Gold has influenced human society for centuries as a currency and medium for art and jewellery. In modern times, gold plays a critical role in modern electronics, chemical synthesis, aerospace technologies, and biomedical science. Unfortunately, mining for gold is terrible for the environment. Deforestation, greenhouse gas emissions, and the use of highly toxic reagents such as mercury and cyanide are all widespread problems with gold mining.
In large-scale, formalized gold mining, cyanide is the most common reagent to extract gold from ore. The tailings in these operations often have high levels of this toxic reagent. While there are ways to degrade cyanide, there is still a risk to wildlife if they are exposed to cyanide or if a tailings dam is breached.
In artisanal and small-scale mining (ASGM), liquid mercury is mixed with a gold source (such as ore or ore concentrates) to form a dense amalgam. The amalgam is relatively easy to separate from the mixture. The gold is then recovered by heating the amalgam and boiling off the mercury. The simplicity of this process is why amalgamation has been used for centuries in gold mining. Unfortunately, it is still widespread in ASGM and results in harm to the miners’ health and the environment. In fact, the largest source of mercury pollution on the planet is from ASGM.
Our research program has a long-standing goal to provide safer and more sustainable alternatives to cyanide and mercury. We originally focussed on mercury remediation technologies, but soon realized that eliminating the use of mercury would be a more direct solution. To do this, we needed to develop techniques that are low-cost, robust and technically simple so they could be used in areas with limited resources.
To be clear, we are not demonizing miners that use mercury. Many of them are mining for gold as a subsistence practice: it is their livelihood. Instead, we aim to support them and provide safer alternatives to mercury.
Our strategy was to identify readily available oxidants that could react with gold metal and render it water-soluble as a gold salt. One of the most effective reagents we identified was trichloroisocyanuric acid (TCCA). This reagent is used in water sanitation and pool chlorination, so it is low-cost and widely available. TCCA alone does not react with gold, but simply adding salt water makes active chlorine species that can oxidize and leach gold. The TCCA can be recycled and regenerated. Alternatively, cyanuric acid (the product formed after its reduction) is degradable. In fact, cyanuric acid is a trimer of urea so it has potential use as a fertiliser. Our dream was to take advantage of the cyanuric acid in tailings to support phytoremediation and land rehabilitation. We are still a long way from realizing that goal, but it is an exciting possibility.
After leaching the gold, we needed a sorbent that could selectively bind the gold and remove it from water. We discovered that polysulfide sorbents are highly selective at binding gold, even in highly complex mixtures. The strong bond between gold and sulfur is known, but what we didn’t anticipate is that these sorbents readily reduce the gold salt to gold metal. They are therefore redox active sorbents and we believe that the reduction of the gold salt provides a thermodynamic driving force for the gold sorption.
A final innovation is to recycle the sorbent. Here we relied on a linear poly(trisulfide) that can be “un-made” by depolymerization. This capability allows simultaneous gold recovery and monomer recycling. We are particularly proud of developing this polymer because it was originally thought to be inaccessible. Our photo-chemical polymerization in a flow reactor overcame this challenge to provide that key material.
We are very grateful to our collaborators that supported this study. Adelaide Control Engineering provided essential support for large scale trials in sorbent production, mining trials, and e-waste recycling applications. We also thank Mercury Free Mining and their leadership and skill in partnering us with a mine in Peru so that we could test our method on ore. I am also in debt to the very talented team of post-doctoral researchers that made this all happen. Drs Max Mann, Thomas Nicholls, Harshal Patel, Lynn Lisboa, Jasmine Pople, Le Nhan Pham, Max Worthington and several other dedicated researchers stuck with this project over many years. Their creativity and endurance made some of our ambitious aims a reality.
Our long-term goal is to work with industry, governments, and philanthropic organizations dedicated to making gold mining and e-waste recycling safer and more sustainable. This study is our first step toward realizing this goal.
