Chemical Dissolution of Metal Oxides

Miguel A Blesa

• Publisher: CRC Press
• Year: 2018
• Print ISBN:
  9781315891415
• eBook ISBN:
  9781351087414
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

The dissolution behaviour of metal oxides has applications in many scientific fields, each with its own jargon and methodological approach. Any scientist interested in this subject should understand the literature from these various areas. This book describe different specialized treatments to surface-controlled metal oxide dissolution reactions and translates them into a unified picture based on surface complexion

Part I: Introduction

Chapter 1

The Impact of Metal Oxide Dissolution in Technology and Environmental Science

1.1 Hydrometallurgy

Hydrometallurgy provides examples of the application of most of the types of oxide dissolution processes that will be described in this book: copper and zinc oxides can be leached from their ores by diluted minerai acids; dissolution in alkaline medium is the key step in the Bayer process for obtaining pure alumina from bauxite; uranium oxides are oxidatively dissolved, either in the presence of acids or complexants (carbonate); iron may be selectively eliminated (and later recovered) from ilmenite by leaching with sulfuric or hydrochloric acid, a process that leads to synthetic rutile. Because of the wide variety of leaching reagents that can be used, the chemical differences between the various components of the ores can be better exploited by hydrometallurgical processes, as opposed, for instance, to pyrometallurgy. Selectivity and product purity are therefore distinct properties of aqueous solvents. The wise use of these properties requires, of course, the profound knowledge of reactivity patterns of metal oxides (and of other raw materials).

A typical flowsheet of a hydrometallurgic process is shown in Figure 1.1, taken from Reference 1. As usuai, both dissolution and its reverse phenomenon, precipitation, are involved. The typical selectivity of hydrometallurgy is implicit in the term “leaching”, as opposed to “dissolution” (see Chapter 4).

Figure 1.2 illustrates the steps in the hydrometallurgy of uranium. A detailed description of this subject may be found in Reference 2.

1.2 Weathering of Minerals

Soils and sediments are formed by the action of naturai waters on rocks;^3,4 this process, known as weathering, is depicted in its context in Figure 1.3. Weathering embodies all processes that transform primary minerals (that are in many cases unstable towards water) into clays and eventually into sedimentary rocks through the dissolving influence of water and atmospheric gases;⁵ the involved minerals may be salts, silicates, or oxides. Chemical weathering controis the global hydrogeological cycle of many elements and influences therefore the properties of soils (such as nutrient content) that have far-reaching consequences. Weathering may involve hydration, ion exchange, oxidation, dissolution, and nucleation and growth of new phases, all influenced by diffusional accessibility and structural relationship between phases. Weathering of oxides and silicates have been studied extensively and illustrate the operation of surface-controlled reactions in the involved dissolution processes. Table 1.1 presents examples of different types of reactions, and Table 1.2 summarizes some of the documented evolutions of minerals through weathering.⁶

FIGURE 1.1 Schematic flowsheet in a typical hydrometallurgic process. (From Habashi, F., Chem. Eng. News, 60, 4, 1982. Copyright ©1982 American Chemical Society. With permission.)

FIGURE 1.2 Schematic flowsheet in the hydrometallurgic processing of uranium ores.

FIGURE 1.3 The formation of soils, sediments, and sedimentary rocks by weathering. (From Stumm, W. and Morgan, J. J., Aquatic Chemistry. An Introduction Emphasizing Chemical Equilibria in Naturai Waters, 2nd ed., Wiley-Interscience, New York, 1981. Copyright ©1981 John Wiley & Sons, Inc. With permission.)

TABLE 1.1
Types of Weathering Reactions

TABLE 1.2
Weathering Sequences of Minerals

1.3 Mobilization of Metal Ions in the Environment

Aquatic systems act as conveyor belts for a huge naturai chemical reactor that processes not only rocks, but also any other materials that come in contact with it, especially wastes of all types. In these systems, the kinetics of the solid/liquid interaction are dominant. Figure 1.4 (taken from Reference 7) stresses the analogy between aquatic systems and chemical reactors.

FIGURE 1.4 The analogy of the ocean and a chemical reactor. (From Siever, R., Sedimentology, 11, 5, 1968. Copyright © 1968 Elsevier-Sequoia S.A. With permission.)

The largest of these aquatic systems are the oceans, where redox reactions are of special importance. Transition metals can often be present in aqueous homogeneous media in more than one oxidation state. The redox potentials of the Fe(III)/Fe(II) and Mn(IV)/Mn(III)/Mn(II) couples are especially adequate to bring about an active redox chemistry that essentially depends on the redox potential of the aqueous medium, E_H. This magnitude is, in turn, determined by the O₂ partial pressure, pO2 and biological activity. The pH of the medium is also highly relevant because the change in the oxidation state of the metal ion usually produces drastic changes in its hydrolytic behavior. For example, the reaction

Fe2++O2→Fe3++O2−(1.1)

is highly endoergonic at pH 1, but much less so at pH > 7, where the stoichiometry is

Fe2++O2+3H2O→Fe(OH)3+HO2−+2H+(1.2)

The limited solubility of Fe(OH)₃ (and of hydrous oxides of metal ions in high oxidation states in general) implies the precipitation of iron from anoxic naturai waters whenp₀ is increased (Equation 1.2). Conversely, iron(III) hydrous oxides formed in oxygen-rich media will tend to dissolve when transported to anoxic waters, either by the action of organic matter or d...

Table of contents

• Cover
• Half Title Page
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Part I: Introduction
• Part II: Phenomenological Approach to Metal Oxide Dissolution
• Part III: Fundamentals of Metal Oxide Dissolution Mechanisms
• Part IV: Review of Selected Cases
• Symbols
• Index