Some physical parameters of extractant

The physical properties and other technical parameters of some extractants and solvents are generally: melting point, boiling point vapor pressure, dielectric constant, dipole moment, relative density viscosity, surface tension, ignition point, flash point, explosion limit, toxicity and solubility in water (or miscibility with water), etc., which is briefly analyzed below.

(1) Melting point

The melting point should not be higher than the lowest temperature where the manufacturer is located to ensure that the solvent is liquid, otherwise it needs to be insulated.

(2) Boiling point

According to the different recovery and regeneration methods of the extraction solvent, the same boiling point range is required, as mentioned above, if the distillation method is used to recover, the boiling point is not too high, and the boiling point is too low, it will increase the evaporation loss, and the water solubility of the low boiling point solvent is generally larger.

(3) Vapor pressure

There is a corresponding relationship between vapor pressure and boiling point, that is, the lower the boiling point, the higher the vapor pressure under the same temperature condition, which will increase the loss of solvent.

(4) Dielectric constant

Dielectric constant is a parameter that reflects the polarity of solvent. High polarization, high dipole moment, low wall mass and high density solvent have high dielectric constant. The high polarity of the extraction solvent is conducive to the extraction of dry polar solutes, and vice versa, so the dielectric constant is also an important reference for the selection of extractants.

(5) The larger the density difference between the two phases, the more conducive to phase separation, and the recommended minimum density value is 0.02g/cm3

(6) Surface tension and interfacial tension between the two phases

The interfacial tension value of the two phases depends on the surface tension of the solvent. The experiment shows that the interfacial tension between 5 and 25mN can have good droplet fragmentation and polymerization, and the interfacial tension is high

The droplet tends to be stable and not easily broken, and the droplet is not easily polymerized when the interfacial tension is low. The recommended minimum interfacial tension value is 2mN/m25 when using the mixed clarifier and extraction column operation, and the centrifugal extraction separation can be quickly separated.

(7) Viscosity

The lower the viscosity of the two phases, the lower the power of the dry dispersed phase drops due to dry action, the faster the separation rate of the two phases, and the lower viscosity of the continuous phase is beneficial to phase separation, so it is possible to use the phase with greater viscosity as the dispersed phase.

The two phase density difference, interfacial tension and phase viscosity are the main controlling factors that determine the two phase separation speed under certain temperature conditions.

(8) Flash point

The flash point is the lowest temperature at which flash occurs. The lower the flash point, the greater the risk of fire.

(9) Ignition point

The ignition point is the ignition point, which is the lowest temperature at which combustibles occur. In general, the burning point of liquids is much higher than the flash point

(10) Explosion limit

It is an important factor to ensure production safety, especially its lower explosive limit should be within the safe range. The flash point, ignition point and explosion limit are very important for the safe operation of liquid-liquid extraction.

(11) Toxicity

Toxicity is another safety control factor, as indicated by the Generic LDo for Toxicity of solvents, which is a semi-lethal dose per unit mass of laboratory animals (rats or mice). The toxicity of the solvent can be divided into different grades according to its size, such as highly toxic, highly toxic, toxic, low toxic, mildly toxic and non-toxic (there are other grading methods), and the application of highly toxic and highly toxic solvents should be strictly prohibited. In order to control production and ensure product safety, the concentration of solvents in the workshop air and the residual amount of solvents in the product are also put forward limiting indicators.

(12) miscibility with water

On the one hand, the solubility of the solvent in water, which is an important factor in the selection of extractants, obviously the lower the water solubility of the solvent, the better, low water solubility is conducive to reducing the loss of the solvent and reduce the energy consumption of recovering the solvent from the extract, and reduce the primary pollution; On the other hand is the solubility in the solvent, too much water in the solvent will affect its yield. It should be pointed out that the solubility data given in the general manual are the solubility under normal temperature (20℃ or 25℃) and pure water conditions, and the actual production process conditions are different, so the influence of temperature changes and changes in water phase composition on the solubility should be noted. In addition, the data cited by different manuals are different, and the solubility data may also be different, so comparison and verification should be carefully carried out when quoting, and experimental verification should be carried out by ourselves if necessary. In addition, there are also molar mass, phase heat capacity, liquid phase heat capacity, solubility parameters are also a reference index to investigate the performance of solvents.