Two-step impregnation in HT-CTMP production

Abstract

There is provided a method of preparing chemithermomechanical pulp (CTMP), said method comprising the steps of: pre-impregnating wood chips with a pre-impregnation liquid to obtain pre-impregnated wood chips; feeding the pre-impregnated wood chips to an impregnation zone comprising an impregnation liquid comprising sulfite and / or alkali using a plug screw such that the pre-impregnated wood chips expand in the impregnation zone and absorbs the impregnation liquid, thereby providing impregnated wood chips; transferring the impregnated wood chips to a heating zone without compressing the impregnated wood chips; heating the impregnated wood chips in the heating zone with steam having a temperature of above 140° C., such as at least 150° C., such as at least 160° C., to obtain pre-treated wood chips; and defibration of the pre-treated wood chips, wherein the residence time in the heating zone is no more than two minutes.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of production of high temperature chemithermomechanical pulp (HT-CTMP) to be used in the manufacture of paperboard.BACKGROUND

[0002] Chemithermomechancial pulp (CTMP) is a high yield pulp which can provide a high bulk and has been used since 1960s. CTMP is produced by mild chemical impregnation of wood chips, followed by a heat treatment to soften the wood. The treated wood chips are then subjected to defibration / refining (typically in several steps) and optionally bleaching. The obtained CTMP typically has comparatively high bulk, preferably in combination with low shives content. The process can be further improved by using higher temperatures during the heat treatment. Using steam of relatively high temperature in the heat treatment typically leads to a decrease in the energy input needed during the pressurized defibration step.SUMMARY

[0003] An objective of the present disclosure is to realize more of the potential of the high temperature (HT) step in production of HT-CTMP, in particular when such production is based on hardwood.

[0004] To meet this objective, there is provided a method of preparing chemithermomechanical pulp (CTMP), said method comprising the steps of:

[0005] pre-impregnating wood chips with a pre-impregnation liquid to obtain pre-impregnated wood chips;

[0006] feeding the pre-impregnated wood chips to an impregnation zone comprising an impregnation liquid comprising alkali and / or sulfite using a plug screw such that the pre-impregnated wood chips expand in the impregnation zone and absorbs the impregnation liquid, thereby providing impregnated wood chips;

[0007] transferring the impregnated wood chips to a heating zone without compressing the impregnated wood chips;

[0008] heating the impregnated wood chips in the heating zone with steam having a temperature of above 140° C., such as at least 150° C., such as at least 160° C., to obtain pre-treated wood chips; and

[0009] defibration of the pre-treated wood chips,wherein the residence time in the heating zone is no more than two minutes.BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIGS. 1-3 illustrate a system for producing HT-CTMP according to embodiments of the present disclosure.DETAILED DESCRIPTION

[0011] The present disclosure provides a method of preparing chemithermomechanical pulp (CTMP).

[0012] The method comprises the step of pre-impregnating wood chips with a pre-impregnation liquid to obtain pre-impregnated wood chips. The pre-impregnation liquid typically comprises alkali such as NaOH.

[0013] The temperature of the pre-impregnation liquid is preferably at least 70° C., such as 70° C.-99° C., such as 80° C.-99° C. At such a relatively high temperature, the viscosity of the pre-impregnation liquid is lower, which facilitates the absorption thereof.

[0014] The pre-impregnation liquid is preferably aqueous.

[0015] In a preferred embodiment, the wood chips comprise hardwood chips, such as birch wood chips or maple wood chips. As an example, at least 55% by dry weight of the wood chips may be hardwood chip, such as birch wood chips or maple wood chips. The average length of the hardwood chips may be below 20 mm to aid impregnation.

[0016] The pre-impregnation step may comprise feeding wood chips to a pre-impregnation zone comprising the pre-impregnation liquid. In such case, a plug screw (or another compressing device) is preferably used to feed the wood chips. Thereby, the wood chips expand in the pre-impregnation zone and absorb the pre-impregnation liquid.

[0017] In one embodiment, the wood chips supplied to the pre-impregnation step has undergone pre-steaming. Accordingly, the method may further comprise a step of pre-steaming the wood chips prior to the pre-impregnation step. The residence time of the wood chips in the pre-streaming step may be at least 10 min, such as at least 15 min, such as at least 20 min, such as at least 22 min. The upper limit may be 60 min, such as 30 min. The steam supplied to the pre-steaming step is typically of atmospheric pressure. The pre-steaming step may be carried out in a steam bin. Typically, the chips are fed to an upper part of the steaming bin and withdrawn from a lower part of the steaming bin. Further, the steam is typically supplied in the lower part of the steaming bin. The temperature in the upper part of the steaming bin is typically 90° C.-99°C.

[0018] In one embodiment, the pre-impregnation liquid comprises alkali (typically in the form of NaOH). In such case, NaOH may be fed to the pre-impregnation step in an amount of 1-8 kg per tonne of dry wood chips, such as 2-8 kg per tonne of dry wood chips, such as 3-5 kg per tonne of dry wood chips. Reducing the amount of NaOH in the pre-impregnation liquid (and impregnation liquid), may result in a CTMP having increased bulk and brightness.

[0019] The step of pre-steaming the maple wood chips may be preceded by a step of washing the maple wood chips with water.

[0020] The method further comprises the step of feeding the pre-impregnated wood chips to an impregnation zone using a plug screw (or another compressing device). A plug screw is however preferred. The impregnation zone comprises an impregnation liquid. The impregnation liquid comprises alkali (typically NaOH) and / or sulfite (preferably added as Na2SO4). The pre-impregnated wood chips expand in the impregnation zone and absorb the impregnation liquid, thereby providing impregnated wood chips.

[0021] The temperature of the impregnation liquid is preferably at least 70° C., such as 70° C.-99° C., such as 80° C.-99° C. At such a relatively high temperature, the viscosity of the impregnation liquid is lower, which facilitates the absorption thereof.

[0022] The impregnation liquid is preferably aqueous.

[0023] In one embodiment, alkali in the form of NaOH is fed to the impregnation zone in an amount of 5-25 kg per tonne of dry wood chips, 9-25 kg per tonne of dry wood chips, such as 9-20 kg per tonne of dry wood chips. In one embodiment, the alkali in the form of NaOH is fed to the impregnation zone in an amount of 5-20 kg per tonne of dry wood chips, such as 5-15 kg per tonne dry wood chips. This is associated with obtaining a satisfactory bulk and brightness of the final CTMP.

[0024] In one embodiment, Na2SO3 is fed to the impregnation zone in an amount of 5-30 kg per tonne of dry wood chips, such as 15-25 kg per tonne of dry wood chips. In another embodiment, Na2SO3 is fed to the impregnation zone in an amount of 5-25 kg per tonne dry wood chips, such as 5-20 kg per tonne dry wood chips, such as 5-18 kg per tonne dry wood chips. It has been found that a CTMP having satisfactory properties can be obtained while having a low sulfite consumption. Furthermore, the addition of sulfite in the impregnation liquid may increase the brightness of the CTMP prior to bleaching and thus a pulp with a higher bulk may be obtained at a given brightness after bleaching.

[0025] The impregnation liquid may comprise a higher concentration of alkali (e.g. NaOH) than the pre-impregnation liquid. Similarly, alkali may be fed at a higher rate to the impregnation zone than to the step of pre-impregnating wood chips. However, it may also be the case that neither the pre-impregnation liquid nor the impregnation liquid comprises NaOH or another type of alkali.

[0026] In one embodiment, the impregnation liquid comprises sulfite and has a pH below 10.9. Such a pH reflects are relatively low (or no) supply of NaOH.

[0027] The method further comprises the step of transferring the impregnated wood chips to a heating zone without compressing the impregnated wood chips. Hence, no plug screw is used for the transfer of the impregnated wood chips. Instead, the transfer of the impregnated chips may comprise lifting the impregnated wood chips out of the impregnation zone using a transport screw and then allowing the impregnated wood chips to fall into a heating zone in which the heat-treatment takes place.

[0028] The method further comprises the step of heating the impregnated wood chips in the heating zone with steam to obtain pre-treated wood chips. The temperature of the steam used for said heating is above 140° C., preferably at least 150° C., such as at least 160° C. Accordingly, the CTMP produced by the method can be referred to as HT-CTMP. An upper limit for the steam temperature may be 190° C.

[0029] The residence time in the heating zone is no more than two minutes. This results in sufficient softening of the chips while maintaining a low energy consumption.

[0030] The method also comprises the step of defibration of the pre-treated wood chips. This step is typically carried out at high consistency, e.g. 35 wt. %-50 wt. %. The high-consistency defibration may be succeeded by low-consistency refining and / or bleaching. Embodiments of such low-consistency refining and / or bleaching is described below in the Examples section.

[0031] The defibration of the pre-treated wood chips is typically carried out under pressure.EXAMPLEExemplary Embodiment of a System for Producing HT-CTMP

[0032] FIGS. 1-3 illustrate exemplary embodiments of a system for producing CTMP.

[0033] A chipper 101 is used to prepare chips from wood. In case of hardwood, in particular maple wood, it is preferable to prepare chips that are relatively short, such as <20 mm, to aid impregnation. Softwood chips are generally easier to impregnate and can hence be longer, such as 22-24 mm. The settings of a conventional wood chipper can be adjusted to achieve such lengths. Shorter chips from such a chipper are also thinner.

[0034] The chips from the chipper 101 are typically stored in at least one silo 102. When different types of wood are mixed in the system, there is typically one silo for each type of wood, such as one hardwood chips silo 102a and one softwood chips silo 102b. When there is more than one type of chips, a chips mixing system 103 is typically arranged downstream the silos 102a, 102b.

[0035] The chips from the chips mixing system 103 are optionally stored in an aerobic environment in a maturation silo 104 for a period of at least 24 h (typically about 72 h). A typical temperature in the maturation silo 104 is 60° C., which can be achieved by feeding low-pressure steam into the maturation silo 104. The treatment of the chips in the maturation silo 104 degrades triglycerides. The degradation products can then be extracted in downstream process steps.

[0036] Another option is to design the chip silos 102a, 102b as maturation silos. A benefit of this option is that the maturation time and temperature can be individually adapted to the respective wood types.

[0037] Yet another option is to place the maturation silo 104 between the chips washing arrangement 106 and the pre-steaming bin 107 described below. It is also possible to omit the maturation step.

[0038] Before being impregnated, the chips are washed in a chips washing arrangement 106. Upstream the chips washing arrangement 106, a conditioning device 105 may be arranged. The conditioning device 105 is typically a chip steaming bin. The purpose of the conditioning device 105 is to provide chips of fairly constant temperature. The conditioning device 105 may also, to some extent, reduce variations in moisture content. During cold winter months, ice on the chips is melted in the conditioning device 105, which facilitates the downstream washing and processing. Hence, the conditioning device 105 may be particularly advantageous when there is no upstream maturation silo. In case there is an upstream maturation silo, the conditioning device 105 may be omitted.

[0039] In the chips washing arrangement 106, the chips are typically soaked and agitated in water and then dewatered. The washed and dewatered chips are then steamed in a pre-steaming bin 107. The residence time of the chips in the pre-steaming bin 107 is typically at least 10 min. The pre-steaming step softens the chips and make them more elastic, which is advantageous in the downstream impregnation steps.

[0040] A plug screw 115 feeds the steamed chips from the pre-steaming bin into a pre-impregnation chamber 116. The steamed chips, which were compressed in the plug screw 115, expands in a bath of pre-impregnation liquid 117 in the pre-impregnation chamber 116. During the expansion, the chips absorb pre-impregnation liquid. The temperature of the pre-impregnation liquid is preferably 80° C.-99° C. The pre-impregnation liquid may comprise alkali. In such case, alkali can be fed to the pre-impregnation chamber in an amount of 3-5 kg per tonne of dry wood chips. From a bulk-saving perspective, it may however be preferred to use warm water without any NaOH or other alkali as the impregnation liquid. The (expanded and impregnated) chips are lifted from the bath of pre-impregnation liquid 117 by means of a transport screw 118. The pre-impregnation step further softens the chips and make them more elastic, which facilitates the downstream steps of impregnation and heat-treatment. In other words, the pre-impregnation step assists in realizing the full potential of the “HT” technology in HT-CTMP production.

[0041] A plug screw 119 feeds the pre-impregnated chips from the impregnation chamber into a reactor 120. The pre-impregnated chips, which were compressed in the plug screw 119, expands in a bath of impregnation liquid 121 in the reactor 120. During the expansion, the chips absorb impregnation liquid, which preferably has a temperature of 80° C.-99° C. The impregnation liquid preferably comprises sulfite and optionally alkali. As an example, Na2SO3 is preferably fed to the reactor in an amount of 10-20 kg per tonne of dry wood chips.

[0042] The (expanded and impregnated) chips are lifted from the bath of impregnation liquid 121 by means of a transport screw 122 and are then allowed to fall over an edge 123 and into steaming area 124 of the reactor 120, in which they are heated by steam having a temperature above 140° C., preferably at least 150° C., such as at least 160° C. The residence time in the steaming area 124 is less than two minutes. The chips treated in the reactor 120 are transferred to the chips defibrator 114 without flashing off any steam on the way.

[0043] In the chips defibrator 114, the dry matter content is about 45%-50% (in case there is no plug screw between the steaming area 124 and the chips defibrator 114, the dry matter content may however be as low as 30%). The pulp from the chips defibrator 114 is subjected to flashing in a steam separator 125 and then pulped in a first pulper 126. The pulp from the first pulper 126 is then treated in a first dewatering press 127. The pressate from the dewatering press 127 contains extractives (and dissolved wood substances and residual chemicals) that are unwanted in the final CTMP product. Separation of extractives by pressing in this position is advantageous since the pulp still has very high freeness (typically >650 ml or even >700 ml) and is thus easily dewatered. Limiting the residence time in the first pulper 126 to below 10 min (typically about 3 min) is advantageous since it limits the time available to the extractives to be adsorbed onto the fibers before the first dewatering press 127.

[0044] The pulp from the dewatering press 127 is subjected to middle consistency (MC) bleaching in a MC bleach tower 128 using unreacted peroxide from the downstream high consistency (HC) bleaching and, if needed, make-up quantities of NaOH and peroxide. MC means 10 wt. %-12 wt. %. The MC-bleached pulp is treated in a second dewatering press 129 also producing a pressate. The pulp from the second dewatering press has a consistency of about 30%-35% and is subjected to high consistency (HC) bleaching in a HC bleach tower 130 using fresh peroxide and alkali (and optionally a peroxide stabilizer, such as a silicate or a non-silicate stabilizer and / or a chelating agent, such as DTPA or EDTA). The HC-bleached fibers from the HC bleach tower 130 are pulped in a second pulper 131 (residence time: <10 min, such as about 3 min) to produce a pulp having a consistency of about 4 wt. %-6 wt. %. This pulp is then subjected to low consistency (LC) refining in LC refiners 132. A third dewatering press 133 then separates a third pressate from the LC-refined pulp. The fibers from the third dewatering press 133 are pulped in a third pulper 134 (residence time: <10 min, such as about 3 min) to produce a pulp having a consistency of 2 wt. %-4 wt. %. Screens 135 are then used to separate a reject from the pulp from the third pulper 134. The separated reject is collected in a reject tank 136.

[0045] The design of the remaining parts of the system depends on if only market pulp is produced (i.e. all CTMP is subjected to flash drying and baling) or if there is an adjacent board-making machine to which at least part of the CTMP is supplied without drying.

[0046] In the former case, which is illustrated in FIG. 2, the pulp from the screens 135 are cleaned in cleaners 137 to provide cleaned pulp and second reject that is collected in a second reject tank 138. The cleaners 137 are preferably cyclones that separate unwanted heavy particles. The cleaned pulp is then filtered in a disc filter 139 and collected in a MC tower 140. From the pulp from the MC tower 140, a fourth dewatering press 141 produces dewatered fibers and a fourth pressate. The dewatered fibers are led to an arrangement for fiber treatment and shredding 142 and then to a flash drying arrangement 143. Finally, bales of the dried fibers from the flash drying arrangement 143 are formed in a baling arrangement 144.

[0047] In the latter case, which is illustrated in FIG. 3, the pulp from the screens is filtered in a disc filter 145 and treated in a fourth dewatering press 146 such that a fourth pressate and an MC pulp are obtained. The MC pulp is collected in a MC tower 147.

[0048] To produce (dried) market pulp, a fifth dewatering press 148 produces dewatered fibers and a fifth pressate from MC pulp from the MC tower 147. The dewatered fibers are led to an arrangement for fiber treatment and shredding 149 and then to a flash drying arrangement 150. Finally, bales of the dried fibers from the flash drying arrangement 150 are formed in a baling arrangement 151.

[0049] To use the produced CTMP in the production of paperboard, MC pulp from the MC tower 147 is led to a board-making machine.

Claims

1. A method of preparing chemithermomechanical pulp (CTMP), said method comprising the steps of:pre-impregnating wood chips with a pre-impregnation liquid to obtain pre-impregnated wood chips;feeding the pre-impregnated wood chips to an impregnation zone comprising an impregnation liquid comprising sulfite and / or alkali using a plug screw such that the pre-impregnated wood chips expand in the impregnation zone and absorbs the impregnation liquid, thereby providing impregnated wood chips;transferring the impregnated wood chips to a heating zone without compressing the impregnated wood chips;heating the impregnated wood chips in the heating zone with steam having a temperature of above 140° C., to obtain pre-treated wood chips; anddefibration of the pre-treated wood chips,wherein the residence time in the heating zone is no more than two minutes.

2. The method of claim 1, wherein the impregnation liquid comprises sulfite and has a pH below 10.9.

3. The method of claim 1, wherein the amount of sulfite (calculated as Na2SO3) fed to the impregnation zone is 10-30 kg, per dry ton wood chips supplied to step b).

4. The method of claim 1, wherein the step of pre-impregnating wood chips comprises feeding wood chips to a pre-impregnation zone comprising the pre-impregnation liquid using a plug screw such that the wood chips expand in the pre-impregnation zone and absorbs the pre-impregnation liquid.

5. The method of claim 1, wherein the wood chips comprise hardwood.

6. The method of claim 1, wherein the step of transferring the impregnated wood chips comprises lifting the impregnated wood chips out of the impregnation zone using a transport screw and then allowing the impregnated wood chips to fall into the heating zone.

7. The method of claim 1, wherein the wood chips supplied to the pre-impregnation step has undergone pre-steaming.

8. The method of claim 1, wherein alkali is fed to the step of pre-impregnating wood chips in an amount of 1-8 kg per ton of dry wood chips.

9. The method of claim 1 claims, wherein alkali is fed at a higher rate to the impregnation zone than to the step of pre-impregnating wood chips.

10. The method of claim 1, wherein alkali is fed to the impregnation zone in an amount of 9-25 kg per tonne of dry wood chips.

11. The method of claim 7, wherein the pre-impregnation liquid comprises no alkali.

12. The method of claim 3, wherein the amount of sulfite (calculated as Na2SO3) fed to the impregnation zone is 15-25 kg per dry ton wood chips supplied to step b).

13. The method of claim 5, wherein the hardwood is birch or maple.

14. The method of claim 7, wherein the pre-steaming is for a time period of at least 10 min.

15. The method of claim 8, wherein alkali is fed to the step of pre-impregnating wood chips in an amount of 3-5 kg per ton of dry wood chips.

16. The method of claim 10, wherein alkali is fed to the impregnation zone in an amount of 9-20 kg per ton of dry wood chips.

17. The method of claim 1, wherein the heating zone has a temperature of at least 160° C.

Images