Plasma dissociation of monazite

Abstract

Monazite is a phosphate mineral which contains a significant amount of rare earth elements (REE) with thorium (Th) and a low concentration of uranium (U). The radioactive components of monazite, Th and U, make it an unattractive source of REE due to the added cost associated with radioactive waste handling. The mineral is also chemically inert requiring the use of corrosive reagents at elevated temperatures to effectively extract REEs. According to thermodynamic calculations, monazite dissociation into oxides of REE and phosphorus (P) is favourable at temperatures greater than 1000°C, while the reverse reaction occurs well below a 1000°C. Rapid cooling is required to prevent the reverse reaction. Achieving these high temperatures, with rapid heating and cooling, is possible by means of thermal plasma treatment such as a radio frequency plasma. This type of plasma system offers superior control of plasma/particle interaction, quenching rates and plasma gas composition. The objective of this study was to determine the parameters for dissociation of the monazite under various conditions to reduce its chemical inertness. Dark and grey powders were obtained under reducing conditions, while lighter powder formed in the presence of an oxygen plasma with increase particle circularity observed after plasma treatment. While SEM-EDS analysis indicated no notable change in powder composition after plasma treatment, dissolution tests proved that monazite was not only more reactive, but also more susceptible chemical reaction after treatment.