Minerals and Metals in Ancient India v1.c26.s6.t4.2

v1.c26.s6.t4.2

The phases identified by X-ray and electron diffraction studies (Biswas et al., 1984: 236) are summarised in Table 18.5. The results show that in the roasting operation prior to retort distillation, a small part of sphalerite was not converted into oxide and remained in the retort as ZnS and ZnSO₄. The presence of goslarite or ZnSO₄, 7H₂O (hydrated at a later stage) was confirmed by the endothermic DTA peaks apart from X-ray and electron diffraction studies. Some ZnO might have remained unconverted in the retort to undergo atmospheric conversion to basic carbonate at a later stage. While most of the ZnO was reduced to metal, a part of it must have been converted at high temperature to phosphate and silicates from which the metal could not be recovered. Biswas et al. (1984) detected a number of zinc-, calcium-zinc, lead-zinc, calcium-lead-zinc and magnesium-aluminium silicates. Later, Freestone et al. reported (1985b: 7) that lead slag contained iron-, calcium-iron, calcium-zinc, magnesium and calcium-magnesium silicates (vide chapter 17). The only silicate phase that Freestone et al. could report in the retort residue was CaMgSi₂O₆ or diopside.