Minerals and Metals in Ancient India v1.c26.s6.t2: Thermodynamics of Smelting
Minerals and Metals in Ancient India v1.c26.s6.t2
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Thermodynamics of Smelting
v1.c26.s6.t2.1
Before we present the composition of the residue in the ancient retorts (left after recovery of zinc by distillation) let us briefly discuss the physico-chemical route possibly adopted in the ancient zinc-smelting process at Zawar.
v1.c26.s6.t2.2
In the Zawar mines, zinc occurs mainly as sphalerite, ZnS, although there are other zinc minerals such as smithsonite, ZnCO₃ etc. in minor proportions. The host rock is dolomitic and arkosic (quartz and feldspar) containing a variety of sulphide minerals such as sphalerite, galena PbS, pyrite FeS₂, pyrrhotite Fe₁₋ₓS, chalcopyrite CuFeS₂, argentite Ag₂S, silver etc. (Straczek et al., 1967)
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.
v1.c26.s6.t2.4
Zinc melts at 432°C and under one atmospheric pressure boils at 913°C, which is below the temperature required to reduce the oxide to metal in the presence of carbon. Though the initiation of reduction of zinc oxide to zinc by carbon begins at 950°C, the minimum temperature necessary for successful extraction of the metal in a retort is of the order of 1150-1200°C. The reaction ZnO + C → Zn + CO Δ H = - 57 kcal. is highly endothermic. Therefore, before the advent of modern high-pressure reduction technology or electrolytic extraction processes, zinc had to be produced inevitably as vapour around 1200°C and then condensed as liquid much below 913°C.
v1.c26.s6.t2.5
Furthermore, to overcome re-oxidation of zinc vapour, the partial pressure of oxygen had to be kept very low. The obvious method of doing this was to use excess of charcoal or other suitable carbonaceous matter in the retort to maintain a strongly reducing atmosphere and excess of carbon monoxide. Temperature-pO₂ relation *Fig.* 18.5 shows that the production of metallic zinc requires extremely reducing conditions below the pO₂ required to reduce wüstite (FeO) to metallic iron. Some metallic iron has indeed been detected in the spent charge. Taking mole fractions of 0.05 FeO and 0.10 ZnO in the glassy portion of the spent charge and making simple thermodynamic assumptions, it has been found (Freestone 1985a: 238) that the FeO-Fe and ZnO-Zn reactions are displaced to the dashed curves (*fig.* 18.5). Both the FeO and ZnO contents of the glassy portion of the residue are consistent with operation of the process 1-2 log units of pO₂ below the Zn-ZnO reaction curve, within the temperature range of the interest.