Iron Making

The blast furnace

Ironmaking consists in winning iron metal from iron chemically combined with oxygen. The blast-furnace process, which consists in the carbothermic reduction of iron oxides, is industrially the most efficient process. From a chemical engineering point of view, the blast furnace can be described as a countercurrent heat and oxygen exchanger in which rising combustion gas loses most of its heat on the way up, leaving the furnace at a temperature of about 200°C, while descending iron oxides are reduced to metallic iron. The blast furnace is a tall, vertical steel reactor lined internally with refractory ceramics such as high-alumina firebrick (45 to 63 wt.% Al2O3) and graphite. Five sections can be clearly identified:

 

(i) At the bottom is a parallel-sided hearth where liquid metal and slag collect. This is surmounted by 

(ii) an inverted truncated cone known as the bosh. Air is blown into the furnace through

(iii) tuyeres, i.e., water-cooled copper nozzles, mounted at the top of the hearth close to its  junction with the bosh.

(iv) A short vertical section called the bosh parallel, or the barrel, connects the bosh to the truncated upright cone that is the stack.

(v) Finally, the fifth and top section, through which the charge is fed into the furnace, is the throat.

The lining in the bosh and hearth, where the highest temperatures occur, is usually made of carbon bricks, which are manufactured by pressing and baking a mixture of coke, anthracite, and pitch. Actually, carbon exhibits excellent corrosion resistance to molten iron and slag in comparison with the aluminosilicate firebricks used for the remainder of the lining.

 

 

The Processing of Blast Furnace

 

 

 

Figure 1. The Process of Blast Furnace

 

 

 

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 sulfur 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 sulfur and other impurities, the blast furnace operator may blend the different stones to produce the desired slag chemistry and create optimum slag properties such as 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 455° C
2) Fe3O4 + CO = CO2 + 3 FeO	Begins at 595° C
3) FeO + CO = CO2 + Fe     or     FeO + C = CO + Fe	Begins at 705° 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 its first reaction as follows:

CaCO₃ = CaO + CO₂

This reaction requires energy and starts at about 870°C. The CaO formed from this reaction is used to remove sulfur from the iron which is necessary before the hot metal becomes steel. This sulfur 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.

During the blast-furnace process, the solid charge (i.e., mixture of iron ore, limestone, and coke) is loaded either by operated skips or by conveyor belts at the top of the furnace at temperatures ranging from 150 to 200°C, while preheated air (i.e., 900 to 1350°C) in hot-blast stoves, sometimes enriched up to 25 vol.% O, is blown into the furnace through the tuyeres. During the process, the coke serves both as fuel and reducing agent, and a fraction combines with iron. The limestone acts as a fluxing agent, i.e., it reacts with both silica gangue materials and traces of sulfur to form a slag. Sometimes fluorspar is also used as fluxing agent. During the carbothermic reduction, the ascending carbon monoxide (CO) resulting from the exothermic combustion of coke at the tuyere entrance begins to react with the descending charge, partially reducing the ore to ferrous oxide (FeO). At the same time the CO is cooled by the descending charge and reacts, forming carbon dioxide (CO2) and carbon black (soot). 

 

This soot is dissolved in the iron, forming a eutectic, and hence decreases the melting temperature. At this stage, the temperature is sufficiently high to decompose the limestone into lime (CaO) and CO2. Carbon dioxide reacts with the coke to give off CO, and the free lime combines with silica gangue to form a molten silicate slag floating upon molten iron. Slag is removed from the furnace by the same taphole as the iron, and it exhibits the following chemical composition: 30 to 40 wt.% SiO2, 5 to 15 wt.% Al2O3,  reduction of ferrous oxide into iron is completed and the main product, called molten pig iron (i.e., hot metal or blast-furnace iron), is tapped from the bottom of the furnace at regular intervals. The gas exiting at the top of the furnace is composed mainly of 23 vol.% CO, 22 vol.% CO2, 3 vol.% H2O, and 49 vol.% N2, and after the dust particles have been removed using dust collectors, it is mixed with coke oven gas and burned in hot-blast stoves to heat  the air blown in through the tuyeres. It is important to note that during the process, traces of   aluminum, manganese, and silicon from the gangue are oxidized and recovered into the slag, while phosphorus and sulfur dissolve into the molten iron.

 In summary, the blast furnace is a counter-current realtor where solids descend and gases ascend. In this reactor 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%
Sulfur (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%

Figure 2. The Blast Furnace Plant

Direct reduction iron

The blast-furnace process is strongly dependent on the commercial availability of coke. For that reason, numerous substitute processes have been investigated since the 1950s to produce a prereduced product for crude steelmaking based on iron ore reduction without using coke as a reductant and to avoid operation of a capital-intensive coke oven plant. These technologies have been especially attractive in countries suffering a coke supply deficit, and hence they are used in Central and South America, India, and  Africa. These processes are grouped under the term direct reduction and smelting reduction.  Direct reduction processes produce solid direct reduced iron (DRI) or hot briquetted iron  (HBI), while smelting reduction processes produce liquid hot metal. However, despite their great promise, these technologies have never superseded the blast furnace, especially in industrialized countries, for the following reasons:

 

(i) Direct reduction is attractive at locations where cheap energy and particularly cheap natural gas is available.

(ii) The development of a market for steel scrap as a raw material acts against direct reduction. Direct reduction can be divided according to the type of reductant used (i.e., natural gas, coal) or the screen size of iron ore (i.e., coarse, fines).

 

 

Table 1. Processes For Direct Reduction

 

 

 

Table 2. Pure Iron Grades