crusher for underground coal mines

Crushers for Underground Coal Mines: An Overview

Within the intricate and demanding environment of an underground coal mine, the efficient reduction of mined material is a critical link in the production chain. Crushers for underground coal mines are specialized machines designed to break down large run-of-mine (ROM) coal into a manageable and consistent size directly at the mining face or in close proximity. This primary crushing step is essential for ensuring smooth material flow on conveyor belts, optimizing bunker storage, and preparing coal for subsequent transport to the surface. Unlike surface mining operations, underground crushers must contend with severe space constraints, stringent safety regulations regarding dust and ignition sources, high reliability demands, and the need for modularity to follow the advancing mine workings. This article explores the types, applications, key selection criteria, and real-world implementations of these vital underground systems.

Types of Crushers Used Underground

The choice of crusher is dictated by coal characteristics (hardness, moisture, presence of rock), required capacity, and spatial logistics. The two predominant types are:

• Roll Crushers: Employ two counter-rotating rolls to crush material via compression. They are favored for their simplicity, compact design, and relatively lower generation of fines. They are effective for crushing brittle materials like coal and can handle moderate levels of rock.
• Sizer Crushers (or Mineral Sizers): Utilize two rotors with teeth or picks that rotate inward at low speed to shear and slice the material. They have become increasingly standard in modern high-capacity longwall mines due to their high throughput, ability to handle wet and sticky material with less clogging, and lower power consumption per tonne.

A comparative overview is provided below:

Feature	Roll Crusher	Sizer Crusher
Crushing Principle	Compression	Shear and Tear
Typical Capacity	Low to Medium	Very High
Fines Generation	Moderate	Lower (typically)
Handling Wet/Sticky Coal	Prone to clogging	Excellent - self-cleaning design
Space Requirement	Compact	Generally longer but can be modular
Primary Advantage	Simplicity, lower capital cost	High capacity, efficiency in tough conditions

Other types like jaw crushers are rarely used in modern coal seams due to bulkiness and higher fines generation.

Key Design & Operational Considerations

1. Safety & Certification: Equipment must be certified for use in potentially explosive atmospheres (e.g., MSHA approval in the USA, ATEX in Europe). This involves flameproof/explosion-proof electrical components and designs that minimize spark risk.
2. Dust Control: Integrated water spray systems or dry fog systems are mandatory to suppress respirable dust at the crushing point.
3. Mobility & Installation: Crushers are often mounted on skids or rugged crawlers to be relocated as sections advance. In longwall operations, they are part of a "beam stage loader" (BSL) system behind the shearer.
4. Robustness & Reliability: Built to withstand extreme shock loads from large lumps of coal and rock (e.g., "hard hats") with minimal downtime.

Real-World Application: Case Study

A prominent example is the implementation of mobile sizer crushers in several Australian longwall mines operating in the Bowen Basin.

• Challenge: Mines extracting high-capacity longwall panels faced bottlenecks at the stage loader area where lump coal from the shearer needed immediate size reduction before entering the belt conveyor system. Traditional crushers struggled with peak loads and varying material conditions.
• Solution: Installation of heavy-duty, electrically driven mobile sizer crushers integrated directly into the BSL setup. These units were designed with high-torque, low-speed rotors capable of handling sudden influxes of large volumes.
• Outcome: The sizers provided consistent product sizing (typically below 250mm), ensuring no blockages on downstream conveyors. Their reliability significantly reduced unplanned stoppages at this critical transfer point, directly contributing to achieving planned longwall advance rates and overall mine productivity. The specific model used was often a tracked mobile unit from manufacturers like MMD or FLSmidth (formerly Sandvik), allowing it to be pulled forward during panel retreat.

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Frequently Asked Questions (FAQs)

Q1: Why can't we simply transport all ROM coal to the surface for crushing?
Transporting oversized material long distances via conveyors is inefficient and hazardous. Large lumps can cause belt damage, spillage, blockages at transfer points, and pose a significant safety risk. Crushing underground creates a uniform product stream that maximizes conveyor capacity and safety.

Q2: What is the single biggest maintenance concern for underground coal crushers?
Wear on crushing elements (roll shells, sizer teeth/picks) is constant due to abrasion from sandstones/shales within the seam. However,unexpected tramp metal or extremely hard rock intrusions ("hard hats") pose a greater risk, potentially causing catastrophic damage like broken shafts or rotors. Modern crushers incorporate shear-pin protections or hydraulic overload systems to mitigate this.

Q3: How does moisture content affect crusher selection?
High moisture leads to sticky clayey materials that can plug chutes and adhere to crusher surfaces.Sizer crushers, with their intermeshing teeth and cleating action,are specifically engineered to handle wetter feeds more effectively than roll crushers, which rely more on compression friction.

Q4: Are there fully automated underground crushers?
Yes. Modern installations are integrated into mine-wide monitoring systems.Automation includes remote start/stop sequences synchronized with conveyors, automatic adjustment of roll/sizer gap for product consistency based on power draw monitoring,and real-time condition monitoring(vibration,temperature)to predict maintenance needs.

In conclusion,crushers in underground coal mines are not merely auxiliary equipment but are productivity-critical assets.The evolution from simple roll crushers to advanced,safe,and highly reliable sizers reflects the industry's drive towards higher efficiency,safety,and output within one of its most challenging operational environments