Acid extraction of alumina from Canadian non-bauxite sources at CANMET

Abstract

CANMET's laboratory work on acid extraction processes for the recovery of alumina from Canadian non-bauxite sources is reviewed. The materials investigated were anorthosite, clays and coal waste. Several processes were used to improve the acid solubility of anorthosite. In one method the rock was melted and quenched as an amorphous glass; more than 95% of the aluminum content was extracted from this material in mineral acids. In another process the extraction of aluminum by hydrochloric acid leaching of finely ground anorthosite was greatly increased - from 307 to over 80% - by adding to the leach a small amount of fluosilicic acid. It was also found that anorthosites from some deposits were soluble in hydrochloric acid without fluosilicic acid; this material approached the composition of bytownite and was composed of unaltered feldspar. Six Canadian clays were examined as alumina sources. The after calcining at temperatures of between 750 and 850 ° C, alumina contents ranged from 23 to 29%. Material from one deposit was upgraded by wet screening to remove coarse quartz and produced a 36% Al 2 03 calcine. Aluminum extraction in either hydrochloric or sulphuric acid was in the 85 to 90% range for all samples. Waste coal from the Hat Creek, B.C. deposit was also examined as a source of alumina. The material was burned in a fluid-bed combustor at 750 to 850 °C to produce an acid soluble ash containing 30.8% Al 203 . Aluminum extraction of 80% was achieved in a 30-min leach in 26 m % HC1. Alumina was recovered from hydrochloric acid leacbates by sparging with HC1 gas to crystallize aluminum trichloride hexahydrate. The crystals were dissolved and recrystallized to purify the product prior to calcining to alumina. It often contained impurities - P, Mg and Cr - in excess of the specifications for cell-grade material. To overcome this problem a process was developed wherein the hydrochloric acid leachate was evaporated and calcined at 400°C to produce a crude alumina. After water leaching to remove soluble chlorides, this material was treated by a modified Bayer process to produce alumina which met cell-grade specifications.