Extraction of Iron using blast furnace and various types of Steel:

Extraction of Iron using blast furnace and various types of Steel:

Iron ore is reduced to iron by heating them with coke (a form of carbon) in blast furnace. As mentioned earlier, common iron ores are hematite (Fe2O3) and magnetite (Fe3O4). The air blown into the bottom of the blast furnace is heated using the hot waste gases from the top. Heat energy is valuable, and it is important to conserve heat energy. The coke burns in the blast of hot air to form carbon dioxide; exothermic reaction releases heat. This reaction is the main source of heat in the furnace.

C + O2 = CO2

At the high temperature at the bottom of the furnace, carbon dioxide reacts with carbon to produce carbon monoxide.

C + CO2 = 2CO

It is this carbon monoxide which is the main reducing agent in the furnace to produce iron.

Fe2O3 + 3CO = 2Fe + 3CO2

In the hotter parts of the furnace, the carbon itself also acts as a reducing agent. Notice that at these temperatures, the other product of the reaction is carbon monoxide, not carbon dioxide.

Fe2O3 + 3C = 2Fe + 3CO

The temperature of the furnace is hot enough to melt the iron which trickles down to the bottom as ‘pig iron’, where it can be tapped off. The limestone is added to convert siliceous impurities into ‘slag’ ( as calcium silicate, CaSiO3), which melts and runs to the bottom. The calcium silicate melts and runs down through the furnace to form a layer on top of the molten iron.

CaCO3 + O2 = CaO + CO2. CaO + SiO2 = CaSiO3

Pig iron – The molten iron from the bottom of the blast furnace is pig iron. It contains 3.5 - 4.5% carbon and varying amount of contamination such as, sulfur, silicon and phosphorus. Pig iron is the intermediate step on the way to cast iron and steel.

Cast Iron – Some time pig iron from the blast furnace can be used as cast iron. It is very impure, containing about 4% of carbon. This carbon makes it very hard, but also very brittle.

Steel - Most of the pig iron is used to make one of a number of types of steel. There isn't just one substance called steel - they are a family of alloys of iron with carbon or various other metals after removal of impurities from molten iron.

Removal of impurities - Impurities in the pig iron from the Blast Furnace include carbon, sulfur, phosphorus and silicon. Sulfur is removed by reacting with magnesium (Mg) as magnesium sulfate (MgS).

Mg + S = MgS

Carbon is removed by blowing oxygen in molten iron. The impure molten iron is mixed with scrap iron (from recycling) and oxygen is blown on to the mixture. The oxygen reacts with the remaining impurities to form various oxides. The carbon forms carbon monoxide. Since this is a gas it removes itself from the iron! This carbon monoxide can be cleaned and used as a fuel gas.

Elements like phosphorus and silicon react with the oxygen to form acidic oxides. These are removed using quicklime (calcium oxide), which is added to the furnace during the oxygen blow. They react to form compounds such as calcium silicate or calcium phosphate which form a slag on top of the iron.

Various steel products: The various steel products used include Wrought iron, Mild steel, High carbon steel and other specialized steel.

Wrought iron: When all the carbon is removed from the molten iron to give high purity iron, it is known as wrought iron. Wrought iron is quite soft and has little structural strength. It was once used to make decorative gates and railings, but these days mild steel is normally used instead.

Mild steel: Mild steel is iron containing up to about 0.25% of carbon. The presence of the carbon makes the steel stronger and harder than pure iron. The higher the percentage of carbon, the harder the steel becomes. Mild steel is used for lots of things - nails, wire, car bodies, ship building, girders and bridges amongst others.

High carbon steel: High carbon steel contains up to about 1.5% of carbon. The presence of the extra carbon makes it very hard, but it also makes it more brittle.

Specialized steel: These are iron alloyed with other metals, such as -

	iron mixed with	special properties	uses include
stainless steel	chromium and nickel	resists corrosion	cutlery, cooking utensils, kitchen sinks, industrial equipment for food and drink processing
titanium steel	titanium	withstands high temperatures	gas turbines, spacecraft
manganese steel	manganese	very hard	rock-breaking machinery, some railway track (e.g. points), military helmets