Surface Mining: Mechanical Extraction Methods Study Notes

Classification and Definition of Surface Mining Mechanical Extraction Methods

Surface mining refers to the process of exploitation in which the extraction of ore, coal, or stone is carried out at the surface with essentially no exposure of miners underground. There are four primary mechanical extraction methods utilized in surface mining: open pit mining, quarrying, open cast (strip) mining, and auger mining. Among these, open pit and open cast mining are cited as the most critical surface mining methods and rank among the eight most important mining methods overall. Most near-surface deposits utilizing open pit or open cast methods employ a conventional mining cycle consisting of rock breakage via drilling and blasting, followed by excavation, loading, and haulage. In contrast, specialized methods like dimension stone quarrying and auger mining often utilize alternative means for breakage, typically avoiding explosives unless necessary to free large blocks or for aggregate production.

The term quarry is applied within the mineral industry to describe mines producing crushed stone or aggregates, such as ballast for rail tracks, road surfacing, or building stones sourced from crushed granite and limestone. Most of these quarries utilize the open pit mining method. However, quarrying is more specifically defined as the mining method associated with the production of intact blocks of rock known as dimension stone. This material is used for architectural and decorative purposes, including polished marble and granite tiles.

Open Pit Mining: Technical Design and Bench Geometry

Open pit mining is the process of extracting near-surface deposits using a surface pit excavated through one or more horizontal benches. This method is predominantly used for metallic or nonmetallic deposits and more sparingly for bedded deposits like coal. The excavation involves removing both the overburden and the ore in benches that typically vary from $9$ to $30\,m$ in height. A single bench may be sufficient for thin deposits and overburden ranging from $15$ to $45\,m$, whereas thicker deposits require multiple benches, often resulting in a pit shape resembling an inverted cone where higher benches are larger than lower ones.

The design of benches serves several purposes: controlling the depth of blastholes, managing the slope of the pit walls, and mitigating the dangers of highwall faces. Benches also provide the necessary face length for sustained production. The technical dimensions of a bench are constrained by the materials-handling equipment; for example, a power shovel can trim a higher bank than a hydraulic excavator or Front End Loader (FEL). The width of the bench must be sufficient to contain fly-rock from blasts and provide maneuvering space for haulage units. Maximum slopes are determined by soil or rock mechanics concerns.

Specific bench dimensions varies by the mineral being mined:

• Copper: Height $12-18\,m$, Width $24-38\,m$, Slope $50-60$ degrees.
• Iron: Height $9-14\,m$, Width $18-30\,m$, Slope $60-70$ degrees.
• Nonmetallic: Height $12-30\,m$, Width $18-45\,m$, Slope $50-60$ degrees.
• Coal: Height $15-23\,m$, Width $15-30\,m$, Slope $60-70$ degrees.
• Overall Range: Height $9-30\,m$, Width $15-45\,m$, Slope $50-70$ degrees.

Open pit mining is a large-scale method responsible for more than $60\%$ of all surface output worldwide. It is highly mechanized and capital-intensive, allowing for the exploitation of low-grade metallic deposits at reduced costs.

Sequence of Development and Pit Infrastructure in Open Pit Mining

The development of an open pit involves hauling moderate to large amounts of waste and ore over long distances at steep grades. Because ore grades are often decreasing, stripping ratios must be kept modest, usually between $0.8$ and $4\,m^3/tonne$. Due to these economic and technological constraints, most open pit mines do not exceed $300\,m$ in depth, unless the deposit is of exceptional grade, size, or stripping ratio.

The major steps in development include obtaining permits, land clearing, and the construction of surface buildings. Strategic planning is required for the location of waste dumps, topsoil stockpiles for reclamation, and ore processing facilities. Initial entry into a bench is achieved via a box cut (or drop cut), which is a wedge-shaped volume of rock removed to establish the first bench face. Drillholes are placed in parallel rows at descending depths to create a ramp of negotiable grade between benches.

Road design is critical for safety and efficiency. Haul roads are designed with specific widths, banking, and safety berms. A typical haul road structure includes a wearing course of $200\,mm$, a base layer of selected blasted waste rock, and a subgrade of recompacted in situ material. In many pits, trucks travel on the left side (or the highwall side) to allow drivers to better gauge their distance from the berm, preventing accidental falls into the pit. Pit slopes are classified as working slopes during exploitation (utilizing a high factor of safety) and ultimate slopes in the final stages of the pit's life.

Cycle of Operations and Auxiliary Tasks in Open Pit Mining

Exploitation involves three main phases: overburden removal, mining the valuable minerals, and auxiliary operations. If the ore and waste are similar in nature, operators often use identical equipment for both to allow for interchangeability during breakdowns or production spikes.

For overburden removal and mining, the drilling methods selected depend on rock strength: augers for weak rock, roller-bit rotary for average rock, and percussion or rotary-percussion for hard rock. Blasting typically uses Ammonium Nitrate and Fuel Oil (ANFO) or slurry, which is loaded via bulk trucks or by hand and fired using electrical caps or detonating cord. Excavation equipment includes power shovels, hydraulic shovels, FELs, dozers, and scrapers. For soil-like materials, bucket-wheel excavators are used. Haulage is managed via trucks, belt conveyors, or rail. In very steep pits, hoisting methods such as high-angle conveyors, skip hoists, or hydraulic pipelines are employed.

Auxiliary operations are essential for safety and efficiency and include slope stability monitoring, dust control, pumping and drainage, waste disposal, surveying, equipment maintenance, and personnel transport. Environmental control and reclamation are prioritized during the mine's lifetime to minimize the final expense of terminating operations.

Amenability and Characteristics of Open Pit Mining

Open pit mining is applicable under various conditions. Ore and rock strength can be of any type. While any deposit shape can be mined, thin deposits parallel to the surface with low dips are preferred. Size is ideally large or thick, and grade can be very low if other factors allow for it. Uniformity is preferred, but blending is easily implemented. Depth is limited by the economic stripping ratio, with shallow to intermediate depths being ideal.

Advantages of open pit mining include high productivity ($90$ to $366\,tonnes$ per employee-shift), low labour requirements, flexibility in output, and nearly $100\%$ recovery with low dilution. It offers safer conditions than underground mining by eliminating underground hazards. Disadvantages include depth and stripping ratio limits, high capital investment for large equipment, and vulnerability to weather. Environmental impacts, such as the potential for acid mine drainage if the pit fills with water, and the need for extensive land reclamation, are also significant.

Quarrying for Dimension Stone

Quarrying for dimension stone refers to the extraction of rectangular blocks of rock, such as marble, granite, or slate, roughly sized for architectural use. These stone products are used for monuments, building slabs, flagstones, roofing, and wall tiles. While quarries resemble open pits, their benches (called faces) are generally lower and vertical. Highwalls in a quarry can reach impressive heights, sometimes approaching $300\,m$. These operations are typically smaller in scale and limited to materials with specific structural or aesthetic properties.

Open Cast (Strip) Mining: Direct Casting and Variations

Open cast mining, often colloquially called strip mining, is primarily used for coal and bedded deposits. It differs from open pit mining in that overburden is not hauled to waste dumps; instead, it is cast directly into adjacent mined-out panels. This process, known as casting, allows excavation and dumping to be performed by a single machine, typically a specialized boom-type excavator like a dragline. This method achieves the highest productivity of all surface methods because it eliminates the need for overburden haulage.

Typical dimensions in open cast mining include a highwall height of $30$ to $60\,m$, a cut width of $23$ to $45\,m$, a highwall slope of $60$ to $70$ degrees, and a spoil pile slope of $35$ to $50$ degrees. The two major variations of open cast mining are area mining and contour mining. Area mining is used on flat terrain with flat-lying seams, where cuts are made in straight, parallel panels. Contour mining is conducted in hilly or mountainous terrain, following the contours of the topography and limited by the economic stripping ratio. Reclamation in these methods often follows immediately after mining.

Development and Operations in Open Cast Mining

The development of an open cast mine involves the creation of an Environmental Impact Assessment (EIA), selecting stripping equipment, and positioning the surface plant. In flat areas, the plant is located centrally to minimize haulage distances. In outcrop areas, it is located on barren land near the outcrop. The initial drop cut is the most difficult phase and may require some haulage of overburden.

The stripping cycle is determined by the overburden nature. If the rock is hard, vertical drillholes are stopped about a metre above the coal to prevent damage. Ripping with a dozer is an alternative for weak rock. The mining cycle for the ore or coal involves cleaning the top of the seam with a rotary brush or dozer, followed by optional drilling and blasting or direct excavation with an FEL, power shovel, or continuous miner. At some phosphate operations, the ore is flushed into a slurry using hydraulic monitors and transported via pipeline. Auxiliary operations prioritize reclamation, as timely restoration can avert sizeable bonding costs.

Open Cast Mining Amenability, Advantages, and Applications

Open cast mining is suited for tabular, bedded deposits that are preferably horizontal or have low dips. It is most effective for continuous deposits of large lateral extent. Advantages include the highest productivity of any coal mining method (average of $9\,tonnes$ per employee-hour), low unit cost (relative cost of approximately $10\%$), and high recovery rates approaching $100\%$. It allows for early production and minimizes the need for highwall support.

Disadvantages include technological depth limits (generally about $90\,m$) and stripping ratio limits (ranging from $1.3$ to $19\,m^3/tonne$). The method has a negative public image due to surface damage, requiring proactive public relations and substantial environmental reclamation. Weather can impede operations, and surface runoff must be managed to prevent acid mine drainage. Common applications include the mining of coal, anthracite, bentonite, lignite, phosphate, tar sands, and uranium.

Auger Mining and Highwall Extraction Methods

Auger mining is a surface mining method that recovers coal from under the highwall once the ultimate stripping ratio of an open cast operation is reached. This is classified as a surface method because the crew remains on the surface to remotely operate the equipment. Two variations exist: traditional auger mining and highwall mining. Traditional auger mining uses drag-bit drilling heads ($0.6$ to $2.4\,m$ in diameter) to bore circular holes $60$ to $90\,m$ long into the highwall, typically recovering $40\%$ to $65\%$ of the coal. Highwall mining involves sending a remotely controlled continuous miner into the highwall to create parallel entries roughly $3\,m$ wide and as high as the seam, reaching depths up to $300\,m$ with a recovery rate of approximately $60\%$.

Auger mining is economically attractive because it extracts additional coal at a much lower cost than primary stripping. However, it requires specific conditions: long highwalls, stable roof conditions, and a bench width of $15$ to $25\,m$. No additional stripping is required unless extracting from an outcrop. The cycle consists of excavation using the augering machine or highwall mining system, with haulage handled by auger flights or conveyor units. The method has been used since the 1940s, while highwall mining became more reliable after the 1950s following the development of the Joy Pushbutton Miner.

Auger Mining Characteristics and Amenability

Auger mining is amenable to tabular, bedded deposits with low or nearly horizontal dips. It is feasible for small or large deposit sizes and can be used on low-grade ore. It is preferred for seams that are uniform in thickness and free of partings. The primary advantages include very high productivity ($22$ to $450\,tonnes$ per employee-shift), the lowest cost of any coal mining method (relative cost around $5\%$), and low capital investment. It preserves surface topography and recovers coal that would otherwise be lost.

Disadvantages include the low overall recovery of the in-place coal and the hazard of methane explosions. The productivity is also entirely dependent on a single extraction unit. Auger mining is rarely an independent method and is almost exclusively used in conjunction with open cast mining. Auxiliary operations are simple, focusing mainly on power distribution and haul road maintenance.