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Miner's Toolbox |
| Mine Backfill Engineering |
| Hydraulic Backfill | |||
Introduction Hydraulic backfill, sometimes known as conventional backfill, slurry backfill, or sandfill, is a mixture of sand sized solid particles, water and binder. The mixture is about 60-75% solids by weight depending on the specific gravity of the solids. The excess water must drain from the fill after placement and pumped from the mine. Hydraulic backfill is usually pumped using conventional slurry pumps or allowed to flow by gravity through the mine if the mine is located below the backfill plant. Hydraulic backfill must be pumped with a high velocity in order to maintain a turbulent flow in the pipeline so the solids remain in suspension. Hydraulic backfill is most commonly prepared from classified mill tailings however there are operations that use sand from alluvial or esker deposits mined on surface. It is frequently the case that the tailings do not have sufficient coarse fraction to meet all of the backfill requirements so the mine may supplement the backfill with borrow sand or may fill parts of the stopes with waste rock where possible. Hydraulic backfill is prepared and placed in the following steps: 1. Desliming of Mill Tails 2. Dewatering of Mill Tails 3.Storage of Classified Tails 4. Repulping of Tails with Water 5. Addition of Binder (optional) 6. Transport to Stope 7. Water Drains from Fill
Determining if a Decant System is Required Timber fill fences can normally be employed successfully provided adequate drainage of the fill can be maintained. Typically, a minimum percolation rate of 10 cm/hr is specified for hydraulic backfill. Depending on the rate of filling and other factors, this percolation rate may or may not be sufficient to drain the stope quickly enough to. Drainage towers can be avoided if the percolation rate of the fill exceeds the rate at which excess water develops in the stope. Hydraulic pouring produces a saturated, settled backfill with an excess layer of free water. The amount of excess water depends on the pulp density, the settled porosity and the specific gravity of the tailings. This quantity is expressed as
Note that in-place porosities are generally found to be between 0.42 and 0.48 for hydraulic backfill and 0.35 to 0.42 for densified backfill. The linear (vertical) filling rate is expressed as:
In order that excess water drain through the fill under the gravitational gradient of unity, thus avoiding decant systems, the percolation rate (P) must be equal to or greater than the rate of build-up of excess water which is given by the product of the previous two equations:
Large open stopes require only small percolation rates (P³ 1.0 cm/hr) for A = 3000 m2, whereas very high percolation rates are required for small pour areas (P³ 50 cm/hr for A = 60 m2).
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