This review discusses the use of polymers in tailings. It contains references to books and papers, conferences that deal with this topic, the fields where the technology is used, and the suppliers and consultants who service the sector.
We hesitate to take on so large a topic and one where we know for certain there are many experts, which we are not. Bear with us: we have spent time searching out and collating information in the hope that it may help you and save you time. And if you can, care, or want to, we would be delighted to have your input, comments, and even updates of this review. In particular, if you have a paper, product, or perspective that you believe should appear here, please let us know and we will move fast to update and include information from you and/or about your company.
description of a polymer is "a large molecule composed of repeating structural units" and while it often is used in reference to plastics, for our purposes we will speak of it as a long chain molecule. The use of this molecule in the mining industry comes about because of its ability to join together minerals or flocs that would not normally coalesce in an economically reasonable time. Much time and effort has been put into this field of tailings research in mining in two areas in particular:
i. Polymer use in Thickeners
ii. Polymer use in Tailings Disposal
The distinction here is the addition of polymer to increase the solids content by flocculating and settling the tailings to the bottom of the tank (thickener) as oppose to increasing the solids content by depositing on a slope, and thereby promoting the expulsion of water and atmospheric drying.
The term flocculant is also used. Here is one definition from this link:
"Flocculant is a chemical designed to increase particle-settling rates in a thickener. Flocculant causes a number of small individual particles to join together to form an agglomerate which settles much faster. Correct flocculation is the single most important factor in the operation of a high rate thickener. The amount of flocculant added to the thickener is dependent on the ore. Harder ores will tend to have a coarser size distribution. They will require less flocculant, as individual particles are heavier and settle faster. Soft clay ores will tend to have a very fine size distribution. They will require larger flocculant additions as individual fine particles settle very slowly".
We are lucky enough to own a copy of Paste & Thickened Tailings - A Guide. As the title suggests, its primary aims are:
"To lay out simple terms the advantages and disadvantages of thickening tailings to a higher concentration or density than that achieved as underflow from conventional plant thickeners prior to deposition, and to provide a technical resource about the application of thickening technology for surface disposal."
Consideration of rheological concepts, high concentration hydraulic transport systems, above-ground and backfill disposal techniques are discussed at length, giving an inclusive overview of aspects of paste and thickened tailings technology. The use of polymer in thickening is the principle topic in the chapter on reagents.
The continual building of the base of knowledge in the industry is most evident in the Paste Conference series, of which we are have all the conference preceedings back to 2006. In that time we have found these two papers to be the most applicable to this technology review.
- Polymer Aids for Flocculation of Oil Sands Tailings, X. Wang, Z. Xu, J.H. Masliyah (2009) which can be purchased here.
- Polymer modified paste fill for isolation of waste disposal sites, M. Fall, J.C. Celetin, F.S. Han (2010)
There also exists the Tailings and Mine Waste conference preceedings that have some publications on polymer amended tailings, including:
"Breakage and Reformation of Flocs in Oil Sands Tailings Slurries: In this paper, mature fine tailings from the oil sands were treated with a number of different polymers in order to assess floc breakage and reformation. These two polymeric phenomena were assessed "rheologically and by dynamic measurements of particle size distribution."
Paste 2013 Seminar:
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
Polymers in tailings are common in thickening, but it is less common to add them to the tailings stream just before deposition. In any case, much study has taken place in both areas to advance the technology. In the InfoMine library these are the papers we found that best represent these advances:
Thickening of Mine Waste Slurries: This paper outlines the process of thickening, including the flocculation of the waste slurry through the addition of polymers. Advances in the field of thickening is not the objective of this paper. Rather it lays out the fundamentals behind thickening and drives toward their promotion of environmental sustainability. If you are unfamiliar with the process then I would highly recommend this paper.
Numerical Modelling of Tailings Thickening: In this paper the authors attempt to model sedimentation and consolidation in the thickening process. "Modelling the dewatering behavior of slurries during thickening is not well understood in geotechnical engineering", this dewatering process is made up of sedimentation and consolidation and hence the drive to model.
Parametric Modelling of Polymer-Assisted Slurry Sedimentation for Nickel Laterite Mining: This publication introduces the 24 factorial design, which essentially means that there were four parameters that were changed in order to look at their interactions, and relationships with each other.
The OneMine website is always a treasure trove of information, but you will have to subscribe to their website to view the publications. There are several publications on the topic since the last century. As a sneak preview, here is what there is to be found:
Polymer Flocculation and Coagulation for Sedimentation of Copper Floatation Tailings: This paper deals with the application of two different types of polymers (cationic and anionic) to a thickener. Their effects on different particle sizes, different rates and syntax of application, and their effect in combination were looked at in this paper.
The Impact of Flocculant Addition on a Tailings Storage Facility: Here described is polyacrylamide based flocculant added to the tailings line immediately prior to deposition. This was done in order to facilitate dewatering which brings about an increase in density and significant improvement in impoundment capacity.
Paste Thickening of Fine Coal Refuse: This paper "describes the paste thickener as an alternative for dewatering a fine coal refuse (tailings) stream for deposit as a paste". It also describes dual flocculants: both anionic and cationic were used.
According to Ron Magill, the author of the EduMine course The Mill Operating Resource - 2: Mineral Recovery, the thickener is a very basic piece of machinery. It simply provides a holding area large enough to allow the concentrate time to settle to the bottom. Once it has settled, there are adjustable rakes that rotate and pull the solids to an exit point (first figure). With the solids settling to the bottom, the liquid, called the liquor, overflows into a launder. Depending upon how the thickener is being used, these two substances, the solids and liquor, will go their separate ways, each being processed in accordance with the needs of the circuit. For more information on the process, here is what Outotec have said in their product information publication.
The benefits of using this technology is the reduction of water content of the waste tailings, which makes for easier disposal, and the increase in the amount of water that can be recycled.
Before the tailings are added to the thickener, a polymer is added to aid in flocculation. This is done to allow the minerals to join up together to form small masses which overcomes its tendency to float. The benefit of adding a polymer to the material going into the thickener is that the aggregates formed as a result of addition of the polymer fall more readily to the bottom of the tank where they may be swept up with the rest of the stream and hence taken from the thickener to the tailings impoundment or the filter presses.
Keep in mind that the effect of the added polymer on the tailings viscosity is probably negated as a result of shearing of the fluid as it flows down the pipe. Hence in practice, the addition of polymer to the tailings as part of its passage through the thickener is relatively insignificant in terms of the performance of the tailings once deposited from a spigot on to a beach. It is for that reason that some are moving to add the polymer to the tailings just prior to the discharge of the tailings from the spigot at the tailings impoundment.
Mine and tailings-specific testing on a range of commercially available polymers is recommended.
Here is a great description of the action of polymers in thickeners. We acknowledge the Thickening Resource Book from which this extract comes.
The schematic below shows the long chain negatively charged flocculant molecule combining with the positively charged ore particles to agglomerate and form a heavier colloid, which settles faster.
The path an agglomerated particle will take when it is introduced into the thickener is mainly dependent on flocculation. This is shown in the diagram below.
If the feed particles are not flocculated then it is probable that the buoyancy force will carry them upward to the overflow stream. This will cause the bed level to rise. Correctly flocculated particles will settle to the base of the thickener and allow clear overflow. Over flocculated feed will sink rapidly and may bog up the thickener rakes (waste of flocculant).
Introduction to Problem
While traditionally thickening is the principle user of polymer in the treatment and disposal of mine tailings. In recent years much research has been put into the area of dewatering tailings related to a number of industries but in particular, the oil sands. Without any further technologic intervention self-consolidation would take centuries. These fine tailings are left for approximately three years and become "mature fine tailings" (MFT) which has a with a high clay content. The constituents of these tailings is further expanded upon in the first part of this publication, while a more comprehensive analysis of the problem and possible solutions is put forward in this publication by BGC in 2010.
Polymer in Tailings
From what we have seen in thickeners, the addition of polymer to the tailings for the purposes of increasing solids content is not new. The difference with these systems is that the polymer is added just prior to discharge, with no other process required. Prior to discharge can mean different things to different oil sands operators. This article gives a rundown of the different technologies each of the oil sands operators are in the process of developing. For a more illustrative description have a look at this presentation by Natural Resources Canada.
One of the technologies being used is "In-line Polymer Injection" which involves the polymer being injected into the tailings immediately before discharge. After the injection stage there are options including centrifuging, or deposition on drying areas. Centrifuging is one avenue being pursued by Syncrude while Suncor and Shell are looking to dry these deposits on "sloped sand banks" to achieve appropriate drying for reclamation. Shell has also used this approach with what it calls "Atmospheric Fines Drying".
With specific relevance to this topic, some of the issues pointed out in this paper presented in the 2011 Mine Waste Conference, - "Breakage & Reformation of Flocs in Oil Sands Slurries", gives an indicator of the complexity of dealing with polymers and the challenges that lay ahead for the oil sands industry.
It is also worth noting that polymer addition to tailings in order to achieve dewatering and increased use of deposition space is not reserved to the oil sand and much work has been undertaken in other industries to realise these same objectives. The paper "The Impact of Flocculant Addition on a Tailings Storage Facility" shows how a very similar approach was used in the coal industry in Australia.
InfoMine lists the consultants who design thickeners and more generally amend tailings with polymer prior to deposition. The following are but a selection of the industry leaders in this authors opinion:
- Paterson & Cooke consult internationally in slurry technology, and have considerable expertise in thickener circuit design.
- WesTech are a big name in the area of paste thickeners and also consult worldwide to the mineral industry. They advertise their HiFlo and HiDensity thickening systems as their main contribution to the area of polymer amended tailings.
- Paste Thick Associates, according to their website they are "We are engineers experienced in the process and mechanical design of solids-liquid handling equipment. We specialize in thickeners and associated equipment that produce paste and thickened tailings."
- SRK Consulting are consultant engineers who have dealt with many thickened tailings projects in the past. Here is one such project on what is said to be the largest thickened tailings project in the world.
There are suppliers of polymers abound on SupplyMine who stock everything to keep the thickeners and other polymer amended tailings systems running. Those listed below are the some of the featured listings, but also provided are the rest of the suppliers.
Zinkan: As you can see from this link, Zinkan supplies polymers for thickening in the forms of anionic, non-ionic, and cationic products.
Qualfloc: Provide polymers are generally used as a flocculant in aqueous media to enhance the solid-liquid separation, these include:
- Anionic PAM
- Cationic PAM
- Non-ionic PAM
- Amphoteric PAM
SNF Flomin: SNF are a major producer of polymers for flocculation in the mining industry. The flocculants are used for density control in thickeners, as a rheology modifier in the concentrate thickeners, to increase solids for shipment, in the slimes dams for dewatering and overall for water reclamation.
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