GoldExtraction.pdf

Gold Extraction and

Recovery Processes

Wong Wai Leong Eugene and Arun S. Mujumdar

Minerals, Metals and Materials Technology Centre (M3TC)

Faculty of Engineering, National University of Singapore

Mar 2009

For Internal Use Only

Not for General Distribution

M INERALS , M ETALS AND M ATERIALS T ECHNOLOGY C ENTRE (M3TC)

N ATIONAL U NIVERSITY OF S INGAPORE

M3TC Report

GOLD EXTRACTION AND RECOVERY PROCESSES

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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M3TC Report

GOLD EXTRACTION AND RECOVERY PROCESSES

Introduction

Gold has been known and highly valued since prehistoric times. It may have been the first metal used by

humans and was valued for ornamentation and rituals [ 1 ]. Approximately 60% of the gold mined today is

held by government and central banks and gold is presently the most significant means of international

payment [ 2 ].

Production and Reserves

Annual world production of gold has been steadily increasing over the years reaching approximately

2,300 metric tonnes in recent years. Based on Table 1 , China, South Africa, Australia and United States

are the largest producers of gold with a total production of approximately 42% of the world’s supply. Gold

abundance in the Earth’s crust is 0.002 per 10 6 atoms of silicon [ 2 ]. Gold average economic abundance in

the ore today is in the range of 0.1 to 0.2 oz/ton or 3.42-6.84 ppm, but deposits yielding grades to the fire

assay which are as low as a tenth of those values are also exploited [ 2 ]. It may be noted that gold

distributions tend to be highly irregular on macro as well as micro levels.

Table 1 Distribution of gold production and reserves according to countries

Source: USGS, Mineral Commodity Summaries, Jan 2009

1 Reserves — That part of the reserve base which could be economically extracted or produced at the time of

determination. The term reserves need not signify that extraction facilities are in place and operative.

Reserves include only recoverable materials; thus, terms such as “extractable reserves” and “recoverable

reserves” are redundant and are not a part of this classification system

2 Reserve Base — That part of an identified resource that meets specified minimum physical and chemical

criteria related to current mining and production practices, including those for grade, quality, thickness, and

depth. The reserve base is the inplace demonstrated (measured plus indicated) resource from which reserves

are estimated. It may encompass those parts of the resources that have a reasonable potential for becoming

economically available within planning horizons beyond those that assume proven technology and current

economics.

The reserve base includes those resources that are currently economic (reserves), marginally economic

(marginal reserves), and some of those that are currently subeconomic (subeconomic resources). The term

“geologic reserve” has been applied by others generally to the reserve-base category, but it also may include

the inferred-reserve-base category; it is not a part of this classification system.

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M3TC Report

GOLD EXTRACTION AND RECOVERY PROCESSES

Gold Minerals

In nature, gold most often occurs in its native state (that is, as a metal), though usually alloyed with silver.

Native gold contains usually eight to ten percent silver, but often much more — alloys with a silver content

over 20% are called electrum [ 1 ].

The average concentration of gold in the world is about 0.005 g/t which is lower than other metals. The

low concentration of gold in primary rocks means that upgrading by a factor of 3000-4000 is usually

required during ore formation processes to achieve commercial concentrations. This may be possible

using natural gravity concentration processes or leaching gold with natural fluids from the host rock. Thus,

by highly oxidizing, acidic and complexing (chloride) solutions, followed by redeposition in a more

concentrate form. Owing to its siderophile properties (weak affinity for oxygen and sulfur, high affinity for

metals) gold tends to concentrate in residual hydrothermal fluids and subsequent metallic or sulphidic

phases, rather than silicates, which form at an early stage of magma cooling. Rocks that are high in clays

and low in carbonates are the best sources of gold, and reprecipitation occurs when the hydrothermal

solutions encounter a reducing environment, such as a region of high carbonate, carbon or reducing

sulphide contain [ 3 ].

The various types of common gold deposits are tabulated in Table 2 [ 2 ].

Table 2 Gold deposits

Deposit

Structural

character

Age

Type locality

Main

minerals

Assoc.

minerals

Typical

elements

Metals

mined

% of world

production

Gold-quartz

lodes

Hydrothermal

veins

Mostly

precambrian

To be

expected in

precambrian

rocks, such as

the Canadian,

Australian,

Brazilian-

Guayana and

African-

Arabian

shields

Low-silver

native Au

Quartz

20-25

Epithermal

deposits

Hydrothermal

veins

Tertiary

Emperor

deposit (Fiji)

High-silver

native Au,

tellurides

Quartz,

carbonate,

barite, fluorite

Te, Bi, Pb,

As, Sb,

Hg, etc.

Au, Ag,

some

base

metals

Very small

Young

placers

Unconsolidated

semiconsolidated

sands and

gravels

Recent

River beds

that assay for

gold

Native Au

Heavy

minerals such

as monazite,

ilmenite,

zircon, apatite,

pyrochlore,

etc

Ti, RE’s,

Au, Ti,

RE’s,

5-10; 25 to

35 in the

past

Fossil

placers

Conglomerates

consisting of

quartz pebbles in

matrix of pyrite

and mica

Precambrian Witwatersrand

To be

expected in

the

precambrian

shields

Native Au

Heavy

minerals

including

uraninite

U, PGE,

etc

Au, U,

PGE

More than

Disseminated

gold

Silty

carbonaceous

dolomitic

limestone

Mesozoic

tertiary

Carlin, Cortez,

Getchel, Gold

Acres

(Nevada type

ores); Bald

Mountain

Very fine

native Au

Silicate,

barite, pyrite

and other

sulfides

Fe, As,

Sb, Hg, C,

very little

Has been

increasing.

Brought

into prod.

In 1935

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M3TC Report

GOLD EXTRACTION AND RECOVERY PROCESSES

Greenstones Metamorphosed

volcanic

Precambrian Kalgoorlie,

Hemlo,

Timmins, Val

D’Or Kirkland

Lake

Native Au,

calaverite

Tellurides of

Au, Hg, Pb,

Cu, Bi,

compounds of

gold with Hg,

Sb, and other

elements,

native

tellurium

Hg, Pb,

Bi, Sb

Has been

increasing

Young

greenstones

Metamorphosed

volcanic

Mesozoic

tertiary

Deposits in

Southeastern

Au California

Native Au

Pyrite, Fe

oxides,

various

silicates

Fe, Si

Has been

increasing

Byproduct

gold

Any gold

bearing base

metal deposit

Native Au,

other gold

minerals

Base metal

minerals

Cu, Pb,

Zn, As,

Sb, Bi

Au,

base

metals

5-10

Gold in

seawater

0.0011-0.05 ppb

in solution

Gold Extraction Methods

Although new process are being proposed on a regular basis, there have in fact been no dramatic

changes in the metallurgical techniques for gold extraction since the introduction of the cyanide process

(cyanide leaching or cyanidation) by McArthur and Forrester in 1887 [ 2 ]. A basic flowchart for the

recovery of gold from its ore is provided in Figure 1 .

Figure 1 Basic flowchart for gold recovery.

The major categories of commercially viable recovery processes include the following [ 4 , 5 ]:

Amalgamation (with mercury)

Gravity Concentration (using jogs, tables, spirals, Reichert cone, moving belt separator, etc.)

Flotation (as free particles or contained in base metal sulfide concentrates)

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