Impact of Bayer Process Liquor Impurities on Causticization

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-hydroxy-2-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.