Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Reexamination Certificate
1998-06-18
2001-11-13
Alvo, Steve (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
C162S048000, C162S088000, C162S089000
Reexamination Certificate
active
06315863
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the bleaching of wood pulp and a method of reducing the amount of barium scale that is formed on equipment used in the bleaching process.
BACKGROUND OF THE INVENTION
For many years chlorine was used predominantly as the primary bleaching agent for wood pulp. When chlorine was used as the bleaching agent in the first bleaching stage, the consistency of this stage was preferably in the range of about 2.0-5.0% solids. Since the process stream coming to the first bleaching stage typically had a consistency much higher than desired for the first bleaching stage, it was often diluted with filtrate from a chlorination washer for the first bleaching stage. This recycle of the washer filtrate reduced water usage and reduced energy costs. There was also an additional benefit in that with the filtrate recycle, the pulp delignification was improved slightly.
As the industry moved away from the use of chlorine and substituted chlorine dioxide, it became necessary to increase the retention time of the pulp in the reaction tower because of the reduced reaction rate of chlorine dioxide compared to reactions involving chlorine. A typical way of increasing the retention time has been to increase the consistency of the pulp stream to about 10 to 12% solids. As a result of increasing the consistency of the pulp stream in chlorine dioxide bleaching processes, it became unnecessary to recycle chlorination washer filtrate for dilution.
Barium is present in the pulp stream and originates from the wood chips that serve as the raw material for the wood pulp.
The bleaching ability of chlorine dioxide depends in part on pH. The bleaching efect of chlorine dioxide is maximized at a pH of about 2.3 to 3.0. Since the pH of the pulp stream into the bleaching stage is typically more basic than 2.3 to 3.0, acid is typically added to the pulp stream. The most common acid has been sulfuric acid because of its relatively low cost and availability. While the addition of sulfuric acid effectively reduces the pH, it also increases the number of sulfate ions which are available to react with barium and precipitate as barium sulfate, forming unwanted scale on equipment. When scale forms on equipment, such as the chlorination washer, the ability of the equipment to remove unwanted materials, reaction products and unreacted chlorine dioxide is reduced. One of the side effects of reducing the ability of a washer to remove unwanted materials is that less calcium is removed in the washer, and thus passes downstream to extraction stages where the calcium is available to react with carbonate ions and precipitate out as calcium carbonate and form additional unwanted scale.
Hydrochloric acid is another acid that has been used in a manner similar to sulfuric acid to acidify the pulp stream; however, when hydrochloric acid finds its way into filtrates, the utility of such filtrates is reduced. Often such hydrochloric acid containing filtrates require expensive treatment in order to render the filtrate stream disposable.
In certain bleaching processes, pH of the pulp stream has been controlled by applying excess amounts of chlorine dioxide to the pulp. Apparently, such larger doses of chlorine dioxide are effective to sufficiently acidify the pulp such that the addition of sulfuric acid is not needed. In those chlorine dioxide bleaching processes that do not or cannot apply excess amounts of chlorine dioxide, and thus employ sulfuric acid as a pH adjustment chemical, the formation of barium scale continues to be problematic.
In view of the costs involved in removing barium scale and calcium scale from pulp bleaching process equipment, the need exists for a solution to reduce or avoid the formation of barium sulfate which can lead to scale formation on process equipment. Such an improvement would reduce the cost of pulp bleaching processes by reducing down time and costs associated with removal of both barium scale and calcium scale from equipment.
As an aid in understanding the prior art as it relates to a typical pulp mill, and more specifically, the bleaching process within a pulp mill, a typical pulp mill operation is described below.
FIGS. 1A-1C
illustrate a typical pulp mill. In the mill, means of transporting chips or pulp from one operation to another will depend upon the consistency of the pulp and the location of the equipment. The transportation may be accomplished by a conveyor or a chute if the consistency is too high for the pulp or chips to be pumped. If the pulp or chips can be pumped, a pipe can be used to transport the material.
Chips
10
, process water
11
, steam
12
and pulping chemicals
13
are placed in digester
14
. Wood chips
10
may optionally be treated prior to entering digester
14
by conventional means such as by pre-steaming the chips in a steaming vessel or impregnating the chips with digestion chemicals in an impregnation vessel. Chemicals
13
that are contacted with the chips will depend on the process being used, e.g., sulfate, sulfite, or soda, and whether or not digester
14
is operated in a batch or continuous mode. In
FIGS. 1A-1C
, a continuous digester is illustrated. Chips are cooked under appropriate conditions within digester
14
. The cooking conditions will depend upon the species of chip and the type of pulp being used and are well known.
The products of the digestion process are delignified or partially delignified wood chips, spent pulping chemicals, and lignin and carbohydrate products which have been removed from the wood chips in the digestion process. Treatment of the chips after cooking, will depend in part on the type of digester being used. A major portion of the spent pulping chemicals and lignin products are removed from the chips prior to further processing by washing. In a continuous digester as illustrated, the chips are washed in a washing section of the digester. This is indicated by process water
15
entering the washing stage of digester
14
and an effluent stream
16
leaving the washing stage of digester
14
. Effluent
16
will consist of the lignin and carbohydrates which have been removed from the chips during the digestion process and spent pulping chemicals. This effluent is carried to a treating facility for processing. If the pulp is Kraft or sulfate, such treating facility would include a recovery system wherein the liquor is burned to recover the pulping chemicals for reuse. Such treatment would not occur in a batch digester where all the washing would occur in the following brown stock washing system.
Following this washing treatment, the chips are passed from digester
14
through a blow line to storage or blow tank
22
. It is customary in pulp mills to have storage tanks between separate processes so that the entire mill will not shut down if one section of the mill is taken off line. Storage tank
22
is located between the digester stage and the subsequent washing or bleaching stages.
Material passing through the blow line comprises a slurry that contains the remaining lignin and carbohydrates, spent digestion chemicals, and fibers formed from the chips as they are blown from the digester. The chips are formed into fibers when the pressure on the chips is partially released, usually at the outlet of digester
14
. The slurry will still be under some pressure to move it through the blow line. If digester
14
is a continuous digester, additional fiberizing may be accomplished by a refiner, or refiners, in the blow line. Such refiners fiberize large particles that have not been reduced to fibers earlier in the process. In
FIG. 1A
, two refiners,
18
and
19
, are illustrated; first refiner
18
does coarse refining and second refiner
19
does fine refining. It should be understood that such refiners are optional and often are encountered in a liner board mill; however, in a bleached pulp mill refiners would normally not be included in a blow line. In addition, such refiners would not be employed if digester
14
was operated in a batch mode.
The blow line is shown in three sections,
Alvo Steve
Christensen O'Connor Johnson & Kindness PLLC
Weyerhaeuser Company
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