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
  • D21C1/00—Pretreatment of the finely-divided materials before digesting
  • D21C1/06—Pretreatment of the finely-divided materials before digesting with alkaline reacting 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)
• • 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
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/02—Pulping cellulose-containing materials with inorganic bases or alkaline reacting compounds, e.g. sulfate 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
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes
  • D21C3/222—Use of compounds accelerating the pulping 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
  • D21C3/00—Pulping cellulose-containing materials
  • D21C3/22—Other features of pulping processes
  • D21C3/26—Multistage processes

Definitions

• the present invention relates to a process for preparing kraft pulp in which lignocellulosic material is treated with recycled process liquors, polysulfide and fresh re-causticized white liquor for removing lignin therefrom. More particularly, the present invention relates to the use of polysulfide white liquor (or another polysulfide source) in the early stages of modern displacement kraft batch cooking, and the advantage thereof in terms of improved pulp yield.
• lignocellulosic material In the various kraft pulping processes, lignocellulosic material, most commonly wood chips, is generally treated at elevated temperatures with alkaline cooking liquor containing sodium hydroxide and sodium hydrogen sulfide, called white liquor. In these processes a significant part of the cellulosic material, especially hemicelluloses, are partly lost due to reactions with the white liquor. In kraft pulping conditions hemicelluloses undergo alkaline peeling reactions, de-polymerization, in which the hemicelluloses are dissolved as organic acids into the cooking liquor. A known iact is that by a polysulfide treatment, hemicelluloses are stabilized and are thereafter more resistant to cooking liquor in kraft cooking conditions, which leads to higher yield, from the processes lignocellulosic substance input.
• polysulfide-containing white liquor is obtained from causticized white liquor by various processes, which partially oxidize the liquor's sodium sulfide into polysulfide. These processes typically use oxidation catalysts to polymerize the white liquor's hydrogen sulfide.. Also, polysulfide can be produced directly by elemental sulftir addition into white liquor. Polysulfide-containing white liquor is called polysulfide white liquor or orange liquor. The use of polysulfide white liquor has been quite straightforward in conventional kraft cooking operations.
• the normal white liquor has been converted to polysulfide white liquor, and that liquor has been used in the same manner as normal white liquor.
• the polysulfide After the impregnation into the wood chips in the digester and during the temperature rise to above 100 °C, the polysulfide has reacted with the carbohydrates making them less vulnerable to alkaline degradation. Later during the cook, when temperature has risen over 140 °C, the rest of the polysulfide has rapidly broken down through thermal decomposition.
• all the conventional cooking systems have used cooking liquors having initial temperatures well below 100 °C and the temperature in the digester has been elevated slowly or in steps, the polysulfide has had the possibility to react and iacilitate higher pulp yield.
• White liquor is introduced into the digester as "hot white liquor", which has been heated up using the heat of the previously generated spent liquor, and steam.
• hot is meant to designate a liquor having a temperature above its boiling point at ambient pressure, i.e. a liquor that must be stored in a pressurized vessel.
• the hot white liquor temperature in typical displacement batch cooking is 140 - 180 °C.
• polysulfide has been an impossible chemical. Polysulfide is unstable at higher temperatures, for instance at those used in kraft cooking; the chemical begins to break down al temperatures above 100°C, and it degrades completely in a few minutes at 150 °C. Thus at the typical process temperature of the hot white liquor, polysulfide white liquor is not stable at all. This fact has disabled the use of polysulfide in the modern displacement kraft pulping processes developed for higher energy and chemical economy and improved pulp quality.
• Characteristic for the displacement kraft batch process is to recover hot black liquor from the end of the cooking by displacement and then re-use the energy in the subsequent batches. Partly this is done by using the hot spent cooking liquor as such, and partly by heating the white liquor with the heat of the hot spent liquor leaving the digester system or collected for re-use at low temperature.
• This process arrangement leads to high temperatures in the input liquor accumulators.
• a low temperature white liquor typically at 80 °C in the storage tank after re-causticization, cannot be used anymore, and the environment for stable polysulfide in the white liquor does not exist anymore.
• a further characteristic feature of a modem displacement batch process is to combine energy efficiency and efficient use of residual and fresh cooking chemicals to achieve, in addition to energy efficiency, high pulp strength and good delignification through hydroxyl and hydrosulfide concentration and temperature profiling at different stages of the cooking cycle (H ⁇ ljanen, Tikka, EP-B 520452, Tikka, U.S. Pat 5,183,535).
• anthraquinone acts by reducing carbohydrate end groups, stabilising them against alkaline peeling and producing anthrahydroquinone, which is alkali soluble.
• Anthrahydroquinone reduces lignin, making it more reactive. In this process, anthraquinone is formed again and may react with carbohydrates.
• a particular problem is fouling of the surfaces of heat exchangers in the evaporation plant, leading to a decrease in heat transfer.
• These evaporator fouling problems are typically related to calcium.
• calcium-containing material dissolves into the black liquor from the Hgnocellulosic material.
• the cook proceeds with heating, the temperature increases and no essential liquor exchange occurs. Then, a major part of the dissolved calcium-containing material in the cooking liquor is broken down, calcium carbonate is formed and as a result, a major part of the calcium is resorbed onto the lignocellulosic material in the digester.
• evaporation of the black hquor can normally he carried out without problems caused by precipitation of calcium.
• the evaporation problems with black liquors originating from the early stages of cooking are typically related to calcium- containing material dissolved in the early stages of a cook.
• the dissolved calcium- containing material has not been degraded, and the amount of calcium bound to the dissolved material in the black hquor is high.
• this problem may be solved by re-introducing liquor having a high calcium precipitating potential into the digester as a displacement liquor at the end of a cook. At this point, the digester temperature is sufficiently high to cause calcium liberation.
• the calcium is then removed together with the pulp, and does not enter the evaporation and regeneration cycle.
• a method for producing pulp using polysulfide white liquor involves a kraft batch process which comprises the addition of polysulfide white liquor in the impregnation and pretreatment in the very 'front-end' of the cooking process, and a finalizing dehgnif ⁇ cation stage with white liquor and optional cooking catalyst addition, which is facilitated by the modified 'front- end' process chemistry resulting in improved pulp yield.
• a volume of polysulfide white liquor is added as the first portion of Hquor to enter the digester, or admixed with the impregnation liquor prior to this being pumped into the digester.
• impregnation liquor means a liquor stored at a temperature below its atmospheric boiling point, used for a first immersion of the lignocellulosic raw material in a batch pulping process.
• lignocellulosic material entering a displacement batch process is thus pretreated with polysulfide- containing process liquors in conditions favourable for pre-treatment and polysulfide stability.
• a set of displacement batch process arrangements capable of taking advantage of the polysulfide white liquor, are presented
• the required dose of fresh polysulfide white liquor is introduced in the initial stage of the process.
• the impregnation stage provides for both impregnation and polysulfide pre-treatment reactions.
• An impregnation liquor circulation collects the displaced impregnation liquor which is reused to fill and pressurize the digester after the first fresh liquor fill.
• the polysulfide concentration and dosage remain at a high level, and only the liquor balance excess is left over to be sent to chemicals recovery or re-use later in the cooking sequence.
• the displacement cooking process sequence then continues with hot fills of black and white liquors, cooking and final displacement.
• -An optional feature in the present invention is a new way to use a cooking catalyst, such as anthraquinone (AQ).
• AQ anthraquinone
• the cooking catalyst can be used to maintain a normal kraft cooking rate and production level.
• the catalyst may be introduced into the digester together with the hot white hquor. By this means it will not escape due to displacements, and on the other hand, it arrives in the hot cooking stage where it is most needed. As a result, the technical and economical benefits of the displacement batch cooking are retained, while the advantages of polysulfide and cooking catalyst can be fully exploited.
• FIGURES Figure 1 shows a block diagram of a kraft displacement batch cooking system according to the invention.
• the top of the figure defines the required tanks, leaving or entering streams denoted by letters.
• the cooking stage sequence is displayed therebelow, chronologically starting from the top and the vertically indicated streams representing the liquors passing through the digester during the various stages.
• the letters denoting the various entering and leaving streams correspond to the source or destination in the tank scheme.
• Figure 2 shows a further embodiment of a kraft displacement batch cooking system according to the invention.
• the kraft batch cook is started by charging the digester with wood chips, or with another lignocellulosic material. After the lignocellulosic material charge, the digester is filled in a stepwise sequence using a volume of fresh polysulfide white hquor XI from polysulfide white liquor tank 6, and impregnation liquor Al from tank 4, topping the digester full of liquor.
• the volume of polysulfide white liquor maybe added as a separate first portion, forming a raising pad of liquor as further liquor is pumped in from below.
• the polysulfide white hquor is mixed into the impregnation hquor.
• An overflow, A2, conducted to the impregnation Hquor tank 4, is preferred in order to remove air and the diluted first front of hquor.
• the digester is pressurized and impregnation is completed during an appropriate period of time.
• the cooking process is then continued by pumping in a volume of hot black Hquor Bl from hot black liquor accumulator 1.
• the cooler impregnation liquor A3, displaced by hot black Hquor, is conducted to impregnation Hquor tank 4 to be re-used in subsequent batches.
• Pumping a volume C of hot white Hquor from tank 3 into the digester then continues the cooking sequence. Together with white liquor C, an amount of cooking catalyst may be dosed into the digester.
• the Hquor Dl displaced by hot Hquor above about the atmospheric boiling point, in conducted to hot black liquor accumulator 2.
• the digester is close to the final cooking temperature.
• the final heating-up is carried out using direct or indirect steam heating and digester re-circulation.
• the spent liquor is ready to be displaced with wash filtrate El from tank 5.
• a volume A4 from impregnation liquor tank 4 may be used as a first portion of displacement Hquor in order to deactivate the potential scale-forming calcium therein, as disclosed in EP 1 242674.
• the first displaced volume B2 displaced by wash filtrate is conducted to black liquor accumulator 1.
• the second portion D2 of displaced black Hquor which is diluted by the wash filtrate but is still above atmospheric boiling point, is conducted to the hot black liquor accumulator 2.
• the digester contents are discharged for further processing of the pulp. The above cooking sequence may then be repeated
• the hot black Hquor from accumulator 2 is conducted to the evaporation plant, first being used to heat white liquor due for tank 3, and/or water, by means of heat exchange.
• Tank 6 is provided for storage of polysulfide white Hquor, supphed from a plant for polysulfide generation.
• a volume of polysulfide white liquor corresponding to 25-90 % of the total active alkali dosage and resulting in 0,5% to 5% polysulfide dosage on abs. dry wood is added into the digester before the impregnation fill is completed. Impregnation is carried out using temperature of between 70 °C and 120 °C, preferably between 85 - 95 °C, and a time of between about 20 to about 120 minutes, preferably between about 30 to about 60 minutes, more preferably between about 25 to about 40 minutes.
• the polysulfide white hquor is added as a separate portion before the introduction of further Hquor.
• the volume of polysulfide white liquor corresponds to 50-75 % of the total active alkali dosage for the relevant batch.
• a cooking catalyst is used
• Preferable catalysts are anthraquinone, anthraquinone sulfonate, and hydroanthraquinone. Any other appropriate redox-catalyst as anticipated by those skilled in the art may be used
• the foHowing fractions displaced from the digester - hot cooking spent liquor fraction B2 and
• hot black Hquor accumulators 1 and 2 of Figure 1 are combined.
• the black liquor flow to the evaporation plant is drawn from the single hot black Hquor accumulator, the stream being used to heat, by means of heat exchange, white Hquor heading for white liquor accumulator 3, and/or water, as shown for tank 2 in Fig 1.
• the stream from impregnation liquor tank 4 to hot black liquor tank 1 may be heated by means of heat exchange with the stream from hot black Hquor accumulator 2 to the evaporation plant.
• one or more additional white liquor charges may be carried out during the cook.
• the principal advantage of the process of the present invention is that the 'front-end' of the kraft displacement batch cooking system is rendered more effective from a chemical point of view, and in regard to process arrangements.
• the polysulfide charge used reflects the improvement in pulp yield from the digested HgnoceUulosic material.
• a hot black liquor pre-treatment stage was commenced by introducing hot black liquor (HBL 1, 150°C, app.20 gEA/1) to the bottom of the digester, displacing the spent impregnation Hquor (DWBL) out from the top of the digester.
• the hot black liquor pre-treatment stage duration was 30 minutes.
• hot white liquor 121 gEA (NaOH)/l, sulfidity 37%) charge was introduced into the digester bottom displacing the corresponding volume of spent hot black Hquor (RHBL) out of the digester.
• RHBL spent hot black Hquor
• a 45 minute heat-up sequence with cooking liquor circulation raised the temperature from 150°C to the cooking temperature of 167°C.
• a white Hquor charge was introduced into the digester bottom displacing the corresponding volume of spent hot black liquor out of the digester (DCBL 1).
• terminal displacement was started by introducing cool DWBL and wash filtrate (WF) in a sequence into the digester bottom displacing the hot spent black liquor out of the digester top (DCBL 2).
• the terminal displacement stage duration was approximately 40 minutes.
• the pulp was disintegrated, washed with deionised water, screened and analysed. The cooking procedure was repeated and thus equilibrium in the cooking process was obtained and the cooks started to repeat themselves corresponding to the industrial batch cooking system.
• a hot black liquor pre-treatment stage was carried out by introducing hot black liquor (HBL 1, 150°C, app. 20 gEA/1) to the bottom of the digester of the digester, displacing the spent impregnation liquor from the top of the digester (DWBL).
• the hot black liquor pre- treatment stage duration was 30 minutes.
• hot white Hquor (121 gEA (NaOH) l, sulf ⁇ dity 37%) charge was introduced, displacing the corresponding volume of spent hot black Hquor from the digester (RHBL). Together with the WL, a cooking catalyst was charged mixed with a small volume of hot black Hquor. A 45 minute heat-up sequence with cooking Hquor circulation raised the temperature from 150°C to the cooking temperature of 167 °C. At 250 H-factors, a further white liquor charge was introduced into the digester bottom displacing the corresponding volume of spent hot black liquor from the digester (DCBL 1).
• terminal displacement was started by introducing a volume of cool DWBL and wash filtrate (WF) in a sequence into the digester bottom displacing the hot spent black Hquor out of the digester top (DCBL 2).
• the first displaced portion of hot black liquor together with the hot black liquor displaced earlier during the cooking sequence (DCBL 2 + DCBL 1) covered the 171 volume needed for the next cook's hot black Hquor (HBL 1).
• the pulp was disintegrated, washed with deionised water, screened and analysed. The cooking procedure was repeated, equilibrium in the cooking process was reached and the cooks started to repeal themselves corresponding to the industrial batch cooking system.
• the reference cook follows the same addition and temperature sequences as the cooks according to the invention. AU cooks were digested to the same target kappa number. A clear yield improvement correlating with polysulfide addition can be observed
• Example 2 The effect of anthraquinone to preserve the rate of deHgnification during cooking It is seen, that the reference cook requires a significantly higher H-factor to reach the target kappa number.
• PS pad AQ 0% 70 % 1,45 1106 19,1 27,1 44,3 0,15
• PS pad AQ 0,1% 70 % 1,45 1096 18,1 27 45,8 0,1

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• 2003
  • 2003-12-31 FI FI20031929A patent/FI120361B/fi not_active IP Right Cessation
• 2004
  • 2004-12-22 US US10/564,373 patent/US20060175029A1/en not_active Abandoned
  • 2004-12-22 CA CA002530704A patent/CA2530704A1/en not_active Abandoned
  • 2004-12-22 DE DE602004019689T patent/DE602004019689D1/de not_active Expired - Fee Related
  • 2004-12-22 EP EP04805227A patent/EP1702101B1/de not_active Expired - Lifetime
  • 2004-12-22 WO PCT/FI2004/050196 patent/WO2005064075A1/en not_active Ceased
  • 2004-12-22 AT AT04805227T patent/ATE423866T1/de active

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