Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/001—Modification of pulp properties
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1005—Pretreatment of the pulp, e.g. degassing the pulp

Definitions

• the present invention relates to a method of treating cellulose pulps according to claim 1.
• Pulp mills have recently attempted to abandon the use of elementary chlorine, and partly also chlorine dioxide, the reasons for this being both aspects of environmental protection and market factors.
• Disadvantages caused by elementary chlorine include both noticeable malodorous gaseous emissions and liquid effluents from chemical pulp mills into atersystems. Affecting primarily atersystems, chlorine dioxide does not cause odour disadvantages on such a large scale.
• AOX number designating the loading on watersystems it can be noted that elementary chlorine is many times more detrimental than chlorine dioxide.
• bleaching of cellulose pulps is based on such bleaching chemicals as oxygen, peroxide or ozone
• removal of heavy metals forms an essential process stage.
• Detrimental metals include manganese, copper and iron, which catalyze reactions harmful to the quality of pulp. They degrade bleaching chemicals, which decreases the efficiency of bleaching and increases the consumption of chemicals.
• heavy metals are primarily bound to carboxyl acid groups.
• the acid stage causes the kappa number to drop in the peroxide stage, whereas no decrease in the kappa number has been found in the acid stage.
• the acid treatment could be effected even at a temperature of 100 °C, but this could result in pulp of poorer quality.
• the acid treatment is effected at a temperature of 10 to 95 "C, most preferably at 40 to 80 °C, and at pH 1 to 6, most preferably 2 to 4.
• the acid treatment is followed by a stage in which suitable alkaline earth metal is added.
• pulp can be treated with a suitable bleaching and/or delignification chemical, such as chlorine dioxide.
• Removal of detrimental metals may be made more efficient by using chelating agents for binding metals in connection with the acid treatment.
• One such method is disclosed in the SE patent 501651, which brings forward an acid treatment similar to that in the above-mentioned EP publication 511695, with the difference of the acid treatment being effected in the presence of a chelating agent.
• chelating agents used for binding metals contribute to rising the bleaching costs.
• the primary aim of the above-described acid treatments of pulp is to achieve such a composition of metals which is preferable for chlorine-free bleaching chemicals.
• the kappa number may be decreased by 1 to 2 units due to a washing and extraction phenomenon.
• the metal composition affects the consumption of bleaching chemicals, the reason for the use of known acid stages being therefore removal of metals from the pulp.
• An object of this invention is to eliminate or minimize the disadvantages of prior art and to achieve a totally new arrangement for bleaching cellulose pulps, in particular cellulose pulps manufactured under alkaline conditions, by means of either totally chlorine-free bleaching chemicals, or chlorine dioxide, which is still significant in pulp bleaching. Further, an object of the invention is to produce cellulose pulp easily bleached, e.g. by means of oxygen and/or peroxide.
• cellulose pulps contain 4-O-methyl- ⁇ -D- glucuronic acid groups (glucuronic acid groups) .
• glucuronic acid groups 4-O-methyl- ⁇ -D- glucuronic acid groups
• sulphate pulps contain in addition to glucuronic acid groups also a significant amount of 4- deoxy- ⁇ - -threo-hex-4-enopyranosyl uronic acid groups (hexenuronic acid groups) bound to xylan. The amount of these groups is in some pulps even substantially greater than the amount of known glucuronic acid groups.
• hexenuronic acid groups consume bleaching chemicals reacting electrophilically, such as chlorine, chlorine dioxide, ozone and peracids (Buchert et al., 3rd European Workshop on Lignocellulosics and Pulp, Swiss, 28.-31.8.1994).
• the hexenuronic acid groups do not affect the consumption of oxygen and hydrogen peroxide used as bleaching chemicals in alkaline conditions, because they do not react with said chemicals.
• no degradation of hexenuronic groups occurs in oxygen and/or peroxide bleaching.
• special problems as regards to pulps bleached with oxygen and/or peroxide are relatively low brightness, and/or a tendency of such pulps to undergo brightness reversion.
• our invention is based on the idea that by selectively removing hexenuronic acid groups from cellulose pulps in connection with bleaching it is possible to reduce the consumption of bleaching chemicals. Surprisingly, it has been discovered that at the same time, the brightness reversion tendency of pulp decreases. Also, bleaching becomes more selective, since the heavy metals can be removed more efficiently. Characteristics of the invention become apparent in the appended claims.
• Said selective removal of hexenuronic acid groups according to the invention is effected by adjusting the water suspensions of cellulose pulps slightly acidic - typically, the pH is set between about 2 and about 5 - and by treating the water suspensions at a raised temperature.
• the temperature is at least 85°C, most preferably at least 90 °C. Utilization of temperatures as high as this has previously been avoided in acid treatment, because it has been assumed that the quality of pulp would suffer.
• the primary purpose of acid treatment has been removal of detrimental metals. In above-described acid treatments, the purpose of which is removal of metals, the temperature does not play a significant role. What is significant is that the pH of the pulp is so low that metals separate from fibers.
• Duration of the treatment does not play a significant role in view of removal of metals, except insofar as it is sufficiently long, typically over 10 minutes. Extra time is not harmful for removal of metals but it naturally causes extra costs to the mill, since long treatment time requires use of larger tanks. Large tanks have been avoided also because it has been feared that the acid stage would harm the strength qualities of the pulp. Thus, long treatment times in connection with acid stages as mentioned in prior art only mean that a long treatment time does not have a harmful effect on removal of metals.
• cellulose pulp is treated in the presence of water at a temperature of at least 85 °C at a pH in the range from about 2 to about 5 (typically at a pH in the range from 2 to 5) in order to remove hexenuronic acid groups from the cellulose pulp.
• the pH value of the water suspension of the cellulose pulp is maintained between 2.5 and 4.
• the lowest pH values (2.5 to 3.5) are preferable for softwood and the highest (3 to 4) for hardwood.
• acids - inorganic acids e.g. mineral acids such as sulphuric, nitric and hydrochloric acid, and organic acids such as formic and/or acetic acid - may be used to set the pH value for slush pulp.
• the acids may be buffered, e.g. with the salts of the acids, such as formiates, in order to keep the pH value as even as possible during the treatment.
• the temperature ranging from 85 °C upwards.
• the temperature is kept at about 90 to 110 °C.
• 100 °C is a natural maximum limit. Even higher temperatures are possible if pressure vessels are used.
• the treatment may be effected in a bleaching tank under pressure of 200 to 500 kPa, at a temperature of 110 to 130 °C.
• the maximum limit of the temperature is usually set to about 180 °C.
• T (°C) is the temperature of the acid treatment.
• the degradation of hexenuronic acid groups is in accordance with the first- order reaction kinetics.
• t is between 5 minutes and 10 hours.
• the treatment is practised under atmospheric conditions.
• the typical treatment time at a temperature of 90 °C is about 1.5 to 6 hours, at 95 ⁇ C about 50 minutes to 5 hours, at 100 ⁇ C about 0.5 to 4 hours.
• the intention is to remove as large a part of the hexenuronic acids as possible, preferably at least about 50 %, especially preferably at least about 75 %, and most suitably at least about 90 %.
• the concept "pulp contains a small amount of hexenuronic acids at the most” means that the amount of hexenuronic acids is 50 % at the most, especially preferably 25 % at the most, and most suitably 10 % at the most of the amount which is present after cooking in corresponding pulp which has not been treated.
• the treatment may be effected as continuous treatment in a flow-through reactor, or as batch treatment. Pulp is treated in the presence of water, in other words the pulp received from the pulp cooking process is slushed into water so that the consistency of the slush in the pre-treat ent according to the invention is about 0.1 to 50 %, preferably about 1 - 20 %.
• the pre-treatment is preferably effected subject to the mixing. In continuous mixing, stationary mixers may be used.
• the arrangement according to the invention may be applied to pulps which are produced by means of a sulphate process or other alkaline methods and contain hexenuronic acid groups.
• sulphate process means a cooking method, the primary cooking chemicals of which are sodium sulfide and sodium hydroxide.
• Other alkaline cooking processes include for example extended cooks based on extending conventional sulphate cooking until the kappa number of the pulp has dropped below the value of approximately 20. These methods typically comprise oxygen treatment.
• Extended cooking methods include for example extended batch cooking ( +AQ) , EMCC (extended modified continuous cook), batch cooking, Super-Batch/0 2 , MCC/0 2 and continuous cooking/0 2 .
• hexenuronic acids form about 0.1 to 10 mol- % of the hydrolysis products of the xylanase treatment of softwood pulp received from said cooking methods. After the pre-treatment according to the invention the concentration of hexenuronic acids will drop to about 0.01 to 1 mol-%.
• the term "in connection with bleaching” means that the acidic pre-treatment is effected either prior to bleaching, during bleaching or, at the latest, after bleaching.
• substances reacting electrophilically e.g. chlorine, chlorine dioxide, ozone or peracids
• the treatment is practised to unbleached pulp in order to change characteristics, e.g. bleachability, of cellulose pulp.
• oxygen gas and/or peroxide in bleaching or bleaching treatment
• the pre-treatment may be effected between the bleaching stages of a bleaching sequence.
• Z ozone treatment (ZQ meaning that complexing agent is added in ozone treatment)
• the pre-treatment according to the invention is effected in a bleaching sequence either prior to an oxygen or peroxide stage, or subsequent to that, but prior to a chlorine dioxide stage r ozone stage or peracid stage (e.g. a formic acid or peracetic acid stage), in order to reduce the consumption of ozone and/or peracids. Since it is possible to improve bleachability of pulps by means of the pre- treatment, the invention enables the consumption of said bleaching chemicals to be decisively reduced, and/or the use of chlorine dioxide, ozone or peracids in bleaching to be eliminated.
• the treatment may be effected either prior to this oxygen stage, or subsequent to it, preferably subsequent to the oxygen stage.
• stage D following stage A may be carried out without washing between the stages, in other words the sequence is hereby 0-AD-E-D.
• ozone consumption caused by hexenuronic acids (HexA) and thereby also the saving in the chemical consumption achieved by means of the method according to the invention can be calculated theoretically by taking into account that the hexenuronic acid consumes an equivalent amount of ozone (1 eq 0 3 /HexA).
• the saving in the consumption is 1 to 3 kg 0 3 per ton pulp.
• furan derivatives forming out of hexenuronic acids consume twice the amount of ozone, and therefore it is preferable to wash the pulp as efficiently as possible after the acid treatment, prior to the bleaching stage.
• Everything above concerns also all other chlorine- free electrophilic bleaching chemicals, such as peracetic acid, persulphuric acid and peroxomolybdates.
• Reducing the consumption of the bleaching chemical by means of the acid treatment is based on the fact that in removal of hexenuronic acids, the amount of reactive acid groups in bleaching is decreased, and thus there will also be less material to be bleached.
• the primary bleaching chemical used is peroxide-containing substance (usually hydrogen peroxide).
• peroxide-containing substance usually hydrogen peroxide.
• the brightness reversion tendency of which, expressed as a pc-number, is smaller than 2.
• the brightness reversion tendency cannot be prevented by any other efficient way than by removing hexenuronic acids. Since in the acid treatment according to the invention also detrimental heavy metal concentrations may be reduced, it is preferable to effect the acid treatment prior to the first P-stage.
• Peroxide treatment is most suitably accompanied by oxygen gas pre- treatment.
• the pH of the slushed pulp treated with oxygen is first set to the value of about 3 - 4 and the temperature of the pulp is raised to 90 - 130 °C, at which temperature it is kept at least 5 minutes, subsequent to which it is treated with hydrogen peroxide under alkaline conditions in order to produce bleached pulp.
• a peroxide-containing substance may be for example Caro's acid or a corresponding substance, which degrades in suitable conditions (e.g. alkaline conditions) forming hydrogen peroxide or peroxo-ions.
• the pre-treatment according to the invention may be effected in the presence of chelates which bind heavy metals.
• EDTA and DTPA may be mentioned as examples of these chelating agents.
• chelating agents are dosed into the pulp in the proportion of about 0,2 % of the pulp. It can be mentioned, though, that one special advantage of the acidic pre-treatment according to the invention is that metals can be removed rather efficiently even without chelating agent treatment, as is disclosed in Example 10.
• the acidic pre-treatment may also be practised to unbleached or bleached pulp to modify characteristics relating to the qualities of paper.
• water retention capacity of the pulp can be decreased, whereby it is possible to produce stiffer pulp applicable for use in packing boards, for example.
• Fig. 1 graphically illustrates the effect of acidity on the hydrolysis velocities of arabinose acid groups and hexenuronic acid groups of pine sulphate pulp at a temperature of 80 °C.
• Theoretical curves have been fitted to experimental points in accordance with the equations illustrated in Example 2 respectively.
• Fig. 2 illustrates the dependency of the time needed for removal of hexenuronic acid groups on the temperature at a scale of 80 to 140 °C, birch sulphate pulp having been treated with acid at pH 3.5. At this pH the reaction velocity is nearly maximal. At higher pH values the retention time shall be longer at a certain temperature.
• the three upper curves illustrate the optimal operating range, wherein 95, 90 and 80 % of the hexenuronic acid groups have been removed.
• the broken line illustrates the lowest limit of the retention time, where 50 % of the hexenuronic acid groups have been removed.
• the kappa numbers of the pulps have been defined according to standard SCAN-C 1:77, the viscosity according to standard SCAN-CM 15:88, and the brightness according to standard SCAN-C 11:75.
• the brightness reversion tendency is measured by means of a dry heating method (24 h, 105 °C). The pc number was counted from the results.
• 4-0-methylglucuronoxylan isolated from hardwood was treated in 1 M sodium hydroxide liquor at a temperature of 160 °C for 2 hours.
• the liquor was cooled and the xylan precipitated from the liquor by adjusting the liquor neutral.
• the precipitated xylan was washed and dried, subsequent to which it was treated with endoxylanase.
• the hydrolysate was fractionated by using anion exchange chromatography and gel filtration. In this way, the oligosaccharide fraction was isolated, which fraction was by means of NMR spectroscopy discovered to contain 4-deoxy- ⁇ -L- threo-hex-4-enuronoxylotriose (80 %) and -tetraose (20 %).
• oligosaccharide liquor was dissolved into 10 mM acetate buffer (pH 3.7) in deuterium oxide. The liquor was inserted into an NMR tube and changes therein were followed by means of l H NMR spectroscopy at a temperature of 80 °C for 17 hours.
• the hexenurosidic linkages may be selectively hydrolyzed under mild conditions without significant hydrolysis of xylosidic linkages.
• glucosidic and mannosidic linkages of cellulose and glucomannan being stronger than xylosidic linkages of xylan, are stable in these conditions.
• Pine sulphate pulp (kappa number 25.9) was incubated in buffered liquors (pH 1.5 - 7.8) at different temperatures (25, 50 and 80 °C) for 2 hours. Subsequent to the treatments, the pulp samples were washed with water. The washed pulps were treated with xylanase, and the hydrolysates were analyzed by means of X H NMR spectroscopy.
• hexenuronic acid groups of the cellulose pulp may be selectively removed under slightly acidic conditions (pH > 2) at a raised temperature. Partial hydrolysis of arabinose groups occurs, but the loss in yield caused by this is diminutive due to the low concentration of arabinose in cellulose pulps (softwood pulps 1 %, hardwood pulps 0 %).
• hexenuronic acid groups can be removed from sulphate pulp, due to which the kappa number used for representing the delignification grade drops significantly.
• a similar reduction can be expected to occur in the consumption of electrophilic bleaching chemicals reacting with hexenuronic acid groups.
• Pine sulphate pulp bleached with oxygen and peroxide (9 g, kappa number 5.3) was treated in 0.06 M formiate buffer ( pH 3.2, 600 ml) at a temperature of 100 °C for 2.5 hours. Degradation of hexenuronic acid groups was followed by means of absorption of light (250 nm, € 8,700) caused by 2- furan-carboxylic acid.
• the total amount of hexenuronic acid groups was calculated to be 48 meq/kg of pulp. All hexenuronic acid groups were removed from the pulp in the reaction time of about 30 minutes. The treated pulp was filtered in a B ⁇ chner funnel, and washed with water. Compared with the original pulp, the treated pulp was infiltrated very easily. The kappa number of the treated pulp was 2.3. The kappa number of sulphate pulp bleached with oxygen and peroxide according to the invention is very low after treatment removing hexenuronic acid groups. The treatment according to the invention significantly improves possibilities to produce full-bleached TCF pulps without ozone bleaching.
• Birch sulphate pulp (100 g, kappa number 11.5) bleached with oxygen was mixed in water (3 1).
• the pH of the suspension was adjusted to the value 3.4 by adding 2 ml strong formic acid.
• the suspension produced in this way was incubated at a temperature of 100 °C for 4 hours.
• the amount of the removed hexenuronic acid groups was calculated to be 54 meq/kg of pulp, which is approximately 98 % of the total amount of hexenuronic acid groups of the pulp.
• the kappa number of the treated pulp was 6.2.
• Chelating with EDTA (0.2 % of the pulp) was carried out to both treated and non-treated pulp at a concentration of 3.5 %.
• the treatment was practised at a temperature of 60 °C, the duration thereof being 45 minutes.
• the brightness reversion tendency of the pre-treated pulp was, expressed as a pc number, over 50 % lower than the brightness reversion tendency of the non-treated pulp.
• Unbleached birch sulphate pulp (kappa number 15.4) was treated with formic acid at a concentration of 5 % so that the pH of the slush was 3.0, 3.5 or 4.0.
• the pulps treated in this way were incubated in 150 ml-pressure vessels at temperatures of 85, 95, 105 and 115 °C for 0.2 - 24 hours.
• Disengagement of hexenuronic acid groups was followed by defining the concentrations of furan derivatives having formed out of hexenuronic acid groups in the filtrate.
• the kappa number and viscosity were defined of the incubated pulps. The decrease in the kappa number was in a linear way dependent on the decrease in the hexenuronic acid concentration.
• the maximal reduction of hexenuronic acid concentration was 60 meq/kg, corresponding to a 6.3-unit reduction of the kappa number. 90 % of the hexenuronic acid groups being removed, the yield of the treatment was 98 % calculated on the basis of TOC.
• the degradation of hexenuronic acid groups was in accordance with the first order reaction kinetics.
• the minimum retention time (reduction of 50 % in the hexenuronic acid concentration) required by the treatment, and the optimal retention time (reduction of 80 - 95 % in the hexenuronic acid concentration) are illustrated by means of curves fitted to experimental points (Fig. 2) . At pH 3.0 - 3.5 the degradation velocity of hexenuronic acid groups was very close to its maximum value. At higher pH values the retention times required are longer due to a slower reaction velocity.
• Birch sulphate pulp (kappa number 10.3) bleached with oxygen was treated under conditions according to Example 8 to remove hexenuronic acid groups. The kappa number after the treatment was 5.4. Both acid-treated and non-treated pulp was bleached with DED sequence using several doses of chlorine dioxide and alkali. Being bleached to the brightness level 88.0 % ISO, the acid-treated pulp consumed 2.5 % chlorine dioxide calculated as active chlorine, and 1.4 % sodium hydroxide. The corresponding consumption percentages of chlorine dioxide and sodium hydroxide by the non-treated pulp were 4.3 and 0.8, respectively. The yield of the DED sequence was 97.1 % for the acid-treated pulp and 95.5 % for the non-treated pulp.
• Chelating with EDTA (0.2 % of the pulp) was carried out to both non-treated and treated pulp at a concentration of 3 %.
• the treatment was effected at a temperature of 50 °C, the duration thereof being 45 minutes.
• the metal concentrations of the pulps were defined with an atomic absorption spectrophotometer.
• the treatment removing hexenuronic acid groups decreased especially iron and manganese concentrations of the pulp (Table 2).
• the decrease in iron was in this case significantly greater than when using chelate treatment, and even the decrease in manganese was as great as when using chelate treatment.
• chelating agents may be replaced either partly or totally with treatment removing hexenuronic acids. If chelating agents are used, it is preferable to add them in connection with the treatment removing hexenuronic acid groups.

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Citations (3)

• 1994
  • 1994-10-13 FI FI944808A patent/FI102301B/fi active
• 1995
  • 1995-10-12 PT PT95934164T patent/PT786029E/pt unknown
  • 1995-10-12 CA CA002160430A patent/CA2160430C/en not_active Expired - Lifetime
  • 1995-10-12 NZ NZ293983A patent/NZ293983A/en not_active IP Right Cessation
  • 1995-10-12 RU RU97107353A patent/RU2126862C1/ru active
  • 1995-10-12 CN CN95196599A patent/CN1075143C/zh not_active Expired - Lifetime
  • 1995-10-12 EP EP95934164A patent/EP0786029B1/en not_active Revoked
  • 1995-10-12 AU AU36559/95A patent/AU3655995A/en not_active Abandoned
  • 1995-10-12 ES ES95934164T patent/ES2147303T3/es not_active Expired - Lifetime
  • 1995-10-12 WO PCT/FI1995/000566 patent/WO1996012063A1/en not_active Application Discontinuation
  • 1995-10-12 DE DE69517532T patent/DE69517532D1/de not_active Expired - Lifetime
  • 1995-10-13 ZA ZA958655A patent/ZA958655B/xx unknown
  • 1995-10-13 SE SE9503595A patent/SE518080C2/sv not_active IP Right Cessation
• 1997
  • 1997-04-10 FI FI971508A patent/FI971508A0/fi unknown
  • 1997-04-11 NO NO971682A patent/NO971682L/no not_active Application Discontinuation

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