A cyclic voltammetric study of the oxidation and reduction of elemental tellurium over the pH range 0-14 has been undertaken with tellurium disk electrodes which could be rotated. In both oxidation and reduction, the behaviour of tellurium is largely that expected from the predictions of the E-pH diagram for this element. Exceptions to this general principle were observed during the oxidation of tellurium at both low and high pH where it was found that tellurous acid was relatively quick to form and slow to dissolve; this led to a type of passivation behaviour. On the reduction side, the system behaved in a complex, fashion between pH 8 and 11 with both HTe - and Te22- being formed.

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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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