The basic rule for a material to have paste fill flow properties is that it has at least 15 wt% of its particles finer than 20 microns. This generally ensures that the material has the required colloidal properties and is not self draining. What this means is that the material, when mixed with sufficient water to produce a 150 mm to 250 mm (6″ to 10″) slump consistency (measured using the North American 12″ high cone slump test), does not have a pipeline critical flow velocity and will retain at least 95 wt% of the water used for transport when placed in the stope.
These properties produce a stiffer, lower porosity fill with better ground support properties. Unfortunately, these properties while beneficial to transport and ground support, make the backfill susceptible to liquefaction if not consolidated with a binder. Consequently, paste fills cannot be placed underground without sufficient binder to consolidate the material and produce the required cohesion necessary to overcome the potential of liquefaction.
Paste backfill for underground mining is generally made from mill tailings which can be categorized by their size distribution into two basic types. Coarse tailings produced from many base metal ores (nickel, copper) contain from 20 to 30 wt% minus 20 microns size particles. These tailings make good paste fills and have relatively favourable paste fill flow properties. This type of tailings can usually be transported via gravity through a 150 mm diameter (6″) borehole/pipeline distribution system at 140 tonnes/hour if the vertical head to horizontal transport distance is at least three to one. As the ratio decreases the placement rate drops proportionately.
Gold ore and some base metal ore (zinc) produce much finer tailings from the milling. The tailing ranges from 30 to 60 wt% minus 20 microns in sizing. The high fines content generally produces a greater resistance to flow in a paste fill and reduces the placement rate to approximately 90 tonnes per hour in the above distribution system. This type of tailings is often passed through a cyclone to remove a portion of the fine fraction before it is used for paste fill at a mine.
A third type of paste fill can be created by blending alluvial sand or mining waste products (mine waste rock and/or smelter slag) with the tailings to produce a specific paste fill size distribution that has minimum flow resistance and/or superior ground support properties. This well graded paste fill generally contains between 13 to 20 wt% minus 20 micron material and can be placed at rates of approximately 180 tonnes per hour in the above gravity flow distribution system.
The type of backfill (rock fill, slurry fill, or paste fill) used at a mine will be determined by the mine geometry and the materials economically available to produce an underground fill. If paste fill proves to be the most economic fill to use at a mine site, the type of paste fill produced will be determined by the economics behind producing the different paste fills and the underground performance required from the backfill in daily production and long term ground support.
The three paste fill plants described in this paper demonstrate the variation in design that accompanies the different types of material that can be used by a mine for paste backfill. It also demonstrates how the use of paste fill over hydraulic slurry or rock fill will allow a mine to utilize more of the different waste materials produced by mining as backfill. These advantages along with the potential for lower binder consumption and improved ground support properties make the development of paste backfills one of the major innovations in mining in the last several decades.
