Furnaces in casting:
Melting furnaces used in the foundry industry are of many diverse configuration The selection of the melting unit is one of the most important decisions foundries must make with due consideration to several important factors including:
· The temperature required to melt the alloy
· The melting rate and quantity of molten metal required
· The economy of installation and operation
· Environmental and waste disposal requirements
Several types of furnaces are most commonly used in foundries:
· Direct fuel-fired furnaces
· Crucible furnaces
· Electric-arc furnaces
· Induction furnaces
Selection of the most appropriate furnace type depends on factors such as the casting alloy; its melting and pouring temperatures; capacity requirements of the furnace; costs of investment, operation and maintenance and environmental pollution considerations.
A cupola is a vertical cylindrical furnace equipped with a tapping spout neat its base. Cupolas are used only for melting cast irons, and although other furnaces are also used the largest tonnage of cast iron is melted in cupolas.
It consists of a large shell of steel plate lined with refractory. The charge, consisting or iron, Coke, flux and possible alloying elements, is loaded through a charging door located less than halfway up the height of the cupola.
The iron is usually a mixture of pig iron and scrap (including risers, runners, and sprues left over from previous castings). Coke is the fuel used to heat the furnace. Forced air is introduced through openings near the bottom of the shell for combustion of the coke.
The flux is a basic compound such as limestone that reacts with coke ash and other impurities to form slag. The slag serves to cover the melt, protecting it from reaction with the environment inside the cupola and reducing heat loss. As the mixture is heated and melting of the iron occurs, the furnace is periodically tapped to provide liquid metal for the pour.
Combustion or Oxidizing zone
It is the zone where combustion takes place. It extends from the top of the tuyeres to a surface boundary below which all the Oxygen of air is consumed by combustion, chemical reaction that takes place in the zone is
C(coke) + 02 (from air) -> C02 + Heat
The temperature in this zone is about 1800°C.
It extends from the top of the combustion zone to the top of the initial coke bed. The CO2 produced in the combustion zone moves up and is reduced to CO. The temperature also drops to 1650°C.
C02 + C2 -» CO – Heat
It includes the first layer of pig iron above the initial coke bed. In this zone, the pig iron is melted. The following reaction takes place.
3 Fe + 2 CO -» Fe3C + C02
It includes all the layers of cupola charges placed above the melting zone to the top of the last charge. The layers of charges are heated by the out-going gases. The temperature in the zone may be up to 1050°C.
It is the zone beyond the pre-heating zone, through which the hot gases go to the atmosphere.