A Study on the Recovery Method of Rare-Earth Elements from REY-Rich Mud toward the Development and the Utilization of REY-Rich Mud

Abstract

Lu) に，同じ第 III 族の Sc，Y を加えた 17 元素の総称である。Sc は他のレアアースと地球化学的な挙動が若干異なるため，本論文 ではランタノイド 15 元素に Y を加えた計 16 元素を REY (rare-earth elements and yttrium) として取り扱う。また，La から Eu までを軽 レアアース (light rare-earth elements: LREE) ，Gd から Lu までを 重レアアース (heavy rare-earth elements: HREE) と区分する。 REY はハイブリッドカーや風力発電の発電機に使われる高性 能高効率モーター，LED・三波長型蛍光灯などに代表される省エ ネルギー製品など，特に我が国がリードするグリーンテクノロ ジー産業には欠かせない資源である。しかし，REY の生産を独 占する中国は近年資源ナショナリズムを強めており，それに伴 う価格高騰や供給不足が顕在化している。このような中，Kato et al. (2011) 1) によって，REY を極めて高濃度で含有する遠洋性深 海堆積物 (レアアース泥 ：REY-rich mud) が，太平洋に広く分布 することが報告された。レアアース泥は， (1) REY，その中でも 特に我が国の産業が必要とする Dy や Tb などの HREE や Eu を 高濃度で含むこと， (2) 開発の障害となる Th や U をほとんど含ま ないこと， (3) 海底面に堆積物として層状に分布しているために 資源量が膨大で，かつ探査が容易であること，さらに， (4) 常温 で短時間，希酸に浸すだけで含有されるほとんどの REY を堆積 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 deep-sea 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, H 2 SO 4 , HNO 3 , NaOH and (NH 4) 2 SO 4) 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.