Waste not, want not: legacy mines as sources of critical metals
Sitting on our global landscape are many thousands of legacy mine waste sites, which could be sources of critical metals, as Dr Anita Parbhakar-Fox* explains.
As a child, I was taught the three Rs – reduce, reuse and recycle. The school I attended gave all students a testing magnet and instructions to test every drinks can we could find to seek those that were aluminium and therefore suitable for recycling.
Today, I reflect and realise this valuable lesson in sustainability was not, at the time, practised by our primary industries. For example, the metal mining industry has produced around 20-25Gt of waste rock and 14 billion tonnes of tailings per annum. Consequently, instead of lush, green rehabilitated mine waste piles, we are left with countless examples showing poor historical waste management practices.
One of them, the oxidation of sulphide bearing solid (such as waste rock) and processed mine wastes (i.e. tailings and slag), can give rise to acid and metalliferous drainage, promoting metal mobility via aqueous pathways. Furthermore, the physical erosion of these wastes can also generate hazardous dusts posing risks to communities living downwind of legacy or active mining operations. The tangible benefits of spending more money upfront to better characterise waste materials is hard to convey, even when armed with financial statistics. However, with the growth of the current breed of Fortune 500 tech companies such as Apple and Tesla, in both popularity and market value terms, a major transformation in mine waste management could be on the horizon.
A new source of metals?
In 2015, Dr Kathleen Smith, a senior geoscientist at the United States Geological Survey, gave an address at the American Chemical Society annual meeting. She explained that to sustain our growing thirst for metals, we need to find them in societal waste. In doing so, we reduce our need for primary resources and offset waste disposal costs. Dr Smith’s team also focused on recovering gold (equivalent to a low-grade ore deposit), silver and rare earth elements from human effluent or sewage, with calculations suggesting that waste from 1 million Americans could contain up to £8.6m worth of metals.
This proposition promotes a more sustainable society, which, as a global community we are striving for with 169 targets to achieve by 2030 listed in the United Nations Sustainable Development Goals (UN SDGs). Apple has announced plans to recover metals from old iPhones in March with the unveiling of their robotic system, called Liam. The system can take apart one iPhone 6 every 11 seconds, recovering aluminum, copper, tin, tungsten, cobalt, gold and silver. The recycling capacity, however, is limited to only a few million phones per year, a small fraction compared to the 74.8 million sold during the final quarter of 2016 alone. While Dr Smith and Apple’s approaches to metal recovery align with the circular economy model defining a secondary metal resource, there is another very significant potential source of metals – mine waste.
Sitting on our global landscape are many thousands of legacy mine waste sites. In Australia alone it is reported that there are 60,000. These waste landforms are potentially economic sources of base, precious and critical metals. Our metallurgical history books state that large-scale volumes of mine waste were produced when the Romans exploited copper, gold, silver lead and zinc bearing deposits across the Iberian Pyrite Belt in southwest Spain and Portugal.
A protracted operational history followed, with many of these ancient deposits finally used in the early 21st century. The remains? Billions of tonnes of sulphidic waste rock at the Earth’s surface. This has since been oxidising to produce sulfuric acid, which has leached into surface and ground waters lowering the pH and permitting metal (cadmium, copper, lead and zinc) mobility.
Other waste landforms in this area, including the 10Mt slag pile at the historic Rio Tinto mine site, Spain, are sources of acid as they too are sulfide bearing. Under the Mediterranean climate, secondary sulphate mineral efflorescence precipitate on the slag pile surface, with chemical analyses showing they contain high Zn (up to 4wt.%), which prompted an in-depth analyses of each mineral group. Initial assessments using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) technology revealed that up to 5wt.% of refractory Zn is present in olivine phases – enough to be considered a low-grade resource. Metal recovery using new bioleaching methods represent an opportunity to break the source-pathway-receptor pollutant linkage chain. Simplistically, through reprocessing the contaminant source, funds could be generated and contribute to a site’s rehabilitation.
Critical metals such as lithium, cobalt and indium are considered hot property and are the very lifeblood for the diversifying technology and electronics industry. The rise of Tesla continues, with an ambitious sales target of 500,000 vehicles a year by end 2018, requiring 27,433 tonnes/year of lithium carbonate.
However, this ambition is limited by the demand for lithium being met.
In 2016, Australia ranked top globally, producing 14,300 metric tonnes of lithium, a much-needed boost to the West Australian mining industry. Maintaining this position is hard, as new projects can cost anywhere between AUD$150m to $2bln. However, new mineral extraction techniques such as the Sileach process, have allowed for the cost-effective processing of spodumene with up to 90% lithium recovered. While acid drainage is not typically associated with these materials, dust generation is of concern, suppression of which costs the industry millions of dollars annually. Therefore, through the development of new processing and recovery technologies, the industry has a game-changing opportunity with respect to mine waste evaluations, as attention can now be given to characterising non-acid forming minerals with a view to identifying additional viable metal sources.
Circular economy mentality
The circular economy mentality is by no means new – indeed, there are examples of the mining industry proactively evaluating mining wastes, typically for the target commodity originally sought, for example the Ergo Project, South Africa, and Renison Bell, Australia.
With the thirst for critical metals, resource definition studies of mine wastes should routinely include these. For example, in Kasese, Uganda, operations to bioleach cobalt from pyritic tailings (copper mining gangue) were established in 1999 with more than 600 tonnes recovered by 2012. Other such resources exist, including the Old Tailings Dam, Savage River iron ore mine, Tasmania, where 38Mt of pyritic tailings were deposited. In 2013, the cost of rehabilitating historic mine waste at this site was a minimum of AUD$120m. While many rehabilitation options have been considered, including flooding the tailings or creating a vegetation cover to restrict sulfide oxidation, geotechnical challenges have prevented them.
Alternatively, microanalysis of pyrite mineral chemistry revealed up to 3wt.% lattice-bound cobalt. Therefore, a theoretical maximum of 1.14Mt of cobalt is present – a resource likely exists.
Bioleaching testwork confirmed 93% cobalt recovery after ten days when using a mixed culture of Acidithiobacillus ferrooxidans, Acidithiobacillus thiooxidans and Leptospirillum Ferrooxidans – bacteria known to accelerate sulphide oxidation in mine waste environments. This demonstrates that through studying geochemical processes, they can be advantageously mimicked to create value. The business case for this example forecasts a contained metal value of US$143m. For the new, chemically inert, waste stream produced after tailings reprocessing there are several options for reuse such as aggregate, fertiliser or as a construction material.
What of indium, the critical metal enabling us to smartly communicate through touch-screen technology? Historic waste rock piles, of which there are tens of thousands in Australia, represent potential repositories.
For example, copper, tin and silver were produced at the legacy Baal Gammon copper-silver-tin mine in Queensland. Microanalysis of sulfidic waste boulders revealed they contain indium, which could be recovered using today’s metallurgical techniques.
Similarly, the Zeehan-lead zinc field in Tasmania contains over one hundred legacy sites. A recent waste rock characterisation study using LA-ICPMS at the University of Tasmania, Australia, revealed that acid-forming sulphides and carbonates are also indium bearing, presenting another critical metal recovery opportunity. In both cases, mining projects have not been established because no one has yet defined resources.
By extracting value from mineral waste, the opportunity for responsible sourcing of critical metals and establishing better socio-environmental practices is tangible. Furthermore, custodians of these sites, typically mine operators or state governments, can reduce rehabilitation costs.
Rick Humphries, a former mine rehabilitation advisor at MMG and Rio Tinto, stated that the industry has a head-in-the-sand approach to mine closure. He identified two common delay-tactics when dealing with mine wastes as offloading assets to smaller companies, or putting mines on care and maintenance.
Viewing mine waste as anthropocene deposits is a necessary step in providing metals to sustain our technological development, as well as reducing the legacy environmental footprints we have inherited. In essence, the UN SDGs are a much-needed catalyst to start a green revolution in mine waste evaluation. The long-term benefit? Launching our industries into a sustainable future through creating products with real green metal credentials.
*Dr Anita Parbhakar-Fox is a postdoctoral research fellow at the University of Tasmania, working for the ARC Transforming the Mining Value Chain research hub. She is the deputy leader of the geo-environmental risk module and leads a team of postgraduates students investigating new methods for characterising existing and future mine waste materials.