Mineral Processing Wastes - Material Description

ORIGIN

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.

Waste Rock

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

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

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

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.
8th Floor
Washington, D.C. 20036

CURRENT MANAGEMENT OPTIONS

Recycling

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⁾

Disposal

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.

MARKET SOURCES

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.

Waste Rock

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

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

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.

Mill Tailings: 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: 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.

MATERIAL PROPERTIES

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.

Physical Properties

Waste Rock: 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.

Mill Tailings: 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 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 1. Particle size distribution of selected samples of mill tailings (percent by weight).⁽⁸⁾

Screen Size (Mesh)	Copper Tailings Kennecott, Magna, UT	Gold Tailings Homestake Lead, SD	Iron Ore Tailings Kaiser-Eagle Mtn, CA	Lead-Zinc Tailings ASARCO Mascot, TN	Molybdenum Tailings Climax Climax, CO	Taconite Tailings Hanna Hibbing, MN
19.1 mm 12.7 mm 9.5 mm 6.4 mm #10 #20 #28 #35 #48 #65 #100 #150 #200 #270 #325 #400	- - - - - - - 99.4 98.0 95.4 92.4 90.2 87.8 N.R. N.R. N.R.	- - - - - - - - - 100.0 97.6 94.6 90.3 82.4 72.1 N.R	99.7 83.4 65.1 46.8 17.6 7.9 5.7 4.1 2.8 1.9 1.4 0.9 0.7 - - -	- - - - - 99.6 N.R. 91.6 N.R. 69.2 58.2 47.4 41.4 N.R. N.R. N.R.	- - - - 100.0 99.5 98.5 95.8 89.5 81.1 70.7 60.3 50.0 44.2 41.5 35.5	- - - 100.0 97.0 92.5 N.R. 86.5 83.0 79.0 74.0 68.0 62.5 53.0 46.0 N.R.
N.R. indicates value not reported.

Table 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.

Coal Refuse: 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 2. Physical properties of copper tailings.*^{(15 ,16 )}

Property	Value
% Passing No. 0.075 mm (No. 200) sieve	31.7
Color	grey
Rainfall Erosion (%)	2.3
Specific Gravity	2.71
AASHTO Classification	A-2-4
Plasticity Index	Nonplastic
* Copper tailings (Duval) from the Sierrita Mine, Duval, Arizona.

Chemical Properties

There are little to no chemical data on waste rock. Data are presented for mill tailings and coarse coal refuse.

Mill Tailings: Table 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 3. Chemical composition of selected samples of mill tailings (percent by weight).⁽⁸⁾

	Copper Tailings Phelps Dodge Ajo, AZ	Gold Tailings Homestake Lead, SD	Iron Ore Tailings Kaiser Eagle Mtn, CA	Lead-Zinc Tailings USSRM Co. Midvale, UT	Molybdenum Tailings Climax Henderson, CO	Taconite Tailings Eveleth Eveleth, MN
SiO2 Al2O3 FeO CaO MgO S Na2O K2O CO2	67.3 16.3 2.1 2.8 N.R. 0.5 N.R. N.R. N.R.	52.8 1.6 34.0 1.0 8.2 N.R. 0.5 N.R. N.R.	48.57 - 18.8 5.74 4.64 0.67 N.R. N.R. N.R.	53.91 2.27 11.4 7.14 2.16 12.0 N.R. N.R. N.R.	75 - 80 7 - 12 0.2 - 3 0.12 - 1.0 0.1 N.R. 0.4 - 4 4 - 8 N.R.	64.34 0.25 11.57 3.57 4.15 N.R. N.R. N.R. 7.57
N.R. indicates value not reported.

Coal Refuse:Table 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 4. Range of Chemical Composition of Coarse Coal Refuse Samples (percent by weight).⁽¹⁴⁾

Constituent	Composition Range
SiO2 Al2O3 Fe2O3 CaO MgO S Na2O K2O	37 - 62 16.4 - 32.4 43 - 29.1 0.1 - 4.6 0.6 - 1.6 0.5 - 7.1 0.2 - 1.3 2.1 - 4.7

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.

Mechanical Properties

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

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⁾

Coal Refuse

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.

REFERENCES

1. Collins R. J. and S. K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction. National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board, Washington, DC, 1994.
2. U.S. Environmental Protection Agency. Report to Congress on Wastes from the Extraction and Beneficiation of Metallic Ores, Phosphate Rock, Asbestos, Overburden from Uranium Mining, and Oil Shale. Report No. EPA/530-SW-85-033, Washington, DC, December, 1985.
3. Dolekzil, M., and J. Reznicek. "Mineral Processing 1989 - Trends and Developments," International Mining, August, 1990.
4. "Gravity Separation," International Mining, September, 1985.
5. Martinez, E. and D.E. Spiller. "Gravity- Magnetic Separation," Engineering and Mining Journal, June, 1991.
6. Murray, Haydn H. Beneficiation of Industrial Minerals Using High Intensity Magnetic Separation, Department of Geology, Indiana University, Bloomington, Indiana.
7. Collins, R. J. and R. H. Miller. "Utilization of Mining and Mineral Processing Wastes in the United States," Minerals and the Environment, Volume 1, No. 1, Surrey, England, April, 1979.
8. Collins, R. J. and R. H. Miller. Availability of Mining Wastes and their Potential for Use as Highway Material, Federal Highway Administration, Report No. FHWA-RD-76-106, Washington, DC, May, 1976.
9. Federal Register Citations, 54 FR 36592, 9/1/89; 54 FR 2322, 1/23/90; 61 FR 2338, 1/25/96; 62 FR 26041, 5/12/97.
10. Evaluation of the Manufacture of Construction Materials from Red Mud and By-Product Sulfates, Final Report to Kaiser Aluminum Company and Allied Corporation, Gramercy, Louisiana, November, 1985.
11. Organization for Economic Co-operation and Development. Use of Waste Materials and Byproducts in Road Construction, OECD, Paris, France, 1977.
12. Collings, R. K. "Current and Potential Uses for Mining and Mineral Processing Wastes in Canada: Standards," ASTM Journal of Testing and Evaluation, Vol. 12, No. 1, 1984.
13. Pettibone, Howard C. and C. Dan Kealy. "Engineering Properties and Utilization Examples of Mine Tailings," Proceedings of the Third Mineral Waste Utilization Symposium, IIT Research Institute, Chicago, Illinois, March, 1972.
14. McQuade, Paul V., W. J. Head, and Robert B. Anderson. Investigation of the Use of Coal Refuse-Fly Ash Compositions as Highway Base Course Material, Federal Highway Administration, Report No. FHWA/RD-80/129, Washington, DC, June, 1981.
15. Sultan, H. A. "Stabilized Copper Mill Tailings for Highway Construction," Transporta-tion Research Record No.734, Transportation Research Board, Washington, DC, 1979.
16. Sultan, H. A. Utilization of Copper Mill Tailings for Highway Construction, Final Technical Report, National Science Foundation, Washington, DC, January, 1978.
17. British Standard Institution. "Methods of Testing Soils for Civil Engineering Purposes," London, B.S. 1377, Test 9, Determination of the total sulphate content of soil, and Test 10, Determination of the sulphate content of ground water and of aqueous soil extracts," 1967.

Mineral Processing Wastes - Asphalt Concrete

INTRODUCTION

Both waste rock and mill tailings have physical properties that are suitable, in most cases, for use in asphalt paving applications. Coarse coal refuse is generally unsuitable for such use.

Waste Rock

Waste rocks derived from most ore processing sources can be considered for use in asphalt paving applications provided they satisfy conventional asphalt paving aggregate requirements. Waste rock should not contain deleterious components and must not be commingled with unsuitable materials. Waste rock from iron ore processing is usually either trap rock or granite, which makes an excellent source of aggregate.

Mill Tailings

Mill tailings have successfully been used as aggregate in asphalt paving applications. Generally, the coarser, sand-size fractions of mill tailings can also be considered for use as coarse aggregates provided there are no harmful or reactive chemical components concentrated from the host rock and the tailings can satisfy the conventional paving aggregate requirements. Despite the fine size of most mill tailings, these materials can be blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range. It is also possible to separate the coarser fraction of tailings by classifying prior to disposal. Depending on the source of the mill tailings, there may be some concern with trace metals remaining after ore processing that could potentially leach from fine-grained tailings, which have a high surface area.⁽¹⁾

PERFORMANCE RECORD

Over the years, there have been numerous examples of the use of mineral processing wastes, notably waste rock and mill tailings, in asphalt paving applications. The current use of mineral processing wastes as aggregate in hot mix asphalt is not a common practice, due in great part to the relatively remote location of many sources of these wastes. Although examples of mineral processing waste utilization are not well documented, it is known that waste rock has been used as coarse aggregate in asphalt paving in at least 8 states and that mill tailings have been used in asphalt paving in at least 13 states.⁽¹⁾ There are also two states (Kentucky and Pennsylvania) where coal refuse has been used to a limited extent in asphalt paving. Table 5 is a summary of the known U.S. mining and mineral processing wastes in asphalt paving in at least 19 different states. A few state agencies have been involved in recent research or field trials, but only five states (Kansas, Missouri, Nevada, New York, and Oklahoma) report any ongoing or upcoming research on the use of mineral processing wastes as aggregate or mineral filler in hot mix asphalt.⁽²⁾

Table 5. Summary of the use of mining and mineral processing wastes in asphalt paving mixtures in the United States.

State	Type of Mining Waste Used	Project Lcation(s)	Estimated Tonnage
California	Gold Dredge Tailings Iron Ore Tailings	Sacramento Freeways County Road near Eagle Mountain	Not known Not known
Illinois	Lead-Zinc Tailings	Local Roads in Northwest Illinois	90,000 tonnes (100,000 tons)
Kansas	Lead-Zinc Tailings (Chert)	Southeast corner of Kansas	Not known
Kentucky	Bituminous Coal Refuse	Low volume roads	Not known
Louisiana	Iron Ore Waste Rock	Location not known	Not known
Minnesota	Coarse Taconite Tailings	Roads and bridge decks in Duluth and Minneapolis-St. Paul areas	Not known, but substantial amounts
Missouri	Barite Tailings (Tiff Chert) Lead-Zinc Tailings (Chert) Iron Waste Rock (Trap Rock) Lead Waste Rock	Local roads in east central Missouri Southwest corner of Missouri Southeast part of Missouri Street paving in St. Louis area	Not known Substantial amounts Not known Not known
Nevada	Barite Tailings (Chert)	I-80 Resurfacing near Battle Mountain	Not known
New Jersey	Iron Ore Tailings	Northwest part of New Jersey	Not known
New Mexico	Molybdenum Tailings and Waste Rock	North central part of New Mexico	Not known
New York	Iron Ore Waste Rock	Essex and St. Lawrence Counties	Not known -- asphalt use since 1930
Oklahoma	Lead-Zinc Tailings (Chert)	Northeast corner of Oklahoma	Substantial amounts
Pennsylvania	Anthracite Coal Refuse Burnt Anthracite Refuse (Red Dog) Iron Ore Waste Rock	4 experimental test sections -- Luzerne Co. Penn. Turnpike N.E. Extension - Luzerne Co. PA Turnpike - Berks & Chester Cos.	Limited amounts Not known Substantial amounts
South Dakota	Gold Waste Rock	Seal Coat Rt. 35 near Lead	Not known
Tennessee	Zinc Coarse Tailings	Eastern part of Tennessee	Not known
Utah	Classified Copper Mill Tailings	Mineral Filler in Salt Lake City area	Limited amounts
Washington	Lead-Zinc Waste Rock	Northeast corner of Washington	Not known
Wisconsin	Coarse Iron Ore Tailings and Waste Rock Lead-Zinc Tailings	U.S. Rt. 141 north of Milwaukee Local roads in southwest Wisconsin	Not known Not known
Wyoming	Coarse Iron Ore Tailings	Southeastern part of Wyoming	Limited amounts

Colorado and South Dakota have used crushed rock waste from gold mining operations in road construction, including asphalt paving. Lead waste rock has been used in bituminous mixtures in Missouri. Lead-zinc waste rock has been used for resurfacing by local and county agencies in Washington. Trap rock from iron ore processing has been crushed to meet standard specification requirements for hot mix aggregate in New Jersey and Pennsylvania.

In Missouri and Illinois, iron waste rock has been used as a skid-resistant aggregate for asphalt paving. In New Mexico, waste rock from molybdenum mining operations has been used as aggregate in asphalt paving with satisfactory performance.⁽³⁾ Most waste rock is generated in the western United States, particularly in copper mining areas such as Arizona and Utah.

Mill Tailings

In Minnesota, taconite tailings improved the frictional resistance of asphalt overlays, and, on this basis, this material is still used in the northern part of the state for hot mix resurfacing. In New Mexico, the coarse tailings from molybdenum mining operations have been used as aggregate in asphalt paving with satisfactory performance.⁽³⁾ At least 12 states, including Alabama, California, Illinois, Kansas, Minnesota, Missouri, Nevada, New Jersey, New Mexico, New York, Oklahoma, and Wisconsin, have used or continue to use mill tailings for asphalt paving applications. Utah reportedly has used the fines from copper mining operations as a mineral filler in asphalt.⁽²⁾

MATERIAL PROCESSING REQUIREMENTS

Waste Rock

Crushing and Screening: Many sources of waste rock are geologically similar to natural sources of construction aggregate, and, therefore, can be crushed and/or screened using conventional aggregate processing equipment. Waste rock from iron ore or taconite processing may be heavier than conventional aggregate.

Mill Tailings

Dewatering: Mill tailings may have to be dried to reduce the moisture content, or may require selective screening and dewatering prior to being introduced into a hot mix asphalt plant. When reclaimed from a tailings pond, stockpiling and air drying for a period of time may be sufficient to reduce the moisture content of some tailings by evaporation, especially in arid areas.

Crushing and Screening: Screening and/or crushing may be required in some cases to produce a suitable aggregate like product from mill tailings or to meet gradation specifications. Crushing is not normally required, with the possible exception of some coarse tailings that may require size reduction of oversize particles. Some fine-sized tailings, such as copper mill tailings, can be classified prior to disposal in order to separate the coarser fraction of the tailings for subsequent reuse.

ENGINEERING PROPERTIES

Waste Rock

Some of the properties of waste rock that are of interest when used in asphalt paving applications include gradation, shape and texture, specific gravity, shear strength, and abrasion resistance.

Gradation: Waste rock is often homogeneous but can vary widely in size from boulders down to gravel, due to variations in ore formation and different mining techniques. In general, most sources of waste rock can be reduced to a desired gradation by normal crushing and sizing methods.

Shape and Texture: Waste rock is coarse, hard, and angular in shape and can vary in size from large boulders or blocks down to gravel.

Specific Gravity: The average specific gravity of waste rock is about 2.65, with a range from 2.4 to 3.6 depending on the nature of the mineral constituents. Specific gravity may be used to determine other important properties such as void ratio, porosity, and degree of saturation.⁽⁴⁾

Shear Strength: Typical values for the angle of internal friction for waste rock materials often exceed 35 degrees and contribute to high bearing capacity and stability.

Abrasion Resistance: Most sources of waste rock are able to satisfy abrasion loss requirements. Waste rocks from the processing of iron ore or taconite are usually quite dense and often have relatively low abrasion loss values.

Mill Tailings

Some of the properties of mill tailings that are of interest when mill tailings are used in asphalt paving applications include gradation, shape and texture, specific gravity, absorption, unit weight, and stripping resistance.

Gradation: Mill tailings are usually very fine-graded, cohesionless materials. They consist of hard, angular siliceous particles with a high percentage of fines. Typically, mill tailings range from sand to silt-clay particle size with 40 to 90 percent passing a 0.075 µm (No. 200) sieve. They are disposed of in slurry form by pumping into large ponds.⁽⁵⁾

Shape and Texture: Mill tailings are uniform in particle shape and texture. Mill tailings typically consist of hard, angular, siliceous particles with a high percentage of fines.

Specific Gravity: The specific gravity of tailings ranges from about 2.0 to 3.5, depending on the mineralogical composition.

Absorption: Water absorption values for lead, zinc, copper and iron ore tailings are typically higher than the standard maximum limit of 1.0 percent for fine aggregate in asphalt paving mixes.^(6,7)

Unit Weight: Iron ore tailings and taconite tailings from northern Minnesota have high unit weight values, up to as high as 2750 kg/m³ (170 lb/ft³). The dry rodded weight of most other tailings sources is expected to range from 1450 kg/m³ (90 lb/ft³) to 2200 kg/m³ (135 lb/ft³), which is comparable to that of most natural aggregates, which are approximately 2000 kg/m³ (125 lb/ft³) to 2300 kg/m³ (140 lb/ft³).⁽⁵⁾

Stripping Resistance: Iron ore and taconite tailings do not appear to be susceptible to stripping. Mill tailings from other sources should be evaluated for stripping potential as part of the normal asphalt paving mix design procedures.

DESIGN CONSIDERATIONS

Mix Design

Waste Rock: Waste rock for use in hot mix asphalt must comply with the requirements for coarse aggregate in bituminous mixtures.⁽⁸⁾ Asphalt mixes containing waste rock can be designed using standard laboratory procedures.

The potential for stripping of asphalt mixes containing waste rock should be assessed in the laboratory as part of the overall hot mix asphalt mix design. Stripping resistance can be enhanced by adding hydrated lime or a proprietary antistripping additive.

Mill Tailings: There are no standard specifications for the use of mill tailings in hot mix asphalt paving. There are, however, a few states that have historically used different types of mill tailings as a fine aggregate, a mineral filler, or in some cases, as a coarse aggregate in asphalt paving mixes. Tailings should meet the appropriate specification requirements for their intended use, either as a source of fine aggregate⁽⁹⁾ or mineral filler.⁽¹⁰⁾ Asphalt mixes containing mill tailings can be designed using standard laboratory procedures. The potential for stripping of asphalt mixes containing mill tailings should also be assessed in the laboratory as part of the overall design.

Structural Design

Waste Rock: Conventional AASHTO pavement structural design methods⁽¹¹⁾ are appropriate for the thickness design of asphalt paving mixtures incorporating waste rock as the coarse aggregate.

Mill Tailings: Conventional AASHTO pavement structural design methods⁽¹¹⁾ are also appropriate for the thickness design of asphalt paving mixtures incorporating mill tailings as the fine aggregate, mineral filler, or coarse aggregate.

CONSTRUCTION PROCEDURES

Material Handling and Storage

The same methods and equipment used to store or stockpile conventional aggregates are applicable for waste rock and mill tailings. However, users of those materials should be aware that such materials usually have an acid potential and that leaching may occur during stockpiling or heating in the asphalt plants.

Placing and Compacting

The same methods and equipment used for conventional pavements are applicable to asphalt pavements containing waste rock or mill tailings. Compaction operations should be visually inspected on a continuous basis to ensure that the specified degree of compaction can be achieved and there is no movement under the action of compaction equipment.

Quality Control

The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing mineral processing wastes. Mixes should be sampled in accordance with AASHTO T168,⁽¹²⁾ and tested for specific gravity in accordance with ASTM D2726,⁽¹³⁾ and in-place density in accordance with ASTM D2950.⁽¹⁴⁾

UNRESOLVED ISSUES

There is a need to establish general environmental criteria for the selection of mining or mineral processing by-products to be used in paving applications. More knowledge is needed concerning the variation in mineral processing operations that can alter the quality of such by-products.

More specifically, there is a need to investigate and analyze the environmental impact of some waste rock and mill tailings sources that may contain inorganic metal and sulfide-based metallic ore constituents to assess the level of leachability, if any, when used in hot mix asphalt. Mill tailings may contain concentrations of certain inorganic metal constituents that may be leachable. Some waste rock and tailings have been leached with cyanide as a means of further ore extraction. Certain sources of taconite are known to contain asbestiform fibers. Uranium mill tailings can be a source of residual radiation, and phosphate rock can be a source of low-level radiation resulting from radon gas.

REFERENCES

1. Collins, R. J. and R. H. Miller. Availability of Mining Wastes and Their Potential for Use as Highway Material, Volume I, Classification and Technical and Environmental Analysis. Federal Highway Administration, Report No. FHWA-RD-76-106, Washington, DC, May, 1976.
2. Collins, R. J. and S. K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction. National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board, Washington, DC, 1994.
3. Collins, R. J. and R. H. Miller. Utilization of Mining and Mineral Processing Wastes in the United States. Minerals and the Environment, Vol. 1, No. 1, Surrey, England, April, 1979.
4. Wright Engineers Limited, Golder, Brawner and Associates Limited, and Ripley, Klohn and Leonoff International Limite., Tentative Design Guide for Mine Waste Embankments In Canada. Technical Bulletin TB 145, Mines Branch Mining Research Centre, Department of Energy, Mines and Resources, Ottawa, Canada, March 1972.
5. Emery, J. J. Use of Mining and Metallurgical Waste in Construction. Minerals and Environment, Paper No. 18, London, England, June, 1974.
6. Rai, M., G. S. Mehrotra, and D. Chandra. Use of Zinc, Iron, and Copper Tailings as a Fine Aggregate in Concrete, International Conference on The Use of Fly Ash, Silica Fume, Slag and Other Mineral By-Products in Concrete, Montebello, Québec, Canada, August, 1983.
7. Hewitt, D. F. Industrial Mineral Resources of the Brampton Area. Ontario Department of Mines, Industrial Mineral Report 23, 1969.
8. ASTM D692-94a. "Standard Specification for Coarse Aggregate for Bituminous Mixtures." Annual Book ASTM of Standards, Volume 04.03, ASTM, West Conshohocken, Pennsylvania, 1996.
9. American Association of State Highway and Transportation Officials. Standard Specification for Materials, Fine Aggregate for Bituminous Paving Mixtures. AASHTO Designation: M29-83, Part I Specifications, 16th Edition, 1993.
10. American Association of State Highway and Transportation Officials. Standard Specification for Materials, Mineral Filler for Bituminous Paving Mixtures. AASHTO Designation: M17-83, Part I Specifications, 16th Edition, 1993.
11. AASHTO Guide for the Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington, D. C., 1993.
12. American Association of State Highway and Transportation Officials. Standard Method of Test, Sampling Bituminous Paving Mixtures. AASHTO Designation: T168-82, Part II Tests, 14th Edition, 1986.
13. American Society for Testing and Materials. Standard Specification D2726-96, "Bulk Specific Gravity and Density of non-Absorptive Compacted Bituminous Mixtures." Annual Book of ASTM Standards, Volume 04.03, ASTM, West Conshohocken, Pennsylvania, 1996.
14. American Society for Testing and Materials. Standard Specification D2950-96, "Density of Bituminous Concrete in Place by Nuclear Methods."Annual Book of ASTM Standards, Volume 04.03, ASTM, West Conshohocken, Pennsylvania, 1996.

Mineral Processing Wastes - Granular Base

INTRODUCTION

Waste rock, mill tailings, and coarse coal refuse can be used as a granular base in pavement construction applications. Burnt coal refuse (or red dog) from banks or piles that have caught on fire has also been used as a granular base material.

Waste Rock

Waste rock derived from igneous or metamorphic rocks, as well as properly consolidated limestones, sandstones, and dolomites are generally suitable for granular base and subbase applications, provided the waste rocks do not contain deleterious components and are not commingled with overburden.

Mill Tailings

Coarser-sized mill tailings can be used in granular base and subbase applications. Generally, the coarser, sand-size fractions of mill tailings can also be considered for use as construction aggregates, provided there are no harmful or reactive chemical components concentrated from the host rock. Despite the fine size of most mill tailings, these materials can be blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range, or can often be classified prior to initial disposal in order to recover the coarser fraction for possible use.

Coal Refuse

Coarse coal refuse can be used as aggregate in granular base applications. Burnt coal refuse (red dog) is also a suitable granular base material. Proper compaction of coarse coal refuse to its maximum dry density is necessary to achieve stability within a pavement structure. Fine coal refuse slurry has little or no load carrying capability and is, therefore, unsuitable for use as a construction material.

The carbonaceous content of coal refuse, its potential for spontaneous combustion, as well as its pyritic or sulfur composition and acidic nature are causes for environmental concern.

PERFORMANCE RECORD

The use of mineral processing waste as a granular base material is not very common, since many mineral processing wastes are not close to urban areas where construction materials are needed. There is little current research or actual reported use of such wastes in granular base construction.

Waste Rock

A review of the published and unpublished reports reveals that at least 13 states have made use of some source of waste rock in their state highway construction programs, sometimes dating back as many as 50 years. However, it does not appear that any state highway agencies or universities have been involved in research for waste rock use as aggregate in granular base or subbase applications.⁽¹⁾

Mill Tailings

At least five states, including Alabama, Alaska, Arizona, Arkansas, and Virginia, have reported using mill tailings as aggregate in granular base course applications, although the Arkansas experience was not considered successful, due to poor performance or economics.⁽¹⁾ Other states that have made some use of mill tailings for granular base or subbase construction in the past include California, Colorado, Idaho, Illinois, Michigan, Minnesota, and Tennessee.⁽²⁾

Coal Refuse

Coal refuse has been successfully used in cement stabilized base applications in Europe. The success of this material for use in this application is reportedly dependent on proper compaction. There has been occasional use of coal refuse in Alabama, Kentucky, Virginia, and West Virginia as an alternative material for bases and subbases.^(1,2,3)

The Pennsylvania Department of Transportation has rejected anthracite refuse usage as aggregate for base and subbase courses because of high percent losses in the sodium sulfate (soundness test).^(4,5) West Virginia is evaluating the use of coal refuse as subbase material.

The Ministry of Transport in the United Kingdom permits the use of incinerated coal refuse (well- burnt, nonplastic shale) as a granular subbase material in Ministry controlled road work.⁽⁶⁾

MATERIAL PROCESSING REQUIREMENTS

Waste Rock

Crushing and Screening: Where the waste rock consists of hard, stable chunks of rock with no overburden or vegetation, granular aggregate material can be produced by crushing. Crushing and screening can be accomplished using conventional aggregate processing equipment.

Mill Tailings

Dewatering: Processing of certain tailings sources (such as dewatering, reclaiming, and selective size classification) may be necessary, although this is not common practice and can be costly. Tailings reclaimed from ponds will normally require a reasonable period of time to dewater, depending on climatic conditions.

Screening: Some tailings materials may contain sufficient coarse sizes (greater than 2.0 mm (No. 10 sieve) or 4.75 mm (No.4 sieve)) that could be classified and separated from the finer fraction for possible use as a granular base material.

Coal Refuse

Separation or Cleaning: The basic coal refuse processing techniques used in coal preparation plants are separation of the coal from the unwanted foreign matter (pyrite and marcasite). The equipment most frequently used in these plants to classify the refuse is based on the difference in specific gravity between the coal and the host rock.

For older refuse banks, additional separation or cleaning may be required in the field to remove and recover the combustible portion of coarse coal refuse for use as fuel, prior to using the remaining refuse material as a granular base or subbase material. Such cleaning also serves to prevent potential spontaneous combustion of the refuse.

ENGINEERING PROPERTIES

Waste Rock

Some of the properties of waste rock that are of particular interest when waste rock is used in granular base applications include gradation, specific gravity, and shear strength.

Gradation: Waste rock is generally coarse, crushed, or blocky material covering a range of sizes, from very large boulders and rocks to sand-size particles and dust. Waste rock can be crushed and screened for use or blended with other aggregates to generate a product suitable for granular base or subbase aggregate.

Specific Gravity: The average specific gravity of waste rock is about 2.65, with a range from 2.4 to 3.6 depending on the nature of the mineral constituents. Specific gravity may be used to determine other important properties such as void ratio or porosity.⁽⁷⁾

Shear Strength: Typical values of the angle of internal friction of most waste rock sources exceed 35 degrees and contribute to relatively high bearing capacity and stability.

Mill Tailings

Some of the properties of mill tailings that are of particular interest when mill tailings are used in granular base applications include gradation, particle shape and texture, unit weight, and moisture-density characteristics. It is difficult to definitively characterize representative samples of mill tailings materials because of the number of sources and the variations in the degree of processing that can be encountered.

Gradation: Typically, mill tailings range from sand to silt-clay in particle size with 40 to 90 percent passing a 0.075 mm (No. 200) sieve. They are normally disposed of in slurry form by pumping into large retention areas/settlement ponds.⁽⁸⁾ Despite the fine size of most mill tailings, these materials can be classified prior to disposal or blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range for use as a construction aggregate.

Shape and Texture: Mill tailings consist of hard, angular, siliceous particles.

Unit Weight: Iron and taconite tailings typically have high unit weight values up to as high as 2700 kg/m³ (170 lb/ft³). The unit weight of most other tailings sources is expected to range from 1500 kg/m³ (90 lb/ft³) to 2200 kg/m³ (135 lb/ft³), which is comparable to that of most natural aggregates, which are approximately 2,000 kg/m³ (125 lb/ft³) to 2300 kg/m³ (140 lb/ft³).⁽⁸⁾

Moisture-Density Characteristics: With the possible exception of iron ore or taconite tailings, most mill tailings have an optimum moisture content in the range of 10 to 18 percent. The maximum dry density of most tailings sources is in the range of 1600 kg/m³ (100 lb/ft³) to 2025 kg/m³(125 lb/ft³).⁽⁹⁾

Coal Refuse

Some of the properties of coal refuse that are of particular interest when coal refuse is used in granular base applications include gradation, particle shape/ texture, moisture-density characteristics, shear strength, permeability, and frost susceptibility.

Gradation: Coarse refuse, which can contain particles that are greater than a 4.75 mm (No. 4) sieve, is generally a well-graded material for particles up to 100 mm (4 in) in size. These particles consist mainly of slate or shale with some sandstone or clay. Most coarse refuse contains particles that may break down under compaction equipment, resulting in a finer gradation following placement.

Shape/Texture: Coal refuse is composed mainly of flat slate or shale particles with some coal, sandstone, and clay intermixed. Such particles may weather or break down easily.

Moisture-Density Characteristics: Based on available data, the optimum moisture content of coarse coal refuse is likely to range from 6 to 15 percent and its maximum dry density may vary from 1300 kg/m³ (80 lb/ft³) to 2000 kg/m³ (120 lb/ft³).⁽⁶⁾

Shear Strength: The shear strength of coarse coal refuse can be highly variable. The angle of internal friction values for coarse coal refuse have been reported to be between 18 and 42 degrees.⁽¹⁰⁾ The shear strength of coal refuse is usually lower than that obtained for granular materials with similar properties, but can be increased by proper compaction.^(6,10,11) Previous experience with coal refuse usage as a construction material has demonstrated that the shear strength of the refuse is acceptable if proper compaction measures are achieved during construction.^(10,12)

Permeability: The permeability of coarse coal refuse can be highly variable and should be determined for each particular source and application. It is related to the composition of the refuse, its degradation during compaction, and the degree of compaction.^(10,13) The permeability of coarse coal refuse is lower than that of other granular materials with a similar grain size distribution. Conventional formulas relating permeability to particle size distribution and uniformity are not applicable for estimating the permeability of coarse coal refuse.⁽¹⁴⁾

Low permeability values are desirable in order to reduce air circulation and to reduce the potential for spontaneous combustion, oxidation of pyrites, and acidic leachate. Fly ash may be added to the refuse for this purpose. The average permeability for coal refuse-fly ash mixtures is significantly lower (10^-6 to 10^-7 cm/sec) than that for the coal refuse alone (10^-3 x 10^-5 cm/sec) because fly ash fills the voids of the coal refuse.^(14,15)

Frost Susceptibility: Coal refuse is susceptible to frost heave, especially burnt coal refuse. Frost damage can reportedly be reduced or eliminated by the addition of cement to the refuse.⁽¹⁶⁾

DESIGN CONSIDERATIONS

Waste Rock

There are no standard specifications for the use of crushed waste rock in granular base applications. Most sources of waste rock are of a quality that is comparable to conventional aggregates used as granular base materials, so specifications applicable to such aggregates can probably be used, provided sufficient compaction is achieved.

Mill Tailings

There are no standard specifications for mill tailings as an aggregate in granular base. The tailings must meet sizing requirements and satisfy standard Proctor moisture-density criteria.⁽¹⁷⁾ Durability testing may also be required. Most tailings sources may have an excess amount of material finer than the 4.75 mm (No. 4) or 2.00 mm (No. 10) sieve. This will either limit their use in granular base course applications or necessitate separation and use of the coarser fraction of the tailings.

Mill tailings to be used in granular base should also be tested in accordance with AASHTO test methods T234⁽¹⁸⁾ and T236⁽¹⁹⁾ to determine the shear strength characteristics and to define the angle of internal friction and cohesion of the material tested. The California bearing ratio (CBR) test (AASHTO T193)⁽²⁰⁾ can also be used to evaluate the subgrade bearing capacity.⁽²²⁾

Coal Refuse

Tests for combustion potential and standard Proctor moisture-density criteria should be carried out for all coal refuse that is considered for use in granular base construction. Leaching and swelling indexes, porosity, freeze-thaw tests, and wet-dry swelling tests are also recommended. Water soluble sulfate testing methods and specifications for determining the amount of sulfate found in coal refuse and measures used to overcome such sulfate content are available from the British National Coal Board.⁽²¹⁾ The introduction of fly ash to coal refuse may help to neutralize the acidity of the refuse, increase its moisture-holding capacity, increase its pore space volume, and reduce its erodability.⁽²²⁾ Lime and/or cement added as a binding agent with the fly ash produces a pozzolanic reaction, which can provide added strength and durability to the coal refuse/fly ash mixture.⁽²²⁾

Coal refuse to be used in granular base should also be tested in accordance with AASHTO test methods T234⁽²³⁾ and T236⁽²⁴⁾ to determine the shear strength characteristics and to define the angle of internal friction and cohesion of the material tested. The CBR test (AASHTO T193)⁽²⁵⁾ can also be used to evaluate the subgrade bearing capacity.⁽²²⁾

CONSTRUCTION PROCEDURES

Material Handling and Storage

Waste Rock and Mill Tailings: The same methods and equipment used to store or stockpile conventional aggregates are applicable for waste rock or mill tailings.

Coal Refuse: Prior to using the refuse to construct a granular base, the bank should be cleaned or processed to recover the residual coal or combustible matter. This ordinarily involves a screening of the refuse, which also removes oversize and deleterious materials.

Placing and Compacting

Waste Rock: The same methods and equipment used to place and compact conventional aggregate can be used for the placement of waste rock. As with any other oversize rock placement, compaction operations must be inspected on a continuous basis to ensure that the specified degree of compaction can be achieved, or that there is no movement under the action of compaction equipment.

Mill Tailings: No modifications to normal construction equipment or procedures are needed, except that tailings may need to be dried to near optimum moisture content prior to placement and compaction.

Coal Refuse: No significant modifications to normal construction procedures are needed, except that possible material breakdown under compaction equipment requires more repetitive testing in the field. Proper compaction of coal refuse reduces air voids to less than 10 percent, and can reduce the permeability to less than 10^-5 to 10^-6 cm/sec, which is very low. Under such conditions, the material is sufficiently dense for base course construction and the potential for ignition is substantially reduced.⁽⁶⁾

Quality Control

Waste Rock and Mill Tailings: The same test procedures used for conventional aggregate are appropriate for waste rock and mill tailings, although waste rock may have particles that are too large for certain in-place density tests. The same field test procedure used for conventional aggregate are recommended for granular base applications when using waste rock or mill tailings. Standard laboratory and field test methods for compacted density are given by AASHTO T191,⁽²⁶ T205,⁽²⁷⁾ T238, ⁽²⁸⁾and T239.⁽²⁹⁾)

Coal Refuse: Strict compaction control testing must also be performed when using coal refuse as a base. A determination of the sulfate levels leached from the coarse refuse materials is required in order to design for the protection of any adjacent concrete structures. The pH value of the refuse in water should also be determined for proper selection of type of underdrain or other drain pipes.

UNRESOLVED ISSUES

Relatively little is known about how variations in mineral processing operations can alter the quality of mineral processing wastes.⁽³⁰⁾

Waste Rock and Mill Tailings

There is also a need to determine whether specific sources of such materials are environmentally suitable for use in granular base construction. In addition, there is a need to develop engineering data on the design properties and performance of potential waste rock and/or mill tailings used in granular base applications.

Coal Refuse

There is a need to further evaluate the environmental concerns regarding the potential for acidic leachate from coarse coal refuse used in granular base applications. The production of such leachate is caused by the oxidation of pyrite and marcasite with the presence of high sulfur content. If acidic leachate were to be produced over time, it could contaminate groundwater, adversely impact the ecosystem, and cause deterioration or corrosion of underdrains or other drain pipes.

More information may be needed to completely mitigate concerns associated with the spontaneous combustion potential of coarse coal refuse.

REFERENCES

1. Collins, R. J. and S. K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction. National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board, Washington, DC, 1994.
2. Collins, R. J., and Miller, R. H. Availability of Mining Wastes and their Potential for Use as Highway Material. Federal Highway Administration, Report No. FHWA-RD-76-106, Washington, DC, May, 1976.
3. Wilmoth, R. C., and R. B. Scott. "Use of Coal Mine Refuse and Fly Ash as a Road Base Material," Proceedings of the First Symposium on Mine and Preparation Plant Refuse Disposal. Louisville, Kentucky, October, 1974.
4. Luckie, P. T., J. W. Peters, and T.S. Spicer. The Evaluation of Anthracite Refuse as a Highway Construction Material. Pennsylvania State University, Special Research Report No. SR-57, July, 1966.
5. Collins, R. J., and R. H. Miller. Availability of Mining Wastes and Their Potential for Use as Highway Material: Executive Summary. Federal Highway Administration, Final Report No. FHWA-RD-78-28, Washington, DC, September 1977.
6. Maneval, D. R. "Utilization of Coal Refuse for Highway Base or Subbase Material," Proceedings of the Fourth Mineral Waste Utilization Symposium. IIT Research Institute, Chicago, Illinois, May, 1974.
7. Wright Engineers Limited, Golder, Brawner and Associates Limited, and Ripley, Klohn and Leonoff International Limited. Tentative Design Guide for Mine Waste Embankments In Canada. Technical Bulletin TB 145, Mines Branch Mining Research Centre, Department of Energy, Mines and Resources, Ottawa, Canada, March, 1972.
8. Emery, J. J. "Use of Mining and Metallurgical Waste in Construction," Minerals and Environment, Paper No. 18, London, June, 1974.
9. Sultan, H.A. Utilization of Copper Mill Tailings for Highway Construction. Final Technical Report, National Science Foundation, Washington, DC, January, 1978.
10. McQuade, P. V., P. E. Glogowski, F. P. Tolcser, and R. B. Anderson. Investigation of the Use Of Coal Refuse-Fly Ash Compositions as Highway Base Course Material: State of the Art and Optimum Use Area Determinations. Federal Highway Administration, Interim Report No. FHWA-RD-78-208, Washington, DC, September, 1980.
11. Bishop, C. S. and N. R. Simon. "Selected Soil Mechanics Properties of Kentucky Coal Preparation Plant Refuse," Proceedings of the Second Kentucky Coal Refuse Disposal and Utilization Seminar. Lexington, Kentucky, May, 1976.
12. Tanfield, R. K., "Construction Uses of Colliery Spoil," Contract Journal, January, 1974.
13. Zook, R. L., B. J. Olup, Jr., and J. J. Pierre. "Engineering Evaluation of Coal Refuse Slurry Impoundments," Transactions of the Society of Mining Engineers. AIME, Volume 258, March, 1975.
14. Drenevich, V. P., R. J. Ebelhar, and G. P. Williams. "Geotechnical Properties of Some Eastern Kentucky Surface Mine Spoils," Proceedings of the Seventh Ohio River Valley Soils Seminar, Lexington, Kentucky, October, 1975.
15. Moulton, L. K., D. A. Anderson, R. K. Seals, and S. M. Hussain. "Coal Mine Refuse: An Engineering Material," Proceedings of the First Symposium on Mine and Preparation Plant Refuse Disposal. Louisville, Kentucky, October, 1974.
16. Kettle, R. J., and R. I. T. Williams. "Frost Action in Stabilised Colliery Shale," Presented at the 56th Annual Meeting of the Transportation Research Board, Washington, DC, January, 1977.
17. American Association of State Highway and Transportation Officials. Standard Method of Test, "The Moisture-Density Relations of Soils Using a 5.5-lb [2.5 kg] Rammer and a 12-in. [305 mm] Drop," AASHTO Designation: T99-86, Part II Tests, 16th Edition, 1993.
18. American Association of State Highway and Transportation Officials. Standard Method of Test, "Strength Parameter of Soils by Triaxial Compression," AASHTO Designation: T 234-85, Part II Tests, 16th Edition, 1993.
19. American Association of State Highway and Transportation Officials, Standard Method of Test, "Direct Shear Test of Soils Under Consolidation Drained Conditions," AASHTO Designation: T236-84, Part II Tests, 16th Edition, 1993.
20. American Association of State Highway and Transportation Officials. Standard Method of Test, "The California Bearing Ratio," AASHTO Designation: T193-81, Part II Tests, 16th Edition, 1993.
21. British Standard Institution. "Methods of Testing Soils for Civil Engineering Purposes, B.S. 1377, Test 9. Determination of the total sulphate content of soil, and Test 10, Determination of the sulphate content of groundwater and of aqueous soil extracts." London, 1967.
22. Pierre, J. J., and C. M. Thompson. User's Manual Coal-Mine Refuse in Embankments. Federal Highway Administration, Report No. FHWA-TS-80-213, Washington, DC, December, 1979.
23. American Association of State Highway and Transportation Officials. Standard Method of Test, "Strength Parameter of Soils by Triaxial Compression," AASHTO Designation: T 234-85, Part II Tests, 16th Edition, 1993.
24. American Association of State Highway and Transportation Officials. Standard Method of Test, "Direct Shear Test of Soils Under Consolidation Drained Conditions," AASHTO Designation: T236-84, Part II Tests, 16th Edition, 1993.
25. American Association of State Highway and Transportation Officials. Standard Method of Test, "The California Bearing Ratio," AASHTO Designation: T193-81, Part II Tests, 16th Edition, 1993.
26. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Sand Cone Method," AASHTO Designation: T191-86, Part II Tests, 14th Edition, 1986.
27. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Rubber-Balloon Method," AASHTO Designation: T205-86, Part II Tests, 14th Edition, 1986.
28. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T238-86, Part II Tests, 14th Edition, 1986.
29. American Association of State Highway and Transportation Officials. Standard Method of Test, "Moisture Content of Soil and Soil Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T239-86, Part II Tests, 14th Edition, 1986.
30. Lin, I. J., "Seasonal Effects on Processing Plants." International Mining, January, 1989.

Mineral Processing Wastes - Embankment or Fill

INTRODUCTION

Several different types of mineral processing wastes, particularly mill tailings and coarse coal refuse, have been successfully used to construct highway embankments. To a lesser extent, waste rock has also been sporadically used as a fill material in highway construction.

Waste Rock

Waste rock derived from igneous or metamorphic rocks, as well as properly consolidated limestones, sandstones and dolomites, are generally suitable for use in embankment or fill construction applications, provided the rocks do not contain deleterious components and are not commingled with overburden. Prior to use, some consideration should be given to the leaching potential of waste rock from sulfide-based ore bodies (such as lead, zinc, or silver) or waste rock subjected to heap leaching.

Mill Tailings

Mill tailings have been used previously in embankment and fill applications by some state and local highway agencies. Generally, the coarser, sand-size fractions of mill tailings can also be used as a construction aggregate, provided there are no harmful or reactive chemical components concentrated from the host rock. Despite the fine size of most mill tailings, these materials can be blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range, or can often be classified prior to initial disposal in order to recover the coarser fraction for possible use. However, the metal leaching potential of these materials can be a cause for environmental concern and should be thoroughly investigated prior to embankment use.

Coal Refuse

Coarse coal refuse can be used in embankment applications. Proper compaction of coarse coal refuse to its maximum dry density is necessary to achieve stability and to minimize the potential for spontaneous combustion. Burnt coal refuse (red dog) is also a suitable embankment or fill material. Fine coal refuse slurry has little or no load carrying capability and is, therefore, unsuitable for use as a construction material.

Potential problems with spontaneous combustion associated with the carbonaceous content of coal refuse, its pyritic or sulfur composition, and acidic nature are causes for environmental concern.

PERFORMANCE RECORD

Mineral processing wastes have been used in a number of states during the past 30 to 40 years for embankment applications where such materials have been available, acceptable, and economical.

Although uses of mining and mineral processing wastes for highway embankment construction have not generally been well documented over the years, it is known that these materials have been used for such purposes in at least 14 different states. There are also at least two other examples (feldspar tailings in North Carolina and coal refuse in Ohio) where these materials have been used on a local basis as fill and approved for highway construction. Table 6 is a summary of the known uses of mining and mineral processing wastes in embankments in these 16 different states.⁽¹⁾

Waste Rock

Waste rock derived from all sources may be used for embankment applications provided it satisfies applicable specification requirements for rock base. Waste rock should not contain deleterious components and must not be commingled with unsuitable materials. Currently, New York is the only state that is reportedly using waste rock as highway material. It is being used as stone fill for embankments and as rip rap for bank and channel protection. Performance has been acceptable in each application.(2) Other states that have made some use of waste rock for embankment construction in the past include Arizona, Colorado, Michigan, and Washington.⁽¹⁾

Mill Tailings

The coarser, sand-size fractions of most mill tailings ordinarily make acceptable embankment construction materials, provided there are no harmful or reactive chemical components contained in the tailings. Despite the fine size of most of tailing materials, they can be readily classified or blended with coarser materials, such as natural gravel, to bring the overall fines content to a more acceptable range.

In the last 15 to 25 years, some very large highway embankments have been constructed using mill tailings. Copper tailings have been used in Utah and Michigan, lead-zinc tailings in Idaho, feldspar tailings in North Carolina, and gold tailings in California and Colorado. Although some embankment applications may not have been well documented, their performance has been generally described as good to very good for this type of application.⁽¹⁾

In 1994, it was reported that at least 11 local or state agencies in Alaska, California, Colorado, Idaho, Michigan, Minnesota, Missouri, North Carolina, South Dakota, Utah, and

Table 6. List of mining waste embankment or structural backfill projects constructed in the United States.

State	Type of Mining Waste Used	Project Location(s)	Estimated Tonnage or Volume
Alaska	Mill tailings	Location not known	Not known
California	Gold dredge tailings	Sacramento area	Substantial amounts
Colorado	Gold mill tailings Coal mine wastes	North central part of California Location not known	Not known Not known
Idaho	Lead-zinc tailings Gold dredge tailings	I-90 near Kellogg Forest Road in Custer Co.	>765,000 m3 (>1 million yd3) Not known
Illinois	Coal refuse	I-57 in Franklin Co.	Not known
Indiana	Coal overburden	Two interstate highways	Not known
Michigan	Copper waste rock Copper stamp sands Iron waste rock	US Rte 45 in Military Hills U.S. Rte 41 near Houghton U.S. Rte 2 near Ironwood	350,000 m3 (460,000 yd3) 46,000 m3 (60,000 yd3) 130,000 m3 (250,000 yd3)
Minnesota	Taconite tailings	Northeast part of Minnesota	Not known
Missouri	Iron waste rock	Southeast part of Missouri	Not known
New York	Iron waste rock	Northwest part of New York	Not known
North Carolina	Feldspar tailings	Western part of North Carolina	Probably small amounts
Ohio	Bituminous coal refuse	Southeast part of Ohio	Probably small amounts
Pennsylvania	Anthracite coal refuse Anthracite coal refuse Bituminous coal refuse	Cross Valley Expressway near Wilkes Barre I-81 near Hazleton US Rte 219 relocation near Ebensburg	115 million m3 (1.5 million yd3) Substantial amounts 145,000 m3 (190,000 yd3)
South Dakota	Gold mill tailings	Western part of South Dakota	Not known
Utah	Copper mill tailings Copper mill tailings	I-215 west of Salt Lake City Other roadways near Salt Lake City	3 million (3.3 million tons) 2 million (2.2 million tons)
Washington	Lead-zinc waste rock	County roads in Metaline Falls area, northeast corner of Washington	Not known

Washington have been involved, at one time or another, in mill tailings use in embankment applications.⁽²⁾

Pennsylvania has had successful experiences with the use of coarse coal mine refuse in at least four embankment construction projects. Both anthracite and bituminous coal refuse have been used.⁽³⁾ Other states with some experience in coal refuse embankment construction include Illinois, Indiana, Maryland, and Ohio.⁽²⁾ Also, some local usage of coarse coal refuse as a fill material has been previously indicated in Colorado and Kentucky.⁽¹⁾ The Federal Highway Administration has prepared a manual recommending the appropriate methods for using coal refuse to build highway embankments.⁽⁴⁾

Great Britain has been a forerunner in the utilization of coal mine refuse in highway construction. The Ministry of Transport in Great Britain permits the use of coal refuse (well-burnt nonplastic shale) as an alternative material source for the construction of embankments.⁽⁵⁾

MATERIAL PROCESSING REQUIREMENTS

Waste Rock

Crushing: Crushing and sizing is the only processing required to make use of oversize waste rock in embankments. Waste rock should be free of overburden and vegetation before crushing..

Mill Tailings

Dewatering: For certain mill tailings sources, some processing (such as dewatering, reclaiming, and selective size classification) may be necessary, although this is not common practice and can be costly. Tailings reclaimed from ponds will normally require a reasonable period of time to dewater, depending on climatic conditions.

Screening: Some fine tailings can be size classified to recover a coarser fraction (between the 4.75 mm (No. 4) and 0.075 mm (No. 200) sieves) for use as an embankment construction material.

Coal Refuse

Separation or Cleaning: Various mineral processing techniques are used to separate the coal from the unwanted foreign matter in coal preparation plants. The equipment most frequently used in these plants to classify the refuse is based on the difference in specific gravity between the coal and the host rock.

Additional separation or cleaning may be required in the field in order to remove and recover the combustible portion of coarse coal refuse for use as fuel, prior to placing the remaining refuse material in an embankment. This is particularly the case for older refuse banks.

ENGINEERING PROPERTIES

Waste Rock

Some of the properties of waste rock that are of particular interest when waste rock is used in embankment or fill applications include gradation, specific gravity, and shear strength.

Gradation: Waste rock is generally coarse, crushed, or blocky material covering a range of sizes, from very large boulders to fine sand-size particles and dust. Waste rock can be processed and blended with other aggregates to generate a product suitable for use in embankment construction.

Specific Gravity: The average specific gravity of waste rock is about 2.65, with a range from 2.4 to 3.6 depending on the nature of the mineral constituents. Specific gravity may be used to determine other important properties such as void ratio or porosity.⁽⁶⁾

Shear Strength: Typical values of the angle of internal friction of most waste rock sources exceed 35 degrees and contribute to relatively high bearing capacity and stability.

Mill Tailings

Some of the properties of mill tailings that are of particular interest when mill tailings are used in embankment or fill applications include gradation, particle shape and texture, moisture-density characteristics, and unit weight. The chemical composition of the tailings should also be known prior to its use. It is difficult to definitively characterize representative samples of mill tailings materials because of the number of sources and variations in the degree of processing that can be encountered.

Gradation: Typically, mill tailings range from sand to silt-clay in particle size, with 40 to 90 percent passing a 0.075 mm (No. 200) sieve. They are usually disposed of in slurry form by pumping into large retention areas or settlement ponds.⁽⁷⁾ The coarser, sand-size fractions, if any, of mill tailings are more highly recommended for embankment construction. Mill tailings can be classified prior to disposal or blended with coarser materials, such as gravel, to bring the overall fines content to an acceptable range, preferably less than 35 percent passing a .0075 mm (No. 200) sieve.

Shape/Texture: Mill tailings consist of hard, angular, siliceous particles.

Moisture-Density Characteristics: With the possible exception of iron ore or taconite tailings, most mill tailings have an optimum moisture content in the range of 10 to 18 percent. The maximum dry density of most tailings is in the range of 1600 to 2025 kg/m³ (100 to 125 lb/ft³).⁽⁸⁾

Unit Weight: Iron ore and taconite tailings typically have high unit weight values, up to as high as 2700 kg/m³ (170 lb/ft³). The unit of weight of most other tailings sources is expected to range from 1500 to 2000 kg/m³ (90 to 135 lb/ft³), which is comparable to that of most natural aggregates.⁽⁷⁾

Coal Refuse

Some of the properties of coarse coal refuse that are of particular interest when coarse coal refuse is used in embankment or fill applications include gradation, particle shape/ texture, moisture-density characteristics, strength, permeability, durability, resistance to wetting/drying, and frost susceptibility.

Gradation: Coarse coal refuse, which is greater than 4.75 mm (No. 4 sieve), is a well-graded material (can vary in size from 100 mm (4 in) to 2 mm (No. 10 sieve)) consisting mainly of slate or shale with some sandstone or clay. Most coarse refuse contains particles that may break down under compaction equipment, resulting in a finer gradation following placement.

Shape/Texture: Coal refuse is composed mainly of flat slate or shale particles with some coal, sandstone, and clay intermixed. Such particles may weather or break down easily.

Moisture-Density Characteristics: Based on available data, the optimum moisture content of coarse coal refuse is likely to range from 6 to 15 percent and its maximum dry density may vary from 1300 to 2000 kg/m³ (80 to 120 lb/ft³).^(3,5)

Shear Strength: The shear strength of coarse coal refuse can be highly variable. The angle of internal friction values for coarse coal refuse have been reported to be between 18 and 42 degrees.⁽⁹⁾ The shear strength of coal refuse is usually lower than that obtained for other granular materials with similar properties, but can be increased by proper compaction.^(5,9,10) Previous experience with coal refuse usage as a construction material has demonstrated that the shear strength of the refuse is acceptable if proper compaction is achieved during construction.^(9,11)

Permeability: The permeability of coarse coal refuse can also be highly variable and should be determined for each particular refuse source. It is related to the composition of the refuse, its degradation during compaction, and the degree of compaction.^(9,12) The permeability of coarse coal refuse is less than that of other granular materials with a similar grain size distribution. Conventional formulas for estimating the permeability of coal refuse on the basis of size distribution and uniformity are not applicable for this material.⁽¹³⁾

It is preferable to attain lower permeability to reduce the air circulation, and void ratio and to eliminate spontaneous combustion, oxidation of pyrites, and acidic leachate.⁽¹⁴⁾ The permeability can decrease rapidly when the percentage of particles minus 4.75 mm (No. 4 sieve) increases. However, at some point, as more minus 4.75 mm is added, the coarse particles may be displaced or pushed apart, creating higher permeability.⁽¹⁵⁾

Durability: If durability is a concern in the top layers of an embankment, fly ash can be added to the refuse to neutralize acidity of the refuse, increase its moisture-holding capacity and pore space volume, and reduce its erodability.⁽⁴⁾ Lime and/or cement used as a binding agent with the fly ash produces a pozzolanic reaction, providing added strength and durability to the coarse coal refuse.⁽⁹⁾

Resistance to Wetting/Drying: Coal refuse begins weathering immediately after it has been placed in an embankment. Increases in the soluble sulfur content can induce oxidation of the pyrite. However, once the material is sealed within the embankment, oxidation is limited and weathering is greatly reduced. Water penetration is virtually eliminated, along with the degradation resulting from intermittent wetting and drying.

Frost Susceptibility: The top layers of coal refuse (especially burnt refuse) may be susceptible to damage from frost. Frost damage can be reduced or eliminated by the addition and mixing of cement into the top 1 meter (3 ft) of refuse.⁽¹⁶⁾

DESIGN CONSIDERATIONS

Waste Rock and Mill Tailings

The design requirements for waste rock or mill tailings in embankment construction are the same as for conventional aggregates or soils. These materials must meet appropriate sizing requirements and satisfy standard Proctor moisture-density criteria according to AASHTO T99.⁽¹⁷⁾

Structural design procedures to be employed for embankment or fill construction containing waste rock or mill tailings are essentially the same as design procedures that are used for conventional embankment materials. An analysis of the slope stability and consolidation characteristics of the embankment must be completed prior to construction. Some tailings sources may have an excessive amount of fines (greater than 35 percent passing the 0.075 mm (No. 200) sieve) which could necessitate prior classification or separation and use of only the coarse fraction of the tailings in an embankment or fill.

Coal Refuse

The design requirements for coarse coal refuse in embankment or fill construction are essentially the same as for conventional aggregates or soils. However, tests for standard Proctor moisture-density and spontaneous combustion potential should be carried out for all coal refuse that is considered for use in highway construction. Leaching and swelling indexes, porosity, freeze-thaw tests and wet-dry swelling tests are also required. Water-soluble sulfate testing methods/ specifications for determining the amount of sulfate found in coarse coal refuse and measures used to overcome such sulfate content are available from the British National Coal Board.(18)

Design procedures for embankments or fill containing coal refuse are the same as design procedures for conventional embankment materials. Slope stability and settlement analyses should be conducted to ensure that the coal refuse embankment is stable at the design slope and will not settle excessively. The potential for weathering and frost heave must also be considered.

Coal refuse for use in embankments should be tested in accordance with AASHTO test methods T234⁽¹⁹⁾ and T236⁽²⁰⁾ to determine the shear strength characteristics of the material tested. AASHTO T216⁽²¹⁾ and T193⁽²²⁾ are also used to determine the consolidation characteristics of the refuse and evaluate its subgrade bearing capacity.⁽⁴⁾

CONSTRUCTION PROCEDURES

Material Handling and Storage

Waste Rock and Mill Tailings: The same methods and equipment used to store or stockpile conventional aggregates are applicable for waste rock and mill tailings.

Coal Refuse: Prior to using the refuse to construct embankments or fills, the bank should be cleaned or processed to recover the residual coal or combustible matter. This ordinarily involves a screening of the refuse, which also removes oversize and deleterious materials.

Placing and Compacting

Waste Rock: The same methods and equipment used to place and compact conventional rock as embankment base or foundation material can be used for the placement of mine waste rock. Compaction operations and methods must be visually inspected on a continuous basis to ensure that the specified degree of compaction can be achieved, or that there is no movement under the action of compaction equipment. The construction of embankment bases or foundations containing rock or oversize materials usually requires a method specification, which describes how to place and compact such materials, but does not include test methods or acceptance criteria.

Mill Tailings: No modifications to normal construction equipment or procedures are needed for placing and compaction of mill tailings, except that mill tailings may need to be dried to near optimum moisture content prior to placement and compaction.

Coal Refuse: No modifications to normal construction equipment or procedures are needed, except that material breakdown under compaction equipment requires more repetitive testing in the field. The key to the success of placing coarse coal refuse is in proper compaction. Proper compaction of coal refuse reduces air voids to less than 10 percent, and can reduce the permeability to less than 10^-5 to 10^-6 cm/sec, which is very low. Well-compacted material is sufficiently dense for embankment construction with minimal potential for ignition because of spontaneous combustion.⁽⁵⁾

Quality Control

Waste Rock and Mill Tailings: The same test procedures used for conventional aggregate are appropriate for waste rock and mill tailings, although waste rock may have particles too large for certain in-place density tests. The same field test procedures used for conventional aggregate are recommended for embankment and fill applications when using waste rock or mill tailings. Standard laboratory and field test methods for compacted density are given by AASHTO T191,^(23) T205,(24) T238,⁽²⁵⁾ and T239.⁽²⁶⁾

Coal Refuse: Strict compaction control testing is necessary when building an embankment with coarse coal refuse. One of the best methods for controlling the compaction of coarse coal refuse embankments is to first place a test strip to determine the most appropriate compaction equipment and number of passes to ensure adequate compaction. The test strip will also assist in identifying the degree of particle breakdown and its effect on moisture-density characteristics for different types of compaction machinery.

The quality control test procedures described above for waste rock and mill tailings are also applicable to coarse coal refuse, except that some refuse particles may be too large for certain in-place density tests.

Special Considerations: A determination of the sulfate levels that may be leached from coarse coal refuse is required in order to design for the protection of any adjacent concrete structures. The pH value of the refuse in water should also be determined for proper selection of type of underdrain or other drain pipes.

UNRESOLVED ISSUES

Waste Rock and Mill Tailings: General specifications and design methods should be developed for waste rock and/or mill tailings use in embankment or fill applications by those agencies where such materials are logistically available in large quantities and are suitable for embankment or fill use.

There is also a need to determine whether specific sources of such materials are environmentally suitable for embankment construction, particularly some sources of mill tailings. Engineering data are needed on the design properties and performance of waste rock and/or mill tailings that have been successfully used in highway embankment or fill applications.

Coal Refuse: There is a need to further evaluate environmental concerns regarding the potential for acidic leachate from coarse coal refuse used in embankments. The production of such leachate is caused by the oxidation of pyrite and marcasite with presence of high sulfur content. If acidic leachate were to be produced over time, it would contaminate ground water, adversely impact the ecosystem, and cause deterioration or corrosion of underdrains or other drain pipes.

REFERENCES

1. Collins, R. J. and R.H. Miller. Availability of Mining Wastes and their Potential for Use as Highway Material. Federal Highway Administration, Report No. FHWA-RD-76-106, Washington, DC, May, 1976.
2. Collins, R. J. and S. K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction. National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board, Washington, DC, 1994.
3. Butler, P. "Utilization of Coal Mine Refuse in Highway Embankment Construction," Transactions of the Society of Mining Engineers. AIME, Volume 260, June, 1976.
4. Pierre, J. J. and C. M. Thompson. User’s Manual - Coal Mine Refuse in Embankments. Federal Highway Administration, Report FHWA-TS-80-213, Washington, DC, December, 1979.
5. Maneval, D. R. "Utilization of Coal Refuse for Highway Base or Subbase Material," Proceedings of Fourth Mineral Waste Utilization Symposium. IIT Research Institute, Chicago, Illinois, May, 1974.
6. Wright Engineers Limited, Golder, Brawner and Associates Limited, and Ripley, Klohn and Leonoff International Limited. Tentative Design Guide for Mine Waste Embankments In Canada. Technical Bulletin TB 145, Mines Branch Mining Research Centre, Department of Energy, Mines and Resources, Ottawa, Canada, March, 1972.
7. Emery, J. J. "Use of Mining and Metallurgical Waste in Construction," Minerals and Environment. Paper No. 18, London, June, 1974.
8. Sultan, H. A. Utilization of Copper Mill Tailings for Highway Construction. Final Technical Report, National Science Foundation, Washington, DC, January, 1978.
9. McQuade, P. V., P. E. Glogowski, F. P. Tolcser, and R.B. Anderson. Investigation of the Use Of Coal Refuse-Fly Ash Compositions as Highway Base Course Material: State of the Art and Optimum Use Area Determinations. Federal Highway Administration, Interim Report No. FHWA-RD-78-208, Washington, DC, September,1980.
10. Bishop, C. S. and N. R. Simon. "Selected Soil Mechanics Properties of Kentucky Coal Preparation Plant Refuse," Proceedings of the Second Kentucky Coal Refuse Disposal and Utilization Seminar. Lexington, Kentucky , May, 1976.
11. Tanfield, R. K. "Construction Uses of Colliery Spoil," Contract Journal. Great Britain, January, 1974.
12. Zook, R. L., B. J. Olup, Jr., and J. J. Pierre. "Engineering Evaluation of Coal Refuse Slurry Impoundments," Transactions of the Society of Mining Engineers. AIME, Volume 258, March, 1975.
13. Drenevich, V. P., R. J. Ebelhar, and G. P. Williams. "Geotechnical Properties of Some Eastern Kentucky Surface Mine Spoils," Proceedings of the Seventh Ohio River Valley Soils Seminar. Lexington, Kentucky, October, 1975.
14. Moulton, L. K., D. A. Anderson, R. K. Seals, and S. M. Hussain. "Coal Mine Refuse: An Engineering Material," Proceedings of the First Symposium on Mine and Preparation Plant Refuse Disposal. Louisville, Kentucky, October, 1974.
15. Stewart B. M., and Atkins, L. A. Engineering Properties of Combined Coarse and Fine Coal Wastes. Bureau of Mines Report of Investigations; 8623, United States Department of the Interior, 1982.
16. Kettle, R. J., and R. I. T. Williams. "Frost Action in Stabilised Colliery Shale," Presented at the 56th Annual Meeting of the Transportation Research Board, Washington, DC, January, 1977.
17. American Association of State Highway and Transportation Officials. Standard Method of Test, "The Moisture-Density Relations of Soils Using a 5.5-lb [2.5 kg] Rammer and a 12-in. [305 mm] Drop," AASHTO T99-86, Part II Tests, 16th Edition, 1993.
18. British Standard Institution. "Methods of Testing Soils for Civil Engineering Purposes, B.S. 1377, Test 9, Determination of the total sulphate content of soil, and Test 10, Determination of the sulphate content of ground water and of aqueous soil extracts," London, 1967.
19. American Association of State Highway and Transportation Officials. Standard Method of Test, "Strength Parameter of Soils by Triaxial Compression," AASHTO Designation: T234-85, Part II Tests, 16th Edition, 1993.
20. American Association of State Highway and Transportation Officials. Standard Method of Test, "Direct Shear Test of Soils Under Consolidation Drained Conditions" AASHTO Designation: T236-84, Part II Tests, 16th Edition, 1993.
21. American Association of State Highway and Transportation Officials. Standard Method of Test, "One-Dimensional Consolidation Properties of Soils," AASHTO Designation: T216-83, Part II Tests, 16th Edition, 1993.
22. American Association of State Highway and Transportation Officials. Standard Method of Test, "The California Bearing Ratio," AASHTO Designation: T193-81, Part II Tests, 16th Edition, 1993.
23. American Association of State Highway and Transportation Officials. Standard Method of Test, " Density of Soil In-Place by the Sand Cone Method," AASHTO Designation: T191-86, Part II Tests, 14th Edition, 1986.
24. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Rubber-Balloon Method," AASHTO Designation: T205-86, Part II Tests, 14th Edition, 1986.
25. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T238-86, Part II Tests, 14th Edition, 1986.
26. American Association of State Highway and Transportation Officials. Standard Method of Test, "Moisture Content of Soil and Soil Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T239-86, Part II Tests, 14th Edition, 1986.