Effect of sample preparation on contaminant leaching from copper smelting slag

Currently standardised leaching tests require grain size reduction, which for large fragments of slags could overestimate the leaching results. To assess the effect of the fine-grained fraction generated by sample crushing, a set of leaching experiments was performed on copper smelting slag from the Zambian Copperbelt: (i) EN 12457-2 batch tests (standardised grain size <4 mm; modified procedure with grain size of 4-0.5 mm simulating exposure of larger fragments on the dumps) and (ii) CEN/TS 14997 pH-static tests (standardised grain size <1 mm simulating the possible wind dispersion scenario near the slag crushing facilities or disposal of fine-grained granulated slag; additional grain sizes <5 mm, 5-0.5 mm and 5-0.5 mm after ultrasonic cleaning). A higher proportion of the fine-grained fraction generally led to greater leaching of Cu, Co and Zn. The metal levels in the leachates under circum-neutral conditions were all below the EU limits for non-hazardous waste. However, at pH 4, the presence of fine dust particles dramatically increased the concentrations of metals in the slag leachates. The greater leachability of Cu and Co from slag particles under acidic conditions suggests a risk of their mobilisation in acidic soils in the Copperbelt area.