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Paste and Thickened Tailings 

 
Authors: Jack Caldwell and Lawrence Charlebois (Robertson GeoConsultants)

December 2012

Summary

This technology review is intended to get you started or bring you rapidly up to date on the topic of paste and thickened tailings. It covers the basic terminology, talks about the issues involved in paste and thickened tailings, lists suppliers and consultants who specialize in these types of tailings, and gives a perspective on the future of tailings.

INTRODUCTION

The literature on paste and thickened tailings is probably more voluminous than the actual number of mines where the methods are used. The idea has been around for a long time and the promise will continue to beguile and fascinate tailings practitioners for a long time hence. For there are indeed successful case histories and there are indeed benefits if you can get it right.

This review is not and never could be exhaustive-the topic is too big and the disciplines to broad. You will have to go to the sources materials to see what has been written on the topic that is applicable to your specific site and its issues. Still what follows will guide you through the thicket of materials.


The Definitive Resources

The book that probably started it all is Thickened Tailings Disposal in the Mining Industry by Eli I. Robinski; it is available at this link. And also at this alternative link.

The definitive book on the topic is Paste and Thickened Tailings by Richard Jewell and Andy Fourie; it is available at this link.

The fastest and easiest read on the web is by Jon Engels at his Tailings.info website. Here are the links to what he posts on thickened tailings and paste tailings. He nicely summarizes the advantages of both methods.

The definitive conference is the annual Paste and Thickened Tailings Seminar held every year since 1999. The proceedings are available in book format from the Australian Center for Geomechanics.

Be sure to attend the 16th International Seminar on Paste and Thickened Tailings - Paste 2013.
It will be held in Belo Horizonte, Brazil, June 17-19, 2013.
It is jointly organized by the Australian Centre for Geomechanics, the Federal University of Minas Gerais – UFMG, IBRAM, InfoMine, and Pimenta de Avila Consultants.

The seminar will cover topics on stacked, thickened, paste and filtered tailings.

The following topics and related themes are the focus for Paste 2013:

  • Hydraulically Placed Dry-Stacks
  • Filter-Pressed Dry-Stacks
  • Thickened Tailings
  • Paste Tailings
  • Environmental Considerations
  • Polymer-Amended Tailings

For more information about Paste 2013 please visit www.paste2013.com

The definitive course is by Patterson & Cooke; see this link for their website where they announce the next course.

And while not definitive, we continue to like the supplement to International Mining on Paste Tailings Management as available at this link. See also the InfoMine Library for the annual International Mining supplements on Paste from 2008 to 2012.

And we have written extensively, if not necessarily informatively or intelligently, about both thickened and paste tailings at the blog I THINK MINING. Go take a look and do a search for the terms that interest you.

There are collections of papers on this topic via a standard Google search-look for the pdf marker and you will probably get a genuine technical paper. At OneMine (a pay to enter site) we found 120 papers going back to 1995 on the topic of paste and thickened tailings. And at the InfoMine Library (a non-pay site) we found plenty more technical papers.


Other Perspectives

Converting to Paste Tailings at the Chevron Mining, Inc. Molybdenum Mine, Questa, New Mexico by Steve Blodgett and James Kuipers is probably the most overly optimistic assessment of the potential benefits of paste tailings we have read. Still it is worth going through for the level of current and new detail (September 2012) is impressive.

Conversely, the realism that pervades the assessment of possible use of paste tailings at the BHP Olympic Dam Expansion is most sobering. See this link for the document entitled Tailings Storage Facility Design Report. Again we recommend reading this volume before giving in to excessive enthusiasm for paste or thickened tailings.

Finally see the section on thickened tailings in the paper on the history of tailings practice at this link. There we quote Geoff Blight as saying:

In certain circumstances a thickened tailings deposition system will prove preferable. In other cases an unthickened tailings system will be superior. In every case the relative merits should be compared fairly and quantitatively and not on the basis of sales slogans, fashion trends, or over-simplified calculations on the back of an envelope.

The Issues

The fastest way to summarize the topics that arise in paste and thickened tailings is to summarize, as we do below, the topics in the Jewell and Fourie book:

  • 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 you 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.

Another good overview of the many topics and issues involved in paste and thickened tailings are the abstracts submitted for Paste 2013 in Belo Horizonte in Brazil. Ninety-six abstracts were submitted. There are abstracts that promise papers that defend thickened and paste tailings against the detractors: those who are concerned about thickener performance & reliability; the development of steep beaches; and the post-earthquake stability of such deposits. There are abstracts for papers that tell of cases histories---an increasing number worldwide. These abstracts extol the virtues of the method. Many abstracts deal with the use of paste tailings underground-how to best use a paste tailings to backfill mine workings and hence to keep stopes and tunnels stable for workers and equipment.


Terminology

The paste and thickened tailings expert will rightly tell you that these materials are but elements of a continuum that starts all the way back at watery, segregating tailings, transitions to non-segregating, thickened tailings, then to paste tailings, and finally arrives at solid cakes of filtered tailings. While there is some distinction based on density and rheology (yield stress, namely), each process engineer, rheologist, and tailings person will have their own "thresholds" on which they base decisions for pump selection, pipeline design, deposition planning, etc.

Here is one set of definitions of the terms - with thanks to Patterson & Cooke.


Here are two slightly different perspectives, with thanks to John Lupo:




Suppliers

There are many manufacturers, suppliers, and consultants offering services and equipment. The list is long, and new folk are always entering, so if the following lists do not include your company and you believe the list should include your company, please contact us and we will add you. From our browsings, we compile the following list of supplier of equipment for thickened tailings:


Then there are the suppliers who sell the chemicals that go into the thickeners to make them work: CIBA is one such. Senmin is another, as is SNF FLOMIN.

The point is that when talking to them, you need to understand the conventional terminology. Here is one example - again thanks to Patterson & Cooke:


Consultants

Most consultants to the tailings sector of the mining industry will be able to help. The list is long and includes:

Also consider Metso for a full service approach.


The Promise

The primary promise, to our mind, of paste and thickened tailings is reduced water consumption. When water is in short supply or expensive, there is no doubt that one approach to reducing water consumption is thickened tailings. Here is a water balance for a typical operation involving thickened tailings.

In practice a lesser, but probably more costly approach to reducing water consumption is to adopt Sand Slime Splitting as shown in the following:

Finally filter pressing your tailings involves less water consumption - see the Technology Review on that subject.


The Problem

The mine develops and expands; different ore types are exploited; the fines content and the nature of the clays change; that is the way of mining. The problem is that the thickeners are designed for one type of ore, usually that anticipated in the first few years of operation; but as the ore changes, the thickeners may not function at optimum, or at all, faced with a totally different ore source and grind.

The trick, the solution, to potential changes in the ore, grind, tailings properties, and functionality of the thickeners, is to design and construct what we call a robust tailings facility. Ideally the tailings facility should be able to accept tailings varying from poorly to effectively thickened tailings. This would involve designing for flatter beaches, higher starter dikes, more or less return water, and potential seismic instability. It can be done, although it will increase the cost of the tailings facility. And you just have to be less optimistic about thickener performance.


The Beach Profile

An all too common mistake in designing a tailings facility for paste or thickened tailings is to get the beach profile wrong. First the assumption is that the beach will form an essentially planar surface of given inclination. In practice the beach usually form a concave up profile and the result is a lot less volume for a given spigot elevation than calculated.

Second is to decide that the inclination will be steep-we have read of predictions up to ten percent. In practice the overall inclination of the concave profile can be very flat---less than one percent. This may be the result of variability in thickener efficiency: when poorly thickened tailings of high water content is flowing, it may erode down the nice steep beaches formed when the thickener is operating well.

The case history of Esperanza is instructive to us all in this regard.


Seismic Response

From the Paste Series of Proceedings are two papers dealing with the question of how a paste or thickened beach will respond in an earthquake.

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.


The Future

Finally this perspective of the future from Andy Robertson, President of Robertson GeoConsultants:

Dammed tailings:   The largest mines have increased their production, tons milled per day, by an order of magnitude each 30 years for the past 120 years. In the late 1800s the largest mines milled a few 100s of tons of ore per day. Today our largest mines mill a few 100,000s of tons per day. Quantities of tailings have grown in proportion, and to store them our largest tailings dams have had to increase in volume from a few millions of tons capacity to several billions. Areas occupied by individual dams have increased from a few hectares to many square kilometres. Maximum dam heights have increased from less than 20 m to approaching 300 m - giants on the landscape.

Damned tailings:   In constructing these tailings dams we are no longer merely doing earthworks - we are terraforming, or changing the face of the earth with gargantuan structures that will remain on our landscape in perpetuity. These huge structures store vast quantities of water and liquefiable solids that could flow if the dams breach. They are long-term hazards, and must remain stable in the landscape long after we are no longer there to monitor and maintain them. In the view of the downstream communities, we are not terraformers, but terror fermenters. And to add insult to potential injury, we steal their water and pollute their environment. In metaphorical terms 'we have tickled the dragon' of society. The dragon is waking and is becoming hostile to the threats we pose. Our tailings dams are increasingly 'damned' and our water usage and seepage abhorred.

Damless tailings:   The dragon's message is becoming increasingly clear - 'become damless or get out of dodge - and no, you cannot have our water either'.

Lessons in damlessness:   Our weapons for survival are filtered paste and thickened tailings, as well as mechanically and hydraulically stacked tailings - all technologies that may offer opportunities to eliminate or reduce the sizes of containment dams, reduce water use and seepage and produce dry, stable landforms at closure.


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