Recent Developments in the Lime-Fly Ash Process for Alumina and Cement

摘要

Recent developments from research of the limestone-fly ash sinter process for the extraction of alumina from coal fly ash and use of the residue in the manufacture of portland cement may result in substantial reductions in processing costs. It has been found that cement kiln dust can be used in place of limestone, and that the addition of coal refuse to the sinter mixture will increase alumina recovery and reduce kiln temperatures required. The soluble compound formed in the sintering reactions has been identified through use of XRD methods. Compound formation conditions have been determined for a range of sinter compositions. A systematic investigation of minor additives to sinter compositions has resulted in the determination of a group of ions which, when present in sintered material, will prevent the auto-disintegration (dusting) of the clinker formed. Several ions, even at very low concentrations, can enter the lattice of the major sinter compound, calcium orthosilicate (C sub 2 S) and inhibit the phase transformation of beta -C sub 2 S to gamma -C sub 2 S and the accompanying lattice expansion that causes clinker disintegration. The understanding gained about clinker disintegration, conditions for formation of specific soluble compounds in the clinker, and the roles of coal refuse-derived iron and sulfur in sinter reactions can lead to improvements in the sinter process and enhanced profitability. Use in the process of other wastes provides further incentive for coal ash processing to meet our nation's requirements for alumina and cement, and provide an alternative form of coal combustion waste disposal.