Today, a significant portion of global steel production is dominated by two methods: Blast Furnace-Basic Oxygen Furnace (BF/BOF) and Direct Reduced Iron-Electric Arc Furnace (DR/EAF). The latter method is notably advancing and developing. The process of steel production based on direct reduction-electric melting involves two stages: production of Direct Reduced Iron (DRI) and its subsequent conversion into steel in an electric arc furnace or induction furnace.
Direct Reduced Iron (DRI) typically contains around ninety percent metallic iron and has a strong affinity for oxygen, exacerbating its susceptibility to reoxidation (spontaneous combustion). Therefore, the risk of reoxidation of sponge iron persists from production to consumption stages. This phenomenon can occur during transportation on conveyor belts within the plant, during long-distance transport by trucks, railways, and ships, as well as during storage in warehouses and silos.Spontaneous combustion of sponge iron, accompanied by its combustion, leads to various problems such as loss, production stoppage and reduction, damage to transportation and storage equipment, and other significant losses. Research indicates that one of the key factors influencing reoxidation and spontaneous combustion of sponge iron is the characteristics of the materials used in its production, an area where fundamental research correlating raw material properties with oxidation behavior of sponge iron has yet to be conducted.
The primary reasons contributing to the spontaneous combustion of sponge iron are as follows:
1. Reoxidation behavior and spontaneous combustion of Direct Reduced Iron are closely related to its characteristics, especially chemical composition, porosity, and distribution of pores, which are primarily influenced by the raw materials used in its production.
2. Direct Reduced Iron produced from low-grade iron ores, due to high gangue content, has a high self-heating temperature. This type of sponge iron also exhibits high initial oxidation rate and becomes highly susceptible to ignition once it reaches its self-heating temperature.
3. Self-heating temperature: Research indicates a critical temperature below which the reoxidation rate is very low, but it sharply increases once the temperature exceeds this critical point. This critical temperature is referred to as the self-heating temperature of Direct Reduced Iron.
4. Direct Reduced Iron derived from hematite iron ores tends to have a higher self-heating temperature compared to sponge iron from magnetite iron ores with similar properties (porosity, gangue content, etc.). Additionally, sponge iron from hematite iron ores exhibits slower initial oxidation rate and lower susceptibility to spontaneous combustion.
5. Direct Reduced Iron produced from iron ores with high sulfur content, due to the effect of sulfur removal during the pelletizing stage on the porosity and rapid oxidation rate in the initial stage, shows a high susceptibility to spontaneous combustion. In this context, the amount of sulfur in minerals (such as pyrite or pyrrhotite) and its distribution are crucial factors.
6. The higher the limestone consumption in pellet production, the greater the continuous porosity in Direct Reduced Iron production, leading to increased susceptibility to reoxidation and spontaneous combustion.
