Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Including recovery of organic by-product
Patent
1996-01-17
1999-11-23
Nguyen, Dean T.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Including recovery of organic by-product
162 16, 162 31, D21C 1106
Patent
active
059893877
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for reducing the chlorine concentration in a sulphate cellulose process, wherein part of the chlorine content in the chemical cycle is separated from the cycle and removed.
BACKGROUND OF THE INVENTION
In the process of a sulphate cellulose mill, chlorine (essentially present in the combined form, primarily as sodium chloride) accumlates in the chemical cycle as the amount of chlorine accumlates the process from the raw material, such as wood, from water, together with substituting chemicals, etc., is greater than the amount of chlorine removed from the process in any form. Even though chlorine is removed together with the pulp and at other process stages, it, however, tends to accumlate in the process. Excessive chlorine causes disadvantageous corrosion and pluggings in the soda recovery boiler and is also otherwise disadvantageous in the regeneration of chemicals. Various attempts have been made to remove chlorine, e.g. together with salts in the form of NaCl. U.S. Pat. No. 3,746,612 discloses a method for removing chlorine from white liquor, where white liquor is treated in an extremely complicated way in order that chlorine contained in the solution could be separated as NaCl. In general, the removal of chlorine from the process is difficult in most cases and requires plenty of extra equipment. In addition, chlorine removal processes are difficult to adjust.
BRIEF SUMMARY OF THE INVENTION
The object of the present invention is to provide a method by means of which chlorine can be removed from the process easily and simply without any major equipment investments. The method according to the invention is characterized in that sulphurous odour gases are introduced into the soda recovery boiler at least in such an amount that the concentration of sulphur oxides in the soda recovery boiler is such that at least part of chlorine separating in gaseous form from the bed is in the form of hydrogen chloride in the upper part of the soda recovery boiler, that the hydrogen chloride is separated from the flue gases by scrubbing the flue gases, whereby the hydrogen chloride (HCl) contained in the flue gases reacts with a washing solution and can thus be removed by separating part of the washing solution of the flue gas scrubber.
An essential feature of the invention is that sulphorous strong and possibly diluted odour gases are introduced into the soda recovery boiler so that a sufficient proportion of chlorine separating from the boiler bed can be converted into hydrogen chloride in boiler conditions, whereby it can be separated from the flue gases by scrubbing with water or an alkali-water solution. Chlorine can thus be removed from the process efficiently while any remaining sulphur dioxide residues can be removed from the flue gases especially when alkali-water solution is used. At the same time the burning of odour gases produces more thermal energy, and the odour problems associated with the process are considerably smaller than previously. Another advantage of the invention is that odour gases can be eliminated by burning them in the soda recovery boiler so that they will not disturb the operation of the lime sludge reburning kiln or power boiler, and no separate odour gas burning apparatus or boiler need to be provided. In this way odour gases can be handled efficiently and economically.
The invention will be described more fully with reference to the attached drawings, in which
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 illustrates schematically the application of the method according to the invention in connection with a conventional soda recovery boiler; and
FIG. 2 shows an equilibrium diagram for the flue gases of a soda recovery boiler obtained from a practical embodiment.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a soda recovery boiler 1 into which black liquor is introduced for burning. The figure also shows a concentration unit 2 associated with an evaporation plant and intended for heating the liquor and delaying it for a predetermined period of tim
REFERENCES:
patent: 3396076 (1968-08-01), Crosby et al.
Warnqvist, Bjorn, et al.: "Chlorides in the Kraft . . . " International Bleaching Pulp Conference, May 2-6, 1976, (Chicago), pp. 13-18.
Warnqvist, B. et al., "Removal of Chloride From Recovery Systems by Reactions with Sulfur Dioxide" Svensk Paperstidning, No. 11, 1975, pp. 409-410.
Alaniemi Maiju
Hamalainen Risto
Harila Pauli
Hyoty Paavo
Imelainen Keijo
Nguyen Dean T.
Tampella Power Oy
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