An examination of the technical and economic feasibility of the combined mining of the rare-earth element-rich mud (REE-rich mud) and ferromanganese nodules (FN) around Minamitorishima (Marcus) Island in Northwest Pacific is introduced. A previous study showed that the mining of REE-rich mud around Minamitorishima Island was not economically feasible. Therefore, in this study, three changes from the previous mining model to improve its economy are proposed. The first one is combined mining with FN in the area. The second one is introducing a pulp-lifting system that can lift both REE-rich mud and FN at high concentrations through a riser pipe. The third one is the reuse of waste mud and processed slag for construction materials. The economic evaluation results show a change from a slightly negative to quite positive economy depending on the mixing ratio of REE-rich mud and FN in the pulp-lifting. In addition, some technical approaches necessary to realize the combined mining method are introduced.

Ferromanganese nodules have been recognized as a potential future metal source for over 50 years. Many research and development efforts have been conducted by many organizations. Most of the efforts have been concentrated into the mining technologies especially for hydraulic lifting through riser pipes with bulk-scale nodule collector. However, no commercial mining venture exists. Uncertainty in the economy of nodule mining is considered to be the reason for this. In order to improve the economy, a mining subsystem based on mechanical lifting and small-scale collectors is proposed and the preliminary economic feasibility is examined in this study. The benefit was at a favorable level compared with that using hydraulic lifting with bulk-scale collector. From the viewpoint of environmental impact assessment, environmental considerations of deep-sea sediment plume are explained.

Rare-earth mud and manganese nodules coexist on the seafloor around Minamitorishima Island. To investigate the feasibility of a pulp-lift system that can ensure economic efficiency by pumping manganese using rare-earth mud as the working fluid, we conducted pulp-lift experiments at a head of 5.0 m using a squeeze pump for mortar pumping. In the study, we used carboxymethylcellulose (CMC) as the working fluid, which is a pseudo-plastic fluid like deep muddy seawater. We investigated the effects of the viscosity of the working fluid and the pump pulsation characteristics on lifting. The results revealed that the drag force acting on the pumped ore increased as the fluid became more viscous and the pulsation period became higher, while the reverse flow rate increased due to the negative pulsation pressure. This suggests that there is an optimum value for the viscosity of the working fluid.