Hydrometallurgy 95 (2009) 297–301

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Precipitation of boehmite in sodium aluminate liquor

B. Dash a, B.C. Tripathy a, I.N. Bhattacharya a,⁎, S.C. Das a, C.R. Mishra b, B.K. Mishra c

a Institute of Minerals and Materials Technology (CSIR), Bhubaneswar-751013, Indiab National Aluminium Company, Bhubaneswar-751013, Indiac Centre of Studies in Surface Science and Technology, Department of Chemistry, Sambalpur University, Orissa, India

⁎ Corresponding author.E-mail address: indranb@yahoo.com (I.N. Bhattachar

0304-386X/$ – see front matter © 2008 Elsevier B.V. Adoi:10.1016/j.hydromet.2008.07.002

a b s t r a c t

a r t i c l e i n f o

Article history:

Gibbsite is generally precip Received 21 May 2008Received in revised form 9 July 2008Accepted 9 July 2008Available online 16 July 2008

Keywords:Boehmite precipitationSodium aluminate liquorTartaric acidAlumina/caustic ratio

itated from sodium aluminate liquor in the presence of huge quantity of gibbsiteseed at a temperature between 60 and 70 °C. This gibbsite is then calcined to produce alumina. Boehmite is astable phase at higher temperature (N100 °C) and formation of boehmite below 100 °C in supersaturatedsodium aluminate liquor is unusual. In the present study an attempt has been made to produce boehmiteunder atmospheric pressure conditions. Effect of various parameters such as temperature, alumina/caustic(Al2O3/Na2O) ratio, seed size, amount of seed, organic additives, precipitation time, etc., on boehmiteformation has been investigated. At higher temperature and low alumina/caustic ratio, boehmite is found tobe precipitated under atmospheric pressure by adding boehmite seed. The optimum quantity of seedrequired for precipitation has been found to be 300 g L−1. Reduction in precipitation temperature could beachieved by using different organic additives. Tartaric acid has been found to be highly effective in reducingthe temperature of boehmite precipitation to 50 °C. Increase in precipitation time increased boehmiteproduction.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Boehmite (Al2O3·H2O) is usually associated with bauxite and usedfor production of alumina through Bayer process. It is used as aprecursor for manufacturing various activated aluminas. The Bayerprocess is mainly adopted to precipitate gibbsite, an aluminatrihydrate, for production of alumina. In Bayer process, sodiumaluminate liquor is decomposed in the presence of large quantity ofgibbsite seed at 65–70 °C to produce gibbsite, which is then calcined toobtain alumina. The energy part of the Bayer process is of majorconcern. It consumes almost 15–20% of the total energy consumptionin aluminium plant. In recent times, the urge to reduce energyconsumption in aluminium industries led to the development of otherprocesses. The precipitation of boehmite has become one of suchalternatives. The precipitation of boehmite was investigated (Hectoret al., 1997; Misra, 1986) in the past through various routes other thanthe Bayer process. However it has not been prepared in view of using itas a source for alumina production in the industry. Boehmite can alsobe prepared by hydrolysis of aluminium salts. Boehmite wasprecipitated by Misra and Shivakumar (1986) from the sodiumaluminate liquor in the presence of large quantity of boehmite gel ata temperature around 120 °C. In recent years notable contributions inthe field were made by researchers of National Technical University ofAthens, Greece (Paspaliaris et al., 2000; Panias et al., 2001; Skoufadis

ya).

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et al., 2003; Filippou and paspaliaris, 1993; Panias and Paspaliaris,2003; Kontopoulos et al., 1998; Elkatatny et al., 1998; Dash et al., 2007;Loh et al., 2005). They have carried out boehmite precipitationisothermally in atmospheric conditions above 85 °C.

It has been observed that boehmite precipitation depends onmainly alumina/caustic ratio of the aluminate liquor and temperature.At higher alumina/caustic (A/C) ratio boehmite can be precipitated athigher temperature and at lower A/C ratio boehmite precipitation ispossible at lower temperature but at the expense of boehmite yield. Atlower A/C ratio alumina supersaturation is low thereby yielding lessboehmite. At A/C ratio of 1.0–1.1, the precipitation temperature ofboehmite is more than 85 °C, which is higher compared to gibbsiteprecipitation temperature in actual plant practice. At lower tempera-ture (65–70 °C) the available supersaturation is high. Boehmiteprecipitation at lower temperature would therefore increase theprecipitation yield. However precipitation of boehmite at lowertemperature is a challenging task because there is every possibilityof gibbsite to be co-precipitated.

The activation energy of gibbsite precipitation in sodium aluminateliquor is 50–59 kJ/mole and that of boehmite is 89 kJ/mole (Skoufadiset al., 2003). Therefore, gibbsite precipitation is kinetically favouredthan the boehmite one. In order to facilitate boehmite precipitation,inhibition of gibbsite nucleation is necessary. This may be achieved byadding different organic additives to the reaction medium. ActualBayer liquor also contains substantial amount of organics.

In the present study, effect of various parameters such as seedquantity, seed size, organic additives, and duration of precipitation onboehmite productionwas investigated. Various organic additives such

Fig. 1. Variation of lowest temperature at which boehmite is precipitated with alumna/caustic ratio. Time=5 h, Al2O3=150 g L−1, seed=100 g L−1.

Fig. 3. Effect of seed size on boehmite yield. A/C=1.0, seed=100 g L−1, time=8 h,Al2O3=150 g L−1.

298 B. Dash et al. / Hydrometallurgy 95 (2009) 297–301

as Polyacrylamide (PAA), Carboxy-methyl cellulose–sodium salt (Na-CMC) and organic acids such as tartaric acid (TA), succinic acid,glutaric acid etc. were investigated. All the experiments were carriedout using synthetic supersaturated sodium aluminate solution.

2. Experimental

2.1. Preparation of boehmite seed

The boehmite seed preparation was carried out in a PARR autoclave(model no. 4542). Boehmite seed prepared through hydrothermal routeis most suitable because of its crystalline nature. To find out the hydro-thermal condition for boehmite seed preparation, a study on optimiza-tion of boehmite seed preparation was carried out. The boehmite seedwas prepared from gibbsite seed material collected from M/s NALCO,Bhubaneswar. The pH of the pulp was maintained at 10. The boehmitetransformation temperature was found to be 195 °C. After each experi-ment in autoclave the pulp was allowed to cool down and then it wasremoved from the autoclave andwasfiltered. After filtration the residuewas dried in an oven at 110 °C for overnight. The dried seed was takenfurther for boehmite precipitation studies. Theoretically, theweight lossduring gibbsite to boehmite transformation is 23%. In the present studyalso a weight loss of about 23% was observed, thus confirming tothe formation of boehmite. Formation of boehmite was also supportedby X-ray diffraction study (Dash et al., 2007).

Fig. 2. Effect of seed quantity on boehmite yield. A/C=1.0, time=8 h, Al2O3=150 g L−1.

2.2. Preparation of synthetic liquor

The supersaturated sodium aluminate liquor was prepared bydissolving required amount of aluminium granules in sodiumhydroxide solution. The liquor was prepared for different alumina/caustic ratios. In general 150 g L−1 Al2O3 solution of A/C ratio 1.0 wasprepared by dissolving aluminium granules (E.Merck, Germany) in150 g L−1 of Na2O solution.

2.3. Precipitation

Precipitation was carried out in a 300 mL capacity stainless steelreactor with lid. The lid had provisions for accommodating a stirrer, athermometer and an opening for sampling. Agitation of the liquor wasperformedwith a laboratory model Remi stirrer having variable speedcontrol. Stirring speed of around 250±25 rpm was maintained ineach experiment to ensure proper suspension of seed particles. Theprecipitation temperature was maintained within a stability rangeof ±0.1 °C using a constant temperature bath (JULABO, Germany). For ahigher scale (400–600 mL) study 1 L reactor was used.

The required volume (100 mL) of aluminate liquor with pre-adjusted Al2O3 and Na2O concentrations was poured into theprecipitation reactor and the reactor was kept in the constanttemperature bath. Once the solution reached the required tempera-ture an appropriate quantity of boehmite seed was added followed byagitation. All the experiments were conducted under isothermalcondition. The precipitation reaction was then continued for thedesired time period. Unless otherwise mentioned all the experimentswere carried out for 8 h. However to observe the effect of precipitationtime some experiments were carried out for 24 h and 48 h asdiscussed in Section 3.5 and 3.6. After the reactionwas over the slurrywas taken out and filtered. The precipitate was then repulped with250–300 mL of water and filtered again. This process was repeated 3

Table 1Effect of PAA on boehmite precipitation

Sl. no. Temperature ( °C) PAA (mg L−1) Precipitated phase

1 90 Nil Boehmite2 85 Nil Boehmite3 80 Nil Gibbsite4 80 125 Boehmite5 70 125 Boehmite6 60 125 Gibbsite

Conditions: A/C=1.0, time=8 h, amount of seed=100 g L−1.

Table 2Effect of Na-CMC on boehmite precipitation

Sl. no. Temperature (°C) Na-CMC (mg L−1) Precipitated phase

1 90 Nil Boehmite2 85 Nil Boehmite3 80 Nil Gibbsite4 80 125 Boehmite5 70 125 Boehmite6 60 125 Gibbsite

Conditions: A/C=1.0, time=8 h, amount of seed=100 g L−1.

Table 4Comparison of precipitated product at different A/C ratios with tartaric acid in mg L−1

level

Sl. no. Tartaric acid (mg L−1) Precipitated phase

A/C=1.0 A/C=1.1

1 50 Gibbsite Gibbsite2 100 Boehmite Gibbsite3 200 Boehmite Gibbsite4 300 Boehmite Gibbsite

Conditions: temp.=80°C, amount of seed=100 g L−1, time=8 h.

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to 4 times until the pH of the filtrate reached to around 7. Finally theprecipitate was dried at 110 °C for 24 h. The filtrate was analysed foralumina content. Yield was measured as the weight of boehmiteobtained in g L−1. At the end of the experiments, the solids were takenfor phase analysis by X-ray diffractometry to know whetherprecipitates are boehmite or gibbsite. Certain experiments were alsocarried out taking 400 or 600 mL solution in a separate reactor.

2.4. Analyses

Particle size measurements were carried out in a Malvern Particlesize analyser model 3600E. All precipitates were analysed by X-raydiffraction (XRD) to know the crystalline phase. XRD was run on aPhilips powder diffractometer model PW 1830 X'pert system using aCu target. Analysis of Al2O3 in liquor was carried out by the EDTA-ZnSO4 method. Alumina was estimated by titrating the residual EDTAwith ZnSO4 taking xylenol orange as indicator. The pH of the solutionwas maintained at 3 to 5 by adding sodium acetate-acetic acid buffer.

3. Results and discussions

Precipitation of boehmite was carried out under atmosphericpressure conditions taking synthetic sodium aluminate liquor ofdifferent A/C ratios. Organic additives were added to find out thepossibility of precipitation of boehmite at lower temperatures whenA/C ratios are higher. Various parameters studied are discussed below.

3.1. Effect of alumina/caustic ratio (A/C) and temperature

Fig. 1 depicts the relationship of precipitation temperature with A/Cratio. Lower A/C ratio, up to 0.95, favours formation of boehmite at lowtemperature, while higher A/C ratio requires higher temperature above85 °C for boehmite formation. Thus low supersaturation of sodiumaluminate liquor is more favourable for boehmite precipitation thanhigher supersaturation of the liquor. The actual supersaturation in theprecipitation process is measured as the difference between theconcentration of alumina in liquor and concentration of alumina insaturated solution at aparticular caustic concentration and temperature.Thuswhen the precipitation temperature is higher, the concentration ofalumina at saturation is also higher and hence the difference betweenactual concentration of alumina and concentration at saturationbecomes low. But at low precipitation temperature, supersaturationwill be high. Therefore, in that condition A/C ratio of the solution shouldbe lowered in order to facilitate boehmite precipitation.

Table 3Boehmite precipitation without additive

Sl. no. Temperature ( °C) Additive (mg L−1) Precipitated phase

1 90 Nil Boehmite2 85 Nil Boehmite3 80 Nil Gibbsite4 70 Nil Gibbsite5 60 Nil Gibbsite6 50 Nil Gibbsite

Conditions: A/C=1.0, time=8 h, amount of seed=100 g L−1.

3.2. Effect of seed amount and seed size

Fig. 2 shows the effect of seed amount on boehmite yield. Theexperiments are mainly carried out at 85 and 90 °C taking A/C ratio ofthe liquor at 1.0. It has been found that with increase in amount ofseed, the yield has increased. It has also been found that 300 g L−1 ofseed may be the optimum value because beyond this amount nosignificant improvement in the yield was observed.

The effect of seed size on boehmite yield (Fig. 3) at A/C ratio 1.0 attwo different temperatures was investigated and it was found thatwith decrease in seed size boehmite yield has been increased.Therefore, amount of seed and the size of seed particles are consideredto be the major controlling factors for boehmite yield. Total surfacearea of particles is enhanced when more seed particles and/or finerseed particles are added facilitating higher yield.

3.3. Effect of PAA and Na-CMC

Tables 1 and 2 depict the effect of PAA and Na-CMC on boehmiteprecipitation respectively. PAA and CMC are the flocculants, generallymixed with the slurry to make larger lumps of the particles. Inpresence of the flocculants (125mg L−1) the precipitation temperaturereduced to 70 °C. This may be attributed to the inhibition of gibbsitenucleation in presence of the additives. Below 70 °C gibbsite isprecipitated. By adding more amount of PAA or CMC boehmiteprecipitation temperature could not be reduced further. Tables 1 and 2also show that at 85 and 90 °C without adding any additive boehmitecould be precipitated but when precipitation temperature wasreduced to 80 °C gibbsite was precipitated.

3.4. Effect of tartaric acid

Tartaric acid was found to be the most effective additive ininhibiting gibbsite nucleation and favouring boehmite precipitation.Initially experiments were conducted at 90 °C with a solution of A/Cratio 1.0 and then temperature was decreased. Without tartaric acid,boehmite was precipitated at 85 °C but below this temperaturegibbsite was precipitated (Table 3). Similar results are obtained withA/C ratio 1.1. Table 4 shows the amount of tartaric acid required forboehmite precipitation. An amount of 100 mg L−1 tartaric acid wasfound to be sufficient for precipitating boehmite from the liquor withA/C ratio 1.0. But the amount is not sufficient to precipitate boehmite

Table 5Boehmite precipitation with Tartaric acid

Sl. no. Temperature(°C) Tartaric acid (g L−1) Precipitated phase

1 90 Nil Boehmite2 80 Nil Gibbsite3 80 0.9 Gibbsite4 80 1.5 Gibbsite5 80 2.25 Boehmite6 70 2.25 Boehmite7 60 2.25 Boehmite8 50 4.0 Boehmite

Condition: A/C=1.1, amount of seed=100 g L−1, time=8 h.

Fig. 4. Effect of precipitation time on boehmite yield. A/C=1.1, temperature=80 °C,TA=6 g L−1, Al2O3=150 g L−1, seed=300 g L−1.

Fig. 6. Effect of A/C ratio on boehmite yield. Temperature=90 °C, time=24 h, seed=100 g L−1

(9 g L−1 TA at A/C ratio 1.2).

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from the liquor with A/C ratio 1.1. As the supersaturation is high in theliquor with A/C ratio 1.1, gibbsite precipitation is favoured. Thereforeprobably more additive is required to suppress gibbsite precipitationat A/C ratio of 1.1. It is well established that in Bayer plants the gibbsiteyield becomes low because of presence of organics which inhibitgibbsite precipitation (Watling et al., 2000). Table 5 shows theboehmite precipitation conditions for liquor having A/C ratio 1.1. Itwas found that the boehmite precipitation temperature could bereduced to 60 °C by adding 2.25 g L−1 tartaric acid. Further increase intartaric acid concentration to 4 g L−1 the precipitation temperaturedecreased to 50 °C.

Various other organic acids such as succinic acid, salicylic acid,glutaric acid, citric acid, formic acid etc. were tried but results werenot satisfactory in reducing the boehmite precipitation temperaturebelow 80 °C.

3.5. Effect of precipitation time on boehmite yield

Experiments were conducted for precipitating boehmite at differ-ent time period starting from 8 h to 48 h to see the effect of time onboehmite yield. Fig. 4 depicts the progress of boehmite precipitationwith time. It was found that by increasing the precipitation time theboehmite yield has increased; however the yield was maximum atabout 45 h and thereafter remained almost constant.

Fig. 5. Effect of seed amount on boehmite yield. Time=24 h, temperature=93 °C,Al2O3=150 g L−1.

3.6. Effect of various parameters on boehmite yield at longer precipitationtime

To find out the effect of various parameters such as seed amount, A/Cratio and temperature on boehmite yield at longer precipitation time aseries of precipitation experiments was carried out taking 400–600 mLof sodium aluminate solution in a stainless steel reactor for 24 h and48 h. From Fig. 5 it is found that the yield is more in the liquor with A/Cratio 1.1. This is because at higher A/C ratio the supersaturation of theliquor is higher and therefore the concentration of alumina available forprecipitation is more. In case of A/C ratio 1.0 with 100 g L−1 of seed, theyield of boehmite was 17.4 g L−1 and with subsequent increase in seedamount the yield was also increased. Similarly in the case of the liquorwith A/C ratio 1.1 and 100 g L−1 seed the yield was 33.1 g L−1 andsubsequently with increase in the seed quantity the yield was alsoincreased. It is also clear fromFig. 5 that in both the cases of A/C ratios 1.0and 1.1 the improvement in the yields with seed amount 300 g L−1and600 g L−1 wasmarginal. This indicates that the seed amount of 300 g L−1

wouldbe sufficient toobtainproper yieldwithhigher precipitation time.Fig. 6 clearly indicates that with increase in the A/C ratio the boehmiteyieldwas increased. It has been observed that substantial increase in theyield was observedwith A/C ratios 0.95 to 1.1, however the increasewasminimal with 1.2. In the case of A/C ratio 1.2 tartaric acid (9 g L−1) wasadded because with A/C ratio 1.2, the supersaturation is very high andthe condition is more favourable for gibbsite precipitation. Fig. 7 shows

Fig. 7. Effect of temperature on boehmite yield. A/C=1.1, seed=300 g L−1, time=48 h,TA=6 g L−1.

301B. Dash et al. / Hydrometallurgy 95 (2009) 297–301

the effect of temperature on boehmite yield. Increasing the temperatureincreases the boehmite yield indicating that the kinetics of boehmiteprecipitation is faster at higher temperature and vice versa.

4. Conclusions

The study carried out on precipitation of boehmite from synthe-tically prepared supersaturated sodium aluminate solutions hasrevealed various aspects of boehmite precipitation. (i) It has beenfound that boehmite precipitation mainly depends on temperatureand the liquor supersaturation. Higher temperature and low super-saturation are favourable for boehmite precipitation. (ii) The seed sizeand amount has positive impact on precipitation of boehmite. Theyield of boehmite precipitation is increased with finer seed size andhigher seed quantity. (iii) Addition of PAA and Na-CMC could reducethe boehmite precipitation temperature to 70 °C but below thistemperature gibbsite is precipitated. (iv) Effect of tartaric acid is foundto be most beneficial. It reduced the boehmite precipitationtemperature to 50 °C at all the A/C ratios tested such as 1.0, 1.1 and1.2. A substantial increase in boehmite yield was obtained whenprecipitation time is increased to 24 or 48 h.

Work on developing an alternative additive which can be used in asmall quantity for boehmite precipitation is in progress. Use of organicacids such as succinic acid, glutaric acid, formic acid etc. could notreduce the boehmite precipitation temperature below 80 °C.

Acknowledgements

The authors are thankful to M/s Nalco, Bhubaneswar, for thefinancial support. They are also thankful to the Director, IMMT,Bhubaneswar, for giving permission to publish this work.

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