US2957795A - Process for making paper pulp - Google Patents

Description

Oct. 25, 1960 H. D. STUCK PROCESS FOR MAKING PAPER PULP Filed June '7, 1956 DILUTION. DIGEISTER.

REF! NER.

INVENTOR. HAROLD D. STUCK.

+7PM ATTORNfKS.

2,957,795 PROCESS roR MAKING PAPER PULP iIarold 'D. Stuck, Audover, Mass., assignor to John W. Bolton & Sons, Inc., Lawrence, Mass., a corporation of Massachusetts Filed June 7, 1956, Ser. No. 589,903 6Claims. (Cl. 16228) This invention relates to a process of making paper pulp from wood chips with minimum injury to the fibres thereof.

.. It has been customary in the paper making art to di- 'g'est wood chips at elevated pressure and temperature for varying periods to soften the encrustant bond holding the cellulose fibres together. Suitable cooking liquor has been introduced into the digester to perform the digestion action, for example alkali, and steam may be applied directly or indirectly to the digester, all depending on the particular process used.

When the cook has been completed, the contents of the digester have been blown through a blow valve into a blow tank or blow pit. The sudden reduction of pressure from over ninety or one hundred pounds per square inch to atmospheric pressure in the blow tank has caused thev cooked wood pieces to explode thus disintegrating the same but also sometimes reducing the length of the fibres. Usually also the inlet from the blow valve directs the hot cooked wood pieces against a target, bafile plate, or the like, in the blow tank to enable the resulting impact to assist in converting the cooked wood pieces to a pulpy mass at the bottom of the tank. Such an impact may also damage the cellulose fibres by reducing the length thereof.

This invention is directed to the production of an improved paper pulp through a process which eliminates such an internal explosion and external impact with its resulting damage to, and shortening of, individual by a controlled, rotary, mechanical disintegration applied *alternately in opposite angular directions when the pieces are at nearly digestion temperature and the bond between fibres is at maximum softness.

A further object of the invention is to rotatively disintegrate cooked wood pieces into smaller sizes, by repeated reversals of the path of movement thereof, when at the high consistency, temperature, pressure and wetness, and at the low content of air, of the completed digestion step thereby reducing foaming, reducing fines and securing a higher yield pulp.

In the drawing:

Fig. 1 is a diagrammatic view of suitable apparatus for carrying out the process of the invention.

Fig. 2 is a view similar to Fig. l but including dilution tank apparatus, and

Fig. 3 is a fragmentary, cross sectional view of one type of apparatus for carrying out the rotative disinte- 1 gration step of the process.

As illustrated in Fig. 1 the first step of the process is the digestion of a charge of wood chips in a digester 20 United States Patentoi p 2,957,795 Patented Oct. 25, 1960 of any well known type. Wood chips are fed to digester 20 from a chip bin 21 and in the alkaline process, a suitable charge of alkali is also included. It will be understood that this invention is applicable to the alkaline, soda, neutral sulfite or other similar processes by selecting suitable conditions, and that the digestion step, while essential to the invention, is well known in the art. A suitable digestion step is described in U.S. Patent 2,591,106 to Sutherland of April 1, 1952, and is preferred for use in this invention. In accordance with the description of said patent the charge of alkali used in the digester is selected in the range between 400 and 450 pounds of alkali (measured in Na O) per ton of air dry pulp. Steam is introduced into the digester 20, through pipe 22 to raise the temperature of the digester charge to 340 F. at a pressure of about 110 pounds per square inch in a time period of about one hour and forty-five minutes. The cook is then held at this maximum tem perature and pressure for a period of about fifty-five minutes at which time it is ready for discharge out of the digester 24).

A conduit 23 leads from the base of digester 20, through a blow valve 24 and normally the conduit 23 would lead directly into a blow tank which is open to atmospheric pressure. It has been customary to open the valve such as 24, wide to permit the elevated pressure within the digester 20 to force the charge into the blow tank with great force and sudden reduction of pressure .to atmospheric.

In the process of this invention, however, the conduit 23 leads directly into a rotary cage type disintegrator 25. The digester charge, made up of cooked wood pieces or chips together with cooking liquor, is not fed to the disintegrator 25 with the explosive force of a full reduction in pressure to atmospheric and the flow thereof is controlled through valve 24 and fed in a thin stream as with the valve passage constricted and only partly open. The disintegrator 25 includes a pressure chamber 26, a central inlet '27 and a circumferential outlet 23 leading to an outlet conduit 29. Thus the digester charge of relatively high consistency, elevated pressure of over 100 pounds per square inch and elevated temperature of over 300 F. upon arrival in the chamber 26 is at a temperature of about 250 F. and a pressure of about fifty pounds per square inch.

The rotary cage type disintegrator 25 is usually termed a cage mill and includes opposed, spaced apart rotors 31 and 32 revolving at high speed around a horizontal axis 33 and each having axially extending rollers such as 34 and 35 extending toward the other and mutually overlapping. The digester charge is fed to the centre of the rotors through central inlet 27 and is then rapidly whirled and/or swirled around within the chamber 26 in substantially concentric paths with each wood piece tending to be relatively gently disintegrated into smaller pulpy pieces by the overlapping rollers 34 and 35.

It should be noted that the soft cooked material entering the inlet 27 is directed against the rapidly rotating plane surfaces 36 or 37 of the rotors 31 and 32 which combined with the controlled and reduced volume passed by valve 24 and the super atmospheric pressure within chamber 26 tends toward disintegration without fibre damage. The material is then centrifugalized outwardly and alternately whirled angularly between the plurality of opposing unidirectional concentric forces created by the oppositely revolving rollers such as 34 and 35 to the outside of chamber 27 for discharge through the outlet 28. The stream of softened cooked pieces is thus initially intercepted by a high speed revolving plane surface which mushrooms and splatters the same into annulus shape and the annulus is subjected to a plurality of opposite circumferentially directed impacts to further disintegrate the components without a shearing or grinding contact.

The effect of such revolving cages, and the alternate,

opposite, mechanical impacting action, thereof, on the woodchips or pieces of the charge has been found to be a reduction in the size of the softened wood pieces with- .out injury to, or undesirable shortening of, the individual gester are in a condition of maximum softness of the bond between fibres.

The cooked wood pieces, for example, may be one to six inches long and made up of fibres a few of which may also be six inches long. The mechanical disintegration, produced directly after the completion of the digestion step, separates such pieces into soft pieces of less lateral dimension but without reducing the length of any elongated fibres threin. In the process shown in Fig. 1, a conduit 38 feeds a suitable supply of hot brown liquor at a temperature of about 170 F. directly into the pressure chamber 26 to dilute the consistency of the material therein to about 3.25% fibre solids. The outlet conduit 29 leads the diluted and disintegrated cook directly into .a suitable refiner, such as the Jordan 39 shown, with the paper stock still at a temperature of about 180 F. and at a pressure of about fifty pounds per square inch.

As shown in Fig. 2 the disintegrated cook discharged through outlet 28 of disintegrator 25 may, if desired, be directed by a conduit 41 into a vat or tank 42 of a Well known type. The tank 42 may have the capacity to accommodate the product of a number of digesters such as 20 and and dilution may be effected therein by means of the conduit 43. The diluted paper stock is then directed by a conduit 44 into a refiner 45 which may be a Jordan, a disc refiner or any other suitable and well known refining device.

It has heretofore been proposed in the art to relieve the pressure in a digester before blowing in order that the low pressure blow will yield a better quality pulp. It has also been proposed to blow at full pressure into a closed diffuser instead of an open blow tank, the pressure vessel retaining the steam and other vapors. However, in the process of this invention, not only is the full explosive blow and violent impingement on a target in a blow tank eliminated, but the cooked wood pieces are still subjected to a mechanical disintegration step while at nearly the temperature of digestion and while the bond between fibres is at maximum softness. Knots are thereby opened up, less foaming results, less screen rejects occur, washing is made easier and a higher yield pulp is produced.

In the usual system wherein the digester charge is blown into a blow tank and the diluted material is then pasesd through a disc refiner, the resulting paper stock may include 6 to rejects. The disc refiner consumes considerable power, however, because of the close clearance of its working surfaces, and as stated above, some of the desirable long fibres may have been shortened by the violence of the disintegration process.

I have found that the step of mechanically disintegrating by whirring the cooked material at high speed while at about the consistency, temperature and pressure of digestion uses low power because of the greater clearance of working surfaces and produces paper stock with about 20% rejects prior to refining. The stock may then be diluted to about 8 to 10% consistency and the rejects screened out prior to refining or if not screened out passage of the stock through a disc refiner results in 1% or less rejects. It is only necessary for the cooked material to remain a short interval, such as a few minutes in the rotary cage type disintegrator 25 for an effective disintegration of the soft wood pieces in view of the high speed of the rotors thereof.

For example, a typical digester may be emptied through a constricted valve opening during a period of about ten minutes or less, thereby maintaining the disintegrator full of material being continuously disintegrated and discharged during that period.

I claim:

1. In a process for making paper pulp from wood, the steps of subjecting pieces of wood to the digesting action of hot cooking liquor at super atmospheric pressure for a time sufiicient to soften the encrustant bond holding the cellulose fibres together and produce a pulp of relatively high consistency; then blowing a thin, valve controlled, high consistency, stream of said cooked and cooking liquor into a pressure tight environment in a constricted flow; then, in said environment, centrifugalizing said cooked wood and cooking liquor while still hot and still at super atmospheric pressure at high speed while subjecting the same to disintegrating impacts alternately in opposite angular directions; then diluting said pulp to a lower consistency with hot cooking liquor at a temperature of about 170 F. and then relieving the super atmospheric pressure on said whirled and cooked pieces.

2. A process for mechanically disintegrating wood pieces digested to a soft pulp of relatively high consistency at super atmospheric temperature, and pressure which process comprises the steps of blowing a thin, valve controlled, high consistency stream of said digested pieces into a pressure tight environment in a constricted flow and impacting said stream on a rotative plane surface; then, in said environment, subjecting said digested pieces to projective, centrifugal force and a plurality of repeated impacts from spaced rolling surfaces moving at high speed in opposite, unidirectional, coaxial paths while still at super atmospheric temperature and pressure until said pieces have disintegrated into component parts; then reducing the consistency of said pulp to about 3.25% fibre solids and then moving said digested and disintegrated component parts into an atmospheric environment.

3. A process for mechanically disintegrating digested Wood pieces which comprises the steps of blowing a thin, valve controlled, high consistency, stream of digested wood pieces into a pressure tight environment under a pressure of about p.s.i. and at a temperature of over 300 F.; then centrifugalizing said pieces while alternately, subjecting said pieces to mechanical, spaced apart, impacts first in one angular direction and then in the opposite angular direction around a common axis in said pressure tight environment while reducing said pressure to about 50 p.s.i., said temperature to about 250 F. and said consistency to about 3.5% until said pieces are disintegrated and then delivering said pieces from said environment while reducing said pressure and temperature to substantially atmospheric pressure and temperature.

4. In a process for making paper pulp from wood, the steps of subjecting pieces of wood to the digesting action of hot cooking liquor at about 340 F. and at a pressure of about pounds per square inch for a time sufficient to soften the encrustant bond holding the cellulose fibres together; then blowing the cooked wood and cooking liquor directly into a pressure chamber in a constricted flow and into impact with a rotative plane surface; then centrifugalizing and repeatedly impacting in opposite circumferential directions said cooked wood pieces within said pressure chamber, into pieces of much reduced size without injury to individual fibres while still at a temperature of about 250 F. at a pressure of about 50 pounds per square inch and at the consistency of digestion; then reducing said consistency and then discharging said pieces from said pressure chamber directly into a refiner.

5. In a process for making paper pulp from wood, the steps of subjecting pieces of wood to the digesting action of hot cooking liquor at super atmospheric pressure of about 100 psi. for a time sufiicient to soften the encrustant bond holding the cellulose fibres together; then forming a thin, pressurized, high speed stream of digested material, blowing said material, under constricted valve control, into a pressure tight disintegrating zone and reducing said pressure to about '50 p.s.i. and reducing said temperature to about 250 F.; then centrifugally forming said digested material into an annular extending transverse to the axis of said stream by intercepting said stream with a rapidly rotating plane surface; then repeatedly impacting the digested material in said annulus, without shearing or grinding the fibres therein, by rotary, mechanical disintegration applied alternately in opposite angular directions, said centrifugalizing and impacting steps being accomplished while said cooked wood and cooking liquor are maintained at said temperature of about 250 F. and said pressure of about 50 psi. in said disintegrating zone and then directing said material out of said disintegration zone and further reducing the temperature and pressure thereof to substantially atmospheric temperature and pressure.

6. A method of preparing pulp from wood which comprises the steps of subjecting pieces of wood to the digesting action of hot cooking liquor at a pressure of about 100 psi. for a time suificient to soften the encrustant bond holding the cellulose fibres together; then blowing a thin, valve controlled, high consistency stream of said digested material into a pressure tight environment in a constricted flow thereby reducing said pressure to about p.s.i.; then maintaining said pressure while cage milling said digested material until said fibres have separated from each other without substantial reduction in length thereof and then further reducing said presure well below 50 psi. for completing the separation of said fibres.

References Cited in the file of this patent UNITED STATES PATENTS 850,384 Mallam Apr. 16, 1907 1,827,710 Leyst-Kuchenmeister Oct. 13, 1931 1,970,330 Mason Aug. 14, 1934 2,323,194 Beveridge et al. June 29, 1943 2,591,106 Sutherland Apr. 1, 1952 2,616,802 Kehoe et al. Nov. 4, 1952 2,662,821 Muench Dec. 15, 1953 2,709,652 Plunguian May 31, 1955 2,718,821 Compston Sept. 27, 1955 2,730,931 Reister Jan. 17, 1956 2,791,503 Lyon May 7, 1957

Claims (1)

1. IN A PROCESS FOR MAKING PAPER PULP FROM WOOD, THE STEPS OF SUBJECTING PIECES OF WOOD TO THE DIGESTING ACTION OF HOT COOKING LIQUOR AT SUPER ATMOSPHERIC PRESSURE FOR A TIME SUFFICIENT TO SOFTEN THE ENCRUSTANT BOND HOLDING THE CELLULOSE FIBRES TOGETHER AND PRODUCE A PULP OF RELATIVELY HIGH CONSISTENCY, THEN BLOWING A THIN, VALVE CONTROLLED, HIGH CONSISTENCY, STREAM OF SAID COOKED AND COOKING LIQUOR INTO A PRESSURE TIGHT ENVIROMENT IN A CONSTRICTED FLOW, THEN, IN SAID ENVIRONMENT, CENTRIFUGALIZING SAID COOKED WOOD AND COOKING LIQUOR WHILE STILL HOT AND STILL AT SUPER ATMOSPHERIC PRESSURE AT HIGH SPEED WHILE SUBJECTING THE SAME TO DISINTEGRATING IMPACTS ALTERNATELY IN OPPOSITE ANGULAR DIRECTIONS, THEN DILUTING SAID PULP TO A LOWER CONSISTENCY WITH HOT COOKING LIQUOR AT A TEMPERATURE OF ABOUT 170*F. AND THEN RELIEVING THE SUPER ATMOSPHERIC PRESSURE ON SAID WHIRLED AND COOKED PIECES.

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