In the 1800’s the use of mercury to extract gold from ore (called Amalgamation) was the most commonly used method by small scale prospectors for this purpose. It has been a highly controversial method of extraction to say the least because of both the physical and environmental health risks that mercury poses.
Individual prospectors will have trouble finding mercury to use in the Amalgamation process today, because it is no longer sold in supply shops that cater to small scale prospectors. Additionally, newer methods to extract fine gold from concentrates have eliminated the need for the average gold prospector to use mercury. Still, it is very commonly used in many developing countries around the world such as Africa and Latin America, where access to more advanced mining equipment is limiting.
This article will look at some facts about this historically significant gold extraction method and briefly describe how it works.
The History of Mercury and Gold Extraction
The first use of mercury in a large scale mining process was 1828 but evidence suggests that the use of mercury to extract gold from ore may have been first used over a thousand years ago. It is popular among small scale prospectors because it is cheap and simple to use. This was not necessarily the case for larger scale mining operations.
Few large-scale mining operations use mercury today, or if they do it is in a much more controlled environment. Today’s use of mercury is most common among artisanal miners in less developed countries.
Mercury is hazardous to the human body because we have no way of getting rid of it efficiently so it collects in various organs. It has long been known to cause such things as nervous conditions in the body; the famous phrase “mad hatter” came from the use of mercury extensively in the hat making business in the 18th century.
Small scale prospectors that still use it must be careful they don’t breathe the vapors when handling the metal because it evaporates quite readily at room temperature and is easily absorbed by the lungs.
In the old days mercury was used in an open system by means of using such devices as sluices and drywashers. This method is highly frowned upon and dangerous to use these days because the waste mercury tailings are deposited directly into the environment, and get introduced into waterways.
How Amalgamation Works
Mercury was chosen for the gold extraction process because of its ability to form amalgams with all metals except platinum and iron. The drawbacks to using mercury in the gold extraction process (not including the hazardous nature of handling the mercury itself) is that it takes large amounts of water to do it efficiently and mercury is very sensitive to such things as oily residues and will not bind with the gold if anything like that is present in the ore or the support stages of the process.
The process of gold extraction which uses the chemical binder Mercury has been shown to be most effective on ore that is from 100 to 325 mesh; this allows for maximum surface exposure to the gold.
Once the ore has been crushed to this size it is then placed in barrel type rollers. The rollers turn slowly to coat the ore well with the mercury compound. Sometimes the rollers will also have grinders placed inside them to further refine the ore. The barrels rotate slowly to prevent small mercury particles from forming that hurt the process.
As was mentioned a large amount of water is used during the process to speed the separation of the waste ore and amalgams. Once this part of the process is complete, the amalgam is then placed in what is known as a retort device (distillation of the mercury). Once the amalgam is heated the mercury becomes vaporized and the free gold separates out. The vaporized mercury is then collected in a separate container for reuse.
*Again, it must be strongly emphasized that mercury has been proven to be a highly toxic substance to both humans and the environment and is not recommended for use at the current time for the separation of gold from ore.