Specific features of scandium behavior during sodium bicarbonate digestion of red mud

Abstract

Bauxite ore contains ~65% of world Scandium recoverable reserves. From Bayer extractable alumina >96% of Scandium merges into red mud (RM). In smelter grade alumina produced from the bauxites Sc2O3 content ranges from 1.2 to 2.3 ppm. Sc content in initial bauxite has little influence on its content in alumina. Scandium that is present in red mud after processing of Middle Timan bauxite and Nothern-Ural bauxite has two forms. Part of Scandium (~65%) can be readily extracted from boehmite and diaspore. Sc2O3 content in Al monohydrates amounts to 80–100 ppm. During autoclave digestion Sc goes into red mud as extractable form (Scav) and is adsorbed on the RM surface in form of oxyhydroxide ScO(OH). Sorbed scandium can be present on the surface of any RM phase or mineral. And due to this property concentrating of Scav during RM treatment is inefficient. Scav can be easily digested with sodium bicarbonate solution under “mild” conditions. Chemically bonded Scandium in the structure of mainly iron-containing minerals of bauxite (30 ± 15%) can be extracted by “severe” acid leaching with breaking of crystal structure of hematite and other minerals. When bauxite is digested in autoclave (Bayer process) new phases are formed (calcium hydrogarnet (HG) and sodium hydroalumosilicate (DSP). Hydrogarnets and DSP do not bind Scandium chemically and it indirectly confirms Scandium sorbing properties in RM. Tests on development of sodium bicarbonate digestion modes of scandium from RM showed significant range of values of Sc extraction into the solution (from 10 to 65%). One of the most important factors affecting Sc extraction is Sc secondary losses due to formation of new phases (incl. aragonite, dawsonite, etc.) These phases are in equilibrium under such conditions. The mechanism of Sc secondary losses during sodium bicarbonate digestion of red mud was analyzed, and some practices to increase of extraction values were offered.