The
use of cupola plant for recycling considerable quantities of steel scrap
containing non-metallic material is not new. However, the use of cupola
technology as the heart of an integrated reprocessing system for automobile
body shells would result in a low cost bulk metal supply route for large
foundry operations where there is a readily available source of suitable
feedstock.
Over
recent years the increasing pressures to reduce waste and improve reprocessing
opportunities has resulted in many automobile manufacturers claiming a higher
degree of recyclability for their products. This, together with increased use
of valuable recoverable material, is likely to lead to more selective
dismantling. Nevertheless, significant quantities of organic materials in the
form of paints, lubricants and plastic components
and the trim are likely to remain in the resulting car body shell. Zinc coated
steel will almost certainly feature in the body construction, while wiring
looms and electric motors may be a source of unwanted copper contamination
unless removed prior to reprocessing. Car body shells can be prepared by either
baling or fragmentation. Fragmentation scrap with appropriate magnetic
separation and screening can be rendered clean and at adequately low levels of contamination.
However, its generally small size and thin section do not make for an ideal
cupola feedstock. Newer, modified cupola techniques to avoid excessive
oxidation could improve this situation and lead to greater acceptability.
Baled scrap from known sources
has been fairly widely used for cupola charges, although in smaller furnaces
the tendency to scaffolding problems has limited its use. The use of hot blast
and a tapered shell arrangement will alleviate the scaffolding difficulty and
reduce oxidation problems with such feedstock.
Ancillary
Equipment
Ü Environmental Control
The
design of effective emission control equipment for cupolas is more difficult
than for other foundry processes due to the wide range of gas temperatures
(100–1200ºC), particle sizes (<1 mm to 10 mm) and flow rates involved. In
addition, the presence of sulphur dioxide can cause corrosion problems and any
combusted volatile matter may create condensation difficulties with filter
media. The high levels of carbon monoxide can be hazardous and unacceptable
from a discharge point of view unless burnt.
Most
modern emission control plant for cupolas is based on the use of dry bag filter
systems. Cold blast operation can result in effluent gas temperatures which
vary during the campaign and are generally well in excess of that capable of
being handled by economical fabric filters. Gas cooling is therefore required
to prevent damage to the filter bags. However, too low a gas temperature may
result in condensation of both volatile matter and water vapour leading to
adverse effects on plant performance. These include blinding of the filter
fabric, corrosion problems in the plant itself, and risk of fire or explosion
damage due to flammable deposit ignition.
In
order to heat the blast in hot blast cupolas, the off-take gases are burnt in a
recuperator but the system actually used may vary depending on the condition of
the gases to be handled.
Ü Dust and Waste Injection Systems
Much
research and development effort has been expended over the years in attempting
to perfect methods for introducing particulate materials in cupolas and these
can be categorised as follows:
exploitation of swarf and borings
concentration of metallic elements, in particular
zinc, for subsequent recovery
recycling of waste materials such as collector waste, used sands, etc.
Most
injection systems have been based on lancing systems incorporation in the
tuyeres but it has been found that the cooling effect involved has restricted
the opportunities for significant continuous injection because of the need to
melt the material collected in the coke bed. It is, therefore, of considerable
interest to increase the continuous injection rates and to establish practical
feed rates for various waste materials.
Some
work undertaken in Germany has been carried out using oxy-fuel burners in the
tuyeres of a hot blast cupola. The technology should be transferable to most
types of cupolas.
A
mixture of natural gas and oxygen is supplied to the burner with dust being injected
through the burner pipe from a specially designed transporter unit and ejected
through the burner outlet.
Oxy-fuel
burners result in very high flame temperatures compared to fuel-air systems,
since no nitrogen has to be heated.
Each
type of particle influences the overall melting process in a different way, for
instance the carbon containing flue ash/cupola dust acts as a supplementary
fuel. In these trials the injection rates were restricted by iron quality
considerations rather than blockage of the coke bed in front of the tuyeres.
Other
advantages of tuyere oxygen injection, such as charge coke reduction, reduced
blast air requirements, better carbon pick-up and metal temperatures can be
achieved by use of the burners when injection is not being employed. This makes
for increased furnace flexibility. Recycling of dry bag filter dusts by cupola
injection can result in the concentration of zinc such that it can be sold to a
zinc reclaimer.
Ü Waste Heat Recovery
The
exhaust gases leaving a cupola are at a high temperature and contain
combustible elements which could be exploited as an energy source. Clearly this
energy is utilised in hot blast cupolas to good effect but even in these cases
less than 50% of the energy available is employed The available waste heat could be used for a variety
of other applications by employing suitable heat exchanger systems. Indeed a
few examples do exist of hot water and steam production for space heating and
process heat. Space heating and hot water production for ablutions and other
uses may not be required consistently but one interesting example of a process
heat application is for steam generation to power an electricity generating set
and compressor. Foundries usually require both electricity and compressed air
during their operating hours and therefore there is a continuous outlet for the
waste heat from the cupola plant. It is claimed that the use of this system on
a 50 tonnes/hr hot blast facility results in an energy utilisation approaching
100%.
Other
possible applications of cupola exit gas energy could be for charge material
and coke drying and preheating duties to prevent condensation in bag filter
outlet chimneys, dust extraction systems and shake-out drums.
The need for high efficiency gas
cleaning equipment on modern cupola installations provides the possibility of
waste heat recovery, which requires evaluation.
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