Extraction of Rare-earth-elements from Phosphate Ores of Different Grades Using Roasting, Leaching and Precipitation

Tarek M Mohammed

The growing demand for sustainable, renewable energy technologies using electric vehicles and wind turbines, has led to an increased interest in the recovery of rare earth elements (REEs) from mineral resources of different grades. For example, over 250-million-ton phosphate rock of low grade (0.05-0.10 wt.% REE), typically processed annually for phosphate fertilizer could provide over half of the annual REEs produced worldwide, warranting the development of new processing methods. The current practice of processing high/medium grade RE phosphate ores using sulfuric acid baking results in the loss of REEs and phosphorus as REPO4 during neutralization/purification steps and generates highly polluting wastewater. This thesis focused on the processing of three REE mineral resources of high, medium, and low grade. Among some salts tested, iron sulfate addition to the sulfuric acid baking of high-grade (44.8 wt.% REE) phosphate ore at 480 °C and subsequent leaching achieved high recoveries of REEs (> 95%) and separated iron phosphate (~90%) in the residue which has the potential for further purification to high value material for lithium iron phosphate battery. Roasting of medium-grade (25.0 wt.% REE) phosphate ore containing 17.6 wt.% Fe, with sodium hydroxide produced water leachable Na3PO4 as a potential fertilizer by-product. Subsequent dry chlorination of dephosphorised ore followed by water leaching selectively recovered over 95% REEs, and merely 2% iron. A marketable rare earth oxide product of high purity and precipitation efficiency of 99% was accomplished for both ores. The use of phosphoric acid among other organic/mineral acids to process low grade (0.1 wt.% REE) rare earth phosphate ore effectively enriched REEs by three times in the leach residue for subsequent processing. Suite of standard characterisation techniques including XRF, XRD, SEM-EDS, and FTIR, and analytical methods including ICP/OES-MS and TGA /DSC were used to analyse solids and solutions where appropriate. Thermodynamic modelling using HSC software package facilitated the interpretation of results. The outcome of this thesis could represent a significant step forward in meeting the growing global demand for REEs while also being more environmentally friendly.

Extraction of Rare-earth-elements from Phosphate Ores of Different Grades Using Roasting, Leaching and Precipitation

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