CA1174013A - Method at mechanical cellulose pulp manufacture - Google Patents

Abstract

Abstract of the Disclosure The invention relates to a method of manufacturing cellulose pulp free of resin in a closed system. The resin is dispersed in a refiner step and in a subsequent alkaline bleaching step and is washed-out in a counterflow-process with a washing effect of at least 70% and successively decreasing ion strength.

Description

11740~3 The present invention relates to cellulose pulp manuacture and in particular to thermomechanical or chemimechanical pulp substantially free from resin.
Bleached mechanical pulps of the kind groundwood pulp, refiner p~lp and thermomechanical pulp ~so-called TMP) and chemically modified thermomechani-cal pulp (so-called CTMP), i.e. pulps with yields abo~e 90 %, have been intro-duced for hygiene products of more qualified nature, such as fluff and tissue.
For obtaining good absorption properties, a low resin content is required. For the highest qualities, thus, the DKM extract content of the pulp must be below Q.2 %.
This is achieved relatively easily in an open system where the pulp in one way or the other is washed with substantial amounts of water between every process step, and where the pulp is bleached with peroxide. When there is suf-ficient water supply, it is even possible to work with a common backwater system, which is quite usual for practical reasons.
The resin does not dissolve in the washing liquid, but is dispersed in colloidal state or as coarser particles in the liquid which is preferably alkaline. There have been serious apprehensions with respect to the possibility of re-using the backwater to a high degree only at low resin concentrations.
T~erefore, it has been industrial practice for products of low resin contact not to have a water system which uses less than a total of at least 15 to 20 m3 ~ater ptm90. At lower water consumption levels, apparently, the low resin cont-ent could not be maintained.
A lower water consumption, however, is of interest, especially when waste water purification methods are concerned, but also when heat from the pro-cess is to be recovered. Interesting results are obtained when the water con-sumption can be reduced to below 10 m3 ptm90.

- 2 -1~740~ 3 Counterflow washing is a well-known and widely applied method for re-covering conce~trated waste liquors from different digestion processes. Material dealt with in this way is almost enti~rely~dissolved substance, which per se does not give rise to any washing-out difficulties. For sulphate pulps from wood rich in resin, however, colloidal resin can be salted-out under certain circum~
stances and this renders washing difficult. This problem, however, generally is solved by washing-out remaining resin in a subsequent suitably adapted multi-step bleaching operation, provided with proper washing plants with relatively large water consumption. A direct coupling between the technique in the manufacture of mechanical pulps and the like and counterflow washing methods for the recovery of waste liquors from different digestion processes has not been considered realistic, because it has been difficult to predict how the relatively coarsely dispersed resin will behave in mechanical pulp manufacture when being repeatedly filtered through a pulp web.
It has now been found, however, that such a coupling is possible, pro-vided that the conditions are adapted to the special demands of the resin in the case of mechanical pulp.
The mechanical pulps, TMP or CTMP~ generally are defibered in slightly acid or neutral environment, which does not bring about a stable dispersion of 2Q resin even when the mechanical processing in connection with the fibre exposure can be ideal therefor. When, for example, peroxide bleaching in an alkaline environment fpllows, the alkaline environment facilitates the resin dispersion.
Washing carried out between these two process steps can be of great interest, because the lower resin content level in the peroxide solution, which per se is favourable, reduces the risk of coagulation of resin particles and there~y gives greater stability to the resin removal. When, however, bleaching waste liquor is simultaneously recirculated in the bleaching step, which is of i~74013 interest both for reducing the water consumption and for more efficiently utiliz-ing the bleaching chemicals, salting-out concentration seems to be obtained sur-prisingly rapidly. It is then important to relatively quickly wash the resin thus salted-out with washing water having an ion strength which decreases succes-sively to pure water. The washing should be carried out substantially by dis-placement and can be driven to a high displacement degree for solved substance.
For the lowest water consumption, a washing should be carried out prior to the peroxide step and in series with washing after the peroxide step. The washing liquid should be moved in a consequent counterflow.
~t has been found quite surprisingly, that by such a method pulps with acceptably low resin contents can be manufactured with very low water consumption.
According to the present invention, there is provided method of manu-facturing a cellulose pulp substantially free from resin and in a yield above 9Q %, by mechanical defibration of wood material and subsequent bleaching, said method being conducted in a substantially closed system corresponding to an amount of waste water of at most 15 m3 per ton pulp, characterized in that back-water from each of the defibration stage and the bleaching stage is maintained separated, that excess backwater from the process is withdrawn at a washing stage arranged after the defibrating stage, that excess backwater from the bleaching 2Q stage is used for diluting or washing of the pulp after the defibrating stage and that the washing effect of dissolved substance amoun~s to at least 70 %.
Particular embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which:
Plgure 1 graphically illustrates the resin content of pulp treated by varying amounts of fresh water after chip refining in a chip refiner but before dewatering of the resulting pulp in a press;

1~74(~ 3 Pigure 2 diagrammaticall~ illustrates steps according to the process of the present invention; and Pigure 3 diagrammatically illustrates a modification of the process of Flgure 2.
Unbleached CTMP pulp was manufactured in a pilot plant by impregnating spruce chips containing about 1.2 % resin with sodium-sulphite solution, whereby the chips absorbed about 20 kg Na2S03/ton bone-dry chips. The chips were pre-heated with steam to 110C and then refined in a chip refiner. The pulp con-centration after the chip refiner was measured and found to be about 30 % by weight. The pulp was diluted to about 4 % pulp concentration and then dewatered ~n a press to ab~ut 30 % pulp concentration. The resin content of the pulp after the press was analysed.
Several experiments were carried out, in which the pulp after the re-finer but before the press was diluted with water or a mixture of water and back-water. By using an eYer decreasing amount of fresh water and an ever increasing amount of backwater for diluting the pulp, the closing degree of the system could be increased.
Analysis of the resin content of the pulp showed the relation illus-trated in Figure 1 between the resln content in the pulp and the amount of fresh 2a water supplied to the system expressed in m3 fresh water/ton pulp.
By establishing material balances for resin at every experiment, it was possible to calculate how much of the resin is found dispersed in the back-water after refining and can be removed by dewatering and washing and, respec-tively, how much resin cannot be removed in this way. The following results were obtained.

1~7~01;:3 Fresh water amount added Resin amount removable by m3/ton pulp washing or dewatering, %
0.25 0.61 The results shown in Figure 1 illustrate one of the observations, on which the present invention is based. In the manufacture of unbleadhed CTMP
pulps, increased closing of the water system implies that an ever smaller amountof the wood resin is being solved or dispersed, and the resin content of the pulps increases. Similar investigations in the manufacture of thermomechanical pulps have yielded similar results. The results can be interpreted in that with increased closing an increased salting-out o~ resin solved or dispersed at re-fining is obtained. In the manufacture of unbleached mechanic pulps for applica-tion fields requiring a low resin content of the pulps, it has previously not been possible to close the systems rigidly.
Pulps from the aforementioned pilot plant were then subjected to al-kaline peroxide bleaching, which a.o. has the following objects:
- it increases the brightness of the pulp - it lowers the resin content of the pulp.
The experiments were carried out so that unbleached TMP and CTMP pulps after the press were diluted with fresh water or backwater from the bleaching step. Bleaching chemicals were admixed to the pulp suspension at about 12 %
pulp concentration, whereafter the pulp suspension was passed to a bleaching tower. After bleaching, the pulps were diluted with fresh water or backwater to 4 % pulp concentration and thereafter pressed to about 35 % pulp concentration.
The resin content of the pulp samples was determined after the press.

117~013 The Table below shows some examples of pulps used ln the bleaching experiments~ The Table also includes the chemical charges made in the bleaching step.

Pulp Resin Presh water Resin Bleaching chemical contentadded before content charges, kg/t pulp before bleaching in pulp press plant after H22 ~aOH Na EDTA
% m3/t pulp press sili-% cate -CTMP 10.84 7.1 0.49 37 25 40 3.0 CTMP 20.81 23.9 0.27 25 18 50 3.8 CTMP 30.88 26.6 0.32 42 30 42 3.1 TMP 1.04 26.3 0.48 46 31 43 3.2 By establishing material and resin balances in the bleaching step it has been possible also here to calculate how much of the ingoing resin was dis-persed and, thus, could be washed-out and, respectively, how much resin could not be washed-out. The pulps studied in the above Table yielded the results as follows.

PulpPresh waterResin Resin Undispersed Temperature added in content contentresin content in bleaching in pulp in pulpin bleached bleaching step and before a~ter pulp Osctep before bleaching bleaching %
press % and pres-m3/~t pulp ing CTMP 15.5 0.49 0.16 0.05 70 CTMP 221.1 0.27 0.15 0.13 70 CTMP 318.4 0.32 0.15 0.13 60 TMP 23 3 0.48 0.19 0.13 85 1~7~V~3 At the bleaching, the main part of the remaining resin is solved out.
The alkaline peroxide bleaching thereby transfers the resin in such a form that it does not precipitate out at increased closing, at least not whcn the closing is held within reasonable limits. The water consumption at the closing experi-ment with C'~MP pulp 1 corresponds to about 6 to 10 m3/t pulp.
The experimental investigations of the manufacture of bleached mechani-cal pulp showed the following results.
1. In the manufacture of unbleached mechanical, thermomechanical and chemically modified thermomechanical pulps, part of the resin in the wood is dissolved or dispersed. The dispersed resin can be removed by washing.
2. B~ closing of the backwater system for unbleached pulp an ever smaller part will be dispersed. The resin content of the unbleached pulp increases with increased backwater closing.

3. Upon the bleaching of these pulps the resin is transferred in such a ~orm that the main part of the resin is dispersed. The resin has been modified By the bleaching so that it no longer precipitates~i~tincreasing closing.

4. The resin amount not dispersible in peroxide-bleached mechanical pulp is only 0.05 to 0.15 % of the pulp amount.
These discoveries render it possible to manufacture bleached mechanical 2a pulp ~ith a very low resin content at low water consumption, when the pulp is washed before and after the bleaching and when the backwater is moved in a counter~low to the pulp.
An embodiment of the i~vention is shown in Figure 2, but the invention, of course, is not restricted to this embodiment.
The system comprises a refiner step 1, a dewatering or washing step 2, a bleaching step 3, a washing step 4, a dewatering and/or washing step 5 and a drying step 6 where the bleached pulp is dried.

1~7~()13 Chips are supplied to the refiner step 1 and may be untreated, steamed and/or impregnated with chemlcals. The pulp after the refiner 1 is passed to the ~ashing step 2, which ma~ be a washlng filter or preferably a dewatering or washing press. The pulp suspension is diluted with back~ater from the bleaching step prior to the washing step and, if necessary, with backwater from washing step 2.
The unbleached pulp after washing step 2 is supplied with bleaching chemicals and diluting water, which comes from washing step 4 and is passed to the bleaching tower 3.
After bleaching, the pulp is diluted with backwater from washing step 4 and washed in washing step 4. As displacement liquid is used backwater from washing and dewatering step 5. The pulp is dewatered before the drier 6 in the dewatering step 5, which-preferably can be a dewatering or washing press. The backwater leaves the system via washing step 2. rhe washing liquid is supplied in connection to washing step 5.
The resulting effect can be illustrated as follows. CTMP chips are refined in step 1. The pulp concentration after the refiner is 30 %. The pulp ls dewatered to 48 % pulp concentration in dewatering press 2. The pulp there-after is bleached at 12 % pulp concentration, washed on the washing filter 4 and flnally dewatered on the dewatering press 5 to 48 % pulp concentTation before drier 6.
By means of an automatic model and the experimental results obtained, resin balances~for the system were calculated at different closing degrees.
The following relation was obtained between the amount of backwater leaving the system, the washing efficiency degree of resin dissolved out or dis-persed in the bleaching step and the resin content in bleached pulp. The con-tent of indispersible resin in the pulp after the bleaching step is assumed to 1~7~3 be 0.13 %. The Tesin content in the pulp after the refining is assumed to be 0.9 %.

Outgoing backwaterWashing efficiency Resin content amount degree of resin in bleached pulp m3/t pulpi dissolved out in bleaching step % by weight _ 1 0.77 0.31 3 0.$7 0.20 o.g2 0.16 0.95 0.15 The calculation shows that it is possible by rigid closing to manufac-ture bleached mechanical pulp with low resin content, provided that the pulp is washed in counterflow.
Peroxide bleaching of mechanical pulp or the like ~pulps with yields 90 %) yield under normal conditions a very great amount of residue chemlcals, ~hich cannot be utilized in a simple way By tightening up the bleaching condi-tions, but which to a more or less high degree can be utilized when the residue solution of the chemicals after the bleaching is returned to unbleached pulp.
To carry out such return operation requires an apparatus equipment better than used normally and a better control of the water systems. Experiments with such return have been carried out with relatively good success, but the efficiency degree has proved low, probably because diluting phenomena and other disturbing effects have influenced the procedure in a relatlvely irrational way.
It has now been found entirely by surprise, that by accurately control-ling conditions, an efficiency of almost 100 % of the bleaching with these residue chemicals can be obtained. The prerequisite condition is that the chemicals are returned _ 10 .

il~4013 with only insignificant dilution, and that backwater from the refiner step, be-fore bleach~ng, is permltted to pro~ide dilution only to a small extent. This in its turn requires an accurately balanced backwater system, at which the back-water of the refiner step and of the bleaching step are well separated.
The best way of separating the two process steps, wlthout risk of back-waters being mixed together, and at the same time to provide the conditions pre-requisite for maximum return of residue chemicals from the bleaching plant, has proved to be the arrangement of a press immediately ~efore the bleaching plant.
Only a small amount of llquid from the preceding step then is supplied to the bleaching plant water system, and at a limited washing water amount after the bleaching plant only a small amount of bleaching plant backwater must be ejected.
The smaller the ejection is, the greater is the proportion of residue chemicals to be utilized. By reduced dllution into the system also the risk of distur-bances at the bleaching reactions of the residue chemicals is reduced.
In view of the aforesaid, the bleaching process can be designed as ollows.
Immediately before the bleaching plant, the pulp is pressed to at least 20% dry content, possibly in combination with washing. Higher dry contents are to be preferred, and normally at least 30 %, at maximum about 50 % should be attained. The pulp is thereafter diluted with insignificantly diluted bleaching plant backwater, which contains residue chemicals from the bleaching, to a pulp concentration normal for the bleaching and possibly varying, according to circum stances, between about 5 % and 15 %. Thereafter fresh peroxide solution and necessary auxiliar~ chemicals are admixed in known manner to the pulp, and the pulp is bleached during a suitable time adapted to the temperature prevailing in the system. ~ormal times are 0.1 to 4 h, normal temperatures are 45 to 85C.
At the beginning of the bleaching, pH is high and decreases gradually to the ~1740~3 level 8 to 9. The bleaching alway~s is finished before the chemicals have been consumed completely~, ~ecause no al~aline miscolouring should occur. In order to obtain high brightness values, the amount of residue chemicals normally must be rather high. The pulp thereafter is passed to a washing step with high washing effect, for example a washing filter, where the residue solution is recovered.
In order not to dilute the residue solution, the washing must be carried out with a small washing liquid amount, i.e. with a low dilution factor. This solution then is returned for dilution immediately before the bleaching plant.
The method is illustrated by the following laboratory experiment and experiments in a semi-technical pilot plant:
The effect of directly returned residue solution from peroxide bleach-ing is illustrated in a laboratory experiment, at t~l~ch a thermomechanical pulp at first was bleached with fresh chemicals, whereafter the residue solution was titrated on peroxide and made stronger with fresh peroxide, sodium hydroxide, sodium silicate and magnesium sulphate to the same total charge as in foregoing bleaching experiments, whereafter the pulp was bleached analogous with the new bleaching solution. This was repeated once more, and the residue solution from the latter experiment was returned in order to control that the bleaching was not deteriorated when the proportion of organic substance in the recirculated residue 2a solution was increased. As appears from the Tables below, the different variants yield substantially the same result. This was obtained when the bleachings were carried out at 12 % pulp concentration and at a high peroxide content in the resi-due solution.
At later analogous experiments where the residue solution was returned more diluted, and the bleaching i~s carried out at somewhat lower pulp concentra-tion, a slightly inferior brightness was obtained for the charged peroxide ~about 2 units lower than at the highest charge).

:1174013 Conventional peroxide bleaching with resh bleachlng chemicals.
Experiment 1 Peroxide bleaching Pulp concentration % 12 Temperature aC 40 Time min 120 Hydrogen peroxide kg/ton 40 Na~silicate kgfton 30 Mg~04 kg~ton 0-5 NaOH kg/ton 28 32 36 Initial pH 11,5 11.7 11.9 Final pH 10.3 10.5 10.6 Residue peroxide g/l 2.92 3.02 2.99 Residue peroxide kg/ton 21.4 22.1 21.9 Consumed peroxide kg~ton 18.6 17.9 18.1 Analyses:
Brightness %ISO 77.4 77.4 77.8 .
Peroxide bleaching with addition of 4.66 m3 per ton bleaching waste liquor from experiment 1, and make up in the form of fresh chemicals.

117~ 3 Experiment 2 Peroxide bleaching Pulp concentration % 12 Temperature QC 40 Time min 120 Hydrogen peroxide kg/ton )26.4+ ~13.6~=40 Na-silicat0 kg/ton 10.7+(19.3)=30 MgS04 kg~ton 0.06+(0.44)=0.5 NaOH kg/ton23+~5~ 27+(5)31+(5) Initial pH 11.5 Flnal pH 9,9 10.1 10.3 Residue peroxide g/l 2.7g 2.74 2.69 Residue peroxide kg/ton 20.5 20.1 19.7 Consumed peroxide kg/ton 19.5 19.9 20.3 Analy~es:
Brightness %ISO .77.3 77.5 76.2 X)Make up chemicals xx~ Chemicals in residue solution Peroxide bleaching with addition of 4.62 m3 per ton bleaching waste liquor from experiment 2, and make up chemicals in the form of fresh chemicals.

Experiment 3 Peroxide bleaching Pulp concentration % 12 Temperature C 40 Time min 120 Hydrogen peroxide kg/tonx)27 9+Xx)~12.1)=40 Na-silicate kg/ton 11.0~(19.0)=30 MgSO4 kg/ton 0.06~(0.44)=0.5 NaOH kg/ton27.7+(0.3)31.7+(0.3)35.7~(0.3) lQ Initial pH 11.5 11.6 11.7 Final pH 9.9 10.0 10.3 Residue peroxide g~l 2.69 2.57 2.48 Residue perox~de kg/ton 19.7 18.8 18.2 Consumed peroxide kg/ton 20.3 21.2 21.8 Analyses:
Brightness %ISO 78.1 78.4 78.7 x) Make up chemicals xx) Chemicals in residue solution The effect of washing before the peroxide bleaching for preventing the backwater from the refiner step from disturbing the bleaching was illustrated 2Q in a laboratory experiment.
A mechanical pulp was manufactured in a pilot plant from spruce chips, in that the spruce chips first were impregnated with sodium sulphite solution so that the chips absorbed about 20 kg Na2S03/ton bone-dry wood. The impregnated chips were preheated with steam and refined in a chip refiner to pulp. The pulp after the refiner had a pulp concentration corresponding to about 25 % and contained about 3 ton aqueous solution per ton bone-dry pulp. The aqueous solu-1~74013 tion contained organic substance sol~ed out from the wood and anorganic substance,sodium sulphite and sodium sulphateJ from the impregnation solution. The bright-ness of the pulp was measured and found to be 60.1 % ISO.
From the pulp obtained two samples were taken. One pulp sample was washed well with water, so that substantially all organic and inorganic substance was removed from the pulp. The washed and the unwashed pulp samples were bleached in the laboratory with alkaline peroxide in known manner.
Experiments were also carried out in a semi-technical equipment in-cluding the possibility of pressing-out the refiner step water in a press between refining and bleaching for a chemical modified thermomechanical pulp (yield 95 to 96 %). The results of the laboratory studies interpolated to brightness 70 and, respecti~ely, 75 % lSO are reported in the Table below.
~ashing before bleaching Unwashed Washed Brightness %ISO 70 75 70 75 Peroxide bleaching Pulp concentration % 12 12 12 12 Temperature C 40 40 40 40 Time min 120 120 120 120 Hydrogen peroxide kg/ton 24.0 43.5 24.0 43.5 Na-silicate kg/ton 30 30 30 30 EDTA kg/ton 3 3 3X) 3X) MgS04 kg/ton 0.5 0.5 0.5 0.5 NaOH kg/ton 19.2 30.5 19.2 30.5 Residual peroxide kg/ton 3.0 9.0 8.0 17 Consumed peroxide kg/ton 21.0 34.5 16.0 26.5 -x~ Added in connection with washing before bleaching 1~74013 The results from the experlments on a semi-technical scale were as follows:

Washing before ~leaching Unwashed Washed-pressed pulp Peroxide hleaching Pulp concentration % 12 12 Tempera~ure C 50 50 Time min 100 100 Hydrogen peroxide kg/ton 39.3 45.5 Na-silicate kg/ton 41 42 MgS04 kg/ton 0.5 0.5 EDTA kg/ton 6.2 3.1 NaOH kg/ton 29 30 Residual peroxide kg/ton 7.3 22.6 Consumed peroxide kg/ton 32.0 22.9 Analysis:
Brightness 72.5 78.3 -Analogous laborator~ exper~ments for pure thermomechanical pulp resulted in the following:

Washing before bleaching Unwashed Washed -2aBrightness 7Q 75 70 75 Peroxide ~leaching Hydrogen peroxide ~g~ton 25 40 25 40 NaOH kg/ton 20 28 20 28 Residue peroxide kg/ton 3.0 6.0 7.0 12.0 Consumed peroxide kg/ton 22.0 34.0 18.0 28.0 1~74013 The other bleaching conditions agreed with the ones shown previously.
These experiments show, that the solved wood substance from the re-fining step consumes great peroxide amounts when pulp from the refining step is supplied to the bleaching step without solved substance having been removed, and they further show, that peroxide bleaching of a mechanical pulp requires great peroxide excess for obtaining optimum bleaching.
The experimental work, thus, has yielded the results as follows:
The backwater from refining mechanical thermomechanical and chemical modified thermomechanical pulps contains substances, which consume peroxide and alkalî at alkaline peroxide bleaching.
- Bleaching of such mechanical pulps requires a high peroxide excess at bleaching to high brightness values.
- The peroxide excess in the backwater can be re-utilized for bleaching when the backwater is returned to ths bleaching step.
On the basis of these results different system couplings were studied by means of mathematic models. It was now found possible to combine in a suit-able manner equipment and operation conditions so, that a transfer of too much backwater from the refining step to the bleaching step, where the backwater con-sumes bleaching chemicals, is prevented, and that simultaneously peroxide contain-2Q ing backwater after the bleaching step is returned with high utilization degreeto the bleaching in order thereby efficiently to utilize this peroxide.
An example of an embodiment of the invention is shown in Figure 3.
Mechanical pulp from the refining step 1 is passed to the press 2, where backwater from the refining is washed out.
Before the bleaching step 3 fresh bleaching chemicals are added via flow 5, and residue solution from the peroxide bleaching is added via backwater flow.6. The pulp suspension is passed to bleaching step 3, which preferably can 1~74V13 be a bleaching tower. From the bleaching step 3 the pulp suspension is passed to a washing step 4 where washing is carried out in an efficient washing equip-ment, preferably a washing filter, to which washing liquid 7 is supplied. The washing liquid 7 ma~ be pure water or backwater from subsequent operations, for example dewatering.
Calculations have shown that at increased dry content after press 2 ever increasing amounts of chemical-consuming backwater are pressed off from therefining step, and at the same time more chemical-containing backwater 6 can be returned to the bleaching step. By introducing a washing step 4 after the bleaching and designing the washing step as a displacement step while simultane-ously adding washing liquid in an amount corresponding to a low dilution factor,a great part of the active chemicals remaining after the hleaching can be re-turned to the bleaching step.
The effect obtained by a method according to Figure 1 compared with a method, where all backwater of the refiner step is supplied to the bleaching step and no backwater from the bleaching plant is returned, has been simulated in a mathematic process model according to Figure 1. The pulps, viz. CTMP pulps, are assumed having been well washed before the bleaching and require a peroxide charge of 40 kg H202/ton pulp. The pulps are bleached to 75 % ISO. Considering the chemical prices of to-day, being about SwCr 4:-~kg H2O2 and SwCr 0.7:-/kg NaOH, the following chemical savings in relation to the present state of art areobtained.
By installing a press step 2 in Figure 1 the following chemical savings at dlfferent pulp concentrations after the press step are obtained:

Pulp concentration Chemical saving after press step 2, % SwCr/ton pulp The chemical saving in the above Table applies to a dilution factor in washing step 4 corresponding to 1 m3/ton pulp.
By~using a normal washing filter instead of a press in position 2, a chemical saving corresponding to about SwCr 27:-/ton pulp is obtained.
The chemical saving obtained is very great and implies a substantial cost saving at the manufacture of bleached mechanical pulp.
As~ upper limit for the pulp concentration should be at about 50 % pulp concentration. Above this concentration lt is difficult to dewater pulp, and the pulp also will be difficult to defibre before the bleaching step.
The dilution factor 1 m3/ton pulp yields a somewhat better result than 2 m3/ton, which in its turn yields better results than dilution factor 3 m3/ton pulp.
Gomplementary investigations have shown, that similar results are ob-tained irrespective of the bleaching conditions being chosen. Correspondingbleaching studies, for example, have been carried where the pulp concentration was varied between 5 and 15 %, the temperature in the bleaching step was varied between 40 and 85C, and the corresponding staying time between 0.1 and 4 hours.
The invention is not restricted to the embodiments described, but can be varied within the scope of the idea of the invention.

_ 20

Claims (16)

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

1. Method of manufacturing a cellulose pulp substantially free from resin and in a yield above 90 %, by mechanical defibration of wood material and subse-quent bleaching, said method being conducted in a substantially closed system corresponding to an amount of waste water of at most 15 m3 per ton pulp, charac-terized in that backwater from each of the defibration stage and the bleaching stage is maintained separated, that excess backwater from the process is with-drawn at a washing stage arranged after the defibrating stage, that excess back-water from the bleaching stage is used for diluting or washing of the pulp after the defibrating stage and that the washing effect of dissolved substance amounts to at least 70 %.

2. Method according to claim 1 wherein said amount of waste water is at most 10 m3 per ton pulp.

3. Method according to claim 1 wherein said washing effect of dissolved substance amounts to at least 80 %, or at least 85 %.

4. Method according to claim 1, characterized in that the bleaching stage consists in an alkaline bleaching stage.

5. Method according to claim 4 wherein the bleaching stage is a peroxide bleaching stage.

6. Method according to claim 1, characterized in that the defibration stage consists in a refiner stage.

7. Method according to claim 1, characterized in that respective said backwaters from the defibration stage and the bleaching stage are maintained separated from each other by arranging an intermediate washing stage between the stages.

8. Method according to claim 7 wherein said intermediate washing stage is in the form of a washing press.

9. Method according to claim 7, characterized in that the washing stages before or after the bleaching stages are arranged in series and that the washing water is moved in a consequent counterflow against the pulp flow.

10. Method according to claim 4, characterized in that the bleaching is conducted at a temperature above 60 C.

11. Method according to claim 10 wherein the bleaching is conducted at 70 - 85 C.

12. Method according to claim 1, characterized in that the pulp, after the mechanical defibration, is pressed to a pulp consistency of 25 - 50%, whereafter the pulp is diluted substantially with residual bleaching liquor from the bleaching stage.

13. Method according to claim 12 wherein the pulp is pressed to a consistency of 30 - 45%.

14. Method according to claim 12, characterized in that excess bleaching liquor from the bleaching stage is recovered in concentrated form in a washing stage arranged after the bleaching stage, whereby the dilution factor in the washing stage is limited to 0-2 m per ton pulp.

15. Method according to claim 14 wherein said dilution factor is 0.5 -1.5 m3 per ton pulp.

16. Method according to claim 1 for the manufacture of thermomechanical or chemimechanical pulp.