New Chemical Complexation Agents for Precious Metals

Large-scale mining of precious metals often involves the use of toxic materials and also consumes large amounts of energy and water. In addition, the tailings produced by a mine site typically contain significant amounts of the sought-after metal, though it is usually not economically feasible to treat the tailings to obtain the remaining metal.
Similarly, waste streams from factories, recycling plants and other industrial operations are often contaminated with precious metal waste. These metals can be damaging to waterways and other portions of the environment if released. Although these precious metals have high value, the high cost and difficulty involved with water treatment severely limits any current practical plan to fully extract the value of these streams. Accordingly, more efficient and greener methods of recovering precious metals from waste streams would be beneficial to mining operations, wastewater treatment plants, and municipal agencies.
Over the years a lot of applied research and development has examined a range of classical separation technologies such as precipitation, liquid extraction, resin adsorption, and membrane filtration, as a way to improve the recovery of precious metals. In many cases, this has led to commercially-viable methods that have been employed at specific mining sites around the world. But the molecular basis of the underlying technologies has not changed very much over the last few decades. For example, most commercially-available ion-exchange resins are derived from 50-year-old materials whose ion affinity is based a few simple molecular properties such as the ionic charge and molecular polarity.
This lack of intellectual progress is curious because over the same time period there has been an explosion in fundamental understanding of the weak interactions between molecules - a field of scientific research that is known as supramolecular chemistry. Indeed, the intellectual legitimacy of supramolecular chemistry has been recognized twice with the Nobel prize in chemistry, once in 1987 and again in 2016. The field has developed a sophisticated understanding of how molecules interact with each other, and this has enabled researchers to design molecular complexation agents that can recognize and capture a target molecule based on subtle features such as the molecular shape.
As a result, modern molecular complexation agents can recognize their targets with extraordinary selectivity and form complexes that greatly change the chemical and physical properties of the target. This new understanding has enabled the invention of new methods of chemical separations that can be used for water purification, pollution control and recovery of high-value chemicals. In the case of precious metal mining there is a high possibility of inventing novel classes of chemical additives that can greatly improve the efficiency and lower the cost of metal recovery methods.
The potential of these innovations is illustrated by two separate discoveries recently reported by two academic chemistry laboratories in the US that may have a major impact in gold mining. Currently, most of the world’s gold mining relies on a 125-year-old method that treats gold-containing ore with large quantities of poisonous sodium cyanide, which is extremely dangerous for mine workers and can cause environmental issues.
Many alternative lixiviants have been explored over the years as potential replacements for cyanide, especially oxidizing solutions that convert the ore into gold chloride or gold bromide. While these alternative methods are environmentally more benign, there are various technical challenges due in large part to the lower stability of gold chloride and gold bromide, which complicates the purification steps. The discovery of new gold precipitation agents has the potential to greatly simplify the processes that separate and purify gold chloride or gold bromide from a leach solution, and keep it in a very stable crystalline form until the reduction step that produces the pure gold metal. This technology is likely to have financial potential in mining situations where cyanide is not feasible.
In 2013, a research group led by J.F. Stoddart at Northwestern University reported a new method of precipitating gold bromide from a leach solution using cyclodextrin, a non-toxic starch-like molecule that can be derived from potatoes or corn.
The technology is currently licensed by Cycladex, a small private company, and has led to several pilot plant demonstrations of gold recovery from the tailings of old gold mines. Cycladex claims its process reduces operating costs by approximately 30-50 percent and capital costs by 35-50 percent. It also has the major advantage of using environmentally-friendly chemicals instead of toxic and expensive sodium cyanide. The Cycladex precipitation method is highly selective for gold bromide and will not work for other gold complexes like cyanide, chloride or thiosulfate. Thus, its value is entirely based on the feasibility of using a lixiviant that produces gold bromide.
In 2018, a separate research group led by myself at the University of Notre Dame reported a very different series of chemical additives that precipitate gold chloride or gold bromide from either an acidic leach solution or from an organic solvent after leach extraction. In all cases, the chemical additives form a gold-containing crystalline complex that can easily be separated by filtration. In addition to selective precipitation, the chemical additives can be incorporated into a range of green separation processes such as liquid extraction or membrane filtration. They may be especially useful in urban mining processes that recycle gold or extract it from the waste streams of manufacturing plants that use gold. Another important aspect of the Notre Dame technology is that it also can be used to capture and separate other precious metals, such as platinum, palladium or nickel, and there is high potential for application in recycling of the precious metals that are within the catalytic converters in motor vehicles.
Looking to the future, it is likely that these recent academic discoveries in selective gold complexation and precipitation will catalyze further research and development efforts to improve the current gold extraction and recovery processes that are used around the world. Furthermore, the general supramolecular chemistry principles that underlie the selective complexation can be applied to other precious metals beyond gold. It is highly likely that the advanced molecular level understanding and new technologies emerging from the field of supramolecular chemistry will have increased impact in the mining and recovery of precious metals.