Recovery of rare earth elements and sulfuric acid solution from phosphate byproducts via hydrofluoric acid conversion

Abstract

The recovery of high-added value products from phosphate byproducts presents currently a great economic and environmental interests. Rare earth elements (REEs) are present in different byproducts of the fertilizer industry including phosphate tailings, phosphogypsum and phosphoric acid sludge (PAS). This latter contains significant amount of REEs, which could be regarded as a possible alternative resource of REEs. In this article, we addressed the enrichment of REEs in calcium fluoride (RE/CaF2) phase by a conversion reaction of calcium sulfate (RE/CaSO4) sourced from phosphoric acid sludge using hydrofluoric acid. The conversion rate and the enrichment factor of REEs were evaluated. It was demonstrated that the conversion rate reached over 97% with 100% transfer of REEs to CaF2 phase. The REEs were selectively enriched in CaF2 with an enrichment factor of over 42%. Characterizations using XRD, FTIR, TGA and SEM-EDS were conducted to confirm the obtained results. More importantly, the speciation and the conversion mechanisms of REEs were highly discussed. The leaching of REEs from CaF2 reached 48% by aqua regia solution, allowing the production of REEs solution for further precipitation of REEs products. Additionally, the process generates calcium fluoride and sulfuric acid solution as high-added value co-products.