mining methods

GDACE Mining and Environmental Impact Guide Chapter 6: Mining Methods

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6. MINING METHODS 6.1. Introduction In the previous chapter, an overview was given of the most common prospecting and exploration methods utilized to prospect for the mineral commodities present in Gauteng. This chapter contains a description of the mining methods currently employed in Gauteng. Mention is made of possible environmental impacts of these mining methods and their rehabilitation; however the reader is referred to Chapter 9 for a more detailed description. The next chapters, Chapter 7 and 8, contain descriptions of the currently mined, economic mineral deposits and other uneconomic mineral commodities respectively. Mining is the process of extracting mineral resources from the earth. The mining method used depends heavily on the physical and chemical properties of the mineral, the physical form in which it occurs, as well as the geometry and depth of the ore body. 6.2. Surface Mining When a mineral occurs fairly close to the surface in a massive or wide tabular body, or where the mineral itself is part of the surface soil or rock, it is generally more economic to mine it by means of surface mining methods. Strip mining, open-pit, opencast mining and quarrying are the most common mining methods that start from the earth's surface and maintain exposure to the surface throughout the extraction period. For both access and safety, the excavation usually has stepped or benched side slopes and can reach depths exceeding 600m. Complete disruption of the surface always occurs, which affects the soil, fauna, flora and surface water, thereby influencing all types of land use (See Figure 6-1). If the operation extends to depths below the water table, it will affect the near-surface groundwater. An understanding of the pre-mining environment is therefore essential. It is also important to understand the mining method employed so that surface rehabilitation, an essential component of this type of mining can be meaningfully planned. 6.2.1. Open-pit mining This method of mining is used if the near-surface ore body is massive and when it occurs in a steeply dipping seam or seams, or a pipe, or makes up the country rock. Here, the whole ore body is mined with no overburden being put back into the void. The Premier Diamond Mine at Cullinan and crusher stone quarries are good examples. A modern open-pit mine is shown in Figure 6-2. In open-pit mining the barren rock material covering the ore body normally requires drilling and blasting to break it up for removal. A typical mining cycle consists of drilling holes into the rock in a pattern, loading the holes with explosives, or blasting agents, and blasting the rock in order to break it into a size suitable for loading and hauling to the mill, concentrator, or treatment plant. There the metals or other desired substances are extracted from the rocks.


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Figure 6-1: Aerial view of an open pit coal mine showing disruption of the earth surface

Figure 6-2: A modern open-pit mine with benches (Source: Wells et al., 1992) The ore body is traced deeper and deeper into the ground using a series of benches for both access and safety (Figure 6-3). Sometimes rock surrounding the ore has to be removed so that the sides of the pit do not become dangerously steep. This waste rock, and waste that is separated from the ore during processing, is dumped on the surface away from the pit onto a waste dump.


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The opportunities for land use following open-pit mining are often limited, because it is often very expensive to fill the pit. The main objective is usually to make the pit walls safe and to landscape the waste rock dumps, but many innovative solutions have been used, such as using the pit as a waste disposal site, filling it with water with the intention of creating an ultimate recreation/water supply/nature conservation end use or simply fencing it in and leaving it as a tourist attraction.

Figure 6-3: Map and cross section of an open-pit mine (Source: Terezepoulos, 1993) The residual impact of open-pit mining is usually a completely different land use. With few exceptions (coal, sulphidic ore), ore bodies that lend themselves to open-pit mining are not usually prone to causing water pollution (although the tailings resulting from subsequent


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mineral processing may be) and therefore water accumulating in the rehabilitated pit can usually be used for a number of purposes. Open cast mines generally mine horizontally contiguous ore bodies such as coal mines. The waste overburden is often cast into mined voids using explosives and a progression of mined allows for the waste rock to be replaced in the mined void once the ore has been removed. Soil can then be replaced facilitating progressive rehabilitation. See the description for strip mines. 6.2.2. Quarrying Quarrying is the open, or surface excavation of rock to be used for various purposes, including construction, ornamentation, road building or as an industrial raw material. Quarrying methods depend mainly on the desired size and shape of the stone and its physical characteristics. A typical granite quarry is shown in Figure 6-4.

Figure 6-4: Photograph of a granite quarry in South Africa (Source: Trade International, 2008)

In some cases (e.g. clays) the material is soft enough to be removed by mechanical means. However, when the rock is solid and hard (as in the case of stone aggregates and limestone for cement), drill and blast techniques are used. The rock is shattered using explosives positioned in a series of holes drilled in the rock in a pattern designed to yield the greatest amount of fracturing. Processing is usually limited to a further reduction in size by crushing and sorting according to size by screening.


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For building or dimension stone, where the rock needs to be extracted in large homogeneous rectangular blocks, blasting cannot be used. Several methods are used to break out the blocks, including splitting, diamond saws and diamond wire cutting. These methods are described in Box 6-1. Box 6-1: Quarrying Equipment and Methods

QUARRYING EQUIPMENT AND METHODS

• Splitting a line of holes For splitting a line of holes is drilled perpendicular to the joints or cleavage planes in the rock and wedges are inserted into the holes and hammered until the stone splits off. Much quarrying of ornamental stone today is done by using pneumatically operated splitters. After the vertical cuts have been made, horizontal cuts are made working on the same principle. Wedges are then used to split off the long blocks, which are subdivided and removed. • Diamond saws

Diamond saws are large diamond-impregnated circular blades up to 2 m in diameter that are used to form vertical cuts in the rock by moving the machine along a guideline or rail. Extremely accurate cuts can be made in this way.

• Wire saws

Wire saws are also used. These consist of several pulleys over which passes an endless carborundum or diamond-impregnated steel wire. Holes are drilled in the rock, each hole being made large enough to accommodate a pulley and the shaft to which it is attached. The wire, extending from one pulley to another, presses down against the rock between them. As the cut is deepened by the constantly moving wire the pulleys are continuously lowered into the holes. Diamond dust or fine silica sand, depending on rock hardness, is often introduced along the cutting surface to aid penetration.

6.2.3. Borrow Pits A borrow pit refers to an open pit where material (soil, sand or gravel rock) is removed for use at another location. Borrow pits are usually used in earthworks operations, which involves the movement of large quantities of soil, sand or gravel for use in the construction of roads, dams, embankments, bundings, berms, dikes and other structures, or the manufacture of bricks and concrete. A borrow pit differs from conventional quarries in that they are generally shallower and located in close proximity to the area where the material will be used. Therefore, a borrow pit has the advantage of eliminating the potential adverse effects brought about by the transportation of the excavated materials along public roads. This includes the loss of materials during transportation, vehicle entrainment of material


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on roads and resultant emissions of the material into the atmosphere. There are several potential negative environmental impacts associated with borrow pits. Firstly, the establishment of a borrow pit results in the loss of land that could be used for other land uses such as agriculture, human settlements, or recreation. This impact is usually temporary, however if not adequately rehabilitated, the impact could be long term. Secondly, soil erosion and deposition of the eroded material into nearby water bodies could occur, thereby having an adverse impact on water quality. Thirdly, in cases where the borrow pit is not properly cordoned off, people could accidentally fall into the pit, or dump general and toxic waste into the pit. If illegal dumping occurs and the water table is exposed, a potential risk of groundwater contamination exists. Fourthly, borrow pits could have negative impacts on the biological environment in that the natural habitat is destroyed and an artificial habitat is created that attracts unwanted plant and animal species, including weeds and mosquitoes. Suitable mitigation measures exist to manage the negative environmental impacts of borrow pits. The potential for other land uses should be carefully assessed before a borrow pit is established at any given location. In order to reduce potential effects of sedimentation, it should be ensured that borrow pits are not located in close proximity to surface water bodies. In order to ensure the health and safety of the environment and persons living close to the borrow pit, the site should be cordoned off and access to the site should be controlled. Mitigation measures that can be implemented after the decommissioning of a borrow pit include the backfilling of the borrow pit with soil, sand and gravel, followed by re-vegetation; the establishment of a pond or small dam for recreational use; or the use of the decommissioned pit for landfill or waste disposal, including necessary liners and waste management principles. A rehabilitated borrow pit is shown in Figure 6-5.

Figure 6-5: Emerging wetland formed from an old borrow pit (Source: Biebighauser, 2008)


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6.2.4. Strip mining Strip mining is mostly used when the deposit is horizontal or gently dipping and within about 60 m of the surface, such as shallow-lying South African coal seams. The method, shown in Figure 6-6, involves removing and stockpiling the top soil, drilling and blasting the rock (overburden) above the coal seam, removing the blasted overburden by draglines in long parallel strips (hence ’strip mining’) to uncover the coal. Then, depending on the coal’s hardness, either scraping or drilling and blasting are used to remove the coal. The removed overburden is placed in rows of spoil piles in the preceding strip from which the coal has been removed.

Figure 6-6: Strip mining with concurrent rehabilitation (Source: Wells et al., 1992) As soon as the mining strip (or pit) has been moved out of the way, the spoil piles can be landscaped — the start of the rehabilitation process. Once the desired shape and slope have been achieved, top soil previously stockpiled or sometimes brought directly from the unmined side, is replaced. The new ground is then treated in a conventional agricultural way by fertilising, liming and sowing to pastures. Sowing to pastures, as the first step in revegetating, is very important. It contributes significantly to erosion control and it allows the re-establishment of the soil’s micro-organisms, which are required for nutrient cycling. During the mining operation, considerable volumes of groundwater may be encountered and rainwater also falls onto the pit and spoil piles; therefore there is considerable potential for water pollution. This potential is controlled by installing separate clean and dirty water collection circuits. Clean water running off unmined and rehabilitated land is channelled, where possible, into nearby streams. Dirty water from the pit, haul roads and plant areas is collected and re-used for activities that do not require good-quality water, such as dust control and coal washing. The most important residual impacts remaining after rehabilitation are: • The box-cut spoil mound (the overburden from the first strip which does not have a

mined-out strip to go into) which forms a low hill in the new topography;


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Did you know?

Because most of South Africa’s coal suitable for strip mining occurs in Mpumalanga, about 50% of which is high-potential farm land, the main objective of rehabilitation is to return the land to productive agriculture. Considerable success has been achieved during the rehabilitation of strip-mined land both overseas and in South Africa. High-yielding pastures are an immediate result and they can be used for hay production or grazing. After a number of years under pasture, those areas rehabilitated to an arable standard can be, and have been, returned to grow crops.

• The final strip (called the final void) becomes a depression because there is no

overburden to fill it. This can become a lake or vlei area which, depending on the water quality in it, can be of benefit to the ultimate land user;

• The ramps, which can be rehabilitated only at the end of the mine’s life because they

continue to be used to remove coal from the pit, also become low-lying areas. They can serve as storm water runoff control drains, directing runoff from rehabilitated areas into the final void. If these ramps can be filled during the mining period, they have no residual impact different from other rehabilitated areas;

• The whole of the new landscape could be higher in altitude than the surrounding unmined

land due to the overburden swell after blasting being greater than the thickness of the coal seam removed;

• The groundwater table in the new landscape will eventually recover to a level dictated by

the surrounding unmined rock types and topography. Depending on what type of overburden there was in the mine, there is a potential for this groundwater to be more salty than before mining. This residual impact is not yet fully understood and is being researched.

If the shovel-and-truck method of strip mining is used, the box-cut spoil can sometimes be placed economically in the final void and the ramps can be progressively filled, obviating all residual topographic impacts. Unfortunately, this flexibility is not possible using a large dragline because of its mode of operation.

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6.2.5. Dump reclamation The mineral extraction processes from past mining eras were not as efficient as those used today and often mineral prices have increased dramatically from the time the orebody was first mined. Therefore the tailings generated at these old mines often still contain payable values of mineral, especially the sand and slimes dumps at old Witwatersrand gold mines. ’Dump reclamation’ refers to the reprocessing of these dumps. Typically, the material in the old dump is monitored (squirted with a very high-pressure jet of water which erodes the dump material away into a sluice). The sluice gravitates the dump material to a low point where it is collected and pumped, via a pipeline, to the treatment plant which could be located some distance away. Figure 6-7 shows a typical monitoring operation. The main environmental protection activities during reclamation are to keep storm water away from the working areas, to prevent rainwater and the process water used for the monitoring that has fallen on the site from leaving it in an uncontrolled fashion and to prevent dust pollution during dry, windy conditions. In the Witwatersrand, monitoring is the primary method of dump reclamation. In many cases, water control practices are not well adhered to, resulting in pollution of the local surface water environment (Figure 6-8).

Figure 6-7: A typical slimes monitoring operation (Source: Wells et al., 1992)


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636737.54 1815

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WaterpH 2.25U 6366 ug/lAs 66 ug/lNi 16238 ug/lCr 1570 ug/lSedimentU 371 ppmHg 8 ppmAu 2.5 ppmCd 1.1 ppmAs 660 ppm

WaterpH 3U 2360 ug/lAs 16 ug/lNi 14151 ug/lCr 222 ug/lSedimentU 71 ppmHg 0.02 ppmAu 0.2 ppmCd 0.6 ppmAs 103 ppm

Figure 6-8: Ikonos satellite image of an East Rand tailings dam undergoing reclamation

(Note the high levels of contamination recorded in water and sediment samples collected outside the paddocks and across the road from the dump).

A practice of concern in the Witwatersrand is the partial reclamation of slimes dams and the subsequent sale to another operator to avoid responsibility for final rehabilitation. This process can be effectively countered by realistic estimation of the rehabilitation liability before such a sale is allowed to proceed. If the old dump material is coarser than slime, such as found on a sand or coal dump, it is often recovered by a front-end loader and transported to the plant by conveyor. Once the whole dump has been reclaimed down to the original soil level under the dump, reclamation stops and rehabilitation of the site begins. The options available for different land uses on these sites are varied. In an urban area they are usually earmarked for urban development, office and industrial parks, residential, etc. In a rural area they can be returned to agriculture. In many cases the top part of the soil profile at these sites has been contaminated with acid water seepage from the dump. This has to be ameliorated with agricultural lime. Radium is often transported into the soil immediately below the dump. The radium will decay to radon, a radioactive inert gas, which can pose a hazard to living beings. Rehabilitation must reduce the radium activity to acceptable levels and action should be taken to prevent the emanation of radon from the soil. If buildings are constructed on these sites, special ventilated foundations are required to prevent high levels of radon gas accumulating within the closed buildings. Radium contamination is likely in the zone below any Witwatersrand dump, even where the ore mined had a relatively low uranium concentration, as it is concentrated at the redox and pH boundary in the near surface soil. In all these cases, a site-specific investigation is called for. The most common method of reducing radioactivity at reclaimed dump sites is to transport the material containing the elevated radiation levels away from the site for


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processing through a gold treatment plant, deposit it on a slimes dam or use it in an area where it is safe to do so. The main residual impact of reclaiming precious-metal dumps is not at the site of the reclaimed dump but rather impacts related to the slimes dam which has to take the same volume of material that was in the original dump. This could be on a new dump or on a recommissioned old dump. When coal dumps are reclaimed, a much smaller volume of material has to be redumped, but this waste may be more offensive due to an increased pyrite concentration. Extra precautions against acid seepage and spontaneous combustion may have to be taken to minimise the residual impact from this type of dump. 6.3. Underground mining Under certain circumstances surface mining can become prohibitively expensive and under-ground mining may be considered. A major factor in the decision to operate by means of underground mining rather than surface mining is the strip ratio or the number of units of waste material in a surface mine that must be removed in order to extract one unit of ore. Once this ratio becomes large, surface mining is no longer attractive. The objective of underground mining is to extract the ore below the surface of the earth safely, economically, and with the removal of as little waste as possible. These cost need to be weighed against the extraction of the ore. In open pit mine up to 90-95% of the ore body can be removed. In underground mining generally more ore has to be left behind as it is used to support the mine roof. The entry from the surface to an underground mine may be through an adit, or horizontal tunnel, a shaft or a declined shaft (Figure 6-9). A typical underground mine has a number of roughly horizontal levels at various depths below the surface and these spread out from the access to the surface. Ore is mined in stopes, or rooms. Material left in place to support the ceiling is called a pillar and can sometimes be recovered afterward. A vertical internal connection between two levels of a mine is called a winze if it was made by driving downward and a raise if it was made by driving upward.

Figure 6-9: Idealised cross-section of a mine (Source: Scoble, 1993) A modern underground mine is a highly mechanised operation requiring little work with pick and shovel. Rubber-tired vehicles, rail haulage and multiple drill units are commonplace. In order to protect miners and their equipment much attention is paid to mine safety. Mine ventilation provides fresh air underground and at the same time removes noxious gases as


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well as dangerous dusts that might cause lung disease, e.g. silicosis. Roof support is accomplished with timber, concrete or steel supports or, most commonly, with roof bolts, which are long steel rods used to bind the exposed roof surface to the rock behind it. Shafts, which are generally vertical, but may be inclined depending on the orientation of the ore body, can be distinguished from adits and tunnels, which are horizontal. Shafts and adits are the main access routes through which men, supplies, ore and waste are transported. They are the chief service openings during the development and operation of a mine, and provide space for compressed-air pipes or electric cables. By law at least two access points to the ore body are required for adequate ventilation and safety concerns.

High-productivity deep mines usually sink vertical shafts. As a rule, the shafts have big cross-sectional areas in order for large quantities of air to be supplied underground, as well as to provide a cage with enough space to carry large pieces of equipment and a big workforce into and out of the mine. The ore skips are usually large and travel at high speeds in the shaft. To provide this capacity, these shafts are often circular in shape and up to 10 m in diameter, though rectangular shafts are also used. Shafts are usually reinforced with steel and lined with concrete. Little difficulty is experienced in shaft sinking through solid rock, which contains little water. When loose, water-bearing strata, such as dolomites in western Gauteng gold mines have to be contended with, careful sealing of the shaft lining becomes necessary, and pumping facilities are needed. When there is an excessive quantity of water, cast-iron tubing is sometimes used. This tubing consists of heavy cast-iron rings made in segments, with flanges for connecting, and bolted together in place. Cement grout is forced into the space between the outside of the tubing and the surrounding earth to form a seal. In the grouting method, liquid cement is forced into the water-bearing earth under very high pressure. On mixing with the water, the cement solidifies the adjacent area, and it is removed by drilling and blasting as with rock. In general, the only direct environmental effects of deep underground mining methods are on groundwater. These impacts may be highly significant, both during mining when dewatering decreases the amount of available groundwater for other activities, and after mining, when the water table rebounds and may be recharged by highly polluted minewater. Indirectly,

Did you know? AngloGold Ashanti’s Mponeng Gold Mine in Carletonville is currently South Africa’s deepest underground mining operation, at a depth of 3.777km below ground level.

(Source: AngloGold Ashanti, 2008)

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environmental impacts are associated with mine residue deposits, surface subsidence as a result of dewatering and the disposal of water pumped from underground to enable mining to take place safely. 6.3.1. Bord-and-pillar mining This method is sometimes called room-and-pillar mining. It is commonly used for flat or gently dipping bedded ores or coal seams. Pillars are left in place in a regular pattern while the rooms are mined out. In many bord-and-pillar mines that are nearing closure, the pillars are taken out, starting at the farthest point from the mine haulage exit, retreating, and letting the roof come down upon the floor. Room-and pillar-methods are well adapted to mechanisation This mining method is employed in near-surface Gauteng and Mpumalanga coal mines. Figure 6-10 and Figure 6-11 show typical layouts.

Figure 6-10: Typical bord-and-pillar layout (Source: Wells et al., 1992)

Figure 6-11: Cross section of typical bord-and-pillar layout (Source: Scoble, 1993)


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Before the advent of modern pillar design in 1967, or the adoption of special precautions when mining at depths shallower than about 40 m, little was known about what size of pillars to leave behind. Sometimes, in their eagerness to extract the maximum amount of coal, the old miners left pillars too small to support the roof indefinitely. In addition, they sometimes ’robbed’ the pillars on their retreat from the exhausted coal faces. The result of this was that, some time after the mines closed down, certain areas of the roof collapsed into the bords and into underground roadways and intersections. In places, this collapse continued right to the surface. This allowed air to enter the old workings and to start a spontaneous combustion reaction in the residual coal (Figure 6-12). Underground fires resulted which often further weakened the pillars, causing even more collapses to take place.

Figure 6-12: Collapse in old shallow bord-and-pillar workings (Source: Wells et al., 1992)

The environmental and residual impacts are significant and numerous. For instance, in the Witbank area these fires are still burning. Random and unplanned surface subsidence occurs, rendering the land unsafe. Dangerous ground results where collapse has not yet taken place but may do at any time. Near-surface aquifers may also break, draining water into the collapsed workings. Rain falling on these areas goes straight into the workings, adding to the water load. This water often becomes acidic and has a high concentration of salts. Because many of these workings occur in places where the coal seam outcrops on the surface, acidic seepage emerges along the outcrop or from adits, causing severe water pollution in streams draining the area. Also, significant air pollution occurs due to the sulphurous fumes. Rehabilitation of these old workings is difficult (Figure 6-13), because of the danger of collapse and because rehabilitation was not planned as part of the original mining operation. In some cases it is not possible until the fires have burned out. Many attempts have nevertheless been made. These include filling the collapsed areas with rubble, bulldozing the sides of the collapses to safe slopes, backfilling the workings with non-combustible material such as gravel, soil or ash and flooding the workings. None of these methods has yet solved the water pollution problem, although some have been partly effective in stopping the fires. Trenching around the burning areas and backfilling the trench has been attempted to stop the fires spreading with only limited success, since the fire often jumps to the other side of the trench. Forced collapsing of the roof of the old mine by blasting has also been attempted, but was proved unsuccessful due to the unpredictable outcome of the blast. Remining these old areas, if at all possible, before collapse or burning occurs, is undoubtedly the best overall solution, since it removes the source of both combustible material and water


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pollution. Unfortunately this is often not feasible economically. Another option is to fill the whole area with power station ash. Most coal contains pyrite, leading to the potential for the formation of acid mine drainage. This can contaminate both ground water and surface water. Many coal deposits have high sulphur contents, leading to a significant hazard. Coal dumps are also important acid generators and sites of spontaneous combustion if not constructed properly.

Figure 6-13: Collapsed old bord-and-pillar workings near Witbank (Source: Wells et al., 1992)

6.3.2. Other shallow underground mining Shallow mining has taken place in many parts of the country, mainly in pursuit of gold and other metals. In Gauteng this has been limited to small gold prospects in the Magaliesberg/Krugersdorp area and silver/lead mines at The Willows, Edendale and Union Mine, east of Silverton. At these sites, a number of open shafts have been identified, some of which have been used as a water source. While galena (lead sulphide) does not have as high an acid generation potential as pyrite, its oxidation will cause some acid mine drainage and lead poisoning. In the case of the small gold mines, although they are small, they can have large impacts, for example the Chinese Shaft on Harmony Gold’s Randfontein Estates property appears to have become the first decant point for mine water from the Western Basin of the Witwatersrand. In most cases however, the miners who made these early excavations were usually in search of oxidised ore and visible gold, neither of which is usually associated with pyritic material. Significant acid pollution is therefore unlikely where isolated near-surface gold mining may have happened in the past. The main environmental impacts of these operations are the residual shafts, pits and rock dumps and mercury contamination where gold was extracted by amalgamation.


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6.3.3. Longwall mining A limitation of bord-and-pillar coal mining is that a significant quantity (up to 40%) of coal is left behind in the pillars that support the roof. Various methods of pillar extraction have been developed to remove these pillars so as to optimise coal recovery. Other mining methods, such as rib pillar extraction, shortwall mining and longwall mining (Figure 6-14) have been developed with the aim of directly recovering the maximum amount of coal.

Figure 6-14: Longwall mining The predominant method of longwall mining is the longwall retreat system (Figure 6-15). In retreat longwall mining, two sets of entries are driven between 100 to 250 m apart. When the entries have been driven a predetermined length, say two kilometres, they are connected and a rectangular longwall block is outlined. The longwall face is then installed and as mining continues into the panel, back to the original development, the entries are allowed to collapse behind the face line. Generally the main gate contains the belt conveyor and the pantechnicon for facilitating power and logistics to the longwall face. The main environmental concerns with longwall mining relate to the lack of roof support following mining. The impacts include subsidence of the surface, the cracking of the strata between the coal seam and the surface, and the subsequent dewatering of aquifers in this zone. However, the subsidence in this case is predictable both in time and extent. It is thus possible to design rehabilitation of the surface in advance, divert streams around areas that will subside, and provide alternative supplies to landowners dependent on near-surface aquifers for their water supply. Roads, houses and, indeed, water reservoirs have been successfully undermined using these methods.


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Figure 6-15: Retreat longwall mining The water from the overlying aquifers flows into the mine and, depending on its quality, can often be pumped out and discharged to surface streams. Where the water is naturally salty or becomes polluted in the mine, it can usually be used as mine service water and may involve a certain amount of treatment. The residual impacts may include all or some of the following: • A change in agricultural cropping practice in subsided areas if they become waterlogged; • An undulating topography, resulting from subsided land over the mining panels and non-

subsided land over the barrier pillars and roadways which are left between panels as shown in Figure 6-16. Theoretically it is possible to mine out this coal to allow the whole surface to settle evenly. However, there are many practical difficulties which are being investigated by the industry to minimize this residual impact and increase coal recovery;

• When this method is used to mine below a depth of about 60 m, the weight of the strata

above the coal seam may be sufficient to close the cracks in the strata overlying the seam and thus allow recharge of near-surface aquifers, enabling them to be used again. This is dependent on the geology (presence of faults, dykes, sills, etc.) but if the cracks do not reseal, a residual impact would occur.

6.3.4. Wits gold mining The methods used to mine the conglomerates of the Witwatersrand basin are varied and depend largely on the mining depth, reef geometry, reef dip, degree of folding and faulting, rock hardness and temperature gradients. The generally consistent nature of the ore bodies, and the continuity of the narrow, tabular reefs around a large proportion of the basin rim, has made it possible to optimise mining operations by standardising many of the procedures across the entire region. Mining takes place from the surface to depths of more than 4 000 m


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and several gold-mine lease areas include reefs whose reserves have been extended beyond this depth.

Figure 6-16: An example of undulating topography that could (Source: Wells et al., 1992) result following longwall mining

Capital-intensive mining has made the extraction of such deep ore possible in recent years, but there are many factors, including rock stability, degree of faulting and rock temperature that increase the costs of operation and place real limits on the depths of operation. Virgin-rock temperatures increase almost linearly with depth as a result of the heat which flows from the earth's interior. At surface, virgin-rock temperature is around 20 °C compared to 52 °C at 4 km depth in the Central Rand. The extreme hardness and abrasiveness of the quartz arenites and conglomerates severely restrict the cost-effective use of mechanical methods of rock breaking and place a finite life on rock handling and transportation equipment. The surface effects of deep gold mining are generally very little with effect on surface being almost negligible if the depth to surface is more than a few hundred metres. With the shallower mining occurring near the outcrops there may be limitations to development on the surface unless geotechnical stability can be guaranteed. A typical layout of a West Wits gold mine is shown in Figure 6-17. 6.4. Environmental impact and rehabilitation of mining operations The ultimate aim is to make certain that environmental impacts are minimal and to ensure that rehabilitation and closure of the mine site is carried out in such a manner that the effects of mining will not adversely affect the surrounding environment in the long term. Mining is generally different from other activities in that the impacts can last for centuries. The environmental impacts associated with surface mining are not only restricted to the surface infrastructure, but also the mine pit itself. With underground mining the environmental


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impacts are generally less on the surface. Underground operations can adversely affect groundwater conditions, could cause subsidence on surface and waste disposal sites are required. Several possible environmental impacts have been mentioned in the description of specific surface and underground mining methods above. The discussion below is a more generalised overview of the environmental impact and rehabilitation of mining operations. More detailed explanations are given in chapter 9. A number of environmental considerations have been taken into account in planning and designing the mineral processing plant, waste disposal area and other surface infrastructure, including: • Identification of the best location for the surface storage facilities, taking into account the

status of the land with respect to ownership, geology, archaeological features, flora and fauna;

• The design and construction of the embankments, considering short- and long-term

structural stability; • Ways that potentially significant adverse effects such as noise, dust, visual effects, acid

drainage and cyanide can be avoided, remedied and mitigated; • The surface and subsurface drainage systems required to ensure that potentially

contaminated water can be collected and managed; • The water management and water treatment facilities required to ensure that there are no

significant adverse effects on the surrounding rivers and streams; and • The requirements for rehabilitation and closure of the site. Where possible progressive rehabilitation of disturbed areas should be undertaken as this offers a number of advantages: • Improving the visual appearance of the disturbed areas; • Establishing a cover to provide erosion control; • Improving runoff water quality by minimising silt loads; and • Controlling dust. While some disturbed areas can be rehabilitated on a progressive basis while the mine is operating, some areas cannot be rehabilitated until mining is complete. For this reason, although a significant amount of progressive rehabilitation will have been completed by the time the mine closes, some rehabilitation will need to be carried out after that time.


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Figure 6-17: Idealised layout of a typical West Wits gold mine (Source: Whiteside et. al., 1976)

A – Dolomite B – Black Reef C – Ventersdorp Lava D – Ventersdorp Contact Reef E – Kimberley Shale F – Main Reef G – Carbon Leader H – Jeppestown Quartzite 1 – Reduction plant 2 – Head gear 3 – Waste dump 4 – Stoped-out areas with mat packs 5 – Ventilation shaft 6 – Footwall cross-cut with box holes 7 – Cross-cut to reef 8 – Stope box holes 9 – Footwall haulage 10 – Cross-cut to VCR 11 – Shaft station 12 – Main vertical shaft 13 – Cross-cut to Carbon Leader 14 – Cross-cut to VCR 15 – Cross-cut to VCR 16 – Settlers and water pumps 17 – Subvertical hoist chamber 18 – Subvertical shaft 19 – Ore passes 20 – Cross-cut to Carbon Leader 21 – Cross-cut to Carbon Leader 22 – Raise to Carbon Leader


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Remedial initiatives to minimise environmental impact during and after mining include: • Containment and treatment of contaminated water; • Proper storage and removal of hazardous materials; • Removal of surface infrastructure (buildings, plant, etc.); • Earthworks and contouring the mine area to as close as possible to the original. This

includes filling pits, trenched and small excavations; making pit sides safe and covering surface area with subsoil and topsoil as necessary; and

• Revegetation of the pit slopes, slimes dams and waste rock dumps. 6.5. Mining Waste Management Plans The mining operations create different types of mining waste materials, depending on their mining methods, as well as other waste products associated with generic mining related activities. Each waste product requires a unique strategy and disposal facilities suitable for the classification of that waste. The management of waste generated and the disposal of this waste is regulated under the following legislation: • The Environment Conservation Act, 1989 (Act No. 73 of 1989 and its amendments); • The National Water Act, 1998 (Act No. 36 of 1998); • The National Environmental Management Act, 1998 (Act No. 107 of 1998); • The Minerals and Petroleum Resource Development Act, 2002 (Act No. 28 of 2002); • The White Paper on Integrated Pollution Control and Waste Management; • Hazardous Substances Act, 1973 (Act No. 15 of 1973); and • Health Act, 1977 (Act No. 63 of 1977). 6.5.1. General Waste Management The practice of mining generates wastes, residues, polluted waters and air emissions. The waste and residues from various mines falls into a number of categories, and these can include: • Waste that is collected within the settling, slimes, slurry dams; • Tailings produced from the ore processing; • Waste rock from the mining process;


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• Overburden, cover, and / or “soft” material; • Other non specification waste such as discard, parting (coal) and sub-economic lower

grade ore; • Industrial waste (i.e. including hazardous wastes and oils and greases); • Domestic waste (i.e. waste that are generated from the plant offices and laboratories); • Waste water (i.e. including process water and water from sanitation processes, as well as

sewage sludge); and • Air emissions, dust, particulate matter and even odour.

Regardless of the mineral being mined, when reviewing a waste management plan, authorities should ensure that they can extract the following data from the waste management plan: • What is the source of the mine waste? • What are the volumes of the mine waste? • What is the quality and hazardous nature of the mine waste? • How long does the waste remain on site? If it remains on site for longer than three

months, it is necessary to have the storage facility licensed with the DWAF; • Can, and is any of, this waste being recycled? If so, how? • Are there Material Safety Data Sheets (MSDS) for products requiring these, stored on the

site? • Where will the safe disposal certificates be kept on site? • How will each of the mine residue sites and waste storage facilities be managed?

Did you know? There is also the potential waste in the form of radioactivity as there are background levels of radiation that naturally occur in minerals such as coal, coal ash and granite. DME are developing a policy for the management of all radioactive wastes that originate in the mining sector as well as the associated management measures, however, this is not yet available.

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Although it is not possible to provide all information required for the management of all waste products generated by the mining industry, a basic outline of the management of mine related waste is given in this section. For the purpose of this manual, waste management guidelines have been provided for mine residue deposits, solid waste, liquid waste and air emissions. 6.5.2. Mine Residue Deposits Mine Residue Deposits (MRD’s) are legislated in terms of the Regulations 527 of the Minerals and Petroleum Resources Development Act, Act No. 28 of 2002 (MPRDA). These regulations have been based upon the SABS Code of Practice for Mine Residue and the Chamber of Mines Guidelines for Environmental Protection. In order to ensure that a waste management plan covers all aspects applicable to the management of mine residue, an example of a checklist has been generated which the authorities can use to evaluate Mine Residue Waste Management Plans. Appendix 6.1 provides the checklist. When utilising this checklist, it is important to note that many of the aspects will also consider the safety of the residue deposit. Although this report focuses on Best Practice for environmental management, ensuring the safety of a mine residue facility will often reduce the potential for environmental impacts. Therefore, it is important to ensure safety requirements are outlined in a waste management plan for mine residue deposits. 6.5.3. Solid Waste In order to ensure that a waste management plan covers all aspects applicable to the management of solid waste, an example of a checklist has been generated which the authorities can use to evaluate Solid Waste Management Plans (Appendix 6.2). An outline applicable to the management of typical / generic solid waste products generated by mining industries has been provided as Appendix 6.3. 6.5.4. Liquid wastes In order to ensure that a waste management plan covers all aspects applicable to the management of liquid waste, an outline applicable to the management of typical / generic liquid waste products generated by mining industries, has been provided in Appendix 6.4. 6.5.5. Air emissions The management of air quality in South Africa is legislated under the National Environmental Management: Air Quality Act, Act No. 39 of 2004 (NEM:AQA), with the applicable South African National Standards (SANS) for common air pollutants and monitoring guidelines being published in SANS 1929:2004. Section 32(b) of the NEM:AQA states that the minister may prescribe steps to be undertaken to prevent nuisance dust. Although the minister has not yet prescribed these steps, the spirit of the legislation indicates that dust emissions are not desirable and must be controlled.

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