A C C E P T E D M A N U S C R I P T C was conducted using particles of size range 150-180 µm, 5% (w/w) solids and a stirring rate of 1100 rpm. The similarities and differences in leaching efficiencies of metal ions are rationalised on the basis of proton activity of acids and the participation of anions due to complexation with metal ions. The leaching efficiency of calcium, phosphate, fluoride, sodium and strontium reached 80-100% after 5-10 min in hydrochloric, perchloric and nitric acid compared to lower leaching efficiencies (<40%) in phosphoric acid. Despite the low solubility products of phosphates of iron(III) and calcium (pK SP ≈ 24, 31), the higher pH, and lower proton activity of phosphoric acid, the higher leaching efficiencies of calcium and iron (≥80%) suggest the formation of complex species of these metal ions with phosphate ions. The leaching efficiencies of lanthanum, cerium and neodymium were low in all acids and showed a descending order: HClO 4 (54-63%) > HCl (21-13%) > HNO 3 (5-7%). The RE leaching efficiency in HClO 4 remained relatively unaffected with time, but the low leaching efficiencies in other acids after 30 min (<20%) indicates the precipitation of RE-phosphates and prospect of selective leaching of FAP. The slope of linear correlation of leaching efficiency of REs was close to unity in HClO 4 , HCl and HNO 3 indicating similar behaviour. A higher slope for La-Ce leaching efficiency correlation of 1.8 compared to 0.9 for Nd-Ce in H 3 PO 4 warrants further studies. ACCEPTED MANUSCRIPT

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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)PO 4), florencite ((La,Ce)Al 3 (PO 4) 2 (OH) 6), xenotime (YPO 4) and cheralite ((Ca,Ce)(Th,Ce)(PO 4) 2). The treatment of fluorapatite for rare earth extraction commonly requires a pre-leach stage with a mineral acid. Calcium, sodium, magnesium, aluminum, potassium, iron, manganese. A range of other metals such as 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/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.

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Bandara, A.M.T.S. and Senanayake, G. (2018) Dissolution of calcium, phosphate, fluoride and rare earth elements (REEs) from a disc of natural fluorapatite mineral (FAP) in perchloric, hydrochloric, nitric, sulphuric and phosphoric acid solutions: A kinetic model and comparative batch leaching of major and minor elements from FAP and RE-FAP concentrate. Hydrometallurgy