The Modern Cokeless Cupola

The modern cokeless cupola

It is worth reminding the reader that a cupola is a continuous melting shaft furnace which has by its inherent design considerable advantages over batch type melters such as electric furnaces or rotary furnaces. A cupola can accept a wide range of raw. materials including oily, wet and contaminated scrap. These materials are unsuitable for electric furnaces for safety reasons and because of the contamination their use is also often limited for metallurgical reasons. In cupola melting there is a degree of refining as the metal forms droplets during melting before collecting in the well. Many contaminants are lost or reduced in value in this process whereas when melting in electric furnaces or rotary furnaces whatever is in the charge material finishes up in the liquid.
Additionally, the cupola is a counterflow vertical shaft furnace and offers the high possibility of good melting efficiency compared with batch type melters. Low top gas temperatures mean a large proportion of the available heat goes into the metal whereas in electric furnaces there are losses not only from the surface of the metal but at least 25% of the energy input goes into the water in the induction coil. In rotary furnaces high waste gas temperatures, even with recuperators mean high losses.
The cokeless cupola has further advantages over the conventional coke cupola. The waste gas has a low CO content and there is only 1% before dilution which means the maximum heat is being released to the metal compared with 12% to as high as 20% CO in' some coke operations which is a considerble loss to the process. Eliminating coke removes the major source of pollution and as there is no free oxygen in the cokeless in the cokeless cupola no metallurgical fume is formed. Some coke cupolas particularly with oxygen enrichment or oxygen injection produce considerable volumes of metallurgical fume which then requires large filtration plants to remove it.
The cokeless cupola can operate with no emission controls and still meet the new environmental regulations providing the charge is clean and would be of a similar cleanliness to that which is often required for electric melting. The cost of such materials is often higher but can be justified to reduce the capital investment of pollution control.
If cheaper contaminated scrap is used some emission control equipment will be necessary but at a much lower cost. Even if a bag filter is required to collect zinc fume from galvanised scrap a much smaller one can be used as the volume and temperature of the waste gas is much lower than with a conventional coke cupola. The cokeless cupola is an efficient melter which can solve the environmental problem and additionally as there is no sulphur pick up in melting it makes an ideal unit for the manufacture of base iron suitable for ductile iron production.
Modern approach
The modern approach to cokeless melting is to use the cupola as efficiently as possible and hence it is a lined cupola although a water cooled shell is employed so it can be operated on a long campaign basis.

The cupola is operated at relatively low tapping temperature to extend the refractory life and even with 40% steel in the charge tapping temperatures of around 1,400oC are employed and the metal then superheated and recarburised in a suitable electric furnace. Such plants incorporating a single long campaign cokeless cupola duplexing through electric furnaces are presently in use in Germany, Spain, Japan and Korea with further installations going ahead in Austria and Korea.
Economics
The economics of operating these plants are obviously of considerable interest and the reader can be assured that they have not been installed without the overall economics being attractive. The main costs involved in cokeless melting are the fuel, refractory spheres, electricity for superheating, plus lining refractory and recarburiser. In the duplex operation gas consumptions of around 50cu m/tonne are typical which in the UK means around £5-£6/tonne depending on the gas price. Sphere consumptions below 1% have been achieved with typical consumptions around 1.2% with 30% steel in the charge. This results in a melting cost of between £5 and £6.50/tonne. The electricity for superheating at 50-70kWh/tonne adds between £2 and £3.50 depending on the amount of superheating required and recarburiser to be added. The re-lining of the cupola adds a further £3-4/tonne and the recarburiser £2-£5/tonne depending whether petroleum coke or graphite is used and the actual addition. This means the cost of simple grey iron melting can be as low as £17/tonne and up to £24/tonne for ductile iron production with a high steel charge and graphite for recarburising. These figures compare favourably with conventional coke melting and are very attractive compared with electric or rotary furnace operations.
For the UK foundry which has existing cupolas a conversion can often be carried out to keep the capital cost down compared with installing a complete new melting shop if the cupola is abandoned. Obviously a new cokeless cupola plant can be built and may offer the best efficiency but at a higher capital cost. Fig 1 shows a 12/14 tonnes/hour plant operating in Germany. This single cupola duplexes through a small high powered channel furnace where up to 1% carbon is added and the metal superheated 200oC. Fig 2 shows a new 6 tonne/hour plant in Japan which duplexes through two 3 tonne coreless induction furnaces. Fig 3 is a 4/5 tonne/hour cupola operating in Korea which feeds metal by ladle transfer to existing electric furnaces. The cupola is not 'dead' in spite of the bad press it occasionally receives. Its competitors try and make out it is old fashioned but a modern approach means it still has a significant part to play in providing liquid iron for foundries. By adopting a modern cokeless cupola pIant a foundry can reduce its melting cost at the same time as solving the environmental problem that exists with conventional coke cupolas. The existing charge make-up can often be continued without the need of going to clean high quality materials which are usually recommended for electric melting particularly if they are being installed without emission controls. A cokeless cupola plant is an efficient, clean, well controlled melting system to give the foundry good flexibility of operation at low cost. 'Long live the cupola', especially the modern cokeless cupola.

Cokeless Cupolas Limited, The Hayes, Stourbridge, West Midlands DY9 8NH; Tel: 01384 896448, Fax: 01384 893272.

Fig.1 shows a 12/14 tonnes/hour plant operating in Germany	Fig.2 shows 6 tonne/hour plant in Japan which duplexes through two 3 tonne coreless induction furnaces	Fig.3 is a 4/5 tonne/hour cupola operating in Korea which feeds metal by ladle transfer to existing electric furnaces