A new operating concept for the Inco flash furnace was investigated. A supernatant coke layer would be added onto the surface of the molten bath to establish a reducing barrier between the slag and the freeboard. The partially oxidized sulfide mineral particles would thus be reduced while percolating through the coke layer to yield sulfur deficient (partially metallized) matte. The iron activity of sulfur deficient mattes is higher than that of regular mattes, and this coke barrier should lead to higher nickel and cobalt recoveries. The main challenge for the success of the new operating concept is to create conditions that optimize the rate of reduction without affecting furnace throughput and without requiring changes in furnace geometry. To achieve this objective it is necessary to develop an in depth understanding of the mechanism and basic chemistry of coke reduction of the flash flame products.Experiments were conducted in a miniplant flash furnace to investigate the feasibility and merits of this process scheme. The miniplant flash furnace test work also ties the results of graphite reduction tests with flash smelting operation. The test work suggests that coke reduction of partially oxidized sulfide mineral particles takes advantage of the fast reduction rate of matte, thus resulting in magnetite-coke reduction, with the production of metallic iron prior to the formation of slag. As a result, the reduction rate is optimized, sulfur deficient (metallized) matte is produced, and a significant improvement in matte/slag cobalt partition is achieved. All these objectives were met in the miniplant tests while flash smelting at a throughput per unit area of molten bath similar to that of the commercial Inco flash furnace.A complete investigation of the reduction mechanism was conducted utilizing all of the test results, thermodynamic calculation, samples of burner flame particles, samples of the matte and slag, SEM and XRD analysis, multiphase CFD modeling and TGA test work.Coke reduction of the flash flame products, i.e. partially oxidized sulfide mineral particles, was simulated by graphite-oxysulfide and graphite-matte reduction test work in this study. Graphite-slag reduction was also conducted for comparison. The graphite reduction tests were divided into two separate parts, X-ray visualization tests and crucible graphite reduction tests. The X-ray visualization tests confirmed that (1) oxysulfide and matte have much better wettability of graphite than slag; (2) the slag reduction product gas is discharged through a gas film on the surface of graphite; (3) the rate of reduction of oxysulfide and matte is visually faster than that of slag. The crucible graphite reduction tests measured the rate of graphite reduction of oxysulfide, matte, slag and a mixture of matte + slag at various temperatures utilizing an on-line gas analyzer. The results clearly show that the rates of reduction of matte and oxysulfide are several times faster than that of slag.