Each ZnS grain roasts by the classic shrinking-core model: oxygen diffuses through the porous ZnO shell that has already formed and reacts at the shrinking unreacted-sulfide surface:
2 ZnS + 3 O2 → 2 ZnO + 2 SO2
For reaction-controlled shrinking cores, the unreacted core radius rc obeys
1 − (1−X)^(1/3) = k·t / r0
k = k0 · [0.3 + 0.7·(O2/100)] · exp(−Ea / R·T)
so conversion X (desulfurization) climbs fastest for small grains (low r0), hot beds and oxygen-rich blast — exactly the temperature and oxygen sliders above. A grain's soluble-Zn content tracks its own core conversion, because the ZnO shell is what a dilute-acid leach dissolves; unconverted ZnS at the core is not.
Push the bed too hot, though, and some of that freshly-formed ZnO reacts with iron-oxide gangue instead of staying as leachable oxide:
ZnO + Fe2O3 → ZnFe2O4 (zinc ferrite, acid-insoluble)
- Higher temperature / more oxygen — faster shrinking core, higher desulfurization, more SO2 for the acid plant — but past ~1300 °C, ferrite formation starts eating into soluble Zn even as desulfurization keeps climbing.
- Higher-grade concentrate — less iron gangue per tonne of ore, so the same over-firing produces less ferrite; richer feed is more forgiving of a hot bed.
- Watch the gap between the desulfurization and soluble-Zn readouts: a wide gap at high temperature is ferrite loss, not unreacted sulfide.