Making Deep Sea Mining Cleaner and Greener

This paper describes concepts to minimize the plume generated by unwanted sediments collected along with manganese nodules during hydraulic mining operations. The concept consists of two novel technologies: separating all sediment from the collected nodule slurry to eliminate sediment from entering the riser and lift system, thereby reducing, or eliminating a midwater plume, and subsea electrocoagulation (EC) to create rapidly settling flocs of sediment being discharged from the seafloor collector. The first approach involves designing a gravity separator (hopper) whereby the larger particles fall through an outlet and are entrained with clean water before entering the riser, while all the sediment and water collected with the nodules exits at the hopper overflow. To prevent sediment from being entrained with the nodules, a "reverse hydrocyclone" and secondary hopper is incorporated in the underflow circuit to help maintain a positive pressure differential between the riser inlet and the hopper during operations. The second approach employs the marinization of proven wastewater treatment EC technology to create large metalliferous flocs which attract the sediment causing rapid settling when the slurry is discharged aft of the collector. Work reported here includes numerical (CFD) confirmation of the separator performance, and experimental demonstration of the EC application, as well as techno-economic assessments of the cost impacts of implementing these technologies. This work was supported by the U.S. Department of Energy, Advanced Research Projects Agency – Energy (ARPA-E), and with contributions of sediment for the EC tests by The Metals Company from the Clarion Clipperton Zone and Ocean Minerals, LLC from the Cook Islands EEZ.