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Comparative solubility and leaching studies relevant to hydrometallurgical processing of rare earth phosphate concentrates containing fluorapatite

Bandara, A.M.T.S. (2017) Comparative solubility and leaching studies relevant to hydrometallurgical processing of rare earth phosphate concentrates containing fluorapatite. PhD thesis, Murdoch University.

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Highly technological applications and scarcity of supply have inspired the recovery of rare earth elements (REEs) from various resources. Many rare earth phosphate mineral concentrates are associated with fluorapatite (FAP). An acid pre-leach is required to selectively leach the FAP fraction as calcium salts, phosphoric acid and hydrofluoric acid prior to the sulphuric acid digestion to avoid the interference of calcium and other impurities in the downstream processing steps of the acid baked product, rich in REEs. Hence, investigation of the dissolution kinetics of FAP and solubility of trivalent rare earth metal ions and other major and minor impurities in different acid-sulphate solutions systems are important areas of current research.

This thesis focused on (i) the solubility of REEs using pure perchlorate/sulphate and mixed RE-carbonate salts in acid-salt mixtures and synthetic process liquors and (ii) dissolution kinetics of phosphate, fluoride and major, minor and trace metal ions from a rotating disc of FAP and particles of FAP and a RE-phosphate concentrate. The strong and weak acids tested included perchloric, hydrochloric, sulphuric, nitric and phosphoric acids as well as weak organic acids. The feed materials, leach liquors and residues were characterized using standard analytical methods and the results were explained on the basis of reaction stoichiometry, equilibria and kinetic models.

The main findings from the solubility studies are: (i) the precipitation of hydrated RE(III)-sulphate and Na-RE(III)-sulphate salts in different solution media, (ii) the formation of sulphate complexes of RE(III), aluminium and iron(III) based on Raman spectroscopic studies, and (iii) the negative impact of high pH and high temperature on the solubility of REEs in sulphate media. The rotating disc dissolution of FAP and particle leaching of FAP and RE-phosphate concentrate highlighted that the FAP dissolution is affected by the proton activity and the association of calcium ions with anions to form complex species and insoluble product layers, which in turn affect the diffusion or chemically controlled dissolutions of cations and anions from FAP and the phosphate concentrate. The comparative study based on different acid concentrations, temperatures and pulp density showed the advantages of using phosphoric acid for pre-leaching FAP from the phosphate concentrate to minimize REE leaching.

Keywords: Rare earth phosphate, natural fluorapatite, characterization, acid pre-leach, solubility, complex species, kinetic models

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Engineering and Information Technology
Supervisor(s): Senanayake, Gamini, Jayasekera, Sunil and Königsberger, Erich
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