While we all know that mining is an existential part of the precious metals industry, the sourcing side of things has been somewhat overshadowed in recent years by often glitzier news of spot prices and market valuations. But with mining headlines picking as of late, (read: the industry blockbuster known as the Barrick-Randgold merger), our interests in the sector have undoubtedly been refreshed.
So what better time to explore even more of the gold sourcing sector? To start, we’re tackling one of its most fundamental questions: How is gold extracted?
How is Gold Extracted?
While this may seem like a straightforward topic, the truth is, it’s not. Extracting gold is a multi-step process that can be approached in different ways depending on how a company is prospecting and mining its gold supply.
For example, placer mining, which was popular during the 19th-century gold rushes, relies on a gravity-separation extraction process. In these cases, water is used to separate gold particles from loosely constructed placer ores by tactics like panning or sluice boxing. Picture the classic Gold Rush images of eager frontiersmen and women sifting through river beds with handheld pans.
Hard-rock mining, which makes up the majority of today’s commercial gold sourcing, requires a more in-depth, chemical-reliant extraction process. The most used approach, in this case, is gold cyanidation, which, in short, uses a cyanide-based solution to separate precious metals from encapsulating rock.
Because of its widespread use, we’re focusing today on the ins-and-outs of the cyanidation method specifically.
The approach was discovered in the late 18th century by Carl Wilhelm Scheele. However, more than 100 years would pass before it would be used on a large-scale basis.
In 1887, John Stewart MacArthur and brothers Dr. Robert and Dr. William Forrest touted 96-percent-pure gold yields from the cyanidation process, which cemented its destiny for commercial-grade deployment. In 1890, the process officially kicked into high gear in the Rand of South Africa, leading to an investment spike in that region’s mining operations.
Shortly after that, American metallurgist Charles Washington Merrill and engineer Thomas Bennett Crowe enhanced things further by adding a vacuum mechanism and zinc dust to the process. The new-and-improved method would come to be known as the Merrill-Crowe approach, which is still used today.
The process of gold cyanidation is in-depth and multi-faceted.
After gold-containing ores are mined via the hard-rock method, they are crushed into smaller, gravel-sized bits, which are then cycled through rotating drums where steel balls grind them down even further. Eventually, a fine powder-like material results, to which water is added, creating a “slurry” or “pulp” substance.
Next, the eponymous cyanide comes into play in a process known as “leaching.” A sodium, potassium, or the more cost-effective calcium-cyanide mixture is added to the water-powder pulp to dissolve it down even further.
After the pulp is leached, the gold particles themselves have to be “recovered” for further processing. There are a few different recovery methods to choose from:
The carbon-in-pulp option involves introducing activated carbon into the leached pulp, which attracts and absorbs gold particles, much like a sponge to water.
Because the gold-bearing carbon is much larger than the rest of the pulp matter, it can be easily parsed out by screening the substance through a wire mesh. Once carbon particles are separated, the pulp is “eluted,” or bathed in a solvent to wash away the non-gold-bearing, and therefore now unnecessary, matter.
The eluted carbon is then processed through “electrowinning,” where water and cyanide circulate through the bathed carbon, now separating gold from the carbon itself.
The gold is then deposited on a cathode – a wire wool apparatus – by electrolysis. The water-cyanide solution passes back through the gold-loaded carbon, which extracts more of the precious metal. The process continues until all gold has been stripped from carbon.
The now gold-plated cathodes are placed in acid, which dissolves the cathodes themselves.
The acid is no longer needed and subsequently drained off, leaving just the desired gold sediment. The sediment is washed several times with water, before being sent into the smelting process.
In addition to carbon-in-pulp, there is also the aforementioned Merrill-Crowe method of gold recovery. This process uses a vacuum to remove air from the leached solution and mixes zinc dust in with the gold, which allows it to more easily separate from the rest of the solution. The separated gold-zinc-dust combo is then mixed with sulfuric acid to dissolve the zinc, and the solution is filtered again, leaving behind just the desired gold pieces.
Once carbon-in-pulp, electrowinning and/or the Merrill-Crowe method is complete, the extracted gold is smelted into doré bars, which are sent off to refineries for final processing before consumer consumption.
Gold has enjoyed widespread use across many industries. We’re of course all familiar with the investment use of the precious metal, but perhaps less known is its application in electronics.
As we wrote about in this blog post, gold and silver are used in the building of many an electronic device, including computers, fax machines, cell phones, and printers, just to name a few.
Given the benefits of precious metals sourcing, a burgeoning industry is venturing into extracting gold, silver and other metal components from spent devices. Specifically, the tech giant, Apple, reportedly reclaimed more than one ton of gold from its discarded products, yielding potentially $40 million in recoveries.
These non-traditional extraction methods come with warnings, however. As we pointed out in our previous posts, un-protected extraction is “incredibly dangerous to human health,” and can also be environmentally hazardous if extreme precautions are not taken to contain toxins.
So there you have it – the process of extracting gold from its raw earth environments. We told you it wasn't straightforward, but hopefully, you enjoyed learning about the ins-and-outs of this fascinating process. Stay tuned for more deep dives into the world of precious metals sourcing!