WO1998051855A1 - Procede de reduction des problemes lies au tartre dans la production de pate a papier blanchie, a l'aide d'un circuit ferme de liquide de lavage - Google Patents

Procede de reduction des problemes lies au tartre dans la production de pate a papier blanchie, a l'aide d'un circuit ferme de liquide de lavage Download PDF

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Publication number
WO1998051855A1
WO1998051855A1 PCT/SE1998/000879 SE9800879W WO9851855A1 WO 1998051855 A1 WO1998051855 A1 WO 1998051855A1 SE 9800879 W SE9800879 W SE 9800879W WO 9851855 A1 WO9851855 A1 WO 9851855A1
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Prior art keywords
stage
suspension
pulp
liquid
washing
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PCT/SE1998/000879
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English (en)
Inventor
Jan Georg LIDÉN
Staffan Lars Sune Magnusson
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Mo Och Domsjö Aktiebolag
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Priority to AU74639/98A priority Critical patent/AU7463998A/en
Publication of WO1998051855A1 publication Critical patent/WO1998051855A1/fr

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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
    • D21C9/002—Modification of pulp properties by chemical means; preparation of dewatered pulp, e.g. in sheet or bulk form, containing special additives
    • D21C9/004—Modification of pulp properties by chemical means; preparation of dewatered pulp, e.g. in sheet or bulk form, containing special additives inorganic compounds
    • 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
    • D21C11/00—Regeneration of pulp liquors or effluent waste waters
    • D21C11/0021—Introduction of various effluents, e.g. waste waters, into the pulping, recovery and regeneration cycle (closed-cycle)
    • D21C11/0028—Effluents derived from the washing or bleaching plants
    • 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
    • D21C9/008—Prevention of corrosion or formation of deposits on pulp-treating equipment
    • 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
    • D21C9/02—Washing ; Displacing cooking or pulp-treating liquors contained in the pulp by fluids, e.g. wash water or other pulp-treating agents

Definitions

  • the invention relates to a method of minimizing scaling problems in certain critical positions in the manufacture of bleached cellulose pulp where washing liquid (suspension liquid) is passed counter-currently, such that the manufacturing process will be essentially fully closed with respect to the flow pattern of washing liquid (suspension liquid).
  • washing liquid suspension liquid
  • the motive lying behind the latter is to both reduce the consumption of fresh water and to reduce the amount of organic material released to the recipient when manufacturing bleached cellulose pulp.
  • cellulose pulp is meant primarily chemical pulp, i.e. where lignocellulosic material of any kind whatsoever is chemically digested by means of an acid or an alkaline process.
  • the sulphite method is an example of an acidic digestion process, while the sulphate method is an example of an alkaline digestion process.
  • alkaline digestion processes are the polysulphide process and processes of the type soda ( sodium hydroxide) process in which catalysts are used, such as any quinone compound.
  • Falling within the term sulphate method is, for instance, the use of high sulphidity, the use of counter- current cooking where white liquor is also delivered during an advanced stage of the cook, and use of a chemical treatment of the lignocellulosic material prior to the actual sulphate cook.
  • Cellulose pulp is produced from a very large number of different lignocellulosic materials.
  • a very common lignocellulosic material is wood, originating from both deciduous trees and coniferous trees, which is normally chopped to chip form prior to the pulp manufacturing, for instance the digestion (cooking) process.
  • the cellulose pulp is normally bleached in several stages, with the use of both oxygen-based bleaching agents and chlorine dioxide (D).
  • oxygen-based bleaching agents are oxygen gas ( 0 ) , ozone ( Z ) and some per-compound (P), for instance hydrogen peroxide, sodium peroxide and different per-acids.
  • P per-compound
  • the per-compound can be activated by polyoxoanions, for instance in the form of molybdate, in an acid environment.
  • Q sequestering or chelating agent treatment
  • typical chelating agents are ethylenediaminetetraacetic acid (EDTA), diethylenetriamine pentaacetic acid (DTPA) and nitrilotriacetic acid (NTA).
  • the pulp is liberated in a far going way from undesirable substances, by washing for example, and the washing liquid (the suspension liquid) is passed essentially strictly counter-current, such that the pulp manufacturing process will be essentially fully closed with respect to the flow pattern of the liquid.
  • the washing liquid the suspension liquid
  • Degradation of the manganese complex in any position so as to cause manganese, for instance, to be reprecipitated on the pulp is prevented by bringing the pH of the suspension liquid subsequent to oxygen gas delignification and forward in the pulp treatment chain to the bleaching process with the non chlorine-containing oxidative bleaching agent, for instance hydrogen peroxide, to a value of at most 10, and by bringing the carbonate concentration of the suspension liquid to a value which is equal to or which exceeds a given lowest value, depending on the position in the cellulose pulp treatment chain. If the amount of manganese that accompanies the pulp into the aforesaid third treatment stage, i.e. the peroxide stage for instance, is not successfully reduced to a substantial degree, the pulp cannot be bleached in the aforedescribed way because the major part of the bleaching agent charged will immediately be destroyed by the manganese.
  • the non chlorine-containing oxidative bleaching agent for instance hydrogen peroxide
  • the most critical position is where the pulp suspension is washed after the Q-stage, preferably in two stages.
  • the pH value of the pulp suspension is normally about 5 during the Q-stage and retains this pH value when the pulp suspension is delivered, for instance, to a subsequent first washing filter.
  • the washing liquid is conveyed strictly counter-current, meaning that after the P-stage, the pulp is washed on, e.g., a washing filter either with fresh water or with suspension liquid emanating from a subsequent treatment stage in the described treatment sequence.
  • the washing liquid (suspension liquid) collected in a storage tank underneath the aforesaid washing filter will also be relatively strongly alkaline, with a pH value of at most 10.
  • Part of the suspension liquid in the mentioned storage tank is conveyed counter-currently and used as washing liquid in a second wash filter downstream of the described Q-stage.
  • the aforesaid washing liquid (suspension liquid) is collected in a storage tank underneath the washing filter concerned.
  • Part of the suspension liquid in this storage tank is conveyed counter- currently and used as washing liquid in the first washing filter mentioned by way of introduction, downstream of the Q- stage.
  • a storage tank is also provided underneath this washing filter, for collecting the washing liquid or the suspension liquid, part of this liquid being conveyed counter-currently, and so on.
  • the pulp suspension delivered to the first washing filter after the Q-stage has a pH value of about 5.
  • the pulp suspension contacts in the washing filter a suspension (washing) liquid whose pH is below 10.
  • the extent to which the pH value of this suspension liquid will be below 10 is dependent on a number of circumstances.
  • a weakly acid pulp suspension and an alkaline suspension liquid meet at this washing filter.
  • Scales are formed in and in the vicinity of the washing filter concerned, for reasons described in direct connection with what has just been stated. A certain degree of scales can also be formed in the subsequent second washing filter.
  • a position that is particularly subjected to scaling is the washing stage immediately prior to the Q- stage, particularly if one in the Q-stage uses a pH signi- ficantly below 5.
  • Scales can also be formed in the washing stage or stages immediately after the alkaline oxygen gas bleaching of the pulp. This is quite unusual, however.
  • the starting material used for the manufacture of the cellulose pulp i.e. the lignocellulosic material in the form, e.g., of wood
  • Some of the original calcium can disappear from the pulp during its way to the bleaching plant, where the first bleaching stage in the described case is comprised of an alkaline oxygen gas bleaching stage. Both the pulp entering the oxygen gas bleaching stage and the pulp that leaves said stage contain significant quantities of calcium.
  • the pH of an alkaline pulp suspension e.g.
  • a suspension as well from a sulphate cook as from an oxygen gas bleaching stage is lowered, some of the calcium present in and on the pulp fibres will be released and dissolved in the suspension liquid.
  • the amount of calcium released is a function of the total calcium content of the pulp and of the pH, and the main rule is that the lower the pH, the more calcium released. If it in different positions will be so, that the suspension liquid besides a high content of calcium also contains high oxalate content and/or sulphate content and/or carbonate content, precipitation of calcium oxalate, calcium sulphate and calcium carbonate respectively will take place. Large quantities of oxalate are formed from the pulp in both acidic and alkaline, oxidizing bleaching stages.
  • Sulphate most often originates from the sulphuric acid that is supplied to the pulp suspension or the suspension liquid as a matter of routine when desiring a low pH value in a pulp treatment stage.
  • stages are the Q-stage, D- stage and Z-stage. High concentrations of carbonate can also occur in certain positions.
  • the scales formed in washing apparatuses immediately after acidic treatment stages or bleaching stages consist of precipitates of primarily calcium oxalate, and possibly also of precipitates of calcium sulphate to some extent.
  • the scales are composed of precipitates of calcium carbonate.
  • washing filter that includes a wire cloth
  • the precipitate will first fasten to the threads of the wire cloth and it is successively growing so as to gradually reduce the size of the open areas of the wire cloth. As the size of these open areas diminishes, it will be more and more difficult for the suspension liquid or washing liquid to pass through the wire cloth and it is ultimately necessary to switch-off the filter and clean the more or less clogged wire cloth with the aid of a chemical agent and/or a mechanical device.
  • washing presses or presses are used to liberate the pulp from undesirable substances, the large number of round holes included in the presses and their rollers and through which the washing liquid or suspension liquid shall pass gradually become blocked or clogged in an analogous manner.
  • the scaling problems can become so serious as to cause even pipes or tubing of significant dimensions through which suspension liquid passes in the proximity of any washing apparatus to become completely blocked.
  • Described in Swedish Patent Specification 9401125-1 ( 502 706 ) is a method for producing bleached cellulose pulp that coincides to a great extent with the aforedescribed, patented method.
  • the primary difference is in the actual treatment/bleaching sequence.
  • the cellulose pulp can serially in the bleaching plant be subjected to oxygen gas delignification (0), jointed chelating agent treatment and chlorine dioxide bleaching
  • the pulp may first be treated with a chelating agent (Q) and the pulp then be bleached with chlorine dioxide (D).
  • Q chelating agent
  • D chlorine dioxide
  • Scaling problems have also occurred in one and sometimes more position( s ) in a sulphate pulp mill in which this method is applied.
  • a Q/D-stage When a Q/D-stage is used, scaling problems occur primarily in the wash immediately after this acidic treatment stage.
  • the Q-stage When the Q-stage is followed by the D-stage, scaling problems occur primarily in the wash immediately after the Q- stage, which most often also is acidic. It has been shown to be a common feature, that when more than one acidic stage is included in a treatment/ bleaching sequence, it is in the washing stage after the first acidic stage that scaling problems primarily occur. This is, in itself, natural, since the suspension liquid in this position normally includes the maximum amount of released calcium.
  • Scaling problems can also occur in washing apparatus located immediately upstream of an acidic treatment/bleaching stage. In this case, acidic/neutral and calcium-rich suspension liquid (washing liquid) is delivered to the normally alkaline pulp suspension in the washing apparatus, which results in the precipitation of calcium o
  • the arosen concentrate can be dealt with in two ways. One way is, that the concentrate is mixed with the remaining quantitatively dominating spent liquor in a position during its evaporation from weak black liquor to heavy black liquor, which then is burned in the recovery boiler. The other way is that the concentrate is destroyed separately by burning in a furnace separated from the recovery boiler.
  • the suspension liquid concerned will most often contain oxalate and/or sulphate.
  • the solubility of calcium oxalate and/or calcium sulphate can be exceeded when the water content of the suspension liquid is lowered by evaporation. This results in scaling problems in the evaporation plant instead.
  • Another proposal involves precipitating calcium from acidic suspension liquid with the aid of an alkaline carbonate-containing liquid and to subsequently separate the formed precipitate.
  • the purified suspension liquid is then returned to the liquid cycle.
  • a third proposal consists of separating divalent cations from acidic suspension liquid by means of ion exchange technique.
  • a fourth proposal consists of adding substances that prevent the re-precipitation of calcium released in the suspension liquid.
  • substances are different types of polymers, and simple inorganic substances in ionic form, such as phosphate.
  • the content of carbonate ions is high in the E-filtrate and P-filtrate. At an enlargement of the contents around these stages, e.g. as a result of an increased closing of the system, primarily it will be calcium carbonate which precipitates, since pH > 8. A neutralization of these filtrates to apH ⁇ 8, e.g. by mixing with acidic neutral filtrate rich in calcium ions may cause calcium oxalate to precipitate. Calcium oxalate can also be expected to precipitate in the vicinity ofDI- and Z -stages, since these stages have high contents of both calcium and oxalate ions. The risk of precipitation in connection with the Q-stage also exists in a QP-sequence in strictly counter -current when the P-filtrate rich in oxalate ions is mixed with the Q-filtrate containing calcium ions.
  • the first alternative is to operate a completely open system as in the case of "yesterday's bleaching plant", which is no durable solution.
  • the other way is to eliminate one of the dangerous components calcium ions or oxalate ions. It will probably be necessary to reduce the content of both calcium ions and oxalate ions. For instance, the calcium ions can be dissolved from the pulp in an acidic wash and the acidic filtrate rich in calcium ions and other metal ions is removed for purification (“kidney”) (Caron, Williams, 1996) or recirculation to the smelt dissolver in the chemical recovery process (Ulmgren etal, 1994).
  • Another way of preventing process problems is to control the precipitation to a specific site in the process where the precipitate can be readily dealt with, or by precipitating calcium oxalate in the pulp and, in this way, removing the precipitate from the process. In this latter case, however, problems can occur in the paper mill due to the carry over.
  • scales tend to form e.g., in those washing apparatus that are in the immediate proximity of the stages, meaning that the washing apparatus must be cleaned frequently and therewith limit the production of bleached cellulose pulp.
  • the present invention provides a solution to the aforementioned problem and relates to a method for minimizing calcium-caused scaling problem when washing and/or increasing the consistency of and further handling of the cellulose pulp suspension which in at least two stages, including at least one bleaching stage, is treated alternately in a neutral/ acidic and alkaline environment and wherein washing liquid (suspension liquid) is passed counter-currently so that the treatment process will be essentially completely closed with respect to the flow pattern of washing liquid ( suspension liquid), characterized in that the major part of the calcium present in and on cellulose pulp fibers entering the treatment process is prevented from dissolving in the suspension liquid by adding a water soluble, oxalate- containing chemical to the suspension liquid downstream of the neutral/acidic treatment stage and/or to the pulp suspension immediately prior to or in the neutral/acidic treatment stage.
  • the invention is based on the idea concept of trying to retain the calcium in and on the pulp fibres to the greatest possible extent and to counteract the calcium leaving the pulp fibres and dissolving in the surrounding suspension liquid, even when there are used in the treatment process stages that range from a neutral to a strongly acidic environment.
  • a pH range this means a pH from 7.5 down to 2.
  • alkaline environment is meant that the cellulose pulp is treated (bleached) at a pH value of 8 or higher. This is achieved by adding a substance, i.e. a water soluble, oxalate-containing chemical, that to a high extent blocks the release of the calcium from the pulp fibres.
  • Suitable water soluble, oxalate- containing chemicals are oxalic acid and sodium oxalate. Of these two chemicals, oxalic acid is preferred because of its low price. Naturally, any suitable water soluble,oxalate- containing chemical can be used.
  • the water soluble,oxalate-containing chemical shall be added in an amount such that the mole ratio between oxalate and calcium in each position will be 0.8 or greater.
  • oxalate is formed from the pulp in both acidic and alkaline, ozidizing bleaching stages, and often in significant quantities.
  • process associated oxalate is present in the suspension liquid.
  • both the process associated oxalate and the added oxalate are included. Because the addition chemical concerned, preferably oxalic acid, commands a considerable kilo price, it is important to keep the amount added as small as possible.
  • the cellulose pulp suspension can be treated in series with oxygen gas ( 0 ) , chelating agent ( Q ) and peroxide ( P ) , or with some other non chlorine-containing oxidative bleaching agent. Impurities are removed from the pulp suspension after all given treatment stages, by washing and/or by increasing concistency in at least one stage.
  • Any known apparatus can be used, and here exemplified by single- stage or two-stage diffusers, pressurized or not, belt (flat) washer, washing (drum) filter, washing press and other presses.
  • the washing liquid (the suspension liquid) is conveyed counter-currently, so that the treatment process will be essentially fully closed with respect to the liquid flow pattern.
  • suspension liquid which can also be called bleaching spent liquor
  • substantially all suspension liquid which can also be called bleaching spent liquor
  • the mixture is passed in the form of weak black liquor to the evaporation plant of the mill and is thereafter conveyed in the form of heavy black liquor to the recovery boiler of the mill for combustion and final destruction.
  • both the oxygen gas bleaching process and the peroxide bleaching process are effected under pronounced alkaline conditions in accordance with known technology, whereas the chelating agent treatment is effected in a neutral or acidic environment.
  • the pH range is suitably 4 to 7 and the use of a pH of about 5 is probably the most common in practice.
  • the water soluble, oxalate- containing chemical shall be added preferably either to the suspension liquid at a position downstream of the Q-stage, or to the pulp suspension immediately prior to or in the Q- stage.
  • the chemical may, of course, be added to the system in both of these positions.
  • oxygen gas bleaching stages may be used initially, with or without intermediate washing of the pulp.
  • the peroxide stage can be a pressurized stage.
  • oxygen gas is normally delivered at an overpressure and such a stage normally being designated PO.
  • the cellulose pulp suspension is treated in series with oxygen gas (0), chelating agent plus chlorine dioxide (Q/D) and peroxide (P) or some other non chlorine-containing oxidative bleaching agent.
  • the water soluble, oxalate-containing chemical shall be delivered to the suspension liquid in the described sequence downstream of the Q/D-stage and/or be delivered to the pulp suspension immediately prior to or in the Q/D-stage. This additional procedure is analogous with the procedure described with reference to the first mentioned treatment sequence.
  • the cellulose pulp suspension is treated in series with oxygen gas (0), chelating agent ( Q ) , chlorine dioxide ( D ) and peroxide ( P ) .
  • This sequence includes two acidic stages, i.e. both the Q- stage and the D-stage.
  • the water soluble, oxalate-containing chemical shall be added to the suspension liquid downstream of the Q-stage and/or the D-stage and/or shall be added to the pulp suspension immediately prior to or in the Q-stage and/or the D-stage.
  • the precise manner in which the chemical is introduced into the system in this case is determined partially by the pH value that prevails in both the Q-stage and the D-stage.
  • a very low pH value in the chlorine dioxide stage should be avoided (as is known, the addition of chlorine dioxide to the pulp suspension will cause the pH value to drop) and the pH value can be increased, for instance, to the level 3-4 by means of an alkali addition.
  • a fourth conceivable sequence is 0-Z-Q-P, where both the ozone (Z) stage and the chelating agent (Q) stage is acidic. Reference is made to the statement just above with regard to the positions at which the chemical is added. Too strongly acidic treatment stages shall be avoided and it is not completely clear what happens if the ozone bleaching is carried out at a pH of about 2. In such a case, it would seem appropriate to supply the pulp suspension with an alkaline solution after completion of the ozone bleaching process and prior to conveying the pulp suspension to the following washing stage. The pH value of the pulp suspension may be increased to 4, for instance.
  • the aforedescribed treatment sequences produce bleached cellulose pulp having brightnesses that are sufficient for the intended final product in many cases, usually paper (or paperboard) of different kinds.
  • the invention enables such bleached pulp to be produced with a fully closed liquid flow pattern.
  • desiring pulp of very high brightness, such as 90% ISO or thereabove at least one further bleaching stage is necessary, for example two, possibly together with a further chelating agent stage.
  • a choice can be made between also allowing these treatment stages being included in a totally closed liquid flow system or of allowing said treatment stages to be included in an open system, i.e.
  • the invention also finds application in treatment sequences that lack chelating agent (Q) stages and which thus include solely bleaching stages and possibly extraction stages.
  • Examples of such sequences are 0-D-EO and D-EO, where EO designates an oxygen gas reinforced extraction (e.g. sodium hydroxide) stage.
  • the water soluble, oxalate- containing chemical shall be added to the suspension liquid downstream of the acidic chlorine dioxide stage (D) and/or be added to the pulp suspension immediately prior to or in the D-stage.
  • excessively high chlorine dioxide charge shall not be used, since .a high chlorine dioxide charge can result in plugging problems in the recovery boiler due to the occurrence of high chloride contents in the chemical recovery system when the liquid system is substantially fully closed.
  • One solution to this problem may be to separate sodium chloride from the electrostatic precipitator dust, although this means a costly investment.
  • the invention can also be applied when, for instance, choosing to utilize the suspension liquid (washing liquid) that is recovered in the washing stage after an acidic stage, for instance a Q-stage, and to treat that liquid flow separately in the aforedescribed manner, by evaporating the liquid and later burning the concentrate in a separate furnace distanced from the recovery boiler, or burning the concentrate in the recovery boiler mixed into the heavy black liquor.
  • the water soluble oxalate-containing chemical is preferably added to the pulp suspension immediately prior to or in the Q- stage.
  • the chemical added will prevent scaling in the evaporating apparatus in addition to standard prevention of scaling in the washing stage immediately after the Q-stage.
  • scaling is prevented only in the evaporation apparatus.
  • the invention enables a substantially fully closed liquid flow pattern to be used in the manufacture of bleached cellulose pulp with a final brightness that is sufficient for the intended final product in many instances.
  • One of the advantages obtained with fully closed liquid flow pattern is that the amount of fresh water required is reduced to a minimum and that the emission of contaminated and oxygen consuming materials to the recipient disappears .
  • the unique advantage with the invention is that the scaling problems that are automatically dependent on a fully closed liquid circulation system are reduced to a minimum and possibly eradicated. This enables bleached cellulose pulp to be produced persistently in a highly environmentally friendly manner, i.e. day after day, week after week and possibly month after month without interruption.
  • Samples of coniferous sulphate pulp having a kappa number of 17 measured in accordance with the standard method SCAN-C1:77, and a viscosity of 950 cm 3 /gram measured according to the standard method SCAN-C15:62 were taken at a consistency of 15% from a washing filter in a sulphate pulp mill.
  • the pulp was screened and thereafter washed first on a washing filter and then in a washing press.
  • the pulp was then oxygen gas bleached under alkaline conditions and washed on two washing filters in series and in a subsequent washing press.
  • the pulp was then stored in a storage tower for a given period of time, and then washed on a washing filter where pulp samples were taken in accordance with what has been said in the introduction.
  • the pulp was transferred to a laboratory, where it was centrifuged to a pulp consistency of about 33%. Eight beakers having a volumetric capacity of 3 litres were each placed in order.
  • Sodium oxalate was also added to four of these beakers so as to obtain a total content of 5 mmole oxalate per litre.
  • One molar hydrochloric acid (HC1) solution was added in different quantities to these two series of pulp suspensions, each series including four beakers and its contents, so as to obtain a variation in pH of between 6 and 3.
  • Birch wood chips were digested in batch digesters with sulphate cooking liquor, so as to obtain cellulose pulp.
  • the pulp was initially screened and then subjected to the treatment stages apparent from the flowsheet in Figure 1.
  • the screened pulp was conveyed to a belt washer 1, where the pulp was washed with suspension liquid passed counter-currently.
  • the pulp suspension was then passed to a washing press 2, where the consistency of the pulp was raised to about 30%, simultaneously with liberating the pulp from impurities to a certain extent.
  • the pulp was supplied with alkali in the form of sodium hydroxide (NaOH) in an amount corresponding to 45 kg per tonne of pulp, and magnesium in the form of magnesium sulphate (MgS0 4 ) in an amount corresponding to 0.25 kg per tonne of pulp.
  • NaOH sodium hydroxide
  • MgS0 4 magnesium in the form of magnesium sulphate
  • pulp is meant here and in the remainder of this example in connection with addition figures a pulp having a dryness of 90%.
  • the pulp was diluted with suspension liquid immediately prior to entering the oxygen gas bleaching reactor 3, to obtain a pulp consistency of 10%. 14 kg of oxygen gas were charged to the pulp suspension in the reactor 3 for each tonne of pulp.
  • the temperature was about 110°C, the time 90 minutes, and the pressure 0.3 Mpa at the top of the reactor.
  • the pulp had a kappa number of 15 and a viscosity of 1100 cm /g prior to the oxygen gas stage. The kappa number fell to 9 and the viscosity to 780 cm /g after the oxygen gas delignification (bleaching). In this position, the pulp had a brightness of 60% ISO (measured in accordance with the standard method SCAN-C 11:75).
  • the oxygen gas bleached pulp was passed to a washing press 4 and, after having been purified, was passed from there to a storage tower 5. Suspension liquid was supplied immediately upstream of the inlet to the storage tower 5, so as to obtain a pulp consistency of 10%. Subsequent to having passed the storage tower 5, the pulp was passed to a washing filter 6.
  • the pulp was passed to a bleaching tower 7 in which the pulp was treated with both chlorine dioxide ( C10 2 ) and a chelating agent in the form of EDTA.
  • Chlorine dioxide was added in an amount corresponding to 11 kg per tonne of pulp, calculated as active chlorine.
  • EDTA was added in an amount corresponding to 2 kg per tonne of pulp.
  • 1.1 kg of sulphuric acid (H 2 S0 4 ) was also added for each tonne of pulp, so as to obtain a pH value of about 4.7 in the pulp suspension.
  • the pulp consistency was 10% during this treatment stage, and the temperature of the incoming pulp was 70°C and the time 2 hours and 30 minutes. After this joint treatment stage, the pulp was passed to two serial washing filters 8 and 9.
  • the pulp suspension was then passed at a pulp consistency of 10% to a terminating peroxide bleaching stage 10.
  • 12 kg hydrogen peroxide (H 2 0 2 ) per tonne of pulp and 7.5 kg sodium hydroxide (NaOH) per tonne of pulp were added to the pulp suspension prior to said suspension entering the bleaching tower 10.
  • the temperature was 82°C and the time 3 hours.
  • the pulp was passed from the bleaching tower 10 to a washing filter 11. In this position, i.e. after the washing, the pulp had a brightness of 85% ISO.
  • the description hitherto has primarily been concentrated on the transport of the pulp suspension through the bleaching plant, in the flow sheet from left to right.
  • the transport of the washing (suspension) liquid counter-currently through the bleaching plant, in the flow sheet from right to left will now be described.
  • Suspension (washing) liquid was delivered to the forwardly moving pulp suspension through the branch line 19 in accordance with what has earlier been described.
  • the liquid recovered from beneath the washing press 4 was passed to the washing press 2 through the line 20.
  • Suspension (washing) liquid was delivered to the forwardly moving pulp suspension through the branch line 21 in accordance with what has earlier been described.
  • the liquid recovered from beneath the washing press 2 was passed to the belt washer 1, where the washing liquid meets the pulp suspension containing cooking spent liquor counter-currently.
  • the recovered mixture of cooking spent liquor and bleaching spent liquor was passed in the form of weak liquor through the line 23 for evaporation, to obtain heavy black liquor that was later burned in the recovery boiler ( these latter stages are not shown in the Figure ) .
  • the test, or trial, was commenced by adding to the pulp suspension after it had left the washing filter 6, i.e. immediately upstream of the acid stage Q/D in position 7, an oxalic acid (H 2 C 2 0 4 ) solution in a quantity corresponding to 5 kg oxalate (C 2 0 4 2 ⁇ ) per tonne of pulp, through the line 24.
  • the addition of sulphuric acid to the pulp suspension in order to obtain the desired pH value in the Q/D-stage was stopped when the supply of oxalic acid to the pulp suspension was started.
  • a high initial oxalic acid addition safely allows a stoichiometric excess of oxalate in relation to calcium to be obtained. This following lower oxalic acid addition then continued for 17 hours, whereas the fully closed suspension (washing) liquid flow pattern was maintained for a further 5 hours, i.e. after the supply of oxalic acid ceased.
  • pulp samples and suspension liquid samples were taken for determining calcium and oxalate contents.
  • the calcium content and the oxalate content of wet pulp from the washing filter 8 were determined by mixing a known quantity of pulp with water and hydrochloric acid ( HCl ) in a beaker so as to obtain a pH lower than 1.
  • the beaker was heated to a temperature of 60°C for 60 minutes, whereafter the solution was filtered through a 0.45 ⁇ m membrane filter.
  • Final determination of calcium and oxalate (C 2 0 4 2 ⁇ ) was effected with atomic absorption spectrometry and ion chromatography respectively.
  • Corresponding suspension liquid contained 3.2 mmole calcium and 0.69 mmole oxalate per litre.
  • three hours after commencing the addition of oxalic acid each litre of suspension liquid had a calcium content of 0.8 mmole, i.e. a significant drop, and an oxalate content of 1.4 mmole, i.e. twice as much.
  • the calcium content of the suspension liquid remained at 1 mmole per litre, or lower, during the whole of the oxalic acid addition period.
  • the mole ratio between oxalate and calcium in the pulp was close to 1 during the addition period, which indicates that calcium was bound to oxalate in and on the pulp fibres to a great extent.
  • Twentyfour hours after starting the trial and four hours after terminating the oxalic acid addition it was observed how the content of calcium in the pulp had fallen and the content of calcium in the suspension liquid had risen.
  • the analysis of oxalate in the same pulp samples showed that the mole ratio oxalate/calcium had fallen markedly below 1, which indicates that the substance quantity oxalate in the Q/D- stage had been too low (caused by the exclusion of an oxalic acid addition) to prevent calcium release.
  • the calcium content of the suspension liquid was 1 mmole per litre, or lower, which means 40 mg or less per litre of suspension liquid. This indicates that an at least five-fold safety margin was achieved when applying the invention during the trial period, despite the liquid flow pattern having been fully closed. This indicates that it should be possible to produce bleached cellulose pulp in accordance with the invention with a fully closed counter- current liquid flow pattern persistently, i.e. in the absence of scales in, e.g., washing apparatus.

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Abstract

Des problèmes de tartre apparaissent, par exemple, dans les appareils de lavage lors de l'utilisation d'un circuit de liquide de lavage entièrement fermé dans la production de pâte à papier blanchie. L'invention permet de réduire les problèmes de tartre dus au calcium lors du lavage et/ou de l'augmentation de la consistance des suspensions de pâte à papier, et lors d'autres traitements de la pâte à papier selon lesquels la suspension de pâte est traitée en au moins deux temps, comprenant au moins une opération de blanchiment, alternativement dans un environnement neutre/acide et alcalin, et selon lesquels le liquide de lavage (suspension liquide) circule à contre-courant de manière que le traitement se fasse sensiblement entièrement en circuit fermé par rapport au circuit du liquide de lavage (suspension liquide). Selon l'invention, la majeure partie du calcium présent dans et sur les fibres de la pâte à papier prise pour le traitement ne se dissout pas dans la suspension liquide, grâce à l'addition à la suspension liquide d'un produit chimique soluble dans l'eau et contenant un oxalate, en aval de l'opération de traitement neutre/acide, et/ou l'addition de ce produit chimique à la suspension de pâte juste avant ou pendant l'opération de traitement neutre/acide.
PCT/SE1998/000879 1997-05-14 1998-05-12 Procede de reduction des problemes lies au tartre dans la production de pate a papier blanchie, a l'aide d'un circuit ferme de liquide de lavage WO1998051855A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU74639/98A AU7463998A (en) 1997-05-14 1998-05-12 Method of minimizing scaling problems in the manufacture of bleached cellulose pulp while using an essentially fully closed washing liquid flow pattern

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
SE9701778A SE509444C2 (sv) 1997-05-14 1997-05-14 Förfarande för minimering av kalcium orsakat inkrustproblem vid framställning av blekt cellulosamassa
SE9701778-4 1997-05-14

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WO1998051855A1 true WO1998051855A1 (fr) 1998-11-19

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AU (1) AU7463998A (fr)
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Cited By (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1402092A1 (fr) * 2001-04-24 2004-03-31 Weyerhaeuser Company Pate alcaline a sciure a faible degre moyen de polymerisation et procede de production correspondant
EP1443144A1 (fr) * 2003-01-31 2004-08-04 Kvaerner Pulping Ab Procede de blanchiment de pates cellulosiques et sa appareil
WO2013135957A3 (fr) * 2012-03-12 2013-11-14 Upm-Kymmene Corporation Procédé et système de traitement des flux liquides dans une usine de pâte à papier chimique
EP2753750A1 (fr) * 2011-09-05 2014-07-16 Stora Enso Oyj Procédé pour le traitement de liqueur noire de bois de feuillus et liqueur noire de bois de feuillus traitée selon le procédé

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1995009944A1 (fr) * 1993-10-01 1995-04-13 Union Camp Patent Holding, Inc. REDUCTION DES DEPOTS PAR REGULATION DU pH ET DE LA CONCENTRATION DE SELS
WO1996035015A1 (fr) * 1995-05-02 1996-11-07 Sunds Defibrator Industries Ab TRAITEMENT DE PATE A PAPIER AVEC UN AGENT CHELATEUR SUIVI D'UN BLANCHIMENT A UN pH SUPERIEUR A 9
WO1997008381A1 (fr) * 1995-08-30 1997-03-06 Sunds Defibrator Industries Ab Procede pour empecher l'accumulation de metaux dans un processus de blanchiment de pate a papier en circuit ferme

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO1995009944A1 (fr) * 1993-10-01 1995-04-13 Union Camp Patent Holding, Inc. REDUCTION DES DEPOTS PAR REGULATION DU pH ET DE LA CONCENTRATION DE SELS
WO1996035015A1 (fr) * 1995-05-02 1996-11-07 Sunds Defibrator Industries Ab TRAITEMENT DE PATE A PAPIER AVEC UN AGENT CHELATEUR SUIVI D'UN BLANCHIMENT A UN pH SUPERIEUR A 9
WO1997008381A1 (fr) * 1995-08-30 1997-03-06 Sunds Defibrator Industries Ab Procede pour empecher l'accumulation de metaux dans un processus de blanchiment de pate a papier en circuit ferme

Cited By (9)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US6861023B2 (en) 1996-08-23 2005-03-01 Weyerhaeuser Company Process for making lyocell fiber from sawdust pulp
US7090744B2 (en) 1996-08-23 2006-08-15 Weyerhaeuser Company Process for making composition for conversion to lyocell fiber from sawdust
EP1402092A1 (fr) * 2001-04-24 2004-03-31 Weyerhaeuser Company Pate alcaline a sciure a faible degre moyen de polymerisation et procede de production correspondant
EP1402092A4 (fr) * 2001-04-24 2004-08-04 Weyerhaeuser Co Pate alcaline a sciure a faible degre moyen de polymerisation et procede de production correspondant
CN100343427C (zh) * 2001-04-24 2007-10-17 韦尔豪泽公司 具有低平均聚合度值的木屑碱法浆粕和其生产方法
EP1443144A1 (fr) * 2003-01-31 2004-08-04 Kvaerner Pulping Ab Procede de blanchiment de pates cellulosiques et sa appareil
EP2753750A1 (fr) * 2011-09-05 2014-07-16 Stora Enso Oyj Procédé pour le traitement de liqueur noire de bois de feuillus et liqueur noire de bois de feuillus traitée selon le procédé
EP2753750A4 (fr) * 2011-09-05 2015-04-22 Stora Enso Oyj Procédé pour le traitement de liqueur noire de bois de feuillus et liqueur noire de bois de feuillus traitée selon le procédé
WO2013135957A3 (fr) * 2012-03-12 2013-11-14 Upm-Kymmene Corporation Procédé et système de traitement des flux liquides dans une usine de pâte à papier chimique

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Publication number Publication date
SE509444C2 (sv) 1999-01-25
SE9701778D0 (sv) 1997-05-14
AU7463998A (en) 1998-12-08
SE9701778L (sv) 1998-11-15

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