Gold amalgam has proven to be an effective method for extracting gold fines, especially when hydro-mechanical methods are insufficient. In placer mining, where deposits are primarily composed of decomposed granite slurry, large amounts of mercury were historically used. This involved using “riffle boxes” in long runs, with mercury poured in at the beginning of the process. The resulting gold amalgam is a solid mass with a dull gray color. Unfortunately, the use of mercury in 19th-century placer mining in California, now banned, has led to significant pollution issues in riverine and estuarine environments, persisting to the present day. Amateur miners using engine-powered water vacuum/dredge setups may discover substantial slugs of amalgam in downstream river and creek bottoms while searching for gold nuggets.
Gold extraction through amalgamation involves heating the obtained amalgam in a distillation retort to recover mercury for reuse, leaving behind the gold. However, this process releases mercury vapors into the atmosphere, posing health risks and contributing to long-term pollution. In developed nations, alternative methods have replaced mercury amalgamation due to the hazards associated with mercurial toxic waste. Despite this shift, small-scale gold placer miners, often operating illegally, continue to use mercury amalgamation, especially in developing countries.
An amalgam probe is employed to detect the presence of mercury salts in water. These salts, more toxic than mercury metal and amalgams, can form an amalgam with copper. A nitric acid solution containing salts is applied to copper foil, and mercury ions present leave silvery-colored spots of amalgam. This method aids in distinguishing mercury from silver. The redox reaction involved in this process is: Hg2+ + Cu → Hg + Cu2+.