There can be little argument that the drosses leaving most aluminum melting operations, are some of the highest value waste materials generated in this industry. Their aluminum alloy content varies from a high of up to 95% in skimmings taken directly off transfers or pouring ladles, to a low of about 40%. The contained alloy are removed from charging wells or dry hearths where dirty or contaminated materials are introduced to melting furnaces.
Over the years, most aluminum melting operations have become accustomed to having the material hauled away by recycling firms for remote processing. These recovery operations incur significant costs which begin with the cost of transport to the remote facility. They continue with the costs of remelting the materials, processing it to recover a portion of the contained alloy, the melting losses associated with the operation, the casting costs to convert it into a useful charge material, the disposal of any waste or landfill material remaining, and the final transport back to the foundry or die cast operation producing the original dross. All of these recovery operations are supported by major capital investments in process equipment, energy costs to fuel the furnaces and equipment, labor costs to perform the recovery tasks, and finally a profit to support the organizations involved.
All furnaces melting aluminum produce drosses with the same potential recoverable value. This is whether they are in foundries, die casters, or melting furnaces supporting extrusions, forgings, or rolled products. The service industries performing the recycling have created the ethic that their contribution is just a normal factor in the business cycle, and is no longer even considered to be a recoverable expense. The drosses are shipped out by either direct sale or recycled under contracts that pay for return of the recovered metal. What is often over looked or forgotten is the true value of the materials leaving for recycling, and the net value difference of the material that has been returned for reentry into the foundry’s process loop.
Equipment has been developed over the years to address this need, but it has been a continuing problem because of the quantity of material that requires processing. In many cases, the quantities have been too small to make economic processing viable (e.g. ladle skims), or too large to effectively deal with the handling problems of heat retention, mixing and recovery of the metallics. These problems have, for the most part, driven operations to methods that attempt to reduce the volume and content of dross by separation processes performed on the surface of the aluminum bath. As described earlier, they can be effective if performed properly, but the task is labor intensive and performed in high heat and less than desirable levels of smoke and fume. Further, under the best processing conditions, most of the drosses removed still have more than 50 % contained alloy.
Mix Master Separators
One of the very first effective systems is pictured in Figure 1, which processed the large volumes of drosses removed from the hearths of reverberatory furnaces. The method required a pre-heated refractory lined ladle, a quick transfer of the hot dross, and addition of flux followed by mixing with paddles to develop the gravity separation of alloy and foreign material. The recovered alloy was drained out of a bottom tap hole and returned to the generating furnace. At this time, your authors are not aware of any operating units in this country, and it is assumed that the combined problems of maintenance, environment, and operating costs caused the demise of the process.
In the late nineties, another process called the “Dross Press” surfaced to recover metal from the hot drosses, and is shown in Figure 2. This equipment package consisted of large, heavy walled steel molds, mounted in a very powerful press. In operation, the dross was transferred into the mold and compressed to squeeze the recovered metal out of bottom drain holes. The heavy walls were designed to cool thermiting dross, which added measurably to the recovery, but also trapped a lot of metal that could not be extruded through the drain holes.
This produced an end product with a lot of encapsulated metal in the dross cake, which still required outside processing since it was far from a suitable charge with the dross, flux, and metal admixture. The drain metal was clean, and a desirable product, but was usually less than 25% of the weight of dross charged. The process was also burdened with very high initial capital cost, and expensive mold maintenance charges. While still quite successful in some operations, it has not developed a large following.
Dross Boss™ Process
Around the turn of the century, another process appeared on the market as a mini version of the early Mix Master units. The units have been designed to service aluminum melters producing dross in quantities ranging from 10 to 500 lbs, with levels of automation ranging from manual stirring to full automatic operation where all the operator has to do is load the hot dross and start the cycle.
The designs of the smaller units feature portability, and may be moved from furnace to furnace to serve every style of furnace, from crucibles to small reverberatory furnaces. They are unique in that they do not require preheat of the equipment before the drosses may be processed for recovery, and merely utilize the heat of the directly transferred dross with a supplement from an exothermic drossing flux. The operator is required to add a stopper to the drain hole before transferring the dross, add a small quantity of flux, and stir the contents for a few minutes. This produces a gravity separation of the contents of the Reaction Vessel, with the molten aluminum beneath the demetallized dross floating above. Opening the drain hole with a prod permits the recovered metal to drain into an ingot pan, which may then be transferred directly back to the original melter. The chemical composition of the recovered metal is approximately equal to the original charge, with only a small loss of magnesium and strontium. Further, the recovered metal has been confirmed to be free of oxides and inclusions as a result of the intensive mixing of flux and unprocessed dross. ¹ ²
The total operation for recovery takes a little over ten minutes, and will normally recover up to 80% of the contained alloy in the initial dross. The demetallized dross still contains approximately 25-30% aluminum and has value, which may be recovered by the usual recycling operations. In general, typical drosses have been found to vary in initial analysis of approximately +70% aluminum, and will yield an average of 50 – +65% of the initial weight charged. Some rich skims may be as high as +90% contained alloy, and will generate higher recoveries, typically in the 80% range.
All process units with capacities above 50 lbs. are processed with powered mixing. These include units that return the recovered molten metal directly back into the generating furnace, as well as the larger units (300-500 lbs.) that have portable Reaction Vessels which are filled at the furnace and then transported to a Mixing Tower for processing. These volumes are typically drained back into large sows, again, which may be charged to the generating furnace while still at elevated temperature.
A 200# unit with powered mixing is pictured in Figure 3, which has been operating at a sand foundry producing Almag castings for a number of years. They would remove approximately 200+# of prefluxed Almag dross at the end of their single operating shift, process the material in the unit for about six minutes and then drain an average of 140# of recovered metal for transfer back to their melter. ³
¹ D.E. Groteke, “Dross Reclamation at Aluminum Melting Furnaces”, AFS Transactions, 2000, p 579-583.
² D.E. Groteke, “Aluminum Melting Process Cost Reductions”, AFS 6th Int. Conference on Molten Aluminum Processing, Nov. 11-13, 2001.
³ Product Innovations, “Sand Caster Cuts Metal Loss From Dross”, Modern Casting, January 2008, p60.