PROCESS FOR BLEACHING OF LIGNO-CELLULOSIC MATERIAL

Abstract: The present invention relates to a process for bleaching of ligno-cellulosic pulp digested by a chemical process for reduced formation of chloro-organic compounds, while preserving the final brightness and strength properties of the pulp, by treating the pulp in a first stage with only chlorine dioxide at a charge factor of up to 2.0, whereupon the pulp is treated in an alkaline extraction stage reinforced with from 3.0 kg of hydrogen peroxide/ton of pulp. After the two-stage treatment the pulp can be finally bleached to the desired brightness in one or more chlorine dioxide stages.

Description: Process for bleaching of liqno-cellulosic material The present invention relates to a process for bleaching of ligno-cellulosic pulps for reduced formation of chloro-organic substances while preserving the final brightness and strength properties of the pulp, where,in a first treatment stage, only chlorine dioxide is used at a low charge factor, whereupon, in a second stage, the treatment with hydrogen peroxide is carried out under alkaline conditions with ' increased charging of peroxide. After the two-stage treatment the pulp can be finally blea¬ ched to the desired brightness in one or more chlorine dioxide stages. The term ligno-cellulosic pulp refers to chemical pulps from softwood and/or hardwood, delignified according to the sulphite, sulphate, soda, or organosolv process, or modifications and/or combinations thereof. Before the bleaching sequence with chlorine-containing chemicals according to the invention, the pulp may also have been delignified in an oxygen stage. Background In the production of chemical pulp of high bright¬ ness, wood chips are first cooked to separate the cellulose fibres. During the cooking part of the lignin holding the fibres together is degraded and modified such that it can be removed by subsequent washing. However, in order to achieve sufficient brightness, further lignin must be removed, together with brightness impairing (chromophoric) groups. This is frequently effected by delignification with oxygen, followed by bleaching in several stages. Bleaching of chemical pulps is mainly carried out with chlorine-containing bleaching agents, such as chlo¬ rine, chlorine dioxide and hypochlorite, and in addition to this in intermediate extraction stages with alkali and hydrogen peroxide and/or oxygen. The chlorine-containing substances react with the lignin and render it soluble, either in the same stage or in the subsequent alkaline extraction stage. The reaction products which are formed in the bleach- ing stages using chlorine-containing chemicals, result in discharges detrimental to the environment. These chloro- organic compounds can be measured, inter alia, as AOX (= adsorbable amount of organic halogen) , and the level can be estimated by the formula AOX= i x (C + D/k 2 ) (l) wherein C = total amount of charged Cl 2 D = total amount of charged CIO2 calculated as active chlorine ki = about 0.1, k 2 = about 5 To be able to compare the bleaching effect of the chlorine-containing substances, the amount of active chlo¬ rine is indicated, which is the oxidation potential multi¬ plied by the proportion of chlorine in the molecule. There- by, 1 kg of chlorine dioxide corresponds to 2.63 kg active chlorine. As can be seen, the influence of chlorine dioxide on the AOX formation is but a fifth of that of molecular chlorine. As the environmental legislation gets more strin- gent, a change to a higher proportion of chlorine dioxide has occurred iri the last few years. It is to be expected that the permissible discharge level will be further redu¬ ced, which makes it desirable to find chlorine-free bleach¬ ing processes, which still satisfies customers desire regarding the brightness of the final product. A normal bleaching sequence for softwood treated in accordance with the sulphate process is (C + D) El D E2 D wherein (C + D) = addition of chlorine (C) and chlorine dioxide (D) in the same stage D = chlorine dioxide stage El = alkaline extraction stage, optionally with addition of peroxide (EP) and/or oxygen (EPO and EO, respectively) E2 = alkaline extraction stage, optionally with addition of peroxide (EP) The (C + D) and El stage is defined as a prebleaching sequence. The sequence D E2 D is called final bleaching. In the present application, final bleaching may also be rest¬ ricted to one or two D stages. The above-mentioned oxygen delignification is normally regarded as part of the diges¬ tion process. The amount of active chlorine used/required in the (C + D) stage is defined as the charge factor CF according to CP = total active chlorine in kg/ton of pulp/ (2) kappa number before the (C + D) stage The amount of chloro-organic compounds formed is reduced by a combination of a changeover from molecular chlorine to chlorine dioxide and/or reduced charge factor according to formulae 1 and 2, respectively. As stated above, it is also possible to reduce the discharge of sub¬ stances detrimental to the environment by reducing the kappa number which is a measure of the amount of lignin, the organic substance to be dissolved out. Therefore, one also strives to further reduce, by different pretreatments and prebleaching stages, the amount of lignin that must be removed by chlorine bleaching. Changes in the prebleaching, such that mainly chlo¬ rine dioxide is used in the (C + D) stage, normally causes a decrease in the brightness of the ready-bleached pulp and thus a decrease in quality. Lowering the charge factor makes it even more. difficult to achieve sufficient final brightness. It is known from, for example, Germgard et al, Nordic Pulp and Paper Research Journal, 3(4), 166-171 (1988) to use a few kg of hydrogen peroxide per ton of pulp in the El and E2 stages to be able to reduce the con¬ sumption of active chlorine with preserved final brightness (90% ISO). The bleaching sequence (D90 CIO) El D E2 D was used on a sulphate pulp of birch, at a charge factor of 2.4, which is a conventional level allowing no major limi- tation of the AOX formation. US 3,720,577 and DE 2,754,949 relate to the use of 100% chlorine dioxide in the first bleaching stage. In the first document, a charge factor of 2.7 is used in Example l which is a far too high value as stated above. The D stage is followed by a peracetic acid stage and optionally another D stage. The peracetic acid reduces the viscosity of the cellulose and yields acetic acid as a by-product, which substantially increases the COD-value (Chemical Oxygen Demand) which is another important measure of the discharges to the environment. In DE 2,754,949 a low charge factor is used in the D stage but there is no indication that the subsequent extraction stage must be reinforced with hydrogen peroxide to reach acceptable concentrations of AOX. Furthermore, these documents do not indicate the importance of using a substantially higher proportion of the chlorine dioxide in the final bleaching to get maximum benefit from the bleaching capacity and still produce a pulp with low impact on the environment. Alfthan et al, Svensk Papperstidning, 88(13), 24-27 (1985), have made tests where the El and, ' optionally, the E2 stage have been reinforced with hydrogen peroxide in a bleaching sequence in which the first stage is a D stage. According to laboratory experiments and mill trials con¬ ducted on birch sulphate pulps with a kappa number of 18, the total chlorine dioxide consumption during the final bleaching can be reduced by up to 20 kg, calculated as active chlorine, per ton of pulp. Peroxide charges of more than 2.5 kg H 2 0 2 per ton of pulp in the first extraction stage may have caused difficulties regarding the purity of the ready-bleached pulp. Furthermore, laboratory experi¬ ments conducted on sulphate pulp of softwood have shown that the effect of H 2 0 2 was considerably better in the second extraction stage (during the final bleaching) than during the prebleaching. However, such changes in the final bleaching affect the AOX discharges to a far lesser extent than the corresponding changes during prebleaching. Furthermore, it is known from Jiri Basta et al, TAPPI Proceedings, 1989 Pulping Conference, Book 2, pp 427-436, that the pulp must reach a kappa number of 3.5 or less already after the first extraction stage to make it possi¬ ble to achieve an acceptable final brightness. This res- triction reduces the possibility to use a high proportion of chlorine dioxide and a really low charge factor, as is shown in Fig. 9. Technical Problem It is previously known to reduce the discharge of chloro-organic compounds from bleach plants for chemical pulp by substituting chlorine dioxide for chlorine. This has been carried out to a large extent already, in the final bleaching. Thus, a normal bleaching sequence for a chemically digested ligno-cellulosic pulp consists today of (C + D) E D E D wherein the extraction stages optionally are reinforced. Prior art treatment techniques to further reduce the discharge of AOX (= adsorbable amount of organic halogen compounds) primarily consisted of costly external processes because of the limited success in achieving an acceptable final brightness by process modifications. It would of course be desirable if one could reduce the amount of AOX by substituting chlorine dioxide for chlorine also in the prebleaching and simultaneously to reduce the charged amount. However, it has so far not been possible to modify the subsequent extraction and final bleaching stages in such a manner that an acceptable final brightness of the pulp can be obtained. The Invention The present invention relates to a treatment process by which the discharge of AOX is greatly reduced, and the brightness and the strength properties after the final bleaching are simultaneously preserved. This treatment is carried out in two stages in which the pulp is first bleached with chlorine dioxide at a low charge factor, and a reinforced treatment with hydrogen --peroxide and, op¬ tionally, oxygen is carried out in the second " stage. This two-stage treatment results in a bleaching process which is far less detrimental to the environment in that the amount of chlorine-containing chemicals in the bleaching process is reduced. The invention thus relates to a process for treating ligno-cellulosic pulp as disclosed in the appended claims. 6 The invention relates to a process in the prebleaching of pulp, in which the combination of a low charge factor, a high proportion of chlorine dioxide, and reinforcement of the extraction stage with hydrogen peroxide jointly give the intended AOX reduction. The main difference in relation to prior art tech¬ nique is that the delignification in the chlorine-contain¬ ing stage of the prebleaching is not carried as far as has previously been deemed : necessary. By reducing the amount of chlorine dioxide per ton of pulp at the beginning of the bleaching sequence, the total AOX discharge can be re¬ duced to approximately 0.5 kg/ton of pulp, which is a con¬ siderable improvement compared to prior art technique. However, a prerequisite to obtain this AOX level, is the presence of larger amounts of hydrogen peroxide than have been used before, in order to obtain adequate brightness. Chlorine dioxide as the only chlorine-containing bleaching agent of stage 1, relates to chlorine dioxide produced by conventional techniques, without external addi- tion of chlorine, so-called technical chlorine dioxide. In other words, the chlorine dioxide may contain chlorine formed during the production and dissolved in the absorp¬ tion water. One example of industrial processes in which a certain amount of chlorine is formed, is the reduction of chlorate with chloride. Other chlorate reducing agents, such as sulphur dioxide and methanol, give but minor amounts of chlorine. The chlorine dioxide water from such essentially chlorine-free processes is especially prefer¬ red. In the two-stage process according to the invention, the first stage is carried out by treatment with technical chlorine dioxide at a charge factor of up to 2.0, pre¬ ferably within the range of from 0.6 to 1.8. The most preferred range for the charge factor is from 0.75 to 1.25. At the end of the first stage, the pH may lie within the range of from 1 to 4. The time and temperature are adjusted such that the concentration of residual chlorine is from 0.1 to 0.5 kg/ton of pulp. The pulp concentration may be from 1 to 40% by weight, preferably from 5 to 15% by weight. In the second stage, the pulp is treated with hydro¬ gen peroxide at a pH above 7, preferably at a pH in the range of from 7 to 13. Most preferably the pH lies in the range of from 8 to 12. The second stage can be reinforced with oxygen up to 10 kg/ton of pulp, but more peroxide remains unreacted after the extraction if no oxygen is used. This peroxide may continue to react with the pulp and may thus improve the properties and the economy. Hydrogen peroxide is charged in an amount of from 3.0 kg per ton of dry pulp. The upper limit is not critical and can for economical reasons be set to 20 kg per ton of dry pulp. ' The charge of hydrogen peroxide lies preferably in the range from 4.0 to 10 kg per ton of dry pulp. pH at the end of the second stage may lie within the range of from 8.5 to 13, preferably from 10 to 12. The residence time " may be from about 30 min. to about 6 h at a temperature of from about 50 to 130°C, preferably from 80 to 100°C. The pulp concen- tration may be from 1 to 40% by weight, preferably from 5 to 15% by weight. Ligno-cellulosic pulp relates to chemical pulps from softwood and/or hardwood digested according to the sul¬ phite, sulphate, soda or organosolv method, or modifica- tions and/or combinations thereof. The method according to the invention is applicable to such bleaching of the treated pulp where the bleaching sequence is preceded by delignification in an oxygen stage. After the two-stage treatment, the pulp can be finally bleached in known manner to the desired brightness with chlorine dioxide in the sequence D E D, D (EP) D, D D, D, or other combinations of chlorine dioxide stage (D) and extraction stage (E), optionally reinforced with hydrogen peroxide (P). A preferred embodiment of the invention is a bleaching sequence, comprising a first stage with chlorine dioxide at a charge factor of up to 2.0, a second stage with hydrogen peroxide at a pH above 7 and with a charge of from 3.0 kg per ton of dry pulp, and at least a third stage with chlorine dioxide, optionally with an intermediate extraction stage. Most preferred is a final bleaching sequence of two chlorine dioxide stages with intermediate extraction, optionally reinforced with hydrogen peroxide. According to the invention, it has also been shown that the reduction of AOX is improved when the ratio be¬ tween the amount of chlorine dioxide charged in the pre¬ bleaching sequence and the amount of chlorine dioxide charged in the final bleaching sequence, lies within the range of from ' 1:1 to 1:5, preferably from 1:1 to 1:3. Upon inspection of the quality of pulps made accor¬ ding to prior art technique and according to the present invention, it was found that the pulp viscosity number, which is a measure of the chain length of the carbohydra- tes, was unchanged. The high viscosity implies a high chain length and, thus, a strong pulp. Furthermore, the kappa number of the finally bleached pulp is at the same low level as the reference pulp, which means that the amount of undissolved lignin is low. The brightness after the final bleaching can be brought to about 89.5% ISO, which is a sufficient and acceptable level for this type of pulp. The brightness, measured as % ISO, is a standardized technique generally accepted by the cellulose industry. The invention and its advantages are illustrated in more detail by the following Examples which are intended only to illustrate the invention, without restricting it. In the Examples, use is made of an oxygen-delignified sulphate pulp of softwood, with kappa number 17.0, visco¬ sity 1040 dm 3 /kg, and brightness 33.3% ISO after the oxygen stage. The kappa number, viscosity and brightness of the pulp as well as the amount of AOX were determined throug¬ hout according to SCAN Standard Processes. The amount as well as the proportion of chlorine dioxide are calculated as active chlorine. Example 1 This Example is intended to show, for a chemical pulp, the effect of the proportion of chlorine dioxide (D) in the (C + D) stage on the amount of AOX formed in the (C + D) stage, the subsequent El stage, and in the final bleaching sequence D E2 D. The charge factor was 1.8 throughout. Treatment conditions: (C + D) stage: Final pH about 1.5 at 15 and 50% D and 2.5+/-0.3 at 100% D; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp El stage: Final pH 11.0+/-0.3; 90°C; 60 min; 2 kg H 2 0 2 /ton of pulp; 5 kg oxygen/ton of pulp D stage: Final pH 2.5+/-0.3; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp . ,;..•.-,.. -: E2 stage: Final pH 11.0+/-0.3; 60°C; 60 min; 2 kg H 2 0 2 /ton of pulp D stage: Final pH 3.4+/-0.2; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp. TABLE I Propn of D in (C + D) AOX (kg/ton of pulp) (%) C + D El D E2 D Total 15 1.5, 1.4 0.2 3.1 50 1.3 0.75 0.15 2.2 100 0.7 0.15 0.15 1.0 As is apparent from the Table, the greater part of the AOX discharge is generated in the prebleaching..In view hereof, it would seem more important to modify the chlorine-containing stage here rather than in the D stage of the final bleaching. According to formula l, the amount of AOX formed is reduced if the proportion of chlorine di¬ oxide in the first stage is increased. Surprisingly, the effect upon the AOX formation is far greater in the El stage than in the (C + D) stage. With 100% of charged chlorine dioxide, the amount of AOX in the first extrac¬ tion stage will be on a level . with the discharge from the entire final bleaching. In order to get below 1.0 kg AOX/- ton of pulp totally for the entire bleaching sequence, a change to 100% chlorine dioxide is not sufficient, but fur¬ ther measures are required, as illustrated in Example 2. Example 2 This Example is intended to show, for the oxygen- delignified pulp, the relationship between the. charge factor (CF) according to formula 2, the charging of hydro¬ gen peroxide in the El stage, the discharge of AOX from the entire bleaching sequence, and quality properties. The bleaching sequence was D El D E2 D where the two first stages are included- in the prebleaching. 100% chlorine dioxide in varying amounts was used in the D stage of the prebleaching, as shown in the Table. A total of 32 kg chlorine dioxide/ton of pulp, calculated as active chlori¬ ne, was used in the final bleaching. Treatment conditions: D stage: Final pH 2.5+/-0.3; time and temperature adjusted to give a " content of residual chlorine of 0.1-0.5 kg/ton of pulp El stage: Final pH 11.0+/-0.3, 90°C; 60 min; 5 kg oxygen/- ton of pulp D stage: Final pH 2.5+/-0.3; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg of pulp E2 stage: Final pH 11.0+/-0.3; 60 "C; 60 min; 2 kg H 2 0 2 /ton of pulp D stage: Final pH 3.4+/-0.2; time and temperature adjusted to give a content of residual chlorine of 0.1-0.5 kg/ton of pulp. TABLE II Test CF H 2 0 2 Total AOX Final brightness kg/ton kg/ton % ISO 1 2.0 0 1.0 89.6 2 1.5 2 0.74 89.2 3 1.0 0 0.67 85.7 4 1.0 6 0.54 89.4 As can be seen from test 1 and 3, reducing the charge factor from 2 to 1 results in a major reduction of the total AOX discharge, but the brightness will be un¬ acceptable if the subsequent extraction is but an oxygen- reinforced alkali stage. If the bleaching process is de¬ signed in accordance with the invention (test 4), with substantial reinforcement of the subsequent El stage by a hydrogen peroxide charge of 6 kg/ton of pulp, the pulp obtained receives a final brightness acceptable to the market. This is despite the fact that the kappa number after this extraction stage is as high as 5.7, which accor¬ ding to Basta et al is an insufficient delignification. Thus, it has surprisingly proved possible to reduce the delignification in a pure D stage, while preserving the quality of the final product. At the same time,. ' the AOX formation obtained is clearly the most propitious. The final viscosity of the pulp varied in all tests within the range of from 916 to 920 dm 3 /kg, which shows that the process according to the invention does not adversely affect the strength properties (measured as vis¬ cosity) . In test 2 where CF=1.5 and the hydrogen peroxide charge was 2 kg/ton of pulp in the El stage, the AOX formation is still undesiredly high, although the final brightness is acceptable. Example 3 This example illustrates the effect of different charges of hydrogen peroxide in step 2 (El) on the forma¬ tion of AOX in the prebleaching and final bleaching and on the final brightness of the pulp with a process according to the invention where the chlorine factor _is 1.0. The treated pulp consisted of a non-oxygen delignified sulphate pulp of softwood, which, before the treatment, had a kappa number of 26.8, a viscosity of 1329 kg/dm 3 and a brightness of 30.0% ISO. The amount of chlorine dioxide is calculated as active chlorine. Bleaching sequence: DO El DI E2 D2, where DO El makes up the prebleaching sequence and DI E2 D2 the final bleaching sequence. The treatment conditions in DO and the stages of the final bleaching sequence were kept constant throughout test 1 to 5. The treatment conditions were: DO: 100% chlorine dioxide; Charge factor 1.0 corresponding to 27 kg of active chlorine/ton of dry pulp El: 0-20 kg hydrogen peroxide/ton of dry pulp; Reinforced with oxygen. DI: 25 kg of active chlorine/ton of dry pulp E2: 2 kg hydrogen peroxide/ton of dry pulp D2: 7 kg of active chlorine/ton of dry pulp The ratio between the amount of chlorine dioxide charged in the final bleaching sequence and prebleaching sequence, respectively, is 1.19. The results obtained are shown in the Table below. TABLE III Test H 2 0 AOX, kg/ton Final brightness, kg/ton DO El DI E2 D2 Total % ISO 0.50 1.02 85.4 0.42 0.93 86.3 0.34 0.86 87.0 0.30 0.81 88.4 0.19 0.70 90.1 As is apparent from the Table, an increased charge of hydrogen peroxide in the extraction stage reduces the AOX formation in the final bleaching and increases the pulp brightness.

Claims: Claims 1. A process for reducing the amount of chloro- organic substance formed during delignification and bleaching of ligno-cellulosic pulp digested by chemical processes, c h a r a c t e r i s e d in that the pulp, in a first stage, is bleached with chlorine dioxide as the only chlorine-containing bleaching agent with a charge factor of up to 2.0, whereupon, in a second stage, the pulp is treated with hydrogen. ' peroxide at a pH above 7, and with a charge of from 3.0 kg of hydrogen peroxide per ton of dry pulp. 2. A process according to claim 1, c h a r a c ¬ t e r i s e d in that the treated pulp is bleached to the desired brightness with chlorine dioxide in one or more stages, optionally with an intermediate extraction stage. 3. A process according to claim 2, c h a r a c ¬ t e r i s e d in that the treated pulp is finally bleached to the desired brightness with chlorine dioxide in two stages with an intermediate extraction stage, optionally reinforced with hydrogen peroxide. 4. A process according to claim l or 2, c h a r a c ¬ t e r i s e d in that the the ratio of the amount of chlorine dioxide charged in the prebleaching sequence to the amount of chlorine dioxide charged in the final bleaching sequence lies within the range of from 1:1 to 1:5. 5. A process according to claim 1, c h a r a c ¬ t e r i s e d in that the charge factor lies within the range of from 0.6 to 1.8. 6. A process according to claim l or 5, c h a r a c ¬ t e r i s e d in that the charge factor lies within the range of from 0.75 to 1.25. 7. A process according to claim 1, c h a r a c ¬ t e r i s e d in that the amount of hydrogen peroxide lies in the range from 4.0 to 10 kg of hydrogen peroxide per ton of dry pulp. 8. A process according to claim 1, c h a r a c ¬ t e r i s e d in that the second treatment stage is reinforced with oxygen. 9. A process according to claim 1, c h a r a c ¬ t e r i s e d in that the ligno-cellulosic pulp is a chemical pulp from softwood and/or hardwood digested according to the sulphite, sulphate, soda or organosolv method, or modifications and/or combinations thereof. 10. A process according to claim 9, c h a r a c ¬ t e r i s e d in that the ligno-cellulosic pulp after digestion and before the treatment according to claim l is treated in an oxygen-delignifying stage.

Inventors: ANDERSSON LENNART, BLOM CHRISTIAN, ANDERSSON, LENNART, BLOM, CHRISTIAN
Applicants: EKA NOBEL AB
Application year: 1991
Main classification: D21C9
IPC codes: D21C9