By Dr Max Mann, Monash University; and Professor Justin Chalker, Flinders University
A new Australian-led innovation is showing how simple, accessible chemistry could help small-scale miners recover gold safely – without the deadly environmental toll of mercury and cyanide.
Gold has been part of human culture for thousands of years, and plays an integral part in modern technologies like electronics, aerospace and medicine. Unfortunately, gold production comes with an environmental price. Toxic reagents like cyanide are widely used in the formal mining sector, and the heavy metal mercury is prevalent in artisanal and small-scale gold mining. Despite efforts and regulations to contain the tailings and release of these reagents, they still pose a risk to miners and the environment.
The mercury emissions from artisanal and small-scale gold mining are particularly alarming. This sector involves more than 15 million people, often operating in developing regions with few resources. These miners are often subsistence miners: they are mining to make a living. Unfortunately, the use of mercury is a risk to them and the environment.
In these small operations, liquid mercury is mixed with gold ore or gold ore concentrate, resulting in a dense gold-mercury amalgam. To recover the gold, the amalgam is heated and the mercury is boiled off to separate it from the gold. While this process is simple and has been used for centuries, it presents a high risk to the miner’s health and accounts for up to 40 per cent of annual global mercury emissions. Motivated by the Minamata Convention on Mercury, an international treaty designed to minimise the release and emission of mercury, we wanted to develop safer, more sustainable solutions to eliminate the use of toxic reagents in gold mining. Our aim is to provide support for the artisanal mining sector, support their livelihood, and protect the wellbeing of the miners and environment.
To achieve this ambitious goal, our approach was to develop an integrated process that includes all the stages of gold recovery, including initial gold concentration, leaching, gold recovery and reagent recycling. Concentration is already commonly done by gravimetric methods, including the use of pans, sluice boxes, shaker tables or centrifugal concentrators. The next step after gold concentration is the leaching process.
For leaching, we wanted to identify an oxidant that would render the gold water-soluble in the form of a gold salt. A reagent like this must be low-cost and widely available. We identified a reagent that is frequently used in pool chlorination and water sanitation, and widely available at a competitive price. While this reagent – trichloroisocyanuric acid (TCCA) – does not react with gold directly, simple activation with salt water generates a reactive chlorine species that readily oxidises gold.
After leaching, two different strategies can be used to manage the reaction product, cyanuric acid. Firstly, the cyanuric acid can be regenerated and recycled using the same method employed in its manufacture. Alternatively, we envision using the terminal product (cyanuric acid) as a fertiliser – perhaps in land rehabilitation or phytoremediation of tailings or polluted areas.
To recover the gold, a selective sorbent was needed. Through our extensive work with sulphur-rich polymers, we discovered that polysulphides can selectively bind to gold even in highly complex mixtures. More than 95 per cent of gold can be recovered from the solution even in the presence of other metals, such as aluminium, iron, copper, zinc or tin. Our fundamental studies also showed that these polysulphide sorbents convert the gold ion in solution to gold metal, which is a unique feature of these sorbents.
For this project, we developed various polysulphide sorbents to address the challenge of scale and recyclability. One such sorbent is made from an unsaturated triglyceride such as canola oil and elemental sulphur. Both of these reagents are low-cost and readily available. Collaborating with a commercial engineering firm, this sorbent was made on a tonne scale. Another polysulphide sorbent was made by a novel photochemical ring-opening polymerisation, which was invented for this project. The major advantage of this sorbent is that the polymer can be recycled back to its monomer and used to remake the sorbent.
After sorption of the gold onto the polymer, gold can be recovered by pyrolysing the polymer or by the depolymerisation, with the latter enabling sorbent recycling. We also developed a purification protocol for rapid refining of gold. The same TCCA leaching system can be used to re-leach the gold, followed by precipitation using ascorbic acid (vitamin C). This is an effective method that can be used to purify the gold to greater than 99 per cent purity.
The whole, integrated method was then tested on gold ore concentrate from small-scale mining collaborators operating in the Andes Mountains of Peru. This ore concentrate was treated with the TCCA leach solution before the gold was recovered by a polysulphide sorbent. After pyrolysing the sorbent and purifying by re-leaching and precipitation, the gold had a purity of 99 per cent.
The same integrated process was also applied to electronic waste (e-waste). A gold concentrate was first prepared from RAM pins to provide a substrate containing 13 per cent gold and 62 per cent copper. More than 85 per cent of gold was recovered with a purity of 99.9 per cent.
These results are an important step towards safer and more sustainable gold recovery from ore concentrates and e-waste; however, there is still considerable work to be done. The scale-up of the full process is still in development.
Our long-term vision is to construct a containerised demonstration plant in order to develop a model for centralised facilities to support artisanal and small-scale gold mining. Such facilities can purchase ore concentrate from the community and, in doing so, bring them into the formal economy. The centralised plant can then carry out leaching and refining in a controlled environment, safely and sustainably recovering high-purity gold, and ensuring duty of care in recycling and tailings management. We aim to support artisanal and small-scale mining, and the rise of e-waste recycling (‘urban mining’) in a sustainable and profitable way.
