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.
The term “blast” describes the process of forcing or supplying combustion air above atmospheric pressure. We continuously monitor the blast furnace operation, checking and recording temperatures and times.
Typically, a blast furnace measures approximately 30m in height and 10m in diameter at its widest point, although some contemporary blast furnaces surpass these dimensions. 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. The term “blast” describes the process of forcing or supplying combustion air 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.
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How Does a Blast Furnace Work?
A blast furnace is a large, hot furnace with a diameter of 10m, typically 30m high. Heavy steel plates and a thick lining of refractory brickwork, water-cooled to prevent damage, make up the blast furnace.
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. We must maintain the stock level in the furnace and add raw materials 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 flame sweeps through the honeycomb and heats it on the way to the chimney damper, which is closed and the blast stove is put on air, i.e.,
The air blast is driven by powerful turbo blowers, and it passes through and absorbs the stored heat. The air temperature ranges 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.
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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.
Preparation is necessary before smelting iron ore. It enables the furnace to work perfectly and sometimes economically. The furnace requires a consistent and uniform ‘diet’ to eliminate as many unwanted materials as possible.
Either the richness or size and density of the lumps needs to be improved, which is done in several ways:
- by the concentration (upgrading) 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, the iron ore smelting process needs 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 scorching 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 ‘mixers’ 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 hold a stock of metal, but in an active mixer, some refining takes place with the making of slag therein.
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Blast Furnace Maintenance
Blast furnaces normally work day and night for several years. The furnace may shut down for maintenance if the brick lining starts to crumble.
We continuously monitor the blast furnace operation to check and record temperatures and times.
The chemical content of the iron ores received from various mines is checked, and the ores are blended with other iron ore to achieve the desired charge.
We expect to take samples from each pour and check them for chemical content and mechanical properties, such as strength and hardness. This will serve as the basis for the quality control of a blast furnace.