Retreatment of Historic Tailings and Low-Grade Stockpiles

Tailings storage facilities (TSF) are typically classed as ‘waste’ facilities; potential liabilities with significant safety and environmental issues to overcome. This negative perception could be curtailed if the mining industry could view these disused materials not as waste, but as potential resources.

Since the solid phase from tailings can contain remaining valuable metals that were not recovered in the past, they represent potential resources with a growing awareness of opportunities to rehabilitate such facilities whilst also generating revenue.

The potential resource within any tailings facility is dependent on multiple factors including the amount of material, the mineralogy of the original ore, the mineral or minerals targeted in primary processing and the nature of that processing and the efficiency of recovery.

Origin of Tailings
Tailings will be unique for every deposit in terms of scale and composition. The amount of ore processed per unit of commodity produced will vary with the type of deposit, the mineralogy of the deposit, the quality of the ore, the quantity of the ore and the mining methodology chosen that may increase or minimise diluting material sent to the processing plant.

A continuing trend is likely to be that the amount of waste material produced per unit of commodity will increase over time as overall ore grades decline and as lower grade deposits are exploited. This trend of exploiting lower grade deposits is partly to be driven by increasing metal prices and new processing methods.

There are two key drivers that are likely to increase focus on old tailings; the first is that the same increases in price that could allow the exploitation of lower grade in-situ material may also incentivise the reprocessing of low-grade tailings or dumped material if other factors (such as the ability to process that material) are met. The second driver is that changes in focus for industry may also prompt interest in commodities that were previously ignored in the historic tailings. Commodities previously considered waste, or a contaminant, might become sought after as technology develops in light of the green energy transition and drive for electrification.

Focus of Targets
The primary focus for most initial assessments is likely to be the potential to recover high value material by identifying and targeting tailings at deposits mined when commodity prices of the primary material were lower than current prices, or where processing (or certain eras of processing over the mine life) was inefficient by today’s standards, potentially coupled with higher head grades allowing for such inefficiency. Both of these scenarios could have seen material that is of interest today left in tailings or waste dumps and low-grade stockpiles.

In terms of inefficiency, there are multiple factors worth considering: was there a focus on gravity recovery only at a historical gold deposit? Was the recovery of refractory gold not attempted? Or was recovery poor from refractory ores due to the processing methodology chosen?

Rare Earth Elements
An assessment of tailings facilities or dumps/stockpiles for re-exploitation should not simply focus on the primary mineral or minerals originally exploited. As technology changes, and as the green revolution accelerates, minerals seen as waste in the past can become a valuable commodity. Minerals previously not recovered and allowed to pass into tailings can now be recovered and utilized. An example of this is rare earth elements (REE).

A 2016 study assessed the potential for rare earth elements in the Kiruna iron ore tailings (Sweden) to act as a new REE resource for the EU. The study found 1200-1500ppm REE in 50-100Mt of tailings. Whilst economic production was difficult to secure, the tailings might now be considered an important strategic resource.

Lithium
Ambitious plans for targets of electric vehicle usage will require a lot of lithium, a critical component of batteries. Whilst many new lithium focused operations are in the planning stages, a new mine to obtain lithium is not necessarily required. New techniques allow the recovery of lithium from previously dumped, low-grade material at historic lithium mines, as well as mines where the commodity was not considered previously. For example, Rio Tinto explored for gold in borates tailings in the USA. During this exploration phase, they discovered lithium at a higher grade than rival US projects already under development.

Exploration Process
It is probable that process plant records will hold information recording the time period of tailings deposition, volume of material deposited, estimated plant recovery, estimated grades of tailings and process methodology, amongst other data.

Prospects should be treated in a similar way to any prospective in-situ deposit, by drilling using a suitable methodology to gather representative samples to carry out a Mineral Resource estimate and collect material for processing testwork. The drilling methodology chosen will vary depending on, amongst other factors, the depth, accessibility, stability and moisture content of the tailings. Drilling can be from auger drilling, reverse circulation drilling right the way through to sonic drilling.

Mineral Resource Estimation
The Mineral Resource estimation procedure for a TSF will follow a similar procedure to an in-situ deposit but certain aspects will need additional consideration and data.

Domaining of the TSF will require information to delineate the volume of the tailings material. Typically, this would require information to develop 3D surfaces representing the pre-deposition base of tailings, the current surface levels and the position of the confining walls. In an ideal situation, information would be available to code different time periods of tailings deposition within the model. Conducting this task may allow the model to be sub-domained to carry out restricted grade estimation for tailings produced during the mining of ore of different mineralogy or during times of different processing methodology.

Perhaps the most crucial aspect at this stage will be the results of new metallurgical testwork carried out on samples from the TSF. The commodity under investigation was not recovered during primary processing, therefore it must be clearly understood why and what can be done to recover it during reprocessing.

Conclusions
Exploitation of lower grade mineral deposits is becoming increasingly common with the continual depletion of mineral resources coupled with rising demand. By targeting and exploiting historic tailings facilities and stockpiles, sustainable and profitable mining operations can be carried out. These tailings may have been deposited in times of inefficient processing methodologies or incorrect processing techniques for the mineralogy targeted and, may have grades comparable to in-situ deposits exploited today.

Reprocessing of this material will come with challenges; the material was not recovered in primary processing, and it must be clearly understood why not, and what can be done to recover that mineral during reprocessing. The reprocessing of each deposit will require different solutions depending on the mineral targeted, the mineralogy of the primary deposit, the original process methodology and the efficiency of the original processing.

How can we help?
WAI has extensive experience in the assessment of historic tailings and stockpiled materials. WAI can design appropriate exploration programmes to gather information for Mineral Resource estimates in line with international reporting guidelines and advise on collection of samples for metallurgical testwork and, subsequently, complete the Mineral Resource and Ore Reserve estimates with sign-off by appropriate Competent or Qualified Persons. Furthermore, through our in-house laboratory facilities, WAI can provide high quality metallurgical testwork programmes ranging from initial bench scoping investigations to feasibility studies and pilot plant campaigns.