Cr(VI) Generation During Flaring of CO-Rich Off-Gas from Closed Ferrochromium Submerged Arc Furnaces
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ALTHOUGH chemical oxidation states for chromium (Cr) range from 4 to +6,[1], only Cr(III) and Cr(VI) are stable in the ambient environment.[2–4] Cr(III) is considered an essential micro-nutrient that is essential for protein, carbohydrate, and lipid metabolism in animals and humans,[5] while Cr(VI) is generally considered to be carcinogenic, mutagenic, and teratogenic.[6,7] Cr(VI) can be generated and/or released into the environment through various anthropogenic activities, e.g., chromate chemical manufacturing,[8,9] electroplating,[10] leather tanning,[11] cement manufacturing,[12] paint industries,[13] stainless steel welding,[14] and stainless steel production.[15] Of relevance in this paper is the generation of Cr(VI) during ferrochrome (FeCr) production. FeCr is produced from chromite ore and is a relatively crude alloy that consists mainly of Cr and iron (Fe). FeCr is the only source of new Cr units used during stainless steel production for which 80 to 90 pct of all produced FeCr is used.[16] Stainless steel is a vital alloy in the modern society, making FeCr equally important. Ma characterized off-gas dust from a FeCr smelter in South Africa, but did not specifically consider all the production steps that could lead to Cr(VI) formation.[17] Beukes et al.[18] reviewed the generation of Cr(VI) S.P. DU PREEZ, Post graduate student, J.P. BEUKES, Chief Research Scientist and Lecturer, P.G. VAN ZYL, Lecturer, are with Chemical Resource Beneficiation, North-West University, Potchefstroom Campus, Private Bag X6001, Potchefstroom, 2520, South Africa. Contact e-mail: [email protected] Manuscript submitted May 29, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
during FeCr production. According to this paper and references therein, several ferrochrome production steps can lead to Cr(VI) formation. This review[18] also highlighted uncertainties with regard to certain production steps that need to be further investigated in order to enhance the current understanding of Cr(VI) generation during FeCr production. One such process is the flaring of CO-rich off-gas from closed submerged arc furnaces (SAF). In Figure 1, an example of a typical off-gas flare from a closed FeCr SAF burning on a stack is presented. Off-gas from a closed FeCr SAF is usually cleaned with wet venturi scrubbing, which removes 99.9 pct of particulate matter in the off-gas, reducing particulate matter from 35 to 45 g/Nm3 to 50 to 100 mg/Nm3.[19] In certain first-world countries, the remaining particulate matter is further removed with sintered filters leaving less than 1 mg/Nm3 particulate matter in the cleaned furnace off-gas.[19] However, in most developing countries, this last step, i.e., sintered filtering after wet venturi cleaning, has not been implemented. This implies that some Cr-containing particulate matter will pass through the CO-rich off-gas stack flare, which could lead to the possible formation of Cr(VI). This release of Cr(VI) is of great concern, since Cr(VI) is considered to be carcinogenic, particularly for the respiratory track. Curren
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