As an answer to the increasing demand for a stable supply of rare-earth element and yttrium (REY) , Kato et al. (2011) reported the discovery of "REY-rich mud" which has high REY concentration and is distributed widely on a deep seafloor in the Pacific Ocean. In addition to the high REY concentration, the REY-rich mud has great advantages as a mineral resource; (1) its exploration is easy to conduct due to its stratiform distribution as a deepsea sediment, (2) the concentration of radioactive elements such as U and Th which inhibit the development of mineral resource is very low, and (3) the most of REY can be easily extracted/recovered by using dilute acids for a few hours. In this study, we investigate the most appropriate recovery method of rare-earth element from the REY-rich mud by chemical leaching method toward its industrial development and usage in near future. The chemical leaching experiments were performed by five kinds of reagents (HCl, H2SO4, HNO3, NaOH and (NH4) 2SO4) under various leaching time, temperature and leachate concentration to aim the highest recovery ratio. The experiment showed that strong acids such as hydrochloride acid and sulfuric acid are appropriate leachate. Our results also showed that the recovery ratio is at the highest under the condition of the minimum required leachate concentration, minimum required leaching time and normal temperature. In our experiments, the recovery ratio except Ce from JMS-2 represents 88 % in case of hydrochloride acid (0.5 mol/L, 20min, 25 ℃, water-rock ratio 10:1) and 82 % in case of sulfuric acid (0.2 mol/L, 10min, 25℃, water-rock ratio 10:1) . The high recovery ratio of REY (80 - 90 %) even with dilute leachate under low temperature in a short leaching time is a strongly attractive characteristics of REY-rich mud as a new seafloor mineral resource.
