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Miner's Toolbox |
| Mine Backfill Engineering |
| Types of Mine Backfill |
There are 4 types of mine backfill
Dry Fills: -generally consists of surface sand, gravel, open pit waste rock, underground waste rock,smelter slag -generally unclassified except to remove large boulders -usually transported underground by dropping down a raise from surface directly into a stope or to a level where it is hauled to a stope with an LHD or trucks. -Usually contains some adsorbed surface moisture. -Suitable for mechanized cut and fill or avoca or other method where structural backfill is not required.
Cemented Rockfill Generally consist of waste rock mixed with a cement slurry to improve the bond strength between the rock fragments. Methods of placement involve mixing the rock and cement slurry in a hopper before placing in stopes, or percolating a slurry over the rock after it has been placed. The waste rock can be classified or unclassified. CRF contains a mixture of coarse aggregate (<150 mm) and fine aggregate (<10 mm fraction). The ideal gradation is one which minimizes the void space. Cement slurry concentration is approximately 55% by wt. (1.2:1 water:cement ratio) Suitable for longhole open stoping, undercut and fill , and other methods where a structural fill is required.
Hydraulic Sandfill Hydraulic sandfill can consist either of classified mill tailings (Westmin-Myra Falls) or naturally occurring sand deposits mined on surface (Detour Lake Mine). Hydraulic sandfill is prepared by dewatering the mill tailings stream to a pulp density of approximately 65-70% solids (depending on S.G) and then passing it through hydrocyclones to remove the "slimes" and retain the sand fraction for backfill. Slimes are removed to improve the percolation rate of the backfill. The backfill mixture is hydraulically pumped from surface through a network of pipes and boreholes to the stope. Sand obtained from surface borrow pits will be screened prior to use in a backfill plant to remove oversize particles that could plug the backfill line. Sandfill can be cemented or uncemented. Successful sandfills have permeability coefficients in the range of 7x10-8 m/s to 7.8x10-5 m/s corresponding to a medium silt to coarse sand. Hydraulic placement of sandfill results in a loose fill structure with a void ratio of about 0.70. In practice, an apparent cohesion often develops in uncemented sandfill which increases the shear strength of the backfill. Often a vertical face of 3-4 m can be maintained under some mining conditions. Nearby blast vibrations can also act to densify the fill and increase its shear strength To overcome the lack of true cohesion in the sandfill, cement and other binders are added. Note that backfill strength decreases with water content and the water content needed to transport sandfill is far in excess of what is required for cement hydration. Hence, mine operators are moving towards less water in the fill to decrease cement and binder consumption Flow velocities in excess of 2 m/s are required to maintain a homogeneous dispersion of the fill components in the slurry
Paste Backfill Paste backfill is a high density backfill (>70% solids depending on SG). In order to pump material at this density, a component of fines is required. As a general rule, the fines content (<20 micron) should be a minimum of 15% by weight The slump of paste backfills is approximately in the 7-10 inch range Paste backfill is pumped by piston type pumps of the same type used to pump concrete. Whole mill tailings can often be used to make paste backfill. The final product has a lower void ratio so the backfill is denser. Many mines are moving towards paste backfill because it a lower cement content is required to gain equivalent strengths when compared to conventional hydraulic fill.
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