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
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1005—Pretreatment of the pulp, e.g. degassing the pulp
• • 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
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1036—Use of compounds accelerating or improving the efficiency of the processes
• • 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
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• the pulp is bleached in one or more stages.
• mechanical pulps have been bleached with chlorine-free bleaching agents, the intention in this case being to remove chromophoric groups while preserving the content of lignin.
• chlorine-free bleaching agents such as hydrogen peroxide, peracetic acid and ozone
• bleaching with chlorine-free bleaching agents is less effective.
• hydrogen peroxide bleaching in alkaline environment is disturbed by the presence in the pulp of ions of certain trace metals, primarily Mn, Cu and Fe. These metal ions cause degradation of hydrogen peroxide into undesirable products, thereby reducing the effectiveness of the peroxide bleaching and increasing the consumption of peroxide.
• the prior art teaches bleaching of chemical as well as mechanical pulps in the presence of magnesium salts.
• Most of the known bleaching processes do not comprise acid pretreatment, which preserves the content of undesirable trace metals in the pulp.
• bleaching with chlorine-free bleaching agents is normally performed in alkaline environment. As a result, it becomes impossible to retain or reintroduce the especially desirable magnesium ions, since these are precipitated at an alkaline pH and thus cannot diffuse into the pulp to such an extent that a pulp of high brightness and strength can be obtained.
• US-A-3507744 and JP-A-52063402 disclose the bleaching of high-yield pulps and chemical pulps with peroxide solutions where the pulps are subjected to alkaline earth metal pretreatments.
• non of the two documents teach an acidic pretreatment in the presence of a magnesium compound.
• the invention provides a process for treating chemically digested lignocellulose-containing pulp under the conditions-disclosed in the claims, whereby the content of trace metal ions in the pulp is selectively aitered to render more effective the subsequent bleaching with a peroxide-containing compound.
• the invention relates to a process for bleaching of chemically digested lignocellulose-containing pulp, whereby the pulp is treated with about 500 up to about 4000 ppm, based on dry pulp, of a magnesium compound in dissolved form at a pH within the range of from about 2 up to about 6, whereupon the pulp is washed wherein the pH of the washing liquid is equal to or higher than the pH of the treatment with the magnesium compound and subsequently the pulp is bleached with hydrogen peroxide at a pH in the range from 8 up to 12 in the presence of a calcium compound wherein the amount of calcium compound added in the bleaching is in the range from 100 ppm up to 4000 ppm, calculated as part by weight of alkaline earth metal on part by weight of dry pulp.
• the initial treatment may be carried out in the presence of at least one more compound containing an alkaline earth metal.
• the initial treatment is performed at a pH of from 2 up to 6, and preferably from 2 up to 5.
• the initial treatment may also be performed at a pH of from 3.1 up to 4.
• the amount of an optional compound containing an alkaline earth metal added in the initial treatment may be within the range of from about 500 ppm up to about 4000 ppm, calculated as part by weight of alkaline earth metal on part by weight of dry pulp.
• the amount of the compound containing an alkaline earth metal added lies within the range of from 500 ppm up to 3000 ppm, preferably within the range of from 500 ppm up to 2000 ppm.
• the amount of magnesium compound added as well as the other conditions in the initial treatment are so chosen that the magnesium content of the pulp prior to the bleaching with a peroxide-containing compound amounts to at least about 50% of the initial content.
• the amount of magnesium compound added as well as the other conditions are so chosen that the magnesium content of the pulp prior to the bleaching with a peroxide-containing compound lies within the range of from 100% up to 300% of the initial content, preferably within the range of from 130% up to 200%.
• the peroxide-containing compound is hydrogen peroxide.
• Bleaching with a peroxide-containing compound is suitably performed at a pH equal to or higher than the pH in the initial treatment in the presence of a magnesium compound. In this way, the trace metal ions having a positive effect on the bleaching are retained also in the bleaching stage itself.
• the peroxide-containing compound is hydrogen peroxide
• the pulp can be treated at a pH of from about 7 up to about 13, suitably at a pH of from 8 up to 12, preferably at a pH of from 9.5 up to 11.
• Bleaching with the other peroxide-containing compounds indicated above takes place within the normal pH ranges for each bleaching agent, which are well-known to the expert.
• Bleaching with a peroxide-containing compound takes place in the presence of yet another compound containing a calcium compound.
• the compound containing an alkaline earth metal present in the bleaching may be the same compound that was used in the initial treatment or another compound.
• the alkaline earth metal suitably is calcium, magnesium or barium, or a mixture thereof. This further improves the selectivity of the bleaching and reduces the consumption of the peroxide-containing compound.
• the bleaching takes place in the presence of a calcium compound.
• the calcium compound can be added to the pulp suspension outside the bleaching tower, e.g. by introduction into the pipeline leading to the bleaching tower. Also, the calcium compound can be added to the pulp suspension inside the bleaching tower, before the bleaching has commenced.
• the amount of the compound containing an alkaline earth metal added in the bleaching lies within the range of from about 100 ppm up to about 4000 ppm, calculated as part by weight of alkaline earth metal on part by weight of dry pulp.
• the amount of the compound containing an alkaline earth metal added lies within the range of from 300 ppm up to 3000 ppm, preferably within the range of from 600 ppm up to 2000 ppm.
• the calcium compound present in the bleaching with a peroxide-containing compound is suitably calcium chloride, calcium nitrate, calcium sulphate or calcium carbonate.
• Calcium chloride is preferably used.
• the calcium compound is suitably added at such a combination of pH, temperature and concentration of the calcium compound that said compound is in active form when contacted with the pulp.
• the pH lies within the range of from about 7 up to about 11.
• a positive effect is obtained also when the calcium compound is precipitated in the pulp suspension, e.g. as calcium carbonate.
• the washing liquid may be fresh water, optionally containing some added pH-adjusting chemical, or wastewater from one or more bleaching stages or extraction stages, giving a suitable pH in the washing stage. Furthermore, the washing liquid may consist of other types of wastewater, optionally purified, provided that it has a low content of undesirable ions of metal, such as manganese, iron and copper.
• Washing between the initial treatment and bleaching with a peroxide-containing compound relates to methods for removing, more or less completely, the liquid phase from the pulp suspension in order to reduce the content of dissolved trace metal ions in said suspension. Washing may involve raising the pulp concentration, e.g. by sucking-off or pressing the suspension through a filter, or lowering the pulp concentration, e.g. by dilution with a washing liquid. Washing also relates to combinations and sequences in which the pulp concentration is alternatingly raised and lowered, one or more times. In the present process, a washing method should be chosen which effectively removes the trace metal ions released in the initial treatment, aspects of process technique and economy being considered.
• the effectiveness of the washing may be indicated as the amount of liquid phase removed, compared with the liquid phase present in the pulp suspension prior to washing. Washing effectiveness can be at least about 80%, and suitably lies within the range of from 90% up to 100%, preferably within the range of from 95% up to 100%.
• the initial treatment in the presence of a magnesium compound can also be performed in the presence of a bleaching agent.
• Bleaching agents that are active within the pH range suitable for the initial treatment include chlorine dioxide, ozone and acid peroxide-containing compounds.
• Acid peroxide-containing compounds include such organic compounds as peracetic acid and such inorganic compounds as hydrogen peroxide and peroxosulphuric acid (Caro's acid).
• the initial treatment takes place in the presence of ozone or peroxosulphuric acid, preferably ozone, since these bleaching agents affect but to a minor extent the content of ions of alkaline earth metals in the pulp.
• the process according to the invention may also involve an extraction stage before the bleaching with a peroxide-containing compound. This is especially convenient when the initial treatment is performed in the presence of a bleaching agent.
• the extraction is performed at a pH that is equal to or higher than the pH in the initial treatment.
• the pH in the extraction stage lies within the range of from about 7 up to about 11, preferably within the range of from 8 up to 10.
• the pH in a subsequent stage suitably is equal to or higher than the pH in the preceding stage, in order to preserve the content of ions of alkaline earth metals in the pulp.
• the pH in the washing stage should be at least about 4
• the pH in the bleaching stage should be at least about 8.
• lignocellulose-containing pulp relates to pulps containing fibres that have been separated by chemical treatment, or recycled fibres.
• the fibres may be of hardwood or softwood.
• chemical pulp relates to pulps digested according to the sulphate, sulphite, soda or organo-solv process.
• the lignocellulose-containing pulp consists of chemically digested pulp, preferably sulphate pulp of softwood.
• the process according to the invention can be applied to pulps suitably with a yield within the range of from 40% up to 80%, preferably within the range of from 45% up to 65%.
• the process according the invention can be performed at an optional position in the bleaching sequence, e. g. immediately after the making of the pulp.
• an optional position in the bleaching sequence e. g. immediately after the making of the pulp.
• this is preferably delignified in an oxygen stage before applying the process according to the invention.
• the initial treatment can be performed at a temperature of from about 10°C up to about 100°C, suitably from 25°C up to 90°C, preferably from 40°C up to 80°C.
• the initial treatment can be performed for a period of time of from about 1 s up to about 600 min, suitably from 1 min up to 120 min, preferably from 10 min up to 60 min.
• the pulp concentration in the initial treatment can be from about 1% by weight up to about 60% by weight, suitably from 2% by weight up to 40% by weight, preferably from 3% by weight up to 35% by weight.
• the pulp is treated at a temperature of from about 30°C up to about 100°C, preferably from 60°C up to 90°C, and for a period of time of from about 30 min up to about 960 min, suitably from 60 min up to 360 min.
• the pulp concentration can be from about 1% by weight up to about 70% by weight, suitably from 3% by weight up to 60% by weight, preferably from 10% by weight up to 50% by weight.
• Treatment with the other peroxide-containing compounds indicated above takes place within the normal ranges as to temperature, period of time and pulp concentration for each bleaching agent, which are well-known to the expert.
• the amount of hydrogen peroxide lies within the range of from about 1 kg up to about 60 kg per tonne of dry pulp, calculated as 100% hydrogen peroxide.
• the upper limit is not critical, but has been set for economic reasons.
• the amount of hydrogen peroxide lies within the range of from 2 kg up to 50 kg per tonne of dry pulp, preferably from 5 kg up to 40 kg per tonne of dry pulp, calculated as 100% hydrogen peroxide.
• the pulp After the initial treatment in the presence of magnesium and subsequent bleaching with a peroxide-containing compound, the pulp can be used as such for making paper of lower brightness. Alternatively, the pulp can be finally bleached to the desired higher brightness by treatment in one or more stages.
• the final bleaching involves chlorine-free bleaching agents such as the peroxide-containing compounds indicated above, ozone, oxygen or sodium dithionite, optionally with intermediate alkaline extraction stages which may be reinforced with peroxide and/or oxygen. In this way, the formation and discharge of AOX is completely eliminated.
• the general conditions as well as the compound containing an alkaline earth metal were so chosen that the alkaline earth metal present was dissolved in the pulp suspension.
• the pulp was washed at a pH of 6-7, in order to remove those metal ions that have a negative effect on the subsequent chlorine-free bleaching.
• the pulp was bleached with hydrogen peroxide at a temperature of 90°C, a residence time of 240 min, and a pulp concentration of 10% by weight.
• the addition of hydrogen peroxide was 25 kg/tonne dry pulp, calculated as 100% hydrogen peroxide, and the pH was about 10.5.
• the oxygen-delignified sulphate pulp of softwood employed in Example 1 was treated in the presence of a magnesium compound at a pH within the range of from 1.5 up to 11 in order to illustrate the effect of pH in the initial treatment.
• the amount of magnesium added was 1000 ppm, calculated as part by weight of magnesium on part by weight of dry pulp.
• the conditions in the initial treatment were identical with those in Example 1, except the pH variation.
• the conditions in the subsequent hydrogen peroxide bleaching were identical with those in Example 1.
• the pulp was washed at a pH that was about 2 pH units higher than that in the initial treatment. The results of the tests appear from Table II below.
• the oxygen-delignified sulphate pulp of softwood employed in Example 1 was treated in the presence of a magnesium compound at a pH of 2.1 +/- 0.1 in order to illustrate the effect of pH in the subsequent washing stage.
• the pH was adjusted by addition of sulphuric acid.
• the amount of magnesium added was 1000 ppm, calculated as part by weight of magnesium on part by weight of dry pulp.
• the conditions in the initial treatment as well as in the hydrogen peroxide bleaching were identical with those in Example 1.
• the pulp was washed at a pH within the range of from 3 up to about 11 where appropriate by deionised water to which sodium hydroxide had been added. The results of the tests appear from Table III below.
• pulp treatment according to the invention with an intermediate washing stage at a pH within the range of from about 3 up to about 10 results in a considerable increase in brightness and a considerable reduction of the kappa number.
• pulp treatment according to the present invention using hydrogen peroxide as the peroxide-containing compound results in a very high increase in brightness and a considerable reduction of the kappa number.
• the oxygen-delignified sulphate pulp of softwood employed in Example 1 was treated in the presence of a magnesium compound at a pH of 2.1 +/-0.1 in order to illustrate the effect of a calcium compound on the hydrogen peroxide bleaching (Test 2).
• the amount of magnesium added was 1000 ppm, calculated as part by weight of magnesium on part by weight of dry pulp.
• calcium was added in an amount of 1000 ppm, calculated as part by weight of calcium on part by weight of dry pulp.
• the conditions in the initial treatment were identical with those in Example 1, as were the conditions in the hydrogen peroxide bleaching, except the presence of a calcium compound. After the initial treatment, the pulp was washed at a pH of 6-7.
• pulp treatment according to the invention using a calcium compound in the hydrogen peroxide bleaching results in a much higher brightness at a lower consumption of hydrogen peroxide than does bleaching in the absence of calcium.
• the oxygen-delignified sulphate pulp of softwood employed in Example 1 was treated in the presence of a magnesium compound at a pH of 2.1 +/- 0.1, washed and bleached with hydrogen peroxide, in order to illustrate the effect of a selective metal profile in the pulp prior to bleaching.
• the amount of magnesium added was 1000 ppm, calculated as part by weight of magnesium on part by weight of dry pulp.
• calcium was added in an amount of 1000 ppm, calculated as part by weight of calcium on part by weight of dry pulp (Test 4).
• the conditions in the initial treatment were identical with those in Example 1, as were the conditions in the hydrogen peroxide bleaching, except the presence of calcium compound in Test 4.
• the pulp was washed at a pH of about 3 (Test 1) and 6-7 (Tests 2-4), respectively.
• the pulp was treated without any magnesium in the initial treatment and without any calcium in the bleaching stage (Tests 1 and 2).
• Test Metal content before bleaching ppm Brightness after bleaching (%ISO) Mg Ca Mn Fe Cu 0 300 1800 68 8.3 1.5 ---- 1 30 220 6.5 5.8 ⁇ 0.5 45.5 Test Metal content before bleaching, ppm Brightness after bleaching (%ISO) Mg Ca Mn Fe Cu 2 100 640 20 7.0 0.9 45.7 3 550 690 18 6.3 0.7 63.2 4 550 1700 18 6.3 0.7 68.2

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