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Some smelter slags represent a significant environmental hazard

Since the beginning of the industrial age, slag, the glassy material left over when metals are refined from ore, has been considered ugly but harmless. But research conducted at Stanford University has found that some kinds of this voluminous mining waste, which closely resembles volcanic rock, contain high levels of potentially toxic elements and can release them into the environment.

Because it has been considered chemically inert, slag has been mixed with cement and used to construct roadways and railroad beds. It has been used for sand blasting. It has been added to roofing shingles. And it has even been used to sand roads in the winter.

This might not be such a good idea, however, since slag produced in refining copper, zinc, cadmium and other base metals can contain significant concentrations of a number of potentially toxic elements, including arsenic, lead, cadmium, barium, zinc and copper, Michael Parsons, a graduate student in geological and environmental sciences at Stanford, has found. He also has shown that slag can release these elements into the environment under natural weathering conditions and cause pollution of soils, surface waters and groundwater.

Parsons presented the results of his work in a poster paper presented on Wednesday, Dec. 9, at the American Geophysical Union meeting in San Francisco. Collaborators include Professor Marco T. Einaudi and Associate Professor Dennis K. Bird in Stanford's department of geological and environmental sciences and Charles N. Alpers of the U.S. Geological Survey.

Parsons draws a distinction between two main kinds of slag. The type produced in making steel does not contain high levels of toxic elements, but slag produced by smelters that refine copper, lead, cadmium and other base metals contains higher levels of potentially hazardous elements. That is particularly true of smelters that were operating at the turn of the century and before, Parsons says.

He has been studying one such site, the Penn Mine in Calaveras County, Calif. that operated intermittently from the early 1860s to the late 1950s. During that time, the refinery operators drained slag off the molten metal and poured it into bathtub-shaped molds. When it cooled and hardened, they dumped the waste material along the banks of the nearby Mokelumne River. The Penn Mine is currently the site of an environmental restoration project. The project sponsors, however, have assumed that the slag dump does not pose an environmental hazard and so have not included it in their clean-up plan.

Parsons has measured relatively high proportions by weight of a number of toxic elements in the Penn Mine slag: zinc levels as high as 28 percent, copper levels up to 6 percent, lead levels to 11 percent, cadmium levels to 1.4 percent, and arsenic levels approaching 1 percent.

In 1963, a dam was built downstream of the mine. So today the reservoir it created, Camanche Lake, which is used for drinking water and irrigation, floods the slag dump for about six months a year. Through a combination of field and laboratory studies Parsons has determined that the slag dump, which contains about 250,000 cubic yards of material, is a significant source of cadmium, copper and zinc levels in the reservoir that exceed the EPA chronic toxicity guidelines for the protection of aquatic life. These levels are restricted to the immediate vicinity of the dump, and peak when the reservoir is drawn down in the spring. During this period the slag is exposed to groundwater than can be as acidic as vinegar. But the elements leached from the slag rapidly combine with the lake sediments so they don't remain in the water for long, he has determined.

Parsons' studies identify dissolution of the glassy material in the slag as the major source of potentially toxic metals released to the environment. Ironically, many modern smelters use water jets to quench their slag to form a fine-grained, glassy material that is convenient for disposal or resale. This makes it more likely that the slag will release the toxic elements that it contains to the environment. For that reason, future decisions concerning the management of this metallurgical waste should be based on scientific knowledge of the mechanisms that control its environmental impact, not ease of disposal or secondary market value, Parsons says.


Related materials: Michael Parsons' web page

By David F. Salisbury

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