Optimized Conditions for Extracting Fe–Ni–Co–Cu Alloy from Polymetallic Nodules

Polymetallic nodules are strategic resources rich in metal elements, such as Mn, Fe, Ni, Co, and Cu. However, the low Fe contents in the nodules require high-temperature melting processes to separate the alloys from manganese slag. Herein, iron oxide was added to aggregate Ni, Co, and Cu for effective and convenient extraction of Ni, Co, and Cu from polymetallic nodules, as well as generate Fe–Ni–Co–Cu alloy and manganese-rich slag at lower temperatures. The crystal phase transformations of slag and liquidus temperatures were predicted by thermodynamic studies. The data revealed an improved slag phase by increasing the FeO content in the slag. With the decrease of Mn/Fe ratio in raw material, the liquidus temperature first declined and then increased. The diameter of alloy particles first rose and then declined as a function of carbon content. At the carbon-to-oxygen ratio from 0.6 to 1.0, the diameter of alloy particles increased from 200 μm to 3000 μm, respectively. Samples without iron oxide displayed mostly aggregated Ni, Co, and Cu in the iron phase, but were still attached to the manganese slag. The addition of iron oxide to an Mn/Fe ratio of 1.6 resulted in numerous large-diameter alloy particles with a slag phase composed of MnO, FeO, and tephroite. The most effective separation of slag and metal was recorded at Mn/Fe ratio of 1.6, carbon-to-oxygen ratio of 1.0, and reduction temperature of 1250 °C, Ni, Co and Cu are almost completely separated from manganese minerals. Overall, new insights into low-temperature extraction of Fe–Ni–Co–Cu alloy from polymetallic nodules were provided, promising for future low-temperature metal recycling. Graphical Abstract