Merrill-Crowe process
Encyclopedia
The Merrill-Crowe Process is a separation technique for removing gold
from a cyanide
solution
.
The solution is separated from the ore by methods such as filtration
and counter current decantation
(CCD) and is then clarified in special filters, usually coated with diatomaceous earth
to produce a clarified solution. Oxygen is then removed by passing through a vacuum deaeration column. Zinc dust is then added to the clarified, deaerated solution which precipitates the gold, zinc having a higher affinity
for the cyanide ion
than gold. Silver
and copper
are also precipitate if present.
The gold precipitate is then filtered out of the solution, mixed with fluxes and smelted to form crude and impure bars which are sent to a refinery to remove the copper and silver, the process used depending on the impurities in the gold.
The basic process was discovered and patented by Charles Washington Merrill
around 1900, then later refined by Thomas B. Crowe, working for the Merrill Company.
In more recent years the EMEW technology has started to replace this process through the use of electrowinning
Gold
Gold is a chemical element with the symbol Au and an atomic number of 79. Gold is a dense, soft, shiny, malleable and ductile metal. Pure gold has a bright yellow color and luster traditionally considered attractive, which it maintains without oxidizing in air or water. Chemically, gold is a...
from a cyanide
Cyanide
A cyanide is a chemical compound that contains the cyano group, -C≡N, which consists of a carbon atom triple-bonded to a nitrogen atom. Cyanides most commonly refer to salts of the anion CN−. Most cyanides are highly toxic....
solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
.
The solution is separated from the ore by methods such as filtration
Filtration
Filtration is commonly the mechanical or physical operation which is used for the separation of solids from fluids by interposing a medium through which only the fluid can pass...
and counter current decantation
Decantation
Decantation is a process for the separation of mixtures. This is achieved by carefully pouring a solution from a container in order to leave the precipitate in the bottom of the original container...
(CCD) and is then clarified in special filters, usually coated with diatomaceous earth
Diatomaceous earth
Diatomaceous earth also known as diatomite or kieselgur/kieselguhr, is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 1 micrometre to more than 1 millimetre, but typically 10 to...
to produce a clarified solution. Oxygen is then removed by passing through a vacuum deaeration column. Zinc dust is then added to the clarified, deaerated solution which precipitates the gold, zinc having a higher affinity
Chemical affinity
In chemical physics and physical chemistry, chemical affinity is the electronic property by which dissimilar chemical species are capable of forming chemical compounds...
for the cyanide ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
than gold. Silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
and copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
are also precipitate if present.
The gold precipitate is then filtered out of the solution, mixed with fluxes and smelted to form crude and impure bars which are sent to a refinery to remove the copper and silver, the process used depending on the impurities in the gold.
The basic process was discovered and patented by Charles Washington Merrill
Charles Washington Merrill
Charles Washington Merrill was an Americanmining metallurgist. He was born in Concord, New Hampshire to Sylvester and Clara L. Merrill. He attended elementary and high school in Alameda, California and then attended the College of Mining of the University of California, where he received a...
around 1900, then later refined by Thomas B. Crowe, working for the Merrill Company.
In more recent years the EMEW technology has started to replace this process through the use of electrowinning
Electrowinning
Electrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution or liquefied. Electrorefining uses a similar process to remove impurities from a metal. Both processes use electroplating on a large scale and are important techniques...
See also
- Other Gold cyanidationGold cyanidationGold cyanidation is a metallurgical technique for extracting gold from low-grade ore by converting the gold to a water soluble coordination complex. It is the most commonly used process for gold extraction...
techniques:- Carbon in pulpCarbon in pulpCarbon in Pulp is an extraction technique for recovery of gold which has been liberated into a cyanide solution as part of the gold cyanidation process....
- ElectrowinningElectrowinningElectrowinning, also called electroextraction, is the electrodeposition of metals from their ores that have been put in solution or liquefied. Electrorefining uses a similar process to remove impurities from a metal. Both processes use electroplating on a large scale and are important techniques...
- Resin in pulp
- Carbon in pulp