The modern cokeless cupola
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.
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
Cokeless Cupolas Limited, The Hayes, Stourbridge, West Midlands DY9 8NH; Tel: 01384 896448, Fax: 01384 893272.
shows a 12/14 tonnes/hour plant
operating in Germany
shows 6 tonne/hour plant
in Japan which duplexes
through two 3 tonne coreless
is a 4/5 tonne/hour cupola
operating in Korea which
feeds metal by ladle transfer
to existing electric furnaces