Fines produced from degradation of polymetallic nodules: implications for deep-sea mining

Polymetallic nodules in abyssal plains have long attracted attention as potential resources for some critical metals. Thier exploitation may cause a serious disturbance to the marine environment, and proper environmental impact assessments must be made beforehand. Discharge of the fluid-particle mixture tailings after hydraulic lifting is one of the poorly understood disturbances in the mining of polymetallic nodules, with fines generated by collisions between nodules and pipe walls making up an important part of the discharge. In order to better understand the formation process and properties of these nodule fines, degradation experiments were conducted on two types of polymetallic nodules from the Clarion-Clipperton Fracture Zone, eastern Pacific. During the experimental degradation in a planetary ball mill, the portion of fine particulates in the size of several to few tens of micrometers gradually increased. This confirms that hydraulic lifting will generate significant amount of nodule fines that are too small to be recovered and are likely to be discharged into the ocean. Processes such as abrasion and attrition are likely responsible for their production. The fines generated have higher contents of Al, K, and Fe and lower contents of Mn, Co, Ni, As, Mo, and Cd compared to the average composition of the original nodules. Release of sediment paticles encapsulated in the nodules is largely responsible for the difference, but uneven contribution of ferromanganese minerals and irregular behaviors of some elements such as Pb were also observed. The collected results show that the fines generated during lifting of polymetallic nodules are not the same as simple nodule powder and have significant compositional differences depending on the particle size, and a more detailed approach is needed for the environmental impact assessment.