role of boehmite/solution interface in boehmite precipitation from supersaturated sodium aluminate...

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Hydrometallurgy 74 (2004) 203–212

Role of boehmite/solution interface in boehmite precipitation

from supersaturated sodium aluminate solutions

D. Panias*

Laboratory of Metallurgy, School of Mining and Metallurgical Engineering, National Technical University of Athens,

Zografos Campus, Athens 15780, Greece

Received 21 January 2004; received in revised form 13 March 2004; accepted 27 April 2004

Abstract

This work presents the developments made in the Laboratory of Metallurgy of the National Technical University of Athens

in the field of boehmite precipitation from supersaturated sodium aluminate solutions under atmospheric conditions. Initially,

the boehmite process is described by giving emphasis to kinetic problems that reduce the process efficiency and make it less

attractive. Moreover, it is demonstrated that the physicochemical properties of the boehmite/solution interface are of great

importance for the understanding of the boehmite precipitation mechanism. Therefore, the physicochemical properties of the

boehmite/solution interface are theoretically determined giving emphasis to the surface speciation diagrams and the surface

potential that are presented as a function of pH and the boehmite seed concentration in the solution. Finally, the theoretical data

are correlated to the mechanism of boehmite precipitation and utilized for the understanding of alternative precipitation methods

that improve the process efficiency.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Boehmite/solution interface; Boehmite precipitation; Supersaturated sodium aluminate solutions

1. Introduction of the existing Bayer process, or of micro-sized

A novel process for the production of boehmite has

been developed in the Laboratory of Metallurgy of the

National Technical University of Athens (Panias and

Paspaliaris, 2003). The process consists of a precipi-

tation stage in which pure crystalline boehmite is

precipitated from a supersaturated sodium aluminate

solution under atmospheric conditions. This process

can be used for the production either of smelter grade

alumina, after modification of the precipitation stage

0304-386X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.hydromet.2004.04.001

* Tel.: +30-210-7722276; fax: +30-210-7722168.

E-mail address: [email protected] (D. Panias).

crystalline boehmite which is the main precursor for

g-Al2O3 which has numerous applications in technical

ceramics, thin solid films, catalysis, etc. (Kaya et al.,

2002).

The thermodynamic analysis of the Al2O3–

Na2O–H2O system (Panias and Paspaliaris, 1999)

has shown that boehmite can be formed at temper-

atures as low as 50 jC depending on the solution

composition. However, kinetic analysis of the

boehmite precipitation from supersaturated sodium

aluminate solutions has shown that boehmite cannot

be precipitated from such solutions under atmospher-

ic conditions in the absence of boehmite seeds

(Skoufadis et al., 2003a). In the presence of boehmite

D. Panias / Hydrometallurgy 74 (2004) 203–212204

seeds, boehmite can be precipitated at temperatures

as low as 90 jC. Moreover, the kinetic analysis has

shown that the precipitation reaction has high activa-

tion energy (89 kJ/mol) and revealed strong kinetic

inhibitions demonstrating the role of NaOH as an

inhibitor and the role of boehmite seed concentration

as an accelerator.

The scope of this work is the study of the

physicochemical properties of the boehmite/solution

interface, their correlation to the mechanism of

boehmite precipitation and the development of alter-

native precipitation methods that improve the process

efficiency.

Fig. 1. Boehmite precipitation at 90 jC from a supersaturated

sodium aluminate solution with 132 g Al2O3/L, 120 g Na2O/L and

seed concentration 230 g/L.

2. Experimental procedures and apparatus

The experiments were performed in an Inconel

autoclave reactor with a maximum capacity of 600

ml equipped with all the necessary items for the

control of temperature and stirring rate.

The autoclave was loaded with 420 ml super-

saturated sodium aluminate solution with pre-ad-

justed Al2O3 and Na2O concentrations and the

appropriate amount of boehmite seed. The pulp

was rapidly heated up to the required temperature

and each experiment was performed under isother-

mal conditions. At the end of each experiment, the

pulp was cooled rapidly down to 50 jC and then

filtered. The solids were characterized mineralogi-

cally by thermogravimetric analysis and X-ray dif-

fractometry in order to verify that the precipitated

solids were pure crystalline boehmite. The filtrates

were analyzed for their Al2O3 content using the

EDTA/ZnSO4 titrimetric method (Skoufadis et al.,

2003a).

The boehmite seed was produced by hydrother-

mal transformation of alumina trihydrate to boehm-

ite at 300 jC (Skoufadis et al., 2003a). The

supersaturated sodium aluminate solutions were

prepared with dissolution of pure alumina trihydrate

in a sodium hydroxide solution at 160 jC (Skou-

fadis et al., 2003a). The chemicals that were used

for the production of the boehmite seed and the

preparation of the supersaturated sodium aluminate

solutions were pure hydrargillite (Al2O3�3H2O) and

sodium hydroxide commercially available from

Merck.

3. Boehmite precipitation at 90 jC

The typical boehmite precipitation curve in the

presence of boehmite seed under atmospheric con-

ditions (90 jC, 1 atm) is shown in Fig. 1. The

alumina (Al2O3) concentration in the supersaturated

sodium aluminate solution is plotted against the

precipitation time and is compared with the boehmite

solubility (Panias et al., 2001a) under the same

experimental conditions. The rate of boehmite pre-

cipitation gradually decreases and an apparent

equilibrium is achieved at which the alumina concen-

tration in the solution is almost twice the boehmite

solubility under the same conditions. The observed

precipitation behaviour is non-typical, as the rate of

boehmite precipitation has been reduced to such a

level that the process seems practically to have ceased

while the remaining supersaturation degree is very

high.

All the above observations have been attributed to

very strong kinetic inhibitions, which have been

correlated with the sodium hydroxide content and

the initial boehmite seed concentration in the solution

(Skoufadis et al., 2003a). Taking into account, these

results as well as the experimental evidence (Panias et

al., 2001b) that boehmite cannot be precipitated from

supersaturated aluminate solutions under atmospheric

conditions in the absence of boehmite seeds, it can be

concluded that the physicochemical properties of the

boehmite/solution interface are of great importance

Table 1

Model parameters

D. Panias / Hydrometallurgy 74 (2004) 203–212 205

for the understanding of the boehmite precipitation

mechanism.
Parameter Literature source
log Ka1int = 7.83 Stumm, 1987

log Ka2int =�9.36 Stumm, 1987

log KNaint =�8.6 Davis et al., 1978;

Komulski, 1999; Rowlands

et al., 1997

Surface sites density

of boehmite = 9� 1018 sites/m2

Wood et al., 1990

Boehmite specific

surface area = 2.88 m2/g

Skoufadis et al., 2003b

4. Modeling of the boehmite/solution interface

The surface of dry boehmite is characterized by

the presence of low-coordinated aluminium cations

that can act as Lewis acid surface sites. When

boehmite is exposed to an aqueous environment,

water molecules are chemically bonded to these

coordinatively undersaturated aluminium centres.

Dissociation of protons from the bound water mole-

cules and their transfer to the neighbouring oxide ions

result in the formation of the surface hydroxyl groups

(ZAlOHB) that are known also as aluminol groups

(Stumm, 1987). The presence of two lone electron

pairs and a dissociable hydrogen ion on aluminol

groups indicates that these groups are ampholytes.

They behave as acids or bases taking part in proton-

ation and deprotonation surface reactions and forming

positively (ZAlOH2+) and negatively (ZAlO�)

charged surface species according to the following

reactions:

ZAlOHB þ HþfZAlOHþ2 K int

a1 ð1Þ

ZAlOHBfZAlO� þ Hþ K inta2 ð2Þ

Deprotonated surface aluminol groups exhibit

Lewis base behaviour and therefore in sodium hy-

droxide solutions, adsorption of sodium cations takes

place according to the following surface reaction:

ZAlOHB þ NaþfZAlONaB þ Hþ K intNa ð3Þ

Taking into account the above surface reactions,

the Gouy–Chapman electrical double-layer theory

(Dzombak and Morel, 1990) and the parameters

shown in Table 1, the surface speciation diagram

(Fig. 2) as well as the surface potential of boehmite

particles (Fig. 3) have been calculated as a function

of pulp concentration and initial solution pH at

25 jC and 1 M ionic strength (Skoufadis et al.,

2003b).

The surface potential of boehmite particles is

negative in mild to strong alkaline solutions as is

shown in Fig. 3. This result can be explained from

the surface speciation diagram shown in Fig. 2. At

pH 8.6, the concentrations of positively (ZAlOH2+)

and negatively (ZAlO�) charged surface species

are equal and the majority of surface sites are

occupied by the neutral surface species ZAlOHB.

Therefore, the point of zero charge in the system

boehmite/water is at pH 8.6 in accordance with the

literature (Parks, 1965). As the solution pH

increases, the concentration of negatively charged

surface species increases while the concentrations of

positively and neutral surface species decrease.

Thus, an excess of negative charge is developed

on the boehmite surface and the surface potential

becomes negative. As the difference in the concen-

tration of the surface potential determining species

(ZAlO�, ZAlOH2+) increases, the surface potential

takes more negative values reaching a maximum

value of � 110 mV in the case of 10 g/L pulp

concentration. This general trend of surface poten-

tial is inverted at the point where the surface

sodium complex ZAlONaB becomes the dominating

surface species. This point is at pH 11.5 in the case

of 10 g/L pulp concentration. Above this point, the

concentration of the ZAlONaB surface species

increases continuously at the expense of ZAlOHB

and ZAlO� surface species. Therefore, the concen-

tration of ZAlO� species gradually decreases and

the surface potential becomes less negative in

highly alkaline solutions. The results show that

sodium cation sorption onto the surface of boehmite

particles plays an important role in the determina-

tion of the surface potential of the boehmite/solu-

tion interface acting as a strong and efficient

surface charge modifier especially in high alkaline

solutions.

Fig. 2. Boehmite surface speciation diagram as a function of pulp concentration at 25 jC and 1 M ionic strength.


As the boehmite concentration in the aqueous

suspension increases from 10 to 1200 g/L, the curves

in the speciation diagram shift to the higher pH

region shown in Fig. 2. The most interesting feature

is that this region, where the concentrations of

positively and negatively charged surface species

are almost equal, extends to higher pH values and

generally the difference in their concentrations is

better controlled now. For this reason, the surface

potential becomes less negative under the same pH

conditions as the boehmite concentration increases

(Fig. 3) with the exception of the extremely high pH

values where the surface potential is independent of

boehmite concentration. The results show that the

boehmite surface potential can be controlled by the

boehmite concentration in the aqueous suspension

Fig. 3. Boehmite surface potential as a function of initial pH a

revealing its role as an efficient surface charge

modifier, especially in intermediate to high alkaline

solutions.

5. Development of alternative boehmite

precipitation methods

As it has been reported in the previous sections,

kinetic analysis of the boehmite precipitation process

has revealed the role of NaOH as a process inhibitor

and the role of boehmite seed concentration as a

process accelerator. Therefore, two alternative precip-

itation processes have been tested: (a) precipitation

with a constant concentration of free sodium hydrox-

ide in the solution and (b) high seed precipitation.

nd pulp concentration at 25 jC and 1 M ionic strength.

Fig. 5. High seed boehmite precipitation method versus typical

precipitation method (90 jC, 132 g/L Al2O3, 120 g/L Na2O).

allurgy 74 (2004) 203–212 207

5.1. Precipitation with a constant free sodium

hydroxide concentration (CFSH)

During boehmite precipitation from supersaturated

sodium aluminate solutions as described by the

general reaction (4), hydroxide anions are continu-

ously liberated increasing the free sodium hydroxide

concentration in the solution thereby causing addi-

tional kinetic inhibitions. Therefore, an alternative

precipitation method was designed in order that the

free sodium hydroxide content of the aluminate

solution was kept constant during the whole boehm-

ite precipitation process. The control of NaOH con-

tent was achieved by neutralization of the sodium

hydroxide released during boehmite precipitation

with carbon dioxide, the neutralization was complet-

ed within well-specified time intervals. Following

this procedure, the concentration of free sodium

hydroxide during the whole precipitation process

was kept practically constant at 38 g/L Na2O which

is the free sodium hydroxide concentration in the

initial aluminate solution.

AlðOHÞ�4ðaqÞfAlOOHðsÞ þ OH�ðaqÞ þ H2O ð4Þ

The experimental results are shown in Fig. 4

where the new alternative precipitation method

D. Panias / Hydromet

Fig. 4. Boehmite precipitation method under constant free NaOH

content versus typical precipitation method. (90 jC, 132 g/L Al2O3,

120 g/L Na2O, initial boehmite seed concentration 230 g/L).

(CFSH) is compared with the typical boehmite

precipitation process under the same experimental

conditions. The kinetic inhibitions responsible for

the low efficiency of the typical boehmite precipi-

tation process are absent from the new CFSH

precipitation process as it is clearly shown in Fig.

4. Boehmite is precipitated with high rate and

after 24 h of precipitation the process efficiency is

60 g/L Al2O3. This efficiency is almost twice the

achieved efficiency with the typical precipitation

process under the same conditions (retention time

24 h) and higher than the 48 g/L Al2O3, which is

the achieved efficiency after 96 h of continuous

precipitation.

5.2. High seed precipitation process (HSP)

It is well known that the seed ratio, or equiva-

lently, the seed concentration in the aluminate solu-

tion affects the rate at which the precipitation

reaction reaches equilibrium. In addition, kinetic

analysis has shown that the seed concentration

affects also the attained equilibrium level. This is

an unusual result indicating that boehmite seed plays

an important role during the precipitation process

and also affects the phenomena that are responsible

for the self-deceleration of the precipitation process.

Therefore, an alternative precipitation process was

developed according to which the initial boehmite

Fig. 6. Combined HSP/CFSH boehmite precipitation process

(132 g/L Al2O3, 120 g/L Na2O, initial seed concentration 1200 g/L,

90 jC).


seed concentration in the solution increases from 230

up to 1200 g/L. This process was called high seed

precipitation process (HSP).

Comparison between the HSP process and the

typical one is shown in Fig. 5. The boehmite precip-

itation rate and the process efficiency increase sub-

stantially as the initial boehmite seed concentration in

the aluminate solution increases from 230 g/L (typ-

ical precipitation) to 1200 g/L. The achieved effi-

ciency of the HSP process with 1200 g/L initial

boehmite seed concentration after 24 h of precipita-

tion is 60 g/L Al2O3 which is almost the same with

the corresponding one of the CFSH process under the

same conditions (retention time 24 h, 90 jC, 132 g/L

Al2O3, 120 g/L Na2O, seed concentration 230 g/L).

The HSP process efficiency increases to 73 and 76 g/

L Al2O3 as the precipitation time is prolonged to 72

and 96 h, respectively.

A new kinetic inhibition appeared after the first

24 h of boehmite precipitation with the HSP process

as seen in Fig. 5. The mean precipitation rate during

the first day is 2.35 g Al2O3/L h (HSP process with

1200 g/L initial seed concentration), while the mean

rate during the next 2 days (from 24 to 72 h) is

substantially smaller, 0.25 g Al2O3/L h, and during

the last day (from 72 to 96 h) is even smaller still,

0.15 g Al2O3/L h. Taking into account that the free

sodium hydroxide concentration in the solution after

the first 24 h of HSP process with 1200 g/L initial

seed concentration is twice the initial one, this new

inhibition could be probably overcome by control-

ling the free sodium hydroxide content in the solu-

tion. For this reason, a combined experiment was

designed according to which boehmite was precipi-

tated following the HSP process with 1200 g/L

initial seed concentration for the first 24 h; then

the 50% of the released amount of free sodium

hydroxide was neutralized with carbon dioxide and

the boehmite precipitation continued for 2 more

days. The results of this combined experiment are

shown in Fig. 6.

The results show that the decrease of the free

sodium hydroxide concentration in the solution after

the first 24 h of boehmite precipitation is beneficial

for the evolution of precipitation. The mean rate

of the boehmite precipitation from 24 to 72 h

increases from 0.25 g Al2O3/L h (HSP process) to

0.5 g Al2O3/L h (combined HSP/CFSH process) and

the process efficiency after 72 h of precipitation

reaches the level of 85 g/L Al2O3.

6. Discussion–conclusions

The classical crystallization theory (Nielsen, 1964)

has to be firstly utilized in the interpretation of the

experimental data. During the CFSH boehmite pre-

cipitation process (Fig. 4), the total sodium hydroxide

concentration in the solution decreases due to the

neutralization of the released sodium hydroxide.

Therefore, the boehmite solubility decreases, and thus,

the supersaturation degree increases. According to

classical crystallization theory, the rate of boehmite

precipitation should increase as the driving force (the

supersaturation) increases,. Indeed, this is evident in

Fig. 4. Although the observed experimental data

seems to be explained with the classical crystallization

theory, a detailed analysis reveals that the reason for

the process acceleration is not the increase of the

supersaturation, as seen in Fig. 7. The CFSH process

takes place under almost the same supersaturation

(during the first 9 h of precipitation) or under sub-

stantially lower supersaturation (from 9 to 24 h of

precipitation) with significantly higher precipitation

rates (Fig. 4).

Fig. 8. Effect of the total sodium hydroxide concentration in the

sodium aluminate solution on the rate and efficiency of the HSP

process with 600 g/L initial boehmite seed concentration at 90 jC.

Fig. 7. Evolution of the supersaturation degree under the typical and

the CFSH precipitation processes.


This observation is attributed to the inhibitive effect

of sodium hydroxide on boehmite precipitation. A

previous work on boehmite precipitation from super-

saturated sodium aluminate solutions had revealed that

the rate of boehmite precipitation is inversely propor-

tional to the term CNa2O�1,8 (Skoufadis et al., 2003a).

Therefore, as the total sodium hydroxide concentration

in the solution decreases during boehmite precipita-

tion, the precipitation rate increases using the CFSH

process.

During the HSP process (Fig. 5), an increase in the

seed concentration in the solution causes an increase in

the available surface area, and thus, the rate of boehm-

ite precipitation is expected to increase according to

the classical crystallization theory. Actually, the first

part of the precipitation curves shown in Fig. 5 (almost

the first 24 h of precipitation) can be interpreted by the

classical crystallization theory but there are two ex-

perimental results which cannot be explained. The first

one is a common observation in any attempt at

boehmite precipitation from supersaturated sodium

aluminate solutions. The boehmite precipitation pro-

cess is effectively over despite the precipitation driving

force (the supersaturation) remaining very high (Figs.

1, 5, 7 and 8). The second one is more impressive.

Kinetic analysis (Skoufadis et al., 2003a) has shown

that the rate of boehmite precipitation from supersat-

urated sodium aluminate solutions can be expressed in

term of an apparent equilibrium rather than of a real

equilibrium. This is evident in all experiments (Figs. 1,

5, 7 and 8). The system attains an apparent equilibrium

instead of a real one and the attained apparent equi-

librium shifts closer to the real one as the initial

boehmite seed concentration increases (Fig. 5). This

is an unusual result, seed concentration is known to

affect the rate at which precipitation reaction reaches

equilibrium but should not affect the equilibrium level.

Therefore, a different interpretation of experimental

data is necessary.

As has been reported, boehmite precipitation from

supersaturated sodium aluminate solutions under at-

mospheric conditions is a typical heterogeneous pro-

cess that takes place only in the presence of boehmite

seeds. Therefore, the understanding of precipitation

mechanism and the explanation of experimental data

require the correlation of the physicochemical prop-

erties of boehmite/solution interface with the chemical

properties of the sodium aluminate solution. It is well

known (Gerson et al., 1996; Panias et al., 2001a) that

in the high alkaline sodium aluminate solutions, dis-

solved aluminium exists almost exclusively in the

form of tetrahydroxoaluminate anion Al(OH)4�.

Therefore, the first step in the precipitation mecha-

nism is the transfer of the tetrahydroxoaluminate

anion from the aqueous solution to the charged

boehmite/solution interface. This process can be de-

scribed by the following schematic reactions taking


into account the surface species on boehmite/solution

interface that are shown in Fig. 2.

ZAlONaB þ AlðOHÞ�4 ðaqÞ

¼ Z tAlONaB . . . AlðOHÞ�4 b ð5Þ

ZAlO� þ AlðOHÞ�4 ðaqÞ

¼ Z tAlO� . . . AlðOHÞ�4 b ð6Þ

ZAlOHB þ AlðOHÞ�4 ðaqÞ

¼ Z tAlOHB . . . AlðOHÞ�4 b ð7Þ

ZAlOHþ2 þ AlðOHÞ�4 ðaqÞ

¼ Z tAlOHþ2 . . . AlðOHÞ�4 b ð8Þ

All the above schematic surface reactions are

energetically activated because they include the trans-

fer of a negatively charged aqueous species from the

solution to the negatively charged surface of boehmite

seed particles. The energy barrier for the above

surface reactions equals the required electrical work

(W) for the transfer of a tetrahydroxoaluminate anion

from the solution to the boehmite seed surface which

is proportional to the tetrahydroxoaluminate anions

charge (QAl(OH)4�) and the boehmite surface potential

(W) as is shown with the following equation.

W ¼ QAlðOHÞ�4 � W ð9Þ

The energy barrier for the surface reactions

increases as the surface potential (W) increases.

The boehmite surface potential in extremely high

alkaline solutions (pH>12.5) decreases as the solu-

tion pH increases (Fig. 3), and thus, the precipitation

process seems to be favoured under those conditions

if the kinetic problems are energetic in nature. This

cannot be true since considerable experimental data

reveal the negative effect of hydroxide ions on the

boehmite precipitation process. For that reason, the

transfer of the tetrahydroxoaluminate anions to the

boehmite seed surface seems not to be controlled by

energetic factors, and therefore, it is necessary to

examine the other physicochemical properties of

boehmite/water interface in order to interpret the

experimental results.

Taking into account the surface speciation diagram

(Fig. 2), the boehmite seed particles are exclusively

covered with only two kinds of surface species,

(ZAlONaB and ZAlO�), in highly alkaline sodium

aluminate solutions. If both kinds of surface species

are accessible for the sorption of the tetrahydroxoalu-

minate anion (Eqs. (5) and (6)), the rate of the

boehmite precipitation should not be affected by

changes in the hydroxide ions content of the solution.

On the contrary, the precipitation rate decreases as the

free sodium hydroxide content in the solution

increases (Figs. 4 and 6). An increase in the free

sodium hydroxide content of the solution causes an

increase in the surface concentration of the ZAlONaB

species and, consequently, a decrease in the surface

concentration of the ZAlO� species. Therefore, it

could be concluded that these surface species are not

equivalent during boehmite precipitation and only the

ZAlO� surface species are accessible for sorption of

the tetrahydroxoaluminate anions in extremely alka-

line sodium aluminate solutions. If this is true, the

positive effect of the seed concentration on the

boehmite precipitation process can be explained. In

extremely alkaline solutions, an increase in boehmite

seed concentration causes a small but not negligible

increase in the surface coverage with the ZAlO�

species (Fig. 2). The most important change in the

precipitation system is related to the total concentra-

tion of ZAlO� surface species in the solution (grions

of ZAlO� surface species per volume of solution)

that is given by Eq. (10).

tZAlO�btotal ¼ SC � SSD � SSA � BSC ð10Þ

where, SC is the % boehmite surface coverage with

the ZAlO� species, SSD (sites/m2) is the surface site

density of boehmite, as given in Table 1, SSA (m2/g)

is the specific surface area of boehmite seed, BSC (g/

L) is the boehmite seed concentration.

The total concentration of ZAlO� surface species

increases linearly with the boehmite seed concentra-

tion (BSC). Assuming that the rate of boehmite

precipitation depends on the total concentration of

ZAlO� surface species in the solution, an increase in

the boehmite seed concentration will increase the

Fig. 9. Precipitation of boehmite from supersaturated sodium

aluminate solutions with 65 g/L Na2O and 500 g/L initial seed

concentration under various temperatures.


precipitation rate as shown by Fig. 5. Moreover, when

the kinetic inhibition attributed to the increase of the

free sodium hydroxide content in the solution

becomes important (Figs. 5 and 6), the HSP process

with higher boehmite seed concentration will be

closer to the corresponding thermodynamic equilibri-

um as is observed in Fig. 5.

Taking into account the preceding discussion, it is

reasonable to infer that the HSP process should be

improved if the total sodium hydroxide concentration

in the sodium aluminate solution is decreased. Indeed,

this was proven correct as is seen in Fig. 8. The rate of

boehmite precipitation is improved when there is a

lower total sodium hydroxide concentration and the

process efficiency has also increased.

The most impressive experimental result was

obtained when both the precipitation temperature

and the total sodium hydroxide concentration were

decreased. The results are shown in Fig. 9 where

boehmite precipitation from a solution with 65 g/L

Na2O was studied. Pure crystalline boehmite can be

precipitated from supersaturated sodium aluminate

solution with the HSP process at temperatures lower

than 90 jC which is the temperature limit for boehm-

ite precipitation from a solution with 120 g/L Na2O.

The precipitation process follows the same mode as in

the case of precipitation with 120 g/L Na2O (Figs. 1

and 5). Kinetic inhibitions are also observed as the

free hydroxide ions concentration increases during the

course of precipitation and the temperature decreases

from 95 to 70 jC. The potential for boehmite precip-

itation from supersaturated sodium aluminate solu-

tions at temperatures lower than 90 jC is not

obviously correlated with the physicochemical prop-

erties of the boehmite/solution interface. It has to be

mentioned here that the substantial decrease of the

total sodium hydroxide concentration to such a level

that the solution pH is in the region 10–12 with the

simultaneous very high initial seed concentrations,

creates favourable conditions for boehmite precipita-

tion because the surface potential takes less negative

values (Fig. 3) and the surface is covered mainly by

ZAlOHB and less extensively by ZAlO� and

ZAlOH2+ (Fig. 2).

As a conclusion, the preceding analysis has corre-

lated the physicochemical properties of boehmite/

solution interface with the mechanism of boehmite

precipitation from supersaturated sodium aluminate

solutions and elucidated the role that sodium hydrox-

ide and boehmite seed play during the precipitation

process.

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role of boehmite/solution interface in boehmite precipitation from supersaturated sodium aluminate...

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