The paper presents the results of rare metal collection experiments conducted off the shores of Okinotorishima (Okinotori Islands) and Iriomotejima (Iriomote island), Japan, in 2007 and 2008. The adsorbent materials used in the experiments were developed originally for uranium by the Japan Atomic Energy Agency and for lithium by the National Institute of Advanced Industrial Science and Technology. The purpose of this study is to investigate various factors affecting the adsorption efficiency of rare metals from seawater, e.g., soaking duration, seawater temperature, depth in seawater, and mooring system for adsorbent components. The results presented in the paper focus primarily on the adsorption efficiency involved with uranium and lithium.
About 70% of all elemental ions can be found in seawater, albeit at low concentrations. The concentration of typical rare metals in seawater is shown in Table 1. The increasing worldwide demand for rare metals is being driven by the economic development of emerging countries as well as by hightech industrialization, e.g., lithium for high-power/long-lasting batteries, uranium for nuclear electric power generation, vanadium compounds for catalysts, and molybdenum additive for alloy steels. For the last two decades, the Japan Atomic Energy Agency and the National Institute of Advanced Industrial Science and Technology have been studying adsorbents for the extraction of uranium and lithium, respectively, from seawater. Various adsorbents have been developed, and rare metal extraction efficiencies have improved.
Since previous studies by the Japan Atomic Energy Agency have shown that higher seawater temperatures result in greater rare metal collection efficiencies, experiments were conducted in May 2007 in Okinotorishima (Okinotori Islands), the southernmost island in Japan (1,740 km south of Tokyo), and from November 2007 to January 2008 in Iriomotejima (Iriomote island), located 2,100 km southwest of Tokyo, as shown in Fig. 1.