Direct / indirect iron reduction

Direct / indirect iron reduction

The direct and indirect iron reduction process is necessary to remove the Fe2O3 oxides and convert it to the FE (HRD or Sponge Iron) required for steelmaking.

In the steel industry, there are two methods of reducing iron ore:

Indirect reduction: Iron ore is transformed using a blast furnace (BF) to obtain pig iron, which is then refined to produce steel. This is the process used in blast furnaces.

Direct reduction: Iron ore in pellet form and in its natural state, is deoxygenated to obtain the metallic iron used to manufacture steel.

Indirect reduction: Blast furnace

A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, usually pig iron, but also others such as lead or copper. “Blast” refers to “forced” or supplied combustion air above atmospheric pressure.

Indirect Reduction Process Chemistry

In a blast furnace, fuel (coke), ores, and flux (limestone) are continuously supplied through the top of the furnace, while hot air (sometimes oxygen-enriched) is injected into the bottom section from the furnace through a series of pipes called nozzles or burners, so that chemical reactions take place throughout the kiln as the material falls downward. The end products are usually the molten metal and slag phases drawn from the bottom, and the fuel gases exiting the top of the furnace. The downflow of ore and the upflow of hot, carbon monoxide-rich fuel gases is a process of countercurrent exchange and chemical reaction:

  1. 3 Fe2O3 + C –> 2 Fe3O4 + CO
  2. Fe3O4 + C –> 3 FeO + CO
  3. FeO + C –> Fe + CO

The end result of the indirect reduction in BF is called the pig iron. The pig iron has to go through a refining process to reach the required quality.

Direct reduction

Directly reduced iron (DRI), also called sponge iron, is produced from the direct reduction of iron ore (in the form of lumps, pellets, or fines) by a reducing gas or elemental carbon produced from natural gas or coal. Many minerals are suitable for direct reduction.

Reduced iron derives its name from the chemical change that iron ore undergoes when heated in a furnace at high temperatures in the presence of gases rich in hydrocarbons, carbon monoxide, or elemental carbon. Direct reduction refers to processes that reduce iron oxides to metallic iron at temperatures below the melting point of iron. The product of such solid-state processes is called directly reduced iron (DRI).

The reducing gas is a mixture of gases, mainly hydrogen (H2) and carbon monoxide (CO). The process temperature is typically 800 to 1200 °C.

Direct reduction process chemistry

1. Hemetite is converted to magnetite by reduction with carbon monoxide and hydrogen.

2. Magnetite is converted to ferrous oxide by reduction with carbon monoxide and hydrogen.

3. Ferrous oxide is converted to sponge iron by reduction with carbon monoxide and hydrogen.

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