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Articles from MATERIALS HANDLING (211 Articles), ENVIRONMENT PRODUCTS (186 Articles), SAND PROCESSING EQUIPMENT (157 Articles)

An example of a flocculated conventional tailings slurry. Conventional low density deposition of tailings still has a part to play for the right site at the right time.
An example of a flocculated conventional tailings slurry. Conventional low density deposition of tailings still has a part to play for the right site at the right time.
 










Tailings management: Laying the groundwork

The ideal tailings management system produces the correct volume of process water at the best quality and tailings with the right characteristics, while minimising the impact of waste solids on the site’s function. The key to getting it right, Stewart Reeve explains,
is to understand the nature of your tailings.
An inappropriate tailings management system can be costly and stressful, wasting capital, personnel time, power, water, consumables and space. Access to reserves can be compromised, the viability of expansions jeopardised and quarry closure timeframes and expenses may inflate beyond initial expectations.

From top left: Clear graduated measuring cylinder, pH meter, marsh funnel and fine sieves. Even simple tools can help operators better monitor and manage a tailings system.
From top left: Clear graduated measuring cylinder, pH meter, marsh funnel and fine sieves. Even simple tools can help operators better monitor and manage a tailings system.
An inadequate system can increase an operation’s environmental and OHS risk profile, which has corporate, social and political ramifications.

Good tailings management maximises the quarry’s long term potential and represents value to the business by allocating operational and capital resources to reasonably minimise the tailings risk profile.

A tailings management system is conventionally defined as the equipment and infrastructure that dewaters, transports and stores the tailings solids. However, because tailings typically place high demand on a site’s water circuit and have great bearing on its water balance and water management compliance, an integrated approach to water and tailings management has real merit.

Incorporating the tailings management and water management systems into a site-wide water quality management plan allows a quarry to capitalise on these systems’ infrastructural and procedural synergies and further reduce its risk profile.

UNDERSTANDING THE RISK PROFILE
Before selecting a tailings management system, it is useful to identify elements of risk. The broad components are:
•    Capital – processing, transport and storage infrastructure, capital budget.
•    Operational – management plans and procedures, process consumables, operational budget.
•    Environmental and safety – regulatory obligations and
best-practice systems.
•    Social/political – responsibility to and relationship with the community, actual and perceived issues.
•    Regulatory – existing and planned regulation, possible changes to regulation.
These factors overlap with and influence each other and need to be appropriately weighted according to the site’s unique context, with foresight to accommodate for possible change.
By using these values to define design parameters, the resulting tailings management system will be:
•    Relatively low risk.
•    Able to facilitate nominal production requirements.
•    Robust enough to cope with reasonable process variations.
•    Flexible enough to adapt to changes in nominal expectations.
The best tailings management systems are site and business specific. Consequently, the “right” system does not necessarily reflect industry trends or replicate what currently works at another site but is designed to practically and strategically serve the quarry’s interests in the short, medium and long term.

Figure 1. Mineralogies affect water chemistries differently, even for the same tailings load.
Figure 1. Mineralogies affect water chemistries differently, even for the same tailings load.

Figure 2. Changes in dissolved water content from mineralogies or a smaller water circuit can affect key environmental water parameters, such as pH.
Figure 2. Changes in dissolved water content from mineralogies or a smaller water circuit can affect key environmental water parameters, such as pH.

Figure 3. An example of the possible linear and non-linear consequences of process variations.
Figure 3. An example of the possible linear and non-linear consequences of process variations.


DEFINING THE “NOMINAL CASE”
When developing a tailings management system, either for a greenfield or brownfield site, it is common to obtain nominal quantification of the following tailings slurry parameters:
•    Type of tailings solids.
•    Volumetric or mass flowrate.
•    Slurry density.
•    Particle size distribution.
•    The pH of the process water.
Diligent designers may also measure various geotechnical parameters, such as beach angle and permeability of the settled solids. Many suppliers and consultants are able to design and provide cost estimates for unit processes based on the above figures alone. However, to achieve a truly relevant, reliable tailings management system, it is important to appreciate these additional factors:
•    The “shifting nominal case”.
•    Range of variation.
•    Rate of variation.

Rapid dewatering and modified rheology: a chemically aggregated clay slurry.
Rapid dewatering and modified rheology: a chemically aggregated clay slurry.
Variations and their potential impacts

Variation from the nominal is the bane of otherwise well designed systems. Resulting periods of under- and over-performance can drain operational resources, decrease product quality, undermine productivity and profitability and even prompt revisions of mine plans and estimations of quarry viability.

Though variations cannot be eliminated, it is possible to ascertain their nature, anticipate their effects and account for them in the design of process infrastructure and control procedures. That said, the potential impacts of variations are not always readily apparent at the conceptual design stage; often, they are revealed in the running and troubleshooting of the system, when small operational problems accumulate into larger, strategic issues.

The factors and possibilities discussed below are frequently overlooked in the design of a tailings management system. They are worthy of serious consideration.

THE MINE PLAN

Mineralogy and handling: a sodium bentonite slurry standing tall at only 10 per cent solids!
Mineralogy and handling: a sodium bentonite slurry standing tall at only 10 per cent solids!
The method and pattern of extraction directly influences the frequency and extent of variation in tailings and process water. More selective extraction, producing geologically consistent feed, can enhance tailings management efficiency and efficacy. Feed blending may be a suitable compromise for some sites.

Mineralogy
The geology and geochemistry fed to the beneficiation plant characterise the nature of the tailings. Thorough prior analysis of the reserves being extracted will facilitate better design and operation of the tailings management system. Pay attention to anomalies, such as particularly acidic, alkaline or salty profiles, or sections containing heavy metals, higher silt loads or particles that are finer than usual.

Consolidation profile
Rules of thumb exist for slurry dewatering rates and limits, but every site is geologically and operationally different. Since the assumptions employed will resonate throughout the entire system, they must be well founded. Variations originally considered “acceptable” may affect the system and/or production more than projected. Tailings reagents can markedly affect the rate and extent of consolidation. Site- and mineralogy-specific tailings consolidation data can be generated via a range of basic empirical tests.

Tailings load
The fraction of feed reporting to tailings can vary substantially through the profiles of a site’s reserves, even when the mineralogy is fairly consistent. Tailings management systems should be designed to handle not only nominal conditions but also significant, sustained excursions in the mass flow through the system. Such excursions can modify water chemistry and generally have exponential and cascading consequences that can feedback into the process, affecting production and mine planning.

A Clarometer FAS thickener settling time monitor and automatic flocculant control system (photo courtesy of Oscillation Pty Ltd).
A Clarometer FAS thickener settling time monitor and automatic flocculant control system (photo courtesy of Oscillation Pty Ltd).
Tailings particle size

The changing nature of the particle size distribution is one of the chief parameters that should be considered in the design of a tailings management system. Small changes in tailings particle size profiles can cause large changes in the performance of dewatering, transportation and storage units, often with disproportionately negative results.

Water chemistry
Water chemistry plays a critical role in the performance of tailings and water management systems. It also affects production efficiency and product quality. It dictates the scaling and corrosion potential of process water and heavily influences a tailing’s consolidation profile. Water chemistry is altered by the process material’s mineralogy, the tailings reagents used, the chemical profile of make-up water, and the volume of the recirculating water circuit.

Rheology and transportation
Rheology, the flow nature of a slurry, is affected by many mineralogical and process factors. A tailing’s rheology determines the efficiency and efficacy of its transportation, irrespective of mode. It is prudent to conduct at least some preliminary rheological studies, to better understand the site-specific nominal case and to identify how variations in mineralogy, process conditions and reagents could affect slurry transportation.

Instrumentation and process control
Many units struggle when faced with even mild excursions from nominal values, requiring operator intervention and causing production rate loss or downtime. Most tailings management units can be automated in some fashion. Even basic instrumentation can greatly enhance a manually operated system. Investment in process control usually pays off in the short term.

Social and political perception
Whether you agree with them or not, the views held by politicians, bureaucrats and the community have genuine implications for your tailings and water management system. Factoring reasonable social and political concerns into the design of your tailings management system yields a number of valuable tangible and intangible short and longer term benefits.
The pH vs settling time profile for a low density sodium bentonite  slurry, an example of varying water chemistry affecting a slurry’s consolidation profile.
The pH vs settling time profile for a low density sodium bentonite slurry, an example of varying water chemistry affecting a slurry’s consolidation profile.

Hybrid systems
Use convention as a guide – but don’t let it dictate. A novel combination of tailings units may be exactly what your site needs.

Staged implementation
Weigh up the necessity of purchasing and installing everything up front. Staged implementation spreads out capex and opex burdens and, in some cases, can better suit a quarry’s mine plan and resources.

A high rate thickener and pressure filtration housing is one of the options to be explored and utilised for tailings management (photo courtesy of Minroc).
A high rate thickener and pressure filtration housing is one of the options to be explored and utilised for tailings management (photo courtesy of Minroc).
KNOW YOUR OPTIONS

Clarifiers, thickeners, pressure filters, vacuum filters, centrifuges, tailings dams, co-disposal, water dams, tanks, pumps, pipes, mixers, trucks, conveyors, engineered channels, coagulation, flocculation, chemical aggregation, binders, filter aids, geo-textiles, screens, hydrocyclones, automated, semi-automated and manual control systems, instruments, used equipment, hired equipment, existing infrastructure — these are just some of the generic options to be explored and utilised.

Owners and managers generally know their site and operation well and have a good sense of what can and cannot be achieved. With that as your starting point, seek advice and get informed about the many options and combinations available for the management of your tailings. Investigate their practical and strategic implications.

Be creative and entertain your quarry’s ideal outcomes; there is a way to make most ideas work.

Stewart Reeve is the senior process engineer for Victory Engineering. Email: stewart@victoryeng.com

















Saturday, 20 April, 2019 10:57am
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