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October 2010www. .com

TAILINGS

Tailings dams are used to manage and contain mining waste, and are crucial to the success of any mine. But, writes Paul Moore, they also play an important role in water recovery and conservation

At the tail end

“External specialist skills and experience are purchased to design and advise on the construction, operation, monitoring and closure of TMFs”

The safe and efficient disposal of mine waste is key to any operation. While the vast majority of tailings

dams operate as planned, it only takes one significant failure, such as Marcopper (1996), Los Frailes (1998) or Baia Mare (2000), to blacken the name of the entire industry for years.

Most recently, in October, a major red mud spill killed four people and injured 120 when a tailings dam, managed by Magyar Aluminium, in the town of Ajka, Hungary, failed.

Therefore the correct approach to long-term tailings dam (often referred to as a tailings storage facility or tailings management facility –TSF/TMF) management is crucial to the success, and reputation, of mining as a whole.

As with the issue of mine closure (see page 32), tailings dam design/strategy generally involves a combination of mining company expertise and the input of a specialist consultancy that can offer the necessary multi-disciplinary aspects. A large number of factors have to be taken into account, from topography to seismicity and hydrology.

Ciaran Molloy, technical director of engineering for AMEC’s UK environmental business, says: “In-house mining company expertise is usually very limited in this area. This is often because the deposition of tailings is generally viewed by mining companies as a cost and not an asset, and their focus is on mineral development rather than construction of tailings dams. For this reason, external specialist skills and experience are purchased to design and advise on the construction, operation, monitoring and closure of TMFs.”

Jamie Spiers, tailings engineer at SRK, comments: “From conceptual to feasibility study levels of a project, specialist consultants are generally employed to ensure all pertinent design features and project risks are considered at an early design stage. Tailings consultants also specialise in geotechnical foundations investigation – essential for the design of retaining embankment structures, drainage provisions and associated infrastructure.

“TSF design requires a multi-disciplinary approach, including civil/environmental engineering, hydrology, hydrogeology, geology, mechanical/process engineering, geoechemistry and mine-planning input. It is essential that the tailings design team understand and apply these factors to any design.”

Larger organisations with stakes in operating mines may have tailings specialists within their mine-planning team; however, many companies still rely upon specialist consultant input for quality assurance/control and monitoring from construction through to closure.

Wherever possible, local regulatory guidelines and legislation, as well as international directives, should be used in the design. International examples include the IFC, Mine Waste Directive, the EC Seveso II directive, the Equator Principles and, of course, any other similar tailings projects. The major consultants develop their tailings approaches in accordance with the best available techniques, which are based on global guides developed by the International Commission on Large Dams (ICOLD), the Canadian Dam Association (CDA) and others, as well as industry best practice.

PROJECT APPROACHThe strategy for planning tailings disposal varies, depending on the situation, customer and consultancy concerned. Han Ilhan, vice-president and global mining business director at URS, comments: “The URS approach to an overall tailings management plan is to first develop a comprehensive strategy evaluation of the various thickening, conveyance, deposition and storage options with the mine’s operational management.

“The main objective in this process is to select the option that is fully aligned with the mine life-cycle goals, and acceptable to the regulatory agencies and other outside stakeholders. URS promotes and facilitates a risk-based alternatives evaluation process as a decision-making tool for mine operations in selecting the ‘right’ tailing-management option. Finally, detailed design activities are developed to optimise start-up capital and operational costs without compromising safe operation of the tailings storage facility.”

While every project is unique with respect to its location, geomorphology, climate and geology, a series of international best practice design steps for a tailings management facility must be addressed during the pre-feasibility, feasibility and detailed design stages.

Mr Molloy of AMEC states: “Appraisal of the respective design elements and their subsets will lead to the development of a set of unique design plans, which are economically appraised to develop capital and operational cost estimations for inclusion within the overall mine project financial model.”

Looking towards the tailings area at LKAB Kiruna, Sweden

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October 2010 www. .com

40 TAILINGS

The facility must also be designed for closure to ensure it can be closed effectively at the end of its working life.

The design of tailings storage structures also varies considerably for each specific project. Factors that influence their design include foundation conditions, land use patterns, seismic hazards, watershed hydrology, groundwater quality and the availability of local fill for construction. Early consultation with local authorities is also essential to prevent unnecessary delays at the permitting stage.

MONITORING TAILINGS FACILITIESDuring actual operation, a number of methods are used to monitor tailings dams. Ideally, all permanent structures should have standpipe piezometers installed around the embankment to allow water levels to be measured monthly. Should the position of the phreatic surface within a particular embankment rise above an acceptable level, this may pose a stability risk to the outer embankment structure and remedial measures should be taken immediately.

Mr Spiers at SRK adds: “Groundwater monitoring bores should be installed around the dump perimeter to ensure seepage from the TSF does not adversely affect local groundwater supplies. Water samples should be tested on a monthly basis to ensure pH levels, sulphides and selected metal concentrations are within internationally recognised safe limits.

“Finally, visual inspections of the structure should be carried out on a daily basis during the operational phase to ensure all drainage structures and embankments are intact and functioning correctly. This is essential to ensure water flows, both above and within the TSF, are managed efficiently and safely.”

Leon Botham, principal and senior geotechnical engineer at Golder Associates, comments: “There are many ways to monitor tailings dams, and similar methods are used for monitoring

water-supply dams as well. Monitoring could include the installation of instrumentation, such as piezometers, slope indicators, settlement cells and survey stations, but, more importantly, monitoring will include a regular visual inspection programme.”

Mr Molloy at AMEC adds: “A mine waste-management plan must be developed as the design progresses. In accordance with best practice, this document will be used to develop an operation, maintenance and surveillance (OMS) manual, which will address facility description, roles and responsibilities, TMF operation, failure modes and effects, maintenance, dam surveillance and emergency response plans.

“The OMS manual is a live document that is regularly updated as the TMF cycle advances. Depending on the stage of the life cycle of the TMF, whether active or closed, statutory inspection reports by an independent competent person is also required at regular intervals,” he adds.

Earthquakes represent a very real hazard to tailings dams in certain regions. A method commonly used to determine the effects of a design seismic event on a particular site is to assume the earthquake occurs on the closest, potentially active fault, which is selected on the basis of previously conducted geological studies.

Attenuation tables are used to estimate the magnitude of the force of a possible earthquake occurring on the selected fault.

A high seismic area will require a downstream embankment configuration that is effectively drained and not prone to failure by liquefaction. In a low seismic-risk active area, an upstream embankment may be appropriate, again fully drained.

CONTAMINANTS & CONTAINMENTAcid rock drainage and metal leaching represent major risks to environmental receptors in proximity to mining sites. Most contaminants from these operations arise through the mobilisation of water and ingress of oxygen into stored waste.

Mr Botham at Golder says: “Potential contaminants need to be identified at an early stage so that an assessment of potential barriers or other mitigation measures can be evaluated within the design. In some cases, the natural geology may preclude any requirement for a liner system, such as with low hydraulic conductivity materials.

“In other instances it may be beneficial to engineer the tailings to provide a low hydraulic conductivity tailings mass, which can then be disposed of in a mined-out open pit or other feature, where the hydraulic conductivity of the surrounding rock is significantly higher than the tailings mass. This hydrodynamic containment is successfully applied at uranium mines in northern Saskatchewan, Canada.”

Barriers can take several forms. Jamie Spiers at SRK notes: “Groundwater protection, particularly after closure of a TSF structure, depends upon diversion of fluids from the encapsulated waste. This is achieved by using composite cover systems, which ensure the majority of precipitation is diverted away from stored wastes.

“As a general rule, natural liner systems at the base of the TSF structure are preferable to geo-membranes for long-term storage as the design life of these materials is a maximum of 30 years.”

• Site selection to minimise the potential environmental impact• Elevation/storage capacity optimisation and sequential development planning• Classification of the facility as high or low-level risk• Tailings discharge and transport system with

respect to the waste produced by the mine plant• Geochemical classification of the waste and construction materials• Seepage analysis to appraise the requirement for either a natural mineral basin liner or a modified soil/geosynthetic liner to protect

natural groundwater effectively• Geotechnical classification of foundation and construction materials• Stability analysis under both static and seismic loading conditions to confirm the optimum embankment cross-section (downstream, centre line or upstream)• Water balance, addressing inflows, retention and abstraction• Environmental/structural risk analysis• Reporting

Source: AMEC

Typical issues addressed in tailings management facility design

“During actual operation, a

number of methods are used

to monitor tailings dams”

Thickened tailings stack

at Peak Gold, Australia

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TAILINGS

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TOPOGRAPHY & VISCOSITYThe topography of a selected site will probably be the biggest factor in determining the overall cost of the tailings facility. The topography of a selected disposal site is effectively utilised in initially sizing the facility’s footprint by taking full advantage of natural hills and valleys in order to optimise sequential embankment-building works, minimise the footprint and reduce the impact on the environmental. For example, on a flat plane, a four-sided, paddock-type facility may be required, which could require significantly more civil works than a cross-valley facility in steeply incised terrain.

The beach slope angle of the deposited tailings can have important ramifications on the capacity of the tailings facility. Once this angle has been ascertained (which is a function of particle size distribution, water content and tailings density), modelling software can ascertain flow paths within a particular topographic constraint quickly.

This, in turn, allows efficient starter embankment placement, spigotting plans and drainage provisions to be costed accurately. Spigotting is similar to cycloning and involves the separation of tailings particles according to grain size.

The selection of the tailings discharge viscosity (which is related to pulp density, expressed as a percentage) is a pre-requisite in selecting the method of delivering tailings to a surface facility and the deposition technique. Low pulp-density (low viscosity) tailings can be centrifugally pumped to the tailings dam and discharged sub-aerially over a wide beach area.

As viscosity increases, a different pumping method may be required, such as positive displace-ment, with the tailings exhibiting a ‘thickened’, toothpaste-like state when released. If the pulp density is increased further, the tailings become a ‘dry’ product, and viscosity no longer plays a part in delivery and deposition.

Mr Botham at Golder comments: “The nature of the waste will also play a role. In particular, if a waste material is benign and will not leach, there is really no reason to provide a barrier, except for water conservation. If the waste will leach metals, a barrier system may have to be included.

“But, by engineering the waste, it may be possible to segregate the tailings into separate streams such that there is a large stream that is generally benign, while there is a small stream that may have special handling considerations. This smaller stream may require a special facility that can be constructed at a much lower cost than if the total tailings was disposed in a single facility.”

SOFTWAREBefore the advent of many software programs, engineers had to use their engineering judgement and experience to decide the critical conditions for the design, and then perform one or two analyses.

Today, it is possible to let software packages determine the critical conditions. The consultant then uses engineering judgement to evaluate whether that critical condition is reasonable, and then modifies the design and analyses to optimise the solution. Designers can now look at many more

options in a relatively short period of time in order to develop an optimal solution for a project.

With recent developments in 3D digital terrain modelling, tailings deposition scenarios can be assessed quickly in both rugged valley and paddockstyle tailings dam structures. Specialised software is frequently used at all stages of the design cycle to model a range of deposition techniques and

embankment construction methods in order to ensure the use of the most cost-effective, efficient solutions. This software also assists with the visualisation of design scenarios – essential for reporting and presenting to project team members, clients and investors alike.

Software also forms part of slope stability evaluation packages, which allow many slip surfaces and material variations to be appraised in a

Thickener at Codelco Andina, Chile

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42 TAILINGS

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shorter timeframe, and in the use of hydrogeological modelling to appraise the potential for basin and embankment seepage, leading to robust interception/mitigation design.

WATER CONSERVATIONWater management is a common challenge for all mining operations. By their nature, tailing storage facilities contain large quantities of process water, either in free form as tailing ponds or in the pores of deposited tailings. Therefore, the management of process water is a key factor in developing design criteria.

Mr Molloy at AMEC tells Mining Magazine: “Water reclaim is a core issue within the design and operation of a TMF facility and there are very few operations throughout the world that do not reclaim water from tailings. In arid countries it may be more efficient to reclaim water at the process plant and discharge the tailings as a ‘dry’ product, while, in areas of higher rainfall, tailings may be partially

thickened, slurried and pumped to the TMF at a target pulp density. The TMF is then operated as a natural thickener, allowing water to be reclaimed from its supernatant pond back to the process plant. Recovery of water from a slurried TMF is essential in controlling the size of the pond to prevent uncontrolled releases reporting to the environment.”

In water-drainage terms, surface water hydrology factors generally favour water diversion around the tailings dam and minimising water flowing into the impoundment, unless one of the objectives is to collect water for plant operations. In general, these flows are minimised for normal and flood conditions.

If possible, this will be achieved by locating the tailings dam as close to the head of the drainage basin as possible to minimise the cost of building surface water-diversion structures.

As location, topography and hydrologic considerations/constraints are evaluated relatively easily, they assume great importance in the site-screening process.

Extreme rainfall or arid conditions will have a marked influence on the water balance of a tailings dam, thus site-specific water-management structures will need to be installed to ensure surface water flows (or lack of) are managed appropriately.

Climatic and topographical conditions are used by designers to understand the potential hydrologi-cal impact on a tailings dam as early as possible, so this can be managed effectively in the facilities design, operation and closure plans.

For example, hydrological analysis and water balance are used to understand the potential magnitude of natural slope run-off to the tailings and supernatant pond, leading to the establishment and sizing of peripheral water-diversion drains to limit excess fresh water into the dam.

THICKENED TAILINGS DISPOSALThe surface or underground disposal of thickened tailings is being increasingly applied where water resources for processing are scant, and climatic conditions are such that thickened tailings are deemed appropriate.

Mr Spiers at SRK tells Mining Magazine: “Numerous research studies are under way on SRK projects in Australia, Central Africa and Chile regarding thickened tails and paste technologies. The advantages of thickened tails include water conservation, efficient run-off management, minimisation of containment-dam construction costs, enhanced stability and minimisation of infiltration of supernatant fluids and oxygen, which could lead to acid generation. The increased processing costs incurred by thickening tailings can often be offset by reduced capital costs for embankment construction and water-treatment facilities.”

Mr Botham says: “Surface disposal of thickened and paste tailings is considered on almost every project now, whereas a few years ago these options were considered to be too expensive. In particular, any location where water supply is limited, these options are seriously considered. However, as with every other design, we need to evaluate all options. There are situations where thickening tailings are simply an added cost and will provide little to no overall benefit for the project.”

At more arid sites, further filtration to form a ‘cake’ is gaining momentum, leading to surface co-disposal with waste rock and/or ‘dry’ stack construction. Although the market continues to discuss the environmental advantages/disadvantages for surface-thickened tailings disposal under variable climates, there is an opinion that thickened tailings for underground disposal may be appropriate under all climatic conditions.

“Thickened tailings for

underground disposal may be

appropriate under all climatic

conditions”

View of tailings impoundment at

Aitik mine, Sweden

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