Cyanide in Gold Mining
We all like gold, right? It’s bright, shiny, and expensive. However, the chemical processes of extracting gold for commercial use has its downsides.
The biggest gold mining companies in the world produce millions of ounces of the precious metal every year. Yet gold cyanidation, the process used to leach gold out of ores, is a great concern to the environment and human health. The more mining takes place, the more cyanide is used. As humans continue to pollute the air, water, and earth, it’s important to realize the effects of some of our operations. Cyanidation is an old tradition that has proved effective in the past, but the world is changing faster than ever, and perhaps it’s time to change this one method as well.
How is cyanide used in gold mining?
Cyanide (CN-), is a nitrile, which is any organic compound consisting of a carbon atom (C) triple bonded to a nitrogen atom (N). The specific nitrile compound commonly used in gold mining is the white, water-soluble solid, sodium cyanide (NaCN). Unfortunately, NaCN is highly toxic to humans and the environment.
Through the process of gold cyanidation, gold (Au) is extracted from ore by treating it with a dilute alkaline NaCN solution in the presence of oxygen. This solubilizes the gold in the chemical reaction 4Au + 8NaCN + O2 + 2H2O → 4NaAu(CN)2 + 4NaOH. The dissolved gold solution is then precipitated in leaching vats by adding zinc to the mixture, which causes this reaction; 2Au(CN) + Zn = 2Au + Zn(CN)4–2. At this point, companies are able to distribute the isolated form of gold for commercial profits. However, it’s not always this straightforward. The use of cyanide is very often complicated because of the chemical composition differences in ores.
Unfavourable “extra” elements in the mineral compounds will alter the chemical reaction for gold cyanidation and pose difficulties for the mining process itself, as well as safety. For example, the rather common presence of copper (Cu) will dissolve in cyanide solutions. This leads to an increased quantity of CN- in the solution and even restricts the dissolution of gold. Furthermore, a trend among cheaper and ill-maintained mining and processing facilities has shown them saving money by reducing alkalis in the leaching process. Without the use of alkalis such as calcium oxide (CaO), the cyanide mixed in the solution will decompose to form hydrogen cyanide gas (HCN). Alarmingly, HCN is widely recognized for being extremely poisonous to all organisms. Obviously, this is a concern for the staff at mining sites.
The implications of the dangers of sodium cyanide in gold mining spread beyond just workplace safety, however, with cyanide spills proving catastrophic for business as well. For example, Barrick Gold Co. — one of the largest gold mining companies in the world — was required to invest a surplus of nearly $500 million in 2018 to compensate for 3 cyanide spills in 3 consecutive years at the Veladora mining site in Argentina.
Environmental and Health Concerns
Cyanide spills can occur in any operating sites, directing large amounts of the compound into pre-balanced ecosystems. For instance, runaway cyanide will react with water in rivers or lakes to produce sodium hydroxide (NaOH) and hydrogen cyanide (HCN), demonstrated through the equation NaCN + H2O → NaOH + HCN. Sodium hydroxide is corrosive to all organ tissues, which makes it deadly for aquatic organisms. Similarly, hydrogen cyanide is extremely toxic to humans and animals.
In fact, after a spill in Romania just 21 years ago, 3.5 million cubic feet of cyanide-contaminated waste was led into the Tisza and Danube rivers. This killed fish and poisoned water supplies for about 250 miles downriver in Hungary and Yugoslavia.
This is not only an aquatic issue though, as CN- in the air quickly breaks down into numerous compounds, one of which is nitrate (NO-3). Nitrate reacts in soil and water and is converted into nitrous oxide (N2O) through denitrification. Nitrous oxide is a potent greenhouse gas contributing to global warming, which is one of the most relevant issues for corporations, especially.
Unfortunately, the complications do not end at the environment, as cyanide has terrible biological effects on organisms as well. CN- binds to the iron (Fe) atom in cytochrome C oxidase in the mitochondria of cells, inhibiting its ability to transport electrons to oxygen in aerobic cellular respiration, seen in the equation C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP. Without oxygen, mitochondria cannot produce the energy carrier adenosine triphosphate (ATP) causing tissues and nerve cells to die. This chain of reactions is commonly known as cyanide poisoning, and is rapidly deadly for humans.
So, what now?
Its not enough to highlight an issue and not provide at least a suggestion. So here’s what I’ve found based on my research.
The best direct replacement for cyanide in gold leaching is the chemical compound thiosulphate (S2O2–3). Thiosulphate leaching has been steadily increasing in popularity, and luckily, it is a very similar process to cyanidation as it involves the same sort of technology.
The chemistry of thiosulphate leaching possesses a higher degree of complexity than cyanide, however the benefits are plentiful. Simply, the use of thiosulphate is safer than cyanide as the former is non-toxic at concentrations tested for leaching and it does not pose immediate, serious threats to the environment.
From a business standpoint, thiosulphate is clearly the better option as well. It is cheaper in unit cost, it leaches gold quicker under regular conditions, and it can recover more gold than cyanide from ores with higher copper concentrations.
Change isn’t easy. But sometimes it’s necessary. And though I’m no expert, if a little bit of research can raise an eyebrow, then maybe there’s a fix just waiting to happen.
