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
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds
  • D21C9/163—Bleaching ; Apparatus therefor with per compounds with peroxides

Definitions

• the present invention relates to a method for bleaching mechanical and chemithermomechanical pulp (CTMP).
• CMP mechanical and chemithermomechanical pulp
• SGW groundwood pulp
• TMP thermomechanical pulp
• SGW groundwood pulp
• thermomechanical pulp roundwood logs are usually chopped into a great amount of small wood pieces, named chips, and the true free-laying of the fibres is carried out by means of one or more refiners.
• chips Large parts of the chemithermomechanical pulp manufacturing process are similar to the thermomechanical pulp manufacturing process.
• the main difference is that one in a prestage treats the lignocellulose material, normally wood chips, with a sodium sulphite solution at a certain temperature and during a certain period of time. Consequently, the pulp yield will usually be one or some percent lower than in the case of thermomechanical pulp.
• Any lignocellulose material whatsoever can be used as starting material for the pulps mentioned.
• Examples of such materials are bamboo, straw, bagasse, kenaf and wood.
• Wood is the preferred starting material, and both hardwood and softwood can be beneficially used, either separately or in combination.
• lignin-preservative bleaching When bleaching mechanical pulp and CTMP lignin-preservative bleaching is used. Among bleaching agents that can be used for lignin-preservative bleaching there are as well reducing bleaching agents as oxidizing bleaching agents.
• dithionite usually sodium dithionite, also called hydrosulfite, is the most common from a commercial point of view.
• hydrogen peroxide is the most common from a commercial point of view.
• the absolutely most common bleaching agent in the described connection seems to be hydrogen peroxide.
• This agent is a very effective, i.e. a strongly brightening, bleaching agent.
• the bleaching with hydrogen peroxide demands that a separate bleaching tower and also other bleaching equipment have to be used, which leads to that the fixed costs at the hydrogen peroxide bleaching will be and are high.
• Dithionite usually sodium dithionite
• a bleaching tower also can be added directly to the pulp suspension, for example in a storage tower or earlier in the pulp manufacturing process, for example just after the free-laying of the fibres, without the use of bleaching towers and other bleaching equipment.
• the last mentioned means a decrease of the fixed costs.
• This with adding a reducing bleaching agent, for example sodium dithionite, directly to the advancing pulp suspension just after the free-laying of the fibres is known from the Swedish patent application 9900816-1.
• Transition metals are detrimental to the bleaching of mechanical pulp and CTMP with, e.g. as well hydrogen peroxide as dithionite.
• the presence of mangane ions in significant quantities is particularly serious when bleaching of such pulp with hydrogen peroxide.
• An increasing amount of manganese ions leads to an increasing decomposition of the added amount of hydrogen peroxide which means, that the part of the hydrogen peroxide which is decomposed comes not to a use for brightening the pulp.
• the decomposed products are in themselves harmful, because they form chromophore groups in the pulp, which works against that which is tried to attain, namely a brightening of the pulp.
• transition metals are normally removed from or neutralized in the pulp and the pulp suspension by complex-binding the transition metals with a complexing agent, for instance in the form of ethylene diamine tetraacetic acid (EDTA) and/or diethylene triamine pentaacetic acid (DTPA).
• a complexing agent for instance in the form of ethylene diamine tetraacetic acid (EDTA) and/or diethylene triamine pentaacetic acid (DTPA).
• EDTA ethylene diamine tetraacetic acid
• DTPA diethylene triamine pentaacetic acid
• a reducing chemical such as sodium hydrosulphite or sodium sulphite for instance, is added to the pulp suspension in addition to a complexing agent.
• Success has also been achieved by treating the starting material, usually wood chips, with a complexing agent solely or with both of the aforesaid chemicals.
• EP-A-0 187 477 and EP-A-0 311 356 disclose processes for bleaching mechanical pulp according to the conventional bleaching technic, i. e. the pulp is treated in certain bleaching steps or stages.
• the present invention provides a solution to this problem and relates to a method for bleaching mechanical and chemithermomechanical pulp including that the advancing pulp suspension obtained after that the fibres are laid free is prebleached without use of a bleaching tower by adding reductive bleaching agent to the flowing pulp suspension in a location just after the fibres are laid free, that the prebleaching is carried out under given conditions in the form of high temperature and minimized oxygen access at said location and immediately downstream of said location and that the prebleached pulp is subjected to at least one further bleaching treatment including that the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.
• an oxidizing bleaching agent preferably peroxide bleaching agent, reinforced with borohydride.
• TMP is a dominating pulp type from a quantitative point of view.
• the process for laying the fibres free is usually carried out either in one refiner or in two subsequent refiners.
• a steam separator is situated, ususally a cyclone of some kind, through which the obtained pulp suspension is brought to pass.
• the pulp suspension is thereafter transported usually to a slusher (which however is not absolutely necessary), for example by means of a screw conveyor, and further on to a storage vessel (latency chest) and therefrom to a screening department and from there to a bleaching treatment.
• a slusher which the pulp suspension is transported through a conduit to the storage vessel by means of a pump, located just after the slusher.
• the bleaching agent can, for example, be added to the pulp suspension in the earlier mentioned screw conveyor.
• One other adding location and a preferred one is the earlier mentioned pump.
• White water is normally delivered to the slusher and the bleaching agent can be introduced in that white water, which later on is introduced into the pulp suspension. It is also possible, to introduce the bleaching agent directly into the slusher. It is of course possible to divide the charge of bleaching agent and to add the bleaching agent to the pulp suspension, for example in two or more of the stated locations.
• the advancing pulp suspension used in the foregoing and also in the main claim shall be given a wide meaning. This phrase shall not solely be seen to mean when the pulp suspension flows forwards in a conduit or pipe, but also when the pulp suspension is held in a vessel and container, for instance in the form of a slusher and storage vessel since even in these latter cases the pulp suspension still moves forwards in the sense that it is fed into the vessel at one location and exits from said vessel in another location.
• Any known reducing bleaching agent can be used.
• bleaching agents are dithionite (which is sometimes called hydrosulfite and which is preferred), borohydride, hydrazine and formamidine sulfuric acid.
• Dithionite is commercially available primarily as sodium dithionite, i.e. Na 2 S 2 O 4 .
• the bleaching agent concerned is introduced into the pulp suspension primarily in the form of an aqueous solution, the concentration of which will suitably lie within the range of 20-120 g/l.
• the amount of bleaching agent added will depend, among other things, on the difficulty in bleaching the pulp in question and how big increase in brightness of the pulp which is desired in the prebleaching in comparison with the increase in brightness of the pulp totally.
• Pulp bleaching parametres such as temperature, time, pulp consistency, pH, etc.
• TMP thermomechanical pulp
• the temperature will, of course, be very high, e.g. 80-95°C, and the consistency of the pulp normally low, e.g. 2-4%.
• the bleaching time will be short as a result of this very high temperature among other things, and will probably range from a time span of some seconds up to some minutes.
• the bleaching time will probably also depend partly on the rate at which the pulp suspension flows at the location where the bleaching agent is added.
• the pH-value will naturally lie within the range of 4-7.
• the pH-value should lie from 4.5 and upwards, in order to achieve an optimal bleaching result.
• a pH-value as high as 8.5 can be used for bleaching purposes, a pH of this magnitude is less suitable for other reasons.
• the amount of oxygen in the pulp suspension shall at the addition of the bleaching agent be as low as possible, preferably zero.
• the pulp suspension is protected from air contact and air dispersion in a considerable degree from the location of laying the fibres free and forward some length in the chain of pulp treatment. Besides that the temperature of the pulp suspension is very high in that area, which also contributes to an effectively bleaching of the pulp.
• CTMP is very similar to manufacture of thermomechanical pulp, which has been described briefly above and the locations for addition of the bleaching agent at prebleaching of such a pulp coincident to a high degree with the above described.
• the pulp suspension When manufacturing groundwood pulp the pulp suspension is collected in a grinder pit after that the fibres are laid free, wherefrom the pulp suspension is transported forward. Suitable locations for addition of the bleaching agent are the grinding pit and just subsequent to that. It is especially suitable to carry out the bleaching in question in a system pressurized with steam, i.e. at pressure grinding.
• the pulp is subjected to at least one further bleaching treatment.
• the pulp is bleached with an oxidizing bleaching agent, preferably peroxide bleaching agent, reinforced with borohydride.
• an oxidizing bleaching agent preferably peroxide bleaching agent, reinforced with borohydride.
• the pulp is treated with borohydride in a pretreatment stage, i.e. before the true peroxide bleaching stage, as that borohydride is added to the pulp some time during the true peroxide bleaching stage, including that the borohydride is added to the pulp suspension together with the peroxide bleaching agent and that the borohydride is added to the pulp suspension at the end of the peroxide bleaching stage.
• the peroxide bleaching stage can in itself be conventional (except a possible addition of borohydride) and any known peroxide bleaching agent can be used. Examples of such are hydrogen peroxide, sodium peroxide, peracetic acid, performic acid and peroxosulphuric acid (Caro's acid). Hydrogen peroxide is the preferred peroxide bleaching agent.
• any known alkali metal borohydride can be added to the pulp.
• the most common is sodium borohydride. This can be added as such to the pulp.
• the commodity Borol which is a water solution containing 12% sodium borohydride and 40% sodium hydroxide, is the most used. It is an advantage to use one or more completion chemical(s) to the borohydride during the pretreatment stage.
• the commodity Borol is used as a borohydride source it is not always necessary to add an alkali, e.g. sodium hydroxide, to the pulp, in view of that the commodity contains a considerable amount of sodium hydroxide.
• it is usual to add variable amounts of alkali to the pulp for example together with the commodity Borol.
• sodium borohydride as such is added to the pulp it is necessary with an addition of alkali also; e.g. sodium hydroxide.
• Further suitable completion chemicals are water glass (sodium silicate) and some magnesium compound, as magnesium sulfate.
• the pretreatment of the pulp with at least borohydride takes place during a short period of time, for example at least 15 seconds and at most 500 seconds. Optimal result is obtained within the interval 60 to 180 seconds. The time period in question is counted from when the borohydride is added to the pulp up to that the peroxide bleaching agent is added to the pulp in the subsequent peroxide bleaching stage.
• the borohydride counted as sodium borohydride
• Suitable temperature is 20-95°C.
• the pulp consistency is not directly critical, but it is preferred with from medium consistency and higher, i.e. from 6% to 38%. While alkali always is present during the pretreatment of the pulp via the addition of the commodity Borol or via a direct addition or via both the ways the pretreatment will be carried out under alkaline conditions, for example within the pH-interval 9-13.
• pulp suspension borohydride and any completion chemical at the end of, for example a conventional peroxide bleaching stage, for example, when the peroxide bleaching has been run in 100-120 minutes.
• the charge of borohydride, counted as sodium borohydride, can also in these two cases lie within the interval 0.05-0.3%, counted on bone-dry pulp.
• pulp suspension just before the bleaching with peroxide bleaching agent reinforced with borohydride can be bleached in some other way in one or more stage(s).
• prebleached pulp is bleached with peroxide bleaching agent at medium consistency
• the subsequent peroxide bleaching stage reinforced with borohydride is carried out at high pulp consistency and furthermore the spent bleaching liquor from the high consistency bleaching stage, which contains residue peroxide like residue hydrogen peroxide, shall be delivered backwards in the pulp refining process and be mixed into the pulp just before or in the beginning of the medium consistency bleaching stage with e.g. hydrogen peroxide.
• Thermomechanical pulp was manufactured in a way, which schematically is presented in Figure 1 in the Swedish patent application 9900816-1, to which is referred once more.
• the starting material for the pulp manufacturing was considerable fresh, barked spruce logs of Scandinavian origin.
• a water solution containing sodium dithionite in a concentration of 60 g/l is added in such a flow, that the charge of the bleaching agent was 6 kg per tonne of bone-dry pulp.
• the temperature of the pulp suspension in that location was 88°C, their pH-value 4.6 and their pulp consistency 3%.
• the pulp prebleached in that way was screened in the screening department 19 so that an accept pulp flow and a reject pulp flow were obtained.
• the reject pulp flow was treated in accordance with that presented in Figure 1 in the patent application mentioned and a water solution containing sodium dithionite in a concentration of 60 g/l was added to the reject pulp suspension flow in the pump (not shown in the Figure) situated just after the slusher 31 in such a flow, that the charge of the bleaching agent was 6 kg per tonne dry pulp also in that location.
• the temperature of the pulp suspension in that location was 85°C, their pH-value 5.1 and their pulp consistency 3%.
• the pulp On the disc filter 21 pulp was picked out for further transportation to the laboratory. In that location the pulp, i.e. the prebleached pulp, had a brightness of 71% ISO and a metal content, determined according to standard analysis methods, of 12.1 ppm (or mg per kg bone-dry pulp) manganese (Mn), 11.2 ppm iron (Fe) and 3.8 ppm copper (Cu).
• Mn manganese
• Fe iron
• Cu 3.8 ppm copper
• the pulp was subjected to a conventional hydrogen peroxid bleaching in the way that the 20 grams of pulp was divided finely by hand so that the pulp become fluffed. Thereafter the pulp was put down into a plastic bag.
• a plastic bag To this plastic bag a certain amount of a water solution containing hydrogen peroxide in a charge of 4%, counted on bone-dry pulp, sodium hydroxide in a charge of 3.5%, counted on bone-dry pulp, and water glass (sodium silicate) in a charge of 3.12%, counted on bone-dry pulp was delivered.
• the content of the plastic bag i.e. the pulp sample
• was kneaded by hand so that the chemicals were distributed equal in the pulp amount stated.
• the pulp consistency was 30% after mixing the chemicals into the pulp.
• the plastic bag with its content was placed into a water bath, with a temperature of 75°C, during a time period of 90 minutes.
• the bleaching was interrupted by picking the plastic bag up from the water bath and its content was moved to a glass vessel, in which the pulp sample was diluted with deionized water down to a pulp consistency of 0.6%. A sulphuric acid solution was also added so that the pH-value of the pulp suspension was decreased to 5. During the dilution of the pulp sample and before the addition of sulphuric acid a specimen of the spent bleaching liquor was taken out in which the end-pH and the content of hydrogen peroxide residue were determined according to conventional analysis technic. It was manufactured a sheet by hand of the pulp sample according to SCAN-CM 11:75 on which the brightness was determined according to the analysis method SCAN-P 3:93. All the brightness values mentioned in this working example are measured in the just described way.
• Pr at first a water solution containing sodium borohydride and sodium hydroxide in the form of the commodity Borol in an amount regarding sodium borohydride of 0.1%, counted on bone-dry pulp, plus an extra amount of sodium hydroxide of 1.55%, counted on bone-dry pulp, and water glass in an amount of 3.12%, counted on bone-dry pulp, was added to the pulp sample in the plastic bag. After thoroughly dividing of the chemicals by means of hand kneading the sealed plastic bag was placed in the earlier mentioned water bath, which held a temperature of 75°C.
• the plastic bag was picked up from the water bath and opened and a water solution containing hydrogen peroxide in an amount of 4 %, counted on bone dry pulp, and sodium hydroxide in an amount of 1.55%, counted on bone-dry pulp, was delivered. After this addition of the water solution the pulp had a pulp consistency of 30%. The plastic bag was sealed and again placed in the water bath mentioned and was kept there for a time period of 90 minutes. Also in this trial the handling of the pulp sample was continued in the way described above. The characterizing thing for this trial was obviously that the sodium borohydride was added to the pulp in a pretreatment stage before that the true hydrogen peroxide bleaching of the pulp was carried out.
• a third trial according to the invention named Pe
• the pulp sample was treated in accordance with the reference trial with the important difference, that after 80 minutes of the hydrogen peroxide bleaching of the pulp the plastic bag was picked up from the water bath and opened.
• the commodity Borol was delivered so, that sodium borohydride was added to the pulp in an amount of 0.1%, counted on bone-dry pulp and furthermore deionized water was added so that the pulp consistency was decreased to 5%.
• Sulphuric acid was also added so that the pH-value in the pulp suspension was decreased to the interval 6.5-7.0.
• the plastic bag was sealed and the chemicals were equilized within the pulp sample by means of hand kneading.

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• Chemical And Physical Treatments For Wood And The Like (AREA)

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• 2000
  • 2000-02-11 SE SE0000431A patent/SE515708C2/sv not_active IP Right Cessation
• 2001
  • 2001-02-08 AT AT01906451T patent/ATE343674T1/de not_active IP Right Cessation
  • 2001-02-08 CA CA002395824A patent/CA2395824A1/en not_active Abandoned
  • 2001-02-08 EP EP01906451A patent/EP1266074B1/en not_active Expired - Lifetime
  • 2001-02-08 ES ES01906451T patent/ES2269348T3/es not_active Expired - Lifetime
  • 2001-02-08 US US10/203,202 patent/US20030121625A1/en not_active Abandoned
  • 2001-02-08 JP JP2001558530A patent/JP2003522846A/ja active Pending
  • 2001-02-08 DE DE60124081T patent/DE60124081T2/de not_active Expired - Fee Related
  • 2001-02-08 WO PCT/SE2001/000260 patent/WO2001059205A1/en active IP Right Grant

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