TY  - JOUR
AU  - KE, Wenli
AU  - LIAO, Ning
AU  - XIE, Fubo
AU  - LI, Yawei
AU  - CAI, Guoqing
AU  - QIN, Jiantao
PY  - 2025
TI  - Effect of Titanium Oxide/Iron Oxide on Stability of Mullite-Based Refractories Under H2+CO Conditions
JO  - Journal of the Chinese Ceramic Society
SN  - 0454-5648
SP  - 2586
EP  - 2596
VL  - 53
IS  - 9
AB  - IntroductionThe global imperative to reduce carbon emissions has catalyzed the development of hydrogen-based metallurgical technologies, with hydrogen shaft furnaces emerging as a promising low–carbon alternative for ironmaking. These reactors demand refractory linings with an exceptional stability to withstand synergistic thermal, chemical, and mechanical stresses when operating under extreme reducing environments (i.e., H2/CO atmospheres at 1050 ℃, 0.5–1.0 MPa). Conventional aluminosilicate refractories, while widely used, suffer from deleterious reactions in H2-rich atmospheres, including silica volatilization, carbon deposition via the Boudouard reaction, and structural degradation induced by iron oxide (Fe2O3) reduction. Recent studies highlight that co-doping Fe2O3 with TiO2 can mitigate these issues, yet systematic investigations into the role of TiO2/ Fe2O3 phase chemistry and pretreatment strategies remain scarce. This study was to elucidate the impact of TiO2/ Fe2O3 composite oxides on the reduction stability of mullite-based refractories under simulated hydrogen shaft furnace conditions (i.e., H2/CO = 5:2). We were to investigate the phase evolution and microstructural engineering influence resistance to H2/CO induced degradation via varying the TiO2/Fe2O3 molar ratio (0.5–4.0) and employing pre-sintering treatments.MethodsHigh purity electromelting mullite (Al2O3 >70% (in mass fraction), 3–1 mm, 1–0 mm and 200 mesh), plate corundum (Al2O3 >99.5%, 325 μm), α–Al2O3 (Al2O3 >99.5%, 2–5 μm), Fe2O3, TiO2 powder (analytically pure), silicon powder (2 μm), and high viscosity yellow dextrin (200 μm) were used as raw materials. Fe2O3 and TiO2 powder were proportioned according to the mass ratio m(TiO2):m(Fe2O3) of 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0, respectively, and were placed into a planetray ball mill according to a mass ratio of m(zirconium ball):m(water):m(mixed powder) of 2:1:1, The ball mill was operated at 300 r/min for 3 h. After milling, the slurry was taken out and put it in an oven for drying at 110 ℃ for 24 h. The ground sample was screened by a sieve with 200 mesh, thus obtaining the composite oxide of titanium oxide and iron oxide without pre-burning. The sample of ϕ50 mm×10 mm was prepared with the sifted powder at 100 MPa, and then heated in air at 1300 ℃ for 3 h. After cooling to room temperature, pre-burned titanium-iron oxide composite oxide was obtained. The dextrin aqueous solution was prepared according to a ratio of m(dextrin):m(warm water) of 1:1. The mixture was trapped at 25 ℃ for 12 h, and the cylindrical sample was shaped into ϕ50 mm×50 mm at 150 MPa, and dried in an oven at 110 ℃ for more than 24 h. Finally, the dried sample was heated in air to 1600 ℃ for 3 h, obtaining the titanium-iron oxide composite mullite refractory. The temperature increasing rates of this experiment were 5 ℃/min at 0–1000 ℃, 3 ℃/min at 1000–1400 ℃, and 2 ℃/min at 1400–1600 ℃, respectively.Results and discussionA large amount of Al2TiO5 and a small amount of Fe2TiO5 are formed in mullite refractory via the introduction of non-pre-fired titanite composite powder, which is not a dense Mosaic structure. In particular, the porosity increases during the formation of Al2TiO5, weakening the protective effect of iron oxide under reduction conditions, and resulting in more pores and cracks formed after the reduction atmosphere treatment. The stability cannot improve as TiO2/Fe2O3 ratio increases. After reduction, the compressive strength of the sample is only 32.2–38.6MPa, showing a significant downward trend, and the attenuation rate is as high as 17.3%–26.5%. Fe2TiO5 solid solution is formed in the pre-fired powder at 1300 ℃, and a large number of titanium oxide is coated iron titanate complex, forming a dense Mosaic structure in the corundum mullite matrix. Fe2TiO5 is formed during a progressive reduction of ferrous compound FeTiO3, greatly inhibiting the reduction of iron oxide. Thus, the anti-reduction stability of the material is significantly improved. When the mullite refractory with m(TiO2):m(Fe2O3) > 2.0 is added, the reduction stability is improved. The compressive strength remains 74.3–84.5 MPa at the reduction temperature of 1050 ℃, and the compressive strength attenuation rate at room temperature is only 2.6%–5.8%.ConclusionsThe results indicated that non-pre-sintered titanium-iron oxide composite samples exhibited a significant structural degradation after being treated at 950 ℃. This degradation was characterized via the formation of numerous micron-sized pores in the matrix and a weakening of the aggregate-matrix interface, leading to a peak compressive strength of only 38.6 MPa and a strength decay rate of 17.3%–26.5%. In contrast, pre-fired titanium-iron oxide composite samples maintained a good structural integrity after reduction at 1050 ℃. When a molar ratio of TiO2/Fe2O3 exceeded 2, the compressive strength remained at 71.9 MPa. The microstructural analysis revealed little cracks in the samples, and the strength attenuation rate was controlled within 10%. In the non-pre-sintered titanium-iron oxide composites, the composite phase of Al2TiO5 and Fe2O3, which was introduced without pre-firing, could not effectively inhibit the reduction reaction of Fe2O3. This resulted in the precipitation of metallic iron and subsequent volume shrinkage. However, in the pre-fired composites, Fe2TiO5 was preferentially transformed into the ferrous compound FeTiO3 during the reduction process. The resistance of the material to H2/CO reduction erosion was enhanced via constructing a cooperative stable structure of Fe—O—Ti. This finding could provide a theoretical basis for the development of high-stability refractories for hydrogen-based shaft furnaces.
UR  - https://doi.org/10.14062/j.issn.0454-5648.20250085
DO  - 10.14062/j.issn.0454-5648.20250085