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The major players in this arena are facilities that carry out secondary smelting of
The physical and chemical properties of these elements have a few similarities and many differences. Each of the individual sections below will first summarize what the processes carried out by facilities specializing in each of these metals have in common, and will then analyze the different impacts associated with each metal in parallel sections.
From an environmental standpoint, the main advantages that secondary smelting offers over primary smelting from ore are that:
There are nevertheless many downstream processes that are basically the same for both primary and secondary smelting, with comparable impacts. In addition, separation of recovered materials from the products into which they have been incorporated involve some impacts unique to the secondary sectors.
As these impacts are enumerated, it may seem that the secondary processes begin to lose some of the environmental high ground they may be accorded due to their key position as converters of "brown" to "green" matter in the industrial ecology food chain. In fairness, one should compare the impacts not just with those of the primary smelters of the respective metals, but should include the impacts from mining the ores as well. In this more comprehensive context, some of the "green" luster may be restored.
The metals share a few similarities . Most are relatively low-melting, for example (copper being somewhat higher, although still far below the melting point of iron). But their differences are more striking. Magnesium, zinc, and aluminum are well tolerated by the body in appropriate amounts -- magnesium and zinc are, in fact important nutrients -- whereas lead is notoriously toxic. Magnesium and aluminum are among the lightest metals used in structural applications, while lead in one of the heaviest substances in common use. Magnesium and aluminum are highly chemically active (relying on thin, protective surface layers of oxide to keep them from quickly corroding), while lead is inert in most common environments. Zinc and copper tend to fall in the middle along most of these spectra. But these are bulk properties. As fumes and dusts, some of the similarities re-emerge.
Not surprisingly, the impacts and risks associated with each of these metals mirror their particular suites of properties, as outlined below.
Environmental impacts and risks
Effects of existing and future regulations on impacts
The secondary smelting of individual nonferrous metals is grouped into a common industrial category by the SIC system (SIC 3341, Secondary Smelting and Refining of Nonferrous Metals). In the NAICS system, the different metals are distinguished in the fifth and sixth digits for aluminum (NAICS 331314, Secondary Smelting and Alloying of Aluminum) and copper (NAICS 331423, Secondary Smelting, Refining, and Alloying of Copper), but the rest are grouped together into a common category (NAICS 331492, Secondary Smelting, Refining, and Alloying of Nonferrous Metal (except Copper and Aluminum)).
The basic sequence of operations carried out by secondary smelters of each of the metals is superficially similar. First, the metal-containing parts are mechanically separated from the more easily removed non-metallic constituents of the products that have been recovered from the post-consumer waste stream. (Post-industrial sources, such as manufacturing scrap, may not require this step.) Then the metals are separated from more tightly bound contaminants, generally by a sequence of processes of increasing temperature. The first step may volatilize or burn off organics, leaving the desired metals along with the more refractory constituents. Then the desired metals will melt away from the remaining materials (a process referred to as "sweating", a term which seems to be in common use across all of the processors of the respective metals other than copper), leaving the higher melting metals, such as copper or iron, behind to be dealt with by other means. Finally, the metals recovered in the sweating process are refined until they can re-enter commerce with the appropriate materials specifications as raw materials for subsequent manufacturing into finished products.
But there are a few fundamental differences that change the nature of the impacts associated with each metal significantly. The differences stem from a combination of chemistry and economics. The most active of the metals, aluminum and magnesium, are so active that they will displace hydrogen, acquiring its oxygen from water, if given a chance. If they are oxidized during the process of recovery, they are essentially lost. To recover them in metallic form would require a difficult and expensive reduction step, that must be carried out by electrolysis in an nonaqueous electrolyte. One would be essentially repeating the processes involved in primary production of the metal -- any advantage in having recovered the metals from the waste stream would be nullified. Therefore, in secondary recovery, they are always kept to the greatest extent possible in metallic form, and are never resmelted. It is not scarcity that makes aluminum and magnesium valuable -- they are abundant in the earth's crust -- it is the energy that must be put into turning their oxides into metal that adds the value.
On the other end of the activity spectrum, copper is sufficiently "noble" (inactive) that it is sometimes found in nature in metallic form (native copper). Oxides of copper can be converted to the metal simply by heating. It is also relatively easy to reduce dissolved copper electrolytically, in a water solution, a process which produces a very pure form of the metal. Furthermore, in contrast to the situation with aluminum and magnesium, copper ores are relatively scarce. Thus it is economically advantageous to recover copper because of the intrinsic value of the material, not because of the energy invested in converting its chemical form. Secondary copper recovery can therefore repeat many of the processing steps of primary recovery and still remain an economically viable proposition.
Lead and zinc resemble copper in relative scarcity, though they are somewhat more active as metals. Lead and zinc smelting do not rely on heat alone, but instead generally use carbon as a reducing agent (typically in the form of coke). If the zinc is being recovered from metal, and reduction is not needed, it can be processed in a retort furnace, using what is essentially a distillation process -- zinc is actually heated until is vaporized, and the vapor is recondensed, producing an acceptably pure metal. Lead, on the other hand, is typically smelted in a blast furnace, with a reduction step necessary, apparently because much of recovered lead comes from recycled batteries, in which much of the lead is present in oxidized form. Rotary kilns can also be used in lead recovery -- if a large fraction of the recovered material is in metallic form, it can be melted directly, avoiding the need for a reduction step. The remaining oxidized fraction of the lead is then put through the blast furnace.
The table below lists the proportion of the U. S. metal supply that is derived from secondary processing. Different sources give different figures, but these numbers should be adequate for a rough indication of the relative importance of secondary processing for each metal. It is interesting, and perhaps a bit disheartening, to note that the amount of recycling has actually been on the decline in recent years in some cases.
Table. Percent of U. S. metal supply derived from secondary sources
| Metal |
Percent secondary |
Year | Notes | Information Source |
| Aluminum | 32.2% | 2000 | Down by 6.6% from 1999 level (34.5%?) | Aluminum Association |
| Copper | 33% | 1999 | Down from peak of 50% in early 1980s | Copper Development Association |
| Lead | 63% | 1998 | Apparently down from 72% in 1993 | 63%: USGS; 72%: EPA |
| Magnesium | 33% | 1998 | Depends strongly on aluminum recycling rate | USGS |
| Zinc | 27% | 1998 | May have been higher; AZA claims > 33% | USGS |
A good source for recycling information, providing useful short descriptions of both the processes involved and market factors influencing recycling rates, can be found in a series of documents issued by the U. S. Geological Survey (USGS), each document dealing with a specific metal. A summary document is also available. In addition to the metals analyzed here, it may be of interest to list the recycling rates for several other metals, for comparison purposes. Data from the USGS documents indicate that in 1998, the recycling rate (as percent of apparent supply) for iron and steel was 41%, for manganese 37%, for tin 22%, for molybdenum 33%, for chromium 20%, for cobalt 32%, for gold 29%, and for platinum 16%.
Most of the trade organizations for the primary metal producers claim to represent the secondary smelters as well. (The quotations below have been taken from the respective association websites, with emphasis added.) Older sources, such as the EPA-OECA Sector Notebook on the Nonferrous Metals Industry, list organizations that represent the recyclers specifically. There seems to be little or no trace of these organizations on the web -- they may have recently merged into the larger organizations, or they may be organizations with a staff of one that tend to be active as opportunity permits.
Aluminum Association: "The Aluminum Association, Inc. is the trade association for producers of primary aluminum, recyclers and semi-fabricated aluminum products, as well as suppliers to the industry.
Copper Development Association: This is a fairly sizeable organization, headquartered in New York, that lists 33 different staff members (in nine different area codes) on their contacts page. There is remarkably little information about the organization itself apparent on the website (most of which is devoted to copper-related technical and marketing information, news items, and the like).
Lead Industries Association : Another website with some information (primarily promotional) about lead, but virtually nothing about the organization. The contacts page lists an 800 number and a Milwaukee address.
Battery Council International is a trade group for recyclers of lead batteries.
International Magnesium Association: "the mission of the International Magnesium Association (IMA) is to promote the use of the metal magnesium in material selection and encourage innovative applications of the versatile metal. IMA's members consist of primary producers of the metal, recyclers, foundries, fabricators, end-users and suppliers."
Air emissions data for certain key criteria pollutants (ozone precursors) are available from the National Emission Trends (NET) database (1999), and hazardous air pollutant emissions data are available from the National Toxics Inventory (NTI) database (1996 is the most recent year for which final data are available). For the secondary nonferrous metals sector as a whole (SIC code 3341), the total emissions are:
Unfortunately, since the SIC designation groups all nonferrous smelters together, it is not possible from these data to compare the relative contributions from the individual metals. The emissions tend to be of the same rough order of magnitude as those from the primary metals industries (with the exception of iron and steel production, whose emissions are an order of magnitude larger).
The primary risk associated with secondary nonferrous smelting operations would be from air emissions in the vicinity of the facility. Metal-bearing dust particles could be associated with any metal smelting operation. Because of the toxicity of lead, the risk would be particularly acute from lead smelting operations. Zinc is somewhat less toxic to humans, although heavy exposure to zinc fumes can have significant health consequences -- more often, zinc production has been linked with heavy impact on vegetation in the area surrounding production facilities.
Dioxin emissions are most likely associated with the roasting of organic contaminants in the presence of chlorine. The elevated dioxin emissions associated with the secondary copper smelting operation noted above is presumably due to the inclusion of polyvinyl chloride in the insulating materials from copper wire -- this would serve as a chlorine source, as well as a source of organics.
Secondary lead smelters are covered by a NESHAP finalized in 1995. An EPA Fact Sheet claims that the rule "will reduce emissions of air toxics from these facilities by 1,400 tons per year, a 72 percent reduction from the preregulated levels emitted by these facilities. In addition, the rule is expected to reduce emissions of particulate matter (which can cause serious respiratory problems) from these facilities by 150 tons per year, and carbon monoxide (which can cause adverse health effects, including fatigue, nausea, and respiratory problems) by 88,000 tons per year". (The Fact Sheet was last updated in 2002, but the actual effect of the NESHAP, as opposed to the projection, is not cited.) Another Fact Sheet, from 1994, lists the 23 secondary lead smelting facilities to be affected by the 1995 rule. A more recent list would be useful, but does not seem to be easy to come by. Because all secondary smelters of nonferrous metals other than aluminum and copper are grouped into the same NAICS code, the Economic Census numbers are uninformative as to the number of establishments still in business that perform secondary lead smelting specifically. The Battery Council International website provides detailed information on lead-acid battery recycling laws on a state-by-state basis, and provides some general information on where consumers can take batteries to be recycled, but does not appear to provide any information on which facilities actually do the recycling.
Secondary aluminum smelters are covered by a NESHAP finalized in 2000. The rule was originally intended to apply to aluminum die casters and foundries as well as to recyclers of aluminum metal, but was amended in 2002 to exclude die casters and foundries.
A NESHAP was issued in 2002 for primary copper smelters, but there is apparently nothing in the works for secondary copper smelters.
An Effluent Limit Guideline (ELG) for Nonferrous Metals Manufacturing appears in Title 40 of the Code of Federal Regulations, Chapter 1, Part 421, and covers secondary recovery of metals from recycle wastes.
The EPA Office of Enforcement and Compliance Assurance (OECA) has prepared a Sector Notebook on the "Nonferrous Metals Industry" that includes information on secondary recycling, available at http://www.epa.gov/compliance/resources/publications/assistance/sectors/notebooks/nfmetlsn.pdf
The aluminum industry is one of the "Industries of the Future" in the program administered by the Office of Industrial Technologies (OIT) of the Department of Energy. The aluminum home page is at http://www.oit.doe.gov/aluminum/
A detailed and useful resource available from the DoE-OIT site is a July, 1997 document, Energy and Environmental Profile of the U. S. Aluminum Industry, available at http://www.oit.doe.gov/aluminum/pdfs/alprofile.pdf
Information on the EPA Voluntary Aluminum Industrial Partnership (VAIP) is available at http://www.epa.gov/highgwp1/vaip/
A Technical Report issued by the Copper Development Association, The U.S. Copper-base Scrap Industry and Its By-products, 2001, is available at http://environment.copper.org/pdf/scrap_report01.pdf
The U. S. Geological survey has issued a number of short documents summarizing the recycling of a number of different metals in 1998, including: