Suspension calcination and alkali leaching of low-grade high-sulfur bauxite: Desulfurization, mineralogical evolution and desilication

Abstract

To enable the utilization of low-grade and high-sulfur bauxite, the suspension calcination was used to remove the sulfur and the activate silica minerals, and the calcinated bauxite was subjected to a desilication process in NaOH solution under atmospheric pressure. The desulfurization and desilication properties and mineralogical evolution were studied by X-ray diffraction, thermogravimetry-differential thermal analysis, scanning electron microscopy, and FactSage methods. The results demonstrate that the suspension calcination method is efficient for sulfur removal: 84.21% of S was removed after calcination at 1000°C for 2 min. During the calcination process, diaspore and pyrite were transferred to α-Al₂O₃, magnetite, and hematite. The phase transformation of pyrite follows the order FeS₂ → Fe₃O₄ → Fe₂O₃, and the iron oxides and silica were converted into iron silicate. In the alkali-soluble desilication process, the optimum condition was an alkali solution concentration of 110 g/L, a reaction time of 20 min, and a reaction temperature of 95°C. The corresponding desilication ratio and alumina loss ratio were 44.9% and 2.4%, respectively, and the alumina-to-silica mass ratio of the concentrate was 7.9. The Al₂O₃·2SiO₂, SiO₂, and Al₂O₃ formed during the calcination process could react with NaOH solution, and their activity decreased in the order of Al₂O₃·2SiO₂, SiO₂, and Al₂O₃.

Keywords

low-grade bauxite / sulfur / suspension calcination / alkali-soluble desilication / Bayer process

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