Foam technology has the potential to revolutionise surface tailings disposal and backfilling by providing a less expensive and environmentally safe alternative to traditional methods. Placing waste back underground as backfill cannot be used to dispose of all the tailings produced, and some surface disposal is usually still required.
Traditional tailings transportation in pipes use water as the transport medium, foam technology instead, involves the introduction of micro-air bubbles, which replace the water as the main transport medium. This air can be removed after placement if necessary, by the addition of a defoaming agent. This could produce an under-saturated stable cake.
The costs associated with dam rehabilitation have increased dramatically. For example, a single large tailings dam at the now closed Elliot Lake mine cost over $100 million to rehabilitate at the end of the life of the mine. There is a move toward dry stacking of tailings (or thickened tailings disposal). This method involves the densification of the tailings, usually using some form of high rate thickener, prior to disposal. This produces a near homogeneous material with some self-supporting capability.
The potential advantages of such a method are:
* The placed tailings consolidate much faster and the chance of liquefaction is significantly reduced. There is no need for a settling pond
* More process reagents can be recovered at the metallurgical plant
* The reduced water in the tailings reduces seepage and the pollution potential
* Water recovery is much higher and this could become critical in certain dryer areas
* Rehabilitation can be concurrent with the stack operation
This method is not new and has been successfully used by Kidd Creek in Canada since 1974, when the first tailings thickener was installed. Other mines using this method include the Ekati diamond mine in Canada, Bulyanhulu gold mine in Tanzania, Kubaka gold mine in Russia, and Hindustan Copper in India.
Dry stacking does require a totally different construction design. Foam technology could benefit this disposal method. It could reduce the cost of transporting this higher density tailings product (and this is a significant proportion of the costs). It could produce almost any desired final beach angle on the dry stack after deposition. This would be achieved in practice by altering the dosage of defoamer at the discharge nozzle. Many mines are already considering high density (paste) disposal, but if the foam concept works, it will be more effective.
There are a number of areas that require further research, before foaming technology can be considered. The first is a new concept for transport and discharge design with foam. Work is also required on the rheology of the material during piped transportation to the deposition site. Furthermore there will be a need for a long-term rain water disposal system, although the system used by Kidd Creek may also be acceptable if foam is used.
The use of tailings in backfill has many potential advantages for underground mines, such as:
* Reduced tailings on surface
* Reduced surface subsidence
* Increased underground extraction
* Reduced rockburst (seismic) damage
* Improved support for mining excavations and/or working surfaces
* Increased worker safety.
Backfill will normally be prepared on surface and there are four main methods used to transport the fill underground. Using hydraulic pipelines, the fill will be a slurry (low density) or paste (high density). This is the most common method. Mechanical methods would employ conveyor belts or trucks. Pneumatic pipelines transportation requires dry (or damped) material. This method is seldom used today because of dust concerns, high energy requirements and high wear rates. The other alternative is to use gravity via raises and rock passes. The criteria and requirements for backfill vary, depending on the mine's specific needs. The following are some of the more common and important:
* The fill should be placed at the lowest possible cost
* The risk of fill failure (for example liquefaction) must be minimised
* Early strength development needs to be adequate
* Long-term strength should be sustainable
* Delivery volumes must be reliable and adequate
* After placement, dimensional stability must be achieved
* Segregation should be minimised
Many mines are moving towards high density (paste) fill. This material is typically over 78% solids by weight as opposed to hydraulic fills that are usually a maximum of 70% solids by weight. Total tailings can be used, minimising the problems associated with the surface disposal of the remaining material. However, all the tailings produced cannot be backfilled due to the different densities of in situ and broken rock. At best only about 60% can be returned underground, leaving the rest for surface disposal. After placement, the high density tailings produce little or no bleed water resulting in reduced post filling shrinkage, reduced pumping (as the exuded water is minimised) from underground and more efficient cement hydration. Another advantage is a faster mining cycle because stopes adjacent to filling areas can be mined sooner after the fill placement. Pipe wear is reduced if the backfill is transported underground in pipelines.
However, there are also problems, such as more difficult rheology as the paste has higher pressure losses per unit length of pipe as compared to hydraulic (low density) backfills. The fill is not self levelling, having a higher beach angle when placed in a stope. The system also suffers from a relatively high operational and capital cost (on most mines the backfill operational cost of between $3.00 and $6.00 typically is the single highest mining input cost).
MBT is already involved in selling admixtures for use in backfilling, and although cost-effective, a far better solution seems to exist for some applications. This solution is the use of foam technology as it can produce a backfill with the combined benefits of both a paste and a hydraulic fill because the foam produces a 'pseudo' low density slurry making pipeline pressures low and reducing pipe wear rates during transport. Also, after placement the foam is destroyed producing a paste-like material with desirable physical properties.
MBT's test work has enabled the company to establish certain advantages of foamed fill:
* Foam enables/improves the transport properties of relatively dry, coarse materials
* Foam reduces and in most cases eliminates segregation of the solids/water in high density backfills during transport
* Foam lubricates plug flow by forming an air annulus between the plug and the pipe wall
* Foamed material will compress under pressure but expand upon release of pressure but remains unproven in a full-scale pipeline of significant length
* There is a significant decrease in compressive strength of backfill material that has not been subjected to de-foaming
* The amount of foam generated during mixing increases with the degree of gap grading or void ratio of the material being foamed
* The addition of foam to material that is being pumped, regardless of particle size gradation, significantly decreases the pumping pressure.
The case studies using foam technology are limited at present but do illustrate the potential for this technology. For instance, a medium sized Canadian gold mine, operating at depths of about 1,350 m, has been using hydraulic tailings backfill but is consolidating its milling operations with another mine owned by the same company. With the closure of the first mine's milling operations, its tailings backfill source is also disappearing. It was therefore decided to investigate the use of alluvial sand as a substitute for the tailings. In order to keep the hydraulic sand fill front settling during
transport, the fill was designed at 68% solids. To enable attaining the required backfill strength of 1.35 MPa in the stopes, it is necessary to add up to 6% cement as a good portion off the cement will leave the stope in the decant water. By using a pre-generated foam (using a specific foam generator), MBT was able to attain better flow properties with the same sand at 90% solids, and take a 50% reduction in cement to attain the target strength.
The tailings of a US soda ash mine are very fine and form a paste with a 125 mm slump at 37% solids. During pumping trials at the mine, the addition of a powdered foaming agent at the pump reduced pump pressures to 75% of the non-foamed material. Had it been possible to add the foaming agent at the mixer, the pressures may have been even further reduced. These trials were to investigate the future potential surface deposition of the tailings material.
In another example, a potash operation in Northern England was using subaqueous deposition for tailings disposal but due to increasingly more stringent environmental regulations is now disposing of a large portion in the mined out underground areas. The mine is not using any cement in this backfill as it is not intended for support but as a means of disposal. The current transport of the tailings underground requires a maximum pulp density of 62% solids. This leaves a great deal of troublesome brine to be returned to surface. During laboratory trials, the solids content of the backfill was increased to 75% using a pregenerated foam that was injected into the mixer, and rheological properties were obtained that surpassed those of the untreated 62% solids material.
Research plans have been drawn up to investigate transport and placement of foamed tailings materials both for backfill and surface deposition. For surface deposition, roller compaction of the foamed material is thought to be a method of de-foaming the material. Also, tailings that have been treated with a foaming agent often tend to consolidate to a higher final density. One of the major foamed fill research tasks now underway at the MBT's US research centre in the US is investigating using foamed tailings backfill in a Canadian underhand stoping mine operation. The current tailings backfill has a high bulk density. It is thought that the bulk density of the fill can be reduced to between 30 and 40% of the current fill using a pre-generated foam. As the fill is only used to prevent sloughing of the stope walls above the mining, a lighter density material would be easier to support.
This article is based on the paper The Potential Use of Foam Technology in Underground Backfilling and Surface Tailings Disposal by A J S Spearing (E-mail: email@example.com), technical director, and D Millette and F Gay, senior scientists, UGC Americas
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|Date:||Aug 1, 2001|
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