CA1246304A - Process and apparatus for bleaching of pulp - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A process for the rapid bleaching of pulp in which chlorinated pulp is formed into a sheet on a moving foraminous surface and subjected to chlorination stage washing, a rapid first alkaline extraction, and further washing after the extraction. The treatments involve displacement of one liquid by another. The first alkaline extraction may be performed in two steps with the alkaline liquor used in the second of the steps comprising a mixture of a) recycled alkaline liquor displaced from the sheet by the washing water, b) washing water which has passed through the sheet, and c) fresh caustic. The rapid alkaline extraction replaces a complete alkaline extraction stage which would include a steam mixer, a retention tower and a washer.

Description

Background of the Invention 1. Field of the Invention This invention relates to improvements in the alkaline extraction of wood pulp and to improvements in the washing immediately prior to and/or immediately following said alkaline extraction. The alkaline extraction is an inter-mediate stage in the delignification and bleaching of chemical pulps produced by the kraft, sulfite, alkaline sulfite, soda, etc. cooking processes. Its purpose is to remove highly coloured degradation products of lignin that have been made soluble by oxidizing agents such as chlorine dioxide, combin-ations of chlorine and chlorine dioxide, hypochlorites, and oxides of nitrogen or ozone, ahead of a subsequent oxidizing chemical required to achieve a given brightness target.

2. Description of the Prior Art Wood pulp as it comes from the digester, whether hardwood or softwood, contains residual colouring matter which must be removed by bleaching if the pulp is to be used for printing or writing paper or paper which is to be dyed~
Furthermore bleaching may be required to remove impurities if the pulp is to be used as the raw material for the production of rayon, gunpowder and other cellulose products.
Depending upon the nature of the raw pulp and the end use to which the pulp will be employed, various chemical bleaching stages and various sequences of these stages have been used heretofore. Among the principal chemical bleaching stages which have been used are the chlorination stage (designated "C"?, the alkali extraction stage (designated "E") the hypochlorite stage (designated "H") and the chlorine ~--dioxide stage (designated "D"). In addition both chlorine :~L2~6;~
and chlorine dioxide may be used in the same chlorination stage, desigrated (IlCD'' or ~Dc~). Various combinations of the above stages have been employed depending upon the specific conditions and bleaching requirements. For example, common bleaching sequences may include the following: CEH, CEHD, CEHED, CEHDED AND DEDED. Of these the CDEDED AND CDEHDED are the recognized standard sequences for providing market pulps in the range 88 to 90 + GE brightness.
Oxygen delignification at low, medium and high consistency has been practiced commercially for several years.
The oxygen stage in sequences such as OCEHD, OCEHED, ~ND
OCEDED is used to partially delignify the unbleached pulp prior to chlorination.
It is normal practic~ in conventional bleacheries to have a wash after each bleach stage in order to remove the impurities solubilized in that stage. However, the need for a wash following a bleach stage differs depending upon the stage. The need for washing is greatest after the chlorination and first extraction stages, whereas there are advantages to eliminating the wash completely after some other stages as taught by U.S. Patent 4,238,281 which issued December 9, 1980 to the assignees of this application. Thus it is possible to ha~e bleach sequences such as CDE~HD), DCE(DED)l CDE(HDED) where there is no wash between the stages within the brackets.
In commercial practice large amounts of caustic soda are wasted due to poor washing after chlorination with the resultant carryover of HCl into the first extraction stage, and large amounts of hypochlorite and/or chlorine dioxide are wasted due to poor washing after the first extraction stage with the resultant carryover of highly coloured organic ~.~4L6~3~4 matter into the hypochlorite and/or chlorine dioxide stages.
The first extraction stage is normally carried out in a slurry at 8 to 16% consistency and at tim~ ranging from 1 to 2 hours and at temperaturesranging from 50 to 80C.
The mill scale equipment to carry out a first extraction stage includes a steam mixer to mix caustic with the pulp and heat it up to the operating temperature, a thick stock pump to pump it to the top of a one to two hour retention tower, a filtrate pump to lower the consistency at the bottom of the retention tower to about 4% so that the pulp can be pumped out of the tower and a pump to dilute the pulp further to about 1% consistency before it enters the vat of the extrac-tion stage drum washer.
W.H. Rapson teaches, in "Dynamic Bleaching", TAPPI, Vol. 49, No. 8 pp. 329-334, 1966, that the limiting factor n the bleaching process is not the chemical reaction rate but, instead, is the rate of diffusion of the reactant to the reaction sites. Rapson proposed an irnprovement which involved passing the bleaching solution through a layer of pulp, thus displacing the liquid already present in the pulp layer. In this way, the bleaching solution would be in constant motion with respect to the pulp fibre. The rate of mass transfer through the water layer to the fibre wall would be greatly increased, and thus the bleaching time could be reduced from hours to minutes for each stage. In Page 331 of this paper, and in Canadian Patent No. 783,~83, issued April 23, 1968, Rapson suggested carrying out a multi-stage displacement bleaching process using a mo~ing wire or belt. In the paper, 10 successive stages were proposed, and in the patent it was proposed to use 6 stages. With retention times as proposed

3 -63~L
by Rapson, a very large belt would be required for the type of output typical of a modern pulp plant. Wash water recycling with countercurrent flow and recycling of liquor is proposed in Figure 2 of the Rapson paper, but there is no interchange between washing and extraction stages.
A.W. Brinkley, Jr. et al, in U.S. Patent No.
3,575,795 issued ~pril 20, 1971, taught a utilization of Rapson's technique using a series of relatively small cylindrical vessels including radial diffusion equipment and increasing the consistency of the pulp to between about 5%
and about 15%. They thus provided a scheme for the commercial use of the dynamic bleaching technique.
Kamyr Inc. makes use of the principles set out by Brinkley et al in commercially operating displacement bleacheries having extraction stages which range from 10 to 20 minutes retention of which about 2 to 3 minutes is used for dynamic movement of liquor through the pulp per displacement and the rest is static movement of the liquor with the pulp through the tower. These, bleacheries normally have no wash stage between the chlorination and caustic extraction stage so that their caustic consumption in the extraction stage is higher than in conventional bleacheries. Similarly they normally ha~e no wash between the extraction stage and the hypochlorite or chlorine dioxide stage so that the chemical consumption in the third stage is also abnormally high.
Allan G. Jamieson taught in Canadian Patent No.
881,406 issued September 21, 1971, a continuous process for rapid bleaching of a wood pulp by treating wood pulp in an aqueous suspension at a consistency of about 3% to about 6%
by weight with reagents in a bleaching sequence which included L2~

the three stages of (1) treatment with chlorine, (2) alkaline extraction, and (3) treatment with chlorine dioxide, under conditions of continuous agitation in continuously stirred vessels, the residence time of the pulp in each stage not exceeding about 10 minutes, and separating the reaction products and unreacted reagent from the pulp between the stages. This process thus suffered the deficiency of a relatively lengthy alkaline extraction period.
Liebergott in U.S. Patent no. 3,874,992, issued April 1, 1975, teaches that a rapid and thorough extraction of chlorinated pulp can be obtained by mixing the caustic with the pulp in a steam mixer at a consistency of less than 12~ in the temperature range 60 to 80C. The mixing takes less than 5 minutes and the mixture is then pressed to a consistency of about 18 to 40~ in about 1 minute.
Liebergott's process suffers from the deficiency that it raises the consistency to a high level, which is suitable for gas phase bleaching, but which is not suitable for a hypochlorite or chlorine dioxide stage using conventional medium consistency equipment.
Attempts by others to shorten the retention time in the first extraction stage to the range 2 minutes to 10 minutes have either resulted in production of a low brightness pulp or to high bleach chemical consumption and none has succeeded in reducing the time to less than one minute.
3. Summary of the Invention It has been found that if a pulp is well chlorinated, extraction can be performed by the displacement method is a matter of seconds. The term "well chlorinated"
as used herein means chlorinated to a level i3~
of Ool9 x ~appa No. or higher as ~ available chlorine on pulp;
this results in removal. of 57~ to a~out 77% of the lignin in the chlorination stage. Surprisingly, it has been found that as the chlorination level increases the efficiency of the rapid extraction increases. For example, if the extraction is performed in two successive displacements in a counter-current system using highly alkaline liquor at a temperature Of 70c.o 90~, a time of about 5 seconds per displacement is sufficient. This compares with a time of minutes per stage proposed by Rapson. Accordingly, with the present invention the retention time required during extraction can be brought to a level which is compatible with operating the extraction process along with one or more washing stages on a single continuously moving foraminous surface which may form part of . commercially available horizontal or drum washer equipment, and which is not of excessive size. The system of the invention can be made more efficient in terms of energy input and chemical consumption than conventional systems and is ideally suited to bleache,ries where minimum bleach plant effluent v~lume is desirable.
In accordance with one aspect of the invention, a process for the bleaching of digested pulp includes the steps of:-(a) passing the digested pulp into a chlorinationto~er together with chlorine and chlorinating said pulp to a well chlorinated level as herein defined, (b) formi.ng said chlorinated pulp into a layer on a continuous moving mesh surface in a forming zone and removing liquid which drains from the pulp in the forming zone, (c) performing chlorination washing by adding wash water to said layer to wash chlorinated liquid from said layer ~ d~ performing alkali extraction by displacing said wash water with alkaline liquor.
(e) subjecting said pulp to at least one extraction washing step and (f) passing the pulp to a subsequent bleaching stage.
Extraction and subsequent washing may be perfor~ed on a single piece of apparatus; this requires a special coutercurrent system as compared to those used countercurrent washing in bleacheries since the extraction wash water is recycled for use in the extraction. It also differs from counter-current washing since fresh chemical, NaOH, is injected in the middle of the countercurrent washing process.
In accordance with a further aspect of the invention, therefore, in a process for use in the bleaching of pulp wherein chlorinated pulp is subjected to a chlorination wash and an alkaline extraction performed in one or more successive steps while advancing as a pulp layer on a continuously moving mesh surface, and is subsequently washed by passing wash water through the layer to displace alkaline liquor therefrom, the alkaline liquor used in said one step or in the last of said successive steps of said extraction comprises a highly alkaline mixture of:
(a) recycled alkaline liquor displaced from the pulp layer by said wash water, - (b) said wash water after having passed through the layer to displace the alkaline liquor, and (c) fresh caustic.
In one embodiment of the invention, two successive extraction steps and the subsequent washing are performed ~ Eii3~
while the pulp is moving on a single foraminous surface formed by an endless foraminous belt, which may be the wire of a commercial pulp washer. The term "wire" includes woven m.e s h of metal or plastic fibres. In another embodiment, the two successive extraction steps are performed on separate foraminous drums, which may be similar to conventional drum washers. In each embodiment, liquor from the second extrac-tion step which has passed through the sheet may be mixed with alkaline liquor from the first extraction step which has passed through the sheet and recycled for use in the first extraction step.
If the amount of make-up caustic added at the second extraction step is about 2% on pulp, the conditions can be adjusted so that at steady state conditions the concentration of caustic entering the second extraction step can be 3 to 10~ caustic on pulp depending on the countercurrent component of the flow.
Washing water from the chlorination washing stage may be recycled for chlorination washing, or for dilution of the unbleached pulp prior to the chlorination stage. It is desirable that the alkali.ne liquor of the extraction stage should not mix with water bei.ng recycled to the chlorination recycle system. For thi.s purpose, a shower is arranged before the first shower which supplies the extraction liquor, and thi.s shower is supplied with relatively fresh water; i.e.
fresh water or water recycled from a later stage but which is substantially uncontami.nated with strong alkali or alkali deriyati.~es. The drainage means under the sheet are arranged so that a porti.on of the relati.vely fresh water which has passed through the sheet is mi.xed with the first portion of alkaline liquor to pass through the sheet, said relatively fresh water thus serving to prevent extraction stage filtrate from being carried countercurrent into the chlorination stage recycle system where it would consume chlorine in the next cycle.
The process in which extraction is done using two countercurrent displacement steps, combined with the use of a well chlorinated pulp and with good washing before and after the extraction, is found to give efficient rapid extraction. For example, it has unexpectedly been found that when a well chlori.nated and washed pulp is extracted rapidly by passing first a relatively weak alkaline liquor through it followed by a relatively strong alkaline liquor both at temperatures of 60C to 90C in a countercurrent fashion followed by two steps of countercurrent displacement washing,the washed extracted pulp which has had contact with the alkali solutions for a total of from 4 secondsto 30 seconds behaves in the subsequent .hypochlorïte and chlorine dioxide stages similar to pulp which has had a conventional alkali extraction stage of 1 hour at 60 to 80C.
The efficiency of this process is such that chlor-i.nation washing, alkaline extraction in one or two steps, and extraction washi.ng can be performed on a single wire or fabric belt of practical si.ze for a full scale bleaching operation (say of the order of 1,000 tons day). More specifically, it is possible to use a single commercially avai.lable horiz.ontal washer to obtain six succ~ssive displacemen:ts which may be for example two countercurrent chlorination wash steps, two countercurrent extracti.on steps, followed by two countercurrent extractï.on wash steps. or may be one chlorinatïon wash step, ~ ~u -~two countercurrent extraction steps, and three countercurrent extraction wash steps. Experiments indicate that using well chlorinated pulp, satisfactory extraction can be obtained using 2 to 16 seconds (or preferably 3 to 6 seconds) per extraction step, and a total residence time on the wire of less than 1 minute, and preferably about 30 seconds, well within the time limits of a conventional horizontal washer.
As an alternative to a horizontal washer, one or more conventional drum washers may be adapted for use with this process. Preferably, the chlorinated pulp is formed into a sheet on a first drum washer where it is subjected to washing and to a first extraction displacement, and the partly extracted pulp is passed to a second drum washer where it is subjected to second extraetion displaeement and washing.
The mixing of reeyeled wash water with the alkaline liquor of the extraetion stage, and the reeyeling of liquor between the two extraction steps, represent eountereurrent flows of the displaeement system. In a eountereurrent dis-plaeement system the amount of liquid whieh enters the pulp in any one stage is slightly greater than the amount of liquid remaining in the pulp at the end of that stage; this ensures full displaeement of the liquor previously in the pulp. In a direet eountereurrent system.the excess from one stage is mixed with liquid which has passed through the pulp in the preeed.ing stage and reeyeled to this preeeding stage. The amount of liquid whieh moves i.n eountereurrent manner depends on the Displaeement Rati.o, whieh is here defined as the volume of liquor di~splaced into a sheet to the volume of liquor remai.ning i.n the sheet at the conelusion of the displaeement stage. In order to operate a direet eountereurrent displaee-~6~

ment system the displacement ratio must be greater than l.0to l. For example if the displacement ratio is 1.2:1 the countercurrent component of the displacing liquor is 0.2 to l while l.0 volume will be retained in the sheet. Thus 0.2 volume moves countercurrent to the flow of the pulp and l.0 volume moves with the pulp.
Brief Description of the Drawings In the accompanying drawings which illustrate pre-ferred embodiments of the invention:-Figure 1 is a schematic flow diagram of a conven-tional chlorination and extraction stage using con~entional single stage drum washers for washing after both the chlor-ination and extraction stages;
Figure 2 is a schematic flow diagram of a chlorin-ation stage and extraction stage of the present invention wherei.n one belt washer serves the functions of a single stage chlorination washer, a single stage of extraction, and a single extracti.on stage washing;
Figure 3 is a detailed view of the belt washer shown in Figure 2, Figure 4 i.s a schematic flow diagram of a chlorin-ation stage and extracti.on stage of the present invention wherein one belt washer ser~es the functions of a 2 step countercurrent chlorinati.on washer~ 2 stepS of countercurrent extraction and 2 stepS of countercurrent extraction stage washing;
Figure 5 is a detailed.view of the belt washer of Figure 4;
Figure 6 i.s:a diagrammatic view of a system of multi.stage drum.washers arranged to provide a chlorination wash Eii3~

stage, two extraction stages~ and an extraction wash;
Figure 7 is a schematic flow diagram of a bleaching system incorporating this invention and also that of the afore-said U.S. Patent No. 4,238,281, and Figures 8 and 9 are graphs show-ing results obtained with varying displacement time in the extraction steps.
Detailed Description In commercial practice of a conventional first extraction stage the NaOH (caustic) application can vary from 1.5 to 4.5% on pulp, the consistency of the pulp from 3% to 16%, the temperature from 30C to 80C and the retention time from 30 to 120 minutes. Typical conditions would be 3-Q NaO~
on pulp at 11~ consistency and 70C for 60 minutes. Figure 1 shows a typical chlorination and extraction system.
Pulp which has been mixed with chlorine enters the bottom of the chlorination tower 10 at 11, the tower 10 providing the retention time required for the chlorination reaction. The pulp overflows from the top of the tower through line 12 to the vat of a single stage drum washer 13 where a sheet of pulp is formed on the drum and wash water 14 is sprayed on the sheet to displace and wash out soluble impurites. Filtrate from the washer passes through a drop-leg lS to seal tank 16. Since the consistency leaving tower 10 is about 3~ and the consistency required for good sheet formation in the washer vat is about 1% the stock must be d,iluted with filtrate from seal tank 16; this filtrate is circulated by pump 16'. At 3% consistency one ton of fiber contains 32.3 tons of water while at 1% consistency 1 ton of fiber contains 99 tons of water, therefore 99 - 32.3 = 66.7 tons of filtrate must be pumped up about 40 feet to provide the dilution water required at the washer vat. This requires a large consumption of energy.

~6;~
The washed pulp discharged from the drum washer is at about 11~ consistency. NaOH is added at 17 as the pulp falls into a steam mixer 18 where it is heated to the desired temperature by direct contact with steam. The pulp then falls into a thick stock pump 19 which at 11% consistency must pump a total of 9 tons of slurry per ton of fiber to a heightof approximately 80 feet to the top of extraction tower 20 which provides the retention time for the extraction stage. In order to be able to pump the extracted pulp out of tower 20 by way of pump 21 and line 22 to washer 23 it is necessary to dilute it with filtrate from the washer seal tank 24 by way of pump 25 and di.lution ri.ng 26. This along with agitator 27 dilutes and mixes the pulp to about 4% consistency so that it can be remo~ed from the tower. It must be further diluted to about 1% consistency at the ~at of washer 23 by way of filtrate pump 25 and li.ne 28. This tower and vat dilution requires pumpi.ng approximately 90 tons of water per ton of fiber by way of fi.ltrate pump 25 to the height of the washer 23 which consumes a large amount of energy. Pulp discharged from the washer i.s heated in steam mixer 29 and is pumped to the next bleach stage by means of thi.ck stock pump 30.
Fi.gure 2 shows, di.agrammatically, one ~ersion of the presen-t invention. The chlori.nated pulp o~erflows from the chlorinati.on tower 40 by way of pipe 41 to the headbox 42 of a horizontal washer 43 to form a shee-t or mat. The pulp mat is formed from stock at about 3% consistency on the wire and i.s dewatered i.n an i.niti.al formi.ng zone to about 8-14% consist-ency. It then passes through three displacement treatment zones, i.ncluding a first wash z.one 44 to displace products of the chlori.nation stage, a second zone 45 whïch provides a ;

caustic extraction stage, and a wash zone 47 where the dis-solved solids are washed from the extraction stage 45. The pulp at 10 to 20% consistency is discharged from the washer into a steam mixer 48 and then into a thick stock pump 49 from which it enters the next bleach stage.
Figure 3 is a detailed diagram of the horizontal washer shown in Figure 2. The term "hori-zontal washer" is used to denote a piece of equipment gener-ally similar to a conventional brown stock horizontal washer although having special features to be described. A series of showers 55, 59, and 66 successively apply liquids to the sheet of pulp on the wire which liquids displace each other as they pass through the wire with little mixing. Since the wire advances at a steady rate the liquids occupy diagonal zones as shown.
From the headbox 42 to the first showe 55 is a forming zone 51 where the pulp is dewatered raising the con-sistency from about 3% leaving the headbox to about 8to 14% at the first shower. Under the.forming zone there are two filtrate trays 52 and 53. The proportions of these trays may be varied to allow the desired flow to sewer 53' from tray 53. The chlorination wash zone 44 has a shower 55 and a filtrate tray 56; the shower 55 is supplied with recycled water but, as will be explainedr this water is relatively fresh. Filtrate from tray 52 is recycled via outlet line 52' to reclaim fibers lost during sheet formation and filtrate from tray 56 is recycled via outlet line 56l because it has added heat value due to the countercurrent washing system used. The diagonal zone 58 on the wire indicatesthe progress of the wash liquor through the pulp mat. It can be seen that the tray 56 extends far enough 6~

forward that part of the wash liquor from shower 55 will be deposited in this tray 56. This represents the countercurrent component of the wash flow in the first wash stage.
Shower 59 which contains about 3% to 10% NaOH, based on the pulp being treated, and diagonal zone 62 represent the extraction stage 45. There is a tray 61 under this extraction stage which receives approximately one half of the relatively fresh shower water from shower 55 by way of diagonal zone 58 and a countercurrent component of the highly contaminated first extraction stage filtrate, which is removed from the system by being passed to sewer via outlet line 61'. The portion of filtrate from diagonal zone 58 that is divided between trays 56 and 61 provides a barrier to prevent first extraction stage filtrate from entering the chlorination stage where it would cause severe foam problems and cause high chlorine demand in the chlorination stage, and the relatively fresh water in zone 58 provides a similar barrier within the pulp sheet. Although the water supplied to shower 55 may be recycled frcm a later stage, ~s explained below, it must be uncontaminated 20 with any strong alkali or alkali derivatives. It may be noted that the amount of water su~plied by shower 55 may ke relatively small so this does not constitute a full displacement step provided that some of the wash water enters the chlorination recycle system and part goes to the extraction stage sewer.
The extraction stage wash shower 66 uses a counter-current filtrate from a later bleach stage or fresh hot water or a mixture of the two which moves through the sheet in zone 68 displacing alkaline liquor into tray 64. Most of the shower water is retained by the sheet at 8% to 14 % oonsistency~ however a countercurrent component is deposited into the tray 64. Steam is added at 69 to heat the filtrate leaving tray 64 via outlet ~2~

line 64' and about 2~ NaOH on pulp make-up is added at 70.
This low make-up is sufficient to sustain a concentration of ahout 3 to 10% NaOH on pulp in the extraction stage, depending on the displacement ratio. The steam added at 69 is the small amount that is required to heat the countercurrent component of shower water 66 to the desired temperature of the extrac-tion stage which is between 70C and 90C.
A final tray 71 is provided which is optional and is not necessary to the process. However it is possible to discharge the pulp from the washer at a higher consistency than the approximately 8 to 14~ used during the displacement stages. This increase in consistency has two desirable effects;
it improves the washing accomplished by shower 66 and it decreases the amount of water leaving the washer with the sheet so that it decreases the mass that must be heated to the temperature of the next bleach stage. If this option is used the filtrate from tray 71 can be recirculated via line 71l as shown and used as shower water at 55. Otherwise shower 55 can be provided with fresh hot water or filtrate directly from a later hleach stage or a mixture of the two.
A second configuration of this invention, shown in Figures 4 and 5, differs from that of Figures 2 and 3 in that the washing of both the chlorinated pulp and the extracted pulp is carried out in two countercurrent steps, as is the extraction. As seen in Figure 4, the system as a whole is similar to that of Figures 2 and 3 in having a chlorination tower 80, downpipe 81, and headbox 82 which deposits the pulp as a shee-t on the horizontal washer 83 from which the washed - and extracted pulp goes to steam mixer 90 and then into stock pump 91 to be pumped to the next bleach stage. The sheet of - pulp passes through chlorination washing performed in steps 84 and 85, extraction carried out in two steps 86 and 87, and e~traction washing carried out in two steps 88 and 89.
Figure 5 shows in more detail the preferred config-uration of the horizontal washer of Figure 4. Pulp at chlor-ination stage consistency flows directly from the chlorination tower through headbox 82 and into a forming zone 101 where it is thickened to approximately 8 to 14~ consistency. Filtrate from the forming zone is collected in two separate trays 102 and 103 proportioned such that the filtrate from 103 overflows to sewer 103' and that from 102 is recycled via outlet line 102' for dilution and consistency regulation of brown stock entering the chlorination stage. Water from shower 104 displaces chlorination filtrate from the sheet into tray 105 and passes through the sheet in diagonal zone 106. The countercurrent component of the washing flow is deposited in tray 105 from which it is recycled via outlet line 105' to the brown stoc~
for consistency control. The second step of chlorination washing has two showers 114 and 115 and a filtrate tray 116.
The diagonal spaces 117 and 118 on the belt indicate the progress of the two different wash liquors through the pulp mat. Part of the wash liquor from 114 is deposited in tray 116.
This represents the countercurrent component of the wash flow, filtrate from tray 116 being recycled to shower 104 via line 116'. Filtrate comi.ng from shower 115 via diagonal zone 118, which is relatively fresh water, (being supplied by filtrate from a later bleach stage and/or by fresh hot water) is split fairly equally between trays 120 and 121. The function of the additional shower 115 on the chlorination wash and the additional tray 121 connected to sewer via line 121' is to provide a positive barrier to prevent mixing of th~ chlorination was.h water and the highly contaminated extraction stage filtrate to prevent the latter filtrate from entering the chlorination stage via the countercurrent flow system. Filtrate from tray 120 is recycled to shower 114 via line 120'.
Shower 119 and diagonal zone 122 represent the first of two countercurrent extraction steps 86 and 87. The first extraction step filtrate to penetrate the sheet of washed chlorinated pulp enters tray 121 and flows to sewer via line 121'. The major part of this filtrate enters tray 124, which along with its countercurrent component, is recycled to shower 119 via line 124'. Shower i23 and diagonal zone 125 represent the second extraction step. The countercurrent component of this flow which enters tray 124 represents the only source of caustic supply to the first extraction stage. The major component of shower 123 enters tray 127 and along with a small amount of make-up caustic 129 is recycled to shower 123 via line 127'. Shower 126 and diagonal zone 128 represent the first step of the extraction wash. The countercurrent com-ponent enters tray 127 from this zone and the main component enters tray 131 from which it is recycled to shower 126 via line 131', along with steam 133 in an amount required to main-tain the two extraction stages at the desired temperature.
Filtrate from a later bleach stage andjor fresh hot water is supplied to shower 130 for the second step 89 of the extraction wash. Only the countercurrent component of'this wash water passes through the sheet into tray 131, the remainder stays in the washed sheet 132 which proceeds into the next bleach stage.
It will be seen that trays 105, 116, 124, 127 and 131 all extend through one zone to a point just below the shower of the next zone. The flow through the associated shower is such 6~4 that each of these trays receives a minor countercurrent component of the liquid provided by the shower positioned above the front edge of the tray. The amount of counter-current flow in such a system is determined by the Displace-ment Ratio (DR) as explained above.
Since on a wire the sheet can be formed from a 3%
consistency slurry and once formed the consistency remains fairly constant through all the washing steps, the total vol~e of water which must be pumped to supply all of the showers of Figures 4 and 5 is about equal to that required for vat dilution from pump 16' alone in Figure 1 and the pumping energy required for washer 23 is completely avoided. The large energy savings of the process of Figures 2 and 4 are therefore obvious.
As indicated above, it is contemplated that a system as described with reference to Figures 4 and 5 could be carried out on the wire of conventional horizontal washer apparatus having a total retention time of about 30 to 60 seconds for all stages, provided that the pulp is initially well chlorin-ated and provided that concentrations of caustic are usedsimilar to those described.
Although in the preferred embodiment the process is performed on a horizontal washer it can also be carried out on multistage drum washers as shown in Figure 6. These drum washers, also known as vacuum washers or pressure washers, are in common use for washing and have non-rotating internal segments separated by partitions and connected to seal pots by drop legs.
In the arrangement shown, first the pulp is formed into a sheet on the first drum washer 202 where it is ~2~

subjected to a chlorination wash with relatively fresh water supplied from shower 210, and to the first step of extraction by alkaline liquid applied to shower 221.
The second drum washer 228 receives the partially extracted pulp and this is treated to a second extraction step by alkaline liquid from shower 233. Water for the extraction wash is supplied from shower 239. The displacement ratios of the showers and non-rotating internal segments of the washers are chosen to give countercurrent flows as in the previous embodiments.
Figure 7 shows diagrammatically a complete bleaching system which combines the displacement extraction and washing system, in accordance with this invention, with a simplified bleaching sequence as described and claimed in the aforesaid U.S. Patent No. 4,238,281 in which washing is eliminated between later stages in the process. This simplified bleaching procedure uses only three washing s-~ges i.e. 1) subsequent to the initial chlorination stage and/or prior to the second bleaching stage, 2) subsequent to the first extraction stage, and 3) subsequent to the final and chlorine dioxide bleaching stages. This procedure allows bleaching to be done much more quickly than in con~entional procedures which involve washing after each stage while producing pulp of better physical properties than is produced by conventional systems.

~2~ i3~
Various sequences of the simplified bleaching pro~
cedure are described in aforesaid ~.S. Patent No. 4,238,281.
The one illustrated in Figure 7 is the sequence CDE(~HDED), where washing is omitted between the HDED stages.
Referring to Figure 7, 280 is a chlorination tower, from which line 281 leads the chlorinated pulp into headbox 282 so that it forms a sheet on horizontal washer 283, these items being similar to those described with reference to Figure 4. On the belt the pulp is subjected to a first washing stage 284, extraction performed in two steps 285 and 286, and extraction washing in three steps 287, 288, and 289. Liquid for the stages or steps 284 to 289 is supplied through lines 284', 285', 286', 287', 288' and 289' respectively. Displace-ments 284 through 287 operate in similar manner to the system of Figures 4 and 5, except that water for the first chlorination wash 284 would be relatively uncontaminated water recycled from the last wash step 289 which would be supplied with fresh water; wash stage 284 thus serves the same function as showers 104, 114 and 115 of Figure 5. Water for the first extraction wash step 287 has a countercurrent component provided by the succeeding wash step 288. Water for wash step 288 would be filtrate from the last wash stage after the HDED
stage supplied through line 288'. The tray 121a receives a mixture of the fresh wash water and the first part of the extraction stage filtrate and this is passed -to sewer 121a.
Trays 102a, 105a and 124a are equivalent to trays 102, 105 and 124 of Fig~re 5, and line 129a is equivalent to line 129 in Figure 5.
After leaving washer 283, the pulp passes into steam mixer 290, into pump 291, and then into a hypochlorite stage 292, a chlorine dioxide stage 294, a second extraction stage - 296, and into a second chlorine dioxide stage 298. The bleached pulp would then be washed by washer 300, but no 6~
washing would occur between 292, 294, 296 and 298.
It may be noted that the systems of Figures ~ and 5, and of Figure 7, both involve 6 steps on the belt. In the latter case however this is only a single chlorination wash and three extraction washing steps, and arrangement preferred in some circumstances.
The system of Figure 7 is effective in bleaching performances, as shown by reference to Example E below, and is efficient in the use of energy. I~t uses only one thick stock pump compared to five used in a conventional system using the CDEHDED sequence, and can use only two horizontal washers as compared to six arum washers used in the conventional system.
Allowable Ratios and Proportions for Process In order for this process to work there must be a countercurrent component to the extraction stage flows. In other words part of the shower water for the second extraction step must pass right through the sheet into the seal tank or tray below that step. Stated another way, the Displacement Ratio (defined above) must be greater than l.0:1. If the dis-placement ratio is 1.2:1 then 0.2 volumes pass through the sheet and 1.0 volume remains in the sheet. This also applies where washing is done in two steps.
If the displacement ratio were equal to or less than l.0:1 the caustic concentration in the second extraction step would build up to an extremely high level and the highly coloured reaction products would be carried with the pulp into the succeeding bleach stage. Since all of the caustic for the first extraction step is added by way of the countercurrent component of the second step there would be no extraction in the first extraction step and there would be no bleed of ~6~

impurites to sewer through the countercurrent component of the first extraction step. At the other extreme, if the dis-placement ratio is too high, the concentration of caustic in the second extraction step will not build up since most of the caustic applied will be carried forward into the first step. Since the first step would also have a correspondingly high displacement ratio unconsumed caustic would be sent to sewer and thus wasted. For these reasons the displacement ratio should not be less than 1.0:1 or more than 1.6:1 and preferably between about 1.1:1 to 1.4:1, a most preferred range would be 1.2:1 to 1.3:1.
Double Shower'on Chlorination Wash Limitations on the fresh water shower of the chlor-ination wash stage, for example showers 55, 115, and 284' need consideration. This shower could range from a low of about 0.4 displacement ratio for the system shower in Figure 3 to a high of about 1.3 displacement ratio equal to the total shower flow on the chlorination wash stage for the system shown in Figure 7. In Figure 3 all of the wash water is shown split between the alkaline sewer and the chlorination recycle. In Figure 7 there is a separated bleed of chlorin-ation wash water to sewer since the volume of wash water exceeds the total ~olume required by the alkaline sewer and the chlorination recycle system. There should always be some flow to shower 55 since it is required to provide a barrier between the acidic chlorination stage and the highly coloured and alkaline extraction stage.
Double Tray'Under First Extraction Stage There must be a double tray, such as at 56 and 61 in Figure 3, under the first extraction step. Tray 61 must be at least sufficient to accept all of the countercurrent component of the first extraction step. Since in practice perfect displacement is not possible, tray 61 should be sized to accept all of the countercurrent component of the first extraction step plus a part of, but not all of~ the filtrate from shower 55 since part of the flow from shower 55 is required to re-establish a condition of countercurrent flow.
If colour removal from the bleachery effluent were a problem a very concentratea effluent from the first extrac-tion step could be obtained subdividing tray 61 of Figure 3 into two parts, 61A and 61B. Tray 61A would receive predomin-ately filtrate from shower 55 and being relatively clean could be sewered while 61B which would be equal to or slighly greater than the countercurrent component of the first extraction step and would contain most of the pollution load from the extrac-tion in a concentrated low volume which could be more readily treated or utilized in a closed cycle process such as proposed by Rapson.
Number of Countercurrent Extraction Steps Although a single extraction step is possible, for the most efficient extraction in accordance with the invention this is performed in two or more steps operated in a counter-current fashion such that the first extraction step consumes the largestpart of the total alkali consumed and removes most of the readily removable alkali soluble impurities. The second extraction step has a higher concentration of alkali and a relatively impurity-free pulp to operate upon. If there were n countercurrent extraction steps where n is equal to or greater than 2, each successive extraction step would have a progressively purer pulp to extract and a progressively higher ~6~
concentration of caustic to react with the pulp. The upper limit on n would be the economics of capital and operating costs of the process and the size limitations on the equip-ment required.
Experimental Results Example A
Experiments were performed simulating the flows shown in Figure 5 using a progressive batch process except that fresh water was always applied at showers 115 and 130 and chlorination filtrate was not recycled at 102' and 105' since there was only one chlorination lot. A northern softwood kraft pulp chlorinated to 0.23 x Kappa No as % available chlorine on pulp was subdivided into 9 batches which were progressed successively through eight stages of operation simulating what occurs in zones 101, 106, 117, 118, 122, 125, 128, and 132 of Figure 5, the 9 batches giving 9 cycles of operation and producing substantially steady state conditions with respect to the extraction stages and extraction wash stages. Each batch of chlorinated pulp was added to a stain-less steel Buchner funnel attached by valved lines to receptacles simulating the trays of Figures 5~ Each batch was dewatered from 2.5% consistency to 11% consistency simulating zone 101 and during the first cycle was washed with fresh water simulating the flows from showers 104, 114 and 115 for chlorination stage washing. Fresh water,with 3.8% NaOH on pulp added~was used to simulate shower 119 and fresh water with 10.2% NaOH was used to similate shower 123 to provide the first and second extraction steps. Fresh water was used to simulate showers 126 and 130 to provide the two extraction wash steps.
The displacement ratio for each shower was as indicated in Figure 5. For example, the wash water for shower 104 (which operates at a D.R. of 1.2:1) consisted of 1 vol. of the wash water displaced by the wash water of shower 114 in the previous cycle, plus the first 0.2 vols. of the wash water from shower 114 to penetrate through the batch and enter the receiving vessel in the same previous cycle. For cycles 2 to 9 the filtrates from the trays of Figure 5 were recycled to the appropriate showers while 2.0% NaOH on pulp was added as make-up chemical at 129.
The followïng table sets out the conditions of the unbleached pulp, of the chlorination, the temperature, and the conditions of shower water and trays noted at each phase.
Since the lïquid from tray 124 is passed to shower 119 without any addition the condition in this tray in each cycle is reflected by the condition in shower 119 in the next cycle.
Table A-l Unbleached pulp: Northern softwood kraft, Roe No.
5.1, Kappa No. 30.7; Visc. 33 Pa.s.
Chlorination: 6.62% C12 + 0.2~ C102 on pulp, 2.5 consistency, 40C, 90 minutes.
Extraction: at 904C, 11~ consistency pad, NaOH
additions as above.

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The above table shows that during the first 5 cycles the residual in tray 124 decreased from 3.7 to 2.0% ~laOH on pulp and thus the NaOH applied at steady state became 2.0% on pulp instead of 3.8 in the first stage.
The residual in tray 127 remained almost constant for 9 cycles so that the ~aOH at shower 123 also remained almost constant with the constant addition of 2.0% NaOH on pulp for each cycle. The pH and alkalinity in tray 131 incxeased during the first 5 cycles and then remained constant so that at steady state there was almost 1.5% NaOH on pulp being applied at shower 126. The total alkali present in the system at cycle 9 was the same as a.t cycle 1 indicating that 2.0% NaOH
on pulp added each cycle was sufficient to keep the system in balance.
A second batch of chlorinated pulp was prepared and the preceding experiment was repeated except that the displace-ment ratio for shower 104 was 1.3:1. The displace~ent ratios at 11~l, 115 and 121' were 0.7:1, 0.6:1 and 0.6:1 instead of those shown in Figure 5. The condi.tions of the unbleached pulp, the chlorin-ation, and extracti.on were the same as for Table A-l except for the D.R. The reslllts are shown in Table A-2 below:

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~ 28 -L6~

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In this case the residual caustic concentration in Tray 124 increased slightly from~cycle 1 to steady state at about cycle 5. The caustic concentration in Tray 127 decreased slightly during the first 5 cycles and a small residual of caustic appeared in Tray 131. The total alkali in the system remained almost unchanged from cycle 1 to cycle 9 again demonstrating that a make-up of 2.0% NaOH on pulp was sufficient to keep the system in balance. Comparison of Tables A-l and A-2 shows that the pH in Tray 121 going to sewer i.s signifi-cantly lower with a 1.2:1 DR than with a 1.3:1 DR. On the other hand with a 1.2:1 DR the residual alkali in Tray 131 is significantly hi.gher than with a displacement ratio of 1.3:1.
Thus the alkali. content o~ pH of trays 121 and 131 could be used to keep a commercial belt washer operating this process at a displacement ration between 1.2:1 and 1.3:1.
In Tables A-l and A~2 there is no significant change in ei.ther the Kappa No. or viscosity of the extracted pulp between cycle 1 and steady state. This justifies the procedure used in the succeeding examples where fresh chemical was used for all rapi.d extractions.
In order to make a comparï.son of the efficiencies of rapid extraction at 1.2 DR and 1.3 DR with conventional extrac-tion a third batch of pulp was chlorinated and subdivided for extraction according to the conditions shown in Table A-3.
These pulps were all bleached to approximately 91 brightness by the CDE(HD) bleach sequence where there was no wash between the H and D stages; the results are sho~n in Table A-3 below.
The conditions used for Table A-3 include extraction at displacement ratios of 1.2::1 and 1.3:1, both at steady state, usi.ng flows.heet of Figure 5 for the CDE(HD) bleach sequence. The unbleached pulp and chlorination conditions are the same as for Table A~l, and the rapid extraction conditions are the same as conditions in Tables A-l and A-2 for DR = 1.2:1 and DR = 1.3:1 respectively.
The conventional extraction is at 11% consistency, 80C, for 1 hour, washed with 1.3 volumes of fresh water. The hypochlorite state is 6 minutes at 80C, no wash between H and D stages. The chlorine dioxide stage is 9% consistency, 80C, 3 hours.

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6~31D4 -The results in Table A-3 show that the rapid extrac-tion is capable of producing pulp similar in bleached properties to those obtained with a conventional extraction. The slight increase in hypochlorite required is more than compensated for by lower caustic usage in the rapid extraction compared with the 2.5 and 3.0% NaOH applied in the better of the conventional extractions.
Examples_B_and C
Various experiments and calculations were performed to indicate the effects of varying the number of stages, the Displacement Ratio, the chlorination level, and the temperature.
Example B
The Effect of Time and Temperature on Extraction Efficiency Using conditions similar to those described for cycle 1 in Example A except that the displacement ratio was 1.25:1 and the flow sheet of Figure 7 was used, the effects of temp-erature in the range of 20C to 90C and times in the range 5 seconds to 11 seconds per displacement were studied. The results are presented in Table B.Control conventional extrac-tions were a~so performed on the same sample of pulp chlorin-ated at the 0.27 x Kappa No. level.
Table B
Unbleached Pu~e: Northern Softwood kraft, Roe No.
5.1, Kappa No. 29.2, Viscosity, 28.2 m Pa.s Chlorination: 7.64% C12 + 0.2~ C102 on pulp, 3.0% consistency 45C, 120 minutes, Kappa No. 6.9, Extraction (Displacement): 3.0% NaOH on pulp in -first step 7% NaOH on pulp in second step, 8~ consistency, 1.25:1 displacment ratio.

~.2~ 4 . .__ __ . .._ _ Temp Av. Time D Hypo Visc. at on Washer per displacement Kappa No. Applied 70.0 Br sec on pulpm Pa.s 2011.5 4.18 1.05 16.7 30 8.3 4.00 1.10 16.7 40 8.1 4.07 1.06 15.4 50 7.5 3.56 0.81 17.9 55 7.0 3.60 0.87 16.9 60 6.1 3.60 0.84 18.7 70 6.0 3.42 0.86 17.6 80 5.1 3.36 0.76 18.2 90 5.2 3.32 0.77 17.3 Conventional E stage 80C, 3.0% NaOH 0.37 18.2 on pulp, 10% cs, 1 hour E stage wash 1.3:1 DR, Kappa No. 2~57 . _ ._ Conventional E stage 20C, 3.0% NaOH 1.10 15.7 on pulp, 10% cs, 1 hour, E stage wash 1.3-1 DR~ Rappa No~ 4~11 There appears to be a very weak trend towards higher CDE Kappa No. as the temperature is decreased from 90C to 70C, however, the differences are within the experimental error of the Kappa No. test. However, below about 50C the temperature has a significant effect on the chemistry of the lignin removal process as indicated by higer CDE Kappa No. and higher hypochlorite demand to reach 70 Brightness.
Further work was done in which sheet weight and vacuum were varied in order to obtain times in the range of 2 seconds to 16 seconds per displacement. Figures 8 and 9 display times for displacement in this range plotted respectively against the Kappa No. of the extracted pulp and the hypochlorite - 34 ~

;3~

required to reach a final brightness, and show that the time changes in this range had no ePfect on the extraction efficiency.
Example C
This example shows the effects of varying chlorin-ation levels, as well as variations in the number of consec-utive extraction stages, on extraction efficiency, as compared to control pulps treated with conventional extractions.
Table C-l shows conditions for rapid extraction using two displacements, at various chlorination levels at varying displacement ratio, and the table also shows comparative results for the control pulps.
Before conducting the tests shown in this example, numerical simulation methods were used to obtain the steady state caustic concentrations for double extraction stages.
The calculated steady state conditions were used for extractions where the displacement ratio ranged from 1.1 to 2.0 as shown in the tables. The chlorinated and the extracted pulps were washed with 2 displacements of fresh water equal to the displacement ratio used f,or extraction. The flowsheet was similar to Figure 5. The caustic concentrations in these extractions were those which could be obtained in a counter-current system by the addition of only 2~ NaOH on pulp.
The control pulps, after chlorination, were washed with a 1.3:1 displacement ratio of fresh water. They were then extracted at three levels of caustic concentration using conditions typical of commercial operation. After extraction the pulps were washed with a 1.3:1 DR of fresh water in prep-aration of hypochlorite bleaching. It should be noted that very few commercial mills use a displacement ratio as high as 1.3:1 indeed many mills have displacement ratios of 1.0:1 or lower. Therefore by commercial standards these control extrac-tions were very well washed after both ehlorination and extrae-tion.
Table _ Unbleached Pulp: North Eastern Canadian softwood, Kappa No.
30.8, Roe No. 5.1, Chlorination: 3.0% eonsisteney, 40.C (see table for other conditions).

Extraction, displaeement: 11% eonsisteney, two displaeement steps Ea & Eb at80~C.wi~h 3Ø% ~OH in Ea, 7.0% NaOH in Eb, 20 seconds per displae~.ent.
Extraetion, eontrol: 11% eonsisteney, 70C, lhour, El wash 1.3 DR

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The controls show that there was a definite advantage for applying more than 2.0% NaOH on pulp with respect to both CDE Xappa No. and hypochlorite bleach requirements.
The results for the-two extraction steps show that displacement ratios of 1.1 to 1.4:1 gave better results than 2.0:1.
The conditions represented by a chlorination level of 0.166 x KapRaNo and 3.0% NaOH on pulp in a conventional ex-traction are typical of conditions used in most of the bleachedkraft industry for producing softwood pulps with a CDE Kappa No. in the range 6.5 to 5Ø
The following table C-2 shows results from the same seri.es of experiments and with the same pulps, but showing results both for a single and a double extraction stage. The results relate to extraction of 20 seconds each at 80C
with 4.2% NaOH for the first extraction displacement and 10.4%
for the second extraction displacement, and with.6.0% NaO~ in the case of a single extraction stepO The results show efficïency in t.erms of % los.s of Kappa No. duxina extraction compared with conventi.onal extraction as 100% efficient.

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These results show that with a hi~hly chlorinated pulp, rapid extraction performed in two successive steps has almost 95% of the efficiency of conventional extraction.
Example D
This example relates to the CDE (HDED) bleach sequence as described with reference to Figure 7, using countercurrent washing and water re~use throughout the bleach sequence.
To establish steady state conditions, a southern pine kraft pulp was used for the CDE (HDED) sequence which has no wash between the stages shown within the brackets. Filtrate from the (HDED) washer was used as shower water on the last stage 289 of Figure 7. Chlorination filtrate from the first and third trays 102'a andlO5~b, representing 7~% of the total chlorination filtrate, was recycled for the next chlorination. Thus only 22% of the available chlorination filtrate was sewered at 282c.
The conditions of the experiment were as follows:
Unbleached Pulp: Southern pine kraft, Roe No. 5.35, Kappa No. 31.9, Visc. 24.,6 m Pa.s.
Chlorination Stage: 7.04% C12 + 0.2% C102, 50C, 2.5Q6 consistency, recycle of 78% of CD stage filtrate to next cycle of chlorination.
Extraction (Rapid): Similar to flowsheet of Figure 7 except a 1.3:1 displacement ra-tio was used. 2.00% freshmake-up NaOH on pulp was added to each of the seoond through fi:E~h cycles and 2.25%'~1aOH on pulp ~as added - - for each cycie fra~L 6 to'cycle ~4. Total ~aOEI on pulp at start of cycle 1 was 14% and at start of cycle 15 was 11.2%. Acidic (HDED) filtrate was used for the last stage 289 of Figure 7. Extraction was follcwed'by (~3D~) bleachina with no wash l~etween st3ges wit}lin parenthesis, the stages }~eing as follows:-, Hypochlorite Stage: all cycles 1.0% C12 on pulp, 80C, 11% consistency, 6 minutes.
Chlorine Dioxide Stage: all cycles 0.4% C102 on pulp, 80C, 10% consistency, 5 minutes.
Extraction Stage: all cycles 0.55% NaOH on pulp, 80C, 10% consistency, 4.5 minutes.
Chlorine Dioxide Stage: all cycles 0.8% C102 on pulp, 80C, 9.2% consistency, 4 hours maximum.
The results are shown in Table D-l.
Table D-l ~o Cycle No. - ~ 11 15 Total NaO~ in E stage 14.0 11.2 12.0 11.2 at start of cycle, % on pulp Hypochlorite Sta~

Brightness ISO 69.8 59.8 61.0 58.7 `Visc. m Pa.s 16.3 21.9 20.6 Final Dioxide Stage Brightness ISO 90.4 89.0 88.9 89.4 Visc. m Pa.s 14.8 20.0 18.6 l9.0 _ At cycle 1 the total alkali charge to the system was 14.0% NaOH on pulp. During the first 5 cycles a make-up caustic addition of 2.0% on pulp was used. The total alkali in the system gradually decreased to 12% on pulp. At cycle 6 the make-up caustic was increased to 2.25% NaOH on pulp. This was sufficient to keep the system in balance throughout later cycles.
The need for increased caustic make-up was caused by the acidic (HDEDI filtrate used as shower water at stage 289 working countercurrent through the system to neutralize some of the alkali for the extraction stages. The more highly acidic chlorination fiitrate due to chlorination filtrate ~ 41 -6~

recycle may also have contributed to the higher caustic con-sumption. Table D-l shows that after cycle 6 the total caustic in the E steps came lnto balance. During the first 5 cycles, with constant chemical additions in the HDED stages, the brightness after the H stage dropped 10 points and the final brightness after D2 stage dropped 1 point. This is normal for a countercurrent washing system as it is well known that countercurrent washing will decrease brightness or increase the bleach chemical charge required to reach a given brightness.
Table D-2 below shows conditions used to bleach a second, higher Kappa No. unbleached southern pine pulp to market pulp brightness of 91 ISO. A chlorination level of .28 x Kappa No.was used to bleach this pulp by the CDEIHDED) sequence at steady state with respect to the countercurrent washing system. Because of the higher unbleached Kappa No.
and the higher C12 charge for chlorination the caustic charge to the extraction stages was increased to 2.5~ NaOH on pulp.
Also shown in Table D-2 are conditions and results for a control bleach usi~g fresh water for washing after the CD, E (HD) and (ED~ stages of the CDE(HD) (ED) sequence. Two levels of chemical addition to the final bleach stage are shown so that comparisons with the CDE(HDED) sequence can be made at a common brightness by interpolation. It is evident that equi~alent quality pulp can be produced by the CDE(HDED) sequence using rapid extraction and the CDE(HE)(ED) sequence using conventional extraction.
Table D-2 Comparison of rapid extraction in the CDE(HDED) sequence with conventional extraction in the CDE(HD)(ED) sequence Unbleached pulp: Southern pine kraft, R~e No. 6.11, Kappa No.
40.4, Visc. 32.7 m Pa.s.
Bleaching oonditions for the CDE(HDED) sequence with rapid extraction ~ere similar to those in Examples D-1 except chemical additions were as shown in the table below and (HDED) filtrate was used for stage 288 and fresh water for 289 as shown in Figure 7.
Bleaching conditions for the CDE(HD) (ED) sequence with conven-tional extraction were as follows:-Chlorination Stage: 50C, 2.5% consistency, 42 minutes, fresh water used for chlorination. Chlorinated pulp washed by 1.3:1 displacementwith fresh water.
Extraction (oonventional): 10.7% consistency, 703C 1 hour, washed by 1.3:1 displace~ent of fresh water.
Hypochlorite Sta~e: 11% oonsistency, 80C, 6 minutes, no wash.
Chlorine Dioxide Dl: 10% consistency, 80C, 45 minutes, wash with 1.3:1 displacement of fresh water.
Extract on E2: 11% consistency, 80C, 10 minutes, no wash.
Chlorine Dioxide D2: 9.2% consistency, 80PC, 3 hours.

Bleach Sequence CDE(HDED) CDE(HD) (EP) . ~ _ . _ . . _ Type of Extraction Rapid Conventional Cycle 26 CD C12 applied, % on pulp 10.23 8.96 C102 " " 0,23 0~20 El NaOH applied, % on pulp 2.50 3.50 H C12 applied, % on pulp 0.80 0.80 NaOH* " " ~60 1,55 Dl C102 applied, ~ on pulp 0.30 0.50 E2 NaOH applied, % on pulp 0.50 0~50 /oon't) ~246;~
Bleach Sequence CDE(HDED). CDE(HD)IED) Type of Extraction Rapid Conventional Cycle . 26 D2 C102 applied, % on pulp 0.80 Brightness 91.0 90.5 91.6 Reverted Brightness, T-260 87.5 85.8 88.5 Visc. m Pa.s 19.~ 20.5 20.5 * includes combined NaOH used to make the NaOCl Further tests show that with the rapid extraction method of this invention, which has a much more thorough washing after the chlorination and extraction stages than in a conventional bleachery, the increase in chemical consumption at the steady state was actually less than would have been expected wi.th a conventional countercurrent washing system.
This increase is needed to compensate for loss of brightness due to carryover of coloured material into the hypochlorite and/or chlorine dioxide bleaching stages during a countercurrent operation.
Example E
The above examples all relate to kraft pulps. This example is concerned with sulfite pulp and alkaline sulfite pulp cooked in the presence of 0.07% anthraquinone on wood (AS-AQ).
A sample of.sulfite pulp cooked for use in news furnish was semi-bleached to 71 brightness using the CEH
sequence in which the extraction stage was preceded by two displacement washes with fresh water at 70C usi.ng a DR of 1.3:1. The single extraction stage consisted of a displacement of 2.0% NaOH through the pulp at 70C at a DR of 1.3:1 followed by two wash stages at 70C and 1.3:1 DR. The pulp was then bleached to 71 brightness with hypochlorite.
A sample of commercially cooked AS-AQ pulp was semi-bleached by the CEH sequence and fully bleached by the CDE(HD) sequence. Conditions for the extraction steps were 70C, 11%
consistency and 1.3:1 DR. The pulp was bleached with hypo-chlorite and chlorine dioxide to 92.3 brightness. There was no wash between the H and D stages.
Results for both pulps are presented in Table E.
It can be seen that rapid extraction technology can be used to bleach sulfite type pulps to either semi bleached or fully bleached market pulp brightness levels with excellent viscosity levels.

~4~3~
, .
Table E
Demonstration of rapid extraction technology for use with softwood sulfite and alkaline sulf:lte cooked in the presence of anthra~uinonc (AS-AQ).
_ . _ ._ . _ Type of cook sulfite Alkaline sulfite - anthraquinone Roe No. 6.74 8.07 Kappa No. 31.6 44.6 Visc. m Pa.s 52.8 42.9 _ _ _ .
Bleach sequence CEH CEH CDE'(HD) Chlorination:-C12 applied, % on pulp 6.40 8.88 8.32 C102 appl ed, % on pulp NIL NIL 0.20 F,irst_rapid extraction NaOH applîed, % on pulp 2.0 3.3 3.3 Second rapid extraction NaOH applied, % on pulp NIL 7.6 7.6 Kappa No. 2.4 2.8 Hypochlori-te C12 applied, % on pulp 0.42 0.50 0.22 Brightness, ISO 71.0 84.0 __ Visc., m Pa.s , 23.5 30.4 __ Chlorine Dioxide C102 applied, % on pulp 0.70 Brightness, ISO 92.3 Viscosity m Pa.s _ 41.0

Claims (16)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A process for the bleaching of digested pulp including the steps of:-(a) passing the digested pulp into a chlorination tower together with chlorine and chlorinating said pulp to a level of 0.19 x Kappa No. or higher as percentage available chlorine on pulp;
(b) forming the chlorinated pulp into a layer on continuously moving mesh surface means in a forming zone and removing liquid which drains from the pulp in the forming zone;
(c) performing chlorination washing by adding wash water to the pulp layer to displace chlorinated liquid from said layer;
(d) performing alkali extraction by displacing said wash water with alkaline liquor applied to said layer;
(e) subjecting said pulp to at least one extraction washing step; and (f) passing the pulp to a subsequent bleaching stage.

2. A process according to claim 1, wherein said alkaline liquor has a caustic content of at least 3 percent on pulp.

3. A process according to claim 1, wherein said alkali extraction is performed in two steps by displacing said washwater with a first alkaline liquor in a first step and displacing said first alkaline liquor with a second highly alkaline liquor in the second step, the liquor for the first step being a recycled mixture of the first and second liquors which have passed through the pulp layer.

4. A process according to claim 1, wherein said alkali extraction is performed in one or more successive steps while the pulp layer advances on said surface means, and wherein said extraction washing step is performed by passing fresh water through said layer to displace the alkaline liquor therefrom;
and wherein the alkaline liquor used for the extraction in said one step or the last of said successive steps comprises a highly alkaline mixture of:-(a) recycled alkaline liquor displaced from the pulp layer by said wash water, (b) said wash water after having passed through said layer to displace the alkaline liquor, and (c) fresh caustic.

5. A process according to claim 4, wherein said extraction is performed in at least two steps, and wherein alkaline liquor from the last extraction step which has passed through the pulp layer is mixed with alkaline liquor from a previous extraction step which as passed through said layer and the mixture is recycled for use in the previous extraction step.

6. A process according to claim 1, wherein some of the washing water used in said chlorination wash is subsequently passed to a chlorination recycle system, including the step of passing wash water substantially uncontaminated with strong alkali or alkali extractives through the pulp layer prior to the extraction, at least a part of the stream of uncontaminated water after passing through said layer being mixed with the first portion of alkaline extraction stage liquor to pass through said layer with the mixture being removed from the system, said uncontaminated water thus serving to prevent extraction stage filtrate from being carried countercurrent into the chlorination stage recycle system where is would consume chlorine in the next cycle.

7. A process according to any of claims 1, 4 or 6, wherein all the extraction and washing steps set out in said claims are performed where the layer of pulp is supported by single horizontal belt which constitutes said mesh surface means.

8. A process according to any of claims 1, 4 or 6, wherein all the extraction and washing steps set out in said claims are performed in a total time of less than 1 minute.

9. A process according to any of claims 1, 4, or 6, wherein all the extraction and washing steps set out in said claims are performed where the layer of pulp is supported by a single horizontal belt which constitutes said mesh surface means and wherein the residence time on said belt is less than 1 minute.

10. A process according to any of claims 1, 4, or 6, wherein all the extraction and washing steps set out in said claims are performed while the layer of pulp is supported by a single horizontal belt which constitutes said mesh surface means, and wherein the residence time on the belt allows between 2 seconds and 16 seconds per displacement.

11. A process according to any of claims 1, 4 or 6, wherein the displacement ratio, defined as the ratio of the volume of liquid displaced into the pulp layer in each of steps (c) and (d) of claim 1, to the volume of liquid remaining in the pulp layer at the end of each said step is between 1.1:1 and 1.4:1.

12. A process according to any of claims 1, 4 or 6, wherein the displacement ratio, defined as the ratio of the volume of liquid displaced into the pulp layer in each of steps (c) and (d) of claim 1, to the volume of liquid remaining in the pulp layer at the end of each said step is between 1.2:1 and 1.3:1.

13. A process according to any of claims 1, 4 or 6, wherein said continuously moving mesh surface means is the surface of one or more drum washers.

14. A process according to claim 4, further comprising the step of continuously removing from the system a first portion of the extraction stage liquor to pass through the layer and which is the portion most highly contaminated with lignin products.

15. A process according to claim 14 wherein extraction is performed in at least two successive displacements, said first portion of extraction stage lignin being removed from the first of said displacements.

16. A process according to claim 15 wherein alkaline liquor from the last extraction displacement which has passed through the layer is mixed with alkaline liquor from a previous extraction displacement which has passed through the layer and the mixture is recycled for use in the previous extraction displacement.