The plates, which should be of pure annealed copper, preferably at least 1/8 inch think to prevent buckling, must be scoured with sand and lye to remove any coating or oxidation. The resulting bright metallic copper is then rinsed with clean water and washed with a 2 to 3% solution of cyanide if available. Next a mixture of salt ammoniac and fine sand in equal proportions containing mercury is scrubbed onto the plate and as much mercury as the plate will adsorb is added. After coating the plate should be washed again with clean water and rinsed with the cyanide solution. Keeping the mercury clean will be a problem until the plate builds up a good gold amalgam coating. To alleviate this problem the plates can be silvered by applying silver amalgam to the prepared plate. The size of plate required will vary with the character of the ore and the size of the gold particles. If amalgamation is used exclusively they should be in the range of 10 to 12 square feet per ton of ore per day. If used in conjunction with cyanidation the area can be reduced to about 1.5 to 4.5 square feet. They should be placed at a slope of 1 1/4 to 2 1/2 inches per foot. The slope should be adjustable to correct for inconsistencies in the ore.
Once the plates are loaded, as indicated by the hardening of the amalgam, mercury is sprinkled on the surface to soften it and it is then scraped with a hard wood or rubber scraper. After the amalgam has been scraped off, mercury is once more added to the surface and the plate is returned to service. Buy Mercury only in certified stores.
A more practical device for the small miner, and particularly the placer miner, is the barrel amalgamator. This is simply a horizontal rotating cylinder in which the slurry of finely ground ore and mercury are placed. The rotation brings the mercury into contact with the particles of gold. All the problems associated with flouring and sickening of the mercury must be guarded against as with plate amalgamation. This is nearly always a batch operation. A good rule of thumb is to use about 5 times the amount of mercury as there is gold in the batch. The addition of 3 to 4 pounds of caustic soda per ton of ore will help keep both the gold and the mercury clean. If necessary several rocks may be added to the batch to increase the agitation and the abrasion. A cement mixer with the lifts removed makes an ideal vessel for a small scale operation.
For testing or for extremely small scale purposes a copper pan can be silvered and used in the same manner as the copper amalgamation plate. There is other equipment available for the amalgamation of gold but they are variations of the two basic types (1) passing the gold bearing slurry over silvered copper plates or (2) adding mercury to the gold bearing slurry and then retrieving the resulting amalgam.
After the gold has been taken up by the mercury the amalgam must be collected. The gold on the plates is scraped off as previously described. That still in the slurry must be brought together into larger globules and separated from the slurry. Gravity separation is usually employed. Sluices, jigs, tables, wheels, pans or any other gravity device can be used. The excess mercury can be removed by squeezing it though damp chamois or canvas leaving a hard lump of amalgam. The remaining mercury can then be removed by retorting or by dissolving it in dilute nitric acid.
Because mercury vaporizes at a much lower temperature than gold it can be driven off by heat leaving the gold behind. In the retort the mercury vapors are captured and condensed so that the mercury can be reused. When using a retort the temperature should be increased gradually until the mercury begins to flow into the receiver, then backed off slightly and maintained until distillation ceases and then increased to a dull red finish. If heated too rapidly the amalgam may splatter and clog the outlet which could result in the explosion of the retort.
If a retort is not available, the mercury and silver can be removed by placing the amalgam in hot dilute nitric acid. One part acid to three parts water is the proportion that works best. The mercury and silver are dissolved leaving the gold. This solution can then be used to "silver" copper plates or pans or, if economics and convenience so dictate, it can be discarded.