Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Reexamination Certificate
1994-03-14
2001-04-03
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
C162S038000, C162S040000, C162S065000, C162S082000
Reexamination Certificate
active
06210527
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method of producing bleached wood pulp in which wood chips are digested and are then subjected to subsequent bleaching stages that are conducted in the presence of a sodium hydroxide. More particularly the present invention relates to such a method that includes an ozone bleaching stage in which a waste stream produced from an ozone bleaching stage is scrubbed to produce an oxygen containing stream useful in an oxygen delignification stage of the pulping process. Even more particularly, the present invention relates to such a method in which the waste stream is scrubbed by white liquor either in an external stage or during production of polysulfide liquor and then is separately reacted with white liquor to produce oxidized white liquor containing an appreciable amount of thiosulfate species of sulfur (thiosulfate liquor), and fully oxidized white liquor, containing almost no thiosulfate sulfur, to serve as sodium hydroxide in bleaching stages.
In the formation of bleached wood pulp, wood chips are digested in the presence of white liquor, which contains sodium sulfide and sodium hydroxide for such digestion, to produce brownstock pulp and weak black liquor. It is known that pulping with polysulfide liquor has advantages over conventional white liquor cooking in the wood chip digestion stage. The brownstock pulp is then washed and weak black liquor is extracted for reprocessing. The pulp is then subjected to oxygen delignification. The oxygen delignification is conducted in the presence of thiosulfate liquor, oxygen and steam. After the oxygen delignification, the wood pulp is sequentially subjected to an ozone bleaching stage, an extractive oxidation stage, which may be conducted in the presence of peroxide, and a final peroxide or chlorine dioxide bleaching stage. The extractive oxidation stage is conducted in the presence of thiosulfate liquor. Fully oxidized white liquor is a sodium hydroxide source for peroxide based bleaching stages and has advantages in such bleaching stages over thiosulfate liquor.
The ozone feed to the ozone bleaching stage is made in an ozone generator from air or more preferably oxygen. The end result is a mixture of ozone and oxygen containing about 5% ozone if air is used and anywhere from 10 to 14% ozone if the ozone is generated from oxygen. Not all of the feed to the ozone bleaching stage is consumed and as a result, a waste stream is produced that contains ozone, oxygen, carbon dioxide and water. This waste stream is further processed by an ozone destruct unit and a carbon dioxide scrubber to produce oxygen that can be used in an oxygen deliqnification stage. Ozone is destroyed so that the stream may be recycled to the ozone generator after CO
2
removal and drying. Also, some of the waste stream may be vented atmosphere and ozone must be destroyed for industrial hygienic reasons. Carbon dioxide must be removed, otherwise it would consume sodium hydroxide inside the oxygen delignification stage, limiting the extent of lignin removal.
As will be discussed, the present invention provides a method of producing bleached wood pulp in which a waste stream produced from an ozone bleaching stage is scrubbed and then used as a source of oxygen for oxygen delignification. Expensive ozone destruct units are not used and in fact oxygen requirements can be balanced with oxygen recovery from the waste stream. The implication of this is that an oxygen recycle involved in the utilization of the ozone destruct unit can be eliminated together with its attendant capital and power consumption. Additionally, there is no need to further purify the waste stream to remove carbon dioxide and water. Moreover, the present invention advantageously utilizes polysulfide liquor in the wood chip digestion stage, oxidized white liquor in oxygen delignification and extractive oxidation stages, and fully oxidized white liquor in the peroxide bleaching stage.
SUMMARY OF THE INVENTION
The present invention provides a method of producing bleached wood pulp. In accordance with the method, wood chips are digested in a digestion stage to produce brownstock pulp and weak black liquor. The brownstock pulp is washed and the weak black liquor is extracted. The brownstock pulp after having been washed is introduced into sequential bleaching stages, including oxygen delignification and ozone bleaching stages, to produce a bleached wood pulp product. The oxygen delignification stage utilizes an oxygen containing stream and the ozone bleaching stage utilizes an ozone/oxygen containing stream. The ozone bleaching stage produces a waste stream principally containing water vapor, carbon dioxide, ozone, and oxygen. The waste stream is recovered and scrubbed with an aqueous, sodium sulfide and sodium hydroxide containing solution to remove ozone and carbon dioxide from the waste stream and thereby form a scrubbed stream. The oxygen containing stream, used in the oxygen delignification stage, is formed from at least part of the scrubbed stream.
Residual ozone is consumed from the waste stream by oxidizing sodium sulfide to an oxygenated sulfur species such as sulfite, thiosulfate or sulfate. Sodium hydroxide reacts with carbon dioxide to form sodium carbonate. In this manner, the waste stream becomes a scrubbed stream to eliminate the need for an ozone destruct unit. Additionally, since carbon dioxide has been removed, it will not neutralize the alkalinity required in the oxygen deliqnification process. Furthermore, in another aspect of the present invention, that will be discussed hereinafter, the oxygen recovered from the waste stream can be balanced with oxygen usage by utilizing the waste stream as an oxidant in a polysulfide production stage. Such usage will scrub the waste stream and will produce polysulfide that can be advantageously used in the wood chip digestion stage. Furthermore, the resultant scrubbed stream can also be used in oxidizing and fully oxidizing the white liquor in oxidized white liquor and fully oxidized white liquor stages. The fully oxidized white liquor can also advantageously be used in a peroxide based bleaching stage.
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G.H. Homer et al., Ozone and Oxygen Supply for the Bleaching of Pulp, Presented at te Non-Chlorine Bleaching Conference, Hilton Head, SC, Mar., 1993.
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Kirschner Mark J.
Sethna Rustam H.
Alvo Steve
Pace Salvatore P.
The BOC Group Inc.
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