With sulphuric acid I think it should increase with the temperature, as the water is displaced from the reaction and the diffusion velocity from the solid also increases.
When carbonated ore is used, calcine is produced through Waelz process and the following clinkerization process. Calcine obtained by both methods are processed through same hydrometallurgical zinc recovery phases. Zinc recovery from calcine by hydrometallurgical method is conducted by dissolution (leach), solution cleaning (precipitation + cementation), electrolysis etc. The zinc in the calcine which is produced as a result of roasting and clinkerization processes is partially in the form of zinc oxide (ZnO) and zinc ferrite (ZnFe2O4). While zinc oxide can easily be dissolved in sulphuric acidic solutions, high temperatures (90-95C) and a high sulphuric acid concentration (50-150 g/L) is required for dissolution of zinc ferrite. Reactions:
ZnO + H2SO4 = ZnSO4 + H2O
ZnFe2O4 + 4H2SO4 = ZnSO4 + Fe2(SO4)3 + H2O
if PbO exist;
PbO + H2SO4 = PbSO4 + H2O
Due to the fact that zinc oxide dissolves in low temperature and in low H2SO4 concentration while zinc ferrite dissolves in high temperature and in high H2SO4 concentration, leaching process of calcine is conducted at two stages. These stages are named as neutral leaching and hot acid leaching.
When sulfide ore is used, In this leaching system, ZnS and sulfate inter-reacted and generated S, ZnSO4, H2O, etc. In the absence of oxygen, the partial pressure of H2S formed slows down the kinetics of the leaching reaction. The addition of oxygen in the system reduces this effect. The leaching temperature is an important thermodynamic parameter. Generally, with the increase in leaching temperature recovery of zinc and sulfur increase (for pressure leaching of ZnS, temperature :120-160 oC and also, oxygen partial pressure is an important factor.). Reactions:
ZnS + H2SO4 + 1/2O2 = ZnSO4 + S + H2O
ZnS + H2SO4 = ZnSO4 + H2S
H2S + Fe2(SO4)3 = 2FeSO4 + H2SO4 + S
ZnS + Fe2(SO4)3 = 2FeSO4 + ZnSO4 + S
2FeSO4 + H2SO4 + 1/2O2 = Fe2(SO4)3 + H2O
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