Rare earth elements (bio)leaching from zircon and eudialyte concentrates

Abstract

Sands containing placers enriched with heavy minerals such as zircon, garnet, ilmenite, rutile, magnetite and monazite are recovered as construction material from the German Baltic Sea coast. The heavy minerals could be concentrated by gravity and magnetic separation. Rare earth elements (REE) were enriched in zircon concentrates as well as in concentrates of the zirconium-containing mineral eudialyte from a deposit in Greenland. In order to test bioleaching for REE extraction from these concentrates, experiments with cultures of mesophilic as well as moderately thermophilic, acidophilic, lithoautotrophic bacteria and gluconic acid-producing organoheterotrophic bacteria were run in shake flasks or pH-controlled 2 L bioreactors at 30 °C or 42 °C. Elemental sulfur and glucose served as substrate for the autotrophs and heterotrophs, respectively. Chemical leaching experiments as controls for bioleaching by the autotrophs were run without bacteria and adjusted over time to the same pH by sulfuric acid addition as previously measured in the inoculated bioreactors, in order to estimate bioleaching versus chemical leaching at the same pH. Chemical leaching experiments as controls for bioleaching by the heterotrophs were run with added gluconic acid as well as other organic acids. The results showed no significant differences in REE extraction between chemical and biological runs under same conditions (pH, temperature or gluconic acid concentration), indicating that bioleaching was in fact acid leaching based on the enzymatic oxidation of elemental sulfur to sulfuric acid in case of the autotrophs and gluconic acid production from glucose in case of the heterotrophs. The amount of extracted REEs increased with decreasing pH. Mesophiles versus moderate thermophiles did not show remarkable differences. The extraction efficiency for the single REEs was maximum 35% for zircon concentrates but up to about 70% for the eudialyte concentrate. This study allows for a comparison of chemical leaching vs. bioleaching and shows the limitation of bioleaching for processing heavy minerals for REE extraction.