Autoclave breakdown of zircon with ammonium fluorides

Abstract

Autoclave breakdown of zircon with ammonium fluorides was studied. DOI: 10.1134/S1070427206110036 Zircon is one of the main mineral sources of zirconium materials, but the technology of its breakdown and processing is still labor-consuming, and much efforts are made to improve it. The classical industrial methods of breaking down zirconium concentrates are sintering with fluosilicates and chlorination. Numerous studies summarized in [1] were devoted to zircon fluorination; however, it did not gain industrial application. The fluoride technology allows the scheme of the zirconium product processing to be reduced considerably, but high cost and corrosive behavior of HF and F2, and also SiF4 liberation restrain industrial application of these reagents. In this study we examined the autoclave breakdown of zircon with ammonium fluorides. Unlike fluorine and hydrogen fluoride, ammonium fluorides are very convenient to use, as these are solid crystalline substances under normal conditions. Ammonium fluorides can be obtained from metal fluorides by the action of ammonia water, which is the basis for conversion and recycling of ammonium fluorides. Ammonium fluoride is known to react slowly with zircon [2, 3]. At temperatures below 240 C, the reaction rate is negligible and the degree of breakdown is unsatisfactory for the industrial use, whereas at higher temperatures ammonium fluoride evaporates with decomposition. Here we report new data on the zircon breakdown with ammonium fluoride and hydrofluoride. The essence of the method consists in the reaction of zircon with ammonium fluoride or hydrofluoride under isochoric conditions (in an autoclave), after which the reaction products are separated by sublimation to give zirconium tetrafluoride and dioxide. The advantage of ammonium fluorides over other fluorinating agents consists in their convenient physicochemical properties allowing their regeneration and recycling. The experiments were carried out with zircon samples from Tugan deposit of Tomsk oblast. The reactions of zircon with ammonium fluoride and hydrofluoride were studied under isochoric conditions in a 100 cm3 stainless steel autoclave. A weighed portion of zircon with a grain size of 0.074 mm was placed in the autoclave together with a weighed portion of ammonium fluoride taken in a 20% excess in relation to the stoichiometric amount calculated for the reaction ZrSiO4 + 13NH4F = (NH4)3ZrF7 + (NH4)2SiF6 + 8NH3 + 4H2O. The autoclave was heated to 150, 200, 250, 300, 350, and 400 C and kept at these temperatures for 0.5, 1, 2, and 4 h. After opening the autoclave, its content was transferred into a crucible and calcined for 30 min at 400 C to remove by sublimation unchanged NH4F and formed (NH4)2SiF6. The residue in the crucible was a mixture of unchanged ZrSiO4 and NH4ZrF5, to which 5 ml of concentrated sulfuric acid was added, and the mixture was kept until H2SO4 vapor disappeared. Zirconium in the form of ZrOSO4 solution was leached with water from the formed cake, converted to zirconium dioxide, and weighed to determine the degree of conversion. The same procedure was used to study the reaction of ammonium hydrofluoride with zircon 2ZrSiO4 + 13NH4F HF = 2(NH4)3ZrF7 + 2(NH4)2SiF6 + 3NH3 + 8H2O.