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. / MSc (Chemistry), North-West University, Potchefstroom Campus, 2014
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/12084 |
| Date | January 2014 |
| Creators | Van Staden, Yolindi |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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