Statistical Modeling of the Industrial Sodium Aluminate Solutions Decomposition Process
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THE Bayer process accounts for virtually all the commercial processing of bauxite ore to alumina. The crystallization of aluminum hydroxide (Al(OH)3) from caustic aluminate solution is the rate determining step of the Bayer cycle, which is used for alumina production.[1,2] The control of the particle size distribution through the manipulation of solution conditions is required to minimize the generation of fine particles.[3] Because of the complexity of Bayer liquor speciation, the mechanisms of Al(OH)3 crystallization are still not understood completely and are the subject of considerable research effort.[4] During Al(OH)3 crystallization, growth and aggregation of the crystals occur simultaneously. The mechanism of the crystallization process was investigated by many authors who concluded that this highly complex process includes several phases (nucleation, crystal growth, and crystal breakdown). The mechanism of gibbsite agglomeration is still understood poorly on a microscopic scale.[5] Investigations of the gibbsite nucleation from the synthetic sodium aluminate solutions revealed that this process is accelerated in saturated solutions[6] yielding large crystal grains as the result of their aggregation.[4] Also, recent investigations indicated that the introduction of certain ions in the
ZˇIVAN ZˇIVKOVIC´, Professor, IVAN MIHAJLOVIC´, Reader, ISIDORA DJURIC´, Assistant, and NADA SˇTRBAC, Professor, are with the Technical Faculty in Bor, University of Belgrade, 19210 Bor, Serbia. Contact e-mail: [email protected] Manuscript submitted November 24, 2009. Article published online July 20 2010. 1116—VOLUME 41B, OCTOBER 2010
aluminate solution influences the crystallization rate and the size of obtained gibbsite crystals.[7–13] The crystallization mechanism of Al(OH)3 from sodium aluminate solutions was investigated by Li et al.,[4] they indicated that the mineral form of obtained crystals, as well as their size distribution, depends on the concentration of the solution at temperatures ranging between 338 K (65 C) and 343 K (70 C). This was also proved in the investigations published by Dash et al.[14] The kinetics of the gibbsite crystallization from the caustic sodium-aluminate solution, as well as size and the shape of obtained particles, depend on the following parameters of the process: temperature, alumina/caustic ratio, amount and size distribution of the crystallization seeds, stirring speed, and the presence of activation ions added to the solution.[3,7,14,15] Investigations of boehmite (AlOOH) precipitation from the aluminate solutions were reported recently,[14,16,17] with the aim to find sustainable alternative for the current technology of alumina production. In those investigations, boehmite precipitation was facilitated in the highly saturated solutions at temperatures greater than 373 K (100 C).[18] Nevertheless, the Bayer process is still superior compared with such alternatives.[2] Most of the results published recently, concerning gibbsite crystallization from the sodium aluminate solution
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