Coupled effect of SiO₂ content and hydrogen-enriched atmospheres on the reduction behavior and microstructural evolution of fired hematite pellets

Abstract

As demand grows for low-carbon ironmaking, it is essential to understand how hydrogen reduces iron ore pellets under varying gangue compositions and gas atmospheres. In this work, fired hematite pellets with a basicity (mass ratio of CaO to SiO₂) of 0.3 and SiO₂ contents ranging from 1wt% to 4wt% were systematically investigated under three typical shaft furnace atmospheres (Midrex, HYL, and coke oven gas (COG)) as well as under 100% H₂, to clarify the reduction kinetics, reaction mechanism, and microstructural evolution of the fired pellets. The results indicate that a higher hydrogen proportion significantly accelerates the reduction rate of the fired pellets, while an increase in SiO₂ content generally leads to a decrease in the overall reaction rate. However, the effect of hydrogen concentration on the reduction behavior of the fired pellets varied markedly with their silicon content. For the fired pellets containing 1wt% and 2wt% SiO₂, an increase in hydrogen concentration causes deterioration in reduced pellet characteristics, as evidenced by the increase in reduction swelling index from 26.14% to 34.26% and the decrease in cold compressive strength from 110 to 78 N. In contrast, fired pellets with 3wt% and 4wt% SiO₂ exhibit the opposite trend, with the reduction swelling index decreasing from 15.26% to 9.23% and cold compressive strength improving from 179 to 271 N. Kinetics analysis indicates that under 100% H₂, the reduction of fired pellets with 1wt% SiO₂ is governed by a mixed gas-diffusion and uniform reaction model, whereas fired pellets with 4wt% SiO₂ follow an unreacted core model. These differences in reduction kinetics, reduction behavior, and post-reduction properties are closely associated with the formation of more Al-bearing calcium silicate slag phases in high-SiO₂ reduced pellets, which strengthen intergranular bonding, buffer phase-transformation-induced stress, and promote the evolution of metallic iron from whisker-like to granular or layered morphologies.

Keywords

hydrogen metallurgy / fired pellets / silica content / reducing atmosphere / microstructural evolution / iron nucleation behavior

References

[1] L. Ren, S. Zhou, T.D. Peng, and X.M. Ou, A review of CO₂ emissions reduction technologies and low-carbon development in the iron and steel industry focusing on China, Renewable Sustainable Energy Rev., 143(2021), art. No. 110846.

[2] X.Y. Zhang, K.X. Jiao, J.L. Zhang, and Z.Y. Guo, A review on low carbon emissions projects of steel industry in the World, J. Cleaner Prod, 306(2021), art. No. 127259.

[3] J.S. Zhang, J.L. Shen, L.S. Xu, and Q. Zhang, The CO₂ emission reduction path towards carbon neutrality in the Chinese steel industry: A review, Environ. Impact Assess. Rev., 99(2023), art. No. 107017.

[4] Sun MM, Pang KL, Barati M, Meng XY. Hydrogen-based reduction technologies in low-carbon sustainable iron-making and steelmaking: A review. J. Sustainable Metall., 2024, 10(1): 10

[5] Tang J, Chu MS, Li F, Feng C, Liu ZG, Zhou YS. Development and progress on hydrogen metallurgy. Int. J. Miner. Metall. Mater., 2020, 27(6): 713

[6] Zhu DQ, Zhang F, Guo ZQ, Pan J, Yu W. Grate-kiln pelletization of Indian hematite fines and its industrial practice. Int. J. Miner. Metall. Mater., 2017, 24(5): 473

[7] Bararzadeh Ledari M, Khajehpour H, Akbarnavasi H, Edalati S. Greening steel industry by hydrogen: Lessons learned for the developing world. Int. J. Hydrogen Energy, 2023, 48(94): 36623

[8] Tong S, Hong LK, Xu Y, Ai LQ, Sun CJ, Zhou MJ. Iron phase sintering behavior of magnetite pellets by isothermal reduction with pure hydrogen at high temperature. Int. J. Hydrogen Energy, 2024, 110: 609

[9] Silva FR, Lemos LR, de Freitas Nogueira P, Bressan M. Effect of ternary basicity of iron ore-fluxed pellets on melting and softening properties in a blast furnace. Metall. Mater. Trans. B, 2021, 52(1): 69

[10] Yang CC, Zhu DQ, Pan J, Zhou BZ, Xun H. Oxidation and induration characteristics of pellets made from Western Australian ultrafine magnetite concentrates and its utilization strategy. J. Iron Steel Res. Int., 2016, 23(9): 924

[11] Yang ZC, Liu ZG, Chu MS, Gao LH, Feng C, Tang J. Effect of basicity on metallurgical properties of fluxed pellets with high MgO content. ISIJ Int., 2021, 61(5): 1431

[12] Zhang YB, Chen XJ, Su ZJ, et al.. Improving properties of fluxed iron ore pellets with high-silica by regulating liquid phase. J. Iron Steel Res. Int., 2022, 29(9): 1381

[13] Gan M, Ji ZY, Fan XH, et al.. Value-added utilization of waste silica powder into high-quality chromite pellets preparation process. Powder Technol., 2018, 328: 122

[14] El-Geassy AA, Nasr MI, Mousa EA. Influence of manganese oxide and silica on the morphological structure of hematite compacts. Steel Res. Int., 2010, 81(3): 178

[15] Mohanty MK, Mishra S, Mishra B, Sarkar S. Effect of basicity on the reduction behavior of iron ore pellets. Arab. J. Sci. Eng., 2018, 43(11): 5989

[16] Park H, Sahajwalla V. Effect of alumina and silica on the reaction kinetics of carbon composite pellets at 1473 K. ISIJ Int., 2014, 54(1): 49

[17] D. Spreitzer and J. Schenk, Reduction of iron oxides with hydrogen: A review, Steel Res. Int., 90(2019), No. 10, art. No. 1900108.

[18] Zuo HB, Wang C, Dong JJ, Jiao KX, Xu RS. Reduction kinetics of iron oxide pellets with H2 and CO mixtures. Int. J. Miner. Metall. Mater., 2015, 22(7): 688

[19] J.H. Zhang, L.W. Wang, C.H. Wang, S.Y. Ren, and S.Y. Li, Mechanism and kinetics of direct reduction reaction between H₂/CO and hematite powder in a fluidized bed, Chem. Eng. J., 507(2025), art. No. 160476.

[20] Zhao ZC, Tang J, Chu MS, et al.. Direct reduction swelling behavior of pellets in hydrogen-based shaft furnaces under typical atmospheres. Int. J. Miner. Metall. Mater., 2022, 29(10): 1891

[21] Abdelrahim A, Iljana M, Omran M, Vuolio T, Bartusch H, Fabritius T. Influence of H₂–H₂O content on the reduction of acid iron ore pellets in a CO–CO₂–N₂ reducing atmosphere. ISIJ Int., 2020, 60(10): 2206

[22] A.A. El-Geassy, Rate controlling step in the reduction of iron oxides; kinetics and mechanism of wüstite-iron step in H₂, CO and H₂/CO gas mixtures, IOP Conf. Ser. Mater. Sci. Eng., 229(2017), No. 1, art. No. 012002.

[23] Duan BW, Han XL, Si TH, Fang SL, Li FJ, Xu LP. Metallogenic mechanisms of three typical high-basicity sinter: Hematite-type, magnetite-type and vanadium-titanium magnetite-type. Int. J. Miner. Metall. Mater., 2026, 33(4): 1126

[24] Tang CM, Yang CC, Pan J, Zhu DQ, Lu LM, Guo ZQ. Oxidation behavior of high FeO ferrous spinels and its impacts on the induration characteristics of oxidized pellets. Metall. Mater. Trans. B, 2024, 55(5): 3072

[25] Wang P, Wang CQ, Wang HT, Long HM, Zhou TB. Effects of SiO₂, CaO and basicity on reduction behaviors and swelling properties of fluxed pellet at different stages. Powder Technol., 2022, 396: 477

[26] Guo YF, Liu K, Chen F, et al.. Effect of basicity on the reduction swelling behavior and mechanism of limestone fluxed iron ore pellets. Powder Technol., 2021, 393: 291

[27] Li BH, Zhu DQ, Guo ZQ, Song LG, Pan J, Li SW. Impact of SiO₂ content on the hydrogen-based direct reduction of acidic high-grade fired hematite pellets. J. Sustainable Metall., 2025, 11(2): 1078

[28] Sarkar A, Chavan V, Pai NN, et al.. Reduction of iron ore pellets: A microstructural perspective?. Metall. Mater. Trans. A, 2024, 55(2): 537

[29] Y. Shi, D.Q. Zhu, J. Pan, Z.Q. Guo, S.H. Lu, and M.J. Xu, Improving hydrogen-rich gas-based shaft furnace direct reduction of fired hematite pellets by modifying basicity, Powder Technol., 408(2022), art. No. 117782.

[30] A. Vickerfält, J. Martinsson, and S.C. Du, Effect of reduction degree on characteristics of slag formed by melting hydrogen-reduced DRI and partitions of P and V between slag and metal, Steel Res. Int., 92(2021), No. 3, art. No. 2000432.

[31] Lei J, Zhang C, An JS, et al.. Mechanism study on gas-based reduction swelling behavior of ultra-high grade pellets. J. Mater. Res. Technol., 2023, 26: 823

[32] Dwarapudi S, Ghosh TK, Shankar A, Tathavadkar V, Bhattacharjee D, Venugopal R. Effect of pellet basicity and MgO content on the quality and microstructure of hematite pellets. Int. J. Miner. Process., 2011, 99(1–4): 43

[33] Prusti P, Barik K, Dash N, Biswal SK, Meikap BC. Effect of limestone and dolomite flux on the quality of pellets using high LOI iron ore. Powder Technol., 2021, 379: 154

[34] Du Z, Zhu QS, Fan CL, Pan F, Li HZ, Xie ZH. Influence of reduction condition on the morphology of newly formed metallic iron during the fluidized bed reduction of fine iron ores and its corresponding agglomeration behavior. Steel Res. Int., 2016, 87(6): 789

[35] Wang HT, Sohn HY. Effects of reducing gas on swelling and iron whisker formation during the reduction of iron oxide compact. Steel Res. Int., 2012, 83(9): 903

[36] Barustan MIA, Jung SM. Morphology of iron and agglomeration behaviour during reduction of iron oxide fines. Met. Mater. Int., 2019, 25(4): 1083

[37] Wang HT, Sohn HY. Effect of CaO and SiO₂ on swelling and iron whisker formation during reduction of iron oxide compact. Ironmaking Steelmaking, 2011, 38(6): 447

[38] Yang CC, Wei L, Zhu DQ, Pan J, Xia GH, Qu SJ. Hydrogen-based direct reduction behavior of iron ore pellets with iron grades ranging from 59% to 68%. Int. J. Hydrogen Energy, 2025, 138: 787

[39] Sarkar A, Schanche TL, Wallin M, Safarian J. Kinetics study on the H2 reduction of Nchwaning manganese ore at elevated temperatures. Int. J. Miner. Metall. Mater., 2025, 32(5): 1091

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