Here's a link to gold extraction processes. I past

Here's a link to gold extraction processes. I pasted the MC part for an easy read. Article very informative and I have forgotten who sent me the link, perhaps Megashear, but appreciate it. GGB

https://pdfs.semanticscholar.org/7559/67e38c3...921855.pdf

MC part of article:
Merrill-Crowe process
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 [6]. 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 because zinc has a higher affinity for the cyanide ion than gold.
Gold (along with silver) is then precipitated with zinc dust according to the reaction:
2KAU(CN)2 + Zn = K2Zn(CN)4 + 2Au
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 major advantage of carbon-in-pulp recovery over Merrill Crowe recovery is the elimination of
the leached ore solids and liquid separation unit operation. The separation step typically involves
a series of expensive gravity separation thickeners or continuous filters arranged for
countercurrent washing or filtration of the solids. For ores exhibiting slow settling or filtration rates,
such as ores with high clay content, the countercurrent decantation (CCD) step can become cost
prohibitive.
Ores with high silver content will generally suggest that Merrill-Crowe recovery be used. This is
because of the very large carbon stripping and electrowinning systems required for processing
large quantities of silver. The typical rule of thumb states that economic silver to gold ratios of
greater than 4 to 1, will favor installation of a Merrill-Crowe system, but this decision can be
altered if the ore exhibits very slow settling rates.