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"There is considerable demand for consistent quality and quantity of backfill in mining operations," Rob Bradley, manager - mine engineering at Murray & Roberts Cementation, says.
"This need relates to a number of issues, which include safety, ore extraction rates, reduction in ventilation costs as well as the traditional requirement of providing regional support in the stopes," he says.
Murray & Roberts Cementation has extensive experience in this technology and by the very nature of its involvement in the mining industry can offer its comprehensive experience and engineering capability to its clients.
Backfill is a mining method used to fill previously mined-out stopes with a suitable product. It is essentially used to provide regional rock support within a mining excavation in order to enable further operations. The method is believed to have started in South Africa in the 1930s, but capacity was limited. Murray & Roberts’ experience commenced in the 1980s.
"Each backfill operation is unique in terms of the business case, logistics, mining depth, strength required, materials used and the reason for the backfilling operation. We have extensive experience in a wide variety of applications," Bradley says.
Backfilling is mostly used for regional support, and also offers potential advantages of improved ore extraction rates and lower injury risk, as timber support systems constitute a fire risk. It also facilitates improved ventilation; in deep level mines it reduces energy costs by optimising refrigeration and ventilation services and infrastructure.
Backfilling also helps simplify logistics and reducing the risk of materials handling injuries when compared to conventional support systems.
Typically, a mine will define a need for a backfill operation. At this stage, Murray & Roberts Cementation consults with rock engineers and geologists from the mine and in some cases brings in its strategic partner, NS Consultancy. "With an established mine, all the necessary expertise in terms of geology and hanging wall conditions would be on site, as personnel are familiar with the mine and its geology. Greenfield operations require an in-depth understanding of the pumpability and flow characteristics of the available materials (rheology) and necessitate determining optimal mix designs through test work and engineering modeling. This is highly specialised work, requiring specialised instrumented pumping test loops, hence the value of having a close association with an organisation such as NS Consultancy," Bradley comments.
Following these assessments and studies, Murray & Roberts Cementation completes the up front engineering, which takes factors such as client requirements, cost, geology and rheology into consideration. This process determines the specification of the material to be used for the backfill, the flow through the pipes and the strength characteristics of backfill required. Each application requires a specific solution.
"This is not a case of a generic solution fitting all mines," Bradley says.
From the rheological design and type of fill, the company designs the reticulation system and the mixing and pumping plant as well as quality control systems. Murray & Roberts Cementation will then build the plant and, if of value, operate it.
Backfill material is generally pumped into geofabric bags placed underground where the support is needed, which have been attached to the hanging wall. Excess water from the slurry which has been pumped underground through the reticulation system drains out and solids remain in the bag, ultimately providing the support once cured. The product being pumped has custom designed properties which will depend on the strength required according to the rock, the depth and the size of the area.
Murray & Roberts Cementation has been involved in designing and building various backfill systems, some to a depth of 2700 metres below surface and with a reticulation in excess of 34 km.
A recent feasibility study undertaken has a backfill design reticulation of several hundred kms.
"Since this is a safety critical operation, risk assessments are undertaken prior to the design to ensure that all safety levels are met. The backfill solution is tailored to the mining method in place such as longhole mining or drift and fill," Bradley explains.
The predominant and preferred material used for backfill is mine waste tailings often blended with cementitious binders such as Portland cement and slagment. This is an environmentally friendly option which allows the mines to move some of their waste back underground. Where a strong backfill is not required, a lower cost binder such as fly ash could be added. Fills include cyclone-classified tailings, full plant tailings and semi-classified tailings, each with its own unique set of characteristic particle size distributions and self-draining and water reticulation characteristics, which affect pumpability and final strength development.
Types of backfill include:
- Hydraulic fill 55 - 68% solids (1.54 - 1.75 Relative Density)
- High density fill 65 - 72% solids (1.75 - 1.84 Relative Density)
- Paste fill 72 - 85% solids (1.82 - 2.0 Relative Density)
- Tailings aggregate fill (tailings aggregate where small particles are added)
- Rock fill (this is the cheapest option and, although not technically a backfill, it is used)
"Backfilling demands an engineered solution which needs to be supervised. Benchmarking and best practice are always used by Murray & Roberts Cementation," Bradley says.
Test work is always undertaken to ensure that UCS development - Unconfined Compressive Strength - is determined. Tri axial stiffness tests are done to determine the strength of the material to ensure that particles bind correctly, while liquification tests are done to ensure the most appropriate binder content to ensure stability.
Actual pumping tests are also often carried out through pipeline test loops to optimise flow rates and minimize pressure losses to ensure that the appropriate fill, binder and rheology has been found.
Backfill material is generally gravity fed down the shafts, maintaining full flow conditions through the pipelines to minimise the wear.
With careful consideration to maintaining full flow it is possible to have feeds from the stock tanks to more than one region simultaneously. The installation and size of barricades and bulkheads retaining the backfill within excavations depends on the type of fill being used as well as the excavation height. Materials of construction include steel wire rope, geofab material and wooden poles. Full structural designs are supplied at specification stage to ensure safety, and very high structures can be terraced for structural integrity.
"Where necessary, we can also establish pumping stations underground to boost flows to other areas, although this is not ideal. We can also build storage dams underground should it be required," Bradley says.
“Backfill engineering is a complex topic and involves many engineering disciplines and practical smarts, all of which Murray & Roberts Cementation can integrate into a single package, which few others can do," he concludes.
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