Blast furnace is a metallurgical furnace that makes liquid iron known as “hot metal” from iron oxides through physical and chemical reactions. The furnace is fed with iron ore, limestone and coke at the top, with preheated air being blown through the bottom. The air is blown through multiple nozzles, called tuyeres. The air temperature is usually in the 900 to 1250 degree Celsius range. Upon rising, the temperature reaches close to 1650 degree Celsius, which helps produce the slag and liquid iron. The furnace temperature may even reach 2000 degree Celsius.

A blast furnace in broad daylight. Image credit: Flickr
A blast furnace in broad daylight. Image credit: Flickr

A blast furnace may have 12 to 40 tuyeres, depending on the furnace size. The air reaches the top of the furnace in a few seconds (typically 6-9 seconds) after undergoing several chemical reactions. The dumped raw materials need six to eight hours to reach the furnace’s bottom where the end products, liquid iron and slag, are made. These end products could be used for making pig iron, and also for processing copper, lead and similar metals.

Blast furnace usage is generally continuous and production stops only when the brick line must be replaced. This means the furnace could run non-stop for 5-10 years. However, planned maintenance could be performed whenever needed in between, turning to be the only stoppage periods.

Furnace Dimensions and Build

The furnace is a big, refractory brick-lined steel stack. Typically, a blast furnace could be 32 and 10 metres in height and diameter, respectively, with the ability to make anywhere between 1000 to 10000 tons of cast iron daily. In other words, a typical blast furnace can make iron needed for five cars in five minutes. Close to 70 percent of global steel production is courtesy blast furnaces.

Blast furnace diagram. Image credit: Flickr
Blast furnace diagram. Image credit: Flickr

To ensure zero loss of heat, the furnace has thick steel on the sides that are lined using refractory bricks. The furnace can be divided into the following zones:

  • Stack (extending from the furnace’s mid portion or bosh to the top): The stack has a fireclay brick lining to safeguard the furnace shell, and is always full with alternating coke layers during the procedure.
  • Bosh (intermediate zone): The hottest part of the furnace where the reaction between coke and air happens.
  • Hearth or floor (the furnace’s bottom): The place where molten iron accumulates. It has a tap hole for drawing out molten iron, and a slag hole above the tap hole to eliminate the flux and impurities mixture. Usually, the impure liquid slag floats above molten iron, which makes it easier to separate the two.

Raw Materials

  • Iron Oxide 

In a blast furnace, iron oxides are chemically reduced to molten iron courtesy the coke’s carbon monoxide and carbon. The iron oxides used could be raw ore, sinter or pellets. The raw ore (magnetite or hematite) usually needs to be 0.5 to 1.5 inches in size and may contain 50 to 70 percent pure iron. Such iron-rich iron ore can be directly put through the furnace. However, iron ore with lower iron content must undergo processing to increase the iron element.

Such iron ore is grounded and crushed into a powder to remove gangue, a waste material. The remaining powder usually contains 60 to 65 percent of pure iron rolled as balls within the furnace to create marble-shaped, strong pellets. Sinter comes from small coke, fine raw ore, limestone (sand-sized) and several other iron-containing waste materials from steel plants. These materials turn into liquid iron, with the leftover impurities heading to liquid slag.

  • Coke 

Coke is generally a mixture of different coals. In the furnace, coke is burned at the hearth, courtesy the air from tuyeres. Coke comprises 90 to 93 percent carbon, sulfur and some ash, but it’s much stronger than raw coal. This strong energy value of the coke offers gases, heat and permeability essential for reducing and melting iron ore, sinter and pellets.

  • Limestone 

Limestone is a material extracted from the earth through explosives. The limestone entering the furnace could be pure calcium stone, dolomitic (made of calcium magnesium carbonate) stone, or their blends. The stone put into the furnace is usually 0.5 to 1.5 inches in size and acts as a fluxing agent (chemical cleaner or purifier). Limestone is melted down into a slag that removes impurities such as sulfur. The slag also comprises coke ash and other substances.

These raw materials are kept and prepared in a stock house before being put through the furnace from the top, either through belt conveyors or skip cars. Other facilities/ components supporting the operation include a stove for preheating air pushed to the nozzles at the bottom; cast house that comprises troughs that distribute slag and liquid iron for transfer to slag-reclamation spots and steel-making furnaces.