Minerals and Metals in Ancient India v1.c26.s6.t2.3

v1.c26.s6.t2.3

It is necessary to roast sphalerite before it can be smelted, because sphalerite is not decomposed in a reduction reaction. Roasting sphalerite to zinc oxide 2 ZnS + 3 O₂ → 2 ZnO + 2 SO₂ is a partly endothermic reaction requiring a temperature of the order of 900°C. As soon as the oxidation reaction sets in, sulphur in the ore supports further combustion. However, to produce a dead roast, free from sulphur, and also to decompose any zinc sulphate ZnSO₄ formed in the reaction, a higher temperature of the order of 1000°C is necessary. We detected in the retort residue not only a little sulphur (0.35%) but also some ZnSO₄, indicating that zinc ore had been roasted, but not to a dead heat, prior to reduction-smelting in the retort. (Biswas et al., 1984) Barnes (1973) described a medieval method practised in Persia in which the ore used to be roasted at temperature in excess of 1000°C to obtain pure zinc oxide sublimate, free from Ag, Fe and Pb etc., depositing on iron or ceramic frame above the fire. The zinc oxide sublimate was known as tutia in the Persian language and the metal zinc obtained from tutia as tutenaga. The high Ag, Fe and Pb contents as well as presence of sulphur and ZnSO₄ in the retort residue indicate that the Zawar process was one of simple roasting and not the Persian method.