Hydrometallurgy 98 (2009) 298–303
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Hydrometallurgy
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Preparation of sodium aluminate from the leach liquor of diasporic bauxite in
concentrated NaOH solution
Shaotao Cao a,b,c, Yifei Zhang a,b,⁎, Yi Zhang a,b
a
b
c
National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Beijing 100190, China
Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e
i n f o
Article history:
Received 7 March 2009
Received in revised form 26 May 2009
Accepted 26 May 2009
Available online 31 May 2009
Keywords:
Bauxite
Leaching
Concentrated NaOH
Monosodium aluminates hydrate
MAH
Crystallization
a b s t r a c t
A new process to produce monosodium aluminate hydrates (MAH) by fast crystallization from the leach
liquor of a diasporic bauxite in concentrated NaOH solution is presented. The crystallization of MAH was
carried out easily compared to the precipitation of gibbsite and the effect of agitation, initial concentration of
sodium aluminate, seed amount and the presence of red mud were systematically studied in a batch
crystallizer. The apparent kinetics of crystallization followed a second order rate law with an apparent
activation energy for MAH crystallization of 38.0 kJ/mol which implies a surface-diffusion controlled
mechanism. X-Ray diffraction and scanning electron microscopy identified the structure of MAH as
Na2O·Al2O3·2.5H2O with a flake crystal morphology. The molar ratio α of Na2O to Al2O3 in the MAH products
was b 1.2 after a simple wash by dilute sodium aluminate.
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Sodium aluminate is often used as a water purifying agent, an
additive to improve sizing and filler retention and for pitch control in
news-print mills (Misra, 1986). It is also a pH adjustment chemical on
many occasions (Rayzman et al., 1998) and a convenient source of
alumina to prepare zeolites and other catalysts (Kaduk and Pei, 1995).
In the hydro-chemical process for alumina production from
bauxite, the molar ratio of Na2O to Al2O3 (αk) in the concentrated
leach liquor is normally above 10 (Rayzman et al., 1998; Zhang et al.,
2008, 2005). Therefore, hydrated sodium aluminate has to be
crystallized from the liquor, followed by its re-dissolution to obtain
the sodium aluminate solution with αk about 1.5, needed for the
seeded hydrolysis of the solution to precipitate aluminum hydroxide.
The solid sodium aluminate takes on different hydrates or
anhydrous forms in the phase diagram for the Na2O–Al2O3–H2O
system. Anhydrous monosodium aluminate crystallizes from supersaturated sodium aluminate liquor containing 530–550 g/L Na2O at
170 °C whilst MAH (Na2O·Al2O3·2.5H2O) crystallizes from liquors
with 400–620 g/L Na2O at 60–140 °C. At low temperatures,
Na2O·Al2O3·3H2O crystallizes at 5–45 °C and tri-sodium aluminate
is obtained from solutions with high alkali concentration (Misra,
1986). In the phase diagrams for the Na2O–Al2O3–H2O system at
⁎ Corresponding author. National Engineering Laboratory for Hydrometallurgical
Cleaner Production Technology, Beijing 100190, China. Tel./fax: +86 10 62655828.
E-mail address: [email protected] (Y. Zhang).
0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.hydromet.2009.05.016
95 °C and 110 °C, Zhang et al. (2003a) also determined the formation
of 4Na2O·Al2O3·12H2O and the 6Na2O·Al2O3·12H2O in concentrated aluminate solutions.
Anhydrous sodium aluminate can be produced by either sintering
the mixture of sodium carbonate and bauxite or aluminum hydroxide
in rotary kilns at 1000 °C (Misra, 1986), or drying sodium aluminate
solution in fluidized bed (Misra, 1986; Pevzner et al., 1981). It was
reported that anhydrous sodium aluminate can also be produced by
thermal decomposition of sodium dawsonite (Contreras et al., 2006).
However, hydrated sodium aluminate is often manufactured by the
crystallization of the supersaturated sodium aluminate solution
(Hudson and Swansiger, 1976; Misra, 1986; Rayzman et al., 1998),
which is obtained through the evaporation of the leach solution from
the Bayer Process, or from the Sintering Process or from the hydrochemical process of alumina production.
The sintering method using solid materials and the evaporation of
sodium aluminate solution to produce anhydrous sodium aluminate
are both energy-intensive. Furthermore, the thermal decomposition
of sodium dawsonite is unable to produce pure sodium aluminate,
which significantly influences the usefulness of the product (Misra,
1986). On the other hand, crystallization from the bauxite leach
solution in the Bayer Process is potentially an economical process to
produce solid sodium aluminate in the industry. The Bayer Process is
more efficient compared with the Sintering Process because of less
energy consumption (Loh et al., 2002), but it is not as economical as
the hydro-chemical process to produce hydrated sodium aluminate.
This is because the concentrated alkali leach solution used to treat
S. Cao et al. / Hydrometallurgy 98 (2009) 298–303
bauxite in the hydro-chemical process can be directly used for the
crystallization of hydrated sodium aluminate without further evaporation (Rayzman et al., 1998; Sazhin, 1958; Zhang et al., 2005). The
leaching of diasporic bauxite, which constitutes over 98% of the
bauxite in China (Liu et al., 2007), must be carried out under a
pressure of 6.0 MPa at 250 °C in the Bayer Process, while in the hydrochemical process it is carried out at pressures under 0.5 MPa below
190 °C (Zhang et al., 2008, 2005). Therefore, this research investigates
for the first time the crystallization of MAH from the leach liquor of
diasporic bauxite with concentrated alkali solution used in the hydrochemical process, without evaporation.
Unlike the precipitation of aluminum hydroxide in the Na2O–
Al2O3–H2O system, which has been intensively investigated (Li et al.,
2005; Livk and Ilievski, 2007; Sweegers et al., 2002), there are few
systematic studies published on the crystallization of sodium
aluminate. Sazhin (1958) studied the crystallization of solid sodium
aluminate from synthetic solution with the initial Na2O concentration
below 525 g/L and the initial αk above 3.7. Rayzman et al (1998)
reported the crystallization of MAH from aluminate solution made
from leaching nepheline rocks, where the molar ratio αk of Na2O to
Al2O3 in the solution was 10–11 and the crystallization ratio of MAH
was accordingly less than 65%. But none have reported on the kinetics
of crystallization and the characterization of MAH.
This research therefore focuses on the parameters affecting the
apparent kinetics of MAH crystallization from the concentrated NaOH
leach liquor of diasporic bauxite, including the effect of red mud. It
was considered that if the red mud in the leach liquor insignificantly
influenced the crystallization of MAH, then filtration of the red mud
would be an unnecessary step since the crystal MAH must be redissolved and de-silicated to make sodium aluminate solution in the
hydro-chemical process (Zhang et al., 2008).
2. Experimental
2.1. Experimental procedure
Leaching of the bauxite was carried out in a stirred autoclave, and the
filtrates were sometimes evaporated to obtain the required supersaturated aluminate solution for the crystallization of MAH. The time of
crystallization was recorded after adding seed when the solution
reached to the preset-temperature. The obtained crystals were filtered
and washed with dilute sodium aluminate solution; then dried for
chemical and phase analysis and identification of their morphology.
The compositions of the initial supersaturated solutions and the
solutions during crystallization at various times were analyzed to
calculate the crystallization rate of MAH. The solutions were sampled
from the crystal slurry using a syringe fitted with sintered titanium
micro-filter.
2.2. Experimental apparatus
The 1 L autoclave used for leaching diasporic bauxite was made of
stainless steel, but parts of the reactor in contact with the leach
solutions as well as the beaker used for evaporation of the filtrate were
made of nickel metal. The MAH crystallizer consisted of a 350 mL
Teflon vessel heated by circulating oil from a thermostat bath within
±0.5 °C and stirred with a Teflon impeller driven by a multi-variable
speed motor under auto-controlled agitation. The crystallizer was
sealed to prevent the evaporation of water from the solution and the
influence of dust during the crystallization.
2.3. Materials
The diasporic bauxite, whose composition is listed in Table 1, was
provided by Yulian Co. Henan Province, China and was ground below
0.1 mm. Reagents of NaOH and Al(OH)3 were of analytical grade, and
299
Table 1
Chemical composition of the diasporic bauxite and the typical leach liquor.
Components
Na2O
Al2O3
SiO2
Fe2O3
CaO
TiO2
A/S
Diasporic bauxitea (wt.%)
Typical leach solutionb (g/L)
–
427
57.97
244
11.68
5.9
5.20
1.0
0.59
0.12
2.68
–
4.96
41.4
a
b
On dry basis.
Leached by concentrated NaOH solution with α of 22 at 180 °C for 2 h.
the water was purified using a Millipore Milli-Q system. MAH seed
used in the experiments was prepared by the crystallization of a leach
solution of diasporic bauxite at 60 °C for 18 h, where the initial Na2O
concentration was 446 g/L. The red mud used for investigating the
effect of red mud on the crystallization of MAH was fresh filter cake
from the leach liquor of bauxite.
2.4. Analytical methods
The chemical compositions of the solution samples were analyzed
by Optima 5300DV ICP-AES. X-Ray diffraction patterns of the crystals
were identified on the powder diffractmeter (XRD, X’Pert MPD Pro,
PanAnalytical, Netherlands) with Cu Kα (λ = 0.15408 nm) radiation at
room temperature. Field-emission scanning electron microscopy
(FESEM, JSM-6700F, JEOL, Japan) was used to analyze the microstructure of the crystal samples which had been previously ultrasonicated and well dispersed in absolute alcohol.
The crystallization ratio (ηt) of MAH from the supersaturated
sodium aluminate solution was calculated by Eq. (1), which is similar
to that of gibbsite precipitation (Zeng et al., 2008).
ηt =
αt − α0
αt − 1
ð1Þ
where α0 and αt denote the initial molar ratio of Na2O to Al2O3 and that at
time t in solution, respectively. Both are calculated by Eq. (2) as follows:
α = 1:6454
CN
CA
ð2Þ
where CN (g/L) is the concentration of Na2O and CA (g/L) the
concentration of Al2O3.
3. Results and discussion
3.1. Leaching of diasporic bauxite
The leaching of diasporic bauxite in concentrated NaOH solution
with αk of 22 was carried out at 180 °C under 0.35 MPa for 2 h with
stirring at 700 rpm. The mass ratio of NaOH solution to bauxite was
2.5:1, except for the experiment investigating on the effect of initial αk
on the crystallization of MAH. The typical composition of the leach
solution is also listed in Table 1.
3.2. Isothermal crystallization of MAH
The chemical structure of the aluminate species in concentrated
solution has been studied over several decades, but it is still not well
understood. The main species in dilute liquor of the Bayer Process is
widely accepted as Al(OH)−
4 , but it was suggested that the dehydrated
species existed in the sodium aluminate solution with
Al2O(OH)2−
6
increasing alkaline concentration (Moolenaar et al., 1970; Watling,
1998). Whilst there are no published references investigating the
aluminate species in concentrated NaOH (N14 M), the main species is
also postulated as Al2O(OH)2−
6 and the general chemical equation for
MAH crystallization can be expressed as follows:
þ
2−
4Na + 2Al2 OðOHÞ6
= 2Na2 O·Al2 O3 ·2:5H2 OðMAHÞ + H2 O:
ð3Þ
300
S. Cao et al. / Hydrometallurgy 98 (2009) 298–303
Table 2
Experimental conditions of the crystallization of MAH from solution.
Exp. no.
T
(°C)
NaOH
(g/L)
α0
Stirring speed
(rpm)
MAH seed
(g/L)
Red mud
(g/L)
1
2
3
4
5
6
7
8
9
10
11
12
13
60
60
60
60
60
60
60
60
60
60
80
100
60
575
425
469
580
600
574
577
594
570
566
583
568
580
2.84
2.94
2.78
4.88
2.82
2.73
3.02
2.85
2.87
3.00
2.97
2.81
2.92
300
300
300
300
100
200
500
300
300
300
300
300
300
40
40
40
40
40
40
40
0
20
80
40
40
40
No
No
No
No
No
No
No
No
No
No
No
No
158
Fig. 2. Effect of initial α0 on the crystallization of MAH.
The investigated conditions of MAH crystallization in this study are
listed in Table 2 and include stirring speed, initial concentration of
supersaturated sodium aluminate solution, seed amount and
temperature.
3.2.1. Effect of stirring
The results of experiments 1, 5, 6 and 7 under the conditions listed
in Table 2 showed that the crystallization ratio of MAH from the
solution was little changed by varying the agitation speed from 200 to
500 rpm and was only slightly lower at 100 rpm. Therefore, all other
experiments were carried out at 300 rpm, and the influence of
diffusion in the supersaturated solutions can be ignored.
3.2.2. Effect of the initial concentration of sodium aluminate
The rate and extent of MAH crystallization from the supersaturated
solution increased with the initial alkali concentration from 425 to
575 g/L Na2O (Fig. 1), but decreased with the initial αk from 2.8 to 4.9
(Fig. 2).
According to Zhang et al., (2003a) and Agranovsky (1970), the
solubility of MAH in sodium aluminate solution decreases with an
increase of NaOH concentration in the concentrated alkali region of
the phase diagram of Na2O–Al2O3–H2O. Therefore, the driving force,
and accordingly the crystallization of MAH increases with NaOH
concentration and initial alumina concentration, but the alumina
concentration is more important as shown by Fig. 2.
Fig. 1. Effect of initial NaOH concentration on the crystallization of MAH.
3.2.3. Effect of seed amount
With unseeded sodium aluminate solution (α0 = 2.9), the induction time of MAH nucleation at 60 °C was about 1 to 2 h. However,
when 20 to 80 g/L MAH seed was added, the induction time for
crystallization was negligible and there was little difference in the rate
or recovery of MAH as shown in Fig. 3. Clearly, only low amounts of
seed (b20 g/L) are needed to initiate fast crystallization unlike
gibbsite precipitation.
3.2.4. Effect of crystallization temperature
The results of MAH crystallization at 60, 80 and 100 °C with 40 g/L
seed in solutions with α0 of 2.9 are shown in Fig. 4. Clearly, the higher
temperatures increase both the kinetics and the solubility of MAH in
solution (Zhang et al., 2003a; Agranovsky, 1970), but the increased
solubility lowers the supersaturation and driving force for crystallization, so the overall rate and recovery of MAH decreases. In this
case, with excess seed the kinetics are quite fast.
3.2.5. Regression of the crystallization kinetics
The apparent kinetics of MAH crystallization from the supersaturated sodium aluminate solution can be expressed as follows
(Skoufadis et al., 2003):
dCA
4 n
= − k CA −CA
dt
ð4Þ
where CA (g/L) is the concentration of Al2O3 in solution while CA⁎ (g/L)
the equilibrium concentration, n is the reaction order, and k is the
reaction rate constant.
Fig. 3. Effect of seed amount on the crystallization of MAH.
S. Cao et al. / Hydrometallurgy 98 (2009) 298–303
Fig. 4. Effect of temperature on the crystallization of MAH.
Fig. 6. lnk versus 1/T of the kinetic equation for MAH crystallization.
As discussed above, CA⁎ is related to the Na2O concentration and
the temperature of crystallization, and can be obtained from
references to the phase diagram of the Na2O–Al2O3–H2O system
(Zhang et al., 2003a; Agranovsky, 1970). Based on Eq. (4), if the
reaction order n was 2, the integration formula of CA can be linearly
correlated to the crystallization time t, and expressed as follows:
1
1
−
= kt
4
CA − CA4
CA;0 − CA;0
301
ð5Þ
⁎ (g/L) are the actual and the saturated concentrawhere CA,0 and CA,0
tions of alumina in the initial solution, respectively.
According to the data in Fig. 4, the linear relation of CA and t was
regressed as shown in Fig. 5, based on Eq. (5). This confirmed that the
reaction order of MAH crystallization at 60, 80 and 100 °C was 2,
which is the same as the precipitation of aluminum hydroxide
(Skoufadis et al., 2003; Watling et al., 2000). Based on the data in
Fig. 5, the empirical constants k for MAH crystallization at 60, 80 and
100 °C were further regressed and found to vary linearly with the
reciprocal of the temperature (1/T), as shown in Fig. 6. According to
the Arrhenius equation, the apparent activation energy for MAH
crystallization was calculated from the slope of the line as 38.0 kJ/mol,
which indicates that the crystallization of MAH was distinctly surfacediffusion controlled (Mullin, 2001).
Fig. 5. Regression of the crystallization rate by the kinetic equations of second order.
3.3. Effect of red mud
The effect of red mud on the crystallization of MAH was
investigated in order to study the feasibility of omitting the filtration
operation before MAH crystallization. The red mud remaining in the
leach solution was 158 g/L and could act as the solid agent for the
desilication of the liquor which is the next step in the hydro-chemical
process to produce alumina.
Fig. 7 shows that red mud slightly depresses the crystallization
ratio of MAH from the solution which could be attributed to either a
lower number of moles of aluminate ion in the slurry relative to the
clear solution or to an increase of the solubility of MAH in the sodium
aluminate solution.
It is well known that the impurities in the solution can considerably
affect crystal nucleation and growth (Mullin, 2001). In this research, it
was found that the concentration of silicate ion (SiO2−
3 ) in the 60%
NaOH leach solution could be over 20 g/L, which is much larger than
that in pregnant Bayer liquor that contains about 15% NaOH and b1 g/L
2−
content in supersaturated sodium
SiO2−
3 . The increased SiO3
aluminate solution has been found to restrain the crystallization of
MAH (Sazhin, 1958; Zhang, 2003). Red mud generally contains over
50% aluminosilicate; therefore if the solubility product of aluminosilicate remains reasonably constant in the liquor, the increased
in solution would then decrease of the
concentration of SiO2−
3
Fig. 7. Effect of the existence of red mud on the crystallization of MAH.
302
S. Cao et al. / Hydrometallurgy 98 (2009) 298–303
(Misra, 1986). However, many operating conditions, impurities and
even reactor hydrodynamics can affect the morphology (Garnier et al.,
2002; Mirza et al., 2008) and further investigations are needed on
which factors are significant.
For practical purposes, the shape as well as the particle size of the
crystalline MAH significantly influences the efficiency of washing the
spent solution away from the filter cake. High washing efficiency is
needed to obtain a product with low αk. In these experiments, the
molar ratio αk of the MAH obtained was below 1.2 after a simple wash
by dilute sodium aluminate solution (180 g/L Na2O and 100 g/L
Al2O3).
3.5. Contrast of the crystallization behavior of Al(OH)3 and MAH
Fig. 8. The XRD patterns of solids crystallized from the sodium aluminate leach solutions.
concentration of aluminate ion. This, in turn, enhances the solubility of
MAH (Zhang et al., 2003b) and decreases of the extent of crystallization of MAH.
3.4. Characterization of crystalline sodium aluminate (MAH)
The crystalline phases obtained from the experiments were
basically MAH as verified by the XRD spectra shown in Fig. 8. The
morphology of the MAH crystallized from the supersaturated sodium
aluminate solution mostly consisted of separate flakes with some
twinning and composite round flakes (Fig. 9) which is different from
the morphology of octagonal platelets presented by the references
Aluminum hydroxide (or gibbsite) and MAH are two typical
equilibrium solid phases in the Na2O–Al2O3–H2O system, but display
very different characteristics in their crystallization processes from
the respective supersaturated sodium aluminate solutions. It is well
known that the precipitation of aluminum hydroxide is very slow, and
a large amount of seed is necessary in the alumina industry, but the
crystallization of MAH with a much less seed is quite fast as shown in
this study.
Watling (1998) showed that the main aluminum-containing ions
2−
in sodium aluminate solution, i.e. Al(OH)−
4 and Al2O(OH)6 , are 4coordinate. As a result, the transformation of these ions to the 4coordinate Al atoms in the MAH crystals (Kaduk and Pei, 1995) needs
less activation energy than transformation to 6-coordinate Al atoms in
aluminum hydroxide or gibbsite. The activity energy of gibbsite
precipitation is between 50 and 59 kJ/mol and that of boehmite
precipitation is 89 kJ/mol (Skoufadis et al., 2003), which are both
much higher than 38 kJ/mol for MAH crystallization found in this
work.
Fig. 9. SEM images of MAH crystallized from the sodium aluminate leach solutions. (a, b) MAH seed; (c, d) MAH product.
S. Cao et al. / Hydrometallurgy 98 (2009) 298–303
4. Conclusions
The crystallization rate and recovery of MAH from the leach liquor
of diasporic bauxite in concentrated NaOH solution was high, unlike
the slow precipitation of gibbsite, and increased with the initial
concentration of Na2O and Al2O3 in the solution. However, the overall
recovery decreased slightly with increasing the crystallization
temperature and without filtration of the red mud in the liquor. The
amount of seed required for the crystallization of MAH was much less
than that for the precipitation of gibbsite. The crystallization rate of
MAH showed a second order dependence upon the supersaturation of
sodium aluminate, and the apparent activation energy for MAH
crystallization was determined to be 38.0 kJ/mol, which implies that
the mechanism of MAH crystallization was distinctly surface-diffusion
controlled.
The MAH products were confirmed by the XRD analysis as
Na2O·Al2O3·2.5H2O and crystallized as mostly thin flakes with some
twinning and composite round flakes. The molar ratio αk of Na2O to
Al2O3 in the filtered product was below 1.2 after a simple washing by
dilute sodium aluminate solution.
Acknowledgements
We acknowledge the National Basic Research Program of China
(973 program, NO. 2007CB613501), and the National Key Technologies R&D Program (NO. 2006BAC02A05) for funding this work.
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