WO2024133425A1 - Two-step impregnation in production of ctmp from maple wood - Google Patents

Abstract

There is provided a method of preparing chemithermomechanical pulp (CTMP) from maple wood, said method comprising the steps of: - pre-impregnating maple wood chips with a pre-impregnation liquid comprising alkali to obtain pre-impregnated chips; - feeding the pre-impregnated chips to an impregnation zone comprising an impregnation liquid comprising alkali using a plug screw such that the pre- impregnated chips expand in the impregnation zone and absorb the impregnation liquid, thereby providing impregnated chips; - heat-treating the impregnated chips to obtain pre-treated chips; and - defibration of the pre-treated chips, wherein the pre-impregnation liquid comprises alkali in an amount of 1-8 kg per tonne of dry wood chips, such as 2-8 kg per tonne of dry wood chips.

Description

TWO-STEP IMPREGNATION IN PRODUCTION OF ( I MP FROM MAPLE WOOD

TECHNICAL FIELD

[0001] The present invention relates to the field of production of chemithermomechanical pulp (CTMP) to be used in the manufacture of paperboard. In particular, it relates to such production using maple wood as a starting material.

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] The present disclosure provides a method of preparing chemithermomechanical pulp (CTMP) from maple wood. The method comprises the steps of:

- pre-impregnating maple wood chips with a pre-impregnation liquid comprising alkali and/or sulfite to obtain pre-impregnated chips;

- feeding the pre-impregnated chips to an impregnation zone comprising an impregnation liquid comprising alkali and/or sulfite using a plug screw such that the

- pre-impregnated chips expand in the impregnation zone and absorb the impregnation liquid, thereby providing impregnated chips;

- heat-treating the impregnated chips to obtain pre-treated chips; and defibration of the pre-treated chips, wherein the pre-impregnation liquid comprises alkali in an amount of 1-8 kg per tonne of dry wood chips, such as 2-8 kg per tonne of dry wood chips. BRIEF DESCRIPTION OF THE DRAWINGS

[0004] Figures 1-3 illustrates a system for producing CTMP according to exemplary embodiments of the present disclosure.

DETAILED DESCRIPTION

[0005] There is provided a method of preparing chemithermomechanical pulp

(CTMP) from maple wood.

[0006] The method comprises the step of pre-impregnating maple wood chips with a pre-impregnation liquid to obtain pre-impregnated chips. The preimpregnation liquid is preferably aqueous.

[0007] The pre-impregnation liquid may comprise alkali and/or sulfite. The alkali is typically NaOH. Sulfite is typically added as Na₂SO₃. The chemical form of the sulfite depends on the pH. However, the pre-impregnation liquid may also be water without such additives.

[0008] The temperature of the pre-impregnation liquid is preferably at least 7O°C, such as 7O°C-99°C, such as 8o°C-99°C. At such a relatively high temperature, the viscosity of the pre-impregnation liquid is lower, which facilitates the absorption thereof.

[0009] The average length of the maple wood chips may be below 20 mm to aid the impregnation.

[0010] A result of the pre-impregnation step is that the chips are wettened and softened before the impregnation step, which significantly increases the liquidabsorbing capacity of the chips in the latter step.

[0011] In one embodiment, the maple wood chips are provided in mixture with chips of another type of wood, preferably softwood, such as spruce. Accordingly, the maple wood chips and the chips of another wood type are co-processed in this embodiment of the method.

[0012] The step of pre-impregnating maple wood chips may comprise feeding said maple wood chips to a pre-impregnation zone comprising the pre-impregnation liquid using a plug screw (or another device compressing the chips) such that the maple wood chips expand in the pre-impregnation zone and absorb the preimpregnation liquid. [0013] Alkali (typically in the form of NaOH) is fed to the step of preimpregnating maple wood chips 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. Using a small amount of alkali in the pre-impregnation liquid, such as 1-8 kg per tonne chips, is associated with an increase in bulk and brightness of the final CTMP.

[0014] The pre-impregnation step may be preceded by a step of steaming the maple wood chips for a period of at least 10 min, such as at least 15 min. The upper limit may be 60 min, such as 30 min. The steam supplied to such a steaming step is typically of atmospheric pressure. The steaming step may be carried out in a steaming 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.

[0015] The step of steaming the maple wood chips may be preceded by a step of washing the maple wood chips with water.

[0016] The method further comprises the step of feeding the pre-impregnated chips to an impregnation zone comprising an impregnation liquid using a plug screw (or another compressing device) such that the pre-impregnated chips expand in the impregnation zone and absorb the impregnation liquid, thereby providing impregnated chips. The impregnation liquid is preferably aqueous.

[0017] The temperature of the impregnation liquid is preferably at least 70°C, such as 7O°C-99°C, such as 8o°C-99°C. At such a relatively high temperature, the viscosity of the impregnation liquid is lower, which facilitates the absorption thereof.

[0018] In one embodiment, the impregnation liquid comprises sulfite, which is typically added as Na₂SO₃. Sulfite may be added to the impregnation liquid 5-30 kg per tonne dry wood chips, such as 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. In one embodiment, sulfite may be added to the impregnation liquid in an amount of 9-25 kg per tonne of dry wood chips, such as 9-20 kg per tonne of dry wood chips. 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 can be obtained at a given brightness after bleaching. The impregnation liquid may further comprise alkali (typically NaOH).

[0019] The impregnation liquid may have a higher concentration of alkali (e.g. NaOH) than the pre-impregnation liquid (if alkali is used at all in these stages). However, it may also be the case that alkali is only included in the pre-impregnation liquid.

[0020] Alkali (e.g. NaOH) may for example be fed to the impregnation zone in an amount of 4-25 kg per tonne of dry wood chips, such as 4-20 kg per tonne of dry wood chips, such as 4-15 kg per tonne of dry wood chips. Further, alkali (e.g. NaOH) can fed at a higher rate to the impregnation zone than to the step of pre-impregnating wood chips. The rate may be measured as kg/h or kg/tonne dry wood chips.

[0021] However, in another embodiment, no alkali is fed to the impregnation zone. Accordingly, it may be the case that neither the pre-impregnation liquid nor the impregnation liquid comprises alkali (e.g. NaOH).

[0022] In one embodiment, the impregnation liquid has a pH below 10.9. Such a pH reflects a relatively low (or no) supply of NaOH, while Na₂SO₃ may still be included.

[0023] The method further comprises the step of heat-treating the impregnated chips to obtain pre-treated chips.

[0024] In one embodiment, the impregnated chips are transferred from the impregnation zone to the heat-treatment step without compressing the impregnated chips. Hence, no plug screw is used for the transfer of the impregnated chips in this embodiment. Instead, the transfer of the impregnated chips may comprise lifting the impregnated chips out of the impregnation zone using a transport screw and then allowing the impregnated chips to fall into a heating zone in which the heattreatment takes place.

[0025] The heat-treatment step may comprise heating the impregnated chips with steam having a temperature of above 140 °C, such as at least 150 °C, such as at least 160 °C. Accordingly, the method of the present disclosure may be a method of preparing high temperature CTMP (“HT-CTMP”). An upper limit for the steam temperature may be 190 °C. [0026] The residence time in the heat-treatment step is preferably no more than two minutes.

[0027] The method also comprises the step of defibration of the pre-treated chips. This step is typically carried out at high consistency, e.g. 35 wt.% - 50 wt.%. The defibration of step d) is typically carried out under pressure.

[0028] The pulp obtained from the defibration step may be subjected to refining (such as low consistency refining) and/or bleaching. Embodiments of such refining and/or bleaching are described in the examples section below with reference to figures 1-3. The consistency of said low consistency refining is typically 3 wt.% - 7 wt.%, such as 4 wt.% - 6 wt.%.

EXAMPLES

Exemplary embodiment of a system for producing CTMP

[0029] Figures 1-3 illustrate exemplary embodiments of a system for producing CTMP from maple wood, which is a relatively dry and dense type of wood that is comparatively difficult to impregnate.

[0030] A chipper 101 is used to prepare chips from the maple wood and optionally another type of wood, such as softwood. The maple wood chips prepared by the chipper 101 are preferably 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. However, it may be considered to more practical to cut all chips to the shorter length (< 20 mm). The settings of a conventional wood chipper can be adjusted to the desired lengths. Shorter chips from such a chipper are also thinner.

[0031] 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 maple wood 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.

[0032] 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 6o°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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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, typically by supplying atmospheric steam to the 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.

[0037] 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 preimpregnation chamber 116. During the expansion, the chips absorb pre-impregnation liquid. The temperature of the pre-impregnation liquid is preferably 8o°C-99°C. The pre-impregnation liquid may comprise alkali (e.g. fed to the pre-impregnation chamber in an amount of 3-5 kg NaOH per tonne of dry wood chips), but is preferably water without any alkali or sulfite. 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.

[0038] 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. The impregnation liquid is water to which sulfite and optionally alkali may have been added.

[0039] The temperature of the impregnation liquid is preferably 8o°C-99°C.

[0040] 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. The chips treated in the reactor 120 are transferred to the chip defibrator 114 without flashing off any steam on the way.

[0041] In the chip defibrator 114, the dry matter content is about 45 wt.% - 50 wt.% (in case there is no plug screw between the steaming area 124 and the chip defibrator 114, the dry matter content may however be as low as 30 wt.%). The pulp from the chip 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 first 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.

[0042] The pulp from the first 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 129 has a consistency of about 30 wt.% - 35 wt.% 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 nonsilicate stabilizer and/or a chelating agent, such as DTPA or EDTA). The HC-bleached pulp 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.

[0043] 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.

[0044] In the former case, which is illustrated in figure 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.

[0045] In the latter case, which is illustrated in figure 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.

[0046] 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.

[0047] 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) from maple wood, said method comprising the steps of:

- pre-impregnating maple wood chips with a pre-impregnation liquid to obtain preimpregnated chips;

- feeding the pre-impregnated chips to an impregnation zone comprising an impregnation liquid using a plug screw such that the pre-impregnated chips expand in the impregnation zone and absorb the impregnation liquid, thereby providing impregnated chips;

- heat-treating the impregnated chips to obtain pre-treated chips; and

- defibration of the pre-treated chips, wherein the pre-impregnation liquid comprises alkali in an amount of 1-8 kg per tonne of dry wood chips, such as 2-8 kg per tonne of dry wood chips.

2. The method of claim 1, wherein the impregnation liquid comprises sulfite.

3. The method of claim 1 or 2, wherein sulfite is fed to the impregnation zone in an amount (calculated as Na₂SO₃) of 9-25 kg per tonne of dry wood chips, such as 9-20 kg per tonne of dry wood chips.

4. The method of any one of the preceding claims, wherein the impregnated chips are transferred from the impregnation zone to the heat-treatment step without compressing the impregnated chips.

5. The method of claim 4, wherein the transfer of the impregnated chips comprises lifting the impregnated chips out of the impregnation zone using a transport screw and then allowing the impregnated chips to fall into a heating zone in which the heattreatment takes place.

6. The method of any one of the preceding claims, wherein the heat-treatment step comprises heating the impregnated chips with steam having a temperature of above 140 °C, such as at least 150 °C, such as at least 160 °C.

7. The method of any one of the preceding claims, wherein the step of preimpregnating wood chips comprises feeding said maple wood chips to a preimpregnation zone comprising the pre-impregnation liquid using a plug screw such that the maple wood chips expand in the pre-impregnation zone and absorb the preimpregnation liquid.

8. The method of any one of the preceding claims, wherein the pre-impregnation step is preceded by a step of steaming the maple wood chips for a period of at least 10 min, such as at least 15 min.

9. The method of claim 8, wherein the steam supplied in the steaming step is atmospheric.

10. The method of claim 8 or 9, wherein the step of steaming is carried out in a steaming bin in which the temperature is 90-99 °C.

11. The method of any one of claims 8-10, wherein the step of steaming the maple wood chips is preceded by a step of washing the maple wood chips with water.

12. The method of any one of the preceding claims, wherein the average length of the maple wood chips is below 20 mm.

13. The method of any one of the preceding claims, wherein no alkali is fed to the step of pre-impregnating maple wood.

14. The method of any one of the preceding claims, wherein the impregnation liquid comprises no alkali.

15. The method of any one of the preceding claims, wherein the maple wood chips are provided in mixture with chips of another type of wood, preferably softwood, such as spruce.