Smelter

Smelters are industrial facilities that are used to treat metal ores or concentrates with heat, carbon , and oxygen in order to produce a crude-metal product, which is then sent to a refinery to manufacture pure metals.

In many cases, smelters process sulfide minerals, which yield gaseous sulfur dioxide , a significant waste product. Other smelters, including some that process iron ores, do not treat sulfide minerals. Similarly, secondary smelters used for recycling metal products, such as used automobile batteries, do not emit sulfur dioxide. However, all smelters emit metal particulates to the environment , and unless this is prevented by pollution control devices, these emissions can cause substantial environmental damages.

The earliest large industrial smelting technique involved the oxidation of sulfide ores using roast beds, which were heaps of ore piled upon wood. The heaps were ignited in order to oxidize the sulfide-sulfur to sulfur dioxide, thereby increasing the concentration of desired metals in the residual product. The roast beds were allowed to smoulder for several months, after which the crude-metal product was collected for further processing at a refinery. The use of roast beds produced intense, ground-level plumes filled with sulfur dioxide, acidic mists, and metals, which could devastate local ecosystems through direct toxicity and by causing acidification .

More modern smelters emit pollutants to the atmosphere through tall smokestacks. These can effectively disperse emissions, so that local air quality is enhanced, and damages are greatly reduced. However, the actual acreage of land affected by the contamination is enlarged because the tall smokestacks broadcast emissions over a greater distance and acid rain may be spread over a larger area as well.

Emissions of toxic chemicals can be reduced at the source. Metal-containing particulates can be controlled through the use of electrostatic precipitators or baghouses. These devices can remove particulates from flue-gas streams, so that they can be recovered and refined into pure metals, instead of being emitted into the atmosphere. Electrostatic precipitators and baghouses can often achieve particle-removal efficiencies of 99% or better.

It is more difficult and expensive to reduce emissions of gases such as sulfur dioxide. Existing technologies usually rely on the reaction of sulfur dioxide with lime or limestone to produce a slurry of gypsum (calcium sulfate) that can be disposed into landfills or sometimes manufactured into products such as wallboard. It is also possible to produce sulfuric acid by some flue-gas desulfurization processes. At best, removal efficiencies for sulfur dioxide are about 95%, and often considerably less.

The types of smelters that do not treat sulfide metals, such as iron ore smelters, logically do not emit sulfur dioxide. They do, however, spew metal particulates into the atmosphere. For example, a facility that has been operating for centuries at Gusum, Sweden, has caused a significant local pollution with copper and lead , the toxic effects of which have damaged vegetation. The surface organic matter of sites close to the Gusum facility has been polluted with as much as 2% each of zinc and copper. Secondary smelters also do not generally emit toxic gases, but they can be important sources of metal particulates. This has been especially well documented for secondary lead smelters, many of which are present in urban or suburban environments. The danger from these is that people can be affected by lead in their environment, in addition to long-lasting ecological effects. For example, lead concentrations in soil as large as less than 5% dry weight were found in the immediate vicinity of a battery smelter in Toronto, Ontario. In this case, people were living beside the smelter. Garden soils and vegetables, house dust, and human tissues were all significantly contaminated with lead in the vicinity of that smelter. Similar observations have been made around other lead-battery smelters, including many that are situated dangerously close to human habitation.

One of the best-known case studies of environmental damage caused by smelters concerns the effects of emissions from the metal processing plants around Sudbury, Ontario . This area has a long history as a mining community. For many years, roast beds were the primary metal-processing technology used at Sudbury. In fact, in its heyday, up to 30 roast beds were operating in Sudbury. Unforturnately, this process saturated the air and soil with sulfur dioxide, nickel , and copper. Local ecosystems were devastated by the direct toxicity of sulfur dioxide and, to a lesser extent, the metals. In addition, dry deposition of sulfur dioxide caused a severe acidification of lakes and soil. This caused much of the plant life to die, which in turn started soil erosion . Naked bedrock was exposed, and then blackened and pitted by reaction with the sulfurous plumes and acidic mists.

When the devastating consequences of the roast bed method became clear, the government prohibited their use. The processors turned to new technology in 1928 when they began construction of three smelters with tall stacks . Since these emitted pollutants high into the atmosphere, they showed substantial improvements in local air quality. However, the damage to vegetation continued; lakes and soils were still being acidified, and toxic contaminants were spread over an increasingly large area.

Over time, well-defined patterns of ecological damage developed around the Sudbury smelters. The problems that had occurred in the roast beds were being repeated on a large scale. The most devastated sites were close to the smelters, and had concentrations of nickel and copper in soil in the thousands of parts per million ; they were very acidic with resulting aluminum toxicity, and frequently fumigated by sulfur dioxide. Such toxic sites were barren, or at most had very little plant cover. The few plants that were present were usually specific ecotypes of a few widespread species that had evolved a tolerance to the toxic effects of nickel, copper, and acidity. Aquatic lake habitats close to the smelters were similarly affected. These waterbodies were acidified by the dry deposition of sulfur dioxide, and had large concentrations of soluble nickel, copper, aluminum, and other toxic metals. Of course, the plant and animal life of these lakes was highly impoverished and dominated by life forms that were specifically adapted to the toxic stresses associated with the metals and acidification.

It is well recognized that increased linear distance from the point source of toxic chemicals reduces deposition rates and toxic stress. Correspondingly, the pattern of environmental pollution and ecological damage around the Sudbury areas decreases more-or-less exponentially with increasing distance from the smelters. In general, it is difficult to demonstrate damages to terrestrial communities beyond 9–12 miles (15–20 km), although contamination with nickel and copper can be found at least 62 miles (100 km) away. In comparison, lakes that are deficient in plant life have little acid-neutralizing capacity and can be shown to have been damaged by the dry deposition of sulfur dioxide at least 25–30 miles (40–48 km) from Sudbury.

To improve regional air quality, the world's tallest smokestack, at 1,247 feet (380 m), was constructed at the largest of the Sudbury smelters. This "superstack" allowed for an even wider dispersion of smelter emissions. At the same time, a smelter was closed down, and steps were taken to reduce sulfur dioxide emissions by installing desulferization, or flue-gas scrubbing , equipment and by processing lower-sulfur ores. In aggregate, these actions resulted in a great improvement of air quality in the vicinity of Sudbury.

The improvement of air quality allowed vegetation to regenerate over large areas, with many existing species increasing in abundance and new species appearing in the progressively detoxifying habitats. The revegetation has been actively encouraged by re-seeding and liming activities along roadways and other amenity areas where soil had not been eroded. Aquatic habitats have also considerably detoxified since 1972. Lakes near the superstack and the closed smelter have become less acidic, metal concentrations have decreased, and the plants and animals have increased biomass .

However, there is important controversy about contributions that the still-large emissions of sulfur dioxide from Sudbury may be making towards the regional acid rain problem. The superstack may actually exacerbate regional difficulties because of the wide dispersal of its emissions of sulfur dioxide. It is possible that height of the superstack may be decreased in order to increase the local dry deposition of emitted sulfur dioxide and reduce the long range dispersion of this acid-precursor gas.

The Sudbury scenario is by no means an unusual one. Many other smelters have substantially degraded their surrounding environment. The specifics of environmental pollution and ecological damage around particular smelters depend on the intensity of toxic stress and the types of ecological communities that are being affected. Nevertheless, broadly similar patterns of ecological change are observed around all point sources of toxic emissions, such as smelters.

In most modern smelters, a reduction of emissions at the source can eliminate the most significant of the ecological damages that have plagued older smelters. The latter were commissioned and operated under social and political climates that were much more tolerant of environmental damages than are generally considered to be acceptable today.

See also Air pollutant transport; Electrostatic precipitation; Water pollution

[Bill Freedman Ph.D. ]

RESOURCES

BOOKS

Freedman, B. Environmental Ecology. San Diego: Academic Press, 1989.

——, and T. C. Hutchinson. "Sources of Metal and Elemental Contamination of Terrestrial Ecosystems." In Metals in the Environment. Vol. 2. Edited by N. W. Lepp. London: Applied Science Publishers, 1981.

Nriagu, J., ed. Environmental Impacts of Smelters. New York: Wiley, 1984.