Production method of CTMP having a low extractives content

Abstract

The present disclosure relates to a method for producing chemithermomechanical pulp, CTMP, comprising: a) providing wood chips comprising softwood, hardwood or a mixture thereof; b) storing the chips at a temperature of 45-75° C. for at least 24 h in an aerobic environment to obtain matured chips; c) impregnating the matured chips with an impregnation liquid to obtain impregnated chips; d) transferring the impregnated chips to a heating zone; e) heating the impregnated chips in the heating zone with steam to obtain pre-treated chips; f) defibrating the pre-treated chips to obtain a pulp; and g) dewatering the pulp.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a production of chemitheromechanical pulp (CTMP) having reduced extractives content.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 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.

[0003] Pulp, such as CTMP, comprise wood extractives such as triglycerides and fatty acids. These wood extractives may cause problems in final applications due to taste and odour issues. This is especially a problem when using pulp in food packaging applications such as in liquid packaging board (LPB). There is, thus, a desire to minimize the amount of extractives in the final pulp in order to minimize problems with taste and odour in the final applications.SUMMARY

[0004] The present inventors have realized that exposing wood chip to an aerobic environment at increased temperatures for at least 24 h facilitates the production of a CTMP having a low extractives content.

[0005] The present disclosure related to a method for producing chemithermomechanical pulp (CTMP) comprising:

[0006] a) providing wood chips comprising softwood, hardwood or a mixture thereof;

[0007] b) storing the chips at a temperature of 45-75° C. for at least 24 h in an aerobic environment to obtain matured chips;

[0008] c) impregnating the matured chips with an impregnation liquid to obtain impregnated chips;

[0009] d) transferring the impregnated chips to a heating zone;

[0010] e) heating the impregnated chips in the heating zone with steam to obtain pre-treated chips;

[0011] f) defibrating the pre-treated chips to obtain a pulp; and

[0012] g) dewatering the pulp.

[0013] Storing the chips at a temperature of 45-75° C. for at least 24 h in an aerobic environment facilitates the production of a CTMP having a low extractives content. The storing enables oxidative degradation of triglycerides that are present in the wood chips. The triglycerides degrade to form free fatty acids which are easier to wash out of than triglycerides. In addition to being more easily removed than triglycerides, the free fatty acids may aid in the removal of other extractives which may be hard to remove otherwise. Hence, the storing of wood chips at a temperature of 45-75 ° C. for at least 24 h in an aerobic environment facilitates the removal of extractives from the wood and enables the production of a CTMP with a reduced extractives content.

[0014] The chips may be stored at a temperature of 50-70° C. such as 55-65° C. and / or for 48-96 h. A longer storing time may enable more of the triglycerides to degrade into free fatty acids.

[0015] The produced CTMP may be a high temperature CTMP (HT-CTMP) and may be produced by 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. HT-CTMP may have a lower shives content compared to CTMP and a more efficient chip defibration due to a more effective impregnation and heating.

[0016] The matured chips may be pre-steamed prior to being impregnated. The pre-steaming of the chips removes air from the chips and replaces it with steam which may improve impregnation.

[0017] The matured chips may further be washed with water prior to being pre-steamed. The washing removes dirt and gravel from the matured chips.

[0018] The impregnation liquid is typically aqueous and may comprise Na2SO3 and / or NaOH.

[0019] 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.

[0020] The method may comprise a bleaching step after defibration of the pre-treated chips in order to improve the brightness of the pulp. The bleaching step may be a two-step process wherein the first bleaching step occurs at a medium consistency of 10-12 wt. % and the second bleaching step occurs at a high consistency of 30-35 wt. %.

[0021] The method may further comprise a refining step after the defibration of the pre-treated chips wherein the refining occurs at a consistency of 3-8 wt. %, such as 3-7 wt. %, such as 4-6 wt. %. The refining step may be performed in at least two steps, such as at least three steps. Performing a refining step may enable the production of a CTMP having a higher tensile strength. The refining step may occur after the bleaching step or prior to the bleaching step. The refining step is advantageously preformed after to the bleaching step as bleaching may facilitate the second refining step.

[0022] Step g) may comprise the sub-steps of: diluting the pulp with water to obtain a diluted pulp, e.g. using a pulper; and dewatering the diluted pulp. Accordingly, the pulp obtained after defibration of the pre-treated chips may be diluted in a pulper in which the residence time is less than 15 min, such as less than 10 min. Using a pulper to dilute the pulp at this stage may enable a fast dilution and hence less time for free extractives such as fatty acids to re-adsorb to the fibers prior to dewatering.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Aspects and embodiments are now described, by way of example, with reference to the accompanying drawings, in which:

[0024] FIG. 1 schematically illustrates a system for producing bleached (HT-)CTMP.

[0025] FIG. 2 schematically illustrates a continuation of the system in FIG. 1 designed for producing market (HT-)CTMP.

[0026] FIG. 3 schematically illustrates a continuation of the system in FIG. 1 designed to produce market (HT-)CTMP and supply an adjacent board-making machine with (HT-)CTMP.

[0027] FIG. 4 shows the total amount of extractives over time present in spruce chips exposed to storage in an aerobic environment at different temperatures.

[0028] FIG. 5 shows the total amount of extractives over time present in birch chips exposed to storage in an aerobic environment at 60° C.

[0029] FIG. 6 shows the total amount of esters / ketones (dark grey) and triglycerides (light grey) over time present in birch chips exposed to storage in an aerobic environment at 60° C.DETAILED DESCRIPTION

[0030] The present disclosure relates to a method for preparing a CTMP having a reduced amount of extractives in order to improve the use of the pulp in applications such as food packaging.

[0031] Wood comprises different extractives which are typically embedded in the cell wall. Triglycerides and fatty acids constitute some of the wood extractives and play a vital role in the wood. However, these extractives may also pose a problem during application of the pulp in e.g. food packaging applications as the extractives may cause taste and odour issues. The taste and odour issues may be especially pronounced for hardwood pulp, such as birch, as it generally contains more extractives. In addition to issues with taste and odour, the extractives may interfere with sizing agents such as alkyl ketene dimer (AKD) and deteriorate their effect.

[0032] Storing wood chips at a temperature of 45-75 ° C. for at least 24 h in an aerobic environment leads to oxidative degradation of triglycerides into free fatty acids. The dispersibility of free fatty acids in the water phase is better than the dispersibility of the triglycerides which means that they may be more easily removed from the pulp in a subsequent washing step. In addition to being easier to remove, the free fatty acids may also facilitate the removal of other extractives by acting as emulsifiers. The effective removal of extractives leads to the production of a CTMP having less issues with taste and odour while simultaneously improving the effect of sizing agents such as AKD.

[0033] In order for the degradation of triglycerides to occur, oxygen needs to present during storing as the degradation is an oxidative process. By storing the wood chips in an aerobic environment for a longer period of time, such as 48-96 h, a higher yield of the degradation products can be obtained as the degradation reactions are allowed to proceed for a longer time. The degradation rate decreases significantly after 96 h and any additional storing will not lead to a noticeable reduction in extractives.

[0034] The temperature during storing needs to be over ambient temperature as the degradation reaction is promoted by an energy input. However, increasing the temperature over 75 ° C. requires a lot of energy and is not energy efficient. In addition, using temperatures above 75 ° C. may lead to discoloration of the chips.

[0035] The storing process may be used on various wood types in order to decrease the amount of extractives in the final CTMP. As example, hardwood such as birch, maple and aspen and / or softwood such as spruce and pine may be used to produce a CTMP of the present disclosure. An effective removal of extractives may be particularly suitable when using hardwood, such as birch, as raw material as they are known to have more pronounced issues concerning taste and odour when used in e.g. food packaging applications.

[0036] The storing may take place in one or more maturation silos. Using more than one maturation silo may be advantageous when preparing CTMP comprising a mixture of different wood types. The storing can then be adapted for the respective wood types.

[0037] The aspects of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown.

[0038] FIGS. 1-3 illustrate exemplary embodiments of a system for producing bleached HT-CTMP.

[0039] The system may comprise a chipper 101 used to prepare chips from wood. In case of hardwood, in particular maple, it is preferable to prepare chips that are relatively short, such as <20 mm, to aid impregnation. Softwood chips are generally easier to impregnate and can hence be longer, such as 22-24 mm. The settings of a conventional wood chipper can be adjusted to achieve such lengths. The thickness of the chips is proportional to its length, i.e. the shorter the chips are, the thinner they will be.

[0040] The chips from the chipper 101 are typically stored in at least one silo 102. When different types of wood are mixed in the system, there is typically one silo for each type of wood, such as one hardwood chips silo 102a and one softwood chips silo 102b. When there is more than one type of wood chips, a chips mixing system 103 is preferably arranged downstream of the silos 102a, 102b.

[0041] The chips from the chips mixing system 103 are stored in a maturation silo 104 for a period of at least 24 h, typically for 72 h. A typical temperature in the maturation silo 104 is 45-75° C. such as 60° C., which can be achieved by feeding low-pressure steam into the maturation silo 104. The environment in the maturation silo is aerobic. The degradation products produced during the storing are extracted in downstream dewatering steps.

[0042] It is possible 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.

[0043] Another option is to place the maturation silo 104 between the chip washing arrangement 106 and the pre-steaming bin 107 described below.

[0044] Before being impregnated, the chips may be washed in a chip washing arrangement 106. Upstream the chip 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 the moisture content of the chips. During cold winter months, the conditioning device 105 may be used to melt the ice on the chips, which facilitates the downstream washing and processing. Hence, the conditioning device 105 may be particularly advantageous when there is no upstream maturation silo.

[0045] In the chip washing arrangement 106, the chips are typically soaked and agitated in water and then dewatered. Dirt and gravel are removed in the chip washing arrangement 106. 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 such as 15-20 min. The pre-steaming of the chips removes air from the chips and replaces it with steam which may improve impregnation downstream.

[0046] The steamed chips from the pre-steaming bin 107 are subjected to impregnation in one or two steps.

[0047] In case of one-step impregnation, a plug screw 108 may feed the steamed chips into a reactor 109. The steamed chips, which were compressed in the plug screw 108, expand 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 80° C.-99° C.. 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 a heating zone 113 of the reactor 109, in which they are heated by steam. The steam may have a temperature of above 140° C. if HT-CTMP is produced. The transferring of the impregnated chips from the bath of impregnation liquid 110 to the heating zone 113 may occur without any compression of the chips. The impregnation liquid is thereby not pressed out of the chips prior to reaching the heating zone 113. The chips treated in the reactor 109 are transferred to a chip defibrator 114 without flashing off any steam on the way.

[0048] In case of two-step impregnation, a plug screw 115 may feed 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 is water that may comprise NaOH and optionally Na2SO3. 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 may then feed the pre-impregnated chips into a reactor 120. The pre-impregnated chips, which were compressed in the plug screw 119, expand 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 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 a heating zone 124 of the reactor 120, in which they are heated by steam having a temperature above 140° C. The transferring of the impregnated chips from the bath of impregnation liquid 121 to the heating zone 124 occurs without any compression of the chips. The impregnation liquid is thereby not pressed out of the chips prior to reaching the heating zone 124. The chips treated in the reactor 120 are transferred to the chip defibrator 114 without flashing off any steam on the way.

[0049] The impregnation liquid, in both the one-step and the two-step impregnation, may comprise NaOH and / or Na2SO3. The NaOH softens the wood chips by modifying carbohydrates such as hemicellulose and is typically used more for hardwood. The Na2SO3 softens the wood by modifying lignin by sulphonation making it more hydrophilic and is typically used more for softwood. The impregnation liquid may be adjusted as to optimize the softening of the chips which may occur differently depending on the wood composition.

[0050] In the chip defibrator 114, the dry matter content is 40-50 wt. % such as 45-50 wt. %. It is advantageous for the solid content to be high in order to use as much of the energy for defibration of the chips rather than vaporizing the water in the impregnated chips. However, too high solid content, >50 wt. %, may lead to burnt fibers. The pressure during defibrating may be ≥2 bar. If HT-CTMP is produced, the pressure may be ≥5 bar.

[0051] The pulp obtained from the chip defibrator 114 may be subjected to flashing in a steam separator 125 and then pulped in a first pulper 126. The pulp may be diluted in the pulper 126 to a consistency of 3-5 wt. % and homogenized for 10 min or less, typically for 2-5 min. Using a pulper 126 in this position enables a short residence time which in turn limits the time available for re-adsorption of the extractives to the fibers. The water phase at this stage in the process (after defibration) comprises a high amount of extractives which is why it is advantageous to reduce the time between the defibration step and removal of the water phase. The pulp from the first pulper 126 is thereby treated in a first dewatering press 127. The pressate from the dewatering press 127 which contains the extractives is removed from the process. Separation of extractives by pressing in this position is even more advantageous since the pulp still has a high freeness (typically >650 ml or even >700 ml) and is thus easily dewatered.

[0052] The pulp from the first dewatering press 127 is subjected to middle consistency (MC) bleaching in a MC bleach tower 128 using residual peroxide from a downstream high consistency (HC) bleaching. Fresh alkali such as NaOH may be added to the MC bleaching if necessary. MC means 10-12 wt. %. The MC-bleached pulp may be dewatered in a second dewatering press 129 also producing a pressate. The pulp from the second dewatering press has a consistency of about 30-35 wt. % and is subjected to HC bleaching in a HC bleach tower 130 using fresh peroxide and alkali. It is advantageous that the peroxide is present in excess compared to the alkali during the entire bleaching process in order to obtain a high brightness. The HC-bleached fibers from the HC bleach tower 130 may be pulped in a second pulper 131 (residence time is preferably <10 min, such as about 3 min) to produce a pulp having a consistency of about 3-8 wt. %. This pulp is then subjected to low consistency (LC) refining in LC refiners 132. The LC refining may be performed in multiple stages, typically three LC refiners 132 are used. A third dewatering press 133 may then separate a third pressate from the LC-refined pulp. The fibers from the third dewatering press 133 may be pulped in a third pulper 134 (residence time is preferably <10 min, such as about 3 min) to produce a pulp having a consistency of 2-4 wt. %. Screens 135 may then be used to separate a reject from the pulp from the third pulper 134. The separated reject is collected in a reject tank 136.

[0053] The design of the remaining parts of the system depends on if only market pulp is produced (i.e. all (HT-)CTMP is subjected to flash drying and baling) or if there is also an adjacent board-making machine to which at least part of the (HT-)CTMP is supplied without drying.

[0054] In the former case, which is illustrated in FIG. 2, the pulp from the screens 135 are cleaned in cyclones 137 to provide cleaned pulp and a second reject that is collected in a second reject tank 138. 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.

[0055] In the latter case, which is illustrated in FIG. 3, the pulp from the screens 135 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.

[0056] To produce (dried) market pulp, a fifth dewatering press 148 produces dewatered fibers and a fifth pressate from the 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.

[0057] To use the produced (HT-)CTMP directly in the production of paperboard, MC pulp from the MC tower 147 is led to a board-making machine.EXAMPLESExample 1

[0058] A laboratory study simulating the storage process of wood chips was performed using spruce as raw material.

[0059] Spruce chips were stored at three different temperatures, 40° C., 60° C. and 75° C. for a total of 7 days. Samples were taken after 0, 1, 2, 3, 4 and 7 day / s. The amount of extractives was analysed using HPLC.

[0060] FIG. 4 shows the amount of extractives in the wood chips at different storage times and temperatures. A significant decrease in the amount of extractives in the wood chips was observed after 1 day when storing at 60° C. and 75° C. The amount of extractives continued to decrease for 7 days, however, the decrease was considerably smaller.

[0061] The initial amount of extractives, measured prior to storing, in the chips to be stored at 60° C. was slightly lower compared to the initial amounts in the chips to be stored at 40° C. and 75° C. Hence, the decrease in extractives after 1 day was larger for 75° C. than for 60° C. After the first day, the chips stored at the two different temperatures (75° C. and 60° C.) followed the same trend.

[0062] Storing the chips at 40° C. led to an increase in the amount of extractives after 1 day, however, this is probably due to an error during measurement. The amount of extractives decreased after 2 days and continued to decrease after 3 days. Storing the chips for 3 days at 40° C., decreased the amount of extractives but not to the same extent as the higher temperatures. This indicates that temperatures above 40° C. are desirable in order to obtain a significant reduction in the amount of extractives.Example 2

[0063] A second laboratory study simulating the storage process of wood chips was performed using birch as raw material.

[0064] Birch chips were stored at 60° C. for a total of 7 days. Samples were taken after 0, 1, 2, 3, 4 and 7 day / s. The amount of esters / ketones, triglycerides and the total amount of extractives was analysed using HPLC.

[0065] FIG. 5 shows the total amount of extractives at different storage times. It can be seen that the total amount of extractives decreases from the start and continues to decrease for 7 days. However, the decrease between day 4 and day 7 was considerably smaller indicating that most of the extractives have been removed prior to 4 days.

[0066] FIG. 6 shows the amount of triglycerides and the amount of esters / ketones. It can be observed that the amount of esters / ketones and triglycerides decreases rapidly over the 7 days. The largest decrease in triglycerides can be observed already after 1 day, at which point the amount of triglycerides had been reduced to 50 wt. %. The amount of triglycerides continued to decrease during the seven days, however the decrease between day 4 and day 7 was the smallest. The amount of esters / ketones decreased more evenly over the course of the seven days of storage with the smallest reduction occurring between day 4 and day 7.

[0067] The observations made in example 1 and 2 indicate that storing wood chips at increased temperatures for at least 1 day in an aerobic environment, decreases the amount of extractives present in the wood chip and hence in the CTMP.

Claims

1. A method for producing chemithermomechanical pulp, CTMP, comprising:a) providing wood chips comprising softwood, hardwood or a mixture thereof;b) storing the chips at a temperature of 45-75° C. for at least 24 h in an aerobic environment to obtain matured chips;c) impregnating the matured chips with an impregnation liquid to obtain impregnated chips;d) transferring the impregnated chips to a heating zone;e) heating the impregnated chips in the heating zone with steam to obtain pre-treated chips;f) defibrating the pre-treated chips to obtain a pulp; andg) dewatering the pulp.

2. The method of claim 1, wherein the chips are stored at a temperature of 50-70° C.

3. The method of claim 1, wherein the chips are stored for 48-96 h.

4. The method of claim 1, wherein the CTMP is high temperature CTMP, HT-CTMP.

5. The method of claim 4, wherein the steam used for heating the impregnated chips has a temperature of above 140° C.

6. The method of claim 1, further comprising a bleaching step after dewatering of the pulp in step g).

7. The method of claim 6, wherein the bleaching is a two-step process, and the first bleaching step occurs at a medium consistency of 10-12 wt. % and the second bleaching step occurs at a high consistency of 30-35 wt. %.

8. The method of claim 1, further comprising a refining step after dewatering of the pulp in step g) wherein the refining occurs at a consistency of 3-8 wt. %.

9. The method of claim 1, wherein the refining step occurs after the bleaching step.

10. The method of claim 1, wherein the pulp obtained after defibration of the pre-treated chips is diluted in a pulper with a residence time of less than 15 min.

11. The method of claim 1, wherein the matured chips are pre-steamed prior to being impregnated.

12. The method of claim 11, wherein the matured chips are washed with water prior to being pre-steamed.

13. The method of claim 2, wherein temperature is 55-65° C.

14. The method of claim 5, wherein the steam has a temperature of at least 160° C.

15. The method of claim 8, wherein the refining occurs at a consistency of 4-6 wt. %.

16. The method of claim 10, wherein the residence time is less than 10 min.

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