Improve profitability by reducing aluminum melting costs.

By zeroing in on the often overlooked areas of energy and melt loss, aluminum foundries can increase their furnace efficiency for bottom line results.

As profit margins shrink in response to low-cost global competition and the growing cost of regulatory compliance, cost control is priority one. Today's aluminum foundry produces daily reports on production costs, scrap rates, delivery performance, sand control and disposal, and maintenance. One area often overlooked by foundry managers in their quest for cost reduction, however, is the energy cost in aluminum melting and the melt loss.

The total energy cost of melting and the total cost of melt loss are simple figures to determine. Melting cost is the energy used per day (kW) vs. the amount of metal melted (lb). Melt loss is calculated by the metal purchased (lb) minus the castings shipped (lb) (over a month or year to account for the remelt of gates, risers and scrapped castings). By monitoring these costs and controlling them when possible, a significant savings and increased melt performance can be achieved.

Energy Costs

As foundries upgrade their equipment and increase capacities, they often replace old furnaces with newer energy-efficient models to control melting energy costs and increase efficiency (Table 1). This step alone, however, isn't enough. Regardless of the type or age of melting equipment, foundries must be aware of their furnace melt rates, energy usage and waste to control melt energy costs.

Any increases in melt rate or reduction in energy use is a direct profit margin enhancement. The best way to increase the melt rate of a furnace is to make sure that the furnace is operating at full efficiency with minimal heat loss. Figure 1 shows the primary sources of heat loss during melting. It is important to note that only a percentage of the heat generated by the furnace is available heat for melting, and that this percentage can be increased through control measures. These control measures include:

Walls - The heat loss through the walls is a direct result of the refractory lining efficiency, maintenance and installation. Regular repair and maintenance of this refractory will help minimize heat loss. Refractory walls of melting furnaces must be checked regularly for cracks and spalling, and repaired as necessary, and the furnace lining must be replaced as needed. Also, check with your refractory supplier about options for lining the furnace. Newer refractory materials continually are being developed in an attempt to increase refractory life. A change in lining or a new design also can increase efficiency and decrease this heat loss factor.

Opening - Whenever the furnace dip well is open, the furnace is losing heat. If the furnace does not have a cover, this heat loss is constant. Reverberatory furnace dip wells and crucible furnaces should be covered to reduce heat loss. In one test, a foundry decreased its melt time by more than 25% by adding a refractory cover to an uncovered crucible furnace during melting. The time a furnace is open during degassing, fluxing and melting also contributes to the heat loss. This heat loss can be minimized by performing these processes under a cover.

Flue loss - Although some heat loss through the exhaust flue is inevitable, it can be minimized by keeping the burners properly "tuned" for maximum burner efficiency. The proper fuel-air ratio can produce greater heat (a hotter flame) with less fuel, which allows the furnace to melt more quickly and decrease the melt time. The local gas utility can be a resource bringing burners to maximum efficiency, and many offer incentives and cost-sharing for repairs or new equipment that save energy. Other options are new furnace designs and aftermarket equipment that capture flue loss and reuse it as an additional source of heat for melting.

Pyrometer care - Although the pyrometer is not part of the furnace, it is the only means of melt temperature measurement. The pyrometer should be kept clean and calibrated as a mistake of 50F results in a foundry using at least 2% more energy per melt.

Burner Control - High energy bills may warrant the consideration of new furnace controls or burners. The newest burners are designed for maximum fuel economy and offer high-efficiency combustion methods, low-flame cycling options and other controls that older equipment may not have.

Melt Loss

Melt loss is another cost factor that is rarely quantified by foundries, and, as a result, is rarely targeted for reduction. Aluminum foundry melt loss averages 8% per year, with some shops experiencing up to 15%. An opportunity to improve foundry profit margins exists if the sources for those losses are identified and reduced or eliminated (Table 2).

Five major areas of melt loss exist in a foundry: dross and furnace skimmings; metal spills and mixed metal; saw kerfs or cuttings; dust collector fines; and spilled sand and core butts that contain metal. These "waste materials" often are disposed of through smelters, metal suppliers and scrap brokers. This "hassle-free" convenience of having the materials hauled away often clouds the basic issue that a large potential source for profit improvement is disappearing out the door with little or no management control.

Furnaces vary widely in the amount of melt loss and waste they generate. For reverberatory furnaces, oxide films caused by direct flame impingement on the molten aluminum and charge materials generate high metal loss rates. This oxidation is minimized through proper burner maintenance. Crucible furnaces, meanwhile, should run with an oxidizing flame and with exhaustion of combustion gases away from the molten metal surface.

The condition of the charge material also is a source of metal loss. Foundries typically melt 30-lb ingot bars and "returns" of gates, risers and scraped castings. Home returns, however, are frequently contaminated with residual green sand adhering to the surface and lubricants from the cutoff and cleaning operations. In addition, they may contain glass screens or ceramic filters that were utilized in the gating systems. While clean and dry charge materials sustain a minimal loss of material when remelted, wet or contaminated constituents charged into the crucibles or open wells of reverberatory furnaces result in excessive aluminum-rich dross from the reaction of the moisture and organic compounds with molten aluminum. In addition, ingot and returns that have oxidized due to contact with moisture and humidity also react, resulting in dross formed from the oxides. Foundries can combat this excess oxide by keeping all ingot and scrap indoors and dry and by preheating returns.

Table 1. Efficiencies of Aluminum Melting Furnaces

Furnace                      Melt Loss     Efficiency        Btu/lb

Gas reverberatory                 2-12%      12-28%       1500-5000
Electric reverberatory               1%      70-75%             820
Stack melter                       1-3%      40-60%        850-1250
Crucible (gas)                     2-3%       7-17%       1800-3300
Electric resistance        less than 2%      70-90%             700

Note - This information was collected from various furnace
manufacturers.

The largest percentage of waste (melt loss) is in the removal of dross. Most aluminum melters generate thousands of pounds of dross each year and dispose of it to the highest bidder. Because most dross lots are small shipments (incurring high freight and handling charges), foundries are typically compensated for dross at only 20% of its true value. Foundries accept this cost penalty because they incorrectly believe they have little control over it.

Table 2. High Cost of Aluminum Melt Loss Based on Metal Usage

Melt      Metal        Metal     Aluminum     Daily     Annual
loss   melted (lb)   loss (lb)   cost ($)   loss ($)   loss ($)

1%        7215           72        0.90         65      16,900
2%        7289          146        0.90        131      34,060
4%        7441          298        0.90        268      69,680
6%        7599          456        0.90        410     106,600
8%        7764          621        0.90        559     145,340

The amount of metal remaining in the dross is a direct result of fluxing efficiency. When properly performed, the dross should be loose, granular material with minimal metal. The treatment of the dross varies from operation to operation as some perform a fluxing and cleansing process on the melt while others simply skim the untreated melt. This dross results in a 1-2% metal loss for clean and dry charge materials, but can reach up to 10% for contaminated and wet charge materials of light section thickness.

If machine turnings are part of the charge, melt losses can exceed 15% because of oxidized material Efficient foundries or diecasting shops typically ship out dross for reprocessing with metal contents up to 50%, while shops doing a poor job of in-house fluxing and treatment may be shipping out dross with metal contents approaching 85%. If the dross contains 70% metal and the alloy cost is $0.90/lb, then a foundry is losing the equivalent of $63 per 100 lb of dross (minus the payment received from the collection/dross processor).

Dross recovery units designed to remove the metal from the furnace skimmings offer a means of reducing metal loss for smaller foundries. These units vary in style and method, but are designed to minimize the amount of metal in the final dross that leaves the foundry for recycling.