Surface Mining

Topic 6: Mining Methods Part II: Surface Mining-Planning and Design of Open Pit Mining

Hassan Z. Harraz [email protected] 2015- 2016

This material is intended for use in lectures, presentations and as handouts to students, and is provided in Power point format so as to allow customization for the individual needs of course instructors. Permission of the author and publisher is required for any other usage. Please see [email protected] for contact details.

Prof. Dr. H.Z. Harraz Presentation

Outline of Topic 6: 1)

Definition of Open pit Mining Parameters: 1.1.) Basic Concept 1.2) Open pit Mining method 1.3) Bench 1.4) Open Pit Bench Terminology 1.5) Bench height 1.6) Cutoff grade 1.7) Open Pit Stability: i) Pit slope ii) Pit wall stability iii) Rock strength iv) Pit Depth v) Pit diameter vi) Water Damage vii) Strip Ratio (SR) 1.8) Open-pit mining sequence 1.9) Various open-pit and orebody configurations

2) Limit or Ultimate Pit Definition: 2.1) Introduction 2.2) Manual Design 2.3) Computer Methods 2.4) Learchs-Grossman method 2.5) Floating cone method 3) Open pit Optimization: 3.1) The management of pit optimization 3.2) A simple example 3.3) The effects of scheduling on the optimal outline 4) Optimum production scheduling 5) Materials handling Ex-Mine 6) Waste disposal: 6.1) Dump design 6.2 ) Stability of mine waste dumps 6.3) Mine reclamation  Example of Open Pit Mining Methods

We will explore all of the above in Topic 6.

February 2, 2016


1) Definition of Open Pit mining parameters

February 2,


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1.1) Basic Concept  Although the basic concept of an open pit is quite simple, the planning required to develop a large deposit for surface mining is a very complex and costly undertaking.  At one mine, it may be desirable to plan for blending variations in the ore so as to maintain, as nearly as possible, a uniform feed to the mill.  At another operation it may be desirable to completely separate two kinds of ore, as for example, a low- grade deposit where one kind of "oxide" ore must be treated by acid leach, but a second kind of "sulfide" ore must be treated by different methods.  The grade and tonnage of material available will determine how much waste rock can be stripped, and there is often an ultimate limit to the pit that is determined more by the economics of removing overburden than a sudden change in the ore deposit from mineral to non-mineral bearing material.  The ultimate pit limit and the slope of the pit walls are therefore determined as much by economics and engineering as by geological structure. Material that is relatively high grade may be left unmined in some awkward spot extending back too deeply beneath waste  The typical large open pit mining operation that has been in production for 10 years and more is operating under conditions that could not possibly have been foreseen by the original planners of the mine.  Metal prices, machinery, and milling methods are constantly changing so that the larger operations must be periodically reevaluated, and several have been completely redeveloped from time to time as entirely different kinds of mining and milling operations.

February 2, 2016


 Sometimes the preliminary stripping of the waste overburden is contracted to firms specializing in earthmoving. Mining is usually done by track-mounted electric shovels in the large operations, and by rubber-tired diesel front-end loaders in the smaller operations. Scrapers are sometimes used in special situations.  Large bucket-wheel excavators of the kind used in European coal mines have not been applied to metal mining, because this equipment is best adapted to softer bedded, relatively flat-lying strata.   

Many factors govern the size and shape of an open pit. These must be properly understood and used in the planning of any open pit operation. The following are the key items affecting the pit design: 1) Topography, 2) Geology, 3) Grade, 4) Localization of the mineralization, 5) Extent of the deposit, 6) Property boundaries, 7) Production rates, 8) Road grades, 9) Mining costs, 10) Processing costs, 11) Metal recovery, 12) Marketing considerations, 13) Bench height, 14) Pit slopes, 15) Cutoff grade, 16) Strip Ratios (SR).

2 February 2016

Prof. Dr. H.Z. Harraz Presentation Mining Methods, Surface mining

1.2) Open pit Mining method  Mine working open to the surface.  It is usually employed to exploit a nearsurface deposit or one that has a low stripping ratio.  Operation designed to extract mineral deposits that lie close to the surface.  It is used when the orebody is near the surface and little overburden (waste rock) needs to be removed.  Large hole exposes the ore body.  Waste rock (overburden) is removed.  It often necessitates a large capital investment but generally results in high productivity, low operating cost, and good safety conditions.  2nd cheapest method, but has the largest environmental impact. Why?

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 Funnel shaped hole in ground, with ramp spiraling down along sides, allows moderately deep ore to be reached.  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 mined  Mining rate > 20,000 tons mined per day (tpd).  Design issues:  Stripping overburden  Location of haul roads  Equipment  size of trucks and fleet  Pit slope angle and stability


Classic Open Pits Characterized by Oval Shape, Benches, spiraling roads  Characterized by a series of stair-step like benches that each act as a working area  Pit shapes tend to be more configured to geology of the deposit more than equipment needs/convenience  Many pits are ovals  Fits the geometry of disseminated metal deposits  Pits tend to be wider relative to length  Pits tend not to move like strip mine – pit develops in place

These pits expand without Moving and generally Target a vein or steeply Dipping stock on ore

Surface Mining methods (Open pit Mining method)

Figure shows Open pit Mining method

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open pit mining: funnel shaped hole in ground, with ramp spiraling down along sides, allows moderately deep ore to be reached. Initial mining for zinc at Franklin and Ogdensburg, New Jersey-USA.

Photo I took at Bingham. 4 km in diameter 1 km in depth, at its zenith 400000 tons of rock per day

Overburden Removal

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Removing Overburden 2 February 2016


Some photos and machinery used in open-pit mining

A Dragline Shovel

Loading ore in pit

Haulage is usually by truck, although railroads, inclined rails, and conveyor belts have been used. The conveyance unloads directly into a primary crusher and crushed material is stored in coarse ore bins prior to shipment to the mill.

Blastholes are usually drilled vertically by selfpropelled, track-mounted pneumatic or rotary drills. Bulk explosives are loaded in the holes and large volumes of ore are broken in a single blast. Sometimes the drill holes are routinely sampled and assayed to help plan the position of the shovels in advance of mining. Blasthole assay control is especially desirable when exploration data are incomplete or lacking as in the case in the older pits which have long been mined past the limits of "ore" used in original planning. 2 February 2016


Mining Trucks

Crushing in pit

Drilling in pit

*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... 2 February 2016


1.3) Benching   

Benching is used to properly patch or extend a slope Benching is also used to temporarily support equipment for other work elements Bench detail must be wide enough to support a dozer % slope in towards the roadway to resist sliding

Width of dozer

6%

Benching Detail Bench level intervals are to a large measure determined by the type of shovel or loader used, and these are selected on the basis of the character of the ore and the manner in which it breaks upon blasting and supports itself on the working face. 2 February 2016


1.4) Open Pit Bench Terminology

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Parts of a bench

Trucks parked awaiting the call for their next loads of ore...!!! 2 February 2016


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Terms in Open Pit Benches   

Quarries in strong rock can sustain about 80 to 85o toe to crest slopes. Geology determines limits but about 58 to 72o is a common range for toe to crest in open pit metal. Over-all slopes often more conservative  Frequently less than 45o.  Cannanea Mexico is nearly 60o

Crest Toe Catch Bench Berm

Toe to Crest Slope Bench

Final Pits Slopes allow Benches to be wide enough to Catch rocks and accommodate A berm. (This is often less than Than 10 m).

Note: that the toe to Crest slope is much Steeper than the over-all

Over-all Pit Slope Localized single bench failures from a steep toe to crest slope are much more Tolerable than an over-all pit slope failure over the entire side of a pit.

Fig.1: Bench cross sections

Fig.2: Example of pit slopes varying in a deposit

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Pit

Bench

Beam

Floor

Weight

Weight

Angle

Width

Width

Overall slope

Slope

Slope

crest

Interval

Top

Figure showing typical open-pit bench terminology 2 February 2016


Outside Dump

Typical Non-haul Road bench

Typical Bench Wall

Catch Berm

Typical Haul Road

Top of Main Ramp Out of Open Pit

Drill rig Drilling Out a New Pattern

Loaded Haul Truck Going to Run of Mine Stockpile

Shovels loading haul trucks

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Drilled out pattern about to be charged with explosivesProf. Dr. H.Z. Harraz Presentation

Typical Open Pit Mine

Mining Methods, Surface mining

Empty haul truck returning to shovel 23

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1.5) Bench height

The bench height is the vertical distance between each horizontal level of the pit.  The elements of a bench are illustrated in Fig. 1.  Unless geological conditions dictate otherwise, all benches should have the same height. The height will depend on : i) The physical characteristics of the deposit; ii) The degree of selectivity required in separating the ore and waste with the loading equipment; the rate of production; iii) The size and type of equipment to meet the production requirements; and iv) The climatic conditions. The bench height should be set as high as possible within the limits of the size and type of equipment selected for the desired production. The bench should not be so high that it will present safety problems of towering banks of blasted or un-blasted material. The bench height in open pit mines will normally range from 15m in large copper mines to as little as 1 m in uranium mines.

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1.6) Cutoff grade  Cutoff grade is any grade that for any specific reason is used to separate any two courses of action.  The reason used in setting a cutoff grade usually incorporates the economic characteristics of the project.  When mining, the operator must make a decision as to whether the next block of material should be mined and processed, mined and stockpiled, mined (to expose ore) and sent to the waste dump, or not mined at all.  The grade of the block is used to make this decision.  For any block to be deliberately mined, it must pay for the costs of mining, processing, and marketing. The grade of material that can pay for this（the costing, processing and marketing) but for no stripping is the breakeven mining cutoff grade.  A second cutoff grade can be used for blocks that are below the mining cutoff grade and would not be mined for their own value.  These blocks may be mined as waste by deeper ore blocks.  The cost of mining these blocks is paid for by the deeper ore.  The final destination of these blocks is then only influenced by costs for the blocks once they have been mined.  The blocks can be processed at this point if they can pay for just the processing and marketing costs.  In mining phase the cutoff grade calculation would include the drilling, blasting, loading, and hauling costs.  The processing costs would cover crushing, conveying, grinding, and concentrating costs.  The marketing costs could include concentrate handling, smelting, refining and transportation.  Additional direct costs for royalties and taxes would also be included.  Overhead costs should also be added to the calculation.  Depreciation is used in the calculation for the purpose of setting the pit size.

Table 1 is the calculation of the mining cutoff grade for a copper project with the following parameters: 30 kt/d (33000 st pd) of ore mined for 20years $300,000,000 capital cost (include replacement capital) $1.00 mining cost per ton of ore $0.95 mining cost per ton of waste $3.00 processing cost per ton of ore $1.00 general and administrative (G&A) cost per ton of ore $0.75 freight, smelter, and refining (FSR) cost per kilogram of copper 85% overall copper recovery Note that The cutoff grade will increase as the costs increase is shown in Fig. 8. The difference between the mining cutoff grade and the milling cutoff grade is shown in Fig.9. 2 February 2016


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Geometry of a Working Bench

Shovel Length

Truck

Truck Width

Shovel

Berm width Back-up

Truck Turning radius

A bench big enough To accommodate Equipment working is Much wider than one Only intended to catch Rolling rocks.

Impact of a Working Bench

Over-all Slope

The over-all slope of the pit is drastically Reduced if one must accommodate wide Working benches.

1.7) Open Pit Stability The following are the key items affecting the Open Pit Stability: i) Pit slope ii) Pit wall stability iii)Rock strength iv)Pit Depth v) Pit diameter vi)Water Damage vii)Strip Ratio (SR)

February 2, 2016


i) Pit Slopes  The slope of the pit wall is one of the major elements affecting the size and shape of the pit.  The pit slope helps determine the amount of waste that must be moved to mine ore.  The pit wall needs to remain stable as long as mining activity is in that area.  The stability of the pit walls should be analyzed as carefully as possible.  Rock strength, faults, joints, presence of water, and other geologic information are key factors in the evaluation of the proper slope angle.  The physical characteristics of the deposit cause the pit slope to change with rock type, sector location, elevation, or orientation within the pit.  Pit 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.

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Pit slopes  Fig. 2 illustrates how the pit slopes may vary in the deposit.  A proper slope evaluation will give the slope that allows the pit walls to remain stable.  The pit walls should be set as steep as possible to minimize the strip ratio.  The pit slope analysis determines the angle to be used between the roads in the pit.  The overall pit slope used for design must be flatter to allow for the road system in the ultimate pit.  Fig 3 and Fig 4 show the need to design the pit with a lesser slope to allow for roads:  Fig. 3 has been designed with a 450 angle for the pit walls.  The pit in Fig. 4 uses the same pit bottom and the 450 inter-ramp slope between the roads, but, a road has been added. So the overall pit slope is lesser the inter-ramp slope.  In the example, almost 50% more tonnage must be moved to mine the same pit bottom.  In the early design of a pit a lesser pit slope can be used to allow for the road system.  The pit in Fig. 5 was designed with an overall slope of 380.  The overall slope to use will depend on the width, grade, and anticipated placement of the road.  Fig. 6 shows a vertical section of a pit wall from Fig.4.

 The inter-ramp angle is projected from the pit bottom upward to the original ground surface at point B.  The overall pit slope angle is the angle from the toe of the bottom bench to the crest of the top bench.  Point A shows the intercept of the overall pit slope angle with the original ground surface. 2 February 2016


Fig.3: Pit designed with a 45o pit slope

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Fig.4: Pit designed with a 45o inter-ramp slope and a road system

Fig.5: Pit designed with a 38o overall slope to allow for a 45o inter-ramp slope and a road system

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Original ground surface A surface intercept of pit wall if roads are included. B surface intercept of pit wall if roads are not included

Slope angle between roads

• •

Average pit slope angle Fig.6: Vertical section through a pit wall

•

Quarries in strong rock can sustain about 80 to 85 degree toe to crest slopes. Geology determines limits but about 58 to 72 degrees is a common range for toe to crest in open pit metal. Over-all slopes often more conservative  Frequently less than 45 degrees  Cannanea Mexico is nearly 60

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Slope Stability Function of the natural angle of repose, density, surface and subsurface water flow Early stabilization of surfaces is critical i.e. seeding, mulching, erosion blanket Upward tracking of slopes slows sheet flow Eliminate points of concentrated flow using berms or using slope drains as outlets Slopes can be “softened” if space permits Difficult slopes may require riprap, gabions, or other measures for permanent stabilization

THE SLOPE EFFECT What happens if we Change the slope Angle?

What just happened to the overburden volume? What just happened to our stripping ratio?

Conclusion – Pit Slope Makes a Big

Difference in Open Pits

Implications for Slope Effect

• In long area strip mines where things broke down to 2 dimensions slope did not impact stripping ratio • Here in this static 3D pit geometry the impact is huge • Obviously having a steeper slope improves economics

Limiting Slopes • One limit is geologic – having the pit slide in on you is bad for investment (and possibly your health if you are at the bottom) • One exercise commonly taught in rock mechanics courses is plotting fractures on stereo net  Illustrates how many fractures are opened up by benches

Daylighted fracture Offers an opportunity To slide off.

Non-Daylighted fracture offers little Risk

Probability of Failure • Not all daylighted fractures will slip • Not every non-daylighted fracture will hold • More major extensive daylighted fractures more likely a major failure is  One New Mexico mine lost entire pit as slide slipped in over several months

Significance of Failure • Some small failures will take a few hours to clean up – can risk these to save money • Larger regional failures are fatal, probably cannot endure much risk

Can tolerate daylighted Fractures on benches

Daylighted fractures on over-all Pit slope are another matter

The Equipment Considerations  Why benches? • Benches stop rolling rocks (a rock rolling down 600 ft and hitting you in the head will split your scull – even if there are no brains)  Benches act as rock catchers – they need to be wide enough for this – with the aid of a berm (around 10-15 feet)

• Benches match equipment digging height

Woops! Bigger shovels allow bigger bench Height – but require bigger trucks

• Flat area on benches provides room for equipment to move  Bigger trucks have bigger turning radius Truck

Shovel Plan view of bench work area

Grade Control and Limits on Bench Heights Usually have to dig whole bench toe to crest:  Cannot select ore. Some Mining Depends on selecting only best ore for processing:  Can loose selectivity as bench height increases.

Economics and Advantages of Bench Height

Maintaining bench area involves a cost: • Less bench area = less cost. • Higher benches are cheaper (usually).

In drilling for blasting it takes time to set up for every hole drilled: • Higher benches allow larger more accurate holes. • Allow greater spacing – uses drill time more effectively.

The Pit Slope Problem Our Ultimate Pit Was Calculated at Our Final Pit Slope:  A final pit slope has benches wide enough to catch falling rock and allow for a road to get equipment out.  A bench 10 to 15 meters wide will usually catch falling rock  But that may be just barely enough for a truck to drive forward if the bed drags the highwall and the tire runs over the berm  No room for maneuvering the truck for production.

 A Final Pit Slope cannot account for equipment in operation.

Open-pit slope failure –structural problems Pre-mining geological structures, particularly fault planes, represent zones of potential weakness in the rock mass, and are therefore zones of potential slope failure, and should be taken into account when designing the mine.

Fault planes dipping towards the pit (as shown in the figure) present a greater risk than faults dipping away from the pit. Faults planes often provide passage-ways for water movement, and these waters, through the process of weathering and chemical alteration of minerals, may reduce the strength of the rocks on either side of the fault plane, and reduce the “coefficient of friction” along it. The coefficient of friction (the “traction” or “grip”) along the fault will determine whether failure and slippage of rock down the fault plane is likely. The coefficient of friction may change with time:  as water-flow patterns are affected by mining  as faults are exposed by the removal of rock, opening fluid pathways into faults  by the reduction of the mass of the rock located above the fault plane. 2 February 2016


ii) Pit Wall Stability o Crack measuring o Failure warning o Movement of the walls

 Stable  Instable: o Underlying fracture or fault o Magma o landslide

Most 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 3

Figure 2 2 February 2016


Figure 4 shows a landslide that occurred recently following rain storms. A berm was created at the base of the slide to protect

the main haul road.

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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.


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Pit slope versus rock strength

Greater rock strength can support greater bench heights resulting in: i. a steeper pit, ii. a lower stripping ratio and iii. less waste rock. 2 February 2016

Pit depth versus pit diameter

A greater final pit depth requires a larger diameter pit (assuming rock strength and pit slope remains unchanged) . resulting in i. a higher stripping ratio and ii. more waste rock.


Figures from Spitz and Trudinger, 2009

The Depth Effect • Note that as a pit goes deeper the stripping ratio increases until it reaches an economic limit. • Rule 1 : as slope decreases S.R. increases • Rule 2 : as depth increases S.R. increases

vi) Water Damage

 Pit most keep dry  Dewatering also helps to keep the slopes dry and more stable.

Figures shows: 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. 2 February 2016


What happens when water accumulates? On October 9, 2003, a major landslide occurred, causing perhaps eight fatalities at the Grasberg Mine, Indonesia (Figures 8 and 9).

The accident was related to heavy rainfall and accumulation of water in the soil layer at the top of the pit. 2 February 2016


Open-pit slope failure –case study –groundwater problems A 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 surface.  A strong layer of younger clay sediments overlies weaker weathered bedrock.

Seepage and mineral precipitation

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). Figures modified from Speight, 2002. 2 February 2016


vii) Stripping Ratio  Stripping ratio is defined as the ratio of the Overburden/Cover Rock / Waste Rock to the orebody.  One of the ways of describing the geometrical efficiency of a mining operation is through the use of the term 'Stripping Ratio' (SR).  It refers to the amount of waste that must be removed to release a given ore quantity.  Stripping ratio is defined as the ratio of the Overburden/Cover Rock / Waste Rock to the orebody.  Stripping ratio or strip ratio refers to the ratio of the volume of overburden (or waste material) required to be handled in order to extract some volume of ore.  The strip ratio is the ratio of the number of tons of waste that must moved for one ton of ore to be mined.  The results of a pit design will determine the tons of waste and ore that the pit contains.  When speaking of an open pit mine, the term strip ratio is used.  The ratio of waste and ore for the design will give the average strip ratio for that pit.  The ratio is most commonly expressed as:

Waste (tons)

SR = ---------------Ore (tons)  Strip Ratio (SR) is the mass of waste to be mined to obtain one unit mass of ore.

Examples:

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 For example, a 3:1 stripping ratio means that mining one cubic meter of ore will require mining three cubic meters of waste rock. • Syncrude Tar Sands: SR = 1.2 - 1.4 • Highland Valley: SR ~0.5 • Bagdad mine: SR ~1.2 • Cortez mine: SR = 1.6 Prof. Dr. H.Z. Harraz Presentation Stripping Ratios

1.8) Open-pit mining sequence

Figure 2.8 Open-pit mining sequence (for pipe-like orebody) 2 February 2016


1.9) Various open-pit and orebody configurations Flat 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.

Ore Control Process Layout Drill Holes 3D

Electronically Log Holes

Map Geology with DGPS

Fusion Ore Controller Reporting System

Estimate Grade

Design Ore Blocks (Manual & MRO)

Model Geology & Ore Zones

CENTRAL DATABASE

Mining / Ore Control (Modular Mining & Reporting system)

Reconciliation

Controls of Gold Mineralization - NY

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Controls of Gold Mineralization (GH Cut1)

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Mining Process Flow Chart • Exploration Geologists (Resource Model):i) Drilling:  Reverse Circulation (RC) Drilling.  Diamond Core Drilling. ii) Surface Grab Samples. iii) Geological Mapping. • Mine Planning Engineers. • Mine Production Engineers. • Mine Geologists: i) Grade Control Plan & Drilling. ii) Design Ore Blocks. • Drill and Blast Engineer. • Mining Contractor.

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Example: Geita Mining Production Process Ore grade control drilling

Blast hole drilling

Charging of blast holes

Firing of blast holes

Loading of blasted rock

Ore hauled to run-of-mine (rom) stockpiles Waste material hauled to dump Nyankanga Pit

Grade control drilling Loading of blasted rock

Stockpiled ore loaded into primary crusher Blast hole drilling

Crushed ore stockpile

Waste material tipped onto dump

Reshaping waste dump for rehabilitation


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2) limits or Ultimate Pit Definition

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2.1) Introduction  As the first step for long or short-range planning, the limits of the open pit must be set.  The limits (ultimate pit) define the amount of ore minable, the metal content, and the associated amount of waste to be moved during the life of the operation.  The size, geometry, and location of the ultimate pit are import in planning tailings areas, waste dumps, access roads, concentrating plants, and all other surface facilities.  Knowledge gained from designing the ultimate pit also aids in guiding future exploration work.  The material contained in the pit will meet two objectives.  A block will not be mined unless it can pay all costs for its mining, processing, and marketing and for stripping the waste above the block.  For conservation of resources, any block meeting the first objective will be included in the pit.  The result of these objectives is the design that will maximize the total profit of the pit based on the physical and economic parameters used. As these parameters change in the future, the pit design may also change.

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2.2) Manual Design The usual method of manual design starts with the three types of vertical sections shown in Fig.1 i) Cross sections spaced at regular intervals parallel to each other and normal to the long axis of the ore body. These will provide most of the pit definition and may number from 10 to perhaps 30, depending on the size and shape of the deposit and on the information available. ii) Longitudinal section along the long axis of the ore body to help define the pit limit at the ends of the ore body. iii) Radical sections help define the pit limits at the end of the of the ore body.

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Each section should show ore grades, surface topography, geology (if needed to set the pit limits), and structural controls (if needed to set the pit limits), and any other information that will limit the pit (e.g. ownership boundaries). The stripping ratio is used to set the pit limits on each section. The pit limits are placed on each section independently using the proper pit slope angle. The pit limits are placed on the section at a point where the grade of ore can pay for mining the waste above it. When the line for the pit limit has been drawn on the section, the grade of the ore along the line is calculated and the lengths of the ore and waste are measured. The ratio of the waste and ore is calculated and compared to the breakeven stripping ratio for the grade of ore along the pit limit.  If the calculated stripping ratio is less than the allowable stripping ratio, the pit limit is expanded.  If the calculated stripping ratio is greater, the pit limit is contracted. This process continues on the section until the pit limit is set at a point where the calculated stripping ratio and breakeven stripping ratio are equal. In Fig. 2, the grade on the right side of the pit was estimated to be 0.6% Cu. At a price of $2.25 per kg of copper, the breakeven stripping ratio from Fig. 3 is 1.3: 1. 2 February 2016


The line for the pit limit was found using the required pit slope and located at the point that gave a waste: ore ratio of 1.3:1. At the limit Length of waste (XY) 1.3  Length of ore ( YZ) 1

On the left side of the section, the pit limit for the 0.7% Cu grade was similarly determined using a breakeven stripping ratio of 2.7:1. Fig. 2, Fig. 3 If the grade of ore changed as the pit limit line was moved, the breakeven stripping ratio to use would also change. The pit limits are established on the longitudinal section in the same manner with the same stripping ratio curves. The pit limits for the radial section are handled with a different stripping ratio curve, however. As shown in Fig. 4, the cross sections and the longitudinal section represent a slice along the pit wall with the base length as the surface intercept. The radial section represents a narrow portion of the pit at the base and much wider portion at the surface intercept. 2 February 2016


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The next step in the manual design is to transfer the pit limits from each section to a single plan map of the deposit. The elevation and location of the pit bottom and the surface intercepts from each section are transferred. The resultant plan map will show a very irregular pattern of the elevation and outline of the pit bottom and of the surface intercepts. The bottom must be manually smoothed to conform to the section information. Starting with the smoothed pit bottom, the engineer will develop the outline for each bench at the point midway between the bench toe and the crest. The engineer manually expands the pit from the bottom with the following criteria:  The breakeven stripping ratios for adjacent sections may need to be averaged.  The allowable pit slopes must be obeyed.  If the road system is designed at the same time, the inter- ramp angle used.  If the preliminary design does not show the roads, the outline for the bench midpoints will be based on the flatter overall pit slope that allows for roads.  Possible unstable patterns in the pit should be avoided. Simple geometric patterns on each bench make the designing easier. When the pit plan has been developed, the results should be reviewed to determine if the breakeven stripping ratios have been satisfied. 2 February 2016


2.3) Computer Methods  As should be appreciated, the manual design of a pit gets the planning engineer closely involved with the design and increases the engineer’s knowledge of the deposit.  The procedure is cumbersome, though, and is difficult to use on large or complex deposits.  Drawback of manual design is that if any of the design parameters change, the entire process may have to be repeated.  Another drawback to the method of manual design is that the pit may be well designed on each section, but, when the sections are joined and the pit is smoothed, the result may not yield the best overall pit.

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2.4) Learchs-Grossman method The two-dimensional Learchs-Grossman method will design on a vertical section the pit outline giving the maximum net profit. The method is appealing because it eliminates the trial-and-error process of manually designing the pit on each section. The method is also convenient for computer processing. The results must still be transferred to a pit plan map and manually smoothed and checked. The example in Fig. 5 represents a vertical section through a block model of the deposit. There are three steps in Learchs-Grossman method: Step 1: Add the values down each column of blocks and enter these numbers into the corresponding blocks in Fig. 7. Step 2: Start with the top block in the left column and work down each column. Step 3: Scan the top row for the maximum total value. For example the optimal pit would have a value of $13. This is the total net return of the optimal pit. The Fig. 7 shows the pit outlined on the section. 2 February 2016


Return

2.5) Floating cone method If the grade of the base is above the mining cutoff grade, the expansion is projected upward to the top level of the model as in Fig. 8. The resulting cone is formed using the appropriate pit slope angles. If the total revenues are greater than the total costs for the blocks in the cone, the cone has a positive net value and is economic to mine. A second block is then examined, as shown in Fig. 9. Each block in the deposit is examined in turn as a base block of a cone.

3) Open pit Optimization

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3.1) The management of pit optimization The first thing to realize is that any feasible pit outline has a dollar value which can, in theory, be calculated. To calculate the dollar value we must decide on a mining sequence and then conceptually，mine out the pit, progressively accumulating the revenues and costs as we go. • The second thing to realize is that in doing this calculation we have, in effect, allocated a value to every cubic meter or to every block of rock. • Current computer optimization techniques attempt to find the feasible pit outline which has the maximum total dollar value.

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3.2) A simple example  Let us assume that we have a flat topography and a vertical rectangular ore body of constant grade as is shown in Fig.1. Let us further assume that the ore body is sufficiently long in strike for end effects to be ignored. In this simplified case there are eight possible pit outlines that we can consider, and the tonnages for these outlines are given in Table 1. If we assume that ore is worth $2 per ton after all mining and processing costs have been paid, and that waste costs $1 dollar per ton to remove, then we obtain the values shown in Table 2 for the possible pit outlines.  When plotted against pit tonnage, these values produce the graph in Fig. 2. with these very simple assumptions the outline with the highest value is the number five.

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3.3) The effects of scheduling on the optimal outline When we schedule a pit, we plan the sequence in which various parts of it will be mined and the time interval in which each is to be mined. This affects the value of the mine because it determines when various items of revenue and expenditure will occur. This is important because the dollar we have today is more valuable to us than the dollar that we are going to receive or spend in a year’s time. In what we will call “worst case” mining, each bench is mined completely before the next bench is started. Waste at the top of the outer shells is mined early. In what we will call “best case” mining, each shell is mined in turn and thus related ore and waste is mined in approximately the same time period. In this case, the optimal pit is usually close to the one obtained by simple optimization. Unfortunately if we try to mine each shell separately, mining costs usually increase and cancel out some of the gains. 2 February 2016


In Fig 5, there are three small ore bodies and their corresponding waste volumes, with their values and costs shown. A floating cone program will examine A and will find that the corresponding cone has a total value of ( 40-20-30) = -10, and so is not worth mining. It will then examine B, will find a cone of value( 200-8030)=+90 and will convert it to air, leaving the values shown in Fig. 6. The cone for C has a total value of (40-50+40-20)= +10, so that the program mines it. This should not happen, because some of the value of ore body A is being used to help pay for the mining of waste (50 region) which is below it. The true optimal pit in this case includes A, B, but not C.

4)Optimum production scheduling

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 The objective of production scheduling is to maximize the net present value and return on investment that can be derived from the extraction, concentration, and sale of some commodity from an ore deposit.  The method and sequence of extraction and the cutoff grade and production strategy will be affected by the following primary factors. i) Location and distribution of the ore in respect to topography and elevation; ii) Mineral types, physical characteristics, and grade/tonnage distribution; iii) Direct operating expenses associated with mining, processing and converting the commodity into a salable form; iv) Initial and replacement capital costs needed to commence and maintain the operation; v) Indirect costs such as taxes and royalties; vi) Commodity recovery factors and value; vii) Market and capital constraints; viii) political and environmental considerations.

The procedure used to establish the optimal mining schedule can be divided into three stages. The first defines the extraction order or mining sequence, the second defines a cutoff grade strategy that varies through time and will be optimal for a given set of production parameters, and the third defines which combination of production rates of mine, mill, and refinery will be optimal, within the limits placed by logistical, financial, marketing, and other constraints. Fig.1 shows the internal phases: typical pit cross section 2 February 2016


5) Materials handling ExMine

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 This section will outline design consideration for the development of a hard rock mine in-pit crushing and conveying system.  The obvious trend in the nonferrous metals mining industry is toward the mining of lower grade ores at increasing tonnage rates. With the progressive development of larger and more efficient milling equipment and alternative processing techniques the definition of ore, low grade, and waste varies at each operation.  In the past , the primary crushing, fine crushing, and mill complex tended to be located in relative close proximity and the majority of the horizontal and vertical travel distances from the ore source to the crushing station was handled by truck haulage.  Due to rising fuel and maintenance costs, economic conditions have forced the pit designer to minimize the distance the trucks have to transport the ore, and to bring the primary crusher closer to the source and thus utilize conveyors to perform a much larger proportion of the ore transport requirements. Data generated from actual installations have shown that properly designed crushing and conveying system compare to truck haulage systems as follows: Significantly lower operating and maintenance costs. Higher initial capital costs, but with lower present value costs when compared to the life of the operation. Improved foul weather operating conditions. Can provide comparable operating flexibility in certain circumstances. 2 February 2016


Table 1 shows the economic benefit comparison in-pit crushing vs. truck haulage. Table 2 shows crusher system characteristics desired by mine personnel. Fig.1 shows the portable crusher and feeder at Sierrita Mine. Fig.2 shows the schematic of snake sandwick conveyor. Studies are currently underway to determine what can be done to improve the technology of in-pit belt conveying. Consideration is being given to the loop belt concept. This method works on the principle that depends on the radial forces generated by putting S curves in the profile of a sandwicked conveyor. By continually varying the curvature, a lift is achieved within “geometrical constraints” conforming to specified mine slopes. ( see Fig.2 and 3) Many high angle conveying concepts have been studied and the sandwick belt conveyor seems the most promising. 2 February 2016


6) Waste disposal Planning and environmental protection aspects

6.1) Dump design A waste dump is an area in which a surface mining operation can dispose of low grade and/or barren material that has to be removed from the pit to expose higher grade material. The first step in designing a dump is the selection of a site or sites that will be suitable to handle the volume of waste rock to be removed during the mine’s life. Site selection will depend on a number of factors, the most important of which are: i) Pit location and size through time. ii) Topography. iii) Waste rock volumes by time and source. iv) Property boundaries. v) Existing drainage routes. vi) Reclamation requirements. vii) Foundation conditions. viii) Material handling equipment. Fig. 1 shows a waste dump. 2 February 2016


2 February 2016


6.2 ) Stability of mine waste dumps The overall stability of mine waste sumps is depend on a number of factors such as: i) Topography of the dump site. ii) Method of construction. iii) Geo-technical parameters of mine waste. iv) Geo-technical parameters of the foundation materials. v) External forces acting on the dump. vi) Rate of advance of the dump face. All of these factors combine in various ways during the life of a mine waste dump to aid in the stability of the dump or to contribute to its instability

2 February 2016


6.3) Mine reclamation The purpose of reclamation is to upgrade the physical character of all or part of a mining area after the mineral values have been removed and, thereafter, to protect the surrounding environment from contamination. In surface mining operations, the three largest areas that are reclaimed are the mine extraction, the mine waste dump and the mill tailings areas. This section pertains to the first two items. If the commodity extracted is a bedded deposit of large extent and of relatively show depth such as in coal mines, the backfilling of worked-out areas is a common method of waste disposal and reclamation. Waste material removed from the initial box cut or pit either be stockpiled and later transported to fill the final excavation or the stockpile could be reclaimed and not moved and the last pit left with little reclamation effort applied. In most surface operations in commodities other than coal, the amount of backfilling is restricted of totally impractical. Therefore, most of the reclamation effort is directed toward the waste disposal area.

In most surface mining operations, the waste material removed from the pit is deposited on an adjacent area. The area required for waste disposal is usually equal to or greater than the pit area because the disturbed waste mater has a greater volume than in-situ, a lower slope angle than the pit walls, and rarely can the material be stacked as high as the pit is deep. In designing waste dumps, particular consideration has to be given to reclamation needs if the cost is to be minimized. If the overall slope of the sump face has to be reduced to prevent erosion and to allow placement of top soils and vegetation, then the dump design should consider terracing to minimize the amount of material re-handling. As illustrated in Fig. 2, the cost of re-handling decreases in proportion to the square of the reciprocal of the number of terraces into which a dump can be broken. Therefore a dump constructed using three terraces will have only one-ninth the rehandle dozing costs of a dump of similar height with no terracing. In order to facilitate reclamation efforts, a berm should be left on each terrace level. This will lower costs by providing easier access to the faces for equipment spreading topsoil and for re-vegetation efforts. The berms can also serve as erosion protection and drainage diversions, if necessary. The main hazards to a reclamation project will be erosion and leakage of contaminated waters that will hamper re-vegetation or be hazardous to life. Both of these problems usually can be corrected through proper drainage control and treatment. Drainage channels will need to be rock-lined if the channels are to remain in the same location without excessive bank erosion.

2 February 2016


Example of Open Pit Mining Method

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Open-pit mine: Chuquicamata copper mine, Región de Antofagasta, Chile Benches

http://upload.wikimedia.org/wikipedia/commons/2/2a/Chuquicamata_panora ma.jpg

Access ramps

Dust Locality: Región de Antofagasta, Chile. Slope failure Pit dimensions: 4.3 km long x 3 km wide x 850 m deep. Mining dates: 1915 -present Total production: 29 million tons of copper to the end of 2007 (excluding Radomiro Tomić 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). 2 February 2016


Highland Valley Pit, British Columbia  Porphyry copper  137,000 tons mined/day (tpd)  296 Mt reserves:  0.42% Cu  0.008% Mo  Cu, Mo concentrates with gold and silver Byproduct

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• •

• • • •

The Highland Valley Copper mine, located in British Columbia, 60 km southwest of Kamloops is 97.5% owned by Teck Cominco. Operations at Highland Valley began over 20 years ago by predecessor companies. The open pit contains two in-pit crushers feeding a 12,000 ton/ hour conveying system that delivers ore to stock piles at the mill. The operation is expected to shut down in 2009. There is a plan to expand the Valley pit and salvage ore from the west wall of the Lornex pit. If some rock slope stability problems can be overcome in these pits, the mine life could be extended to 2013.

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Example: Palabora, South Africa The Palabora orebody is an igneous, pipe structure containing:-

 Copper  Magnetite (iron oxide),  Vermiculite (used for insulation),  Zirconium,  Titanium, and  Uranium.  The mine opened in 1964.  It is a full mine-to-smelter complex.  Open pit operations ended in 2002.  The pit used to be the deepest and steepest in the world.  The mine is now an underground block-caving operation with a 20 year mine life.  Proven reserves are 225 Mt at 0.7% copper.

2 February 2016


Example: Tyrone Copper Mine is situated near Silver City, New Mexico, USA

 The open pit Tyrone Copper Mine is situated near Silver City, New Mexico, USA.  Silver City was founded as a mining town, and the nearby mining operations of Phelps Dodge are still the basis for the local economy.  In 2006, the Chino and Tyrone mines produced 125,400 long tons (127,400 t) of copper.  Mine employment was 1,250, with wages and salaries totaling $73 million.

2 February 2016


Surface Mining

Surface Mining

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