Because molten salt is a common material in metallurgical industry, physical chemistry of molten salt has become an important branch of physical chemistry in metallurgical process.
structure of molten salt molten salt is composed of cations and anions. The interaction between ions includes electrostatic force (which is a long-range force obeying Coulomb's law), short-range repulsive force and van der Waals force. As a primary approximation, the structure of molten salt can be described by electrostatic hard sphere model. That is to say, anion and cation are charged hard balls with a certain radius, and van der Waals force is ignored or taken as a correction term. Due to the electrostatic effect, every ion in molten salt is surrounded by ions of different numbers. The X-ray diffraction experiment results show that the nearest distance between anions and cations in molten salt is slightly reduced rather than increased compared with the crystal structure, but the number of the first neighbor (coordination number) of each ion is significantly reduced compared with that in the crystal. This shows that there are gaps or vacancies in molten salt with irregular distribution. The molten salt solution formed after the two kinds of molten salts are mixed with each other has similar structure. According to the potential energy equation of ion interaction, the movement and arrangement of ions in molten salt can be simulated by computer, and then many physical and chemical properties of molten salt or molten salt solution can be calculated.
the study on the physical and chemical properties of molten salt and phase diagram of molten salt and molten salt solution not only helps to understand the structure of molten salt and molten salt solution, but also provides a basis for finding a useful molten salt system in production technology. The selection of suitable molten salt electrolyte is a key to the success of molten salt electrolysis process. The properties of molten salt system, such as melting point (phase equilibrium), density, surface tension or interfacial tension, viscosity and conductivity, have important influence on electrolytic production. The study of molten salt phase diagram is very important for understanding the interaction between molten salts and making molten salt electrolysis process. The commonly used measurement methods of molten salt phase diagram are visual inspection, variable temperature method and differential thermal analysis. The calculation of molten salt phase diagram by thermodynamic function with the help of computer has become an auxiliary means of molten salt phase diagram measurement. The type of molten salt phase diagram is related to the type of interaction between molten salts. Some molten salts with similar valence and ionic radius form an almost ideal solution in the liquid phase, and form a continuous solid solution after solidification. Such as potassium chloride-rubidium chloride system. When the valence type or ion radius is quite different, the phase diagram with low melting point will be formed. Such as potassium chloride-lithium chloride system. Some molten salt phase diagrams have stable or unstable intermediate compounds. A few molten salts are not completely miscible, forming a liquid layered system.
except for molten salts with valence type and close ionic radius, which often form an almost ideal solution, the mixing heat of most molten salt systems is not zero. Thermodynamic properties of many molten salt solutions can be calculated by regular solution theory.
Dissolution of metal and gas in molten salt Many molten salts and liquid metals have certain mutual solubility. The solution of metal in molten salt is sometimes called "metal fog" because this solution was mistaken for colloidal solution in the past. "Metal fog" is extremely unfavorable to electrolytic smelting, because it makes the metal precipitated from the cathode dissolve and lose, thus reducing the current efficiency. The solubility of different metals in different molten salt systems varies greatly. Alkali metals, calcium, rare earth metals, cadmium and bismuth have great solubility in their own halide molten salts, while gallium, thallium, tin and lead have little solubility.
many gases can also be dissolved in molten salt. The dissolution of anode gas and its interaction with cathode metal is an important factor affecting the current efficiency in molten salt electrolysis.
electrochemical study of molten salt the electrochemical properties of molten salt are very important for molten salt electrolysis technology. Conductivity of molten salt, electrode potential and electrochemical sequence of metal in molten salt, mechanism and electrode process of molten salt electrolysis, etc. are all research contents of molten salt electrochemistry. The measurement of electrode potential of molten salt is an effective means to study the thermodynamic properties of molten salt solution. It is also an important basis for studying molten salt electrolysis and corrosion of metals in molten salt. The measurement of conduction mechanism and migration number of molten salt, the study of diffusion and polarization on the surface of molten salt electrolysis electrode, and the study of crystallization process of solid metal during cathode precipitation are all important aspects to understand and master the principle of molten salt electrolysis. Anode effect is a characteristic phenomenon of molten salt electrolysis. When the electrolytic composition and current density reach a certain threshold, the anode effect makes the cell voltage suddenly rise sharply, accompanied by some special appearance signs. In the case of industrial electrolysis of molten salt, anode effect causes electric energy loss, but it can also be used as a symbol of electrolytic cell work. At present, there is no unified view on the mechanism of anode effect.