氢基预还原铬铁矿的还原进程与固结机理研究

Reduction process and pellet consolidation mechanism of hydrogen based pre-reduced chromite

  • 摘要: 铬铁矿预还原是铬铁合金生产过程中减污降碳的重要手段,当前铬铁矿预还原仍采用配碳球团氧化焙烧的方法,能耗和碳排放居高不下,因此在“双碳”背景下,铬铁矿氢基预还原工艺路线的开发意义重大。本研究通过在水平管式炉中对铬铁矿球团进行不同条件下的恒温还原,深入分析了氢基还原焙烧铬铁矿过程中球团的还原进程和固结机理,结果表明:在铬铁矿还原中,还原温度、还原时间、H2:CO比率与铁的金属化率和脱氧率成正比;当铬铁矿球团在温度为1300℃的纯H2条件下还原3小时后,Fe的金属化率达到85.9%。XRD和SEM结果显示,铬铁矿矿相结构为复杂尖晶石结构(Mg, Fe)(Cr, Fe, Al)2O4,其中Fe3+位点的Fe原子优先还原,而仅有少量的Cr原子被还原,表明H2对Cr的还原能力较弱。球团强度方面,强度随着铁金属化率的增加呈现先升高后降低的趋势,研究表明铁金属化率的升高降低了粘结相中Fe2+的比率,使球团中粘结相的熔点升高,颗粒间粘结力下降,导致球团强度降低。球团强度的提升主要是由高温条件下熔融金属Fe和粘结相的粘结作用所致。当还原温度保持在1200℃以上时,球团的平均强度高于1000N,满足球团入炉的强度要求。该研究有望为铬铁矿氢基预还原工艺路线的开发提供理论支撑。

     

    Abstract: Ferrochrome alloy is a critical raw material in the production of stainless steel, corrosion-resistant steels, and high-temperature alloys, with global demand continuously rising. However, its production is highly energy-intensive, primarily due to the high-temperature reduction requirement of Cr2O3 (>1600?°C) and the elevated melting point of chromium-containing melts (1900~2050?°C). Pre-reduction of chromite ore is an effective approach to lowering energy consumption and carbon emissions in ferrochrome production. At present, carbon-bearing pellet oxidation roasting remains the mainstream method, which results in high energy use and CO2 emissions. The advancement of hydrogen metallurgy offers a promising alternative, and under the “dual carbon” policy framework, developing a hydrogen-based pre-reduction process for chromite holds significant strategic value. In this study, pre-reduction experiments were conducted on chromite pellets using a horizontal tube furnace. A self-developed proton flow meter in combination with a multi-gas mixing system was employed to precisely control the furnace atmosphere. The pellets were first heated to the target temperature under an argon atmosphere, followed by isothermal reduction in a H2–CO mixed gas atmosphere. Upon completion of reduction, argon was reintroduced during cooling to room temperature to prevent reoxidation. The effects of reduction parameters—including the H2/CO ratio, temperature, and time—on iron metallization rate, Fe2+ conversion, and compressive strength were systematically investigated. Post-reduction, the compressive strength of pellets was tested using a universal testing machine in accordance with the national standard GB/T 14201-2018. Coupled with chemical analysis, XRD phase identification, and SEM microstructural characterization, the mineral phase transformation and microstructural evolution mechanisms of the pellets were clarified, and the synergistic mechanism between reduction and consolidation was revealed. The results show that the reduction temperature, time, and H2:CO ratio are positively correlated with the iron metallization and deoxygenation rates. Under pure H? at 1300?°C for 3 hours, the Fe metallization rate reached 85.9%. XRD and SEM analyses revealed that the chromite phase exhibits a complex spinel structure of the form (Mg, Fe)(Cr, Fe, Al)2O4. Fe atoms at Fe3+ sites were preferentially reduced, whereas only a limited amount of Cr was reduced, indicating the relatively weak reducing ability of H2 for Cr. In terms of mechanical performance, pellet strength first increased and then decreased with rising iron metallization. The decline in Fe2+ content in the bonding phase at higher metallization levels led to an increase in the bonding phase's melting point, resulting in poor fluidity and reduced softening and solidification ability. Strength enhancement was mainly attributed to the bonding effect between molten metallic Fe and the bonding phase at high temperatures. When the reduction temperature exceeded 1200?°C, the average compressive strength of the pellets remained above 1000?N, meeting the strength requirements for furnace charging. However, further increases in the metallization degree continued to reduce Fe2? content in the bonding phase, elevating its melting point, weakening interparticle bonding, and thus decreasing pellet strength. The reduction process of chromite pellets can be divided into four distinct stages: preheating, solid-state reduction, softening and consolidation, and over-reduction-induced weakening. This study is expected to provide theoretical support for the development of hydrogen-based pre-reduction processes for chromite ore.

     

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