Meticulous Planning, Desired Output

A specialist from Knight Piésold Africa states that detailed planning and thorough implementation of key elements in the dewatered tailings disposal process is essential to achieving the expected moisture content.

In the present mining atmosphere, cost, environment, and reputation are the primary factors influencing decisions on the tailings disposal technique employed. Regarding meeting these requirements, mounting evidence indicates that theviability of dry stack disposal (dewatered tailings) is increasing.

However, attaining the desired residual moisture content threshold in dewatered tailings can be a complex task. Therefore, meticulous planning and effective implementation are crucial.

This insight comes from Andrew Copeland, Technical Director (Mining) at Knight Piésold Africa, as part of the company’s regular updates to mining companies in a changing regulatory environment.

Knight Piésold is a highly regarded international consulting company that offers engineering and environmental services across the mining, power, water, transportation, and construction sectors.

Achieving the Desired Moisture Content

Drawing from Knight Piésold’s extensive experience in developing sustainable solutions for clients, Copeland is eager to advise mining companies on the practical approach of achieving the desired moisture content in dewatered tailings. Specifically, he highlights the following aspects: the impact of the fine fraction on dewatering; the significance of sampling tailings; evaluating dewatering systems during pre-feasibility; and the business case for dry tailings.

1.

No Clay Presence

Relating to moisture content in tailings without clay presence, Copeland expounds, “In tailings from hard rock sources where no clay is present, consistently achieving a filter cake moisture content of as low as 15% should be achievable with well-operated equipment.”

• Clay Presence

On the other hand, in cases such as weathered rock-originated tailings, kimberlite, or mineral sands ore bodies, clays can result in a moisture content of 30 to 40%. Generally, with these materials, achieving a filter cake moisture content of 30% residual moisture can be highly challenging, as Copeland indicates. Inevitably, this can lead to a substantial volume of water remaining in the tailings and being deposited on a dry stack.

Given this challenge, some argue that the industry should refrain from labelling dewatered tailings as “dry stacks,” as liquefaction might still be possible. Thus far, this remains the bone of contention within the industry.

2. Sampling Tailings

Due to the effect of the fine fraction on moisture content, Copeland emphasises the importance of obtaining tailings samples before designing the filter plant, transport system, and dry

The fine fraction, particularly the clay fraction (presence of clay in tailings), determines the moisture content in dewatered tailings. Two common scenarios arise with tailings: one without clay presence and another with clay presence. www.knightpiesold.com stack. This is because a slight change of 5% in moisture content could significantly impact the number and type of required filter units, the choice of transportation method, and the distinction between a stack that might liquefy or not (with or without compaction).

3. Assessing Feasibility of Dewatering and Disposal Systems

The feasibility of dewatering and disposal systems can be dictated by the cost of accessing water. For example, in situations where water scarcity is severe or water treatment is expensive, Copeland illustrates: “In semi-arid to arid climates where water is scarce, expensive, or obtained through seawater desalination, the drive to maximise water recovery through tailings dewatering is substantial. Thus, during the pre-feasibility stage of a project, the team must assess the feasibility of various dewatering and disposal systems.”

The Business Case for Dry Stack Disposal

From the above mentioned, the process of fully dewatering tailings to form a cake and transporting it to a “dry-stack” facility seems convoluted. One would ask: Why not dewater through high-rate thickeners and pump as highdensity slurry to a wet disposal facility? There could be a business case for the latter from the cost perspective.

In response to this, Copeland refers to a recent study the Knight Piésold’s team conducted. The team examined the merits of both wet and dry disposal facilities. The results highlighted the relevance of the dry ‘stack disposal’ facility in ensuring that tailings disposal at a mining company ticks all necessary boxes in sustainable lifecycle.

Copeland states that it is important to look beyond cost to appreciate merits of the dry stack disposal technique. “The dry stack disposal option is costly but appealing due to water-related factors (cost, environment, and reputation). On the other hand, the wet disposal option is cost-effective but must be weighed against water permits or high supply costs,” he explains.

On the intermediate option of paste (very high density slurry) disposal could be another option. However, it is often less favourable due to cost and operational challenges, potentially not significantly better than the high-density case. This reinforces the relevance of dry-stack disposal.

Engaging an experienced consulting firm

Overall, given the intricacies involved in achieving the desired moisture content in dry stack tailings, Copeland stresses the importance of engaging an experienced consulting firm that can coordinate all the specialist parties. “An experienced consulting firm can provide a range of advice, design experience, with the support of other parties. This is advantageous for a client who would otherwise have to solicit specialised work from multiple parties and tender processes. Responsibilities are distributed between parties, and a consultant with a wealth of experience can apply insights from one project to another. Quick consultations with colleagues for advice and internal peer reviews are invaluable, particularly when cost-effective risk mitigation is crucial. GISTM stipulates that while an EoR (Engineer of Record) may be an individual, having a company or team to support that individual is equally important due to skill shortages and the need for an EoR to oversee multiple TSFs.”

The team at Knight Piésold are dedicated to developing relevant solutions for comprehensive dewatered tailings disposal for clients in various mineral commodities in Africa.

Dry Stack Tailings Management

On GISTM Compliance and ESG Reporting

Adopted after the Vale Brumadinho Dam disaster, the 2020 Global Industry Standard on Tailings Management (GISTM) sets a benchmark for safe tailings facility management, striving for zero harm.

In the context of contemporary design, construction, and management of Dry Tailings Storage Facilities (TSF), an inherently complex task, Andrew Copeland, Technical Director of Knight Piésold, explains, “While GISTM covers the entire spectrum of good tailings management practices, its primary focus is on ensuring the safety and stability of facilities. It offers specific guidelines for assessing failure modes, the risk of liquefaction, and brittle behaviour.”

Many clients recognise the need for compliance. One notable response is the shift away from designing upstream tailings dams, even though dry-stack facilities are designed and constructed in an upstream manner. Alternatively, clients acknowledge that buttressing of TSFs might be necessary and more cost-effective than building new ones.

ESG Reporting

Compliance with GISTM and ESG reporting go hand in hand. Adhering to GISTM standards can enhance a mining company’s ESG (Environmental, Social, and Governance) score reporting.

Regarding ESG reporting, the following are the primary areas of focus: reducing water consumption, minimising environmental impact by controlling seepage through geochemical testing and barrier system design and siting TSFs away from communities or downstream infrastructure and land use.

Grey Area

However, one aspect of ESG that is often overlooked is energy consumption. Thus far, this remains a grey area, laments Copeland. “The process of tailings dewatering, whether through vacuum or pressure filtration, demands significant power, which is typically not accounted for. Hence, if the power source is fossil fuels, air emissions offset gains in water recovery.”

Knight Piésold specialises in tailored solutions to manage the complexities trade-offs. Copeland notes, “We can conduct trade-off studies and collaborate with vendors to assess entire systems, not just isolated components of the disposal chain. This is crucial, and GISTM highlights the importance of life-cycle design and costing, particularly closure design. KP can handle design, permitting, environmental aspects, and closure considerations.”

On how the impact of IoT on TSFs management

Big data and the Internet of Things (IoT) have brought forth incredible efficiencies from exploration to mine closure and rehabilitation. Concerning tailings storage facilities, Copeland observes that, in recent times, one of the greatest areas of advancement have been online and remote instrumentation, as well as satellite monitoring (movement in particular).

Online and remote instrumentation has opened a wide range of opportunities in the sphere of monitoring. “The installation of automated monitoring devices in tailings dams such as piezometers, inclinometers, water levels, rain gauges, flow rates and many others, with alert levels, allow for hourly or daily dashboards to be generated and updated (depending on reporting requirements or status). These dashboards provide immediate indicators of any change/ deterioration in integrity and can be reacted upon as per the Trigger Action Response Plans (TARPs).”

The opportunities for unlocking more data from satellites is being explored, especially changes in moisture content that may be an early warning of instability (coupled with movement monitoring). However, Copeland believes that it is only a matter of time before advances in this area open further opportunities.