Beneficiation of a commercial kaolin from Mar de Espanha, Minas Gerais, Brazil: Chemistry and mineralogy

Raw materials used to produce a commercial kaolin from Minas Gerais, Brazil, and the intermediate and final products after processing were studied to trace the fate of iron during industrial beneficiation. X-ray diffraction shows kaolinite to be the principal phyllosilicate in all samples. Sample treatments include size fractionation and a chemical treatment with sodium bisulfate, metallic zinc, and sulfuric acid, which remove all associated goethite and most hematite but do not significantly affect structural iron in the phyllosilicates. Scanning electron micrographs show that individual kaolinite grains are approximately 0.3 μm in size and have an almost hexagonal morphology in a compact arrangement. Tubular halloysite was also occasionally observed. Mössbauer spectra measured at 298° and 100 K evince the presence of hematite and paramagnetic trivalent iron in all samples, whereas goethite, which is abundant in raw and intermediate samples, is absent in the final kaolin. The industrial processing removes almost two-thirds of the total iron content (essentially iron oxides) of the raw material during size fractionation, so the intermediate sample contains 0.42% Fe3+. The final marketable material contains 0.12% Fe3+ in hematite (0.07%) and phyllosilicates (0.05%), rendering it suitable as paper filler.