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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Mineral processing wastes are referred to in the Resource Conservation and Recovery Act (RCRA) as wastes that are generated during the extraction and beneficiation of ores and minerals. These wastes can be subdivided into a number of categories: waste rock, mill tailings, coal refuse, wash slimes, and spent oil shale. The mining and processing of mineral ores results in the production of large quantities of residual wastes that are for the most part earth- or rock-like in nature.
It is estimated that the mining and processing of mineral ores generate approximately 1.6 billion metric tons (1.8 billion tons) of mineral processing waste each year in the United States.(1) Mineral processing wastes account for nearly half of all the solid waste that is generated each year in the United States. Accumulations of mineral wastes from decades of past mining activities probably account for at least 50 billion metric tons (55 billion tons) of material.(2) Although many sources of mining activity are located in remote areas, nearly every state has significant quantities of mineral processing wastes.
Large amounts of waste rock are produced from surface mining operations, such as open-pit copper, phosphate, uranium, iron, and taconite mines. Small amounts are generated from underground mining. Waste rock generally consists of coarse, crushed, or blocky material covering a range of sizes, from very large boulders or blocks to fine sand-size particles and dust. Waste rock is typically removed during mining operations along with overburden and often has little or no practical mineral value. Types of rock included are igneous (granite, rhyolite, quartz, etc.), metamorphic (taconite, schist, hornblende, etc.) and sedimentary (dolomite, limestone, sandstone, oil shale, etc.). It is estimated that approximately 0.9 billion metric tons (1 billion tons) of waste rock are generated each year in the United States.(1)
Mill tailings consist predominantly of extremely fine particles that are rejected from the grinding, screening, or processing of the raw material. They are generally uniform in character and size and usually consist of hard, angular siliceous particles with a high percentage of fines. Typically, mill tailings range from sand to silt-clay in particle size (40 to 90 percent passing a 0.075 mm (No. 200) sieve), depending on the degree of processing needed to recover the ore.
The basic mineral processing techniques involved in the milling or concentrating of ore are crushing, then separation of the ore from the impurities.(1) Separation can be accomplished by any one or more of the following methods including media separation, gravity separation, froth flotation, or magnetic separation.(4,5,6)
About 450 million metric tons (500 million tons) per year(1) of mill tailings are generated from copper, iron, taconite, lead, and zinc ore concentration processes and uranium refining, as well as other ores, such as barite, feldspar, gold, molybdenum, nickel, and silver. Mill tailings are typically slurried into large impoundments, where they gradually become partially dewatered.
Coal refuse is the reject material that is produced during the preparation and washing of coal. Coal naturally occurs interbedded within sedimentary deposits, and the reject material consists of varying amounts of slate, shale, sandstone, siltstone, and clay minerals, which occur within or adjacent to the coal seam, as well as some coal that is not separated during processing.
Various mineral processing techniques are used to separate the coal from the unwanted foreign matter. The equipment most frequently used in these plants is designed to separate the coal from reject materials, and incorporates methods that make use of the difference in specific gravity between the coal and host rock. Most of the coal that is cleaned is deep-mined bituminous coal. The reject material is in the form of either coarse refuse or fine refuse.
Coarse coal refuse can vary in size from approximately 100 mm (4 in) to 2 mm (No. 10 sieve). The refuse is discharged from preparation plants by conveyor or into trucks, where it is taken and placed into large banks or stockpiles. Fine coal refuse is less than 2 mm (No. 10 sieve) and is usually discarded in slurry form. Approximately 75 percent of coal refuse is coarse and 25 percent is fine. Coarse coal refuse is referred to as colliery spoil in the United Kingdom.
Some 109 million metric tons (120 million tons) of coal refuse are generated each year in the United States. There are more than 600 coal preparation plants located in 21 coal-producing states. The largest amounts of coal refuse can be found in Kentucky, West Virginia, Pennsylvania, Illinois, Virginia, Ohio, and Delaware.(1) As the annual production of coal continues to increase, it is expected that the amount of coal refuse generated will also increase.
Wash slimes are by-products of phosphate and aluminum production. These wastes are generated from processes in which large volumes of water are used, resulting in slurries having low solids content and fines in suspension. They generally contain significant amounts of water, even after prolonged periods of drying.( ) In contrast, tailings and fine coal refuse, which are initially disposed of as slurries, ultimately dry out and become solid or semi solid materials. Approximately 90 million metric tons (100 millions tons) of phosphate slimes (wet) and 4.5 million metric tons (5 million tons) of alumina mud (wet) are generated every year in the United States. These reject materials are stored in large holding ponds. Because of the difficulty encountered in drying, there are no practical known uses for wash slimes.
Spent Oil Shale
Oil shale is mined as a source of recoverable oil. Spent oil shale is the waste by-product remaining after the extraction of oil. It is a black residue generated when oil shale is retorted (vaporized and distilled) to produce an organic oil-bearing substance called kerogen. Spent oil shale can range in size from very fine particles, smaller than 0.075 mm (No. 200 sieve ), to large chunks, up to 230 mm (9 in) or more in diameter. The coarse spent oil shale resembles waste rock because of its large particle size. The material, when crushed to a maximum size of 19.0 mm (3/4 inch), can be characterized as a relatively dense, well-graded aggregate.(8)
The oil shale industry in the United States initially developed in the early 1970's primarily in northwest Colorado with a series of pilot retorting plants that operated for a number of years. Because of unfavorable economics and a lack of sustained government support, the commercial oil shale industry has never developed. Consequently, there is little or no current production of oil shale, and the only spent oil shale available is from the pilot plant operations, which have since been suspended.
Additional information on the production, location, quantities, and characteristics of various types of mineral processing wastes can be obtained from:
National Mining Association
1130 Seventeenth Street, N.W.
Washington, D.C. 20036
CURRENT MANAGEMENT OPTIONS
Although many sources of mineral processing wastes are remotely located, large quantities of these materials have historically been used as highway construction materials whenever it has been economical and appropriate to do so. At least 34 different states have reportedly made some use of one or more types of mineral processing wastes for highway construction purposes.(1)
The mining industry has traditionally made use of its own waste materials, either by reprocessing to recover additional minerals as economic conditions become more favorable, or by using them for internal construction purposes. It is regarded as standard practice in the North American mining industry to utilize mine waste materials in the construction of dikes, impoundments, and haul roads on the mining property, and in mine rehabilitation such as cemented mine backfill. Nevertheless, internal usage of mining and mineral processing wastes consumes only a small percentage of the millions of tons of such wastes that continue to be generated every year.(8)
Many mineral processing waste materials have limited potential for use as aggregates because of their fineness, high impurity content, trace metal leachability, propensity for acid generation, and/or remote location (i.e., away from aggregate markets). However, when the location and material property characteristics are favorable, some sources of waste rock or coarse mill tailings may be suitable for use as granular base/subbase, railroad ballast, Portland cement concrete aggregate, asphalt aggregate, flowable fill aggregate or fill, and engineered fill or embankment.
The EPA is presently in the process of assessing the regulatory status of several mineral processing wastes that could pose environmental problems if not managed in an appropriate manner. These evaluations could affect the feasibility of using these materials in beneficial use applications.(9)
Coarse coal refuse has been successfully used for the construction of highway embankments in both the United States and Great Britain. Coarse coal refuse has also been blended with fly ash and used in a number of stabilized road base installations. Burnt coarse refuse (often referred to as red dog because of its reddish color) has also been used as an unbound aggregate for shoulders and secondary roads. Fine coal refuse (culm or gob) has been recovered for reuse as fuel and is being burned in many cogeneration facilities now operating in the United States.
Because of the low solids content and associated handling difficulties, no practical uses have as yet been found for wash slime materials.(10)
The processing of ores typically involves grinding and the addition of water and chemicals in the ore treatment refining plant, with a large portion of the resulting waste leaving the plant in the form of a slurry. Usually this slurry is impounded to permit settling of the solids, with any free water accumulated in the pond pumped back to the plant or allowed to discharge from the pond to an adjacent water course. Other waste rock (gangue) excavated from the ore body, and any coarse wastes separated during processing are stored in waste piles or in the base of tailings dam embankments.(11,12) By far, the major fraction of mining waste such as waste rock are disposed of in heaps (or piles) at the source.
Coarse coal refuse is typically removed from the preparation plant and disposed of in large piles or banks. Such deposits of refuse are sometimes referred to as carbon banks (anthracite) or gob piles (bituminous). Sometimes, refuse in these banks or piles can ignite and burn because of spontaneous combustion.
Mineral processing wastes are available from mining and mineral processing operations, most of which are located near the mine source and operated by mining companies. The quality of mineral processing wastes can vary widely and is highly dependent on the specific source. To properly assess these issues, each source of mineral processing waste must be separately investigated. Of particular interest are the environmental properties associated with these waste materials and their potential impacts if used in recycling applications.
Depending on the mineral waste processing operations and parent rock involved, acidic leachate from sulfide-based metallic ores, low-level radiation from uranium host rock, or radon gas generation from uranium and phosphate rocks may be environmental concerns. In addition, some waste rock from copper, gold, and uranium mining is leached to recover additional ore. Since cyanide is used for leaching, such waste rock should not be reused without first conducting careful testing. Finally, some iron ore waste rock may contain traces of residual iron, which could cause red staining if exposed for a prolonged period. Such waste rock sources should usually be avoided in applications where aesthetic concerns may be a consideration.
Mill tailings from gold mining may typically contain cyanide, whereas tailings from uranium processing may be radioactive and, if so, should not be used in construction applications. Mill tailings from processing of sulfide ores may contain heavy metals such as arsenic. Some sources of taconite tailings have been found to contain asbestos fibers.
Mill tailings consisting of quartz, feldspars, carbonates, oxides, ferro-magnesium minerals, magnetite, and pyrite have been used in the manufacture of calcium silicate bricks, and have also been used as a source of pozzolanic material.
Coal refuse usually contains some sulfur-bearing minerals, notably pyrite and marcasite, which could result in an acidic leachate. Pyrites can be removed by sink-float techniques during coal processing. Prior to use in embankment construction, coarse coal refuse banks are usually cleaned to remove any residual coal content for use as fuel, especially if the refuse is in an old bank.
HIGHWAY USES AND PROCESSING REQUIREMENTS
Asphalt Concrete Aggregate, Granular Base, and Embankment or Fill
Waste Rock Some waste rock has successfully been used as aggregate in construction applications, especially in asphalt paving and in granular base courses. Waste rock has also been used as riprap for banks and channel protection, and as rock fill for embankment construction. Where additional sizing of waste rock is necessary, in order to meet specification requirements, most, if not all, sources can be crushed and/or screened in the same way that a conventional rock source is crushed and screened.
Coarse tailings, which are generally considered those tailings that are larger than a 2.0 mm (No. 10) sieve, have been used as aggregate in granular base course, asphalt pavements, chip seals, and, in some cases, concrete structures. Fine tailings have been used as fine aggregate in asphalt paving mixes, particularly overlays, and as an embankment fill material. There are numerous examples of the use of mill tailings in local and state highway construction projects throughout the United States.(7) Conventional crushing and screening techniques can be used for sizing mill tailings.
Coal refuse has been used as embankment fill, with some coarse coal refuse also used in stabilized base applications. Most older coal refuse embankments/stockpiles contain a fairly high percentage of carbonaceous material, which because of poor disposal practices in the past, can ignite spontaneously. As mentioned previously, coal refuse banks are cleaned prior to use in order to remove the carbonaceous material. In addition, modern coal refuse disposal practices mitigate this problem by placing the refuse in thin, well-compacted layers and covering all exposed surfaces with several feet of earth fill in order to reduce or eliminate the presence of oxygen needed to initiate or support combustion.
Spent Oil Shale
Spent oil shale has some potential for use as fine aggregate or mineral filler in asphalt paving. Coarse spent oil shale requires crushing and sizing prior to use.
The material properties of the various categories of mineral processing wastes are influenced by the characteristics of the parent rock, the mining and processing methods used, and the methods of handling and/or disposing of the mineral by-product. The physical, chemical, and mechanical properties of waste rock, mill tailings, and coarse coal refuse are presented in the following sections.
Waste rock results from blasting or ripping and usually consists of a range of sizes, from large blocks down to cobbles and pebbles. Waste rock can be processed to a desired gradation by crushing and sizing, like any other source of aggregate.
The hardness of the waste rock is determined by the rock type. For example, iron ores are often found in hard igneous or metamorphic rock formations, so waste rock from iron or taconite ore processing is usually hard and dense. For the most part, lead and zinc ores are found in limestone and dolomite rock, so the waste rock from processing these ores will have characteristics much like other carbonate aggregates.
The specific gravity or unit weight of most sources of waste rock will be in approximately the same range as the specific gravity or unit weight of conventional aggregates. However, the specific gravity or unit weight of waste rock from the mining of iron ore and taconite will be considerably higher than that of conventional aggregates. The specific gravity of waste rock can be expected to range from 2.4 to 3.0 for most rock types and from 3.2 to 3.6 for waste rock from iron ore and taconite minings.
The grain size distribution of mill tailings can vary considerably, depending on the ore processing methods used, the method of handling, and the location of the sample relative to the discharge point in the tailing pond. In general, the lower the concentration or percentage of ore in the parent rock, the greater the amount of processing needed to recover the ore and the finer the particle size of the resultant tailings. Some ores, such as iron ore, are found in relatively high percentages and are fairly easy to separate. Therefore, the resultant tailings are coarser than those from other ores, such as copper, which is found in very low percentages, and requires very fine grinding for separation. Hence, copper tailings are usually quite fine-grained.
Table 9-1 presents a comparison of the particle size distribution of selected samples of copper, gold, iron, lead-zinc, molybdenum, and taconite tailings. These examples represent a cross-section of the varied size distributions of mill tailings. With the exception of some iron ore tailings, it is probable that most mill tailings will be very fine-grained materials with 50 percent or more of the particles passing a 0.075 mm (No. 200) sieve.
Other physical properties of mill tailings include specific gravity, unit weight, and moisture content. There is a scarcity of published information on these properties for most types of mill tailings. The specific gravity of mill tailings, based on limited data, appears to range between 2.60 and 3.35, with most tailings having values under 3.0 except for iron ore and taconite tailings. The dry rodded weight of most mill tailings is likely to range from 1450 to 2200 kg/m33 (90 to 135 lb/ft3).(13) The moisture content of mill tailings is highly variable, depending on the particle sizing of the tailings and the percent solids of the tailing slurry. Mill tailings are almost always nonplastic.
Table 9-1. Particle size distribution of selected samples of mill tailings (percent by weight).(8)
Table 9-2 provides some published physical property data on a copper tailings sample from Arizona. These data are probably representative of the physical characteristics of most fine-grained tailings materials, especially from the processing of metallic ores.
Coarse coal refuse is a well- graded material with nearly all particles smaller than 100 mm (4 in). Differences in the range of particle sizes can be attributed to variations in the processing methods used at different coal preparation plants. The predominant portion of coarse coal refuse is from a fine gravel to a coarse to medium sand, with from 0 to 30 percent passing a 0.075 mm (No. 200) sieve.(14) The specific gravity of coarse coal refuse normally ranges from 2.0 to 2.8 for bituminous coal refuse and from 1.8 to 2.5 for anthracite coal refuse. The specific gravity is directly proportional to the plasticity index of the refuse. As the plasticity index increases, the specific gravity also increases. The plasticity index for coarse coal refuse can range from nonplastic up to a value of 16. The natural moisture content of coarse coal refuse has been found to range from 3 percent to as high as 24 percent, but is usually less than 10 percent.(14)
Table 9-2. Physical properties of copper tailings.*(15 ,16 )
There are little to no chemical data on waste rock. Data are presented for mill tailings and coarse coal refuse.
Table 9-3 provides chemical composition data for selected samples of copper, gold, iron, lead-zinc, molybdenum, and taconite tailings. As seen from these data, most tailings are siliceous materials. Besides iron ore and taconite tailings, gold and lead-zinc tailings samples also contain fairly substantial percentages of iron. Although pH readings are not reported, some sources of mill tailings, especially those with low calcium and magnesium contents, could be acidic.
Table 9-3. Chemical composition of selected samples of mill tailings (percent by weight).(8)
Table 9-4 provides composite chemical composition data for a total of 14 different samples of coarse coal refuse that were analyzed as part of an investigation of the possible use of coal refuse-fly ash blends as base course material.(14) There is no typical chemical composition for coarse coal refuse and the sulfur content of the refuse is related to that of the coal from which it was derived. Like mill tailings, coarse coal refuse is a siliceous material, but it has considerably more alumina than tailings. Coarse coal refuse is almost always acidic.
Because of its low pH and presence of pyritic sulfur, there are several concerns related to the chemical composition of coarse coal refuse, including the following:
Corrosivity: The pH value of the refuse in water should be determined for proper selection of type of underdrain or other drain pipes. Extremely acidic refuse in the subgrade will require the use of special compositions of coatings on pipes to avoid deterioration or corrosion of the pipe.
Deleterious Substances: The oxidation of the pyrite and marcasite in coal refuse is deleterious and produces an acid discharge upon contact with water. Bituminous coal refuse composed of poorly consolidated siltstone can have high tendency toward weathering and can disintegrate under environmental conditions.
Table 9-4. Range of Chemical Composition of Coarse Coal Refuse Samples (percent by weight).(14)
Sulfate Content: Chemical parameters of coal refuse should be considered when using it for embankment or granular base applications. The determination of the sulfate levels leached from the refuse materials is required to design for the protection of concrete structures. The following tests have been used to determine the sulfate levels: British Standard 1377, Methods of Test for Soils for Civil Engineering Purpose; Test 9 - Determination of the total sulfate content of soil; and Test 10 - Determination of the sulfate content of ground water and of aqueous soil extracts.(17) Typically, the sulfate content of the refuse is in the range of 0.01 to 4.7 percent.
The mechanical properties of most interest with respect to waste rock, mill tailings and coarse coal refuse are shear strength, moisture-density characteristics, and permeability. A limited amount of data on these properties are available for waste rock, but the properties would be expected to be similar to those of conventional mineral aggregates of similar rock type and composition.
Mill tailings are virtually cohesionless materials with internal friction angles that can range from 28 to 45 degrees. Maximum dry density values may range from 1600 to 2300 kg/m3 (100 to 140 lb/ft3), with optimum moisture content values that may be between 10 to 18 percent. Permeability values ranging from 10-2 to 10-4 cm/sec have been reported, with most values in the 10-3 cm/sec range.(13)
The shear strength of coarse coal refuse is derived primarily from internal friction with comparatively low cohesion. Friction angles have been found to range from 25 to 42 degrees, with anthracite refuse normally having lower friction angles than bituminous refuse. The optimum moisture content may range from 6 to 15 percent, while the maximum dry density can range from 1300 kg/m3 (80 lb/ft3) to 2000 kg/m3 (120 lb/ft3). A wide variety of moisture-density curves have been developed for coarse coal refuse because of the variability of the material, although most moisture-density curves are relatively flat. Permeability values of compacted coarse coal refuse can vary over a fairly wide range from 10-4 to 10-7 cm/sec,(14) depending on the gradation of the refuse before and after compaction.