Steel Making with Coal

Some 70% of total steel production is based on the smelting of iron ore in blast furnaces and the subsequent refining of the iron into steel, mainly in Basic Oxygen Furnaces (BOF). A blast furnace typically uses iron ore, coke (made from coal), small quantities of limestone, and, where Pulverised Coal Injection (PCI) is employed, pulverised or granulated thermal coal.

Source: World Coal Institute

Iron ore, mined in many countries, is a mineral containing iron oxides. Commercial ore grades usually have an iron or ferrous content of at least 58%. Most of the ore fed to the furnace is finely ground and then mixed and heated with coke fines to form 'sinter'. Smaller quantities of natural lump or pelletised ores are then added.

Coke is made from coking coals, which have certain physical properties that cause them to soften, liquefy and then resolidify into hard but porous lumps when heated in the absence of air. Coking coals must also have low sulphur and phosphorous contents.

Coal is carbonised in batteries of coke ovens. The coal blend, crushed to a maximum size of 3 mm, is poured into the top of the ovens and heated to above 1200°C over a period of 18-20 hours. The volatile contents of the coal are driven off as coke oven gas, which is first cleaned to remove impurities and yield by-products such as tar and benzole; then used to heat the ovens themselves and as fuel elsewhere in the steelworks. The red-hot coke is pushed out of the ovens, cooled and screened to remove the smaller sizes. The larger sized material - typically above 30 mm - goes to the blast furnace, where it:

• supplies carbon as a reducing agent, removing the oxygen from the ore;
• provides heat to melt the iron;
• acts as a load-bearing but permeable layer, supporting the burden whilst allowing the reducing gases to pass through.

Ore, coke and limestone are fed into the top of the furnace. The hot air blast and, if PCI is installed, the pulverised coal, are injected through nozzles into the base of the furnace. The pulverised coal injected in this way is used as a less expensive source of carbon and fuel. The molten iron or hot metal are periodically tapped from the bottom of the furnace and taken directly to the Basic Oxygen Furnace. Steel scrap and more limestone are added, and oxygen is blown onto the liquid metal, which is 93-95% pure iron at this stage. The reaction with the oxygen raises the temperature to 1600-1650°C and oxidises the impurities to leave almost pure liquid steel.

Blast furnaces with PCI require 350-400 kg of coke, made from 525-600 kg of coking coal, plus 100-200 kg of cheaper PCI coal - around 700 kg of coal for each tonne of hot metal produced. Furnaces without PCI use more coke, also equivalent to some 700 kg of coal, but all of it the more expensive coking coal. As each tonne of steel uses approximately 90% hot metal and 10% scrap, about 630 kg of coal are used per tonne of steel.

About 630 kg of coal are used to produce 1,000 kg of steel.

Some 30% of world steel is produced in Electric Arc Furnaces, which melt scrap iron and steel. Much of the electricity used in arc furnaces is generated in coal-fired power stations.

New processes are being developed for the direct reduction of iron (DRI), eliminating blast furnaces and coke ovens and the need for coke. For examples of such technologies in Australia, see: Coal Use and the Environment - Reducing the Environmental Impacts of Coal Use.

However, DRI plants will still use coal as a fuel and a reductant, and will account for only a small percentage of the world steel output for many years.

For the foreseeable future, coal will remain indispensable to the production of steel.

[Source: World Coal Institute]

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