The effect of 3,4 dihydroxy benzoic acid (3,4 DHBA) on the precipitation of alumina trihydrate

Minerals Engineering, Vol. 9, No. 5, pp. 557-572, 1996

Pergamon PII:S0892-6875(96)00043-X

Copyright O 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0892--6875/96 $15.00+0.00

T H E E F F E C T O F 3,4 D I H Y D R O X Y B E N Z O I C A C I D (3,4 D H B A ) ON THE PRECIPITATION OF ALUMINA TRIHYDRATE T. T R A N , M . - J . K I M a n d P . L . M . W O N G Centre for Minerals Engineering, The University of New South Wales, Sydney 2052, Australia (Received 2 October 1995; accepted 6 February 1996)

ABSTRACT In this study, a second order rate equation was used successfully model the rate of nucleation and crystal growth during the precipitation of alumina trihydrate. In unseeded liquors, 2~,4 DHBA reduces the rate of nucleation and crystal growth significantly. It hinders the agglomeration of nuclei and crystals, producing less dense products compared to those precipitated from pure liquors. It was also found that alumina hydroxide nuclei need to crystallise before crystal growth occurs, and 3, 4 DHBA delays the crystallisation process. In seeded liquors, 3, 4 DHBA causes an exponential increase in the number of secondary nuclei and a decrease in the rate of crystal growth. It also causes an increase in induction time. Provided this induction time is known the whole system can be characten'sed by the second order equation. Accumulated kinetic and induction time data can be used for the simulation of the precipitation process.

Keywor&~ Oxide ores; bydrometallurgy; reaction kinetics; particle morphology; particle size

INTRODUCTION Natural organic matter is present in bauxite in varying amount. A portion of the organic matter is broken down into lower molecular weight organics and becomes soluble in sodium aluminate liquor during the digestion stage of the Bayer process. The soluble organics can build up to an extent that they can affect the economy of the Bayer process by decreasing yield and product quality. Over the last decade, there has been considerable interest in developing various methods of removing organics from Bayer liquor which has been reported in a review by The [1]. Considerable work on the effect of organic impurities on crystal growth, with respect to the quality of the product, has also been reviewed by Armstrong [2]. The effect of organic impurities on the kinetics of nucleation and crystal growth in seeded liquors has been studied by Alamdari et al. [3] and Calalo et al. [4], using mannitol and sodium oxalate as model organic compounds, respectively. Although the precipitation kinetics were characterised by a particle population balances model developed by Misra and White [5], the effect of organic impurities on the induction time for seed growth as observed by Brown [6] was not discussed. This study aims to investigate the effect of 3,4-dihydroxy benzoic acid (3,4 DHBA) on the kinetics of nucleation and crystal growth during the precipitation of alumina trihydrate, with particular reference to the induction time and morphological changes. As plant liquors contain a huge range of organic species, most experimental studies were conducted using model organic compounds aiming at achieving a more fundamental understanding. 3,4-DHBA was chosen as a model organic due to its significant effect on production yield as identified by Armstrong [2]. Precipitation in unseeded liquors was also carried out to

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gain an insight into the effect of the model organic on the nucleation and crystallisation stages. Aldcroft and co-workers [7] have carried out work on the crystallisation of pure amorphous alumina trihydrate in water and water-alcohol mixture, but not in strong alkali solutions.

EXPERIMENTAL All chemicals and reagents used in this study were of analytical grade. All solution were prepared using distilled water. Pure sodium aluminate liquor were prepared by digesting 120 g of analytical grade gibbsite in one litre of concentrated sodium hydroxide, giving a final alumina (expressed as g/1 Al203) to caustic (expressed as g/1 Na2CO 3) ratio (A/C) of 0.7. Precipitation tests were carried out using polypropylene plastic bottles held by a variable speed motor rotator and immersed in a thermostatically controlled water bath. Samples required for kinetic studies were taken at pre-determined time intervals. The alumina and soda content of the solution samples were analysed by sodium gluconate method [12]. For transmission electron microscope (TEM) study samples were prepared by a method described by Green et al. [8]. Initially, a drop of the precipitate suspension was drawn from the plastic bottle and dried on a copper specimen grid which has been coated with a thin layer of carbon supporting film. However, once the specimen is subjected to the vacuum in the TEM (Joel 2000 FX) a mixture of sodium aluminate crust with no regular structure and nuclei of < 10 nm were observed. At a later stage, 100 ral of samples were drawn and centrifuged at 2000 rpm for 30 minutes and the clear supernatant liquid was discarded. The precipitate was re-suspended by vibrating the sample in 70 °C distilled water using Whidi Mixer for 30 minutes. The procedure was repeated three times and the suspension was later dried on the copper grid. The technique was found to be successful, and nuclei or crystal of alumina trihydrate could be clearly photographed. Seeds with surface re-coated were used in the experiments. The original seeds were obtained from an industrial supplier. The seeds were coated with a thin layer of pure alumina trihydrate by suspending them at 100 g/1 in a sodium aluminate solution ( initial A/C of 0.44 at 85°C and caustic concentration of 200 g/1 Na2CO3) for 20 hours. The size of the final seeds ranges from 20 #m to 38/~m, with an average size of 30 #m measured by a Malvem Mastersizer and from micrographs obtained from scanning electron microscopy. The coated seeds look smoother than the uncoated ones. The BET surface area measured by BET method is found to be 0.837 m2/g.

RESULTS AND DISCUSSION Previous modelling work on the kinetics of precipitation in seeded liquor usually used the particle population balances model (second order) developed by Misra and White [5]. Alamdari et al. [3] and Calalo et al. [4] used this particle population balances model to study the effect of mannitol and oxalate, respectively, on the kinetics of precipitation in seeded liquors. This method has one drawback. As it is based on particle size measurement, secondary nuclei grown on a seed are all counted as one particle, and thus the induction time can not be observed clearly using the particle population model. Furthermore, the change in size for individual particles is minimal during the induction period. According to Brown [6], the induction time is the time taken to reach an equilibrium m o u n t of nuclei and is generally identified graphically as the first linear section of the second order plot. To account for this problem a second order model (specified in the appendix) basing on the change of A/C ratio is used. This model is more sensitive to secondary nucleation as it measured the overall drop in the A/C ratio rather than the changes in the size of an individual particle. The K value is particularly useful for comparing the kinetics of precipitation in the presence and absence of an organic impurity. For comparing the kinetics of precipitation of alumina in organic-contaminated liquors to those of pure liquors, the alumina to caustic ratio at equilibrium (A/Ceq) for pure liquors is taken as the point of reference. The A/Ceq for pure liquors is obtained by extending the precipitation experiments up to 14 days.

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Individual A/Ceq value for each temperature is shown together with the results in their respective figures (most of these values are lower than the ones predicted from an empirical equation given by Misra and White [11]). The surface area of the seed (or precipitate) changes as a function of time, and an in situ measurement of the area is usually very difficult. However, it was found in this study that the rate of reaction is directly proportional to the initial seed surface area (to be discussed later), and therefore the initial seed surface area is used in the second order equation to characterise the system.

Kinetics of precipitation in unseeded liquors Figure 1 shows the kinetics of precipitation of alumina trihydrate in unseeded liquors in the presence and absence of 3,4 DHBA, which fit well into a second order model. There are two distinct regions, namely the nucleation and crystal growth. For pure liquors, higher nucleation rate and lower nucleation time were observed compared to those of organic-contaminated liquors. The K values for nucleation and crystal growth for both liquors are presented in Table l(a). It is clear that 3-4 DHBA has a significant impact on the rate of precipitation and yield.

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InitialA/Cratio0.7,Caustic200 g/l,75 °C,AICeq= 0.335 Fig. 1 Kinetics of precipitation in unseeded liquors Figures 2 (a) and (b) show the transmission electron micrographs of the precipitate after 2 and 4 hours of precipitation, respectively. The amorphous material consists of agglomerates of nuclei less than 10 rim. It was very difficult to obtain precipitate samples for contaminated liquors after 2 and 4 hours as there was very little precipitate formed. After 6 hours of precipitation, the precipitate in pure liquors became more crystalline as shown in Figure 3 (a). In this particular figure, one hexagonal alumina trihydrate crystal is observed, and two others on the left hand side show irregular shape. At this point, it important to note than there appears to be a missing link between the amorphous material and the crystalline material. The most possible explanation is that the amorphous material have crystallised to form the crystalline material. The hexagonal crystal is gibbsite which was also observed by Schoen and Roberson [9] in slightly alkaline solutions. The irregular crystals are probably the precursors to gibbsite formation as their frequency of appearance reduce dramatically in the later stage of precipitation. The precipitate from contaminated liquors after six hours of precipitation, shown in Figure 3 (b), consists of agglomerated nuclei; however, the nuclei appears to be more crystalline than the ones observed in Figure 2 (a) and (b). The gaps between the aggregated nuclei imply that the organic somehow hinders the agglomeration of nuclei. RE 9-5-F

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TABLE 1 (a) K values for unseeded liquors calculated from Figure 1, (b) the effect of 3,4 DHBA on K values at different temperature (calculated from Figure 7) (c) the effect of 3,4 DHBA on activation energy E 0 (calculated from Figure 8) and rate constant !%.

g/1 Organic 0 1

K(Nucleation) 1/hr 0.0300 0.0034

K (Growth) 1/hr 0.120 0.073

(a) Temperature 65 °C 75 °C 85 °C

K(Induction) 1/hr 0g/1DHBA I~DHBA 0.89 0.37 1.23 0.62 2.12 1.08

K (Growth) 1/hr lg/1DHBA 0g/1DHBA 0.48 0.378 0.57 0.425 0.64 0.47

(b) DHBA

og/1 lg/1

Induction Eo(J/mole) ko(1/hr.m 2) 49.64x103 412.79×106 45.91x103 574.66×105

Growth ko(1/hr.m 2) = Eo(J/mole) 712 13458 10973 225

(c)

After twelve hours of precipitation, the gibbsRe crystals shown in Figure 4 (a) started to grow and cemented together. The transition period from nucleation to crystal growth can he observed in Figure 1. The precipitate from contaminated liquors at 12 hours shown in Figure 4 (b) indicates an increase in packing density although the gaps within the aggregate can still be observed. Figures 5 (a) and (b) show the micrographs of the crystals in pure and contaminated liquors, respectively, after 18 hours of precipitation. The crystals in pure liquors show an increase in size, whereas the agglomerates in contaminated liquors show sign of crystallisation of the agglomerated nuclei into gibbsite crystals. This transition period can be seen in Figure 1. After 24 hours, the crystals become too large to be observed by scanning electron microscope. Instead optical microscope is used, and the resulting photographs are shown in Figure 6 (a) and (b). Crystals from uncontsminated liquors become polycrystalline whereas those from contaminated solutions are not well cemented together. The kinetic studies on unseeded liquors have shown that 3,4 DHBA had a significant effect on the rate of precipitation, the crystallisation process, the cementation of nuclei or crystals and the final morphological structure. The following discussion concentrates on the effect of 3,4 DHBA on the rate of alumina precipitation in seeded liquors, which is of interest to industry.

Effect of 3,4 DHJ3Aon precipitationof alumina trihydrate

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1P-l

Fig.2 Transmission electron micrograph of precipitate after (a) 2 hours (b) 4 hours of precipitation from pure liquors

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Fig.3 Transmission electron micrograph of precipitate after 6 hours of precipitation from (a) pure (b) contaminated (lg/l 3,4 DHBA) liquors

Effect of 3,4 DHf3A on precipitation of alumina trihydrate

Fig.4 Transmission electron micrograph of precipitate after 12 hours of precipitation from (a) pure (b) contaminated (lg/l 3,4 DHBA) liquors

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Fig.5 Transmission electron micrograph of precipitate after 18 hours of precipitation fi (a) pure (b) contaminated (lg/l 3,4 DHBA) liquors

Effect of 3,4 DHJ3Aon precipitationof alumina trihydrate

Fig,.6 Optical micrographs of precipitate after 24 hours of precipitation flrom (a) pure (b) contaminated (lg/l 3,4 DHBA) liquors

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566 Kinetics of precipitation in seeded liquors

Figures 7 (a), (b) and (c) show the effect of 3,4 DHBA on the kinetics of crystal growth at 65°C, 75°C and 85°C respectively. From these figures, it can be generalised that the higher the temperature is the faster the rate of precipitation will be with respect to their A/Ceq, for both pure and contaminated liquors.

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Seed 100 g/l, Initial MC ratio 0.7, Caustic 200 g/l, 85 0(2,AICeq 0.404 (c)

Fig.7 The effect of 3,4 DHBA on the kinetics of crystal growth at different temperatures (a) 65oc (b) 75oc (c) 85oc

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In other words, given a constant supersaturation value, alumina will precipitate from the solution at a faster rate at a higher temperature. This has also been observed by Misra and White [5]. High temperatures ( > 85°C), however, are not used in industry because their main focus is to precipitate maximum amount of alumina from the pregnant liquors, and the best way to do that is to decrease the temperature of the liquors so that the alumina content remained in the liquors is as low as possible. From Figure 7 it can also be generalised that the higher the temperature is the shorter the induction time will be whether an organic is present or not. The shorter induction time may also be due to the decrease in A/Ceq. Brown [6], however, found that decreasing supersaturation would have increased the induction time. The decrea.~e in induction time in this case, therefore, is mainly due to the increase in temperature, implying that temperature is a more dominant factor in controlling the induction time. Figure 7 also shows that the presence of 3,4 DHBA causes an increase in induction time. This is due to the increase number of secondary nuclei formed on the seed surface which can be seen in Figure 8 (b). Figure 8(a) shows the morphology of the original seeds used. Brown [6] has shown that a dramatic increase in the formation of secondary nuclei on the seeds has significant impact on the induction time.

(a)

03) F i g 8 Scanning electron micrograph of crystals after 24 hours of precipitation (a) pure (b) cont~uninated (lg/1 3,4 DHBA) liquors, 100g/1 seed, A/C 0.7, caustic (200g/l as Na2CO3)

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Values of K, k0 and activation energy (Eo) can be calculated from Figure 7 and Figure 9 (Arrhenins plot) and tabulated in Table 1 Co) and (c). The k0 values and activation energy are calculated on the assumption that the K values is proportional to initial seed surface area. The Arrhenlus plot of In K vs I/T in Figure 9 shows a linear relationship, which verifies that the assumption is valid. Note that the activation energy calculated here is different from those reviewed by Audet and Larocque [10] as a different driving force expression was used. In the presence of 3,4 DHBA, the second order rate constant for both induction and crystal growth decrease significantly whereas the activation energy is only slightly reduced. The lower activation energies imply that secondary nucleation is more spontaneous in contaminated liquors than pure liquors.

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Fig.9 Arrhenlus plot of log K vs 1/T for (a) nucleation Co) crystal growth

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Figure 10(a) shows that an increase in seed concentration increases the K values, listed in Table 2(a), and therefore the rate., of precipitation. The increase in seed concentration also causes a reduction in induction time. This is probably due to the lower equilibrium number of nuclei needed to be formed as observed by Brown [6]. An a'verage k o value for the second order equation of crystal growth is calculated to be 739 + 19 h r - l.m2 (Table 2(c)), basing on the activation energy calculated before. Similarly, the average k o value for the induction period can be calculated from the K values for this period. The k 0 and activation energy (E) are significant in characterising the nucleation and crystal growth. To express k o values in terms of 1/m2 of initial seed area is important as these values can be used for various amount of seeds to calculate K values or the rate of reaction.

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Co) Fig. 10 The effect on the rate of precipitation and induction time due to variation in (a) seed concentration (b) 3,4 DHBA concentration

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TABLE 2 (a) K values for different seed concentrations (calculated from Figure 9(a)), (b) K values for seeded liquors with different 3,4 DEIBA concentrations (calculated from Figure 9(b)), (c) k o values for different seed concentrations as specified in Figure 9 (a).

Seed 0 30 60 90

K(Induction) 1/hr 0.402 0.799 1.120

K (Growth) 1/hr 0.199 0.182 0.354 0.520

K(lnduction) 1/hr 1.130 0.640 0.520 1.102

K (Growth) 1/hr 0.513 0.469 0.374 0.920

0.038

(a) g/1 Organic 0 0.5 1.0 2.0

(b) Seed 30 60 90 Average

ko(Induction) 1/hr.m2 4.47x108 4.44x10 s 4.35x10 s 4.42x10 s

ko (Growth) 1/hr.m2 721 757 737 738

(c) The effect of 3,4 DHBA concentration on the rate of precipitation is shown in Figure 10 (b) and the K values are calculated and presented in Table 2(b). In the presence of 3,4 DHBA, the Arrhenius plot still applies (Figure 9 (a) and (b)), implying that the whole system can be characterised provided that the induction time is known. The effect of other organic impurities may, therefore, be characterised in the same manner. Once enough data is collected for all the important organic species in a particular process then the system can be simulated or predicted with computation techniques.

CONCLUSION This study has shown that the various stages of the precipitation of alumina trihydrate in the absence and presence of 3,4 DHBA can be characterised by second order rate equation:

d(AIC), =ko e-~jRr S [AICt-AICj 2 dt or

a(AIC), •K S [AIC,-AIC, q]2 dt

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The whole precipitation system can be simulated or predicted later provided the induction time is characterised, which depends very much on the liquor chemistry. 3,4 DHBA was found to have a significant impact on product yield, the rate of nucleation, the rate of crystallisation, the rate of crystal growth, agglomeration of nuclei and crystals, induction time and the activation energies of induction and crystal growth.

APPENDIX

Second order equations used

-d(AIC)t=ko e -EdRTS [A/Ct-A/Ceq]2 dt

(1)

or

d(AIC)t=K S [A[Ct-AICeq]2 dt

(2)

where

S= A/Ceq =

alumina concentration as A1203 g/l soda concentration as Na2CO3 g/l time, hr :second order rate constant, hr-1.m-2 surface of the seed, m2 equilibrium A/C ratio

K=

koe -e,,/RrS

h

C= t=

Upon integration with respect to t: 1

1

[A/C,-AIC.q]

[A/co-x/CJ

(3)

REFERENCES °

2. 3. 4.

5. 6.

The, K., Recent advances in organics--oxalate control in, and oxalate disposal arising from the Bayer process, Proceedings, Alumina Quality Workshop, Gladstone, Australia (1988). Armstrong, L., Bound soda incorporation during hydrate precipitation, Proceeding of the Third International Alumina Quality Workshop, Hunter Valley NSW, Australia, 282-292 (1993) Alamdari, A., Raper, J. & Wainwright, M., Poisoning of the precipitation of alumina trihydrate by manni~Iol,Light Metals 1993, 143-149 (1993). Calalo, R. & Tran, T., Effects of sodium oxalate on the precipitation of alumina trihydrate from synthetic sodium aluminate liquors, Light Metals 1992, 25-133 (1992). Misra, C. & White, E.T., Kinetics of crystallisation of aluminum trihydrate from seeded caustic aluminate solutions, Chemical Engineering Progress Series, 67(107), 53-65 (1971). Brown, N., A quantitative study of new crystal formation in seeded caustic aluminate solution, J. Crystal Growth, 29, 309-315 (1975).

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Aldcroft, D., Bye, G.C., Hughes, C.A., Crystallisationprocessesin aluminium hydroxide gels,

J.AppI.Chem,, 19, 167-172 (1969). 8. 9. 10. 11. 12.

Greene, R.S.B., Murphy, P.J.,Posner, A.M. & Quirk, J.P.,A preparativetechniquefor electron microscopic examination of colloidalparticles,Clays and Clays Minerals, 22, 185-188 (1974). Schocn, R. & Robcrson, C.E., Structuresof aluminium hydroxidc and geochemical implications, The American Mineralogist,55, 42-77 (1970). Audct, D.R. & Larocquc, J.E., Dcvclopmcnt of a modcl for predictionof productivityof alumina hydrate precipitation,Light Metals 1989, 21-26 (1989). Misra, C., Solubilityof aluminium trihydroxidc(hydrargiUitc)in sodium hydroxide solutions, Chemistry and Industry,May, 619-623 (1970). Watts, H.L. & Utlcy, D.W., Sodium gluconatcas a complcxing agent in the volumetric analysis of aluminium compounds, AnalyticalChemistry, 28(11), 1731-1735 (1956).