Mist precipitators, electrostatic process

Primary copper processing results in air emissions, process wastes, and other solid-phase wastes. Particulate matter and sulfur dioxide are the principal air contaminants emitted by primary copper smelters. Copper and iron oxides are the primary constituents of the particulate matter, but other oxides, such as arsenic, antimony, cadmium, lead, mercury, and zinc, may also be present, with metallic sulfates and sulfuric acid mist. Single-stage electrostatic precipitators are widely used in the primary copper industry to control these particulate emissions. Sulfur oxides contained in the off-gases are collected, filtered, and made into sulfuric acid. 

This subcategory involves phosphoric acid (dry process), phosphoms pentoxide, phosphoms pentasulfide, phosphoms trichloride, and phosphoms oxychloride. In the standard dry process for phosphoric acid production, liquid phosphoms is burned in the air, the resulting gaseous phosphoms pentaoxide is absorbed and hydrated in a water spray, and the mist is collected with an electrostatic precipitator. Regardless of the process variation, phosphoric acid is made with the consumption of water and no aqueous wastes are generated by the process. 

If a wet method for collection is selected, such as a wet electrostatic precipitator, fiber-type self-draining mist eliminator, or wet scrubber, ammonia can be regenerated from the salt solution by reaction with a readily available metal oxide such as lime or zinc oxide with formation of a stable sulfur product for disposal. These metal oxides, however, as well as their reaction products, are insoluble and could cause deposition on heat transfer surfaces and/or clogging in the regenerating equipment. Therefore, as indicated in Figure 2, to ensure continuity and reliability of the process, a soluble metal oxide was utilized (in the form of sodium hydroxide solution) to regenerate the ammonia in the experimental work described. This procedure also allows more eflFective utilization of the metal oxide the soluble oxide (NaOH) can be regenerated in batch equipment outside the continuous portion of the process by reaction with either the aforestated insoluble reactants, lime, or zinc oxide. Better control is aflForded in a batch reactor with more eflBcient use of reactants. However, in full-scale equipment undersirable deposition of reactant and product may be controllable so that batch operation may not be necessary. 

Hydration, too, releases some additional heat. The properties of phosphorus pentoxide and the absorption process inevitably leave as much as 25 % of the oxide plus a phosphoric acid mist in the exit gases from the absorption tower. These are captured on passage through an electrostatic precipitator. By variations of the process details and equipment, grades (concentrations) of phosphoric acid from 75-105% (ortho, or superphosphoric acid) H3PO4 may be made in this manner [28]. 

Electrostatic precipitation is perhaps the most versatile and cost effective of all particulate collecting devices and can be applied to any process where there is a need to remove solid particulate and mist of fume sized particles from the gas stream, whether it be for recovery or pollution control duties. It can be designed to deliver any efficiency for any gas flow rate and temperature and has a low pressure drop and a life span of more than 20 years. 

The process gas is further cooled in the condensing tower (4) by circulating weak acid which is cooled externally in impervious graphite heat exchangers (5). Entrained droplets of acid mist are removed from the gas in electrostatic precipitators (6). Drips from the precipitators are returned to the gas cooling tower. 

All the pyrolysis vapors produce an oil mist following rapid quenching, but their polar character allows the easy utilisation of electrostatic precipitation in the recovery system of a pyrolysis process. 

Following scrubbing and cooling, suspended droplets of scrubbing solution or mist are removed in wet electrostatic precipitators. The cleaned gas may stiU contain unacceptable levels of mercury vapour, which must be removed before transfer of gas to the acid conversion plant. There are a nnmber of mercnry removal techniques but the most commonly used is the Boliden-Norzink mercury removal process, which uses mercuric chloride solution to scrub the gas and absorb mercury according to Eqnation 4.7 (Dyvik, 1985) ... 

In these cases formation of mist may be avoided by selecting conditions so that the volatility of certain reactants is r uced. In all processes where mist is liable to be formed, scrubbers or electrostatic precipitators will have to be installed in the gas stream coming from the reactor. 

Wet precipitators are used where aerosol mixtures must be separated with a high efficiency from waste air, circulating air or process gas. These will have drip trays below the collection plates. A typical plant-scale horizontal wet electrostatic precipitator is shown in Figure 7.20, for which applications include tar separation from coke-oven gas, separation of recondensed lubricant and resin vapours, paints mist and oil mist. 

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