Non-toxic mining for gold
A safer and more sustainable way to extract and recover gold from ore and e-waste is reported at Flinders University, Australia.
The researchers use the reagent trichloroisocyanuric acid (TCCA), which they say is widely used in water sanitation and pool chlorination, and is low-cost and benign.
Professor Justin Chalker says, 'We discovered that if TCCA reacts with salt water, it forms an active oxidant that can react with gold and convert it into a water-soluble salt.'
The gold can then be selectively bound to a sulphur-rich polymer made by the team. They claim the process’ selectivity allows gold recovery even in highly complex mixtures.
Heating the polymer to around 140°C breaks the S-S bonds in the 'polymer backbone'. The monomer can be separated from the gold using distillation, so it can be reused and the gold can be recovered.
One of the polymers used is made from elemental sulphur and canola oil, which the researchers say they can make on a multi-tonne scale. The recyclable sorbent is made using a different process that converts a cyclic trisulphide into a polymer using light.
They collaborated with a team from the US and Peru to validate the method in an ore, as small-scale mines otherwise rely on toxic mercury to amalgamate gold, with gold mining being one of the largest sources of mercury pollution. Gold mining typically uses highly toxic cyanide to extract gold from ore, the team says, with risks to the wildlife and the broader environment if it is not contained properly.
'We demonstrated that gold can be leached and recovered from alluvial ore, provided by our collaborators from Mercury Free Mining,' Chalk says. 'The yield and purity of the gold were very encouraging in these trials.'
The paper on Sustainable gold extraction from ore and electronic waste in nature sustainability shows how they were able to recover 412mg of gold at 98.8% purity from 1kg ore using their TCCA-NaBr system and their poly-2 sorbent. They were also able to recover 59mg of gold, of more than 99% purity from 200g of ore.
The team says they have used this method for many other metal sources, from printed circuit boards in discarded computers to trace gold in scientific waste streams.
The paper reads, 'Printed circuit boards and other components of discarded electronics can contain 200-900mg of gold per kilogramme, which exceeds gold concentrations in ore.'
The leaching reagent reacts with other metals like copper, not just gold. They crush and mechanically concentrate RAM cards and central processing units, with the gold and copper leached via the TCCA method. The gold can then be recovered using the sulphur-rich polymer, with copper recoverable by electroplating.
Chalk explains, 'For example, for an e-waste-derived mixture consisting of 3.9g gold and 18.6g of copper, we used 178g of TCCA to leach both gold and copper.'
The TCCA is converted to cyanuric acid, which can be recycled and converted back to TCCA. They are also exploring the use of cyanuric acid as a fertiliser in future work.
They plan to work with miners and e-waste recyclers to trial the method on a larger scale.
Chalk adds, 'To scale up the process, we envision making an integrated and automated system that can take gold concentrates (for ore or e-waste) and leach and recover the gold. This process will also be coordinated with copper recovery, TCCA regeneration, and water recycling. These are the goals in the next phase of this project to provide a centralised plant that can accommodate larger-scale gold recovery across all steps of the leaching and recovery process.'
