[thedoc]

An important resource that we find in deep dark places are the metals and minerals we need for industry and everyday life, which means that mining minerals like copper and platinum is a multi-billion pound industry.  Dave and Meera have been out exploring how you separate the metal we do want from the rocks we don’t...
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[Bass]

Besides floating, there are several other common techniques used to separate ore from waste. Among gravity methods are spirals, bowls, jigs and tables- all of which work well on ores with high density differences.  Common chemical means used on oxide ores include solvent extraction and cyanide leaching.  The old Berkeley Pit in Butte, Montana (large copper mine) is now full of water and a toxic brew of minerals.  Quite a bit of research is currently being done on cleaning the water and recovering minerals in solution.  One successful method used is shredded steel cans (#10 cans used in restaurants)- the metal laden water is run through the shredded steel resulting in the iron being replaced by copper and other elements.

[Boogie]

To expand on what Bass said, the most common means of separating metals from waste material is gravity separation. Prospectors who are out to find gold, for example, will first classify the material using screens in order to try to get the particles the same size. Gold is 19 times heavier than water and also much heavier than most of the waste material. When the particles are the same size and subjected to aeration in water, the heavier particles (gold) will sink to the bottom of the container, allowing the lighter material to wash out. The most common example of this is called "panning". Panning is one of the oldest and still the most reliable means of separating metals from waste material. There are several tools prospectors use for gravity separation. The pan, sluice, panning wheel, "blue bowl", "gold cube" and shaker table. Googling these items should help you to visualize and help understand them. With the modern day gold rush that is being driven by the high gold prices currently, there are several new designs in gold recovery products being developed, tested and implemented in the field. In all processes, the final step to clean up is still the good old gold pan. Adaquate classification and aeration is the key to success.

The floatation process is also very effective but requires all the material to be crushed (pulverized) and chemicals are added to cause the precious metals to float where it can be collected and separated from the waste material. This type of process is usally done by large mining companies with a large facility.

In the old days, mercury was used to recover very fine gold. Gold has an affinity towards mercury (it is attacted to it). Prospectors would use mercury to collect gold particles and then retort the mercury (heat it until mercury vaporizes) and the gold would be left in the container. This is a VERY dangerous process today, and using mercury to capture gold is illegal. Google gold amalgamation.

Diamonds are a bit different. They are heavy too, but not as heavy as gold. They are not as easy to separate using the gravity separation process. For diamonds, grease is implemented. Diamonds have an affinity towards grease and will stick to it. I'm not sure if this will also work for corundum minerals.

[bigblock]

Everything Boogie said was correct except one part, using mercury to recover gold is not illegal. With the right permits this can be done legally on a large scale. There are other ways to separate gold from mercury besides vaporizing the mercury. I actually find mercury to be a very productive method for separating gold from ore concentrate. The key to this is safety for the environment and yourself. When done correctly this method proves to be the best way in my opinion.

[Boogie]

Thanks for clearing that up, bigblock. I didn't make it very clear that I was referring to using mercury in the field to capture gold. The old timers would use copper plate and mercury to capture fine gold as mercury will stick to the copper plate, and gold will stick to the mercury. I believe that sort of use is concidered illegal. 

Anyway, yes, I have some friends that clean up cons (concentrated heavy material containing gold or other precious metals.) with mercury. You really need to be cautious and know what you're doing though. I very strongly suggest that newbies avoid it unless you have someone experienced to run some cons with you first. A person can become toothless, mad, and then dead pretty quick by inhalling the vapors.

What is the name of the alternative process that doesn't involve a retort?

[bigblock]

You can dissolve the mercury with nitric. It would leave the gold behind while the mercury is in solution. You can also recover the mercury from the nitric by adding aluminum to the solution although I cant seem to get this to work for me. I have heard of people pressing the mercury through a type of cloth which filters out the gold.

Edit: Boogie was correct about the method the old-timers used to recover gold with mercury is illegal in a open circuit. He was also correct to say that recovery with mercury should not be attempted by someone new.

[evan_au]

Most diamonds fluoresce under X-Rays, so some mines crush the ore and run it under an X-Ray tube. Cameras detect the glow, and the diamonds are separated into a different stream by blasts of air.

This sure beats one old-time method, where a diamond deposit in a sandy desert was mined by paying the locals to lie down on their stomachs in the hot sun, and sift through the sand, a handful at a time.