Note: Descriptions are shown in the official language in which they were submitted.
10152025CA 02264423 1999-03-05-2-Method for treatment of pulpThe present invention relates to a method for treatment of pulp in an alkalinepulping process of chemical pulp during which process pulp is treated in achlorine dioxide stage including an acid treatment at a temperature of over80°C.Pulp mills have recently attempted to abandon the use of elementarychlorine, and partly also chlorine dioxide, the reasons for this being bothaspects of environmental protection and market factors. Disadvantagescaused by elementary chlorine include both noticeable malodorous gaseousemissions and liquid effluents from chemical pulp mills into water systems.Liquid chlorine dioxide does not cause odour disadvantages on such a largescale, it affects primarily water systems. When comparing these chlorinechemicals with each other by means of the AOX number designating theloading they incur on water systems, it can be noted that elementarychlorine is many times more detrimental than chlorine dioxide. The AOXnumber of chlorine is of the order of 4 - 7 and that of chlorine dioxide is 1 -1.5, or even below one.However, sequences utilizing chlorine dioxide are still popular andenvironmentally feasible. There are many reasons for this. The price of chlorinedioxide is very competitive compared with that of other chemicals. Also thestrength and brightness of pulp obtained by dioxide bleaching are good, in factat least approximately of the same order as the ones obtained by usingperoxide at the same consumption of chemical (kg/admt).Chlorine dioxide bleaching (D) is a well-known and widely used bleachingmethod. Conventional chlorine dioxide bleach treatment is usually performedat a temperature of 50 - 80°C and a with a chlorine dioxide dosage of about10 - 30 kg per ton of pulp. Sequences used are for example D,,ED1ED2 andOD°ED,ED2 and various modifications of these. The treatment time in a Dostage is usually shorter than in other D stages, being for example 30 - 9010152025l iCA 02264423 2002-05-17-3-minutes. The pH of the pulp decreases to the range of 1 - 3 towards the end of theDo stage. In the D1 and D2 stages the treatment time is 2 - 3 hours and the pH a littlehigher than in the Do stage. The chlorine dioxide stage is usually performed at atemperature of about 70°C while the treatment time in the Do stage is 0.5 - 2 hoursand in the D1 and D2 stages 2 - 3 hours. Higher temperatures have been avoided asthe low final pH in the range of 1 - 3 in the D stage combined with a high temperatureand long treatment time deteriorates the strength properties of the fibers.Beside chlorine dioxide bleaching, also bleaching processes utilizing ch|orineâfreechemicals, such as peroxide and ozone, have been introduced today. As pulpbleached with chlorine dioxide has good properties, the development of this processhas been continued in addition to the new processes mentioned. One of the mostimportant objects is to reduce the amount of chlorine dioxide needed in the bleachingand thus to make the method more acceptable in view of the environmental impact.Finnish patent application no. 944808 published on April 14, 1996 and WOpublication no. 96/12063, published on April 25, 1996 disclose an acid treatmentmethod for pulp by means of which the consumption of chlorine dioxide can bereduced. This treatment removes from the pulp hexenuronic acids which can reactwith chlorine dioxide and thus increase chemical consumption. The acids may beremoved by adjusting the pH of the pulp to the range of 2 - 5, preferably to 2.5 - 4,and the temperature to over 80°C, preferably to 90 - 110°C, and by retaining the pulpin these conditions typically for 30 - 300 minutes. During the treatment thekappa number of the pulp reduces typically by 2 - 9 units.When the sequence O-A-D-E-D has been used, the consumption of chlorine dioxidein the bleaching of hard wood pulp has reduced by 30 -40 % at the brightness ofISO 88 %.by 10 - 20 %.bleaching without an A stage.The corresponding consumption with soft wood pulp has reducedIn both cases, the yield remained nearly unchanged compared with1015202530CA 02264423 1999-03-05-4-The object of the present invention is to provide an alternative method ofcombining a hot acid stage with a bleaching sequence utilizing chlorine dioxide.A further object of the invention is to provide a method which may be appliedin an economical way particularly in the bleaching plants of existing pulp mills.In other words, the object of the invention is to fit chlorine dioxide bleachingof pulp into a bleaching plant in an overall economical and environmentallyacceptable way.To achieve the objects mentioned, the method of the invention is characterizedin that the chlorine dioxide stage comprises successively at least a first chlorinedioxide step and an acid treatment step and that the pH of the first chlorinedioxide step is adjusted so that the final pH of the step is over 4. Theconditions during the first chlorine dioxide treatment are such that thehexenuronic acid groups in the pulp do not react with chlorine dioxide.It has been disclosed above that hexenuronic acid groups are removed by hotacid treatment prior to the chlorine dioxide treatment. According to theinvention we have now discovered that the ClO2 treatment can be performedin the bleaching sequences also before the hot acid treatment if reactionsbetween the hexenuronic acid groups of the pulp and the chlorine dioxide areprevented. Thus, the D stage comprises successively a D step and an A step.According to the method of the invention, the chlorine dioxide step of the DAstage is performed so as to have a final pH of over 4, preferably over 5. Thehighest pH value is normally 7. Conventionally, the final pH of the first or thesecond chlorine dioxide bleaching stage is less than 4, typically 1 - 3.5.Surprisingly, it has been found out that hexenuronic acids do not react withchlorine dioxide at the pH range of over 4 and thus no chemical is consumedby reactions with chlorine dioxide. Chlorine dioxide is reduced to chlorite butit does not decompose further. The chlorine dioxide dose in this step ispreferably 0.1 - 1.5 % active Cl (1 - 15 kg active Cl/admt), preferably over 0.5- 1.0% active Cl (5 - 10 kg active Cl/admt).In the acid treatment (A step), the conditions are typically as follows:10152025CA 02264423 1999-03-05- pH 2 - 5, preferably 2.5 â 4â temperature over 80°C, preferably 90 - 110°Câ time 30 â 300 minutes, preferably at least t minutes,t = 0.5 exp(10517/(T+273) - 24) (t = 0.5 eââ°5â7"â273ââ24â), in whichT °(C) is the temperature of the acid treatment.According to the invention, the temperature in the D step of the DA stage ispreferably over 70°C, preferably 80 - 100°C, which is higher than thetemperature of a conventional D stage. Thus, the temperature in the D and theA steps is essentially the same and there is no remarkable need to cool or toheat the pulp between the steps which is advantageous for the energyeconomy. However, the invention is not limited to the high temperature butthe D step may be performed also at the conventional D step temperature ofbelow 70°C.The treatment time in the D step of the invention is short, less than 10minutes, preferably 30 seconds to 3 minutes. Conventionally, the chlorinedioxide treatment time is over 30 minutes, even 120 minutes depending on thetemperature and therefore the treatment requires a reactor of its own. The DAstage according to the invention may be practiced by performing the acidtreatment in the reactor tower but because of the short retention time the Dstep preceding the A step may be arranged to take place for example in thefeed line of the A tower. The pulp flowing in the line is heated to the desiredtemperature, for example 90°C, and chemicals, such as chlorine dioxide, andalkali or acid if necessary to adjust the pH, are mixed into it. The pulp flowsunder these conditions for the required period, e.g. one minute, andsubsequently, acid is added to the pulp and it is supplied into the acid tower.By providing the chlorine dioxide treatment before the acid treatment, the aciddemand in the A step reduces as the reactions of the chlorine dioxide with thelignin produce hydrochloric acid and organic acids as byproduct.1015202530CA 02264423 l999-05- 13.5.it is characteristic of a preferred embodiment of the invention that the chlorinedioxide stage as described above comprises in addition to the D and A stepsdescribed above also a second D step, i.e. the stage in question comprises afirst chlorine dioxide step, a hot acid treatment step and a second chlorineD/A/D).performed in acidic conditions, in other words the pH may be less that 2 butdioxide step (DAD, could be marked also The second step ispreferably it is 2 â 4. Thus, no essential adjustment of the pH is needed afterthe A step. Adjustment of the temperature is not necessary, either, but theentire chlorine dioxide stage may be effected essentially at the sametemperature as the A step and no essential adjustments of the temperature areneeded between the steps. Thus facilitates remarkably practicing the step asfar as process technology is concerned.The treatment time in the second D step is preferably also short, less than 10minutes, preferably 1 - 5 minutes and it may be performed for example in thedischarge line of the A step prior to the washer following the DAD stage.Chlorine dioxide is added to the pulp flowing in the discharge line and thetemperature and the pH are adjusted if necessary. The required chlorinedioxide dose is typically 0.5 â 2.0 % active Cl (5 - 20 kg active Cl/admt). Theacid treatment removes hexenuronic acids from the pulp and therefore theamount of chlorine dioxide chemical required is smaller than with pulp fromwhich hexenuronic acids have not been removed.In this (DAD) embodiment of the invention, the chlorine dioxide treatment maybe considered to have been divided into two relatively short partial treatments.Both steps may be practised at a high temperature; therefore the temperatureand the treatment time must be chosen so as to cause as little viscosity lossesin the pulp as possible. Two partial treatments contribute to a morehomogenous brightness of the pulp.The DA stage may be practiced also by arranging for the pulp a chelatingtreatment with EDTA, DTPA or a corresponding compound after the A step inorder to remove harmful metals; in this case the stage is DAQ. Providing a10152025CA 02264423 1999-03-05-7-chelating treatment in connection with the DA stage is advantageousparticularly if the bleaching sequence comprises later a bleaching stage with achemical such as peroxide which would be disturbed by heavy metals. The 0step may be performed at essentially the same temperature as the precedingD and A steps. The pH may be in the range of 3 - 6.The total amount of chlorine dioxide needed in the DA stage is not greater thanthe amount needed in an AD stage although the first D step is performedbefore the acid treatment. The final pH of over 4 in the first D step preventsreactions between the CIOZ and the hexenuronic acids. A DA stage hasproduced pulp of at least as good quality as an AD stage. An advantageprovided by a DA stage or a multistep DAD stage is that, compared with an ADstage, a DA or a DAD stage may be effected with simpler apparatus as eachstep does not require a tower of its own. However, the invention may bepractised also so that one or both of the D steps are performed in towers oftheir own.A disadvantage of the AD step may be the release of malodorous gases in theacid step if the pulp still after washing contains adequate amounts of sulfurcompounds from the digestion. When the D step precedes the A step,malodorous gases are oxidized by the bleaching chemical during the DA stage.The method of the invention is applicable for pulps manufactured by the sulfateprocess and other alkaline methods which pulps contain hexenuronic acidcompounds. The pulp to be treated has preferably been oxygen delignified.The DA stage according to the invention may be at the beginning of thebleaching sequence for example after the oxygen delignification or later in thesequence for example after a peroxide stage. Therefore the invention isadvantageously applied for example in connection with the following bleachingsequences or partial bleaching sequences:digestion - O - DAD - E;10152025CA 02264423 1999-03-05-3-digestion - O - DAD - E - DN- D; ordigestion - O â Q - OP - D/A/Q - P0, in which- indicates a wash between the stages,0 indicates a delignification stage utilizing at least oxygen,0 indicates removal of metals by chelating,A indicates a hot acid treatment,E indicates an alkaline stage, andOP or PO indicate a bleaching stage based on oxygen and/orperoxide, possibly at least part of the steps being pressurized and0 indicating oxygen chemical and P peroxide, and the first letterin each step signifying the main effective bleaching chemical andthe second letter possibly used the bleaching chemical supportingthe bleaching reaction.The method of the invention will be described more in detail in the followingwith reference to the enclosed drawing figure which illustrates by way ofexample a preferred apparatus for carrying out the method of an embodimentof the invention. Pulp is treated in a DAD stage.Pulp is transferred from a preceding treatment stage 10 by means of a highconsistency pump 12 to an acid tower 18. The consistency of the pulp is 6 -25 %, preferably 8 - 18 %. The preceding stage may be a wash followingdigestion or more often an oxygen delignification following digestion or a washfollowing oxygen delignification.Alkali or acid is added to the pulp in order to adjust the initial pH for thechlorine dioxide treatment or the first D step so that the final pH is over 4,preferably over 5, in order to prevent reactions between chlorine dioxide andhexenuronic acids. The alkali and the chlorine dioxide may be added directlyto the pump 12, it may be injected into duct 15_between the pump 12 and thetower, or to a particular mixer 14 provided for this purpose. The typical chlorinedioxide amount added is 0.5 -1.5 % active Cl.10152025l 1 1CA 02264423 2002-05-17-9-A Steam is added to the pulp in order to raise the temperature typically up to 80 - 100°C.Steam may be added prior to the pump 12 in a steam mixer (not illustrated) or mixed inthe duct 15 following the pump 12. Alternatively, pulp may be heated indirectly in theduct 15.Thus, the first D step is performed in the feed line 15 of the acid tower while the pulpflows in the line. The treatment time in this step is typically 30 seconds to 3 minutes.The D step is followed by an acid treatment in the tower 18. After the chlorine dioxidetreatment the pH of the pulp is decreased to the level 2 - 5 required by theacid treatment by adding acid (sulfur acid, hydrochloric acid, waste acid from theproduction of chlorine dioxide or any suitable organic acid) to the pulp in the mixer 14.Also the temperature may be adjusted if necessary but according to the invention thesteps of the DAD stage are preferably effected at the same temperature, for example95°C, whereby no essential temperature adjustment is required between the steps.The feed and flow of pulp to the acid tower should be as even as possible by using adistributor16 or a doctor. The distributor mentioned is disclosed for example inU.S. patent no. 4,964,950, published on October 23, 1990, andabove purpose is described in Finnish patent publication no. 94442, published onMay 31, 1995.its use for theIf the device 16 has adequate mixing properties and if desired, acid may be addedthrough it and thus even avoid the use/purchase of the device 14. Pulp flows from thedevice 16 to the reactor tank 18 dimensioned for the treatment time required by theacid treatment, for example 120 minutes at 95°C. Using the distributing feederdescribed in the FI publication no. 94442 mentioned ascertains that all portions of thetower are filled evenly and that the pulp column rises up evenly in the tower so thatharmful chanelling can not occur. Correspondingly, the top of the tower has beenprovided with a discharger 20 or a discharge doctor to guide pulp to the dischargeline 24 of the tower.10152025CA 02264423 l999-05- 13-10-The second D step is performed in the discharge line 24 in order to bleach thepulp. Chlorine dioxide may be added to the pulp via the acid tower discharger20 of a pump 22 in the discharge line. Also a separate mixer (not illustrated)may be arranged in the line 24 for addition of chemical which is about 0.5 â2.0% active CI.The addition of chemicals to the pulp is fitted so that a proper treatment timefor carrying out the D step in the discharge line 24 before a washer 26 isreached. The retention time is less than 10 minutes, preferably 1 - 5 minutes.The treatment temperature of this D step is preferably the same as the one inthe previous D and A steps; thus no essential adjustment of the temperatureafter the A step is needed here either. If desired, pulp may be heated or cooledin a heat exchanger or by direct feeding of steam. A higher temperature thanconventionally is, however, required in order to obtain adequate bleachingduring the short treatment time mentioned.The pH of the second D step is lower than that of the first D step. Typicallythe pH is about 2 - 4; thus adjustment of the pH is usually not needed after theA step. If necessary, the chemical adjusting the pH may be added in the samestage as the chlorine dioxide.Pulp has been bleached in a DAD stage the way described above andsubsequently the pulp is treated in the washer 26 and is transported to furthertreatment. Typically the subsequent stage is an E stage.Example:Oxygen-bleached pulp was tested in a laboratory, the kappa number of whichwas 11.9, viscosity 1061 ml/g and the ISO brightness 49.2 %. The pulp wastreated with the following sequences:1_-_D_A1o-_DN;QD: - CIOZ dose 1.0 % as active Clâ time 1 minute1015202530CA 02264423 1999-03-05-11-final pH 5.3temperature 95°Ctime 180 minutespH 3.5temperature 95°CCIOZ dose 1.25 % as active Cltime 2 minutesfinal pH 2.5temperature 95°C85°C, 60 minutes, 1.25 % NaOH, O2 4 bar, final pH 11.7kappa number after treatment 2.3viscosity after treatment 890 mg/Ibrightness after treatment 70.2 % ISO75°C, 180 minutes, 1.8 % CIOZ as active CI, 0.3 % NaOH, finalpH 3.3N (neutralization) pH 8.375°C, 180 minutes, 0.8 % CIOZ as active CI, 0.1 % NaOH, finalpH 5.1Bleached pulp: viscosity 868 ml/g, brightness 89,9 % ISOin the second test the same pulp was treated2. AD - E _;_lQNâ_D A: -time 180 minutespH 3.5temperature 95°CCIOZ dose 2.25 % as active Cltime 1 minutefinal pH 2.1temperature 95°C85°C, 60 minutes, 1.25 % NaOH, O2 4 bar, final pH 11.6kappa number after treatment 2.7viscosity after treatment 890 mg/l1015CA 02264423 1999-03-05-12-â brightness after treatment 69.4 % ISODN â 75°C, 180 minutes, 1.8 % CIOZ as active CI, 0.3 % NaOH, finalpH 3.4- N (neutralization) pH 8.3D â 75°C, 180 minutes, 0.8 % CIOZ as active CI, 0.1 % NaOH, finalpH 5.1Bleached pulp: viscosity 866 ml/g, brightness 89,9 % ISO.Based on the test we may state that a DA stage and an AD stage according tothe invention produce pulp of the same quality. Some practical advantagesdescribed above may be obtained by effecting the chlorine dioxide and the acidtreatment according to the invention.' While the invention has been herein shown and described in what ispresently conceived to be the most practical and preferred embodiment, itwill be apparent to those of ordinary skill in the art that many modificationsmay be made thereof within the scope of the invention, which scope is tobe accorded the broadest interpretation of the appended claims so as toencompass all equivalent methods.
