Introduction
Figure 1: Underground backfill at Cleveland Potash, Boulby Mine, UK (Paterson & Cooke Consulting Engineers (Pty) Ltd)
Tailings can be stored below ground in previous worked out voids. The tailings are generally mixed with a binder, usually cement, and then pumped underground to fill voids and help support an underground mine. For example a ‘room and pillar’ mining operation that uses backfill will be able to extract the insitu pillars containing ore. This is possible due to the cemented backfill acting as a support and preventing heading collapse and problems with subsidence. The backfill tailings are generally mixed on the surface with the cement and then piped either down a decline, shaft or surface borehole into the area of the mine that requires backfilling.
Advantages:
- The tailings are stored underground and thus prevent surface disturbance. This is more environmentally friendly as areas of land don’t have to be used for a surface tailings storage area.
- Ore rich pillars and supports can be extracted.
- The backfill helps to support the mine
- Backfill reduces the risk of rock bursts occurring as pressures are not focused on pillars and supports.
- Improves the ventilation circuit in the mine.
- Prevents roof falls from blasting (Air Over Pressure (AOP))
- Binders help to minimise groundwater contamination
Disadvantages:
- High costs, particularly if binders are used.
- The tailings need to be dewatered, increasing costs.
- Hold ups in extraction strategies.
- Risks of liquefaction of the tailings if saturation levels are high, and a trigger (seismic vibration) are present.
- Seepage of tailing effluent into groundwater, thus contamination.
- Extra manpower and equipment.
- Ore dilution from poor quality fills.
Figure 2: Pipeline from the plant depositing backfill tailings down a surface borehole
Binders (cementing) help to prevent groundwater contamination as the backfill experiences chemical and physical characteristic changes. For pyritic tailings the cement will reduce oxidation and acid generation of the fill, thus resulting in reduced mobilisation of metals. This is particularly useful if an underground void is below the water table, as when pumping ceases the cemented fill will be in direct contact with groundwater. Problems with fill migration, liquefaction and slump are prevented.
Problems with backfill
There are many problems with backfill. The main ones being:
- Plugging of pipe line – A big problem if binders are used as the pipeline may need replacing.
- Borehole plug – If the backfill is pored down a borehole there is a chance it can get plugged.
- Backfill sloughing – The backfill forms a crust that can shed allowing fresh backfill to migrate.
- Pipe line burst
- Bulkhead failure
- Pipe hammering
- Liquefaction of the backfill
- Backfill segregation
- Plugged sump
- Rat holing
- Pump failure
Types of backfill used in underground mines
There are four types of backfill used.
- Paste backfill
- Hydraulic sand fill
- Cemented fill
- Dry rock fill
- Paste backfill is similar to surface paste deposition. The tailings are dewatered to >75% (by weight) solids and pumped underground, generally by piston pumps. The paste has a homogenous appearance and produces a measurable slump (visible when released from a cone shaped slip mold). When the paste is deposited underground it is normally free standing and the contained water tends not to migrate out.
- Hydraulic sand fill is used when the tailings are cycloned to produce separate slimes and sand fractions. The slimes are disposed extracted due to their poor permeability and are generally stored in a surface impoundment. The sands are hydraulically pumped underground into the voids and can be mixed with binders if need be. As the sands settle and consolidate the excess water is bled off. This can be aided by the AOP from underground blasting. High levels of AOP can result in a liquefaction event.
- Cemented fill consists of tailings and waste rock deposited in underground voids. It is used when storage of waste rock is required and the excess void spaces need filling. Cement slurry can be poured over the waste rock to fill and bind these voids. This is useful when low volumes of cement slurry are available (cost implications) to bind the backfill.
- Dry rock fill is rock waste, surface sands, gravels, or dried tailings. The fill is either dropped down a raise, or tipped into an open stope by a Load Haul Dump (LHD) or dump trucks. The dry rock fill is most suited for cut and fill mining.
