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 which 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.
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 required, which is a major operating cost of a backfill operation
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 which 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 intailings 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 which are relatively non-settling and non- segregating and which have a higher concentration and viscosity than that 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 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% solids Newtonian slurry in conventional or high-rate thickeners. Conversely, the same tailings stream could be thickened to 65–70% solids non-Newtonian suspension using a paste thickener.
Increased water recovery is important to many countries through out the world due to the scarcity of fresh water. The more water that can be recovered from a tailing stream and, in turn, used 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.
Surface stacking is the disposal of tailings on the surface of the ground. As an on-settling, non- segregating suspension of solids, minimal water is released into the impoundment and the stacked tailings dries 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.
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.