Cupola Furnace | Diagram and Working Of Cupola
It consists of a cylindrical steel shell with its interior lined with heat resisting fire bricks. There is a drop door at the bottom after closing which proper sand bed could be prepared. This sand bed provides proper refractory bottom for molten metal & coke. Above the sand bed, there is a metal tapping hole which will be initially closed with clay known as ”bot”. Opposite & above the metal tapping hole, there is a slag hole where slag is trapped. Above the slag hole, there is a wind box which is connected to air blowers. Air enters to the cupola through the tuyeres. Above the charging platform, there is a charging hole through which charge is put into the cupola. The charge consists of the pig iron, scrap iron, coke and fluxes.
First the drop door at the bottom is closed. Sand bed with slope towards tap hole is rammed. Coke bed of suitable height is prepared above the sand bed and is ignited through the tap hole. After proper ignition, alternate layers of charge, flux & coke are fed through the charge door. Then the charge is allowed to soak in the heat and the air blast is turned on. Within 5 to 10mins, the molten metal is collected through the tap hole. When enough metal is collected in the well of the cupola, the slag is drained off through the slag hole. Then the molten metal is collected in the ladles and is transported to the moulds with a minimum time loss.
Fluxes are added in the charge to remove the oxides & other impurities present in the metal. The flux commonly used is lime stone (CaCO3) in a proportion of 2 to 4% of the metal charge. Others fluxes used are dolomite, sodium carbonate, calcium carbide. Flux reacts with oxides to form compounds having low melting point and lighter so that it will float on the metal pool.
When charge comes through the coke bed, some amount of carbon is picked up by the metal depending on the temperature and the time when the metal is in contact with the coke. It is of the order of 0.15% carbon.
It is Oxidised in the cupola and there will be a loss of 10% silicon. It may be as high as 30%. To increase the silicon content, ferrosilicon is added to the metal.
There is a loss of 15 to 20% manganese during melting process. The content of manganese can be increased by the addition of ferromanganese.
There will a sulphur pick up in a range of 0.03 to 0.05%.
Other furnaces include
- Open hearth furnace
- Rotary furnace
- Crucible furnace
- Immersion heated furnace
Based on the source of heating, they can be classified as
- Electrical heating furnace (arc, resistance or induction)
- Fossil full fired furnace (solid, oil/gaseous fuel)
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