Mining isn’t just about extracting minerals; it’s about addressing what’s left behind. In ‘room and pillar’ and ‘cut and fill’ mining operations, backfilling becomes critical. By using cemented backfill, in-situ pillars containing ore can be extracted. This is possible as the cemented backfill acts as a support, preventing heading collapses and subsidence issues. Typically, tailings for backfill are mixed with a binder-like cement on the surface in a small processing plant. The mix is then transported down a decline, shaft, or through boreholes into the sections of the mine needing backfill.
What is Cement Paste Backfill?
Cement Paste Backfill (CPB) is a blend of mine tailings, water, and binders like cement. Once prepared, it’s pumped into the underground voids left by mining operations. The inclusion of cement helps the mixture solidify, offering a firm mass that prevents rockfalls and subsidence.
Why is CPB Important?
Advantages:
- Environmental Preservation: Storing tailings underground minimizes surface disturbance. This approach is more eco-friendly as there’s no need to designate vast land areas for surface tailings storage.
- Mitigation of Surface Issues: By using underground backfill, a range of surface problems are mitigated, including:
- Dust generation
- Visual disturbances
- Surface water contamination
- Inundation risks due to tailings facility failure
- Extraction of Valuable Ore: With backfill in place, ore-rich pillars and supports can be extracted without jeopardizing the mine’s structural integrity.
- Enhanced Mine Support: The backfill provides additional support to the mine, reducing risks associated with voids or empty spaces.
- Reduced Risk of Rock Bursts: Storing tailings underground helps distribute pressures evenly, minimizing the risk of rock bursts, which can otherwise occur if pressures concentrate on pillars and supports.
- Improved Ventilation: The process enhances the mine’s ventilation circuit, ensuring a safer environment for workers.
- Protection Against Roof Falls: The use of backfill can prevent roof falls that might result from blasting, particularly those induced by Air Over Pressure (AOP).
- Groundwater Protection: Binders in the backfill not only solidify the tailings but also minimize groundwater contamination risks.
- Reduced Acid Rock Drainage (ARD): The oxidation rates for pyritic tailings, which can lead to ARD, are substantially reduced when they are stored as cemented backfill.
- Optimized Water Recovery: Storing tailings underground often results in better water recovery before storage compared to traditional surface disposal methods. This can be both cost-effective and environmentally beneficial.
Disadvantages:
- Tailings Processing and Crushing: To prepare tailings for backfilling, they often need to be processed and crushed to achieve the desired grain size. This adds to operational complexity, costs, and energy consumption.
- High Costs: Using binders and other necessary materials can be expensive.
- Intensive Dewatering: The tailings need to be dewatered to a paste consistency, leading to high operational expenses.
- Pumping Challenges: Expensive positive displacement pumps may be required for high-density tailings discharge.
- Operational Delays: Hold-ups in extraction and mine development strategies can occur.
- Liquefaction Risks: If saturation levels of the tailings are high and a triggering event like seismic vibration occurs, there’s a risk of liquefaction. This necessitates the building of barricade walls in some cases.
- Potential Groundwater Contamination: While binders minimize this risk, there’s still a possibility of tailings effluent seepage into the groundwater.
- Increased Operational Requirements: Managing an independent backfill plant requires extra manpower and equipment.
- Ore Dilution: This can result from poor quality fill placement or extraction management.
- Infrastructure Issues: Problems can include pipeline plugging, borehole blockages, backfill sloughing, pipeline bursts, bulkhead failures, and more.
- Environmental Concerns: Despite the mitigation of surface disturbances and associated risks, other environmental challenges can emerge, like potential groundwater contamination and issues related to the binders used.
Incorporating Fibers
To address some of these challenges, particularly the high binder content, the industry is exploring the use of fibers in the backfill mix. Fibers can enhance the strength and cohesion of the backfill, potentially reducing the required amount of costly binders like cement. By doing so, this can lead to cost savings and potentially more environmentally friendly backfill compositions.
Types of Backfill in Underground Mines
There are multiple backfill types utilized, depending on the specific needs of the mine:
- Paste Backfill: Similar to surface paste deposition, it has >65% solids (by weight) and displays a homogenous appearance.
- Hydraulic Sand Fill: Used when tailings are separated into slimes and sand fractions.
- Cemented Fill: A mix of tailings and waste rock. Useful when only small volumes of cement slurry are needed.
- Dry Rock Fill: Suitable for the cut-and-fill mining method, it employs rock waste, surface sands, gravels, or dried tailings.
Where is CPB used today?
The use of cement paste backfill (CPB) has become a staple in many mining operations worldwide due to its safety, environmental, and economic advantages. Here are a few real-life examples where CPB has been employed:
- Kidd Creek Mine, Canada: One of the pioneers in the use of CPB is the Kidd Creek Mine in Timmins, Ontario. Operated by Glencore, the mine has been utilizing CPB since the 1970s. Their early adoption and development of this technique have made them a reference in the mining industry. The system they developed involved dewatering the tailings, mixing them with cement, and then pumping the paste backfill underground. Their success has been a model for other mines across the world.
- Lundin Mining’s Eagle Mine, USA: Located in Michigan, this nickel and copper mine adopted the use of CPB to address environmental concerns and improve stability. By using CPB, the Eagle Mine has minimized surface tailings storage, reducing the potential environmental footprint. The backfill process also allows them to maximize ore extraction by providing support to mined-out areas.
- Bulyanhulu Gold Mine, Tanzania: Operated by Barrick Gold Corporation, the Bulyanhulu mine uses CPB to stabilize the mine and reduce the environmental impact of its operations. The use of CPB has enabled the mine to safely extract ore from areas that would otherwise be too risky to mine. The mine’s commitment to safety and sustainability has made it a key player in the Tanzanian mining sector.
- Mount Isa Mines, Australia: Owned by Glencore, the Mount Isa Mines complex is one of the oldest and most expansive mining operations in Australia. Over the years, the mine has faced challenges in managing its tailings. However, with the introduction of CPB technology, they’ve been able to reutilize tailings, stabilize mined-out zones, and improve the overall safety and efficiency of the operation.
- Zinkgruvan Mine, Sweden: Operated by Lundin Mining, the Zinkgruvan mine is a significant producer of zinc, lead, and silver. The mine has adopted CPB as a method to enhance the stability of underground excavations and to maximize ore recovery. The use of CPB has also minimized the mine’s environmental impact by reducing the need for surface tailings dams.
Challenges and Innovations
Like any other process, the use of CPB is not without its challenges:
1. Mix Design: Getting the right mixture of tailings, water, and cement is crucial. Too much water can lead to a weak backfill, while too little can make the mixture unworkable.
2. Pumping: Pumping a viscous mixture like CPB requires specialized equipment. The backfill must be delivered uniformly and consistently to be effective.
Despite these challenges, innovations are continually emerging. Advanced rheology tests are being developed to understand the flow behaviour of the CPB better. Moreover, new binder formulations are being explored to improve the strength and durability of the backfill.
A Glimpse into the Future
With the increased emphasis on sustainable mining practices, the use of CPB is expected to grow. Researchers are exploring additives that can make the backfill more resilient and environmentally friendly. There’s also interest in understanding how CPB can be used in conjunction with other mining methods to enhance efficiency.
Moreover, as digital technology advances, we may soon see mines using real-time sensors to monitor the quality and consistency of the backfill. This would ensure that every inch of the mined void meets safety and environmental standards.
Conclusion
Cement paste backfill might sound like a niche topic, but it’s at the forefront of making mining safer, more sustainable, and more efficient. As we dig deeper into the Earth in search of resources, technologies like CPB ensure that we do so responsibly and innovatively.