5094
Ind. Eng. Chem. Res. 2007, 46, 5094-5099
Impact of Bayer Process Liquor Impurities on Causticization Max Wellington*,†,‡ and Franklin Valcin† Department of Science and Engineering, Atlantic International UniVersity, Honolulu, Hawaii 96813, and Process Engineering & Control Department, Alumina Partners of Jamaica, Nain, St. Elizabeth, Jamaica, West Indies
Spent liquor from a high-temperature Jamaican alumina refinery was analyzed to determine levels (g/L) of the following impurities (p ) 0.05): oxalate, 2.74 ( 0.52; sulfate, 23.23 ( 3.00; chloride, 6.36 ( 0.50; fluoride, 2.49 ( 0.05; thiosulfate, 2.47 ( 0.62; ethanoate, 5.16 ( 0.72; and organic carbon, 23.68 ( 1.99. GC/MS analysis of the liquor revealed the following relative contents of organic constituents: dibutyl and ethylhexyl phthalates, 50%; 2-hydroxyphenylethanone, 10.2%; (1H-pyrrol-2-yl)ethanone, 8.7%; 4-hydroxy2-methylacetophenone, 6.5%; 2,4-dimethylphenol, 6.1%; 3-methylphenol, 5.0%; methyl-2-pyrollidinone, 4.8%; 1-methoxy-3-phenoxybenzene, 4.4%; 5-amino-1-naphtol, 2.2%; and tri-, hexa-, and heptadecene, 2.1%. Tests done to examine the impact of some of these impurities on causticization using both synthetic Bayer liquor and caustic soda revealed fluoride as having the greatest negative impact on the causticization reaction (37% and 28%, respectively). Alumina was also shown to reduce causticization in synthetic liquors (∼40%). Oxalate, sulfate, chloride, thiosulfate, and 4-methylphenol did not appear to have any significant effect on the causticization reaction, whereas pthalate and 2-hydroxyphenylethanone appeared to slightly improve causticization (4-6%). Despite enhancing Bayer liquor causticization, lithium hydroxide did not prove viable when used as an additive for optimizing this reaction, as it appeared to react by replacing the sodium in sodium aluminate rather than by sequestering fluoride ions from the Bayer liquor. Introduction The Bayer process is used industrially for the production of alumina from bauxite ore. In the Bayer process, bauxite is subjected to high-temperature digestion (140-250 °C) in a concentrated caustic solution (∼3 M NaOH) in high-pressure reactors. The resulting liquor, termed pregnant or green liquor, which is supersaturated in sodium aluminate, is then clarified and filtered to remove mud and other insoluble residues. The clarified pregnant liquor is then cooled (65-75 °C) and seeded to precipitate aluminum hydroxide, which is calcined to yield the alumina product. The resulting spent liquor is concentrated by evaporation and recycled for further bauxite digestion. The composition of bauxite used in the processing dictates the digestion temperature. Gibbsitic bauxites (Al2O3‚3H2O) can be processed in low-temperature Bayer refineries (140-150 °C), whereas boehmitic bauxites (Al2O3‚H2O) require higher temperatures for digestion (250-255 °C). The organic liquor compositions of low-temperature Bayer refineries differ somewhat from those of high-temperature refineries, where the exposure to high temperatures and pressures and strong sodium hydroxide concentration degrades the complex organic species to low-relative-molecular-mass (
