There have been several methods developed by miners to extract gold particles from ore over the last hundred or so years. Of those methods, the ‘Cyanide Leaching Process’ (Cyanidation), is the method that is commonly used the most often to do this.
As the name implies, the main component in the process is a sodium cyanide solution. This article will give a general overview about how this important, 120 year old gold mining process works.
Safety and Environmental Issues
Yes we are talking about the same chemical that is well-known as a deadly toxin in both its solid and gaseous form. With that being said, it must be noted that the cyanide content in the extraction solution is kept at an extremely low level (100 to 500 parts per million); this is done for safety, environmental and economic reasons.
Many people do not realize that cyanide is found naturally occurring in several of the foods they eat every day but the body tolerates it quite well in small doses.
The biggest health concern with the use of cyanide in mining is the effect that it can have if it leaches into surface water. Fish are said to be over a thousand times more sensitive to cyanide than humans are, and pollution can have a detrimental effect on native ecosystems if used improperly.
Therefore, a reasonable amount of scrutiny of this process to ensure proper use of this chemical is certainly justified.
How the Extraction Process Works
Cyanidation is not used to extract large size gold nuggets from ore because the process is deemed too slow and is not cost effective. In mining operations that contain larger, free-milling type gold deposits, stamping and traditional gravity separation methods are more effective.
Cyanide is used to extract gold from ore where it is spread throughout the ore in fine amounts.
It is also usually done in conjunction with a process such as grinding, milling or a gravity separation that increases the surface area of the ore to expose as much gold as possible; this is done because the sodium cyanide solution has no effect on the gold during the extraction process if it does not come into direct contact with the gold particles.
The ore that is left over after grinding, milling or gravity separation is commonly known as a ‘heap’.
There are two common ways that the heap is processed:
1. Controlled Mills
This process which is also known as ‘vat leaching’ most often takes place in a specialized mill that is located on the mining premises. This process works by exposing the surface of the finely crushed ore to the cyanide salt solution. Once exposed to the gold, the solution binds with ions in the gold that allows them to separate out into solution and then be screened for collection.
It is well-known that the introduction of more oxygen speeds up the leaching process but it is not often done in a mill out of cost concerns. Different elements in the ore itself can wreak havoc on the process and significantly hurt the effectiveness of the process.
Also Read: How Mercury is Used to Recover Fine Gold
2. Heap Leaching
This type of gold separation technique works very similar to mill extraction but is much more cost effective; the drawback is it is harder to control what happens to the toxic cyanide solution after it has done its job. It also has the advantage of adding oxygen to the mix naturally. It is most often used on the very lowest grade of ore or to reprocess ore that is a byproduct of the other extraction steps.
The Cyanide Leaching Process has been the most widely used and most cost effective method of extracting gold from ore since the 1970’s. Most open-pit gold mining operations around the world use a heap leaching to extract ores from these types of deposits. Consider that most of the largest gold mines in the world are mining what would be considered low-grade ore by most standards. It would be impossible to profitably work most of these ores without use of cyanide.
Many commercial mining operations have found it profitable to rework old mine tailings from last century. Significant gold was left behind in these tailings due to ineffective mining methods during the early days, but they can be reworked profitably now.
Until researchers come up with a more efficient and cost effective method of extraction, it looks like it will continue to remain the most popular gold extraction method for the foreseeable future.