Paper making and fiber liberation – Apparatus – Digester
Reexamination Certificate
2001-11-15
2003-05-27
Alvo, Marc S (Department: 1731)
Paper making and fiber liberation
Apparatus
Digester
C162S246000, C162S251000
Reexamination Certificate
active
06569289
ABSTRACT:
BACKGROUND AND SUMMARY OF THE INVENTION
Co-pending patent application Ser. No. 09/248,009 [10-1265], filed on Feb. 10, 1999 (the complete disclosure of which is included by reference herein), discloses a method of treating comminuted cellulosic fibrous material with a beneficial additive prior to chemical digestion. This additive is preferably a strength- or yield-enhancing additive, such as athraquinone [AQ] or polysulfide [PS] and their derivatives or equivalents. The present invention comprises a further method and apparatus for effecting the pretreatment of comminuted cellulosic fibrous material, typically wood chips (though the invention is equally applicable to the treatment of other forms of cellulose), to improve the properties [e.g. strength] of the resulting pulp or to improve the effectiveness [e.g. yield] of the pulping process.
It has been discovered that the pretreatment of comminuted cellulosic fibrous material, for example, wood chips, can be more effective if, among other things, the heating and cooking of the chips after pretreatment is essentially isolated from the pretreatment process. That is, a more effective pretreatment can be obtained if the pretreatment process is performed at a cooler temperature, with or, preferably substantially without, the presence of alkali, and the heating of the chips to cooking temperature is performed after the pretreatment (impregnation of the cellulose material) is essentially completed. In one aspect of this invention the additive is added earlier in the treatment process, and the content of cooking chemical, for example, kraft white liquor, is reduced or diverted from this earlier stage of pretreatment and introduced during later stages of pretreatment or in the formal cooking treatment. Thus, according to the present invention, possibly longer, cooler, less alkaline pretreatment is provided so that the cooking additive more effectively treats the chips prior to heating to cooking temperature, that is, to a temperature greater than 140° C.
According to one aspect of the present invention there is provided a method of continuously producing chemical cellulose pulp from a comminuted cellulosic fibrous material slurry, comprising substantially continuously: (a) Impregnating the material with a solution containing yield or strength-enhancing additive at effective alkali and temperature conditions so that substantially no alkali degradation of the cellulose occurs, and so that little or substantially no acid hydrolysis occurs. And, (b) after (a), treating the material with an alkaline cooking liquor, at cooking temperature, to produce a chemical cellulose pulp with higher yield or strength than if (a) were not practiced.
In the method (a) may be practiced so that there also is substantially no dissolution of lignin from the material. At the end of (a), the material has been impregnated with the yield or strength enhancing additive (such as AQ or its derivatives or equivalents) so that the problems associated with conventional higher temperature additive impregnation are avoided. For example specifically with respect to AQ, it is a large molecule and needs a longer time to diffuse into the wood chips, or like cellulose material, than does for example, alkali, and it needs to be reduced in order to dissolve so that it can diffuse. Therefore sometimes AQ is used in its reduced form (commonly referred to as SAQ). However also typically about 80% of the AQ reacts with dissolved lignin and thus is not capable of performing its intended yield and strength enhancing function, leaving only about 20% of the AQ for performing the desired functions. By utilizing the invention a much higher percentage of the AQ (up to substantially all) that is added actually impregnates the wood chips and performs its yield or strength enhancing function, and the AQ may or may not be added in reduced form. When the AQ successfully impregnates the chips it keeps the hemicellulose from being dissolved during cooking, and thus increases yield, and utilizes other mechanisms to enhance strength. Other materials may also be utilized to facilitate penetration of the chips with the additive, such as the use of a surfactant.
In the method as described above (a) may be practiced at an alkali concentration between 0-less than 10 g/l expressed as NaOH, and at a temperature of between about 80-130° C., typically less than about 120° C. and preferably about 110° C. or less. The amount of alkali is most desirably substantially zero, but preferably at most less than about 5 g/l expressed as NaOH.
In the method (a) may be practiced in a feed system of a continuous digester and (b) in a continuous digester. Exactly where in the feed system (a) is practiced may be widely variable. For example a chip bin could be located in a wood yard, and the additive could be sprayed or otherwise applied to the wood chips even prior to entry into the chip bin, or while they were entering the chip bin, or while they were in the chip bin. Then the wood chips could be pumped from the wood yard to the digester using primarily or substantially exclusively water as the slurrying medium (with no intentional significant alkali addition) so that the chips would be at desirable low temperature, low alkali, impregnation-facilitating conditions for a significant period of time. In a typical situation impregnation in (a) will take at least about 20 seconds, e.g. between about 2-60 minutes at superatmospheric pressure (which superatmospheric pressure may be provided in any conventional manner including by a level of liquid above the chips, pumping, and/or in a pressure vessel). Alternatively the additive may be introduced after the chip bin and before a pump and/or high pressure feeder, in a separate treatment vessel such as an impregnation vessel, or any other location in the feed system that proves advantageous for any reason.
The invention may also comprise substantially immediately after impregnation in (a) adding between about 35-100% of the alkali used to treat the material, including in (b), to the material. Alternatively the alkali can be added far downstream. Also the method may further comprise (c), between (a) and (b), treating the material in a second zone with a solution containing additive at an effective alkali concentration of about 5—less than 15 g/l expressed as NaOH and greater than in (a), and at a temperature of between about 110-130° C. and higher than in (a).
In the method (a) may be practiced using as the additive at least one of AQ or its derivatives or equivalents, polysulfide or its derivatives or equivalents, or sulfite in the form of sulfur dioxide, NaHSO
3
, or Na
2
SO
3
. A suitable amount of additive may be used; for example if AQ or its derivatives or equivalents are used, typically (a) is practiced with a total additive concentration of between about 0.02-0.5% on wood, typically between about 0.02-0.1% on wood.
Additive may also be added during the digesting process, as is conventional.
According to another aspect of the present invention there may be provided a method of treating comminuted cellulosic fibrous material comprising substantially continuously: (a) Treating a slurry of comminuted cellulosic fibrous material with a first liquid comprising primarily or substantially exclusively water containing a beneficial additive at a maximum effective alkali concentration of less than 10 g/l, and at a first temperature of about 80-130 degrees C., for at least about 20 seconds, e.g. between about 2-60 minutes under superatmospheric pressure. And, (b) after (a), treating the material in the slurry with a second liquid having an initial effective alkali concentration greater than 10 g/l (preferably greater than 15 g/l and most desirably greater than 20 g/l) expressed as NaOH, and at a second temperature greater than 130° C. (e.g. greater than 140° C.), to produce a chemical cellulose pulp.
The method as described above may further comprise (c), between (a) and (b), treating the slurry with a third liquid containing the ben
Phillips Joseph R.
Stromberg C. Bertil
Alvo Marc S
Andritz Inc.
Nixon & Vanderhye P.C.
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