MANUFACTURE OF IRON:
PIG IRON AND CAST IRON.
The manufacture of iron is based on the principle of reduction of iron oxide with carbon monoxide. This reduction process is carried out in a furnace called ‘blast furnace’.
The iron ore is first washed concentrated roasted in order to remove impurities such as sulphur and phosphorus as oxides. The roasting process also decomposes carbonates to oxides and also oxides sulphides. The roasted oxide ore is introduced from the top to the blast furnace along with sufficient amounts of limestone and coke. The blast furnace is provided at the top with cup and come arrangement in order to drop the ore while furnace is on.
The blast furnace is made of steel and lined with fire bricks with cylindrical shape. The furnace is about 100 feet and 23 feet in diameter. Hot air is forced into the furnace through openings or ‘twyers’ present between six to eight feet from bottom of the furnace. The bottom of the furnace is provided with outlets to remove molten iron and slightly above it is a slag hole (Figure 1.1 ) different temperature zones are shown in furnace.
When the mixture of are, coke and limestone descends through the top of the furnace, a blast of air comes across it is the upward direction. The mixture gets heated up and reactions take place in v varions zones of the blast furnace. The coke is first oxidized by the hot blast of are at bottom to give CO and CO2 with the liberation of large amount of heat which brings the temperature in zone f at about 1500ºC The excess of CO is produced in the furnace which reduces Fe2O3 or Fe3O4 to Fe.
Near the middle of the furnace limestone decomposes to give lime, CaO and CO2 . CaO acts as flux and combines with silica present as gangue in the ore to form a slag of calcium silicate which is removed from slag hole. Iron meets near the bottom of the furnace and removed through an outlet.
The chemical change which take place in the blast furnace during smelting of iron ore are somewhat complex. However, the following reactions are simplified representations.
Chemical reaction involved.
- The reduction action between CO and iron oxide starts at 300ºC to 600ºC In zone a and b the following reactions take place:
- In zone c at about 750ºC the following reactions occur:
- In zone d at 1000ºC CaCO3 decomposes to give CaO and CO2
- At about 1300ºC CaO (flux) combines with SiO2 (present in the ore) to from calcium silicate (slag).
- At 1500ºC or above the iron descending from zone c meets and gets collected at the bottom of the furnace Carbon (coke) burns in zone f to from CO which reduces the iron oxide while crossing it.
- The exhaust or flue gases which come out from the top of the blast furnace still contain large amounts of carbon monoxide . These gases are burnt and used to preheat the air blast.
It is prepared from cast iron by puddling I.e., heating in special type of reverberatory furnace provided with doors DD. The furnace is lined with oxides of iron in haematite or magnetite. The iron oxide oxidises impurities present in iron thus C,S and P are removed as corresponding volatile oxides.
Heat is reflected and radiated from the roof of the furnace upon. The molten iron etc , while blowing hot gases through it. The iron melts and is stirred or puddle by iron rods for through contact with furnace lining. The pasty mass of iron is collected in balls ‘blooms’ and removed from the furnace. Small amounts of slag remain present in wrought iron which gives fibrous structure to it but is helpful during welding. The wrought iron may be easily hammered and draw into wires.