Sulfurization of Fe-Ni-Cu-Co alloy to matte phase by carbothermic reduction of calcium sulfate

Abstract

The calcium sulfate (CaSO4) was proposed as an alternative sulfur source to convert the Fe-Ni-Cu-Co alloy to matte phase. Solid carbon was used as a reducing agent and the influence of oxide fluxes on the sulfurization efficiency at 1673 K (1400 oC) under CO-CO2-SO2-Ar atmosphere was investigated. When the CaSO4 was equilibrated with the Fe-Ni-Cu-Co alloy without any reducing agent, it was reduced by Fe in liquid alloy, resulting in the formation of FeS. The sulfurization efficiency was about 56 pct, even though an excess amount of CaSO4 (gypsum equivalent, Geq = 1.7) was added. The equilibration time was significantly shorten from 36 to 3.5 hours and the sulfurization efficiency was highly increased from 56 to 91 pct by addition of solid carbon as a reducing agent, even though the amount of carbon was lower than the theoretical equivalent for carbothermic reduction of CaSO4, viz. Ceq = 0.7. Although the CaS (not FeS) was formed as a primary reaction product, it was continuously reacted by CaSO4, forming the CaO-rich slag. The carbothermic reduction time as well as the sulfurization efficiency was not affected by addition of Al2O3 (-SiO2) fluxes, whereas the equilibration time was more shorten to 2.5 hours by addition of Al2O3-Fe2O3 flux. This originated from the fact that the calcium silicate and the calcium aluminate, of which melting point is relatively high, were mainly formed in former systems, while the low melting point calcium ferrite was formed in the latter system. Consequently, the calcium sulfate (waste gypsum) can replace the expensive pure sulfur as a new raw material for sulfurization process of Fe-Ni-Cu-Co alloy with proper amount of carbon and the small amounts of iron oxide (Fe2O3) as fluxing material. And then, the non-ferrous by-products were applied as substitutes of oxide flux to reduce the process cost. The present results can be used for improving the recovery process of rare metals such as Ni and Co from deep sea manganese nodules