Understanding limitations to monazite concentrate dissolution in oxalic acid

Prior works with organic acids have shown dissolution of rare earth element (REE) bearing phosphate minerals with oxalic acid, solubilising phosphate and depositing rare earth oxalates that can be solubilised in a second stage. Previously published information shows local maxima in recovery and hypothesised reaction control mechanisms. The current article investigates reaction limitations (mineralogical and thermodynamic) in greater depth to inform process development. Monazite concentrate was treated with oxalic acid at temperatures in a range from 30 °C to 95 °C. Various factors were found to influence the extent of phosphorus dissolution. Previous studies achieved <32 % phosphate dissolution at 65 °C, this work has demonstrated the ability to achieve in excess of 65 % conversion using a multi-stage, cross flow leach at 45 °C with a large reduction in simultaneous iron dissolution. Approximately 65 % of the phosphorus was solubilised using a multi-stage, cross-flow leach system. Two different monazite compositional signatures, indicative of variations in REE and phosphorus contents, were identified displaying significantly different reactivity. One form readily dissolved in oxalic acid, the other experienced little or no reaction. Calculated oxalate losses to the reprecipitated salts and complexation with solution components was used to estimate availability of free oxalate, this was supported by the concentration of elements with low oxalate solubility (Ca and Ce). It was demonstrated that free oxalate is a limiting factor in dissolution. It is therefore evident that under different conditions, there are both mineralogical and solution thermodynamic drivers for reaction rate and extent achievable. • >65 % phosphate dissolution achieved in multi-stage leach at 45 °C. • Oxalate consumption estimated and is key where P, Fe and Ca dissolution is high. • Ash layer present but rare, most recrystalised particles are fully liberated. • Mineralogy shows multiple monazite phases with varied reactivity. • Electron microscopy displays vastly varied behaviour of different particles. • Two response styles of impurity elements discovered, Th follows rare earths.