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Plasma torch heats up interest in the cupola; General Motors Corp. and Westinghouse have teamed up to develop a cleaner, more efficient plasma melting system at GM's Central Foundry Div.

Plasma Torch Heats Up Interest in the Cupola

General Motors Corp and Westinghouse have teamed up to develop a cleaner, more efficient plasma melting system at GM's Central Foundry Div.

A plasma arc torch, originally developed for the space program, has been harnessed to the venerable cupola melting furnace, successfully pairing a 200-year-old iron melting process to a space-age technology. It happened this summer when General Motors Corp's Defiance, OH, Central Foundry Div formally accepted for production a cupola melting system that uses six electrically-powered plasma torches as the heat source for a 45-ton-per-hour continuous melting furnace producing gray iron.

Capping nearly six years of research and development work, the multi-million dollar plasma melter is said to reduce the furnace's operating costs while improving metal quality. Supplied by Westinghouse Environmental Systems & Services, and co-developed with the Electric Power Research Institute and Modern Equipment Co, the melter uses a 13-foot diameter cupola furnace that now can effectively remelt lower cost charge materials such as scrap iron, unbriquetted iron chips and borings, compressed bundles of excess sheet metal, small steel fragments and recyclable sprue from the foundry's operation. The system also uses less coke than cupolas without plasma technology.

The Central Foundry system relies on the super heat generating capabilities of the plasma torches to provide much of the energy required to melt charge materials, which are added via a special material handling system. Melting occurs above the coke bed where temperatures exceed 2800F.

As the iron melts, it seeps through the coke bed, flowing out through a tap hole located below the tuyeres into a 90-ton holding furnace. Here, the slag is removed and the metal is transported to the molding line for pouring.

Improved melting efficiency and operating flexibility is provided by a process-wide, computer-controlled quality assurance capability. This allows total control over the melt zone process and enables operators to take immediate corrective action should the molten iron deviate from preset melting parameters.

Plasma Technology

In conventional cupola melting, air enters through the tuyeres at the base of the cupola to aid in the combustion of the coke which provides the heat to melt the iron charge. With the plasma system, a portion of the air is preheated by the plasma torch, which is capable of reaching 10,000F, thus, reducing the combustion energy required from the coke and, in turn, the amount of air required through the cupola.

Inside the plasma torch are two tubular electrodes placed end to end. During operation a process gas, such as nitrogen or air, is injected into a small gap between the electrodes. A spark between the electrodes initiates an arc when the high voltage power supply is energized. The arc is moved to the inside diameter of the electrodes by the incoming gas.

This electric arc ionizes or disassociates the incoming gas to produce a plasma inside the torch. The plasma superheats the cold incoming gas and the superheated gas then exits the torch at temperatures three times higher than that achievable with oil or natural gas systems.

The plasma torch accomplishes two things. First, the reduced air volume through the cupola allows the charge to contain large quantities of fine, unbriquetted scrap metal, thus saving briquetting costs and time. Second, particulate emissions are reduced because the lower velocity blast air keeps borings and chips from being blown out the top of the melter.

And, because plasma melters require less coke than is necessary in a conventional cupola operation, carbon monoxide, sulfuric acid and sulfur dioxide emissions also are reduced significantly. This saves on the amount of particulate materials that foundry exhaust and its pollution control system must handle.

The use of plasma torches follows work that CFD did in the early 1980s using coke breeze and other additives to boost cupola operating efficiency, reduce emissions and lower costs while increasing iron production and quality. The Defiance plant, largest of GM's six domestic foundries, manufactures 400,000 tons of automotive castings annually for use in all of the automaker's cars and trucks.

PHOTO : The intensely hot Westinghouse plasma torch (capable of reaching 10,000F) provides most of

PHOTO : the heat to more efficiently melt iron at General Motors Corp's Central Foundry Div,

PHOTO : Defiance, OH.

David P. Kanicki, Publisher/Editor
COPYRIGHT 1989 American Foundry Society, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
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Author:Kanicki, David P.
Publication:Modern Casting
Date:Dec 1, 1989
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