HYDROMETALLURGY

HYDROMETALLURGYFundamentals and Applications

MICHAEL L. FREE

Copyright © 2013 by The Minerals, Metals & Materials Society. All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.Published simultaneously in Canada.

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Library of Congress Cataloging-in-Publication Data:

Free, Michael.Hydrometallurgy : fundamentals and applications / by Michael L. Free.

pages cmIncludes bibliographical references and index.

ISBN 978-1-118-23077-0 (hardback)1. Hydrometallurgy. I. Title.TN688.F74 2013669.028'3–dc23

2013011235

Printed in the United States of America.

10 9 8 7 6 5 4 3 2 1

CONTENTS

PREFACE xi

1 INTRODUCTION 1

1.1 The Importance of Metals / 11.2 Mineral Deposition / 21.3 Importance of Water / 101.4 Aqueous Processing and Utilization of Metals / 121.5 Overview of Fundamentals and Applications / 17References / 19Problems / 20

2 CHEMICAL FUNDAMENTALSOF HYDROMETALLURGY 21

2.1 General Reactions / 212.2 Chemical Potential / 242.3 Free Energy and Standard Conditions / 282.4 Free Energy and Nonstandard Activities / 302.5 Equilibrium / 312.6 Solubility Product / 332.7 Relationships Amongst K , pK , pKa, and pH / 342.8 Free Energy and Nonstandard Temperatures / 362.9 Heat Generation due to Reactions / 40

v

vi CONTENTS

2.10 Free Energy and Nonstandard Pressures / 412.11 Equilibrium Concentration Determinations / 412.12 Activities and Activity Coefficients / 422.13 Practical Equilibrium Problem Solving / 472.14 Electrochemical Reaction Principles / 522.15 Equilibrium and Electrochemical Equations / 56References / 62Problems / 62

3 SPECIATION AND PHASE DIAGRAMS 65

3.1 Speciation (or Ion Distribution) Diagrams / 653.2 Metal-Ligand Speciation Diagrams / 693.3 Phase Stability Diagrams / 72Reference / 83Problems / 83

4 RATE PROCESSES 84

4.1 Chemical Reaction Kinetics / 844.2 Biochemical Reaction Kinetics / 924.3 Electrochemical Reaction Kinetics / 944.4 Mass Transport / 1064.5 Combined Mass Transport and Reaction Kinetics / 1144.6 Models for Reactions Involving Particles / 1174.7 Combined Mass Transport and Electrochemical

Kinetics / 1284.8 Crystallization Kinetics / 1304.9 Overview of Surface Reaction Kinetics / 131References / 134Problems / 135

5 METAL EXTRACTION 137

5.1 General Principles and Terminology / 1375.2 Bioleaching/Biooxidation / 1595.3 Precious Metal Leaching Applications / 1655.4 Extraction from Concentrates / 171References / 179Problems / 181

CONTENTS vii

6 SEPARATION OF DISSOLVED METALS 183

6.1 Liquid–Liquid or Solvent Extraction / 1846.2 Ion Exchange / 2006.3 Activated Carbon Adsorption / 2076.4 Ultrafiltration or Reverse Osmosis / 2116.5 Precipitation / 212References / 214Problems / 215

7 METAL RECOVERY PROCESSES 218

7.1 Electrowinning / 2187.2 Electrorefining / 2297.3 Cementation or Contact Reduction / 2327.4 Recovery Using Dissolved Reducing Reagents / 235References / 236Problems / 237

8 METAL UTILIZATION 239

8.1 Introduction / 2398.2 Batteries / 2408.3 Fuel Cells / 2458.4 Electroless Plating / 2478.5 Electrodeposited Coatings / 2498.6 Electroforming / 2518.7 Electrochemical Machining / 2518.8 Corrosion / 253References / 262Problems / 263

9 ENVIRONMENTAL ISSUES 264

9.1 Introduction / 2649.2 United States Environmental Policy Issues / 2669.3 Metal Removal and Remediation Issues / 268References / 275Problems / 276

viii CONTENTS

10 PROCESS DESIGN PRINCIPLES 277

10.1 Determination of Overall Objectives / 27810.2 Determination of Basic Flow Sheet Segments / 27810.3 Survey of Specific Segment Options / 27910.4 Overall Flow Sheet Synthesis / 28510.5 Procurement of Additional Information / 28710.6 Selected Industrial Flow Sheet Examples / 289References / 310Problem / 311

11 GENERAL ENGINEERING ECONOMICS 312

11.1 The Effects of Time and Interest / 31211.2 Return on Investment (ROI) / 32211.3 Cost Estimation / 32311.4 Discounted Cash Flow Economic Analysis / 32511.7 Evaluating Financial Effects of Risk / 337References / 344Problems / 345

12 GENERAL ENGINEERING STATISTICS 348

12.1 Uncertainty / 34812.2 Basic Statistical Terms and Concepts / 35312.3 The Normal Distribution / 35412.4 Probability and Confidence / 35512.5 Linear Regression and Correlation / 37312.6 Selecting Appropriate Statistical Functions / 37512.7 Hypothesis Testing / 38212.8 Analysis of Variance (ANOVA) / 38412.9 Factorial Design and Analysis of Experiments / 38712.10 The Taguchi Method / 390References / 396Problems / 397

APPENDIX A ATOMIC WEIGHTS 399

APPENDIX B MISCELLANEOUS CONSTANTS 401

APPENDIX C CONVERSION FACTORS 402

CONTENTS ix

APPENDIX D FREE ENERGY DATA 403

References / 410

APPENDIX E LABORATORY CALCULATIONS 412

E.1 Background Information / 412E.2 Solution Preparation Principles / 413E.3 Solution Preparation Calculations / 413Problem / 415

APPENDIX F SELECTED IONIC SPECIES DATA 416

APPENDIX G STANDARD HALF-CELL POTENTIALS 418

APPENDIX H GENERAL TERMINOLOGY 420

APPENDIX I COMMON SIEVE SIZES 423

APPENDIX J METALS AND MINERALS 424

INDEX 429

PREFACE

This book provides a college-level overview of chemical processing of met-als in water-based solutions. It is an expanded version of a previous textbook,Chemical Processing and Utilization of Metals in Aqueous Media, with two edi-tions written by the author. The information in this book is relevant to engineersusing, producing, or removing metals in water. The metals can take the formof dissolved ions, mineral particles, or metal. The material in each chapter inthis textbook could be expanded into individual textbooks. It is clearly not com-prehensive in its coverage of relevant information. Other resources, such as thefour-volume series Principles of Extractive Metallurgy by Fathi Habashi, pro-vide more details for specific metal processing methods. Thus, this text presentsa condensed collection of information and analytical tools. These tools can beused to improve the efficiency and effectiveness with which metals are extracted,recovered, manufactured, and utilized in aqueous media in technically viable,reliable, environmentally responsible, and economically feasible ways.

The author expresses gratitude to his family, colleagues, teachers, and studentswho have contributed in various ways to the completion of this text.

The author has used his best efforts to prepare this text. However, the authorand the publisher make no warranty of any kind, expressed or implied, withregard to the material in this book. The author and the publisher shall not beliable in any event for incidental or consequential damages in connection with,or arising out of, the use of the material in this book.

Michael L. FreeSalt Lake City, Utah

xi

CHAPTER 1

Metals form the foundation forour modern standard of living.We would be living with StoneAge technology if we did not

have metals.

INTRODUCTION

Key Chapter Learning Objectives and Outcomes

Understand the importance of metalsKnow some important uses of metalsKnow the value of metalsUnderstand how metal-bearing ore bodies can formUnderstand how metals are naturally cycled in different forms on earthUnderstand the importance of water in metal processingIdentify basic methods needed to produce metal from metal-bearing ore

1.1 THE IMPORTANCE OF METALS

Metals are needed for survival. Our bodies rely on metals to perform manyvital functions. Iron is used to transport oxygen to cells. Calcium is needed forbones. Sodium and potassium are needed to regulate many necessary biologicalfunctions. Many other trace metals are needed for a variety of critical activitieswithin our bodies.

Metals form a foundation for our modern standard of living. We use largequantities of metals to build transportation vehicles that range from bicycles andcars to ships and airplanes. We rely on metals for structural support for buildings,bridges, and highways. We need metals to build computers and electronic devices.

Hydrometallurgy: Fundamentals and Applications, First Edition. Michael L. Free.© 2013 The Minerals, Metals & Materials Society. Published 2013 by John Wiley & Sons, Inc.

1

2 INTRODUCTION

Metals are also necessary to generate electricity. Metals are the foundation forthe myriad of motors that mechanize our factories and homes. Metals are criticalto our way of life. We would be living with Stone Age technology if we did nothave metals.

Countless things are made of metal or have metal in them that is critical totheir function. Metal items are found in nearly every part of the world, and metalsare produced in a variety of locations around the world. A summary of metalsand its uses is presented in Table 1.1. Worldwide production of metals is shownin Table 1.2.

The total annual value of raw metal produced in 2011 was estimated to bemore than US$2 trillion. This is only the value of the raw metal. The value to theworld economy is several times that amount when the value of the final productsthat use metal is considered.

Metals undergo continual cyclical processing on our planet as shown inFigure 1.1. Metal comes from the earth. Metals are continuously transported andtransformed by geological processes.

Metals usually originate as minerals that are discovered, mined, and trans-formed into metal. All metal in use today is undergoing a very gradual or rapidprocess of corrosion or degradation. The process of corrosion is essentially achemical transformation from the metallic form to the mineral or ion form.Atmospheric corrosion of metals leads generally to the formation of metal oxideminerals. The oxygen in the atmosphere and water causes the oxidation. Oxidemineral products of corrosion are often the same as the minerals in ores. Thus,metals are found in various forms depending on their position in the metal cycle.However, the time between metal cycle events can be enormous.

Metals usually originateas minerals that are discovered,

mined, and transformedinto metal.

The processing paths used to produceindividual metals are often very differ-ent. Many processing methods involvewater or water-containing (aqueous)media. Thus, the term hydrometallurgyis often used to describe this topic. Thistextbook covers the fundamental princi-ples of chemical metallurgy. It also presents many important methods of process-ing, utilizing, and evaluating metals and metal processes in aqueous media. Thisbook includes many associated topics such as metal extraction, electrodeposition,power storage and generation, electroforming, environmental issues, economics,and statistics.

1.2 MINERAL DEPOSITION

Metals generally originate in the earth’s crust as metal oxides, sulfides, and otherminerals. Many metals such as aluminum, iron, calcium, sodium, magnesium,potassium, and titanium are abundant as shown in Table 1.3. In some locations,mineralization processes have concentrated specific metals. Metals such as

MINERAL DEPOSITION 3

TABLE 1.1 Comparison of Selected Metals and Common Usesa

Metal Symbol Common Uses

Actinium Ac Thermoelectric power, source of neutronsAluminum Al Alloys, containers, aerospace structures, outdoor

structures, also in ceramics as Al2O3Antimony Sb Semiconductors, alloys, flame proofing compoundsBarium Ba Used mostly in nonmetallic forms such as barite and

barium sulfate, specialty alloysBeryllium Be Specialty alloys, electrodes, X-ray windowsBismuth Bi Specialty alloys, thermocouples, fire detection, cosmeticsCadmium Cd Specialty alloys, coatings, batteries, solar cellsCalcium Ca Used mostly in nonmetallic forms, reducing agent,

deoxidizer, specialty alloysCerium Ce Used mostly in nonmetallic forms, catalysts, specialty

alloysCesium Cs Catalyst, oxygen getter, atomic clocksChromium Cr Stainless steel, coatings, catalyst, nonmetallic forms in

colorants, corrosion inhibitorsCobalt Co High strength, high temperature alloys, magnets,

nonmetallic forms as colorantsCopper Cu Electrical wiring, tubing, heat transfer applications,

alloys such as bronze and brassDysprosium Dy Specialty magnets, specialty alloys, nuclear control

applicationsErbium Er Nonmetallic forms as colorantsEuropium Eu Specialty products in nonmetallic forms and alloysGadolium Gd Specialty products in nonmetallic forms and alloysGallium Ga Solar cells, semiconductors, neutrino detectors, specialty

alloys, coatingsGold Au Jewelry, bullion/investment tool, decoration, specialty

coatings, alloys, coinsHafnium Ha Specialty alloys, nuclear control rodsHolmium Ho Specialty alloys, filaments, neutron absorptionIndium In Specialty alloys, solar cells, thermistors, solderIridium Ir Specialty alloys and coatings, jewelry, electrodesIron Fe Steels, cast iron, many alloys—most widely used, lowest

cost metalLanthanum La Specialty products in nonmetallic forms and alloysLead Pb Batteries, radiation shielding, cable covering,

ammunition, specialty alloysLithium Li Heat transfer and specialty alloys, batteries, nonmetallic

forms in glasses, medicineLutetium Lu Catalysts, specialty alloysMagnesium Mg Specialty alloys, reducing agent, pyrotechnics, many

nonmetallic form usesManganese Mn Steel and specialty alloys, nonmetallic form uses in

batteries, colorants, chemistry

(continued )

4 INTRODUCTION

TABLE 1.1 (Continued)

Metal Symbol Common Uses

Mercury Hg Chlor-alkali production, amalgams, specialty usesMolybdenum Mo Primarily used for steel alloys, catalysts, heating

elementsNeodymium Nd Specialty magnets, lasers; also nonmetallic forms such

as glass colorantsNeptunium Np Neutron detectionNickel Ni Many specialty alloys, batteries, tubing, coatings,

magnets, catalystsNiobium Nb Specialty alloys, magnetsOsmium Os Specialty, high cost, hard alloysPalladium Pd Alloys (decolorizes gold), catalysts, used for hydrogen

gas purificationPlatinum Pt Catalyst, jewelry, thermocouples, glass making

equipment, electrochemistryPlutonium Pu Nuclear weapons and fuelPolonium Po Neutron source, satellite thermoelectric powerPotassium K Reducing agent, most uses in nonmetallic forms such as

fertilizerPraseodymium Pr Specialty alloys, nonmetallic forms such as glass

colorantRadium Ra Neutron sourceRhenium Re Specialty alloys, thermocouples, catalystsRhodium Rh Specialty alloys, thermocouples, catalysts, jewelryRubidium Rb Specialty alloys, catalysts, specialty glasses (nonmetallic)Ruthenium Ru Specialty alloys, catalystsSamarium Sm Magnets, specialty alloys, catalystsScandium Sc Specialty alloysSilver Ag Jewelry, silverware, solder, batteries, antimicrobial

applications, coinsSodium Na Reagent in chemical reactions, batteries, used mostly in

nonmetallic formsStrontium Sr Specialty alloys, zinc refining, fireworks, glass colorantTantalum Ta Specialty alloys, capacitors, surgical appliancesTechnetium Tc Radioactive tracers,Thallium Tl Specialty alloys, photovoltaic devicesThorium Th Specialty alloys, portable gas light mantles, catalystsThullium Tm Specialty alloys, radiation source if previously exposedTin Sn Specialty alloys, solder, coatings, semiconductorsTitanium Ti Aerospace alloys, corrosion resistant alloys, implants,

paint pigment (TiO2)Tungsten W Filaments, alloys, tool steels and hard materialsTurbium Tb Dopant in solid-state devices, specialty alloysUranium U Nuclear fuel, nuclear weaponsVanadium V Specialty steels, catalysts, nuclear applications

MINERAL DEPOSITION 5

TABLE 1.1 (Continued)

Metal Symbol Common Uses

Ytterbium Yb Specialty alloys, radiation sourceYttrium Y Alloys, catalysts, nonmetallic colorant

applicationsZinc Zn Galvanized metal coatings, alloys such as

brass, solder, batteries, coinsZirconium Zr Nuclear fuel canisters, corrosion resistant

tubing, explosive primers

aReferences 1–3.

TABLE 1.2 World Metal Production, Price and Value Estimates Basedon Data from USGS Mineral Commodity Summaries 2012a

Estimated Estimated Estimated TotalWorldwide Metal Price Value (Price ×

Production 2011 (2011), ProductionMetal (metric tons) US$/kg in US Dollars)

Aluminum 44,100,000 2.71 119,585,969,708Antimony 169,000 13.45 2,272,757,336Beryllium 240 447.54 107,409,775Bismuth 8,500 25.57 217,376,926Cadmium 21,500 2.75 59,125,000Chromium 24,000 14.50 348,000,000Cobalt 98,000 39.68 3,888,974,625Copper 16,100,000 8.82 141,978,438,678Gallium 95 700.00 66,500,000Gold 2,700 51, 440.73 138,889,966,710Indium 640 720.00 460,800,000Iron/steel 1,100,000,000 1.32 1,455,058,533,036Lead 4,500 2.49 11,210,565Magnesium 780,000 5.18 4,041,094,380Mercury 1,930 100.00 193,000,000Molybdenum 250,000 76.94 19,235,432,880Nickel 1,300,000 22.71 29,520,051,148Niobium 63,000 41.00 2,583,000,000Palladium 207 23, 470.54 4,858,400,758Platinum 192 55, 301.04 10,617,800,216Rhenium 49 2, 000.00 98,000,000Silver 23,800 1, 108.93 26,392,557,155Tin 253,000 25.35 6,414,383,033Titanium 186,000 22.71 4,223,638,087Zinc 12,400,000 2.20 27,337,463,348Zirconium 1,410 64.00 90,240,000

aReference 1.

6 INTRODUCTION

Metal dissolution/separation

Metal transport

Mineral (or metal) formation

Mineral/metal separation/concentration

Metal extraction

Metal recovery

Metal refining/purification

Metal shaping/forming

Metal use

Metal waste/corrosion

Metalrecycling

Chemical separation

Figure 1.1 Overview of the general metal cycle.

manganese, barium, strontium, zirconium, vanadium, chromium, nickel, copper,cobalt, lead, uranium, tin, tungsten, mercury, silver, gold, and platinum aregenerally scarce. Economic extraction of scarce metals requires a suitable oredeposit.

Ores are natural materials containing a concentrated resource. An ore depositor ore body contains a large volume of ore. Some rare metals and mineralsmust be concentrated by a factor of 1000 or more above their average naturalabundance to form an economically viable ore body as indicated in Table 1.4(see Appendix A for atomic weights to convert to an atomic basis).

The method of concentration within the earth’s crust varies widely. It is depen-dent on the metal and associated mineral(s) as shown in Table 1.5. Some metals

MINERAL DEPOSITION 7

TABLE 1.3 Abundance of Elements in the Earth’s Crusta

Element Abundance, % Element Abundance, %

Oxygen 46.4 Vanadium 0.014Silicon 28.2 Chromium 0.010Aluminum 8.2 Nickel 0.0075Iron 5.6 Zinc 0.0070Calcium 4.1 Copper 0.0055Sodium 2.4 Cobalt 0.0025Magnesium 2.3 Lead 0.0013Potassium 2.1 Uranium 0.00027Titanium 0.57 Tin 0.00020Manganese 0.095 Tungsten 0.00015Barium 0.043 Mercury 0.000008Strontium 0.038 Silver 0.000007Rare Earths 0.023 Gold <0.000005Zirconium 0.017 Platinum metals <0.000005

aReference 4.

TABLE 1.4 Enrichment Factors Necessary for Ore BodyFormationa

Approximate EnrichmentFactor from NaturalOccurrence Level to

Metal Form Economical Ore Body

Aluminum 4Chromium 3000Cobalt 2000Copper 140Gold 2000Iron 5Lead 2000Manganese 380Molybdenum 1700Nickel 175Silver 1500Tin 1000Titanium 7Tungsten 6500Uranium 500Vanadium 160Zinc 350

Ores are naturalmaterials containing

a concentratedresource. An ore bodyis a local region in the

earth’s crust containingdesired minerals in

concentrationssufficient forcommercialextraction.

aReference 5.

8 INTRODUCTION

TABLE 1.5 Methods of Metal/Metal-Bearing Mineral Depositiona

Mag. Sub V. Hydro. Weath. Sed. Plac. Metas.

Aluminum XChromium X XCobalt XCopper X X X X XGold X X XIron X X X X XLead X X X XManganese X X X XMolybdenum X XNickel X X XSilver X X XTin X X XTitanium X XTungsten X X XUranium X XZinc X X X X

Water can play animportant role in orebody formation andmetal processing.

Mag., magma crystallization;

Sub V, submarine volcanic exhalative processes;

Hydro., hydrothermal solution deposition;

Weath., weathering;

Sed., sedimentation;

Plac., placer deposition;

Metas., contact metasomatism.aReference 5.

such as chromium are concentrated by precipitation or crystallization in magma.Precipitation is dependent on solubility, which is dependent on temperature. Asthe magma cools, different local temperature zones are created. Each temperaturezone may correspond to a specific mineral (or set of similar minerals) formationzone. Thus, temperature zones can create mineral zones or veins. This processis similar to the creation of ice crystals on a cold windshield. In such cases, apure component leaves a mixture (air and water vapor) to form a pure depositin locally enriched regions. A comparison of crystallization from magma and anair/water vapor mixture is illustrated in Figure 1.2.

Ore-grade aluminum deposits are formed by the dissolution and removal ofcontaminants such as silicates through extensive weathering. Weathering is mostrapid in tropical environments. Correspondingly, most commercial aluminumdeposits are in tropical regions).

Other metals such as silver and tin are concentrated hydrothermally. Theyare first dissolved in hydrothermal solutions within the earth’s crust. Next, theyare transported to regions where solution conditions change. Solution conditionscan change due to changes in geological formations. Disruptions in rock layers

MINERAL DEPOSITION 9

Coldwindshield

Water vaporand airmixture

Ice crystals(frost)

Hot magma

Dissolved mineral

constituents

Coolrocklayer

Mineral crystals(a) (b)

Vein

Figure 1.2 Illustrations of (a) mineral crystal formation near a hot magma source fromdissolved mineral constituents and (b) ice crystal formation from a water vapor–air mix-ture on a cold windshield. The ice is pure H2O in a more locally concentrated formthan the air–water vapor mixture from which it came. Similarly, mineral crystals containconcentrated metal.

and related aquifers can force water upward to cooler zones. Changes such astemperature decrease cause changes in solubilities. If a dissolved metal is nolonger soluble, it precipitates. Precipitation on a large scale can create an orebody. An ore body is a local region in the earth’s crust containing desired mineralsin concentrations sufficient for commercial extraction.

Metals such as gold can be transported as suspended particles. Particles settleto form “placer” deposits where water flows slowly. Thus, some gold ores areplacer deposits.

Metals have some solubility in water as ions or ion complexes. Consequently,hydrothermal ore formation processes are very important. In fact, four depositionmethods in Table 1.5 involve water. Hydrothermal processing is the most commonore body formation method in Table 1.5. Correspondingly, metals can often bedeposited from solution. Conversely, they can also be dissolved and processedusing appropriate solutions. However, extreme processing conditions are oftenrequired for metal extraction.

Metal ore body formation is theresult of large-scale

concentrated mineral deposits inspecific areas.

Metals such as platinum and goldrequire highly oxidizing environments aswell as complexing agents to make theirdissolution in water possible. Other met-als such as magnesium tend to dissolvemore easily.

Aqueous processing of metals hasbeen performed for several centuries. Copper was recovered from acidic minewaters using metallic iron as early as the sixteenth century in Europe [6].Evidence of metal dissolution due to bacterial activity in Rio Tinto, Spain, wasreported as early as 1670 [7]. The dissolution of precious metals by cyanide

10 INTRODUCTION

TABLE 1.6 Historical Events Involving Chemical Processing of Metalsin Aqueous Media

Event Reference Year Entity or Place

Separation of silver from gold leaf 10 1556 EuropeElectroless deposition of Cu 11 1670 Rio Tinto, SpainDiscovery of precious metal cyanidation 8 1783Invention of electrochemical battery 12 1800 Pavia, ItalyPrecipitation of copper using iron 11 1820 Strafford, VTInvention of electroforming 13 1838 St. Petersburg, RussiaInvention of aqueous-based fuel cell 14 1839 Swansea, G.B.Electrolytic reduction of copper 12 1869 SwanseaCation exchange on zeolites 10 1876 (Lemberg)Cyanidation of precious metals patented 10 1887 (MacArthur)The Bayer caustic leach process for aluminum 10 1892 (Bayer)Alkaline rechargeable battery patented 15 1916 United StatesLarge-scale electroforming of reflectors 16 1924 Newark, NJLarge-scale electrowinning 10 1926 ArizonaSynthesis of anion-exchange resins 10 1935 (Adams, Holmes)Recovery of uranium by ion exchange 10 1948 (Bross)Ammoniacal leaching/gaseous reduction of Ni 11 1954 (Sherrit-Gordon)Large-scale uranium solvent extraction 10 1955 Shiprock, NMPressure sulfuric acid leach of oxide Ni ore 10 1958 Mao Bay, CubaLarge-scale bacterial dump leaching 10 1960 Bingham, UTAqueous fuel cells US Space Program 17 1965 United StatesLarge-scale Cu solvent extract, /electrowinning 11 1968 Bluebird, AZPachuca leach of Cu ore with sulfuric acid 10 1974 (Nchanga LTD)Commercialization of pressure oxidation of

zinc sulfide10 1981 (Cominco)

Commercialization of biooxidation of gold-oreconcentrate

7 1984 (Gencor)

Demonstration of 11 MW aqueous fuel cell 18 1991 JapanPressure oxidation of copper concentrate 19 2002 Bagdad, AZ

has been known since 1783 [8]. However, widespread application of aqueousmetal processing is relatively recent. Worldwide hydrometallurgical copperproduction is increasing steadily. It increased from 13% to 18% from 1996 to2000 [9]. This increase indicates that hydrometallurgical processing of metals isgrowing. In addition, most of the world’s zinc, uranium, silver, gold, and copperis purified by aqueous means. Historical events associated with aqueous metalprocessing and utilization are listed in Table 1.6.

1.3 IMPORTANCE OF WATER

Water makes aqueous processing of metals possible. Water is an unusualsubstance from a chemical perspective. Most substances with similar molecular

IMPORTANCE OF WATER 11

weights, such as methane and ammonia, are gases at room temperature. However,water is a liquid at room temperature. Water’s unusual properties are dueprimarily to hydrogen bonding effects. These effects are related to the tendencyof hydrogen to donate electrons and the tendency of oxygen to accept electrons.Consequently, water molecules tend to orient themselves accordingly. Theoxygen in water molecules has more electron abundance and maintains a slightlynegative charge. Hydrogen maintains a slightly positive charge. Electrostaticattraction helps the hydrogen atoms to associate with the oxygen atoms ofadjacent molecules. The networked structure of associated molecules is nearlytetrahedral (4.4 nearest neighbors [20]). The resulting structure has open poreslarge enough for water molecules. The open pores accommodate ion diffusion.In fact, the structure is so porous that water is compressible. High pressures cancreate a solid (ice VII) with a density of 65% greater than that of liquid water[21].

Another important property of water is its role in electrochemical reactions.Molecules of water can break apart into oxygen and hydrogen. This molecularbreakage is sometimes referred to as hydrolysis. However, the term hydrolysisis more commonly used to describe the decomposition of water by associa-tion with another compound. Water splitting occurs at either very high or verylow oxidation potentials. At high potentials, oxygen gas and hydrogen ionsform. At low potentials, the products are hydrogen gas and hydroxide ions.The associated chemistry is presented in Equation 1.1 and Equation 1.2. Elec-trons are liberated by electrolysis at high oxidation potentials. Electrolysis is theremoval or acquisition of electrons caused by the application of an electrochem-ical potential or voltage. In contrast, electrons are consumed at low oxidationpotentials.

2H2O ↔ 4H+ + 4e− + O2 (1.1)

2H2O + 2e− ↔ H2 + 2OH− (1.2)

Equation 1.1 or its chemical equivalent form that involves hydroxide ratherthan hydrogen, 2H2O + O2 + 4e− ↔ 4OH−, is crucial in many metal oxida-tion/reduction reactions. The reverse reactions are also critical. The hydroxideequivalent of Equation 1.1 in the reverse direction is fundamental to most atmo-spheric corrosion. The reverse of Equation 1.1 is necessary for metabolism inoxygen-breathing organisms.

Water molecule structure is important to chemical reaction events. Cement andplaster harden in response to events such as hydration. Hydration is the processof acquiring water. Hydration increases effective ion size and decreases mobility.All ions hydrate to varying degrees. In other words, multiple water moleculespartially bond with each ion. In addition, many interfacial phenomena are alteredby hydration.

12 INTRODUCTION

1.4 AQUEOUS PROCESSING AND UTILIZATION OF METALS

Metal atoms can be found in a wide variety of forms. They can exist as dissolvedions and ion complexes. They are also found as pure metals and metal-bearingminerals. Minerals can be transformed from minerals to metallic metals byhydrometallurgical processing. These transformations occur through dissolution,concentration, and recovery processes.

Metals must first be obtained from the earth. Consequently, metal extractionis discussed in the first sections of this text. After metals are extracted, purified,and recovered, they can be utilized. Thus, metal utilization is discussed afterextraction, purification, and recovery.

Minerals can be transformedfrom minerals to metallic metals

through hydrometallurgicaldissolution, concentration, and

recovery processes.

Metal atoms are metallically bondedin pure metals. They have no net chargein the metal lattice. Metal with no netcharge is sometimes written with a super-script “o” (Mo) (often, no “o” is written).Metal atom removal from a solid piece ofmetal nearly always requires the loss ofat least one electron (Mo ↔ M+ + e−).

Electron removal converts a metal atom to an oxidized state. An oxidized metalis deficient in electrons. Oxidized metal can be found as ions or in compounds.Examples of oxidized metal ions are Fe2+ and Fe3+. Metal ions, as well as othertypes of molecules with a net charge, are generally soluble in water. Examples formetal-bearing compounds that are ions include Fe(OH)+, CuCl3

−, and WO43−.

Examples of oxidized metal in compounds are FeO, Fe3O4, and Fe2O3. Ionscan combine with other ions to form ionic or nonionic compounds. Examplesinclude Fe3+ + SO4

2− ↔ FeSO4+ and Fe2+ + SO4

2− ↔ FeSO4. Oxidized metalmay also remain as metal ions.

Chemical reactions are neededto extract metals from minerals.

In contrast, metal can be removedfrom some compounds without elec-tron loss. An example of this is 2H+ +CuO ↔ Cu2+ + H2O. In this example,copper is oxidized to its divalent (+2)state in CuO and Cu2+. No electron transfer is needed for this exchange reac-tion. Metals found as metal-bearing minerals are already oxidized. Consequently,metal removal from a mineral may not require electron removal. Some metals inminerals require additional oxidation to be extracted. However, in order to removeor extract a metal from a mineral, a chemical bond must be severed. Thus, chem-ical reactions are needed to separate metals from minerals. In order to break achemical bond, a more favorable alternative must be provided. In the case ofthe copper oxide reaction with hydrogen shown previously, the oxygen is moresatisfied in the water molecule product than in the copper oxide reactant. Thus,the copper–oxygen bond was severed to form a more favorable set of products.

AQUEOUS PROCESSING AND UTILIZATION OF METALS 13

10 Ne

18 Ar

36 Kr

2 He

54 Xe

86 Rn

118 Uuo

9 F

17 Cl

35 Br

53I

85 At

117Uus

71Lu

103 Lr

8 O

16 S

34 Se

52Te

84 Po

116Lv

70Yb

102No

7 N

15 P

33 As

51Sb

83 Bi

115Uup

69Tm

101Md

6 C

14 Si

32 Ge

50 Sn

82 Pb

114Fl

68 Er

100Fm

5 B

13 Al

31 Ga

49 In

81 Tl

113Uut

67 Ho

99Es

30 Zn

48Cd

80 Hg

112 Cn

66Dy

98Cf

29 Cu

47 Ag

79 Au

111Rg

65 Tb

97Bk

28 Ni

46Pd

78 Pt

110Ds

64 Gd

96Cm

27 Co

45Rh

77Ir

109Mt

63 Eu

95Am

26 Fe

44Ru

76 Os

108Hs

62 Sm

94Pu

25 Mn

43Tc

75 Re

107Bh

61 Pm

93Np

24 Cr

42Mo

74 W

106Sg

60 Nd

92 U

23 V

41 Nb

73Ta

105Db

59 Pr

91Pa

22Ti

40Zr

72Hf

104 Rf

58Ce

90 Th

21Sc

39 Y

57-71

89-103

57 La

89Ac

4 Be12Mg

20 Ca

38 Sr56 Ba

88 Ra

3 Li

11 Na

19 K

1 H

37 Rb55 Cs

87 Fr

GroupPeriod

1

2

4

3

5

6

7

Lanthanoids

Actinoids

1 2 3 4 5 6 7 8 15 16 17 189 10 11 12 1413

Figure 1.3 Periodic table of the elements.

Reactivity of metals is basedon atomic structure.

Reactivity of metals is based onatomic structure. Atomic structure isa function of electron orbitals andthe number of protons and neutrons.The basic structure is most commonly

described using a periodic table. An example of a periodic table is shown inFigure 1.3. Metal elements on the far left, such as sodium and potassium, arevery reactive. Their high reactivity limits their use in metallic form to minor,specialty applications. Metals in the second column from the left, which includesmagnesium, are also very reactive, but they can be used in metallic form. Metalsin other columns have a variety of properties based on their atomic structure.Metals in the lower sections are generally more dense.

One key to hydrometallurgy is contact between the metal and the water. Thiscan occur in a variety of ways. Metals and metal-bearing minerals can contactwater in chemical reactors, lakes, rivers, oceans, ponds, etc. They can also contactwater through condensation, rain, or water vapor. Contact can occur above orbelow ground level.

After contact, a reaction is needed to solubilize the metal. Reactions involv-ing electron transfer require an electron donor and an acceptor. Other chemicalreactions require a reactive species. The reactive species must exchange with ortransform the nonmetal part of the mineral. The reaction converts the metal to anion form to make it soluble. After the metal is solubilized, it can be processedin solution. Dissolved metals are then concentrated, purified, or removed. Theseaqueous processing steps for metals are presented in Figure 1.4.

Concentration processes include liquid–liquid or solvent extraction, ionexchange, carbon adsorption, reverse osmosis, and precipitation. In most

14 INTRODUCTION

Recycling (often aftersize reduction and sorting)

Return to aqueouscontact step

Aqueous contact withmetal/mineral

Cathodic reduction(electrorefining)

Concentrationand

purification

Removal byprecipitation

or Ion-exchangeprocess

Metal extraction by(Electro)/chemical

reaction

Recovery bycementation

or electrowinningManufacturing/electroforming

Utilization, batteries,fuel cells, etc.

Metal refining Metal production/utilization Environment metalremoval

Figure 1.4 Flow sheet possibilities for metal dissolution/extraction, concentration, recov-ery, production, utilization, and processing in aqueous media. (Items in bold font are themain processing steps.)

industrial processes, unwanted ions are dissolved along with the desired ions.Consequently, the desired ions must be separated from unwanted ions using aconcentration process. Concentration processes can also involve the separationof solvent from desired ions. Solvent or solution extraction is a commonindustrial concentration process. Concentration processes are also referred to aspurification processes.

AQUEOUS PROCESSING AND UTILIZATION OF METALS 15

Concentration processesinclude liquid–liquid or solvent

extraction, ion exchange, carbonadsorption, reverse osmosis,

and precipitation.

Solvent extraction consists of contact-ing the metal-bearing aqueous solutionwith an organic compound. The organiccompound is dissolved in a nonaque-ous medium. The nonaqueous mediumis vigorously mixed with the aqueousmedium. Vigorous mixing provides inti-mate contact between the media. Thus,the metal ions can be transferred readily from aqueous to organic media.

The solvent extraction process is designed to be selective. Consequently,unwanted entities remain in the aqueous phase. After extraction of the metalinto the organic or nonaqueous phase, the nonaqueous medium is separated fromthe aqueous medium by settling. The extracted metal is then stripped from thenonaqueous phase. Stripping is performed in a smaller volume to concentrate theions. Stripping solutions are maintained with low levels of impurities.

Ion exchange is essentially the same as solvent extraction, except that theactive organic compound is anchored to a stationary medium such as resin beads,rather than dissolved in a nonaqueous medium. Carbon adsorption is often treatedas a specialized form of ion exchange. Reverse osmosis is a process of molecular-level filtration.

Precipitation consists of adding chemical compounds that react with dissolvedentities. The resulting reactions result in precipitate formation. Precipitates mayconsist of waste or valuable materials. Precipitates are separated from solutionby solid–liquid separation processes.

After metal has been concentrated in aqueous media, it can be recovered as apure metal. Often, pure metal is the desired form of recovered metal. However,it can also be manufactured into products such as coatings and electroformedobjects. In addition, it can be utilized in devices such as batteries, electrodes,and sensors. In some applications such as batteries and fuel cells, metals and/ormetal compounds are used as electrodes in aqueous media.

Recovery of metals from concentrated solutions is made by electrochemicalreduction. In electrolytic reduction, the dissolved metal ions are plated on acathode. Electrolytic reduction results from an impressed or applied voltage. Thevoltage is applied between anodic (anode) and cathodic (cathode) electrodes. Inother forms of reduction, the electrons necessary to reduce the dissolved metal toits elemental or metallic state are provided by a reductant. The reductant can besupplied through solution species (noncontact reduction) or surface compounds(contact reduction).

Recovery of metals fromconcentrated solutions is madeby electrochemical reduction.

An example of extraction, concen-tration, and recovery for copper ispresented in Figure 1.5. The extractionstep for most metals involves acid,oxidizing agents, and/or complexingagents. The concentration step is oftensolvent extraction or ion exchange,

16 INTRODUCTION

2H+

2H+2H+

2H+

Cu2+

Cu2+

Fe2+

Fe2+

Mineral impurities

Malachite 2CuCO3(OH)2

2CuCO3(OH)2 + 4H+ ↔ 2Cu2+ + 4H2O + 2CO2

Loading: 2RH + Cu2+ ↔ R2Cu + 2H+

RH

RH Fe2+

R2Cu

2H+ Fe2+

R2Cu

Cu2+

Cu2+

Cu2+

2H+

2H+

Stripping: R2Cu + 2H+ ↔ 2RH + Cu2+

Cu2+

Cu2+

RH

− +Cu2+

2e−

Cu

2H+

H2O

0.5O2

Cu2++2e− ↔ CuH2O ↔ 2H+ + 2e− + 0.5O2

R2Cu

Extraction (leaching from ore)

Concentration (Chemical separationby solvent extraction)

Recovery(by electrowinning)

Figure 1.5 Metal extraction, concentration, and recovery example for copper from min-eral to metal by hydrometallurgical processing. (See plate section for color version.)

which releases acid or other ions back into solution. The recovery step provideselectrons needed to reduce the metal to its metallic state.

Refining or additional purification of metals is often performed electrolytically.This technique, referred to as electrorefining, is utilized to purify metals. Highpurity metal can be obtained by electrorefining. Electrorefining is the electrolyticprocess of refining an impure metal. It occurs by electrically forcing metal disso-lution and redeposition. Dissolution occurs at the anode and redeposition occurson the cathode. Often the contaminants are not dissolved and remain as fineparticles. Less reactive metals tend not to dissolve. More reactive metals tend toremain solubilized. Thus, the final product is purer than the initial material.

Removal of metal impurities is accomplished by many techniques. Most metalcontamination removal is performed by precipitation. Metals can also be removedby ion exchange processes. Alternatively, carbon adsorption is commonly used