Polymetallic nodulesPolymetallic nodules containing useful metalsMetals such as Co, Ni, Cu, and Mn are widely found on the seafloor at water depths ranging from 4000 to 6000 m. SmeltingSmelting of these nodules produces matteMatte with a Ni, Co, and Cu content of 30%, 3%, and 20%, respectively, which is difficult to process in Ni matteMatte refineries owing to the high Ni-to-Cu ratio. Hence, this matteMatte needs to be separated into Cu-rich and Co–Ni-rich mattesMatte to be processable at existing smeltingSmelting and refining facilities. This can be performed via a slow-cooling process for liquid–liquid separation and subsequent growth of separate matteMatte grains for their easy separation into the two types of matteMatte. This paper describes the investigation of the effect of the cooling rate and the composition of the starting matteMatte, which was simulated to match that expected for a matteMatte obtained from polymetallic nodulesPolymetallic nodules, on the matteMatte obtained via slow coolingSlow cooling. The precipitates were roughly classified into three phases: copperCopper sulfide, nickel sulfideNickel sulfide, and mixed sulfideMixed sulfide phases. The grain size of the copperCopper sulfide phase increased with decreasing cooling rate. The results suggest that the matteMatte can be separated into multiple concentrates via mineral processingProcessing.