Hydrometallurgy E-Book

Michael L. Free

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 571

Veröffentlichungsjahr: 2013

Beschreibung

As the first book to compile the fundamentals, applications, reference information and analytical tools on the topic, Hydrometallurgy presents a condensed collection of information that can be used to improve the efficiency and effectiveness with which metals are extracted, recovered, manufactured, and utilized in aqueous media in technically viable and reliable, environmentally responsible, and economically feasible ways. Suitable for students and researchers, this college-level overview addresses Fundamentals of Chemical Metallurgy in Aqueous Media, Speciation and Phase Diagrams, Rate Processes in Aqueous Metal Processing, Aqueous Metal Extraction and Leaching, Fundamentals of Metal Concentration Processes and more.

Leseprobe

Table of Contents

Title Page

Copyright

Preface

Chapter 1: Introduction

1.1 The Importance of Metals

1.2 Mineral Deposition

1.3 Importance of Water

1.4 Aqueous Processing and Utilization of Metals

1.5 Overview of Fundamentals and Applications

References

Problems

Chapter 2: Chemical Fundamentals of Hydrometallurgy

2.1 General Reactions

2.2 Chemical Potential

2.3 Free Energy and Standard Conditions

2.4 Free Energy and Nonstandard Activities

2.5 Equilibrium

2.6 Solubility Product

2.7 Relationships Amongst K, K, K, AND H

2.8 Free Energy and Nonstandard Temperatures

2.9 Heat Generation due to Reactions

2.10 Free Energy and Nonstandard Pressures

2.11 Equilibrium Concentration Determinations

2.12 Activities and Activity Coefficients

2.13 Practical Equilibrium Problem Solving

2.14 Electrochemical Reaction Principles

2.15 Equilibrium and Electrochemical Equations

References

Problems

Chapter 3: Speciation and Phase Diagrams

3.1 Speciation (OR ION DISTRIBUTION) Diagrams

3.2 Metal-Ligand Speciation Diagrams

3.3 Phase Stability Diagrams

Reference

Problems

Chapter 4: Rate Processes

4.1 Chemical Reaction Kinetics

4.2 Biochemical Reaction Kinetics

4.3 Electrochemical Reaction Kinetics

4.4 Mass Transport

4.5 Combined Mass Transport and Reaction Kinetics

4.6 Models for Reactions Involving Particles

4.7 Combined Mass Transport and Electrochemical Kinetics

4.8 Crystallization Kinetics

4.9 Overview of Surface Reaction Kinetics

References

Problems

Chapter 5: Metal Extraction

5.1 General Principles and Terminology

5.2 Bioleaching/Biooxidation

5.3 Precious Metal Leaching Applications

5.4 Extraction from Concentrates

References

Problems

Chapter 6: Separation of Dissolved Metals

6.1 Liquid–Liquid Or Solvent Extraction

6.2 Ion Exchange

6.3 Activated Carbon Adsorption

6.4 Ultrafiltration or Reverse Osmosis

6.5 Precipitation

References

Problems

Chapter 7: Metal Recovery Processes

7.1 Electrowinning

7.2 Electrorefining

7.3 Cementation or Contact Reduction

7.4 Recovery Using Dissolved Reducing Reagents

References

Problems

Chapter 8: Metal Utilization

8.1 Introduction

8.2 Batteries

8.3 Fuel Cells

8.4 Electroless Plating

8.5 Electrodeposited Coatings

8.6 Electroforming

8.7 Electrochemical Machining

8.8 Corrosion

References

Problems

Chapter 9: Environmental Issues

9.1 Introduction

9.2 United States Environmental Policy Issues

9.3 Metal Removal and Remediation Issues

References

Problems

Chapter 10: Process Design Principles

10.1 Determination of Overall Objectives

10.2 Determination of Basic Flow Sheet Segments

10.3 Survey of Specific Segment Options

10.4 Overall Flow Sheet Synthesis

10.5 Procurement of Additional Information

10.6 Selected Industrial Flow Sheet Examples

References

Problem

Chapter 11: General Engineering Economics

11.1 The Effects of Time and Interest

11.2 Return on Investment (ROI)

11.3 Cost Estimation

11.4 Discounted Cash Flow Economic Analysis

11.5 Evaluating Financial Effects of Risk

References

Problems

Chapter 12: General Engineering Statistics

12.1 Uncertainty

12.2 Basic Statistical Terms and Concepts

12.3 The Normal Distribution

12.4 Probability and Confidence

12.5 Linear Regression and Correlation

12.6 Selecting Appropriate Statistical Functions

12.7 Hypothesis Testing

12.8 Analysis of Variance (ANOVA)

12.9 Factorial Design and Analysis of Experiments

12.10 The Taguchi Method

References

Problems

Appendix A: Atomic Weights

Appendix B: Miscellaneous Constants

Appendix C: Conversion Factors

Appendix D: Free Energy Data

References

Appendix E: Laboratory Calculations

E.1 Background Information

E.2 Solution Preparation Principles

E.3 Solution Preparation Calculations

Problem

Appendix F: Selected Ionic Species Data

Appendix G: Standard Half-Cell Potentials

Appendix H: General Terminology

Appendix I: Common Sieve Sizes

Appendix J: Metals and Minerals

Index

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:

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

pages cm

Includes bibliographical references and index.

ISBN 978-1-118-23077-0 (hardback)

1. Hydrometallurgy. I. Title.

TN688.F74 2013

669.028'3-dc23

2013011235

Preface

This book provides a college-level overview of chemical processing of metals in water-based solutions. It is an expanded version of a previous textbook, Chemical Processing and Utilization of Metals in Aqueous Media, with two editions written by the author. The information in this book is relevant to engineers using, producing, or removing metals in water. The metals can take the form of dissolved ions, mineral particles, or metal. The material in each chapter in this textbook could be expanded into individual textbooks. It is clearly not comprehensive in its coverage of relevant information. Other resources, such as the four-volume series Principles of Extractive Metallurgy by Fathi Habashi, provide more details for specific metal processing methods. Thus, this text presents a condensed collection of information and analytical tools. These tools can be used 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 students who have contributed in various ways to the completion of this text.

The author has used his best efforts to prepare this text. However, the author and the publisher make no warranty of any kind, expressed or implied, with regard to the material in this book. The author and the publisher shall not be liable 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. FREE

Salt Lake City, Utah

Chapter 1

Introduction

Metals form the foundation for our modern standard of living. We would be living with Stone Age technology if we did not have metals

Key Chapter Learning Objectives and Outcomes

Understand the importance of metals

Know some important uses of metals

Know the value of metals

Understand how metal-bearing ore bodies can form

Understand how metals are naturally cycled in different forms on earth

Understand the importance of water in metal processing

Identify 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 many vital functions. Iron is used to transport oxygen to cells. Calcium is needed for bones. Sodium and potassium are needed to regulate many necessary biological functions. Many other trace metals are needed for a variety of critical activities within our bodies.

Metals form a foundation for our modern standard of living. We use large quantities of metals to build transportation vehicles that range from bicycles and cars 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. Metals are also necessary to generate electricity. Metals are the foundation for the myriad of motors that mechanize our factories and homes. Metals are critical to our way of life. We would be living with Stone Age technology if we did not have metals.

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

Table 1.1 Comparison of Selected Metals and Common Usesa

Metal

Symbol

Common Uses

Actinium

Ac

Thermoelectric power, source of neutrons

Aluminum

Al

Alloys, containers, aerospace structures, outdoor structures, also in ceramics as Al

2

O

3

Antimony

Sb

Semiconductors, alloys, flame proofing compounds

Barium

Ba

Used mostly in nonmetallic forms such as barite and barium sulfate, specialty alloys

Beryllium

Be

Specialty alloys, electrodes, X-ray windows

Bismuth

Bi

Specialty alloys, thermocouples, fire detection, cosmetics

Cadmium

Cd

Specialty alloys, coatings, batteries, solar cells

Calcium

Ca

Used mostly in nonmetallic forms, reducing agent, deoxidizer, specialty alloys

Cerium

Ce

Used mostly in nonmetallic forms, catalysts, specialty alloys

Cesium

Cs

Catalyst, oxygen getter, atomic clocks

Chromium

Cr

Stainless steel, coatings, catalyst, nonmetallic forms in colorants, corrosion inhibitors

Cobalt

Co

High strength, high temperature alloys, magnets, nonmetallic forms as colorants

Copper

Cu

Electrical wiring, tubing, heat transfer applications, alloys such as bronze and brass

Dysprosium

Dy

Specialty magnets, specialty alloys, nuclear control applications

Erbium

Er

Nonmetallic forms as colorants

Europium

Eu

Specialty products in nonmetallic forms and alloys

Gadolium

Gd

Specialty products in nonmetallic forms and alloys

Gallium

Ga

Solar cells, semiconductors, neutrino detectors, specialty alloys, coatings

Gold

Au

Jewelry, bullion/investment tool, decoration, specialty coatings, alloys, coins

Hafnium

Ha

Specialty alloys, nuclear control rods

Holmium

Ho

Specialty alloys, filaments, neutron absorption

Indium

In

Specialty alloys, solar cells, thermistors, solder

Iridium

Ir

Specialty alloys and coatings, jewelry, electrodes

Iron

Fe

Steels, cast iron, many alloys—most widely used, lowest cost metal

Lanthanum

La

Specialty products in nonmetallic forms and alloys

Lead

Pb

Batteries, radiation shielding, cable covering, ammunition, specialty alloys

Lithium

Li

Heat transfer and specialty alloys, batteries, nonmetallic forms in glasses, medicine

Lutetium

Lu

Catalysts, specialty alloys

Magnesium

Mg

Specialty alloys, reducing agent, pyrotechnics, many nonmetallic form uses

Manganese

Mn

Steel and specialty alloys, nonmetallic form uses in batteries, colorants, chemistry

Mercury

Hg

Chlor-alkali production, amalgams, specialty uses

Molybdenum

Mo

Primarily used for steel alloys, catalysts, heating elements

Neodymium

Nd

Specialty magnets, lasers; also nonmetallic forms such as glass colorants

Neptunium

Np

Neutron detection

Nickel

Ni

Many specialty alloys, batteries, tubing, coatings, magnets, catalysts

Niobium

Nb

Specialty alloys, magnets

Osmium

Os

Specialty, high cost, hard alloys

Palladium

Pd

Alloys (decolorizes gold), catalysts, used for hydrogen gas purification

Platinum

Pt

Catalyst, jewelry, thermocouples, glass making equipment, electrochemistry

Plutonium

Pu

Nuclear weapons and fuel

Polonium

Po

Neutron source, satellite thermoelectric power

Potassium

K

Reducing agent, most uses in nonmetallic forms such as fertilizer

Praseodymium

Pr

Specialty alloys, nonmetallic forms such as glass colorant

Radium

Ra

Neutron source

Rhenium

Re

Specialty alloys, thermocouples, catalysts

Rhodium

Rh

Specialty alloys, thermocouples, catalysts, jewelry

Rubidium

Rb

Specialty alloys, catalysts, specialty glasses (nonmetallic)

Ruthenium

Ru

Specialty alloys, catalysts

Samarium

Sm

Magnets, specialty alloys, catalysts

Scandium

Sc

Specialty alloys

Silver

Ag

Jewelry, silverware, solder, batteries, antimicrobial applications, coins

Sodium

Na

Reagent in chemical reactions, batteries, used mostly in nonmetallic forms

Strontium

Sr

Specialty alloys, zinc refining, fireworks, glass colorant

Tantalum

Ta

Specialty alloys, capacitors, surgical appliances

Technetium

Tc

Radioactive tracers,

Thallium

Tl

Specialty alloys, photovoltaic devices

Thorium

Th

Specialty alloys, portable gas light mantles, catalysts

Thullium

Tm

Specialty alloys, radiation source if previously exposed

Tin

Sn

Specialty alloys, solder, coatings, semiconductors

Titanium

Ti

Aerospace alloys, corrosion resistant alloys, implants, paint pigment (TiO

2

)

Tungsten

W

Filaments, alloys, tool steels and hard materials

Turbium

Tb

Dopant in solid-state devices, specialty alloys

Uranium

U

Nuclear fuel, nuclear weapons

Vanadium

V

Specialty steels, catalysts, nuclear applications

Ytterbium

Yb

Specialty alloys, radiation source

Yttrium

Y

Alloys, catalysts, nonmetallic colorant applications

Zinc

Zn

Galvanized metal coatings, alloys such as brass, solder, batteries, coins

Zirconium

Zr

Nuclear fuel canisters, corrosion resistant tubing, explosive primers

a References [1–3].

Table 1.2 World Metal Production, Price and Value Estimates Based on Data from USGS Mineral Commodity Summaries 2012a

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

Metals undergo continual cyclical processing on our planet as shown in Figure 1.1. Metal comes from the earth. Metals are continuously transported and transformed by geological processes.

Figure 1.1 Overview of the general metal cycle.

Metals usually originate as minerals that are discovered, mined, and transformed into metal. All metal in use today is undergoing a very gradual or rapid process of corrosion or degradation. The process of corrosion is essentially a chemical transformation from the metallic form to the mineral or ion form. Atmospheric corrosion of metals leads generally to the formation of metal oxide minerals. The oxygen in the atmosphere and water causes the oxidation. Oxide mineral 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 originate as minerals that are discovered, mined, and transformed into metal.

The processing paths used to produce individual metals are often very different. Many processing methods involve water or water-containing (aqueous) media. Thus, the term hydrometallurgy is often used to describe this topic. This textbook covers the fundamental principles of chemical metallurgy. It also presents many important methods of processing, utilizing, and evaluating metals and metal processes in aqueous media. This book 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 other minerals. 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 manganese, barium, strontium, zirconium, vanadium, chromium, nickel, copper, cobalt, lead, uranium, tin, tungsten, mercury, silver, gold, and platinum are generally scarce. Economic extraction of scarce metals requires a suitable ore deposit.

Table 1.3 Abundance of Elements in the Earth's Crusta

Ores are natural materials containing a concentrated resource. An ore deposit or ore body contains a large volume of ore. Some rare metals and minerals must be concentrated by a factor of 1000 or more above their average natural abundance 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).

Table 1.4 Enrichment Factors Necessary for Ore Body Formationa

Approximate Enrichment

Factor from Natural

Occurrence Level to

Metal

Form Economical Ore Body

Aluminum

4

Chromium

3000

Cobalt

2000

Copper

140

Gold

2000

Iron

5

Lead

2000

Manganese

380

Molybdenum

1700

Nickel

175

Silver

1500

Tin

1000

Titanium

7

Tungsten

6500

Uranium

500

Vanadium

160

Zinc

350

Ores are natural materials containing a concentrated resource. An ore body is a local region in the earth's crust containing desired minerals in concentrations sufficient for commercial extraction.

a Reference [5].

The method of concentration within the earth's crust varies widely. It is dependent on the metal and associated mineral(s) as shown in Table 1.5. Some metals such as chromium are concentrated by precipitation or crystallization in magma. Precipitation is dependent on solubility, which is dependent on temperature. As the magma cools, different local temperature zones are created. Each temperature zone may correspond to a specific mineral (or set of similar minerals) formation zone. Thus, temperature zones can create mineral zones or veins. This process is similar to the creation of ice crystals on a cold windshield. In such cases, a pure component leaves a mixture (air and water vapor) to form a pure deposit in locally enriched regions. A comparison of crystallization from magma and an air/water vapor mixture is illustrated in Figure 1.2.

Table 1.5 Methods of Metal/Metal-Bearing Mineral Deposition

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

Water can play an important role in ore body formation and metal processing.

Mag., magma crystallization;

Sub V, submarine volcanic exhalative processes;

Hydro., hydrothermal solution deposition;

Weath., weathering;

Sed., sedimentation;

Plac., placer deposition;

Metas., contact metasomatism.

Reference [5].

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

Other metals such as silver and tin are concentrated hydrothermally. They are first dissolved in hydrothermal solutions within the earth's crust. Next, they are transported to regions where solution conditions change. Solution conditions can change due to changes in geological formations. Disruptions in rock layers and related aquifers can force water upward to cooler zones. Changes such as temperature decrease cause changes in solubilities. If a dissolved metal is no longer soluble, it precipitates. Precipitation on a large scale can create an ore body. An ore body is a local region in the earth's crust containing desired minerals in concentrations sufficient for commercial extraction.

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

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

Metal ore body formation is the result of large-scale concentrated mineral deposits in specific areas.

Metals such as platinum and gold require highly oxidizing environments as well as complexing agents to make their dissolution in water possible. Other metals such as magnesium tend to dissolve more easily.

Aqueous processing of metals has been performed for several centuries. Copper was recovered from acidic mine waters using metallic iron as early as the sixteenth century in Europe [6]. Evidence of metal dissolution due to bacterial activity in Rio Tinto, Spain, was reported as early as 1670 [7]. The dissolution of precious metals by cyanide has been known since 1783 [8]. However, widespread application of aqueous metal processing is relatively recent. Worldwide hydrometallurgical copper production is increasing steadily. It increased from 13% to 18% from 1996 to 2000 [9]. This increase indicates that hydrometallurgical processing of metals is growing. In addition, most of the world's zinc, uranium, silver, gold, and copper is purified by aqueous means. Historical events associated with aqueous metal processing and utilization are listed in Table 1.6.

Table 1.6 Historical Events Involving Chemical Processing of Metals in Aqueous Media

1.3 Importance of Water

Water makes aqueous processing of metals possible. Water is an unusual substance from a chemical perspective. Most substances with similar molecular weights, such as methane and ammonia, are gases at room temperature. However, water is a liquid at room temperature. Water's unusual properties are due primarily to hydrogen bonding effects. These effects are related to the tendency of hydrogen to donate electrons and the tendency of oxygen to accept electrons. Consequently, water molecules tend to orient themselves accordingly. The oxygen in water molecules has more electron abundance and maintains a slightly negative charge. Hydrogen maintains a slightly positive charge. Electrostatic attraction helps the hydrogen atoms to associate with the oxygen atoms of adjacent molecules. The networked structure of associated molecules is nearly tetrahedral (4.4 nearest neighbors [20]). The resulting structure has open pores large enough for water molecules. The open pores accommodate ion diffusion. In fact, the structure is so porous that water is compressible. High pressures can create 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 molecular breakage is sometimes referred to as hydrolysis. However, the term hydrolysis is more commonly used to describe the decomposition of water by association with another compound. Water splitting occurs at either very high or very low oxidation potentials. At high potentials, oxygen gas and hydrogen ions form. At low potentials, the products are hydrogen gas and hydroxide ions. The associated chemistry is presented in Equation (1.1) and Equation 1.2. Electrons are liberated by electrolysis at high oxidation potentials. Electrolysis is the removal or acquisition of electrons caused by the application of an electrochemical potential or voltage. In contrast, electrons are consumed at low oxidation potentials.

1.1

1.2

Equation 1.1 or its chemical equivalent form that involves hydroxide rather than hydrogen, , is crucial in many metal oxidation/reduction reactions. The reverse reactions are also critical. The hydroxide equivalent of Equation 1.1 in the reverse direction is fundamental to most atmospheric corrosion. The reverse of Equation 1.1 is necessary for metabolism in oxygen-breathing organisms.

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

1.4 Aqueous Processing and Utilization of Metals

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

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

Minerals can be transformed from minerals to metallic metals through hydrometallurgical dissolution, concentration, and recovery processes.

Metal atoms are metallically bonded in pure metals. They have no net charge in the metal lattice. Metal with no net charge is sometimes written with a superscript “o” () (often, no “o” is written). Metal atom removal from a solid piece of metal nearly always requires the loss of at least one electron (). Electron removal converts a metal atom to an oxidized state. An oxidized metal is 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 other types of molecules with a net charge, are generally soluble in water. Examples for metal-bearing compounds that are ions include Fe(OH)+, CuCl3−, and WO43−. Examples of oxidized metal in compounds are FeO, Fe3O4, and Fe2O3. Ions can combine with other ions to form ionic or nonionic compounds. Examples include and . Oxidized metal may also remain as metal ions.

Chemical reactions are needed to extract metals from minerals.

In contrast, metal can be removed from some compounds without electron loss. An example of this is . In this example, copper is oxidized to its divalent (+2) state in CuO and Cu2+. No electron transfer is needed for this exchange reaction. Metals found as metal-bearing minerals are already oxidized. Consequently, metal removal from a mineral may not require electron removal. Some metals in minerals require additional oxidation to be extracted. However, in order to remove or extract a metal from a mineral, a chemical bond must be severed. Thus, chemical reactions are needed to separate metals from minerals. In order to break a chemical bond, a more favorable alternative must be provided. In the case of the copper oxide reaction with hydrogen shown previously, the oxygen is more satisfied 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.

Reactivity of metals is based on atomic structure.

Reactivity of metals is based on atomic structure. Atomic structure is a function of electron orbitals and the number of protons and neutrons. The basic structure is most commonly described using a periodic table. An example of a periodic table is shown in Figure 1.3. Metal elements on the far left, such as sodium and potassium, are very reactive. Their high reactivity limits their use in metallic form to minor, specialty applications. Metals in the second column from the left, which includes magnesium, are also very reactive, but they can be used in metallic form. Metals in other columns have a variety of properties based on their atomic structure. Metals in the lower sections are generally more dense.

Figure 1.3 Periodic table of the elements.

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

After contact, a reaction is needed to solubilize the metal. Reactions involving electron transfer require an electron donor and an acceptor. Other chemical reactions require a reactive species. The reactive species must exchange with or transform the nonmetal part of the mineral. The reaction converts the metal to an ion form to make it soluble. After the metal is solubilized, it can be processed in solution. Dissolved metals are then concentrated, purified, or removed. These aqueous processing steps for metals are presented in Figure 1.4.

Figure 1.4 Flow sheet possibilities for metal dissolution/extraction, concentration, recovery, production, utilization, and processing in aqueous media. (Items in bold font are the main processing steps.)

Concentration processes include liquid–liquid or solvent extraction, ion exchange, carbon adsorption, reverse osmosis, and precipitation. In most industrial processes, unwanted ions are dissolved along with the desired ions. Consequently, the desired ions must be separated from unwanted ions using a concentration process. Concentration processes can also involve the separation of solvent from desired ions. Solvent or solution extraction is a common industrial concentration process. Concentration processes are also referred to as purification processes.

Concentration processes include liquid–liquid or solvent extraction, ion exchange, carbon adsorption, reverse osmosis, and precipitation.

Solvent extraction consists of contacting the metal-bearing aqueous solution with an organic compound. The organic compound is dissolved in a nonaqueous medium. The nonaqueous medium is vigorously mixed with the aqueous medium. Vigorous mixing provides intimate 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 metal into the organic or nonaqueous phase, the nonaqueous medium is separated from the aqueous medium by settling. The extracted metal is then stripped from the nonaqueous phase. Stripping is performed in a smaller volume to concentrate the ions. Stripping solutions are maintained with low levels of impurities.

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

Precipitation consists of adding chemical compounds that react with dissolved entities. The resulting reactions result in precipitate formation. Precipitates may consist of waste or valuable materials. Precipitates are separated from solution by solid–liquid separation processes.

After metal has been concentrated in aqueous media, it can be recovered as a pure metal. Often, pure metal is the desired form of recovered metal. However, it can also be manufactured into products such as coatings and electroformed objects. 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/or metal compounds are used as electrodes in aqueous media.

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

Recovery of metals from concentrated solutions is made by electrochemical reduction.

An example of extraction, concentration, and recovery for copper is presented in Figure 1.5. The extraction step for most metals involves acid, oxidizing agents, and/or complexing agents. The concentration step is often solvent extraction or ion exchange, which releases acid or other ions back into solution. The recovery step provides electrons needed to reduce the metal to its metallic state.

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

Refining or additional purification of metals is often performed electrolytically. This technique, referred to as electrorefining, is utilized to purify metals. High purity metal can be obtained by electrorefining. Electrorefining is the electrolytic process of refining an impure metal. It occurs by electrically forcing metal dissolution and redeposition. Dissolution occurs at the anode and redeposition occurs on the cathode. Often the contaminants are not dissolved and remain as fine particles. Less reactive metals tend not to dissolve. More reactive metals tend to remain solubilized. Thus, the final product is purer than the initial material.

Removal of metal impurities is accomplished by many techniques. Most metal contamination removal is performed by precipitation. Metals can also be removed by ion exchange processes. Alternatively, carbon adsorption is commonly used for metal removal. Reverse osmosis or membrane filtration is another important removal technique.

After metals have been utilized in the form of consumer products such as wires, batteries, vehicle parts, and electronic devices, the parts can be recycled by the same means that allowed their creation, although recycling of consumer products generally requires sorting and size reduction before dissolution as indicated in Figure 1.4.

1.5 Overview of Fundamentals and Applications

The main steps in hydrometallurgical processing are extraction, concentration, and recovery. A discussion of specific applications for these steps has been provided. Next, a discussion of underlying fundamentals will be provided. The fundamentals of hydrometallurgy are related to the application.

Thermodynamics provides quantitative answers regarding chemical stability and reaction viability.

A determination of whether or not reactions occur spontaneously is made using thermodynamics. Effectively, thermodynamics provides answers regarding chemical stability and reaction viability. Thermodynamic calculations can determine whether or not the addition of a specified amount of a compound will dissolve or precipitate. Such calculations can be used to determine whether or not leaching of a specific mineral is favorable under a specified condition. Thermodynamics can be used to determine the voltage needed for recovery of a desired metal under specified conditions. The heat generated by a reaction can also be determined using thermodynamics. Thus, thermodynamics is a critical tool in hydrometallurgical processing. Thermodynamics is discussed in Chapter 2. Chapter 3 discusses speciation and equilibria, which are subsets of thermodynamics.

Rates of reactions are critical to safety and economics in hydrometallurgy.

The rate of reactions is important to hydrometallurgy. If a reaction occurs rapidly, it may lead to an explosion. If a reaction takes 10,000 years to complete, it may be of no commercial interest. Reaction kinetics is an evaluation of the rates of reactions. Factors that affect rates such as temperature, concentration, and area are considered in reaction kinetics calculations. Overall reaction rates are often dominated by mass transport of reacting species. Species must be transported in solution by processes such as diffusion and convection in order to arrive at a reacting surface. Consequently, reaction kinetics is coupled with mass transport. Thus, rate processes such as reaction kinetics and mass transport are discussed together in Chapter 4.

Metals are used in a wide variety of products. Their response to their environment, such as corrosion, often determines their useful life.

Metal extraction, which has already been discussed in the hydrometallurgical context, is presented in Chapter 5. Concentration and recovery, which have also been discussed, are presented in Chapters 6 and 7, respectively.

Utilization of metals in aqueous media, which is important to many areas of technology, is discussed in Chapter 8. Metals are commonly exposed to outdoor environments including water. Exposure of metals to water often leads to corrosion. In batteries, energy is harvested from metal corrosion reactions. Correspondingly, the utilization chapter discusses these topics as well as some niche metal manufacturing in aqueous media.

Appropriate responses to environmental issues are critical to hydrometallurgy.

Hydrometallurgical processing has important environmental ramifications. In order for hydrometallurgical processing to be performed in a sustainable way, it must be performed in an environmentally sensitive way. Chapter 9 discusses environmental regulations and issues related to hydrometallurgy.

Hydrometallurgical processes must be technically and economically viable.

The production of metal from minerals in large commercial settings requires appropriate equipment and design for organized treatment of material. Chapter 10 discusses principles associated with designing hydrometallurgical processes.

Commercial hydrometallurgical processing must be performed in an economically viable way. Engineers need to understand how their technical solutions impact the overall process economics. Engineers are often required to do preliminary economic assessments for technical solutions they offer to corporate management. Solutions to engineering challenges must be economically viable as well as technically sound in order to have them applied on a large scale. Chapter 11 discusses important economic principles that can be applied to hydrometallurgical processes.

Statistical analyses are important tools for evaluating data and making decisions.

All hydrometallurgical engineers are faced with data collection and analysis. Statistics provides robust and trusted methods for data collection and analysis. Chapter 12 discusses statistical principles and methods that have application in hydrometallurgical data collection and analysis.

References

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21. L. Pauling, “General Chemistry,” Dover Publications Inc., New York, 1970.

Problems

1.1. What is the metal cycle and how is it related to hydrometallurgy?

1.2. Name three methods of ore body formation that involve hydrometallurgy.

1.3. Discuss briefly the importance of water in hydrometallurgy.

1.4. List the main metal processing steps.

1.5. List five metals that have large production value.

Chapter 2

Chemical Fundamentals of Hydrometallurgy

Chemical reactions provide the foundation for all hydrometallurgical processing.