Blast furnace

A blast furnace is a Four with internal combustion, intended for the manufacture of the cast iron starting from the iron ore. This cast iron is refined thereafter by heating (Décarburation) what makes it possible to produce Acier and ferrous derivatives.

History

During the 14th century, the hydraulic force is applied for the ventilation of the hearths or low hearth X used to extract iron from the ore. The use of Wheel S with paddles or cups to replace the man power makes it possible to increase the power of the winds. This allowed the increase height of the furnaces until reaching four to five meters. With a furnace this height and the temperatures permitted by the new bellows, iron once reduced combined with carbon, producing cast iron, whose melting point (approximately 1200°) is definitely lower than that of pure iron. One thus obtained liquid cast iron with the bottom of the furnace, and either the magnifying glass of pasty iron which it was necessary until there extracting from the furnace to lead it to forge.

The place and the exact date of appearance of the blast furnaces are not established yet with certitude  ; they seem to have been known at the 14th century in the area of Liège  ; structures excavated in Sweden and the 13th century could also be of the furnaces of this type. They spread at the 15th century in most of Northern Europe.

The major consequence of the production of cast iron is that the blast furnaces can function uninterrupted, the cast iron being periodically run, whereas the low hearth must be stopped to extract the magnifying glass which was formed there. At the 18th century, the duration of lighting of the blast furnaces generally went from five to nine months according to the availability of energy hydraulique  ; they were reloaded by the loud-voiced person out of charcoal, ore and limestone (a calcareous additive allowing a better evacuation of the impurities in the form of a dairy )   ; the cast iron was cast with twice a day.

In 1709 takes place first run with the coke with the the United Kingdom with Coalbrookdale (Shropshire) by Abraham Darby (1678 - 1717). The coke replaces the Charcoal in the food of the fourneau  ; it is produced by desulphuration of the Houille. The process with coke is improved by the son and small son of Abraham Darby (who are also called Abraham Darby). However, the cast iron produced this manner is initially unsuitable with refining (decarburization to produce wrought iron), which explains why at that time the use of coke does not spread. In 1760, the United Kingdom counts only seventeen blast furnaces with coke and it is only in 1780 that its use spreads.

In France, the first tests with coke take place only in 1769 with Hayange (today in the French department of the the Moselle) before are built, under the direction of the British William Wilkinson, blast furnaces with the coke of the Creusot. This production however pains to compete with the traditional production with the coal of bois : it exceeds it only in 1853, with the favor of the explosion of the request due to the development of the railroad. In 1860, still a third of the French cast iron is produced in blast furnaces with the charcoal.

At the 19th century the use of coke causes a radical change in the technique. The height of the blast furnaces reaches thirty meters. The bellows are replaced by cast iron pistons actuated by the vapor.

In 1828, the Neilson British with the idea to heat the winds (air injected into the bottom of the blast furnace). In 1829, it produces the first industrial machine which heats the air with 150°C (with Clyde, in the United Kingdom). This makes it possible to save a third of coal compared to the process of the time. The process is quickly adopted. In 1870, the Cooper British and Whitwell develop a process using hot and combustible gases rejected by the blast furnace to heat the winds.

Capacities and production

As an indication, in 1806, the production of cast iron in a blast furnace is 4 tons/day, in 1850 it passes to 15 tons/day. In 1974, blast furnace 4 of Arcelor Dunkirk, with its 14 m in diameter to the Creuset (this diameter is the most significant data to judge the capacity) allocates the European record. It can provide more 10  000 ton S of cast iron per day.

However, today, the majority of the blast furnaces, have a size slightly smaller, even if each restoration of a Creuset is generally an opportunity to increase its size. A diameter of 11 m for a day laborer production of 6  400 t/jour is characteristic of a large modern blast furnace. This tonnage corresponds well to the capacities of the tools located downstream, with the Aciérie.

Parallel to the increase in the size, the abandonment of the raffs, like the Lorraine Iron ore, allowed, equal dimension, almost to double the production of a blast furnace.

Among the Coproduit S resulting from a blast furnace, one can quote:

• the dairy of blast furnace, developed in public works. For a blast furnace functioning with high-grade iron ores, one generally reaches a proportion 1 ton of dairy for 2 tons of produced cast iron.
• gas recovered with Loud-voiced person, which corresponds to the hot Air injected with the Creuset, whose Oxygène became Carbon monoxide, is a good Combustible.

It is also necessary to mention the blast furnace like a production equipment of the Manganèse. Currently, 30% of the production of Manganèse are resulting from this die (the remainder being elaborate with the electric furnace).

Constitution and operation

• the tank, of cylindrical form widened in the 1/4 of its base, consists of brick S refractory S supported by a reinforcement external of metal beams.

• the loading is carried out by the top (Minerai of Fer, ferrous waste, coke or “Charbon”).
• the recovery of the cast iron takes place by Coulée with the bottom of the furnace.
• an injection of Air is carried out with broadest furnace, in order to maintain the Combustion coal, thus allowing the fusion of all the elements.
• Contrary to the point of run cast iron one operates a casting of Laitier, recovery of waste of fusion, or Scorie S.
• the temperature is variable according to the height in the tank (from top to bottom):
• 300 °C on the level of the Loud-voiced person, phase of Dessication;
• 400 °C with 800 °C, phase of Reduction;
• 900 °C with 1.200 °C, phase of Carburation;
• 1.800 °C, phase of fusion;
• 1.600 °C, phase of Liquefaction, place of castings.

Taking into account the strong presence of carbon during the process, the product obtained is an alloy iron-carbon of the type cast iron (carbon rate higher than 2.1%).

Chemical reactions

The blast furnace is a chemical engine, whose operation with counter-current (the gases go up whereas the solid matter goes down) ensures an excellent thermal efficiency to him.

Total reactions

The principle is to reduce by carbon monoxide iron oxides present in the iron ore metal.

Production of the reducing agent CO (Carbon monoxide):

The total reaction is the following one:

$\backslash \; mathrm\; \{C\; +\; \backslash \; frac\; \{1\}\; \{2\}\; O\_2\; \backslash \; longrightarrow\; CO\}$

Taking into account the excess of carbon and temperature, there is conversion of the totality of oxygen into carbon monoxide.

It is in fact produced by the succession of the two following reactions:

$\backslash \; mathrm\; \{C\; +\; O\_2\; \backslash \; longrightarrow\; CO\_2\}$

then

$\backslash \; mathrm\; \{C\; +\; CO\_2\; \backslash \; longrightarrow\; 2CO\}$ (reaction of Boudouard)

From there, the reaction of reduction of iron oxides is the following one:

$\backslash \; mathrm\; \{Fe\_2O\_3\; +\; 3CO\; \backslash \; longrightarrow\; 2Fe\; +\; 3CO\_2\}$

Coke thus has two functions:

• by combustion, it produces the reducing agent by combustion in particular at exit of the conduits. The reaction is strongly exothermic, one reaches temperatures of 2200°C.
• It consumes the Carbon dioxide (CO₂) produced by the reduction of iron oxides to regenerate the reducing agent (CO) iron oxides.

The reduction of iron oxides

The iron oxides are reduced according to the following sequence:

$\backslash \; mathrm\; \{Fe\_2O\_3\; \backslash \; longrightarrow\; Fe\_3O\_4\; \backslash \; longrightarrow\; FeO\; \backslash \; longrightarrow\; Fe\}$

The sequence of temperature on the level of the tank are the following (on the basis of the top of the tank according to the temperature:

• T > 320 °C
  $\backslash \; mathrm\; \{3Fe\_2O\_3\; +\; CO\; \backslash \; longrightarrow\; 2Fe\_3O\_4\; +\; CO\_2\}$

• 620°C < T < 950°C
  $\backslash \; mathrm\; \{Fe\_3O\_4\; +\; CO\; \backslash \; longrightarrow\; 3FeO\; +\; CO\_2\}$
• T > 950°C
  $\backslash \; mathrm\; \{FeO\; +\; CO\; \backslash \; longrightarrow\; Fe\; +\; CO\_2\}$

in the bottom of the tank, there is CO regeneration by the reaction of Boudouard at a temperature of approximately 1  000 - 1  050°C.

See too

• Iron and steel industry
• dairy Iron
• Manufacture of steel
• History of the production of steel
• Steel-works

External bonds

• Images of many blast furnaces
• Historical of the iron and steel industry, site of the commune of Poisson (Haute-Marne) (52)
• blast furnaces of the 14th century with Spa, by Georges Plunger.

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