An open-cast mine is a surface excavation or cut made to extract ore, remaining open to the surface throughout the mine's operational lifespan. The process involves exposing and mining ore, typically requiring the excavation and relocation of significant amounts of waste rock. In commercial mining operations, the primary goal is to exploit the mineral deposit at the lowest feasible cost, ultimately maximizing profits.
A bench in the context of open-cast mining refers to a ledge representing a single operational level. Above this ledge, mineral or waste materials are mined, creating a distinct bench face. Successive layers, each forming a bench, are removed to extract the mineral or waste. Multiple benches may operate simultaneously in different parts and elevations of the open-cast mine.
The bench height is the vertical distance from the highest point, or crest, to the toe of the bench. It is typically determined by the specifications of operating machinery, such as drills and shovels, and is subject to government mining regulations.
The bench slope is the angle, measured in degrees, between the horizontal plane and an imaginary line connecting the bench toe and crest.
Pit limits define the vertical and lateral boundaries within which economically viable open-cast mining can occur. The decision on pit limits is often influenced by the balance between the cost of removing overburden or waste material and the minable value of the ore. Additionally, factors such as existing surface infrastructure (e.g., townships, rivers) play a role in determining pit limits.
To enhance slope stability and ensure safety, berms are often left within the pit. A berm is a horizontal shelf or ledge incorporated into the ultimate pit wall slope. The specifications for berm interval, berm slope angle, and berm width are determined by the geotechnical characteristics of the slope. These safety measures contribute to the overall stability of the pit slope.
The overall pit slope angle in open-cast mining is the angle formed by the wall of the pit, measured between the horizontal plane and an imaginary line connecting the top bench crest to the bottom bench toe.
To facilitate access into the pit throughout the mining operation, a haul road is essential. A spiral system involves arranging the haul road in a spiral pattern along the perimeter walls of the pit, ensuring a relatively uniform gradient from the top to the bottom of the pit. In contrast, a zigzag or switchback system involves the road zigzagging to surmount steep grades on the footwall side of the pit. The choice between a spiral and zigzag system depends on factors such as the shape and size of the ore body, truck economics, and pit slope stability.
The width of the haul road is determined by the required capacity and the type of haulage unit being used. The grade of the road is defined as the inclination in terms of degrees from the horizontal or as a percentage of rise to the horizontal.
The angle of repose, or angle of rest, represents the maximum slope at which a heap of loose material will remain stable without sliding. The sub-outcrop depth is the depth of waste material that must be removed before exposing any ore. This waste is often referred to as preproduction stripping.
Open-cast mining is a surface mining method used to extract any near-surface deposit by creating a surface pit with one or more horizontal benches. This method is commonly employed in mining metallic or nonmetallic deposits and occasionally in coal and other bedded deposits. The process involves removing both the overburden (if present) and the ore in benches ranging from 9 meters to 30 meters in height. Additional benches can be added to extract a pit of any desired depth. In the case of thick deposits, many benches are needed, resulting in a shape resembling an inverted cone.
The purpose of the benches is to control the depth of blastholes, the slope of pit walls, and the safety of highwall faces. Benches also provide sufficient face length for sustained, uninterrupted production. Stripping and mining activities are coordinated to ensure that ore revenues cover waste costs, while long-term objectives are met.
The design of individual benches takes into account the materials-handling equipment used. The height of the bench is limited by the reach of the excavator, with power shovels capable of trimming higher banks than front-end loaders or hydraulic excavators. The width must be adequate to contain flyrock from bench blasts and allow maneuvering room for excavators and haulage units. The slope of the bench and the pit itself is determined by rock or soil mechanics considerations.
Open-cast mining is a large-scale production method, allowing the extraction of mineral resources at lower costs, making it feasible to mine ever-decreasing grades of most metallic deposits. This approach enables the use of highly mechanized, capital-intensive, and labor-saving mass production equipment. The versatility of open-cast mining is illustrated in figures, showcasing its applicability on flat-lying seams and in areas with multiple seams, such as certain iron or coal measures, as well as other deposit types.
Open-cast mining typically involves the transport of substantial amounts of waste and ore over relatively long distances with steep grades. Due to the usually low ore grades, and the decreasing trend in most commodities, ratios of waste to ore must be kept at modest levels (usually ranging from 0.8 to 4 cubic meters per tonne). As a result, most open-cast mines are less than 300 meters in depth unless associated with deposits of better-than-average grade, stripping ratio, or size.
The major steps in developing an open-cast mine include clearing the land after obtaining all necessary permits, constructing surface buildings, and strategically locating waste dumps. If surface reclamation is necessary, topsoil stockpiles are also established. Ore storage, processing, and storage facilities are situated with consideration for the ultimate pit limit and external access. Equipment is selected and acquired as required, followed by advanced stripping of overburden to facilitate timely ore exploitation. Stripping and mining operations are carefully coordinated in line with short-range and long-range mining plans.
Establishing the first bench and subsequent benches in waste or ore is a crucial operation. The initial entry into a bench is known as the box cut (or drop cut), representing a wedge-shaped volume of rock that must be removed to create a new bench face. Drillholes are strategically placed in parallel rows in descending order of depth, allowing for a negotiable grade ramp from the upper to the lower bench after blasting and excavation of the box cut.
Proper bench and haul road design is a significant concern in pit development. The height and width of working benches are typically determined by the dimensions of the equipment used, with excavator reach being a key factor. Working slopes are often established with a relatively high factor of safety to ensure stability during the deposit's exploitation. In the final stages of pit development, slopes may be steepened to achieve a lower stripping ratio.
The design of haul roads is critical for safety and efficiency. Well-managed pits incorporate proper haul road width, banking, safety berms, and curve designs to ensure trucks can operate safely with low rolling resistance. In many pits, haulage trucks travel on the left side of the road to help drivers gauge their position relative to the berm, reducing the risk of misjudging and accidentally driving off the road.
This excavation method, often characterized by a cone shape (although adaptable to various shapes based on ore body size and form), is employed for ore bodies with characteristics such as being pipe-shaped, vein-type, steeply dipping, stratified, or irregular. While commonly linked to metallic ore bodies, it is versatile enough to be applied to deposits of any type that align with the required geometric considerations.
The essential steps for exploiting an ore body encompass stripping the overburden, extracting valuable minerals, and conducting auxiliary operations to ensure the safe and efficient progression of the operation.
Stripping is the process of removing overlying material to expose the deposit and excavating overburden within the pit boundaries once the ore is revealed. The specific nature of the overburden dictates the sequence of operations. Softer materials may not necessitate breakage, while more consolidated rocks require explosives for fragmentation. Materials-handling equipment is then chosen to meet operational requirements.
Drilling
Auger (for weak rock)
Blasting
Ammonium nitrate-fuel oil (ANFO) or slurry
Excavation
Power shovel
These alternative methods provide flexibility in executing each stage of the operations cycle, allowing adaptation to the specific characteristics of the materials and operational conditions.
Certainly! Let's delve deeper into the various aspects of the mining operations cycle, including the processes of drilling, blasting, and excavation:
In open-cast stripping and mining, the selection of equipment and operational cycles is heavily influenced by the nature of the ore and waste materials. Similarities or even identical equipment and cycles can be employed when the ore and waste exhibit significant resemblance. The advantage of using the same equipment and cycle lies in the interchangeability of machinery during breakdowns or unexpected changes in production or stripping demands. The mining cycle of operations and the associated equipment typically involve the following components:
The key consideration in selecting equipment and cycles is the degree of similarity between ore and waste. This similarity allows for increased operational flexibility and efficiency, particularly in situations where equipment interchangeability is advantageous.
In both stripping and mining operations, auxiliary tasks play a crucial role. In the context of open-cast mining, specific auxiliary operations of concern include:
Successful open-cast operations are influenced by various natural, spatial, and geologic conditions. The most logical circumstances under which the open-cast mining method can be applied include:
These conditions collectively define the practical and economic feasibility of employing the open-cast mining method.
Opencast (strip) mining is a surface exploitation method primarily utilized for coal and other bedded deposits, resembling open-cast mining with a distinctive characteristic: the overburden is not transported to waste dumps but cast directly into adjacent mined-out panels. This casting process involves excavation and dumping into a final location, performed as a single unit operation by a single machine. The method's uniqueness lies in casting within the pit, contributing to high productivity and often lower costs. Open-cast mining is considered a large-scale and widely popular surface method.
The attractiveness of open-cast mining goes beyond haulage replacement with casting. Spoil (overburden) deposited in mined-out areas concentrates mining activity in a relatively small space, allowing immediate reclamation. Additionally, the short duration the pit remains open permits steeper slopes in the overburden bank, known as a highwall, with typical dimensions ranging from 30 to 60 m in height and slopes between 35o to 50o.
The key to productivity in open-cast mining lies in the output of the stripping excavator. Utilizing some of the world's largest land machines reduces the number of active faces in the mine, enhancing overall productivity. However, a drawback is the reliance on a single excavator for the entire production, with major breakdowns posing significant challenges. Presently, the focus in open-cast mining has shifted towards seeking versatility and reliability over merely increasing the size of stripping machines.
Unlike open-cast mining, the open-cast method typically involves distinct equipment for stripping overburden and mining coal or minerals. Specialized boom-type excavators are employed for casting overburden, while conventional loading and haulage equipment is used for mining. The differences in overburden (soil or blasted rock) and the minerals mined (usually coal, whether blasted or not) necessitate the use of different equipment for each operation.
In planning a large open-cast mine in a flat or low-relief area, the surface plant is strategically positioned at or near the center of the reserve. This central location is chosen to minimize haul distances and provide convenient access to all parts of the reserve. In cases where the deposit outcrops, an alternative approach might be to place the plant adjacent to the outcrop, preferably on barren land to avoid interference with mining activities. The choice of surface transportation (such as trucks, rail, or water) can also have a significant impact on the plant's location.
Open-cast mining, by its nature, relies heavily on equipment selection. When comparing overburden shovels and draglines in terms of cost per cubic meter (Rs/m3), one might expect overburden shovels to be the preferred choice for most open-cast mines. However, the reality is often the opposite. The reasons for this preference are not solely dependent on the costs associated with a fixed volume of overburden.
The key activities in the development of an open-cast mine commence with the clearing of the land, followed by the establishment of a surface plant. Emphasizing the significance of reclamation in this method, special attention is dedicated to determining the location and ensuring the maintenance of topsoil stockpiles. Environmental and restoration procedures are meticulously planned to align logically and efficiently with the mining activities. Strategic placement of coal dumping, storage, processing (if applicable), and transport facilities is carefully considered in relation to the mining operation.
Once the equipment is selected, the initial pit development phase is initiated. Similar to open-cast mining, the first cut in this process may be a drop cut. This initial cut is often challenging, potentially requiring some haulage of the overburden, and progress may be relatively slow. However, maintaining a highwall after completing the first cut is standard practice, rendering subsequent cuts more efficient. With the establishment of the highwall, simultaneous development and exploitation can progress in a normal and systematic fashion.
The sequence of operations during stripping in open-cast mining is primarily dictated by the characteristics of the overburden. The excavation of soil and decomposed rock may not require prior breakage, while stiff soil or weak rock can be ripped before excavation, and hard rock necessitates drilling and blasting. The drilling method is also tailored to the overburden material, with auger drills suitable for soil or soft rock, roller-bit rotary drills for intermediate rock, and percussion or rotary-percussion drills for hard rock. Drilling is typically conducted using a pattern of vertical holes that terminate about a meter above the coal to prevent excessive damage to the coal seam. Generally, the cycle of operations in stripping involves the following steps:
Drilling
This systematic approach ensures the efficient and safe removal of overburden in open-cast mining, employing various equipment and techniques based on the specific characteristics of the materials being excavated.
The nature of the coal or ore being mined plays a significant role in determining the operational cycle. The process may involve direct loading, ripping and loading, or drilling and blasting. Coal seams, in particular, often allow for direct excavation without prior preparation, utilizing equipment such as power shovels or front-end loaders. The typical mining cycle includes the following steps:
Auxiliary Operations in open-cast mining encompass various crucial tasks, including:
In open-cast mining, efficient reclamation practices can significantly impact costs, making it a key focus in the overall mining operation.
The deposit conditions amenable to open cast mining vary significantly with the type of mineral extracted and the geologic conditions that exist in the deposit area.
The ideal characteristics for a viable ore deposit include versatile ore and rock strength, accommodating various deposit shapes with a preference for tabular and bedded structures. Additionally, the deposit dip is acceptable at any angle, although a preference is expressed for horizontal or low dip configurations. A desirable deposit should exhibit a substantial lateral extent with a preference for continuous formations. While ore grades can be on the lower side, the favorability of other conditions compensates for this. Uniformity in ore distribution is preferred, and the depth of the deposit is ideally shallow to moderate, allowing for effective control of the stripping ratio. These combined attributes contribute to the overall feasibility and viability of the ore deposit.
These combined attributes make the selected coal mining method a favorable choice, balancing productivity, cost-efficiency, and safety considerations.
Despite its advantages, the chosen coal mining method and associated equipment encounter certain limitations and considerations:
Considering these factors is crucial for a comprehensive understanding of the method's limitations and potential challenges, allowing for informed decision-making and effective management of the mining operations.
The removal of overburden in strip mining is a method well-suited for specific geological and topographical conditions. Its applications are optimized under the following favorable circumstances:
These specified conditions are pivotal for the successful and efficient application of strip mining. By adhering to these criteria, mining operations can achieve economic feasibility, optimize stripping ratios, and sustain prolonged and productive mining activities over an extended period.
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