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, said method comprising the steps of: a) providing maple wood chips having an average length below 20 mm, such as below 19 mm; b) washing the maple wood chips with water; c) steaming the washed maple wood chips for a period of at least 15 minutes to obtain pre-steamed chips; d) impregnating the pre-steamed chips to obtain impregnated chips; e) heating the impregnated chips to obtain pre-treated chips; and f) defibration of the pre-treated chips.

[0004] Preferably, the chips in step c) are steamed for a period of at least 20 minutes, such as at least 22 minutes. BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Figures 1-3 illustrates a system for producing CTMP according to embodiments of the present disclosure.

DETAILED DESCRIPTION

[0006] A method of preparing chemithermomechanical CTMP from maple wood.

The method comprises the step of: a) providing maple wood chips having an average length below 20 mm, such as below 19 mm.

[0007] Such comparatively small chips facilitate the downstream impregnation, which in turn results in a more efficient method and an end product of higher quality. Furthermore, chips having an average length below 20 mm can more easily form a plug in a downstream feeding step (typically a plug screw) and thus lead to improved efficiency of the feeding step.

[0008] Step a) may comprise separating and diverting over-sized wood chips.

[0009] 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.

[0010] Preferably, the wood chips used in the present disclosure are fresh wood chips. Fresh wood chips require a lower amount of bleaching chemicals (e.g., hydrogen peroxide and/or NaOH) to reach a given brightness compared to old wood chips.

[0011] In one embodiment, the wood chips may have a dry content of 45-65 wt.%, such as 50-60 wt.%. This enables the formation of an elastic plug when feeding the wood chips to an impregnation zone and thus improves the impregnation of the chips.

[0012] The method further comprises the steps of: b) washing the maple wood chips with water; and c) steaming the washed maple wood chips for a period of at least 15 minutes, such as at least 20 minutes, such as at least 22 minutes, to obtain pre-steamed chips. [0013] An upper limit for the steaming period of step c) may for example be 60 min or 30 min.

[0014] In one embodiment, the maple wood chips are also steamed prior to step b). However, such steaming is typically less extensive than that of step c).

[0015] The steam that is applied to the chips in step c) is typically of atmospheric pressure. The steaming step c) may be carried out in a steaming bin. The steaming bin should be large enough for a residence time of at least 15 min, preferably at least 20 min, more preferably at least 22 min. 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 in the range of 90°C to 99°C.

[0016] The method further comprises the step of: d) impregnating the pre-steamed chips to obtain impregnated chips.

[0017] In one embodiment, step d) comprises feeding the pre-steamed chips to an impregnation zone comprising an impregnation liquid. A plug screw (or another compressing device) is typically used to feed the pre-steamed chips such that the presteamed chips expand in the impregnation zone and absorb the impregnation liquid.

[0018] 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.

[0019] The impregnation liquid is preferably aqueous.

[0020] In another embodiment, step d) comprises:

- feeding the pre-steamed chips to a pre-impregnation zone comprising a preimpregnation liquid using a plug screw (or another compressing device) such the presteamed chips expand in the pre-impregnation zone and absorb the preimpregnation liquid, thereby providing pre-impregnated chips; and

- 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.

[0021] In this embodiment, the temperatures of the pre-impregnation liquid and the impregnation liquid are preferably at least 70°C, such as 7O°C-99°C, such as 8O°C-99°C. At such temperatures, the viscosity of the liquids is lower, which facilitates the absorption thereof.

[0022] The pre-impregnation liquid is typically water to which NaOH may be added.

[0023] Alkali (e.g. NaOH) may be fed to the pre-impregnation zone in an amount of 1-8 kg per tonne of dry maple wood chips, such as 2-8 kg per tonne of dry maple wood chips, such as 3-5 kg per tonne of dry maple wood chips. However, the preimpregnation may also be carried out without NaOH. In such case, the preimpregnation liquid comprises no NaOH.

[0024] It is preferred that the impregnation liquid comprises sulfite (typically added as Na ₂ SO ₃ ). 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.

[0025] Sulfite may be fed to the impregnation zone in an amount (calculated as Na ₂ SO ₃ ) of 4-30 kg per tonne of dry wood chips. Preferably, the amount is 9-30 kg per tonne of dry wood chips, such as 15-25 kg per dry tonne wood chips. In one embodiment, the sulfite is fed to the impregnation zone in an amount (calculated as Na ₂ SO ₃ ) of 4-25 kg per tonne of dry wood chips, such as 4-18 kg per tonne dry wood chips.

[0026] In one embodiment, NaOH is fed to the impregnation zone in an amount of less than 15 kg per tonne dry wood chips, such as less than 10 kg per tonne dry wood chips.

[0027] Furthermore, the impregnation liquid may have a pH below 10.9. Such a pH reflects are relatively low (or no) supply of NaOH.

[0028] The method further comprises the step of: e) heating the impregnated chips to obtain pre-treated chips.

[0029] Step e) typically comprises applying steam to the impregnated chips. The steam may have temperature of above 140 °C, such as at least 150 °C, such as at least 160 °C. Accordingly, the CTMP produced by the method may be high temperature CTMP (HT-CTMP). An upper limit for the steam temperature maybe 190 °C. [0030] In one embodiment, the impregnated chips are transferred from step d) to step e) 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 heat -treatment takes place.

[0031] The residence time in step e) is preferably no more than two minutes.

[0032] The method further comprises the step of: f) defibration of the pre-treated chips.

[0033] The defibration of step f) is typically carried out under pressure.

[0034] Step f) is typically carried out at high consistency, e.g. 35%-5O%. The high- consistency defibration may be succeeded by low-consistency (e.g. 3%-7%, such as 4%-6%) refining and/or bleaching. Embodiments of such low-consistency refining and/or bleaching are described in the examples section below with reference to figures 1-3.

EXAMPLES

Exemplary embodiment of a system for producing CTMP

[0035] Figures 1-3 illustrate exemplary embodiments of a system for producing CTMP from maple wood. The present inventors have realized that maple is a relatively dry and dense type of wood that is comparatively difficult to impregnate.

[0036] A chipper 101 is used to prepare chips from maple wood and optionally another type of wood, such as softwood. The maple wood chips prepared by the chipper 101 are relatively short, such as < 20 mm (preferably 18 mm), to aid impregnation. Softwood chips are generally easier to impregnate and can hence be longer, such as 22-24 mm. Also, all chips may be of the same lengths. The settings of a conventional wood chipper can be adjusted to the desired lengths. Shorter chips from such a chipper are also thinner.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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. In more detail, chips are typically fed to a top portion 107a of the bin 107 and then withdrawn from a bottom portion 107b of the bin by means of a screw 107s or another transport arrangement. Further, steam is typically supplied to the bottom portion 107b of the bin 107. The temperature in the top portion iO7t of the bin 107 is typically in the range of 9O°C to 99°C, such as 93°C to 98°C. The residence time of the chips in the bin 107 is at least 15 min, such as 20 min, such as 22 min, to improve the downstream impregnation. The pre-steaming step softens the chips and make them more elastic, which is advantageous in the downstream impregnation steps.

[0043] The steamed chips from the pre-steaming bin 107 are subjected to impregnation in one or two steps.

[0044] In case of one-step impregnation, a plug screw 108 feeds the steamed chips into a reactor 109. The steamed chips, which were compressed in the plug screw 108, expands in a bath of impregnation liquid 110 in the reactor 109. During the expansion, the chips absorb impregnation liquid. The temperature of the impregnation liquid is preferably 8o°C-99°C. The impregnation liquid typically comprises sulfite (supplied as Na2SO ₃ or NaHSO ₃ , typically Na2SO3) and optionally alkali (e.g. NaOH). The (expanded and impregnated) chips are lifted from the bath of impregnation liquid 110 by means of a transport screw 111 and are then allowed to fall over an edge 112 and into steaming area 113 of the reactor 109, in which they are heated by steam, which preferably has a temperature above 14O°C. The chips treated in the reactor 109 are transferred to a chip defibrator 114 without flashing off any steam on the way.

[0045] In case of two-step impregnation, a plug screw 115 feeds the steamed chips into a pre-impregnation chamber 116. The steamed chips, which were compressed in the plug screw 115, expands in a bath of aqueous pre-impregnation liquid 117 in the pre-impregnation chamber 116. During the expansion, the chips absorb preimpregnation liquid. The temperature of the impregnation liquid is preferably 8o°C- 99°C. The pre-impregnation liquid may comprise alkali and optionally sulfite. The (expanded and impregnated) chips are lifted from the bath of pre-impregnation liquid 117 by means of a transport screw 118. A plug screw 119 then feeds the preimpregnated chips 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 8o°C-99°C. The impregnation liquid typically comprises sulfite (supplied as Na ₂ SO ₃ or NaHSO ₃ , typically Na ₂ SO ₃ )) and optionally alkali, e.g. NaOH. 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, which preferably has a temperature above 14O°C. The chips treated in the reactor 120 are transferred to the chip defibrator 114 without flashing off any steam on the way.

[0046] In the chip defibrator 114, the dry matter content is about 45/6-50% (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 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 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 dewatering press 127.

[0047] 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 IO%-12%. 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 3O%-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%-6%. 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%- 4%- 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] To use the produced CTMP in the production of paperboard, MC pulp from the MC tower 147 is led to a board-making machine.