Table of Contents
The Administration’s proposed Mined Areas Protection Act suggests that each State in the United States be encouraged to develop within 2 years its own program for the regulation of mining activities. In the event individual States prepare inadequate proposals, Federal regulations would be applied. By implementation of this law, land, air and water adjacent to mining operations would be protected and regulatory inconsistencies between the States would be eliminated. The preplanning for waste disposal occasioned by this, Act would ensure that mined land protection would be considered as an integral part of the mining and processing industry.
The U.S. Bureau of Mines has been conducting research on the utilization and stabilization of mineral wastes since passage of the Solid Waste Disposal, Act of 1965. Considerable research has been done on methods of producing plaster and wallboard from waste gypsum from the phosphate industry; rock wool from fly ash and other mineral- wastes; concrete and ceramic products from various tailings and slags; artificial soils from tailings and sewage sludge or municipal refuse compost; and bricks from many waste sources. Research on the stabilization of mining and milling wastes has been done for elimination of air and water pollution, improvement of the appearance of waste accumulations, or conservation of the wastes for potential future use. Some type of reclamation is required regardless of the ultimate use of the processing wastes—whether for retreatment or as sites for pastureland, cropland, airports, golf courses, or industrial or residential development. This paper reports on various methods and approximate costs of achieving stabilization of fine-sized milling wastes. These fine wastes represent the most difficult materials to stabilize of those deriving from the mining and milling industry. The information contained herein may serve as a guide to alternative procedures for restoration of disturbed mining and milling lands.
The principal methods for stabilization of milling wastes include:
- Physical—the covering of the tailings with soil or other restraining materials.
- Chemical—the use of a material to interact with fine-sized minerals to form a crust.
- Vegetative—the growth of plants in the tailings.
The vegetative procedure is preferred in that esthetics of the area are improved while obtaining stabilization. Also, if a mineralized waste is to be conserved for possible later retreatment or if the area is to be used later for residential construction or recreational purposes, it is beneficial to stabilize the area with vegetation. Vegetation does not hinder retreatment procedures as much as covering the tailings with other foreign materials.
Methods have been developed and applied in many areas of the country using physical, chemical, vegetative, and combined procedures. Several milling companies, either independently or in cooperation with the Bureau of Mines, have applied various stabilization techniques to differing types of wastes and environments. The principal aim of these companies has been to achieve effective low-cost stabilization requiring minimal maintenance. Research has indicated that, with the exception of highly acidic, basic, or saline tailings, satisfactory stabilization can be obtained by many methods- at costs of less than $400 per acre. Continuing research, therefore, is directed to devising effective procedures for stabilizing exceptional problem areas.
A summary of various procedures tested is given in the following subsections on physical, chemical, and vegetative stabilization.
Many materials have been tried for physical stabilization of fine tailings to prevent air pollution. Other than water for sprinkling, perhaps the most used material is rock and soil obtained from areas adjacent, to the wastes to be covered. The use of soil often has a dual advantage in that effective cover is obtained and a habitat is provided for local vegetation to encroach.
Crushed or granulated smelter slag- has been used by many companies to stabilize a variety of fine wastes, notably inactive tailings ponds. On active tailings ponds, however, the slag-covered portions are subject to burial from shifting sands. Slag has the drawback, unlike soils or country rock, of not providing a favorable habitat for vegetation. Furthermore, suitable slag, like soil and rock, must be locally available.
Other physical methods of stabilization evaluated include (1) the use of bark covering and (2) the harrowing of straw into the top few inches of tailings.
Chemical stabilization involves reacting a reagent with mineral wastes to form an air- and water-resistant crust or layer which will effectively stop dusts from blowing and inhibit water erosion. Chemicals have the drawback of not being as permanent a stabilizing means as soil covering or vegetation but can be used on sites unsuited to the growth of vegetation because of harsh climatic conditions or the presence of vegetable poisons in the tailings or in areas that lack access to a soil-covering material. Chemical stabilization is also applicable for erosion control on active tailings ponds. Chemicals can be effectively used on portions of these ponds to restrict air pollution while other portions continue to be active.
Seventy chemicals have been tested in the laboratory, and selected materials have been tested in field plots. Optimum conditions and effective rates of application for all materials tested have not been, determined, and testing is continuing. The more effective chemicals of those tested are listed below in order of effectiveness based upon the cost in cents for the amount of reagent required to stabilize 1 sq yd. to overlapping of costs resulted because some of the reagents were required in greater quantities to stabilize different tailings. Thus, although only comparative data are available, the general conclusions presently derived are as follows:
- Coherex a resinous adhesive, furnished good wind-resistant tailings surfaces when applied in quantities costing as little as $0.01 per sq yd, but good resistance to water jet testing was not achieved until reagent costing about $0.10 per sq yd was applied.
- Calcium, ammonium, and sodium ligninsulfonates, as well as redwood bark extracts, were all effective surface stabilizers at costs of about $0.02 per sq yd.
- Cement and milk of lime additives were effective in stabilizing surfaces when applied in amounts costing $0.03 per sq yd.
- Paracol S 1461 (a blend of wax and resin) and Paracol TC 1842 (a resin emulsion) were effective as stabilizers in quantities ranging in cost from $0.04 to $0.10 per sq yd on various tailings samples tested.
- Potassium silicates having SiO2-K2O ratios of 2.5 were effective stabilizers when applied at a rate of $0.07 to $0.15 per sq yd.
- A cationic neoprene emulsion, and Rezosol, an organic polymer, effectively stabilized tailings at costs approximating $0.08 per sq yd.
- Sodium silicate applied in quantities of 4.5 pounds per sq yd and at a cost of $0.11 per sq yd was an effective stabilizer. Calcium chloride was an effective additive to the sodium silicate, whereas ferrous sulfate was not. Addition, of 6 percent by weight of CaCl2 to the sodium silicate permitted reduction of the cost of sodium silcate used from $0.11 to $0.02 per sq yd.
- Peneprime (a bituminous base product), selected amines, dicalcium silicate, and the elastomeric polymers (Compound SP-400, Soil Gard, and DCA-70) produced wind- and water-resistant surfaces at costs of $0.10 per sq. yd and up.
- Pyrite treated with sulfuric acid, Aerospray Binder 52 (a synthetic resin), Landscape (solution of combined sulfur in water-soluble oil), or Water Mate (an organic, non-ionic product) were ineffective stabilizers oil the tailings used in the testing.
The successful initiation and perpetuation of vegetation on fine wastes involves ameliorating a number of adverse factors. Mill wastes usually (1) are deficient in plant nutrients, (2) contain excessive salts and heavy metal phytotoxicants, (3) consist of unconsolidated sands that, when windblown, destroy young plants by sandblasting and/or burial, and (4) lack normal microbial populations. Other less easily defined problems also complicate vegetative procedures. The sloping sides of waste piles receive greatly varying amounts of solar radiation depending on direction of exposure. Studies by Gates have indicated that, contrary to popular belief, photosynthesis of plants is not continuous while the sun is shining; under high temperature conditions, photosynthesis may almost stop. Furthermore, most accumulations of mill tailings are light in color and may reflect excessive radiation to plant surfaces, thus intensifying physiological stresses. For these reasons, vegetation that may be effective on northern and eastern exposures may not be suitable for southern or western exposures.
Ideally, vegetative stabilization should produce a self-perpetuating plant cover directly or foster entrapment and germination of native plant seeds which will form a self-regenerating community. In the latter case, on ecological succession would be established leading to a vegetative covering so completely in harmony with the environment that irrigation or special care would be unnecessary. If the area were not cropped or grazed, only an initial fertilization should be required because the essential nutrients would be largely cycled in place.
Research indicates that, other than the excessive acidity, basicity, or salinity, perhaps the greatest problem to be overcome in establishing vegetation is that of windblown sands. Several approaches have been postulated for preventing windblown sands from covering or cutting off the growing plants. Extensive water sprinkling while the plants are growing, covering the tailings with soil or country rock, hydroseeding using excelsior-filled matting as a cover directly over the tailings, and a combination chemical-vegetative procedure developed by the Bureau of Mines are suggested procedures. Sprinkling, soil covering, hydroseeding, and matting have all been proved on various types of wastes, and the chemical-vegetative procedure has proved effective during the past 4 years on six different tailings ponds.
The hydroseeding procedure as normally used encompasses blowing a slurry of wood chips or paper pulp with admixed seeds and fertilizer over the surface to be stabilized. After the seeds germinate, the wood chips or paper pulp serve as protection for the seedlings and inhibit blowing of sands. Matting serves the same purpose as the wood chips for that method. Usually the area is planted with seeds and fertilizer and then 3-foot-wide strips of excelsior-filled matting are staked continuously or at 3-foot intervals over the planted area. Matting is used most commonly on the sloping surface of tailing dikes to inhibit slippage of the sands.
The Bureau-developed chemical-vegetative procedure involves the application of a small amount of chemicals to newly planted tailings to achieve several worthwhile goals. Sand blasting of the plants is minimized. Moisture is retained in the tailings. Germination is promoted and wilting minimized by creating a dark, heat-absorbing, nonreflecting surface.
The procedure ultimately developed from laboratory testing involved planting the tailings with a mixture of fertilizer and grass, legume, and grain seeds, watering the plot, and applying a stabilizing chemical in water solution. Early fertilization tests showed that all tailings required nitrogen (N) and phosphorus (P) additions, but that nitrogen in quantities of more than 45 pounds per acre seriously hampered legume seed germination. Hence, most tailings were fertilized with 30 and 75 pounds per acre, respectively, of N and P2O5. A few tailings required potassium (K), in which case up to 40 pounds of K per acre were added. All seeds used for planting were selected for compatibility with the particular climatic environment in which the tailings are located. A suitable grain was added to the seed mixture to provide early growth for assisting the chemical in stabilizing the surface. The chemical, preferably thus far a resinous adhesive type, was applied to the moistened tailing in amounts costing 1 cent or less per sq yd. This stabilization procedure generally is best applied in the fall of the year so as to achieve some growth before the onset of winter, thus allowing the root system to develop under a cover of snow and with good moisture conditions.
Mining companies have tested many of the outlined procedures for stabilizing and reclaiming mineral wastes. Stabilization costs using various procedures are shown in table 1. In general, costs for reclaiming sloping dike areas have been about 25 percent greater than costs for flat pond areas. The costs given in table 1 are estimated for a tailings accumulation consisting of 80 and 20 percent, respectively, of pond and dike areas. These costs, although broadly generalized, provide some comparison of different methods.
These data indicate that several methods are available for stabilizing mill tailings at costs of less than $400 per acre. The chemical-vegetative procedure appears to be the most economical procedure for establishing vegetation on tailings and wherever applicable should be a preferred method. Where prevention of blowing sands is the major problem in achieving stabilization, both the chemical-vegetative and hydroseeding methods appear to be economically preferable to the matting or soil-covering procedures. However, for difficult areas these latter methods may be mandatory.