Seafloor sediment rich in rare earth elements and Y (REY-rich mud) has received attention as a new resource for REY. During research cruise KR13-02 of R/V Kairei, mud containing more than 5,000 ppm total REY was collected near Minamitorishima Island, northwestern Pacific Ocean. We conducted a series of chemical leaching experiments on this material, varying the acid concentrations, leaching times, and temperature to determine the optimum conditions of REY leaching from REY-rich mud. The highest extraction 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 amount 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. Although further research is needed, the leaching properties we determined are generally favorable for industrial exploitation of REY-rich mud.
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