Introduction to Mine Backfill

Paste Backfill

De-watering
De-watering is normally achieved in three ways: hydrocyclones, thickening, or filtering. Hydrocyclones are an inexpensive means of de-watering however they will cause some loss of fines which are necessary to pump the paste. Hydrocyclones can only be used when there are excess fines in the tailings to remove a portion of the fines and thereby increase the filtration rate of the tailings. This is known as partial classification. It is also possible to use a thickener to de-water slimes from a cyclone overflow and then combine the thickener underflow with the cyclone underflow.

Both conventional and high density thickeners have been used in paste backfill plants to thicken backfill to a pulp density of approximately 65% solids prior to filtration. Using high capacity thickeners to produce a paste directly from a dilute tailings slurry has been investigated and determined to be technically feasible. To date, however, no plants have been constructed that use this technique.

The Canadian Centre for Mining and Energy Technology (CANMET) is researching a process to produce paste in settling silos that maintain the suspension by injecting compressed air into the paste with special nozzles. No operating plants use this technique, however a pilot plant is being constructed.

Filtering is carried out using disc filters, drum filters or belt filters. The selection of a filter is usually based on a consideration of capital and operating costs.

Mixing
Mixing of the components of a paste backfill mixture can be either a batch or continuous process. The filter cake, cement, water, and aggregate must be accurately weighed and rapidly supplied to the mixer. It is generally recognized that a batch process is easier to control than a continuous system. Screw type mixers are used for continuous systems while high intensity mixers of the type used in the concrete industry are used in batch plants.

Pumping, Transport, and Placement

After mixing, the paste can be either discharged into a concrete pump hopper or through a borehole to the lower mine workings. Typically, paste will flow under gravity at an angle greater than 30° , making selection of the backfill plant location very important. In designing a backfill system, the plant should be located where a series boreholes and short lateral runs could carry the paste to all stopes under gravity. For such plants, a positive displacement pump would normally be installed for clearing line blockages and for cleaning out the pipeline. In most installations, however, this is not possible and a positive displacement pump operates continuously to transport the paste.

Sufficient space should be provided around surface and underground borehole collars to allow a drill to be moved over it to ream the hole should a blockage occur. One mine has reported using liners in the boreholes, however this is not common practice. Borehole and pipeline diameter is a function of the expected pressure losses in the system. Practical pumping distance ranges up to 1 km laterally and unlimited distance vertically depending on the pipe pressure rating. Flow velocity in the pipeline is usually less than 1 metre per second. Figure 9 shows typical pressure losses for a medium slump paste in a 150mm diameter pipeline.

The placement of the pipeline outlet in the stope is important because the paste will not beach out. Rather it will form a moderate angle of repose (30° ), requiring that the outlet be moved periodically depending on the size of the stope being filled.

Figure 1:  Typical Paste Backfill Plant Flowsheet

Case Histories
Three case histories that are well documented in the literature are the Grund Mine, the Louvicourt Mine, and the Lucky Friday Mine. 

Grund Mine
One of the first reported applications of paste technology in the mining industry was at the Grund Mine in Germany. A simplified flowsheet of the Grund Mine backfill plant is shown in Figure 10. Flotation tailings were first de-watered using hydrocyclones followed by thickening. The thickener underflow was pumped to a vacuum belt filter. The filter cake and aggregate from a heavy media separation circuit were mixed continuously to produce a paste with an 88% pulp density. The paste was pumped 80 metre to a shaft and allowed to flow by gravity down 500m. Stopes were located 400 to 2300 m away from the shaft and so an intermediate pumping station was required for more distant stopes. Portland cement was conveyed pneumatically in a separate pipeline to the backfill site. It was injected into the backfill line approximately 50m from the outlet. The backfill developed a strength of 2 MPa with a cement addition rate of 3-4%.

The capital cost of the Grund backfill plant was CDN $7.2 million in 1985 partly due to the large amount of research and development required to implement the system. Table 1 summarizes the key operating parameters for the plant and characteristics of the paste backfill.

Table 1: Summary of the Grund Paste Backfill Plant

Table 2: Summary of the Louvicourt Paste Backfill Plant

Louvicourt Mine
The Louvicourt Mine near Val d’Or, Quebec, commissioned a paste backfill plant in August, 1994. The capacity of the plant is 188 tph of backfill which is greater than the mill production rate of 135 tph.  Laboratory testwork for the plant indicated that an operating range of 78-82% solids was required. An average binder addition rate of 4% was used based on a 60:40 mixture of cement:flyash.

A simplified flowsheet for the Louvicourt backfill plant is presented in Figure 11. Unclassified mill tailings are thickened in a 15.25m diameter high density thickener. The thickener underflow is pumped to a 3,100 m3 agitated surge tank, equivalent to a typical 14-hour backfill pour. The surge capacity was installed so that backfilling could continue during mill shutdowns. The thickener underflow is then pumped from the surge tank to disc filters. The filter cake is discharged onto a conveyor belt which transfers the cake to a twin screw mixer. Fly ash and cement are also added to the mixer on individual weigh belts. A moisture measurement at the mixer discharge is used to control make-up water addition to achieve the final paste moisture content. A positive displacement pump, capable of developing 3.5 MPa, is used to pump the paste into cased boreholes to the 475 Level where connection is made to a series of vertical and lateral runs that cover the mine. The vertical boreholes were 20 cm in diameter, allowing the paste to flow by gravity. Lateral runs were reduced to 15 cm diameter pipe. The pressure loss in the 15 cm portion was 16 kPa per metre at a flow velocity of 1 m/s.

The total cost of the plant including building, boreholes, equipment, and installation was CDN $6.2 million. Table 2 summarizes the key operating parameters for the plant and characteristics of the paste backfill.

Lucky Friday Mine

The Lucky Friday Mine in Idaho was the first mine in North America to develop a paste backfill system. A stiff backfill was necessary for the mechanized underhand cut and fill stoping technique employed by the mine. The backfill plant has an average capacity of 134 tonnes per hour. The paste backfill produced by the plant has a pulp density of 85% (wt) and a slump of 9.5 to 10.5 inches. The cement addition rate is 6-8%.

Paste backfill is prepared by de-watering flotation tailings from a lead-zinc concentrator. The tailings are partially classified using a hydrocyclone to increase thickening and filtration rates. The classified tailings are thickened in a 9.1m diameter conventional thickener. The thickener underflow is pumped at 65% solids to a drum filter that produces a filter cake with 13% moisture. The filter cake is stacked in a covered bunker and reclaimed with a bucket chain excavator when the backfill plant is operating. A conveyor belt transports the filter cake to a weigh hopper that loads the cake into a mixer. A PLC-controlled batch process is used to mix the cement, filter cake and water components. The paste is pumped to the shaft with a Putzmeister positive displacement pump. The paste travels by gravity down the shaft more than 1500m and laterally to the stopes as far as 600m. The pipeline diameter varies from 100 to 150 mm. In the 125 mm pipeline, the average velocity is 2.4 m/s and the friction loss is 8 to 10 kPa per metre.

Table 3 summarizes some of the key operating parameters for the plant and characteristics of the paste backfill.

Table 3: Summary of Lucky Friday Mine Paste Backfill Plant

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