PRECIPITATION AND CALCINATION OF MONOHYDRATE ALUMINA FROM THE BAYER PROCESS LIQUORS

Transcription

1 PRECIPITATION AND CALCINATION OF MONOHYDRATE ALMINA FROM THE BAYER PROCESS LIQORS I. Paspaliaris*, D. Panias, A. Amanatidis, Jacques Mordini, D. Werner, G. Panou, D. Ballas *Author for correspondence. Lab. of Metallurgy, National Technical niversity of Athens, GR Zografos, P.O. Box 6456, Greece ABSTRACT The Bayer process is universally applied for the production of alumina from bauxites. The process has three main steps: Digestion, where bauxite is digested with caustic soda at elevated temperature and pressure to produce aluminate liquor; precipitation, where the aluminate liquor is hydrolysed at 55-6 C to produce crystalline alumina trihydrate, (gibbsite, Ah3.3H2) and calcination, when gibbsite is calcined at 9-12 C to produce anhydrous alumina (Ah3). The present project aims at the development of a highly innovative variation of the Bayer process whereby, at the precipitation step, alumina monohydrate (boehmite, AI23.H2) rather than trihydrate will be precipitated at atmospheric conditions and subsequently calcined to produce anhydrous alumina. Boehmite can be produced economically and in substantial quantities under conditions and with equipment similar to these used for gibbsite precipitation in the current practice of the Bayer process, resulting in significant energy savings during the calcination stage. The amount of energy saving is estimated to be 1.8 GJlt Ah3. This amount represents 6% reduction over the current calcination practice or 15% total energy saving in the Bayer process. 1. INTRODCTION Bayer process is used almost exclusively for the extraction of alumina from bauxites. Approximately 9% of anhydrous alumina produced by the Bayer process is used for the production of primary aluminium[l] (metallurgical grade alumina). The remainder (non-metallurgical grade alumina) is used for the production of refractories, abrasives, ceramics, cement, whiteware, aluminium chemicals, flame retardants, detergent zeolites, adsorbents and fillers. The essential steps of the Bayer process are: I. Digestion of bauxite by using sodium hydroxide at elevated temperatures (22-26 C) in autoclaves. 2. Separation of insoluble impurities (red mud) from the pregnant solution. 3. Precipitation of aluminum hydroxide (gibbsite, AI23.3H2) from the pregnant liquor. Precipitation is effected by cooling the solution to 58 C, thus causing supersaturation, and by introducing small gibbsite particles as seeds. The crystalline gibbsite precipitate is separated into a fine and a coarse fraction; the fine fraction, representing approximately 3% by weight of the coarse, is recycled to the precipitators as a seed to promote crystal-

2 EROTHEN 2 -Lisbon Jan lization; the coarse fraction represents the precipitation product; it is separated from the liquor by filtration. 4. Calcination of gibbsite at about 9-12 C to produce smelter-grade alumina. It is estimated that calcination of gibbsite consumes 3 GJlt of alumina produced. The production of boehmite has been investigated in the past, but not as a precipitation product from the sodium aluminate liquors. The following methods have appeared in the literature for boehmite production: Hydrothermal treatment of gibbsite Hydrothermal conversion of aluminum salts Byproduct of alcohol production Boehmite produced by the two latter methods is gelatinous and contains significantly greater quantity of water than the stoichiometric. None of the above processes is applicable in this case. The idea of producing boehmite rather than gibbsite as the precipitation product in the Bayer process has never been mentioned in the literature, except in a paper[2] of researchers of NTA and in the work performed in the framework of a previous project, supported by the European Commission under the JOLE III program (JOE3CT953). The work performed under the previous project has demonstrated that boehmite can be precipitated at atmospheric conditions (temperatures lower than 1 C). This comprises a highly innovative variation of the Bayer process, since for first time boehmite can be produced under the conditions and with equipment similar to that used for gibbsite precipitation. Therefore, there is no need to use autoclaves in the precipitation stage and the proposed process can be easily applied to the existing alumina plants without major and expensive modifications. Based on these results, rights relevant to a Greek patent No have been transferred to NTA, LB and ELVA. Also, the same partners have made an application (No PCT/GR 98119) for an international patent. As the proposed process is entirely new there is no data concerning boehmite precipitation in the literature except those presented in the reports of the partners of the previous project to the E and which have remained unpublished so far. The most important technological characteristics of the proposed process as identified by the previous research of NT A and LB are:. Boehmite can be precipitated from supersaturated sodium aluminate solutions in the presence of boehmite seed at elevated temperatures, higher than the boiling point of aluminate solution, and also at atmospheric conditions, temperatures lower than 1 C.. Boehmite can be precipitated in two ways; (a) isothermally at temperatures higher or equal to 9 C (b) non isothermally with the "flash cooling" procedure at temperatures lower to 9 C. The yield of boehmite precipitation is 35-4%. This yield is very close to this achieved in the current gibbsite precipitation practice. In order to scale-up the process additional research should be carried out to optimize the precipitation conditions particularly the grain size of the precipitate and the efficiency of the precipitation. It has to be emphasized that this project deals with an entirely innovative process which has been conceived, studied and developed and hopefully will be industrially applied by research institutes and companies working in the European nion. The project, joining the resources of one industrial company world leader in the alumina production (Aluminium Pechiney), one company intending to build an alumina plant (ELVA), one leading engineering company (Lurgi) and two universities, will create a new route for

3 542 EROTHEN 2 - Lisbon Jan. 2 alumina production, which will help the European alumina industry to reduce its energy cost and therefore, become more competitive. 2. OBJECTIVES 2.1 Industrial objective The industrial objective of the project is to develop a highly innovative variation of the Bayer process whereby, at the precipitation step, alumina monohydrate (boehmite, Ah3.H2) rather than trihydrate will be precipitated at atmospheric conditions and subsequently calcined to produce anhydrous alumina. The process is a technological breakthrough since for the first time with this variation of the precipitation step, boehmite can be produced economically and in substantial quantities under conditions and with equipment similar to these used for gibbsite precipitation in the current practice of the Bayer process. The expected achievement is an energy reduction of 1.8 GJ/t Ah3 in the calcination stage (6% reduction over the current industrial practice and more than 15% total energy savings in the Bayer process) for the following reasons: a) There is a significant difference in the enthalpy, about l.l GJ/t Ah3, of the respective dehydration reactions of gibbsite and boehmite. AI2).3H2 ~ AI2) +3H2 All = 187 kj/mole Ah3 AI2).H2 ~ AI2) + H2 All = 72 kj/mole Ah3 b) As boehmite contains one mole of water against three for gibbsite, the amount of the material to be calcined will be reduced by 36 kg/t of alumina produced. This will result in additional energy savings of the order of.7 GJ/t Ah3 as well as in increase in the calciner productivity. Energy costs constitute about 35% of the alumina price. Since most of the alumina plants use either oil or natural gas to meet their energy demands, they are subjected to the rapidly increasing prices of the energy sources. Therefore, the reduction of the energy costs in the coming years is the most important task for the alumina industry. The energy saving in calcination of boehmite is estimated to be 34.8 kg/t of alumina for fuel oil and 7.2 kwh/t of alumina for electricity. This corresponds to 4-5% less fuel or electricity consumption compared with gibbsite calcination for the same alumina production. For a typical Bayer process plant with a capacity of 1.. tpa alumina, this represents energy savings of 35 t of fuel oil per year. The development of a cost-effective process for the production of boehmite instead of gibbsite in the current Bayer process, is of substantial strategic importance to the European nion because it enables:. The development of innovative technologyfor the production of aluminium, a metal of great importance to industrial development.. The European alumina producers to maintain the position they had among the world alumina producers before the productivity reduction of the last decade.. Europe to compete with the big alumina producers outside the E (South and Central America, Oceania), by producing alumina at reduced cost and promoting innovative and cost-effective technology.. The development of an environmentally friendly process, since the consumption of the fuel oil required for alumina calcination will be significantly reduced.

4 EROTHEN2 - Lisbon Jan Size and potential of the markets Approximately 9% of anhydrous alumina produced by the Bayer process is used for the production of primary aluminium[l] by the Hall-Heroult process (metallurgical grade alumina). The remainder (non-metallurgical grade alumina) is used for the production of refractories, abrasives, ceramics, cement, whiteware, aluminium chemicals, flame retardants, detergent zeolites, adsorbents and fillers. Therefore, the demand for alumina depends mainly on the trends of the aluminium sector. The world alumina and primary aluminium production is estimated to be approximately. 45 and 2 million tons, respectively [1.3].12.5% of the alumina production and 18% of the aluminium production are produced in E (Table 2.1). The market corresponding to this world alumina and aluminium production is 3 billion Euro; E accounts for approximately 1 billion Euro (Table 2.2). p to 2, the demand for metallurgical grade alumina is expected to rise at a rate of 3% per year, while this for non-metallurgical alumina is expected to increase at a rate higher than 4% per year. This increase will mainly concern alumina-graphite refractories for continuous casting processes, low-cement monolithics, alumina-rich monolithic ladle linings, fused alumina abrasives and micronised reactive alumina for ceramics. Finally, the production of alumina for detergent zeolites is estimated to increase by 12% per year. These estimates have already stimulated alumina industry to increase their production capacity. It is expected that this increase in production capacity will be focused in Australia, South America and Asia, where the alumina production cost is low. Table 2.1: World production of alumina and primary aluminium (1993) Area Alumina(tons) Aluminium(tons) E Oceania N. America C. America 8 19 S.America - 2 Asia 4 37 I 6 Rest of Europe 6 26 Africa 68 6 Table 2.2: Estimated Global and E Markets (GEuro) Sector Alumina Aluminium E Rest of World Global The potential energy savings from the application of the proposed process are 1.8 GJ/t Ah3, which are equivalent to 43 kg fuel oivt Ah3 or 11 Euro/t Ah3. As the E annual alumina production is approximately 5.6 Mt. alumina, the potential energy savings are estimated to be 61.5 MEuro annually.

5 544 EROTHEN 2 - Lisbon Jan RESL TS TO-DATE 3.1 Optimization of the efficiency of the boehmite precipitation process The most important difficulty the industrial application of the boehmite precipitation process under atmospheric conditions is the strong deceleration of the precipitation rate observed after the first 12 hours of precipitation (figure 3.8). This deceleration significantly reduces the precipitation efficiency because the aluminate liquor reaches an apparent equilibrium stage rather than the real equilibrium one. The alumina concentration at the apparent equilibrium stage is 1.8 times higher than the one at the real equilibrium stage, so the achieved process efficiency is only the 65% of the theoretical one. Extensive experimental work has been carried out in order to develop a suitable technology to overcome this problem. Three techniques have been tested. Two of them are based on the establishment of the appropriate physicochemical properties on the surface of the boehmite particles and the third is using ultrasounds to activate the particle surface Control of the surface properties of boehmite particles Fundamentals of the "Seed Poisoning" Theory Seed poisoning is a kind of chemical deactivation of the surface of the boehmite seed particles taking place during the precipitation of boehmite. According to this theory, the precipitation mechanism changes gradually from "surface precipitation" to "bulk precipitation". Initially, the predominating mechanism is "surface precipitation", which involves the binding of the aluminate ions on the surface of the boehmite seed followed from the formation of a solid precipitate on the surface and the agglomeration of the particles. As a result the mean particle size of the solid particles increases continuously at a high rate. Gradually, the surface of the boehmite particles is deactivated and the mechanism from "surface precipitation" changes to "bulk precipitation", which then becomes the predominating mechanism. Bulk precipitation mechanism involves homogeneous precipitation of boehmite particles by primary nucleation. The primary nucleation pathway is generally very slow and produces new extra fine boehmite particles. This change in the precipitation mechanism results in the substantial decrease of the precipitation rate and also in the decrease of the grain growth and particles agglomeration rate. As a consequence, new particles of submicron size are produced and the mean particle diameter decreases, as shown in figure 3.1. In order to better understand the reason for the poisoning of the boehmite seeds during precipitation a theoretical analysis based on the acid-base chemistry of the surface of boehmite particles was performed.

6 EROTHEN 2 - Lisbon Jan E 16 ;::I..: 14 C1.) Q) 12 E tis 1 :.a s:: 8 tis C1.) 6 E C1.) E 4 ;::I 2 " > Precipitation time, hours Figure 3.1. The volume mean diameter of the particles of the boehmite precipitated at 9 C from a supersaturated sodium aluminate solution containing 132g/1 Ah3, 12g/1 Na2 with seed concentration 23g/1 as a function of the precipitation time Physicochemical properties of the surface of boehmite particles in aqueous solutions Acid-Base titrations of suspensions of boehmite particles in aqueous solutions have been performed in order to study the physicochemical properties of the boehmite particle-solution interface. The results are shown in figure 3.2.,16 Me :c,14 u ~,12 ~ l!,1 8,8 <E:! 3,6 CI) ~,4 'u g,,2 CI), ~ The points are experimental results ~ The solid line represents the pre- p<;: Figure 3.2. Acid-Base titration data of suspensions of boehmite particles with concentration IOg/1in aqueous 1M sodium nitrate solutions at 2SoC. In figure 3.2, the surface charge of suspended boehmite particles with concentration IOg/1in 1M sodium nitrate solution at 25 C is shown as a function of ph. The experimental data concern the ph range 4-9 because it is not feasible to obtain experimental data in more acidic or basic solutions. A theoretical model, based on acid-base chemistry of the boehmite particlesolution interface has been developed to predict the physicochemical properties of this interface. The parameters used for the model development are given in table 3.1.

7 546 EROTHEN 2 - Lisbon Jan I 1" 1-" 3.73x1 The theoretical values of the specific surface charge as they are calculated from the model and the corresponding experimental data are presented in figure 3.2 The resulting curves are in close agreement. The physicochemical properties of the boehmite particle-solution interface in high acidic (ph<4) and basic (ph>9) solutions have been estimated by extrapolation of model predictions in the above ph ranges. The results are shown in figures 3.3, 3.4 and 3.5.,15 >,1 :l'! E,5 ' Q ,5... =' en -.1 -,15 Figure 3.3. Surface potential of suspended boehmite particles with concentration IOg/1in 1M sodium nitrate solution at 25 C versus ph o o p<; 8 -&-%>A1H ~%>A1H2+ ~o/o>a1- Figure 3.4. Specific Surface charge of suspended boehmite particles with concentration IOg/1 in IM sodium nitrate solution at 25 C versus ph

8 EROTHEN 2 - Lisbon Jan ,25 NS...,2 e,15 ~,1... l!,5 8 <.f:!... -,5 ~ t/) u -,1 t+:: '(3 -,15 (1) J; -,2 -,25 2 Figure 3.5. Surface Speciation Diagram of suspended boehmite particles with concentration IOg/1in 1M sodium nitrate solution at 25 C. The surface potential and charge take high negative values in alkaline solutions with ph higher than 9. This is attributed to the substantial increase of the concentration of negatively charged surface species and to the substantial decrease of the concentrations of positively and neutral charged surface species above ph 9 as it is shown in figure 3.5. In high alkaline solutions, the required electrical work necessary to attach the negatively charged aluminate ions on the surface of boehmite particles is high, due to the high negative surface potential of the boehmite particles. Therefore, the activation energy of the boehmite precipitation process is rather high, Kcal/mol, and this explains the fact that the precipitation of boehmite is favored at temperatures higher than 9 C. 9:..,9,8, ~95C -J!r-82oC,-e-68oC,-- ~ - _ ;- - -.,.- - -,6 -t -, Duration, hours Figure 3.6. Boehmite precipitation from supersaturated sodium aluminate solutions with 65 g/l Na2 and 5g/1 seed concentration as a function of time and temperature. Reduction of the boehmite particle surface charge by decreasing the Na2 concentration in the initial supersaturated solution makes the precipitation of boehmite possible at temperatures lower than 9 C, as shown in figure 3.6. Pure boehmite can be precipitated under atmospheric conditions at temperatures as low as 68 C, provided that the initial concentration of sodium hydroxide is reduced to 65g/1 Na2. Such conditions cannot be applied in the current

9 548 EROTHEN 2 - Lisbon Jan. 2 industrial practice, because the concentration of sodium hydroxide used to precipitate gibbsite by the common Bayer process is about three times higher. The most important result drawn from the above analysis is that sodium hydroxide concentration significantly retards the rate of boehmite precipitation. Hydroxide ions are continuously liberated during boehmite precipitation resulting in the additional increase of the particle surface charge and also in the activation of additional kinetic inhibitions. There are two possible ways to reduce the negative particle surface charge and therefore to eliminate the effect of the kinetic inhibitions on the boehmite precipitation. a. Neutralization of the excess of hydroxide ions generated in the solution during boehmite precipitation. b. Increase of the concentration of boehmite particles suspended in the solution as it is shown in figure 3.7. "'8 --.,J:) Concentration of Boehmite Particles, g/i ~ ~.J:: <S S -12 ) u -14 t.;:::.~ ) -18 Figure 3.7. Effect of concentration of boehmite particles, suspended in 1M NaN3 solution with ph 11, on specific surface charge at 25 C. The results of boehmite precipitation experiments under constant concentration of the free sodium hydroxide are shown in figure 3.8. The constant free caustic precipitation curve consists of eight batch precipitation experiments. At the end of each experiment, the amount of sodium hydroxide released during the precipitation was neutralized by carbon dioxide and the resulting solution was used for the next precipitation experiment. Following this procedure, the concentration of free caustic during precipitation was kept practically constant at 38.55g/1 NazO. It is clearly shown in figure 3.8 that seed poisoning, the inhibition responsible for the lowefficiency of the normal boehmite precipitation process is not appearing during the first 24 hours of precipitation. The results show that the reason for the reduced precipitation rate is the caustic released during the boehmite precipitation process. The boehmite precipitation under constant concentration of sodium hydroxide in solution proceeds with high rate and after 24 hours the amount of precipitated boehmite is 61g/1 Alz3. This amount is greater than the

10 EROTHEN 2 -Lisbon Jan g/1 Al23 which is the amount of precipitated boehmite with the normal precipitation procedure at 9 C after 96 hours. Although this modification of the usual precipitation procedure results in satisfactory process efficiency, it cannot be applied in industry. The continuous neutralization of the released sodium hydroxide during precipitation consumes the large amount of the sodium hydroxide in the solution and makes the final solution improper for recycling in the digestion reactor due to its high content in sodium carbonate. <::: d S: "=i - CI) 1.5 s:::::.s: ::: 6 CI) u s::::: s::::: 's =s 4: Normal Precipitation Precipitation under constant free caustic _ I Boehmite solubility at 12g/l Na2 2 o ō Time, hours Figure 3.8. Comparison of the normal boehmite precipitation procedure with the boehmite precipitation procedure under constant concentration of sodium hydroxide in solution. The precipitation in all cases were performed at 9 C from a supersaturated sodium aluminate solution with 132g/1 Ah3, 12g/1 Na2 and an initial seed concentration 23g/1. The results of boehmite precipitation experiments with high seed boehmite concentration in the solution are presented in figure 3.9. The rate of boehmite precipitation at 9 C increases substantially as the concentration of the boehmite in the suspension increases from 23g/1 to 12g/1. The rate of the boehmite precipitation at the concentration of 12g/1 boehmite in the suspension is about the same with the rate of precipitation under constant free sodium hydroxide concentration. After the first 24 hours of precipitation a new kinetic inhibition appears, hindering the rate of boehmite precipitation. The mean boehmite precipitation rate during the first day is 2.35g/h, while during the next two days the precipitation rate is substantially lower.3g/h. The curves of the boehmite precipitation at 9 and 12g/l initial boehmite concentration in the solution are almost parallel after the first 24 hours. This means that the rate of boehmite precipitation is quite constant and does not depende on the boehmite concentration. This observation indicates that the surface of boehmite particles has been deactivated. The precipitation mechanism after the first day has changed from "surface" to "bulk" precipitation and the rate of precipitation has been rapidly decreased.

11 55 EROTHEN 2 - Lisbon Jan e 23gl1 d' 12,!2 6gl1 ' gl1 1,E 8 12gl1 c::.!2 8 (;j E 6... Q) (,) c:: 4 «S c:: 's 2 I Boehmite Solubility at 12gll Na2 ;3 < Time, hours Figure 3.9. Effect of the concentration of the boehmite in suspension, gll, on the rate of the boehmite precipitation from a supersaturated sodium aluminate solution with 132g/1Ah3 and 12g/1Na2 at 9 C. A combined experiment, high seed (12g/l) boehmite precipitation and neutralization of the 5% of the sodium hydroxide released during the first 24 hours of precipitation, has been perfonned in order to verify experimentally the "seed poisoning" effect after the first day of precipitation. The results are shown in figure 3.1. The decrease of the free sodium hydroxide concentration in the solution after the first 24 hours of boehmite precipitation is beneficial for the evolution of precipitation. The mean rate of the boehmite precipitation after the first day increases from.3g/h to.5g/h Ah3 and the process efficiency after 72 hours of precipitation reaches the level of 85g/l Ah3. --

12 EROTHEN 2 - Lisbon Jan " b.~ 12 :; ~ 1.S e. 8 ij 6 o c o 4 '" c '6 2 ::! < Boehmite Solubility at 12g/1 Na:O, 9.C ~ 12g/ g/l,Neutralisation o o Time, hours Figure 3.1. Effect of the neutralization of the 5% of the sodium hydroxide that has been released during the first 24 hours of boehmite precipitation from a supersaturated sodium aluminate solution with 132g/1 Ah3 and 12g/1 Na2 at 9 C on the rate of precipitation. All the experiments performed show that the control of the concentration of free sodium hydroxide in the solution is the key factor to increase the precipitation rate and the precipitation efficiency. The high seed precipitation process can be industrially applied. The maximum boehmite precipitation efficiency achieved, without neutralization of sodium hydroxide during precipitation, is 72g/1 Ah3. In the case of the neutralization of the 5% of the released sodium hydroxide during the first day of the boehmite precipitation, the efficiency has increased to g/1Ah ltrasonic Precipitation ltrasounds have been also employed to accelerate of the rate of boehmite. The results are shown in figure The normal precipitation and the high seed precipitation curves are shown in the same figure for comparison with the ultrasonic precipitation curve. The sonication of the suspension was carried out in periods with 5 minutes duration. A silence period with also 5 minutes duration intervened between two successive sonication periods. The sonication period was not continuous. It was carried out with the.5s/s-pulsed mode at 45W. This means that I-minute sonication is equivalent with.5-minute continuous sonication at 45W and.5 minute silence.

13 552 EROTHEN 2 - Lisbon Jan. 2 'Bb 14.~ 12 '5 ~ ].::.2 8 E ~ 6 (,) c 8 4 <IS C ' 2 ::s :;;: o o Boehmite Solubility at 12g/l Na gl1 -e- 12gl] ~Sonication ] Time, hours Figure Effect of the application of ultrasounds on the boehmite precipitation at 9 C from a solution with 132g/1 Ah3, 12g/1 Na2 and 5g/1 initial seed concentration. It is clearly observed that the boehmite precipitation process is accelerated with the application of ultrasounds. The effects of ultrasounds and the high seed concentration on the rate of boehmite precipitation are comparable. The mechanism of acceleration of the rate of boehmite precipitation with the application of ultra sounds is non-well understood. Possibly, the application of ultra sounds accelerates the diffusion of aluminate ions from the solution to the surface of boehmite particles and therefore increases the surface concentration of adsorbed aluminate ions. This technique has two problems. The first one is related to the cost of the application of ultra sounds in large precipitators and the second is related to the effect of ultrasounds on the size of precipitated solids. ntil now, there are not enough data to make the assessment of this techique. 3.2 Optimization of the precipitated boehmite quality The specific targets of this project are on one hand the production of monohydrate alumina with the mineralogical form of boehmite with precipitation from superasaturated sodium aluminate solutions under atmospheric conditions and on the other hand the production of smelting grade alumina with calcination of produced boehmite. The feed of calciners for production of smelting grade alumina, normally the precipitated boehmite, must meet some strict specifications. The most important specification concerns the particle size distribution of boehmite. Smelting grade alumina must be sandy. Therefore, it is necessary the production of sandy type boehmite. This means that the 75% of produced boehmite must be coarser than 45um. The particle size of precipitated solids increases under the action of two different mechanisms, net crystal growth and agglomeration of particles.

14 EROTHEN 2 - Lisbon Jan, In figure 3.12 is shown the mean particle diameter of precipitated solids as a function of precipitation cycle when the precipitated solids are recycled continuously in the precipitator. Each precipitation cycle was performed under the optimum experimental conditions (132g11 Ab3, 12g/1Na2, 12g/1 seed concentration, 9 C and 5min-1 agitation rate) and had a duration 24 hours. The cycle zero represents the initial boehmite seed with a mean particle diameter 7.46um. The cycle one represents the produced solids after the first utilization of this seed. The cycles two, three, four and five represent the produced solids after the first, second third and fourth recycling of the solids respectively. e \8 =' B 14 CI,) e 12 «S is \ -TI 8 ' 6 «S p., 4 r:: a'3 2 ~ o T t j---- -i Precipitation Cycle Figure Mean particle diameter of produced boehmite after the end of each precipitation cycle. The experimental conditions are 12gl1 Na2, 132g11Ah3, agitation rate 5 min-i, 12gll initial seed concentration, 9 C and 24 hours. It is clearly observed in figure 3.12 that the mean particle diameter of produced solids increases almost linearly within each precipitation cycle. The increase of the mean particle diameter is about 1.74um per precipitation cycle. As the duration of each precipitation cycle is 24 hours, the rate of increase of the mean particle diameter in each cycle is.725um/h. The increase of the mean particle size of precipitated solids is attributed to the agglomeration mechanism. Although the mean particle size of precipitated solids at the end of fifth precipitation cycle is about twice the size of the initial seed, the produced boehmite is characterised as floury type. As the target of the project is the production of sandy type boehmite, a new experimental precipitation series was performed utilizing coarser boehmite seed and recycling continuously the produced solids. The results are shown in figure The results obtained from these experiments are quite impressive and are shown in figure Although the mean particle diameter of the initial seed is relatively large (32.67um), a high degree of agglomeration between coarse particles has been achieved. The mean particle diameter increases continuously and after two precipitation cycles the produced solids have a mean particle diameter 81.32um. The produced boehmite is coarser than the specifications for sandy product as it is seen in figure The 81% of the particles of produced boehmite has mean diameter larger than 48.27um. The agglomeration is favored not only between fine particles but and between coarse particles with size in the range 3-5um as it is seen in figure 3.15.

15 554 EROTHEN 2 - Lisbon Jan, 2 E! ;:I u..: 9 8 E! 7.,s au 6 '.,s u 4 ::E 3 "_ --+ I 1- I- I Precipitation Cycle Figure Mean particle diameter of produced boehmite after the end of each precipitation cycle. The experimental conditions are 12g/1Na2, 132g/1 Ah3, agitation rate 5 min-t, 12g/1 initial seed concentration, 9 C and 24 hours. The mean diameter of initial seed is 32.67um u N 6 "u s:: 5 :> 4 '$ r-:- I Produced I Boehmite I -Limit for I Sandy Product I Particle size, urn Figure Particle size distribution of the produced solids from the third precipitation cycle versus typical particle size distribution for a sandy product.

16 EROTHEN 2 - Lisbon Jan en <1> 8 ' 7 CtI p.. <+- 6 <1> bl) CtI... s::: <1>... <1> p.. '2f?- EISeed I C Cycle I mcycle 2 1.!ICycle3 M... M en... M V) N N V) v if ;' - '" M v V... V) V) en V V V en en en en en V '" V V V V V en - M... V... M V) N M..,: ;' N M... - V) Size Classes, umn Figure Classification of the boehmite particles in size classes for the initial seed and the produced solids of the first, second and third precipitation cycles. As a conclusion, high degree of agglomeration between boehmite particles is achieved under a wide variety of experimental conditions. The agglomeration process takes place not only between fine boehmite particles but also between coarse particles. Moreover, the production of sandy type boehmite is a relatively easy target because the optimum conditions for agglomeration between particles and for boehmite precipitation with high rate and efficiency are the same. 3.3 Calcination of boehmite The boehmite produced during the precipitation stage was calcined to produce anhydrous alumina. The effect of the most significant parameters, such as temperature, time and calcination procedure, on the calcination process are investigated. The calcination tests were carried out in a lab scale fluidized bed calciner consisting of 5 mm glass tube with a porous fritte at the bottom. The reactor tube was heated up in the furnace to 8 C / 75 C respectively ~of material was filled into the reactor. After that fluidization gas was increased to.95 m Ih according to.53 m/s superficial velocity. The temperature was held constant. After 6 / 3 minutes the heating was switched off and the reactor was taken out of the furnace and cooled down with ambient air. The product was weight and analyzed for L.O.I., particle size distribution, specific surface and XRD. The table 3.2 gives an overview of the results.

17 556 EROTHEN 2 - Lisbon Jan. 2 Table 3.2: Calcination test results Particle Particle Specific Surface Specific Surface L.O.I. Test <21m <45i m BET Fisher Material (%) (%) (m2/g) (m2/g) (%) Nr. - Original I Calc. At 8 C, 6 min Calc. At 74 C, 3 min n.a Calc. At 8 C, 3 min n.a Metal grade alumina Max 2 Max <1. The tests showed that the specific sample of pure boehmite can be calcined at 8 C without large destruction of particles. The product quality however did not meet the specifications for smelter grade alumina because it was too fine and the specific surface was much too low. 4 CONCLSIONS The most important conclusions drawn until now are the following: ~ Boehmite can be precipitated at a reasonable rate and with satisfactory efficiency, using the high seed precipitation technique. ~ The application of ultrasounds during the precipitation process has a beneficial effect on the process efficiency. ~ The production of "sandy" type boehmite seems to be a realistic target. The optimum conditions at which high precipitation rate, high process efficiency and particle agglomeration are taking place are almost the same. ~ Preliminary calcination tests have shown that the material behaves well during calcination but the resulting specific area does not meet the required standards for the production of smelter grade alumina. 5 LIST OF RELEVANT REFERENCES I. D. Filippou, I. Paspaliaris: "Precipitation of boehmite from alkaline solutions of aluminium (Bayer solutions)", Mining and Metallurgical Annals, No , 1989, pp H.E. Barner, R.V. Scheuerman: "Handbook of themiochemical data for compounds and aqueous species", J. Wiley & Sons, New York, "The economics of aluminium", Roskill Information Services Ltd., Sixth edition, 1995.