US3486619A - Method of removing impurities from a fiber suspension - Google Patents

Description

Dec-=30 1969 NR. GRUNDELIU S EfAL 3,485,619

METHOD OF REMOVING IMP URITIES FROM A FIBER SUSPENSION Filed Jan. 24, .1968 I 2 Sheets-Sheet 1 BY Ham MAM Dec. 30, 1969 N. R-GRUNDELIUS EI 'AL. 3,486,6 9

' METHOD OF REMOVING IMPURITIES FROM A FIBER SUSPENSION Filed Jan. 24, 1968 2 Sheets-Sheet 2 INYENTORS m5 950mb km Imus .5 Km.- Gama. "Lanna No A ML:

KTTQINQYs United States Patent US. Cl. 209211 5 Claims ABSTRACT OF THE DISCLOSURE Method of removing shives and other undesirable lightweight particles from an aqueous fibrous suspension in a hydroclone operated under a substantially normal through-flow volume by maintaining a desired relation of effective areas of the outlets and/ or of pressures in the outlets.

The present invention refers to a method of liberating, at least substantially, from a suspension of fiber material such as a paper pulp suspension solid impurities difficult to separate such as elongate or flattened light Weight particles, especially particles of plastic material, so-called shives or similar particles often present in paper pulp suspensions.

In modern production of chemical pulp very high de mands are made on the purity of a finished product. A large colored bark particles or fiber bundles. These fiber bundles may be caused by the fact that certain parts of the wood due to especially tight structure thereof are insufliciently impregnated during the pulping or cooking process, or that the pulping of the wood in the digester becomes uneven. By the knot separating operation usually following the cooking, knots possibly surrounded by layers of wood, and bigger chips of wood which are not so Well delignified that they can be easily defibrated, are normally separated. During the subsequently following screening to which pulp generally is subjected, further wood fragments and still present bark particles are separated. Depending upon how well the screening is carried out, the pulp may be more or less completely free of the above-mentioned impurities, but economical factors, of course, limit how far the cleaning of the pulp through screening may be carried on. In practice, a pulp which has been screened with utmost care still contains a greater or smaller number of fiber bundles of dilferent size, the color of which may vary from yellow brown to nearly black.

Fiber bundles occurring in the purified unbleached pulp tend to cause specks in paper manufactured therefrom and to contaminate often also bleached pulp, as these fibers bundles will not be completely bleached 'by normal bleaching operations.

A method generally practiced in modern factories is to process the pulp further in hydrocyclone separators or purifiers to improve the purity thereof. In a separator of conventional type with an entirely or partly conical separation chamber the suspension is tangentially fed to the wider end of the separation chamber and separated into two fractions, one light and one heavy. The light fraction, which is centrally discharged from the wide end of the separation chamber, consists of the cleaned fiber suspension. A smaller part of the suspension, mainly containing relatively easily separable impurities such as bark particles, short fiber bundles and other comparatively heavy particles, such as grit, is centrally discharged as heavy fraction at the narrow end of the separator. It has been found that conventional cyclone separators are "ice not capable of removing in this manner the bulk of the above-mentioned ditficult to separate light-weight impurities consisting, for instance, of not well digested or defibrated bundles of longfibers and similar particles elongate in one or two directions. Impurities of this type, which are generally called shives, will therefore tend to follow the light fraction.

The method of the invention makes it possible in a simple and economic manner to liberate the suspension from the last-mentioned kind of impurities.

- The normal through-flow volume of a separator installation is determined by the nominal capacity of the components included in the installation, that is, preferably the nominal capacity of the cyclone separators at which the optimal separation result is obtained.

Broadly, the method of the invention resides in providing a cyclone separator having two outlets, one being a light fraction outlet and the other a heavy fraction outlet, feeding suspension containing good fibers and undesirable particles diflicult to remove into the separator while maintaining in the outlets thereof relative flow conditions which cause the suspension to be separated into a first fraction containing the bulk of shives and other undesirable lightweight particles which due to shape and-density are diflicult to separate and a minimum of the good or acceptable fibers and a second fraction containing the bulk of the good or acceptable fibers and a minimum of the shives and other undesirable particles, and discharging the first fraction through the light fraction outlet and the second fraction through the heavy fraction outlet.

' The relative flow conditions necessary to obtain the afore stated discharge of the fractions is obtained by providing an appropriate relation of the efiective areas of the outlets and/or an appropriate relative pressure relation in the outlets while maintaining a through-flow volume substantially normal for the separator. A required effective area relation may be obtained by making the effective area of the heavy fraction outlet the larger one, and the required pressure relation by providing conventional and suitably set throttling means, for instance, throttling valves, in one of the outlets preferably the light fraction outlet. Suitable throttling means are shown, for instance, in Patent 2,897,972.

The specific data for the necessary effective area and/ or pressure relations can be easily determined for each type of separator with which the method of the invention is to be carried out as the calculation necessary for the purpose are well known to men skilled in the art. It is common practice to regulate the concentrations of the fractions obtained from cyclone separators by varying the area and/or pressure relations at the outlets as is described, for instance, in Patents 2,550,341 and 2,649,963.

Data suitable for practicing the invention with specific separators are stated hereinafter by way of example.

The method according to the invention also makes it possible substantially fully to utilizethe capacity of the separators.

It might be expected that the shives will tend to follow the heavy fraction when the fiber fiow to the outlet for the heavy fraction is increased in accordance with the invention as the shives have substantially same specific weight as the smaller bits of fiber bundles which are moved to this outlet. However, applicant was found by extensive tests that surprisingly the major portion of the shives is in the light fraction, even if a larger part of the good fibers which are normally part of the light fraction, is taken out as heavy fraction. These tests have also shown that it is possible to separate and discharge -90% of the shives present in the suspension as light fraction together with only 3-5 of the whole fiber flow, as is evident from the following test examples:

TRIAL A 1,000 grams of bone-dry sulphite pulp containing 1,000 shives were treated in accordance with the aforedefined method of the invention in a single cycle separator using a 0.5% pulp concentration in such a manner that 950 grams of the fiber flow were obtained as heavy fraction while the balance of 50 grams were obtained as light fraction. The number of shives in the two fractions was counted.

TRIAL B A corresponding counter test was made in accordance with the the aforedefined conventional method. 50 grams of the fiber flow were taken out of a single separator as heavy fraction while the balance of 950 grams was taken out as light fraction. The number of shives in the two fractions was counted.

The result of the two tests appear from Table I below.

TABLE I [Material supplied: 1,000 grams bone-dry sulphite pulp with 1,000 extended shives per kilo bone-dry pulp at 0.5% pulp concentration} As appears from Table I for Test A, 85% of the number of shives supplied to the separator were discharged through the outlet for the light fraction but only of the quantity of supplied fiber. However, it was found that it is possible to obtain such shive separation degree only under optimal conditions. A more practical value of the shive separation percentage for a single cyclone separator of conventional design appears to be 70% with a fiber content of in the light fraction. Such a shive separation represents a relatively high fiber loss, which is acceptable only in some special cases.

By carrying out the separation by the method according to the invention and in several stages by means of cyclone separators combined in hereinafter described arrays, the fiber loss can be reduced to the extent required in practice without decreasing to a corresponding extent the shive separation capability, as will appear from the following.

In the accompanying drawing several embodiments of the invention are shown by way of illustration and not by way of limitation.

In the drawing:

FIGS. 1 to 4 show diagrammatically cyclone separator installations in which several separators are interconnected or piped for carrying out the method according to the invention; and

FIG. 5 is a vdiagram showing curves indicative of the extent of the shive separation obtained with the installations according to FIGS. 1 to 4, and also the shive concentration obtained by a single stage operation.

In FIGS. 1 to 4, C symbolizes the fiber flow entering the installation; D symbolizes the one final fraction which is substantially free from shives, and E the other final fraction' containing a considerable portion of the shives.

More specifically, FIG. 1 shows the use of three cyclone separators 5, 6 and 7 serially connected so that the light fraction from the cyclone 5 becomes the supply stock for the cyclone 6 and the light fraction from the cyclone 6 becomes the supply stock for the cyclone 7, with the heavy .4 fractions (containing a preponderance of accepted fibers) being connected to a common discharge pipe D. The effluent obtained through pipe D may and often does contain in addition to the accepted fibers usual impurities such as sand, that is impurities which have higher specific weight than the accepted fibers and the shives. Such heavy impurities can be readily removed by a subsequent conventional separator operation. The rejected light fraction (containing the bulk of the shives and a minimum of acceptable fibers) is discharged from the cyclone 7 as at E.

FIG. 2 shows a more efiicient arrangement using three cyclone separators 8, 9 and 10 connected in a recirculating system in which the light fraction from the cyclone 8 becomes the supply stock for the cyclone 9, and the light fraction from the cyclone 9 becomes the supply stock for the cyclone 10. The heavy fraction from the cyclone 10 (containing a preponderance of accepted fibers) is recirculated into the supply stock for the cyclone 9, the heavy fraction from the cyclone 9 (containing a preponderance of accepted fibers) is recirculated into the supply (uncleaned) stock for the cyclone 8 and the heavy fraction from the cyclone 8 (containing a preponderance of accepted fibers) is discharged therefrom through the pipe D. The rejected light fraction (containing the bulk of the shives and a minimum of acceptable fibers) is disharged from the cyclone 10 as at E.

FIG. 3 shows a still more efficient system also using three cyclone separators 11, 12 and 13 in which the light fraction from the cyclone 11 becomes the supply stock for the cyclone 13 and the heavy fraction from the cyclone 11 (containing a preponderance of accepted fibers) becomes the supply stock for the cyclone 12. The light fraction from the cyclone 12, and the heavy fraction from the cyclone 13 (containing a preponderance of accepted fibers) are recirculated into the supply (uncleaned) stock for the cyclone 11. The heavy fraction from the cyclone 12 (containing a preponderance of accepted fibers) is discharged through the pipe D and the light fraction from the cyclone 13 (containing the bulk of the shives and a minimum of acceptable fibers) is discharged therefrom as at E.

FIG. 4 shows another highly eflicient arrangement using five cyclone separators, 14, 15, 16, 17 and 18 connected in a combined serial and recirculating system in which the light fraction from the cyclone 14 becomes the supply stock for the cyclone 15 and the light fraction from the cyclone 15 becomes the supply stock for the cyclone 16. The heavy fraction from the cyclone 16 is recirculated into the supply stock for the cyclone 15 and the heavy fraction from the cyclone 15 is recirculated into the supply (uncleaned) stock for the cyclone 14. The heavy fraction from the cyclone 14 becomes the supply stock for the cyclone 17, the heavy fraction from which becomes the supply stock for the cyclone 18 and its heavy fraction (containing a preponderance of accepted fibers) is discharged as at D. The light fractions from the cyclones 17 and 18 are recirculated with the heavy fraction from the cyclone 15 into the supply (uncleaned) stock for the cyclone 14. The light fraction from the cyclone 16 (containing the bulk of the shives and a minimum of acceptable fibers) is discharged therefrom as at B.

After the shives have thus been removed as a cumulative light fraction with a minimum loss of acceptable fibers, the resultant heavy fraction (containing a preponderance of accepted fibers) is further processed to remove the easily separable heavy particles such as sand and grit.

The graphs I to IV in FIG. 5 each refer to a respective one of the combinations according to FIGS. 1 to 4, and indicate by the use of symbols a, b, p and r, the functional relation between a and p and also between b and r. More specifically, there is indicated by:

a: the relation between the number of shives in the final fraction E and in the supplied fiber fiow C;

p: the shive separation ratio per cyclone separator, that is, the relation between the number of discharged shives in the respective separator outlet for the light fraction and the number of shives supplied through the respective separator inlet;

b: the relation between the quantity of good fiber in the final fraction E and in the supplied fiber flow C;

r: the fiber separation ratio per cyclone separator, that is, the relation between the quantity of discharged good fiber in the respective separator outlet for the light fraction and the quantiy of supplied good fiber in the respective separator inlet.

The values of a, p, b and r are given as percents by way of example.

By 'way of example the subsequent tabulation lists for a separation in accordance with the invention in a single cyclone separator the corresponding values of inlet pressure pi, accept outlet pressure pa, reject outlet diameter di, through-flow capacity Q and the relation of p and r as hereinbefore defined.

Case kpJem. kpJcmJ mm. lJmin percent percent pi/pa A corresponding graph for the relation between a and p, and b and r when separation is effected in one stage is represented by the straight graph V.

In the graphs of FIG. 5 the aand b-values correspond ing to different values of p and r as hereinbefore defined are assumed to be p=70% and r=20%. These values, as previously stated, may be considered to be realistic values. Hence, there is obtained theoretically, that is, without taking into account possible losses in the installation.

According to FIG. 1 (diagram curve I): a=34 and b=0.8 which means 34% of the shives in the final fraction E containing 0.8% of the supplied fiber flow C;

According to FIG. 2 (diagram curve II): a='59 and b=1.2 which means 59% of the shives in the final fraction E containing 1.2% of the supplied fiber flow C;

According to FIG. 3 (diagram curve III): a=84 and [1:59 which means 84% of the shives in the final fraction E containing 5.9% of the supplied fiber flow C;

According to FIG. 4 (diagram curve 1V): a=94 and b=1.8 which means 94% of the shives in the final fraction E containing 1.8% of the supplied flow C.

The invention is not limited to the treatment of paper pulp. As is evident from the previous description, the in vention permits in a simple and economical manner liberation of elongate shives and similar solid impurities from suspensions of chemical, semi-chemical or mechanical pulp by means of cyclone separators in such a Way that the shives and other impurities are concentrated in a light fraction containing only a small portion of the useful total fiber content.

The fiber containing flow which in accordance with the present invention has been obtained as light fraction enriched with elongate shives may be subsequently screened or defibrated in a known manner.

We claim: 1. A method of liberating an aqueous fibrous suspension from shives and other undesirable lightweight particles which due to shape and density are difiicult to separate, said method comprising the steps of:

feeding the suspension to a cyclone separator having two outlets, one being a light fraction outlet and the other a heavy traction outlet, for separating said suspension into a first fraction containing a preponderance of shives and other undesirable lightweight particles and a minimum of acceptable fibers and a second fraction containing a preponderance of acceptable fibers and a minimum of shives and other undesirable lightweight particles;

maintaining in the outlets relative flow conditions at which the suspension is caused to separate into said first fraction and said second fraction while maintaining a through-flow volume substantially normal for the separator; and

discharging the first fraction through the light fraction outlet and the second fraction through the heavy fraction outlet.

2. The method according to claim 1 wherein said relative flow conditions are maintained as relative area conditions.

3. The method according to claim 2 wherein the elfective out-flow area of the heavy fraction outlet is larger than the effective out-flow area of the light fraction outlet.

4. The method according to claim 1 wherein the relative flow conditions are maintained as re ative pressure conditions.

5. The method according to claim 1 and also comprising the steps of providing a second cyclone separator as defined in claim 7, recycling the effluent from the light fraction outlet of the first separator through the second separator, and feeding the effluents from the heavy fraction outlets of both separators to a common point of utilization.

References Cited .UNITED STATES PATENTS 2,377,524 6/ 1945 Samson 209211 2,897,972 8/1959 Cannon 210--5 12 2,975,896 3/1961 Hirsch 209-211 3,096,275 7/1963 Tomlinson 2O9-211 X 3,306,444 2/ 1967 Troland 209-21 1 3,352,745 11/1967 Malm 162-55 FRANK W. LUTTER, Primary Examiner Patent N D t d December 30,

Nils Roland Grundelius et al. Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 31, after "large" insert part of the impurities in a finished product is due to dis- Column 4, line 8, "separator" should read separation Column 6, line 41, "claim 7" should read claim 1 Signed and sealed this 30th day of November 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Acting Commissioner of Patents USCOMM-DC 60376-P69 DRM PO-IOSO (10-69) u.s. GOVERNMENY rnmnms ornce: nu o-sn-su

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