Cyanide and Mercury

CYANIDE

Most of you know that cyanide is poison. So why, you ask, do they use it when they mine gold?

The answer is simple. Sodium cyanide (the processing chemical which mining companies use) is a colorless solid which smells a bit like bitter almonds. Cyanide likes to combine with gold- up to 97% of the gold in crushed ore- which makes it one of the most efficient ways to extract the precious metal. This process is called cyanide leaching and has been popular since the U.S. Bureau of Mines recommended it over the use of mercury in the 1960's.

Heap leaching is common in Nevada. Very low grade gold and silver ores are blasted, usually but not necessarily crushed, then stacked in layers on a heap. Cyanide solution is then applied to the pile using drip feeders like those used in farming. Solution with dissolved metal drains from the bottom. If it has mostly gold it will be mixed with activated charcoal (carbon)  but if there is a lot of silver it is mixed with zinc dust and the gold and silver precipitate. Zinc dates to the discovery of using cyanide for gold but activated charcoal was pioneered in the 1970's.
 
When the ore is richer it is ground to a powder and mixed with cyanide solution. That mixture can then be mixed with carbon to transfer the gold. If there is a lot of silver the solids and solution must be seperated before zinc is added.
 
In some deposits in Nevada the gold is dissolved in other minerals that must be digested before the gold can be recovered with cyanide.

By the way, a teaspoon of a 2% cyanide solution can kill you in an hour.

Cyanide breaks down in sunlight- unfortunately it's not always exposed to sunlight, so I wouldn't be using any old cyanide cans to boil my stew, if you know what I mean and I think that you do.

MERCURY

Cyanide, as much as the tree huggers hate it, is much much better than its predecessor, mercury. Mercury is just plain evil.

Way back when, like around the mid-nineteenth century, gold and silver were extracted from the ores using the mercury amalgamation process. Amalgamation is the alloying and collection of fine gold-silver particles in mercury. The mercury is then collected and heated (evaporated) away from the precious metals and recondensed.

One was always losing mercury- it's estimated there are 15 million pounds in the Carson River alone, from the 200 mills working the Comstock. Interestingly, since mercury is not as efficient as cyanide, they estimate much gold and silver was lost with the mercury- as high as 3 million ounces of gold and 64 million ounces of silver.

While mercury is a poison too, it doesn't go away as easily as cyanide. High levels still exist over a hundred years after the fact, and most of the bodies of water in this area- Washoe Lake, Carson River, and Lake Lahontan- are polluted with it.

Someday, some young genius may discover a way to recover precious metals using mother's milk- until then, we're probably stuck with what we got.

SALT

A lot of you might be asking yourselves, "Why did they mine so much salt, when pretzel consumption was relatively low?" Well, the answer to that is, salt was also used during the refining process.

All around Nevada, there are old abandoned salt-mining operations, although salt is still actively mined east of Fallon for other purposes. As early as the 1500's in Mexico, salt was used to extract silver from the ore, in what is known as "the Patio Process." Salt, water, mercury, and roasted copper and iron sulfides were added to the crunched up ore. After about four or five weeks, the salt reacted with the silver compounds to form silver chloride, which then reacted with the mercury to form silver metal. Eventually the process was improved to shorten the time to hours. Another process called "The Washoe Process" also used some salt in the latter stages.

The requirement of salt is disputed, as some think the heat, crushing, and mercury were the prime components of the process. Indeed, there seems to be quite a bit of controversy as to how much salt was required, and it varied substantially from mill to mill. Some variations of the process even called for tobacco juice and sagebrush.

After the cyanide process gained favor in the 1890's, the pan process was forgotten.

SOURCE:

KC Armstrong
Personal correspondence (Thanks for the cyanide clarification!)

Cyanide: Gold’s Killing Companion -
Project Underground
http://www.moles.org/ProjectUnderground/reports/goldpack/goldpack_i.html

Mercury Contamination of the Carson River -
Paul J. Lechler, Chief Chemist/Geochemist - Nevada Bureau of Mines and Geology
http://www.nbmg.unr.edu/dox/nl/nl17a.htm

The Science of the Comstock
Nevada Bureau of Mines and Geology / UNR
Collette Craig, Beth Price, John Fuetsch, Lindsay Craig, Jon Price - UNR
http://www.nbmg.unr.edu/comstockscience/home.shtml

Salt and Silver
http://www.nbmg.unr.edu/dox/nl/nl7a.htm
---J. V. Tingley, Economic Geologist
Nevada Geology
Quarterly Newsletter of the Nevada Bureau of Mines and Geology