Hybrid Mining of Rare-Earth Elements-Rich Mud and Polymetallic Nodules by Air-Lift Pump in Deep Sea around Minamitorishima Island

Polymetallic nodules containing manganese, nickel, cobalt, and other metals occur together with Rare–earth elements–rich mud (REE-rich mud) at 5500–5700 m water depth around Minamitorishima Island, Japan. We have proposed a multi–task type vertical transport system using an air-lift pump (an air-lift system) used for transporting both REE-rich mud and polymetallic nodules as a hybrid mining from the deep sea around Minamitorishima Island. Polymetallic nodules are fragmented and degraded due to collisions with the interior of the pipe, the separator, and neighboring nodules during transport. This degradation affects the performance of the air-lift system due to the change in the drag force of degraded nodules. We conducted experiments, i.e., a free-fall collision experiment and a free-settling experiment, to examine the degradability and the drag force of polymetallic nodules. From the free-fall collision experiment, we developed empirical equations that derive the parameter applied to estimate the degradation of polymetallic nodules. For a kinetic energy of 100 J/kg with collisions during transport, polymetallic nodules on the seabed with D50: 62 mm, was estimated to be those with D50: 25 mm on the mining ship. Also, the drag coefficient of polymetallic nodules and their fragments was evaluated by the free-settling experiment. It was found that the drag coefficient of polymetallic nodules increases as the degradation proceeds. The arithmetic average of polymetallic nodules on the seafloor was 0.75; that of the fragments was 1.33, on the other hand. Numerical simulations using the model of a pilot test showed that the production rate for transporting the fragments is about 10% higher than that of polymetallic nodules on the seabed, as those are. These estimations should be verified by analyzing data from a pilot test, which is scheduled for the near future.