What is open-pit mining?
Open-pit mining, also known as opencast mining, is a surface mining technique that extracts minerals from an open pit in the ground. Open-pit mining is the most common method used throughout the world for mineral mining and does not require extractive methods or tunnels. This surface mining technique is used when mineral or ore deposits are found relatively close to the surface of the earth. Open-pits are sometimes called ‘quarries’ when they produce building materials and dimension stone.
Benefits of open-pit mining include:
- Ease-of-use for mass production
- Small shut-down expense
- Ability to mine selectively for certain grades of ore
- Comparatively small crew size
- Elimination of safety hazards that can accompany complex underground mining operations
- Easy drainage of subsurface water
- No machinery restrictions - even heavy and bulky machinery can be utilized
- Lower capital and operating costs
Materials commonly extracted from open-pit mines include:
- Bitumen
- Clay
- Coal
- Copper
- Coquina
- Diamonds
- Iron
- Gold
- Gravel and Bedrock
- Granite
- Gritstone
- Gypsum
- Limestone
- Marble
- Molybdenum
- Silver
- Uranium
- Phosphate
Open-pit Mining Operations
Open-pit mines are dug on benches that are between four and sixty-meters in size, depending on the size of machinery used to excavate. The walls of most open-pit mines are dug at an angle and include steps to prevent avalanches from occurring inside the build site. The incline section of the wall is called the ‘batter,’ and the flat part of the step is called the ‘bench’ or ‘berm.’
In some cases, de-watering bores are drilled horizontally into the walls to relieve water pressure, which can destroy the walls if not properly addressed.
A haul road is dug at the side of the pit to form a ramp for ore-carrying trucks to haul material to and from the mining site.
After the physical mine infrastructure has been built, production activities include drilling, blasting, excavation, loading and transporting broken ore.
Waste rock is piled up near the edge of the pit and spreads both horizontally and vertically. This is known as the ‘waste dump’. The waste dump is also tiered and stepped, to keep rocks from falling into other parts of the site.
Processed ore, known as ‘tailings,’ is pumped into a settling pond until the water evaporates. These tailings ponds are often toxic due to the presence of unextracted sulfide minerals.
After the mining operations are complete, what happens to an open-pit mine?
After mining finishes, the mine area must undergo rehabilitation to minimize environmental damage. This step in the mining process is critical to ensuring the sustainability of the land for future use.
First, waste dumps are contoured to flatten them out and stabilize them. If the ore contains sulfides, it is covered with a layer of clay to prevent rain and oxygen from oxidizing the sulfides into sulfuric acid, which is also known as acid mine drainage.
Then, the waste dump is covered with soil, vegetation is planted, and the area is fenced to prevent livestock from eating the newly planted vegetation. This layer will eventually erode but in the meantime, it will allow the leaching of heavy metals to occur slowly enough for the surrounding environment to absorb them.
The open pit is also fenced off to prevent access, and over time fills up with groundwater unless the groundwater levels are excessively deep.
