Gold

I studied process engineering, and one of the most interesting topics in that field is metallurgy. One of my first assignments was on aluminum ore processing, specifically bauxite. It was a pretty comprehensive presentation, covering everything from mining to tailings management and remediation along with potential uses for aluminium. It’s a fascinating field, as every mineral has its own quirks and characteristics, which dictate the applicable upgrading and reduction processes, where oxides or salts are converted to pure metal.

I’ve worked with primary steel producers on projects to reduce the carbon footprint of their DRI furnaces (which are already much cleaner than the old coal-driven blast furnaces), and with startups developing lithium leaching methods to improve wastewater and fume management. Being close to these processes has kept my interest in metallurgy alive. I’ll even confide that I attempted a few times to lateral into the field, but I was competing with more experienced candidate and at that time it didn’t pan out (that’s a gold panning joke right there).

Today, I want to focus on a metal that has been prized for centuries. Not only is it a symbol of wealth and status, but it is also crucial to modern industries such as electronics, aerospace, and medicine. While its market price can fluctuate, it has performed well in recent years, making it a key focus for mining operations. In this post, I want to talk about gold, including where it is found, how it is extracted and processed, and a little about the chemistry behind it.

Where Is Gold Found?

Gold is a naturally occurring element found in the Earth’s crust, often in the form of nuggets or veins of gold ore. It is typically associated with quartz veins, particularly in regions with a history of volcanic activity. The largest sources of gold include:

• Alluvial deposits: These are gold deposits that have been eroded from their original source and carried by rivers or streams. They are often found in riverbeds and are relatively easy to extract, making them a historical source of gold. This is nearly raw gold, which is what you see in gold panning.
• Hard rock mining: This involves extracting gold from rock formations, where it is typically found in ore, often combined with other minerals such as quartz, pyrite, or copper.
• Deep-sea mining: In some cases, gold is also found in the ocean as particulate gold that has been carried from rivers and settled on the seabed. This is not a commercially widespread method. Deep-sea mining remains highly controversial because of its potential environmental impact.

The largest gold-producing countries in the world are China, Australia, and Russia, though there are many other regions with substantial deposits, such as South Africa and Canada. There are gold mines all over the world; I am just highlighting the biggest producers. I also have a friend working in much of West Africa, mostly dealing with gold processing.

How did Gold get in the ground in the first place?

You may imagine a geological mechanism that generated the gold, but gold is primarily formed through stellar nucleosythesis. Basically, violent, cosmic events like exploding supernovas or colliding neutron stars (and some other similarly energetic events). This absolutely blew my mind when I took astrophysics (with an amazing professor). Something like half of atomic nuclei heavier than iron are formed via the “r-process”; uranium is another heavy element formed in similar mechanism. These violent events force atomic nuclei to rapidly capture neutrons and become these heavier elements. Earth-bound gold was once part of star debris and material that coalesced and condensed to form the planet, but much of that dense material would not be accessible at the surface today, so more material was likely added from meteorites bombarding the early earth.

Gold is often found in the crust, but usually in very low concentrations. Once the element was on Earth, there were a few mechanisms of concentration that created the economically viable deposits that are exploited today.

a) Hydrothermal Processes

• Hot fluids circulate through cracks in the Earth’s crust.
• These fluids dissolve gold from surrounding rocks (often sulfide minerals or other metals).
• As the fluids cool or react with different rocks, gold precipitates out, forming veins or lodes.
• Most high-grade gold mines are in quartz veins deposited this way.

b) Magmatic Processes

• Gold can also concentrate directly from magma.
• In some cases, as magma cools, gold and other heavy elements settle or become trapped in certain mineral phases.
• This is less common but is important in some porphyry and sulfide deposits.

c) Placer Formation

• Once gold is exposed at the surface through erosion, it can be transported by water.
• Because gold is very dense, it accumulates in riverbeds, alluvial deposits, or beaches, forming nuggets and flakes.
• This is the source of historical gold rushes and panning operations.

d) Metamorphic Processes

• Gold can also migrate and concentrate during metamorphism, where existing rocks are subjected to heat and pressure.
• This can remobilize gold and form new veins in different locations.

Following this, gold tends to be associated with: Gold tends to be associated with:

• Quartz veins (hydrothermal gold) (one of the most common)
• Greenstone belts (ancient volcanic rocks) (the other most common)
• Porphyry deposits (associated with granitic intrusions)
• Sedimentary placer deposits (river or glacial sediments)

How is Gold Extracted and Processed?

Ore is physically extracted from the ground through mining, but its gold content varies, and the choice of extraction method depends on the ore composition and which approach is most appropriate and efficient. Collected ore undergoes a comminution step, including crushing, grinding, and milling, to reduce particle size and increase surface area before further treatment to concentrate the gold. There are several common methods for processing gold ore:

• Gravity Separation: Used primarily for alluvial deposits, gravity methods rely on the fact that gold is denser than most other materials. Gold particles are separated from other materials by shaking or panning, allowing them to settle at the bottom. This simple method is used for placer gold, where gold is already relatively pure. It takes advantage of gold’s high density to collect the heavier particles.
• Froth Flotation: Gold ore often occurs with sulfide minerals such as pyrite and arsenopyrite. In this process, ground ore is mixed into a slurry of water, and reagents are added that make gold-bearing minerals water-repelling (so-called collectors, like xanthates), along with frothing agents, such as pine oils, which stabilize the air bubbles and the froth. Conditions are maintained to control the pulp chemistry, including acidity. Air is blown through the slurry, and fine bubbles attach to the hydrophobic mineral particles, forming a froth on the surface that is skimmed off for further processing. The waste rock, called gangue, remains in the water and is discharged as tailings. This concentration step moves gold from low-grade ore into a smaller, richer concentrate, making subsequent processing cheaper. The chemistry can be adjusted to achieve good selectivity for polymetallic gold ores. Chemical extraction is still required to further isolate and purify the gold.
• Cyanidation: This is the most common method for extracting gold from hard rock ores. The gold ore is mixed with a cyanide solution, usually sodium cyanide (NaCN). The cyanide reacts with gold to form a gold-cyanide complex, which is then separated from the ore using activated carbon (surface adsorption) or zinc dust (the Merrill-Crowe process, especially useful for ores with high silver content). The reaction is represented by the Elsner equation:

$$ 4AU_{(s)}+8NaCN_{(aq)}+O_{2}+2H_{2}O\rightarrow 4Na[AU(CN){2}]{(aq)}+4NaOH_{(aq)} $$ This process is effective, but cyanide is toxic and requires careful handling and waste disposal. Mining remains an environmentally challenging industry, and in regions with limited regulations, environmental stewardship is often minimal.

• Mercury Amalgamation: This older method uses mercury to form an amalgam with gold. After amalgamation, the mercury is heated to evaporate it, leaving behind the gold. This method is less commonly used today due to mercury’s toxicity and environmental impact, which has led to regional bans, and because cyanidation is more efficient, especially at scale. Cyanidation allows recovery of up to 90% of gold, compared with roughly 30% for mercury. Its efficiency enabled exploitation of lower-grade deposits and contributed to the shift toward open-pit mining, making even deposits with low parts-per-million gold concentrations economically viable.
• Smelting: Smelting is a high-heat process used as an intermediate step between ore extraction and final refining. It can be applied to gold-bearing sludge or carbon after leaching and involves heat and fluxing agents to melt out gold from impurities, producing around 90% pure gold. This gold is not yet market-ready. Smelting is common in recycling gold scrap, coins, and jewelry but is generally not a primary extraction method for raw ore.
• Bioleaching: An emerging and potentially more sustainable method, bioleaching uses bacteria to break down minerals surrounding gold ore and release the gold. This process is slower than conventional methods but can be more eco-friendly. To my knowledge, it has not yet reached commercial-scale application, but it is an interesting development in gold processing.

Refining

Once gold has been extracted, it typically undergoes a refining process to purify it and remove impurities such as copper, silver, and other metals that may have carried over from extraction.

• Miller Process: In this simpler method, gold is heated to high temperatures (around 1,064°C) until it melts, then chlorine gas is introduced and passed over the melt. The chlorine reacts with impurities to form chloride compounds that float to the surface and are removed, leaving behind gold with about 99.5% purity. This process is quick and cost-effective but does not produce ultrapure gold.
• Wohlwill Process: This is a more advanced electrolytic method that uses a gold chloride acid solution to further purify gold to very high purity, typically 99.99%. Electrolysis is used to deposit the gold from the solution onto a cathode, leaving impurities behind. I have seen this process used to recover metals from e-waste as well. The highly pure gold is then collected, melted, and cast into bullion, coins, or precursor materials for electronics.

Conclusion

This overview introduces a range of methods used for gold extraction and refining, many of which are also relevant to other minerals and metals. Gold extraction is a complex process involving multiple steps and chemical reactions. From discovery in alluvial deposits to the highly efficient but environmentally challenging cyanidation process, each stage presents unique challenges and opportunities.

Advances in mining and metallurgy continue to improve gold extraction methods, making them more efficient and sustainable. As demand for gold remains strong, these innovations are likely to continue shaping the future of the industry.

Some sources: https://www.lanl.gov/media/news/0325-star-dissolution https://ui.adsabs.harvard.edu/abs/2021RvMP…93a5002C/abstract

I leaned heavily into Markdown formatting in a learning effort to get better with the syntax. I am not sure if it makes the post more readable, but certainly easier to segment at a glance.