Extraction of alumina from clays and high-silica bauxites.

Two processes for extracting alumina from low-grade ores are described, one applicable to both clays and high-silica bauxites, and the other only to bauxites. In the former process, a line-containing sinter is prepared and annealed by cooling it slowly through the range of 1,3000 to 1,2000 C. The annealed sinter dusts to a powder which requires no grinding. This material is extracted with a solution containing about 200 g of Na2C03 and 150 g of NaCI/liter, and an extract is obtained in which the concentration of Al20 3 is 70 to 80 g/liter and that of Si02, 1 to 2 gfliter. By boiling the extract with a seed charge of synthetic sodalite, 3Na20·3AI20 3·6Si02·2NaCI, its silica content is reduced to 0.1 percent of the alumina content or less. Alumina, suitable for the manufacture of aluminum by electrolytic reduction, is precipitated by passing carbon dioxide into the desilicated solution. About 95 percent of the alumina in the clay is recovered by this method. The other process developed in this investigation involves the extraction of alumina from high-silica bauxites with sodium hydroxide-sodium chloride solution and recovery of soda and alumina from the residues by a modified sodalime-sinter method. Recoveries in excess of 90 percent of the alumina in high-silica bauxites are obtained. In cyclical operation of each process, the spent solution from the aluminaprecipitation step is used in the treatment of a fresh batch of material. When the bauxites are extracted with solutions containing mixtures of sodium hydroxide and sodium carbonate, sodium sulfate, sodium bromide, or sodium nitrate, the quantities of silica present in the extracts are lower, the higher the concentration of the salt. This is caused by the formation and increased stabilization of slightly soluble compounds related to sodalite. Comparisons of the X-ray diffraction patterns of various sodalite type compounds are presented. By the extraction of bauxite with sodium hydroxide solution, in the absence of added salts, a relatively soluble hydrated nephelite, Na20·AI203·2Si02·2H20, is formed, which has a crystal structure different from that of sodalite. CONTENTS Page