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 met ...

from the

solution Solution may refer to: * Solution (chemistry), a mixture where one substance is dissolved in another * Solution (equation), in mathematics ** Numerical solution, in numerical analysis, approximate solutions within specified error bounds * Soluti ...

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 Leaching is the loss or extraction of certain materials from a carrier into a liquid (usually, but not always a solvent). and may refer to: * Leaching (agriculture), the loss of water-soluble plant nutrients from the soil; or applying a small amou ...

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 Merrill may refer to: Places in the United States * Merrill Field, Anchorage, Alaska * Merrill, Iowa *Merrill, Maine * Merrill, Michigan *Merrill, Mississippi, an unincorporated community near Lucedale in George County *Merrill, Oregon *Merrill, W ...

). The solution is separated from the ore by methods such as filtration (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 le ...

. 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 periodi ...

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 Affinity may refer to: Commerce, finance and law * Affinity (law), kinship by marriage * Affinity analysis, a market research and business management technique * Affinity Credit Union, a Saskatchewan-based credit union * Affinity Equity Par ...

for the cyanide

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

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 pinkis ...

, will also precipitate, if present. Automated membrane filtration 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 Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

to dissolve the zinc. The solution is filtered, and the remaining solids are

smelted Smelting is a process of applying heat to ore, to extract a base metal. It is a form of extractive metallurgy. It is used to extract many metals from their ores, including silver, iron, copper, and other base metals. Smelting uses heat and a c ...

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 Charles Washington Merrill (December 21, 1869 – February 5, 1958) was an American mining metallurgist. Biography He was born in Concord, New Hampshire to Sylvester and Clara L. (née French) Merrill. He attended elementary and high school in ...

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.

References

{{DEFAULTSORT:Merrill-Crowe Process Gold Metallurgical processes

See also

References