Cupolas are vertical units which are charged with coke, limestone and ferrous scrap to produce molten metal at 1250-1650° C. The process emits smoke, grit and metal oxide fume. It is now subject to the local authority air pollution control system introduced by the Environmental Protection Act 1990. The industry has been objecting to the costs of complying with the new rules (ENDS Report 194, pp 13-16).
The low fume process, developed by BOC and Supercast Foundries, uses oxygen-enriched burners near the top of the cupola to aid combustion. Oxygen injection near the base of the vessel raises the final temperature of the melt to 1500° C.
The improved combustion achieved is claimed to cut carbon monoxide emissions to 1% by volume, compared to 12% in conventional operation. BOC claims that the higher burn-out of particulates and fume renders the plume "virtually invisible", ensuring compliance with one new legal requirement.
Supercast has tested the process on small cupolas at its West Bromwich site. Charles Brookfield, Pollution Control Manager at Sandwell Metropolitan Borough Council, has been impressed with the reduction in visible emissions. Previously, fume from the site had caused visibility problems on the neighbouring M5 motorway.
However, particulate emissions from the cupola have yet to be measured. This will be crucial to BOC's plans for the process. A guidance note issued under the 1990 Act requires particulate emissions from cupolas with a melting capacity over 4 tonnes per hour to be below 115mg/m3 for existing plant and 100mg/m3 for new plant. For cupolas below 4 tonnes per hour, no limit is specified but simple arrestment plant must be fitted. Full tests on a large-scale unit will be needed to assess whether the new technology could remove the need for abatement equipment.
The foundry industry has been considering changing to electric melting to avoid installing abatement equipment such as venturi scrubbers or bag filters. But the capital cost of electric melting - £250-500,000 for a 4 tonnes per hour unit - has deterred most companies. BOC claims that the low fume technology can be retrofitted to an existing cupola at less than half the capital cost of an electric melting unit.