Surface Mining: An Introduction

Introduction to Surface Mining

• Surface mining extracts minerals from the earth's crust at-surface with miners not exposed to underground conditions.
• Surface mining is the predominant exploitation method used worldwide.
• The history of surface mining dates back to prehistoric times -- the Paleolithic man mined at surface for stone implements in 450,000 B.C.
• Minerals have uniquely met human needs through securing food, shelter, defense, hunting, jewelry, transport, heat, and power systems.
• Most ages of cultural development are associated with minerals like the Stone Age, Bronze Age, Iron Age, and Steel Age.
• Post-World War II saw rapid technological advancements like fertilizer, AN-FO explosives, rotary drilling, truck haulage, increased truck and excavating equipment capacity, auxiliary and labor-saving devices, and innovations in mineral beneficiation; this all improved surface mining productivity.
• The main types of mining methods are surface mining methods, when mineral deposits are on or near the surface of the Earth, and underground mining methods, when minerals cannot be extracted economically or technically with surface mining methods.
• Metallic minerals, industrial minerals, and fossil minerals can be mined via surface mining.

Advantages of Surface Mining

• Greater mechanization possibilities are a key advantage, utilizing large, powerful, and highly efficient mining equipment is easier.
• An example is the use of 400 short tons (363 ton) capacity trucks, 220 yd³ (170 m³) bucket capacity draglines, 140 m³ dipper capacity shovels, 20 yd³ (15 m³) hydraulic shovels, and 8400 m³/hr bucket capacity bucket wheel excavators (BWEs).
• Surface mining is more efficient with lower accident rates than underground mining because underground rates are much higher due to ventilation, roof control, and cramped operations.
• Less mine development time, it typically takes 3–5 years underground versus 1–2 years in surface mining, mostly based on infrastructure and lead time on equipment.
• Surface mining enables high recovery of the resource, yielding 85–90% recovery as opposed to 60-70% recovery in underground mining.
• Surface mining offers higher labor efficiency with a less skilled workforce, less danger money, and easier recruitment.
• Better working conditions can be achieved with improved hygienic and working conditions, freedom of movement, sufficient light and air, and no need for support or filling.
• Less energy demand because the equipment is more efficient, no ventilation is required, and there is no water pumping needed, and no need for lights.
• Surface mining uses 5–10 kW/t while underground mining uses 20–50 kWh/t.
• Mechanized, mass production equipment can be utilized that that is capital intensive but labor conserving.
• Surface miners invest in equipment rather than shafts, and trucks can be sold whereas shafts cannot.

Disadvantages of Surface Mining

• Surface mining requires considerable original capital outlays for purchasing equipment and conducting stripping operations.
• Climatic conditions like rain, snow, and severe cold impact surface mining where open pits make work difficult and labor and machine efficiency drop.
• Depth limitations can impact a surface mine's profitability.
• Some minerals are not viable to be worked via surface mining methods.
• Selectivity in surface mining may not be as effective as it is with underground mining.
• Environmental factors are a major disadvantage, destroying the natural beauty of the land by scars left from open pits and accumulation of rejected waste which draws strong pressure from the public.
• The negative environmental impacts include disturbance of landscape and eco-system, diesel emissions into the atmosphere, noise, impact, air and vibration from surface blasting, accumulation of rejected waste, pollution of streams by acid and solids, and pollution of the atmosphere by dust.

Factors Affecting Surface Mining

• Materials variables, such as fragmentation, density, swell factor, diggability, and surficial characteristics
• Equipment variables, such as availability and utilization, economic useful life, unit ownership and operating costs, theoretical capacity and fill factor, cycle time, and equipment matching
• Layout variables, such as rolling and grade resistances, equipment bunching and queueing, multiple service layouts, equipment service layouts, equipment maneuverability, bearing capacity of road beds, and haulage road efficiency and safety
• Labor variables, such as operating schedule, operating efficiency, acquiring and managing technology, and managerial efficiency

Future of Surface Mining

• Mine productivities will increase as a result of innovations and technological developments like full automation and robotics, off-highway truck design and performance, excavator reliability and durability, computer-aided controls in mine operations and design, and blasting agent utilization and detonation efficiencies.
• Factors opposing surface mining include uneconomical extraction when the deposit is too small, irregular, and deeply buried, and also deeper mineralization means increased overburden depth which imposes economic limits.
• Public pressure, developments like electronics, automation, rock-boring machines which have provided significant contribution to underground mining, and research in rapid excavation, principles, techniques, and equipment and storing strategic supplies may also limit the future of surface mining.
• Surface mining methods will be retained and expand wherever surface mining operators can put large quantities of mineral on the market at costs that cannot be matched by underground operators.
• Factors opposing surface mining and favoring U/G mining are increasing in densely populated, industrialized areas of the world, and the trend toward surface mining in these areas is expected to reverse in the future.

Global and National Surface Mining Operations

• A few examples of international surface mining operations:
  • Kışladağ Gold Mine (Turkey), operated by TÜPRAG
  • Kaymaz Gold Mine (Turkey), operated by Koza Gold Corp.
  • Afşin Elbistan Coal Mine (Turkey), operated by Park Teknik
  • Tunçbilek Lignite Mine (Turkey), operated by GLİ
  • Twin Creeks Gold Mine, Nevada, operated by Newmont Corporation
  • Sun Rise Dam Gold Mine, Australia, operated by AngloGold Ashanti
  • Bingham Canyon Cu Mine (USA), operated by Rio Tinto
  • Coal Creek Mine (USA), operated by Arch Resources Inc.
  • Escondida Copper Mine (Chile), operated by BHP, Rio Tinto and JECO Corp.
  • Finsch Diamond Mine (South Africa), operated by De Beers Corp.
  • Palabora Copper Mine (South Africa), operated by Palabora Mining Company
  • Argyle Diamond Mine (Australia), operated by Rio Tinto (World's largest single diamond producer)
  • Super Pit Gold Mine (Australia), operated by Northern Star
  • Mir Diamond Mine (Russia), operated by ALROSA, 525 m deep, 1200 meters in diameter; the air zone within this mine is closed for helicopters because a few accidents occurred when they were "sucked in" by downward air flow

Surface Mining Method Classifications

• Classification I – Extraction Methods
• Classification II – Excavation & Haulage

Extraction Methods Classification: Mechanical Extraction Methods

• Open Pit Mining is utilized for extraction of metalliferous ore.
  • Excavation is made from the ground surface to extract ore in a series of benches.
  • The objective is to extract the ore at the lowest possible cost and to derive maximum profit.
  • The planning is strongly influenced by geology and mining technology and equipment.
  • Major factors: equipment selection, stripping ratios, production rates, ore body geometry, ore dispersion, allowable slopes, topography, and location
• Open Cast/Strip Mining is utilized for coal extraction, and is used primarily for mining of relatively flat tabular deposits; involves the use of draglines.
  • The overburden is usually placed back in the pit and normally directed into the previously mined cut.
  • The shape or format of the strip can take several forms, each which gives its name to method.
  • The mine is usually developed immediately to the full depth of its mining face and progresses horizontally as the orebody is mined.
• Quarrying is utilized for aggregate and non-metallic materials and has certain advantages and disadvantages:
  • Advantages: Low capital investment, suitable to small deposits, very stable walls and benches, high selectivity, good health and safety
  • Disadvantages: Limited by depth, very low productivity, highest mining cost, skilled labor intensive, inflexible with no extensive mechanization, complicated and costly breakage
• Auger Mining:
  • Advantages: High productivity, low mining cost, little development, low labor requirement (only 3 persons/shift), low capital investment, no reclamation, recovers coal that will be lost, good health and safety
  • Disadvantages: Strictly limited, limited coal recovery, single unit production

Extraction Methods Classification: Aqueous Extraction Methods

• Depend on the type of solvent (water or another liquid e.g. dilute sulfuric acid, weak cyanide solution, or ammonium carbonate).
• Placer mining is used to exploit loosely consolidated deposits like common sand and gravel or gravels containing gold, tin, diamonds, platinum, titanium, or coal.
• Solution mining includes both borehole mining, such as the methods used to extract sodium chloride or sulfur, and leaching of either through drill holes or in dumps or heaps on the surface.
• Placer and solution mining are among the most economical of all mining methods, but can only be applied to limited categories of mineral deposits.

Placer Mining - Hydrolicking

• Utilizes a high-pressure stream of water that is directed against the mineral deposit (normally but not always a placer) undercutting it and causing its removal by the erosive actions of the water.
• Advantages: Fairly high productivity, low mining cost, intermediate production rate, low capital cost, simple equipment and cycle, simple to automate operations
• Disadvantages: Potential environment damage, extensive water requirements, limited to unconsolidated deposits, inefficient cutting action

Placer Mining - Dredging

• Performs extraction of the minerals from floating vessels, mechanically or hydraulically.
• Advantages: Most productive of acqueous methods, lowest mining cost, high production rate, low labor requirements, good recovery, continuous operations
• Disadvantages: Potential environment damage, moderate water requirements, limited to unconsolidated deposits, high capital investment in dredges, inefficient cutting action

Solution Mining - Borehole Mining

• Advantages: High productive, moderate production rate, low mining cost and labor requirements, applicable to deep deposits, reduced development time and cost, continuous operation
• Disadvantages: Limited to deposits that dissolve, melt or slurry in water, moderate water requirements, unselective, high dilution, low recovery, groundwater contamination

Solution Mining - Leaching

• Advantages: High productivity and production rate, low mining cost, applicable to deep, low grade deposits, supplements primary mining, performs beneficiation and extraction, good health and safety
• Disadvantages: Limited to soluble leachable minerals, moderate water requirements, unselective, high dilution, low recovery, groundwater contamination

Excavation and Haulage Classification

• Cyclic Methods include shovels-truck method, front end loaders (FEL) – truck methods, and dragline mining methods.
• Continuous Methods include BWE-conveyor method and in-pit-crushing-conveyor method.
• Hybrid Methods include shovel/truck/conveyor, shovel/truck/slurry pipelines, and shovel/in-pit crushing/slurry pipelines.

Basic Concepts and Terminology

• Bench (Benching Bank or Bank) (Basamak) is a ledge that forms a single level of operation above which waste and ore are mined back to a bench face.
  • Several benches may be in operation simultaneously in different parts and at different elevations in the mine.
• Bench Face (Basamak Aynasi) is the exposed face between two bench levels which forms the actual working face for the mining
• Bench Height (Basamak Yüksekliği) is the vertical distance between the highest point of the bench or CREST and the bottom or TOE of the bench face, which represents the distance between two benches and is dictated by equipment and regulatory considerations.
• Bench Width (Basamak Genişliği) is determined by the equipment needed for mining operations and whether it is also to be used as a haul road.
• Bench Slope (Basamak Açısı) is the angle in degrees to horizontal of line joining crest and toe of bench face.

More About Dimensions of a Bench

• Dimensions include its height and angle of slope.
• To determine the dimensions, consider equipment used, the nature of ground, and conditions of work.
• A correct relationship between the height of bench and the working dimensions of the excavator ensures the most efficient, safe, and economical operation of the equipment.
• For non-solid rocks, the Bench Height should be approximately the Digging Height of the Power Shovel.
• For hard rocks, the Height of the Broken Rock Pile should be no more than than 1.5 x Digging Height of Power Shovel.
• For selective loading of different grades of ore or loading of ore separately from waste, the Height of Bench should be no more than the Digging Height of Excavator.
• In Practice (Open Cast Work) bench height should be 3 ~ 20 m (with electric shovel) for waste (10 ~ 15 m), and for ore (8 ~ 12 m).
• Slope Angle (α):
  • Friable and Soft Rocks: α ≤ Angle of Repose
  • Very Hard Igneous & Met. Rocks: α = 70° - 80°
  • Sedimentary Rocks: α = 50° - 60°
  • Semi-ledge & Dry Sand Grounds: α = 40° - 50°
  • Argilleous Rocks: α = 35° - 45°
• Stability of Slopes depends on shear parameters, structural defects, and groundwater conditions.
• Berm (Emniyet Basamağı) is a horizontal shelf or ledge built into an embankment or sloping wall of an open pit or quarry to break the continuity of a long slope for the purpose of strengthening and increasing the stability of the slope or to catch or arrest slope slough material.
  • A berm may also be used as a haulage road or serve as a bench for excavation.
  • Berm interval, berm width and berm slope angle are determined by the geotechnical investigation.
• Overall Pit Slope Angle (Nihai Ocak Eğim Açısı) is the angle measured from the bottom bench toe to the top bench crest.
  • It is the angle at which the wall of an open pit stands and is determined by rock strength, geologic structures, and water conditions.
  • The overall pit slope angle is affected by the width and grade of the haul road.
• Haul Roads (Taşıma Yolları) are roads maintained in the walls of the pit to access the working benches from the surface, that may be straight, or spiral/zigzag in deeper pits.
• Haul Road - Spiral System (Spiral Sistem) is a road constructed around the pit walls in a spiral fashion to provide a uniform grade and minimal abrupt changes in direction, which usually gives them the longest route from pit but access to all benches.
• Haul Road - Switchback System (Geri Dönümlü Sistem) is a road constructed usually in the footwall to handle steep gradient in pit wall, which changes direction frequently, but bends determined by turning radius of truck with bend usually made as wide as possible and zigzag pattern on one side of the pit.

More Terminology

• Haul Road Width is a function of the capacity of the road and the size of the equipment, so the haul road width must be considered in the overall pit design, using where HW = (1.5 * n +0.5) * X and HW is the minimum haul road width and X is the width of the largest truck on haul road.
• Angle of Repose (Doğal Durma/Yığın Açısı) is the maximum slope of the broken material.
• Subcrop or Ore Depth is the depth of waste removed to reach initial ore.
• Pre-Production Stripping is stripping done to reach initial ore.
• Ultimate Pit Limits (Nihai Ocak Sınırları) is the vertical and lateral extend of the economically mineable pit boundary determined on the basis of cost of removing overburden or waste material vs. the mineable value of the ore.
• Pit Scheduling is when material may be mined from the pit either in sequential pushbacks or conventional pushbacks.
• Stripping Ratio (Örtü Kazı Oranı) is expressed in m³ of waste to tons of ore in hard rock open pit operations, and it is a critical and important parameter in pit design and scheduling.
• Average Strip Ratio is the total waste divided by total ore within the ultimate pit.
• Cut-Off Stripping Ratio is when the costs of mining a ton of ore and associated waste equals the net revenue from the ton of ore.