Liberation of chromium from ferrochrome waste materials utilising aqueous ozonation and the advanced oxidation process

Abstract

During ferrochrome (FeCr) production, three types of generic chromium (Cr) containing wastes are generated, i.e. slag, bag filter dust (BFD) and venturi sludge. The loss of these Cr units contributes significantly to the loss in revenue for FeCr producers. In this study, the liberation of Cr units was investigated utilising two case study waste materials, i.e. BFD from a semi-closed submerged arc furnace (SAF) operating on acid slag and the ultrafine fraction of slag (UFS) originating from a smelter operating with both open and closed SAFs on acid slag. A detailed material characterisation was conducted for both case study materials, which included particle size distribution, chemical composition, chemical surface composition and crystalline content. Cr liberation was achieved utilising two methods, i.e. aqueous ozonation and the advanced oxidation method. Various advanced oxidation processes could be applied. However, the advanced oxidation processes considered in this study was the use of gaseous ozone (O3) in combination with hydrogen peroxide (H2O2). Controlling parameters such as the influence of pH, ozonation contact time, waste material solid loading, gaseous O3 concentration and temperature on Cr liberation were investigated for the aqueous ozonation process. The influence of pH, volume H2O2 added and the method of H2O2 addition were considered for the advanced oxidation process. Results indicated that with aqueous ozonation, limited Cr liberation could be achieved. The maximum Cr liberation achieved was only 4.2% for BFD by varying the process controlling parameters. The Cr liberation for UFS was significantly lower than that of the BFD. The difference in the results for the two waste materials was attributed to the difference in characteristics of the materials. The Cr content in BFD was mostly related to chromite and/or altered chromite particles, while the Cr content of the UFS was mostly related to FeCr particles. It is possible that the Cr(III) present in the chromite and/or partially altered chromite might be more susceptible to oxidation to Cr(VI) than the metallic Cr(0) present in the FeCr. During ozonation, aqueous O3 spontaneously decomposes to form hydroxyl (OH•) radicals, which are very strong oxidants in water. The above-mentioned Cr liberation observed was related to the formation of the OH• radicals during the spontaneous decomposition of aqueous O3. This was indicated especially by enhanced Cr liberation at higher pH values, which was attributed to the acceleration of the spontaneous decomposition to OH• radicals at higher pH levels. The advanced oxidation method gave significantly higher Cr liberation results for both case study materials considered, achieving Cr liberations of more than 21%. The advance oxidation processes improve normal oxidation methods. In this study, the H2O2 used in combination with O3 enhanced the formation of the OH• radicals that are responsible for the oxidation of Cr. The Cr liberation levels achieved are possibly not high enough to be feasible for industrial purposes. However, a further investigation of the advanced oxidation process could optimise the process to yield even higher Cr liberation.