Waste slag dumps constitute a large potential source of chromium contamination of groundwaters, particularly under the enhanced leaching action of acid precipitation. Laboratory leaching experiments were carried out to determine the composition factors controlling the chromium leachability of waste slags when exposed to acidic solutions designed to simulate acid precipitation.
Two critical slag composition factors were found to control the chromium leachability: (1) the calcium oxide (CaO)/silicon dioxide (SiO2) ratio and (2) the magnesium content. It was determined that above a 2.0 CaO/SiO2 ratio, chromium exists in slag as calcium chromite, which can be vulnerable to leaching by acid precipitation, especially if it is oxidized to calcium chromate when exposed to the environment over an extended period.
Experimental results showed that maximum chromium leachability from industrial slags occurred when the composition of the slag was at about a 2.0 CaO/SiO2 ratio. Between a 1.0 and 2.0 CaO/SiO2 ratio, in the presence of sufficient magnesium, picrochromite (MgO · Cr2O3) was formed, which is very stable towards oxidation and was resistant to dissolution by simulated acid precipitation.