Effect of basicity on consolidation and hydrogen-based reduction swelling behavior of low-silica high-alumina pellets

The hydrogen-based shaft furnace-electric arc furnace short process represents one of the most promising pathways for the green and low-carbon development of the steel industry. To address the challenges of poor consolidation and unclear reduction swelling behavior of low-silicon high-alumina hematite pellets used in hydrogen-based shaft furnaces, the effect of basicity on the consolidation strength and reduction swelling performance of such pellets was systematically investigated under a hydrogen-based atmosphere, along with the underlying mechanisms. Furthermore, the reduction swelling behavior was regulated by adjusting the reduction temperature and atmosphere. The results indicate that the consolidation strength of low-silica, high-alumina pellets does not monotonically increase with increasing basicity. Instead, a trough in consolidation strength occurs at a basicity of 0.8. This is primarily attributed to the substantial formation of the Ca₃Fe₂Si₃O₁₂ phase, which possesses a relatively high melting point. Consequently, the amount of liquid phase generated in the pellets is minimized, and combined with the low degree of Fe₂O₃ crystallization and high porosity, the consolidation strength remains low. As basicity increases, the reduction swelling index (RSI) first increases and then decreases. Within the basicity range of 0.4-0.8, the RSI reaches a maximum exceeding 17%, defining a "basicity forbidden zone" for reduction swelling. This phenomenon occurs because the elevated basicity causes Si⁴⁺ ions, which would otherwise occupy Fe_xO vacancies, to be "captured" by Ca²⁺ ions, thereby enhancing the nucleation of iron whiskers at active sites and exacerbating reduction swelling. When regulating the consolidation strength and reduction swelling of low-silicon high-alumina pellets by increasing basicity, the range around a basicity of 0.8 should be avoided. An increase in temperature from 900 ℃ to 1 000 ℃ aggravates the reduction swelling of low-silica high-alumina pellets. Under three different process atmospheres, the effect of basicity on reduction swelling varies, and adjusting the H₂/CO volume ratio can narrow the "basicity forbidden zone" for reduction swelling. These findings provide technical support for broadening the range of pellet feedstocks suitable for hydrogen-based shaft furnaces and promote the advancement of green and low-carbon ironmaking in the steel industry under the "dual carbon" goals.