Surface Mining

Slide 1

Day 3: Mining

Methods

Part I-Surface

mining1Mining MethodsSurface mining and underground miningThe mining cycleEquipmentIs bigger better?Grade control2Why do we mine minerals?3Anatomy of a mine: Grasberg, West PapuaFigure from Spitz andProTfr. uDrd. Hin.Zg.eHra,rr2a0z P0r9esentationMining Methods, Surface mining4Mineral extraction: from mining to metalMineralConcentrate:Concentration of ore minerals: removes the ore mineral from the waste.MetalMiningSmelting: removes themetal from the ore mineral by a variety of waysFigure from Spitz and Trudinger, 2009Grinding of ore to allow separation of the ore minerals5Mining Methods, Surface miningMining methods:Surface miningMechanical excavation methodsOpen-pit (or Open-cut or Open-cast or quarry) mining.Terrace mining.Strip (flat terrain) miningContour strip (hilly terrain) miningAqueous methodsPlacer mining.In-situ leaching (ISL)/ solution mining .Underground miningUnsupported Mining Methods:Room and Pillar mining.Stope and pillar mining.Shrinkage Stoping.Supported Mining Methods:Cut and fill mining.Stull stoping.Caving Mining (or Bulk) methods:Longwall stoping.b) Vertical crater retreat.Sublevel caving.Block caving.6Used for laterally extensive deposits.Overburden cast directly back into mined out panels.Rehabilitation keeps pace with mining.Used for soluble ores: uranium, salt, potash. Minimal waste production: only water wastes, no solids.

Reduced waste rock production. Poor safety record.Schematic of common mining methodsSimple in concept, highly engineered for efficiency. Very high waste rock volume.Better safety record.Figure from Spitz and Trudinger, 20097The choice of mining method depends on many factors, including:Shape of the orebody: tabular, cylindrical, spherical.Orientation of the orebody: sub-horizontal, sub-vertical.Continuity of the orebody.Size of the ore body.Distribution of ore-bearing minerals within the orebody: massive ordisseminated (with a cut-off grade).Depth to the orebody.Depth of overburden.Strength of the orebody and overburden/host-rocks rocks.Area of land available for waste disposal open-pit mines cover a largersurface area and generate a greater volume of wastes.Impacts on surface: environnemental, surface drainage and sub- surface aquifers, land-use changes, social.Rehabilitation concerns.Projected production rates.Capital costs, rate of (financial recovery), cash-flow.Safety concerns: surface mining methods have a better safety record.Choice of mining method:What determines the type of mining?8Traditionalminingmethodsfallintotwobroadcategories based on locale: surface or underground.Surface mining: includesMechanical excavation methods {such as Open-pit (or Open-cut or Open-cast); Terrace; and Strip mining}.Aqueous methods {such as placer and solution mining}.Underground mining: is usually classified in three categories of methods: unsupported, supported, and caving.9Figure shows morphology of surface(Open Pit) and Underground methods.10Comparison of waste production for surface and underground mining:Data are for USA in 1997 (from Hartman and Mutmansky, 2002), in million tons.Pit excavation initially generates huge volumes of waste rock that must be removed to allow access the orebody, and to allow stable pit slopes to be developed.Underground miningWaste = 7% of total rock tonnage extracted 9% of ore tonnage extractedSurface miningWaste = 73% of total rock tonnage extracted266% of ore tonnage extractedSurfaceUndergroundAll MiningCommodityOreWasteTotalOreWasteTotalOreWasteTotal(million tons)Metals1,29018633,153643671,3541,8663,220Nonmetals27784493,22712301232,9014493,350Coal6691030310,972421454661,09010,34811,438Total4,73712,61517,352608486565,34512,66318,008Processes and ConsiderationsSurfaceUndergrounddisturbs large areadisturbs muchsmaller surface

produces large amounts of spoil relatively safecheapermore efficient

areaspoil often left inminedangerousexpensiveless efficientUS Mining Trends13Surface mining:This is the traditional cone-shaped excavation (although it can be any shape, depending on the size and shape of the orebody) that is used when the ore body is typically pipe-shaped, vein-type, steeply dipping stratified or irregular.Although it is most often associated with metallic orebodies, (e.g., Palabora copper, Mamatwan and Sishen iron-ore), it can be used for any deposit that suits the geometry most typically diamond pipes (e.g., Venetia, Koffiefontein and Finsch).Surface mining is the predominant exploitation method worldwide.In the USA, surface mining contributes about 85% of all minerals exploitation (excluding petroleum and natural gas). Almost all metallic ore (98%) and non-metallic ore (97%), and 61% of the coal is mined using surface methods in the USA (Hartman and Mutmansky, 2002).Surface mining requires large capital investment(primarily expensive transportation equipment), but generally results in:High productivity (i.e., high output rate of ore).Low operating costs.Safer working conditions and a better safety record than underground mining.14Controls of Gold Mineralization - LCSSurface Mining methods1516Strip out overburden (becomes spoils)

Traditional surface mining methods fall into two broad categories based on locale:Surface mining: includesMechanical excavation methods {such as Open-pit (or Open-cut or Open-cast); Terrace; and Strip mining}.Aqueous methods {such as placer and In-situ leaching (ISL)/ solution mining }.

Clean up (reclamation)Steps of Surface MiningOperation171. Mechanical Extraction Methoda thick deposit is generally mined in benches or steps,although thin deposits may require only a single bench orface.

Of all the variations of mechanical surface excavation mining methods available, the three most common methods only will be described here, namely:Open-pit (or Open-cut or Open-cast or quarry) mining.Terrace mining.Strip (flat terrain) mining.Contour strip (hilly terrain) mining.Mine working open to the surface.Operation designed to extract minerals that lie close to the surfaceIt is used when the orebody is near the surface and littleoverburden (waste rock) needs to be removed.It is usually employed to exploit a near-surface deposit or one that has a low stripping ratio.Waste is first removed, then the ore is broken and loaded.Generally low grade, shallow ore bodies.Non-selective all high and low grade zones minedMining rate > 20,000 tons mined per day (tpd).It often necessitates a large capital investment but generally results in high productivity, low operating cost, and good safety conditions.Design issues: Stripping overburden Location of haul roads Equipment size of trucks and fleet Pit slope angle and stabilitySurface Mining methods (Open pit Mining method)1.1. Open pit Mining method18Open-pit mine: Chuquicamata copper mine, Regin de Antofagasta, ChileLocality: Regin de Antofagasta, Chile.Pit dimensions: 4.3 km long x 3 km wide x 850 m deep.Mining dates: 1915 -presentTotal production: 29 million tons of copper to the end of 2007 (excluding Radomiro Toi production). For many years it was the mine with the largest annual production in the world, but was recently overtaken by Minera Escondida (Chile). It remains the mine with the largest total cumulative production.Production 2007: 896,308 fine metric tons of copper (Codelco, 2007).Mining cost in 2007: 48.5 US per kg (2006), 73.0 US per kg (2007) (Codelco, 2007).Employees: 8,420 as of 31st 2007 (Codelco, 2007).Pre-tax profits: US$ 9.215 billion (2006), US$ 8,451 billion (2007) (Codelco, 2007).DustSlope failureBenchesAccess rampshttp://upload.wikimedia.org/wikipedia/commons/2/2a/Chuquicamata_panorama.jpg1920Overburden Removal21A Dragline ShovelLoading ore in pit22Some photos and machinery used in open-pit miningMining Trucks*To the left is a photograph of a Liebherr 360 ton (327 metric ton) haul truck. This unit is powered by a 2750 horse power engine and weighs 443,000 pounds (177 tons) empty...Crushing in pitDrilling in pit23Figure showing typical open-pit bench terminologyPitBenchBeamFloorWeightWeightAngleWidthWidthOverall slopeSlopeSlopecrestIntervalTop2624Figure 2.8 Open-pit mining sequence (for pipe-like orebody)2725Various open-pit and orebody configurationsFlat lying seam or bed, flat terrain (Example platinum reefs, coal).

Massive deposit, flat terrain (Example iron-ore or sulphide deposits).

Dipping seam or bed, flat terrain(Example anthracite).Massive deposit, high relief (Example copper sulphide).

Thick bedded deposits, little overburden, flat terrain (Example iron ore, coal).Figure from Hartman and Mutmansky, 2002.28261.2. Open Pit Stabilityi) Pit SlopesPit slopes are cut into benches to aid stability and contain any slope failures.Rock most be stronger than sand so the angle of repose can be larger.45 is usually the maximum slope.Pit slopes are benched.

The revenue from ore must pay for the cost of excavating waste from the pushback and for excavating the ore.The slope cannot exceed 45 and remain stable, so at some point it becomes impossible and/or uneconomic to continue mining.

i)Pit slopePit wall stabilityRock strengthPit DepthPit diameterWater DamageStrip Ratio (SR)27ii) Pit Wall StabilityMost orebodies are related to faulting in the earth's crust.Fault generates stresses in the host rock, rupturing it and causing faults in the rock (Figure 2).Faults are typically long linear features so that if a circular pit is used to mine an orebody (Figure 3), it is likely to intersect a fault at two points, which leads to instability in at least two parts of the pit slope.Figure 2Figure 3StableInstable:Underlying fracture or faultMagmao landslide28Figure 4 shows a landslide that occurred following rain storms. A berm was created at the base of the slide to protect the main haul road.Figure 5 shows a major instability. The likely cause is an underlying fracture or fault. The mine wishes to do a major pushback on this pit wall in order to gain access to more ore. This could be a challenging task.29iii) Water DamagePit most keep dryDewatering also helps to keep the slopes dry and more stable.In order to keep the pit dry, There are 40 dewatering pumps around the Cortez pit pumping water out of the ground at a total rate of 30,000 gallons per minute (Figures 6 and 7).Prof. Dr. H.Z. Harraz Presentation Mining Methods, Surface mining3414 November 201130On October 9, 2003, a major landslide occurred, causing perhaps eight fatalities at the Grasberg Mine, Indonesia (Figures 8 and 9).What happens when water accumulates?The accident was related to heavy rainfall and accumulation of water in the soil layer at the top of the pit.35Mining Methods, Surface mining14 November 2011Prof. Dr. H.Z. Harraz Presentation31A slope failure occurred at the Cleo Open Pit (Sunrise Dam Gold Mine, Western Australia) in December 2000. At the time of failure the pit-floor was at 100 m depth below surface.

Two critical factors played a role in the failure:The top of the water table is at a very high level: only 30 m below surfaceA strong layer of younger clay sediments overlies weaker weathered bedrock.The failure is thought to be due to very high pore fluid pressures in the weathered bedrock that created an instability at the interface between the bedrock and the overlying clays, allowing a slippage to occur (Speight, 2002).Open-pit slope failure case study groundwater problemsSeepage and mineral precipitationFigures modified from Speight, 326002.Mining Methods, Surface mining14 November 2011Prof. Dr. H.Z. Harraz Presentation32Examples of Open Pit Mining MethodHighland Valley Pit, British Columbia

Porphyry copper137,000 tons mined/day (tpd)296 Mt reserves: 0.42% Cu 0.008% MoCu, Mo concentrates with gold and silver

By-product331.3. Strip MiningStrip mining is ideally applied where the surface of the ground and the ore body itself are relatively horizontal and not too deep under the surface, and a wide area is available to be mined in a series of strips.Typical examples of this type of mining are the larger tonnage coal mining operations inMpumulanga.

Favourable conditions are:Relatively thin overburden (0-50m maximum otherwise stripping ration and cost of stripping becomes too high)Regular and constant surface topography and coal layers (not more than 20 variation from horizontal on the coal seam topography can vary more since pre- stripping can be used to level it but this is expensive to apply)Extensive area of reserves (to give adequate life of mine (LOM) and to cover all capital loan repayments typically more than 20 years life at 4-14mt per annum production)34Area Strip Mine (Coal)35Figure shows Strip mining with dragline (on overburden) and rope shovel (below, loading coal)36Highwalls1.4. Contour strip (hilly terrain) mining or Contour (Bench) Strip MiningProf. Dr. H.Z. Harraz Presentation4837Drill machines (rotary/percussive)|SMS, emulsion, Primer, Nonel, etc|Shovels, Draglines, etc|Front-end loader, etc|Dumpers, Conveyors, etc|Coal washeriesMining ProcessDrilling| Blasting|Loading| Hauling| Transporting|Processing/WashingOPEN CASTDRAG LINESURFACE MINERIN-PIT CRUSHING & CONVEYING38Selection of Mining EquipmentStripping Ratio in case of Opencast.Life of the mine.Infrastructure available.Proposed annual output.Technology available.39