Metalwork - Materials - Blast Furnace

Iron Ore is smelted in the Blast Furnace in order to remove unwanted impurities such as rocks, clay and sand, and also to seperate the Iron from the Oxygen. The result is Iron which is about 95% pure. The remaining impurities are other elements which can be removed later if necessary. A Blast Furnace is about 100ft. high and produces abut 1000 tons of molten Iron a day. It is made from steel. On the diagram below of the Blast Furnace, just click on the labels for a more detailed description. If you scroll past the diagram you can read about the Blast Furnace Process.

The Blast Furnace Process

1.   The Iron Ore, Coke and Limestone, (the Charge), is conveyed to the top of the Furnace.

2.   The Charge is stored in Bells until the timing is right for the charge to be dropped into the Furnace.

3.   Hot air is then blown through pipes called Tuyeres, to fire the mixture.

4.   The Coke burns to increase the temperature in the Furnace.

5.   The Limestone attracts the impurities in the Iron Ore and forms Slag. This Slag is lighter than the molten Iron and so floats on top of it.

6.   As the Furnace fills, the molten Iron is Tapped off. The Slag is also tapped off at regular intervals.

Most Iron is taken straight from the Blast Furnace to the Steel Mill, but some is poured into buckets called Pigs. This Iron is called Pig Iron and is used to make Cast Iron.

The Charge

The Charge consists of 3 parts :

1. Iron Ore
2. Coke
3. Limestone

Iron Ore

The Iron Ore is first mined and then brought to the Blast Furnace. It contains impurities which have to be removed. Haematite (Hematite) and Magnetite are the most common ores. It takes about two tons of Iron Ore to produce one ton of iron, but this varies with different types of ores.

Coke

Coke is made by heating soft coal in the absence of air. As Coke is burned in the Blast Furnace it raises the temperature to about 2000°C which is enough to melt the Iron Ore. The Carbon in the Coke chemically reacts with the Oxygen in the Iron Ore to form Carbon Dioxide(CO₂, and Carbon Monoxide(CO), which escapes through the Gas Outlet.

Limestone

The Limestone is mined then crushed before being brought to the Blast Furnace. It combines with the impurities in the Iron Ore to form Slag. A material which removes unwanted materials or cleans another material is called a Flux.

The Exhaust Gas Outlet

The Exhaust Gas Outlet collects any gaseous emmisions from the chemical reactions that are taking place in the Furnace. As you know Carbon Dioxide and Carbon Monoxide are not exactly condusive towards healthy living and so they cannot be let to escape into the atmosphere as they are. Firstly because these gases are hot they are reused to save energy. They are piped to nearby Stoves in order to heat them. The gases are then 'cleaned' before being let into the atmosphere.

Charging Bells

You may notice that the Charging Bell system in the above diagram looks more complicated than those you see in books. The reason is that you are looking at a more accurate representation. You can understand that the manufacturers of Iron want to conserve as much energy as possible, not to do so would cost money and be mad! There are in fact two Bells in the system called, the Small Bell and the Large Bell. The Small Bell is filled directly from the Conveyor System, and when it is close to being full it is opened to allow the Charge drop into the Large Bell. The Large Bell is then opened when it is nearly full and the Charge can drop into the Furnace. Using this system greatly reduces the amount of heat that is lost to the atmosphere.

Gas Outlet

The Gas Outlet is simply an array of holes in the Furnace that allows the escaping gases to get to the Exhaust Gas Outlet.

Melting Iron Ore, Coke and Limestone

At the top of the Furnace the Iron Ore, Coke and Limestone is at a temperature of about 200°C. At this stage the materials are goin through the pre-heating stage. Close to the middle of the Furnace the temperature has increased to approximately
480°C, where the raw materials have started to melt. The temperature increases rapidly to about 2000°C at the bottom of the Furnace where the molten Iron is situated, waiting to be removed.

Taphole

The Taphole is used to draw off the molten Iron at regular intervals of about 5 to 6 hours. You should notice that the Taphole is located below the Slaghole. This is because the Slag is lighter than the molten Iron and so sits on top. The molten Iron leaves the Taphole and is either poured into moulds called 'Pigs' or sent to other areas for further refining.

Slag Hole

The Slaghole, which is situated above the Taphole, because Slag is lighter than molten Iron, is used to draw off the waste Slag. The Slag is scraped off every 3 or 4 hours and is then used for road beds, fertilizer or cement.

Bustle Pipe

The Bustle Pipe is a large diameter pipe that circles the base of the Furnace. It carries the hot air from the Stoves, where the air is heated, to the Tuyeres which allow the hot air to enter the Blast Furnace.

Refractory Lining

You might ask, that if the Blast Furnace is made from Steel and there is molten Iron inside the Furnace, how come the Furnace does not melt ? That would be a good question, and the answer is quite simple. Inside the Steel shell of the Furnace there is a layer of Fire Brick called the Refractory Lining. This Refractory Lining reflects the heat bach into the Furnace. You have seen a Refractory Lining before. If you look at the back of a fireplace, (preferably one that does not have a fire burning in it at the time), you will see a reddish cement. This is Fire Brick and causes the heat generated by the fire to be reflected back into the room, rather than be absorbed by the wall at the back of the fireplace.

Conveyor System

The Conveyor System takes the Charge from the area where it is maxed together to the top of the Blast Furnace. The Charge is carried in Skip Cars which run on a rail track.