CA1176408A - Process for the oxygen delignification of pulp - Google Patents

Abstract

PROCESS AND APPARATUS FOR THE
OXYGEN DELIGNIFICATION OF PULP

Abstract of the Disclosure A medium consistency oxygen delignification process and apparatus are provided utilizing one or more substantially horizontal agitated tubular reac-tion zones. The process produces rapid delignifica-tion rates at low alkali charges, minimizes oxygen requirements, and yields pulps having high viscosi-ties. The use of rotary screws or paddles in the reaction zone or zones provides the agitation required to enable good mixing of oxygen with the medium consistency pulp and alkaline chemicals as well as controlling the pulp retention time in each reaction zone.

Description

B FN 7 0 8 9 1~

PROCESS AND APPARATUS FOR THE
OXYGEN DELIGNIFICATION OF PULP

This application is related to Canadian application Serial No. 365,411 filed November 25, 19B0, and entitled "Apparatus and Method or Medium Consistency oxygen DeligniEication of Pulp."
Background of the Invention This invention relates to a process and apparatus for the oxygen delignification o Eibrous materials, and more particularly to the medium-consistency oxygen delignification o bleachable grade pulp and other fibrvus materials using a series o tubular reaction zones.
Conventional processes Eor chemical pulpin~ of Eibrous raw materials have in the past utilized sulur-containing compounds while conventional bleaching processes have utilized chlorlne containing compounds.
Today, environmental considerations have resulted in a search or nonpolluting processes which can ofEer the desired pulp yields and qualities. Much attention has been devoted to the use of oxygen in combination with alkaline chemicals to delignify pulp and other fibrous materials.
For example, several workers have investigated oxygen deligniication of high consistency pulp (i.e., 20-30% consistency). See, Eachus, ~APPI~ Volume 58, p.
151-154 (Sept. 1975) and Hasvold, 1978 International Sulfite Conference, Montreal, Canada (September 13, 1978). Other workers have utilized ~,., oxygen delignification in low consistency (i.e., 1-5~ consistency) pulping or bleachinc~ processes.
See, Paper Trade ~ournal p. 37-39 (~uly ]5, 197~).
Recently, workers have also investigated processes for the oxygen deligni~ication of pulp mill screen rejects and knots. Such screen rejects and knots have often been heretofore unusable and had to be dewatered and then burned or dumped.
However, ~irschner, Paper Trade Journal, p. 32 (November 15, 1978), has reported the use of a low-consistency oxygen delignification process for kraft and sulfite screen rejects which produces a bleachable grade of pulp. }lasvold, 197~
International Sulfite Conference, Montreal, Canada (September 13, 1978), has reported an oxygen process whlch deligniEies sulfi~e knots at a ~5~ pulp con-sistency.
While most workers have utili~ed either high or low consistency oxygen delignification processes in working either with pulp or with screen rejects and knots, both of these processes s~ffer from several disadvantages. Low consistency opera-tion re~uires a large reactor volume to maintain an acceptable retention time for the pulp. Operating at low consistency also produces large power demands for pumping large volumes of pulp and a high steam usage to heat the pulp in the reactor. Additional-ly, the low concentration of dissolved solids in the spent li~uor increases evaporation costs for chemi-cal recovery processes. Operation at high consis-tency, on the other hand, usually re~uires special dewatering e~uipment to attain the higher consisten-cy. It is also known that high consistency opera-tion of an oxygen delignification system can result in overheating of the pulp due to the exothermic delignification reaction, as well as pulp degrada-tion and even combustion o~ the pulp.

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BFN 7089 -3~

Carrying out oxygen delignification of pulp at medium consistency (i.e., 8-20~ consistency) would be advantageous in that much existing mill equipment, including pulp washing and thickening S e~uipment, is designed to operate in that consisten-cy range and no special e~uipment would be re~uired to attain that range. Some workers have reported satisfactory results operating at medium consistency on a laboratory scale using rotary autoclaves with no internal means of mixing (See, e.g., Annergren et al, 1979 Pulp Bleaching Conference~ Toronto, Canada, June 11-14, 1979; Saukkonen et al, TAPPI Volume 58, p. 117 ~1975); and Chang et al TAPPI Volume 56, p.
97 (1973)). However, such e~uipment is not suitable for scale-up ~o handle large tonnages of pulp on a commercial scale~ Other workers have encounterecl serious problems even on a small laboratory scale.
For example, Eachus, TAPPI Volume 58, p. 151 (1975), reported that oxygen delignification at medium con-2Q sistency was not practical because of a high alkalire~uirement, oxygen starvation, and a limited delig-nification.
Chang et al, TAPPI 57, p. 123 (1974), con-cluded that operation at medium consistency produced a considerably lower delignification rate than high consistency operation and also resulted in nonuni-form delignification. Although the authors sugges-ted that these problems could be overcome through the use of higher oxygen pressures in the reaction vessel, use of such higher press~res has several disadvantages. These include greater costs for a thicker-walled reaction vessel, greater difficulty in feeding pulp against the higher pressure, and an increased danger of gas leakage.
Vertical tube oxygen reactors operating at medium consistency have been constructed for trial purposes. (See Annergren et al, 1979 Pulp Bleaching Conference, Toronto, Canada, June 11-1~, 1979, and Rleppe et al, T~PPI Vol. 59, p. 77 (1976).) Ho~ever, such vertical tube designs have serious de~iciencies, including channeling of gas and pulp up through the tower and also the re~uirement for a high speed mechanical mixer to disperse oxygen into the pulp slurry. Such high speed mixiny can lead to pulp deg~adation and additionally re~uires substan-tial power input.
As can be seen, there is a need in the artfor a simple and efficient process for oxygen delig-nification of ~ibrous materials including pulp as well as screen rejects and knots which avoids the 5 problems which have plagued the prior art.
Summary of the Invention _ ~ ccordin~ to one aspect oE the present invention, medium consistency pulp at a consistency of from 8 to 20% along with alkaline materials are introduced into a substantially horizontal reaction`
zone. Oxygen is added to delignify the pulp while the mixture of oxygen, pulp, and alkaline materials is agitated and transported through the reaction zone. Apparatus for delignifying the pulp includes a tubular reaction zone, means for introducing oxygen gas and alkaline materials into the reaction zone, pump means for introducing pulp into the reac-tion zone, and means to transport and agitate the mixture o pulp, oxygen, and alkaline materials through the reaction zone.
The present invention provides a medium consistency process and apparatus utilizing one or more substantially horizontal agitated tubular reac-tion zones which produce rapid oxygen delignifica-tion rates at low alkali charges, minimize oxygenre~uirements, and yield pulps having high viscosi-'76~ 8 ties. The use of rotary screws or paddles in the one or more reaction zones provides the agitation re~uired to enable good mixing of oxygen with the ~edium consistency pulp and alkaline chemicals as well as controlling the pulp retention time in each reaction zone.
By "medium consistency" it is meant that the consistency o the pulp supplied to and main-tained in the reaction zone is from 8-20% and preferably 10-15%. This is to be distinguished from prior high (above ~0% and preferably 25~30%) and low ~less than ~% and preferably 1-5%) consistency delignification systems. The oxygen delignification system of the present invention can be used to delignify any type of pulp including mechanical pulps, thermomechanical pulps, semichen~ical or modified mechanical pulps, chemical pulps, and secondary fiber. Additionally, straw, flax, and bagasse can also be delignified as well as pulp mill screen rejects and knots. Preferably, the starting materials for the process are unbleached wood pulps such as softwood kraft pulps having Kappa numbers between 20 and 50 or hardwood kraft pulps having Kappa numbers between 10 and 30, high yield pulps (i.e., 55-60~ yield) cooked to near the point of fiber liberation such as softwood kraft pulps having Kappa numbers between 50 and 80 or hardwood kraft pulps having Kappa numbers between 25 and 50, or fiberized pulp mill screen rejects and knots.
In accordance with the invention, the pulp or other fibrous material may be sent directly from the blow tank of a chip or raw material digester or cooker to brown stock washers which are typically operated in the medium consistency range. In instances where an initially high ~appa number pulp such as a high yield kraft pulp is utilized, the ~1'7~4~

pulp may, optionally, be sent to a further refining stage before or after leaving the brown stock washers. In instances where the pulp has been screened, the screen rejects and/or knots removed S from the pulp stream may be fiberized in a further refining staye and then recombined with the main pulp stream for the oxygen delignification process.
The pulp is then introduced, at a medium consistency of between 8 and 20~, and preferably 10-15%, into a substantially horizontal tubular reaction vessel where it is contacted with oxygen gas and alkaline chemicals. A thick stock pump is used to feed the pulp into the reaction vessel. Use of the thick stock pump prevents the loss of gas lS pressure from the vessel and does not severely com-pact the pulp so that uniEorm oxygenation and delig-nification can occur.
Ox~qen may be introduced into the delignif-ication system either at one injection point or multiple injection points. Typically~ oxygen gas will be in~ected on the lower side of the reaction vessel. Partially spent gas may, optionally, be removed from the delignification system by venting to the atmosphere or it may be collected for recycle. Additionally, the partially spent gas may be drawn off and utilized for lime kiln enrichment, waste water treatment, or other suitable uses. Any organic compounds or carbon monoxide formed during the delignification reaction may be removed by pass-ing the gas through a catalyst bed before reuse.
Alkaline pulping chemicals are also intro-duced into the reaction vessel to aid in the delig-nification. Examples of such alkaline chemicals which are suitable for use in the practice of the present invention include sodium hydroxide, sodium carbonate, sodium borate compounds, ammonial oxi-~ ~64~`~

dized kraft white liquor~ and mixtures thereof.
Preferably, at least a portion of the total charge of alkaline chemicals is added to the pulp prior to its passage through the thick stock feed pump into the first reaction zone. This insures that the pulp has an alkaline pH when the pulp enters the first reaction zone and also lubricates the pulp for easier pumping. An additional portion of the total charge is added to the first reaction zone rom one or more injection points along the top of the vessel. Magnesium sulfate or other known protector chemicals or catalysts for preserving the viscosity and stren~th of the pulp may be introduced into the pulp either before or after the thick stock ~eed pump.
Steam is also added to the pulp prior to its entr~ into the thick stock Eeed pump. The steam aids in expelling excess air from the pulp prior to delignification. Additional steam may be injected into the reaction vessel as needed in order to main-tain the desired reaction temperature, although the exothermic delignification reaction supplies a sub-stantial fraction of the heat reguirement.
As the pulp at 8-2~% and preferably lC-15%
consistency is introduced into the reaction vessel through the thick stock pump, a rotary screw or series of paddles agitates the pulp, oxygen, and alkaline chemical mixture. It has been found that a solid flight helical screw extending the entire length of the reaction zone produces the gentle agitation necessary for uniform and rapid delignifi-cation. Satisfactory delignification is achieved by rotating the screw at a speed of less than about 15 rpm and preferably 1-6 rpm. In another embodiment of the invention, one or more additional substan-tially horizontal tubular reaction vessels are uti-~FN 7089 -8 lized to achieve ~n additional amount of delignifi-cation of the pulp.
The reaction ~emperature, alkali charge, type of alkaline chemical, oxygen partial pres~ure~
and retention time depend on the type of material being treated and the desired degree of delignifica-tion. Typically, temperatures may range from 80 to 1~0C, alkaline chemical charges from 1 to 20~
calculated as Na2o on ov~n dry material, and oxygen partial pressures from 30 to 200 psi.
Appropriate retention times have been found to be 5 to 120 minutes.
Accordingly, it is an object of the present invention for uniformly and rapidly deligniEying pulp at medium consistencies while minimizing alkali dosages and oxygen requirements to provide a pulp having high strength properties. This and other objects and advantages of the invention will become apparent from the following description, the accom-panying drawings, and the appended claims.
Brief Description of the Drawin~s Fig. 1 is a schematic flow diagram illus-trating the overall process of the present invention;
Figs. 2a and 2b are schematic flow diagrams illustrating alternative embodiments of the inv~n-tion;
Fig. 3*is a graph of pulp viscosity versus Kappa number for medium consistency oxygen delignif-ication of pulp in accordance with the practice of the invention;
Fig. 4 is a graph of pulp viscosity versus Kappa number for different pulp consistencies;
Fig. 5 is a graph of the change in Kappa number versus alkaline chemical charge for agitated and nonagitated delignification processes, and * On same sheet as Fig.1 ~' !

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Fig. 6 is a graph of alkaline chemical charge versus Kappa number reduction for different pulp consistencies.
Description of the PreEerred Embodiments 5As illustrated in Fig. 1, pulp at from 8-20% consistency and preferably 10-15~ consistency from the brown stock washers is introduced into a first horizontal reaction vessel or tube 10 by a thick stock pump 12. Inclined reaction t~bes may also be employed, but the angle of incline should not exceed approximately 45 degrees to avoid compression and dewatering of the pulp in the lower end of the tube, ~hich will interfere with uniEorm mixing of oxygen. Additionally, while the reaction ves5el is illustrated as a cylindrical reactor tube, noncylindrical tubes such as a twin-screw system ma~
be utilized.
Pump 12 may be a Moyno progressing cavity pump available from Robbins & Myers, Inc., Springfield, Ohio~ Alternatively, pump 12 may be a Cloverotor pump availa~le from the Impco Division of Ingersoll-Rand Co., Nashua, New Hampshire, or a thick stock pump manufactured by Warren Pumps, Inc., Warren, Massachusetts.
25It has been found that these pumps are capable of feeding the pulp into the reaction tube against the pressure in that tube without severely compacting the pulp and without any gas losses from the tube. Other feeding devices such as rotary valves or screw feeders are not desirable for use in this invention. A rotary valve allows substantial gas loss from the reaction tube due to the rotation of valve sections which are alternately exposed to the high oxygen pressure in the reactor and then to atmospheric pressure external to the reactor. Use of a screw feeder results in the severe compression ~7~4~3 BFN 70~9 -10-and dewatering of pulp so that efficient oxygenation at the proper consistency range cannot occur.
Prior to inteoducing the pulp into thic~
stock pump 12, steam may be injected into the pulp via line 14. The steam aids in expelling excess air from the pulp and also raises the temperature oE the pulp somewhat. Additionally, it is desirable to add at least a portion of the total amount of the charge of alkaline material prior to the introduction of the pulp into thick stock pump 12. This addition of alkaline material can be made through line 16. The alkaline material serves to lubricate the pulp for easier pumping as well as to insure that the pulp will have an alkaline pH when it enters reac~ion tube 10, Alternatively, all of the charge may be added at this point.
Generally, the total alkaline material charge will amount to from 1 to 20% by weight calcu-lated as Na2o of the oven dry weight of the raw fibrous material. Examples o alkaline materials suitable for use in this invention include sodium hydroxide, sodium carbonate, sodium borate com-pounds, ammonia, oxidized kraft white li~uor, and mixtures thereof although other known alkaline pulp-ing ~i~uors may also be used.
Once introduced into reaction tube 10, thepulp undergoes an oxygen delignification reaction.
Oxygen gas is introduced into reaction tube 10 through line 18. Alternatively, oxygen may be introduced at a number of points along the length of tube 10. Typically, the oxygen partial pressure maintained in the system is from about 30 to ~00 ps ig .
Spent gas may be removed from the system by venting it to the atmosphere. Alternatively, it may be recovered for recycle to the reaction tubes or ~7~8 BFN 708~

may be used for other purposes such as lime kiln enrichment or ~aste ~ater treatment. Any organiG
vapors or carbon monoxide produced during the delignification reaction can ~e removed by passing the gas through a catalyst bed.
The delignification reaction is carried out by mixing the pulp, oxygen, and alkaline li~uor which is injected through line 20 and sprayed over the pulp along the length of the tube. By adding the alkaline li~uor gradually along the length of the tube rather than all at once as is conventional in high consistency (i.e., 20-30~ consistency) oxygen delignification, better pulp viscosity and strength is achieved. Another advantage to gradual-ly adding the alkaline li~uor is that the exothermicdelignification reaction is more easily controlled and the risk of localized overheating i5 diminished.
Satisactory gentle agitation can be achieved by rotating screw 22 with drive means 23 at a rate of less than about 15 rpm and preferably 1-6 rpm. Preferably, the system is operated so that a gas space remains at the top of reaction vessel 10 and the vessel is less than full of pulp. Total retention times of the pulp in the system may vary depending upon the nature and condition of the pulp and the desired amount of delignification to be accomplished. Retention times of bet~een 5 and 120 minutes have been found to be satisfactory. Steam may be injected into the reaction vessel through line 46 to maintain the temperature within the preferred 80-160C range.
Upon completion of the delignification reaction, the pulp exits vessel 10 through outlet 26 and is passed to blow tank 28. The pulp is then discharged using a conventional blow wiper discharg-er.

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In another embodiment of the invention illustrated in Fig. 2a, where like components are indicated by like reference numerals, pulp from washer 50 is sent through refiner 52 for further S fiberization before being fed to thick stock pump 12. Since the consistency of the pulp le~ving washer 50 will be in the medium consistency range, the pulp can be refined and then fed to the reaction vessel at the same consistency without any need for any dewatering~ Refiner 52 may be utilized in instances where delignification is to be carried out on pulp having an initially high Kappa number such as high yield soft wood kraft pulp having an initial Kappa number greater than a~out 50.
Also illustrated in Fig. 2a is the use of one or more subse~uent su~stantially horizontal reaction vessels such ~s vessel 30 to carry out Eurther delig~ ication on the pulp. AS shown, pulp exiting one end of vessel 10 drops into vessel 30 where it is transported along the length o the vessel with gentle agitation by rotary screw 32 hav ing solid helical flights 34 and driven by a suita-ble drive means 33. Steam may be added through line 48 to maintain the temperature in vessel 30 within the preferred range of 80-160C. Additional oxygen may be injected through line 18a i~ re~uired.
Yet another embodiment of the invention is illustrated in Fig. 2b in which like components are represented by like reference numerals. In this embodiment, pulp is transported from an initial cooking or digestion stage through line 54 to screens 56 where oversize slivers, shives, knots, and other impurities are removed. The accepted pulp passes through line 58 into pulp washer 50 while the rejected material is sent to refiner 52 for further fiberization before being recombinedlwith the main ~7~ 8 BFN 7089 ~-13-pulp stream thro~gh line 60. This combined pulp stream is then washed and oxyyen delignified as described ahove to yield a bleachable grade pulp.
In order that the invention may be better understood, reference is made to the following non-limiting examples.
Example 1 A northeastern softwood kraft pulp having an initial Kappa number of 29.3 and a viscosity of 26.3 centipoise ~cps) was oxygen delignified in accordance with the process of the invention. The reaction conditions were 10~ pulp consistency, 100 psig total gas pressure, and a 3~ sodium hydroxide dosage by weight based on dry pulp. Retention time in the reaction zone was varied from 8 to 16 to 3~
minutes hy varying the speed of the rotary screw in tha reac~or. ~he pulp feed rate was set at either 1.7 ton/day (T/D) or 5.0 T/D.
The results are illustrated in Fig. 3.
That graph shows a linear relationship between pulp viscosity and Kappa number at up to 60% delignifica-tion, where % delignification=Initial Kappa No.-Final Kappa ~o. x 100 Initial Kappa No.
This result is surprising because high pulp viscosi-ties, which are indicative of high pulp strength, were obtained at a relatively high percentage of delignification. Commercial high consistency oxygen delignification systems are limited to about 50%
delignification due to sever~ losses in pulp strength ~measured as greatly lowered pulp viscosi-ties) beyond that point.
Thus, utilizing the medium consistency oxygen delignification process of the present inven-tion with substantially continuous gentle agitationof the pulp, more lignin can be removed from the ~6~
BFN 708~ -14-pulp without loss of pulp strength. This can result in significant reductions in operating and capital costs over high consistency processes because of reduced bleaching costs and the elimination of the need for a conventional chlorine bl~aching stage.
Example 2 Medium (15~) consistency oxygen delignifi-cation was carried out on a softwood kraft pulp having an initial viscosity of 29.5 using the process of the present invention. The delignifica-tion reaction was carried out for 20 minutes at 110C and at a total gas pressure of 150 psig.
For comparison purposes, the same pulp was deligni-fied under the same conditions with the exceptiorl that in one instance the pulp was maintained at a low (2%) consistency throughout the reaction and in another instance was maintained at a high (28~) consistency throughout the reaction.
The results are illustrated in Fig. 4. As shown hy that graph, for the same Kappa number, the medium consistency delignified pulp exhibited higher viscosities than both the high and low consistency pulp.
Example 3 Softwood kraft pulp having an initial Kappa number of 29.5 was oxygen delignified in a 2 liter autoclave at 110C and an oxygen gas pressure of 150 psig for a time sufficient to achieve a final Kappa number of 18~5. Several tests were run with the consistency of the pulp varied from 2% to 15% to 28%. The results are reported in Table I below.

BFN 7089 -lS-Table I
Hydrocarbon Consistency % 2 Consumed ~ CO Evolved (as Methane) Based On Based On to CO
~Wei~ht of Pulp Weight of Pulp Ratlo(Moles) 5 23.2 0.010 0.2 151.5 0.015 0.2 281.5 0.024 0.2 For a working system, it is necesary to provide venting of the reactor gases in order to remove combustible reaction products such as carbon monoxide and hydrocarbons. The resulting dilution of the gas in the reactor with oxygen maintains a safe condition.
15 Using the data from Table I, material balance calculations ~ere made to determine the amount of oxygen re~uired to maintain the reactor in a safe condition of 30~ of the lower explosive limit (LEL~ of combustibles. The results are reported in Table II below.
Table II
Consistency 2 Consumed* 2 Re~uired*
% %
252 3.2 3.58 1.5 2.06 28 1.5 2.42 *based on weight of pulp.
The results show that the medium consistency process has lower oxygen re~uirements.
Example 4 A high yield softwood kraft pulp near the point of fibèr liberation and having an initial Kappa numher of 59.4 was oxygen delignified in an ~764~i~

autoelave at temperatures ranging from 100-130C
and at a total gas pressure of 8.~ Kg/cm2 (120 psig). The pulp was maintained at a medium consis-teney for an approximately 15 minute reaction time as the eharge o~ alkaline (caustie) chemicals was varied rom ~-6~ by weight based on oven dry pulp.
As shown in Fig. 5, the curve labeled A in whieh the pulp was eontinuously gently agitated in the autoclave shows a greater reduction in Kappa number ~indicative of a greater delignifieation rate) than the eurve labeled B in which no agitation was performed. The results show the importance o~
gentle agitation of pulp when deli~nifying at medium eonsistency to improve the rate of delignifieation lS of the pulp.
Example 5 Tests were made using a so~twood kraft pulp having an initial ~appa number oE 29.5 to determine the effect of pulp consisteney on the extent of delignifieation for a given alkaline chemieal (eaustie) dosage and reaetion time. The tests were earried out in a 2 liter autoelave at llooc and 10.5 Kg/em2 150 psig gas pressure for 20 minutes.
Low (2%) eonsistency tests were done under condi-tions of vigorous agitation (rotation of stirrer at1250 rpm) while the medium (15~) and high (28%) ennsistency tests were condueted without agitation.
Tbe results are shown in Fig. 6.
As can be seen, surprisingly the extent of delignifieation for the medium and high consistency tests were nearly identical at a given caustic eharge~ The low consistency tests resulted in sub-stantially less delignificationO Therefore, longer reaetion times would be re~uired for a low consis-teney proeess to aehieve the same reduction in Kappanumber as for either a medium or high consisteney proeess.

`8 While the methods and apparatus herein described consitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise methods and appara-tus, and that changes may be made in e.ither withoutdeparting from the scope o the invention, ~hich is defined in the appended claims.
What is claimed is;

Claims (17)

CLAIMS:

1. A process for the continuous oxygen delig-nification of medium consistency pulp comprising the steps of introducing pulp at a consistency of from 8 to 20% and alkaline materials into a substantially horizontal reaction zone, adding oxygen to said reaction zone to delignify said pulp, and transpor-ting the pulp through said reaction zone while agitating the mixture of pulp, oxygen, and alkaline materials for a time sufficient for delignification to occur.

2. The process of claim 1 in which the temper-ature in said reaction zones is maintained at from about 80°C to 160°C.

3. The process of claim 2 in which steam is injected into the pulp prior to its introduction into said reaction zone.

4. The process of claim 1 in which the partial pressure of oxygen in said reaction zone is from 30 to 200 psia.

5. The process of claim 1 in which said alka-line materials are selected from the group consis-ting of sodium hydroxide, sodium carbonate, sodium borate compounds, ammonia, oxidized kraft white liquor, and mixtures thereof.

6. The process of claim 5 in which the charge of alkaline materials present in the reaction zone is from 1 to 20%, calculated as Na2o on an oven dry basis of raw materials.

7. The process of claim 1 in which at least a portion of said alkaline materials are added to the pulp prior to its introduction into the reaction zone.

8. The process of claim 1 in which the consis-tency of the pulp is from 10 to 15%.

g. The process of claim 1 in which said pulp is retained in said reaction zone for from about 5 to 120 minutes.

10. The process of claim 1 in which the pulp is transported and agitated by a rotary screw rotating at less than about 15 rpm.

11. The process of claim 1 in which said mix-ture of pulp, oxygen, and alkaline materials is passed to one or more subsequent substantially hori-zontal agitated reaction zones for a time sufficient for further delignification to occur.

12. The process of claim 1 in which said pulp is fiberized immediately prior to being introduced into said reaction zone.

13. The process of claim 1 in which said pulp is screened and the screen rejects are fiberized and recombined with said pulp immediately prior to being introduced into said reaction zone.

14. Apparatus for continuous oxygen delignification of medium consistency pulp comprising in combination, a substantially horizontal tubular reaction vessel including means for agitating and transporting said pulp through said reaction vessel, means for introducing oxygen gas into said reaction vessel, means for introducing alkaline chemicals into said reaction vessel, and pump means for introducing pulp at 8-20% consistency into said reaction vessel.

15. The apparatus of claim 14 including means for fiberizing said pulp prior to its introduction into said reaction vessel.

16. The apparatus of claim 14 including means for screening said pulp, means for fiberizing the screen rejects from said screening means, and means for recombining the fiberized rejects with said pulp prior to its introduction into said reaction vessel.

17. The apparatus of claim 14 in which said agitating and transporting means include a rotary screw running substantially the entire length of said reaction vessel.