Viscosity of molten salts

For measurements of viscosity of molten salts and glasses at high-temperatures, several methods were proposed. The selection of a particular method depends in general on the viscosity of the liquids to be measured. A broad dispersion of experimental results reflects substantial experimental difficulties connected with viscosity measurement. In general, in the measurement of viscosity of molten salts the method of torsional pendulum is most frequently used, while in the measurement of viscosity of liquids, such as molten glasses, the falling body and the rotational methods are most suitable. Methods for viscosity measurement of liquids with a very high viscosity (above 10 Pa s) will not be described here. 

The viscosity of molten salts determines their appUcatimis in many areas, and their conductivity depends reciprocally rai it. Therefore the viscosity of many molten salts has been measured in the range of their thermal stability, mostly at ambient pressures. Two methods have been mainly employed the capDlaiy flow method and the oscillation method. The latter comprises an oscillating disk or sphere, and in recent years the oscfllating cup has found many supporters [252]. These methods are generally adequate up to 1400 K. 

Table 3.18 The viscosity of molten salts expressed by the and parameters oftj = A,exp(B,/ RT) [214], and their fluidity expressed by the B and I/q parameters of (p = r] = —B + (B/Vo)V [256], The Vo values in parentheses are from Potapov et al. [253], those in italics are the sum of the incompressible ionic volumes from Bockris and Richards [137]...

Sharma SK, Jotshi CK, Singh A (1984) An empirical correlation for viscosity of molten salt hydrates. Can J Chem Eng 62 431 33... 

As in die case of die diffusion properties, die viscous properties of die molten salts and slags, which play an important role in die movement of bulk phases, are also very stiiicture-seiisitive, and will be refeiTed to in specific examples. For example, die viscosity of liquid silicates are in die range 1-100 poise. The viscosities of molten metals are very similar from one metal to anodier, but die numerical value is usually in die range 1-10 centipoise. This range should be compared widi die familiar case of water at room temperature, which has a viscosity of one centipoise. An empirical relationship which has been proposed for die temperature dependence of die viscosity of liquids as an AiTlienius expression is... 

Many plastics because they are organic are flammable incorporate flame-retardants. Additives that contain chlorine, bromine, phosphorous, metallic salts, and so forth reduce the likelihood that combustion will occur or spread. Lubricants like wax or calcium stearate reduce the viscosity of molten plastic... 

Some 30 years ago, transport properties of molten salts were reviewed by Janz and Reeves, who described classical experimental techniques for measuring density, electrical conductance, viscosity, transport number, and self-diffusion coefficient. 

The formation of complex ions is an important problem for the study of the structure and properties of molten salts. Several physicochemical measurements give evidence of the presence of complex ions in melts. The most direct methods are the spectroscopic methods which obtain absorption, vibration and nuclear magnetic resonance spectra. Also, the formation of complex ions can be demonstrated, without establishing the quantitative formula of the complexes, by the variation of various physicochemical properties with the composition. These properties are electrical conductivity, viscosity, molecular refraction, diffusion and thermodynamic properties like molar volume, compressibility, heat of mixing, thermodynamic activity, surface tension. 

It has just been argued that the conductivities of simple ionic liquids, on the one hand, and liquid sihca and water, on the other, are vastly different because a fused salt is an unassociated liquid (it consists of individual particles) whereas both molten silica and water are associated liquids with network structures. What is the situation with regard to the viscosities of fused salts, water, and fused silica Experiments indicate that whereas water and fused NaCl have similar viscosities not far above the melting points of ice and solid salt, respectively, fused silica is a highly viscous liquid (Table 5.46). Here then is an interesting problem. 

Structural studies in fused salts by means of careful and thorough high-temperature measurements of electrical conductivity, density, viscosity, and laser- Raman spectroscopy have been reviewed. Four problem areas are discussed (1) melting mechanisms of ionic compounds with large polyatomic cations, (2) salts as ultra-concentrated electrolyte solutions, (3) structural aspects and Raman spectroscopy, and (4) electrolysis of molten carbonates. The results in these areas are summarized and significant contributions to new experimental techniques for molten-salt studies are discussed.275 The physical properties and structure of molten salts have also been reviewed in terms of operational (hole, free volume, partly disordered crystal) and a priori (intermolecular potential) models.276 Electrochemistry... 

One of the most used and also most suitable methods of viscosity measurement of molten salts is the torsional pendulum method, which could be developed by the introduction of automatic and computational methods. The pendulum is realized in general by an arbitrary rotational body suspended from a torsional wire. However, the exact... 

Janz, G. J., Thermodynamic and Transport Properties of Molten Salts Correlation Equations for Critically Evaluated Density, Surface Tension, Electrical Conductance, and Viscosity Data, J. Phys. Chem. Ref. Data, 17, Suppl. 2,1988. 

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