Maple HT-CTMP

Abstract

There is provided a method of producing a high temperature chemithermomechanical pulp (HT-CTMP), said method comprising the steps of: —impregnating maple wood chips with an impregnation liquid comprising sulfite and NaOH to obtain impregnated chips; —heating the impregnated chips from the impregnation step by applying steam having a temperature of at least 150° C., such as at least 160° C. to obtain pre-treated chips; and—defibration of the pre-treated chips from the heating step to obtain a pulp, wherein the amount of NaOH supplied to the impregnation step is less than 10.0 kg per dry tonne of chips, such as less than 5 kg per dry tonne of chips. A HT-CTMP formed from maple wood is also provided.

Description

TECHNICAL FIELD

[0001] The present invention related to the field of pulps to be used in the production of paperboard and in particular chemithermomechanical pulp (CTMP).BACKGROUND

[0002] Chemithermomechanical 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 heat-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 producing a high temperature chemithermomechanical pulp (HT-CTMP), said method comprising the steps of:

[0004] impregnating maple wood chips with an impregnation liquid comprising sulfite and NaOH to obtain impregnated chips;

[0005] heating the impregnated chips from the impregnation step by applying steam having a temperature of at least 150° C., such as at least 160° C. to obtain pre-treated chips; and

[0006] defibration of the pre-treated chips from the heating step to obtain a pulp, wherein the amount of NaOH supplied to the impregnation step is less than 10.0 kg per dry tonne of chips, such as less than 5 kg per dry tonne of chips.

[0007] The present disclosure further provides a high temperature chemithermomechanical pulp (HT-CTMP) formed from maple wood, said HT-CTMP having a shives content measured according to TAPPI T275 of below 0.5% and a bulk measured according to ISO 534:2011 after sheet forming according to ISO 5269-1:2005 of above 5.0 cm3 / g.BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIGS. 1-3 show exemplary embodiments of a full-scale system for producing HT-CTMP according to the present disclosure.DETAILED DESCRIPTION

[0009] As a first aspect of the present disclosure, there is provided a method of producing a high temperature chemithermomechanical pulp (HT-CTMP).

[0010] The method comprises the steps of:

[0011] impregnating maple wood chips with an impregnation liquid comprising sulfite and NaOH to obtain impregnated chips;

[0012] heating the impregnated chips from the impregnation step by applying steam having a temperature of at least 150° C., such as at least 160° C. to obtain pre-treated chips; and

[0013] defibration of the pre-treated chips from the heating step to obtain a pulp, wherein the amount of NaOH supplied to the impregnation step is less than 10.0 kg per dry tonne of chips, such as less than 5 kg per dry tonne of chips.

[0014] Using a small amount of alkali in the impregnation liquid, such as less than 10 kg per tonne chips, is associated with a satisfactory bulk and brightness of the final HT-CTMP.

[0015] The pulp obtained from the defibration step may for example be the HT-CTMP of the second aspect (described below).

[0016] The maple wood chips are typically washed and then pre-steamed before the impregnation step. Embodiments of the washing and pre-steaming as well as other preparatory steps are described in the examples section below.

[0017] The amount of sulfite (calculated as Na2SO3) supplied to the impregnation step may for example be from 4.0 to 30.0 kg per dry tonne of chips, such as 5.0 to 25.0 kg per dry tonne of chips, such as 5.0 to 18.0 kg per dry tonne of chips. It has been found that a HT-CTMP having satisfactory properties can be obtain while keeping the sulfite consumption low. Furthermore, the addition of sulfite in the impregnation liquid may increase the brightness of the HT-CTMP prior to bleaching and thus a pulp with a higher bulk can be obtained at a given brightness after bleaching.

[0018] Further, the amount (by weight) of sulfite (calculated as Na2SO3) supplied to the impregnation step is preferably higher than the amount of NaOH (by weight) supplied to the impregnation step. In one embodiment, the amount of sulfite (calculated as Na2SO3) is at least 100% higher than the amount of NaOH.

[0019] In one embodiment, the impregnation liquid has a pH below 10.9. Such a pH reflects a relatively low (or no) supply of NaOH.

[0020] 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.

[0021] In the impregnation step, the chips may be fed to an impregnation zone comprising the impregnation liquid using a plug screw (or another compressing device) such that the chips expand in the impregnation zone and absorb the impregnation liquid, thereby providing the impregnated chips.

[0022] In another embodiment, the impregnation step comprises:

[0023] feeding the chips to a pre-impregnation zone comprising a pre-impregnation liquid using a plug screw (or another compressing device) such that the chips expand in the pre-impregnation zone and absorb the pre-impregnation liquid, thereby providing pre-impregnated chips; and

[0024] feeding the pre-impregnated chips to an impregnation zone comprising the 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 the impregnated chips.

[0025] In this embodiment, the temperatures of the pre-impregnation liquid and the impregnation liquid may be at least 70° C., such as 70° C.-99° C., such as 80° C.-99° C. At such temperatures, the viscosity of the liquids is lower, which facilitates the absorption thereof.

[0026] The pre-impregnation liquid is preferably water to which alkali may be added. It is however preferred to use an aqueous pre-impregnation liquid to which neither alkali nor sulfite has been added.

[0027] An alternative to a pre-impregnation step is to soak the chips in water for a period of at least 10 hours.

[0028] In one embodiment, the impregnated chips are transferred to the heating 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 liquid using a transport screw and then allowing the impregnated chips to fall into a heating zone in which the steam-based heat-treatment of step c) takes place.

[0029] The defibration step is preferably high consistency defibration, e.g. defibration at a consistency above 30%, such as above 40%.

[0030] The method may further comprise bleaching and / or low consistency (LC) refining of the pulp.

[0031] As a second aspect of the present disclosure, there is provided a high temperature chemithermomechanical pulp (HT-CTMP) formed from maple wood. The HT-CTMP has a shives content measured according to TAPPI T275 of below 0.5% and a bulk measured according to ISO 534:2011 after sheet forming according to ISO 5269-1:2005 of above 5.0 cm3 / g.

[0032] High temperature chemithermomechanical pulp (HT-CTMP) is defined as CTMP produced according to a process in which impregnated chips are heated with steam having a temperature of at least 150° C.

[0033] The HT-CTMP of the second aspect may have a Canadian Standard Freeness (CSF) of above 640 ml, which CSF is measured according to ISO 5267-2:2001 after disintegration according to ISO 5263-3:2004. An upper limit may be 700 ml.

[0034] The HT-CTMP of the second aspect may be unbleached.EXAMPLESPilot Trial

[0035] Maple wood chips (average length=18 mm) were soaked in water overnight.

[0036] After the soaking period, the chips were subjected to pre-steaming. The pre-steamed chips were then divided into different batches that were impregnated with different aqueous impregnation liquids comprising NaOH and Na2SO3. The washed and pre-steamed chips were fed to the impregnation vessel using a plug screw such that the chips expanded in the impregnation liquid. The amounts of NaOH and Na2SO3 supplied in the impregnations were as follows:

[0037] Batch 1, 4.5 kg NaOH and 6.3 kg Na2SO3 per tonne dry chips;

[0038] Batch 2, 9.7 kg NaOH and 11.9 kg Na2SO3 per tonne dry chips;

[0039] Batch 3, 20.8 kg NaOH and 20.8 kg Na2SO3 per tonne dry chips; and

[0040] Batch 4, 8.7 kg NaOH and 8.9 kg Na2SO3 per tonne dry chips.

[0041] The impregnated chips were then heated by the application of steam. For batches 1 and 4, the temperature of the steam was 165° C. For batches 2 and 3, the temperature of the steam was 140° C. The residence time in the steaming step was less than 2 minutes. The pretreated chips from the steaming step were subjected to defibration at a consistency of about 30 wt. %. Pulp samples were obtained after different degrees of defibration. The freeness, shives content and bulk of the pulp samples were then measured (see table 1 below).

[0042] Table 1. Pulp properties. “Deg. of defib.” means degree of defibration and is the net specific energy consumption (kWh / dry tonne wood chips) in the defibration step. “Shives” means 0.15 mm shives (Sommerville). CSF means Canadian Standard Freeness measured according to ISO 5267-2:2001 after disintegration according to ISO 5263-3:2004.ChemistryBulk(NaOH / )SteamDeg. ofCSFShives(cm3 / BatchNa2SO3temp.defib.(ml)(%)g)10.45% / 0.63%165° C.313.17704.283.945.0110.45% / 0.63%165° C.374.95677.412.22N / A10.45% / 0.63%165° C.488.62648.940.345.2310.45% / 0.63%165° C.584.88640.800.185.1010.45% / 0.63%165° C.699.48565.520.024.6520.97% / 1.19%140° C.280.06726.595.18N / A20.97% / 1.19%140° C.354.58694.232.085.2720.97% / 1.19%140° C.469.25662.121.505.1920.97% / 1.19%140° C.527.80633.760.545.1420.97% / 1.19%140° C.633.52601.040.164.7032.08% / 2.08%140° C.279.31691.944.304.9432.08% / 2.08%140° C.350.52638.331.444.5732.08% / 2.08%140° C.431.23586.050.644.3832.08% / 2.08%140° C.509.68577.250.464.1932.08% / 2.08%140° C.625.05521.330.084.1440.87% / 0.89%165° C.336.97731.203.22N / A40.87% / 0.89%165° C.447.27678.770.66N / A40.87% / 0.89%165° C.557.57644.570.22N / A40.87% / 0.89%165° C.656.74639.340.08N / A40.87% / 0.89%165° C.704.30443.730.063.32

[0043] As shown in the table above, a HT-CTMP having a shives content below 0.5%, a bulk above 5.0 cm3 / g and a CSF above 640 ml could be obtained with batch 1, which was treated by steam having a temperature of 165° C. after impregnation with a liquid comprising NaOH and sulfite. This combination of properties could however not be obtained with batches 2 and 3, which were treated by steam having a temperature of 140° C. after impregnation.

[0044] Unfortunately, a lot of bulk data is missing for batch 4, which was also treated by steam having a temperature of 165° C. after impregnation with a liquid comprising NaOH and sulfite. It is still notable that a HT-CTMP having a shives content of 0.22% (i.e. well below 0.5%) and a CSF of 644 ml was obtained.Exemplary Embodiment of a Full-Scale System for Producing HT-CTMP

[0045] FIGS. 1-3 illustrate exemplary embodiments of a system for producing HT-CTMP.

[0046] A chipper 101 is used to prepare chips from maple wood and optionally another type of wood, such as softwood, e.g. spruce wood. It is preferable to prepare maple wood 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. However, the softwood chips may also have the same length as the maple wood chips. The settings of a conventional wood chipper can be adjusted to achieve desired chip lengths. Shorter chips from such a chipper are also thinner.

[0047] The chips from the chipper are stored in at least one silo 102. If a second type of wood is also used, a silo for each type of wood may be provided, such as a maple wood chips silo 102a and softwood chips silo 102b. In case of more than one wood type, a chips mixing system 103 may be arranged downstream the silos 102a, 102b. Accordingly, a chips mixture having a desired ratio of maple wood chips to softwood chips may be provided.

[0048] The chips from the chips mixing system 103 are optionally stored in a maturation silo 104 for a period of at least 24 h (typically about 72 h) in an aerobic environment. 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.

[0049] Another option is to design the chips silo(s) 102, 102a, 102b as maturation silo(s). A benefit of this option is that the maturation time and temperature can be individually adapted to the respective wood types.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] The steamed chips from the pre-steaming bin 107 are subjected to impregnation in one or two steps.

[0054] 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 aqueous impregnation liquid 110 in the reactor 109. During the expansion, the chips absorb impregnation liquid. The temperature of the impregnation liquid is preferably 80° C.-99° C. The impregnation liquid comprises sulfite and alkali (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 having a temperature above 150° C. The chips treated in the reactor 109 are transferred to a chips defibrator 114 without flashing off any steam on the way.

[0055] 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 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 aqueous pre-impregnation liquid may comprise NaOH and / or sulfite. However, it may be preferred to use water without such additives as the pre-impregnation liquid. 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 pre-impregnated 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 80° C.-99° C. The impregnation liquid comprises sulfite and alkali. The (expanded and pre-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 150° C. The chips treated in the reactor 120 are transferred to the chips defibrator 114 without flashing off any steam on the way.

[0056] In the chips defibrator 114, the dry matter content may be 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 defibrated chips 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 dewatering press 127.

[0057] The pulp from the first dewatering press 127 has undergone chemical treatment, heat treatment by high temperature steam and mechanical treatment (i.e. defibration) and is thus a HT-CTMP. This pulp may be used in the production of paperboard without further chemical treatment or refining. I may also be treated further as described below.

[0058] 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%-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 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 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%-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.

[0059] 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.

[0060] 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.

[0061] 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 47.

[0062] 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.

[0063] 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 producing a high temperature chemithermomechanical pulp (HT-CTMP), said method comprising the steps of:impregnating maple wood chips with an impregnation liquid comprising sulfite and NaOH to obtain impregnated chips;heating the impregnated chips from the impregnation step by applying steam having a temperature of at least 150° C. to obtain pre-treated chips; anddefibration of the pre-treated chips from the heating step to obtain a pulp,wherein the amount of NaOH supplied to the impregnation step is less than 10.0 kg per dry ton of chips.

2. The method of claim 1, wherein the amount of sulfite (calculated as Na2SO3) supplied to the impregnation step is from 4.0 to 30.0 kg per dry ton of chips.

3. The method of claim 1, wherein the impregnated chips are transferred to the heating step without compressing the impregnated chips.

4. The method of claim 1, wherein the maple wood chips are compressed when fed to the impregnation step such that the maple wood chips expand in the impregnation liquid.

5. The method of claim 1, wherein the impregnation step is preceded by a pre-impregnation step in which the maple wood chips are pre-impregnated with an aqueous pre-impregnation liquid.

6. The method of claim 5, wherein the maple wood chips are compressed when fed to the pre-impregnation step such that the maple wood chips expand in the pre-impregnation liquid.

7. The method of claim 1, wherein the defibration step is high consistency defibration, e.g. defibration at a consistency above 30%.

8. The method of claim 1, further comprising low consistency (LC) refining of the pulp.

9. The method of claim 1, further comprising bleaching of the pulp.

10. A high temperature chemithermomechanical pulp (HT-CTMP) formed from maple wood, said HT-CTMP having a shives content measured according to TAPPI T275 of below 0.5% and a bulk measured according to ISO 534:2011 after sheet forming according to ISO 5269-1:2005 of above 5.0 cm3 / g.

11. The HT-CTMP of claim 11 having a Canadian Standard Freeness (CSF) of above 640 ml, which CSF is measured according to ISO 5267-2:2001 after disintegration according to ISO 5263-3:2004.

12. The method of claim 1, wherein the steam temperature is at least 160° C.

13. The method of claim 1, wherein the amount of NaOH supplied to the impregnation step is less than 5 kg per dry ton of chips.

14. The method of claim 2, wherein the amount of sulfite is 5 to 18 kg per dry ton of chips.

15. The method of claim 7, the defibration is at a consistency of above 40%.

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