Influence of chromium containing spinels in an electric arc furnace slag on the leaching behaviour

Electric arc furnace slags are suitable construction and paving materials for roads due to their physical properties (e.g. volume stability). However, concerns are being raised in regard to the leaching behaviour of slags, especially in terms of the element chromium and its toxicity in the hexavalent state. The leaching behaviour is significantly controlled by the mineral phases and their solubility whose formations depend mainly on the process conditions (i.e. additives, temperature, cooling rate). In this contribution, the content, the element distribution and the mobility of chromium for an electric arc furnace slag were investigated and assessed. The extensively analysed material was a slag from a steel production plant in Austria using unalloyed scrap as raw material. XRF and electron microprobe analyses were carried out as well as leaching tests with initial acid/base addition to influence the pH value. XRF results gave a total chromium content of 1.7 mass% Cr. The mineralogical investigation by electron microprobe analyses showed that chromium ions are bound within stable spinel phases. Two different types of spinels occur in the slag: aluminium-chromium-magnesium- and chromium containing aluminium-magnesiumspinels (type I) as well as chromium-manganese-iron-spinels (type II). The chromium content in the accompanying phases (e.g. gehlenite) is insignificant. The natural occurrence of the mentioned type I spinel is quite common and they are found in mafic and ultramafic rocks and soils produced by weathering of these rocks. The leaching tests showed that the extraction of spinel bound chromium of the investigated electric furnace slag is unlikely, even at low pH values.