Effect of the value of x in NH4F•xHF on the digestion of plasma-dissociated zircon

Ammonium add fluoride (NH4F center dot xHF) has been identified as an alternative fluorinating agent for zircon, and may provide a method by which a wide variety of anhydrous metal fluorides can be synthesized. Data on the kinetics of the reaction of NH4F center dot xHF with plasma-dissociated zircon, combined with the thermodynamic parameters, are essential for the development of an industrial process for the production of a precursor for the manufacture of zirconium metal, viz. anhydrous ZrF4. The reaction yields (NH4)(3)ZrF7(s) and (NH4)(2)SiF6(s), the latter forming volatile products at relatively low temperatures, affording easy separation of silicon from the zirconium compound. Another useful aspect of the process is the possibility of recycling, since the ammonia and hydrogen fluoride in the waste stream can be recombined, making them available for re-use as the acid fluoride. It was found that the reaction constant k '' varies exponentially with temperature at a set value of x, but linearly with changes in x at a set temperature. The activation energy for the digestion is between 20 and 47 kJ.mol(-1) for the values of x investigated. (C) 2016 Elsevier B.V. All rights reserved.