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Development of Supercritical Fluid Micronization Technology

Both supercritical rapid expansion technology and supercritical antisolvent technology prepare particles based on the solubility of supercritical fluids.

RESS and SAS

Both rapid expansion of supercritical solution (RESS) technology and Supercritical Anti-Solvent (SAS) technology prepare particles based on the solubility of supercritical fluids.Both processes are physical processes.

In recent years, more and more researches have been made on the preparation of microparticles by chemical reactions in supercritical fluids, especially the application of supercritical water and supercritical CO2 in the process of chemical reactions.

Using the properties of supercritical fluids can transform heterogeneous reactions into homogeneous reactions. While improving the mass transfer rate and reaction rate, the reaction process and separation process can also be well combined to improve yield and selectivity. .

Hydrothermal synthesis method in supercritical water

The continuous change of electrolyte and the change of solvent density caused by temperature and pressure can change the reaction rate and balance, thereby making supercritical water an ideal hydrothermal synthesis medium.The hydrothermal reaction in which the hydrothermal reaction has a high reaction rate can be prepared. Obtain ultra-fine particles with smaller particle size.

Operation process

  • Operation process of the hydrothermal synthesis method in supercritical water is as follows:
  • The metal salt solution is heated and boosted.
  • Boosted metal salt solution meets with the supercritical water at the mixing point, and undergoes rapid heating and reaction.
  • Solution left the reactor and cooled rapidly, and the precipitated particles were removed through the sieve plate.

Supercritical hydrothermal synthesis granulation method

Cabanas et al successfully prepared Ce-Zr, O2 composite oxide nanoparticles from the mixed solution of cerium ammonium nitrate and zirconium acetate by supercritical hydrothermal synthesis granulation method, and the average particle size of the particles after calcination at 1000℃ for 1h It is 145mm.

At the same time, Fe3O1 nanoparticles with a diameter of about 50mm and various Fe-Ni-Zn composite oxide particles m were obtained by continuous hydrothermal synthesis.

Prepare ZnO nanoparticles

Viswanathan and Gupta used zinc acetate solution as raw material, and successfully prepared ZnO nanoparticles in a continuous tubular reactor by changing the flow rate and feed concentration.
Laser scattering and TEM test results show that the particle size is about 39nm and 120nm respectively.

Equipment requirements

Although the supercritical water hydrothermal synthesis granulation method can be used to prepare ultrafine nanoparticles, the critical temperature (374°C) and pressure (2MPa) of water make the process more demanding on the material of the equipment.

Followed by supercritical water is a strong oxidant, which not only corrodes equipment, but also requires the prepared material to have good heat resistance.

Reverse micellar method of supercritical CO2 in water

The traditional method of recycling particles is likely to cause the increase in particle size and particle size distribution due to the aggregation of particles.

At the same time, a large amount of surfactant will remain on the surface of the nanoparticles. Supercritical CO2 has many properties similar to organic solvents, and can be used as an anti-solvent or solvent trap to remove liquid solvents in the recovery stage, so that thousands (solvent-free) and non-aggregated nanoparticles can be obtained in one step.

Severse micelle method

Han et al adopted the reverse micelle method, using AgNO3 and KBH4 as reactants, isooctane as the continuous phase, and sodium succinate (AOT) and tetraethylene glycol dodecyl ether (CE) as surfactants and auxiliary agents, respectively. Surfactant successfully synthesized nano Ag particles.

Observation results show that the particle size of Ag nanoparticles will vary between 2 and 20 nm with different pressure. They also synthesized ZnS and TiO2 nanoparticles by reverse micelle method, and pointed out that by controlling the temperature, pressure and surfactant concentration and other process parameters, the particle size can be controlled in the range of 1~100m.

Ohde et al contacted two reverse micelles containing Ag+ and X in CO2 water, respectively, and caused the exchange of components by means of the concentration difference between the two microdroplets, and finally successfully obtained AgI and AgBr nanoparticles. It is believed that the interdiffusion between the micelle core and supercritical CO2 is the rate control step in the formation of nanoparticles.

Summary

Traditional micronization method

Although the traditional micronization method is relatively mature, it has shortcomings that are difficult to overcome.

Chemical method

For example, the shortcomings of the chemical method are low yield, high cost and complicated process, so the method is limited to the preparation of some special functional materials, such as super Fine rutile titanium dioxide powder, superfine magnetic iron oxide powder, etc.

Physical method

The disadvantage of the physical method is that the purity, fineness and morphology of the product cannot be guaranteed, which affects the physical properties of the powder, which limits the application of these methods to a certain extent.

Supercritical fluid micronization

As the supercritical fluid micronization method as a simple and environmentally friendly new method overcomes the shortcomings of the traditional micronization method, the micron and nanometer particles that meet the requirements have been successfully prepared, showing good development prospects.

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