Types of Underground Mining Methods

Introduction:
Underground mining is the extraction of minerals and ores that are located beneath the earth's surface. This method is used when mineral deposits are too deep to be economically mined by open-pit methods. Underground mining methods are diverse, tailored to the specific characteristics of the ore body, the surrounding rock, and economic factors. In this article, we will explore the different underground mining methods, their applications, and key considerations. We will also discuss the advantages and disadvantages of each method, providing a detailed analysis of the best-suited scenarios for each mining technique.

1: Room and Pillar Mining

Room and pillar mining is one of the oldest and most commonly used mining methods in the United States. It involves excavating rooms (areas of ore) while leaving behind pillars (structures of ore) to support the mine roof. The size and arrangement of these rooms and pillars depend on the strength of the ore and surrounding rock. This method is mainly used in coal mining but can also be applied to other minerals such as potash, salt, and trona.

Process:

• The ore is removed by cutting out "rooms" while leaving "pillars" to support the roof of the mine.
• The size of the pillars is calculated based on the strength of the ore and the depth of the mine.

Advantages:

• Relatively simple and cost-effective.
• Can be mechanized for increased efficiency.

Disadvantages:

• Significant ore is left behind in the form of pillars.
• Ground control issues can arise if the pillars are too small or poorly placed.

Applications:
This method is commonly used in coal and flat bedded deposits like potash and limestone. It is most effective when the ore body is relatively horizontal, shallow, and has regular dimensions.

2: Longwall Mining

Longwall mining is another underground mining method primarily used for coal extraction. It involves the use of large mechanical shearers that remove coal along a wide wall, typically ranging from 150 to 300 meters in length.

Process:

• Large panels of coal are divided into blocks.
• A longwall shearer extracts the coal as the face progresses, and hydraulic supports hold up the roof until mining is completed.

Advantages:

• High extraction rate, often recovering up to 90% of the ore.
• Highly mechanized, reducing the need for manual labor.

Disadvantages:

• High initial capital costs for equipment.
• Requires significant geological stability.

Applications:
Longwall mining is widely used in coal mining, particularly for seams that are too deep or too thin for open-pit mining. It is efficient in extracting coal seams that are flat and extensive.

3: Cut and Fill Mining

Cut and fill mining is a versatile and highly selective underground mining method used for ore bodies with irregular shapes and steep inclines. It involves mining the ore in horizontal slices, filling the void with waste rock, tailings, or concrete after each slice is removed.

Process:

• The ore is excavated in horizontal slices.
• Once a slice is mined out, the void is filled with backfill material to provide support for the next layer.

Advantages:

• Suitable for irregular and steeply dipping ore bodies.
• Offers excellent ground support and stability.

Disadvantages:

• Labor-intensive and slower than other methods.
• Relatively expensive due to the backfilling process.

Applications:
Cut and fill mining is used in narrow ore bodies, high-grade deposits, and areas where selective mining is required. It is common in underground gold and silver mining.

4: Block Caving

Block caving is a bulk underground mining method often used for low-grade, massive ore bodies. It involves undercutting the ore body, causing it to collapse under its weight and break into manageable pieces for extraction.

Process:

• A series of tunnels are dug under the ore body.
• The ore is undercut, and as it collapses, it is collected and transported to the surface.

Advantages:

• Low operating costs due to minimal drilling and blasting.
• Suitable for massive ore bodies with weak rock.

Disadvantages:

• Requires extensive pre-development.
• Can lead to surface subsidence.

Applications:
Block caving is commonly used in the extraction of copper, diamonds, and molybdenum. It is particularly suited for low-grade, large-scale deposits that are not economical to mine by other methods.

5: Sublevel Caving

Sublevel caving is a variation of block caving where the ore body is divided into sublevels for mining. The ore is extracted in a top-down manner, with the rock collapsing into the void created by the previous extraction level.

Process:

• Ore is blasted and extracted from sublevels while allowing the overlying rock to collapse into the void.
• This process is repeated as mining progresses downward.

Advantages:

• Continuous production with less downtime.
• Suitable for steeply dipping ore bodies.

Disadvantages:

• High dilution rates due to collapsing waste rock.
• Potential for ground control problems.

Applications:
Sublevel caving is used in mining steeply dipping orebodies, particularly in iron ore and copper mining. It is effective for ore bodies with moderate to poor stability.

6: Shrinkage Stoping

Shrinkage stoping is a mining method that involves mining upward in horizontal slices, allowing the broken ore to accumulate in the stope. This broken ore supports the stope walls until the extraction is complete, after which the ore is drawn off.

Process:

• The ore is mined in successive horizontal slices.
• Broken ore is left in place to support the walls and is only removed at the end of mining.

Advantages:

• Simple and cost-effective for high-grade ore bodies.
• Provides good ground control during mining.

Disadvantages:

• Labor-intensive with slow production rates.
• Requires strong, stable rock conditions.

Applications:
Shrinkage stoping is typically used in narrow veins, gold, and silver mining, where high-grade ore is the target, and the ore body is steeply dipping.

7: Vertical Crater Retreat (VCR) Mining

VCR mining, also known as vertical crater retreat, is a method used for steeply dipping ore bodies. It involves drilling large-diameter holes in a vertical pattern and blasting the ore into a series of voids created by previous blasting.

Process:

• Large-diameter holes are drilled in a vertical retreat pattern.
• Blasting is used to fragment the ore, which is then collected from draw points at the base of the stope.

Advantages:

• High recovery rates with minimal dilution.
• Efficient for steeply dipping ore bodies.

Disadvantages:

• Requires precise drilling and blasting techniques.
• Can lead to ground control issues if not properly managed.

Applications:
VCR mining is commonly used in base metal mining (such as copper, lead, and zinc) and is particularly effective for steeply dipping ore bodies with good rock conditions.

Conclusion:

Underground mining methods vary significantly in their application and suitability based on the ore body characteristics, rock stability, economic considerations, and environmental impacts. While methods like room and pillar and longwall mining are efficient for flat-lying coal seams, techniques such as cut and fill and block caving are better suited for irregular and massive ore bodies. Selecting the right method is critical for ensuring economic viability and minimizing environmental impact. Understanding the advantages and limitations of each method allows for more strategic planning and successful mine operations.