Reaction kinetics of the microwave enhanced digestion of zircon with ammonium acid fluoride

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. In this study zircon was treated with ammonium acid fluoride (NH4F·1.5HF) by means of microwave assisted digestion. Reaction times ranged from 15 to 260 min at temperatures between 120 °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. 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. The derived diffusion coefficient corresponds to a solid–liquid case.