Selective thorium and uranium extraction from monazite: II. Approaches to enhance the removal of radioactive contaminants

Abstract

In the first paper of this series, the selective leaching of thorium and uranium from ground monazite with ammonium oxalate was found to depend, inter alia, on their solubility in the lixiviant, which depended on the solution pH, phosphate concentration and temperature. These actinides were released from the monazite matrix during the solution-mediated transformation of the rare earths to oxalates. The latter appeared to eventually passivate the monazite, limiting thorium and uranium leaching. The present article examines the effect of different pretreatments, leach additives, and sonication on the thorium and uranium extraction, and the recovery of the rare earth elements contained in the monazite. Different approaches were explored to suppress the re-precipitation of thorium as phosphate: strongly complexing ligands (citrate and EDTA) and the addition of ferric or aluminum ions that were expected to selectively precipitate phosphate from solution. However, none of these proved successful in experimental testing. Pretreatment with concentrated sodium hydroxide solutions, which removed most of the phosphate, yielded a residue that was amenable to selective leaching of thorium and uranium by oxalate and subsequent extraction of the rare earths with citrate.