Physico-Chemical Analysis of Molten Electrolytes

Vladimir Danek †

• Publisher: Elsevier Science
• Year: 2006
• ISBN:
  9780080464961
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Physico-Chemical Analysis of Molten Electrolytes includes selected topics on the measurement and evaluation of physico-chemical properties of molten electrolytes. It describes the features, properties, and experimental measurement of different physico-chemical properties of molten salt systems used as electrolytes for different metal production, metallic layer deposition, as a medium for reactions in molten salts.The physico-chemical properties such as phase equilibria, density (molar volume), enthalpy (calorimetry), surface tension, vapor pressure, electrical conductivity, viscosity, etc. are the most important parameters of electrolytes needed for technological use. For each property the theoretical background, experimental techniques, as well as examples of the latest knowledge and the processing of most important salt systems will be given.The aim of Physico-Chemical Analysis of Molten Electrolytes is not only to present the state of the art on different properties of molten salts systems and their measurement, but also to present the possibilities of modeling molten salt systems, to be able to forecast the properties of an electrolyte mixture from the properties of the pure components in order to avoid experimentally demanding, and in most cases also expensive measurements.This book fills a substantial gap in this field of science. Also documententing the latest research in molten salts chemistry and brings new results and new insights into the study of molten salts systems using the results of X-ray diffraction and XAFS methods, Raman spectroscopy, and NMR measurements.* This book fills a substantial gap in this field of science* Serves as a invaluable reference for all people working in the field of molten salts chemistry* Describes fundamentals of the various properties of molten electrolytes

Chapter 1

Introduction

Publisher Summary

This chapter introduces a book that focus on the measurement and evaluation of physicochemical properties of molten electrolytes. It also characterizes the properties and the structure of different types of molten salt systems. Inorganic ionic melts represent a group of waterless solvents and solutions. Molten electrolytes are used in galvanic metal plating in melts. Molten mixtures of alkali metal halides and zirconium, thorium, beryllium, and uranium are used as heat-bearing medium in primary circuits of nuclear plants. Mixtures of molten alkali metal halides and hydroxides have a potential use in energy storage, where the relatively high value of the enthalpy of fusion is used. In addition to this, direct methods of measurements and different methods of processing the measured data are discussed.

Inorganic ionic melts represent a group of waterless solvents and solutions, which is interesting both from the point of view of fundamental research as well as with regard to their present and prospective use in technical practice. At present we find them used, e.g. in the metallurgical production of aluminum, magnesium, alkali, and refractory metals, where they are used as electrolytes; in black metallurgy, where they form slags gathering unwanted admixtures and reaction products in iron and steel production. Molten electrolytes are used in galvanic metal plating in melts, e.g. in aluminum plating, boriding, or in the deposition of refractory metal layers (Ti, Mo, Nb, etc.). Molten mixtures of alkali metal halides and zirconium, thorium, beryllium, and uranium are used as heat-bearing medium in primary circuits of nuclear plants.

Mixtures of molten alkali metal halides and hydroxides have a potential use in energy storage, where the relatively high value of the enthalpy of fusion is used. Molten carbonates of alkali metal halides are used as electrolytes in molten carbonate fuel cells. A big industrial field, where oxide melts are predominantly used, is the glass industry. Here, the high affinity of these melts to under-cooling and glass formation is exploited.

Recently, there has also been an increase in the importance of melts in their use as a reaction medium for chemical and electrochemical synthesis of compounds for functional and construction ceramics, e.g. double oxides with spinellitic and perowskite structure and binary compounds with prevailing covalent bond character, mainly borides and carbides of transition metals.

In order to decide the suitability of a certain melt in technical practice, an in-depth knowledge of its physico-chemical properties is unavoidable. The present database of the properties of inorganic melts is relatively broad. Many properties are known, such as phase equilibria, enthalpies of fusion, heat capacities, density, electrical conductivity, viscosity, surface tension, emf of galvanic cells of many molten systems, the measurement of which was stimulated first by their technological application.

Nevertheless, the published data on the physico-chemical properties of the molten systems are often incomplete and in many cases the results given by different authors may more or less differ. The reason is that the experimental measurement of the physico-chemical properties of inorganic melts is sometimes inadequate. First, because of the shortage of expensive construction material. Second, due to the relatively high costs connected with the construction of unique measurement devices.

It is obvious that the choice of a suitable melt for a concrete application, which is often a multi-component mixture, is given by the requisite optimum physico-chemical properties at a given temperature. With regard to the experimental difficulty of direct measurement, in choosing a concrete melt it is much cheaper to use convenient structural models that enable us to forecast the values of the physico-chemical properties of multi-component molten systems on the basis of knowledge of the properties of the pure components. It is then necessary to know the mathematical description of the functional property – composition and property – temperature dependences. The definite form of such dependences is given first by the structure (ionic composition) of the molten systems, which in many cases is still not satisfactorily known. Recently, research on the structure of inorganic ionic melts has rapidly advanced. A substantial improvement in the experimental techniques, especially in high-technology electronics and computer techniques, has contributed to this. However, the present knowledge of the physico-chemical properties of molten systems is in many cases on a higher level than the possibility of their interpretation. First, it follows from the lack of an adequate knowledge of their structure. High-temperature X-ray diffraction analysis applied to the liquid phase has not principally contributed to the classification of the structure of melts, mainly due to the lack of suitable approaches in structural analysis. More success was attained recently by the exploitation of sophisticated methods of high-temperature infrared and Raman spectroscopy, NMR, MAS NMR, and some numerical methods, especially methods of quantum chemistry and of molecular dynamics.

While for the last mentioned methods of investigation the measuring devices could be acquired, with only small adaptations, devices for measurement of physico-chemical properties are not available on the market. The measurement, especially its precision depend on the skills of the scientists and the workers in the laboratory. For instance, the construction of a high-temperature torsional pendulum viscosimeter requires in-depth knowledge of many features of the technique. It should also be mentioned that for scientific purposes, the accuracy and precision of results must be at least one order higher than for industrial purposes. On the other hand, using generalized knowledge many industrial measurements could be avoided as necessary data could be estimated with good accuracy.

Generally, any electrolyte is composed of a mixture of alkali metal halides, which serve as solvent, and the compound of the deposited metal. In addition, there may be other additives, which may improve the properties of electrolyte or enhance the metal deposition.

For a certain purpose a concrete molten system must be used. For example, in the electro-deposition of metals from molten salts several types of molten systems were tested as electrolytes. From the analysis of literature and on the basis of the electro-active species used, they can be divided into two principal groups:

• systems containing halo-complexes of deposited metals,

• systems containing oxides or oxy-complexes of deposited metals.

In all of the investigated systems, one of the most important tasks to be solved is to find the proper composition of the electrolyte with regard to both the suitable physico-chemical properties and the desired character of the electrodeposited product. Both problems are closely related to the actual structure, i.e. the ionic composition of the melt.

Quite recently attention was paid to the role of oxides, either as electro-active species, as impurities or as additives in the electro-deposition of transition metals. This may be demonstrated, e.g. in the case of electro-deposition of molybdenum, where the electrolysis of neither pure K₂MoO₄, nor the KF–K₂MoO₄ mixture yields a molybdenum deposit. However, introducing small amounts of boron oxide, or silicon dioxide to the basic melts, smooth and adherent molybdenum deposits may be obtained. Also, in the case of niobium and tantalum deposition, the presence of oxygen either from the moisture or added on purpose leads to the formation of oxohalo-complexes, which due to their lowered symmetry and thus lower energetic state, decompose easier at the cathode yielding pure metal.

It is thus the aim of the present book to serve as a guide in the measurement of the physico-chemical properties of molten salts and to characterize briefly the properties and the structure of different types of molten salt systems. In this book, only direct methods of measurements and different methods of processing the measured data are discussed. Computer simulation methods are not considered.

Chapter 2

Main Features of Molten Salt Systems

Publisher Summary

This chapter discusses the main features of molten salt systems. Thermodynamic principles to the melting process of substances arose approximately at the time of the formulation of the second law of thermodynamics. The main advantage of classical thermodynamics is that it is not necessary to know the structure of the investigated system. It enables the most important classical thermodynamic conclusions on the melting process to be made before the general idea of the atomic structure of a substance was accepted. At high temperature, molten salts are miscible in any ratio whereas, a limited miscibility is observed at lower temperatures. The molar volume of molten salt mixtures is not influenced by structural changes, as the changes in the ionic ordering are not impressive. In molten salts, the positively and negatively charged ions are in close contact and the interaction forces are also strong. The chapter also discusses structure of melts containing metals of various valences.

The transition from the solid to the...

Table of contents

• Cover image
• Title page
• Table of Contents
• Publisher’s Note
• Preface
• Chapter 1: Introduction
• Chapter 2: Main Features of Molten Salt Systems
• Chapter 3: Phase Equilibria
• Chapter 4: Enthalpy
• Chapter 5: Density
• Chapter 6: Surface Tension
• Chapter 7: Vapor Pressure
• Chapter 8: Electrical Conductivity
• Chapter 9: Viscosity
• Chapter 10: Direct Methods of Investigation
• Chapter 11: Complex Physico-chemical Analysis
• REFERENCES
• Index