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Mar 2009 


Gold Extraction and 
Recovery Processes 


 Wong Wai Leong Eugene and Arun S. Mujumdar 

M I N E R A L S ,   M E T A L S   A N D   M A T E R I A L S   T E C H N O L O G Y   C E N T R E   ( M 3 T C )  
N A T I O N A L   U N I V E R S I T Y   O F   S I N G A P O R E  

Minerals, Metals and Materials Technology Centre (M3TC)
Faculty of Engineering, National University of Singapore

For Internal Use Only

Not for General Distribution

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Executive Summary

Various methods such as gravity concentration, flotation, panning, pyrometallurgy, cyanidation etc are
available for the extraction of gold metal from its ores. Amongst these methods, cyanidation is the most
common method used in the leaching of gold from the ore. This process involves the dissolution of gold
containing ores in dilute cyanide solution in the presence of lime and oxygen. For refractory ores such as
sulfide ores and carbonaceous ores which are not susceptible to direct cyanide leaching, various
methods such as pressure oxidation, roasting, chlorination, biooxidation etc are available to treat these
ores to expose the gold particles before cyanidation. Thioura leaching was developed as a potential
substitute to cyanide leaching due to its lower toxicity and greater rate of gold and silver dissolution. Other
alternative lixiviants to cyanide such as bromides (acid and alkaline), chlorides and thiosulfate are also
being developed.

The common processes for recovery of gold solution includes: (i) Carbon adsorption, Merrill-Crowe
process, (iii) electrowinning and (iv) ion-exchange / solvent extraction. Traditionally, Merrill-Crowe process
was used to remove gold from a cyanide solution by using zinc dust to precipitate gold from its solution.
Carbon adsorption is increasing using in newer plants for gold recovery. Carbon in Pulp (CIP) technique
involves contacting the leached pulp with granular carbon in a series of agitating tanks with a sufficient
retention time.

Selected summary of patented gold recovery processes and examples of commercial mining and
recovery gold processes are also provided in this report.

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The major concern in thiourea leaching is the degradation of the reagents. Uncontrolled oxidation of
thiourea not only leads to loss of reagent but also the formation of elemental sulfur which covers the gold
particles and prevents its leaching [5]. The dissolved precious metals from pregnant solution can be
recovered by suitable adsorptions such as activated carbon, strong acid cation exchangers and
thio/resins or using electrowinnig, cementation and gaseous reduction [5]. Most of the work done to
recover gold with thiourea is on the leaching part of the process. Few results are available on the
recovery of gold and silver from the pregnant solution [2].

Gold Recovery from Solution

The common processes for recovery of the solubilized gold from solution are (certain processes may be
precluded from use by technical factors) [5,6]:

 Carbon Adsorption
 Merrill-Crowe process
 Electrowinning
 Ion-exchange/Solvent Extraction

(i) Carbon Adsorption

The process known as carbon in pulp, or charcoal in pulp or CIP controls the gold precipitation
from the cyanide solution by use of activated charcoal (carbon). Activated carbon can be
manufactured from wood, nuts shells, coal, petroleum coke and a variety of organic products.
Coconut shell carbon is preferred because of its commendable durability and high adsorption
capability for gold and silver cyanide [5]. Other modifications include Carbon-in-Leach (CIL) and
Carbon-in-Column (CIC).

The technique involves contacting the leached pulp with granular carbon (about -8 to +20 mesh)
in a series of gently agitating tanks with a sufficient retention time [2]. The carbon is recycled
through the circuit to build up the loading to 8-10 per cent by weight. The loaded charcoal is then
separated from the pulp on a suitable vibrating screen, coarse enough to retain the carbon, but
fine enough to allow the pulp to pass through. The carbon is next sent to the stripping column for
desorption and regeneration. The technique is used on low grade gold and silver ores.

Leached pulp and carbon are transferred in a counter current flow arrangement between a series
of tanks, usually numbering 4 to 6 [1] as shown in Figure 2. In the final tank, fresh or barren
carbon is put in contact with low grade or tailings solution. At this tank the fresh carbon has a high
activity and can remove trace amounts of gold (to levels below 0.01 mg/L Au in solution).

As it moves up the train, the carbon loads to higher and higher concentrations of gold, as it

comes in contact with higher grade solutions. Typically concentrations as high as 4000 to 8000
grams of gold per tonne of carbon (g/t Au) can be achieved on the final loaded carbon, as it
comes in contact with freshly leached ore and pregnant leach solution (PLS). This can be
measured by comparing the amount of gold extracted from the carbon to the amount of carbon
used. The final loaded carbon then is removed and washed before undergoing "elution" or
desorption of gold cyanide at high temperature and pH. Hot caustic-cyanide (1%NaOH, 0.1 to
0.2%NaCN) solution at nearly boiling temperature in 36 to 72 hours [5]. The elution rate may be
accelerated by adding 10% alcohol at temperatures around 60 to 80°C with a stripping time of 4
to 6 hours. The elution of loaded carbon may also be speeded by using pressure elution (120 to
130°C at 70 psig) in 6 to 8 hours [5].

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Figure 2 Process flow for carbon in pulp [1].

The rich eluate solution that emerges from the elution process is passed through electrowinning
cells where gold and other metals are precipitated onto the cathodes. After precipitation, the
product is treated with dilute sulfuric acid to dissolve residual zinc and most of the copper (if any
is present) [2]. The residual is washed, dried and melted with fluxes. The remaining gold and
silver alloy, called dore’, is cast into molds for assay. Refining is accomplished by electrolysis
during which silver and any platinum group elements are also separated and recovered. Another
method of separating gold from silver dore’ is by parting, where hot concentrated sulfuric or nitric
acids are used to differentially dissolve the silver, while the gold is recovered from the residue.

(ii) 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

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.

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Flowsheet for US Patent 4,578,163 Gold Recovery Process.
Appendix B

Flowsheet for US Patent 7,498,006B2 Process for extracting gold in arsenic-containing concentrate of

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2 Claudia Gasparrini, Gold and other precious metals: From ore to market, Springer-Verlag, 1993.
5 Gold: Advances in Precious Metal Recovery, Nathaniel Arbiter and Kenneth N. Han (Editors), Gordon

and Breach Science Publishers, 1990.
7 A.E. Sanchez-Chacon and G.T. Lapidus, Model for heap leaching of gold ores by cyanidation.

Hydrometallurgy, 44 (1997): 1-20.
8 G. Chi, M.C. Fuerstenau, J.O. Marsden, Study of Merrill-Crowe processing: Part I and Part II, Int. J.

Miner. Process. 49 (1997), 171-192.
9 J. Bujalski, R. Tiller-Jeffery, H. Watling and M.P. Schwarz, CFD modeling and comparison with

experimental residence time distributions in single and two-phase porous flow, 3rd International
Conference on CFD in the Minerals and Process Industries, CSIRO Melbourne Australia, 10-12
December 2003, 463-468.

10 M.J. Leahy, M.R. Davidson and M.P. Schwarz, A two-dimensional CFD model for heap bioleaching of
chalcocite, ANZIAM J. 46 (2005) 439-457. 


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