Direct reduction

In the iron and steel industry, direct reduction is a set of processes for obtaining iron from iron ore, by reducing iron oxides without melting the metal. The resulting product is pre-reduced iron ore.
Historically, direct reduction was used to obtain a mix of iron and slag called a bloom in a bloomery. At the beginning of the 20th century, this process was abandoned in favor of the blast furnace, which produces iron in two stages.
However, various processes were developed in the course of the 20th century and, since the 1970s, the production of pre-reduced iron ore has undergone remarkable industrial development, notably with the rise of the. Designed to replace the blast furnace, these processes have so far only proved profitable in certain economic contexts, which still limits this sector to less than 5% of world steel production.

History

Bloomery

Historically, the reduction of iron ore without smelting is the oldest process for obtaining steel. Low-temperature furnaces, unable to reach the melting temperatures of iron alloys, produce a bloom, a heterogeneous agglomerate of metallic iron more or less impregnated with carbon, gangue, and charcoal. This process was gradually succeeded, from the 1st century in China and the 13th century in Europe, by the blast furnace, which simultaneously reduces and melts iron.
Elaborate low furnaces, such as the tatara or the Catalan forge, survived until the early 19th century. Compared with the indirect process, these processes only survived when they enjoyed at least one of the following two advantages:

ability to process ores that are incompatible with blast furnaces ;
a more "reasonable" size than that of giant plants and their constraints.
Modern direct reduction

More advanced direct reduction processes were developed at the beginning of the 20th century, when it became possible to smelt pre-reduced ores using the Martin-Siemens process or the electric arc furnace. Based on this technical and economic model, a number of processes were industrialized before World War II. They remained confidential, however, and their profitability was generally debated.
Modern direct reduction processes, based on the use of natural gas instead of coal, were studied intensively in the 1950s. On December 5, 1957, the Mexican company Hylsa started up the first industrial production unit of this type in Monterrey, with the pre-reduced ore obtained destined for smelting in an electric arc furnace. As the production of pre-reduced ore with natural gas was economically viable, several plants were built in the late 1960s. As a cheap supply of natural gas was essential to their profitability, most plants were located in countries with gas deposits, in Latin America and in the Middle East.
In 1970, worldwide production of pre-reduced iron ore reached 790,000 tonnes. The processes then in operation were the HYL process, an SL/RN unit, a Purofer unit, and the first plant to use the Midrex process.
Although profitable and innovative, the processes invented did not ultimately prove to be a technological revolution capable of supplanting the traditional blast furnace-based process. However, the quantity of steel produced from pre-reduced materials grew steadily, outstripping world steel production:

in 1975, NML played a significant role in developing a ‘Direct Reduction Technology’ for producing sponge iron with solid fuel like non-metallurgical coal. This formed the basis of the first commercial sponge iron plant of India.
in 1976, installations in service totalled less than 5 Mt;
in 1985, annual production was 11 Mt for an installed capacity of around 20 Mt, the difference being explained by fluctuations in energy costs;
in 1991, production reached 20 Mt.
in 1995, worldwide production of prereducts passed the 30 Mt mark for the first time.
In 2010, 70 Mt were produced, 14% from HYL processes and 60% from the Midrex process. The latter accounts for most of the growth in natural gas-fired production of pre-reduced products, although since 2005 coal-fired processes have been making a strong comeback, mainly in India.

Packaging of pre-reduced iron ore is evenly divided between sponge iron and briquettes. Sponges are a highly porous metallic product, close to the original ore but highly pyrophoric, which limits their transport. They are therefore often subjected to hot compaction, which improves both product density and handling safety. In 2012, 45% of prereducts were transformed into briquettes in this way.

Chemical reactions

Iron oxide reduction

s are reduced in the following sequence:
Each transition from one oxide to the next is due to two simultaneous high-temperature reduction reactions by carbon monoxide CO or dihydrogen H₂:
These temperatures differ from those predicted by the Ellingham diagram. In reality, there is a coupling between carbon monoxide reduction and dihydrogen, so that these reactions work together, with hydrogen significantly improving the efficiency of CO reduction.

Reducing gas production

Coal-fired processes

In coal-fired processes, part of the fuel is first burnt to heat the charge. The product of this combustion is CO₂. When the temperature reaches 1,000 °C, the CO₂ reacts with the unburned carbon to create CO:
CO2 + C ⇌ 2 CO when T > 1 000 °C
The production of H₂ cannot be achieved by the thermal decomposition of water, as the temperatures involved are too low. Hydrogen is in fact produced along with carbon monoxide by the reaction:
H₂O + C → H₂ + CO when T > 1 000 °C
These two reducing gas production reactions, which consume 172.45 and 131.4 kJ/mol respectively, are highly endothermic and operate by limiting charge heating.

Natural gas processes

The reducing atmosphere, rich in CO and H₂, can be created from the high-temperature cracking of natural gas at around 1100-1150 °C, in the presence of oxidized gases from ore reduction reactors.
The system that generates the reducing gases is called a "reformer". In the Midrex process, it consists of tubes heated by the combustion of a portion of the gas from the reactor.

Procedures

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. 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.

Tank processes

In these processes, iron ore is brought into contact with reducing gases produced and heated by a separate plant in a closed enclosure. As a result, these processes are naturally suited to the use of natural gas.

Cyclic processes

In these processes, the ore is fed into a tank, where it remains until it is completely reduced. The vessel is then emptied of its pre-reduced ore, and filled with another charge of untreated ore. These processes can therefore be easily extrapolated from laboratory experiments. What's more, their principle, based on batch production, facilitates process control.

Natural gas processes

In natural gas cyclic processes, a unit produces hot reducing gas, which is injected into the reactor. To ensure continuous operation of the unit converting natural gas into reducing gas, several tanks are operated in parallel and with a time lag.
The best-known of this type is HYL I and its improved variant, HYL II. This is the oldest industrial direct gas reduction process, developed in Mexico in 1957 by the Hylsa company.

Retorts

These are exclusively coal-fired processes, with the reducing gases generated inside the reduction vessel. The ore is charged with coal into a closed container. This is then heated until the oxygen present in the ore combines with the carbon before being discharged, mainly in the form of CO or CO₂. This production of gas by heating a solid material means that the reactor belongs to the retort category.
The principle is an ancient one: in northern China, the shortage of charcoal led to the development of processes using hard coal before the 4th century. To avoid any contact between iron and sulfur, the brittle element provided by coal, China developed a process that involved placing iron ore in batteries of elongated tubular crucibles and covering them with a mass of coal, which was then burned. This process survived into the 20th century.
More recently, other historic processes have come to the fore, such as that of Adrien Chenot, operational in the 1850s in a number of plants in France and Spain. Successive improvements by Blair, Yutes, Renton, and Verdié are not significant. Among the processes developed is the HOGANAS process, perfected in 1908. Three small units are still operational. Not very productive, it is limited to the production of powdered iron, but as it is slow and operates in closed retorts, it easily achieves the purities required by powder metallurgy.
Other retort processes were developed, such as KINGLOR-METOR, perfected in 1973. Two small units were built in 1978 and 1981.