Plants for the production of pre-reduced iron ore are known as direct reduction plants. The principle involves exposing iron ore to the reducing action of a high-temperature gas (around 1000 °C). This gas is composed of carbon monoxide and dihydrogen, the proportions of which depend on the production process.
Generally speaking, there are two main types of processes:
* processes where the reducing gas is obtained from natural gas. In this case, the ore is reduced in tanks;
* processes where the reducing gas is obtained from coal. The reactor is generally an inclined rotary kiln, similar to those used in cement plants, in which coal is mixed with limestone and ore, then heated.
Another way of classifying processes is to distinguish between those where the reducing gases are produced in specific facilities separate from the reduction reactor - which characterizes most processes using natural gas - and those where the gases are produced inside the fusion reactor: coal-fired processes generally fall into this category. However, many "gas-fired" processes can be fed by gasification units producing a reducing gas from coal.
In addition, since the melting stage is necessary to obtain alloys, reduction-melting processes have been developed which, like blast furnaces, produce a more or less carburized liquid metal. Finally, many more or less experimental processes have been developed.