Reducing Aluminum Melting Costs via Process, Equipment Control.
Aluminum foundries are facing numerous factors that have had a major impact upon their profitability. From rising energy, material and labor costs to the threat of additional government regulations, the focus of aluminum foundries is divided between issues related to metalcasting and the real-world realities of today's manufacturing.
For many operations, these increases in operating costs have come at a time when production volumes are falling and customers are demanding lower selling prices for castings. As a result, foundries are looking to every department for cost reduction opportunities.
This article examines aluminum foundry melt room cost centers and suggests specific, practical methods for cost improvement. While some of the changes require a nominal capital investment, most can be implemented with existing melting units to develop real economies for aluminum foundries.
The energy required to bring metal to the pouring temperature is the sum of three quantities--the energy required to raise the metal from room temperature to the melting point (typically 55% of the total); the heat of fusion of the alloy or the energy required to convert it from solid to liquid metal (30% of the total); and the energy to superheat the metal to the pouring temperature (15% of the total). If the melting operation is carried out in a separate furnace and the alloy is transferred to a holder at a casting line, then an additional amount of energy must be added to make up for temperature losses associated with the transfer.
The problem facing aluminum casters is a small portion of the energy input to any melting furnace winds up heating the molten alloy. Energy lost to flue gases and radiation through the melt surface, furnace shell and refractory walls, as well as through operation make up the bulk of the inefficiencies contributing to melting cost escalation.
Compounding these losses is the fact that any direct saving made by process improvements must be multiplied by the efficiency factor of the furnace being used. Thus, if the furnace is operating at 25% efficiency, the benefit of process improvement must be multiplied by a factor of 4 to assess the impact upon fuel consumption.
Figure 1 shows the impact of these losses upon the available energy that can be transferred to the aluminum melt. These losses are the impetus for the upsurge in use of stack melters and improved "clam shell" furnace designs.
If aluminum melting furnaces are heated by a combustion process, then the process should be optimized for the burner design and ambient atmospheric conditions present. This will improve the efficiency of the melting operation, reduce the volume of fuel consumed and lessen the impact upon the environment.
Most burner systems receive their oxygen supply from blower systems that provide a constant volume of air. Unfortunately, the volume of oxygen in each cubic foot of air will vary dramatically with the season due to ambient temperatures and humidity. While seasonal adjustments are not always required, if the combustion conditions are allowed to drift outside of the optimal range, significant variations in the combustion reactions will result.
A cubic foot of air will contain fewer oxygen molecules on a hot summer day with high humidity than on a dry, winter day, and will result in a more reducing (less efficient) combustion. In extreme cases, unburned fuel can be discharged in the flue gases in the form of carbon monoxide and hydrocarbons. Under these conditions, hydrogen gas pickup in the melt will be influenced in addition to the impact upon energy costs.
Conversely, operating in an overly oxidizing condition will promote inefficiencies in combustion through a lowering of flame temperatures because of the quench effect of the excess nitrogen and unburned oxygen. This condition also will lead to increased melting losses and the formation of deposits of corundum on the furnace walls.
The normal losses associated with melting aluminum alloys are directly associated with the mass and surface area of the charge, and general cleanliness of the material. Thin melt stocks will have a disproportionate melt loss if exposed to even moderate temperatures for any amount of time before melting. Additionally, the presence of moisture or organic films will result in the direct conversion of valuable metallics to aluminum oxides and carbides.
Aluminum alloys are susceptible to oxidation at all temperatures, but the losses can be exaggerated by heating the metal above minimum temperatures required for the process. For example, if a base temperature of 1225F (663C) is assumed to represent a normal oxidation loss, increases in the holding temperatures to 1400F (760C) will add 20% to the loss, with a more rapid logarithmic increase occurring above that temperature. On the same basis, the loss at 1500F (815C) is more than 200% of the loss at 122SF. Continued superheating above 1500F ultimately will reach the temperature at which aluminum alloys will bum in air, similar to magnesium alloys.
Another important consideration regarding surface drosses formed during charging or turbulent transfers of molten alloy is that these filmy drosses usually entrap a high percentage of good aluminum alloy in the mixture. Relatively clean skims taken from pouring ladles or holding furnace wells may contain up to 90% of useable alloy, while those from the charge wells of wet-well furnaces receiving a typical mix of ingot and process returns will contain up to 70%.
The entrained air-oxide-aluminum mix is quite sensitive to a more rapid rate of continued oxidation than a normal aluminum volume, and if allowed to enter the firing hearth area of the melter, may be converted to a white ash of aluminum oxide in a process known as "therrnitting"--the exothermic reaction of aluminum burning to oxide. Some melt room operators believe that white ash is a beneficial indicator of low melt loss when observed after removal from the hearth of their furnaces, but in reality it could be an indicator of poor practice, depending upon when and how the ash was formed.
Surface Energy Transfers
Molten aluminum surfaces are efficient at radiating energy and represent an additional area for improvements in controlling energy costs. A molten aluminum surface at a base alloy temperature of 1225F will radiate 9200 Btu/sq ftlhr, with the losses rising rapidly to 20,000 Btu/sq ft/hr at a temperature of 1500F.
Conversely, a blanket of dross and oxide can be an unwanted insulator when present on the molten aluminum surface of the melting hearth. Figure 2 depicts the loss of heating efficiency associated with an increasing layer of dross thickness and confirms the benefit from a frequent removal of this material as one of the best ways to maintain the melting rate of primary melters. Many cases of below-normal melt rates have been resolved by the simple expedient of removing hearth dross to expose a clean melt surface to the hearth chamber atmosphere.
While one of the easiest methods of measuring improvement in melt room operations is to look at the total consumed therms reported on the utility bills at the end of the month, short-term methods also are available. The easiest is an observation of the furnace in operation (if direct reading meters are not available). Depending upon the firing train and burner combinations used on the melters, most units will switch between an on/off mode or a high-fire/ low-fire mode. The ratio of time on high-fire and low-fire, or the time-on versus time-off will be a direct measurement of energy consumed.
Another method, though less effective than measurement of firing times, is the measurement and recording of stack gas temperatures. An increase or decrease in temperature from some optimized level (2100F to 2500F, for example) means that more energy is going up the stack and represents a 10% efficiency loss of the available melting energy. Thus, an early indication would be available to signify a need for improved charging practices, or even furnace or burner maintenance.
Process Cost Reductions
The first step in melting cost reductions is to optimize the melting process at your foundry by ensuring that all equipment is functioning at peak performance.
Optmizing Burner Combustion--The requirement for burner adjustments and calibration has been discussed and points to the need for periodic maintenance in this area. Many operations have found significant cost reductions available in fuel savings and increased performance through this means. With today's burner systems and firing trains, a level of expertise and equipment are required to fine-tune the combustion process for the full range of operation. However, visual indicators are available that can suggest a need for corrective action.
Charge Material Preheat--For furnaces with open charge wells, the practice of preheating charge materials by suspending them over the open well is a practice that can yield big benefits. With 60% of the total melt and superheat energy expended in raising the material to the melting point, any free energy recovered over the well is a positive benefit. Preheating to temperatures of 600-700F will save more than 30% of the total energy requirement. Many operations do achieve some form of preheat by extending the ingot over the charging ledge, but the construction of semi-permanent racks for support and exposure of more ingot surface area while preheating is beneficial (Fig. 3). For those operations charging returns with drop bottom or dump hoppers, it may even be possible to suspend the container over the charging well to obtain the preheat benefit.
If the primary melters are designed with dry hearth charging systems, the materials should be charged in a manner that will allow as much exposure of charge surface to the flue gases as possible. The placement of solid bundles of ingot on the hearth should be avoided, and if sows are charged, the space above the sow should be filled with returns. In addition to the benefit in energy savings, a direct benefit in melt loss reduction also will be derived if the charge materials contain moisture or organics.
Covering Open Wells--The amount of radiant heat losses from molten aluminum surfaces confirms the need to cover any inactive furnace well surfaces with covers or refractory blankets to minimize the loss. An uncovered surface will emit energy at an estimated cost of more than $0.30/hr/sq ft if the furnace efficiency is 30% and energy costs are $6/mcf.
While many operations cover their holder wells, they neglect the wet-charging wells of their primary melters. These frequently are sized to facilitate charging and furnace cleaning, and have a lot of area that is unneeded during large portions of the production day. If these surfaces cannot be converted to a preheating area, an effort should be made to rig removable covers for the surfaces when access is not required (Fig. 3).
Operating Temperatures--High operating temperatures are another area for potential cost reduction. While the amount of savings will be influenced directly by the efficiency of the furnace, an extra 100F of temperature may well add 10% to the energy required to superheat and then hold the metal at the elevated temperature.
Reducing furnace set temperatures during non-operating times will produce direct savings in both melt loss and energy consumption. Furthermore, the reasons for high holding temperatures should be re-examined based upon current needs. Are the high temperatures believed to be required to balance transfer losses between the furnace and mold? If so, the use of ceramic pouring ladles with less heat sink is beneficial. Alternately, transfers between a primary melter and a remote holding furnace should be made with a preheated transfer ladle with an insulating cover. This is a far less expensive option than carrying 20,000 lb of metal in the primary melter at an elevated temperature.
Many operators mistakenly believe that higher set temperatures will promote faster melting. The reality is that the melting rate is controlled by the capacity of the burners, and the ability of the hearth to accept the energy supplied. Layers of accumulated dross will inhibit the heat transfer and further increase melting loss as the mixed layer of metallics and oxide is converted to aluminum oxide by the high hearth temperatures.
Furnace Considerations--Another area often neglected is the amount of time that the furnace remains open for charging or maintenance cleaning The time required to perform these operations should be studied for possible reductions and minimized by means of equipment changes and personnel education.
Modern furnace designs are quite concerned with the amount of energy lost through open flues during the non-firing period of furnace operation (Fig. 4). Designers of the newer furnaces have made efforts to match the flue opening to the firing rate, and to seal the flue during periods when the burners are off. This will eliminate the chimney effect of the open flue, which is greatly enhanced by air leaks around poorly sealed doors. Improved maintenance on door seals as well as the possible installation of automatic flue covers can produce a quick return on investment.
Process Equipment Options
While most operations are reluctant to consider making capital investments during periods of economic slow-down or low profitability, this actually is the time when such investments are most needed.
In-House Dross Recovery and Processing--Aluminum drosses and skims are a co-product of any aluminum melting operation and will vary greatly from operation to operation depending upon the charge materials and the fluxing and process methods. With clean charge materials, the rate of generation will be 1-2% of the weight charged, but if light gage materials with contained water or organics are included in the charge, the generation rate could be as high as 10-15% of charge weight. Traditionally, these co-products can be disposed through recycling channels with the melt shop receiving compensation at a small percentage of the contained value.
Recent advances allow the recovery of metallics in dross at the generating melting furnace. Equipment models are available to process dross volumes from 20-300 lb with either manual or fully automatic separation of the materials. In practice, the hot drosses are transferred directly from the furnace to the process equipment where a small quantity of flux is added and mixed with the dross to achieve a gravity separation of the metal and non-metallics. Once separated, the aluminum values are transferred back to the melter in either molten or preheated ingot form to retain the latent energy.
For foundries doing an average job of treating dross materials, they would ship out material with up to 70% metallics. After processing with dross recovery technology, up to 80% of those metallics are recovered for immediate transfer back to the generating melter. If the average metallic content of the drosses was 70% before processing, the producing operation is losing the equivalent in dross of $45 per hundredweight of new alloy priced at $0.65/lb (less any payment received from the collection/dross processor). After an 80% recovery of the metallics, the contained value of the material leaving the melt shop has been reduced to under $9 per hundredweight (and still has value to the recycler).
Transfer Through Heated Launders--For those operations able to accept the associated limited flexibility of alloy changes and process temperatures, the use of heated launders for direct transfer of molten alloy from the primary melter to the holder at the casting line can be a major boon to their operation. The economic savings are due to:
* the operational temperature in the primary melter dropping to the same level as the holder (with attendant savings in both fuel and melt loss);
* the drosses and oxides in the melt that are generated by the turbulent transfer of the molten alloy being eliminated;
* the range of pouring temperatures in the holder being reduced with a direct benefit upon short fills and die soldering;
* the elimination of the transfer labor and crucible maintenance;
* the improved safety with the elimination of ladle transfers of the molten alloy.
Recent improvements in refractory and heating elements of these systems have reduced the power consumption to 0.56 KW/ft/hr, which is more than offset in cost with the other advantages of the equipment.
Other Equipment Options--A number of other equipment options can offer less direct cost reductions, but they may be applicable to only some operations. For example, if the furnace size is large enough, the use of circulation pumps has been documented to benefit by improving melt rates, lowering energy consumption per pound of material processed, reducing melt losses through elimination of hot spots in the furnace, and stabilizing chemistry.
With the rapid and dramatic increases in the cost of energy units, the economics of oxygen enrichment are being re-examined by many operations, as are the options to use waste heat values currently being discharged to the atmosphere through the stacks of the aluminum melters. Recuperating burners also can offer advantages to some operations, but do add considerably to the initial capital investment for the melting equipment.
This article was adapted from an upcoming presentation at the 6th International AFS Molten Aluminum Processing Conference.
Fig. 1 Pictured are the distribution of energy losses in a typical hearth furnace. Typical recoveries in the melt are only 20-30% of the input energy. Burners 1% Doors 3% Radiation 6% Charging 12% Waste Gas 58-48% Input Energy = 100% Energy Losses 80-70% Utilized Energy in Conventional Furnaces 20-30%
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|Comment:||Reducing Aluminum Melting Costs via Process, Equipment Control.|
|Author:||Groteke, Daniel E.|
|Date:||Oct 1, 2001|
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