Gas separation: processes – Liquid contacting – And degasification of a liquid
Patent
1995-03-24
1996-12-10
Bushey, C. Scott
Gas separation: processes
Liquid contacting
And degasification of a liquid
55228, 552571, 95182, 95195, 95233, 95234, 96201, B01D 300
Patent
active
055826342
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for cleaning a gas containing pollutants, such as hydrogen chloride and heavy metals, by contacting the gas with an aqueous washing liquid which takes up the pollutants. More specifically, the invention relates to a method and an apparatus for cleaning flue gases formed during the combustion of waste, such as household refuse, and recovery of the pollutants removed during the cleaning as valuable products, such as hydrochloric acid, solid chlorides, heavy metals in pure form, and/or minimising the volume of the pollutants removed.
DESCRIPTION OF CONVENTIONAL ART
During the combustion of waste, such as industrial waste and household waste (refuse), the flue gases will contain, inter alia, hydrogen chloride, heavy metals and other pollutants. The hydrogen chloride is formed e.g. when the burnt waste contains sodium chloride from food products or chloride-containing plastics, such as polyvinyl chloride. Before the flue gases are emitted into the atmosphere, hydrogen chloride, heavy metals and other pollutants must be removed by washing the flue gases with an aqueous solution in a so-called scrubber or washer, which results in a diluted aqueous solution of hydrochloric acid. In addition to hydrochloric acid, the aqueous solution contains other pollutants from the flue gases, such as heavy metals, i.e. mainly zinc, lead and mercury, inert solids, as well as bromides and fluorides. The hydrochloric acid concentration is about 2-15% by weight depending on, inter alia, the design of the scrubber and the concentration of hydrogen chloride in the flue gases. Here and in the following, the hydrochloric acid concentration is indicated in per cent by weight of HCl, based on the total amount of HCl and water.
It is known to remove the heavy metals from the washing liquid by adding chemicals which with heavy metals form sparingly soluble compounds. One example is the adding of sodium sulphide, whereby the heavy metals are precipitated as heavy metal sulphides. According to today's technique, the heavy metals are not utilised, but the heavy metal sludge produced is a waste product to be disposed of.
Moreover, it is known to remove the absorbed hydrochloric acid from the washing liquid by distillation. Since hydrochloric acid and water form an azeotrope, a hydrochloric acid is obtained during the distillation, having a maximum concentration of about 20% by weight. Instead of removing the hydrochloric acid in the washing liquid by distillation, a prior art measure is the cleaning of the washing liquid in a wastewater treatment plant and the adding of a chloride-forming substance to the washing liquid and evaporate this for recovery of the chloride of the washing liquid as a solid chloride salt. In these contexts, "chloride-forming substance" means substances which form chlorides with chloride ions. More precisely, oxides, hydroxides or carbonates of alkali metals or alkaline earth metals are intended. Among chloride-forming substances preferred, NaOH, NaCO.sub.3, CaO, Ca(OH).sub.2 and CaCO.sub.3 may be mentioned.
As an example of prior art technique in the field, PCT 92/15519 may be mentioned, which relates to wet treatment of a gas containing hydrogen chloride, the gas being contacted with a washing liquid which contains a chloride-forming substance, such as calcium carbonate. After an optional preevaporation step, the washing liquid is conducted to a distillation unit and distilled, after adding sulphuric acid for recovery of highly concentrated hydrochloric acid.
EP 0 393 402 is a further example of prior art technique, which discloses cleaning of hydrochloric-acid-containing flue gases by absorption in water. The absorption is performed in several steps, the concentration of hydrochloric acid being increased to above 50 g of HCl/l. The thus obtained hydrochloric-acid-containing aqueous solution is distilled to separate the hydrochloric acid. As also appears from EP 0 393 402, it is known to add carbonates or hydroxides of alkali metals
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patent: 3929968 (1975-12-01), Taub
patent: 4384875 (1983-05-01), Batteux et al.
patent: 4539190 (1985-09-01), Shinoda et al.
patent: 4874591 (1989-10-01), Jeney
patent: 5352420 (1994-10-01), Kurzinger et al.
ABB Flakt AB
Bushey C. Scott
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