The Blast Furnace

The purpose of a blast furnace is to chemically reduce and physically convert iron oxides into liquid iron called "hot metal". The blast furnace is a huge, steel stack lined with refractory brick, where iron ore, coke and limestone are dumped into the top, and preheated air is blown into the bottom. The raw materials require 6 to 8 hours to descend to the bottom of the furnace where they become the final product of liquid slag and liquid iron. These liquid products are drained from the furnace at regular intervals. The hot air that was blown into the bottom of the furnace ascends to the top in 6 to 8 seconds after going through numerous chemical reactions. Once a blast furnace is started it will continuously run for four to ten years with only short stops to perform planned maintenance.

The Process

Iron oxides can come to the blast furnace plant in the form of raw ore, pellets or sinter. The raw ore is removed from the earth and sized into pieces that range from 0.5 to 1.5 inches. This ore is either Hematite (Fe₂O₃) or Magnetite (Fe₃O₄) and the iron content ranges from 50% to 70%. This iron rich ore can be charged directly into a blast furnace without any further processing. Iron ore that contains a lower iron content must be processed or beneficiated to increase its iron content. Pellets are produced from this lower iron content ore. This ore is crushed and ground into a powder so the waste material called gangue can be removed. The remaining iron-rich powder is rolled into balls and fired in a furnace to produce strong, marble-sized pellets that contain 60% to 65% iron. Sinter is produced from fine raw ore, small coke, sand-sized limestone and numerous other steel plant waste materials that contain some iron. These fine materials are proportioned to obtain a desired product chemistry then mixed together. This raw material mix is then placed on a sintering strand, which is similar to a steel conveyor belt, where it is ignited by gas fired furnace and fused by the heat from the coke fines into larger size pieces that are from 0.5 to 2.0 inches. The iron ore, pellets and sinter then become the liquid iron produced in the blast furnace with any of their remaining impurities going to the liquid slag.
The coke is produced from a mixture of coals. The coal is crushed and ground into a powder and then charged into an oven. As the oven is heated the coal is cooked so most of the volatile matter such as oil and tar are removed. The cooked coal, called coke, is removed from the oven after 18 to 24 hours of reaction time. The coke is cooled and screened into pieces ranging from one inch to four inches. The coke contains 90 to 93% carbon, some ash and sulphur but compared to raw coal is very strong. The strong pieces of coke with a high energy value provide permeability, heat and gases which are required to reduce and melt the iron ore, pellets and sinter.
The final raw material in the ironmaking process in limestone. The limestone is removed from the earth by blasting with explosives. It is then crushed and screened to a size that ranges from 0.5 inch to 1.5 inch to become blast furnace flux . This flux can be pure high calcium limestone, dolomitic limestone containing magnesia or a blend of the two types of limestone.
Since the limestone is melted to become the slag which removes sulphur and other impurities, the blast furnace operator may blend the different stones to produce the desired slag chemistry to create optimum properties such as has a low melting point and a high fluidity.
All of the raw materials are stored in an ore field and transferred to the stockhouse before charging. Once these materials are charged into the furnace top, they go through numerous chemical and physical reactions while descending to the bottom of the furnace.
The iron ore, pellets and sinter are reduced which simply means the oxygen in the iron oxides is removed by a series of chemical reactions. These reactions occur as follows:

1) 3 Fe2O3 + CO = CO2 + 2 Fe3O4	Begins at 450° C
2) Fe3O4 + CO = CO2 + 3 Fe O	Begins at 600° C
3) FeO + CO = CO2 + Fe     or     FeO + C = CO + Fe	Begins at 700° C

At the same time the iron oxides are going through these purifying reactions, they are also beginning to soften then melt and finally trickle as liquid iron through the coke to the bottom of the furnace.
The coke descends to the bottom of the furnace to the level where the preheated air or hot blast enters the blast furnace. The coke is ignited by this hot blast and immediately reacts to generate heat as follows:
 

C + O₂ = CO₂ + Heat

Since the reaction takes place in the presence of excess carbon at a high temperature the carbon dioxide is reduced to carbon monoxide as follows:
 

CO₂+ C = 2CO

The product of this reaction, carbon monoxide, is necessary to reduce the iron ore as seen in the previous iron oxide reactions.
The limestone descends in the blast furnace and remains a solid while going through it s first reaction as follows:
 

CaCO₃ = CaO + CO₂

This reaction requires energy and starts at about 875°C. The CaO formed from this reaction is used to remove sulphur from the iron which is necessary before the hot metal becomes steel. This sulphur removing reaction is:
 

FeS + CaO + C = CaS + FeO + CO

The CaS becomes part of the slag. The slag is also formed from any remaining Silica (SiO₂), Alumina (Al₂O₃), Magnesia (MgO) or Calcia (CaO) that entered with the iron ore, pellets, sinter or coke. The liquid slag then trickles through the coke bed to the bottom of the furnace where it floats on top of the liquid iron since it is less dense.
Another product of the ironmaking process, in addition to molten iron and slag, is hot dirty gases. These gases exit the top of the blast furnace and proceed through gas cleaning equipment where particulate matter is removed from the gas and the gas is cooled. This gas has a considerable energy value so it is burned as a fuel in the "hot blast stoves" which are used to preheat the air entering the blast furnace to become "hot blast". Any of the gas not burned in the stoves is sent to the boiler house and is used to generate steam which turns a turbo blower that generates the compressed air known as "cold blast" that comes to the stoves.
In summary, the blast furnace is a counter-current reaction where solids descend and gases ascend. In this reaction there are numerous chemical and physical reactions that produce the desired final product which is hot metal. A typical hot metal chemistry follows:
 

Iron (Fe)	= 93.5 - 95.0%
Silicon (Si)	= 0.30 - 0.90%
Sulphur (S)	= 0.025 - 0.050%
Manganese (Mn)	= 0.55 - 0.75%
Phosphorus (P)	= 0.03 - 0.09%
Titanium (Ti)	= 0.02 - 0.06%
Carbon (C)	= 4.1 - 4.4%