With increasing focus on efficient water use by mines, we read more and more of paste tailings as a way to reduce water consumption. I recently received two books on paste tailings from the Australian Centre for Geomechanics :

1) Paste and Thickened Tailings - A Guide (Second Edition)

2) Paste 2006

The need for water conservation is best illustrated by the paper A holistic approach to optimize process water retention and residue disposal for Orapa Mines by Busani, Copeland, and Keevy. They write [Caveat: I edit this and all other quotes in this piece for context and focus.]:

Orapa Mine is situated 240 km west of Francistown, in Botswana’s arid central district. Water is a scarce non-renewable resource. It is currently supplied from an aquifer system around the mines that has negligible recharge. A combination of unfavorable factors including low rainfall, high evaporation rates and a flat topography contribute to this situation.

The Orapa mines are currently operating at an average raw water consumption of 0.5 m3/t of head feed for the treatment plants at Orapa, Letlhakane, and Damtshaa. In order to extend the life of the existing aquifer, Orapa must reduce raw water consumption by 50% within the next two decades.

Orapa is planning to replace one of its older plants with a new generation plant and will also build a dump treatment plant at Lethlhakane Mine. These plants are likely to use more water as they will be designed to produce a finer product to enhance liberation of diamonds.

Orapa Mine is projected to use upto 22 million cubic meters of water per annum by 2012, when the new plant at Orapa and the Letlhakane dump treatment plants are commissioned. This increase is not sustainable as the groundwater sources are currently being mined due to low rates of natural recharge. Improved water retention technology is therefore a necessity for Orapa mines.

They describe a study of different ways to save water, including co-thickened slurry, slimes paste, co-thickened paste, and “dry” slimes disposal. They conclude that paste thickening is the most cost-effective way to reduce water use, and report that they have started a prototype paste thickened project to confirm water savings and cost estimates.

Thickened tailings started out, as most in the mining industry surely know, in Canada when Eli Robinsky applied the technique at the Kidd Creek Mine. Robinsky writes in Forward to the Paste and Thickened Tailings – a Guide

It is no longer necessary to ask “Who has adopted Thickened Tailings Disposal,” or “where can I see an active high-density thickened tailings operation?” as there are now many in operation around the mining regions of the world.

We have now established basic design criteria. The thickening and rheological concepts are better defined and better understood. Detailed characterization of thickened tailings is now a mandatory part of design. Transport and pumping technology has advanced significantly. Paste backfill is now in use by many mines.

However not all problems are solved. Two are: we still await large-capacity thickeners; and the effect of additives needs more study. This latter because an increase in pH may increase the viscosity of acidic tailings, permitting a steeper slope. A reduction in pH may reduce the viscosity of basic tailing, resulting in lower pumping costs.

Proof of Robinsky’s observation of the “main-streaming” of paste tailings is not only the two books I note above, but the many manufacturers, suppliers, and consultants offering services and equipment. This includes supplier of equipment for thickened tailings such as Ciba, Outotec, or Dorr-Oliver. Consultants and equipment suppliers include PasteThick Associates, Pipeline Systems Inc, Golders, WesTech, and Klohn Crippen Berger.

Even the magazines are promoting the news, no doubt because of the potential advertising revenue for suppliers. For example, International Mining has Paste Tailings Management. This excellent twenty-pager is available for free download and is worth ever cent and minute of your time. Another interesting survey and comparison of thickened tailings versus paste tailings is in the Outokumpu Technology newsletter. Finally there are the websites like Tailings.info where you can read case histories and more.

If you are interested in or charge with implementing a water reduction program potentially involving thickened tailings, then get both the books I note from the Australian Center for Geomechanics. Start with the Guide. It is beautifully produced and a joy to peruse. You are lead logically and painlessly through the topics that count:

  • The rheology of thickened tailings including how to measure viscosity with vanes and slump test, and how the resultant viscosity affect pumping, discharge, and ultimately beach angle at the material spreads and flows.
  • Material characteristics, including the old geotechnical stand-bys of gradation, pH, shear strength, and susceptibility to liquefaction when the earthquake comes (more on that later).
  • Slurry Chemistry and reagents, including what you may want to add or remove to make the tailings more amenable to pumping and discharge, to control acid generation, and to limit erosion by wind and water.
  • Thickening and filtration including a detailed description of the thickeners and filtration equipment you may have to evaluate. The authors include folk from Dorr-Oliver EIMCO, Paste Thick Associates, and Outokumpu, so your are getting the best, most objective advice available.
  • Transportation Systems including the pumps and pipelines needed to move and distribute the thickened tailings. The author is Angus Paterson of Paterson and Cooke in South Africa.
  • Above Ground Disposal including a detailed discussion of some of the problems associated with thickened tailings deposits. The issue of earthquake-induced liquefaction is touched on in his conclusion that thickened tailings deposits are susceptible to liquefaction, particularly if there is the right trigger mechanism. The technical procedures for quantifying liquefaction susceptibility are touched on, but not addressed in detail. Then there are the also unresolved issues of slat-crust formation that inhibits drying, too dry a surface and too much dust, and finally erosion when the big precipitation event occurs. Andy Fourie, the author notes that much thinking remains to be done to fully address these issues.
  • Mine backfill introduces the subject of the use of thickened tailings in mine backfill, but ultimately directs you to the more comprehensive publication also from the Australian Center for Geomechanics, Handbook on Mine Fill.
  • Reclamation including these lists of advantages and disadvantages of thickened tailings deposits vis a vis long-term reclamation:
    • Advantages: Early consolidation and traffickability; lower embankments; and less supernatant water.
    • Disadvantages: a highly compacted profile resistant to root development; poor water storage capacity; large surface area; dust; erosion control difficulties.

Which takes us back to the conference proceedings of Paste 2006 where you will find more personal perspectives. Here are some that interested me.

Liquefaction potential of surface deposits of high-density thickened tailings by A. B. Fourie. He concludes a detailed look at liquefaction of thickened tailings in these words: “The perception that thickened tailings deposits may pose a hazard during earthquake loading, because they are not confined behind retaining embankments, must be addressed. While valuable information is available from previous research on liquefaction of natural sands and silts and mine tailings, there is a need to extend this work to include the field of thickened tailings, which inevitably will have aspects of response to dynamic loading that have not been covered in existing studies.” A sobering thought; but contrast this with the next paper, noted below.

Stability of tailings beach slopes by S. Barrera and C. Riveros. They conclude: “The yield stress of the deposited tailings is greater than that of the slurry that generated the slope in the first place. A seismic event that could liquefy the tailings would only have energy to generate a displacement of the liquefied tailings during the seismic event. But once the earthquake shaking ends, there would not be enough energy to continue the displacement.” In short, as we know happens with landfills, everything moves downslope in the earthquake a foot or two and then stops moving when the ground stops shaking. Dramatic but not catastrophic.

Surface paste disposal of high-sulfide tailings by R. Verberg et al. They write:” Since proposed paste implementation will involve maintaining a fresh paste cover (3 to 6 months maximum exposure), the issue of runoff quality is important. Significant emphasis is required on surface water management during paste placement to avoid standing water and ensure collection of run-off. It will be control of run-off, not seepage, that will govern placement protocol.”

Evaporation from surface deposited thickened tailings by Simms, Grabinsky, and Zhan. They note that the rate of evaporation from thickened tailings is an important parameter: both because evaporation densifies the tailings causing a gain in strength and because desaturation increases the rate of acid generation. They describe laboratory test to quantify the rate of tailings surface evaporation hence drying. Compare to the paper Buyanhulu Mine paste tailings facility where the authors conclude that deposition of thicker beaches with longer waiting periods between deposition is preferable because less sulfur oxidation occurs in thicker lifts. They recommend “the optimum deposition interval is that in which a layer of tailings reaches the moisture contents below which there is no further shrinkage.”

I can do no better to conclude this review than quote from a paper by Gordon McPhail of Metago, who writes:

While it is true to say that there are few high density TSFs in operation it is also true to say that they are gaining ground for a number of reasons, including:

  • Major improvements in technology, including:
    • Thickening in terms of both cost and reliability
    • Pumping in terms of equipment and slurry rheology modifiers
    • Design accuracy
    • Understanding and prediction of depositional behaviour
    • Geometric modelling of the high density TSF landform

  • Acceptance of the fact that not only is it unnecessary to achieve paste densities but it is also undesirable since it will enable:
    • The use of conventional thickening equipment
    • The use of centrifugal pumps
    • Easier TSF operation as the tailings will flow naturally into position.

But before I end, I must replicate his warnings regarding management of surface water on high-density tailings deposits:

The mounded form of the placed high density tailings mass means that all stormwater runoff reports to the toe areas where it increases the risk of overtopping and erosion of the toe wall. It is vital to plan for the provision of storm control trenches and spillways/decants which should be positioned at appropriate locations around the perimeter. A typical arrangement is illustrated in Figure 6 and Figure 7.

Figure 6: Typical layout of a storm management system on a high density TSF

Figure 7: Section showing locations of storm control trenches

Notable features of the above figures are:

  • The grading and alignment of the trenches along the toe
  • The locations of spillways at positions where the topography is best suited for these structures