HT-CTMP Formed from a Maple Wood / Softwood Mixture

Abstract

There is provided a high temperature chemithermomechanical pulp (HT-CTMP) formed from a mixture of maple wood and softwood, wherein the dry weight ratio of maple wood to softwood in said mixture is between 92:8 and 50:50. A method of producing HT-CTMP 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] 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 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 high temperature chemithermomechanical pulp (HT-CTMP) formed from a mixture of maple wood and softwood, wherein the dry weight ratio of maple wood to softwood in said mixture is between 92:8 and 50:50.

[0004] The present disclosure further provides a method of producing a HT-CTMP comprising the steps of:

[0005] a) mixing maple wood chips and softwood chips to obtain a mixture, wherein the dry weight ratio of maple wood to softwood in said mixture is between 92:8 and 50:50;

[0006] b) impregnating the chips of the mixture with an impregnation liquid (that preferably comprises sulfite) to obtain impregnated chips;

[0007] c) applying steam having a temperature of at least 150° C. to the impregnated chips to obtain pretreated chips; and

[0008] d) defibration the pretreated chips.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] FIGS. 1-3 shows a full-scale system for producing HT-CTMP according to embodiments of the present disclosure.

[0010] FIG. 4 shows results obtained in Pilot trial 1 described below. “100maple” denotes the reference batch. “85maple” denotes Mixture (i). “70maple” denotes Mixture (ii) impregnated with an impregnation liquid comprising NaOH.

[0011] FIG. 5 is a plot of shives content values (%, y axis) at different CSF values (ml, x axis) based on results obtained in Pilot trial 2 described below. Solid lines represent a steam temperature of 140° C. Dotted lines represent a steam temperature of 165° C. Circles represent batch 1. Squares represent batch 2. Diamonds represent batch 3. Triangles represent batch 4.

[0012] FIG. 6 shows results obtained in Pilot trial 1 described below. “100maple” denotes the reference batch. “70maple” denotes Mixture (ii) impregnated with an impregnation liquid comprising NaOH. “70maple / o NaOH” denotes Mixture (ii) impregnated with an impregnation liquid lacking NaOH.

[0013] FIG. 7 shows results obtained in Pilot trial 3 described below for pulps bleached to an ISO brightness of about 80. “100Map / repeat / Br80” denotes the reference batch. “70Map / Br80” denotes Mixture (ii) impregnated with an impregnation liquid comprising NaOH. “70Map / o NaOH / Br81” denotes Mixture (ii) impregnated with an impregnation liquid lacking NaOH. “85Map / Br80” denotes Mixture (i).

[0014] FIG. 8 shows results obtained in Pilot trial 3 described below for pulps bleached to an ISO brightness of about 75. “100Map / repeat / Br75” denotes the reference batch. “70Map / Br75” denotes Mixture (ii) impregnated with an impregnation liquid comprising NaOH. “70Map / oNaOH / Br76” denotes Mixture (ii) impregnated with an impregnation liquid lacking NaOH. “85Map / Br75” denotes Mixture (i).DETAILED DESCRIPTION

[0015] As a first aspect of the present disclosure, there is provided a high temperature chemithermomechanical pulp (HT-CTMP) formed from a mixture of maple wood and softwood, wherein the dry weight ratio of maple wood to softwood in said mixture is between 92:8 and 50:50. The softwood is for example spruce wood.

[0016] 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.

[0017] In one embodiment, the dry weight ratio of maple wood to softwood in said mixture is between 90:10 and 60:40. In one example, the dry weight ratio of maple wood to softwood in said mixture is between 90:10 and 75:25. In another example, dry weight ratio of maple wood to softwood in said mixture is between 75:25 and 60:40.

[0018] As shown in the Examples section below, the present disclosure enables the production of an unbleached or bleached HT-CTMP of according to the first aspect, which has a bulk measured according to ISO 534:2011 of at least 4.0 cm3 / g after sheet forming according to ISO 5269-1:2005.

[0019] As also shown in the Examples section below, the present disclosure enables the production of an unbleached or bleached HT-CTMP according to the first aspect, which has a tensile index measured according to ISO 1924-3:2005 of at least 4.0 Nm / g after sheet forming according to ISO 5269-1:2005.

[0020] In case the HT-CTMP of the first aspect is bleached, it may be bleached to a brightness of at least 74%, such as at least 78%, as measured according to ISO 2470-1:2016 and ISO 3688:1999.

[0021] In embodiments of the first aspect, the HT-CTMP is bleached and has at least one, preferably at least two, more preferably at least three of the following features:

[0022] a brightness of at least 74% as measured according to ISO 2470-1:2016 and ISO 3688:1999;

[0023] a bulk measured according to ISO 534:2011 of at least 3.9 cm3 / g after sheet forming according to ISO 5269-1:2005;

[0024] a tensile index measured according to ISO 1924-3:2005 of at least 9.0 Nm / g after sheet forming according to ISO 5269-1:2005; and

[0025] a freeness of at least 510 ml, which freeness is measured according to ISO 5267-2:2001 after disintegration according to ISO 5263-3:2004.

[0026] In a particularly interesting embodiment, the bleached HT-CTMP has all four features listed in the previous paragraph.

[0027] An even more desired embodiment of the first aspect has all four features, but with a brightness of at least 76%.

[0028] In one embodiment, the freeness is at least 540 ml, such as 540-640 ml.

[0029] As a second aspect of the present disclosure, there is provided a method of producing a high temperature chemithermomechanical pulp (HT-CTMP), such as a HT-CTMP of the first aspect.

[0030] The method comprises the step of:

[0031] a) mixing maple wood chips and softwood chips to obtain a mixture, wherein the dry weight ratio of maple wood to softwood in said mixture is between 92:8 and 50:50.

[0032] Various embodiments of the mixture are described in connection with the first aspect above. The average length of the maple wood chips is preferably below 20 mm.

[0033] The method further comprises the step of:

[0034] b) impregnating the chips of the mixture with an aqueous impregnation liquid to obtain impregnated chips.

[0035] The chips from step a) are typically washed and then pre-steamed before being impregnated in step b). Embodiments of the washing and pre-steaming as well as other preparatory steps are described in the examples section below.

[0036] 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.

[0037] In step b), 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.

[0038] In another embodiment, step b) comprises:

[0039] 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

[0040] 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.

[0041] In this embodiment, the temperatures of the pre-impregnation liquid and the impregnation liquid are preferably 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.

[0042] 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.

[0043] An alternative to a pre-impregnation step is to soak the chips in water for a period of at least 10 hours.

[0044] In one embodiment, the impregnation liquid comprises no NaOH, such as no alkali at all.

[0045] In another embodiment, the impregnation liquid comprises NaOH. In such an embodiment, it is preferred that less than 15 kg NaOH per dry tonne wood chips is supplied to step b). Using a low amount of NaOH or no NaOH may result in an increase in bulk and tensile index of the final HT-CTMP.

[0046] The impregnation liquid preferably comprises sulfite (typically added as Na2SO3). As an example, the amount of Na2SO3 supplied to step b) is 4-30 kg per dry tonne wood chips supplied to step b), such as 4-25 kg per dry tonne wood chips supplied to step b), such as 4-20 kg per dry tonne wood chips supplied to step b), such as 5-15 kg per dry tonne wood chips supplied to step b). In one embodiment, the amount is 11-30 kg per dry tonne wood chips supplied to step b). 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.

[0047] In one embodiment, the impregnation liquid has a pH below 10.9. Such a pH reflects are relatively low (or no) supply of NaOH.

[0048] The method further comprises the step of:

[0049] c) applying steam having a temperature of at least 150° C. to the impregnated chips to obtain pretreated chips. In one embodiment, the temperature is at least 160° C. An upper limit may be 190° C.

[0050] In one embodiment, the impregnated chips obtained in step b) are transferred to step c) 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.

[0051] The residence time in step c) is preferably no more than two minutes.

[0052] The method further comprises the step of:

[0053] d) defibration of the pretreated chips. The pulp obtained from step d) 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 FIGS. 1-3.

[0054] If the pulp obtained from step d) is subjected to bleaching, the total NaOH consumption in the whole method is preferably below 50 kg / tonne dry wood chips, such as below 30 kg / tonne dry wood chips. Lower total NaOH consumption typically means higher bulk and higher yield.EXAMPLESPilot Trial 1

[0055] Maple wood chips (average length=18 mm) and spruce wood chips (average length=18 mm) were provided. Before being used in the pilot trial, the maple wood chips were soaked in water overnight. Subsequently, two mixtures were prepared:

[0056] Mixture (i) comprising 85 dry wt. % maple wood chips and 15 dry wt. % spruce wood chips; and

[0057] Mixture (ii) comprising 70 dry wt. % maple wood chips and 30 dry wt. % spruce wood chips.

[0058] Further, a batch of 100% maple wood chips (average length=18 mm) was used as a reference.

[0059] After washing and pre-steaming, the chips were impregnated with an impregnation liquid comprising NaOH, Na2SO3 and DTPA in an impregnation vessel of a pilot plant. 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 amount of NaOH, Na2SO3 and DTPA supplied to the impregnation vessel was 10, 20 and 2 kg per tonne dry chips, respectively.

[0060] An additional impregnation was carried out with Mixture (ii) in the same way as described above, but without NaOH in the impregnation liquid.

[0061] The impregnated chips were then heated by the application of steam having a temperature of 165° C. The residence time in the steaming step was less than 2 minutes. The pretreated chips from the steaming step were subjected to defibration / refining at a consistency of about 30%. Pulp samples were obtained after different degrees of refining. Properties of the pulp samples were then measured (see table 1 below). Further, sheets were formed from the pulp according to ISO 5269-1 and properties of the sheets were measured (see table 1 and FIGS. 4 and 6).TABLE 1Pulp and sheet properties. “Deg. of ref.” means degree ofrefining and is the specific energy consumption (kWh / dry tonne woodchips) in the refining step. “F. length” means MAP-Q Fiberlength (mm). “Shives” means MAP-Q shives (an optical methoddesigned to give the same result as a Sommerville measurement).Mixt.Mixt.Mixt.Mixt.Mixt.Mixt.Ref.Ref.(i)(i)(ii)(ii)(ii)(ii)NaOH (kg)10101010101000Maple (%)100100858570707070Spruce (%)00151530303030Deg. of ref.497644558750532700235877CSF (ml)706659700598722636764595F. length0.710.690.800.790.910.871.010.90Shives (%)0.740.321.220.292.090.3818.550.24Bulk (cm3 / g)4.64.414.564.344.814.195.854.38

[0062] As shown in FIG. 4, the tensile index for a given bulk is higher for the pulps comprising a portion of spruce fibers than for the reference pulp comprising only maple fibers. Interestingly, the pulp showing the highest bulk value is that comprising 30% spruce. This is also the pulp providing the highest tensile index values. It is also of interest to note that the pulp showing the steepest increase in tensile index is that comprising 15% spruce, which means that this pulp developed a lot of strength at a comparatively low loss of bulk during refining.

[0063] As shown in FIG. 6, the tensile index for a given bulk can be further improved by omitting the NaOH from the impregnation liquid.Pilot Trial 2

[0064] A mixture of 85 dry wt. % maple wood chips (average length=18 mm) and 15 dry wt. % spruce wood chips (average length=18 mm) was prepared. Before preparation of the mixture, the maple wood chips had been soaked in water overnight.

[0065] After washing and pre-steaming, chips of the mixture was divided into different batches that were impregnated with different 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:

[0066] Batch 1, 21.8 kg NaOH and 22.1 kg Na2SO3 per tonne dry chips;

[0067] Batch 2, 10.0 kg NaOH and 8.8 kg Na2SO3 per tonne dry chips;

[0068] Batch 3, 10.6 kg NaOH and 11.2 kg Na2SO3 per tonne dry chips; and

[0069] Batch 4, 5.0 kg NaOH and 20.8 kg Na2SO3 per tonne dry chips.

[0070] The impregnated chips were then heated by the application of steam. For batches 1 and 2, the temperature of the steam was 140° C. For batches 3 and 4, the temperature of the steam was 165° C. The residence time in the steaming step was less than 2 minutes. The pretreated chips from the steaming step were subjected to defibration / refining at high consistency. Pulp samples were obtained after different degrees of refining. The freeness and shives content of the pulp samples were then measured (see table 2 below).TABLE 2Pulp properties. “Deg. of ref.” means degree of refiningand is the specific energy consumption (kWh / dry tonne woodchips) in the refining step. “Shives” means 0.15mm shives (Sommerville). “CSF” means Canadian Standard Freeness.ChemistrySteamDeg.CSFShivesBatch(NaOH / Na2SO3)temp.of ref.(ml)(%)12.18% / 2.21%140° C.278.07634.831.9412.18% / 2.21%140° C.352.87551.730.4212.18% / 2.21%140° C.441.50590.280.6612.18% / 2.21%140° C.514.68543.070.5012.18% / 2.21%140° C.633.39485.830.2421.00% / 0.88%140° C.302.52717.855.9021.00% / 0.88%140° C.383.43712.006.0821.00% / 0.88%140° C.472.25650.981.3621.00% / 0.88%140° C.566.35638.871.0421.00% / 0.88%140° C.665.73600.620.1431.06% / 1.12%165° C.339.03715.531.6031.06% / 1.12%165° C.445.04686.700.6231.06% / 1.12%165° C.559.05619.760.1031.06% / 1.12%165° C.633.06610.950.1431.06% / 1.12%165° C.771.07582.920.0240.50% / 2.08%165° C.315.99722.272.2640.50% / 2.08%165° C.413.57698.640.7440.50% / 2.08%165° C.509.30614.370.0640.50% / 2.08%165° C.580.86607.650.0440.50% / 2.08%165° C.N / A600.130.02

[0071] As shown in FIG. 5, the batches treated with the higher steam temperature (165° C.) show higher CSF at a given shives content than the batches treated with the lower steam temperature (140° C.), independent of the chemistry.Pilot Trial 3

[0072] Maple wood chips (average length=18 mm) and spruce wood chips (average length=18 mm) were provided. Before being used in the pilot trial, the maple wood chips were soaked in water overnight. Subsequently, two mixtures were prepared:

[0073] Mixture (i) comprising 85 dry wt. % maple wood chips and 15 dry wt. % spruce wood chips; and

[0074] Mixture (ii) comprising 70 dry wt. % maple wood chips and 30 dry wt. % spruce wood chips.

[0075] Further, a batch of 100% maple wood chips (average length=18 mm) was used as a reference.

[0076] After washing and pre-steaming, the chips were impregnated with an impregnation liquid comprising NaOH, Na2SO3 and DTPA in an impregnation vessel of a pilot plant. 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, Na2SO3 and DTPA supplied to the impregnation vessel are set out in the tables 3a and 3b below.

[0077] An additional impregnation was carried out with Mixture (ii) in the same was as described above, but without NaOH in the impregnation liquid (see table 3b).

[0078] The impregnated chips were then heated by the application of steam having a temperature of 165° C. The residence time in the steaming step was 2 minutes. The pretreated chips from the steaming step were subjected to defibration / refining at a consistency of about 30%. Pulp samples were obtained after different degrees of defibration / refining. The pulp samples were then bleached to an ISO brightness of about 75 or about 80. Properties of the pulp samples were then measured (see tables 3a and 3b below). Further, sheets were formed from the pulp according to ISO 5269-1 and properties of the sheets were measured (see tables 3a and 3b below).TABLE 3aMixturerefrefrefref(i)(i)(i)(i)Maple (wt. %)10010010010085858585Spruce (wt. %)000015151515ImpregnationNaOH (kg / tonne)999910101010Na2SO3 (kg / tonne)1919191920202020BleachingISO brightness (%)7580768075807681NaOH (kg / tonne)2036203622392239Total NaOH (kg / tonne)2945294532493249AnalysisCSF (ml)758749643637695686596574Bulk (cm3 / g)4.764.524.524.274.373.764.033.37Tensile index (Nm / g)1.42.63.44.85.89.68.313.6TABLE 3bMixture(ii)(ii)(ii)(ii)(ii)(ii)(ii)(ii)Maple (wt. %)7070707070707070Spruce (wt. %)3030303030303030ImpregnationNaOH (kg / tonne)101010100000Na2SO3 (kg / tonne)2020202019191919BleachingISO brightness (%)7581758076807782NaOH (kg / tonne)2544254425432543Total NaOH (kg / tonne)3554355425432543AnalysisCSF (ml)703688585569756760555542Bulk (cm3 / g)4.423.213.743.015.644.583.923.47Tensile index (Nm / g)6.413.911.618.21.64.011.015.1Tables 3a and 3b in combination with FIGS. 6 and 7 show that HT-CTMPs having the combination of the following features can produced according to the present disclosure: a brightness of at least 74%; a bulk of at least 3.9 cm3 / g; a tensile index of at least 9.0 Nm / g; and a freeness of at least 510 ml.Pilot Trial 4

[0080] Mixture (ii) from pilot trial 3 was treated as in pilot trial 3 with an impregnation liquid comprising NaOH (9 kg / tonne dry wood chips), Na2SO3 (19 kg / tonne dry wood chips) and DTPA, but the steam temperature was 175° C. instead of 165° C.

[0081] In the defibration / refining of the chips treated in this manner, the energy consumption was only 190 kWh / tonne dry wood chips. The pulp obtained thereby was then bleached to 81%. The NaOH charge in the bleaching was 45 kg / tonne dry fibres.

[0082] Despite consuming only 190 kWh / tonne dry wood chips in the defibration / refining, the bleached pulp obtained a tensile index of 8.5 Nm / g (in combination with a bulk of 3.42 cm3 / g and a CSF of 769 ml).Exemplary Embodiment of a Full-Scale System for Producing HT-CTMP

[0083] FIGS. 1-3 illustrate exemplary embodiments of a full-scale system for producing HT-CTMP.

[0084] A chipper 101 is used to prepare chips from maple wood and softwood, such as 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.

[0085] The maple wood chips and the softwood chips from the chipper 101 are stored in a maple wood chips silo 102a and softwood chips silo 102b, respectively. A chips mixing system 103 is arranged downstream the silos 102a, 102b to prepare a chips mixture having the desired ratio of maple wood chips to softwood chips. This ratio is in the range of 92:8 and 50:50 (based on dry weight).

[0086] The chips from the chips mixing system 103 are optionally stored in a maturation silo 104 in an aerobic environment 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.

[0087] 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.

[0088] 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.

[0089] 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.

[0090] 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.

[0091] The steamed chips from the pre-steaming bin 107 are subjected to impregnation in one or two steps.

[0092] 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 typically comprises sulfite and optionally alkali. 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 of at least 150° C. The chips treated in the reactor 109 are transferred to a chips defibrator 114 without flashing off any steam on the way.

[0093] 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 pre-impregnation liquid may comprise alkali and / or sulfite. However, it may be preferred to use water without such additives as the pre-impregnation liquid. The (expanded and pre-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 alkali and / or sulfite. 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 of at least 150° C. The chips treated in the reactor 120 are transferred to the chips defibrator 114 without flashing off any steam on the way.

[0094] 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 / refined chips from the chips defibrator 114 are 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 dewatering press 127.

[0095] The pulp from the first dewatering press 127 has undergone chemical treatment, heat treatment by high temperature steam and mechanical treatment (i.e. refining) and it 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.

[0096] 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 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.

[0097] 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.

[0098] 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 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.

[0099] 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.

[0100] 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.

[0101] 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 high temperature chemithermomechanical pulp (HT-CTMP) formed from a mixture of maple wood and softwood, wherein the dry weight ratio of maple wood to softwood in said mixture is between 92:8 and 50:50.

2. The HT-CTMP of claim 1, wherein the dry weight ratio of maple wood to softwood in said mixture is between 90:10 and 60:

403. The HT-CTMP of claim 1, wherein the dry weight ratio of maple wood to softwood in said mixture is between 90:10 and 75:25.

4. The HT-CTMP of claim 1, wherein the dry weight ratio of maple wood to softwood in said mixture is between 75:25 and 60:40.

5. The HT-CTMP of claim 1, which has a bulk measured according to ISO 534:2011 of at least 4.0 cm3 / g after sheet forming according to ISO 5269-1:2005.

6. The HT-CTMP of claim 1, which has a tensile index measured according to ISO 1924-3:2005 of at least 4.0 Nm / g after sheet forming according to ISO 5269-1:2005.

7. The HT-CTMP of claim 1, wherein said softwood is spruce wood.

8. The HT-CTMP of claim 1, which is bleached to a brightness of at least 74%, as measured according to ISO 2470-1:2016 and ISO 3688:1999.

9. The HT-CTMP of claim 8, which has:a bulk measured according to ISO 534:2011 of at least 3.9 cm3 / g after sheet forming according to ISO 5269-1:2005;a tensile index measured according to ISO 1924-3:2005 of at least 9.0 Nm / g after sheet forming according to ISO 5269-1:2005; anda freeness of at least 510 ml, which freeness is measured according to ISO 5267-2:2001 after disintegration according to ISO 5263-3:2004.

10. A method of producing a high temperature chemithermomechanical pulp (HT-CTMP) comprising the steps of:a) mixing maple wood chips and softwood chips to obtain a mixture, wherein the dry weight ratio of maple wood to softwood in said mixture is between 92:8 and 50:50;b) impregnating the chips of the mixture with an impregnation liquid to obtain impregnated chips;c) applying steam having a temperature of at least 150° C. to the impregnated chips to obtain pretreated chips; andd) defibration of the pretreated chips.

11. The method of claim 10, wherein the dry weight ratio of maple wood to softwood in said mixture is between 90:10 and 60:40.

12. The method of claim 11, wherein the dry weight ratio of maple wood to softwood in said mixture is between 90:10 and 75:25.

13. The method of claim 11, wherein the dry weight ratio of maple wood to softwood in said mixture is between 75:25 and 60:40.

14. The method of claim 10, wherein the impregnated chips obtained in step b) are transferred to step c) without compressing the impregnated chips.

15. The method of claim 10, wherein the impregnation liquid comprises sulfite.

16. The method of claim 10, wherein less than 15 kg NaOH per dry ton wood chips is supplied to step b).

17. The method of claim 10, wherein the amount of Na2SO3 supplied to step b) is 4-30 kg per dry ton wood chips supplied to step b).

18. The method of claim 10, wherein said softwood is spruce wood.

19. The HT-CTMP of claim 6, wherein the tensile index is at least 8.0 Nm / g after sheet forming.

20. The HT-CTMP of claim 8, wherein the brightness is at least 78%.

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