Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21H—PULP COMPOSITIONS; PREPARATION THEREOF NOT COVERED BY SUBCLASSES D21C OR D21D; IMPREGNATING OR COATING OF PAPER; TREATMENT OF FINISHED PAPER NOT COVERED BY CLASS B31 OR SUBCLASS D21G; PAPER NOT OTHERWISE PROVIDED FOR
  • D21H21/00—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties
  • D21H21/14—Non-fibrous material added to the pulp, characterised by its function, form or properties; Paper-impregnating or coating material, characterised by its function, form or properties characterised by function or properties in or on the paper
  • D21H21/32—Bleaching agents
• • 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
  • D21C9/166—Bleaching ; Apparatus therefor with per compounds with peracids

Definitions

• the present invention relates to a method for the bleaching of chemical pulp, wherein the pulp is treated in a plurality of different steps and a solution containing a peracid is used in at least one of the steps.
• the invention additionally relates to the use of a peracid-containing bleaching solution in a process step according to the invention.
• a raw pulp obtained by digesting cellulose-containing materials with suitable digestion chemicals is subjected to a treatment wherein the lignin is removed and a bleaching which involves the use of oxidative chemicals is carried out.
• the purpose of conventional bleaching of chemical pulp is to bring to completion the removal of lignin from a raw pulp obtained from a digestion procedure.
• This bleaching can be carried out by using chlorine or chlorine dioxide and thereafter alkaline extraction steps in which the lignin is dissolved out of the pulp.
• the bleaching of a chemical pulp is increasingly often carried out by bleaching methods using no elementary chlorine or chlorine compounds.
• the first-mentioned bleaching is called ECF (elementary chlorine free) bleaching and the latter TCF (totally chlorine free) bleaching.
• ECF elementary chlorine free
• TCF totally chlorine free
• the chemical pulp is bleached in several successive steps.
• the bleaching is often started with an oxygen delignification, whereafter further bleaching can be carried out by various methods.
• delignification can be continued, for example, by using ozone, peracetic acid or hydrogen peroxide in acid or alkaline conditions.
• ECF bleaching chlorine dioxide steps and thereafter alkaline extraction steps are used.
• Peracetic acid can be prepared most simply by mixing vinegar and hydrogen peroxide, whereupon a so-called equilibrium solution is obtained which contains, in addition to peracetic acid, also unreacted initial substances. Also a pure peracetic acid solution can be prepared from this reaction solution by distillation. There are also known other methods for preparing peracetic acid.
• Peracetic acid is a highly selective bleaching chemical by the use of which good strength properties of the pulp can be maintained. When peracetic acid is used, the optimum pH is approx. 4-7. At a lower pH peracetic acid is a highly selective delignifying chemical. At a higher pH its bleaching action is greater but its selectivity decreases.
• U.S. Patent 5 552 018 describes multistage processes for bleaching and delignification of pulp. These include a sequence with a final peracetic acid step as well as a sequence with a peracetic acid step with use of a magnesium salt and a phosphonic acid to stabilize the peracetic acid, followed by a final alkaline extraction step. Delignification has been demonstrated by reduced kappa numbers and increased brightness.
• U.S. Patent 4 222 819 describes a bleaching sequence including a peracid step with magnesium sulfate, followed by alkaline extraction of dissolvable lignin as the final step of the sequence.
• Agents suitable for post-bleaching include peracetic acid and dithionite.
• the use of hydrogen peroxide is limited by the fact that in order to function properly it would require alkaline conditions.
• dithionite and peracetic acid are highly suited for post-bleaching.
• Dithionite is used commonly in the bleaching of mechanical pulps, but normally it is not used in the bleaching of chemical pulp. The sulfur present in it may also cause problems.
• Post-bleaching with peracetic acid is a very effective method of increasing brightness. The consumption of chemicals for bleached pulps is low, and in addition the reaction is rapid even at low consistency. Furthermore, the optimum pH range for bleaching is precisely suitable for post-bleaching.
• the peracetic acid post-bleaching of chemical pulps involves the problem that, within the pH range optimal for the bleaching result, selectivity is no longer at its best.
• selectivity is no longer at its best.
• the use of peracetic acid damages the fibers, causing the breaking down of carbohydrates and the lowering of pulp strength. This also causes dissolving of organic matter out of the pulp, a factor which may disturb the running of the paper machine.
• the bleaching method according to the invention which solves the problems mentioned above, is characterized in that a peracid is used in the post-bleaching which is the last step of the bleaching process carried out on delignified pulp having a kappa number of 4 or less, the post-bleaching taking place in the presence of one or more earth-alkali metal compounds and being carried out after bleach plant steps in a pulp flow pipe, a storage tower and/or a paper machine the peracid turning colorless chromophoric groups in the pulp.
• a post-bleaching such as this can advantageously be carried out as a separate step at the paper mill to which the pulp has been transferred from a bleach plant.
• the earth-alkali metals such as magnesium and calcium, used simultaneously with a peracid in the method according to the invention stabilize carbohydrates so that the reducing of the pulp strength is prevented. In other words, the strength of the pulp remains good and the dissolving of organic matter is insignificant.
• the brightness and whiteness of the pulp are also higher than without the use of earth-alkali metals. This decreases, for example, the need for using optical brighteners in the paper machine. When this method is used, the pH of the stock to be turned into paper, and the brightness and:whiteness of the pulp are more even, in which case the runnability of the paper machine is considerably improved.
• magnesium protects the viscosity of the pulp.
• delignification which is carried out on a pulp having a high kappa number. This delignification step is followed by a separate bleaching during which the brightness is raised to its final level.
• transition metals such as Fe and Mn are released from the pulp, and magnesium prevents their detrimental effects.
• the addition of magnesium is used for improving the ratio of magnesium to transition metal, which improves especially the running of the subsequent alkaline peroxide step.
• the invention concerns post-bleaching which is carried out on an already delignified pulp having a low kappa number of 4 or less and being free of transition metals, the purpose being to raise the brightness and whiteness of the pulp to the desired level.
• the peracetic acid turns colorless the chromophoric groups in the pulp, mainly in carbohydrates.
• paracetic acid dose is considerably smaller.
• a very small peracetic acid dose 0.1-7 kg/tp, preferably 0.5-3 kg/tp, is sufficient, larger doses being even detrimental.
• the peracid used in the method of the present invention is preferably peracetic acid.
• Other usable peracids include performic acid, perpropionic acid or some longer-chain percarboxylic acid.
• the peracid may be a so-called equilibrium solution, i.e. a reaction mixture containing the said acid, a peracid and hydrogen peroxide, or a pure peracid solution can be used.
• the method by which the peracid solution is prepared does not restrict the use of the method; the peracid may be prepared, for example, by distilling an equilibrium solution or from an anhydride and hydrogen peroxide.
• the peracid may also be a monopersulfuric acid (Caro's acid) or a mixture of Caro's acid and a percarboxylic acid, e.g. peracetic acid.
• Persulfates may also be used, either as such or as a mixture with any of the above-mentioned peracids.
• the earth-alkali metal used in the post-bleaching according to the invention may be in particular calcium or magnesium. These may be added to the bleaching solution in the form of sulfate. acetate, carbonate, oxide or any other compound. It is possible to use in the method either magnesium or calcium alone or both of them together at any ratio. Calcium has over magnesium the advantage that calcium will not precipitate extract-based and resin-based substances which may be present at the wet end of the paper machine.
• the post-bleaching according to the invention may be preceded by any known bleaching reaction known from bleaching sequences.
• the post-bleaching is especially advantageous for use after chlorine dioxide or peroxide bleaching.
• acidification possibly taking place after the bleaching does not restrict the application of the method; it can be used on both non-acidified and acidified stock.
• the said acidification may be carried out with, for example, SO 2 , sulfuric acid or any acid suitable for this purpose.
• the functioning of the invention is not affected by a possible wash of the pulp before or after the post-bleaching, or by the absence of a wash.
• the consistency of the pulp to be after-bleached may be 1-30 % and the temperature during the post-bleaching may be between 30 and 100°C.
• the after-bleached pulp may be taken directly to the paper machine or be dried to produce baled pulp.
• the method is suited for use on sulfate and sulfite pulps made both from softwood and from hardwood, and on various organosolv pulps.
• the said prior art methods in which a peracid and magnesium are used or in which a peracid has been present in the last steps of the bleaching sequence are characterized in that they are carried out in the normal manner in the bleach plant of the pulp mill.
• the post-bleaching according to the invention in which the bleaching reaction is very rapid even at a low consistency, is not tied to the bleach plant and thus also does not require investment in equipment for the bleach plant.
• the method according to the present invention concerns post-bleaching carried out after the bleaching sequence of the bleach plant, outside the actual bleaching, in a pulp flow pipe during the transfer of pulp, during pulp storage in a storage tower or in a paper machine. Instead of the bleach plant, the place at which the post-bleaching according to the invention is carried out is typically a storage tower for bleached pulp or the paper mill.
• the earth-alkali metal compounds used can be added to the bleaching solution in any suitable process step. They can be added to the circulating waters before the water arrives in the post-bleaching step, or they may, for example, come in the dilution waters from the paper machine, where they may be added in any form. It is also possible to use chelators simultaneously with earth-alkali metals.
• the invention comprises the use of a solution which contains a peracid and an earth-alkali metal for post-bleaching of chemical pulp delignified to a kappa number of 4 or less, the post-bleaching being carried our after the bleach plant steps at a paper mill.
• a birch sulfate pulp which had been bleached using a sequence consisting of an oxygen step, chelation, an oxygen + peroxide step, a chlorine dioxide step, a peroxide extraction, a chlorine dioxide step (-O-Q-Op-D-Ep-D) was subjected separately to an post-bleaching at 50 °C with a 30 min retention at a consistency of 5 % with a peracetic acid dose of 3 kg/tp at two different pH levels (approx. 6.5 and approx. 4.5).
• the peracetic acid used had been distilled.
• Calcium was added to the pulp in the form of acetate, and magnesium in the form of sulfate.
• the viscosity of the pulp can be maintained clearly higher.
• the viscosity determined by the copper-ethylene diamine method is directly proportional to the strength of the pulp, especially when the same pulp sample is concerned. It is well-founded to assume that the higher the viscosity of the pulp, the better its strength. A higher viscosity also means that the yield is better and that less organic matter has been dissolved out of the pulp.
• ISO brightness was improved by approx. 1 % ISO by the earth-alkali addition.
• a birch sulfate pulp which had been bleached by the sequence -O-Q-Op-D-Ep-D according to Example 1 was subjected to an post-bleaching at 50 °C with a 30 min retention at a consistency of 5 % with a peracetic acid dose of 3 kg/tp at different pH levels.
• the magnesium sulfite addition was 1 kg/tp.
• the peracetic acid used had been distilled.
• the brightness of the pulp before the post-bleaching was 90,3 % ISO and viscosity 930 dm 3 /kg.
• a TCF-bleached softwood sulfate pulp which had been bleached using oxygen, ozone and hydrogen peroxide was after-bleached at 70 °C with a 240 min retention at a consistency of 10 % by using two different peracetic acid doses.
• the conditions and results of the post-bleaching are shown in Table 3.
• the brightness of the pulp before the post-bleaching was 86.8 % ISO and viscosity 642 dm 3 /kg, and the kappa number was 1.7.
• a TCF-bleached softwood sulfate pulp which had been bleached using oxygen, ozone and hydrogen peroxide was after-bleached at 70 °C with a 240 min retention at a viscosity of 10 % by using two different peracetic acid doses.
• the conditions and results of the post-bleaching are shown in Table 4.
• the brightness of the pulp before the post-bleaching was 87.6 % ISO and viscosity 623 dm 3 /kg, and the kappa number was 1.8.

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• Life Sciences & Earth Sciences (AREA)
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• 1997
  • 1997-12-19 FI FI974575A patent/FI112958B/fi not_active IP Right Cessation
• 1998
  • 1998-12-18 DE DE69839097T patent/DE69839097T2/de not_active Expired - Lifetime
  • 1998-12-18 AU AU16743/99A patent/AU1674399A/en not_active Abandoned
  • 1998-12-18 ES ES98961279T patent/ES2302362T3/es not_active Expired - Lifetime
  • 1998-12-18 AT AT98961279T patent/ATE385526T1/de active
  • 1998-12-18 PT PT98961279T patent/PT1040222E/pt unknown
  • 1998-12-18 JP JP2000525621A patent/JP4499280B2/ja not_active Expired - Lifetime
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  • 1998-12-18 WO PCT/FI1998/000993 patent/WO1999032710A1/en active IP Right Grant
  • 1998-12-18 BR BRPI9813747-6A patent/BR9813747B1/pt not_active IP Right Cessation
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