Handbook of Extractive Metallurgy

Edited by Fatbi Habasbi

Volume III: Precious MetalsRefractory MetalsScattered MetalsRadioactive MetalsRare Earth Metals

~WILEY-VCHWeinheim . Chichester New York Toronto Brisbane Singapore

Professor Fathi HabashiUniversite LavalDepartement de Mines et de MetallurgieQuebec G1K 7P4Canada

This book was carefully produced. Nevertheles, the editor, the autors and publisher do not warrant theinformation contained therein to be free of errors. Readers are advised to keep in mind that statements,data, illustrations, procedural details or other items may inadvertently be inaccurate.

Editorial Directors: Karin Sora, lise BedrichProduction Manager: Peter J. BielCover lilustration: Michel Meyer/mrnad

Library of Congress Card No. applied forA CIP catalogue record for this book is available from the British Library

Die Deutsche Bibliothek - CIP-EinheitsaufnahmeHandbook of extractive metallurgy I ed. by Fathi Habashi.-Weinheim ; New York; Chichester; Brisbane; Singapore; Toronto:WILEY-VCH ISBN 3-527-28792-2Vol. 1. The metal industry, ferrous metals. - 1997Vol. 2. Primary metals, secondary metals, light metals. -1997Vol. 3. Precious metals, refractory metals, scattered metals, radioactive metals, rare earth metals. - 1997Vol. 4. Ferroalloy metals, alkali metals, alkaline earth metals; Naroe index; Subject index. -1997

VCH Verlagsgesellschaft mbH - A Wiley company,D-69451 Weinheim, Federal Republic of Germany, 1997Printed on acid-free and low-chlorine paperAll rights reserved (including those of translation into other languages). No part of this book may bereproduced in any form - by photoprinting, microfilm, or any other means - nor transmitted or translated intoa machine language without written permission from the publishers. Registered naroes, trademarks, etc. used inthis book, even when not specifically marked as such, are not to be considered unprotected by law.Composition: Jean Fran~oisMorin, Quebec, CanadaPrinting: Strauss Offsetdruck GmbH,D-69509 MorlenbachBookbinding: Wilhelm Oswald & Co., D-67433 NeustadtlWeinstraBePrinted in the Federal RepUblic of Germany

Preface

Extractive metallurgy is that branch of met-allurgy that deals with ores as raw material andmetals as finished products. It is' an ancient artthat has been transformed into a modern sci-ence as a result' of developments in cheinistryand chemical engineering. The present volumeis a collective work of a number of authors inwhich metals, their history, properties, extrac-tion technology, and most important inorganiccompounds and toxicology are systematicallydescribed.

Metals are neither arranged by alphabeticalorder as in an encyclopedia, nor according tothe Periodic Table as in chemistry textbooks.The system used here is according to an eco-nomic classification which reflects mainly theuses, the occurrence, and the economic value ofmetals. First, the ferrous metals, i.e., the pro-duction of iron, steel, and ferroalloys are out-lined. Then, nonferrous metals are subdividedinto primary, secondary, light, precious, refrac-tory, scattered, radioactive, rare earths, ferroai-loy metals, the alkali, and the alkaline earthmetals.

Although the general tendency today inteaching extractive metallurgy is based on toefundamental aspects rather than on a system-atic description of metal extraction processes,it has been found by experience that the twoapproaches are complementary. The studentmust have a basic knowledge of metal extrac-tion processes: hydro-, pyro-, and electromet-allurgy, and at the same time he must have athis disposal a description of how a partiCUlarmetal is extracted industrially from differentraw materials and know what are its importantcompounds. It is for this reason, that thisHmldbook has been conceived.

The Handbook is the first of its type for ex-tractive metallurgy. Chemical engineers havealready had their Perry's Chemical Engineers'Handbook for over fifty years, and physicalmetallurgists have an impressive l8-volumeASM Metals Handbook. It is hoped that the

present four volumes will frll the gap for mod-ern extractive metallurgy.

The Handbook is an updated collection ofmore than a hundred entries in Ullmann sEn-cyclopedia ofIndustrial Chemist1y written byover 200 specialists. Some articles were writ-ten specifically for the Handbook. Some prob-lems are certainly faced when preparing sucha vast amount of material. The following maybe mentioned: Although arsenic, antimony, bismuth, bo-

ron, germanium, silicon, selenium, and tel-lurium are metalloids because they havecovalent and not metallic bonds, they are in-cluded here because most of them are pro-duced in metallurgical plants, either in theelemental form or as ferroalloys.

Each chapter contains the articles on themetal in question and its most important inor-ganic compounds. However, there are certaincompounds that are conveniently describedtogether and not under the metals in questionfor a variety of reasons. These are: the hy-drides, carbides, nitrides, cyano compounds,peroxo compounds, nitrates, nitrites, silicates,fluorine compounds, bromides, iodides,sulfites, thiosulfates, dithionites, and phos-phates. These are collected together in a spe-cial supplement entitled Special Topics, underpreparation.

Because oflimitation of space, it was not pos-sible to include the alloys of metals in thepresent work. Another supplement entitledAlloys is under preparation.

Since the largest amount of coke is con-sumed in iron production as compared toother metals, the articles "Coal" and "CoalPyrolysis" are included in the chapter deal-ing with iron.I am grateful to the editors at VCH Verlags-

gesellschaft for their excellent cooperation, inparticular Mrs. Karin Sora who followed theproject since its conception in 1994, and to

Part Four

PartThree

vi

Jean-Franyois Morin at Laval University forhis expertise in word processing.

The present work should be useful as a refer-ence work for the practising engineers and thestudents of metallurgy, chemistry, chemical en-gineering, geology, mining, and mineral benefi-ciation. E>..iractive metallurgy and the chemicalindustry are closely related; this Handbook will

Handbook ofExtractive Metallurgy

therefore be useful to industrial chemists as well.It can also be useful to engineers and scientistsfrom other disciplines, but it is an essential aidfor the extractive metallurgist.

Fathi Habashi

Table ofContents

volume I

Part One The MetaJ. Industry1 The 'Economic Classifica-

tion of Metals .. ; . , .' 12 Metal Production 153 Recycling of Metals 214 By-Product Metals 23

Part Two Ferrous Metals5 Iron 296 Steel 2697 Ferroalloys 403

Volume II

Primary Metals8 Copper 4919 Lead 581

10 Zinc 64111 Tin 68312 Nickel 715

Secondary Metals13 Arsenic 79514 Antimony 82315 Bismuth 84516 Cadmium 86917 Mercury 89118 Cobalt 923

Part Five Light Metals19 Beryllium, 95520 Magnesium 98121 Aluminum 103922 Titanium 1129

Volume III

Part Six Precious Metals23 Gold 118324 Silver 121525 Platinum Group

Metals . . . . . . . . . ... 1269

PartSeven

Part Eight

Part Nine

Part Ten

PartEleven

PartTwelve

Refractory Metals26 Tungsten 132927 Molybdenum 136128 Niobium 140329 Tantalum ,;' .. 141730 Zirconium 143131 Hafnium 145932 Vanadium 147133 Rhenium 1491

Scattered Metals34 Germanium 150535 Gallium 152336 Indium 153137 Thallium 154338 Selenium 155739 Tellurium I57l

Radioactive Metals40 GeneraL 158541 Uranium 159942 Thorium 164943 Plutonium 1685

Rare Earth Metals44 GeneraL 169545 Cerium 1743

Volume IV

Ferroalloy Metals46 Chromium 176147 Manganese......... 181348 Silicon 186149 Boron 1985

Alkali Metals50 Lithium 202951 Sodium 205352 Potassium , 214153 Rubidium 221154 Cesium 2215

viii

PartThirteen

55 Alkali SulfurCompounds 2221

Alkaline Earth Metals56 Calcium 224957 Strontium 232958 Barium 2337

Handbook ofExtractive M etalil/rgy

Part Six

Precious Metals

Au~ors 2355

Name Index 2375

Subject Index 2379

H 'He

. Li Be B C N 0 F Ne

Na Mg AI Si P S Cl AI

K Ca Sc Ii V Cr Mn Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Cd In Sn Sb Ie I Xe

Cs Ba Lat ill Ia W Re Hg II Pb Bi Po At RD.

Fr Ra Act

23 Gold

HEIlMANN RENNER (WHOLE CHAPTER EXCEPT 23.4.3); MARK W. JOHNS ( 23.4.3)23.1 History 118323.2 Properties : 1186

23.2.1 Physical ' 118623.2.2 Chemical ' ' .. 1186

23,3 Occurrence 118823.3.1 Abundance 1188

.23.3.2 GoldDeposits 118823.3.3 Gold Reserves and Resources 1189

23.4 Production : 118923.4.1 Ore Treatment 118923.4.2 Cyanidation 119023.4.3 Recovery of Gold with Carbon 119023.4.3.1 Adsorption ofGold byCarbon 119123.4.3.2 Carbon-in-PlIlpProcess 119223.4.3.3 Carbon-in-LeachProcess 1194

23.5 Gold Refining 119423.5.1 Chemical 119423.5.2 Electrolytic 1196

23.6 Recovery of Gold from SecondaryMaterials 119723.6.1 Recovery from Gold Alloys 119723.6.2 Recovery from Sweeps 119723.6.3 .Recovery from Surface-Coated

Materials 119823.7 Compounds 1199

23.7.1 Potassium Dicyanoaurate(l) 1199'23.7.2 Tetrachloroauric(III) Acid 1201

23.1 HistoryGold is the first element that humans recog-

nized as a metal. Towards the end of the Mid-dle Stone Age and the onset of the NeolithicAge (ca. 8000 B.C.), the world's climatechanged greatly. Large areas became arid, ne-cessitating the establishment of permanentsettlements in river valleys such as the Euph~rates, the Tigris, and the Nile. The earliest ar-cheological fmds that can be reliably datedwere made in predynastic Egypt (ca. 4000B.C.) and Mesopotamia; which the Sumerianssettled in 3000 B.C. Outstanding fmds (ca.3000 B.c.) were made close to modem Varna,on the Bulgarian shores ofthe Black Sea. Goldwas first mentioned in literature in the Indian

23.7.3 Sodium Disulfitoaurate(l) 120123.7.4 Miscellaneous Gold Compounds .. 1202

23.8 Alloys 120223.9 Quality Specificatioll.sand ,"

Analysis 120223.9.1 Quality Specifications 120223.92 Sampling .................... 120323.9.3 Quantitative Analysis 120323.9.4 Purity Analysis 120323.9.5 Trace Analysis 1204

23.10 Uses of Gold and Gold Alloys 120423.10.1 Coins, Medals, Bars 120423.10.2 Jewelry 120423.10.3 Electronics and Electrical

Engineering 120723.10.4 Solders 120723.10.5 Pen Nibs '" 120723.10.6 Chemical Technology \.120823.10.7 Dental Materials , 120823.10.8 Coatings 120823.10.9 Gold Leaf 120923.10.10 Catalysts 1209

23.11 Economic Aspects 120923.12 Toxicology and Occupational

Health 121123.13 References 1211

Vedanta (before 1000 B.C.), the writings ofHerodotus (484--425 B.C.), and the Old Testa-ment (1000 B.c.).

Egypt was the principal gold country in thepre-Christian era and maintained that statusuntil ca. 1500 B.C. Gold production reachedits peak about 1300 B.C. when the first legalfoundations for the production of gold werelaid, awarding the Pharaoh the absolute mo-nopoly. In ca. 2700 B.C. gold rings were intro-duced as means of payment; the first goldcoins appeared around 600 B.C.

The origin of the gold used by the Egyp-tians is unclear. Substantial portions appear tohave come from Nubia in Upper Egypt (nub ==gold), but considerable quantities were proba-

1184

bly imported following the frequently men-tioned expeditions to "Punt".

The special status enjoyed by gold in Egyptalso exerted an influence in neighbouringcountries. The country of Ophir mentioned bySolomon as the origin ofhis gold may be iden-tical with Punt, but India may also have beenthe supplier. The Egyptian gold trade ex-panded particularly under the impetus of theseafaring Phrenicians and Greeks.

Although Egypt was the principal goldcountry until about I B.C., gold was alsofound and utilized in other regions includingIndia, Ireland, Bohemia, the CarpathianMountains, Gaul, on the Iberian peninsula,and in the Caucasus.

Even in ancient times the ownership of gold. shifted from one ruler to the other throughconquests and the collection of tributes. Alex-ander the Great obtained possession of Indiangold as well as considerable portions of thePharaohs' treasure. The Romans had little ofthe metal in their own regions but their mili-tary expeditions netted them major amounts inthe form of booty; they also exploited the min-eral wealth of the countries they had con-quered, especially Spain, where up to 40 000slaves were employed in mining. The state'saccumulation of gold bars and coins was im-mense. Later, however, more and more goldwas used in luxury goods and towards the endof the Roman empire a gold shortage was ex-perienced.

The advent of Christianity in Europe in theMiddle Ages reduced the general striving forgold. Moreover, until the beginning of theMiddle Ages no dominant political power ex-isted for organizing large-scale gold produc-tion. In Europe, only the deposits in theSudeten mountains, the Carpathians, and theAlps were of any significance. Outside Eu-rope, gold was produced in India, Japan, andSiberia.

Following the discovery of America at theend of the fifteenth century, the Spaniardstransferred considerable amounts of gold fromthe New World to Europe. Although the con-quistadors found a highly developed miningindustry in Central America, their efforts to in-

Handbook ofExtractiveMetallllrgy

crease gold production were largely unsuc-cessful; most of the finds consisted of silver. Itwas not until the discovery of deposits in Bra-zil that there was a noticeable increase inworld gold production. These deposits wereexploited from 1725 to about 1800.

Since about 1750 gold has been mined on amajor scale on the eastern slopes of the Uralmountains. In 1840, alluvial gold was discov-ered in Siberia. The Russian deposits were ex-ploited by the Czars and the land owners, whohad to pay their taxes in gold. Russia producedabout one fifth of the world's gold produc~on,a proportion maintained until the present day.

The discovery of gold in California in 1848increased gold production greatly. The speciallaws issued in the Western parts of the UnitedStates allowed private mining thanks to theright to stake claims. This type of workingcontinued when gold deposits were found inEastern Australia (1851), Nevada (1859), Col-orado (1875), Alaska (1886), New Zealandand Western Australia (1892), and WesternCanada (1896). However, these deposits soonlost much of their importance.

The strongest impetus was given to goldproduction through the discovery of the gold-fields of the Witwatersrand in South Africa in1885. This extremely rich deposit appeared toguarantee steady exploitation far into the fu-ture. South African gold soon occupied a com-manding position in the world market.Production grew continuously except for ashort interruption by the Boer War (1899-1902). In the 1970s, gold production largelystabilized in South Africa and in the rest of theworld. In South Africa, more than 300 000people are now employed in the production ofgold.

The discovery of large deposits of gold inBrazil in the 1970s stimulated prospecting ac-tivities. New production centers have been es-tablished in the Sierra Pelada of Brazil,Canada, Australia, Venezuela, and NewGuinea (Ok Tedi), causing a pronounced shiftin the geographic distribution of world goldproduction.

Gold mining in Ghana (Gold Coast) onlybegan to play a role, if a modest one, in the

Gold

twentieth century, although the deposits werealready known in the Middle Ages. Gold pro-duction in Zimbabwe and the neighboringeastern part of the South African bush veldform a moderate but not insignificant part inthe overall production of southern Africa.

Total gold production in antiquity can onlybe assessed approximately. Up to the fall ofthe Roman Empire, production ,may. haveamounted to 10 000 t.

A total production figure of 2000--3000 thas been quoted for the Middle Ages. By thetime of the discovery of America, annualworld production had reached about 5 t.

An annual production of ca. lOt wasreached in ca. 1700, rising to ca. 15 t in 1800and to 40 t by 1848, the year the Californiandeposits were discovered. As early as 1852,more than 200 tJa were mined, but productionsubsequently decreased until 1890. Thereafter,the increased output of the South Africanmines raised annual world production to 500 tin 1904, 700 tin 1907, and 1000 t in 1936. Atpresent that figure has reached 1700 t, ofwhich Eastern-bloc countries contribute ca.300 t.

The total world production of gold to dateexceeds 105 t. More than a third of the present-day gold inventory is held by the central banksof the Western industrialized nations as cur-rency backing. An even larger proportion is inprivate hands, much of it in the form of jew-elry. Krugerrands alone account for 2000 t,and smaller quantities are circulating in indus-try.

The shares of individual states in the over-all production until now are divided amongSouth Africa (40%), the United States (15%),the ancient empires (ca. 10%), the former So-viet Union (ca. 10%), Australia (ca. 10%),and Canada (ca. 5%).

In antiquity, gold grains obtained by wash-ing river sands were cold-worked into the de-sired objects. From ca. 3900 B.C., alluvialgold could be fused into larger lumps. The an-cient Egyptians were the first to quarry gold-bearing rocks. Comminution of the rocks andwashing were often preceded by heat treat-ment.

1185

Analyses of archeological finds show that,in Egypt, the separation of gold from silverand copper was feasible, possibly as early as2000 B.C. Silver was removed by annealingwith common salt to give silver chloride [8].The naturally occurring gold-silver alloy,electrum magicum, was separated into its con-stituents. The slagging of copper by addinglead followed by cupellation was also known.

In Spain, the Romans developed the tech-nique of flush mining, by which vast massesof rock were dropped from a height, commi-nuted, and moved by currents of water. Amal-gamation presumably originated at that time,although it was first mentioned in the litera-ture in the II th century A.D.

In the Middle Ages, fusion with lead andcupellation were developed further. Water-powered crushing machines were also intro-duced for pulverizing ore, and miners leamedto process arsenic-containing gold ores byroasting.

The alchemists endeavored to manufdcturegold by transmutation of base metals. It wasnot until the end of the 18th century that theentire concept was finally rejected as false.However, these endeavors led to a better un-derstanding of chemical processes and to thebirth oithe true natural sciences.

The 17th century saw the discovery of sep-aration by inquartation, i.e., the separation ofgold and silver by nitric acid, and of affma-tion, i.e., separation by sulfuric acid. With ad-vancing industrialization in the 19th centurynew methods replaced the old, but some of thelatter have retained some importance to thisday. Production of gold as a by-product ofother metallurgical processes, (e.g., the refill-ing of copper, zinc, and lead), played an in-creasingly important role in Germany(RammelsberglHarz). In 1863, Plattner'smethod of chlorination was introduced in theUnited States and shortly afterwards in Aus-tralia. In 1867, MILLER succeeded in refininggold with chlorine: Refining by electrolysisaccording to WOHLWILL was introduced in1878 and is still used for all fille gold of 9995and 9999 purity. Since 1888, cyanide leachinghas permitted economical beneficiation of the

1186 Handbook ofExtractiveMetallurgy Gold 1187

23.2 Properties [1,20,24-31]The distinction between noble and base

metals is in many ways arbitrary, and is gener-ally determined by practical considerationsand tradition. Gold is the classic noble metaland complies with all the criteria for this groupof elements: resistance to air, humidity, and tonormal wear. Gold is also remarkable amongthe metals in that it occurs in nature almost ex-clusively in its elementary state.

23.2.1 PhysicalGold, atomic number 79, atomic mass

196.96654, has only one naturally occurringisotope, 197Au. Its most important radioiso-tope, which is used in medicine, is 195Au; itemits E and yrays and has a half-life of 183 d.The electronic configuration ofgold is [Xe]4j45d106s1 Its atomic radius is 0.1439 nm.The ionic radius for coordination number 6 is0.1379 nm for Au+, and 0.085 fill for Au3+.

Some physical properties of gold are as fOl-lows:

Witwatersrand ores, which were less amena-ble to other methods due to the extremely fmedistribution of the gold. Since 1970, conven-tional cyanide leaching has been supersededby the carbon-in-pulp process, which dis-penses with fIltration of the leached rock pow-der. More recently, the ecological problemscaused by cyanide leaching have been over-come by treating the cyanide in the wastewa-ter with hydrogen peroxide.

The recent expansion of gold production isalso due to the mechanization of ore transpor-tation and beneficiation. In South America,however, manual mining of ores is once againbeing resorted to. This politically motivatedmeasure, taken with a view to creating jobs,made employment possible for 500 000 peo-ple.

Solvent extraction is being investigated as anew method for the rapid and effective refin-ing offine gold.

100

.J::.

'!:j

~.

"~:::>0 10::la

0.1 0 20 40 60 BO 100 120Temperature, 0(_

5000

1000

Figure 23.1: Rate of dissolution offme gold sheet metalin various oxidizing agents: a) Aqua regia, 6 mol/L; b)HCi, 6 mollL + Br" 0.2 moUL; c) NaCN, 0.45 mollL + 4-nitrobenzoic acid, 0.1 mollL + NaOH, 0.2 mol/L; d) HCI,6 moUL + Cl! (saturated); e) HCI, 6 moUL + H,D" 0.22moUL; f) NaCN, 1 moUL + air, g) NaCN, 0.45 mol/L +NaOH, 0.2 moUL + air, h) NaCN, 0.006 mol/L +Ca(OH)" 0.04 moUL + air.

sulfur, and hydrogen sulfide under normalconditions.

Sulfuric acid, hydrochloric acid, hydrofluo-ric acid, phosphoric acid, halide-free nitricacid (except in very high concentrations), andpractically all organic acids have no effect ongold, either in concentrated ordilute solutionsand at temperatures upto the boiling point. If ahydrohalic acid is combined with ~n oxjdizingagent, such as nitric acid, a halogen, hydrogenperoxide, or chromic acid, gold will dissolve.Gold can also be dissolved in a combination ofwater and a halogen (the Plattner process) andin selenic acid. Figure 23.1 shows the dissolu-tion rates of gold in the most important indus-trial agents used for its dissolution.

Aqueous solutions of alkali metal hydrox-ides, alkali metal salts of the mineral acids,and alkali metal sulfides do not attack gold.However, gold dissolves in solutions of alkalimetal cyanides in the presence of oxygen (Fig-ure23.l) or other oxidizing agents, such as cy-anogen bromide (the Diehl process), 4-nitrobepzoic acid (Figure 23.1) and 3-ni-trobenzenesulfonic acid, provided they do notrapidly destroy the cyanide. Gold is also 'at-tacked by sodium thiosulfate solutions in thepresence of oxygen, and by alkali metalpolysulfide solutions.

Fused caustic alkalis do not attack gold,provided air and other oxidizing agents are ex-cluded. Gold reacts vigorously with alkalimetal peroxides to form aurates. It is inert tothe alkali metal phosphates and borates, and tothe alkali metal salts of the mineral acids,which can therefore be used as slagging agentsfor removing metallic impurities from gold.

Gold reacts readily with dry chlorine. Themaximum reactivity occurs at 250C, and theminimum at 475 e. Above 475C the reac-tivity increases with increasing temperatureup to and beyond the melting point.

Gold can be recovered from solution byelectrolytic deposition or by chemical reduc-tion. If the tetrachloroaurate(III) complex ispresent, then iron(ll) salts, tin(II) salts, sulfurdioxide, hydrazine, hydrazonium salts, oxalicacid, or ascorbic acid can be used as reducingagents.

23.2.2 Chemical [32]

at 900C 18.32at 1000 C 18.32at 1065 C 17.32at 1200 C 17.12at 1300 C 17.00

Vapor pressure at 1064 C 0.002 Paat 1319 C 0.1at 1616 C 10at 1810 C 100at 2360 C 10 000

Atomic volume at 20C 10.21 cm3/molElectrical resistivity at 0 C 2.06 X 10-6 acmThermal conductivity at DoC 3.14 Wcm-1K-1Specific heat 0.138 Jg-1K-1Enthalpy offusion 12.77 kJ/molEnthalpy ofvaporization 324.4 kJ/molTensile strength 127.5 N/mm'

The melting point of gold has been a fixedpoint on the temperature scale since 1968.

The unit cell of gold is face-centered cubic(type AI), with a lattice constant (ao> of0.40781 nm. Gold as it occurs in nature usu-ally does not have a very crystalline appear-ance. It exhibits threadlike, leaf-shaped, andspherical forms, on which cubic, octahedral,and dodecahedral surfaces can sometimes beseen. When large amounts of molten gold so-lidity, a characteristic pattern of concentricrings appears on the surface.

Pure gold that has not been mechanicallypretreated is very soft. Its hardness on theMohs' scale is 2.5, and its Brinell hardness is18 HB. Gold is the most ductile of all metals.It can be cold drawn to give wires of less than10 ~m diameter, and beaten into gold foil witha thickness of 0.2 ~. Because of its softness,gold can be highly polished; this, togetherwith its noble characteristics and brilliantcolor, gives it its yellow luster. The color ofutility gold is less rich and varies considerablyaccording to its alloy composition. Very thingold foil is translucent; transmitted light ap-pears blue-green.

The physical properties of gold and its al-loys have been thoroughly investigated be-cause of their significance for modemtechnology. For detailed information see [24].

Gold does not react with water, dry or hu-mid air, oxygen (even at high temperature),ozone, nitrogen, hydrogen, fluorine, iodine,

1064.43C2808C19.32 g1cm3

/liPbpDensity at 20C

1188

The very stable dicyanoaurate complex re-quires stronger reducing agents such as zinc.Anion exchangers, which are used for the re-covery of gold from solutions, sometimes re-duce this complex to metallic gold. Similarresults are achieved with activated carbon.

The standard potential of AufAu?r+ is+1.498 V, of AufAu+ +1.68 V, and of Au+JAu3++1.29 V

Gold can be alloyed with many other met-als. In classic metallurgical processes (e.g.;thelead blast furnace process and the reverbera-tory furnace process for copper ore), gold andsilver follow the same route. Zinc, lead, andcopper act as collecting agents for. goldthrough the formation of alloys. Gold exhibitsthe greatest affInity for zinc, followed by lead,and then copper. Zinc is used to remove goldfrom molten lead in the Parkes process. Thereadiness with which gold takes up lead, tellu-rium, selenium, antimony, and bismuth is adisadvantage, particularly with regard to sub-sequent mechanical processing. Gold alloysreadily with mercury at room temperature toform an amalgam. The mercury can be dis-tilled out by heating. This property is utilizedin the amalgamation process, and in fIre gild-mg.

Colloidal gold forms hydrosols of an in-tense red or violet color, which are relativelyresistant even without protective colloids.

23.3 Occurrence [1,2,6,20,34-39]

23.3.1 AbundanceGold is distributed very unevenly in the

Earth's crust, mainly due to enriching pro-cesses that have taken place near the surface.Its average abundance is very low and is esti-mated at ca. 0.005 ppm, although widely vary-ing fIgures are given.

The gold content of ocean water also variesgreatly, depending on the location. Gold con-tents of 0.008-4 mgJm3 (Ppb) have been re-ported.

Handbook ofExtractive Metallurgy

23.3.2 Gold Deposits [7,11,38]

The gold deposits which are most easilyrecognized, and which were the earliest to bediscovered, are enriched veins and deposits ofgold particles. These particles were originallypresent in primary rock that was worn downby weathering Enrichment then followed withpartial consolidation due to flowing water.Such deposits are known as placer or second-my deposits. Typical examples are the rela-tively small gold deposits in the Rhine Valley,California, and Alaska. The abundance ofgoldin placer deposits fluctuates greatly especiallyas the gold particles may be concentrated invery small areas, e.g., a stream bed. Under fa-vorable conditions, placer deposits containingas little as 1 ppm gold may be successfully ex-ploited.

Quartz veins containing gold are oftenfound along the fault plane of rock fractures.As the gold particles have remained at theirplace of origin, these are tenned primaIy de-posits. In general, it can be assumed that thisgold has been fonned hydrothennally, i.e., ithas been through an intennediate stage inaqueous solution. Such deposits are found inEast Africa Australia Canada, and the formerSoviet Union. Their gold content variesgreatly.

The Witwatersrand goldfIelds in South Af-rica (Transvaal and Orange Free State) werealso formed by sedimentation. These are sandand shingle deposits that have been compactedto form massive rock, in which the gold is dis-tributed as very fine particles. This type ofgold deposit is .known as a conglomerate de-posit. The average gold content of the orewhenseparated from the gangue is ca. 12 ppm.Mining reaches a depth of 4000 ill. To dateabout 30 000 t of gold, i.e., about one-third ofthe total world gold production, have comefrom this ore. The waste extracted sand whichis found, for example, around Johannesburg,contains about 1500 t gold (0.5 ppm), mainlycontained in pyrites (FeS2) which is not dis-solved by cyanide treatment. This material canalso be processed economically.

Gold

SulfIdic copper ores may have gold inclu-sions which can become highly concentratedas a result of weathering. In the outer oxida-tion zone, hydrothermal reactions take place,such as

H,O3Fe3+ + Au ~ Au3+ + 3Fe2+while in the underlying cementation zones, thecorresponding' back reaction occurs. Such de-posits are found in Papua New Guinea (OkTedi) and in Brazil. The Ok Tedi deposit con-tains about 4 ppm of gold in the cementationzone.

Copper sulfide ores nonnally contain onlya small proportion of gold 1 ppm); how-ever, they can be a significant gold source.During smelting, gold accompanies silver andcan be separated in the copper anode slimes.Practically all silver ores also contain somegold.

23.3.3 Gold Reserves andResources

The term reserves denotes those resourceswhose existence has been established by pros-pecting and for which mining is economicallyviable. Today, world gold ore reserves are as-sessed at 70 000 t, or more than 40 .times theworld annual primary production In 1970,gold reserves were calculated to be one-fIfthof this amount. At that time, the extensive Bra-zilian deposits had not been discovered.

Of the reserves known today, 40% arefound in South Africa, 35% in Brazil, and15 % in the Soviet Union. These are followedby the United States, Canada, Australia, Zim-babwe, and Ghana, with 1-3% each.

23.4 Production [1,2,20,23,31,33,39-49,115]

23.4.1 Ore TreatmentIn many places, gold is still mined by indi-

viduals and converted on the spot into market-able raw gold using simple manual andmechanical processes, such as panning (grav-

1189

ity separation), milling, and amalgamation.Amalgamation is carried out by allowing aslurry of ground gold-containing ore to flowover mercury-coated copper plates. The re-sulting gold amalgam is periodically removedby scraping. Very fine gold particles cannot berecovered by these methods, and in manycases, especially in Brazil, the use of cyanida-tion to extract the residual gold has been pro-posed. . .",

Where gold is found in river sands coveringa large area, the ore is often mined and pro-cessed in floating dredgers. This type of min-ing is found, for example, in Siberia, and inthe north of the American continent.

In the conglomerate gold deposits. in Wit-watersrand, South Africa, most of the gold oc-curs as very fine particles. This means thatmechanical enrichment and amalgamation areimpossible, and the gold must be converted toa soluble form by reaction with sodium cya-nide. For this purpose, the gold particles arefirst released from the rock material by meansof breakers, wet ball mills, and classifiers. Innewer plants, this milling process takes placeunderground.

Ground gold ore that contains large goldparticles or sulfides may be unsuitable for cya-nidation. Pretreatment, consisting of gravityconcentration, generally followed by amal-gamation, is therefore nearly always neces-sary; this also allows up to 50% of the gold tobe extracted faster than by the cyanidationprocess.I Gravity concentration was fonnerly carriedout using a cord cloth. The cloth was laid on asuitable support, and a water slurry of groundore was passed over it, the grooves in the clothbeing arranged at right angles to the directionof flow. The denser particles were retained inthe grooves while the lighter quartz particlesflowed away with the water.

The cord cloth has now been replaced bycorrugated rubber (thickness 10 mm, groovedepth 3 mID, distance between grooves 6 mm).Modern mechanical equipment has endlessbelts (width 1.5 m, length 7.2 m), tilted at anangle of 11. These advance at a speed of 0.4m/min against the direction of flow of the ore

1190 Handbook ofExtractive Metallurgy Gold 1191

Table 23.1: Factors influencing the adsorption of gold bycarbon.

23.4.3.1 Adsorption of Gold byCarbon

Activated carbon has a porous structure.The following theories have been proposed forthe mechanism by which activated carbonloads gold cyanide: Complete reduction to metal A chemical precipitation mechamsm in-

volving gold, carbon monoxide, and cya-nide

Physical adsorption of sodium dicyanoau-rate(I)

Adsorption of the dicyanoaurate(I) ion Ion-exchange adsorption of the dicyanoau-

rate(I) ion Adsorption of a neutral complex whose na-

ture is pH dependent Electrostatic interaction between the dicy-

anoaurate(I) ion and positively charged sites A physisorption process.. A two-step process in which an ion pair is

adsorbed onto carbon and then reduced to anunidentified species.The last-mentioned theory is now generally

accepted.The adsorption of gold cyanide onto acti-

vated carbon is reversible. Thus, an equilib-rium exists between the gold in solution andthe gold loaded on the carbon. Factors whichaffect the rate of gold adsorption and thosewhich affect the equilibrium loading of goldare listed in Table 23.1.

increase nonedecrease none

none

increasedecreasedecreasedecrease

decrease

on rate

Effect of increasing the factoron equilibriumloading of gold

slight decreaseslight decreaseslight decreaseslight increasedecreasedecrease

pHIonic strengthFree cyanideTemperatureBase metalsCarbon particle sizeMixing intensityPulp density

Factor

CI Materials that are difficult to filter orthicken can be treated successfully.

In the 1940s; a carbon ofhigher activity andgreater abrasion resistance was developedfrom fruit pips and, in 1952, an elution proce-dure involving the use of sodium hydroxideand cyanide (the caustic-eyanide procedure)was developed. In 1960, a plant using carbonwas erected in Canada;and the first major car-bon-in-pulp (CIP) plant to treat ~e fractionfrom which the coarse material has been re-moved (slimes) was built in the United StatesatHomestake in 1973 to treat 2200 tid.

Major developments in CIP continued inSouth Africa, for treatment of the total cya-nided pulp. By 1976, a small pilot plant was inoperation and, by 1978, a plant processing 250tid was on line. The CIP process is now thepreferred method worldwide for the recoveryof gold from cyanided pulp. The only excep-tion appears to be the former Soviet Union,where the resin-in-pulp process is used. TheCIP process is used for the treatment of a vari-ety of feed materials ranging from run-of-mine ore to dump materials and roaster-bedproducts.

The advantages of the CIP process overzinc cementation are:

Capital costs are lower. Operating costs are lower... The ability of carbon to adsorb gold is not

affected by any of the common constituentsof leach liquors.

Carbon is added directly to the cyanidedpulp, and therefore the need for the expen-sive filtration and clarification stage isavoided.

The losses of soluble gold are usually lowerthan in the zinc cementation process.

Ores that contain carbonaceous material canbe processed without loss of gold to the car-bonaceous fraction.

23.4.3 Recovery of Gold withCarbon [50-62]

The first mention of the ability ofcarbon toadsorb precious metals was made in 1847 In1880, it was found that gold can be recoveredfrom chlorinated leach liquors by wood char-coal. McARlHUR and the FOREST brothers dis-covered that cyanide was a good lixiviant forgold in 1890 and, in 1894, charcoal was usedto recover gold from cyanide solutions. Thecharcoal was prepared from wood and did notpossess the high surface area and porosity ofcarbon today. As no elution procedure wasknown, the gold was recovered from the car-bon by smelting. The use of carbon reached ahigh point of efficiency in Australia in 1917when fine carbon was used to recover goldfrom pregnant cyanide solution, but, as thezinc cementation process advanced, so interestin the use of carbon dropped off.

of the more expensive sodium cyanide. Theaddition of calcium oxide ensures that the so-lution remains slightly alkaline. Dissolutiontakes place according to the following reac-tion:4Au + 8NaCN + 2Hp + O2 -7 4Na[Au(CNh] + 4NaOH

The dead powdered mineral is filtered offin large rotary vacuum filters. The filter cakecontains less than a tenth of the original goldcontent of the ore. The filtrate is treated withzinc chips, which are preactivated in lead ace-tate solution, to precipitate the gold:2 Na[Au(CN);) + Zn -7 N~[Zn(CN)41+ 2AuThe raw gold is treated with sulfuric acid to re-move excess zinc, dried, and then roasted inair at 800C to oxidize lead, zinc, and iron. Aflux, usually borax, is added, and the materialis melted down to raw gold, with a gold con-tent of 80-90%.

An ecological problem is caused by thepresence of sodium cyanide in the cake ofdead rock material and in the wastewater.However, when exposed to air and sunlight,the cyanide is converted to nontoxic cyanate,and subsequently carbonate.

23.4.2 Cyanidation [42]The cyanidation process has been used in

South Africa since 1890. In this process, thepowdered mineral slurry, which contains ca.10 ppm gold in the solid matter, is treated withan aerated 0.03 % sodium cyanide solution.Black cyanide (Ca(CN)2 containing carbonand sodium chloride as impurities), a productof American Cyanimid, is often used instead

slurry. The concentrate is sprayed off with wa-ter and sent to the amalgamation plant. Inplace of endless belts, slowly rotating cylin-ders lined with corrugated rubber are some-times used (length 3.6 m, diameter 0.9 m,inclination 3.75).

Concentrates from gravity separation pro-cesses cannot be directly melted down intogold bars, because they contain considerableamounts of iron pyrites and metallic iron.Gold and silver are therefore generally sepa-rated from these components by amalgam-ation. The concentrate, which has a watercontent of about 70%, is filled into a cast irondrum (length 0.9 m, diameter 0.6 m) contain-ing steel balls (diameter 50 mm). The drum isrotated for 12 h, after which the gold particlesare free from all impurities. Mercury is thenadded, and the drum is rotated for a further2 h. The resulting amalgam is separated fromthe other components in a hydrocyclone (di-ameter 200 mm, inclination 20); water and

.excess mercury are removed in a filter press.Remaining mercury is removed by distillation,leaving an impure mass of spongy gold, whichis melted down into gold bars.

A flotation process is often used beforegravity concentration in cases where the goldis closely associated with pyritiferous materi-als.

Roastingof ores in air is a secondary pro-cess which is sometimes used after gravityseparation or flotation. The resulting oxidesare then washed and treated by cyanidation.Gold ores containing sulfidic minerals canalso be treated in a bioleaching process, whichdissolves the sulfides, exposing the gold parti-cles for subsequent cyanidation.

Figure 23.2: Schematic of the carbon-in-pulp circuit: a) Prescreening; b) Final screening; c) Regeneration; d) Elution; e)Electrowinning.

Adsorption circuit

1192

Feed pulp

Certain materials poison activated carbonfor gold adsorption. Calcium carbonate canform in the pores and is detrimental to adsorp-tion, but is removed by acid-washing. Organicmaterials (e.g., machine lubricants, detergents,flotation reagents) also poison carbon to someextent, but are removed during reactivation.Lower adsorPtion efficiencies are attainedwhen a pulp containing calcine, shale, or clayis used, since these finely divided minerals canblock the pores. Copper can decrease effi-ciency of adsorption by competing with goldfor adsorption sites, particularly at low con-centrations of cyanide.

23.4.3.2 Carbon-in-Pulp ProcessOres containing 0.25-100 g/t gold are pro-

cessed in CIP plants at tonnages from lOa-106 t per month. The density of the pulp variesfrom 1.3 to 1.45 g/cm3, depending on its vis-cosity.

A schematic of a CIP plant is shown in Fig-ure 23.2. The cyanided pulp is prescreened toremove coarse material that would otherwisemove with the carbon granules, and might

Handbook ofExtractiveMetallllrgy

Barren pulp

later block the screens in the CIP circuit. Thepulp then flows through a series of six or eightflat-bottomed, cylindrical, agitated tanks. Theresidence time of the pulp in each stage is ca.1 h. Reactivated carbon is added to the laststage, and is moved countercurrent to the flowof pulp. The carbon is h~ld in each stage by in- .terstage screens. The residence time of the car-bon in each stage is 2 d. The carbonconcentration in a CIP circuit is 15-30 g per li-ter of pulp. The barren pulp leaving the circuitis screened to remove fine carbon.

The loaded carbon, which contains 200-20 000 g of gold per ton, is removed periodi-cally from the first stage and eluted with acaustic-cyanide solution. The carbon iswashed with acid to remove calcium carbon-ate, and then reactivated at high temperature ina kiln. The gold in the eluate is generally re-covered by electrowinning.Screening. The total pulp fraction has to beprescreened at a smaller aperture (0.6 mm, 28mesh) than that of the interstage screens. Pre-screening removes coarse material to avoidblocking of the screens further downstream.Two types of screen are used: (1) vibrating

Gold

screens with woven wire or polyurethanemesh, and (2) linear moving-belt screens.

Wood chips constitute a small fraction ofthe incoming pulp, and cause problems furtherdownstream when vibrating screens are used.Another prescreening device, such as adummy tank, is then r~

1194

efficient electrolysis. Its disadvantages in-clude the need for high-quality water, moreexpensive equipment than that used by theZadra process, and higher reagent consump-tion.

Elulion with Organic Solvents. The addi-tion of, for example, a 20% solution ofethanolto a caustic-

1196 Handbook ofExtractive M etallllrgy Gold 1197

~.

iii~

'"EcC>.~o

u...

23.6.2 Recovery from SweepsThe noble-metal processing industry pro-

duces large amounts of waste dust and debris,known as sweeps (e.g., slag, ash, soot, precipi-tation and emulsion residues, and sweepings).The gold content of these materials is mostlyin the range 0.5-10%. Usually they are firstcarefully ground, screened, and, if necessary,dried or burned. The fine nonmetallic fraction

process is the most economical method. Thisalso applies to the treatment of anode slimesfrom the electrolytic refming of silver, provid-ing these have a high gold content and can bemelted.

For small quantities of gold-rich alloys, auseful treatment is dissolution in hot aqua re-gia, or in hydrochloric acid (usually at 20-50C) containing a halogen or hydrogen per-oxide. Silver precipitates as sparingly solublesilver chloride. Relatively pure gold is recov-ered frpm the solution by.selective reduction.If copper and 'lead ions are present, the best re-ducing agents are sulfur dioxide or hydrazine;if platinum-group metals are present, oxalicacid is used.

In the case of gold-copper alloys, whichalso may contain silver and nickel, treatmentwith aerated dilute sulfuric acid allows theseparation of noble metals from base metals.Gold can only be separated from base metalsin this way if the gold content of the alloy isless than ca. 65%, and the material has been

. finely milled or powdered.Two historically important processes for

treating gold-silver alloys are rarely used to-day. The silver may be dissolved with nitricacid, preferably after realloying to a silver-gold ratio of 3: I (inquartation); or hot concen-trated sulfuric acid can be used (affination).Silver-eopper alloys with a low gold contentare not suited to the above process, but can betreated in noble-metal works by the lead cu-pellationprocess. Lead is removed as litharge,PbO, which also takes up copper as CuO, andother metal and metalloid oxides, leaving be-hind dore silver. This is subjected to electro-lytic refming; gold collects in the anode slime.

means that the electrolyte can only be used fora limited period of time. Therefore, the matecrial to be refmed should have a high gold con-tent (generally> 95%) and it is often best touse gold which has already been through theMiller process. The gold quality of the cathodedeposits depends onth.t; condition of the elec-trolyte, and on the amount of anode slime thathas collected in the cell. A purity of 99~9% isconsidered to be a minimum, which is thequality of Russian commercial gold. Today,99,99% can be considered as the norm, ratherthan 99.95%.

The great advantage of the Wohlwill elec-trolysis process is the high purity of the goldproduced. In addition, the by-products, espe-cially platinum-group metals, are also rela-tively easy to isolate. A disadvantage is thefairly long period during which the noblemetal is tied up, leading to considerable fman-ciallosses.

World capacities for Wohlwill electrolysisare very large. In South Africa, approximatelya quarter of all gold from primary production,after it has gone through the Miller process,undergoes fmal purification to fme gold byWohlwill electrolysis. In the former SovietUnion, all gold is treated by electrolysis, as itnormally contains platinum-group metals.Practically all noble-metal parting workswhich recover gold from secondary materialsare dependent on electrolytic refining, againbecause platinum-group metals are normallypresent. Today, almost exclusively fine gold(99.99%) is used, both for industrial purposes,and for investment; trading countries such asSwitzerland therefore have large refining ca-pacities, mainly producing gold of a qualitybetween "good delivery" (99.5%) and 99.9%.

23.6.1 Recovery from Gold AlloysFor the recovery of gold from alloys with a

gold content of more than 30%, the Miller

23.6 Recovery of Gold fromSecondary Materials [1,3,20,23,31,40,41,67-72]

StirrerAnodeCathodeAnodeslime

+

,..:: r:-

Heating mantel

gold of quality ~ 99.9% (in practice usually99.95% and 99.99%).

The Wohlwill process uses an electrolytecontaining 2.5 mollL of hydrochloric acid and2 mollL of tetrachloroauric acid. Electrolysisis carried out with agitation at 65-75 DC. Theraw gold is introduced as cast anode plates.The cathodes, on which the pure gold is de-posited, were for many years made of finegold of thickness 0.25 mrn. These have nowlargely been replaced by sheet titanium cath-odes, from-which the thick layer of fme goldcan be peeled off. In a typical electrolysis cell(Figure 23.4), gold anodes weighing 12 kg andhaving dimensions 280 x 230 x 12 mm areused. Opposite them are conductively con-nected cathode plates, arranged two or threeon a support rail. One cell normally containsfive or six cathode units and four or five an-odes. The maximum cell voltage is 1.5 V andthe maximum anodic current density 1500AJm2 At the anode, the reactionAu + 4C1- --7 [AuC14r + 3e-takes place, and at the cathode the reverse re-action. Anodes and cathodes are normally re-placed every two days. About 10% of theanode gold, especially parts located above theelectrolyte, is remelted to form new anodes.The anode slime is collected in a trough in thebath. In addition to silver chloride, it containsrhodium, iridium, ruthenium, and osmium,which can be recovered. Platinum and palla-dium can be recovered from the electrolyte,which also contains copper, iron, and nickel.

Figure 23.4: Wohlwill electrolysis cell.The accumulation of metallic impurities in

the electrolyte, and of anode slime in the cell,

~.c

'"~s SO

1'00 =0::---..

23.5.2 Electrolytic [23,65]The Wohlwill electrolytic refining process

for gold was developed in 1878 at the Nord-deutsche Affinerie in Hamburg. It is still indis-pensable for the industrial production of fme

0l----l.1o~--~20=----3~O::=:::==40Chlorination time, min-

Figure 23.3: Decrease in the concentration of impuritiesin gold as a function ofchlorination time. * Expressed as apercentage of the initial metal content. Typical startingconcentrations, %: Ag 9.0, Cu lA, Pb 0.35, Fe 0.18, Zn0.06.

Platinum-group metals cannot be separatedby the Miller process, as their chlorides, likethose of gold, do not exist at the reaction tem-perature. However, this is not a disadvantagefor gold from the Witwatersrand deposits, asthis is free from platinum-group metals.

The great economic advantage of the Millerprocess, in addition to low production costs,lies in the fact that the gold leaves the refmeryin a marketable form, thus minimizing the fi-nanciallosses due to tying up of the metal.

Tetrachloroauric(III) acid can be extractedfrom aqueous solution by many organic sol-vents [66]. Solvents which can form metalcomplexes (e.g., ethers and esters) are most ef-fective. Total separation from the usual ac-companying elements cannot, however, beachieved.

With the exception of dibutyl carbitol, nosystems have yet been discovered which canbe applied commercially. Yet a system with ahigh degree of separation efficiency and sim-ple process management, including strippingthe pure gold, could offer considerable advan-tages over currently used methods.

1198 Handbook ofExtractiveMetallurgy Gold 1199

1000800

600

I

23.7.1 PotassiumDicyanoaurate(I)Properties.' Potassium dicyanoaurate(I),

K[Au(CN)~, p 3.452 g/cm3, forms colorlesscrystals which are readily soluble in water andalcohol, but insoluble in acetone and ether.The solubility in water is strongly dependenton the temperature (Figure 23.5). The effect ofpotassium cyanide concentration on the solu-bility of potassium dicyanoaurate is shown inFigure 23.6. The complex is neither air norlight sensitive and is stable in aqueous solu-tion above pH 3.

c'

~ttl'-~COJUCoU

-0

;3 60 0'--..1----'20-----''--4"-0-----'---'-60-----''---8"-0--1.---'100Temperature. 0(_

Figure 23.5: Solubility of potassiiun dicyanoaurate(I) inwater as a function oftemperature.

400

the 6s level differ only slightly in energy.Therefore, ionization of the 6sJ electron toleave a 5s2p 6dJO valence shell, i.e., the reactionAu ~ Au+, is energetically not particularly fa-vorable. Gold(III) compounds with the 5s2p6cfvalence shell exhibit a tendency to fIll the 5dleveL Thus, the electron-acceptor effect isstrong, and the compounds are strong oxidiz-ing agents. The simultaneous participgtion ofboth outer electron shells gives rise to"proper-ties characteristic of the transition metals,such as variable oxidation state, colored com-pounds, and the tendency to form complexes.

1500

23.7 Compounds [1,12,20,32,39,63,73-77]

Almost all gold compounds occur in the ox-idation states 1+ and 3+. The oxidation states2+ and 5+ are also known. Bimetallic com-pounds of gold(II) are usually mixed valencecompounds containing gold(I) and gold(III).

The generally very low stability of binarygold(I) and gold(III) compounds based onionic bonding can be ascribed to the structureof the two outer electron shells. The ten elec-trons of the 5d level and the single electron of

Gold-plated metallic wastes can also be di-rectly deplated by electrolysis, preferably inelectroplating drums used as anodes. The goldis recovered in metallic form from the cathodein a single step. The method is, however, notvery well suited to the recovery of gold frombulk material in anode baskets, because theelectric field is screened ~ff from the materialat the center of-the load, and as a result not alltht;; gold is removed.

Gold can be extracted from solutions with avery low gold content (~O.l gIL) by anion-ex-change resins (e.g., Lewatit M 500) [71] or ac-tivated charcoal [72]. However, elution iscostly and usually incomplete; the gold-loaded carriers are therefore most frequentlyashed, and the gold recovered from the ash.The solutions are not returned to the deplatingprocess because they contain degradationproducts, especially from organic nitro com-pounds, which would interfere with the pro-cess.

The methods described above can also beused to treat other cyanide gold solutions, e.g.,unusable electroplating baths.

Gold-plated materials are occasionallysmelted together with copper ores in largecopper-smelting works. In addition to silver,these ores always contain some gold. A disad-vantage of this method is the long time thegold has to stay in the process. Gold collects inthe anode slimes during electrolytic refiningof the copper and can be recovered from these.All gold concentrates recovered using the pro-cesses described here still require refining.

account for a gold content between a fewtenths percent and a few percent.

The most economic solution is usually toremove only the gold coating, and to send theunderlying base material directly to a recy-cling process. Mechanical pretreatment is of-ten required to expose the gold surface.Depending on the particular combination ofmaterials and the degree and type of finishingemployed, this can be carried out with a shred-der, jaw crusher, or edge runner. Frequently,this is followed by air separation, gravity sepa-ration, or magnetic separation to remove theballast materials. It is normally not advisableto bum off plastic materials because the golddiffuses into the metallic base, and it is thenimpossible to remove it alL For this reason it ispreferable to decompose the plastic materialby pyrolysis.

To remove the gold from the metallic base,the material is usually agitated in alkaline cya-nide solutions (10-20 gIL NaCN). Air is onlyrarely used as an oxidizing agent, because ofthe relatively low rate of dissolution. Instead,water-soluble aromatic nitro compounds(e.g., nitrobenzoic acid or nitrobenzene-sulfonic acid) are normally employed. Potas-sium peroxodisulfate and hydrogen peroxideare also suitable, but they have a greater ten-dency to oxidize the cyanide ion. The basematerials (e.g., iron, nickel, and cobalt) are at-tacked only to a limited extent during the dis-solution process. When the base material iscomposed of copper, or a copper-rich alloy,zinc cyanide or lead salts are usually added asinhibitors to keep them from being attacked.

The gold is recovered from cyanide solu-tion by adding zinc powder, after any excessoxidizing agent has been reduced with hydra-zine or formaldehyde. The gold can also be re-covered by electrolysis, using insolublegraphite or magnetite anodes (100 AJm2 cath-ode, 3-4 V), or platinized titanium anodes.Excess oxidizing agent should also be reducedto obtain optimum current efficiency andspeed of deposition. In addition to the reduc-tive deposition of gold, thermal decomposi-tion of the dicyanoaurate(I) complex is alsopossible.

Surface coatings containing gold comemainly from the electronics industry, plus asmall proportion from the jewelry industryand electrical engineering. The base materialis essentially metallic; the gold coatings areoften only a few micrometers thick, but may

23.6.3 Recovery from Surface-Coated Materials

and the coarse metallic fraction, which canusually be melted, are analyzed separately.

The usual method for treating gold-contain-ing sweeps is to melt the material down, to-gether with silver-containing sweeps, in a leadshaft furnace. Depending on the compositionof the sweeps, calcium carbonate, silicic acid,lead oxide, carbon, and materials containingsulfur are added. The mixture of powderedcomponents is normally briquetted, pelletized,or sintered before being fed into the shaft fur-nace. The metallic phase (alloy) produced inthe furnace contains lead and almost all of thenoble metaL This alloy is then converted todore silver (a few percent Au; < 0.1 % Pb; therest Ag) in a cupel furnace by oxidizing lead tolead(II) oxide. The silicate slag from the shaftfurnace contains a negligible quantity of noblemetal, and can usually be discarded. The slag-ging process may have to be repeated, for ex-ample if the noble-metal content exceeds 300ppm. In the sulfidic phase, which forms be-tween the metallic phase and the slag, almostall the copper collects with the nickel to formcopper matte. The relatively high content ofsilver and valuable nonferrous heavy metals inthe copper matte makes special treatment nec-essary; this is usually carried out at coppersmelting works.

When raw gold is refined, the. resulting res-idues contain more gold than silver. Thesesubstances are treated in a shaft furnace andthen in a refining furnace, together with rawsilver from t4e Miller process. Unlike otherpyrometallurgical recycling methods for silverand gold, a copper matte phase is not formedin the shaft furnace. The Au:Ag ratio is nor-mally 1:4.

1200 Handbook ofExtractive M etallllrgy Gold 1201

Figure 23.6: Dependence of the solubility of potassiumdicyanoaurate(I) in water upon the concentration ofpotas-sium cyanide at 20 DC and at 0 DC.

Preparation [63, 78]. Potassium dicyanoau-rate is prepared from fulminating gold, fromgold(III) hydroxide, and by electrolysis.

From Fulminating Gold. Fulminating goldis precipitated from a solution of tetrachloro-auric(III) acid by addition of excess aqueousammonia; the precise composition of the pre-cipitate is not known. The chloride content ofthe precipitate decreases with increasing ex-cess of ammonia.

r::;"

~~c:l1Juc:8-c"0L:l

200

20 D (

4

oDe2

10 2 4 6 B 10Potassium cyanide concentration, molll_

Fulminating gold must not be allowed todry, since it is highly explosive in this state.The fulminating gold must be thoroughlywashed with deionized water to remove chlo-ride, since chloride ions interfere in electro-plating processes. After washing, theprecipitate is dissolved in a small excess of po-tassium cyanide solution. This is followed byfiltration, concentration, and crystallization.The mother liquor is repeatedly reused in theprocess.

Electro~vsis. When a gold anode is dis-solved in aqueous potassium cyanide, potas-sium dicyanoaurate(I) is formed withhydrogen being liberated at the cathode. Toprevent the dicyanoaurate(I) ion from beingelectrolyzed at the cathode with deposition ofgold, a diaphragm is used. Fluoropolymer-based ion-exchange membranes are used forthis purpose. The mother liquor is reused orworked up following crystallization of the po-tassium dicyanoaurate(I). This can be carriedout under particularly mild conditions by us-ing the continuously-cooled crystallizationprocess. Cyanide and chloride are the majorimpurities. Electrolysis produces potassiumdicyanoaurate(I) with a particularly low chlo-ride content, which depends solely on thequality of the potassium cyanide and of thewater.

From Gold(III) Hydroxide. Instead of ful-minating gold, gold(III) hydroxide may be re-acted with potassium cyanide solution.Precipitation of gold(III) hydroxide from tet-rachloroauric(III) acid by addition of an alkalimetal hydroxide does not always go to com-pletion. Aging of the gold(III) hydroxide con-siderably reduces its rate of dissolution inpotassium cyanide solution.Uses and Economic Aspects. Potassium di-cyanoaurate(I) serves as the gold componentin the baths commonly used for electroplatingand to a lesser extent in electroless plating.

With increasing utilization of gold-platedelectronic components, the demand for goldplating baths, and hence for potassium dicy-anoaurate(I), has grown considerably. In 1987,a little under one tenth of the 1300 t of gold

processed annually in the world was used bythe electronics industry. In the jewelry indus-try, too, that demand has increased as a resultof rolled gold being superSeded by electroplat-ing with hard gold.

23.7.2 Tetrachloroauric(llI) AcidProperties. Tetrachloroauric(III) acid,H[AuC14], crystallizes as a tetrahydrate'in theform of light yellow, deliquescent crystals. Itis readily soluble in water, soluble in alcoholand ether, and is corrosive.

Preparation [63]. Tetrachloroauric acid isprepared by dissolving gold in warm aqua re-gia. In order to remove residual nitric acid, thesolution is concentrated by evaporation withrepeated addition of hydrochloric acid, and theresulting melt is poured into porcelain dishes.Moisture must be excluded while the meltcools, solidifies, and is fmally powdered.Crystallization from the aqueous mother li-.quor produces an extremely hygroscopic ma-terial. Instead of nitric acid, other oxidants(e.g., chlorine) may be added to the hyd~ochloric acid. The high rate of dissolution ofaqua regia is only rarely attained (Figure23.1), but the pollution caused by it is lowerand control of the reaction is simpler, espe-cially under industrial conditions. Care mustbe taken to assure removal of any excess oxi-dants and their reaction products. Tetrachloro-auric(III) acid can also be prepared by anodicdissolution of gold in hydrochloric acid, butthis requires the use of a diaphragm.

Uses [1]. Tetrachloroauric(III) acid is used toprepare other gold compounds. It is also usedto make gold ruby glass, gold purple (purpleof Cassius), and purple colorants for enamel-ing of ceramics.

23.7.3 Sodium Disulfitoaurate(I)Properties. Sodium disulfitoaurate(I),Na3Au(S03h is relatively unstable in thesolid state, and is therefore not isolated ascrystals for industrial use. It is stable in

weakly alkaline solutions above pH 8.5, evenupon heating and when exposed to light.

Preparation. Sodium disulfitoaurate(I) is pre-pared from fulminating gold or from gold(III)hydroxide.

From Fulminating Gold. Fulminating goldis dissolved with stirring in a dilute solution ofsodium hydrogen carbonate; sodium sulfite isthen added. The reaction mixture is ~tirred at70 DC until it clears. The pH must be main-tained above 9. This is achieved by addition ofsodium hydrogen carbonate solution. In orderto remove ammonia, the solution is heated toabout 90 DC with the simultaneous introduc-tion of air. The gold content of the solution iskept at ca. 100 gIL by evaporation or by dilu-tion with deionized water. This concentrationis desirable for electroplating pur'poses. In-stead of sodium sulfite, gaseous sulfur dioxideand a correspondingly higher quantity of so-dium hydroxide solution may be used; the pHis controlled as described above. \

From Gold(IlI) HydroXide. Instead of ful-minating gold, gold(III) hydroxlde may beused. The reaction takes place under approxi-mately the same conditions as with fulminat-ing gold. The disadvantages are the same as inthe preparation of potassium dicyanoaurate(I)from gold(III) hydroxide.Uses. Sodium disulfitoaurate(I) is sometimesused in preference to potassium dicyanoau-rate(I) in electroplating baths. These baths areespecially advantageous for the production ofductile and wear-resistant coatings and inwhite-gold and rose-gold electroplating.Their drawbacks are difficult handling, shorterservice life, and higher price. Their low toxic-ity is counterbalanced by problems in wastewater disposal, e.g., those caused by the ethyl-enediaminetetraacetic acid (EDTA) com-plexes of the alloying elements.

Sodium disulfitoaurate(I) is increasinglybeing replaced by the corresponding ammo-nium salt, which provides special advantageswith Au-Pd-Cu baths, such as the facility toproduce very dense coatings.

1202 Handbook ofExtractive M etallllrgy Gold 1203

23.9 Quality Specificationsand Analysis [20,91,92]

23.9.4 Purity AnalysisPurity tests for commercial gold are today

almost exclusively carried out by physicalmethods, the most important being emissionspectrography, plasma emission spectrogra-

23.9.3 Quantitative AnalysisQuantitative analysis forms the basis for

calculating the gold content of end products,intermediate products, gold ores, and recy-cling materials. The most important methodfor gold, as for silver, is still the centuries-olddocimastic analysis, or docimasy (Greek: test-ing, assay), more commonly known as the fireassay. In this process, noble metals afu takenup by molten lead, while base metals are re-moved by slagging. Two methods are used. Inthe crucible assay, a mixture of lead oxide, areducing agent (e.g., carbon), and a flux ismelted at ca. 1250 e. In the scarification as-say, the sample is subjected to oxidative smelt-ing with grain lead and borax at ca. 1000 e.In each case, a bead of lead containing goldand other noble metals is formed. The lead isthen oxidized at ca. 800C to litharge (FbO)on a magnesia bed (cupellation process). ThePbO melt is absorbed by the porous magn,esiaand a drop of noble metal, which later s~lidifies to a grain, remains behind. If the graincontains platinum-group metals in addition togold, then a chemical or physical analysis fol-lows. Reductive precipitation followed byweighing the gold is the most commonmethod. X-Ray fluorescence analysis of thegrain has the advantage of greater speed andeconomy [94, 95].

In the gold trade, docimasy is the generallyaccepted method for determining gold. An-nual mercantile transactions worth ca. DM 40X 109 are based on the results of docimasticanalyses. Determination of the gold content ofhigh-percentage alloys using physical meth-ods has not been accepted, because it is notsufficiently accurate. The gold content of finegold can be determined far more accuratelyfrom the sum of the impurities than by directdetermination of the noble metal content.

Table 23.2: Marke~blegold qualities..

Designation Gold content, Content ofother~Ia metals, ppm"Good delivery" gold ~ 99.5 Any metals (total)

S; 5000Fine gold ~99.99 Ag S; 100

Cu S; 20others S; 30total S; 100

Fine gold, chemically ~ 99.995 Ag S; 25pure others S; 25

total S; 50Fine gold, high purity ~ 99.999 Ag S; 3

Fe S; 3Hi S; 2AI S; 0.5Cu S; 0.5Ni S; 0.5Pd+Pt S; 5.0total S; 10

23.9.2 Sampling [93]

g and 1 short tori =907.184 kg. Other units are1 dwt = 1 pennyweight = 0.05 oz = 1.555 gand 1 troy grain = 1/24 dwt = 0.0648 g. 1 tola=0.375 oz is an Indian measure for gold.

In general, marketable grades of gold aresubject to the purity standards given in Table23.2.

Accurate determination of gold depends onthe use of correct sampling methods.

Exact procedures have been laid down fortaking samples from ingots of raw gold, orfrom material separated in recycling pro-cesses. The samples may be taken by drilling,cutting, or sawing. As a rule, they. are takenfrom several places in an ingot, which helps toavoid errors arising through possible segrega-tion effects.

Powdered material is usually first screenedto separate the fine from the coarse materiaLThese are then subjected separately to a me-chanical or manual sampling process.

Recycled electronic material usually con-tains only a few percent gold together withplastic and base metal components. Mechani-cal size reduction, e.g., in a shredder, or bycold grinding using liquid nitrogen, often pro-duces material from which samples can betaken.

[u

80-- 18Ct \

60 ~- - 140 ~

100 ";..40 "*"

Whitish

Ag 20 40 60 80Copper, wt%-

Figure 23.7: Gradation of color of gold--silver-

1204 Handbook ofExtractiveM etallllrgy Gold 1205

Table 23.3: Coinage alloys.Coin Country Mintage period Fineness Carat Gross weight, g Gold content

20 mark German Reich 1871-1915 900 21.6 7.964 7.16810 roark 900 21.6 3.982 3.5845 mark 900 . 21.6 1.991 1.7921 ducat German upto 1871 986.1 23.7 3.490 3.441 g

ConfederationKrugerrand' South Mrica since 1967 916.6 22 33.931 lozMaple Leaf' Canada since 1979 999.9 24 31.103 lozAmerican Eagle' ~nited States since 1986 916.6 22 33.931 lozBritannia' Great Britain ;ince i987 916.6 22 33.931 FezNugget' Australia since 1987 999.9 24 31.103 lozTscherwonez Former USSR since 1975 900 21.6 8.60 7.74g

Also in 'I"~ '/ and '/'0 ozgold

Table 23.4: Composition ofgold alloys for jewelry.Gold

content, % Content ofalloy components, %

Colored gold alloys used in the jewelry ~dustry are mostly based on the ternary alloysystem Au-Ag-Cu, allowing a wide variety ofcolors. The workability and resistance to wearof an alloy depend on its mechanical proper-ties; these, and its resistance to corrosion, can

.be controlled by adding zinc. The classifica-tion ranges for colored gold alloys used in thejewelry sector are shown in Table 23.4. Todesignate the different qualities, special codesare used, most of which are specific to particu-lar firms. The properties cover a broad rangeof values to meet all practical requirements(Table 23.5). In 1966 in Germany, an indus-trial standard, DIN 8238 "Gold colors" (in-cluding white gold), was created tostandardize colors in gold alloys and to pro-vide manufacturers with a better means of mu-tual understanding. This standard closelyreflects similar specifications in Switzerland

Gold Solders. The most frequently usedmethod of making joins in the manufacture ofjewelry from gold alloys is hard soldering. Inaddition to fme gold, colored gold solders alsocontain silver, cadmium, copper, and zinc.Their melting temperature is always lowerthan that of the material to be soldered. Theadditives are adjusted so that the solders havegraduated working temperatures. Three sol-ders are generally sufficient, with graduationsof ca. 50 DC in their working temperatures.Some exaI;Ilples of colored gold solders aregiven in Table 23.7.

White gold alloys were first developed in theearly 1900s, in an effort to replace platinum bya cheaper material with identical properties.White gold also differs from colored gold inhaving a higher melting range and is usuallyharder. Nickel and palladium are the only suit-able additives to give gold a color approachingwhitish-gray. The demand for white gold hasfallen in the last few years in favor of coloredgold and platinum. Information about whitegold alloys is given in Table 23.4.

and France. A combination of values repre-senting tone (T), saturation level (S), anddarkness (D), measured using spectrophoto-metric methods, is attributed to each color (Ta-ble 23.6).

Ag 0-16.7, Cu 0-16.7Ag 0-20, Cu 5-25Ag 8-34, Cu 7.5-33.5Ag 8qP35, Cu 30-55, Zn 0-20

Cu 4--8, Ni 10-18, Zn 3--6Pd 10-20, Cu + Zn 5, rest AgCu 15-25, Ni 10-16, Zn 5-8

767559

88.37558.533.3

Colored gold20 carat18 carat14 carat8 caratWhite gold18 carat18 carat14 carat

23.10.2 Jewelry [98]Fine Content of Jewelry Gold. The goldvalue of jewelry alloys is determined by theirgold content (fmeness). In most countries,laws govern the terms used in designating thefineness of gold jewelry for manufacturers,processors, and dealers.

Only alloys.. with a minimum gold contentof 585/000 are sufficiently tarnish resistant forjewelry. Better quality jewelry customarilyhas a gold content of750/000. These alloys of-fer optimum color and mechanical properties.For less expensive jewelry, alloys with a lowgold content are often used. In Germany,333/000 fine is common, and in the UnitedStates 4171000. These alloys tarnish under un-favorable conditions, and cracks may form asa result of stress corrosion. The highest gradealloys are used only occasionally in the manu-facture of jewelry, due to their low strength.However, no alloy can match the fine color ofpure gold.

They are now traded as collectors' items andas a form of investment. With the liberaliza-tion of private gold trading in the 1950s, thedemand for gold increased. A successful newenterprise in this market was the minting andmarketing of the krugerrand (gold content of 1oz troy) by South Africa. Between 1967 and1985, a total of 2000 t (ca. 65 X 106 oz) weresold. Following the repressive measureswhich have been inflicted on the krugerrand,new coins were introduced by other countries(Table 23.3).

Gold was already stored in the form of barsin antiquity, especially in Rome. Today, goldbars weighing ca. 400 oz (ca. 12 kg) of gooddelivery quality (99.5%) are the main formused in public and institutional investment.Gross weight, gold content, the manufacturingfirm and reference number are stamped on thebars, and provide direct and binding informa-tion as to their value. Smaller gold bars aremanufactured for private investors.

The difference between the buying and sell-ing price in banks (broker's commission) isnormally ca. 3--4 % plu~ value added tax.

23.10 Uses of Gold and GoldAlloys [6, 12, 19,20,24,31,33,39,41,96]

In antiquity, gold was considered an objectof value and used as a means of exchange, atfirst in the form of nuggets or flattened disks,and from 650 B.C. as minted coins. The com-position of the metal used varied from alloyscontaining a high proportion of gold, to elec-trum which had a relatively high silver con-tent. The confusing variety of gold coins, andthe great variation in fme gold content, wasfirst limited in the 19th century, when legalregulations were introduced for coinage,which specified nominal values of gold coins,their weight, and gold content. After WorldWar I, gold coins were no longer legal tender.

23.10.1 Coins, Medals, Bars [97]

23.9.5 Trace AnalysisThe determination of trace amounts of gold

is important in ore prospecting and in the eval-uation of residues and waste materials fromthe metallurgical industry. The gold content ofthese materials may be very low (ca. 1-1000ppm). The solids can be dissolved and the re-sulting solution analyzed by atomic absorp-tion spectroscopy. The solution can beenriched prior to the determination, if re-quired, by solvent extraction. Alternatively,metallic gold can be collected in molten cop-per or lead. The resulting alloy can then be dis-solved and the solution analyzed by atomicabsorption spectroscopy. The solid alloy canbe analyzed by neutron activation,. total reflec-tion X-ray fluorescence, and inductively cou-pled plasma mass spectroscopy. Fire assayingis also still significant in this field.

phy, and atomic absorption spectroscopy. Inspecial cases, mainly for ultrapure materials,mass spectroscopy, glow discharge, and neu-tron activation are used. Physical methodshave largely replaced chemical and colorimet~ric methods in the determination of trace im-purities for reasons of economy and speed.

1206 Handbook ofExtractiveMetallurgy Gold 1207

Table 23.5: Properties and applications ofjewelry gold alloys.

Colored gold18 ct 750/2 red 880-865 15.0

750/4\1, reddish 875-850 15.3750150 pale yellow 900-850 15.5750130 yellow 890-850 15.5750/10 pale yellow 970-900 15.8

14 ct 585/2 red 920-880 13.0585/4 reddish 905-860 13.158515 M reddish yellow 860-790 13.2585/10 deep yellow 845-825 13.6585/13 pale yellow 840-800 13.7585/15 pale yellow 890-820 13.7585/17 green yellow 980-940 13.8

8 ct 333/4Y2 reddish 930-890 11.0333/6 pale yellow 845-770 10.9333/9 yellow 860-750 ILl333/16 pale yellow 810-750 11.5

White gold18 ct 760H white 950-875 14.8

containsNi750M white 1170-1040 16.0

contains Pd750S white 1170-1040 15.6

contains Pd14 ct 590H white 1000-870 12.8

contains Ni590M white 1120-1060 14.1

contains Pd590 S white 1150-1050 14.0

contains Pd

Table 23.6: Gold colors according to DIN 8238.

Swiss and Colorimetric Examples"0 German measures accord- ofapproxi-.0

classifi- French ing to DIN 6164 mately cor-~ c1assifica-tf.l cation tion T S D

respondinggold alloys

N weill 1.2 0.9 1.6 590H8N gn1ngelb 24.8 1.6 1.2 585/17IN blallgelb j anne pale 1.7 1.7 1.2 585/152N hellgelb jaune pale 1.8 1.8 1.3 75011503N gelb jaune 2.0 1.8 1.3 750/1304N rose rose 2.4 1.6 1.4 750/4\1,5N rot rouge 2.6 1.5 1.4 750/2

Table 23.7: Working temperatures of colored gold sol-ders.

Nibs for fountain pens are usually madefrom Au-Ag-Cu yellow gold alloys which arerelatively hard; occasionally white gold alloysare also used. The alloys must be able to with-stand the very corrosive ferro-gallic inks; only14 or 18 carat alloys are suitable. The nibpoints must be made of hard metal alloys,which usually contain Ru. as, Jr, W, or Co.

23.10.5 Pen Nibs

Gold-manganese alloys are used for wire-wound resistance thermometers. The thermo-couples Cu + Au 99.4/Co 0.6 and Pt 90/Ir 10+Au 60IPd 40 are suitable for the temperatureranges 0 to -240C and 0 to 700C, respec-tively; the gold alloys form the negative leg.

23.10.4 Solders [101]The eutectics of the following systems are

used as solders for joining materials in transis-tor production technology: gold-tin (25% Sn,771p 280C), gold-silicon (30% Si, mp370C), and gold-germanium (26% Ge, mp350C).

Alloys of gold with tin or silicon are used tomake hard solders with a low melting point,high corrosion resistance, good thermal andelectrical conductivity, and high mechanicalstrength. Heat-sensitive components are sol-dered using these materials.

Certain types of apparatus have cOIflPO-nents made of iron and nickel alloys whichhave to withstand high vacuums and high tem-peratures. To join these materials, vacuumhard solders are used, made of either fIne gold,or gold-copper, gold-silver-copper, gold-nickel, gold-copper-nickel, and gold-palla-dium alloys. Soldering is carried out in a vac-uum furnace or in a protective gas (hydrogen,cracked gas).

Hard silver solder is normally used to joinstainless steels. However, if these joints do notexhibit sufficient corrosion resistance, gold-nickel-zinc alloys similar to white gold, withca. 80% gold, 15% nickel, and 5% zinc, areoccasionally used.

23.10.3 Electronics and ElectricalEngineering [99, 100]

Modern electronics require the use of noblemetals, especially gold, particularly in the ar-eas of information processing, telecommuni-cations, and military arid space electronics. Itis used in active components (diodes, transis-

. tors, integrated circuits, semiconductor mem-ories), assembly and connection engineering(packages, thick-film circuits, printed boards,and plugs) and, to a lesser extent, for passivecomponents (~apacitors and resistors).

The great advantage of gold is its high re-sistance to oxidation and corrosion, and itshigh conductivity which give it excellent con-tact properties. Gold plating is usually carriedout by electrochemical deposition. Thin goldcoatings can also be produced by fIring ofgold-containing pastes, usually coated on ce-ramics. Gold is very malleable, so that it canbe worked into very thin bonding wires, usu-ally with a diameter of ca. 25 1lJIl. Fine goldwires can easily be welded to each other, or toother metals, by pressure or by a combinationof heat and pressure. These microweldedjoints can easily be made on microelectroniccircuits at high speed.

Modern methods used in bonding chips re-quire bumps on the contact surfaces of thecrystal; these bumps are made from gold.

The gold used in electronics, with the ex-ception of gold solders, is practically alwaysfIne gold of purity 99.99 or 99.999%. Veryfew parts are made ofmassive gold for reasonsof economy.

Because of its high price, gold is not used agreat deal in electrical engineering. Roll-bonded gold claddings, or gold coatings madeby electrodeposition, are occasionally used forspecial contact problems. Gold-nickel andgold-silver alloys are used in weak-currentengineering, as contact materials for very lowvoltage switches, and where the contact forcesare low (relays, plugs, measuring instru-ments). Micromigration of these alloys is verylow, and there is little tendency for.insulatinglayers to form.

Brinell hardness(after soft anneal-

ing), kglmrn2

18 L 750/3 820L 750/1: 750L 750/1 700

14 L 585/7 780L585/8 720L 58513\1, 670

8 L 333/15 700L 333/10\1, 640

Gold content, Solder designation Working temper-carat (Degussa) ature, C

130 medium hard or hard

95 soft

150 hard

110 medium hard or hard

95 soft

185 very hard or hard

140 hard125 medium hard120 medium hard125 medium hard85 soft

105 soft125 medium hard120 medium hard160 very hard140 hard125 medium hard100 soft100 soft90 soft

120 medium hard125 medium hard

Dens~,g1crn

Meltingrange,OCColorCarat Designation

1208

Nibs made of stainless, ink-resistant nickelchromium steel are sometimes coated with athin layer of gold; however, this does not im-prove their ink resistance.

23.10.6 Chemical Technology [102]Gold-platinum alloys containing 50-70%

gold, which can be age-hardened, are used tomake spinnerets used in the production ofman-made fibers. Their fine-grained structureis of great advantage in making the necessaryfine holes (diameter 25-120 11m).

Gold alloys are sometimes used for sealsand rupture disks that come into contact withcorrosive substances. A gold-silver-palla-dium alloy (pallacid) containing 30% goldand 30% palladium is resistant to strong min-eral acids, is considerably cheaper than gold,and also has greater high-temperaturestrength.

A gold alloy containing 10% platinum isused to make crucibles for analyticallaborato-ries, e.g., for ash determination of flour andother phosphorus-containing foods. Unlikeplatinum crucibles, it is resistant to corrosionby phosphorus compounds when red-hot.

23.10.7 Dental MaterialsGold alloys are of great importance in pros-

thetic dentistry, for solid parts such as goldfillings, crowns, bridges, cast dentures, clasps,anchorage pins, and metal bases for dental ce-ramics. These materials have to meet a num-ber of requirements. They must be resistant tonormal conditions in the mouth, of a suitablecolor, of different strengths, and be easy towork. Today almost all alloys used are of acomplex composition, containing high pro-portions of gold, palladium, and platinum. Theadvantage of these alloys mainly lies in theirvery fme-grained and homogeneous structure.

23.10.8 Coatings [103-105]The technical and decorative properties of

gold can be combined with a variety of cheap

Handbook ofExtractiveMetallllrgy

base materials, by applying a thin layer ofgoldto base metals, ceramics, glass, or plastics.Electroplating is by far the most frequentlyused method. In most cases, gold is separatedfrom an electrolyte containing potassium di-cyanoaurate. Occasionally, electroplatingbaths containing sodium disulfitoaurate or cy-anide complexes of trivale~tgold are used.Electroforrning is used to manufacture cheaphollow jewelry. Gold up to a thickness of 0.2mm is deposited on a mandrel. The mandrel isthen removed; wax mandrels can be meltedout. A self-supporting gold layer is left, whichcan be reinforced by a filler material.Bright Gold. Ceramic materials, especiallyhigh-quality porcelain and glass, are oftengilded by firing on preparations of bright goldand burnished gold. The essential componentsof these lacquer-like paints are gold sulfore-sinates, mixed with natural oils and resins.The colors are applied either by hand, or byscreen or offset printing processes. Firing iscarried out at 500-1250 DC. Gold coatings fortechnological applications also may be ap-plied by this method.Rolled gold is still used for spectacle framesand gold-plated watches. However, it has lostits former significance in favor of electroplat-ing. Rolled gold is fabricated by solderinggold sheets, usually 14 carat, to blocks madeof copper alloy or stainless steel. These arethen rolled or drawn to the desired shape, pro-ducing strips and wires with a largely non-po-rous gold coating, usually 510 11m thick.Fire Gilding. In this process, the part to begilded is painted with gold amalgam. The mer-cury is evaporated by heating, leaving a rela-tively thick gold layer. The process is veryproblematical with regard to industrial hy-gIene.Vapor Deposition of Gold. Glass panes canbe insulated against loss of heat, through re-flection of infrared rays, by applying very thinlayers of gold in a sputtering process (cathodeevaporation). Plastic components can bethinly gilded by sputtering in a vacuum.

Gold

Gilded films are also used in space technol-ogy, e.g., in space suits, to produce reflectivecoatings and thus protect against heat.

23.10.9 Gold LeafGold leaf is usually made of fme gold or

Au-Ag-Cu alloys with,a very high gold con-tent. The crystal plane (100) in beaten goldleaf lies in the 'plane of the leaf. Gold leaf isused to gild wooden statues, book edges, andfabric prints. In certain beverages (DanzigerGoldwasser), and recently also in some foods,it is used to achieve visual effects.

In former times, small rolls of gold leafwere used to fill cavities in teeth.

23.10.10 CatalystsGold is of almost no significance as a cata-

lytically active metal. It is occasionally usedas an additive in platinum-group metal or sil-ver-based catalysts.

Platinum vapor, which forms from plati-num-rhodium catalysts during the oxidationof ammonia to nitric acid can be retained by agold or gold-palladium gauze. This processhas long been in industrial use.

23.11 Economic Aspects [2,4,18-20,33,34,41,106-111]

Production. Gold represents a significant por-tion of the total world economy. In South Af-rica, it occupies in terms of value the highestposition among all industrial goods. Over500 000 people are employed worldwide ingold production. The value of the gold fromprimary production in 1987 was DM 32 X 109.Thus it occupies third place in the metal trade,after pig iron (DM 200 X l09) and aluminum(DM 45 x 1O~, and before copper (DM 22 x109).

At present, almost half of the gold obtainedfrom ores is produced in South Africa. Brazil,the United States, Canada, and Australia arecurrently expanding production. Since the dis-

1209

covery of the large Brazilian deposits around1970 and the new activities of other producercountries, the supply is more balanced and lessdependent on political constellations than itused to be. Table 23.8 shows the primary pro-duction of gold in the various producer coun-tries.

Supply and Demand. As a consequence ofthe free market, the supply and demand forgold are on the whole quite well balanced.High prices are regulated by reduced con-sumption., especially in the jewelry sector, andin investment purchases, as well as throughthe replacement of noble metals used in tech-nology by other materials. The price is also re-duced by the expansion of primary productionand recycling, as well as by reductions instocks. Low prices then cause an increase indemand, renewed reductions in primary andsecondary production (often because profit-ability sinks too low), and accumulation Qf re-serves.

The supply of gold on the market in 1987was about 2000 t/a (Figure 23.8). Most of this(65%) came from Western mine production,20% came from recycling processes, and 15%was sold by Eastern-bloc countries. Buyingand selling on the part of government mone-tary authorities can have a strong effect on themarket; at the present time, however, this ef-fect is insignificant. The same applies to thesale of private stocks.

Figure 23.9 shows the demand for gold ac-cording to applications.

Western-worldmine production

Sales from Eastern-bloc countries

Recycling (ca. 400 tlFigure 23.8: Gold supply in 1987 (ca. 2000 t).

1210 Handbook ofExtractive Metal/urgy

Table 23.8: Primary gold production in various producer countries, t.Country 1900 1930 1940 1950 1960 1970 1975 1980 1985 1986 1987

South Africa 10 333 437 363 665 1000 713 675 672 640 6