Characterization of an iron smelting slag from Zimbabwe by Raman microscopy and electron beam analysis

A sample of archaeological iron slag from Northern Zimbabwe was characterised by Raman microscopy (RM) and electron beam analysis (EBA). The phases identified in a slag found at an ironmaking site not only reflect the chemical composition of the slag but can also provide crucial information regarding the reconstruction and interpretation of the metallurgical operations, such as the prevailing redox conditions in a furnace at the time of cooling. Free iron oxides, such as haematite, magnetite and wustite, are important indicators of these redox conditions. But while classical techniques of phase identification can be used to identify the different iron oxides, they cannot distinguish between dissimilar oxidation states due to their non-stoichiometric nature, and also the effect of substituting elements in iron oxides. RM proved invaluable in both respects. By combining the results provided by RM and EBA, it was possible to identify (1) the free iron oxides, haematite and magnetite in the original ore, and magnetite and wustite in the slag, (2) oxidised magnetite and wustite, characterized by a shift to higher wavenumbers of their intense characteristic Raman bands and (3) Al3+-substituted magnetite and a solid solution between magnetite and hercynite (FeAl2O4). This pilot study aims to develop a model that enables close identification of the redox conditions by analysing the free iron oxides from a variety of smelting processes. It is hoped that this will provide an independent and quantifiable criterion to distinguish smithing slags (more oxidising) from smelting slags (more reducing), and to understand better the actual smelting process that transforms highly oxidised iron ore to fully reduced iron metal. Copyright (C) 2011 John Wiley & Sons, Ltd.