Microwave digestion of zircon with ammonium acid fluoride: Derivation of kinetic parameters from non-isothermal reaction data

Zircon is notoriously inert. Aggressive processing methods such as alkaline fusion are conventionally used to extract zirconium values from the mineral. In this study results are presented with respect to microwave digestion of zircon using ammonium acid fluoride (NH4F·1.5HF). Two distinct experimental methods were used. In the first method the reaction was followed in the temperature range of 100 to 200 °C, for a full reaction period of 60 min at each temperature. The second method comprised successive digestion periods interrupted by washing and re-introduction of fresh acid fluoride at a series of reaction temperatures varying between 120 and 240 °C. Complete digestion was achieved with the latter technique. The kinetics is shown to be controlled by product-layer diffusion. The two data sets were combined to derive consistent temperature-dependent kinetic parameters. A facile numerical technique for extracting these parameters from the experimental data is used.

► Zircon can be totally digested by microwave irradiation in ammonium acid fluoride. ► The rate-limiting step for the process is diffusion through the product layer. ► Kinetic parameters are derived from a large non-isothermal data set. ► Full digestion is only achieved with intermittent washing of the product layer.