Separation of Uranium and Thorium from Rare Earths Ore for Production - A Review - 2015

Rare Earth Deposits & Mining

Zhu and 2 co-authors reported in 2015 that rare earths play critical roles in the applications of advanced materials. Recently, the recovery of rare earths from a variety of resources has gained much interest. Radioactive elements of uranium and thorium are usually associated with rare earth deposits. The separation of uranium and thorium from rare earths is often a big concern in rare earth industry in order to reasonably manage the radioactive nuclides. This paper reviews the technologies used for separating uranium and thorium from rare earths in rare earth production, particularly in China. Some potentially applicable methods, such as precipitation and solvent extraction for the separation of uranium and thorium from rare earths in different media were also reviewed.

The authors concluded that large amounts of thorium are commonly associated with rare earth ores. Thorium is rejected into a radioactive solid waste by forming thorium pyrophosphate at high temperature (>500 C) for processing Bayan Obo ores, the world largest rare earth deposit, using sulphuric acid treatment in China. Selective precipitation of thorium or selective dissolution of rare earths in chloride solutions is also commonly used to separate thorium from rare earths for processing other kinds of rare earth ores. In the pH range of 4–5, thorium is locked in solid precipitates or solid residues while rare earths stay in the solution. Very small amounts of uranium are found in rare earth ores processed in China and no special measures were taken in relation to the uranium control.

Double salt precipitation of rare earths is widely adopted for some new process development. Large amount of thorium co-precipitation but less uranium⁺⁶ co-precipitation is reported. Therefore, double salt precipitation cannot be used to separate rare earths from thorium and uranium ⁺⁴, but possibly from uranium ⁺⁴. Oxalic acid can be employed to precipitate thorium and rare earths and separate them from uranium in sulphate solutions at high temperatures. Hot sodium carbonate solution can be used to selectively dissolve thorium and to separate it from rare earths.

Solvent extraction is effective to separate uranium and thorium from rare earths. Tertiary amines can be used to selectively extract U⁺⁶ and to separate it from Th⁺⁴ and RE⁺³ while primary amines are highly selective for Th⁺⁴ extraction over U⁺⁶ and RE⁺³ in sulphate solutions. TBP can be used to separate U^+6, Th⁺⁴ and RE⁺³ effectively based on the extraction order of U⁺⁶ > Th⁺⁴ > RE⁺³. Tertiary amines and quaternary amine salts are effective to extract uranium and separate it from thorium and rare earths in solutions with high HCl concentrations.

Organophosphorus acids including D2EHPA and EHEHPA could be used to extract thorium and separate it from rare earths in chloride solutions. Carboxylic acid extractants can be possibly used to separate uranium and thorium from rare earths.