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Filter-pressed, Dry-stack Tailings 

 
Authors: Fergus Murphy and Jack Caldwell (Robertson GeoConsultants)

October 2012

Summary

In this review, we recount the history of filter-pressing tailings, talk of some of the projects where the tailings are filter-pressed, and list and review resources on the web.

INTRODUCTION

More and more mines are choosing to filter press their tailings and place the tailings in a stack. Filtering tailings is not new. The practice is, however, increasing for these reasons: the need to reduce water consumption; the need to limit seepage from the tailings; and the imperative to build a stable stack not subject to slope failure or flow. The increasing price of mined commodities makes the increased costs of filtering feasible.

HISTORY

Here are interesting points from some papers, but do go and read the whole paper-they are fascinating:

A Proposed Filter-press Slimes Plant by Francis L. Bosque (1903). He writes: "The following paper embodies a report which I made on the filter-press treatment of slimes at the Liberty Bell mine, Telluride, Colo. At the time this report was submitted, the management deemed it wiser to defer the installation of so costly a plant as the one required, in the hope that some of the many investigators in the field might soon evolve a cheaper and more perfect method of slimes-treatment, and one better suited to the conditions at Telluride."

The Importance of Fine-grinding in the Cyanide-treatment of Gold-and silver-ores by Frederick C. Brown (1906). He writes: "My experience has shown that the average loss of undissolved gold and silver in the slimes is small; but in some mills using the decantation-process, a very considerable loss takes place in the final solution carried away with the slimes which are discharged after treatment. This loss in the slimes could be obviated by the use of filter-press, but the necessary plant is very expensive to install, and, unless a mine has assured large ore-reserves, it is sometimes risky to expend he capital."

The Disposal of Residues at Kalgoorlie by Harry Adams (1909). He writes in one magnificent sentence: "Originally the conditions in this district varied mainly in the position of the mill, some mills having an elevated site with plenty of dumping space for the time being, whilst others had a flat site, and very little dumping space near at hand; but in most cases the residue discharged was in the form of filter-press cakes, suitable for conveying on belts, or in trucks, and stacking in a high dump; and, while a portion of the residue was used for underground filling, the dump was regarded as valuable asset, to be re-treated later."

There are more, but this is not a history lesson. Let me only reproduce this picture from 1914 of filter presses at the Treadwell Mine in Alaska. They were not used to produce a dry tailings, but were intended rather to squeeze out as much fluid as possible to get at the gold.

Installation of Merrill Gold-presses, Showing Method Adopted to Prevent the Draining of Presses.

The first I heard of filter pressing of tailings was way back in the early 1980s. We had concluded that a conventional tailings facility would not work at the proposed Greens Creek Mine on Admiralty Island, in the Alaska panhandle. There was too much water from rain and too large an earthquake to use conventional hydraulic deposition. Andy Robertson suggested use of filter presses to make a dried---or at least low moisture content solid.

There was much skepticism at his suggestion. He persisted, telling us that the technology was used at some coal mines and was being evaluated for uranium mines. Credit for support and ultimately implementation of Andy's idea must go to the two co-authors of the paper The Production and Handling of Dry Uranium and Other Tailings by A. MacG. Robertson, J.W. Fisher, and Dirk van Zyl. Here are quotes from the paper that give some idea of the thinking of the day:

Because of concerns regarding pollution and contamination migration from uranium tailings impoundments, there is a continued search for innovative, more secure methods of tailings disposal.

Pressure filtration using either belt or filter presses increase hydraulic gradients through thin cakes and this can be used to dewater the finest of tailings. Positive controls that can be applied in pressure filters make them an effective and flexible de-watering device.

For the purposes of this paper, tailings 'are considered to be 'dry' if they have been de-watered sufficiently that they can stand in a substantial pile and be handled by conventional earth moving equipment without liquifying (or slopping about). Non-cohesive tailings would generally attain this state at relatively low moisture contents of 10% to 20%. Cohesive tailings could have moisture contents in excess of 35% and still be considered 'dry'.

In the summer of 2010 I returned to Greens Creek to see that the filter-presses are pressing away and the dry stack is into its seventh expansion. The system works well and I was able to demonstrate to a senior official from the regulators in Guatemala the benefits of the system. Now two mines in Guatemala are using filer-pressed tailings disposal.

In fall of 2011, I went to a distant mine where the thickener had failed to thicken and the mine was closed. I recommended the use of filter-pressed tailing. The next week I lunched with the environmental officer of a big mining company; I was told they are now reluctant to contemplate a new mine at which they cannot use filter-pressed, dry-stack tailings.

At the Tailings and Mine Waste 2011 Conference in Vancouver, Michael Davies of AMEC noted that he is one of the few talking and writing about filter-pressed tailings, and he noted that this is strange considering that there are more filter-pressed tailings operations than thickened tailings operations-yet thickened or paste tailings have their own conference.


TECHNOLOGY

Here from a recent presentation by Mike Gowan of Golder Associates in Australia is a nice flow chart that illustrates the basic aspects of filtered tailings. Once the tailings have passed through a thickener, they are directed to the filter, which may be either a belt, a disc, or a press. The filter in essence squeezes out the water leaving a low moisture content material. The "dried" tailings is then conveyed by a conveyor or trucks to the stack where it may be placed uncompacted or compacted as necessary to achieve strength and seismic resistance.

General Concepts of Filter Tailings


The Greens Creek Filter-Pressed Dry-Stack
The Greens Creek Filter-Pressed Dry-Stack.

The tailings and water are passed through the thickener to increase the solids content in the fluid. Then the fluid is passed to the filter press. The filter press squeezes the water out of the fluid-this water may be returned to the thickener. The tailing, typically at a moisture content of about ten to fifteen percent, is dropped out from the bottom of the filter press.

The filter-pressed tailings form piles. A front-end loader or other equipment is then used to scoop up the tailings and put them in a truck (preferably with a cover over the back). The truck transports the tailings to the dry-stack where the tailings are dumped from the truck, spread out, and sometimes compacted.

Seepage from the dry stack is collected and sent to the water treatment plant for eventual discharge to rivers or the ocean. I know of some operations where cement is added to the tailings soon after they come out of the filter press and before they are placed on the dry stack. I know of operations where conveyor belts are used in preference to trucks.

A basal drain and liner are placed to limit seepage from the tailings to the foundations. A perimeter berm of erosion resistant rock is constructed as successive series of lifts of tailings is placed. The tailings are placed in this lifts which are compacted in most instances of which I am aware.

Dry-Stack Rock Buttress

Dry-Stack Rock Buttress. This is the detail of the outer facing proposed for the Rosemont Copper Filter-Press Dry-Stack south of Tucson, Arizona. Operations begin with construction of an initial rock starter toe or berm. Behind this the dry tailings are placed in layers. As the elevation of the stacked tailings increases, additional, upstream rock berms would be placed and tailings placement would continue behind these.


EQUIPMENT SUPPLIERS

The key to successful filter pressing of tailings is the equipment. Here are links to some of the companies that supply such equipment.

FLSmith are another supplier of filter press technology, who have also come up with this nice little graphic demonstration on the process and inner workings of a modern filter press. They have recently announced, at this link, that FLSmidth has received a US$31 million order for 14 filter presses to be installed as part of Rosemont Copper Company's planned copper concentrator in Arizona, USA. The announcement goes on to say that:

"FLSmidth's filter press technology will play a central role in Rosemont Copper setting a standard for sustainable practices, including using less than half of the water as traditional mines, and reclaiming the site as permanent open space by re-vegetating throughout the life of the mine. The large, heavy duty, 2 m x 2 m mineral-type filter presses will dewater to an optimal 16% moisture all the tailings produced from processing 68,000 metric tonnes per day of ore through the concentrator. The remaining cake can be dry stacked, which is an integral part of the progressive tailings disposal and reclamation programme being conducted by Rosemont Copper. This eliminates the need for tailings ponds and promotes ongoing reclamation and re-vegetation of the surrounding landscape."

MWH has a nice video on YouTube of a system they designed and installed in Europe in order to reduce the volume of previously deposited tailings. In brief, they dredge the old tailings, filter pressed them, and made a new dry stack of considerable less volume, increasing the life of the tailings pond and reducing the environmental impact of the site.

Diemme Filtration specialize in the sale of filter press equipment. This case study in Mexico shows the advantages of the utilization of a filter press system. The advantages of which being:

  • Filtering the tailings coming from silver production, succeeding in recovering the highest quantity of water to use back in the process
  • The recovery of cyanides (one of the main costs of the Merrill Crowe process);
  • Environmental impact reduction, eliminating the tailing ponds.

Outotec at this link provides the best document we came across on the principles and operation of filter and belt presses. Of course they supply such equipment and you should seek their input and proposal.

TEFSA is another commercial supplier who produces a good overview. See their documentation at this link. In terms of the filtering technology they offer, this link gives a good indication of their widespread presence in the filter press industry.

Phoenix are also touted as having a long history in filter pressing, going back more than twenty years and including serving places like, U.S., Canada, Australia, India and Russia. Here are the steps listed in their filter press process:

  • Preconditioning (Flocculation)
  • Feed Distribution
  • Gravity Drainage
  • Wedge Pressure
  • Compression/Shear
  • Solids Discharge

Andritz have two principle design types for the filter press market. These are the Side-bar design and the Overhead beam design. These different designs have advantages

Side-bar Design

  • from simple manual to fully automated equipment designs
  • maximum degree of automation, also with extremely sticky filter cake
  • comparatively low structure weight

Overhead beam design

  • free access to the plate pack
  • special design for the chemical industry (maximum corrosion protection), also suitable for very aggressive media
  • solid structure for severe operating conditions

An Andritz presentation on the comparison between different mine tailings they filter can be found at this link.

Metso also carry Vertical Plate Pressure Filters VPA, which have been developed for tailings amongst other metallic and industrial minerals. A brochure of their Vertical Plate Pressure Filtered system is available on the site where different stages in filtration are illustrated graphically. These are:

  • Filtration
  • Compression
  • Air Dewatering VPA
  • Cake Discharge
  • Cloth Washing

Hangzhou has a filter press division named Leo Filter Press Co. Ltd. They are a professional filter press manufacturer, designer and exporter. They also offer chamber filter press, membrane filter press, plate and frame filter press. These are all specially designed filter presses according to different industries' requirements. If you thought there was just one type of filter press, then I suggest you let Hangzou enlighten you with their other products.


CONFERENCE

To learn more about these technologies, consider attending the Paste 2013 conference to be held in Belo Horizonte, Brazil. It will be held on June 17-19, 2013.

The Paste and Thickened Tailings Seminar, held every year since 1999, is the leading forum for practitioners in field to meet, deliberate, and exchange knowledge and information.

The objectives of the Seminar series are to disseminate the advances in technology made in the field of paste and thickened tailings, and provide a forum to present and discuss field applications, and obtain feedback from practitioners working at mine sites worldwide.

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

  1. Hydraulically Placed Dry-Stacks
  2. Filter-Pressed Dry-Stacks
  3. Thickened Tailings
  4. Paste Tailings
  5. Environmental Considerations
  6. Polymer-Amended Tailings

For more information please visit their website at www.paste2013.com


PROJECTS

Rosemont Copper is a proposed new mine south of Tucson, Arizona. Because of the shortage of water in the desert, the proposal is to use filter-presses to dry the tailings. Details of their scheme are at this link.

As part of planning the Rosemont Copper project, AMEC prepared a comprehensive report on the current state of practice of filtered tailings dry stacks. This is volume well-worth downloading if the topic is of interest to you. Included are descriptions of the dry stacks at the following mines:

Alamo Dorado, Mexico: As noted in the AMEC report: "The approximate design criterion for the filter plant was to achieve moisture contents about 20 to 25 percent (weight of water divided by weight of solids). The filtered tailings are then transported by conveyor to a tailings stockpile within the adjacent impoundment area. The tailings are subsequently transported by truck for placement, drying to near optimum moisture content and compaction within the impoundment in an unsaturated, dense, and stable tailings stack (dry stack)."

El Sauzal, Mexico: As noted in the AMEC report: "Key components of the dry stack facility include an up gradient storm water diversion/retention dam, the dry stack storage area with an underdrain system located in the major drainages, a rockfill starter dam and a lower sedimentation pond. The dry stack geometry includes a down gradient zone of compacted tailings faced with rockfill and the up gradient zone of tailings being placed with enough compaction to provide adequate access for haulage equipment."

Greens Creek: I have already mentioned Greens Creek, Alaska. They have been using filter-pressing for over twenty years. See this link for a 2002 study on expansion of the tailings area. It contains much fascinating data. The AMEC report notes: "The tailings are de-watered at the mill using pressure filters and either used underground as cemented backfill or placed on surface in dry stacks. The tailings consisting of between 73% and 96% by weight passing the No. 200 sieve are delivered to the surface tailings facility by truck at or close to optimum standard Proctor moisture content (approximately 15%). The tailings are spread into cells and compacted."

La Coipa, Chile: As noted in the AMEC report: "Leach tailings are processed in a vacuum filtration plant then conveyed to the tailings storage area. The tailings storage area consists of a main deposit, the Rahco, and an auxiliary deposit, the Rakito. The Rahco deposit is developed in an upslope direction by placing compacted lifts of conveyed tailings."

Pogo, Alaska: As noted in the AMEC report: "The Dry Stack is a facility where tailings that have been pressure filtered to a moisture content of about 15% are placed into one of two areas used for either winter or summer placement. In the summer period, assumed to be not more than four months, tailings are placed in lifts and compacted in the shell area, and provide a structural shell for the dry stack."

Raglan, Canada: The AMEC report notes: "Approximately 700,000 tonnes/year of tailings are deposited over the estimated remaining life of the mine of about 15 years, with the possibility of increasing the production rate and/or the length of mine life. Tailings solids comprise about 70% silt sizes. Immediately prior to filtering, the tailings stream is thickened, with thickener underflow at 60-65% solids. Filtered tailings are loaded on trucks, hauled to Tailings Repository, dumped, spread and compacted to form a stable stack with 5H:1V side slopes. Tailings solids, having sulphide/sulphur content of 6-8%, are reactive."

Coeur Manquiri: At this link, the following is noted:

Coeur Manquiri, a wholly owned subsidiary of the US-based Coeur d'Alene Mines Corporation, recently purchased three MC Press filter presses from Siemens Water Technologies for its San Bartolome mine in Potosi, Bolivia. The dewatering filter presses will allow improved dry stacked tailings management on site. The MC Press filter presses are expected to become operational later this year.

Located at the base of the Cerro Rico Mountain (Andes Mountains) near Potosi, Bolivia, the San Bartolome mine is one of the world's newest and largest pure silver mines. With a projected 14-year mine life, San Bartolome has proven and probable reserves of 120 million ounces of silver within gravel deposits at the base of the mountain. The mine produced 7.5 million ounces of silver in 2009, its first year of production. Its mountaintop location and limited available land for dewatered material made dry stacking the most viable storage option. Apart from the MC Press filter presses for dewatering, Siemens also supplies some aftermarket parts to help Coeur Manquiri ensure uninterrupted mining operations.

Dewatering tailings to higher degrees than paste produces a dry cake. These unsaturated tailings are usually hauled by truck or via conveyor to a tailings deposit where they are spread and compacted. Known as "dry stacking", this stable form of storage requires a smaller footprint, is ideal for sloped terrain, and it generally allows for easier reclamation at the end of mine life.

Coeur Manquiri also uses J-Press filter presses from Siemens at the silver recovery plant. They were installed years ago as part of the Merrill-Crowe process, which is a zinc dust precipitation method used to separate silver from a cyanide solution.

South African Coal Mines: The paper Dewatering of Ultra-fine Coal with Filter Presses describes filter pressing of fines in the South African coal mining industry. The document includes good illustrations of how he filtration process operates.

Darthbrook, Australia: At this link is a report on the use of filter presses in the Australian coal mining industry. It notes:

Current practice at Australian coal preparation plants is to thicken fine coal tailings with high-rate thickeners utilising high molecular weight polymeric flocculants. The thickener underflow is either dewatered further or disposed of. Disposal is either in fine tailings ponds or together with coarse reject by co-disposal. Commonly belt press filters are used if thickener underflow has to be dewatered further. Decanter centrifuges are no longer common, while membrane filter presses have just been introduced at Dartbrook due to the high clay content and the need to achieve consistent dry cakes for co-disposal.

Filter pressed tailings are planned at the Marlin Mine and the Escobal Mine, both in Guatemala.

There may be other projects. If you know of them, please let us know.


SOFTWARE

In terms of software for the filter press industry, the design of the filter press equipment has had fluid dynamics research to study the flow of the slurry and waste products within the system. However this product development is of little interest to the end user (mine). Hence the effectiveness of modeling software would be in answering the question, How will this lower moisture content material perform when stacked? These types of questions can best be answered by the following products.

SVOffice

Murray Fredlund runs SoilVision. He supplies software for a large range of mine waste modeling. In addition, he is always ready and willing to help you use the codes or modify them to meet project-specific needs. Here is how he describes one of his codes and its use to evaluate filter pressed tailings stacks:

AMEC had purchased the SVFluxTM software in order to model the stacking of filtered tailings and the subsequent seepage emanating from the facility. During the process, they had indicated they wanted to represent initial conditions in the software in a particular way that was not yet implemented. We examined their specifications and realized that their required change could be implemented with a few days of effort. The software was then modified and a new version created in a time period of four days. AMEC subsequently completed the modeling project and has utilized the software on multiple projects since.

MAGAZINES

There is only one that we know of - see Filtration and Separation


CONSULTANTS

AMEC

As should be apparent from the frequent references in this review so far, AMEC is well qualified to design your filter-pressed tailings stack. John Lupo was with them and while there he did some magnificent work on the topic. Now he is with Newmont, but maybe he would still be willing to give you some advice.

MWH

Also with a number of references, MWH have a history of designing and implementing large projects involving filter press tailings in the Port of Antwerp, Belgium. This project involved the dredging of harbour sediment and reducing the moisture content through filter pressing, in order to conserve land which helped port authorities saves millions of dollars a year.

Klohn Crippen Berger

The man to ask for is Rick Friedel. Here is the abstract of a paper he and Len Murray wrote called Cyclic and Post-cyclic Laboratory Test Results on Undisturbed Samples of Filter Pressed Mine Tailings:

Relatively little information is available regarding the cyclic behaviour of consolidated mine tailings. A major issue is that tailings often cannot be reliably classified based on conventional empirical techniques for sands or clays. This technical paper presents results from a laboratory testing program of undisturbed in situ samples of filter pressed mine tailings that were deposited in a "dry-stack" tailings storage facility. The laboratory program included index classification tests, consolidation tests, monotonic, cyclic and post-cyclic simple shear tests. This paper will focus on the results of the cyclic and post-cyclic simple shear tests and compare them to previous tests completed on reconstituted samples of the same material.

Robertson GeoConsultants

This is the company we work for when not writing reviews like this one. We have some experience and would be happy to help you.


Others Consultants

Experience is important when dealing with filter pressed materials and these were the consultants we found that had a proven track record to offer. If your company is not here then please feel free to email me and I'll be sure to update this piece.


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