Iron foundries require metal of controlled composition and temperature, supplied at a rate sufficient to match the varying demands of the moulding line. The metallic charge to be melted consists usually of foundry returns, iron scrap, steel scrap and pig iron with alloying additions such as ferrosilicon. The charge is usually melted in a cupola or in an electric induction furnace. Gas-fired or oil-fired rotary furnaces can also be used, but their use is less common.
The cupola (Fig. 1.2) is the classical iron melting unit and is still the most widely used primary melting unit for iron production due to its simplicity, reliability and the flexibility in the quality of charge materials that can be used because some refining of undesirable elements such as zinc and lead can be achieved. While the cupola is an efficient primary melting unit, it does not adapt easily to varying demands, nor is it an efficient furnace for superheating iron. For this reason it is often used in conjunction with an electric duplexing furnace. The simplest form is the cold blast cupola which uses ambient temperature air to burn the coke fuel. The metal temperature that can be achieved is normally from 1350 to 1450°C but higher temperatures can be achieved through the use of divided blast (as in Fig. 4.1) or oxygen enrichment. The refractory linings of cold blast cupolas have a short life of less than 24 hours, so cupolas are operated in pairs, each used alternately while the other is re-lined. In hot blast cupolas (Fig. 4.2), the exhaust gases are used to preheat the blast to 400–600°C, reducing coke consumption and increasing the iron temperature to more than 1500°C. They may be liningless or use long life refractories giving an operating campaign life of several weeks. ‘Cokeless’ cupolas (Fig. 4.3), have been developed in which the fuel is gas or oil with the charge supported on a bed of semi-permanent refractory spheres. They have advantages of reduced fume emission.
Cold blast cupola operation
The cupola is charged with:
1. coke, the fuel to melt the iron
2. limestone, to flux the ash in the coke etc.
3. metallics, foundry scrap, pig iron, steel and ferroalloys
4. other additions to improve the operation
The cupola is blown with air to combust the coke and the air flow controls the melting rate and metal temperature. The output of a cupola depends primarily on the diameter of the shaft of the furnace and on the metal/coke ratio used in the charge. summarises the operating data for typical cold blast cupolas. A useful measure of the efficiency of operation of a cupola is the ‘Specific Coke Consumption’ (SSC) which is
Annual tonnage of coke 1000 Annual tonnage of metallics charged
= SSC (kg/tonne)
This takes into account both charge coke and bed coke. When the cupola is operated for long enough campaigns, the amount of coke used to form the bed initially can be ignored. However, as the melting period decreases, the role of the cupola bed becomes more important. Table summarises data from 36 cupola installations in the UK in 1989. This table provides a useful reference against which the operation of any cold blast cupola can be compared.
The performance of the cupola is highly dependent on the quality of the coke used.
Typical foundry coke has the following properties:
Moisture 5% max.
Ash 10% max.
Volatiles 1% max. Sulphur 1% max. Mean size 100 mm Undersize <5% below 50 mm
The coke size directly affects coke consumption per tonne of iron melted and also the melting rate. Optimum cupola performance is achieved with coke in the size range 75–150 mm, if smaller coke is used, metal temperature is reduced and a higher blast pressure is needed to deliver the required amount of air to the cupola. Increasing the size of coke above about 100 mm has no beneficial effect, probably because large pieces of coke tend to be fissured and break easily during charging and inside the cupola. Coke usage in the cold blast cupola is typically 140 kg per tonne of iron melted (this is an overall figure including bed coke), it is usual to charge coke at the rate of about 10–12% of the metal charged, but the exact amount used depends on many factors such as tapping temperature required, melting rate and the design of the cupola.