Modification of High Water Content Sediment for Rare Earth Mining in Deep Sea by Surfactants Agents

In recent years, the importance of rare mineral metal resources is increasing and a lot of investigations have been conducted all over the world in order to obtain new sources of rare metal resources. As a result, it was discovered that abundant rare-earth element-rich mud exists on the deep-sea floor in the Japanese Exclusive Economic Zone (EEZ). As one of the effective mining methods, a suction mining method is expected to be applied to the seabed mining. The seabed sediment containing rare-earth element shows the very high water content, more than 100% and the sediment movement during the suction is greatly affected by the water content and liquid limit of the material. In the laboratory testing, we investigate the relationship between the suction behavior of the sediment and these water content and liquid limit and found that the amount of suction of sediment and seabed deformation was described by these two parameters. Therefore, it is important to modify the liquid limit by adding chemical agent to control the sediment behavior. We selected eight different surfactants which can be divided into three types. They are dispersant type, water retention type, and thickener type. We carried out liquid limit test and viscosity measurement of the sediment mixed with the agents. It is found that the water-absorbing polymer increases the liquid limit and viscosity, whereas anionic/cationic surfactant decreases the liquid limit and viscosity. It is possible to control the sediment behavior by adding suitable surfactants.