Phase stability and role of mechanical activation in the chemical beneficiation of red mud

Abstract

This paper focuses on the carbothermic reduction of red mud in the presence of sodium carbonate. The emphasis is on the reduction step in the 'Elgai process' which is central to the chemical beneficiation and carried out in the temperature range where fusing of roasting mass is avoided. The research, aimed at developing a fundamental understanding of the roasting has three different facets: (a) calculation of phase equilibrium in Fe2O3-Al2O3-Na2CO3-C and related systems for the relevant compositions and temperature range (650-950 & DEG;C); (b) simultaneous thermal analysis (TG/DTA) studies based on phase stability; and (c) roasting and leaching of red mud-soda-carbon pellets. Red mud was used in the as-received condition and after mechanical activation in an attrition mill. Red mud and roasted products were characterized by X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) with X-ray microanalysis (EDS). The reduction temperature and carbon requirement were found to depend on the Al2O3 content of red mud, notably the occurrence of aluminogoethite. The interdependence of the decomposition of sodium carbonate, metallization of red mud, sodium aluminate formation, and alumina recovery are elucidated. Mechanical activation improves the degree of metallization and alumina recovery. The significance of Ca(OH)(2) addition as a way forward to reduce the decomposition temperature of sodium carbonate and the consequent reduction in the roasting temperature is highlighted.