What is Blast Furnaces? – Its Diagram & How it Works

In iron and steel making, the basic equipment used is a blast furnace, which generally contains the composite materials for producing a specific type of metal. A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper.

Blast refers to the combustion air being “forced” or supplied above atmospheric pressure. The blast furnace operation is monitored continually in such that, temperatures and times are checked and recorded. A blast furnace is typically about 30m high and 10m in diameter at the widest part, though some modern blast furnaces are even larger than this.  Well, in this reading, we’ll explore what a blast furnace is, its parts, diagram, and how it works.

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What is a Blast furnace?

A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally pig iron, but also others such as lead or copper. Blast refers to the combustion air being “forced” or supplied above atmospheric pressure. The largest consumers of materials and energy in the production of iron and steel are blast furnaces. Their capacity to operate using different metal charges—such as scrap, sinter, or pellets—without significantly affecting the performance makes them extremely versatile.

Additional inputs include energy sources for different phases of the manufacturing process, such as heating air to 1100°C before injecting it into blast furnaces, and coke, which produces carbon monoxide, which is needed to transform iron ore to iron. Because blast furnaces require large amounts of coke, they are frequently used in conjunction with coke ovens, which opens up possibilities for the combined use of byproducts like coke oven gas.

Parts & Diagram of a Blast Furnace

The components of a blast furnace include the Charge, exhaust gas outlet, charging bells, gas outlet, tuyeres, taphole, bustle pipe, slag hole, refractory lining, and conveyor system.

Diagram of a blast furnace

How Does Blast Furnace Works?

A blast furnace is a large, hot furnace with a diameter of 10m, typically 30m high. It is made of heavy steel plates and a thick lining of refractory brickwork, which is water-cooled to prevent damage. The blast is distributed to ten or more nozzles or tuyeres, each about 125-175mm in diameter, which deliver the powerful blast to the middle of the furnace, causing coke to burn fiercely.

The hot furnace gas is then used in the hot blast stove to heat air for the furnaces. The stock level in the furnace must be maintained, and raw materials are added at regular intervals. The charge is spread evenly around the furnace through a rotating hopper, providing a gas seal as the blast furnace cannot be shut off during operation.

Hot Blast Stove

Each blast furnace is usually equipped with three stoves that burn furnace gas to heat the air blast. Each is almost as high as the blast furnace, up to 7 and 8m in diameter. Inside it, there is a honeycomb of fire brickwork and a combustion chamber in which gas is ignited, the frame sweeping through the honeycomb and heating it on the way to the chimney damper is closed and the blast stove is put on air; i.e. the air blast is driven by powerful turbo blowers, pass through and absorbs the stored heat. The air temperature reaches from 650 to 800°C.

Usually, two blast stoves are ‘on gas’ (heating up) whilst the third is on air. When this one cools down, it is put back on gas, and another blast stove is put on the air, all three being used in rotation.

What is Iron Ore Smelting?

Iron smelting is the common way in which metals are produced. It is normally performed in a blast furnace which is the conventional method. Smelting is the production of metal, especially iron, from ore in a process that involves melting and chemical reduction of metal compounds into purified metal. it can also be seen as the various liquids or semi-molten solids produced and used during such production.

Iron ore smelting requires some preparation before it can be done. It enables the furnace to work perfectly and sometimes economically. It needs a regular and uniform ‘diet’ from which as much as possible of the unwanted materials has been removed. Either the richness or size and density of the lumps needs to be improved, which is done in several ways:

  • by the concentration (up-grading) of low-quality ores with the removal of unwanted materials by washing and/or gravity separation or, in the case of magnetic ores, by magnetic separation.
  • by crushing and screening (sieving) to a suitable size. The powdering residue from the screening, known as ‘fines’ is mixed with coal dust or coke breeze, and the sintering (controlled burning) process follows, resulting in the fusion of the fines into hard lumps that can be included in the furnace charge. Sinter is being used more and more nowadays and in some plants, it forms 100% of the charge.
  • the blending of different ores to obtain consistency in quality,
  • and by forming powdery ores into pellets.

As mentioned earlier iron ore smelting process is needed to be performed to improve the efficiency of the furnace and to work economically. It is done by removing unwanted materials and by improving the density of lumps, size, or richness.

Iron Smelting in Blast Furnace

Iron smelting involves two main processes in the blast furnace:

  • the reduction of iron from its compounds and
  • its separation from its physical mixture with gangue materials.

In a furnace, the charge is constantly moving to slow down against the force flow of the very hot gases which bring about the evaporation of moisture from the charge at the top. A little further down, limestone decomposes into lime and liberates carbon dioxide. The main reaction is the reduction of the iron oxides, forming carbon dioxide and leaving the iron-free. Reduction begins quite high in the stack and is progressive as the charge descends and, at the melting zone, both iron and slag trickle down into the hearth. The slag, being lighter floats on the iron.

At regular intervals, as the hearth fills, slag is tapped off, followed by the iron, which goes into huge storage vessels called ‘mixer’, for holding the molten state until required in the steel-making plant, usually quite near the furnace. In addition to holding a reserve of hot metal, mixers serve a useful purpose in that they will compensate for variations in composition between successive additions of hot metal, giving a more uniform product.

An inactive mixer does no more than to hold a stock of metal but in an active mixer, some refining takes place with the making of slag therein.

Blast Furnace Maintenance

Blast furnaces normally work day and night for several years. There is a possibility that the brick lining begins to crumble, and the furnace is then shut down for maintenance. The blast furnace operation is monitored continually in such that, temperatures and times are checked and recorded.

The chemical content of the iron ores received from various mines is cracked, and the ores are blended with other iron ore to achieve the desired charge. Samples are expected to be taken from each pour and checked for their chemical content and mechanical properties which include strength and hardness. Considering these will serve as the quality control of a blast furnace.

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