Abstract

Utilisation of solidified AOD slags from stainless steelmaking is hindered by dicalcium silicate, which causes disintegration and dusting of solidified slag. Utilisation of AOD slags may be improved by changing the slag composition to the composition range in which dicalcium silicate is not formed. Instead of changing the composition during the AOD process — which is not favourable for the process optimization — it is possible to mix AOD slag with other slags in order to obtain compositions more suitable for slag utilisation and hence improve the material efficiency of stainless steelmaking. Integration of ferrochrome and stainless steel production in the Outokumpu Tornio plant enables the mixing of AOD slags with submerged arc furnace (SAF) slags before the slags are solidified. In addition to changing the slag composition to the composition range that favours its utilisation, the combination of two slags enables the reduction of chromium from both slags with a single treatment. Without the recovery of chromium, the chromium losses would be significant especially with the SAF slags. The purpose of this study was to study the behaviour of the slag systems in which AOD and SAF slags are mixed with different ratios. Firstly, the phase compositions of different slag compositions in different temperatures were evaluated. Secondly, reduction of chromium from different slag systems using coke, methane and ferrosilicon as reductants was evaluated in a constant temperature. According to the results the formation of dicalcium silicate is avoided if the amount of SAF slag is more than 30 %. However, the liquidus temperature of the slag mixture increases with increasing SAF slag -content, which defines an upper limit for the amount of SAF slag. Chromium can be reduced from the slag mixtures with all the considered reductants, although the amounts of reductants required for reduction varied greatly.