History of Technology in India, Vol. I: From Antiquity to c. 1200 A.D. v1.c13.s2.t3.44

v1.c13.s2.t3.44

Thus we see that the production of copper from Chalcopyrite is a two step process involving precise control of the thermochemistry. Moesta and Schlick (1989) and Yazawa (1980) have discussed the thermodynamic limits of the roasting and reduction of copper sulphide in great detail. Fig. 7 gives the phase diagram for the Cu-Fe-S-O system as calculated by Yazawa (1980). These studies have confirmed that for dead roasting a temperature higher than 850°C is required and during this process iron acts as sulphur scavenger helping the conversion of CuS into Cu2O and CuO, and during reduction FeO reacts with SiO2 to form 2FeO.SiO2 (Fayalite) slag which has lowest melting point of 1177°C. The calculations have also shown that although at 1000°C copper oxides gets reduced to metal but the slag phase remains solid, and that molten metal separates out only at temperature above 1250°C. Based on the occurrence of 1.08 to 1.88% iron in the copper objects from Ahar, Chakraborti (1983) and Tripathi (1985) have formed the opinion that copper and iron oxides can be simultaneously reduced easily, and in presence of copper the reduction of iron oxide is possible only at very low pressure of oxygen (log PO2 = 10-3 atmosphere) normally not possible to attain in the low shaft furnace. Maddin (1982) and his group have conducted some experiments on the ancient reduction technology and they have found that during the reduction of roasted Fig. 6. Photograph showing an ancient copper mining (a) trench, (b) mouth of shaft at Surda Mines, Singhbhum Fig. 7. Cu-Fe-S-O equilibrium diagram (at 1300°C) indicating the instability of metallic iron in presence of CuO, and ultra low oxygen pressure required for its production