#of chemistry. Once you kind of know the basic properties used to separate mineral species it's easy to extrapolate and

[iamthepulta]

Yeah!!

The history of gold mining is a really interesting topic: all the properties that make gold valuable (non-corrosive, conductive, malleable) also make it a horrible pain to mine. There are roughly five types of gold deposits: Placer, High-sulfidation Epithermal, Carlin, Metamorphic, and Au-Se/Te/Hg. You can also get copper-gold mines, but they're this cranky-child subset of copper deposits that make a lot of money but it's just an extension of copper processing.

Placers are areas where water and erosion have concentrated native gold due to its high density. This is "Gold Rush" gold. People have been mining placer deposits since the Bronze Age since all you need is a pan and a shovel.

[lengthy historical and modern chemical analysis under the cut]

High-Sulfidation/Metamorphic deposits are "veins" of gold. They're typically what weather to form placers. Gold is deposited in native form, usually with calcite (Ca) and quartz (Si); very straightforward.

So when you run out of placer gold, it's obvious to go right to the sources nearby. However, you have to break up the rocks to get the gold for H-S/Meta deposits, which takes a lot of extra time and energy. To shortcut that, the mining process attempts to use a liquid solution as much as possible.

One of the key technological innovations to come out of the Roman Empire, and the funding for their, uh- prolific occupational expansion comes from the discovery of mercury amalgamation: Which is the extremely healthy, not-at-all hazardous process of soaking gold ore in mercury. Gold dissolves in the mercury, and then the solution is placed into the furnace. The mercury forms a gas, leaving a gold nugget in the furnace crucible. (Typically the mercury was sublimated and reused but I'm not sure if that technique was used during the Roman Empire.)

Anyway. Until 1300 AD, this process was one of the most efficient methods, until Aqua Regia was discovered. Royal Water is the fancy term for a hydrochloric and nitric acid solution, which is honestly not much better than mercury. Nitric acid forces the gold into solution, while chlorine ions actually carry the gold where it can precipitate as native gold. However, it's extremely volatile and offgases, so it can only be used very carefully in closed systems.

We're going to put a pin in that and jump back to gold deposits:

[I'm also going to fail to describe this well, so bear with me.] While the first two I mentioned have native gold, without any other elements, Carlin deposits, have microscopic gold that's inside other minerals: usually iron-arsenic-sulfide. These deposits have two problems: you still have to crush the rock to reach the iron-arsenic-sulfides, and then you have to extract microscopic gold without combining the elements around it.

This is where Cyanide comes in. Cyanide (for mining) is formed by combining methane and ammonia and adding salt or potassium. (Na/K(CN)). Cyanide's carbon ion has a slightly negative charge, which allows it to preferentially form compounds with inert metals like Pt, Au, Ag, and Cu under oxidized conditions.

Despite being a dangerous, highly-toxic chemical, Cyanide is still carbon and nitrogen. If you put it in sunlight or add oxygen, it breaks into CO2 and NO3.

There are a few cyanide mining methods. One is heap leaching. An impermeable pad is laid with a drainage system and ore with gold (sometimes crushed sometimes not) is put on top of it. The size of the heap allows for some control over the oxidation conditions of the cyanide, and then the solution is refined and processed further. The cyanide is confined to the heap and waste is rendered inert.

The other is autoclaving (high temp-high pressure furnace) and tank leaching, used for particularly tricky gold ores. The last deposit that I mentioned but haven't talked about - even though I've been typing this for two hours now, lol - was Au-Se/Te/HgS ore. Typically, most metals occur as sulfides. Sulfur is common in nature and it's easy for metal ions to form compounds with it. But sometimes you have an excess of Selenium or Tellurium, and gold precipitates as Au(I). (There's probably a geological/ore deposit reason for this, but as far as I know we're not quite sure. Some deposits just have more Te.)

These compounds require a stronger ligand than cyanide to break though, which makes them a "refractory" ore. [Gold mining gets a bunch of fancy terminology because they think they're special.]

So you can do a couple of things to the refractory ore at this point: The most common method is to separate all Au-Te and minerals by putting them in a (closed) vat of hydrochloric acid. Sulfides sink to the bottom and Te minerals float to the top. This gold concentrate is put into chloric-hydroxide again, oxidized, and then cyanide is added since it's so selective for gold. The cyanide solution is run through a carbon strainer, which precipitates the gold, while the Telluride solution is dried and smelted to produce raw Tellurium.

Alternatively, you can put sulfide and tellurium gold concentrate directly in the furnace and offgas TeO2. The Te and Fe-As-S minerals have typically oxidized and/or become SO4 minerals and can be easily leached with cyanide. This works pretty well, but since Te was with the Au, gold often starts plating the sides of the furnace/autoclave and it needs to be turned off every 6 months to scrape gold off the sides.

Suffice to say, the Romans didn't need tellurium, but I'm sure they would've appreciated the closed pressure vats. 💛

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This has been Lapulta's Mineral Processing TedTalk! Thank you so much for helping me study for my exam tomorrow! xD

As I'm studying, I keep finding fascinating tidbits of mining history/mineral processing that I want to write about. Like Tellurium is highly toxic, and even in low doses, (10 ppm), will give people "garlic breath". That's so cool!!! Gold-Tellurium processing is really cool!!!