Gold

Typical wastewaters

• gold cyanidation (leach solution and tailings) the dicyanoaurate anion Au(CN)₂⁻ in alkaline cyanide solution (300 to 500 mg/L NaCN, pH 10.5) with free and weak acid dissociable cyanide in excess; gold is recovered on carbon before the pulp reaches the pond the effluent problem is the cyanide, not the gold: leachate 120 to 450 mg/L free cyanide before destruction, 0.06 to 1.94 mg/L after
• tailings pond discharge (after cyanide destruction) residual weak acid dissociable and total cyanide, 0 to 2 mg/L, with negligible gold; iron cyanides survive as total cyanide Kittila and Ovacik report no discharge, all water pumped back to the process

1 · Identity

Symbol, number

Au, 79

Oxidation states in water

+1 as the dicyanoaurate anion Au(CN)₂⁻, the only form in which gold is carried in process water; +3 as chloro complexes in aqua regia and chloride leaching; 0 as native metal, insoluble in water (element entry, MWEI BREF).

Note

The element entry covers the metal and its chemistry. The water chapter is the cyanide leach circuit and its effluent.

2 · Occurrence in water

Natural sources

Negligible; native gold in placers and veins; seawater gold is at the nanogram per litre level (element entry).

Anthropogenic sources

Cyanidation of gold and silver ores: leach solutions at 300 to 500 mg/L NaCN (a minimum of about 350 mg/L), heap leach and agitated pulp leach, carbon in pulp recovery; artisanal mercury amalgamation in the ledger.

3 · Speciation

Gold is a noble metal insoluble in water; a complexing agent such as cyanide, which stabilises the gold species in solution, and an oxidant, such as oxygen, are required to dissolve it (MWEI BREF). In alkaline cyanide solution (pH above 10.5 so that HCN does not volatilise) gold is Au(CN)₂⁻, a very stable anion recovered on activated carbon or by zinc precipitation. Chloride, bromide, thiourea and thiosulfate form less stable complexes and need more aggressive conditions.

Solubility

The metal is insoluble; sodium dicyanoaurate is soluble.

Hydrolysis

not relevant

Complexation

Cyanide above all; thiosulfate, thiourea, chloride and bromide as alternatives (MWEI).

Precipitates

Gold cemented on zinc (Merrill Crowe) and eluted from carbon; not water treatment solids.

4 · Role in treatment

5 · Removal and control

6 · Analytics

Sampling pitfalls

Cyanide samples must be preserved with sodium hydroxide to pH above 12 and kept dark and cold; sulfide and oxidants interfere and must be removed.

7 · Regulatory limits

Limits change, and many are set locally. Treat these as the published values to start from, not as your compliance target: check the standard in force at your site and the numbers written into your own permit.

8 · Health and environmental effects

Toxicity

Metallic gold is inert; the toxicity of gold mine effluent is cyanide and the co-leached metals, not gold.

Bioaccumulation

not relevant

Ecotoxicity

not relevant

Flags

• The Elsner equation is written here; the MWEI BREF describes the reaction in words.
• Table 4.27 figures are three European sites reported in the 2009 MTWR BREF and re-quoted in 2018.
• Cyanide destruction chemistries (sulfur dioxide and air, peroxide, alkaline chlorination) were not read this session and are not named as sources.

Gaps

• No source read gives dissolved gold in any water; the seawater figure is the book entry's.
• The cyanide destruction reactions and their reagent doses are not sourced.
• The International Cyanide Management Code limits were not read.
• Thiosulfate leaching effluents and artisanal mercury amalgamation are in the ledger, not here.

Sources

Identity

Name and symbol

Gold, Au

Atomic number

79 protons

Position

group 11 · period 6 · d-block · transition metal

CAS number

7440-57-5

Atomic structure

Atomic mass

196.966 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹ 4f¹⁴ 5d¹⁰
[Xe] 6s¹⁴f¹⁴⁵d¹⁰

Electrons per shell

2, 8, 18, 32, 18, 1

Valence electrons

11 ns and (n-1)d

Physical properties

State at room temperature

Solid

Melting point

1,337.33 K (1,064.18 °C)

Boiling point

3,129 K (2,855.85 °C)

Density

19.282 g/cm3

Appearance

Metallic yellow

Thermal conductivity

318 W/(m·K)

Electrical resistivity

22.14 nΩ·m at 20 °C

Electrical conductivity

45.17 MS/m

Crystal structure

face centered cubic

Molar heat capacity

25.418 J/(mol·K)

Chemical properties

Oxidation states

+3, +1

Electronegativity

2.54 (Pauling Scale)

Ionisation energy

9.226 eV
1st 890.1, 2nd 1,980 kJ/mol

Electron affinity

2.309 eV

Atomic radius

empirical 136, covalent 136, van der Waals 166 pm

Ionic radius

Au⁺ 137; Au³⁺ 85; Au⁵⁺ 57 pm

Reactivity

The most noble of the metals: gold is unaffected by air, water, sulfur at ordinary temperatures and every single acid, dissolving only in aqua regia or in oxygenated cyanide solution; its compounds are mostly linear Au(I) and square planar Au(III), and its relativistically contracted 6s shell is why it is so unreactive.

with water

Does not react with water.

with oxygen, air

Does not react with oxygen at any temperature and resists ozone up to 100 C, which is why gold neither tarnishes nor corrodes.

with acids

Unaffected by nitric, sulfuric or hydrochloric acid alone; aqua regia dissolves it to tetrachloroauric acid: $\ce{Au + HNO3 + 4 HCl -> HAuCl4 + NO + 2 H2O}$, and aerated cyanide solution dissolves it as the dicyanoaurate used in mining.

with halogens

Fluorine attacks it at dull red heat: $\ce{2 Au + 3 F2 -> 2 AuF3}$, powdered gold reacts with chlorine at $\ce{180 C: 2 Au + 3 Cl2 -> 2 AuCl3}$, bromine at 140 C gives AuBr3 and AuBr, and iodine very slowly gives AuI.

Typical compounds

• HAuCl₄ chloroauric acid from aqua regia, the common soluble gold reagent
• AuCl₃ gold(III) chloride red Au2Cl6 dimer, square planar Au(III)
• AuCl gold(I) chloride zigzag polymeric chains with linear gold
• NaAu(CN)₂ sodium dicyanoaurate the soluble form of gold in cyanide leaching
• Na₃Au(S₂O₃)₂ gold sodium thiosulfate gold(I) drug once given for arthritis

Occurrence, production and use

Crustal abundance

4×10-3 milligrams per kilogram

Oceanic abundance

4×10-6 milligrams per liter

Occurrence and sources

It occurs in veins and alluvial deposits, and is often separated from rocks and other minerals by mining and panning operations. About two thirds of the world's gold output comes from South Africa, and about two thirds of the total U.S. production comes from South Dakota and Nevada. The metal is recovered from its ores by cyaniding, amalgamating, and smelting processes. Refining is also frequently done by electrolysis. Gold occurs in sea water to the extent of 0.1 to 2 mg/ton, depending on the location where the sample is taken. As yet, no method has been found for recovering gold from sea water profitably.

• native gold in lode veins and alluvial placers; electrum (gold-silver alloy) China, Russia, Australia, Canada, United States (Nevada, Alaska), Uzbekistan, Kazakhstan, Mexico, Ghana, Indonesia, Peru, South Africa
• by-product gold in porphyry copper ores about 7 percent of US gold; nearly a quarter of undiscovered US gold resources
• gold tellurides (calaverite, sylvanite) Transylvania and other telluride districts
• crustal and oceanic abundance about 0.0013 ppm crust (BGS via RSC); about 4 g per million tonnes of seawater (RSC); 0.004 mg/kg crust and 0.000004 mg/L seawater (PubChem)

Extraction, production

Lode and placer mining, then leaching and refining to dore and bullion

cyanide leaching and heap leaching are the ledger mining sectors for gold and silver extraction; no source in this entry states the cyanidation stoichiometry

Mercury amalgamation (artisanal and small-scale mining)

mercury dissolves alluvial gold and is distilled off to recover it, a practice used since antiquity and still the largest anthropogenic mercury source; no stoichiometry, the amalgam is an alloy

Recycling of new and old scrap

about 90 t in the United States in 2024

Uses

Gold is the most malleable and ductile of all known metals. A single ounce of gold can be beaten into a sheet measuring roughly 5 meters on a side. Thin sheets of gold, known as gold leaf, are primarily used in arts and crafts for gilding. One sheet of gold leaf can be as thin as 0.000127 millimeters, or about 400 times thinner than a human hair.

Pure gold is soft and is usually alloyed with other metals, such as silver, copper, platinum or palladium, to increase its strength. Gold alloys are used to make jewelry, decorative items, dental fillings and coins. The amount of gold in an alloy is measured with a unit called a karat. One karat is equal to one part in twenty-four, so an 18 karat gold ring contains 18 parts pure gold and 6 parts alloy material.

Gold is a good conductor of heat and electricity and does not tarnish when it is exposed to the air, so it can be used to make electrical connectors and printed circuit boards. Gold is also a good reflector of infrared radiation and can be used to help shield spacecraft and skyscrapers from the sun's heat. Gold coated mirrors can be used to make telescopes that are sensitive to infrared light.

A radioactive isotope of gold, gold-198, is used for treating cancer. Gold sodium thiosulfate (AuNa3O6S4) is used as a treatment for arthritis. Chlorauric acid (HAuCl4) is used to preserve photographs by replacing the silver atoms present in an image.

It is used in coinage and is a standard for monetary systems in many countries. It is also extensively used for jewelry, decoration, dental work, and for plating. It is used for coating certain space satellites, as it is a good reflector of infrared and is inert.

• Jewellery, investment and coinage: jewellery (carat alloys), bullion bars, central bank reserves, coins and medals, gold leaf global 2024: jewellery 45 percent, central banks 21 percent, bars 19 percent, coins 7 percent (usgs-mcs2025-gold)
• Electronics: electroplated connectors and contacts; bonding wires in chips; gold-clad base metals electrical and electronics 6 percent of global consumption in 2024 (usgs-mcs2025-gold)
• Chemicals: gold nanoparticle catalysts, including vinyl acetate manufacture
• Medicine: dental alloys; gold compounds for arthritis; artificial joints
• Mining: lode and placer gold mining, cyanide and heap leaching, tailings facilities; mercury amalgamation in artisanal mining

Safety, toxicity

not in sources

Discovery and name

Discovered by

In the Middle East

Discovered

before 6000 BCE

First isolated

not in sources

Named by

not in sources

Origin of the name

from a Proto-Indo-European root meaning 'yellow'

It is estimated that all the gold in the world, so far refined, could be placed in a single cube 60 ft. on a side. Of all the elements, gold in its pure state is undoubtedly the most beautiful. It is metallic, having a yellow color when in a mass, but when finely divided it may be black, ruby, or purple. The Purple of Cassius is a delicate test for auric gold. It is the most malleable and ductile metal; 1 oz. of gold can be beaten out to 300 ft2. It is a soft metal and is usually alloyed to give it more strength. It is a good conductor of heat and electricity, and is unaffected by air and most reagents.