CA1063405A - Apparatus for producing wood pulp from lignocellulose-containing material - Google Patents

Abstract

APPARATUS FOR PRODUCING WOOD PULP
FROM LIGNOCELLULOSE-CONTAINING MATERIAL
ABSTRACT OF THE DISCLOSURE
Apparatus for producing pulp from lignocellulose-containing material in which the material is ground into grist in a grinding space defined between a rotating disc and a stationary disc mount-ed within a pressurized housing. Each disc comprises a series of annularly disposed arched segments having a plurality of radially extending bars and interconnecting transverse ribs defining a plurality of radially arranged pockets which in their radial cross sections have a curved contour. The pockets on one of the discs are offset radially from the pockets on the other disc, to allow the grist in the rotating pockets to be axially deflected by the centrifugal force created by the rotation and collected in an op-posing offset pocket on the other disc. During the rotation of the disc, successive charges of grist deflected by the rotating pockets will force the grist previously collected in the station-ary pockets to slide along the curved contour and be tilted or dumped into the corresponding offset rotating pocket. The portion dumped into the rotating pocket is sheared off by the following bar as it passes the opposing bar on the stationary disc and subse-quently deflected by centrifugal force into a successive offset pocket on the rotating disc and sheared off. The grist thus pro-gresses stepwise towards the periphery of the discs, where it is discharged upon completion of the grinding operation into the grinding housing, from which it is subsequently removed for fur-ther treatment.

Description

BACKGROUND OF THE INVENTION
My invention relates to the production of pulp from fibrous ligno-cellulose materials, such as wood chips and other fibrous vegetable materials and more particularly to pulplng processes which are generally termed "mechanicalprocesses" in which the ligno-cellulose material is fiberized between the grinding discs of a refiner. ^~
1 ~

~6;~4(~5 The refiner discs may be of the counter-rotating kind, or one of the discs may rotate while the other one is stationary.
In the latter instance, the raw material is fed through the center of the stationary disc into the space between the discs, and, by the action of elements affixed to the rotary grinding disc, the material is accelerated and, by centrifugal forces, finally conducted in between the grinding elements of the grinding zone.
The pulp produced is mainly intended for use in the manu-facture of newsprint, cardboard, tissue paper and similar products, and fibrous ligno-cellulose materials from wood or other plants, e.g. bagasse, may be used as raw material. The method used can . .
be classified as a mechanical process and can be carried out under normal atmospheric conditions or in a steam atmosphere at temper-atures above 100C, generally at a temperature between 110~ up to 140C, or, under special conditions, in the temperature range between 150C and 170C. When the mechanical process is carried -out at a temperature above 100C, it is generally classified as "thermomechanical".
S~$MARY OF THE INVENTION
One of the purposes of the invention is to improve the pro-duction of pulp in such a way that the fibers of the raw material are separated from each other without being unduly shortened and so that the fibers are further refined to improve their paper-making properties, this process being accomplished by frictional forces generated within the moist fibrous ligno-cellulose material.
In the following, the material being processed between the grind-ing discs will be called "grist". As the individual fibers in the "grist" have a considerable springiness, the grist will de-velop a considerable volume of capillary voids increasing its capability to hold moisture.
A further purpose of the invention is to reduce the energy required for the pulping process.
During the process of refining, the grist is a moist agglom-

-2-B

634(?5 eration of the processed raw material, which, if it is of conif-erous origin, e~g. spruce, consists mainly of 2mm. to 3 mm. long tracheids with a slenderness ratio of around 100 to 1. The trach-eids in their turn are anatomically built up from three concentric layers of lamellae with a thickness of about one-thousandth of one millimeter and a layer of fibrillae helically arranged around a fourth lamella surrounding the innermost tubular space, the lu-men. The fibril has a slenderness ratio of about 1000 to 1, but, taken together, it is estimated that they constitute about 70% to 80% of the substance of one tracheid. One of the objects of my invention is to separate with a high degree of efficiency the fi-bers from each other and, furthermore, by a surface abrasion pro-cess, to affect the lamellae to such a degree that the fibrils are partially liberated. This improves the papermaking properties of the pulp produced.
In the apparatus according to the invention, the grist is introduced between grinding discs of a refiner. Each disc is e-quipped with a series of pockets separated by radially arranged bars forming pockets displaced leng~hwise relatively to each other ;
in the opposing grinding rings. The pockets have a curved form in cross section, which, at least in the stationary grinding disc, gives the pocket the form of a circle segment. The grist enters the rotating pockets through the gates and is acted upon by centri-fugal forces. At the centrally innermost rotating pocket, the grist is deflected by the rotating surface axially towards the sta-tionary disc to cause the grist to pass through friction surfaces which are formed by the interaction between the bars of the ro-tating and the stationary discs, whereafter the grist is forced into -the opposing pockets in the stationary grinding discs, where it be-comes compacted with the fibers oriented principally parallel in a radial direction at a right angle to the bar. The grist collect-e~ in the stationary pockets will form a relatively compacted body, -a portion of which is caused by successive deposits of grist from ~ -'~

.. ~ : - , . , 1()634~5 the rotating disc to gradually tilt over into the rotating pocket of the rotating disc. The latter portion of the grist collected - in the stationary pocket will then be successively sheared off by the bars of the rotating disc. The friction surfaces generated in the compacted grist by the rapidly moving bar create forces of very high intensity which will act on ~he individual fibers to mechanically refine the grist. The grist will in this manner be conveyed outward stepwise in a radial direction between the rotary and stationary pockets. Each time a portion of the accelerated grist is transferred from a rotating pocket into an opposing sta-tionary pocket, the velocity of the grist is reduced to zero. The kinetic energy of the moving grist is transformed into a mechanical force which will compact the grist in the stationary pocket effic-iently. When the grist comes to a full stop, this kinetic energy will have raised the temperature of the grist 3C-5C depending uopn the angular speed of the refiner disc. This compacting force ~ will greatly enhance the formation of frictional surfaces between `~ the bars and the grist and within the grist. - -~ -The curved bottom of the pockets is shaped so as to efficient-~
. 20 ly change the direction of the flow of the grist axially. The grist in these pockets may move with a tangential velocity of 50 m/sec to 120 m/sec, depending upon the size of the refiner. The frictional surfaces generated within the grist will delaminate and ?, ~ fibrillate the fibers of the raw material.
-~ 25 BRIEF DESCRIPTION_OF THE DRAWINGS
My invention will be understood by reference to the accompany-:
ing drawings, which form a part of this specification and show var-;i ious forms of apparatus by means of which the process may be car-ried out.
Fig. 1 is a vertical view partly in section of a refiner ac-cording to the invention.
-, Fig. 2 is a sectional view of a part of Fig. 1 in a larger ` scale.
.~ 4-B
~ ., .~, .. ~;, , , . ,, . ; . -. .
, ~ , ; . ... . ... .. . . . .
.. ~ . . . . . .

1(~63405 Fig. 3 shows in perspective a part of the grinding disc ac-cording to the invention, with the portion parallel to the central axle of the refiner shown in section~
Fig. 4 shows an axial section through the refiner disc.
Figs. 5A-5C show a section along the line V-V in Fig. 3.
Fig. 6 shows a vertical section through a refiner disc according to a modified embodiment of the invention.
DESCRIPTION OF DIFFERENT EMBODIMENTS OF THE INVENTION
On the drawings the numeral 10 designates the housing of the refiner carrying the stationary grinding disc 12. This grinding - disc works together with the grinding disc 16 fixed to rotating shaft 12 driven by motor (not shown). The shaft 14 is on each side '~.` of the grinding disc 16 supported by bearings which also are not - shown on the drawings. In the stationary grinding disc 12, there are three grinding zones 18, 20 and 50, concentrically arranged. -Likewise, the grinding disc 16 has three concentrically arranged ~ zones 22, 24 and 30. Three grinding zones are thereby formed, `~ namely, one between the elements 18 and 22, in which a prelimin-ary crushing of the admitted raw material, for instance wood chips, takes place. In a second zone formed by the grinding elements 20 and 24, a further coarse defibering takes place before the grist passes into the third grinding zone 30 and 50 of the grinding el-ements according to the invention. The two innermost grinding zones can be equipped with elements and ribs according to formerly ~i 25 known designs.
~, . .
` The raw material, e.g,, chips from wood or other fibrous ligno-cellulose ~aterial, is fed through an opening 32 in the hous-; ing 10 and is conveyed therefrom by a helical screw 34 equippedwith flights 36 through the openings in the central parts of the stationary and rotating grinding discs. A central ring 38 with .
flights 39 is fastened on the rotating disc 16 to convey the chips further towards the first grinding zone.
According to the invention, in the third grinding zone, the :,1 .
; ~ 5 6;~4()S

two grinding elements 30 and 50, preferably carried out as circle rings with the grinding surfaces opposing each other, are equipped with bars for~ing the radially arranged pockets 41 to 46, in the ring 30, and 61 to 66 in the ring 50.
S A minor variation from the radial direction of these pockets can be accepted. The pockets of the rotating disc should be form-ed so as to assist in diverting the grist moving in the pockets sideways from a radial to an axial direction. The curved bottom of the pockets in the stationary discs should preferably have the . lO form of a circle segment.
The two circular rings 30 and 50 of the grinding discs have been machined to form radial or nearly radial grooves. These ;- grooves should preferably have perpendicular radial extensions which are rectangular in cross-section. In circular ring 30 of the : 15 rotating grinding disc, the six pockets are designated by numerals 41-46. They are formed by the bars 47 and grooves recessed in the circular rings. In the embodiment shown, the pocket 41 serves as - an entrance. The number of the following pockets can, if desired, ~-`~ be varied within considerable limits. The pockets of the station-'l 20 ary disc are designated by the numerals 61-66, and the separating bars, with numeral 67. The pockets are walled in at the sides by the flat bars 67. This can be observed in Fig. 3. The bottom of `.i the stationary pockets 61-66 should have the form of a circle seg--, ment. The geometrical center of these segments should be at a -', 25 point which is somewhat sbove the edge of the bars defining the radial sides of the pocket. Otherwise, the movement of the body ~ of the grist will be hampered. The rotating pockets 42-46 can `~ have the same form, but, in the section shown, this pocket has been extended somewhat to increase its cross section, giving it , 30 a larger volume than that of the semi-circular segments of the stationary disc~ This can be seen in Fig. 4. The series of pock-ets in the two grinding discs are radially offset relative to each other, for instance, one-half the length of a pocket. The . 6 ~' .

... . ~ ,..... . .. . .. ~ . ~ . . -~63~05 grist is introduced through a gate 40 which forms the entrance of the innermost pocket 41. The outermost pocket 66 of the stationary grinding element is open to allow the ground fibrous pulp or the girst to be discharged into the housing of the refiner, The bars 47 and 67 radially separating each pocket as shown by the numerals 51 and 52 in Fig. 4 should be made from a material hav-ing considerably higher abrasion resistance than the material of the circular rings 30, 50. These pockets extend to the surface of the narrow edges of the bars 47 and 67. The distance between the grind-ing surfaces of the rings 30 and 50 in axial direction is regulated - by the axial adjustment of the refiner shaft. This is often done by hydraulic servo-motor which in some cases may be adjusted to exert an axial pressure of 30 tons to keep the grinding space constant.
The distance may vary between 0.5 mm down to 0.05 mm, or even less, to obtain the desired refining effect. A distance of 0.8 mm has been used for special pulps. The servo-motor should preferably be able to keep this grinding space constant within a margin ofO.01 mm.
As an example of the actual design of a 50" refiner equipped according to the invention, may be mentioned that the rotating cir-cular rings 30, 50 may have an outer dia~leter of 1270 mrn and an in- --ner diameter of 1016 mrn. The rotating "grinding disc" or the "ro-tor" 16 is designed to be driven with a speed of 1500 rpm to 1800 rpm. The individual pockets 41 to 46 may have a width of 0.33 cm and a cross-section of 1.5 cm2, which gives a volume of 0.57 cm3.
The bars may have a thickness of 3 mrn and a width of 15 mm and a .
length of 127 mm. Such a disc may, around the circumference, have 504 bars arranged in 72 groups of 7 parallel bars, creating scissor-` like cuts instead of the long parallel cuts obtained when the bars are all radially arranged. In the stationary disc 50, the pockets ;~, 30 and bars can be arranged in substantially the same way as in the rotating disc 30e The grouping with parallel bars contributes to a more even distribution of the angular torque during operation. Such `` parallel arrangement of ~he bars in groups also sirnplifies the ma-- chining of the ~rinding elements 30 and 50 by allowing a greater ~7~
:' ., - -.

634~5 number of slots to be milled in one operation.
The refiner operates as follows~ In the first zone formed by the elements 18 and 22, the raw material, e.g., the wood chips, is fragmented with moderate energy consumption. In the second grinding zoneJ the material is further broken down between the el-` ements 20 and 24. The grist may then have a "freeness" of 800 ml determined according to the "Canadian Standard Freeness" method -(CSF). The grist then is introduced through the gate 40 and enters - pocket 41 in the rotating grinding ring 30. The exit part 48 of the -pocket 41 now directs the grist laterally toward the innermost pock-et 61 of the stationary grinding disc 50. The grist then alter-nately passes between the pockets of the rotating and stationary ~"` discs until finally leaving the grinding zone from pocket 66. ~ ~
- In Fig. 4, it is shown how the pockets in both grinding discs -are in open communication with each other corresponding to the po-sition shown in Fig. 5A. When the grinding disc moves in the di-~'J rection indicated by the arrow 31, the bars 47 move from the po-sition shown in Fig. 5A, where the pockets are in full open com-munication with each other. In Fig. 5B, this communication is . ......................................................................... .
closing, and, in Fig. 5C, the communication is fully closed, with ~, the exception of the narrow grinding space set by the hydraulic servomotor controlling the axial position of the rotating shaft.
In this manner, any specific pocket in the stationary disc is passed by 10800 pockets in the rotating disc every second, corresponding ~d 25 to one-tenth of a millisecond for each pocket to pocket communi-^~ cation. Despite this short dwell time, the flow of grist from the 1 pockets in the rotating disc to the pockets in the stationary disc ;~ is not clogged, This can be observed if the flow of chips fed to -, the refiner is shut off and the refiner disc is brought to a stand-still~ When the refiner is opened for inspection, the pockets of .:, the rotating disc are found completely empty. When the propelling action of the grist coming from the rotating disc has ceased, pulp ~-will remain in the stationary pockets.

., .
, ~ .. . ~
- r 1~63405 The grist coming from the pocket 41 moves with very high velocity, around 82 m per second~ and has thus acquired a consid-erable quantity of kinetic energy of motion. When the grist is projected into the stationary pocket, it exerts a considerable mechanical pressure on the grist already collected therein, com-pressing it to a density of about 0.79 g per cm3. The internal friction created when the edge of the next oncoming bar cuts into the body of grist provides the required refining effect. This ac-tion can be modified by the setting of the distance between the grinding discs~ e.g., when pulp for newsprint is produced, a dis-tance between 0.1 mm and 0.2 mm may be used. When tissue paper is produced, a distance of 0.3 mm to 0.5 mm may be used. Pulp for egg containers may be produced with a setting of up to 0.7 mm.
When the grist from the rotating pocket 41 is pressed against the mass of grist collected in the stationary pocket 61, as shown : by the arrow A in Fig. 4, a wedge-shaped portion of this mass will "tilt" into the pocket 42. This portion will then be "shaved off"
by a bar 67 and subsequently accelerated by the centrifugal force and finally deflected into the pocket 62.
Each time the grist is arrested, the kinetic energy of ro-tation is transformed into heat, raising the temperature of the ; grist. As the temperature of the grist affects the quality of the ` pulp, it is of certain importance to counteract these fluctuations.
~ As the whole process of refining is carried out at a temperature -~ 25 very close to the boiling point of water, whether this is done un-^, der atmospheric conditions or under steam pressure in a pressur--,` ized refiner, this heat will also cause evaporation to such an ex-tent that the moisture content of the grist may be reduced. As the intensity of the shearing forces developed in the friction surfaces within the body of grist is highly dependent upon the a-mount of moisture present, the water evaporated must, therefore, ~ be compensated for. This is done by injecting an appropriate a- ~
: mount of water, e.g., through the conduit 55. -The grist diverted from the pocket 41 has a velocity of about ~ B -9- ~ ~
.~

. - ........... . - . .. ..

` ~6~405 80 m/sec, giving it a centrifugal acceleration of 1330 g, and -is pressed against the grist in the pocket 61, where it comes to an instant stop. The stationary pocket 61 is filled with com-pressed grist with the fibers oriented perpendicularly to the bars.
The kinetic energy of the grist is converted into heat, which evaporates moisture. This transfer is repeated each time the pock-ets in the two grinding discs open into each other. When the bars 47 shear through the compacted grist, frictional surfaces are de-veloped between which the fibers are separated from each other and fibrillated. The power from the driving motor is thus trans- -ferred to the grist by means of the bars 47 to kinetic energy of --rotation by centrifugal acceleration of the grist on the order of 1000 g to 1500 g. By directing the flow of grist, it is possible - ~ -.~i to develop the internal frict.onal forces necessary to effect the fiber separating and obtain the desired fibrillation.
It has been mentioned that each time a communication takes place between the pockets 41 and 61 of the rotating and the sta-~ tionary pockets as shown in Fig. 5A, a new quantity of grist is ;.,i transferred to the pocket 61. It has also been mentioned that the grist compacted in the stationary pockets, thanks to its geometri-j cal form of a circle segment, will pivot in the pocket around its -' geometrical center point 49 of the sector, as indicated by the ~, line 69 in Fig. 4. This means that the flow of grist through the refiner is maintained as long as new material is supplied through 1 25 the gate 40 to the pocket 41. This in turn implies that, when -~ grist is continuously supplied to and compacted in the entrance ;
part of the stationary pocket 61, a corresponding quantity of ~-grist is pressed out and sliced off from the exit part of the pocket 61 into the pocket 42 of the refining member 30 of the ro- ~--~ 30 tating disc. During the passage to this pocket, new friction surfaces will be developed within the grist as the rapidly moving bar 47 cuts through the compacted body of fibrous grist which is , prevented from being passed into pocket 42 by the bar 67. In this manner, the grist will alternately move between the moving B
,,, - 10-.. . . ~ ~ ... . . . . .. .. - `
,.. . . . . .~ . . . . .. ... . . . .

6~40S
and the stationary pockets until it finally leaves the stationary member 50 through one of its exits 66 at the periphery of the grinding disc.
Depending upon the adjustment of the distance between the grinding discs and also upon the relative quantity of grist carried in the stationary pockets, a portion of the grist may pass di-rectly radially from the pocket 41, passing through surfaces of friction developed by an edge designated by the numeral 51 in Fig. 4.
In Fig. 5A to 5C, it is shown that, when the bars 47 pass by the bars 67, the passage between the pockets of the rotating and moving members of the refiner is alternatley opened and closed.
In a 50" refiner, which herein has been used as an example, this is repeated for each bar with a frequency of nearly 10,000 times per second. The bodies of grist emerging from the stationary pockets have then been acted upon by forces varying between a magnitude of a few kg per cm2 to zero, which accomplished the transfer of the grist through the pockets. This vibratory effect also assists in compacting the grist so as to make possible the development of frictional forces of high intensity as the grist passes from a sta-tionary pocket to a rotating pocket.
The distance between the opposing narrow edges of the bars : of the rotating and stationary refiner members in Fig. 5A, numbered ` 47 and 67 respectively, corresponds to the "refiner disc distance", an important operating factor of the refiner, depending upon oper-ating conditions, type of raw material and type of fibrous pulp desired. In some cases, when a very finely refined pulp is de-:;
sired, the distance may even be reduced to 0~05 mm, but, in such a case, the capacity of the refiner is reduced. Pulp of a CSF -freeness of down to 2 millimeters may then be produced, ~` The grinding disc distance will in most cases be a multiple of the diameter of a fiber of most vegetable raw materials which, :~ e.g., for the tracheids of spruce generally is around 0.02 mm and 0.03 mm. A direct cutting of the fibers between the surfaces of ,, .- ~ . ................. - . ., .... . - . . - ~
. .

~(~634(~5 ` the leading edges of the bars of the refiner, therefore, is not possible. The grist is refined by the interaction between the edge of the bar and the body of compacted grist. The mechanical effect on the fiber is, therefore~ dependent upon the state of the frictional forces developed within the grist. This is, of course, in turn dependent upon the mechanical stability of the refiner, which should be rigid enough to prevent direct metallic contact ' between any surfaces of the grinding discs.
To accomplish a separation of the fibers from each other and for a further segregation of, e. g., the anatomical parts of a coniferous tracheid and to accomplish the desired refining effect on the pulp, the grist has to be compressed to such a degree that, within the grist, regions of stresses of sufficient magnitude are developed.
The pre-requisite for such conditions is accomplished es-pecially within such regions of compressive stresses which are de-veloped at passages between the rotating pockets 41-46. ~Depend-` ing upon the diameter of the refining discs, the moving grist may be compressed by a centrifugal force of around 500 g, and even up to 1500 g, when it comes to a sudden stop in one of the stationary I pockets 61-66.
The fields of frictional forces developed at the crossings from the stationary grinding disc 50 to the rotating disc 30 have, on the other hand, another character, more like a "slap on the ear". When the grist is accelerated by the bar 47 into the pocket 42, the action is different. The strains there are more like the ~ action of a blunt knife cutting through a piece of cheese. In -~ the compressed grist fo~ced from the stationary grinding disc, ./ .
' 3 the rotating bars will, thus, also create surfaces of friction.
~, 30 Then, in the rotating pocket 42, the grist sliced off and fluffed . by the bars 47 is accelerated to a considerable velocity. and the .~ grist is diverted from the rim 49 in the pocket 42. From there, -it is transferred to the stationary pocket 62 and there again ar-~, ,~

~6a4~5 rested to zero angular velocity with a loss of kinetic energy and a repeated rise in temperature.
The lignocellulose substance is hydrophilic, and, when moist, thermoplastic. When its temperature is raised to the boil-ing point of water (100C), it starts to soften, and, when thetemperature reaches 140C-150C for different lignocellulose mater-ials, the strength of the bond between the fibers is very weak.
The refining process according to the invention is prefer-ably carried out in closed refiners generally called "pressurized refiners", in which the refining process can be carried out under . steam pressure. The desired temperature can then be maintained during the refining process. The energy required to raise the tem-perature to the desired level is obtained from the heat liberated in the refining process. The quantity (amount) of mechanical en-ergy used for the refining process, generally around 800 kWh, up to 2000 kWh per ton of pulp produced, will liberate more heat than is required to keep the grist at the appropriate temperature, e.g., 130C. The heat will then evaporate a portion of the moisture con-.
tained in the grist. Thus, if the mechanical work needed to carry -~
~:: 20 out the refining process according to the invention corresponds to ~`?~ 800 kWh per ton of dry wood substance, the heat used will corres-?~:~
pond to 688,000 kcal per ton of dry substance. If the process is "
carried out at 120C (and at a corresponding steam pressure), to .
. take advantage of the thermoplastic softening of the wood at that temperature, 1309 kg water per ton of wood processed will evaporate.
Assuming that, in the example quoted, the wood chips fed to , the refiner had a moisture ratio of 2:1, the moisture ratio would be reduced to 0.7:1 by evaporation. If a moisture ratio of the grist of 2:1 during the process is necessary for obtaining accept-.j 30 able refining result, a corresponding amount of water should be - -added to the grist in the zone of refining Conduits for such ad--i dition are shown at the numeral 55, in Figs. 1 to 5. ~n the pro--. cess according to the invention, the amount of water added may be .. -13-: .

. ." ~, , . ,~ . ~ . .
- . - -; .. ~ ~ .. . . .... ... , .. . ,.. ,,, .. . ... " .:

automatically controlled.
The water added will be rapidly distributed on the surfaces of the fibers of the grist and will, therefore, influence the state of friction between the fibers and thereby the process of refining. When the moisture ratio of the grist rises, the thick-ness of the liquid film will increase. Too thick a film will act as a lubricant and will lessen the efficiency of the refining pro-cess. ~hen the moisture ratio decreases to 1:1 or less, there is considerable risk of formation of microscopic nodules or bundles of fibers that are difficult to untangle by post-refining. An - increased amount of water will also increase the energy consumed - to accelerate the grist in the pockets of the rotating grinding disc.
When examining microphotographs of a cross-section of spruce wood fibers (tracheids) magnified 50 times, it is easy to recog-nize the cross-sections of the individual fibers and how their size varies within the year rings. A statistical study shows that tracheids generally have a cross-sectional area of 0.03 mm x ~`~ 0.03 mm, corresponding to about 100,000 fibers per cm2. The fiber length is about 2.5 mm, giving a number of about 400,000 tracheids per cm3. The dry weight of spruce wood is about 0.42 g per cm3, which indicates that one g (= 2.4 cm3) of spruce wood contains -aboutl,000,000 fibers and that the combined surface of separated whole fibers is about 3 m2 per g of wood. Refined grist, depend-`~ 25 ing upon its CSF-value, may have a 10-fold combined surface. The ~`~ liquid film distributed on the particles is calculated to be a -:~ -' film of a thickness of less than one-thousandth of one millimeter.
~ However, due to the large su~face involved in refined pulp and the .i .. J~ thin~wate~ fil~ in addition to the~hydrophilic properties of the ~ 30 wood substance, a limited excess of water can be permitted wi~hout s.
;~; seriously di~inishing the desired frictional forces within the grist.
A refiner equipped with grinding discs according to the in-vention creates a greater number of efficient frictional surfaces . . ~.~ . ~

. ~ ~
... . .- - ~ :. ~ ; -- . . .. - , , .

1(~6~05 within the grist than has been possible with any previously known type of refiner. It is also possible in a refiner applying to the invention to use bars more resistant to abrasion than formerly.
This is possible also because of the novel manner in which the bars have been embedded in the body of the disc.
The surfaces of friction which are developed wlthin the grist can also be adjusted in relation to the distance between the surfaces of the rotating bars and the stationary bars. Efficient .~ refining action can, therefore, be accomplished when a greater dis-tance or clearance is used than has been possible with earlier known designs. This means less abrasions of the opposing surfaces of the bars. Such increased distance also counteracts excessive;
- shortening of fibers through cutting.
The bars used in the refiner discs according to this inven-tion will gradually be subjected to wear on the opposing surfaces, which may result in dullness of the leading edge. Worn bars can `: easily be replaced with new bars.
With an axially adjustable ring 56 outside the periphery of the stationary grinding disc 50, it is possible to regulate the ` 20 area of exit from the outermost located stationary pockets 66 to :~ retar~, if desired, the rate of flow of grist between the grinding discs 30 and 50 by means of adjusting nut 57.
The bars 47 and 67 can be made from hard abrasive materials ` like carborundum, sillicon carbide or other ceramic materials. The circular rings 30 and 50 can be made from softer material more ~ easily machined and can, therefore, be made in full circular ring - construction, depending upon type of refiner. The bars are held ,. .
: in machined radial slots, preferably by high-strength synthetic organic glues according to known methods, The bars can also be ` 30 cast into such components, Th~nks to the curved contours of the grooves 41-46 and 61-66, the bars are held with great stability.
The components of the secondary grinding zone of the grind-ing elements shown in Fig. 1, numerals 20 and 24, may also, as an . ' .

: . . .~ . .

1~63405 alternative, be designed similar to the elements 30 and 50 in the same Fig. 1. The working surfaces of the grinding discs should, after assembly, be finished to the highest possible accuracy for best performance when the refiner is in operation.
The first pocket 41 of the grinding disc 30 has an innermost radius of 528 mm. When grist with a moisture ratio of 3:1 is ac-celerated to a velocity of 83 m per second, the required kinetic power input will be 14.5 kWh per ton of pulp calculated as oven dry. When the grist enters the pocket 61 of the stationary grind-ing disc 50, it is retarded to zero velocity, but, when leaving this pocket, it is again accelerated in pocket 42. The grist pass-ing through the refiner is thus accelerated and stopped success--ively six times, one time for each row of pockets. This would - correspond to a power consumption of about 95 kWh per ton of pulp produced. As described above, when the grist in a compacted form emerges from the stationary pockets, surfaces of friction are gen-erated by the edges of the bars. The intensity of this friction - can be modified by setting the grinding disc distance as indicated . .;. . .
. above. The power used for the refining process may, thus, be var- -.. .. .
' 20 ied from a few hundred up to 1000 kWh per ton of ordinary types of pulp. When pulp of special low freeness values is required, up to 2000 kWh per ton of pulp may be used. With increased grind-ing disc distance, a short-cut may be created allowing a portion . .
,f of the grist to pass over the rims 51 directly from a pocket to the next radial pocket. A considerable loss in refining intensity ~, will occur, but the power consumption per ton will decrease. ~:~
;- In the embodiment disclosed in Fig. 6, the pockets 61-66 and `~ 71 of the stationary grinding means 50 in an axial section have the shape of a circle, just as in the preceding embodiment, the ., ~-~ 30 radius of said segment corresponding to the full depth of the pocket or being somewhat larger. Also, the individual pockets of the grinding means 30 are curved in an axial section through the grinding means. In contrast to the embodiment of Fig. 4, the in-,, 1()~3~S
dividual pockets 42-46 and 70 have a gradually sloping wall sec-tion 72 at the radially outer face thereof, said section being plane or approximately plane and, at the botto~ of the pocket, tangentially changing to a wall section 74 extending perpendic-ularly or approximately perpendicularly to the grinding surface formed by the edges of bars 47 and rims 51. The arc-like section 73 may have a radius of curvature approximately corresponding to the full depth of the pocket. In this manner, the rotating pock-ets will form an outline having a relatively long and flat de-flection surface for the grist. At the radially inner side of the grinding means 30, said grist is fed through a port 48 and, at high speed, is hurled into the innermost stationary pocket 61, as " disclosed by arrow A, where the grist will be instantaneously :
stopped and fills up the pocket as a compact plug or body. The latter slides along the surface 68 and is then brought to the in-nermost rotating pocket 42, the plug then passing between the bars 46 and 67, respectively, shearing or friction surfaces then being shaped, in which the separate fibers or fibrilles of the grist are ,~ separated from each other. Due to the circle segment section of the pockets, the plug of the grist collected therein will be-r~o-tated about a center 49, as indicated by line 69, by the subse-.
~.~ quent grist hurled through said port 41. Said plug will then get ,.~
into contact with the rotating bars 47, which disintegrate the - grist while peeling the same. In the rotating pocket, said grist is now instantaneously accelerated to the high speed of the ~ grinding means, the grist then being compacted and, due to the .~ sloping, elongated plane 72 and a favorable distribution of the '~ components of force in the radial and axial direction, also being .~
. passed into the next pocket 62, where, once more, a plug in the shape of a segment of a circle is generated~ The cycle is re-... peated at the alternate passages outwards of the grist between the stationary and rotating pockets, and the obtained final pro-duct is fed out through the last stationary groove 71.
, - ~ -17- -:
.

Claims (5)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a grinding apparatus for producing grist from ligno-cellulose material in which the material is ground in a straight grinding space defined between a rotating disc and a stationary disc arranged within a housing, each of the discs comprising a plurality of substantially parallel radial bars and interconnect-ing transverse ribs defining a plurality of radially disposed pock-ets having a curved profile in cross-section, the pockets in one of the discs being offset radially relatively to the pockets in the other disc to firm a sinuous uninterrupted passage for the grist as it is propelled radially outwards in the grinding space by centrifugal force: the improvement in which the pockets in the respective discs are profiled so that the grist is deflected by a passing rotating pocket into a corresponding offset stationary pocket and retained therein while being compressed by successive charges of grist to a degree sufficient to develop frictional stress forces within the retained grist to promote disintegration thereof and to cause the thus compacted grist to pivot about the profiled surface of the pocket until aligned with a rotating pocket wherein it is fluffed and accelerated into a successive stationary pocket upon being sheared off by the following bar, the grinding space being defined between the free edges of the radial bars, which extend outwardly the full depth of the pockets to the grind-ing space, and the transverse ribs, which radial bars and trans-verse ribs are located in two mutually spaced planes.

2. Apparatus according to Claim 1, in which the bars are composed of material substantially harder than the disc material.

3. Apparatus according to Claim 1, in which the bars in one of the discs have such a width relative to the pockets in the other disc that the radially offset pockets are momentarily closed off from one another during the relative rotational movement between the discs.

4. Apparatus according to Claim 1, in which the pockets of the stationary disc have the profile of a circular segment and the rotating pockets comprise a gradually sloping wall section at the radially outer face thereof, said section being substantially plane and merging tangentially with the curvature at the bottom of the pocket.

5. Apparatus according to Claim 1, in which the operative edges of the bars are located in the plane which defines the width of the grinding space.