Study On Phase Evolution And Self-pulverization Of Converter Steel Slag

Poor grindability of converter steel slag (CSS) seriously restricts its reuse as a resource. To improve grindability of CSS, it is necessary to investigate the relationship of composition, mineral phase and grindability for CSS. In this paper, phase variation and self-pulverization of CSS are studied by composition modification technology. And effects of phase variation and self-pulverization on grindability of CSS are investigated.The CSS have block and rectangular tricalcium silicate (C3S), round and irregular dicalcium silicate (C2S), matrix phase dicalcium ferrite (C2F), RO (a solid solution in the CSS composed of bivalent metallic element such as Fe, Mn), granular periclase (f-MgO), free lime (f-CaO) and metallic iron. Rapid cooling is prone to obtain glass phase, and oxidizing atmosphere is conductive to the formation of C2F in the CSS.Converter slag system of CaO-SiO2-MgO-Fe2O3-P2O5has been synthesized in the laboratory. The chemical composition of the synthetic slag is CaO45-50%, SiO210-15%, MgO10%, Fe2O325-30%and P2O50-5%, respectively. In this paper, the effects of SiO2and Al2O3on the mineral phase of CSS are studied firstly.Influence of SiO2modifer on the mineral phases of the CSS (R=5.0) is investigated in the experiment. The results show that when the amount of SiO2is less than10%, the slag primarily contains C2S, MgO and C2F. When the amount of SiO2exceeds8%, MF firstly forms and gradually increases. When the amount of SiO2is greater than10%, the C3MS2firstly forms and gradually increases. When the amount of SiO2is more than18%, the mineral phases in the slag are changed significantly:(1) C2S in the slag gradually decreases and disappears, and the C3MS2is Si-containing phase mainly.(2) Free MgO gradually decreases and dissolves into C2S and C2F.(3) The Fe-containing phase in the slag changes gradually from C2F to MF. When the amount of SiO2exceeds24%, CMS begins to generate and continually increases, while C2MS2gradually decreases.Influence of Al2O3modifer on the mineral phases of the CSS (R=3.0) is also studied in the laboratory. The results show that when the amount of Al2O3is less than6%, most of Al2O3reacts with C2F to form calcium ferroalumnates (C2AF), and little Al2O3dissolves in other mineral phases. When the amount of Al2O3exceeds4%, MgFeAlO4(MFA) begins to form and increases continually. When the amount of Al2O3is greater than10%, there are two different areas with specific topography in the CSS, namely Ⅰ area and Ⅱ area. The contents of Al and Mg of Ⅰ area are higher than that of Ⅱ area. When the amount of Al2O3increases continually, the proportion of Ⅰ area increases too, and block and mesh MFA will form at its center. However, unlike the case of Ⅰ area, the mineral phase of Ⅱ area has remained little changed.Compared with modification effect of SiO2and Al2O3, SiO2mainly reacts with Si-containing and Mg-containing phases in the CSS, which has good effect on its grindability, and obviously improves its stability for CSS used as a cement material. Al2O3mainly reacts with C2F with high hardness in the CSS, which could improve its grindability.Secondly, the slag system of CaO-SiO2-MgO-Fe2O3has been synthesized in the laboratory, and the influence of SiO2modifer on self-pulverization of CSS is studied in this paper. It is obtained that the compositon has greater effect on self-pulverization of CSS, and when the basicity is about2.0, the composition is very beneficial to form C2S in the CSS. Under this condition, amount of C2S in the CSS is about63.60wt%and its pulverization rate has reached93.60%in the laboratory.Finally, the structure transformations of C2S in the CSS are studied by HT-XRD technology. The results show that when experimental temperature above750℃, γ-C2S begins to change into a’-C2S in the CSS. When the temperature is below650℃, the reaction from a’-C2S to β-C2S gradually starts and ends completely at500℃. Based on the structure changes of pure polycrystalline C2S, the reaction from β-C2S to γ-C2S of CSS should occur at about500℃. Obviously, the temperature condition is a very critical factor for structure transformations of C2S causing self-pulverization in the CSS.From above analysis, self-pulverization rate of CSS is improved greatly by composition modification and temperature control technology. Therefor, to solve poor grindability of CSS is not again a problem. The research provides a new technical route and the scientific basis for efficient use of CSS.