We will discuss the caustic soda recycling process with methanol for two
examples, in relation to the lowest and highest concentrations of caustic solution,
namely the die refreshing process and the Bayer process. It is assumed that the
gibbsite solubility is given by the predictions shown in Figure 2. The caustic solution
generated in the die refreshing process was sampled from a commercial aluminum
forming plant, the caustic solution used was 400 g/L NaOH and 80 g/L A1 in this
process. It is assumed that the caustic solution obtained after the extraction process in
the Bayer process is 160 g/L NaOH and 30 g/L Al. The conditions for these solutions
to be recovered are marked in Figure 2 with the symbols “D” and “G” respectively.
The caustic solution conditions obtained after gibbsite precipitation are estimated on
the assumption that the solutions reach chemical equilibrium after gibbsite
precipitation. The predictions are shown in Table 1 for the die refreshing process, and
in Table 2 for the Bayer process.
The concentrated caustic solution in the die refreshing process cannot be recycled
by normal precipitation after cooling the solution, because the gibbsite solubility is
high at NaOH concentrations above 300 g/L, as shown in Figure 2. In practice, the
commercial aluminum forming plants adopting such a process would need to establish
an economical recycling process. The caustic soda recycling process using methanol
has the disadvantage of an essential methanol distillation step, as shown in Figure 4.
However, it should be noted that the latent heat of methanol (1088 J/g) is
approximately half that of water (2256 J/g). We estimated the heat energy needed to
Methanol as Anti-solventfor Recycling of Caustic Soda Solution
NaOH solution return
Extraction or
dissolution of
Separation of
h impurities
r > ! $ d i n g
of AI(OH),
Separation of
Crystals of
Figure 3. Flowchart of the normal recycling process of caustic soda solution.
Extraction or
dissolution of
MeOH solution
Separation of
Crystals of
Separation of
Figure 4. Flowchart of the new recyclingprocess of caustic soda solution
using anti-solvent methanol.
K. Fukui, K. Maeda. T Yanagisawa,K. Honda and M. Yoshida
recycle the spent caustic soda in two processes, with addition of 55 wt% methanol and
without addition of methanol. In the die refreshmg case, the thermal energy efficiency
for the recycling process using methanol is about one third of that for the hydrolysis
process. Moreover, we expect that the recycling process using methanol should
become more economical for NaOH concentrations higher than 200 g/L in the Bayer
processes with bauxite refining of boehmite- and diaspore-type ores.
The caustic solution used in the Bayer process is about 150-170 g/L NaOH for the
gibbsite-type ore, and the aluminum dissolved in the solution as an aluminate soda
may be about 30 g/L Al. However, following gibbsite crystallization, the aluminum
dissolved in the solution is about 14 g/L Al, as shown in Table 2, indicating that half
of the aluminum extracted from the bauxite solution cannot be separated through the
normal precipitation process by cooling only. If such a solution were recycled to the
extraction process of bauxite, the degree of undersaturation for alumina extraction
may be limited because of gibbsite solubility, and it may cause a reduction in the
extraction rate and amount of alumina in the alumina extraction process. Using the
recycling process with methanol, the caustic solution that is recycled to the extraction
process includes about 7 g/L A1 and 3 g/L A1 for 55 wt% and 71 wt% methanol
addition, respectively. Hence, the methanol anti-solvent process effectively doubles
the yields when compared with the cooling method. However, it should be noted that
health and safety issues associated with implementation of this process need to be
NaOH (g/L)*
A1 (g/L)*
After precipitation
I 55 wt% MeOH
I 71 wt% MeOH
No additive
Table 2. Recycling of the caustic soda solution in the Bayer process at 303K.
After precipitation
No additive
55 wt% MeOH
71 wt% MeOH
NaOH (g/L)*
A1 (g/L) *
Methanol as Anti-solventfor Recycling of Caustic Soda Solution
Effects of Methanol-Addition Methods on CSD
This new technique of gibbsite crystallization with methanol can easily control the
supersaturation by not only supercooling, but also by the amount, mode and rate of
addition into the crystallizer. The anti-solvent effect of methanol is able to naturally
improve fines nucleation in the crystallizer. Another advantage of the methanol
additive method is that the gibbsite crystallizer needs no seeding, and it will be
possible to control the growth and nucleation rates of gibbsite crystals effectively by
optimizing the operating condition of methanol addition.
Some gibbsite precipitation experiments were carried out in batches using a small
0.5-litre crystallizer vessel. The concentrated caustic solution of 400 g/L NaOH and
90 g/L Al, which was sampled from the die-refreshing process in a commercial
aluminum plant, was prepared in a 200-ml crystallizer vessel. The Crystallizer was
maintained at 303K, and 300 cm3 of methanol was added and mixed in the crystallizer
in two different ways: by instantly mixing 200 cm3 of caustic solution with 300 ml of
methanol, and by gradually adding methanol at 30 cm3/day and mixing. The
time-changes of total A1 concentration in the solution and the CSD (crystal size
distribution) were measured for the next ten days. Figure 5 shows the time-change of
the A1 concentration (in g/L) in the solution and NaOH (g/L). In the case of methanol
250 I
I 150
' 0
tir7-E [hl
Figure 5. lime-change of aluminum and caustic soda for gibbsite precipitating
with methanol.
K. Fukui, K. Maeda, Z Yanagisawa,K. Honda and M. Yoshida
Figure 6. Crystal size distributions afer gibbsite precipitating with methanol.
instantly mixed with the caustic solution, then the concentration of A1 in the solution
decreased more abruptly because of violent nucleation. Thereafter, the A1
concentration in the solution gradually decreased as the gibbsite progressively grew in
the crystallizer. However, the aluminum concentration decreased gradually in the case
of gradual addition of methanol. If the methanol addition rate was slow, nucleation
rate and growth rate decreased considerably due to low supersaturation. Another
reason was that the chemical reaction given by Equation (1) occurred at significantly
low rate.
The addition rate of methanol is another operating variable for optimizing the
gibbsite crystal growth rate and fines nucleation rate. The batch process as presented
here is recommended especially for a small-scale plant located at aluminum
processing fields. In the Bayer process, the crystallizers operate continuously, and we
propose a multi-crystallizer system as currently adopted by the study. The crystallizer
vessels are connected in a cascade formation where the methanol concentration in the
vessel increases as one progresses downstream. Figure 6 shows the CSD measured
after eight days. The CSD obtained from gradual mixing shifts to the large region,
compared with the instant mixing method. The gradual mixing method can maintain a
low degree of supersaturation throughout the crystallization process. This method can
suppress the heterogeneous nucleation rate and promote the gibbsite growth rate.
Methanol as Anti-solventfor Recycling of Caustic Soda Solution
The effects of anti-solvent on gibbsite solubility in caustic soda solutions were
investigated in order to improve economical recycling processes for caustic solutions
used in aluminum manufacture. The results obtained here are summarized as follows:
1. Gibbsite solubility is significantly reduced by diluting the caustic solution with
methanol, when compared with normal solubility without additive. The solubility
model of gibbsite in the caustic solution with added methanol was proposed, and
was in good agreement with measured results.
2. The advantage of using methanol as an anti-solvent for crystallization of gibbsite
is shown for both the Bayer process and for the die-refreshing process in an
aluminum forming plant. The growth and nucleation rates of gibbsite crystals can
be controlled by optimizing the mode and rate of addition of methanol.
I . Misra, C., and White E.T. 1971. Kinetics of crystallization of aluminum trihydroxide from
seeded caustic aluminate solutions. Chem. Eng. Prog. Symp. Sex, 67, 53-65.
2. Miki, H. 1994. Crystal size change with elapsed time of aluminum hydroxide in industrial
crystallization. Kagaku Kogaku Ronbunshu, 20,346-35 1.
3. U.S. Patent. 1970. 3,511,606.
4. U.K. Patent. 1965. 1,123,184.
5 . Pitzer, K.S., and Brewer, L. 1961. In Thermodynamics, revised 2nd Edn, by GN. Lewis,
and M. Randall, McGraw-Hill, New York.
6. Tadayon, A., Ang, H.M., Tadt, M.O., and Rohani, S. 2002. Dynamic modeling of gibbsite
precipitation processes: Is secondary nucleation the only major source of fines generation?
The qhAPCChE Confirenee, Christchurch, New Zealand, Paper no.5 12.
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