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Chemical, Physical, and Biological Interaction Daniel K. Dysinger,
Director Montana Tech is at the forefront in development of technologies for prevention and remediation of mining-related pollution. This article provides an overview of issues related to the Berkeley Pit Lake (the Pit) currently under investigation by the faculty and staff at Montana Tech, the Montana Bureau of Mines and Geology, and the Center for Advanced Mineral and Metallurgical Processing (CAMP). Introduction Closing an open-pit mining operation requires careful planning to prevent environmental damage and degradation. Sulfide-bearing open pits that intersect groundwater and develop pit lakes present a specific environment in which many of the normally occurring chemical reactions are accelerated. The Berkeley Pit in Butte, Montana is one such site. In the October 1997 issue of Harper’s magazine, Edwin Dobb, Butte historian and author, succinctly described the present appearance of the Berkeley Pit as being rust-colored, reeking of sulfur, and surrounded by corroded earthen terraces so sterile they appear incandescent in strong light. Arguably, the actual total damage from mineral-related activity is small in relation to the improvements in our quality of life. Yet polluted mine drainage and pit lakes continue to be the mining industry’s worst and most enduring legacies. Much work remains to develop understanding of these chemical systems. Historical Background It is difficult to overstate the importance and contribution of metals produced from the “Richest Hill on Earth” to the growth and security of twentieth-century America. The mines in Butte, Montana provided a consumerist society with vast quantities of metals at the lowest possible cost during a time when the environmental ramifications were poorly understood.
In Butte, selective underground vein mining produced copper, silver, lead, and zinc ore between the 1860s and 1975. In 1955, an area located in Butte’s central zone was stripped of overburden for the initiation of open-cut mining in the Berkeley Pit. The supergene ore body and associated veins were mined until 1981. Water levels in the Pit and the underground workings were controlled by an underground pumping station located 3,900 feet below ground in the Kelly shaft. The Pit and the underground mines were completely shut down in 1982, and the underground workings began to flood. By December 1983, the rising water reached the Pit’s bottom. Chemical Concentrates Acidity in the Pit is caused by oxidation of pyrite and other sulfides exposed to water, oxygen, and naturally occurring bacteria. Copper, iron, arsenic, and other elements are released into the solution by direct oxidation of their sulfide minerals or by secondary reaction with the acidic water. Environmentally, the acidity and metal concentrations in this 25-billion-gallon reservoir are extremely toxic. Metallurgically, the concentrations are too low for economic treatment and recovery. Controlling Chemical Processes The acidity, or pH, of the solution in the Pit is buffered or controlled by various chemical reactions including cycling of iron (II)– iron (III) at the surface, silicate- weathering reactions between solution and wall rock, equilibriation of various solution species such as iron oxysulfates and silica, and solid-phase precipitation. Creation of a Problem Pit-Lake System Water enters the Pit from surface water run-off, groundwater, infiltration, and inter-connected underground workings. Figure 2 provides a cross-sectional view of the Berkeley Pit Lake with its rising water levels shown in profile.
Figure 2. Flooding of the Berkeley Pit. Source: The Berkeley Pit Pulic Education Committee A summary of hydraulic information on the sources and rates of water to the Pit is presented in figure 3. The Pit fill rate has been decreasing annually as the hydraulic gradient is reduced. The current fill rate is approximately 25 feet per year. Based on a 1984 EPA decision, the water depth can reach 1,147 feet from the current estimate of 867 feet before pumping and treatment are required. This level is expected to be reached in 2021 when the Pit will contain an estimated 64 billion gallons of solution.
Metal Concentrates The cycling of iron (II)–iron (III) is controlled by the rate of oxygen diffusion into the water and the photo-catalytic effect of sunlight. In surface water, colloidal iron acts as a catalyst upon which oxygen and/or hydrogen peroxide are produced from water by ultraviolet radiation. Catalysis increases iron oxidation far beyond rates expected by simple atmospheric oxygen diffusion. Precipitation of solids from solution is also important in controlling the acidity and the metal content of the Pit system. Iron and sulfate are believed to be precipitated from solution as jarosite and several varieties of iron oxy-sulfate compounds. In addition, the solution is saturated with gypsum that precipitates readily as calcium is released during interaction with fresh wall rock. The presence of dissolved silica is believed to play an important role in controlling reactions between the water and wall rock, solution acidity, and dissolved solids content. Equilibrium between kaolinite, muscovite, and dissolved silica occurs as a pH buffered to 3.0, roughly the pH of the Pit system. Supergene Enrichment As much as 200 feet of sediments, produced by precipitation of solids from solution and sloughing from the Pit walls, may line the bottom of the Pit. It has been postulated that a supergene enrichment process may be at work below the surface of the sediments as a result of pore water interaction with underlying sulfide minerals and mediation by sulfate-reducing organisms. Evidence of a reducing environment was obtained from the surface of a copper bar that tested positive for sulfur after one month in the sediment at a depth of about 500 feet.
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| Data Point Depth Temperature Eh pH Element As Pb K Na Mn Al Cu Mg Zn Fe SO4 |
Measurement 200.00 13.50 457.00 3.15 Measurement .76 1.39 29.50 68.80 143.00 186.00 199.00 292.00 468.00 983.00 6600.00 |
Units feet centigrade millivolts Units ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm |
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Table 1. A typical analysis of metal concentrations and chemical measurements of water collected at a depth of 200 feet.
Biological Activity Life has been discovered in both the limnetic and benthic zones of the Pit. Microscopic examination of samples revealed the dominant benthic species is Euglena mutabilis Schmitz, an algae often abundant in acid mine drainage waters and genetically well adapted to waters with acidity as low as pH=1 (Nakatsu & Tamm, 1981). Chlamydononas acidiophila Negoro, a phyto-planktonic species, was also identified in Pit water samples. Many other unidentified species of algae, protozoans, and bacteria have been found in the Pit water. Future studies are anticipated to identify all of the biota in the Berkeley Pit and to determine their role in this ecosystem. Conclusion The Pit is an extremely complex system from which much can be learned about remediation and prevention of acidic mine drainage. The Berkeley Pit Lake provides a unique laboratory where many aspects of the various environmental interactons among water, rock, sulfide minerals, and biota can be analyzed. This article describes, in a general way, several of the important interactions. Research at Montana Tech will continue to develop understanding of and solutions to problems associated with past and current technologies for mining metals from the earth. Research and analysis have produced significant improvements in mining technology during the last few decades. However, the evolution of our industrial processes continues to develop evermore efficient and environmentally benign technology for the benefit of all mankind. For a complete copy of the publication, Chemical, Physical, and Biological Interaction at the Berkeley Pit, Butte, Montana, contact: The Center for Advanced Mineral and Metallurgical Processing, Montana Tech of The University of Montana, 1300 West Park Street, Butte MT 59701. Web address: www.mtech.edu/camp.
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