Institute of Geology and Natural Management of Far Eastern Branch of RAS (Blagoveshchensk, Russia):

Rimkevich V. S., Head of Laboratory, Candidate of Geological & Mineralogical Sciences, Senior Researcher, vrimk@yandex.ru (corresponding author)
Pushkin A. A., Researcher, Candidate of Physical and Mathematical Sciences
Girenko I. V., Leading Engineer

Alkaline aluminosilicate rocks and their concentrates serve as a valuable source of raw materials for the manufacture of alumina, aluminum, and various commercial products. Pioneering studies of thermochemical ammonium-fluoride processing of nepheline concentrates of the Kola Peninsula and of the synnyrites of the Sakun massif (Chita region) have been carried out, with preliminary chemical stripping of the amorphous silica contained in the volatile ammonium hexafluorosilicate ((NH₄)₂SiF₆). The complex processing was performed using specifically designed heat treatment, leaching, calcination and regeneration devices, made of teflon and stainless steel. The process included the following main operations: sintering of the feed material with ammonium hydrodifluoride (NH₄HF₂) at 50–200 °C with the holding time of 0.5 to 4.5 hours; powdered cake sublimation to produce volatile ammonium hexafluorosilicate in the temperature range of 350 to 550 °C with the holding time of 0.25 to 1 hours, with the formation of a non-volatile residue consisting of (hexafluoroaluminate ((NH₄)AlF₆), ammonium hexafluoroferrate ((NH₄)FeF₆) and other substances; pyrohydrolysis to obtain iron oxide at temperatures above 600 °C with the holding time of over three hours and its further hydrochloric acid leaching with the recovery of an iron chloride solution (FeCl₃). The water-alkaline treatment of soluble fluoride-ammonium salts of silicon, aluminum, and iron chloride was carried out with the formation of amorphous silica (SiO₂), aluminum hydroxide (Al(OH)₃) and red iron oxide pigment (Fe₂O₃) in the temperature range of 25 to 90 °C, with pH = 7–9 and holding times of over one hour. The output data for the conditioned metallurgical alumina (Al₂O₃) and the conditions for the formation of calcium fluoride (CaF₂) are presented. Additional reagents are also recovered and supplied to the respective stages of the low-waste process. As a result of the studies, an effective innovative method for processing alkaline aluminosilicates with the integrated recovery of various components was developed.

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