An investigation into the fluorination capabilities of ammonium acid fluoride under microwave radiation with respect to zircon

Abstract

South Africa is the second largest producer of zircon (ZrSiO4) in the world, Australia being the largest. Zircon is notorious for its chemical inertness. Extreme processing conditions such as alkaline fusion (NaOH at 600 °C or Na2CO3 at 1200 °C) are used to extract the zirconium values from the mineral. The purpose of this study was to investigate the use of microwave digestion as an alternative process, and to determine the parameters for this technique for the digestion of zircon with ammonium acid fluoride (AAF) under various conditions. Ammonium acid fluoride is more convenient and safer to use than conventional fluorination methods such as HF and F2. In this study zircon was treated with ammonium acid fluoride (NH4F∙1.5HF) by means of microwave assisted digestion. Reaction times ranged from 10 to 330 minutes at temperatures between 100 °C and 240 °C. Successive microwave digestion steps, interrupted by an aqueous wash procedure, resulted in a >99 % conversion of zircon to the water soluble intermediates (NH4.)3ZrF7 and (NH4)2SiF6. XRD and Raman spectroscopy confirm that zircon was the major phase present in the insoluble fraction of the product after washing. Arrhenius rate laws are derived for both reaction control (progressively shrinking particle) and diffusion control by the product layer. Both models show reasonably good agreement with the experimental data, but diffusion control was accepted as the most probable. The derived diffusion coefficient corresponds to a solid-liquid case. Copyright