Removal of impurities from copper and its alloys using ancient refining techniques

Abstract

There were two major objectives of this study both related to purification of impure copper metal using primitive refining techniques believed to have been used in antiquity. First part of the study was to remove iron impurity from bronze that is introduced when iron containing tin (hard-head) is used as source of tin. In the second part, simple refining methods are applied to highly impure copper metal obtained by smelting polymetallic copper ores. "Hard-head" which is a mixture of iron-tin compounds are biproduct of tin smelting process where cassiterite is contaminated with iron oreas. It was not possible to remove all of the tin held in hard-head and some tin is inevitably lost. Hard-head prills were prepared composed of 20 per cent iron ans 80 per cent tin under controller conditions, in clay crucibles. The hard-head prills were then added onto the molten copper metal in the furnace at 1200C. since all compositions of Sn-Fe compounds at 1200C and Cu and Fe does not make an alloy, tin is expected to be extracted by copper while rejecting the iron from the medium. The iron was then oxidized selectively either by the addition of natural fluxes (powdered marble and sand) or by fire-refining method. Experiments were carried out and it showed that, under addition of suitable fluxes, it was possible to remove over 85 per cent iron present in hard-head, leaving the concentration of iron in bronze under 0.5 per cent. The loss of tin, however was less than 20 per cent. It is concluded that unrefinable hard-head can be economically used in the production of bronze. Most primary copper ores are polymetallic in character and contains iron, lead, arsenic, nickel, antimony and zinc in varying amounts. There is ample evidence that such ores were smeltedtogether in antiquity yielding highly impure copper metal containing Pb, Zn, Sb, Ni, and Fe. To use such copper, it must be excessively refined. Fire refining methods were applied on the purification of impure copper that contains 20 per cent Pb, 5 per cent Zn, 5 per cent Sb, 2 per cent Ni, and 3 per cent Fe. Without any flux and using only air blast on the molten alloy, it was possible to remove about all of iron and zinc, and up to 50 per cent of the Pb, Sb, and Ni.