Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Patent
1995-02-23
1997-05-13
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
162 50, 205521, 205536, 205576, D21C 1105, D21C 1112
Patent
active
056288743
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to an environmental-friendly process for reducing the content of chloride in a liquor inventory of a chemical pulp mill. According to the invention, in a recovery system for pulping chemicals containing sulphur and an alkali metal, precipitator dust formed in a recovery boiler is collected and withdrawn, dissolved in water and electrolyzed for production of chlorine or hydrochloric acid in the anolyte. Since the dust normally contains a large amount of sodium sulphate, sulphuric acid and sodium hydroxide can also be produced in the electrolysis. To reduce the content of impurities, before the electrolysis, the pH of the aqueous solution is adjusted to above about 10 to precipitate inorganic substances which are separated-off together with flocculated or undissolved substances.
BACKGROUND TO THE INVENTION
In the production of a chemical pulp, chips of ligno-cellulose-containing material are cooked in an alkaline or acid aqueous solution. This cooking liquor contains inorganic pulping chemicals to improve the dissolution of lignin. The cooking is normally carried out at a temperature above 100.degree. C. to reduce the residence time for the pulp produced. Therefore, the cooking is carried out in a pressure vessel known as a digester.
In the production of sulphate and sulphite pulps with an alkali metal as a base, normally sodium, it is possible to recover the inorganic pulping chemicals in the spent liquor leaving the digester. It is vital both to economy and environment to recover these pulping chemicals to the largest possible extent. This is achieved in the pulping chemical recovery system, which essentially transfers the used inorganic pulping chemicals into a chemical state, where they can be used again for cooking.
An essential part of the recovery system is the recovery boiler, where the spent liquor is burned. Normally, make-up chemicals are added to the spent liquor before the recovery boiler to make up for the chemicals lost during cooking and recovery. The spent liquor is sprayed into the lower part of the boiler, previously at a relatively low temperature to remove free water. Modern recovery boilers operate at a high temperature to reduce the content of sulphur in the flow gases leaving the boiler. Higher up in the boiler, gases and vapors of light hydrocarbons and decomposition products are volatilized. This is known as pyrolysis. Then, the pyrolysis products are burned after mixing with air or oxygen. The solid carbon-based residue which remains after complete pyrolysis of the organics is then heterogeneously burned. The solid particles formed are collected as a dust in precipitators at the top of the recovery boiler, to reduce the release of solid material to the surrounding atmosphere.
A substantial and increasing problem with the pulping chemical recovery system, is the presence of chloride and potassium in the spent liquor entering the recovery boiler. These elements tend to reduce the capacity of the recovery boiler to produce useful chemicals. Thus, chloride and potassium increase the stickiness of carryover deposits and dust particles to the recovery boiler tubes, which accelerate fouling and plugging in the upper part of the recovery boiler. Chloride also tend to increase the corrosion rate of super-heater tubes.
Chloride and potassium are concentrated in the dust formed during the combustion of spent liquor in the recovery boiler. The dust is collected in dry-bottom or wet-bottom electrostatic precipitators. The dust mainly consists of sodium and potassium salts, where sulphate, carbonate and chloride are the dominant anions. The amount of dust corresponds to about 5 to 15% of the sodium entering the recovery boiler, which corresponds to about 50 to 150 kg dust per ton pulp, if the dust is calculated as sodium sulphate.
Today, normally all of the precipitator dust collected and withdrawn from the recovery boiler is recycled to the flow of spent liquor to be burned in the boiler. When the concentration of chloride or potassium is too high, a portion of
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Lindberg Hans
Sundblad Birgitta
Czaja Donald E.
Eka Nobel AB
Nguyen Dean T.
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