Mining Engineering Lecture Notes 1 PDF

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MINING METHODS, MINE DESIGN AND UNIT OPERATIONSSURFACE MININGProf. S. Durucan Department of Earth Science and Engineering Royal School of Mines Imperial College London1 INTRODUCTIONMining is about extracting valuable rock material (mineral) out of the ground and marketing it at a profit. An economic...

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MINING METHODS, MINE DESIGN AND UNIT OPERATIONS SURFACE MINING Prof. S. Durucan Department of Earth Science and Engineering Royal School of Mines Imperial College London

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INTRODUCTION

Mining is about extracting valuable rock material (mineral) out of the ground and marketing it at a profit. An economically mineable mineral is referred to as ore and the measure used to distinguish ore from waste is the cut-off grade, commonly expressed in terms of metal content (e.g. gold in grams per ton or copper in % by weight). In most cases, ore is not sold directly by the mine to its customers. To save transport cost it is usually processed to remove as much waste material as possible. This processing involves crushing and milling, followed by separation of ore mineral from waste minerals. Open-pit, open cast or open cut mines and quarrying, all refer to surface mining operations which are mine workings or excavations open to the surface. Many open pits are started along an outcrop and are continued downward until the thickness of overburden or waste side rock prevents further economic exploitation, Figure 1. Underground mining involves the exploitation of deep seated ore bodies which are reached by access routes (shafts, drifts, etc.) and are extracted using various stoping techniques.

Figure 1.

A typical pit cross section - Phases I, II and III are designed using a decreasing cutoff grade thus increasing the volume of ore mined economically (Modified after Bohnet, 1990).

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1.1 The Choice between Surface and Underground Mining The choice between surface and underground mining methods is usually based on economical, geological, geotechnical and environmental factors. From economical and mine design point of view, the controlling factors that affect the choice of mining method between surface and underground operations are mining cost and ore recovery and dilution. Where a deposit is accessible to surface mining it is often the case of determining the “cut-off limit” when surface operations are no longer economic and underground methods must be adopted. In surface mining, mining cost includes the cost of removing the waste overburden and waste side rock in the slopes of the pit. The ratio of the number of tonnes of waste that must be moved per tonne of ore mined is termed the Stripping Ratio (SR):

SR =

Tonnes of waste removed Tonnes of ore mined

The break-even stripping ratio (BESR) or the economic limit for surface operations in comparison with underground mining is therefore determined as:

BESR =

Underground mining cost / tonne ore - Surface mining cost / tonne ore Waste removal cost / tonne of waste in surface mining

For example, if an underground mining cost of £4.45 per tonne of ore is assumed for a particular orebody and if it was further assumed that the surface mining costs were £0.75 per tonne for the ore removal and £0.75 per tonne of waste removal, then;

𝐵𝐸𝑆𝑅 =

£4.45 − £0.75 = 4.93 £0.75

The above expression is a simple determination of the viability of surface mining operations. In reality, the actual calculation involves other factors such as the grade of the ore and determination of the cut-off grade; mineral types; direct operating expenses associated with mining, processing and smelting; initial and replacement capital costs of plant; taxes and royalties; market and capital constraints; environmental considerations. It may well be that the viability of the surface mining operation is determined not by the break even stripping ratio, but by the grade of the ore. Large, shallow ore bodies with low stripping ratios are generally more economical when mined by surface methods. Deep orebodies with high ratios, on the other hand, are usually mined by underground methods. Plates 1 and 2 show a general view of the open pit and a detailed bench layout at Bingham Canyon copper mine, Utah, respectively.

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Plate 1.

Plate 2.

A general view of the open pit at Bingham Canyon copper mine, Utah.

A detailed bench layout at Bingham Canyon open pit mine, Utah.

The break even stripping ratios for hard rock mines tend to be low, and higher for soft rocks, especially where direct overcasting of waste can be done by the excavator. The lower stripping

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ratios for metalliferous ore deposits by comparison to the mining of coal seams are due to the difference in density of coal and ore, as the more difficult conditions in the exploitation of hard rock deposits (drilling, blasting and transportation of waste over considerable distances). The nature and extent of mechanisation have a considerable bearing on the break even stripping ratio.

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SURFACE MINING

Ease of large scale mechanisation for massive production with enhanced safety and economy has led to wide acceptance for surface mining throughout the world wherever the geological, geotechnical, mining, economical and environmental conditions have been suitable. It is generally considered that surface mining is more advantageous than underground mining in recovery, grade control, flexibility of operation, safety and working environment. There are, however, many deposits that are too small, irregular, steeply dipping, and deeply buried to be extracted by surface methods. Furthermore, even where mineralisation extends to a greater depth in surface mines, the rapidly increasing volume of overburden to be removed imposes economic limits beyond which mining must be abandoned or converted from surface to underground mining. Factors favouring surface mining can be listed as:             

Higher productivity Greater concentration of all operations and simplified management of men and machines Greater output per mine Lower capital cost per annual tonne mined Lower operating cot per tonne Possibility of moving a higher ratio of waste to mineral and exploitation of lower grade reserves Greater geological certainty Less limitation on size and weight of machines Simpler auxiliary operation and services Increased recovery of mineral and less dilution Greater reserves available for mining Simplified planning and control Increased safety

On the other hand, large open pit mines require large areas of land for mining and spoil heaps which are lost, at least temporarily, to agriculture. Increased public concern and environmental constraints are resulting in the reduction or elimination of surface mining operations in many parts of the globe. The main complaints of the public relate to the scars left from strip mining, open pits, quarries and sand and gravel pits, accumulation of rejected waste, pollution of streams and groundwater by acids and solids, and the atmosphere by dust.

Increased

environmental awareness and legislation may mean that the extraction of mineral deposits in

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(or near) areas of natural beauty, national parks and urban settlements by surface mining methods is unlikely to be acceptable. Access to these deposits by underground mining methods or from operating underground mines near the deposit may become the preferred option.

2.1. Mineral Deposits for Surface Mining Many factors control the type, shape and the size of a surface mine. The key items that affect the pit design and shape of the mine are: geology, grade and localisation of the mineralisation, extent of the deposit, topography, production rates, bench height, pit slopes, stripping ratio, and cut-off grade. Deposits suitable for surface mining can be classified as shown in Figure 2.

Figure 2.

Classification of deposits for surface mining.

Pit slope is usually expressed in degrees from the horizontal plane and it is one of the major elements affecting the size and shape of the pit. To minimise the overall stripping ratio, the slope should be as steep as possible and yet still remain stable, see Figure 1. Rock strength, geological features such as joints and faults, presence of water are key factors that should be considered in a geotechnical study in order to determine the ultimate pit slope. Bench height is the vertical distance between each horizontal level of the pit (please refer to Figure 1 and Plate 2). The shape of a mineral deposit fundamentally determines the shape of the mine, and the physical characteristics of the mineral determine the choice of machines used. The major factors affecting the layout are as follows:

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1. the shape and depth of the deposit 2. the properties of the mineral and overburden a. slope angle limitations b. degree of hardness and abrasiveness c. variations in grade d. bearing capacity 3. the geometry of the excavating machinery, particularly dgging height or depth, dumping height, and reach 4. the drainage requirements

2.2 Surface Mining Operations Once it is decided to mine a deposit by surface mining methods, a detailed planning of the various mining operations are carried out for the removal of overburden and the exposed ore or material. The surface mining operations can broadly be classified as: 1. 2. 3. 4. 5.

Ground Preparation Drilling and Blasting Excavation and Loading Transportation Storage and Reclamation

2.2.1. Ground Preparation The ground preparation include necessary civil engineering works like development of approach roads, construction of site offices and other infrastructure facilities for the mining operation to commence.

2.2.2. Drilling and Blasting For all consolidated and semi-consolidated type of deposits overburden drilling and blasting comprises the primary unit operation of surface mining. Drilling of blast holes is done by either of the following methods: 1. 2. 3. 4.

Rotary drilling Percussive drilling Rotary-percussive drilling Down-the-hole drilling

Selection of the type of drilling system for a particular site is made considering a large number of factors such as: 1. 2. 3. 4. 5. 6.

rock properties such as hardness, abrasivity, toughness etc. diameter of the blast hole depth of blast hole speed of drilling cost of drilling type of explosives used

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The present trend of drilling is towards large diameter rotary drilling. For very hard rocks, however, down-the-hole drills with adequate tungsten-carbide-button insert bits are preferred. Plates 3 and 4 show blast hole drilling and charging operations and a blast at Palabora open pit copper mine, South Africa respectively

Plate 3. Blast hole drilling and charging operations at Palabora copper mine, South Africa.

Plate 4. Blast at Palabora copper mine, South Africa.

2.2.3. Excavation and Loading The blasted or un-consolidated rock mass is excavated by different excavators selected on the basis of geotechnical requirements of a specific site. The type of excavators selected depends on the system of mining. The following three systems are currently practised:

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1. Cyclic or intermittent In this system the primary mining operation is carried out by either of the following machines or a combination of them: a. b. c. d.

Rope-shovel Hydraulic excavator, Front end loader, Dragline.

The single bucket excavator deployed in this system works in the following sequence: a. b. c. d. e.

Plate 5.

Crowding to the face Hoisting Swinging Loading to the transport machinery Swinging back

A rope shovel loading a dump truck at Palabora open pit copper mine, South Africa.

This system being highly flexible and easy to deploy with limited skilled labour finds wide application. Plate 5 shows a rope shovel loading a dump truck at Palabora open pit copper mine, South Africa. Front end loaders are used for short- distance hauling of the excavated material by the excavator itself. The system deploying a dragline eliminates deployment of transporting machines as this machine is capable of sidecasting of the material dragged into its large bucket (Plate 6).

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Plate 6.

Plate 7.

A dragline operating at the Optimum coal mine, South Africa.

Krupp bucket wheel excavator with a handling rate of 240,000 m3/day.

2. Continuous Wherever continuous surface mining is found geotechnically feasible, the following equipment can be deployed in conjunction with conveyor belt transport system: a. b. c.

Bucket- wheel excavator Bucket-chain excavator Continuous surface miner

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Bucket wheel excavator has found extensive use in lignite, coal and oil-shale mining. Such mining systems have very high productivity. Due to large capital investment required, these systems are suitable for large deposits where mining life is comparatively longer (Plate 7). Continuous surface miner is comparatively a new system in surface mining and is suitable for thin seam mining deposits which are not highly consolidated and can be mined without blasting. 3. Semi-continuous with in-pit crushers This system tries to blend the advantages of continuous transportation with cyclic excavation. For overburden and minerals like limestone where deployment of continuous transportation by conveyor belt requires reduction of lump sizes of the excavated material, use of a suitable crusher to interface between the excavator and conveyor is necessary. This is achieved by the use of following types of crushing systems: a. in pit fixed crushers b. portable crushers c. mobile crushers In pit fixed crushers are constructed well inside the pit thereby reducing the hauling distance between the face and crushers (Plate 8). Portable crushers are periodically advanced nearer to the advancing face. A mobile crusher, on the other hand, is a mobile machinery housing the required crusher and operates near the face and in conjunction with the face excavator.

Plate 8.

In-pit fixed crusher at the Escondida copper mine, Chile.

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Plate 9.

In-pit fixed crusher and belt conveyor transport at the Chuqicamata copper mine, Chile.

2.2.4. Transportation About 75% of operations in mining involves transportation. In surface mining this is mainly the transportation of bulk solids whereas underground mining involves transportation of man, material and bulk solids. Mine transport systems can mainly be classified as follows: 1. Continuous transportation Continuous transportation practised in mining include the following systems: a. Belt conveyor i. trough belt conveyor, ii. cable belt conveyor, iii. high angle conveyor, iv. cross-pit conveyor, v. pipe belt conveyor, vi. downhill conveyor a. Chain conveyor b. Slurry transportation c. Pneumatic conveying Of these, belt conveyors are very widely used as seen in Plate 10, where ore is transported by belt conveyors for several kilometres at El Abra copper mine in Chile.

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Plate 10. Belt conveyor transport system at El Abra copper mine, Chile.

Conveyor belts over 2,400 mm width with a troughing angle of 40 degrees are extensively used in conjunction with bucket wheel excavators. To eliminate the necessity of frequent shifting of the conveyor belt towards the fast advancing face mobile-transfer conveyors are used between bucket wheel excavators and the pit conveyor. The conveyor system in such a continuous mining system includes the following: a. b. c. d. e.

Bucket Wheel Excavator Mobile transfer conveyor Transfer feeder Series of conveyors Telescopic conveyor or swing conveyor (in multi bench system facilitation change of material transport route) f. Tripper g. Spreader

Plate 11. Bucket Wheel Excavator, beltwagon and a cross-pit spreader working together.

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Slurry transportation proves to be economic in many a long distance situations with difficult terrain conditions. There are a number of successful long distance coal and iron ore slurry transportation systems. This system comprises of: a. b. c. d. e.

Crushing and grinding of material Slurry preparation Slurry pumping stations Dewatering of slurry Pelletisation (optional for ores)

2. Intermittent transportation In the intermittent transportation system the transporting machinery collects the material from a loader and transports it to the delivery point and returns for the next trip. The main systems deployed are: a. Dump trucks a. Trolley assisted dump truck b. Rear discharge dump truck c. Bottom discharge dump truck b. Scraper c. Aerial Ropeways Dump trucks are a widely used means of transportation in surface mines. Off highway trucks are available in wide range and design. Decision on truck capacity requires considerable work prior to selection as their use must be compatible with the excavator used and the fleet size need to be optimised. The limitation of grade negotiation makes dump truck unsuitable for higher gradients and necessitate longer travel time in deeper mines. Trolley assisted systems where the dump truck is assisted by electric power on the slope is implemented in a number of mines. In spite of higher fuel costs, the large capacity dump trucks are popular because of their high flexibility. Scrapers are mainly used for removal of loose overburden or topsoil. They can be assisted by a dozer or two scrapers may work in tandem. Aerial ropeways found use in long distance transportation over difficult terrain. But because of low productivity in newer mining operations they are not selected.

2.2.5

Storage and Reclamation

The transportation system in a surface mine handles two types of material; overburden and ore. The overburden is transported to the dumping area and the ore or coal is transported for the next phase of its use. In most of the cases the mine possess a temporary storage yard to stock buffers and over productions. From this storage system it is reclaimed and dispatched to the concentrator, coal handling plant or the washer. The overburden transported by dump trucks are brought up to the edge of the dump yard and discharged. The dump yard reclamation works involve levelling by dozers. In case of

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continuous transportation by conveyor belts the material is transversely transported to a spreader by means of a tripper. The discharge boom of spreader discharges the overburden to the dump yard which is than levelled by dozers. The stock-piles of coal or ore are constructed by the dumper or conveyor in a similar way but more systematically considering the requirement for blending or quality control. The stockpiles are reclaimed by use of proper reclaimer like scraper reclaimer or bucket wheel reclaimer and then fed to a conveyor system to transport it to the mill or washer.

2.3. Surface Mining Methods The main mining methods may be briefly classified as follows: 1) 2) 3) 4)

Open Cast (Strip) Mining Terrace Mining Open Pit Mining Dredging

2.3.1. Open Cast (Strip) Mining There are two main categories of strip mining: Contour mining - usually employed in hilly terrain where a steep slope limits the width of the area that can be economically mined from the mineral out crop. A bench is excavated in the mountain at the coal seam elevation, allowing for room for the mining equipment and facilities. Drills are used to drill the overburden and place explosives in the holes. They are systematically detonated, blasting the rock where it can be loaded using shovels and loaders and transported to a fill area. Much of the rock will later be used to backfill the mined area.

Figure 3. Contour mining scheme.

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Once the overburden is removed, the coal is mined, in the same method. When the coal is depleated by contour mining, the operation is often followed by augering the exposed mineral bed (particularly coal). Then the holes are filled, the contour bench is reclaimed.

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