1. Introduction
“Environmental Mine Engineering” is traditionally involved with the management of water and waste products to avoid pollution; however, the range of concerns is growing. Today it includes dust control, noise attenuation, reduction of visual impact, recycling, and reclamation. Tomorrow it may include macro-environmental (for example, global warming) and socio-economic considerations.
The interface between the miner and the environmentalist demands an exchange of knowledge and good communications. For this purpose, the miner must become versed in the basics of environmental engineering. The aim of this chapter is to aid this process. The following endeavors are now generally considered to be within the domain of environmental engineering.
• Supply and purification of potable water.
• Supply, treatment, recycling, and disposal of process water.
• Treatment and discharge of mine water, gray water, and black water.
• Treatment and disposal of tailings.
• Stockpiling and replacement of overburden (soil).
• Disposal and treatment of waste rock.
• Control of airborne emissions, including dust.
• Erosion control.
• Diversion channels for storm water runoff and streams.
• Retention pockets for surface pipelines.
• Containment structures for fuel storage tanks.
• Noise attenuation.
• Reclamation and rehabilitation.
The text of this chapter is only a primer to a field of work that is far more extensive than can be adequately addressed in this handbook. Identifying and dealing with environmental concerns often involves complex procedures that are difficult to describe in simple terms. There are exceptions to be found to any general observation and the processes commonly employed are constantly changing due to technical advances and better-defined environmental considerations.
As more stringent environmental regulations are enacted, the required engineering effort becomes more and more significant to hard rock mining. The emphasis should be on prevention rather than reacting after the fact. By anticipating future environmental considerations, environmental impact can be minimized, true costs optimized, and real blunders avoided.
The total environmental effort includes art, science, and engineering. Due to space limitations, this chapter is more or less confined to the engineering applications. Ventilation engineering (including dust control) is discussed separately in Chapter 18 – Ventilation.
2. Rules of Thumb
Environmental Impact Statement
• The cost of an environmental impact statement (EIS) (including base line monitoring and specific previously performed studies) may cost approximately 2.5% of the total pre-production capital cost for a plain vanilla domestic mining project. The cost can increase by 2% for an undertaking that is politically or environmentally sensitive. In the latter case, the cost may increase further if proposals are challenged in the courts. Source: R.W. Corkery
Site Layout
• If the mill (concentrator) is located close to the mine head, the environmental impact is reduced and so are the costs. Pumping tailings from the mill is cleaner, less disruptive to the terrain, and less expensive than to truck haul ore over a similar distance. When pumping water to the mill and hauling concentrate from the mill is considered, the argument is usually stronger. The rule is further reinforced in the case of an underground mine where a portion of the tailings is dedicated for paste fill or hydraulic fill. Source: Edgar Köster
• The mine administration offices should be located as near as possible to the mine head to reduce the area of disturbance, improve communications, and reduce transit time. Source: Brian Calver
• When a mine has a camp incorporated into its infrastructure, the campsite should be as close as practical to the mine to minimize the impact from service and utility lines, decrease the area of the footprint of disturbance, shorten travel time, and reduce costs. Source: George Greer
Site Drainage and Spill Protection
• Drainage ditches to protect the mine plant should be designed to develop peak flow rates based on 100 year, 24 hour storm charts. Source: AASHO
• Dykes around tank farms should be designed to hold 100% of the capacity of the largest tank + 10% of the capacity of the remaining tanks. Source: George Greer
Water Supply
• If a drilled well is to be used for fire fighting without additional storage, it should demonstrate (by pumping test) a minimum capacity of 40 USGPM continuously for two hours during the driest period of the year. Various sources
• Chlorine should be added to water at a rate of approximately 2 mg/litre to render it safe to drink. Source: Ontario Ministry of Health and Welfare
Dust Suppression
• Dust emissions emanating from the transport of ore will not remain airborne when the size of dust particle exceeds 10 µ (ten microns). Source: Howard Goodfellow
3. Tricks of the Trade
• Sub-aqueous deposition of potentially acid generating rock is recommended for nearly all applications, owing to the relative ease of design/construction, and the fact that such an impoundment may be built maintenance free, forever.
• Any surface concrete structure designed for a new mine (or added to an existing mine) should include plastic pipe inserts suitable for loading explosives in order to facilitate ultimate demolition. Source: Peter R. Jones
• Planted tree screens can provide noise abatement and a windshield for dust control. They may also provide a natural snow fence.
• Perimeter earth walls (berms) lessen visual impact and provide some measure of noise absorption.
• Gabions are an aesthetic means of providing slope stability and erosion control
• Drainage ditches with a V-shaped cross section are satisfactory for nearly all applications. (Refer to Section 5.8.)
• Silt fences across ditches help prevent the dispersion of sediment into a natural water course.
• An underground shaft dump feeding a conveyor incline to the mill eliminates a major noise source and permits a shorter headframe.
• Nylon poultry netting over cyanide-laden ponds prevent death to wayward waterfowl. The netting is conveniently supported with a used car tire laid flat on top of each pylon.
• Employing used truck tires to build retaining walls demonstrates that consideration has been given to recycling.
• Stocking fish in a finishing pond provides a natural indicator of purification. Salmonoids, such as trout, sink when they die so it is preferable to use a species of fish that will float providing immediate visual evidence of a problem.
• Drainage ditches for access roads should be designed to develop peak flow rates based on 10 year, 24 hour storm charts. Source: AASHO
• Open pits typically generate 2 to 5 tons of waste rock per ton of ore mined compared to about 0.25 tons of waste rock for an underground mine. Source: Dirk van Zyl
• In the Canadian Shield, the lower limit for acid generating potential can be as little as 0.2% sulfur in some volcanic waste rocks that contain minimal buffering capacity. In this extreme case, any rock with contents of higher than 0.2% sulfur would be classified as potentially acid generating and would be treated accordingly. Source: George Greer
• In the Canadian Shield, basic waste rock (with good buffering attributes) having a sulfur content greater than 0.5% should be considered to be potentially acid generating and treated accordingly.
Source: Tom Lamb
4. Procedures
Environmental concerns are first encountered when applying for an exploration license. The license mandates certain environmental considerations be met or carried out during exploration activity and afterwards. If the exploration proposal encompasses an extensive program, it may be wise to ensure that an underground entry is specified to avoid a second application should the results of the exploration work be promising and an exploration entry (shaft, ramp, or adit) become desirable.
“Base Line Studies” should be initiated as soon as possible once the exploration program indicates success. The base line studies are “Job One” because they must be completed before any change to the environment occurs and because they are a pre-requisite for subsequent efforts. The results of these studies will be the yardstick against which predicted and actual changes to the environment will be measured. The studies always include water monitoring and may include items such as inventories of flora and fauna, background radiation, identification of archeological sites, and a search for applicable hydrological, meteorological, and other previously published relevant data. In special circumstances, the tests may also include blood samples taken from wildlife and local residents to establish original toxicity levels (i.e. levels of mercury or lead).
Background noise measurements may be conducted, where applicable. A weather station should be installed, including a means to measure temperatures, precipitation levels, precipitation pH, wind velocity, and natural dust fall (particularly for remote sites where historic data on climatic conditions is non-existent). If and when the exploration program has advanced sufficiently to indicate that a viable mine may result, it is time to identify the “Major Environmental Constraints.” The list enumerated in the introduction may be used to help identify and categorize the areas of significant concern. From this effort, a short list must be developed and confirmed. It is most efficient to embark straightaway on special studies specifically addressing these concerns, especially when the studies require significant time. For example, a full year may be required to establish migratory patterns for caribou or polar bears. More time may be required to demonstrate that a migratory corridor near the project is wide enough to allow a slight narrowing. Two years or more may be required for complete field test plots to determine weathering kinetics of mine rocks.
A number of proposed mining projects have been aborted [Kitts-Michelin (Labrador), Windy Craggy (British Columbia), Montanore (Montana)] because a major environmental constraint could not be resolved. These misfortunes emphasize the importance of addressing environmental concerns early in the program.
5. Environmental Impact Statement
New mining projects anywhere in Canada or the USA (and elsewhere) are required by federal law to obtain approval based on an acceptable “Environmental Impact Statement.” Because this approval process may become the critical path to production, it is important to initiate its preparation as early as practical. Time can be saved if the pre-qualification process and selection of a firm to carry out the work has been completed before funds are available for its execution. More time can be saved if a detailed schedule of the work to be completed is formatted, agreed upon, and enforced. Managing EIS development is best served with fixed price or target price contracts. Straight cost reimbursable contracts (i.e. cost plus) with environmental consulting and legal firms may invite trivial pursuits adding to cost and time.
Environmental Impact Statement
The EIS (which incorporates previous studies) will describe the mine’s construction, operation, and closure, especially as related to the environment. It should clearly identify all areas of potential concern, provide a measure of the risks, and specify the planned remedies. The EIS is not complete without a “Management Plan” itemizing the procedure to monitor and control environmental concerns during the life of mine and thereafter. This plan will include terms of reference for “Annual Reports” devoted to environmental considerations and provisions for independent “Environmental Audits.”
Mine Closure Study
A Mine Closure Study will be included addressing rehabilitation. This study may identify the need to establish a topsoil stockpile at the outset, compost generating facility, and a nursery for indigenous trees and shrubs. A cost estimate for final rehabilitation may also be included to help determine the amount of the reclamation bond likely required to be posted by the mining company.
Case History
The pre-production cost for the environmental work at the recently completed $700 million BHP Ekati Diamond Mine in the Northwest Territories of Canada was about $14 million (2%), of which approximately $4-5 million was in consulting fees over two years. Source: Canadian Mining Journal
6. Schedule
The following schedule (Table 5-1) provides a case study for the environmental process on two recent projects in Canada. The tabulation demonstrates that timelines can be significantly different for a large, politically sensitive project in a green field region than for a small operation proposed for a brown field project in an established mining district.
Table 5-1 Environmental Process Case Study
7. Acid Rock Determination
At the earliest possible date, a mine rock investigation should be initiated to assess the characteristics of mine grade ores, cut-off grade ores, and adjacent country rocks. Many sulfide-bearing rocks oxidize when exposed to the atmosphere. The products of oxidation include acids and metal compounds that may be harmful on surface and underground.
In certain cases, sulfide ores can cause significant problems. When broken, these ores can heat up significantly (“sinter”) and even disintegrate if stored too long underground or on surface. Moreover, the gangue materials in these ores (which constitute the tailings from the mill) may not be suitable for disposal as backfill.
On surface, for an open pit operation the acid rock studies are desirable to determine the feasibility for “on land” disposal of waste rock and temporary stockpiling of low-grade ore. For an underground mine, the first object of such a study is to assist in initial mine layout concepts. In this case, an attempt is made to confine mine entries and development layouts to areas where the waste rock generated is suitable for on-land disposal.
The second object for an underground mine is to determine in advance whether mine water will be acidic and to what degree. Most underground mines that have sulfide ores not buffered by basic country rock will generate acid mine water with a pH of between 2 and 5. Acid waters pumped to surface require treatment to modify the pH before being released to a natural watershed or recycled as process water for the mill. The neutralization is most often accomplished with the addition of lime (CaO). This neutralization is rarely carried out underground. Instead, the lime is added on surface, usually at a primary sedimentation pond that may bring the pH up to 8 or 9 to initiate precipitation of unwanted metals. At the entrance to a secondary (“polishing”) pond, the water may be dosed with a flocculating agent (alum or polyelectrolyte) and more lime added to complete the precipitation of undesirable solids. If the water is to be subsequently released to the natural environment, it may require the addition of acid to reduce the pH to a mandated level. The actual process for any particular mine may vary from the simplified procedure described above. Table 5-2 provides the pH levels at which the precipitation of common metal ions are optimized.
Table 5-2 Precipitation of Common Metal Ions
8. Drainage Ditches and Culverts
Listed below are general rules for ditch construction.
• Ditches with a V shaped cross section are considered the standard design.
• Ditch side slopes should be no steeper than 2:1, except when excavated in sound rock.
• Ditches should not be constructed at a gradient of less than 0.3% if they are not lined (or 0.2% if smooth-lined) so that the velocity of the water will be self-scouring.
• Ditches running at a gradient of 3% or less may be constructed without benefit of a liner, except in easily eroded material such as fine sand or silt.
• Ditches at a gradient between 3% and 5% should be seeded and protected with a geo-textile mesh to establish a grass lining. Alternatively, the ditch flow can be temporarily diverted (i.e. in a light plastic drainage pipe) until the grass is firmly established.
• For gradients exceeding 5%, the lining should consist of rocks placed on top of a geo-textile membrane. This lining should extend to at least 150 mm (6 inches) above the computed elevation of maximum flow.
Flow Capacity
Table 5-3 provides the flow capacity in cubic feet per second of a standard ditch design (2:1 Vee) for different gradients and depths of water.
Table 5-3 Flow Capacity
Culvert Capacity
Ditches require culverts at road and rail crossings. The culverts should have capacity at least equal to that of the ditch. In temperate climates, no culvert should have a diameter of less than 300 mm (12 inches). For main access roads, haulage roads, and railroads, no culvert should be placed with a diameter less than 450 mm (18 inches).
Table 5-4 provides the entrance capacity in cubic feet per second for culverts flowing full without water backed up at the entrance to the culvert, as well as backed up in intervals of two feet of head over the top of the culvert at its entrance.
Table 5-4 Culvert Entrance Capacity
9. Water Demand
Table 5-5 shows water demand at various mine facilities.
Table 5-5 Water Demand
10. Chlorination of Potable Water
For batch chlorination, Table 5-6 may be used to determine chlorination quantities.
Table 5-6 Chlorination Quantities
For prospecting, exploration, and development projects, the mixing is done in batches and chlorine is typically provided in solution (one gallon plastic bottles of bleach or five-gallon carboys). The chlorine solution must be well mixed with the water. If water is being pumped into a tank, the chlorine may be added during the pumping to allow the action of the incoming water to provide sufficient turbulence for adequate mixing. In the case where water is hauled in tanks, the movement in trucking the water to the site provides mixing. A contact time of minimum 20 minutes is required for the chlorine to be effective. In the case of haulage tanks, this is generally provided in traveling.
For permanent mine sites and associated small town sites, the least expensive source of chlorine solution is made from powdered HTH (“high test hypochlorite”, calcium hypochlorite), which has 70% available chlorine. A 3% solution is made by dissolving the HTH powder in water (preferably with a mechanical agitator). The 3% solution is fed to the water supply with a hypochlorinator. The only reliable hypochlorinator is one that is both electrically driven and has positive displacement. The water pump and hypochlorinator must be electrically interconnected to automatically operate simultaneously.
11. Recycling Mine Water
Traditionally, recycling mine water was confined to providing process water for the underground. No mill desires contaminants, such as nitrates (from explosives) or hydrocarbons (from spilled fuel, spilled lubricants, and waste oil from engine oil changes) in their process water.
Today, better measures are taken by mine operators to reduce this contamination at the source. When it is practical, using mine water as process water can be advantageous because it reduces the total volume of water to be discharged to the environment. Also, if mine water is combined with process water discharged from the mill, the water treatment costs may be reduced because of the pH of the mill water and the reagents it contains may react positively with pollutants in the mine water.
12. Smelter Emissions
Listed below are base case smelter emissions of SO2 (approximate quantities emitted without benefit of scrubbers or flash furnace technology).
• Copper 625 kg/tonne of concentrate (1,250 Lbs./ton of concentrate)
• Lead 330 kg/tonne of concentrate (660 Lbs./ton of concentrate)
• Zinc 265 kg/tonne of concentrate (530 Lbs./ton of concentrate)
Note
McIntosh Redpath Engineering wishes to gratefully acknowledge the assistance given by Mr. George Greer, former employee of the Voisey’s Bay Nickel Company, who provided a significant amount of data, a portion of which is found in the main text of this chapter.