Paste – The Future of Tailings Disposal ?

In the last decade paste technology has progressed from a research based backfill idea to a widely accepted, cost effective backfill method with the potential to radically change the way tailings are disposed of on surface. Paste is simply dewatered tailings with little or no water bleed that are non-segregating in nature. It can be ‘stacked’ on surface and the risks associated with dam failure significantly reduces since there is no liquid containment and therefore no mechanism for the tailings to travel for tens of kilometres downstream in the event of a containment failure. The operating costs for the preparation and transportation of paste may be higher but life-of-mine cost analysis shows comparable costs to conventional disposal with significant environmental benefits. In addition, the eco-political impact of non water-retaining tailings dams could reduce permitting time considerably. Introduction The acceptance of paste backfill plants, especially in bulk mining, has been extensive in the seven years since the first modern plants were constructed in Canada. Extensive research work by INCO in the early 1990s resulted in advances in the preparation and transportation of paste. Since that time, over 30 plants have been built or are under construction. The advantages of paste backfill over hydraulic fill include reduced binder consumption, slimes handling, stope preparation and surface disposal together with productivity improvements associated with an increased mining cycle. The increased utilisation of paste technology has improved the reliability and reduced the cost of preparation and transportation systems. This has led to the possibility of using paste for surface disposal. The final prize of such an endeavour is the introduction of virtually water-free surface tailings storage. This can eliminate the need for large water-retaining dams and remove the associated liability such structures pose to the environment. Such a step appears logical, as regulatory pressures become more demanding in Europe after recent high profile dam failures, increasing the permitting requirements for conventional tailings disposal. The Bulyanhulu Mine in Tanzania, successfully commissioned in March 2001, is the first mine in the world to adopt a total paste solution for all its tailings. A portion will be used underground as backfill and the remainder will be ‘stacked’ on surface as paste. The advantages cited by Barrick Gold in their decision to develop the system include: Reduced capital cost (no large starter dam required) Water conservation (traditional disposal may have led to water scarcity and the need for a dedicated water pipeline from Lake Victoria, ~100 km) Decreased footprint of tailings disposal site This paper discusses the future of paste technology as a surface disposal method and presents a case study for a fictitious mine producing non-acid generating tailings. Conventional slurry disposal is compared with paste and an NPC (Net Present Cost) analysis is completed. Page 2 In addition this paper will present a definition of paste and discuss the current areas of research into paste for surface disposal including tailings dewatering and transportation, current and future regulatory constraints on tailings disposal and the likelihood of successfully applying the technology to acid generating tailings. Paste Definition The definition of paste can become a very scientific and, sometimes, emotive, issue. Golder Associates, who have been working with paste technology since its infancy, has adopted a more empirical definition based on our experience in backfill and, more recently, surface disposal. A paste can simply be defined as a mixture of solids and water that has little or no bleed water when idle. It must also, however, possess some yield stress such that when tested using a conventional concrete testing slump cone, it does exhibit slump. Figure 1 shows two slump tests, one measuring 175 mm of slump and a second, measuring 250 mm of slump, (note: the slump is measured from the top of the cone). Figure 1 – Paste Slump – 175 mm (left) and 250 mm (right) Paste backfill is always placed cemented and the optimum slump that minimises transportation pressure losses and maximises strength gain is often found between 150 and 200 mm. Pumping of paste for backfill is rare but for surface disposal, where cement is not routinely added, pumping makes up a very important part of the paste system. As a result, the optimum slump is higher, between 200 and 275 mm of slump. This reduces the friction losses and allows the paste to be transported 2 to 3 km with one large pump. The difference between ‘backfill’ paste and ‘surface disposal’ paste is subtle but impacts significantly on the preparation method and hence the cost of plant construction. At present, it is not possible to consistently manufacture a ‘backfill’ paste (i.e. a slump of <200 mm) without the use of mechanical dewatering (filtration). Improvements in dewatering technology have enabled the production of higher slump pastes to become possible for some tailings and this has opened up the possibility of low cost paste production, thus bringing environmentally beneficial paste disposal one step closer. Paste Advantages The environmental benefits associated with paste surface disposal can be divided into two main categories the physical characteristics of the paste itself and the operational advantages associated with its placement. The primary advantage is that there is very little free water available to generate leachate. In addition, the relatively low permeability of the poorly sorted full plant tailings limits infiltration, resulting in reduced seepage volume present in the Page 3 deposited paste. Hydraulic conductvity values are not easy to determine for an uncemented paste but values have been determined for weakly cemented samples. These range between 10-7 and 10-8 m/s. The preparation of paste can allow for the material to be ‘engineered’ with the use of additives that can enhance the beneficial properties of the paste. Although not widely used at present, probably due to cost fears, additives hold significant potential for further improving the behaviour of disposed tailings. The most commonly used additive for paste backfill is a pozzolanic binder such as cement, slag, flyash etc. These provide significant strength underground at addition levels of 3 – 6% by weight. For surface disposal, the cost makes them unattractive although tests have shown that 1 wt% cement addition can increase the neutralizing potential by an order of magnitude. It also raises the pH, which can lead to immobilisation of some metals through mineral precipitation. Other additives that require investigation include speciality chemicals, resins and surfactants that could enhance the adsorption of metals; organic carbon and bacteria to aid biofixation and the addition of organic material (topsoil, seed and fertiliser) to assist rehabilitation work. The solids concentration (by mass) at which tailings exhibit paste properties can vary significantly and are a property of the fines content and the colloidal chemistry associated with such fines. For example, high clay content tailings such as aluminium tailings (red mud) can exhibit paste properties at ~45 wt% solids, (wt. Solids / (wt. Solids + wt. Water)). Base metal tailings, especially when combined with sand or waste rock can develop solids contents as high as 85 wt% solids and still be easily transported as a paste. Chemically altered tailings, e.g. from Uranium and Soda Ash processing, are pastes at very low solids content, 20-40 wt% solids. Co-Disposal One of the most exciting opportunities for paste disposal is the wide range of codisposal options that are available. The primary example is that of acid generating waste rock (a common occurrence for many mines) being sent to the same waste facility to be ‘encapsulated’ in tailings paste. This significantly reduces the oxidation risk and reduces the number of waste facilities requiring closure. With the possibility of engineering the paste to become an effective encapsulating material, other wastes can be effectively sealed into the paste, subject to local and european regulatory approval. For some mines, such a concept could be logically extended to include non-mining hazardous waste from other industries. This would also generate revenue streams for the mine. Such a concept would truly result in the mine becoming a waste management company. There are significant permitting and regulatory impacts of such a concept. However, the recent publicity regarding the possible use of a closed open pit in Northern Ontario to take domestic waste from Toronto highlights the possible synergy between mine waste handling and general waste management. In addition, at the time of writing this paper, a company in England is considering reopening the South Crofty Mine (tin) in Cornwall with additional revenue streams from the storage of incinerator ash underground when mixed with tailings for backfill. The concept should certainly be discussed more openly. Paste Preparation – Tailings Dewatering The dewatering method for paste backfill made from run of mine tailings is normally a two-stage process. The tailings are conventionally thickened and then filtered to Page 4 form a wet cake. This material is then re-pulped under closely controlled conditions to accurately prepare the correct consistency paste. Recent equipment development has allowed paste to be prepared in a one-stage process. These systems are called ‘deep tank dewatering systems’ and there are manufacturers that have succeeded in constructing such units. Essentially, the thickeners retain the tailings for longer and are higher, which allows for the added dewatering associated with compression of the tailings. The consistent preparation of paste from such equipment is tailings dependent and testing is required. When Alcan developed a thicke