Mine backfill is soil, overburden, mine tailings or imported aggregate material used to replace excavated zones created by mining operations. Mine fill is an integral component of mines' design and method with many operations utilizing backfill as a means to aid the stabilization of mining-related voids and the disposing of mining wastes. Concrete is a common additive to backfill material to improve strength and stability in underground excavations.
Backfill material can be differentiated into three different categories, hydraulic fill, paste fill, and rock fill, based on water, cement and aggregate content. Transportation and installation varies between types of backfill from pumping through pipes and pouring to hauling with trucks and dumping the fill material in excavated areas. Hydraulic fills are any kind of backfill carried by water through pipelines. Solid particles are sluiced through the water quickly without having the chance to settle until they reach the dumping point. Paste fill is bound with cement to create a very strong product. Much thicker than hydraulic fill, similar to toothpaste, paste fill is also much more uniform in texture after placement. Rock fill can be cemented or non-cemented mine waste rock or aggregate material placed underground by means of trucks, conveyors or raises.
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.
• Around the turn of the century (1900s), mine tailings and other types of waste materials are introduced in underground mines mostly for waste disposal purposes without full knowledge of backfill properties and other consequences.
• The use of hydraulic backfill systems became common in the 1940's and 1950's.
• Following serious mine accidents involving placement of straight tailings in underground openings; removal of the finer portions of mill tailings to control percolation was initiated and implemented. The 100 mm (4”) percolation rate became the industry standard.
• Addition of Portland cement to portions of backfill materials such as scraping floors in cut and fill stopes was initiated in the late 1950s.
• Rock fill and cemented rock fill were introduced in the 1960s. (Mount-Isa, Kidd Creek)
• The 1970s saw a number of significant developments in backfill systems:
The adaptation and improvement of different mining methods as a result of a better understanding of backfill properties. Numerical modeling started to emerge and to bring about a better understanding of the backfill interaction with the surrounding rock masses and the role played by backfill in overall mine support. The introduction of fill in bulk stopesand the requirements for greater free-standing backfill faces necessitated the use of increased quantities of cement with the attendant cost implication. This leads to the investigation and use of alternative binders for fills. Controlled pours and the use of pozzollanslead to lighter bulkhead designs for open stopes.
• The early new century will likely see a more intensive assessment of environmental aspects related to mine backfills.
Engineering proper backfill properties, transportation and installation is critical to the success of the required task.
• Selection of mining methods and backfill requirements
• Outline of required backfill properties through stability calculations, modeling, etc.
• Investigation of sources, quantities and qualities of potential backfill materials
• Testing of fill materials: physical properties, specific gravity of solids, angle of internal friction, porosity of settled fill, percolation rates, strength tests (uniaxial compression strengths with various cement or other binder content at various curing ages, shear strength, compressibility, deformation under load, etc.)
• Design backfill system: fill preparation, storage, cement or fly ash addition storage, transportation to fill plant and to mined out areas, methods of fill placement, bulkhead design, drainage systems, quality control, record keeping and monitoring of the following: quantities of sands poured, water in and out of stopes, cement quantities, fill dilution, fill stability, etc.
• Economic evaluation of various options, costs savings, ROR, etc.
• Detailed engineering, construction, start-up, documentation, routine operation and fine tuning. Run new backfill system with a test stope, implement monitoring of backfill operations, training as required, record keeping: set up various computerized forms, etc., coordination between mine and mill records, follow-up site visits.
Types of Backfill
Hydraulic Fill (slurry and dense slurry fills)
These fills are made of mill tailings and are prepared in surface plants. The fraction of fines is removed through a process known as desliming, whereby the entire fill material is circulated through hydrocyclones and the fine fraction is removed and then sent to the tailings dam. The remaining hydraulic fill fraction is reticulated in the form of slurry through pipelines to underground voids.
The hydrocyclones are used in fill preparation to:
1. Classify: to produce a sands underflow and a slimes overflow, and
2. Dewater: to produce a high density underflow and a low density overflow.
Ws = wight of sands in tonnes
Gs = specific gravity of sands solids
γw = specific gravity of water
n = porosity expressed as a fraction
A = area of stope being filled
Water Monitoring & Removal
As the hydraulic backfill settles water will drain out of the fill material. A water monitoring program and efficient water removal design is essential. Poor water drainage could cause fill liquefaction which is extremely hazardous to mine personnel and operations.
Due to the high water content of hydraulic fill, transportation can be easily managed through pipelines directly to targeted stopes.
Paste fill is comprised of full mill tailings with a typical effective grain size of 5 μm, mixed with a small percentage of binder, in the order of 3–6% by weight, and water. It is the densest form of backfill in the spectrum of thickened tailings placed underground as a backfill material. The acceptance of paste backfill as a viable alternative to hydraulic slurry and rock fill did not truly occur until the mid- to late-1990s with the construction and successful operation of several paste backfill systems in Canada and the BHP Billiton Cannington Mine in Australia.
Three Types of Paste Fill
Coarse tailings: -contain between 15 and 35% minus 20 micron at 7 inch slump, have a pulp density of 78 to 85% solids by weight, will produce fill with at least double the strength of similar slurry fill for same cement content
Medium tailings: contain between 35 and 60% minus 20 micron at 7 inch slump, have a pulp density of 70 to 78% solids by weight
Fine tailings: -contain 60 to 90% minus 20 micron at 7 inch slump, have a pulp density of 55 to 70% solids by weight, high retention of water: good for transport, but bad for strength
Quality control of paste backfill production is of critical importance to the successful transportation and intended required use. A 0.5% change in solids concentration can result in a 2.5 cm change in the slump character of a backfill (Paynterand Dodd, 1997) and increases in paste fill pulp densities of as small as 1% to 2% has demonstrated a doubling of pipeline pressure. Paste backfill is characterized by its slump character or the reclining ‘slump’ paste fill experiences when idle.
Unlike slurry or dense fill paste fill are not transported in a slurry form. Instead a central plug of coarse particles flows through the pipe with an annulus of fines slower moving particles next to the pipe wall acting as a lubricant. A generic “rule of thumb” for the grain size distribution is for a minimum of 15% of the material to be finer than 20 μm, which ensures that the surface area of the grains is large enough to provide adequate surface tension to ensure that the water is held to the solid particles and to provide a very thin, permanent lubricating film.
Slurry fill and paste fill can be placed underground at rates of 100 to 200 tonnes/hour. But for the same volume of fill per hour, paste will be introduced at a higher rate, because there is less water in the paste fill.
Rock Fill (cemented & non-cemented)
Rock fill practice can be divided into two parts for convenience,
1. Traditional practice, in which available materials are used to fill open slopes, with focus on minimization of ground movement problems rather than pillar recovery, and 2. Engineered approach in which cemented rock fills are employed to fill open stopes, with a definite focus towards improving pillar recovery.
Fill within this category is placed dry, with moisture content usually below 5 wt. per cent, to prevent problems resulting from the fill material becoming “sticky”, as at higher moisture contents. Placement usually involves simply passing under gravity from surface or an underground tipple to the stope to be filled. In some cases, however, horizontal distribution involving haulage or conveying may be needed to transport fill from a central pass to the stope to be filled.
Cemented Rock Fill
Dry aggregate material is transported to target excavation then binding material can added in several ways:
2. By tipping rock fill at a central point and introducing the "mortar" constituents contemporaneously around the stope periphery.
3. By tipping rock fill at a central point and introducing the "mortar" around the periphery after rock fill placement.
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