The last step in the recovery of placer gold is when you have concentrated the sand found in a riverbed by removing all the lighter portions of the sand leaving behind what is called black sand. For the most part this black sand consists of two different oxides of iron that are divided into about equal portions of magnetite and hematite as well as other heavy minerals including gold.
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| A horseshoe magnet that has attracted magnetite for the concentration of gold bearing black sands. |
This is called concentrate that must be further concentrated by the removal of the iron oxides. It is a good idea to treat the concentrate with either vinegar or muriatic acid because the gold in the concentrate is often coated with a thin film of iron or manganese oxide giving it the appearance of ordinary black sand. The concentrate should be allowed to pickle in the acid bath for a least a half hour.
then allowed to become dry by placing it in a shallow pan, and gently heating it to dryness. The step of drying the concentrate makes it easier to remove the iron minerals.
Magnetite is affected by an ordinary magnet, and many prospectors used just an ordinary horseshoe magnet that can be often found in the science section of a toy store, or bought through the Internet where there are several suppliers. All you have to do is pass this magnet just above the surface of the dried out black sand, and the magnetite will stick to the magnet further concentrating the black sand.
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| This is a small rare earth magnet. The match shows its size. |
The other iron bearing mineral found in black sand is hematite. This mineral is not attracted to an ordinary magnet, but is attracted to a rare earth magnet along with several other heavy minerals that contain iron like garnet. This is also a mineral that is commonly found in black sands.
In both cases the magnet is passed over the black sand where the magnetite is first removed with an ordinary magnet; the other iron minerals in the black sand are removed with a rare earth magnet. One of the principal manufacturers of these magnets in the United States
Among their products is a complete line of ore dressing equipment based on the use of rare earth magnets. Many of them work on the principle of a conveyor belt with rare earth magnets embedded in a drum on the end of a conveyor belt. The non-magnetic portion of the feedstock drops off of the in of the conveyor, the magnetic portion is carried by the drum underneath the conveyor. Both products drop ,into separate receptacles.
For the smalltime operator all that is required is a single, small rare earth magnet that can be found for sale on the Internet, but rare earth magnets are used in the alternators found in cars, and a visit to the junkyard may just produce some. The use of rare earth magnet separators is well established in the mining industry.