Basics Of Paste Backfill Systems

Paste backfill is a pumpable, flowable, non-Newtonian fluid consisting generally of mine tailings and cement. Paste is prepared from dilute slurries of tailings. This is done by dewatering the slurries with conventional thickening or filtering. Backfill with a paste consistency is prepared by mixing dewatered tailings with cement and water to obtain the desired consistency of medium slump concrete. The paste is pumped with concrete pumps over relatively long (1 km or 0.6 mile) horizontal distances. More commonly, though, it flows by gravity down a mine shaft and then horizontally to the void being filled. Alternatively, cement can be added near the end of the pipeline. A certain proportion of fine particles <20 µm (<625 mesh) must be present in pastes but particles of aggregate can be added without significantly affecting the flow characteristics. Detailed testwork is necessary to design a system to prepare and place paste backfill. Testwork must determine dewatering characteristics, strength of mixes and friction in pipelines. Plants must be well engineered and controlled with PLCs because high quality control is necessary to prevent plugged pipelines and excessive pressures. Relatively small amounts of cement (3% to 5%) produce stiff backfill material 1.5 to 3.5 MPa (217 to 507 psi). The advantages of paste backfill in a mining operation include reduced mine dewatering, labor and materials savings, reduced tailings impoundment requirements, good support properties and reduced spills underground. In certain cases where more than 30% of the weight of the ore is sold as a product, the entire tailings stream can be placed underground. Paste technology can he used for surface or underground placement of tailings.