Arsine, Diborane, and Phosphine: These gases (used as dopant gases in the manufacture of electronic circuitry) have been scrubbed effectively using oxidizing solutions generally stabilized with an alkali to maintain a basic pH. Since the absorption of these compounds is relatively difficult, it is essential that a deep counter current packed bed or, if necessary, two beds in series are used to achieve the low discharge gas concentrations required. Potassium permanganate is used often as the oxidizer but care must be taken when using this oxidizer because of the relatively low solubility of the permanganate itself and some of its reaction products in water.
BF3 and BCl3: These compounds hydrolyze readily and form products of reaction which are water soluble and generally reactive with alkali compounds. They also tend to form extremely fine aerosols (sub-micron sized) which are not effectively removed by conventional wet scrubbers. Although sometimes low in concentration these aerosols produce observable and objectionable plumes.
The Clean Air Group, LLC has developed a multiple staged approach (for low gas flow application) consisting of two Jet Venturi Fume Scrubbers in series or a single Jet Venturi Fume Scrubber followed by a Counter Current Packed Tower with an irrigated mesh mist eliminator section and final fiber bed filter. This ensures complete absorption of the BF3 and/or BCl3 as well as capture of any aerosols formed.
Halogenated Silicon Compounds: SiCl4, SiClx, SiF4 (Case history available): The Clean Air Group, LLC has found that the combination of multiple air pollution control technologies is the most effective way to handle this difficult application. The keys to success are the abilities to scrub acid gases effectively, remove fine particulate efficiently, and handle the gel-like substances formed with low maintenance requirements.
These compounds hydrolyze with water to form gel like compounds and the halogen acid. For instance, when SiCl4 is contacted with water it forms silicon gels and Hydrochloric Acid. The gels will easily adhere to equipment surfaces, spray nozzles, etc. and cause difficult maintenance problems. The use of an aqueous NaOH scrubbing solution results in the formation of more easily handled silicon gel complexes than if in the presence of water only. Of course, the NaOH also reacts with and neutralizes the halogen acids. In addition, if some of the reactions occur in a moist gaseous phase, extremely fine particulate may form.
The Jet Venturi Fume Scrubber first stage is ideal for the initial contact where the gels are formed, removed, and acid gas absorption begins. The extremely open body design and the large open spray nozzle minimizes maintenance as compared to other types of wet scrubbers. The Jet Venturi Fume Scrubber can also be configured to have a top gas inlet and side liquid inlet (the reverse of the standard design) which allows for the use of tangential wash nozzles in the upper portion of the body. This can further reduce down time since the gels can be effectively washed from the body walls.