Inside the many ways gold can be processed


The gold you wear around your neck or around your fingers has a long and fascinating origin story, thanks to the innovative gold-processing techniques used by gold manufacturers around the world.

But people who don’t work in precious metals may be unaware of exactly what goes into turning gold nuggets into the polished product that has turned heads for millennia.

What should you know about the many approaches to making gold? What kind of environmental concerns surround gold processing and which new innovations are trying to evolve the gold processing methods to become more sustainable and move the industry forward?

Gold.TO presents you with an inside look behind the curtain:

From the soil

Gold is often taken from the earth via two ways of mining: placer mining lets you extract gold from sediment such as sand and gravel, and it’s often depicted in Old West movies via someone panning for gold by a riverbed, swirling water around the rim of that pan. As the lighter materials are washed out of the pan, heavier materials such as gold remain on the bottom, allowing the miner to reap the riches from his manual labor.

Lode mining is a different approach than panning: Instead of going through sand and earth in the hope of striking gold, load miners extract gold from rock beneath the crust. This is another process usually portrayed in more large-scale mining operations in media. You’ve likely seen a long tunnel being dug into the side of a mountain or down into the ground, where miners are slowly breaking up earth in search of gold fragments.

Workers dig shafts tunnels branching out, leading to the load deposits. They also have to drill holes for explosives, set off those explosives and then haul all of the resulting blast debris.

What has been common among the mining community is the use of cyanide in the gold-development process. Through cyanidation, metallic gold is oxidized and dissolved in an alkaline cyanide solution, and the oxidant used is atmospheric oxygen, which, in the presence of a solution of sodium cyanide, causes the dissolution of gold and the formation of sodium cyanoaurite and sodium hydroxide, as Encyclopedia Brittanica explains.

And there isn’t just one way to use cyanidation. As the World Council reveals, looking at ores of higher gold content (greater than 20 grams of gold per tonne of ore), cyanidation happens via vat leaching, which involves holding a slurry of ore and solvent in large tanks equipped with agitators.

For firms extracting gold from low-grade ores, heap leaching is commonly used. That involves spraying huge heaps with a dilute solution of sodium cyanide, and this trickles down through the piled ore, dissolving the gold.

When gold dissolution is complete, the gold-bearing solution is separated from the solids.

A drawback to heap leaching is that not only can it be a lengthy process, but only 65% to 85% of the gold in the ore is extracted using this process.

We’ll look later at alternatives to cyanide, which has been down to have serious environmental ramifications.

Refine the way

Refining refers to a processes used to extract and separate the precious metals in mined material.

The smelter is commonly used for refining, whereby the gold and a chemical cocktail called flux – magnesium dioxide, fluoride, silica flour, borax and sodium nitrate – are blended and heated to 1,600 degrees Celsius. This process divides the gold from impurities so that it can be poured into bar-shaped molds. The resulting bars are 80% pure. By refining further, the process can purify the gold to 99.5% fine or better.

The two gold refining methods most commonly employed to derive pure gold are the Miller process and the Wohlwill process.

The Miller process uses gaseous chlorine to extract impurities when gold is at melting point; impurities separate into a layer on the surface of the molten purified gold. The Miller process is quick and simple, but it produces gold of only about 99.5% purity.

The Wohlwill process boosts purity to about 99.99 percent by electrolysis. In this process, a casting of impure gold is placed into an electrolyte solution of hydrochloric acid and gold chloride. Under the influence of an electric current, the gold moves to a negatively charged electrode (cathode), where it is restored to a highly pure metallic state, leaving the impurities as a separate solution.

The Royal Canadian Mint explains the process on their site, with a few more added details:

“Once the rough gold has been melted in a crucible at a high temperature, chlorine gas is injected into the molten mass. Chlorine attacks base metals and silver in preference to gold. A mix of silver chloride and base metals rises to the top of the molten mass in the crucible to form a layer of by-product. These by-products are skimmed from the crucible, leaving only .995 fine gold. All refining by-products contain some gold, which the Mint recovers through both a proprietary internal process and using third party processors.”

What’s the next frontier in gold processing ?

Innovations ranging from nanotechnology to AI tech to cyanide alternatives are shaking up the gold exploration, extraction and processing approaches that have long been established in the gold industry.

The environmental impacts of gold mining have been studied for decades. To look at one example, this study analyzed how several Montana mines in the 1980s degraded the water and soil surrounding it. In 1982, 2,953 liters of cyanide-tainted solution leaked from a containment pond near the The Zortman-Landusky gold mine and “a section of piping used in the mine’s cyanide sprinkling system ruptured and released 196, 841 liters of cyanide solution onto lands and creeks. The tap water revealed cyanide concentration levels above drinking water standards and the community’s local water system was shutdown.”

Due to the polluting effects of cyanide, some mining companies are looking at alternative solutions. Australia-based Clean Mining has opted for a safer, less hazardous chemical agent, thiosulphate. As reports suggest, “this inorganic compound helps dissolve fine gold out of ores into a solution, which can then be recovered through further processing.”

In a statement, Clean Mining’s Jeff McCulloch said, “Eliminating cyanide and the associated tailing dams from the gold recovery process is a game-changer for the sector and, importantly, for the communities where gold miners operate.”

In 2009, Barrick Gold began testing the thiosulfate process in a laboratory and the results were promising enough to merit construction of a demonstration plant the following year. In 2016, Barrick was rewarded for its use of cyanide alternative agents: The Metallurgy and Materials Society of the Canadian Institute for Mining feted Barrick with the MetSoc Innovation Award for successfully developing their thiosulfate leaching circuit at its Goldstrike mine in Nevada.

A specific circuit uses thiosulfate to treat double-refractory ore that can’t be treated using conventional processes.

Nanotechnology is also being applied to the gold-processing chain, as evidenced by new innovation introduced by 6th Wave Innovations in 2016. Because activated carbon is associated with cyanide in the gold-making process, 10% of the carbon is lost during the process, adding to the producer’s input costs. Also, activated carbon is not highly selective, so other minerals in the pregnant leach solution become trapped within the activated carbon – an issue known as organic fouling.

This company developed a solution they call a nanotechnology bead, which is defined as a “molecularly imprinted polymer ion exchange resin.” It works by being imprinted at the molecular level to attract gold and ignore the other elements leached off in mining processes. As a report states: “Unlike conventional ion exchange resins, the IXOS® resin has a long life (>50 loading/unloading cycles), high capacity (~30g/kg) and selectivity for gold (>95%) and greater than 98% gold recovery (no solution losses).”

Machine-learning has become known as one of the key arbiters of rapid change in the technological landscape, in fields ranging from law to urban planning to medicine to investment banking. Gold mining is also influenced by the rise of artificial intelligence, with companies such as Goldspot leading the charge.

This firm has consolidated more than 30 years of historical mining and exploration data into a 3D geological model. Then Goldspot could find several target zones with the highest potential for gold mineralization in Nevada’s Jerritt Canyon district, among several others.

As Forbes writes, “Goldspot seeks to help mining companies cut some of the costs and risks associated with discovering high-quality deposits—something it’s managed to do for a number of its clients and partners, including Hochschild Mining, McEwen Mining and Yamana Gold.

Machine learning has also contributed to the beginning stages of rolling out autonomous vehicles, which so far has only impacted the military industry. But as advances in this area mature, so does the opportunity for self-driving vehicles to be employed in gold mining.

CIM Magazine writes: “The potential benefit of removing the driver from mining equipment is massive from safety, to efficiency and cost reduction. This is particularly true for fly-in fly-out operations where the cost of keeping a human driver on site, fed, warm and happy are increasingly prohibitive.”

Even IBM’s Watson is playing a role in gold mining. A Goldcorp executive wrote about how AI can change the gold-processing game: “Watson can ingest and analyze the data in ways and at speeds we could never accomplish. Our geologists can find patterns and insights from our data that previously remained hidden away like the deepest of ore deposits.”

One of the hottest trends in tech may also infiltrate gold mining. A Deloitte report on the 2019 mining industry discusses the potential for blockchain to impact mining: “Although blockchain can be used for a variety of purposes in the mining industry, provenance is one of its most promising applications. By facilitating asset traceability from origin to destination, blockchain can validate the source of specific commodities across the supply chain to reduce the prevalence of conflict sources, enhance transparency, and improve reporting and regulatory compliance. It can also help mining companies instantly calculate and provide details such as product origin, custodial chain, and the end-to-end carbon footprint involve din producing any particular ton of ore.”

As a fun parting note, in case you were curious about how gold was processed centuries ago, a May 2019 paper found that an unusual but effective method of processing gold came to light: heating a mixture of gold, sand and glass to high temperatures and separating out the gold.

That is a long way from today’s machinery-heavy operations, where AI modeling or blockchain could help geologists and mining firms find large gold deposits and track their products through the supply chain.

Gold is a fascinating object, one that many Gold.TO customers turn to when they want to earn extra money selling their gold jewellery or memorabilia. Learning how gold progresses from deposit to finished product can shed new light on a precious metal that continues to excite people around the world.



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