Direct reduction
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.
[nb 2] 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.
[nb 3][4] As the production of pre-reduced ore with natural gas was economically viable, several plants were built in the late 1960s.
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 (H2O and CO2) from ore reduction reactors.
[15] 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.
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.
[25] 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.
Based on the principle of counter-current piston flow, these processes are the closest to the blast furnace or, more accurately, the stückofen.
This similarity to a blast furnace without its crucible made it one of the first processes explored by metallurgists, but the failures of the German Gurlt in 1857, and the French Eugène Chenot (son of Adrien) around 1862, led to the conclusion that "the reduction of iron ore [...] is therefore [not] possible in large quantities by gas alone".
[9][17] Its direct competitor, the HYL III process, is the result of a research effort by the Tenova Group (de), heir to the Mexican Hylsa pioneers.
[35][36] Technically mature but more complex, they are at a disadvantage compared with equivalent gas-fired processes, which require slightly less investment.
[37]Given that direct reduction is a chemical exchange between gas and solid, the fluidization of ore by reducing gases is an attractive line of research.
However, the changing nature of the constituents, combined with the high temperature and the difficulty of controlling the fluidization phenomenon, make its adoption singularly difficult.
Rotary hearth processes, where the ore rests on a fixed bed and travels through a tunnel, fall into the first category.
[47]This non-exhaustive list shows that, despite the keen interest shown by steelmakers in developed countries during the 1990s, none of these processes met with commercial success.
The tool used then evolved into a long tubular rotary kiln, inspired by those used in cement works, as in the Basset process, developed in the 1930s.
[23] Finally, Indian steelmakers are behind the SIIL, Popurri, Jindal, TDR and OSIL processes, which are simply variants[nb 13] developed to meet specific technical and economic constraints.
[12] Other processes, built on the same principle, failed to develop, such as the Strategic-Udy,[20] consisting of a single plant commissioned in 1963 and shut down in 1964.
[29] One idea is to carry out the entire reduction-melting process in the arc furnace installed downstream of the reduction plant.
All these processes share the electric furnace's advantage of low investment cost, and its disadvantage of using an expensive energy source.
The COREX process, in operation since 1987, consists of a direct-reduction shaft reactor feeding a blast furnace crucible, in which the pre-reduced ore is brought to a liquid smelting state, consuming only coal.
[58] The TECNORED process, studied in Brazil,[59] also performs reduction-melting in the same vessel, but is more akin to a blast furnace modified to adapt to any type of solid fuel.
[60] Of all the processes of this type that have been developed, a single ISASMELT-type industrial unit built in Australia, with a capacity of 0.8 Mt/year,[61] operated from 2005 to 2008[62] before being dismantled and shipped to China, where it was restarted in 2016.
The techno-economic model of the mini-mill, based on flexibility and reduced plant size, was threatened by a shortage of scrap metal, and a consequent rise in its price.
- L'Usine nouvelle, September 1998, La réduction directe passe au charbon.This explains the development of certain reduction-melting processes which, because of the high temperatures involved, have a surplus of reducing gas.
Reduction-melting processes such as the COREX, capable of feeding an ancillary Midrex direct[54] reduction unit, or the Tecnored, are justified by their ability to produce CO-rich gas despite their higher investment cost.
[64] In addition, coke oven gas is an essential co-product in the energy strategy of a steel complex: the absence of a coke oven must therefore be compensated for by higher natural gas consumption for downstream tools, notably hot rolling and annealing furnaces.
In 2007, the breakdown was as follows:[54] China, a country with gigantic needs and a deficit of scrap metal, and Europe, lacking competitive ore and fuels, have never invested massively in these processes, remaining faithful to the blast furnace route.
[30] In addition to reducing CO2 emissions, pure hydrogen processes such as Hybrit are being actively studied with a view to decarbonizing the steel industry.
