Sulfur Incorporation Mineral And Desulfuration From LF Waste Slag By Subcritical Water Leaching

One of the most commonly used and most efficient methods of improving the quality of steel was ladle treatment of the metal with synthetic slag. After refining, the output of waste slag has a sharp increase, which has brought about increasing issues such as environmental pollution and resource spent. The LF waste slag has the characteristics of metallurgical resources because of the chemical composition and can be returned to metallurgical industry as raw material. For these reasons, the reuse of LF waste slag has been paid more attention for a long time.Unfortunately, in spite of its potential, the reuse of LF waste slag was not satisfied and there are only a few waste slag were returned to steelmaking process. This was due to the fact that in the refining process the slag was used as a reservoir of hazardous elements such as sulfur and subsequently used as a source material in steelmaking, the hazardous elements would pollute the steel. So the task of desulfurization to regenerate slag was considerably important before reused.But now there is not suitable method to remove sulfur from the slag, in spite of the technical obstacles, the basic characteristics of LF waste slag were not determined. The crystallized path of minerals and the incorporation of sulfur in refining waste slag during the cooling and solidification, the physical chemistry mechanisms and the changing of metallurgical characteristics during the regeneration were the research bases for the reuse of the LF waste slag. These can proffer foundation theories for the slag's regeneration and guide the desulfurization method's adoption and improving the technologies of reusing the waste slag. So above research works have academic and practical significance to solve the problem of environmental pollution and resource spent caused by LF waste slag.In this subject, a study regarding the typical LF waste slags with micro- and thermodynamics analytical technologies were carried out. The study indicated that the LF waste slag has favorable reuse value in steelmaking process because of the high basicity, low oxidation, high sulfur capacity and good melting characteristic, but the sulfur must be removed before reuse. The crystallized path of minerals was related to their equilibrium temperature, with the temperature drop, Calcium silicate was separated out from the slag at first and Calcium aluminates formed matrix with low melting point, and there were a lot of f-CaO in the waste slag. The amount and dimension of mineral were increased in the low basicity and high Al₂O₃ content slag. The CaO- Al₂O₃-SiO₂ ternary phase diagram and the corresponding basicity value could be used to analyze the thermodynamics equilibrium of the crystallized process of minerals, and the analyze results was very accordant to the microscopic detect results.The sulfur in the slag was existed between the low melting point minerals in a state of dispersing, and the sulfur incorporation mineral separated out from the slag was not by a single phase which was according to the stoichiometry. Ca₁₂Al₁₄O₃₂S was the uppermost sulfur incorporation mineral and it could be separated out from the slag in a broad range of sulfur content and cooling speed. There were CaS and other complicated sulfur mineral separated out when the basicity and composition changed. Ca₁₂Al₁₄O₃₂S was formed by replacing O2- in 12CaO·7Al₂O₃ with S2- in CaS, and the thermodynamics calculation indicated that Ca12Al14O32S can be easily crystallized from the slag and existed stably if only the a(CaS) over 10-4.The regeneration desulfurization reaction of LF waste slag had been done by subcritical water leaching, the results indicated that the reaction was a kind of multiphase reaction between fluid and solid, and the reaction speed and ratio were large increased in advantaged dynamics conditions which offered by the subcritical water. The reaction's resistances were come from mass transfer or interface reaction in different temperature. At the temperature of 373 K⁴73 K, the interface reaction was the control element, and the main resistance changed to mass transfer when the temperature was up to 473 K. There has not any oxidation-reduction reaction in the process and the S2- in the slag entered into the lixivium at the same state. The primary thermodynamics influencing factor of the regeneration desulfurization reaction was temperature and the Unreacted Nucleation dynamics model could be used to characterize the desulfurization reaction. The control element of dynamics process was internal diffusion because of the low apparent activation energy value. At last, the metallurgical capabilities of the refining agent prepared with regenerated slag was researched by laboratory experimentation, theoretical calculation and production application, and the research results indicated that the regenerated slag showed goodish melting capability and the refining agent prepared with regenerated slag had high optical basicity and sulfur capacity. In the laboratary experimentation, the desulfurization reaction speed was quickly andηs approached the equilibrium value at the first 10 minute which was 50%⁷0%. In the production application, using the refining agent prepared with regenerated slag to replace lime, the effect of desulfurization, deoxygenation, inclusion adsorption and alloy recovery were all increased obviously.