Mobile crushers play a crucial role in various industries, including quarrying, ore processing, and recycling of demolition waste  It is essential to provide a safe operation of mobile crushers, emphasizing its relevance to individuals with responsibilities for these machines, including contract managers, supervisory staff, and operators. 

1.Risk Assessment:

A full working method statement and specific risk assessment should be produced for all works involving a mobile crusher. The method statement should cover all the issues raised in this article as well as prevailing site, environmental and regulatory conditions. The key risks are;

    • Machinery Guarding
    • Clearing Blocked or Stalled Crushers
    • Slips and Trips
    • Transport – Potential contact with excavators loading the crusher

1.1 Importance of Proper Planning

    • Well-planned operations are efficient, safe, and productive. Poorly planned operations pose risks of inefficiency, danger, and potential fatalities.
    • Mobile crushers have the capacity to cause fatalities if not operated with caution.

1.2 Typical Layouts

    • Mobile crushers are typically fed by loading shovels, backhoe excavators, or other processing machinery.
    • Standalone machines can discharge directly to a stockpile.
    • For reinforced concrete crushing, consider magnetic separators to prevent metal contamination

2. Feeding Methods

2.1. For excavator loading

  • Make sure the excavator standing pads are stable and positioned at a height that allows the operator to observe the feed hopper from the cab.
  • Provide safe access to the excavator. For wheeled loading shovels, the ramp’s width should accommodate adequate edge protection (minimum 1.5m high) on both sides, considering the machine’s travel. 
  • The ramp’s gradient should not exceed the loader’s capacity (a recommended maximum gradient of 1:10). 
  • The final few meters of the ramp should be level to prevent uphill discharge, facilitating feed monitoring and enhancing loading shovel stability.
  • Ensure the exclusion of pedestrians and obstructions from the bucket operating arc.

2.2. For conveyor loading

    • Ensure proper guarding for all hazardous machinery components. 
    • Neglecting to replace guards following cleaning or maintenance procedures is a significant factor contributing to injuries on these machines.

3. Blocked Crushers

3.1 Reasons for crusher blockages 

  • Stalling caused by:
    • Electrical or mechanical malfunction
    • Material becoming lodged in the chamber, resulting in an overload
    • Excessive material being fed
    • Introduction of tramp metal or wood
    • Accumulation of material in the crash box
    • Accumulation of fine material in the crusher discharge chute
  • Bridging due to:
    • Feed material being larger than specified
    • Abundance of clay or other fines in the crushing cavity, hindering the passage of small material through the crusher
    • Presence of a foreign object in the crusher feed or discharge chamber impeding the flow of material

3.2 Prevention of Blockage 

The primary focus is on preventing the entry of oversize material or tramp metal into the crusher feed hopper through various precautionary measures. The strategies encompass multiple aspects of operation, maintenance, and design:

  • Blast Design in Quarrying: Tailoring quarry blasts to achieve optimal rock fragmentation.
  • Material Reduction Techniques: Employing drop balls or hydraulic hammers to reduce oversize material, particularly in demolition and construction scenarios.
  • Loader Driver Training: Educating loader operators to refrain from loading oversize material.
  • Sizing Bars on Crusher Feeds: Implementing bars on crusher feeds to regulate the size of the material.
  • Housekeeping Practices: Establishing a comprehensive housekeeping program to prevent scrap steel from entering shovel buckets.
  • Bucket Size Management: Ensuring buckets are appropriately sized for the crusher’s capacity.
  • Regular Inspection: Conducting frequent inspections of metal parts, such as bucket teeth and wear plates, to ensure their integrity and prevent breakage.
  • Magnetic and Metal Detection Measures: Strategically placing electrical magnets and/or installing metal detectors to avert tramp metal from entering the crusher.
  • Level Indicators: Employing level indicators for precise control over feed.
  • Drive System Maintenance: Regularly maintaining drive systems to ensure optimal performance.
  • Discharge Chute Cleaning: Removing and adequately cleaning the discharge chute to prevent material blockages.

Photo 1 : Crusher Blockage

 

A well-designed mobile crusher operation should ideally not require personnel to be present on the crusher access platform during routine crushing activities. However, if such presence is unavoidable, it poses various risks, including:

  • Ejection Hazards: Risks of being struck by objects ejected from the crusher, such as stone or metal fragments.
  • Operational Hazards: Potential contact with crusher catches, especially by reinforcing bars.
  • Entrapment Risks: The danger of being pulled into the crushing chamber while attempting to remove contaminants.
  • Loader Bucket Risks: Potential strikes by the loader bucket if the access platform is within the working radius of the loading machine.
  • Fall Hazards: Risks of falling if guardrails and proper access arrangements are lacking.
  • Noise Exposure: Exposure to high levels of process noise, requiring adequate hearing protection.
  • Dust Exposure: Risks of inhaling dust, particularly in concrete or brick crushing operations, leading to serious respiratory diseases.
  • Vibration Exposure: Continuous low-frequency vibration exposure, posing risks of whole-body vibration.

In cases where a person equipped with the appropriate personal protective equipment (PPE) needs to adjust the feed speed, particularly in the absence of remote control facilities, it is crucial to adhere to safety protocols and ensure controlled loading rates into the feed hopper.

3.3 Action when a Crusher Becomes Blocked

Action when a Crusher Becomes Blocked:

    1. Supervision and Assistance:
    • Designate a nominated person to supervise the activity.
    • Call for assistance as necessary.
    • Stop the feed at the earliest opportunity to prevent further blockage.
    • Utilize mechanical means to remove excess material whenever possible.
    • This proactive step aims to address the blockage cause promptly.
    • In certain cases, manual removal may be necessary. When manual removal is employed, ensure that the crusher and associated plant are stopped and isolated.

Hazards Encountered During Blockage Resolution:

    • Poor or Difficult Access: Challenges related to accessing the blocked area, potentially hindering effective resolution.
    • Accidental Start-Up: Risks associated with unintentional activation of the feeder, crusher, or adjacent plant during the unblocking process.
    • Struck by Material: Danger of being struck by material from the feeder, chute, or projected fragments during unblocking.
    • Movement of Material inside the Crusher: Risks associated with the movement of any material present inside the crusher during unblocking.
    • Slipping and Falling: Hazards related to slippery surfaces, posing risks of slips and falls during the unblocking activity.
    • Manual Handling Risks: Dangers associated with the manual handling of rocks and equipment during the removal process.
    • Unexpected Movement of Crusher Components: Risks arising from sudden and unexpected movements of crusher components during unblocking.
    • Damaged Electrical Cable: Potential hazards related to damaged electrical cables, posing electrical safety risks.
    • Noise Exposure: Risks associated with high noise levels during the unblocking process, necessitating hearing protection.
    • Stored Energy Risks: Hazards arising from stored energy in electrical, hydraulic, compressed air, mechanical sources, and gravity.
    • Unsafe Placement of Removed Material: Risks associated with the improper placement of material removed from the crusher, posing additional safety concerns.

By following these actions and being aware of potential hazards, the unblocking process can be conducted safely and efficiently, minimizing the risks to personnel and equipment.

3.4 Clearing Blockages in Bridged Crushers

When dealing with a bridged crusher, it is crucial to employ effective methods to clear blockages safely. 

    • Utilize an excavator with a quick hitch bucket attachment. Choose a static pick or hydraulic hammer for effective blockage clearance.
    • Depending on the risk assessment results, the hydraulic arm method can be employed with or without the crusher still operating. The decision should prioritize safety and efficiency based on the specific situation.
    • In cases where hydraulic arms are unavailable or cranes with hooks prove ineffective, alternative methods become necessary. If a person needs to enter the crusher for positioning hooks or slings, strict safety measures must be followed, including stopping, isolating, and locking off the crusher and feeder. Additionally, wearing a safety harness is imperative.
    • Before considering alternative options, conduct a detailed and thorough risk assessment. Shut off and isolate the crusher before exploring alternative methods.
    • Wedges should be avoided due to the risk of becoming projectiles, which has resulted in fatalities in the past.
    • Some options require specialized expertise and competence. These include:
      • Gas or chemical expansion.
      • Hydraulic ramp plates.
      • Permanently mounted hydraulic breakers (may involve closed-circuit television CCTV for operator assistance).
      • Use of explosives.

By carefully considering these methods and options, and prioritizing safety through risk assessments, the clearing of blockages in bridged crushers can be conducted efficiently and with minimized risks to personnel and equipment.

Photo 2: Clearing Blockage

 

 

4. Safety Protocols for Stalled Crashers

Encountering a stalled crusher poses potential risks, especially if tramp metal is involved, as it may lead to fatal consequences. To manage such situations, strict safety measures and a comprehensive set of procedures are imperative. Here’s a guide on dealing with stalled crushers:

  • Treat a Stalled Crusher with Caution: Consider a stalled crusher as a potential jammed condition with tramp metal, which could be ejected, posing serious risks.
  • Issuing Written Instructions: Provide clear, written instructions to plant operators outlining the steps to take in the event of a crusher stalling.
  • Include the following procedures in the instructions:
    • Isolation of Motive Power: Clearly outline the procedure for isolating the motive power to the crusher and associated plant. 
    • Clear the Area: Direct operators to clear the immediate area of all personnel to minimize potential harm. 
    • Notify Site Manager: Clearly specify the process for notifying the site manager about the stalled crusher.
  • Examination for Causes: After a thorough examination, if no evident electrical or mechanical reasons for the stall are found, it suggests a possible jam caused by tramp metal.
  • Stalled Crusher Permit to Work System: Implement a “stalled crusher permit to work” system to regulate and control any work conducted on the stalled crusher. Ensure that only personnel who are suitably trained and competent carry out the work under this permit.
  • Safe Inspection Practices: Emphasize the importance of inspecting the crushing cavity of a jaw crusher from below, not from above. Remind personnel of past fatal accidents resulting from material being ejected when inspecting from above.
  • Critical Reminder: Stress the importance of avoiding inspections from above, as fatal accidents have occurred due to material ejection.

By adhering to these safety protocols, providing clear instructions, and implementing a permit system, the risks associated with a stalled crusher can be effectively mitigated. Regular training and awareness campaigns further contribute to maintaining a safe working environment around crushers

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