RECOVER ALUMINUM IN-PLANT FROM YOUR OWN DROSSES
One of the frequently over-looked sources of economic loss in most foundry and die cast shops is the general area of melt room costs and metal recovery. In the past these costs have been assumed to be “sunk costs” in the operation, or those over which little control can be exercised. Nothing could be further from the truth, and in this day of increasing competition from more efficient operations, many aluminum-melting departments are taking a very serious look at the total melting operation with the objective of reducing these costs.
Many managers have been surprised to learn that the “metal loss” in their operations will often total 8%, and may even be as high as 12 or even 15% in some cases. Of the total volume of scrap materials going out the door, by far the largest percentage of weight is usually involved in the shipment of drosses and skimmings. As such, these represent the largest opportunity for profit improvement in most operations.
Treatment of the drosses found on any aluminum melt surface varies from operation to operation, with some performing a very careful fluxing and cleansing process on the material, and others merely skimming the material without treatment into a convenient container provided by the local scrap dealer. These drosses constitute a large portion of the “melting loss” sustained by all operating foundries, and will typically be in the range of 1-2% of the weight of clean and dry material, but may be as high as 6-10% for some of the more contaminated charge materials. The drosses generated will thus vary widely from operation to operation: The more efficient foundries or die casting shops ship out drosses for reprocessing with metal contents of 30 – 50%, while those shops doing a poorer job of fluxing and treatment of the drosses in-house (by far the majority) may be shipping out materials with metal contents approaching 75 or even 85%.
Most aluminum melters generate many thousands of pounds of drosses during the course of a year’s operations, and these metal rich materials leave their plants through a variety of channels. Regardless of the departure channel, the drosses and skimmings will typically wind up at a remote processor who will incur container and transportation charges, the cost to process the material and landfill the reclamation salts used in processes to improve the recovery, the unavoidable metal losses contained in the salt wastes, and the transportation costs back to the foundry or die cast shop which will re-use the recovered material in its operation. It is for these reasons and costs that the melt shop generating the original dross will receive typical compensation for the materials shipped out at 10 – 30% of its true value as new alloy.
Dross BossTM Systems:
The market need to minimize these losses has been addressed by the development of a process and equipment whereby much of the metal leaving a foundry or die casting operation as “melting loss” may be reclaimed internally and reused in the process without modification. Units are available to process quantities as small as 10 lbs. of hot dross, and as large as 500 lb., with varying levels of automation.
All Reclaimers are composed of several components that are designed to provide portability, ease of operator use by unskilled labor, and an easy separation of a large portion of the valuable metallics at the point of generation of the drosses. The separation of metallics is accomplished in a reaction vessel which is hinge mounted to permit an easy transfer of de-metallized dross to the storage container. Valuable metallics are recovered and collected in the reaction vessel through a chemical reaction with a small quantity of an exothermic flux added to the mixture, mechanical agitation, and gravity separation. When the reaction and separation are complete, the recovered metallic units are drained through a simple gate valve into either a collection ingot pan or transferred back directly to the melter in molten form. Either transfer method provides a major benefit in energy savings to minimize the cost of remelting the recovered material.
The nature of the drosses being processed has a significant impact upon the amount of aluminum recovered per process cycle. A second factor is the amount and efficiency of the stirring operation, whether it is manual or automated as with the larger systems.
While recoveries of up to 80% of the contained aluminum alloy metallics have been achieved, a more normal recovery will be in the 60 – 70% range of the weight of charged dross. The residual demetallized material will typically have between 25 – 35% contained aluminum, and still has enough value to make it a desirable material for recycling.
Units are currently operating that process dross quantities ranging from 10 lb. of dross skimmings from a transfer ladle to 160 lb. of dross from a chip-melting furnace. In both cases the recoveries exceed 50% of the charged weight of dross, and are PLC controlled with full automation. In the case of the larger unit, the recovered metal is returned directly to the well in molten form, while reducing the volume of dross leaving the plant for outside recycling by more than 70%.
Manually stirred units of 30 to 125 lbs. dross capacity are also in wide use, and are popular with foundries desiring to retain alloy identity from the processed dross. Again, the recovered alloy is returned directly to the originating melter, thus retaining the full value of the individual alloys being processed.
Designs have been completed and quoted for units with 500 lb. of dross capacity, and are currently being considered for 1,000 lb. of dross from larger reverberatory style furnaces. These units will also have the capability of returning the recovered alloy in molten form to the generating furnace in order to develop maximum thermal efficiency.
Reclaimed Metal Quality:
The melt quality of the recovered metal is excellent, with confirmed low levels of both gas and inclusions. This is not an unexpected result, based upon the intimate contact of dross and flux achieved while interacting to separate the oxides from the metal. The melt composition has been measured to sustain minor losses of the oxidizable elements of magnesium, strontium, sodium, etc. during processing, but the amounts are negligible unless the recovered metal is being returned to a very small melt volume
While there are a number of ways to assess the actual savings developed with the Alcovery Process technology, many are intangible and difficult to quantify. The important consideration is that all costs for the recovery operation, whether performed in the generating melt shop or at an outside processor, are borne by the shop producing the original dross. Any containers, freight, process costs to recover aluminum metallics, losses in the recovery operation, transportation to and from the remote processor, landfill or environmental costs associated with disposal of the co-products, etc., in the final analysis are borne by the melt shop making the original dross.
It was estimated earlier that the shops doing an average job of treating their furnaces are typically shipping out materials with 65 – 75%, or more, of contained metallics. If this assumption is accepted, and the alloy cost is $1.25 per pound, then the producing foundry is losing the equivalent of $93 per hundredweight of dross, minus any payment received from the collection/dross processor. On average, users of Dross Boss™ technology are reporting savings ranging from a low of about $0.50/ lb. to a high of more than $1.00/ lb. per pound of dross processed in-plant, depending upon tolling arrangements, remelt costs saved by a molten return of the recovered alloy, value of the alloy, etc.