The Merrill–Crowe Process is a separation technique for removing
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile ...
from the
solution obtained by the
cyanide
Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms.
In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...
leaching of gold ores. It is an improvement of the
MacArthur-Forrest process, where an additional vacuum is managed to remove air in the solution (invention of Crowe), and zinc dust is used instead of zinc shavings (improvement of
Merrill).
The solution is separated from the ore by methods such as
filtration
Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filte ...
(e.g. vertical leaf type clarifier filters) and
counter current decantation (CCD). Afterwards a very clear solution is achieved by using pre-coated filters applying
diatomaceous earth
Diatomaceous earth (), diatomite (), or kieselgur/kieselguhr is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3 μm to ...
. Oxygen is then removed by passing the solution through a vacuum de-aeration column.
Zinc
Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...
dust is added to the clarified, de-aerated solution which
precipitate
In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...
s the gold; zinc having a higher
affinity for the cyanide
ion than gold. Other precious metals,
silver
Silver is a chemical element with the Symbol (chemistry), symbol Ag (from the Latin ', derived from the Proto-Indo-European wikt:Reconstruction:Proto-Indo-European/h₂erǵ-, ''h₂erǵ'': "shiny" or "white") and atomic number 47. A soft, whi ...
, and base metals, like
copper
Copper is a chemical element with the symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductile metal with very high thermal and electrical conductivity. A freshly exposed surface of pure copper has a pinkish ...
, will also precipitate, if present.
Automated membrane
filtration
Filtration is a physical separation process that separates solid matter and fluid from a mixture using a ''filter medium'' that has a complex structure through which only the fluid can pass. Solid particles that cannot pass through the filte ...
offers a cost savings alternative to CCD. Both applications are compared in detail b
K. McGrew, 2016.
The gold precipitate (mixed with zinc dust) is then filtered out of the solution, and the zinc dust and gold are mixed with
sulfuric acid to dissolve the zinc. The solution is filtered, and the remaining solids are
smelted to a gold dore bar. These bars are sent to a refinery to remove the copper and silver, the specific process used depending upon the impurities in the gold.
The basic process was discovered and patented by
Charles Washington Merrill around 1900, and later refined by Thomas Bennett Crowe, working for the Merrill Company.
[Ryder, David Warren, The Merrill story : (being a record of the life and achievements of Charles Washington Merrill, and a history of the Merrill Company and subsidiaries), Merrill Co., 1958.]
See also
* Other
gold cyanidation
Gold cyanidation (also known as the cyanide process or the MacArthur-Forrest process) is a hydrometallurgical technique for extracting gold from low-grade ore by converting the gold to a water-soluble coordination complex. It is the most commonl ...
techniques:
**
Carbon in pulp Carbon in pulp (CIP) is an extraction technique for recovery of gold which has been liberated into a cyanide solution as part of the gold cyanidation process.
Introduced in the early 1980s, Carbon in Pulp is regarded as a simple and cheap process. ...
**
Electrowinning
Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution via a process commonly referred to as leaching. Electrorefining uses a similar process to remove impurities from a ...
**
Filter press
References
{{DEFAULTSORT:Merrill-Crowe Process
Gold
Metallurgical processes