Mine backfill is defined as the material used to fill the cavities (i.e., stopes) created by underground mining. Backfilling can be a means to dispose of sludge and/or tailings that may contain hazardous materials and to reduce surface environmental impacts by storing tailings underground.
Alternately, backfilling with nonhazardous materials can allow for mining productivity improvements. To these materials are added a variety of fillers such as fly ash, course sand, or gravels along with a binder such as cement, which is added to provide structural strength.
Conventional Flow Sheet
A vacuum disc filter, preceded by a high-rate thickener to reduce the hydraulic loading, is typically used to produce the sludge portion of the mine backfill. The filter cake is discharged to a weigh hopper, then to a batch mixing hopper or a continuous mixer, where a measured amount of binder and other materials are added.
The cemented paste is then pumped via high pressure piston pumps below ground or distributed by gravity, depending on the specific site. Most backfill projects in the world use this conventional flow sheet with a vacuum disc filter because there is less water in the filter cake and, therefore, less cement binder (which is a major operating cost of a backfill operation) is required.
Paste Flow Sheet
In underground mining, the WesTech Deep Bed™ paste thickener is an emerging option to the conventional solution of high-rate thickener/vacuum filter for paste backfill applications. There can be a number of factors that make paste thickening an attractive alternative. There are some backfill operations with shallow mines and long distance runs, making the pumping costs of a high-yield stress paste more attractive.
Because a paste is non-settling, the coarse particles do not have to be removed prior to thickening. Paste thickeners can eliminate the need for vacuum filters, which can be expensive to operate, and may not be feasible for high elevation mine sites. This also significantly reduces operator attention.
Alternatively, it is possible to use a Deep Bed paste thickener to feed a vacuum filter. This can reduce the size of the vacuum filter as the feed to the filter is more concentrated than that from a high-rate thickener. Another option would be to use a Deep Bed paste thickener in parallel with a filter. This option allows for the blending of the paste underflow with the filter cake.
The underflow from the paste thickener would be split, sending a portion to the vacuum filter. The paste thickener underflow and the filter cake would then be combined to obtain the desired moisture content for the backfill.
The advantages of paste thickeners in tailings circuits have been well-established for most minerals. The type of paste thickener underflow generally used for surface stacking of tailings is called “thickened tailings.” Thickened tailings are a suspension of solids that are relatively non-settling and non- segregating and that have a higher concentration and viscosity than those of conventional tailings.
Common applications include a thickener with underflow pumps that are typically centrifugal and transfer pumps that can be either centrifugal or positive displacement.
Paste vs. Conventional
Paste thickeners thicken tailings to higher underflow solids concentrations than conventional and high-rate thickeners. For example, a tailings stream in an iron ore application could be thickened to 45-50 percent solids Newtonian slurry in conventional or high-rate thickeners. Conversely, the same tailings stream could be thickened to 65-70 percent solids non-Newtonian suspension using a paste thickener.
Water Recovery
Increased water recovery is important to many countries throughout the world due to the scarcity of fresh water. Recovering water from a tailing stream and using it again upstream in the plant reduces the amount of fresh water a mine needs. The cost of water is frequently one of the primary drivers for implementing a thickened or paste tailings disposal system.
Stacking
Surface stacking is the disposal of tailings on the surface of the ground. As a non-settling, non-segregating suspension of solids, minimal water is released into the impoundment and the stacked tailings dry significantly faster than conventional ponds where fines (slimes) segregate and can seal the surface of the deposit, preventing drying.
Stacking as a method of tailings disposal offers significant advantages over ponding. These advantages include: smaller impoundment area, less water in the pond, improved water or chemical recovery, a lower risk of containment breach, less groundwater contamination, and easier final reclamation of the site.
Extend Pond Life
When tailings ponds reach their capacities, the construction of a new dam is usually the solution for the deposition problem. This solution is very expensive and is not always an option if there are environmental, licensing, and space constraints.
By utilizing a paste thickener, the thickened tailings stream is deposited into the existing pond, increasing the life of the impoundment. The existing free water in the pond will be displaced and available for recovery and the higher concentration and drying properties of the paste will take up less space, extending the life of the pond.
