Recycling and Scrap Melting: Equipment

 

equipment

Courtesy of the Aluminum Association

Equipment
Combustion Technology
Energy Consumption
Process Description
R&D Trends

A variety of furnaces can be used for melting aluminum. The choice of furnace depends upon the desired production rate, the quality and composition of the scrap aluminum to be melted, and whether a batch or continuous mode of operation is desired. Other factors influencing furnace selection are capital costs, refractory lifetime, and metal loss.Lindberg/MPH, Seco-Warwick, Frank W. Schaefer, and Gasmac (Canada) are reported to be the larges suppliers of aluminum melting furnaces. Other manufacturers are active in this market as well. For a complete list, click on the manufacturers database identified by the wrench icon at the bottom of the page.

Reverberatory

Reverberatory furnaces heat the aluminum to melting temperatures with direct fired wall mounted burners. The primary mode of heat transfer is through radiation from the refractory brick walls to the aluminum, but convective heat transfer also provides additional heating from the burner to the aluminum. Reverberatory furnaces are available with capacities ranging up to 150 tons of molten aluminum. Typical aluminum reverberatory furnaces have melting efficiencies of 15%-39%. Recuperation can enhance reverberatory furnace efficiencies to 10-15% but also adds to maintenance costs.

The advantages provided by reverberatory aluminum melters is the high volume processing rate, and low operating and maintenance costs. The disadvantages of the reverberatory aluminum melters is the high metal oxidation rates, low efficiencies, and large floor space requirements.

Electric Crucible

Electric crucible furnaces are similar to gas-fired crucible furnaces, but utilize electric resistance-heating elements to heat the crucible and melt the aluminum. Like their gas counter part, electric crucible furnaces are small and are typically used when alloy flexibility is the most important.

Advantages provided by the electric crucible furnace is the near elimination of emissions and low metal oxidation losses. Disadvantages include increased fuel costs and size limitation.

Rotary

Rotary furnaces are used almost exclusively for reclaiming low grade aluminum scrap and drosses (dross: a mixture of aluminum and aluminum oxide which forms on the surface of molten aluminum. Dross can also contain fluxing agents added to the molten aluminum and other impurities. Dross can contain aluminum content of 35%-95%, depending on how the melt was achieved and the effectiveness of any fluxing agents added to the melt.) into aluminum. The Furnace operates by rotating the charge through the furnace which comes in direct contact with a gas burner or with a refractory wall which was directly heated by the burner. Typical rotary furnaces have holding capacities of 2 to 5 tons and are usually charged with salt which acts as a flux to improve metal recovery and reduce aluminum oxidation.

The advantage provided by rotary furnaces is their ability to process dross and low-grade scrap which is difficult to process in other types of furnaces. The disadvantages are low efficiency, higher maintenance requirements, and considerable salt cake production which must be disposed of as a hazardous waste.

Electric Reverberatory

Electric reverberatory furnaces are used primarily as holding furnaces and are seldomly used as aluminum melters. These furnaces are refractory lined vessels using resistance heating elements mounted in the furnace roof above the hearth. These furnaces are used for smaller melting applications where limitations on emissions, product quality, and yield are of high priority.

Advantages over gas-fired reverberatory furnaces include low emissions, low metal oxidation, and reduced furnace cleaning. Disadvantages include high fuel costs, low production rates, higher capital costs, and frequent replacement of heating elements.

Tower

Tower furnaces are loaded from the top of a vertical tower with aluminum, and burners at the bottom of the tower melt the aluminum. The primary mode of heat transfer in tower furnaces is through convection by direct impingement of the burner on the metal in the lower section of the tower, and by the combustion gases as they travel out the tower past the metal charge. A variation of the tower furnace uses a grate system which lowers the metal through four grates, gradually heating the charge through each grate until on the fourth and final grate, the charge melts and falls into the hearth.

The advantages of the tower furnace are high efficiency (40%-77%) and low oxidation losses. The disadvantages of tower furnaces are their higher capital costs and the furnace size is restricted by height limitations.

Induction

Gas Fired Crucible

Crucible furnaces are small capacity, indirect aluminum melters/holders typically used for small melting applications or exclusively as a holding furnace. The aluminum is placed our poured into a ceramic crucible which is contained in a circular furnace which is fired by a gas burner. The energy is applied indirectly to the metal by heating the crucible.

The advantages of crucible furnaces are their ability to change alloys quickly, low oxidation losses, and their low maintenance costs. Disadvantages include low efficiency, (as low as 12%), high emissions, and size limitations.

Energy efficiency can be improved by 50% by adding a ceramic matrix recuperator to the exhaust system to recover waste heat for preheating the combustion air.

Morgan Thermal Ceramic, Ltd. has introduced a regenerative gas crucible furnace that circulates waste heat to provide both high temperature capability and high efficiency.

Sweating Furnace

A sweating furnace is a dry-hearth furnace used to process aluminum scrap that contains considerable amounts of unalloyed iron and/or zinc. The charge is loaded onto a slanted hearth and heated above the melting temperature of zinc but below that of aluminum. The molten zinc runs off and is collected. Then the furnace temperature is increased above the melting point of aluminum, but below the melting point of iron. The molten aluminum is then collected separately from the zinc, and the solid iron is left behind in the furnace.

Furnace Efficiencies and Capacities

Furnace Type Capacity Efficiency Dross Losses
Reverberatory 1 to 150 tons 1,300-2,500 Btu/lb 3-% Existing, 2% New
Rotary 2 to 5 tons > 2,500 Btu/Lb Dross feedstock 35-95% Al
Tower 10 tons/hour 1,165-1620 Btu/Lb < 1.5 %
Gas Crucible < 1 ton 2,000-2,500 Btu/lb 3%
Recuperated Crucible < 1 ton 1,200 Btu/lb 3%
Electric Crucible < 1 ton 450-550 kWh/ton 2-2.5%
Electric Reverberatory < 1 ton 400-500 kWh/ton 1%
Induction very small to 7.5 tons 450-550 kWh/ton 1%