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Winnowing was the original dry placer method. This process involves screening out all the coarse gravel, placing the fines in a blanket and tossing them in the air, preferably in a good wind. The lighter particles are blown away by the wind and the heavier, usually more valuable minerals fall back onto the blanket. The weave of the blanket tends to hold fine gold. Winnowing is a very primitive method and is not used today. Basically, all it really does is remove the light clay dust particles from the mix
Dry panning is a similar winnowing process, and is only slightly better than blanket winnowing. Shaking the pan while gently winnowing allows the gold to settle in an action similar to a jig. A far better and more successful method is dry washing, also called dry blowing in Australia. The dry washer allows for the processing of gravels at a much higher rate than possible by winnowing or dry panning
In fact, the most widely used piece of dry recovery equipment is the dry washer. For all intents, the dry washer is basically designed to be a short, waterless sluice. It separates gold from sand by pulsations of air coming up through a porous cloth medium. The vibrations and flow of air replace the function of water in the normal sluice, allowing the gold to settle downward. Screened gravel passes down an inclined riffle box with cross riffles. The bottom of the box consists of a thin, light weave canvas or some other fabric. Beneath the riffle box is a bellows, which blows air in short, strong puffs through the canvas. This gives a combined shaking and classifying action to the material. There is also a bellows type which has a battery powered electric motor that operates the bellows for the operator. Other designs employ a motorized blower to provide a continuous flow of air. Dry washing, by its very nature, is an extremely dusty process and the operator is urged to stay up wind as much as possible and even to consider wearing a dust mask when dust cannot be avoided
With both types, the gold gravitates down to the canvas and is held by the riffles, which are set up in the direction opposite of a normal wet sluice. The waste passes over the top of the riffles and out of the end of the sluice section. A basic dry washer is composed of a frame in which a well braced, heavy support screen is covered with fine linen. Above this, riffles made of one-half to three-quarter-inch, L shaped steel or a quarter-round wood molding placed 4 to 6 inches apart. The slope of the box varies from 3 to 6 inches per foot.
A power washer of this type can process up to 21 cubic feet (approximately 0.8 cubic yards) of loose material an hour. Hand-powered washers operated by two men can process 1 or more cubic yards per 8 hours, depending on the size of the material handled. For recovery of gold, the gravels to be processed must be completely dry and well disintegrated. It is critical that the gravels are fully dry because in even slightly damp materials, the clay will clump with the gold preventing good recovery. As a result, any gravel must be thoroughly dry before treatment. During dry times of the year the gravels may be naturally without water, but after stormy periods drying may be required. For small-scale work, sun drying will dry material will usually do the trick. In addition, materials with a heavy clay content, or those bonded together with the caliche will clump up even when dry, so breaking up of the clumps by hand or other methods may be necessary for good recovery. Fully dry, sandy material gives the best recovery results in dry washing
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