A charge of molten Ni-Cu-Fe sulfide matte (≈35% Fe, 30% S, 36.5% Ni+Cu) sits in the vessel. Oxygen-enriched air blown through submerged tuyeres drives, in order:
2 FeS + 3 O2 → 2 FeO + 2 SO2 (slag-forming blow — iron leaves first)
FeO + SiO2 → FeSiO3 (fayalite slag, needs the silica flux)
2 Cu2S + 3 O2 → 2 Cu2O + 2 SO2 (finish blow, only once Fe is gone — undesired if it runs too long)
Iron sulfide has a far higher affinity for oxygen than the nickel and copper sulfides, so while Fe remains it oxidizes preferentially and the freed FeO floats off as slag — this simulation removes Fe mass (and its bonded sulfur) at a rate set by the oxygen slider. Once Fe hits zero the blow enters a finish stage that strips the remaining sulfur toward high-grade "white metal"; keep blowing past that and the oxygen starts consuming Ni and Cu themselves, converting valuable metal into slag — an overblow, the operator's main yield risk.
The silica flux ratio controls slag fluidity: at 1.0× the FeO drains cleanly. Away from 1.0× the slag turns viscous or over-diluted, slowing effective iron removal and trapping (entraining) more metal that is lost whenever the slag is skimmed.
- Start blow — begins the O2 flow through the tuyeres.
- Oxygen enrichment — sets the blow rate and the exotherm driving bath temperature up.
- Silica flux ratio — 1.0× is nominal; drift either way costs efficiency and metal.
- Skim slag — removes the floating slag layer; a small entrained-metal loss goes with it.
- New charge — dumps this batch's finished matte and loads a fresh charge.