An innovative method for processing Egyptian monazite

Abstract

The present work deals with a study of an innovative method for processing Egyptian monazite by alkaline leaching in ball mill autoclaves, where grinding and leaching of monazite take place simultaneously. This was followed by selective separation of thorium and uranium from lanthanides by autoclave leaching of the hydroxide cake obtained with ammonium carbonate–bicarbonate solutions. This method was based on the fact that thorium and uranium form soluble carbonate complexes with ammonium carbonate while lanthanides form sparingly soluble double carbonates. It was found that complete alkaline leaching of Egyptian monazite (99.8%) was attained at 150 and 175 °C within 2.5 and 1 h, respectively (satisfying favorable conditions of the hydroxide cake obtained for subsequent selective carbonate leaching). Monazite leaching was intensified and accelerated in ball mill autoclaves as a result of the grinding action of steel balls, removal of the hydroxide layer covering monazite grains and the continuous contact of fresh monazite surface with alkali solution. The study of selective carbonate processing of the hydroxide cake with ammonium carbonate–bicarbonate solutions revealed that the process must be carried out in an autoclave under pressure. Complete thorium recovery (99.4%) with uranium recovery (94.7%) and their separation from the lanthanides were attained at 70–80 °C during 1–2 h. The extraction of lanthanides in carbonate solution was low and did not exceed 1.5–2%. On the contrary, at atmospheric pressure, the recovery of thorium was low 18.4% at 70 °C. This is due to the instability of ammonium thorium carbonate complex, ammonium carbonate and bicarbonate and their dissociation with the liberation of ammonia, carbon dioxide and water vapor.