Effects of thermal pre-treatment and ore dryness on the recovery of lanthanides from ion-adsorption clays

• Effect of thermal pre-treatment on rare earth recovery from ionic clays was studied.
• Macro- and microstructural changes occurring in clays during heating were identified
• Rare earth recovery increased with the temperature up to 300 °C, then decreased.
• Improved clay permeability due to loss of adsorbed water in the interval 100–300 °C
• Dehydroxylation and structural loss over 500 °C led to REE fixation to the surface.

The present research evaluated lanthanide desorption levels from ion-adsorption clays subjected to thermal pre-treatment at various temperatures in the range 100–900 °C. The authors identified macro- and micro-structural changes occurring in the clays during heating and related the observed trends to the water content and rare earth elements (REE) desorption behavior during subsequent ion exchange leaching, in order to gain a better understanding of how temperature and moisture may affect the overall REE recovery, hence impacting the ore processing route.It was found that the total lanthanide extraction from clays pre-heated in the range 100–300 °C increased with the temperature, then decreased drastically, reaching near-zero levels for the range 700–900 °C. This trend was explained in terms of improved clay permeability achieved upon elimination of pore and adsorbed water molecules in the interval 100–300 °C, which facilitated better lixiviant access to the REE adsorption sites. The subsequent decrease in REE extraction in the interval 300–900 °C was related to the loss of hydroxyl groups from the aluminosilicate matrix (dehydroxylation), resulting in irreversible REE fixation to the surface and major structural changes such as layer collapse, loss of crystal structure and vitrification, with the ultimate result of completely blocking lixiviant access to the adsorbed cations.

Total rare earth extraction increases slightly for clays heated up to 300 °C, then decreases drastically for both ores.As the temperature of thermal treatment increases, the main clay peaks are largely unaffected up to ~ 300 °C, then gradually decrease in intensity up to ~ 700 °C while maintaining same position, due to complete dehydration, followed by dehydroxylation and finally layer collapse. The XRD patterns confirm that REE extraction is adversely influenced by subjecting the clay ores to temperatures in excess of 300 °C, as it indicates increasing structural damage beyond 500 °C, therefore blocked lixiviant access to the REE adsorption sites.Figure optionsDownload as PowerPoint slide