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

Some rare earth phosphate minerals are associated with fluorapatite (FAP) in phosphate ores which make them a potential source of rare earth elements (REEs), to satisfy the increasing demand of REEs, especially for green energy technology and new energy materials. Previous studies have shown that the leachability of different elements which may be categorised as REEs and non-REEs in different acid solutions with the possibility of selective leaching of non-REEs by phosphoric acid to produce a REE rich leach residue which can be processed separately. This study investigates the dissolution of calcium, phosphate, fluoride and REEs from a rotating disc of a natural FAP mineral sample in perchloric, hydrochloric, nitric, sulphuric and phosphoric acid solutions under various leach conditions including different acid concentrations and temperatures and compares the results with batch leaching of natural FAP mineral and RE-FAP concentrate particles. The dissolution rate of REEs, which occur as minor elements in FAP mineral disc, are lower than that of the major elements: Ca, P and F. Higher dissolution rate of calcium from FAP disc in solutions of HCl, HNO3 and H2SO4 compared to HClO4 at high acid concentrations are due to the influence of Cl−, NO3− and SO42− ions, caused by association with Ca2+ ions in disc leaching, but precipitation of CaSO4 retard particle leaching in H2SO4. Despite the lower leaching rates of REEs due to low abundance of REEs on FAP mineral disc surface, the rates in 0.5 mol L−1 acid solutions follow the order H2SO4, HNO3 > HCl > HClO4 > H3PO4 indicating the beneficial effect of ion-association of the type RE(III)-sulfate/nitrate compared to RE(III)-phosphate precipitation. The higher leaching efficiencies of REEs in H3PO4 during first 10 min also decreases with prolonged leaching due to RE(III)-phosphate precipitation. The leaching efficiencies of Ca, Sr, Fe, Al and Mg are in the range 50–100% in 3.25 mol L−1 HCl, HNO3, HClO4 and H3PO4, compared to REEs, Th and U which vary in the range 20–80%, depending upon the formation of complex species or precipitation. A comparison between elemental composition of natural FAP mineral and a RE-FAP concentrate and their similarities in leaching behaviour in different acids is also presented highlighting the exceptional behaviour of phosphoric acid to selectively leach majority of non-REEs leaving REEs and silica in the leach residue. The surface reaction kinetic models and mechanisms are used to show that the dissolution reaction of calcium is mainly controlled by a chemical phenomenon in strong acids and phosphoric acid at the temperature range 25–50 °C. The experimental reaction orders with respect to H+ concentration in HCl, HNO3, HClO4, H2SO4 and H3PO4 in the range 0.46–0.93 for the dissolution reaction of calcium and/or phosphate are explained on the basis of stoichiometric dissolution and the influence of ion-association of anions with H+ and Ca2+