Investigation of alumina discharge into the red mud pond at NALCO's alumina refinery, Damanjodi, Orissa, India

Abstract

Around 14% of alumina gets discharged into the red mud pond from NALCO's alumina refinery. The minerals hosting alumina in red mud have been investigated using Optical microscope, Scanning Electron Microscope (SEM), Electron Probe Micro Analyzer (EPMA) and XRD. The minerals in red mud sample, as identified through integrated techniques, are: gibbsite, boehmite, goethite, silimanite, muscovite, garnet and kaolinite besides minor hematite, sodalite and rutile. The average particle size of red mud is ∼8μm except isolated grains of gibbsite, goethite and garnet which are greater than 20μm in size. The gibbsite invariably contains up to 60 mole % of boehmite. The nodular goethite contains over 30-mol % of 'Al' in its lattice and termed as alumo-goethite. Around 34% of the Al2O3 is present in muscovite/ silimanite/garnet. Studies reveal that most of the minerals in red mud have either alumina in their lattices or are undigested aluminium silicates that do not get dissolved during alumina refining, thus a considerable amount of alumina gets discharged through these phases into the red mud.