Detailed explanation
Direct reduction is a more environmentally friendly and energy-efficient alternative to the conventional blast furnace route of steel production. In this process, iron ores are reduced to sponge iron using reducing agents such as natural gas or coal. Sponge iron is a porous, sponge-like iron product with a high degree of purity. It is used in further processing as a raw material for steel production.
The process takes place in the absence of air, which means that the iron ore does not melt, but only the oxygen is removed. This takes place in various types of industrial furnaces, such as rotary kilns, rotary hearth furnaces, deck furnaces, shaft furnaces and fluidised bed reactors.
Variants of direct reduction
Solids reduction
Coal is primarily used as the reducing agent here. Common processes include the SL/RN process and the DRC process. These processes use rotary kilns or rotary hearth furnaces.
- In rotary kilns, iron pellets are reduced to sponge iron together with coal and other additives. This takes place at temperatures of around 900 to 1,000 °C.
- Rotary hearth furnaces reach temperatures of up to around 1,300 °C. They use various processes such as the Fastmet, Inmetco, ITmk3 or Iron Dynamics process.
- Multi-deck furnaces are mainly used in the Primus process. They utilise several stacked chambers to reduce the iron ore at temperatures of up to 1,100 °C.
Gas reduction
This variant mainly utilises natural gas, which is processed in an upstream reforming process. Common processes include the Midrex process and the HyL/Energiron process.
In shaft furnaces, the reduction gas is introduced into the lower furnace section. At the same time, the iron pellets are moved through the furnace from top to bottom. This results in an effective countercurrent reaction in which the pellets are reduced to sponge iron.
Fluidised bed reactors work according to the principle of the fluidised bed, in which fine solid particles are kept in suspension by a gas flow. In processes such as Fior, Finmet or Circored, the fine iron ores pass through several fluidised bed reactors in succession. They are gradually reduced in the process. Fluidised bed reactors are technologically more demanding and are used less frequently in industry than shaft furnaces.
Advantages of direct reduction
1. More environmentally friendly: A major advantage of direct reduction compared to blast furnaces is that no coke is required. This variant of steel production therefore eliminates the need for a coke oven system. This leads to a reduction in energy consumption and CO₂ emissions.
The environmental friendliness of direct reduction depends largely on the reducing agent used. While the use of natural gas reduces CO₂ emissions compared to the blast furnace process, the use of green hydrogen enables almost emission-free production. This makes direct reduction an important transition technology on the way to climate-neutral steel.
2. Flexibility: Direct reduction can process various forms of iron ore, from fine ores to pellets. This makes the process adaptable and versatile in its choice of raw materials.
3. Efficiency: The process enables continuous production and high purity of the end product. Sponge iron has a high iron content and contains fewer impurities such as sulphur or phosphorus. This leads to a better quality of the subsequent steel.
