The Effect of Iron- and Calcium-Rich Waste Rock’s Acid Baking Conditions on the Rare-Earth Extraction

Abstract

The work investigates the effect of sulfuric acid baking on rare earth element (REE) extraction from two waste rock samples from a phosphate mine. The role of different mineralogical assemblages and the degree of alteration (i.e., weathering), and the behavior of the main impurities–iron, phosphorus, and calcium–on REE extraction are emphasized. For both samples, the sulfuric acid baking at 25 °C, during 15 min, H2SO4:sample (w/w) mass ratio of 0.45:1 is the best condition for achieving the selective leaching of REE. For the iron rich-sample, the increase in temperature reduces REE extraction and increases iron dissolution. The corresponding sulfuric acid consumed by goethite (α-FeOOH) from 25 °C to 160 °C acid baking is ten times higher than that required for the monazite (REEPO4) reaction. Conversely, higher REE and lower iron extraction are observed by increasing (sulfuric acid/sample) mass ratio (0.95:1). Due to the high sulfuric concentration during dissolution, a local saturation zone close to the dissolution front caused the precipitation of iron oxyhydroxides. The calcium-rich sample shows lower REE extraction by leaching (63% maximum) mainly due to the entrapment of REE-bearing minerals by a gypsum layer, and lanthanide’s uptake by calcium sulfate compounds formed during leaching. The results were discussed with the help of a detailed characterization of the residues.