Experimental and theoretical studies on treatment and recycling of pickling sludge generated in stainless steel production

Abstract: The thesis work is one of the on-going research projects within MiMeR (Minerals and Metals Recycling Research Centre) at Division of Process Metallurgy, Luleå University of Technology in Sweden. The major objectives of this thesis work are: i) to characterize raw materials and products obtained from experiments, ii) to study the mechanisms of solid-state reduction and smelting reduction of pickling sludge and iii) to test in laboratory different process alternatives to treat and recycle pickling sludge. The work has been carried out through characterization, thermodynamic calculations, solid-state reduction and smelting reduction tests together with various analyses using thermogravimetry, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, mass spectroscopy and chemical analysis, etc. The major results presented in the thesis are summarized as below: The characterization results show that the dried pickling sludge mainly contains calcium fluoride and hydroxides of metals (Fe, Ni and Cr) which, at temperatures above 600°C, decompose into metal oxides Fe2O3, NiO and Cr2O3. The results obtained from mechanism study on solid-state reduction of Fe2O3, NiO, Cr2O3, Fe2O3-NiO mixture, Fe2O3-NiO-Cr2O3 mixture and pickling sludge show that the formation of transitional compound FeCr2O4 promotes the reduction of Cr2O3 to complete at a temperature lower than that for reducing pure Cr2O3 but hinders the reduction of Fe2O3. There is also a similar interaction between Fe2O3 and NiO through formation of transitional compound (Ni,Fe)Fe2O4, which promotes the reduction of Fe2O3 but hinders the reduction of NiO. The results of isothermal solid-state reduction tests on the pickling sludge show that the reduction of metal oxides in the sludge is completed at 1200° C. The metals in the reduction products exist mostly in the form of Fe-Ni and Fe-Ni-Cr alloys. The CaF2 is stable during the reduction. The results of smelting reduction tests on pickling sludge and on mixtures of pickling sludge with other metallurgical residues, such as oily mill scale and gas cleaning dust, etc., indicate that the metal yield is about 98 - 99 wt-%, and the slag phase mainly consists of CaF2 and CaS, which, after removing sulphur, can be utilized as flux material in steelmaking. The off-gas generated by smelting reduction of pickling sludge mainly contains H2O, CO and CO2, while HF, NO and SO2 are present in low levels. The results from the tests of removal of sulphur and fluorine from pickling sludge in rotary furnace show that the elimination degrees of sulphur and fluorine are about 99 wt-% and 84 wt-%, respectively. Therefore, by using smelting reduction and sulphur removal methods developed in this thesis work, it is technically feasible to recover metals and produce sulphur-free “CaF2-substitute” from the pickling sludge for use as a flux material in the steelmaking process.