Particle removal scrubbers

Double-sided brush scrubbing followed by a spin-rinse-dry step dominates the industry. An example of a double-sided brush scrubber is shown schematically in Fig. 16. The rollers keep the wafer positioned and rotating while the brushes dean debris from both the front and back sides of the wafer. The cleaning solution is delivered through the bristles themselves. Typically, the bristles are porous so that fresh solution can be delivered directly to the wafer and minimal amounts of debris accumulate on the brush. Increasing the down force increases the particle removal efficiency up... 

Both silicon oxide and alumina slurries can be efficiently removed on PECVD TEOS oxide or silicon nitride substrates in a conventional SCI or in a SCI without any water peroxide in the case of outcropping tungsten (see Fig. 5). When water peroxide is not present to continuously regrow a protective oxide layer, OH species can etch the silicon. In the latter case, the backside of the wafer must therefore be protected with a nitride or oxide layer to avoid a severe silicon roughening effect. Nevertheless to achieve the same particle removal efficiency obtained with a scrubber, power mega-sonics also have to be used (see Fig. 18). 

HF-based chemistry is particularly interesting due to its compatibility with all back-end metals and barriers. Unfortunately as the absolute values of tbe zeta potential in the A area of Fig. 13 are lower than in alkaline media, the removal mechanism is even more difficult. Indeed as seen in Fig. 19, the particle removal efficiency in the HF-HCl mixture is almost zero for actual alumina slurries. Very-high-power megasonics performed in a specific HF-compatible bath are absolutely necessary to obtain the same good residual particle level as with the scrubber. 

When using a scrubber, an additional acid treatment must be implemented, preferably afterward. If an HF bath is used prior to the scrubber to improve global particle removal [24], the metallic cleanliness obtained can be preserved if a complexing agent is used in the scrubber process—EDTA in water or choline instead of ammonia. The acid treatment can be introduced in the second scrubber station citric acid or HF. In the latter case, the scrubber has to be especially designed for corrosion and security reasons. 

A tribology study [28] focusing on the coefficient of friction by varying the instrumental parameters of scrubbers suggested that the hydrodynamic drag force is a very important factor for particle removal [29]. Busnaina et al. [30] found that full contact between a brush and a particle is necessary to lift or roll particles smaller than 0.1 mm off a substrate. 

Steelmaking requires 12-15 m /tonne (ca. 3,000-3,700 U.S. gal/tonne) of cooling water and 80-100 L/tonne (ca. 20-25 U.S. gal/tonne) for scrubbing of exhaust gases [37]. Cooling water disposal requires consideration of possible thermal effects. Treatment of scrubber effluent is similar to that used with blast furnace scrubber effluent, except that steelmaking waste streams are usually acidic whereas blast furnace effluent is normally alkaline. After particle removal, pH neutralization can be by stream blending, if the two processes are operated close to each other. 

The mass at constant specific gravity is a function of the cube of the equivalent particle diameter. For example, to capture a 3-micron particle with the same efficiency as a 5-micron particle, the gas velocity must be increased by 2.15 times. Because pressure drop is proportional to the square of the gas velocity, the scrubber must operate at 4.62 times the pressure drop for the 3-micron particle to be captured with the same efficiency as 5-micron particle removal. 

Scrubbers. Scrubbers are designed to contact a liquid with the particle-laden gas and entrain the particles with the liquid. They offer the obvious advantage that they can be used to remove gaseous as well as particulate pollutants. The gas stream may need to be cooled before entering the scrubber. Some of the more common types of scrubbers are shown in Fig. 11.2. 

Dust entrained in the exit-gas stream is customarily removed in cyclone cohectors. This dust may be discharged back into the process or separately cohected. For expensive materials or extremely fine particles, bag collectors may follow a cyclone collector, provided fabric temperature stability is not hmiting. When toxic gases or solids are present, the exit gas is at a high temperature, the gas is close to saturation as from a steam-tube diyer, or gas recirculation in a sealed system is involved, wet scrubbers may be used independently or following a cyclone. Cyclones and bag collec tors in diying applications frequently require insulation and steam tracing. The exhaust fan should be located downstream from the cohection system. 

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

Small solid particles, present in dust and grit emissions, have very low settling velocities (Table 4.4) The collection efficiencies of simple cyclones are tlierefore, as shown in Figure 17.3, relatively low. Fabric filters, electrostatic precipitators or wet scrubbers may be required to remove particles <5 pm in size with an acceptable efficiency. Therefore the cost of pollution control inevitably increases when dealing with particle size distributions skewed towards the lower end. 

Wet scrubbers rely on a liquid spray to remove dust particles from a gas stream. They are primarily used to remove gaseous emissions, with particulate control a secondary function. The major types are venturi scrubbers, jet (fume) scrubbers, and spray towers or chambers. Venturi scrubbers consume large quantities of scrubbing liquid (such as water) and electric power and incur high pressure drops. Jet or fume scrubbers rely on the kinetic energy of the liquid stream. The typical removal efficiency of a jet or fume scrubber (for particles 10 g. or less) is lower than that of a venturi scrubber. Spray towers can handle larger gas flows with minimal pressure drop and are therefore often used as precoolers. Because wet scrubbers may contribute to corrosion, removal of water from the effluent gas of the scrubbers may be necessary. 

The basic principle of wet collectors is to wet the contaminant particles m order to remove them from the gas stream. There is a wide range in scrubber design, cost, and performance." " " Because of this wide variability, scrubbers must be carefully matched to specific applications. 

An impingement scrubber is designed to make the dust particles impinge and adhere to water droplets. Gas is introduced at the bottom and passes up through water-covered perforated trays, where dust is removed by the scrubbing liquid. 

Spray-type collectors In this system water is sprayed or cascaded onto the contaminated air directly or through packed towers, and the fumes or dust are washed away by absorption. These collectors are used extensively on the treatment of fumes of all types and have low pressure drops and hence low power requirements compared to induced spray. A development of this collector is the venturi scrubber, which injects high-pressure water into a venturi through which the fume-laden air is passing. The intimate contact of the two ensures absorption and removal from the air stream. These collectors are used in fume removal and have efficiencies of more than 99 per cent on sub-micron particles. 

Methods of dust removal depend mainly on the particle size of the dust and the temperature and moisture content of the gas. The methods used are broadly divided into dry methods and wet methods. The dry methods involve the use of gravity and baffle chambers, cyclones, filters, and electrostatic precipitators, while the wet methods involve the use of spray towers and venturi scrubbers. In principle, wet cleaning is preferred to dry cleaning because of the excessive wear associated with and the difficulty in handling the fine dusty material removed in the dry methods. The wet methods, however, must be followed by such operations as filtration, drying of filter cakes, and recycling of water. 

In wet scrubbing the dust is removed by counter-current washing with a liquid, usually water, and the solids are removed as a slurry. The principal mechanism involved is the impact (impingement) of the dust particles and the water droplets. Particle sizes down to 0.5 /i.m can be removed in suitably designed scrubbers. In addition to removing solids, wet scrubbers can be used to simultaneously cool the gas and neutralise any corrosive constituents. 

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