Minerals and Metals in Ancient India v1.c26.s6.t2.5

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.