Most of the methods used by the recreational prospector involves granity concentration (i.e. panning, dredging, sluicing, dry washing). These methods concentrate dense material like gold from the surrounding less dense material like sandstone, dolomite and other “rock” materials. Unfortunately, there are other minerals that have densities close enough to gold that these minerals collect in the concentrate formed by gravity concentration. Some common fairly dense minerals include galena (lead sulfide), hematite and magnetite (iron oxides) and carundum (aluminum oxide). These minerals will collect with gold in the bottom of the sluice or pan and form what is commonly known as “black sands”.
The following method for recovering gold from black sands comes from a combination of information found in Gold Mining in the 1990′s by Dave McCracken and How and Where to Find Gold by Verne H. Ballantyne. Both of these books can be order from Barnes and Noble by clicking on the book title.
To recover gold from black sands it is recommended that the recreational prospector only use mechanical separation. Mechanical separation is simply removing the gold from the surrounding materials by using a mechanical approach. This starts out simply by using tweezers to remove the small nuggets and flakes that are easily discernable. Following the removal of the coarse gold, the concentrate should be dried. Drying of black sands should always be done in a well ventilated area with you upwind. During the drying process, it is possible that hazardous vapors could form from natural minerals. It is always better to be safe than sorry.
Following drying, pass the sand through a screen such as window screen. Take the oversize material (particels still on the screen) and pour it on a piece of clean paper. Spread the material out and separate the gold. Sweep the unwanted material off the paper. Then transfer the gold to a storage bottle.
The undersized material from the first screen is then passed through a finer screen such as a tea strainer. The oversized particles are then placed on another clean piece of paper. Again, separate the gold from the surrounding material. This can be accomplished in several ways, including lightly blowing across the sands while holding down the piece of paper. You can also use a small painter’s brush or simply your fingers. Once there is only gold on the paper, transfer to a storage bottle.
The finally undersized material consists of small particles. Spread these particles over another clean piece of paper. Take a magnet wrapped in plastic like cling-wrap and touch the particles. Magnetic particles like magnetite will attached themselves to the magnet. Take the magnet to another clean paper and remove the platic wrap and allow the particles to fall on to the paper. Examine the particles visually and maybe pickup with the magnet wrapped again to insure no gold particles got carried away with the magneted particles.
The remaining particles on the original paper will be fine gold and non-magnetic black sands. Again, the gold can be separated from the black sands by lightly blowing across the paper. Spearation can be encouraged by lightly tapping or vibrating the paper and insuring that only a little concentrate is present on the paper. The remaining fine gold can be placed in a storage bottle.
Other methods exist that will actually recover more gold, but they involve poisonous and/or deadly chemicals. One such method,which has been widely used in the past, involves the cleaning the concentrate with nitric acid then recovering gold by amalgamation with mercury. Nitric acid are dangerous and mercury is poisonous. Another method uses cyanide to dissolve gold into a solution with a pH greater than 10. The gold can then be cemented out of solution by adding zinc powder (The Merrle-Crowe Process). The resulting powder after throughly rinsing can be melted. The zinc will float on top of the gold and can be skimmed off. If the pH of the dissolution solution is not controlled, hydrogen cyanide can form if the pH becomes to low. Hydrogen cyanide is what is used in gas chambers. Finally, I have seen offers by companies claiming to provide environmentally safe recover methods. There are technically viable chemical processing routes using halides, thiourea and/or thiosulfate that would fit these descriptions. Unfortunately, the cost to run these systems may not be worth the amount of gold recovered. The reader is left to make his/her own decision concerning cost versus benefit of such systems.
Here is another method that was submitted to me by CoalBunny. Thanks for the submission! “There is another technique for dealing with black sands. White vinegar and table salt. Acetic acid+sodium chloride=sodium acetate (liquid) and chlorine (gas). The chlorine that is released in a quantity that it’s essentially harmless if at all noticable. The sodium acetate removes the gangue films from the gold. You will find in many locations that the presence of Hg is blamed on the mining community. In the majority of locations this is not as true as the environmental community wishes as it is a natural occurance. I think you know what goes on from there- Gold attracts the mercury, mercury then attracts a myraid of metals that also occur in the area, and they oxidize. End result is gold that doesn’t look like gold. Treat it with the vinegar and salt and it’ll remove the tarnish and the gold with most likely by silver from the Hg. Heat it or treat it with nitric and you have the gold. I prefer the nitric as you can recover the Hg from it.”