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If slag is to be used as construction material, the leaching of some elements, such as chromium must be limited. The leaching of slag depends on the leaching properties of the minerals in the slag. However, the leaching/dissolution properties of individual slag minerals are usually not studied. One common slag mineral that can contribute to the leaching of chromium is magnesiowüstite. The object of this study is to determine whether magnesiowüstite can be modified to avoid chromium leaching. Magnesiowüstite samples with different FeO/MgO ratios with and without chromium content are manufactured. The dissolution is evaluated at pH 7 and 10 using the magnesiowüstite samples without chromium, at size fraction 20–38 μm, by measuring the acid consumption required to maintain constant pH level. The magnesiowüstite samples with chromium content are leached at pH 10; the leachate is analyzed for chromium. The results are unanimous, with increasing FeO content the dissolution of magnesiowüstite and leaching of chromium decrease. At pH 10 the magnesiowüstite, with ≥60 wt% FeO show no sign of dissolution and no chromium leaching could be detected with ≥70 wt% FeO. The results prove that the FeO content can stabilize magnesiowüstite and, thereby, prevent chromium leaching.

Brownmillerite is connected to chromium leaching when present in steel slags. To prevent chromium leaching, brownmillerite in slag should be prevented. Two methods for decreasing brownmillerite content in low-alloy electric arc furnace (EAF) slag were investigated: decreasing the basicity and increasing the cooling rate. The methods were tried on both laboratory scale and in full-scale production. In the laboratory scale experiments, chromium leaching decreased as the basicity decreased until brownmillerite was no longer present, slower cooling resulted in increased chromium leaching, and faster cooling decreased chromium leaching. In full-scale production, basicity modified single batches, with a basicity below 2.2, generally leached less chromium than slag batches with higher basicity, thus verifying the correlation between basicity and chromium leaching seen in laboratory scale experiments. The cooling process in the full-scale experiments was achieved either by letting the slag cool by itself in the air or by water spraying. The water-sprayed slag, which cooled faster, had less chromium leaching than the air-cooled slag. The full-scale production experiments confirmed that both decreasing basicity below 2.2 and increasing the rate of cooling could be used to decrease chromium leaching.