Technical description of blast furnace operation

Diagram af højovn
Operation of a modern coal-fueled blast furnace. The height is most often 20–30 m. Temperatures are shown for isotherms inside the furnace. The reactions which occur inside the furnace are shown at the right; underlined elements are in solution in iron.

This description is largely based on J. G. Peacey & W. G. Davenport, The iron blast furnace: Theory and practice, Oxford 1979, and Terkel Rosenqvist, Principles of extractive metallurgy, New York 1974.
Empirical research provides considerable detail on what happens inside the modern blast furnace, and that is what will be described here. Of large charcoal blast furnaces like those traditionally used in Sichuan it is possible to say that the basic principles are the same but with the major difference that all temperatures are lower. Of small charcoal blast furnaces like those of Dabieshan it is reasonable to assume that the basic principles are approximately the same, but with the possibility of surprising differences.

See the diagram. A modern blast furnace operates continuously for months or years at a time, with coke, ore, and flux being charged in the top, air being blown through numerous tuyères near the bottom, and molten iron and slag being tapped out of tapholes at the bottom. Operation continues until the furnace lining has been so damaged by the high temperatures that it is necessary to repair it.

The ore has been calcined (roasted) before charging, so that the iron in it is entirely in the form of Fe2O3 (ferric oxide, hematite). The fundamental reactions in the blast furnace are the reduction of this by CO (carbon monoxide), first to Fe3O4 (ferrosoferric oxide, magnetite), then to FeO (ferrous oxide, wustite), and finally to metallic iron. Some carbon, typically ca. 4 per cent by weight, dissolves in the iron near the bottom; with this carbon content the melting point of the iron is under 1200°C.

The combustion of coal at the tuyères produces CO2 (carbon dioxide), and this reacts with carbon to produce the necessary CO. The CO reacts with the iron oxides to produce CO2 again, this reacts with carbon to produce CO, and so forth in a cycle. The necessary conditions for each reaction are diagrammed above. What is important is that high temperatures and very high concentrations of CO in the furnace atmosphere are required for the reduction of FeO. The necessary concentration of CO is more readily obtained with charcoal as the fuel than with coke (because charcoal is more reactive), and therefore charcoal blast furnaces operate at lower temperatures.

Iron smelting would be easy if the ore were composed of nothing but iron oxide. In fact all ores contain significant amounts of SiO2 (silica) as well as other minerals: this unwanted material is collectively called the gangue of the ore. If the furnace is to operate continuously the gangue must be removed in molten form, but it will normally have a melting point which is much higher than the temperatures required for the reduction of iron oxides. Therefore a flux, typically CaCO3 (limestone), is charged along with the ore and the fuel. The flux is chosen to form with the gangue a slag with a practicably low melting point (less than 1400°C in modern practice). CaCO3 decomposes in the furnace to CaO (lime) and CO2, and the mixture of CaO and SiO2 has a much lower melting point than either mineral alone. Other minerals in the charge, either incidentally present in the gangue or intentionally added in the flux, may further depress the melting point of the slag. Especially important here is Al2O3 (alumina).

Limestone is not only an excellent flux, it also has the property that it can remove sulphur (S) from the liquid iron by the reaction shown. In modern practice, using coke with fairly high sulphur, large amounts of limestone are used, giving a CaO/SiO2 ratio in the slag as high as 1.2; in charcoal-fueled blast furnaces sulphur is rarely a problem, and much smaller amounts of limestone are used.

In some pre-modern Chinese blast furnaces no flux was used. It seems that in such cases one or more of several special conditions must hold: (1) the ore used may be very rich, i.e. contain only a small amount of gangue; (2) the ore may be ‘self-fluxing’, the gangue containing significant proportions of limestone or alumina; (3) the charcoal may be from a wood which has grown on chalky ground and therefore contains a significant proportion of lime; (4) in operation enough of the furnace lining may be consumed to add significant amounts of alumina or other minerals to the slag; (5) the internal form of the furnace may be arranged in such a way that a small amount of the reduced iron is re-oxidised to FeO near the bottom, providing a very effective flux for silica. In pre-modern furnace operation it also appears possible to tolerate a slag which is rather viscous and does not separate well from the iron: the product is a ‘slaggy’ iron which ironfounders, finers, and puddlers nevertheless are able to use without great problems.

Economies of scale

It can be seen in the diagram that in an iron blast furnace, under normal operation, there is a ‘zone of relatively constant temperature’ near the top of the shaft, in which very little change occurs. It happens that the existence of this zone has great importance, for it acts as a buffer which shields the regions further down in the shaft from outside perturbances. This means that such a furnace is extremely stable in operation, and remains so even when it is scaled up to an enormous size. Consequently the iron blast furnace is the largest machine used in modern industry, with volumes typically in the range 1500–3000 m3, producing typically 2500–5000 tons per day and reaching as high as 10,000 t/day.

A general rule is that for maximum efficiency a blast furnace should be as large as the supply of its raw materials and labour, and the market for its production, will reliably allow. No larger than that, for efficiency also requires that the furnace operate continually for years at a time; the cost of interruption of operation caused by labour or raw materials shortages, or by a failure of the demand for pig iron, will quickly eat up the intended economies of scale.

The principal technical reasons why a larger blast furnace is more efficient appear to be two. First, a larger volume of the whole furnace means less heat lost to the surroundings. Second, a reduction zone with a higher temperature and larger volume means faster reduction. Both factors lead to more efficient use of fuel. In addition, the larger furnace involves a smaller investment relative to production capacity, and requires a relatively smaller labour force.

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