In 2022, humans produced an estimated 62 million tonnes of electronic waste – enough to fill more than 1.5 million garbage trucks. This was up 82 percent from 2010 and is expected to rise to 82 million tonnes in 2030.
This e-waste includes old laptops and phones, which contain precious materials such as gold. Less than one quarter of it is properly collected and recycled. But a new technique colleagues and I have developed to safely and sustainably extract gold from e-waste could help change that.
Our new gold-extraction technique, which we describe in a new paper published today in Nature Sustainability, could also make small-scale gold mining less poisonous for people – and the planet.
Soaring global demand
Gold has long played a crucial role in human life. It has been a form of currency and a medium for art and fashion for centuries. Gold is also essential in modern industries including the electronics, chemical manufacture and aerospace sectors.
Related: Not All Uranium Can Be Used in Weapons. Here’s What ‘Enrichment’ Means.
But while global demand for this precious metal is soaring, mining it is harmful to the environment.
Deforestation and use of toxic chemicals are two such problems. In formal, large-scale mining, highly toxic cyanide is widely used to extract gold from ore. While cyanide can be degraded, its use can cause harm to wildlife, and tailings dams which store the toxic byproducts of mining operations pose a risk to the wider environment.
In small-scale and artisanal mining, mercury is used extensively to extract gold. In this practice, the gold reacts with mercury to form a dense amalgam that can be easily isolated. The gold is then recovered by heating the amalgam to vaporise the mercury.
Small-scale and artisanal mining is the largest source of mercury pollution on Earth, and the mercury emissions are dangerous to the miners and pollute the environment. New methods are required to reduce the impacts of gold mining.
A safer alternative
Our interdisciplinary team of scientists and engineers has developed a new technique to extract gold from ore and e-waste. The aim was to provide a safer alternative to mercury and cyanide and reduce the health and environmental impacts of gold mining.
Many techniques have previously been reported for extracting gold from ore or e-waste, including mercury- and cyanide-free methods. However, many of these methods are limited in rate, yield, scale and cost. Often these methods also consider only one step in the entire gold recovery process, and recycling and waste management is often neglected.
In contrast, our approach considered sustainability throughout the whole process of gold extraction, recovery and refining. Our new leaching technology uses a chemical commonly used in water sanitation and pool chlorination: trichloroisocyanuric acid.
When this widely available and low-cost chemical is activated with salt water, it can react with gold and convert it into a water-soluble form.
To recover the gold from the solution, we invented a sulphur-rich polymer sorbent. Polymer sorbents isolate a certain substance from a liquid or gas, and ours is made by joining a key building block (a monomer) together through a chain reaction.
Our polymer sorbent is interesting because it is derived from elemental sulphur: a low-cost and highly abundant feedstock. The petroleum sector generates more sulphur than it can use or sell, so our polymer synthesis is a new use for this underused resource.
Our polymer could selectively bind and remove gold from the solution, even when many other types of metals were present in the mixture.
The simple leaching and recovery methods were demonstrated on ore, circuit boards from obsolete computers and scientific waste. Importantly, we also developed methods to regenerate and recycle both the leaching chemical and the polymer sorbent. We also established methods to purify and recycle the water used in the process.
In developing the recyclable polymer sorbent, we invented some exciting new chemistry to make the polymer using light, and then “un-make” the sorbent after it bound gold. This recycling method converted the polymer back to its original monomer building block and separated it from the gold.
The recovered monomer could then be re-made into the gold-binding polymer: an important demonstration of how the process is aligned with a circular economy.
A long and complex road ahead
In future work, we plan to collaborate with industry, government and not-for-profit groups to test our method in small-scale mining operations. Our long-term aim is to provide a robust and safe method for extracting gold, eliminating the need for highly toxic chemicals such as cyanide and mercury.
There will be many challenges to overcome including scaling up the production of the polymer sorbent and the chemical recycling processes. For uptake, we also need to ensure that the rate, yield and cost are competitive with more traditional methods of gold mining. Our preliminary results are encouraging. But there is still a long and complex road ahead before our new techniques replace cyanide and mercury.
Our broader motivation is to support the livelihood of the millions of artisanal and small-scale miners that rely on mercury to recover gold.
They typically operate in remote and rural regions with few other economic opportunities. Our goal is to support these miners economically while offering safer alternatives to mercury. Likewise, the rise of “urban mining” and e-waste recycling would benefit from safer and operationally simple methods for precious metal recovery.
Success in recovering gold from e-waste will also reduce the need for primary mining and therefore lessen its environmental impact.
Justin M. Chalker, Professor of Chemistry, Flinders University
This article is republished from The Conversation under a Creative Commons license. Read the original article.
