A brief overview and critical analysis of acid and acid-salt methods for processing alumina raw materials, including high-silica bauxite, nepheline and clay is presented. The basic approaches to the creation of closed and reagent-less or low-reagent-consumption processes are demonstrated. The results obtained in the course of long-term systematic laboratory and bench studies are presented, and they display the prospects for the creation of new technologies for the production of alumina from high silica bauxite and aluminum silicates with the use of different nitrate and sulfate reagents. In the case of using acids, the sorption method of «Acid Retardation» in nano-porous media is shown to be successfully used for recycling them into the process head and to ensure soft conditions for the removal of iron components. This stage is realized with the use of strong base anion exchanger, preliminarily equilibrated with the macro-anion composition of solutions to be processed and washed with water or diluted acid solution. The AR process in three phase systems including organic liquids immiscible with water solutions (“New Chem” method) is also described, a this is also important to provide the complete insulation of concentrated salt solution and to simplify its farther treatment. A circular process with the recovery and consumption of the same amount of salt reagent in each cycle is demonstrated. A comparison of digestion methods is given in terms of the completeness of reagent recycling, the quality of product and the complexity of the process. Advisable conditions for the production of alumina from non-traditional raw with salt-acid decomposition are shown. As it is shown, the most in demand are the processes, which fully or partially can be «inscribed» in modern equipment design and process conditions corresponding to the standard Bayer method or its industrial modifications
Geochemistry and Nd - isotopic composition of high silica rhyolites in the neoarchaean Musoma-Mara greenstone belt, northern Tanzania: evidence for the presence of older continental crust