Phase Formation Processes in Natural Magnesium Silicates of Various Structures by Ammonium Fluoride Treatment
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Vol. 61, No. 2, July, 2020
PHASE FORMATION PROCESSES IN NATURAL MAGNESIUM SILICATES OF VARIOUS STRUCTURES BY AMMONIUM FLUORIDE TREATMENT Sh. M. Sharafeev1 and V. M. Pogrebenkov1,2 Translated from Novye Ogneupory, No. 4, pp. 28 – 35, April, 2020.
Original article submitted January 21, 2020. The processes of forsterite, enstatite, and talc treatment by ammonium fluoride have been studied. The phase-formation processes occurring during firing of fluorinated minerals were studied. It was established that structural silica of the minerals reacts with ammonium hydrogen difluoride with the formation of ammonium hexafluorosilicate, sublimation of which leads to removal of structural silica from the minerals with partial destruction of their structure. The firing of fluorinated minerals leads to their structural transformation with formation of magnesium nesosilicates and fluoronesosilicates. Keywords: forsterite, enstatite, talc, ammonium fluoride treatment, ammonium hexafluorosilicate (HFSA).
An excess of cristobalite in enstatite and forsterite materials has negative effects on the ceramic operating properties. Clayey components and divalent-metal oxides (CaO, BaO, ZnO) introduced into the starting mixtures react with free silica to form low-melting melts that form a glass phase upon cooling [5, 6]. A different magnesia-silicate raw material containing olivines, serpentines, and talc is widely used in forsterite refractory technology. These materials are charged beforehand with MgO or magnesite to increase the forsterite yield by promoting the reaction between MgO and SiO2 to form the minerals during thermal decomposition and to bind impurities (Fe2O3, CaO, etc.) into more refractory phases to produce high-quality materials [7]. The phase composition of magnesia-silicate ceramics produced from natural raw material is difficult to control because of the formation of an excess of SiO2 over the stoichiometric amount of the final desired crystalline phases. This problem is currently solved by preliminary charging of the raw materials with MgO and its salts in addition to glass-forming additives. An alternative method for controlling the phase composition of materials based on silicates is to regulate the SiO2 content in the raw materials. This method is based on the reaction of silicate ores with fused ammonium hydrogen difluoride (HDFA), which reacts readily with quartz impurities and with structural silica of silicates to form complexes such as ammonium fluorosilicates (NH4)2SiF6 and
INTRODUCTION Magnesia-nesosilicate ceramics and refractories with a main crystal phase of enstatite Mg2[Si2O6] or forsterite Mg2[SiO4] in addition to traditional materials based on clays and kaolins are widely used in various fields of science and technology. The numerous varieties of magnesium-silicate and -hydrosilicate minerals and their relatively broad distribution in earth’s crust are responsible for this. Much progress has now been achieved in the production if forsterite and enstatite ceramics based on synthetic raw materials using
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