This study was conducted to explain an inconsistent observation in the previous study that a lignosulfonate-based polymeric dispersant improved copper and gold flotation from high clay ores significantly in laboratory but clear improvement was not observed during the full-scale plant trial (Seaman et al., 2012). In this study the flotation of low and high clay ores after grinding with different grinding media was performed to understand the interaction of lignosulfonate dispersant and grinding conditions equipped with rheology measurements and EDTA (ethylene diamine-tetra acid) extraction. It was found that lignosulfonate dispersant mitigated the negative effect of clay minerals on copper and gold flotation when the amount of iron oxidation products originated from the grinding media was minimised. Mild steel media produced a great amount of iron oxidation products depressing copper and gold flotation and masking the role of the dispersant. (C) 2013 Elsevier Ltd. All rights reserved.