Boehmite Process A New Approach in Alumina Production

Size: px

Start display at page:

Download "Boehmite Process A New Approach in Alumina Production"

Randall Sanders
6 years ago
Views:

1 Boehmite Process A New Approach in Alumina Production Dimitrios Panias, Ioannis Paspaliaris Alumina is produced almost exclusively for more than 1 years by the well-known Bayer process. One of the characteristics of the Bayer process is the production of pure crystalline gibbsite, alumina trihydrate, during the precipitation stage as a precursor for the alumina production. The Boehmite process is a novel variation of the Bayer process. The novelty of this process is related to the production of pure crystalline Boehmite, alumina monohydrate, at the precipitation stage as an intermediate product in the production of alumina. The main characteristic of the new process is that Boehmite is efficiently produced by precipitation from Bayer liquors under atmospheric conditions. The new process can be used for the production of smelter grade alumina, as well as, for the production of several qualities of specialty aluminas. This paper gives a general overview of the Boehmite process. It emphasizes on the phenomena affecting, on the one hand, the efficiency of the Boehmite precipitation process and, on the other hand, the product quality. Finally, the two alternatives, the current Bayer process and the Boehmite process, will be compared and the advantages arising from the application of the new process will be discussed. Keywords: Bayer process Gibbsite Boehmite Precipitation Böhmit-Prozeß: Ein neuer Prozeß für die Aluminiumoxidproduktion Über hundert Jahren wurde Aluminiumoxid durch den wohlbekannten Bayer-Prozeß produziert. Der Bayer-Prozeß führt zu einem reinen Produkt aus kristallinem Gibbsit, Aluminiumtrihydroxid. Die Herstellung von Böhmit, Aluminiummonohydroxid, stellt eine Variante des Bayer- Prozesses dar. Böhmit fällt als Zwischenprodukt aus der Bayer-Lauge unter atmosphärischen Bedingungen aus. Dieser Prozeß ist geeignet zur Produktion von Aluminiumoxid für spezielle Verwendungen. Diese Arbeit beschreibt den Böhmit-Prozesses allgemein. Phänomene, die die Ausbeute und die Qualität von Böhmit beeinflussen, werden dargestellt. Am Ende werden beide Prozesse, d.h. der normale Bayer-Prozeß und der Böhmit-Prozeß, miteinander verglichen und die Vorteile des Böhmit-Prozesses diskutiert. Schlüsselwörter: Bayer-Prozeß Gibbsit Böhmit Ausfällung Le procédé Boehmite Une nouvelle méthode de la production d alumine El proceso de Boehmite Una aproximación nueva en la producción de aluminio Paper presented on the occasion of the International Congress Mining.Metallurgy@3. Millennium M 3, GDMB General Assembly, May 29 to June 1, 22, in Vienna The Boehmite process, a new innovative variation of the Bayer process that is under development in the Laboratory of Metallurgy of the National Technical University of Athens (NTUA), comprises of three main stages. Bauxite digestion with concentrated sodium hydroxide solution followed by boehmite precipitation from the supersaturated sodium aluminate solutions under atmospheric conditions and boehmite calcination to produce anhydrous alumina. The precipitation stage is very critical for the whole process as soon as pure crystalline boehmite has to be produced with high efficiency and standard quality at steady state under atmospheric conditions. 1 Precipitation of Boehmite from synthetic pure sodium aluminate solutions The kinetic analysis of boehmite precipitation [1] from supersaturated sodium aluminate solutions has shown that the activation energy for the precipitation of pure crystalline boehmite is kcal/mol, a value that is almost twice the activation energy value of gibbsite. This result indicates that gibbsite precipitation is kinetically more favorable than boehmite precipitation from the Bayer liquors, especially at temperatures lower than the boiling point of water. In contrast of this conclusion, previous works [2 to ERZMETALL 56 (23) Nr. 2 75

2 6] have shown that pure crystalline boehmite can be isothermally precipitated at temperatures as low as 9 C from supersaturated sodium aluminate solutions with composition similar of that of the Bayer process liquors, when precipitation takes place with the presence of boehmite seed in the liquor. A typical curve of boehmite precipitation at 9 C from synthetic liquor in the presence of 23 g/l initial boehmite seed concentration is shown in Figure 1. The straight line in this Figure represents the solubility of boehmite in a pure caustic solution with 12 g/l Na 2 O at 9 C calculated from a theoretical model [7] that predicts the solubility of boehmite in concentrated 2 to 4.5M sodium hydroxide solutions and in the temperature range 3 to 15 C. As it is observed in Figure 1, the precipitation rate continuously decreases until an apparent equilibrium stage is achieved at which the alumina concentration in the solution is about twice the value of boehmite solubility under the same conditions. This observation reveals a strong kinetic inhibition that significantly reduces the precipitation efficiency. The efficiency after one, two and four days precipitation is only 35, 4 and 48 g/l Al 2 respectively, while the theoretical one is 97 g/l Al 2. This kinetic inhibition has been attributed to a kind of chemical deactivation of the surface of the boehmite seed particles during the precipitation procedure. In aqueous solutions, the boehmite particles are covered with surface aluminol groups (>Al-OH o ) that exhibit ampholytic behaviour according to the following surface reactions (Equations 1, 2). >AlOH o + H + + = >AlOH 2 (1) >AlOH o = >AlO - + H + (2) where, >AlOH 2+, >AlOH o, and >AlO - represent positive, neutral and negative charged surface species. Alumina Concentration, g/l Fig. 1: Time, hours P recipitation Curve Boehmite solubility at 12g/L Na2O Boehmite precipitation curve at 9 C from a supersaturated sodium aluminate solution (132 g/l Al 2, 12 g/l Na 2 O, 5 min -1 and an initial seed concentration 23 g/l) Surface Species % Fig. 2: %>AlOH %>AlOH2+ %>AlO pη Speciation diagram on the surface of boehmite particles (boehmite concentration 1 g/l, background electrolyte 1M sodium nitrate solution, 25 C) The surface potential of the boehmite particles depends on the solution ph and takes negative values in alkaline solutions where an excess amount of negative charged surface species exist as is shown in the surface speciation diagram of Figure 2. As ph increases in the alkaline region, the surface potential takes more negative values and thus, the required electrical work for the attachment of a negative charged aluminate ion on the surface of boehmite particles increases rapidly. This phenomenon is responsible for the observed inhibition during the boehmite precipitation from aluminate liquors due to continuous increase of the free hydroxide ions concentration with a rate that equals the rate of boehmite precipitation according to the following reaction (Equation 3). This kinetic inhibition is eliminated when the negative surface potential on the boehmite particles is properly modified. This can be efficiently achieved with the high seed precipitation technique. - Al(OH) 4 (aq) = AlOOH (s) + OH - (aq) + H 2 O (3) 1.1 High seed precipitation process The high seed precipitation process makes use of the beneficial effect of the increased initial boehmite seed concentration on the surface potential. Indeed, an increase in the concentration of the boehmite particles suspended in an alkaline solution causes a substantial reduction on the specific surface charge of the boehmite particles as is shown in Figure 3. This is a beneficial effect for the process of boehmite precipitation because the repulsion forces between the surface of the boehmite particles and the aluminate ions in the solution are attenuated, making the precipitation process more efficient as is shown in Figure 4. The rate of the boehmite precipitation increases significantly as the initial boehmite seed concentration in the aluminate solution in- ERZMETALL 56 (23) Nr. 2

3 creases from 23 g/l (normal precipitation) to 12 g/l (high seed precipitation). As a result, the achieved efficiency after 24 h retention time is 6 g/l Al 2 and increases to 72 g/l Al 2 if the retention time increases to 72 h. In spite of the improvements in the precipitation efficiency, a new kinetic inhibition is appeared after the first 24 h of boehmite precipitation with the high seed technique. The mean boehmite precipitation rate during the first day is 2.35 g/h Al 2, while the rate during the next two days is.3 g/h Al 2 that is a substantially lower value. This new Specific Surface Charge, mcb/m 2 Fig. 3: Concentration of Boehmite Particles, g/l Effect of the concentration of boehmite particles suspended in 1M NaN solution with ph 11 on the specific surface charge at 25 C inhibition is attributed also to the chemical deactivation of the surface of the boehmite particles due to the excess amount of the free sodium hydroxide in the solution. The concentration of the free hydroxide ions in the solution after 24 h precipitation with 12 g/l initial seed concentration is twice the initial one. Nonetheless, the high seed precipitation process could be industrially applied achieving efficiencies comparable to those of the current Bayer process. 1.2 Physical Characteristics of Precipitated Solids The size of precipitated boehmite particles is related to the size of the initially added seed as is shown in Figure 5 and 6 where the data of two sets of experiments with continuous recycling of solids are given. Each cycle represents a precipitation experiment under high seed concentration using the produced solids of the previous cycle as seed material. The mean particle diameter of the produced solids in the case of fine initial seed (Figure 5) increases almost linearly within each precipitation cycle. The increase of the mean particle diameter is about 1.74 µm per precipitation cycle. In the case of coarse initial seed (Figure 6), Mean Particle Diameter, µm Fig. 5: Precipitation Cycle Mean diameter of the produced boehmite particles as a function of precipitation cycle (12 g/l Na 2 O, 132 g/l Al 2, 12 g/l seed concentration, 9 C, 24 h, 5 min -1, initial seed size 7.46 µm µm Fig. 4: Effect of the boehmite seed concentration on the rate of the boehmite precipitation (132 g/l Al 2 and 12 g/l Na 2 O, 5 min -1, 9 C) Fig. 6: Mean diameter of the produced boehmite particles as a function of precipitation cycle (12 g/l Na 2 O, 132 g/l Al 2, 12 g/l seed concentration, 9 C, 24 h, 5 min -1, initial seed size µm ERZMETALL 56 (23) Nr. 2 77

4 % Undersize Fig. 7: % Percentage of Particles Fig. 8: Particle size, um µm Particle size distribution of the produced solids at the end of the third precipitation cycle vs typical size distribution for a sandy product S<4,88 Seed Cycle 1 Cycle 2 Cycle 3 4,88<S<1,48 1,48<S<19,31 19,31<S<3,53 3,53<S<48,27 48,27<S<56,23 Size Classes, µm um Produced Boehmite Limit for Sandy Product 56,23<S<13,58 Classification of the boehmite particles in size classes 13,58<S the mean particle diameter increases continuously and at the end of the third precipitation cycle the produced solids have a mean diameter of µm and are coarser than the specifications for a sandy product as it is shown in Figure 7. The classification of the particles of produced solids in size classes is shown in Figure 8. The results show that the particles agglomeration mechanism is important in the case of the boehmite precipitation process under high initial boehmite seed concentration and also, the agglomeration is favored not only between fine particles but also between coarse particles with size 3 to 5 µm. 2 Boehmite precipitation from simulated industrial liquors with the high seed process Industrial Bayer liquors usually contain impurities. The most important of them are chlorides, carbonates, sulfates, silicates and organics. Precipitation experiments with simulated industrial liquors, having the typical composition given in Table 1, were performed in order to determine the influence of solution impurities on process efficiency. The results are shown in Table 2. Table 1: Composition of a typical synthetic aluminate liquor Constituent Concentration [g/l] Al Na 2 O (caustic) 155 Na 2 O (carbonate) Chlorides as chlorine ions (NaCl) 2.7 Sulfates as S (Na 2 SO 4 ).78 Organics as C org (CH 3 COONa) 9.3 Silicates as SiO 2 (Na 2 O 3SiO 2 ).93 Table 2: Effect of solution impurities on process efficiency; experimental conditions: 9 C, 5 min -1, 48 h, 12 g/l Composition of Efficiency Mean particle BET specific aluminate liquor [g/l Al 2 ] size [µm] surface area[m 2 /g] Pure liquor (no impurities) Synthetic liquor without organics Typical synthetic liquor (all impurities) Properties of initial boehmite seed The presence of impurities in the aluminate liquor was found to drastically affect the Boehmite process efficiency. This is attributed to the adsorption of impurities, especially organics and carbonates, on the active sites of the surface of boehmite particles that results to a substantial reduction of the available surface active sites for binding the aluminate ions. The mean particle size of the precipitated solids in all experiments has increased from 16,42 µm to about 22,3 µm, due to the particles agglomeration mechanism. BET specific surface area, has also increased from 1,42 m 2 /g to a mean value of 3.47 m 2 /g. This is attributed to the formation of a large number of nuclei from precipitation of boehmite on the surface active sites of seed particles, as it is seen in Figure 9, that create a porous grain morphology. Table 3 gives the results from a set of experiments with continuous recycling of precipitated solids that aims to investigate the stability of the achieved process efficiency, as well as the changes on the physicochemical properties of the produced boehmite. Furthermore, the percentage of sodium bound to the surface of the precipitated solids was ERZMETALL 56 (23) Nr. 2

industrial liquor No REC Efficiency Mass median BET area Sodium bound [g/l] diameter, d 5 [m 2 /g] [%] Seed 6.23 1.41.24 1 71.69 5.9 3.6.26 2 69.12 1.69 7.57.28 3 72.87 9.24 9.31.29 4 71.93 9.28 8.3.37 5 71.

5 Fig. 9: Morphology of grains after boehmite precipitation Fig. 1: Agglomeration and surface precipitation Table 3: Fluctuation of Boehmite process efficiency and physicochemical properties of precipitated solids Conditions: 9 C, 5 min -1, 48 h, 12 g/l, synthetic industrial liquor No REC Efficiency Mass median BET area Sodium bound [g/l] diameter, d 5 [m 2 /g] [%] Seed determined. The process efficiency seems to fluctuate normally around the mean value of g/l Al 2 without a clear tendency for increase or decrease. Furthermore, it must be stated that the observed fluctuations are reasonable, because the composition of aluminate solution was not the same during all experiments. The initial alumina concentration varied in the range 19 to 195 g/l. The mass median diameter d 5 of precipitated solids increases substantially but the BET specific surface area increases explosively. This behavior is attributed to the particles agglomeration mechanism and to the formation of nuclei from newly precipitated boehmite on the active surface sites of the boehmite seed particles, as it is shown in Figure 1. The percentage of the bound sodium gradually increases after each cycle, starting from an initial value of.24 % and reaching the value of.54 % after the 8 th cycle. Sodium ions probably play the role of the modifier of the surface properties of boehmite, just as in the well-known case of gibbsite [8], as soon as they can be adsorbed on the negative charged surface sites, decreasing thus the surface potential and making the boehmite precipitation process more efficient. 3 Discussion and conclusions The typical simplified flowsheets of Boehmite and Bayer processes are shown in Figure 11. The differences between the two processes are focused on the precipitation and calcination stages. Although the precipitation stage gives the innovation to the Boehmite process, the economic profit from the application of the new process arises in the calcination stage. The expected profit is a reduction of 1.8 GJ/t Al 2 in the energy consumption during the calcination stage. This reduction in the energy consumption is attributed to the following reasons: There is a significant difference in the enthalpy, about 1.1 GJ/t Al 2, of the respective dehydration reactions of gibbsite and boehmite. Al 2 3H 2 O = Al 2 + 3H 2 O H = 187 kj/mole Al 2 Al 2 H 2 O = Al 2 + H 2 O H = 72 kj/mole Al 2 As boehmite contains one mole of water against three for gibbsite, the amount of the material to be calcined ERZMETALL 56 (23) Nr. 2 79

6 Bauxite NaOH Bauxite NaOH Digestion Digestion Aluminate Solution Aluminate Solution Gibbsite Seed Boehmite Seed Precipitation 55 6 o C Precipitation 9 o C Gibbsite Boehmite Calcination > 1 o C Al 2 Calcination 7-8 o C Fig. 11: Simplified flowsheets of Boehmite and Bayer processes will be reduced by 36 kg/t of alumina produced. This will result in additional energy savings of about.7 GJ/t Al 2 as well as, in an increase in the calciner productivity. The reduction of 1.8 GJ per tonne of produced alumina represents more than 6 % energy reduction in the calcination stage and more than 15 % total energy savings in the Bayer process. This profit will be achieved if the boehmite precipitation stage becomes comparable to the gibbsite precipitation stage of the current Bayer process. It has to be stated that the boehmite precipitation process is not still comparable to the gibbsite one but the progress made on boehmite precipitation process has eliminated the distance between them. The efficiency of the boehmite precipitation process (6 to 7 g/l Al 2 ) and the retention time (24 to 72 h) are now directly comparable to the ones of the current Bayer process. The precipitation of sandy boehmite is a feasible target although the strength of the produced agglomerates has to be improved. The boehmite calcinations [5] is performed at 2 to 3 C lower temperatures than the gibbsite one proving that there is high potential for significant reduction in the energy consumption during this stage. The produced alumina has a specific surface area that meets the current specifications for metallurgical grade alumina. The L.O.I. and the particle size of the calcined product do not meet the current specifications but there are indications that the produced alumina will meet the specifications for a sandy product in the near future. As a conclusion, the results till now are very encouraging and the Boehmite process could become an attractive alternative of the Bayer process in the near future. 8 Acknowledgements The financial support of the European Commission within the framework of the Brite-Euram III Programme is gratefully acknowledged. (Contract No.BRPR/CT98/728) Literature [1] PANIAS, D. & PASPALIARIS, I. (1999): Precipitation of Boehmite from the Bayer Liquors. An Innovative and Promising Route for Alumina Production. Travaux, 26, No. 3: [2] International Application (PCT/GR98/19) published under the Patent Cooperation Treaty (publication number WO 98/58876). [3] PANIAS, D. & PASPALIARIS, I. (1998): Precipitation of Boehmite An Innovative Route in the Alumina Production: Heft 82 der Schriftenreihe der GDMB Gesellschaft für Bergbau, Metallurgie, Rohstoff- und Umwelttechnik, Vorträge beim 34. Metallurgischen Seminar Fortschritte in der Hydrometallurgie, November 18-2, 1998: pp ; Clausthal-Zellerfeld, Germany. [4] PANIAS, D. & PASPALIARIS, I. (1999): Thermodynamic Determination of the Stability Area of Boehmite in Al 2 -Na 2 O-H 2 O and Al 2 - H 2 O systems. ERZMETALL, 52: [5] PANIAS, D. et al. (21): Boehmite Process: An Alternative Technology in Alumina Production: Light Metals 21, New Orleans, USA, February 21: pp [6] PANIAS, D. et al. (21): Boehmite Process: An Innovative Variation of the Bayer Process: European Metallurgical Conference 21, Friedrichshafen, September 21, Vol. 2: pp [7] PANIAS, D. et al. (21): Solubility of Boehmite in Concentrated Sodium Hydroxide Solutions. Model Development and Assessment. Hydrometallurgy, 59: [8] LOH, J.S.C. et al. (21): Alkali cations Role and effect on gibbsite crystallization: Light Metals 21, New Orleans, USA, February 21: pp ERZMETALL 56 (23) Nr. 2

The Authors Dimitrios Panias is a metallurgical engineer, graduated from the Department of Mining and Metallurgical Engineering of the National Technical University of Athens (NTUA) in 1984.

Since 1989, he works as senior researcher at the Laboratory of Metallurgy of NTUA involving in a large number of projects related to ores and industrial minerals processing and also to industrial

7 The Authors Dimitrios Panias is a metallurgical engineer, graduated from the Department of Mining and Metallurgical Engineering of the National Technical University of Athens (NTUA) in He completed his doctoral thesis on gold pyrometallurgy at NTUA in Since 1989, he works as senior researcher at the Laboratory of Metallurgy of NTUA involving in a large number of projects related to ores and industrial minerals processing and also to industrial wastewater treatment. Now, he is a lecturer at the School of Mining and Metallurgical Engineering of the National Technical University of Athens. Ioannis Paspaliaris is a professor of the Laboratory of Metallurgy, Department of Mining and Metallurgical Engineering, National Technical University of Athens. His research interests include: hydrometallurgical treatment of ores and industrial minerals, metallurgical process development, mathematical modelling and simulation of hydrometallurgical processes, environmental control in the mining and metallurgical industry. Dr. Dimitrios Panias Dr. Ioannis Paspaliaris both: National Technical University of Athens (NTUA) School of Mining and Metallurgical Engineering Laboratory of Metallurgy Zographou Campus GR-1578 Athens Receipt of manuscript: October 2, 22 ERZMETALL 56 (23) Nr. 2 81

I. Paspaliaris*, D. Panias, C. Skoufadis

PRECIPITATION AND CALCINATION OF MONOHYDRATE ALUMINA FROM THE BAYER PROCESS LIQUORS I. Paspaliaris*, D. Panias, C. Skoufadis *Author for correspondence. Laboratory of Metallurgy. National Technical University