Case Studies: Stack Melter vs. Reverberatory Furnace

A Melt Performance Comparison: Stack Melter vs. Reverberatory Furnace
As aluminum foundries expand their operations, the choice of furnace is vital to the efficiency and quality of the melting operation.

One of the most important decisions in the start-up or expansion of a foundry is the choice of melting equipment. It also is one of the most difficult.

The decision first must be based on quantitative reasons such as cost, fuel usage and environmental issues. Then, the decision must incorporate intangibles like melt quality requirements, charge stock available, maintenance requirements, required flexibility of melt rates, availability of parts and field service, and worker attitudes.

A recent expansion of the casting capacity at a large Midwestern foundry presented an opportunity to compare two of the more popular aluminum melting furnaces - slack melters and reverberatory furnaces. After expanding with a casting line supplied by an older reverberatory furnace, the customer desired further capacity, enabling the foundry to install a second casting line. This line, however, would be led by a stack melter. The result was two parallel production lines - one featuring the conventional reverberatory melter with a wet bath charge into an intermittently sealed hearth (see the diagram on the following page). Both furnaces were rated at 3000 lb/hr., were used to melt for an identical product line, and were fed the same melt stock at the same ratios of new metal to returns. Further, the units were operated by the same staff and maintained with procedures and techniques recommended by the equipment manufacturers.

Melt Performance
Side-by-side furnace comparisons were made on the basis of total pounds of material charged, the weight of dross and skimmings generated, the weight of makeup alloy additions and the energy consumed in a day's production. The results are shown below.

After the installation of the stack melter, notable differences existed in the quality of melt produced by the two units.

A number of differences in construction and operation influenced the performance of the two furnace designs. Many factors can contribute to the differences in metal quality. The reverberatory furnace carries a large bath of molten metal in the hearth, has more surface area exposed to oxidation and melt loss, operates at a higher temperature in the melting chamber, and is charged with a bulk addition of ingot and potentially wet shop returns.

A stack melter (or even dry hearth furnace) offers the potential to remove all moisture and organic material from the charge before it can react with molten aluminum to generate oxides (melting loss). The smaller volume of metal in the stack melter hearth reduces both the exposure time and surface area exposed to gas pick-up, while further benefiting melt losses for the same reason. Also as a significant portion of the energy required to superheat the melt to temperature already has been transferred to the charge with heat that would otherwise be lost in the flue gases, the total energy requirement is dramatically reduced and permits lower hearth flame temperatures.

Note: This case history was adapted from a paper presented at the International AFS Conference on Molten Aluminum Processing by Daniel E. Groteke, Q.C. Designs, Inc., St. Joseph, Michigan and John Fieber, Modern Equipment Co., Inc., Port Washington, Wisconsin.