History of the Gold Refining Process

The refining of gold has been a part of human history for a long time. The earliest attempts can be dated as far back as the 4th millennium BC. Although gold refining has been around for 6 thousand years, it didn’t always use the Miller or Wohlwill process that is commonly being used today. It has continually evolved throughout the ages in order to obtain the highest purity possible as knowledge and technology advanced.

 

The earliest attempts at refining gold were found in Nahal Qanah Cave dating back to the 4th millennium BC. Back then, the surface of the gold was refined by about 80-95% while the interior was only about 64-75%. The surface was compacted and heavily burnished which indicated that the depletion gilding technique was used. Most gilding methods are additive, which is the process of adding metals to the surface, where as the depletion gilding method removed metals from the surface. This created a high purity of gold at the surface while the interior would be less pure.

 

The next step in the evolution of gold refining was during the ancient and medieval times. It wasn’t until the invention of coinage that started the practice of gold and silver parting during the 6th century BC. The process that was used involved mixing common salt, argentiferous gold foil and brick dust or burnt clay and placing the mixture into a closed and sealed container. Urine was also added for its acidic properties which aided the decomposition. The container was then heated which led to the silver reacting to the salt and turning into silver chloride, which was then removed and left purified gold behind. Experiments have shown that this procedure could have the gold content be taken from 37.5% to 93%. This was the primary method used until the 16th century.

 

It wasn’t until the Middle Ages that the process started using sulpher, antimony and mineral acids instead of salt.  This method was similar to the salt cementation process but created sulphides instead of chlorides. The impurities of the gold would form metal sulphides while the gold is left unreacted. The antimony process was the same but used stibnite instead of sulpher, which is more stable at a higher temperature. These methods were much quicker than the salt process and gave an even higher purity level of gold, however, it could dissolve some of the gold as well.

 

During the 12th century, the use of acids was introduced. Nitric acid as well as Aqua regia was used for the parting. The use of Aqua regia would dissolve gold into a soluble chloride while also causing silver to become an insoluble chloride. The silver would then be filtered and the gold would be recovered from evaporating the liquid and heating the residue. Nitric acid was suitable for removing small quantities of gold from silver while Aqua regia would be used for removing small quantities of silver from gold.

 

The evolution of the gold refining process has been a long one but with amazing results. With the modern practice of the Miller and Wohlwill methods, both being developed in the mid to late 1800’s, gold can now reach a purity of 99.5% and 99.999%, respectively. There is no doubt that new methods of gold refining will continue to develop as our technology advances, leading to even more efficient processes.