Minerals and Metals in Ancient India v1.c26.s6.t5.9

v1.c26.s6.t5.9

The assumption made by Freestone et al. meant that the retort charge before smelting assayed more than 30 per cent in zinc and metal recovery therefrom was of the order of 90 per cent. We do not know how far this assumption can be justified. There are evidences of careful manual hand-picking and benefication of zinc-rich and lead-rich portions of the rock. (Brooke, 1850: 214) The zinc/lead ratio of the retort charge must have been considerably higher than the average. But it is difficult to believe that the retort charge in the pre-modern period could approach in purity the concentrate (zinc 52 %) now being produced in the modern plant using twentieth century technique of froth floatation. The doubt arises because there are high percentages of insoluble (silica), calcium and magnesium in the retort residue which resembles the ore and the tailing rather than the concentrate in this regard.