Mineralogical phase and formation mechanism of titanium-bearing protective layers in a blast furnace hearth

In theory and practice, TiO₂-bearing iron ores are the preferred raw materials for prolonging blast furnace times due to their protection of the refractory lining of the hearth. Currently, however, a lack of detailed understanding of the mineralogical composition, formation mechanism, and ratio of C to N in the Ti(C, N) solid solution leaves the blast furnace operator unable to employ a scientific and effective measure to deal with abnormal hearth erosion. As a result, frequent hearth breakouts might occur, causing great financial loss to steel companies. In the present work, in an attempt to clarify the essence of longevity blast furnaces, investigations were conducted into blast furnace hearth damage together with dissection analyses, to derive the mineralogical composition and microstructure of titanium-bearing protective layers. The results show that the exact chemical composition of the TiC_xN_1-x which formed in the blast furnace is TiC_0.3N_0.7. Based on thermodynamic analysis, the standard Gibbs free energy of the formation of Ti(C, N) decreases at first, then increases with increasing TiC content. At different temperatures, the proportion of TiC and TiN in the solid solution is different, i.e., more TiC at higher temperatures but more TiN at lower temperatures. At 1423℃, the TiC_0.3N_0.7 is formed in the hot-side of the investigated blast furnace hearth, and the thickness of the titanium-bearing protective layer varies with smelting intensity, temperature, and circulation strength of hot metal. This paper classifies the protective layer into various types based on formation mechanism. Finally, a comprehensive regulatory scheme is presented to act as a basis for extending the lifespan of the blast furnace hearth.