Physical And Chemical Properties As Well As Formation Of The Chromium-Beairng Phases In Istanless Steel Plant Dust

Stainless steelmaking usually generates a lot of dust which contain some leachable toxicsubstances such as hexavalant chromium. As hexavalant chromium-bearing componds havinghigh solubility in water would endanger hunman health seriously and destroy the ecologicalbalance resulting in environmental pollution, they are considered as carcinogens. In addition, thestainless steel plant dust also contains iron, chromium, nickel and other valuable metals whichcan be recycled. However, no matter what kind of recovery process adopted, it would haveplenty of energy consumed as well as secondary pollution waste produced. Therefore, it issignificance to reduce the generation content of the dust and harmful substances. As reported, themain phase in the stainless steel dust is chromium spinel, studying the formation mechanism ofthe chromium-bearing phases is helpful to recognize the process of the main phase in dust whichprovides theoretical guidance to reduce its production.Research has been done on the microstructure, chemical composition, particle sizedistribution and distribution of element at different particle size of the stainless plant dust. Theresults showed that particle size of the stainless steel plant dust is mostly less than5μm andaccumulation into group easily. The primary phases of the dust are ferrochrome spinel, magnetiteas well as a small amount of ZnO. The particle size of the dust formed by the splash of the slag ismainly between30and40μm, and Mn, Zn, Ni, Fe, Cr are concentrated in small particles whichcan be easily inhaled, so it is necessary to strengthen the capture of small particles and controltheir formation.Thermodynamic calculation software Factsage5.2is employed to calculate thethermodynamic balance of Cr-Fe-Zn-Mn-Mg-Al-Ca-Ni-O-Cl and Cr-O₂system, and explorethe influence factors of the chromium-bearing phases formation such as amount of oxygen,alkali metal, Ca, Zn, HCl as well as other metal elements. The results as follows: the balancecomponent of the Cr-Fe-Zn-Mn-Mg-Al-Ca-Ni-O-Cl system is consistent with the formerexperimental results. In the Cr-O₂system, chromium mainly exists in the form of Cr₂O₃stabilized. There are small amounts of CrO₃and CrO₂generated in the high temperature region,and the content increasing with the temperature rise. The higher oxygen potential or oxygencontent is in the exhaust system, the easier to generate CrO₃in the high temperature zone.Various forms of spinel compounds are easily formed in the flue pipe, such as NiFe₂O₄,MgCr₂O₄and FeCr₂O₄. The chromium transformed into a great many of K₂CrO₄or Na2CrO₄inoxidizing atmosphere when alkali metal （K, Na） exsit, while HCl exist, a small amount ofCrO₂Cl₂appeared. In order to reduce the formation of chromium（VI）, controlling the content ofalkali metal in the raw material, the content of HCl formed, avoiding the lime directly involved in the furnace and reducing the content of O₂in the exhaust pipe should be considered.HJ/T299-2007, TCLP and sequential extraction procedure were adopted to study theleaching behavior of heavy metals in the stainless steel plant dust and chemical speciation. It isfound that at least one of the leachable heavy metals exceeds the consultant limit level of China.Pb is the most extractable element in the dusts by the sequential extraction procedure, while Cr ismainly bound to residual, and the chromium in spinel is primary trivalent chromium. Zinc startsto be extracted from the fifth step in the sequential extraction procedure, and it mainly presentsas residual fraction.The research on the surface elemental distribution of stainless steel dust show that SiO₂,CrO₃, Cr₂O₃, FeCl3, FeF2, Fe₂O₃, Fe₃O₄, FeO, CaCl₂, ZnO, ZnF2, NaCl, CaF2exist on the surfaceof EAF dust, while Zn exist probably. The substances exist on the AOD dust are CrO₂, CrO₃,Fe₃O₄, ZnO, SiO₂, Cr₂O₃and Zn, Cr probably. The depth profiles indicate that both trivalentchromium and hexavalent chromium found on the dust surface, the content of trivalentchromium increasing, while hexavalent chromium decreasing with the Ar⁺sputtering timeextend. When the sputtering time is140s, there is only trivalent chromium exist on the dustsurface. Hexavalent chromium primary present on surface of the dust in the form of CrO₃whichsuggest that hexavalent chromium in the stainless steel dust is formed in the exhaust pipeprobably.