Characterisation and liberation of chromium from fine ferrochrome waste materials

Three types of generic chromium (Cr)-containing wastes are generated during ferrochrome (FeCr) production, i.e. slag, bag filter dust (BFD) and venturi sludge. Slag is by volume the largest waste; however, fine FeCr waste materials (e.g. BFD, venturi sludge) are from an environmental perspective the most important. The loss of Cr units to FeCr waste streams represents both an added cost burden (related to disposal/storage) and a loss of revenue in terms of contained Cr units. In this paper, the novel idea of the liberation of Cr units from FeCr BFD and the ultra-fine fraction of slag (UFS) with aqueous ozonation and the advanced oxidation process was investigated. Several techniques were used to characterise both case study waste materials, i.e. particle size distribution, chemical composition, chemical surface composition and crystalline content analysis. Results indicated that limited Cr liberation could be obtained from the waste materials utilising aqueous ozonation. For BFD, only a maximum of 4.2% of total Cr liberation was achieved. However, the Cr liberation of BFD was substantially higher than that achieved for the UFS, which was attributed to the difference in characteristics of the two materials. Cr liberation observed was related to the formation of the OH radicals during the spontaneous decomposition of aqueous O3. Application of the advanced oxidation process by the addition of H2O2 during ozonation increased Cr liberation dramatically. More than 21% of total Cr liberation could be achieved for both the waste materials used in this investigation. Although the afore-mentioned Cr liberation level is unlikely to be commercially viable, the investigation proved that further research could optimise this process.