A Study On The Direct Reduction Characteristics Of Iron Ore-coal Composite Pellets Under Gas-solid Base

Non-blast furnace iron-making technology is one of the main directions of iron and steel industry technology development in recent years. Domestic direct reduced iron production is dominated by coal-base. Iron ore-coal composite pellet has developed rapidly because of its wide raw materials applicability, strong mineral powder granularity adaptability and light environmental pollution. The application of iron ore-coal composite pellets improved the reduction to a certain extent, but there are still some problems in the reduction process, such as high energy consumption, poor product quality and easy bonding. Therefore, research on the use of small amounts of reducing gas to overcome the shortcomings of the existing coal-base direct reduction is necessary. This study carried on a systematic research on the direct reduction characteristics of iron ore-coal composite pellets under the gas-solid base collaborative action, using iron concentrate and red mud as research object and based on the gas-base and coal-base direct reduction. Investigation results provide a foundation for further research and application. The main research work and results include the following aspects.(1) Iron concentrate and red mud were studied using different physical and chemical methods. The basic characteristics such as chemical composition, phase composition and particle morphology of iron concentrate and red mud were understood. The influence of the water content and the amount of binder to the iron ore-coal composite pellets was researched. On this basis, the influencing factors of direct reduction was determined through orthogonal experiment, and the order is reaction temperature> mineral powder particle size> reaction time.(2) The change rule of the reduction characteristics of iron ore-coal composite pellets in N2atmosphere was studied through experiments. The mean deviation and the root-mean square deviation were employed for comparing the experimental data and the calculation results which were taken from different speed control models. The error of calculation results and experimental data was used to determine the rate controlling step. The results show that when the temperature is below900℃, the reduction of both concentrate iron ore-coal composite pellets and red mud iron ore-coal composite pellets is controlled by carbon gasification reaction. When the temperature is higher than900℃, the reduction mechanism changes and three-dimensional gas-phase diffusion turns into the rate controlling step.(3) The change rule of the direct reduction characteristics of iron ore-coal composite pellets in CO atmosphere was studied based on the direct reduction of ordinary pellets in CO atmosphere. The study suggests that the reduction of ordinary pellets is controlled by chemical interface reaction and the reduction of iron ore-coal composite pellets is controlled by three-dimensional gas-phase diffusion. The reductant coal has an effect on the reduction mechanism of iron ore-coal composite pellets. The inlet of CO effectively overcomes the shortcoming of poor local reduction of iron ore-coal composite pellets.(4) The change rule of the direct reduction characteristics of iron ore-coal composite pellets in H2atmosphere was studied based on the direct reduction of ordinary pellets in H2atmosphere. The study reveals that as the temperature increases, the reduction rates of both ordinary pellets and iron ore-coal composite pellets increase rapidly. H2has a very good effect to improve the reduction of red mud iron ore-coal composite pellets. At1100℃, the concentrate iron ore-coal composite pellets and red mud iron ore-coal composite pellets obtain a metallization rate of98.02%and98.37%respectively in H2atmosphere.(5) The change rule of the compressive strength of iron ore-coal composite pellets in different gas atmospheres was studied. It is learned that with the increase of temperature, the compressive strength of both concentrate iron ore-coal composite pellets and red mud iron ore-coal composite pellets presents a trend of increase with the decrease of the first. When the temperature is bellow900℃, the compressive strength changes little. When the temperature is higher than900℃, the compressive strength will increase. The compressive strength in H2atmosphere is significantly lower than in N2and CO atmosphere. Pellets in H2atmosphere have a better reduction characteristic.