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Recovery of rare earths from waste cathode ray tube (CRT) phosphor powder with organic and inorganic ligands

Alvarado-Hernández, L., Lapidus, G.T, González, Federico
Waste management 2019 v.95 pp. 53-58
ambient temperature, cathodes, encapsulation, leaching, ligands, oxides, pH, phosphates, powders, rare earth elements, roasting, sodium hydroxide, solubility, thermodynamics, waste management, wastes, zinc, zinc sulfide
Cathode ray tubes (CRTs) have an appreciable amount of rare earth elements (REEs). In this document, the leaching and recovery of the REEs from CRTs, with different organic and inorganic ligands is presented. Among the complexing agents tested, the pyrophosphate ion was found to be the most advantageous for the extraction of REEs from CRTs, as an alternative to the traditional methods that use highly acidic solutions (pH < 1) and elevated temperatures. Thermodynamic analyses predict the formation of soluble REE-pyrophosphate complexes in a pH range of 2–8. Leaching solutions of 0.1 M Na4P2O7 at pH 6 and room temperature were employed. REE dissolution from the untreated CRT powder under these conditions was extremely low, due to the encapsulation by other components in the powders, such as ZnS (26%), and the high content of phosphates (6%), that severely limited the solubility of the REEs. To increase extraction, pretreatments were employed to alter and remove the passivating species: roasting at 800 °C or contact with concentrated solutions of sodium hydroxide at 95 °C. The combination of these pretreatments completely eliminated the Zn and 79% of the phosphate ion, as well as other base metals, resulting in an improved exposure of REEs for subsequent leaching. Extractions for Y, Eu, Sm, and Ce of 58, 90, 90 and 87%, respectively, were achieved with the pyrophosphate solution at ambient temperature. The REEs were later recovered as oxides by adjusting the solution pH to 11. Subsequently, once the pyrophosphate solution pH is reestablished, it may be reused for leaching.