Electrostatic precipitation particle charging

Precipitate particles grow in size because of the electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution. Ions common to the precipitate are chemically adsorbed, extending the crystal lattice. Other ions may be physically adsorbed and, unless displaced, are incorporated into the crystal lattice as a coprecipitated impurity. Physically adsorbed ions are less strongly attracted to the surface and can be displaced by chemically adsorbed ions. 

The development of electrostatic precipitators was based mostly on empirical work, and it has produced more than 1000 patents covering all aspects of electrostatic air cleaning. From the theoretical point of view, important milestones were papers published by Deutsch as well as White.These papers deal with the collection efficiency of the electrostatic precipitator. The most important early papers dealing with the electrical charging of particles are the ones published by Arendt and Kallmann, Pautheniet and Moreau-Hanot, and White. ... 

The properties of gas ions are of great importance for the electrical performance of an electrostatic precipitator. They also are very important for particle-charging processes. The size of gas ions is normally such that they can be regarded as gas molecules carrying a single elementary charge. It can even be assumed that ions form a gas component with a very low- partial pressure. Thus, the thermal motion of gas ions is assumed to be similar to that of gas molecules. The most important parameters describing the properties of gas ions are... 

The concentration of gas ions significantly influences the particle-charging process. The high ion concentration is essential for the effective charging of fine particles. The distribution of ion concentration in a pipe-type electrostatic precipitator can be approximated by using the equations presented in the previous section. 

According to this approximation, the drift velocity is proportional to the square of the electric field. This is a clear indication of the importance of the electric field inside an electrostatic precipitator. Equation (13.60) is a valid approximation for large particles [dp > 0.5 m), provided that particle charge is close to the saturation level. In the case of small particles, the effect of diffusion charging must be taken into account. 

The two-stage electrostatic precipitators used in light-industry applications are compact devices which can be fitted into the ventilation system. These air cleaners are normally used to clean air from dusts, smokes, and fumes in industrial workplaces. The basic features of these devices are the separate sections for particle charging and collection. The charging section consists of thin metal wires installed between grounded metal plates. The distance... 

W hite, H. J. Particle Charging in Electrostatic Precipitation. Trans. Am. Inst. Electr. F.ng. 70 il951), p. 1189. 

Electrostatic precipitator A filtering system for the removal of particles from an air stream by giving them an electrical charge. The charged particles are attracted to plates of opposite polarity onto which they adhere. The precipitators are classified as... 

Gaseous ion diffusion A method of charging particles in an electrostatic precipitator. 

Figure 4-79. Charging particles in electrostatic precipitator. By permission, adapted after A. Nutting, American Air Filter Co.

Very fine particles such as smokes can be caught by electrostatic precipitation. A high voltage is applied to plates or wires within the filter bank, to impart a static charge to dirt particles. These will then be attracted to earthed plates, and adhere to them. Impurities are generally cleaned off the plates by removing the stack and washing. 

Electrostatic precipitators devices used to trap fine dust particles (usually in the size range 30 to 60 p,m) that operate on the principle of imparting an electric charge to particles in an incoming airstream which are then collected on an oppositely charged plate across a high-voltage field. 

Phossy water, a waste product in the production of elemental phosphorus by the electric furnace process, contains from 1000 to 5000 mg/L suspended solids that include 400-2500 mg/L of elemental phosphorus, distributed as liquid colloidal particles. These particles are usually positively charged, although this varies depending on the operation of the electrostatic precipitators. Furthermore, the chemical equilibrium between the fluoride and flurosilicate ions introduces an important source of variation in suspended solids that is a pH function. Commonly... 

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