Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...
Patent
1979-12-07
1982-05-11
Kratz, Peter F.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Continuous chemical treatment or continuous charging or...
162 30K, 162 51, 162DIG8, 423207, 423480, 423DIG3, D21C 302, D21C 1104, D21C 1108, D21C 1112
Patent
active
043291996
DESCRIPTION:
BRIEF SUMMARY
In the manufacture of cellulose pulp according to the so-called sulfate process, as shown in the block diagram in FIG. 1, the wood chips are digested in the digester house 1 in a cooking liquor, the active constituents of which are sodium hydroxide and sodium sulfide. During the digestion the cellulose fibers are exposed, while the remaining constituents are dissolved and form the so-called black liquor. The cellulose pulp is separated from the black liquor in the pulp washing plant 2. The black liquor, which contains the inorganic chemicals of the cooking liquor together with the dissolved wood substance, is evaporated to a high dry content in the evaporation plant 10, whereafter the so-called thick liquor is led to the recovery boiler 11.
The wood contains some percentage of resin acid and fatty acid. In the alkali digestion, these resin and fatty acids are converted to corresponding sodium soaps, tall soft soap. This soap is difficult to dissolve in black liquor, and its solubility decreases with increasing dry content of the black liquor.
The soap, which has a high economic value, is separated in the pulp washing plant and evaporation plant. The separated soap is treated in the resin boiling plant 18 in a process, which is dealt with later on in this descriptive part.
During the process in the digester house and evaporation plant, volatile evil-smelling sulfur compounds are formed, which are to be blamed for the so-called sulfate smell in the ambient atmosphere of sulfate pulp mills. In recent years the sulfate pulp mills have been charged to collect these smelly gases and destroy them by combustion. This is carried out in the smelly gas combustion plant 20. These smelly gases can be combusted in a separate combustion furnace, whereby the sulfur leaves the system as SO.sub.2 at position 24, or in the lime kiln where part of the sulfur of the smelly gases is absorbed in the lime of the lime sludge.
In the recovery boiler the inorganic chemicals of the thick liquor are converted substantially to sodium carbonate and sodium sulfide. A part of the sulfur supplied together with the thick liquor leaves the recovery boiler as sulfur dioxide together with the flue-gases at position 23. The organic substance of the thick liquor is combusted in the recovery boiler to carbon dioxide and water, which leave the recovery boiler together with the flue-gases at position 23.
In the manufacture of all bleached pulp, it is essential that the conditions during the digestion are such that as much as possible of the lignin is dissolved without degrading the cellulose pulp and affecting its strength. One such operation condition essential for the manufacture is the sulfidity of the white liquor, which is defined by the mole ratio ##EQU1## in the white liquor, where the contents are expressed in grams NaOH/liter. The definition is standardized, see SCAN, 2:63, Svensk Papperstidning 66 (1963), 786. The sulfidity of the white liquor is close to equal to the mole ratio
It was found by experiments, which were carried out by, among others, Annergren and Wilhelmsson, Svensk Papperstidning 77 (1974), 153, that it is very important in the manufacture of bleached sulfate pulp that the sulfidity is higher than 35% and preferably higher than 40%. By additional experiments it was found that, in normal sulfate pulp mills with a sulfidity in the white liquor of 30-40%, the mole ratio S/Na.sub.2 in the white liquor and black liquor system is about equal with the sulfidity. More accurate studies have shown that the mole ratio S/Na.sub.2 is about 1:1.times.sulfidity.
Further other studies, which were carried out in connection with the problem to be solved by the present invention, have proved the existence of a clear relation between the SO.sub.2 -emission with the flue-gases from the recovery boiler and the mole ratio S/Na.sub.2 in the thick liquor supplied from the evaporation to the recovery boiler. This relation is illustrated in FIG. 2 and shown for four different recovery boilers, which were studied in this respect.
It is interesting to
REFERENCES:
patent: 2895801 (1959-07-01), Northgraves et al.
patent: 4053352 (1977-10-01), Hultman et al.
patent: 4098639 (1978-07-01), Noreus
patent: 4122199 (1978-10-01), Dunnery et al.
"Industrial Gas Cleaning", Strauss; 2nd Ed. 1975, Pergamon Press p. 112.
Andersson Per-Erik
Eriksson Fall E. I.
Kratz Peter F.
Sca Development Aktiebolag
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