Some recent developments in the lime-fly ash process for alumina and cement

Abstract

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. The soluble compound formed in the sintering reactions has been identified through use of XRD methods as calcium sulfoaluminate (C 4A 3S). Compound formation conditions have been determined for a range of sinter compositions. In other work, 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) upon cooling of the clinker formed. Several ions, even at very low concentrations, can enter the lattice of the minor sinter compound, calcium orthosilicate (C 2S), and inhibit the phase transformation of beta-C 2S to gamma-C 2S and the accompanying lattice expansion that causes clinker disintegration.