HYDROMETALLURGY METHOD OF REY-RICH MUD - CHEMICAL LEACHING AND SEPARATION -

Kato et al. (2011) reported the discovery of "REY-rich mud" which has high REY (lanthanoid and yttrium) content and is distributed widely on a deep seafloor in the Pacific Ocean. In 2013, mud containing more than 5,000 ppm total REY was found within the Japan’s exclusive economic zone (EEZ) around the Minamitorishima Island (Marcus Island), northwestern Pacific Ocean during research cruise KR13-02 of R/V Kairei. Due to its “extremely high” resource potential, researches for the development system, hydrometallurgical process, and economical evaluation are ongoing through collaboration between industry, academic, and administration.

In this presentation, we report the hydrometallurgical method of REY-rich mud. We conducted a series of chemical leaching experiments on highly REY-rich mud (about 3,000 ppm REY content), varying the acid concentrations, leaching times, and temperature to determine the optimum conditions of REY leaching from REY-rich mud. The highest leaching efficiency of REY other than Ce was 95.1% using hydrochloric acid and 81.3% using sulfuric acid. Extraction efficiency of REY was highest under conditions of relatively low acid concentrations (0.25–0.5 mol/L), short leaching times (2–5 min), and room temperature (25°C). REY extraction levels decreased with increasing acid concentration, leaching time (only in the case of sulfuric acid), and temperature, apparently because of precipitation of calcium sulfate, rare earth phosphate, and rare earth–Na double sulfate. We also conducted chemical leaching experiments with oxalic acid. By using oxalic acid, whole hydrometallurgical process, the extraction of rare earth element from mud and the recovery as oxalate, could be performed in one step and significantly reduce the cost. Our experimental data shows that most of the biogenic calcium phosphate in REY-rich mud, the main host minerals of rare earth elements, was dissolved in oxalic acid. However, rare earth elements are co-precipitated with calcium oxalate, and we cannot separate rare earth oxalate from the precipitation. In this presentation, we also present the results of some experiments aiming the improvement the separation efficiency of rare earth oxalate from the precipitation.