A Dissolution Study of Common Ferrous Slag Minerals

Abstract: Slag is a vital part of metal production since it removes impurities from the metal. As slag is continuously produced, the options are either to dispose slag in landfill or acknowledge slag as a product. Slag can be used in many different applications, ranging from fertilizer to construction material; in some cases, the properties of slag make it a superior alternative to virgin stone materials. The properties and thereby the field of application is determined by the mineralogical composition of the slag. Slag is considered an environmentally friendly material as long as the leaching of certain elements stay below specified thresholds, for leaching of chromium the limit is at 0.5 mg/kg for slag to be considered inert material.The most common leaching approach is to compare leaching analyses from slag samples to deduce which elements and/or phases contribute or prevent leaching of specific elements. With this method each slag need to be investigated separately and the result may only apply for that specific slag type. In this thesis the approach is different: individual minerals are synthesized and dissolved separately at various pH to accurately assess their dissolution capabilities. By studying the dissolution of individual minerals the leaching of any type of slag with known mineralogical composition can be anticipated. Slag leaching can then be tailored, for example, chromium leaching can be eliminated if all chromium containing phases are removed or not capable of dissolving.In this thesis the dissolution of akermanite, β- and γ-dicalciumsilicate, merwinte, monticellite, pseudowollastonite and magnesiowüstite with varying FeO/MgO ratios are studied. Leaching tests of magnesiowüstite with 4 wt% Cr2O3 were also included. The dissolution of each mineral is calculated by the acid addition required to maintain a constant pH with 50 mg of mineral in 100 ml water.As expected, the dissolution decreases as pH increases, with exception of the dicalcium silicates which dissolved completely at pH 4 to 10. The dissolution of the minerals is largely connected to the elemental composition. In the silica based minerals a high Ca ratio promotes dissolution while a high Si ratio impedes the dissolution rate. Both dissolution and chromium leaching of magnesiowüstite depends on the FeO content, with increasing FeO content the dissolution and leaching decreases, magnesiowüstite with at least 60 wt% FeO does not dissolve at pH 10. None of the magnesiowüstite compositions were close to the chromium leaching limit of inert material, 0.5 mg/kg, as the highest leaching sample, with 52 wt% FeO, leached 0.069 mg/kg.