Role of Mineralogy and Origin of Mechanically Activated (Indian) bauxite in Bayer Process

Abstract

This study explored the linkage of mechanical activation (MA) of bauxite ores with their mineralogy and origin. Bauxite ores from Nalco and Indal, widely differing in chemical and phase compositions, were used in the study. Laboratory attrition mill was used for MA and simultaneous milling and leaching(SMILE) experiments. Rapid particle size reduction was characteristic of both Nalco and Indal bauxites. The milling data of both the bauxites fitted very well with kapur's GH equation. Nalco bauxite was found to have faster grinding kinetics and larger red-uction ratio.Irrespective of the chemical and phase compos-ition, zeta potential of the bauxite slurry increased with milling time. However, the increase was more for Nalco bauxite. Increased concentration of reactive singly coor-dinated OH groups on gibbsite surfaces might be respons-ible for this. Gibbsite-boehmite transformation temperat-ure decreased with milling time in both bauxite ores. Beyond 5 minutes of milling, gibbsite in the Nalco bauxite transformed at a lower temperature than that for the corr-esponding Indal bauxite. However, the peak area (which corresponds to the enthalpy of transformation) was less for Indal bauxite indicating increased retention of the absorbed energy. Stuctural changes, progressive decrease of (002)X-ray peak intensity, were also observed in the gibbsite structure with milling. Total alumina as well as the extractable alumina was more in the Indal bauxite. Simultaneous milling leaching (SMILE) was adopted in all the experiments. Effect of SMILE time and rpm were stud-ied in the current study. The studies showed that the recovery of alumina increased with the SMILE time up to about 15-20 minutes whereas the rpm, above 1000, had little effect. The XRD peak of boehmite in the leach residue increased, indicating that the boehmite remained inert during SMILE. This showed that boehmite needed a much stronger activation compared to gibbsite and under-lines the importance of mineralogy.