Processing of rare earth phosphate concentrates: A comparative study of pre-leaching with perchloric, hydrochloric, nitric and phosphoric acids and deportment of minor/major elements

•High leaching of calcium and phosphorus in HClO4, HCl and HNO3, but low leaching in H3PO4•Results consistent with the formation of phosphate/fluoride complex species in solution•Low leaching of RE/uranyl/thorium ions in H3PO4 due to the formation of phosphate precipitates•Selective leaching of calcium and some other metals leaving uranium, thorium and REEs in the residue suitable for acid bake

Phosphate rocks such as fluorapatite often contain significant amounts of rare earth minerals and considered as rare earth ores. They can be processed to produce phosphoric acid as well as rare earth metals. The mineralization, however, is commonly associated with other rare earth minerals such as monazite ((Ce,La,Th,Nd,Y)PO4), florencite ((La,Ce)Al3(PO4)2(OH)6), xenotime (YPO4) and cheralite ((Ca,Ce)(Th,Ce)(PO4)2). The treatment of fluorapatite for rare earth extraction commonly requires a pre-leach stage with a mineral acid. Calcium, sodium, magnesium, aluminium, potassium, iron, manganese and a range of other metals including uranium and thorium may enter the solution depending upon the oxide/phosphate/silicate mineralogy. Further processing may involve partial neutralisation to precipitate any rare earth metals which may have solubilised during pre-leach, acid bake of the residue or precipitate with sulphuric acid, water leach followed by purification and precipitation. This paper describes results from a comparative study conducted on pre-leaching a phosphate rare earth concentrate using perchloric, hydrochloric, nitric and phosphoric acids under various leach conditions including different acid concentrations, temperatures and solid/liquid ratios. Through equilibrium constants and kinetic data including measured leachability of relevant metal ions, the study suggests an alternative process route which involves a selective phosphoric acid pre-leach causing low deportment of rare earth elements, uranium and thorium leading to a potentially more efficient downstream process.