March/April 1992
Near the end of the aluminum and copper recycling line are metal-bearing residues that are sometimes waste, sometimes recyclables. Here's an overview of an area of scrap processing that some have made their special niche.
BY SI WAKESBERG
Si Wakesberg is New York bureau chief for Scrap Processing and Recycling.
When people talk or write about copper or aluminum recycling, they usually focus on the higher-grades, higher-priced material, such as No. 1 copper scrap or used aluminum beverage cans. But their little-discussed scrap cousins—lower-grade materials such as drosses, skimmings, slags, and so forth—also play an important role in the metal-recycling picture. In fact, those that effectively recycle these materials—especially as the metal content drops lower—may be said to be approaching the ultimate in recycling, where nothing of value is lost.
There are industries within industries and there is no doubt that the recovery and reuse of drosses, skimmings, and slags—though a small and relatively underexplored region of metal recycling—may be said to be an industry in itself. And, in many instances, it appears to be a profitable one. The companies that recover these lower-grade items—some for their own use, others for shipment to other consumers—have developed outlets that are vital in reclaiming resources that might otherwise be discarded.
Terms and Origins
Drosses, skimmings, slags—what are these items? Where do they come from? How do they differ from each other? Some industry members tend to lump all these grades together and, it must be admitted, the distinction among them tends to blur when seeking absolute definitions. Webster's Ninth New Collegiate Dictionary, for example, defines dross as "the scum that forms on the surface of molten material." Skimmings and slags are similarly described, though even more vaguely.
"Trading in Metals," by Trevor Tarring and Geoffrey Pinney, provides a bit more specific definitions, describing dross as "partly oxidized scum floating at the top of molten metal." Skimmings, according to the book, are "molten metal which forms an oxidized `skin' on the surface. This is raked off just before pouring and sold, or processed, to recover the metal content." Slag is defined as "metal smelting waste."
Whatever the definition, drosses, slags, and skimmings are residues that arise during smelting and other furnace operations. In the secondary aluminum smelting industry they are generally called drosses and skimmings; in the brass and bronze ingot manufacturing industry, they are mostly referred to as slags. Sometimes, however, the terms are used interchangeably. On top of that, the same nomenclature is used in the secondary lead, zinc, and tin industry. Other names sometimes referred to include residues and ashes.
It should be emphasized that some of these materials are considered to be "high-grade" by marketers in the industry. "We've seen drosses as high as 80-percent metallic," reports one industry official. "But that was about five years ago and, frankly, we haven't seen such high-grade stuff around lately." Other secondary metal executives indicate that a considerable amount of the copper and aluminum drosses and slags now available has a metallic content of as much as 50 to 60 percent. Most recyclers contacted, however, report that the average range of metallic content in drosses runs from approximately 25 percent to 30 percent. Some say the average is as low as 20 percent.
Aluminum smelters generally process the dross produced during aluminum smelting in a salt bath in a rotary furnace, recovering much of the aluminum content from the oxides. What's left is aluminum oxide entrapped in sodium chloride, which is known as salt cake, and reportedly can be further processed to extract the remaining aluminum using new dross-desalination and evaporation technologies. (See "Recycling R&D," beginning on page 137, for details on federal government studies conducted by Argonne National Labs.)
In the brass and bronze industry, slag is recovered through a number of different processes. "Recycled Metals in the 1980s," published by the National Association of Recycling Industries, a predecessor to the Institute of Scrap Recycling Industries, describes some of these processes as follows: "Normally, these items [lower-grade materials] must be processed by milling, screening, and concentrating to obtain a suitable end product which can be charged into the furnace. This is generally accomplished by crushing in a ball mill, rod mill, or hammer mill to remove oxides, dirt, etc., from the metallics and then [removing] the oxides and other nonrecoverable products by screening or air separation to end up with a high metallic `concentrate.' Rich metallic skims normally do not need milling prior to use."
Squeezing Out Profits
There seems to be no consensus among recyclers as to where the boundary lies between low-grade metal residues that are economically recyclable and those that are not.
According to one secondary aluminum executive, his firm considers "anything above 25- to 30-percent metallic content a "better-quality dross" and handles residues containing more than 30-percent aluminum as it would any other scrap within the plant. Drosses with less than 25-percent aluminum, he says, have little value, and are, therefore, problematic."
Other secondary aluminum smelter executives offer differing views. "We can handle material with a metallic content of 15 to 20 percent economically," says one official, noting that "below that, it becomes impractical." Still others also report that they have worked with material as low as 15-percent metal.
But, another smelter representative insists, "You've pretty much gotten what can be squeezed metallically out of aluminum when you're down to 20 percent," casting doubt on the profitability of processing 10- or 15-percent-aluminum material. Another remarks: "When this material gets down to the 10-percent level or thereabouts, we call it `black dross,' which requires significant processing to remove the small fraction of metal in it.
Other aluminum industry members admit that they've had to ship some very-low-grade material for disposal, which can cost $100 a ton or more in many areas. Although industry officials point out that these residues have "not yet" been declared a "hazardous waste"—raising the costs and limiting access to landfills—they note that they are carefully watching developments in this direction since they could establish a new set of economic rules used to define what is viable to recover.
On the red metal side, industry officials say a calculation based simply on the current market value of copper and the copper content of the residued determines whether it will be recovered. Ironically, if an ingot maker does a really good job of refining copper-bearing scrap, the metal content of the slag will tend to be lower, thus decreasing its recovery value. "When copper was $1.50 a pound, it was economical to concentrate the lower-grade slag," says one executive. "When copper falls well below $1, it becomes debatable whether we should continue to concentrate."
Still, some brass and bronze ingot makers indicate, however, that they can sell off all their slag, even when it has a copper content as low as 15 percent, though few recyclers are forthcoming about where they sell these items. "None of it goes to a landfill," however, says one company official.
Finding Buyers
Secondary metal brokers and merchants play an important role in arranging sales of these lower-grade items to smelters, foundries, and refineries, which combine the material with other feedstock in the production of secondary-metal products. Some copper refineries, in fact, "live on this material," says one industry observer. "Slags make up a substantial percentage of their feedstock."
In addition, some foundries, particularly those that produce brass bar stock, digest their own slag—as well as that generated by brass ingot makers—returning it to their furnaces following crushing and concentrating. Others sell their slag, in skimmed form, to copper refineries. Beyond these regional markets, during certain periods—1985 to 1990, for brass skimmings for example—many copper bearing residues are said to have found their way overseas to a ready market in India . " India has traditionally been a big buyer of this lower-grade material," says one ingot maker executive. "But not today. Right now, the Indian market is dead."
There also appear to be many innovative efforts being made by recyclers to market drosses and skimmings of very low nonferrous metallic content. One company has reported developing a product specifically for the cement industry; others indicate shipments to the steel industry.
Most aluminum skimmings and drosses, however, find their way back into the secondary smelting industry for use by smelters. Some secondary aluminum smelters reuse their own skimmings; others sell them off to smelters that can make better use of the material, or they sell to foundries.
Still, lower-grade materials are, in the words of more than one source, "geographically sensitive." According to one aluminum smelter official, "You can't move dross more than a couple of hundred miles without incurring economic losses. Any excessive transportation cost can spell the difference between profit and loss." Nevertheless, some industry executives point out, large primary aluminum companies, such as Alcan and Alcoa, do ship their skimmings over great distances to their own foundries or processing plants. But generally, they say, the shorter the distance covered, the better.
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If there is such a thing as near-perfect recycling, it might be in the recovery and reuse of slags, drosses, and skimmings. Efficient and instinctive recyclers are squeezing not only as much metal as they can, but also a healthy profit, out of these lower-grade materials.
Near the end of the aluminum and copper recycling line are metal-bearing residues that are sometimes waste, sometimes recyclables. Here's an overview of an area of scrap processing that some have made their special niche.