SCIENCE: Cr(VI) containing electric furnace dusts and filter cake from a stainless steel waste treatment plant: Part 2 – Formation mechanisms and leachability
Article in Ironmaking and Steelmaking (2006) Vol 33 No 3 by G. Ma(1) and A. M. Garbers-Craig(2) at (1)Department of Metallurgical Engineering, Wuhan University of Science and Technology, China and (2)Department of Materials Science and Metallurgical Engineering, University of Pretoria, South Africa.
The present study describes the formation mechanisms and leachability of Cr(VI) from electric furnace dust and filter cake from the ferrochromium and stainless steel industries. The results show that stainless steel dust is formed by the entrainment of charge materials, volatilisation of elements and ejection of slag and metal by spitting or bursting of gas bubbles. Ferrochrome dust is formed by the ejection of slag and metal from the electrode holes, the entrainment of charge fines, vaporisation as well as the precipitation of reaction products in the off gas duct. Toxicity characteristics leaching procedure (TCLP) and American Society for Testing and Materials (ASTM) D3987–85 tests show that these wastes are all hazardous materials according to South African legislation, owing to the high degree of Cr (VI) leachability. Cr(VI) in the wastes is easily leached by distilled water, and cannot be stockpiled without prior treatment.
Emissions from ferrochromium and stainless steel plants, which consists of NOx, COx , SOx , organic compounds, particulate dusts and filter cake, pose a potential threat to human health and the environment. The US EPA has classified the dusts as hazardous materials (K061), as it exceeds the Toxicity Characteristics Leaching Procedure (TCLP) test limits for Pb, Cd and Cr,1 which imply that these elements can leach into the groundwater. Filter cake, which is the precipitate after the treatment of waste pickling acid (classified as K062) in stainless steel plants also contains significant levels of Cr(VI) species and inorganic salts.2 These particulate materials therefore need to be treated before it can be stockpiled or land filled.
Between 18 and 25 kg of dust or slurry per ton of ferrochrome produced is collected by the abatement systems in ferrochromium plants, and about 18–33 kg bag house filter dust are generated per ton of stainless steel produced.3–7 The South African ferrochromium industry produces ,100 000 t bag house filter dust and slurry, while the stainless steel industry produces 24 000 t dust annually.4,8,9 According to Cox et al.,10 ,20% of the total chromium in the ferrochrome dust is present as Cr(VI), which is leachable. Table 1 illustrates the maximum acceptable concentrations of Cr species, Zn and Pb in the leachate from these wastes, as specified by different countries.11–16 It shows that the limits and test methods vary from country to country. However, the Cr(VI) species has the lowest limits (0.02– 1.5 mg L21) and is considered to be the most harmful species in the dusts owing to its high solubility in water and carcinogenic properties.
In order to minimise the generation of the wastes and develop the appropriate method to treat them, the formation mechanisms and the leachability of the wastes need to be known and understood. The present paper subsequently describes the formation mechanisms and the leachability of the dust and filter cake. ...
In the present study, the formation mechanisms and leachability of Cr(VI) containing EF dust and filter cake were studied. The following conclusions can be drawn.
1. Stainless steel dust is formed by the entrainment of charge materials, evaporation or volatilisation of elements and ejection of slag and metal by spitting or the bursting of gas bubbles.
2. Ferrochrome dusts are formed by the ejection of slag and metal droplets from the electrode hole, the entrainment of charge materials, vaporisation as well as the formation and precipitation of compounds from vaporised species in the off gas duct.
3. TCLP and ASTM D3987–85 tests show that all the wastes are hazardous materials according to South African legislation. They therefore pose a threat to the environment.
4. Leaching experiments on the stainless steel dust show that ~65% of the Cr(VI) leaches out within 5 min under the current experiment conditions, but that the Cr(VI) leaches out more easily in acidic and basic solutions than in DW.
5. Approximately 72 and 62% of Cr(VI), present respectively in ferrochrome fine dust samples FCD1 and FCD2, leach out by DW in 1 min, while ~57% of the Cr(VI) in the filter cake leaches out under the current experimental conditions in 1 min.
6. Owing to the fact that high concentrations of Cr(VI) are easily leached in short periods of time from the ferrochrome fine dust, these dusts can be put through a washing stage whereby the immediate impact on the environment can be reduced. However, this treatment is only a temporary solution, as Cr(VI) continues to leach (but to a lesser extent), and gradual oxidation of Cr(III) to Cr(VI) occurs under ambient conditions in the presence of Ca containing phases (such as CaO, Ca(OH)2, CaCO3 and CaF2) and alkali oxides.38–40