Investigation of the presence of metallic phases in brazilian tin-slags

The supply chain of Sn presents social and environmental risks that could threaten the availability of cassiterite, while secondary materials, such as tin-slags, stand as an alternative source for smelters. In this context, this work investigated the presence of occluded metallic phases in Brazilian tin-slags. Chemical analyses of different slags revealed that this fact could be related to high ZrO2 levels, which was also supported by different thermodynamic studies. It was indicated that minor additions of CaO could diminish the melting temperature of these slags, contributing towards the precipitation of metallic prills. Smelting experiments were performed between 1773-1573K with a tin-slag concentrate using limestone as a fluxing agent. It was verified that metallic prills were mostly precipitated in the bottom of the crucible above 1673K. However, at 1573K, a mixture of metallic and oxidized phases was obtained, indicating that low working temperatures lead to losses of Sn in the final slag as metallic inclusions. Further additions of limestone improved the coalescence but did not allow efficient precipitation of these metallic prills. Temperature measurements were performed in a 150 kVA EAF during slag processing, and it was verified that tapping of metal/slag occurred partially below 1573K, justifying the presence of metallic prills in the studied slags. Therefore, an optimization of the process relies both on the dilution of ZrO2 content by a controlled addition of fluxing agents, as well as by an increase in working temperatures. Alternatively, magnetic separation can be used to recover occluded metallic particles from final slags.