KTH Industrial Engineering and Management Sulfur and Nitrogen in Ladle Slag
Sweden Akademisk avhandling som med tillstånd av Kungliga Tekniska Högskolan i Stockholm, framlägges för offentlig granskning för avläggande av Teknologie Doktorsexamen, fredagen den 1 april 2016, ABSTRACT The present work deals with some aspects of slags related to secondary metallurgy in the steelmaking process. More specifically the focus is given to sulfur and nitrogen in ladle slags. Even though slags have been fairly well–researched in the past, the available data for these elements in typical ladle slag compositions is rather scarce. In some cases the available data is in discordance. There are also inconsistencies between the literature data and what is commonly observed in the industrial processes. Sulfide capacities were measured at steelmaking temperatures, 1823–1873 K, in ladle slags. The data was found to be in reasonable agreement with the industrial process norms. The sulfide capacity was found to increase with the basic oxides CaO and MgO; and decrease with the acidic components Al 2 O 3 and SiO 2. The sulfide capacity was also found to increase with temperature. The dependence of sulfide capacity on the oxygen partial pressure, for slags containing multivalent elements, was investigated experimentally at 1873 K with a slag containing vanadium oxide. A strong dependence of oxygen partial pressure was observed. The sulfide capacity increase by more than two orders of magnitude when the oxygen partial pressure was increased from 4.6×10-16 atm to 9.7×10-10 atm. The nitrogen solubility and the effect of carbon was investigated in typical ladle slags and the CaO–MgO–SiO 2 system at 1873 K. Carbon increases the nitrogen solubility substantially. In the absence of carbon, the nitrogen solubility is extremely low. Low concentrations of cyanide was detected in the carbon saturated slag. This was much lower than the total nitrogen content and formation of cyanide cannot explain the large increase. The possibility of removing sulfur with oxidation from used ladle slag was investigated experimentally at 1373–1673 K. The sulfur removal of mostly solid slag was found to be a slow process, and would not suitable for industrial practice. At 1673 K the slag was mostly liquid and more than 85% of the sulfur was removed after 60 min of oxidation in pure oxygen atmosphere. ii ACKNOWLEDGEMENTS
Nitrogen solubility in CaO-SiO2, CaO-MgO-SiO2, and BaO-MgO-SiO2 melts
Nitrogen solubility in molten CaO-SiO2, CaO-MgO-SiO2, and BaO-MgO-SiO2 silicate systems was measured as functions of nitrogen and oxygen partial pressures and composition at 1823 K. Experimental findings indicate that nitrogen exists as a free nitride ion or is associated with silicate networks, depending principally on the slag composition. A special analytical technique was developed to separately determine the two species. Its dissolution mechanism, which is similar to that for hydrogen, was proposed, being supported by the detection of a Si-N bond by infrared spectroscopy. Neither the content of free nor incorporated nitrogen was demonstrated to have a very good correlation with the CaO or SiO2 activity in the melts, probably because the activity coefficient of nitrogen is affected by the slag composition.
Analysis of the oxidation reactions of CaS
The chemical reactions of dense pellets of solid CaS in oxidizing atmospheres were studied by using continuous thermogravimetric analysis and iodimetric titration of SO2 in the off-gas. The experiments covered a temperature range of 1223 to 1853 K, and Ar-O2 mixtures varying from 1 to 100 pct O2. The oxidation of CaS was found to be a complex process involving the formation of CaO and CaSO4. Within the experimental conditions stated, three different processes were observed: 1) Weight loss with the formation of CaO and SO2 which occurred at high temperatures and low partial pressures of O2. 2) Weight gain which occurred at low temperatures and high partial pressures of O2. 3) Combination of a) weight gain with the overall formation of CaSO4 and b) weight loss with the overall oxidation of CaS to CaO and the decomposition of CaSO4 leading to an oscillatory behavior which occurred at intermediate temperatures and intermediate to high partial pressures of O2. The boundaries between these three processes are distinct. The rate of oxidation of the sulfide is limited first by the transport of O2 across the gas boundary layer and later by the diffusion of O2 through the porous reaction product. From these results and the Ca-S-0 stability diagram, it is possible to predict the experimental conditions which will produce processes of weight gain, weight loss and oscillatory behavior.