Optimizing your cupola operation.

The cupola furnace generates, worldwide, more than 60% of the liquid iron used for the production of gray and ductile iron castings. U.S. cupola metalcasting facilities produce about five million tons of castings for applications such as automotive, piping, farm machinery and municipal castings.

However, escalating emissions regulations by the U.S. Environmental Protection Agency have given cupola operators much to consider before firing up their furnaces.

A scrap melting device that uses a carbon bearing fuel, the cupola furnace shares the production of liquid iron with furnaces that use electric energy for melting. The cupola was invented more than 200 years ago, but today's version would be unrecognizable to the original inventors. Through the years the cupola has maintained its preeminence in the production of iron for castings because it consumes low cost scrap, uses a low cost fuel and is less susceptible to harmful impurities in the melt.

Most cupolas in operation today produce between 25 and 125 tons of liquid iron pet hour, requiring about 1.5 to 2 tons of liquid iron to produce 1 ton of castings. Fig. 1 illustrates the basics of cupola operation. The cupola furnace melts iron and steel scrap using coke, a fuel derived from coal. Air is injected through ducts near the bottom of the furnace. Combustion of coke generates gas temperatures well in excess of 3,000F (1,649C). The gases rise to melt the scrap. Melting points are between 2,200 and 2,900F (1,204 and 1,593C). Melting produces drops, about 0.25 in. in size, which fall through the cupola and exit the furnace through a tap hole at the bottom.

[FIGURE 1 OMITTED]

Alloy and limestone also are added along with the scrap and coke. Alloy is used to introduce elements such as silicon and manganese that might be lacking in the scrap. The role of limestone is to facilitate the removal of hard-to-melt materials such as dirt and sand that inadvertently enter the cupola with the scrap. In addition coke contains ash, which is difficult to melt. Since the only exit from the cupola for these extraneous materials is the tap hole, they must be converted to a free flowing liquid. Limestone's role is to combine with these materials to produce the necessary liquid.

Judging from the wide range of iron production rates, it is evident that cupolas vary considerably in size. Cupola inner diameters generally are in the range of 3 to 10 ft. Cupola heights vary from 2 to 40 ft. A unique feature of most cupolas is that the furnace is not required to have a refractory lining. Water cascades over the outer wall of the cupola, which prevents the steel shell from melting. This feature cuts down on the furnace's operating costs.

Because cupolas bum large quantities of fuel that generate carbon monoxide and melt large amounts of scrap that contain volatile pollutants, they require emission systems that are much larger than the cupola itself. These systems are costly. As a result, cupola melting has become confined to the largest size cupolas as emission requirements have become more stringent.

The cost of emission control and the rising cost of coke have prompted metalcasting facilities to expend efforts to reduce the consumption of coke by installing refractory linings in cupolas or recovering heat from the exhausting cupola gases. In the latter case, the exiting gases are not only hot but they contain carbon monoxide, which can be burned to produce large amounts of heat. The heat from these sources is directed into a heat exchanger, which preheats the incoming air. This source of energy can boost heat input to the cupola by as much as 50%.

Cupola facilities also are taking steps to operate the cupola in a manner that conserves the very expensive alloy additions to the charge, and they are paying more attention to the type of scrap they melt as it can have a great effect on melting costs. A computer program that accurately simulates cupola operation has the potential to significantly reduce operating costs and is being developed with finances from the Department of Energy, the American Foundry Society and metalcasting industry.

S. Katz, Seymour Katz Associates, Bloomfield, Michigan