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Cyanide glitters for some

Use of the deadly chemical is on the rise in the gold mining industry

by Marc S. Reisch
September 27, 2017 | A version of this story appeared in Volume 95, Issue 39

A photo of electrolytic refining of gold at a cathode electrode foundry Argor-Heraeus in Switzerland.
Credit: Mediacolor's/Alamy Stock Photo
One metric ton of ore can yield as little as 2 g of gold.

Talk about cyanide, and images of a lethal pill used by spies to avoid capture come to mind. Few people outside the gold mining industry know that sodium cyanide is used in most of the world’s production of the precious metal.

The days of panning for gold nuggets in California and Alaska are long gone. The public’s enduring love affair with gold can be satisfied only by extracting tiny amounts from ores with a chemical like cyanide that binds to gold and separates it in a modern-day version of alchemy.

Though controversial because it is deadly if handled improperly, sodium cyanide is the most efficient—and some say the most environmentally safe—way to satisfy growing global demand for gold. More of it is now on the way, and much of it is destined for Mexico.

In February, a joint venture of Grupo Idesa and Evonik Industries’ CyPlus subsidiary opened Mexico’s first cyanide plant, a 40,000-metric-ton-per-year facility in Coatzacoalcos. Then in June, chemical maker Chemours said it would spend $150 million on a new plant in the state of Durango, Mexico. Both projects follow cyanide specialist Cyanco’s 2013 opening of a 55,000-metric-ton-per-year plant in Alvin, Texas, not far from the Mexican border.

Going for the gold

A flow chart showing the extraction of gold from ore using sodium cyanide.
Credit: C&EN/Shutterstock

Crushed rock is mixed into a water slurry and treated with sodium cyanide, which complexes with gold in the ore. Electroextraction removes gold from the solution.

While China is the largest user and producer of sodium cyanide, Mexico is a hotbed for new production because of a burgeoning precious-metal mining industry. “It’s our insecure times that’s led to a ramp-up in gold demand,” says Stefan Schlag Leon Beraud, senior inorganic chemicals director at the consulting firm IHS Markit.

Central banks have mostly given up on the use of gold to support paper currency, Beraud says. Instead it’s the financial industry and institutional investors who look to gold as part of a diversified portfolio. The glittery element serves as a hedge against inflation, deflation, and losses during time of war and political instability, he points out.

Much of the new gold production satisfies demand from the exchange-traded gold funds that have started up in recent years. Many of these funds are backed by physical stores of gold, Beraud notes. The demand has driven up gold prices over the past decade, making it profitable for operators even though they need to process as much as 1 metric ton of ore to extract a mere 2 g of gold.

It’s our insecure times that’s led to a ramp-up in gold demand.
Stefan Beraud, inorganic chemicals director, IHS Markit

Sodium cyanide is not used solely to extract gold from ore. The chemical has a host of other uses, including as a raw material for industrial chemicals such as cyanuric chloride, cyanogen chloride, and various nitriles.

But by far the biggest use is to extract gold. Global output of the gold-capturing molecule is growing at about 2% annually and will reach 1.3 million metric tons by 2020, Beraud estimates. Before the start-up of local cyanide production, Mexico imported 60,000 metric tons from the U.S. annually, he says.

Mexico is now one of the largest sodium cyanide consumers in the world, notes Frank Harenburg, managing director of CyPlus. All the new mine development in the country, helped along by a government more open to the investment, has made the country a larger consumer of cyanide than the U.S. and Canada combined, he says.

“We have a large sodium cyanide plant in Germany and export from there, but we also look to establish production facilities in countries where demand is large,” Harenburg says. Coatzacoalcos is in a region with a history of chemical production and a workforce familiar with handling chemicals safely, he says.

Talks with family-owned Grupo Idesa led to a partnership, Harenburg says. Their joint enterprise, CyPlus Idesa, generates hydrogen cyanide, a toxic gas, and combines it with sodium hydroxide to make a relatively safe-to-handle and easy-to-ship dry briquette form of cyanide.

Neither Chemours, which claims to be the world’s largest producer of dry sodium cyanide, nor Cyanco were eager to talk about their investments in the gold-complexing agent. That reluctance may be due to the dangers associated with the chemical if it is mishandled or spilled.

When a hazardous chemicals warehouse exploded in Tianjin, China, in 2015, fears were widespread that the sodium cyanide stored there could poison people nearby and contaminate the environment.

In 2000, a dam holding cyanide-contaminated water burst at Romania’s Baia Mare mining works. The tainted water flooded local rivers, including the Danube, and killed large numbers of fish. The Romanian spill led to the International Cyanide Management Code. Most of the world’s mine operators, sodium cyanide producers, and shippers have signed on to it.

As long as sodium cyanide is kept away from water and handled with care, it can be safely shipped to local mines, Harenburg says. “If it is spilled as a solid, it can easily be recovered,” he says.

After use, cyanide can be degraded to minimize the hazard it poses to people and the environment, Harenburg adds. Gold mines use a number of processes to turn cyanide into the less hazardous cyanate. Treatment options include hydrogen peroxide and peroxymonosulfuric acid, also known as Caro’s acid. Another alternative is the Inco process, which uses a combination of sodium metabisulfite and air to degrade the cyanide.

In use for more than 100 years, sodium cyanide is more effective than other processes at extracting gold from more types of ore, Harenburg says. The most infamous alternative is mercury, a neurotoxin that has mostly disappeared as a gold-extraction agent. Other options include ammonium thiosulfate and a combination of bromine and a cyclodextrin. They also involve their own hazards and difficult-to-handle waste streams, he says.

Still, everyone would like a safer alternative to cyanide, Harenburg acknowledges. One possibility is using bacteria to extract gold from ore. He says CyPlus parent Evonik is interested in this and other processes that are not yet ready for prime time. As Harenburg sees it, alternative extraction methods will likely be add-ons rather than replacements for cyanide.

For now, says Beraud of IHS, sodium cyanide is firmly entrenched, and “large gold companies have it under control.”

And control is the key. Gold is valued because it does not tarnish. If handled properly, cyanide should avoid tarnishing the precious commodity’s glittery reputation.


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