Kinetics of iron–copper sulphides oxidation in relation to protohistoric copper smelting

This article deals with one specific step of the copper extractive metallurgy process: the roasting of iron–copper sulphides. It aims at shedding light on an archaeological issue: the reconstruction of the copper extractive metallurgy processes during protohistory (IVe–IIe millennium BC). Experimental simulations are performed at laboratory scale by modelizing the conditions of protohistoric furnaces. Kinetic of roasting is studied by thermogravimetry combined with the physico-chemical analysis of synthetic products. The influence of two parameters is studied: (i) the temperature (773, 973 and 1173 K) and (ii) the granularity of the roasted ores (1 mm and 100 μm). In each case, the chemical mechanism governing the oxidation of iron copper sulphide is proposed. Apart from one extreme case (∅ = 1 mm; T = 773 K), it is showed that kinetic is controlled by the transport of molecular oxygen (O2) from the gas to the grain surface. Moreover, we prove that, in some cases where the diffusivity of gaseous oxygen is low, roasting can be accelerated by the presence of an oxide, which constitute an in-situ source of oxygen. Theses experiments support the hypothesis that such a technique could have allowed a roasting process where iron and sulfur were removed by the solid oxygen instead of the gaseous oxygen. These results allow to validate a one-step copper smelting process starting from sulphidic ores, and to identify the experimental parameters of this process.