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Agglomeration of gibbsite particles from carbonation process of sodium aluminate solution
Hydrometallurgy 99 (2009) 163–169
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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
Agglomeration of gibbsite particles from carbonation process of sodium aluminate solution Qiusheng Zhou, Dianjun Peng, Zhihong Peng, Guihua Liu, Xiaobin Li ⁎ School of Metallurgical Science and Engineering, Central South University, Changsha, Hunan Province, 410083, PR China
a r t i c l e
i n f o
Article history: Received 8 February 2009 Received in revised form 31 July 2009 Accepted 31 July 2009 Available online 11 August 2009 Keywords: Sodium aluminate Carbonation Agglomeration Agglomeration model Supersaturation Particle size distribution
a b s t r a c t The particle size distribution of the carbonation products from sodium aluminate solution was analysed based on the relative mass supersaturation and the particle number conversion from the Malvern volume distribution curves. An approximate relationship was developed which showed that the rate of change of particle number in each particle size interval (0–20 μm, 20–45 μm and 45 μm–∞) per unit volume was proportional to the rate of change of the relative mass supersaturation of the solution. The evolution of agglomeration of gibbsite particles in the carbonation process was also studied and the results show that the agglomeration rate constant is mainly influenced by the initial particle number of gibbsite Al(OH)3 under the experimental conditions. The agglomeration between fine grains (b 20 μm) plays a dominant role when the initial particle number is large, while the agglomeration between relatively coarse particles (20–45 μm) occurs with a small initial particle number. A carbonation product with extremely narrow-sized distribution and ideal large average size of 75 μm was produced by regulating the relative mass supersaturation of the solution, the amount and particle size distribution of added seeds and other carbonation parameters such as temperature and stirring rate. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbonation of sodium aluminate solution is one of the important operations of the lime-soda-sinter process in the production of alumina. It directly influences the physical properties of the product, such as the mean particle size, the particle size distribution and morphology. Carbonation has been the key process for producing sandy alumina which can satisfactorily meet the requirements of modern aluminium molten salt electrolysis in the alumina production by the sinter process (Li et al., 2009). As the whole carbonation duration time (5–8 h) is much shorter than that for seeded precipitation (40–70 h) and the gibbsite crystal growth rate is very slow during this time (Veesler and Boistelle, 1993), particle agglomeration is the most important way to rapidly enlarge the particle size needed for sandy alumina. Agglomeration is the process that a number of particles adhere together to form a stable attached polymer (Seyssiecq et al., 1998). Since 1970s, many researchers have studied gibbsite agglomeration in the seeded precipitation of Bayer liquors under various conditions with different methods. Muralidar and Ramkrishna (1985) investigated the mechanism of agglomeration with data from the crystal size distribution based on the crystal population balance equation. Hartel and Randolph
⁎ Corresponding author. Tel.: +86 731 8883 0454; fax: +86 731 8883 04533. E-mail address: [email protected] (X. Li). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.07.015
(1986) and Houslow et al. (1990) also studied the agglomeration mechanism from the aspect of crystallization process. Scott (1963), Misra (1970), Sakamoto and Kanehara (1976), Low (1975), Halfon and Kaliaguine (1976) and Remillard et al. (1980) have all pointed out that agglomeration consisted of collision resulting from the induction of the flow shearing stress and cementation with the binder precipitating from the solution. The result of the collision among particles is to form aggregates which can readily break up. The result of cementation is to form agglomerates which are relatively stable as a chemical bond is formed among particles with the binder of gibbsite precipitating from the solution. However, Marchal et al. (1988) proposed that gibbsite agglomeration process included three steps: the mechanical collision, the interactions between particles and binding resulting from the crystal growth. A considerable amount of work has been reported on the individual influences of crystallizing parameters on the gibbsite agglomeration. Crystallization temperature and supersaturation of the solution play the most important role in the gibbsite agglomeration. Moreover, an increase of the crystallization temperature and supersaturation promotes the gibbsite agglomeration (Seyssiecq et al., 1998). The agglomeration rate increases with the increase of seed charge, but rapidly reaches a plateau before decreasing when the seed charge is too high, while agglomeration continuously decreases with increasing stirring rate (Veesler et al., 1994). As for the relationship between agglomeration rate constant and particle size, there are two quite different opinions. One is that there is a strong dependence of agglomeration rate constant on the particle size
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(Low, 1975); and the other is that there is independence of agglomeration rate constant on the particle size (Ilievski and White, 1994), which is supported by the experiments carried out in the self-designed precipitator operating at constant supersaturation (Seyssiecq et al., 2000; Ilievski, 2001; Ilievski and Livk, 2006). Although a lot of work has been reported in the gibbsite agglomeration literature, consensus has not been reached completely on the intrinsic mechanism and influencing factors of agglomeration. Different researchers have different opinions and established different models. Furthermore, the focus of almost all of the previous research is on the particles with size range from 4 μm to 20 μm. On the other hand, although carbonation and seeded precipitation of caustic aluminate solution follow the same mechanism of gibbsite crystallization (Liu, 2004), there are some differences between carbonation and seeded precipitation, such as much shorter duration time for carbonation, high silicon concentration present in carbonation solution, relatively low concentration and probably different ion structures of aluminium species in the carbonation solution. Furthermore, carbonation is a more complicated process which combines interactions of three phase of CO2-containing gas, liquid and solid, so the gibbsite particle agglomeration may be some different from that of seeded precipitation from Bayer liquor. Up to now, there is little report of agglomeration of gibbsite particles from carbonation. Therefore, this investigation of the agglomeration of gibbsite from the carbonation process, as well as the development of an applicable and efficient agglomeration model, is necessary for effectively and precisely controlling the gibbsite agglomeration. 2. Experimental 2.1. Experimental method Rather pure supersaturated sodium aluminate solution was synthesized in laboratory by dissolving Al(OH)3 powder (from CHALCO, Al2O3 N64.5%, SiO2 b0.02%, Fe2O3 b0.02%) into caustic soda solution (industry grade, NaOH N96.5%) and then filtering for carbonation experiments. Before the carbonation experiments, the required amount of aluminate solution was poured into a 2 L stainless steel carbonation apparatus and the carbonation apparatus was immersed in a thermostat oil bath with temperature precision of ±1 °C. After the solution was preheated for 10 min to the carbonation temperature of 95 °C, accurately weighed gibbsite Al(OH)3 seed (from carbonation of aluminate solution) was added to the carbonation apparatus and at the same time, agitation and ventilation of the mixed CO2 and air gas system were operated to start the carbonation. The CO2 concentration of the gas mixture was measured by the CYES-II CO2 analyzer (Shanghai Analytical Corporation, PR China), and the flow rate of the mixed gas was controlled by a glass rotameter. At prescribed times, 5 ml of the carbonation slurry was sampled and immediately centrifuged to obtain aluminate solution and solid gibbsite Al(OH)3 powder. The resulting aluminate solution was used to determine the concentration of sodium hydroxide as Na2O and the concentration of aluminium species as Al2O3 by titration. The gibbsite Al(OH)3 powder was washed with deionized water and then the crystal size distribution (CSD) was analysed by using a Mastersizer 2000 (Malvern Instruments Ltd., UK). The morphology of the product was observed by scanning electron microscopy (JSM-6360LV, Japan). 2.2. Data processing The total concentration of soda includes caustic soda (NaOH) which gradually converts into sodium carbonate with the carbonation process. Taking into consideration that the volume of the aluminate solution may change with carbonation and that the total amount of soda is a constant in the whole carbonation process, we can calculate
the carbonation rate of sodium aluminate solution by the following equation: η=
Aa −Am × Aa
Nt0 Nt
× 100%
where: η— carbonation rate of sodium aluminate solution, %; Aa— concentration of aluminium species in initial solution, g/L; Am— concentration of aluminium species in carbonated solution, g/L; Nt0— total concentration of soda in initial aluminate solution, g/L; Nt— total concentration of soda in carbonated aluminate solution, g/L. The equilibrium concentration of aluminium species (as Al2O3) in the sodium aluminate solution can be calculated by employing the following equation proposed by E. T. White: 2486:7 1:08753 + Na2 O Ceq = Na2 O exp 6:2106− T T where Ceq = equilibrium concentration of aluminium species, g/L; Na2O=concentration of NaOH as Na2O in solution, g/L; T = temperature, °K. We divide the particle size of the carbonation product into three size intervals 0–20 μm, 20–45 μm and 45–∞, considering that gibbsite particle agglomeration usually takes place among particles sized b20 μm and that the amount of particles of b45 μm is an important criterion for high quality gibbsite product for aluminium smelting. The particle number Ni of each particle size interval per unit volume (where i (i = 1,2,3,4) represents 0–∞, 0–20 μm, 20–45 μm, and 45–∞, respectively), was calculated according to data from the CSD curves of the seed added or the carbonation product at different carbonation stages. In this way, the rate of change of the particle number of each size interval per unit volume solution can be obtained. 3. Results and discussion 3.1. Agglomeration of gibbsite particles from carbonation of sodium aluminate The gibbsite precipitation in the carbonation process is very similar to that of seeded precipitation and includes a series of complex physical and chemical changes such as nucleation, agglomeration, crystal growth, grain grinding and breakage. These sub-processes are usually inter-twined and present in the whole carbonation process. But one of the sub-processes such as crystal nucleation, agglomeration or crystal growth may be dominant in certain stages of the process under the specific carbonation conditions. Fig. 1 shows the variation in particle number for each particle size interval per unit volume solution at different carbonation times with different masses of gibbsite Al(OH)3 seeds added. The particle number of seeds in Fig. 1(a) is relatively large (8.93× 109) and the particles of less than 20 μm account for 84.4%. The particle number of seeds in Fig. 1(b) is much smaller (1.10× 109) compared with that in Fig. 1(a) and the particles between 20 μm and 45 μm account for 79.3%. In Fig. 1(a) and (b), curve 1 shows the change of carbonation rate with different carbonation times, and curve 2 to curve 5 represent the change in particle numbers in the size interval of 0– ∞, 0–20 μm, 20–45 μm, 45 μm–∞, respectively. It can be seen from curve 2 that the total particle numbers are significantly reduced during carbonation processing under the two seed-added systems which clearly indicates that agglomeration has taken place. But there is a little difference between the two seeded systems. Fig. 1(a) indicates that the number of fine grains of 0–20 μm rapidly decreases in the earlier stages of carbonation (Fig. 1(a), curve 3), and the number of particles between 20 μm and 45 μm apparently increases (Fig. 1(a), curve 4), while the number of coarse particles of 45 μm–∞ is almost invariant (Fig. 1(a), curve 5). During carbonation
Q. Zhou et al. / Hydrometallurgy 99 (2009) 163–169
Fig. 1. Variation of particle number in each particle size interval of the product from carbonation with different seed size–∞; 3—0–20 μm; 4—20–45 μm; 5—45 μm–∞; (conditions: 95; (b) Al2O3 128 g/L, L)).
processing, the number of fine grains continues to decrease, but the decrease gets smaller and smaller whilst simultaneously the number of coarse particles is still invariant. When the extent of carbonation reaches 90.8%, the total number of particles is 3.08 × 109, and 84.2% of the particles are those between 20 μm and 45 μm due to agglomeration among fine grains. Relatively coarse particles cannot agglomerate effectively probably due to poor cementation with insufficient binder. In the later period of carbonation (N240 min), the carbonation rate is slow and hardly any agglomeration occurs as a result of low supersaturation of the aluminate solution. With less seed added initially, the results in Fig. 1(b) show that particles b20 μm almost disappeared in the product within 60 min carbonation. However, as the system still maintains a rapid carbonation rate, it can provide sufficient Al(OH)3 binder for particle agglomeration and relatively coarse particles between 20 μm and 45 μm can agglomerate continuously (Fig. 1(b), curve 4). When the extent of carbonation is 87.0%, the total number of particles is 5.1 × 108 and 90.2% have a particle size larger than 45 μm.
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supersaturation of the carbonation solution is larger, especially in the later period of gibbsite precipitation. So it is reasonable to assume the gibbsite crystal growth rate of 1 μm/h in carbonation process. We can calculate the particle number of different particle size intervals of the seeds added to estimate the influence of crystal growth on the change of particle numbers for each size interval by taking into consideration the time interval of sampling of less than 1 h and the assumed crystal growth rate of 1 μm/h. As shown in Table 1, for the fine and coarse seed added in Fig. 1, the number of particles of 19–20 μm that can grow into the next size interval of 20–45 μm is only about 3% of the total particles in the size interval of 0–20 μm. Therefore, the contribution of crystal growth to the change in particle number in the 0–20 μm size interval is negligible compared with the observed N50% change. Likewise, the number of particles of 44–45 μm size in the coarse and fine seeds that grow into the next size interval of 45–∞ is very small and it was calculated that the overall change in particle numbers in the size interval of 20–45 μm due to crystal growth is about 1–1.5%. However, the 44–45 μm particles in the coarse and fine seeds growing into the size interval of 45–∞, constitute 11% and 16% of the total particles, respectively, which may result in relatively large deviations using the change in particle number to characterize agglomeration in this size interval. Fig. 2 shows scanning electron microscopy photographs of the seed added and the carbonation product obtained for different carbonation times, which indicates that primary nucleation and secondary nucleation are rare under the carbonation conditions. Clearly, the change in particle number is mainly attributed to agglomeration and both nucleation and crystal growth are negligible in the carbonation process. Thus the rate of change of particle numbers for the size interval of 0–20 μm and 20–45 μm describes the agglomeration rate of the gibbsite particles from the carbonation. 3.2.2. Definition of relative mass supersaturation for carbonation of aluminate A number of different definitions of supersaturation of aluminate solution have been reported in the literature. They are expressed in terms of the concentration of the aluminium species in the aluminate solution, CA, (as g Al2O3/L) and the total caustic concentration, Cn, (g Na2O/L) which includes the free hydroxide and sodium aluminate concentrations. The main definitions for supersaturation that are used in the gibbsite crystallization are ΔC = C−Ceq
ð1Þ
α=
C Ceq
ð2Þ
σ=
C−Ceq = α−1 Ceq
ð3Þ
S=
C−Ceq Cn
ð4Þ
where, Ceq designates the equilibrium concentration of aluminium species of the aluminate solution.
3.2. Agglomeration model of gibbsite particles from sodium aluminate carbonation 3.2.1. Analyses of the main sub-processes It is well known that the gibbsite crystal growth rate is very small — usually in the range of 0.01–1 μm/h (Veesler and Boistelle, 1993) and the mechanism of gibbsite precipitation in the carbonation process follows that in seeded precipitation. But the gibbsite crystal growth rate may be faster in the carbonation process than the seeded process because the
Table 1 Particle number of different particle size intervals and the total particle number of the coarse and fine seed added in the carbonation process. Seed type
0–20 μm
19–20 μm
20–45 μm
44–45 μm
N 45 μm
Particle total number
Coarse 1.23E + 08 4.02E + 06 8.63E + 08 1.27E + 07 1.18E + 08 1.10E + 09 Fine 7.68E + 09 2.65E + 07 1.37E + 09 3.50E + 06 2.20E + 07 8.93E + 09
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Fig. 2. Scanning electron microscopy of the seed added and the product from carbonation with different carbonation rates and times.
All the supersaturation measures are appropriate for the gibbsite crystal growth. However, the agglomeration process mainly includes collision among particles and binding (or cementation) of gibbsite precipitates. These two sub-processes are closely related to the particles or the total surface area present in the solution. When a large number of particles are present in one unit volume of solution, there are much more chances for the particles to collide in each other to become temporary physically flocculated. However, there is probably poor binding or cementation for forming agglomerates because of a lack of binder if the gibbsite precipitation rate is constant. Based on this consideration, we propose the relative mass supersaturation, σ, which relates to the solid content in the solution and is expressed as the ratio of the absolute supersaturation (concentration difference of aluminium species) to the total solid content in the aluminate solution σ=
C−Ceq ΔC = G0 + ðC0 −CÞ G
ð5Þ
where: σ— relative mass supersaturation of the aluminate solution; ΔC — concentration difference or absolute supersaturation of aluminium species, g/L; G— total solid content, G =G0 +(C0 −C), g/L; G0— the amount of seed added, g/L; — the coefficient of conversion between Al(OH)3 and Al2O3, = 156/102 = 1.53. 3.2.3. Derivation of agglomeration model of gibbsite particles There are two major steps in the gibbsite agglomeration process in the carbonation process: (1) The particles randomly collide to each other and some form loose physical flocculation, which is a reversible process (the physical flocculation can disintegrate to the original grains because of its low mechanical strength). (2) With the carbonation of sodium aluminate solution, Al(OH)3 is precipitated as the binder at the interface among the unbroken
flocculate to cement the original crystals and form relatively substantial agglomerates (so-called crystal coagulation). Therefore, all the factors influencing the agglomeration process can be attributed to collision and binding and the rate of agglomeration of particles in each size interval in one unit volume can be expressed by the following equation −
dNi = gcollision;i ⋅gbind;i dt
ð6Þ
where: −dNi/dt — agglomeration rate of particles of each size interval; gcollision,i — collision function; gbind,i — binding function of supersaturation, solid content in unit volume solution, etc. The collision frequency of particles is proportional to the initial particle number with constant stirring rate. Furthermore, collisions between particles can be divided into effective collisions and ineffective ones and only those which result in temporarily flocculants are the effective collisions. So gcollision,i can be expressed as the product of the original particle number, N0, of the size interval i, and the effective collision factor, fcollision,i. Similarly, cementation effect of particles in agglomeration process can be expressed by the product of the gibbsite precipitation rate (dW/dt) of one unit mass of gibbsite and effective binding coefficient fbind,i. Then Eq. (6) converts to −
dNi dW 1 = N0 ⋅fcollision;i ⋅ ⋅f ⋅ dt dt G0 + W bind;i
ð7Þ
where: N0 —initial particle number in the size interval i per unit volume L; fcollision,i — effective collision factor; G0 — amount of gibbsite seed added, g/L; W—amount of gibbsite Al(OH)3 precipitated, g; dW/dt— precipitation rate of gibbsite Al(OH)3, g/min; fbind,i — effective binding factor.
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According to the definition of the relative mass supersaturation, σ, of sodium aluminate solution, there is σ=
C−Ceq C−Ceq = : G0 + ðC0 −CÞ G0 + W
ð8Þ
Differentiation of Eq. (8) yields dðC−C Þ
ðG0 + WÞ dt eq −ðC−Ceq Þ dW dσ dt = = dt ðG0 + WÞ2
dðC−Ceq Þ −σ dW dt dt
G0 + W
:
ð9Þ
The mass amount of gibbsite precipitated from carbonation, W, can be calculated by W = ðC0 −CÞ:
ð10Þ
less than 0.1 and the sum of 1/ and σ in Eq. (16) approximately equals the value of 1/ (Eq. (17)).
Differentiation of Eq. (10) yields dC 1 dW =− ⋅ : dt dt
ð11Þ
The equilibrium concentration of aluminium species as alumina, Ceq, has a linear relationship with the concentration of sodium hydroxide as Na2O (as shown in Eq. (12)) with carbonation temperature of above 80 °C when the concentration of sodium hydroxide is larger than 170 g/L (Russell, et al, 1955). Ceq ≈aNk
Fig. 3. The relationship between the relative mass supersaturation and the carbonation duration time. (Conditions as Fig. 1).
ð12Þ
where a is a constant. Differentiation of Eq. (12) yields
1 1 1 + σ≈ = = 0:65: 1:53 Then Eq. (16) transforms into dW dσ ≈− ðG0 + WÞ +d : dt dt
ð13Þ
The concentration of sodium hydroxide as Na2O has also a linear relationship with carbonation time with constant concentration of CO2 of mixed gas CO2/air and invariable ventilating rate of the mixed gas (Chen, 2004), as shown in Eq. (14), dNk =b dt
ð14Þ
where b is a constant. Substitution of Eq. (14) into Eq. (13) yields dCeq = ab = d dt
ð15Þ
ð18Þ
Fig. 4 shows the correlation of the gibbsite precipitation rate (dW/dt) to the function (G0 +W) × dσ/dt, which demonstrates that the experiment results are roughly in agreement with Eq. (18). Substitution of Eq. (18) into Eq. (7) and rearrangement yields the final equation −
dCeq dN = a k: dt dt
ð17Þ
dNi dσ = ki + ri dt dt
ð19Þ
where: Ki — agglomeration rate constant, (ki =N0 ⋅fcollision,i ⋅ (−) ⋅fbind); d ri = −N0 ⋅fcollision;i ⋅⋅ G0 + W ⋅fbind;i ; Ni— total particle number in each size interval; σ—relative mass supersaturation; t—carbonation time, min. 3.2.4. The agglomeration model of gibbsite particles and the carbonation data The relationship between − dNi/dt and dσ/dt is plotted in Fig. 5 using the carbonation experiment data under different carbonation conditions. Many experiments were conducted in the laboratory, and only some of them are demonstrated here. By regression of the carbonation experiment data, by employing the least squares method, i = ki dσ + ri with regression we can obtain the correlation− dN dt dt
where d is a constant. Then substitution of Eqs. (11) and (15) into Eq. (9) and rearrangement yields ðG + WÞ dσ +d dW dt =− 0 1 : dt + σ
ð16Þ
In the carbonation experiments, the amount of seed added is 70 g/L. The relative mass supersaturation is high in the early period of carbonation when no gibbsite precipitates. With the process of carbonation, a large amount of gibbsite precipitates from the aluminate solution, which results in the rapid decrease of the absolute supersaturation of the solution and further leads to the rapid reduction of the relative mass supersaturation. Fig. 3 indicates that the relative mass supersaturation, σ, decreases rapidly and becomes smaller and smaller with the processing of carbonation. The ratio of σ to the reciprocal of is
Fig. 4. Correlation of the gibbsite precipitation rate (dW/dt) to (G0 + W) × dσ/dt (conditions as Fig. 1).
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Fig. 5. Correlation of −dNi/dt to dσ/dt with different carbonation conditions. Particle size interval: 1, 0–∞, 2, 0–20 μm, 3, 20–45 μm, 4, 45 μm–∞; particle number of the seed per unit volume solution: a —7.42 × 109; b —1.19 × 1010; c —7.65 × 109; d —1.23 × 1010; e —2.12 × 1010; f —1.1 × 109.
coefficient larger than 0.98, which indicates that the carbonation experiment results are in good agreement with the developed theoretical agglomeration model. Fig. 5 also suggests that the intercepts of the vertical coordinate are very small. This means that the value of ri is invariably small and approximately equals zero under the experimental conditions. So the agglomeration model may further approximately be expressed as the following equation:
−
dNi dσ = ki : dt dt
ð20Þ
3.2.5. Discussion of the agglomeration rate constant The agglomeration rate constant of gibbsite particles from the carbonation of sodium aluminate solution, ki, is mainly influenced by the initial particle number in the system with constant stirring rate.
Fig. 6. Influence of initial particle number on the agglomeration rate constant. Particle size interval: 1—0–∞, 2—0–20 μm, 3—20–45 μm, 4—45 μm–∞; Data from range of experiments.
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Fig. 7. Scanning electron microscopy of the gibbsite product under optimized carbonation conditions.
The initial particle number varies with the crystal size distribution of the seed added when the mass added is a constant 70 g/L. Fig. 6 shows the dependence of agglomeration rate constant on the initial particle number in each size interval of the seed added per unit volume. Clearly, agglomeration mainly occurs rapidly among fine particles when N0 is large and a large number of fine particles are present in the seed. On the contrary, the agglomeration of relative coarse particles plays an important role in the carbonation process when N0 is small and the fine particles only account for a small fraction of the total particles of the seed. Based on the developed agglomeration model of gibbsite particles from carbonation process, a carbonation product with average particle size, d(0.5) of 75 μm and narrow-sized distribution (as shown in Fig. 7) is obtained by controlling the amount and the particle size distribution of the seeds added, with no particles b20 μm or N150 μm. 4. Conclusions An approximate mathematical agglomeration model of gibbsite particles from carbonation of aluminate solution is proposed, which shows that the rate of change of particle numbers in each particle size interval of 0–20 μm, 20–45 μm and 45–∞ is approximately proportional to the change rate of the relative mass supersaturation. The agglomeration rate constant is mainly influenced by the initial particles in the carbonation system under constant stirring rate. The agglomeration of fine grains (b20 μm) plays a dominant role when the initial particle number is large, whereas the agglomeration of relatively coarse particles (20 μm–45 μm) also occurs with a relatively small number of initial particles. A carbonation product with extremely narrow-sized distribution and average particle size of above 75 μm was produced by controlling the amount and the particle size distribution of the seeds added and the relative mass supersaturation of the aluminate solution for carbonation. Acknowledgements The authors gratefully acknowledge the financial support from the National Basic Research Program of China (2005CB6237). The authors sincerely thank the engineering staff from Zhongzhou Branch, China Aluminum Corporation Limited; Xuan Zhang and Yufeng Zhang for experimental assistance; and Yumin Zhang from Chemistry Dept. for help with CSD analyses.
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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
Agglomeration of gibbsite particles from carbonation process of sodium aluminate solution Qiusheng Zhou, Dianjun Peng, Zhihong Peng, Guihua Liu, Xiaobin Li ⁎ School of Metallurgical Science and Engineering, Central South University, Changsha, Hunan Province, 410083, PR China
a r t i c l e
i n f o
Article history: Received 8 February 2009 Received in revised form 31 July 2009 Accepted 31 July 2009 Available online 11 August 2009 Keywords: Sodium aluminate Carbonation Agglomeration Agglomeration model Supersaturation Particle size distribution
a b s t r a c t The particle size distribution of the carbonation products from sodium aluminate solution was analysed based on the relative mass supersaturation and the particle number conversion from the Malvern volume distribution curves. An approximate relationship was developed which showed that the rate of change of particle number in each particle size interval (0–20 μm, 20–45 μm and 45 μm–∞) per unit volume was proportional to the rate of change of the relative mass supersaturation of the solution. The evolution of agglomeration of gibbsite particles in the carbonation process was also studied and the results show that the agglomeration rate constant is mainly influenced by the initial particle number of gibbsite Al(OH)3 under the experimental conditions. The agglomeration between fine grains (b 20 μm) plays a dominant role when the initial particle number is large, while the agglomeration between relatively coarse particles (20–45 μm) occurs with a small initial particle number. A carbonation product with extremely narrow-sized distribution and ideal large average size of 75 μm was produced by regulating the relative mass supersaturation of the solution, the amount and particle size distribution of added seeds and other carbonation parameters such as temperature and stirring rate. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Carbonation of sodium aluminate solution is one of the important operations of the lime-soda-sinter process in the production of alumina. It directly influences the physical properties of the product, such as the mean particle size, the particle size distribution and morphology. Carbonation has been the key process for producing sandy alumina which can satisfactorily meet the requirements of modern aluminium molten salt electrolysis in the alumina production by the sinter process (Li et al., 2009). As the whole carbonation duration time (5–8 h) is much shorter than that for seeded precipitation (40–70 h) and the gibbsite crystal growth rate is very slow during this time (Veesler and Boistelle, 1993), particle agglomeration is the most important way to rapidly enlarge the particle size needed for sandy alumina. Agglomeration is the process that a number of particles adhere together to form a stable attached polymer (Seyssiecq et al., 1998). Since 1970s, many researchers have studied gibbsite agglomeration in the seeded precipitation of Bayer liquors under various conditions with different methods. Muralidar and Ramkrishna (1985) investigated the mechanism of agglomeration with data from the crystal size distribution based on the crystal population balance equation. Hartel and Randolph
⁎ Corresponding author. Tel.: +86 731 8883 0454; fax: +86 731 8883 04533. E-mail address: [email protected] (X. Li). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.07.015
(1986) and Houslow et al. (1990) also studied the agglomeration mechanism from the aspect of crystallization process. Scott (1963), Misra (1970), Sakamoto and Kanehara (1976), Low (1975), Halfon and Kaliaguine (1976) and Remillard et al. (1980) have all pointed out that agglomeration consisted of collision resulting from the induction of the flow shearing stress and cementation with the binder precipitating from the solution. The result of the collision among particles is to form aggregates which can readily break up. The result of cementation is to form agglomerates which are relatively stable as a chemical bond is formed among particles with the binder of gibbsite precipitating from the solution. However, Marchal et al. (1988) proposed that gibbsite agglomeration process included three steps: the mechanical collision, the interactions between particles and binding resulting from the crystal growth. A considerable amount of work has been reported on the individual influences of crystallizing parameters on the gibbsite agglomeration. Crystallization temperature and supersaturation of the solution play the most important role in the gibbsite agglomeration. Moreover, an increase of the crystallization temperature and supersaturation promotes the gibbsite agglomeration (Seyssiecq et al., 1998). The agglomeration rate increases with the increase of seed charge, but rapidly reaches a plateau before decreasing when the seed charge is too high, while agglomeration continuously decreases with increasing stirring rate (Veesler et al., 1994). As for the relationship between agglomeration rate constant and particle size, there are two quite different opinions. One is that there is a strong dependence of agglomeration rate constant on the particle size
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(Low, 1975); and the other is that there is independence of agglomeration rate constant on the particle size (Ilievski and White, 1994), which is supported by the experiments carried out in the self-designed precipitator operating at constant supersaturation (Seyssiecq et al., 2000; Ilievski, 2001; Ilievski and Livk, 2006). Although a lot of work has been reported in the gibbsite agglomeration literature, consensus has not been reached completely on the intrinsic mechanism and influencing factors of agglomeration. Different researchers have different opinions and established different models. Furthermore, the focus of almost all of the previous research is on the particles with size range from 4 μm to 20 μm. On the other hand, although carbonation and seeded precipitation of caustic aluminate solution follow the same mechanism of gibbsite crystallization (Liu, 2004), there are some differences between carbonation and seeded precipitation, such as much shorter duration time for carbonation, high silicon concentration present in carbonation solution, relatively low concentration and probably different ion structures of aluminium species in the carbonation solution. Furthermore, carbonation is a more complicated process which combines interactions of three phase of CO2-containing gas, liquid and solid, so the gibbsite particle agglomeration may be some different from that of seeded precipitation from Bayer liquor. Up to now, there is little report of agglomeration of gibbsite particles from carbonation. Therefore, this investigation of the agglomeration of gibbsite from the carbonation process, as well as the development of an applicable and efficient agglomeration model, is necessary for effectively and precisely controlling the gibbsite agglomeration. 2. Experimental 2.1. Experimental method Rather pure supersaturated sodium aluminate solution was synthesized in laboratory by dissolving Al(OH)3 powder (from CHALCO, Al2O3 N64.5%, SiO2 b0.02%, Fe2O3 b0.02%) into caustic soda solution (industry grade, NaOH N96.5%) and then filtering for carbonation experiments. Before the carbonation experiments, the required amount of aluminate solution was poured into a 2 L stainless steel carbonation apparatus and the carbonation apparatus was immersed in a thermostat oil bath with temperature precision of ±1 °C. After the solution was preheated for 10 min to the carbonation temperature of 95 °C, accurately weighed gibbsite Al(OH)3 seed (from carbonation of aluminate solution) was added to the carbonation apparatus and at the same time, agitation and ventilation of the mixed CO2 and air gas system were operated to start the carbonation. The CO2 concentration of the gas mixture was measured by the CYES-II CO2 analyzer (Shanghai Analytical Corporation, PR China), and the flow rate of the mixed gas was controlled by a glass rotameter. At prescribed times, 5 ml of the carbonation slurry was sampled and immediately centrifuged to obtain aluminate solution and solid gibbsite Al(OH)3 powder. The resulting aluminate solution was used to determine the concentration of sodium hydroxide as Na2O and the concentration of aluminium species as Al2O3 by titration. The gibbsite Al(OH)3 powder was washed with deionized water and then the crystal size distribution (CSD) was analysed by using a Mastersizer 2000 (Malvern Instruments Ltd., UK). The morphology of the product was observed by scanning electron microscopy (JSM-6360LV, Japan). 2.2. Data processing The total concentration of soda includes caustic soda (NaOH) which gradually converts into sodium carbonate with the carbonation process. Taking into consideration that the volume of the aluminate solution may change with carbonation and that the total amount of soda is a constant in the whole carbonation process, we can calculate
the carbonation rate of sodium aluminate solution by the following equation: η=
Aa −Am × Aa
Nt0 Nt
× 100%
where: η— carbonation rate of sodium aluminate solution, %; Aa— concentration of aluminium species in initial solution, g/L; Am— concentration of aluminium species in carbonated solution, g/L; Nt0— total concentration of soda in initial aluminate solution, g/L; Nt— total concentration of soda in carbonated aluminate solution, g/L. The equilibrium concentration of aluminium species (as Al2O3) in the sodium aluminate solution can be calculated by employing the following equation proposed by E. T. White: 2486:7 1:08753 + Na2 O Ceq = Na2 O exp 6:2106− T T where Ceq = equilibrium concentration of aluminium species, g/L; Na2O=concentration of NaOH as Na2O in solution, g/L; T = temperature, °K. We divide the particle size of the carbonation product into three size intervals 0–20 μm, 20–45 μm and 45–∞, considering that gibbsite particle agglomeration usually takes place among particles sized b20 μm and that the amount of particles of b45 μm is an important criterion for high quality gibbsite product for aluminium smelting. The particle number Ni of each particle size interval per unit volume (where i (i = 1,2,3,4) represents 0–∞, 0–20 μm, 20–45 μm, and 45–∞, respectively), was calculated according to data from the CSD curves of the seed added or the carbonation product at different carbonation stages. In this way, the rate of change of the particle number of each size interval per unit volume solution can be obtained. 3. Results and discussion 3.1. Agglomeration of gibbsite particles from carbonation of sodium aluminate The gibbsite precipitation in the carbonation process is very similar to that of seeded precipitation and includes a series of complex physical and chemical changes such as nucleation, agglomeration, crystal growth, grain grinding and breakage. These sub-processes are usually inter-twined and present in the whole carbonation process. But one of the sub-processes such as crystal nucleation, agglomeration or crystal growth may be dominant in certain stages of the process under the specific carbonation conditions. Fig. 1 shows the variation in particle number for each particle size interval per unit volume solution at different carbonation times with different masses of gibbsite Al(OH)3 seeds added. The particle number of seeds in Fig. 1(a) is relatively large (8.93× 109) and the particles of less than 20 μm account for 84.4%. The particle number of seeds in Fig. 1(b) is much smaller (1.10× 109) compared with that in Fig. 1(a) and the particles between 20 μm and 45 μm account for 79.3%. In Fig. 1(a) and (b), curve 1 shows the change of carbonation rate with different carbonation times, and curve 2 to curve 5 represent the change in particle numbers in the size interval of 0– ∞, 0–20 μm, 20–45 μm, 45 μm–∞, respectively. It can be seen from curve 2 that the total particle numbers are significantly reduced during carbonation processing under the two seed-added systems which clearly indicates that agglomeration has taken place. But there is a little difference between the two seeded systems. Fig. 1(a) indicates that the number of fine grains of 0–20 μm rapidly decreases in the earlier stages of carbonation (Fig. 1(a), curve 3), and the number of particles between 20 μm and 45 μm apparently increases (Fig. 1(a), curve 4), while the number of coarse particles of 45 μm–∞ is almost invariant (Fig. 1(a), curve 5). During carbonation
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Fig. 1. Variation of particle number in each particle size interval of the product from carbonation with different seed size–∞; 3—0–20 μm; 4—20–45 μm; 5—45 μm–∞; (conditions: 95; (b) Al2O3 128 g/L, L)).
processing, the number of fine grains continues to decrease, but the decrease gets smaller and smaller whilst simultaneously the number of coarse particles is still invariant. When the extent of carbonation reaches 90.8%, the total number of particles is 3.08 × 109, and 84.2% of the particles are those between 20 μm and 45 μm due to agglomeration among fine grains. Relatively coarse particles cannot agglomerate effectively probably due to poor cementation with insufficient binder. In the later period of carbonation (N240 min), the carbonation rate is slow and hardly any agglomeration occurs as a result of low supersaturation of the aluminate solution. With less seed added initially, the results in Fig. 1(b) show that particles b20 μm almost disappeared in the product within 60 min carbonation. However, as the system still maintains a rapid carbonation rate, it can provide sufficient Al(OH)3 binder for particle agglomeration and relatively coarse particles between 20 μm and 45 μm can agglomerate continuously (Fig. 1(b), curve 4). When the extent of carbonation is 87.0%, the total number of particles is 5.1 × 108 and 90.2% have a particle size larger than 45 μm.
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supersaturation of the carbonation solution is larger, especially in the later period of gibbsite precipitation. So it is reasonable to assume the gibbsite crystal growth rate of 1 μm/h in carbonation process. We can calculate the particle number of different particle size intervals of the seeds added to estimate the influence of crystal growth on the change of particle numbers for each size interval by taking into consideration the time interval of sampling of less than 1 h and the assumed crystal growth rate of 1 μm/h. As shown in Table 1, for the fine and coarse seed added in Fig. 1, the number of particles of 19–20 μm that can grow into the next size interval of 20–45 μm is only about 3% of the total particles in the size interval of 0–20 μm. Therefore, the contribution of crystal growth to the change in particle number in the 0–20 μm size interval is negligible compared with the observed N50% change. Likewise, the number of particles of 44–45 μm size in the coarse and fine seeds that grow into the next size interval of 45–∞ is very small and it was calculated that the overall change in particle numbers in the size interval of 20–45 μm due to crystal growth is about 1–1.5%. However, the 44–45 μm particles in the coarse and fine seeds growing into the size interval of 45–∞, constitute 11% and 16% of the total particles, respectively, which may result in relatively large deviations using the change in particle number to characterize agglomeration in this size interval. Fig. 2 shows scanning electron microscopy photographs of the seed added and the carbonation product obtained for different carbonation times, which indicates that primary nucleation and secondary nucleation are rare under the carbonation conditions. Clearly, the change in particle number is mainly attributed to agglomeration and both nucleation and crystal growth are negligible in the carbonation process. Thus the rate of change of particle numbers for the size interval of 0–20 μm and 20–45 μm describes the agglomeration rate of the gibbsite particles from the carbonation. 3.2.2. Definition of relative mass supersaturation for carbonation of aluminate A number of different definitions of supersaturation of aluminate solution have been reported in the literature. They are expressed in terms of the concentration of the aluminium species in the aluminate solution, CA, (as g Al2O3/L) and the total caustic concentration, Cn, (g Na2O/L) which includes the free hydroxide and sodium aluminate concentrations. The main definitions for supersaturation that are used in the gibbsite crystallization are ΔC = C−Ceq
ð1Þ
α=
C Ceq
ð2Þ
σ=
C−Ceq = α−1 Ceq
ð3Þ
S=
C−Ceq Cn
ð4Þ
where, Ceq designates the equilibrium concentration of aluminium species of the aluminate solution.
3.2. Agglomeration model of gibbsite particles from sodium aluminate carbonation 3.2.1. Analyses of the main sub-processes It is well known that the gibbsite crystal growth rate is very small — usually in the range of 0.01–1 μm/h (Veesler and Boistelle, 1993) and the mechanism of gibbsite precipitation in the carbonation process follows that in seeded precipitation. But the gibbsite crystal growth rate may be faster in the carbonation process than the seeded process because the
Table 1 Particle number of different particle size intervals and the total particle number of the coarse and fine seed added in the carbonation process. Seed type
0–20 μm
19–20 μm
20–45 μm
44–45 μm
N 45 μm
Particle total number
Coarse 1.23E + 08 4.02E + 06 8.63E + 08 1.27E + 07 1.18E + 08 1.10E + 09 Fine 7.68E + 09 2.65E + 07 1.37E + 09 3.50E + 06 2.20E + 07 8.93E + 09
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Fig. 2. Scanning electron microscopy of the seed added and the product from carbonation with different carbonation rates and times.
All the supersaturation measures are appropriate for the gibbsite crystal growth. However, the agglomeration process mainly includes collision among particles and binding (or cementation) of gibbsite precipitates. These two sub-processes are closely related to the particles or the total surface area present in the solution. When a large number of particles are present in one unit volume of solution, there are much more chances for the particles to collide in each other to become temporary physically flocculated. However, there is probably poor binding or cementation for forming agglomerates because of a lack of binder if the gibbsite precipitation rate is constant. Based on this consideration, we propose the relative mass supersaturation, σ, which relates to the solid content in the solution and is expressed as the ratio of the absolute supersaturation (concentration difference of aluminium species) to the total solid content in the aluminate solution σ=
C−Ceq ΔC = G0 + ðC0 −CÞ G
ð5Þ
where: σ— relative mass supersaturation of the aluminate solution; ΔC — concentration difference or absolute supersaturation of aluminium species, g/L; G— total solid content, G =G0 +(C0 −C), g/L; G0— the amount of seed added, g/L; — the coefficient of conversion between Al(OH)3 and Al2O3, = 156/102 = 1.53. 3.2.3. Derivation of agglomeration model of gibbsite particles There are two major steps in the gibbsite agglomeration process in the carbonation process: (1) The particles randomly collide to each other and some form loose physical flocculation, which is a reversible process (the physical flocculation can disintegrate to the original grains because of its low mechanical strength). (2) With the carbonation of sodium aluminate solution, Al(OH)3 is precipitated as the binder at the interface among the unbroken
flocculate to cement the original crystals and form relatively substantial agglomerates (so-called crystal coagulation). Therefore, all the factors influencing the agglomeration process can be attributed to collision and binding and the rate of agglomeration of particles in each size interval in one unit volume can be expressed by the following equation −
dNi = gcollision;i ⋅gbind;i dt
ð6Þ
where: −dNi/dt — agglomeration rate of particles of each size interval; gcollision,i — collision function; gbind,i — binding function of supersaturation, solid content in unit volume solution, etc. The collision frequency of particles is proportional to the initial particle number with constant stirring rate. Furthermore, collisions between particles can be divided into effective collisions and ineffective ones and only those which result in temporarily flocculants are the effective collisions. So gcollision,i can be expressed as the product of the original particle number, N0, of the size interval i, and the effective collision factor, fcollision,i. Similarly, cementation effect of particles in agglomeration process can be expressed by the product of the gibbsite precipitation rate (dW/dt) of one unit mass of gibbsite and effective binding coefficient fbind,i. Then Eq. (6) converts to −
dNi dW 1 = N0 ⋅fcollision;i ⋅ ⋅f ⋅ dt dt G0 + W bind;i
ð7Þ
where: N0 —initial particle number in the size interval i per unit volume L; fcollision,i — effective collision factor; G0 — amount of gibbsite seed added, g/L; W—amount of gibbsite Al(OH)3 precipitated, g; dW/dt— precipitation rate of gibbsite Al(OH)3, g/min; fbind,i — effective binding factor.
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According to the definition of the relative mass supersaturation, σ, of sodium aluminate solution, there is σ=
C−Ceq C−Ceq = : G0 + ðC0 −CÞ G0 + W
ð8Þ
Differentiation of Eq. (8) yields dðC−C Þ
ðG0 + WÞ dt eq −ðC−Ceq Þ dW dσ dt = = dt ðG0 + WÞ2
dðC−Ceq Þ −σ dW dt dt
G0 + W
:
ð9Þ
The mass amount of gibbsite precipitated from carbonation, W, can be calculated by W = ðC0 −CÞ:
ð10Þ
less than 0.1 and the sum of 1/ and σ in Eq. (16) approximately equals the value of 1/ (Eq. (17)).
Differentiation of Eq. (10) yields dC 1 dW =− ⋅ : dt dt
ð11Þ
The equilibrium concentration of aluminium species as alumina, Ceq, has a linear relationship with the concentration of sodium hydroxide as Na2O (as shown in Eq. (12)) with carbonation temperature of above 80 °C when the concentration of sodium hydroxide is larger than 170 g/L (Russell, et al, 1955). Ceq ≈aNk
Fig. 3. The relationship between the relative mass supersaturation and the carbonation duration time. (Conditions as Fig. 1).
ð12Þ
where a is a constant. Differentiation of Eq. (12) yields
1 1 1 + σ≈ = = 0:65: 1:53 Then Eq. (16) transforms into dW dσ ≈− ðG0 + WÞ +d : dt dt
ð13Þ
The concentration of sodium hydroxide as Na2O has also a linear relationship with carbonation time with constant concentration of CO2 of mixed gas CO2/air and invariable ventilating rate of the mixed gas (Chen, 2004), as shown in Eq. (14), dNk =b dt
ð14Þ
where b is a constant. Substitution of Eq. (14) into Eq. (13) yields dCeq = ab = d dt
ð15Þ
ð18Þ
Fig. 4 shows the correlation of the gibbsite precipitation rate (dW/dt) to the function (G0 +W) × dσ/dt, which demonstrates that the experiment results are roughly in agreement with Eq. (18). Substitution of Eq. (18) into Eq. (7) and rearrangement yields the final equation −
dCeq dN = a k: dt dt
ð17Þ
dNi dσ = ki + ri dt dt
ð19Þ
where: Ki — agglomeration rate constant, (ki =N0 ⋅fcollision,i ⋅ (−) ⋅fbind); d ri = −N0 ⋅fcollision;i ⋅⋅ G0 + W ⋅fbind;i ; Ni— total particle number in each size interval; σ—relative mass supersaturation; t—carbonation time, min. 3.2.4. The agglomeration model of gibbsite particles and the carbonation data The relationship between − dNi/dt and dσ/dt is plotted in Fig. 5 using the carbonation experiment data under different carbonation conditions. Many experiments were conducted in the laboratory, and only some of them are demonstrated here. By regression of the carbonation experiment data, by employing the least squares method, i = ki dσ + ri with regression we can obtain the correlation− dN dt dt
where d is a constant. Then substitution of Eqs. (11) and (15) into Eq. (9) and rearrangement yields ðG + WÞ dσ +d dW dt =− 0 1 : dt + σ
ð16Þ
In the carbonation experiments, the amount of seed added is 70 g/L. The relative mass supersaturation is high in the early period of carbonation when no gibbsite precipitates. With the process of carbonation, a large amount of gibbsite precipitates from the aluminate solution, which results in the rapid decrease of the absolute supersaturation of the solution and further leads to the rapid reduction of the relative mass supersaturation. Fig. 3 indicates that the relative mass supersaturation, σ, decreases rapidly and becomes smaller and smaller with the processing of carbonation. The ratio of σ to the reciprocal of is
Fig. 4. Correlation of the gibbsite precipitation rate (dW/dt) to (G0 + W) × dσ/dt (conditions as Fig. 1).
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Fig. 5. Correlation of −dNi/dt to dσ/dt with different carbonation conditions. Particle size interval: 1, 0–∞, 2, 0–20 μm, 3, 20–45 μm, 4, 45 μm–∞; particle number of the seed per unit volume solution: a —7.42 × 109; b —1.19 × 1010; c —7.65 × 109; d —1.23 × 1010; e —2.12 × 1010; f —1.1 × 109.
coefficient larger than 0.98, which indicates that the carbonation experiment results are in good agreement with the developed theoretical agglomeration model. Fig. 5 also suggests that the intercepts of the vertical coordinate are very small. This means that the value of ri is invariably small and approximately equals zero under the experimental conditions. So the agglomeration model may further approximately be expressed as the following equation:
−
dNi dσ = ki : dt dt
ð20Þ
3.2.5. Discussion of the agglomeration rate constant The agglomeration rate constant of gibbsite particles from the carbonation of sodium aluminate solution, ki, is mainly influenced by the initial particle number in the system with constant stirring rate.
Fig. 6. Influence of initial particle number on the agglomeration rate constant. Particle size interval: 1—0–∞, 2—0–20 μm, 3—20–45 μm, 4—45 μm–∞; Data from range of experiments.
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Fig. 7. Scanning electron microscopy of the gibbsite product under optimized carbonation conditions.
The initial particle number varies with the crystal size distribution of the seed added when the mass added is a constant 70 g/L. Fig. 6 shows the dependence of agglomeration rate constant on the initial particle number in each size interval of the seed added per unit volume. Clearly, agglomeration mainly occurs rapidly among fine particles when N0 is large and a large number of fine particles are present in the seed. On the contrary, the agglomeration of relative coarse particles plays an important role in the carbonation process when N0 is small and the fine particles only account for a small fraction of the total particles of the seed. Based on the developed agglomeration model of gibbsite particles from carbonation process, a carbonation product with average particle size, d(0.5) of 75 μm and narrow-sized distribution (as shown in Fig. 7) is obtained by controlling the amount and the particle size distribution of the seeds added, with no particles b20 μm or N150 μm. 4. Conclusions An approximate mathematical agglomeration model of gibbsite particles from carbonation of aluminate solution is proposed, which shows that the rate of change of particle numbers in each particle size interval of 0–20 μm, 20–45 μm and 45–∞ is approximately proportional to the change rate of the relative mass supersaturation. The agglomeration rate constant is mainly influenced by the initial particles in the carbonation system under constant stirring rate. The agglomeration of fine grains (b20 μm) plays a dominant role when the initial particle number is large, whereas the agglomeration of relatively coarse particles (20 μm–45 μm) also occurs with a relatively small number of initial particles. A carbonation product with extremely narrow-sized distribution and average particle size of above 75 μm was produced by controlling the amount and the particle size distribution of the seeds added and the relative mass supersaturation of the aluminate solution for carbonation. Acknowledgements The authors gratefully acknowledge the financial support from the National Basic Research Program of China (2005CB6237). The authors sincerely thank the engineering staff from Zhongzhou Branch, China Aluminum Corporation Limited; Xuan Zhang and Yufeng Zhang for experimental assistance; and Yumin Zhang from Chemistry Dept. for help with CSD analyses.
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