Open-pit mining.

Overview of Open-Pit Mining: Open-pit mining, also referred to as open-cast or open-cut mining, is a surface mining technique employed to extract rocks or minerals from the Earth through an open-air pit, commonly known as a borrow. Unlike methods involving tunnelling into the Earth, such as long wall mining, open-pit mining is conducted on the Earth’s surface.

Key Characteristics:

1. Surface Extraction: Open-pit mining involves the extraction of valuable materials from deposits located near the Earth’s surface.
2. Application: This method is utilized when commercially valuable ore or rocks are found in proximity to the surface. It is especially applicable when the overburden (material covering the deposit) is relatively thin or when the material of interest is structurally unsuitable for tunnelling. Examples include cinder, sand, and gravel.
3. Contrast with Underground Mining: Open-pit mining stands in contrast to extractive methods that require tunnelling into the Earth, as in long wall mining. While open-pit mining is suitable for materials close to the surface, underground mining is employed for minerals found deeper in the Earth’s crust, particularly when they are encased in hard rock.

Open-pit mining is a surface mining technique chosen for its efficiency in extracting commercially valuable materials from deposits near the Earth’s surface. This method is particularly favored when overburden is minimal or when the material of interest is not conducive to underground tunnelling. It represents a widely employed approach in the mining industry for accessing surface-level mineral resources.

Extraction Process in Open-Pit Mining:

To establish an open-pit mine, a systematic process is followed, involving the determination of ore information underground. The extraction process includes the following key steps:

1. Ore Information Gathering: Miners use drill probes to create holes in the ground, mapping each hole’s location on a map. This information provides insights into the vertical extent of the ore body.
2. Benches Planning: The vertical information obtained is crucial for planning the tentative locations of benches in the mine. The grade and economic value of the ore are considered in this planning phase.
3. Quarrying and Bench Mining: Open-pit mines producing building materials and dimension stone are often referred to as quarries. Mining occurs on benches, which are vertical levels of the hole. The interval between benches depends on the deposit, mineral, and machinery size. Large mine benches are typically 12 to 15 meters thick.
4. Progressive Enlargement: Open-pit mines are expanded until either the mineral resource is depleted or the ratio of overburden to ore makes further mining economically unviable.
5. Landfill Conversion: Exhausted mines are sometimes repurposed as landfills for solid waste disposal. Water control measures are required to prevent the pit from turning into a lake, especially in regions with considerable precipitation or where pit layers border productive aquifers.
6. Formation of Artificial Lakes: In some regions like Germany, former open-pit mines are deliberately converted into artificial lakes. Acid mine drainage is a concern in the formation of these lakes.
7. Bench Structure: The walls of the pit are generally mined at an angle less than vertical for safety against rock falls. The walls are stepped, with inclined sections known as batters and flat parts called benches or berms. These steps prevent rock falls from cascading down the entire face of the wall.
8. Ground Support Measures: Additional ground support measures like rock bolts, cable bolts, and shotcrete may be employed where necessary. De-watering bores are used to relieve water pressure, preventing failures in the wall.
9. Haul Road Construction: A haul road, usually located at the side of the pit, forms a ramp for trucks to transport ore and waste rock.

The extraction process in open-pit mining involves careful planning, consideration of geological factors, and implementation of safety measures to ensure efficient and safe mineral extraction.

Impacts of Open-Pit Mining:

Waste:

1. Volume of Waste: Open-pit mines generate substantial waste, with large mines moving almost one million tons of ore and waste rock daily, while smaller mines move a couple of thousand tons.
2. Operations Contributing to Waste: Drilling, blasting, loading, and hauling operations contribute to the creation of waste in open-pit mining.
3. Waste Disposal: Waste rock is transported to waste dumps, either on the surface of the active pit or in previously mined pits.
4. Tailings: Leftover waste from ore processing, known as tailings, often in slurry form, is pumped to tailings dams or settling ponds where water is either reused or evaporated. The presence of unextracted sulfide minerals, toxic elements in the gangue, and cyanide used in gold ore treatment can make tailings dams potentially toxic if proper environmental protections are not in place.

Pollutants:

1. Air Pollution: Open-pit mining disrupts the ground, leading to the creation of air pollutants. Main sources include mineral transportation, drilling, blasting, and the loading/unloading of overburden. These pollutants pose risks to public health, causing respiratory issues and increasing mortality. Additionally, they impact the surrounding flora and fauna.
2. Gold Mining and Health Concerns: Open-pit gold mining, in particular, poses threats to air and water chemistry. The exposed dust may contain toxic or radioactive elements, posing health concerns for workers and nearby communities.

Hydrology in Mining:

1. Groundwater Issues: Open-pit mines operating in areas with heavy groundwater features may face hydrology-related problems. This includes heaving and bursting of the mine floor due to excessive uplift pressure.
2. Groundwater Control: Installation of a groundwater control system is crucial to address hydrology-related problems. This involves dewatering and depressurization wells to manage groundwater pressures. The design of open-pit slopes must adapt based on evolving understanding of rock mass conditions, including groundwater.
3. Open Pit Depressurization: Depressurization is employed to remove tensions or pressure from different areas of a mine, enhancing stability and security. Techniques like annealing, involving slow heating and cooling, relieve internal stress, increasing workability and durability of materials, thereby improving mine safety.
4. Longevity and Expansion: Proper depressurization allows for considerable mine expansions, potentially extending the life of the mine by 10 to 15 years.
5. Prevention of Slope Failure: Horizontal drains are used to accelerate slope depressurization, preventing large-scale slope failure in the mine by lowering pore pressure and enhancing slope stability. Optimization-based control systems are necessary to manage local and regional hydro-geological impacts effectively.