Mechanical Activation of Al-Oxyhydroxide Minerals – Physicochemical Changes, Reactivity and Relevance to Bayer Process

Overview of our research on ‘structure and reactivity’ of gibbsite and boehmite under varied conditions of mechanical activation, e.g. milling energy and presence of a second phase is presented. Bulk and surface changes induced in the solids by milling are characterized in terms of morphology, particle size distribution, specific surface area and nature of porosity, crystallite size and zeta potential. Results on enhanced amorphisation of gibbsite in presence of a second phase (quartz, hematite etc), changes in zeta potential of gibbsite due to loss of texture during milling and anomalous decrease in surface area of boehmite during milling are reported. Reactivity of the activated solids in sodium hydroxide and variation in thermal transformation temperatures is correlated with physicochemical characteristics of the samples and plausible explanation for the observed correlations presented. Significance of the results with specific reference to bauxite and alumina processing in Bayer process is highlighted

Mechanical activation of gibbsite and boehmite: new findings and their implications

Almost the entire metallurgical grade alumina is produced from bauxite using the Bayer process. The leaching conditions in Bayer process depend on various Al-oxyhydroxide minerals/phases present in the bauxite. Among the various bauxite types, gibbsitic (trihydrate) bauxite is easiest to digest. Monohydrate (boehmitic and diasporic) bauxites need more stringent digestion conditions in terms of temperature, pressure and alkali concentration. Mechanical activation during milling, especially high energy milling, results in enhanced reactivity of solids due to physicochemical changes induced by milling. This paper is an overview of our recent research on mechanical activation of bauxite and its constituent phases, gibbsite and boehmite. This paper focuses on the leaching of mechanically activated bauxite ores. Complementary results on mechanical activation of gibbsite and boehmite phases are included for a deeper understanding of mechanical activation of bauxite. Implications of these results are highlighted in the context of the Bayer process.by S. P. Mehrotra, T. C. Alex, G. Greifzu and Rakesh Kuma