Gold Process

8/13/2019 Gold Process

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DRAFT

PROCESS DESCRIPTIONS AND MATERIAL FLOWS

FOR GOLD ORE PROCESSING FACILITIES

Prepared for

U.S. ENVIRONMENTAL PROTECTION AGENCY

Region 975 Hawthorne St.,

San Francisco, California, 94105

Work Assignment No. R0920

EPA Region 9

Date Prepared January 22, 2001

Document Control No. REPA2-0920-002

Contract No. 68-W-99-009

Prepared by Booz Allen & Hamilton, Inc

Contractor Work Assignment Manager Stuart Strum

Telephone No. 210) 244-4233

EPA Work Assignment Manager John Katz

Telephone No. (415) 972-3283


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1.0 Introduction

Gold ores may contain significant amounts of mercury, and processing of the ores to recover gold may

generate mercury air emissions. Most gold ore processing relies on cyanide leaching to extract gold

from the ore. The gold-bearing cyanide solution is then concentrated and the gold is recovered and

further purified. If gold ore contains mercury, air emissions from ore processing may occur undercertain physical and chemical conditions. Factors that control the mercury emission potential of gold

recovery processes include:

Chemical state of mercury in the process: Elemental mercury (Hgo) has an elevated vapor pressure

compared to the oxidized states of mercury (Hg1+, Hg2+). Processes that have chemical and

physical conditions where mercury is stable as Hgoinclude ore roasting, carbon regeneration,

electrochemical gold recovery (electrowinning), gold recovery by zinc precipitation, retorting, and

smelting.

Temperature in the process unit: Mercury is more strongly partitioned to the vapor phase at higher

temperature. Ore treatment processes conducted at elevated temperature include ore roasting,

pressure oxidation (autoclaving), carbon regeneration, electrowinning, retorting, and smelting.

Process configuration: Processes that produce gas emissions that must be vented from the process

have a greater potential for mercury emissions than processes where there is no gas emission

stream, or where the process is performed in a contained or pressurized vessel. Processes

producing gas stream emissions include ore roasting, carbon regeneration, retorting and smelting.

The configuration of processes used to treat ore and recover gold is always site-specific. Ore

processing occurs at mills that may be built at the site where the ore is mined. Mills may also be built at

a central location to process ores from several locations, and the milling and gold recovery processes

may vary at a mill depending on the source of the ore. The process configurations are designed to

maximize gold recovery from the ore with the minimal amount of required handling and processing.

Some processes are generally performed for ore processing and gold recovery at all facilities, while

other ore treatment methods are specifically developed to increase gold recovery based on site-specific

properties of the ore (Ref. 1). The generalized flow process is shown on Figure 1.

2.0 Extraction and Crushing

Almost all gold mining and mineral processing conducted in North America relies on cyanide leaching to

recover gold from the ore. This process was developed in the 1960s and has been implemented worldwide in the mining industry. In order to recover gold using the cyanide leaching process, ore containing

finely disseminated gold is extracted from surface or underground mines using blasting and large-scale

earthmoving equipment. The ore is then sent


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Figure 1. Generalized Process and Material Flow for Gold Extraction and Recovery


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to crushers that break the larger pieces of ore into smaller sizes that can be further reduced in size.

Typically jaw crushers and cone crushers are used in the first size reduction process. Generally, there is

low potential for mercury emissions from these processes because the ore has not been subjected to

chemical reaction or heat treatment that could release the mercury, which is most commonly present as

mercury sulfide (cinnabar).

To reduce particle size to a consistent size that exposes the mineral surfaces for cyanide leaching, the

ore is then ground in a mill. Mills are operated in a variety of configurations, with the most common

being ball mills and semi-autogenous (SAG) mills. In ball mills, the crushed ore is introduced into

rotating drums containing steel balls, which mechanically reduce the ore to a smaller particle size. In

SAG mills, ore particles are crushed against each other, and against steel rods or balls to reduce

particle size. Similar to the crushing process, milling has a low potential for mercury emissions.

Gold can be efficiently recovered from some ores by leaching after the milling process has been

completed. These ores are referred to as free-milling or oxide ores (Ref. 2). Other ores have poor

gold recovery by cyanide leaching unless further processing is performed to increase the extraction of

gold. These are known as refractory ores. Typically refractory ores contain organic carbon thatadsorbs gold from the cyanide leach in a process known as preg-robbing. Gold recovery may also

be suppressed by the presence of gold in sulfide mineral grains that is not available for cyanide

extraction. These ores are typically treated by furnace oxidation (ore roasting) or pressure oxidation

(autoclaving) processes.

3.0 Ore Roasters

Ore roasting is a combustion process where the milled ore is mixed with coal and burned in an industrial

furnace. Additional coal, fuel oil, or natural gas may be injected into the combustion chamber to

complete the oxidation of ore components. Fuel addition is usually performed where the ore has lowheat value (Ref. 3). During the combustion process, ore components that interfere with cyanide

leaching of gold are oxidized. Carbonaceous material in the ore is converted to carbon dioxide, and

sulfide minerals produce sulfur dioxide. The metals in sulfide minerals are typically converted to

oxides, with the exception of mercury. Elemental mercury is stable at combustion temperatures (up to

1,000oF). The oxidation of cinnabar proceeds by the reaction:

Hg2S Hgo+ SO2

Elemental mercury produced during the oxidation in the combustion chamber is emitted in the gas phase

with the roaster off gas, because of the elevated temperature and the chemical state of mercury in theroaster off gas. Removal of mercury from the off gas requires use of pollution control devices that are

effective in removing elemental mercury. Research on mercury emissions at coal-fired boilers using

conventional pollution control devices, such as baghouses, electrostatic precipitators, and sulfur dioxide

scrubbers, has generally shown poor control of elemental mercury emissions (Ref. 4).


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As the ore exits the combustion chamber, it typically enters a quench process, where the temperature is

reduced by contact with cooling water and generation of steam. The steam from the quench process is

used as a heat source in processes at the mill, or may be sent directly to a cooling tower. The process

flow for ore roasters is shown on Figure 2.

4.0 Autoclaves

Autoclaves are pressure oxidation vessels that are used to treat ores to increase gold recovery by

cyanide leaching. The milled ore is mixed with water to form a slurry, and is then acidified with sulfuric

acid. Autoclaving is not used to treat ores with significant alkalinity, as the amount of acid that would

be required for the process is prohibitive. The acidified slurry is then pumped into the autoclave vessel,

where oxygen is used to increase the vessel pressure to over 300 pounds per square inch, and the

slurry is heated to 350oF to 430oF (Ref. 2). The slurry is agitated in the reaction vessel, and is then

discharged to a pressure relief chamber, where the liquid content is flashed to steam that is recovered

and returned to the pressurized segment of the vessel. Because the reaction is performed in a

pressurized oxygen environment at low pH, at least 95 per cent of the sulfide minerals present in the ore

are converted to sulfates. The transformation process for cinnabar, which contains mercury in theslurry, is:

Hg2S + 2O2HgSO4

Some chemical dissociation of mercury from the sulfate may occur in the slurry, as well as reactions that

form other mercury compounds, such as mercuric oxide (HgO). All of these forms of mercury will tend

to partition to the solid and liquid components of the slurry material. Most of the mercury in vapor and

steam emissions from the process will be present as Hg2+. In this form, mercury emitted from the

autoclave in the gas phase can be effectively controlled by wet pollution control devices, such as wetventuri systems used to control particulate emissions. The autoclave process is illustrated on Figure 3.

5.0 Leaching Processes

Gold is extracted from free milling ore and treated refractory ore by cyanide leaching. In heap leaching,

an alkaline cyanide solution is distributed onto stacks or heaps of milled ore. The solution percolates

through the ore, and gold reacts with free cyanide to form soluble gold-cyanide complexes that migrate

with the solution to an impermeable liner beneath the heap and flow to a collection pond. Refractory

ores are generally leached in reaction vessels, referred to as vat leaching. Where the ore has some

ability to adsorb gold-cyanide complexes, carbon adsorbent may be added to the leach vessel in thecarbon-in-leach or carbon-in pulp process. Mercury present in the ore may be leached into the

cyanide solution. The amount of mercury leached from the ore is generally dependent on the cyanide

concentration of the solution (Ref. 2). Chemical compounds may be added to the cyanide solution to

suppress mercury dissolution into the leach solution. These chemical additives must be formulated and

added carefully in order to prevent interference with recovery of gold and/or silver present in the ore.


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There is low potential for mercury emissions from the leaching process because the mercury is present

in solid form in


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Figure 2. Material Flow for Ore Roasters


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Figure 3. Materials Flow for Ore Autoclaves and Carbon Regeneration Kilns


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the ore, or as a soluble cyanide complex in the leach solution, and the processes are not performed at

elevated temperatures. Regional evaluations of cyanide processes in Nevada have shown that

approximately 20 percent of the mercury present in the ore is removed in the leach solution (Ref. 2).

The gold present as cyanide complexes in the leach solution is concentrated by adsorption onto

activated carbon. The gold-bearing solution may be extracted from the leaching process and

introduced into a carbon adsorption column for concentration of the gold content. Carbon may also be

introduced into the leach process to adsorb gold cyanide complexes concurrent with leaching from the

ore. The carbon is then separated by physical separation processes. The carbon contains gold and

mercury as adsorbed cyanide complexes.

6.0 Carbon Stripping

The carbon with adsorbed metal cyanide complexes is treated to recover gold in a concentrated

solution. Adsorbed gold, and adsorbed silver and mercury, depending on the ore composition and

recovery efficiencies of the leaching processes, is removed from the carbon through desorbtion into a

chemical solution. The recovery of gold from carbon, known as carbon stripping or carbon elution, isusually performed using a heated caustic cyanide solution. Alcohol or glycol solutions have also been

used for stripping, although these methods are typically not implemented in North American gold mining

operations.

The stripping process may be performed under pressurized or atmospheric conditions (Ref. 2).

Concentrated caustic cyanide solution may be used to soak the gold-bearing carbon, which is then

flushed with high purity water, using the Anglo American Research Laboratory (AARL) process. A

more dilute caustic cyanide solution may be circulated through the gold-bearing carbon under

atmospheric or pressurized conditions to desorb the gold. This is referred to as atmospheric or

pressure Zarda stripping. The concentrated solution is then sent to recovery processes, while thecarbon is regenerated in a rotary kiln. The amount of mercury remaining on the stripped carbon has

been found to vary widely, depending on the mercury content of the gold-bearing solution and the type

of stripping process used (Ref. 2). In general, Zarda stripping processes leave more residual mercury

on the carbon adsorbent than AARL or organic stripping processes. Some facilities add sodium sulfide

to the gold-bearing solution prior to carbon adsorption. This causes precipitation of mercury sulfide,

which is removed from the solution by filtration before the gold is adsorbed to carbon.

7.0 Carbon Regeneration

After gold has been removed from the activated carbon through the stripping process, the carbon isregenerated before being returned to the adsorption process. Regeneration is performed to increase

the adsorption of gold-cyanide complexes from the leach solution. Rotary kilns are used to regenerate

the spent carbon. Because the carbon can be oxidized in the kiln if air is present in the heating

chamber, steam is introduced to the kiln to maintain a reducing environment. Some carbon is converted

to carbon monoxide during the regeneration process. As the carbon moves through the rotary kiln, it is


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heated, and mercury and other remaining components are desorbed into the gas stream in the kiln.

Regenerated carbon exits the kiln and is captured and quenched, while the gas stream is vented from

the process, along with combustion gas from heating the kiln chamber; most kilns use natural gas or

propane as the combustion fuel. The off-gas, containing steam, carbon monoxide and elemental

mercury vapor, is discharged to pollution control devices. The potential for mercury emissions from

rotary kilns is directly dependent on the mercury content of the stripped carbon. Particulates control

devices, such as venturi scrubbers or electrostatic precipitators, are not effective in controlling mercury

vapor admissions. The mercury can be effectively removed from the off-gas by adsorption in an

activated carbon filter, or in a selenium filter. Carbon adsorption filters are the only mercury-specific

control device used on rotary kilns at mines in the United States. The carbon kiln process is illustrated

on Figure 3.

8.0 Gold Recovery from Concentrated Solutions

Recovery of gold along with co-precipitated metals such as silver and mercury from concentrated

carbon strip solutions is performed by electrochemical or chemical processes. Electrowinning is

performed by applying an electric potential to the solution with a cathod and anode. The metals in thesolution plate out onto the cathode. Gold recovered through electrowinning is usually plated onto steel

wool cathodes, because of the large surface area availabe for gold deposition. The plated cathode, or

sponge, is then removed from the electrowinning cell for futher refinement of the recovered gold.

Gold may also be recovered from solutions by addition of zinc powder. Because zinc has a greater

chemical affinity for cyanide than the other metals in the strip solution, zinc is dissolved and gold, silver

and mercury precipitate as a solid. The fine particulate metals are then recovered by filtration

processes. The potential for mercury emissions from the gold recovery processes may be significant.

Where the strip solutions have significant mercury content, elemental mercury will be present in the

electrowon sponge or the precipitate from the zinc addition process. A plenum device is usuallypresent on the electrowinning cell to remove ammonia and other gases generated at the anode (Ref. 5).

Elemental mercury in the cell can vaporize and discharge with the gases produced in the cell. Because

the cells are typically operated at 90oF to 120oF, carbon adsorption filters will be effective in controlling

mercury emissions in the gas stream from electrowinning cells. Zinc precipitation processes are usually

performed in deoxygenated, enclosed reaction cells; therefore, the potential for mercury emissions

during the precipitation process are low. Recovered solids from both processes should be kept in

covered containers and adequate ventilation should be provided in the areas where these processes are

performed, due to the presence of elemental mercury in the recovered solids.

9.0 Retorting

Recovered gold and other metals in the sponge or zinc preicipitate may contain up to 60 weight percent

gold, depending on the mercury content of the cyanide solution, and the configuration of the gold

recovery process. Solids with significant mercury content are treated in a retort to recover mercury.

The solid material is placed in a pot or tray which is loaded in to a vacuum chamber. Retorting is


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usually done for 12 to 24 hours at 600oC to 700oC, to remove around 99


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per cent of the mercury. The charge is heated under a partial vacuum, and the mercury vaporizes and

is recovered in a water-cooled condenser. The liquid mercury that is bled from the condenser tube is

piped into a collection vessel. The remaining gold and silver at the end of the retorting process typically

contains about 1,000 to 8,000 mg/kg mercury (Ref. 2). The condenser allows some mercury to

discharge in the off gas, and loss of 0.4 to 0.7 per cent of the mercury from the condenser has been

reported. Some facilities operate a carbon adsorption filter between the condenser and the vacuum

pump. The cooled gas downstream of the condenser is expected to allow 95 per cent removal

efficiency for a properly maintained carbon adsorption filter. The layout of the retorting process is

shown on Figure 4.

10.0 Smelting

The retorted gold and silver mixture still contains some impurities, including small concentrations of base

and ferrous metals, and some residual mercury. This material is melted and purified in a refinery furnace

to produce a commercial mixture of gold and silver known as dore. The gold is placed in a crucible and

heated to approximately 1,500oC with a flux material that preferentially absorbs impurities. Most of the

remaining mercury is volatilized in the dore furnace. The dore melt is poured into bars, and any flux slagthat hardens on the bars is removed with a mechanical chipper. The cleaned bars are then shipped to a

commercial gold refinery, where the material is further processed to produce gold bullion (99.5 percent

pure gold). The volatilized mercury is present primarily as elemental mercury. Because most dore

furnace have throughput volumes of less than one ton per hour and use electrical induction as a heat

source, a fume hood is used to vent emissions from the furnace. The fume hood may have a venturi

scrubber or other particulate control device in place. A typical furnace configuration is shown on

Figure 5.


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Figure 4. Typical Process Layout for Mercury Retorts


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Figure 5. Induction Smelting Furnace for Gold Dore Production


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11.0 References

1.1. U.S. EPA, 1994, Technical Resource Document Extraction and Beneficiation of Ores and

Minerals, Volume 2: Gold. Office of Solid Waste, Special Waste Branch. 401 M.Street NW,

Washington, D.C.

2. Menne, D. M. 1998, Mercury in Gold Processing Volume I : Metallurgy.

http://members.iinet.net.au/~menne/hg1.htm

3. Hubbard, G. and J. P. DAcierno, 1999. CFB Roasting for Gold Ore Processing, in Proceedings

of the 15thInternational Conference on Fluidized Bed Combustion.

http://www.etis.net/balpyo/15icfbc/99-0158.PDF

4. U.S. Environmental Protection Agency, 1997 Mercury Study Report to Congress, Chapter 8. An

Evaluation of Mercury Control Technologies and Costs. EPA-452-R-97-010.

5. Van Zyl, D.J.A. and G.M. Eurick, 2000. The Management of Mercury in the Modern Gold MiningIndustry, Nevada Bureau of Mines and Geology.

http://www.unr.edu/mines/mine-eng/mlc/mercurygold.pdf

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