Process for producing kraft pulp from hardwood and softwood mixtures, kraft pulp obtained by the process, and paper products made from the pulp

Description

TECHNICAL FIELD

[0001] The present invention relates to a process for producing Kraft pulp from wood mixtures of hardwood and softwood species. Additionally, the invention relates to paper products produced from the high yield pulp obtained.BACKGROUND ART

[0002] Wood is made of cellulose fibers, hollow cells, constituted by lumen (empty interior space) and cell wall, composed mainly by cellulose, hemicelluloses and lignin (Fengel, D. and Wegener, G., Wood-Chemistry, Ultrastructure, Reactions, 2nd Edition, Walter de Gruyter, Berlin, 1989). Lignin is heterogeneously distributed in the cell wall, and is more concentrated in the so-called middle lamella and cell corners, the outer layer of the fibers, responsible for the cohesion in the three-dimensional structure of wood (Saka, S. R.; Thomas, J.; Gratzl, J. S.; Abson, D., Topochemistry of Delignification in Douglas-fir Wood with Soda, Soda-Anthraquinone and Kraft Pulping as Determined by SEM-EDXA*, Wood Science and Technology, 16, 139-153, 1982).

[0003] The industrial process of cellulose pulp production aims at breaking down the three-dimensional structure of wood and individualizing the cellulose fibers that make up its matrix. Generally, this can be achieved in two ways, the mechanical or the chemical way, or a combination of these two approaches. The mechanical processes consist in grinding the wood or its chips, with yields (pulp mass / wood mass x 100) higher than 90 . In contrast, in chemical cooking, whose purpose is to dissolve the lignin in the wood in order to free the fibers from the matrix, approximately half of the wood constituents, including lignin, is dissolved, giving a pulp production yield of about 45 - 55 %.

[0004] Kraft cooking, or sulfate cooking, is one of the most industrially used chemical processes for the production of cellulosic pulp from wood. This chemical process consists of cooking wood in a cooking liquor, consisting of sodium hydroxide and sodium sulfide, at temperatures of the order of 140 to 180 °C, in pressurized reactors (Ek, Monica; Gellerstedt, Göran; Henriksson, Gunnar; Pulp and Paper Chemistry and Technology Volume 2, Pulping Chemistry and Technology, 2009, De Gruyter, Berlim).

[0005] No chemical method is selective enough to remove all the lignin from the wood, ending the cooking with a residual lignin content of 1 - 5 % in the cellulosic fibers of the produced pulp. The temperature and duration of a cooking determine the degree of delignification and this can be measured by the Kappa Number (KN), which is proportional to the lignin content remaining in the pulp. Another relevant factor for monitoring Kraft cooking is the H-factor, which combines time and temperature variables into a single variable. Numerically, it corresponds to the integral of the relative velocity of delignification in relation to the time variable (Ek, Monica; Gellerstedt, Göran; Henriksson, Gunnar; Pulp and Paper Chemistry and Technology Volume 2, Pulping Chemistry and Technology, 2009, De Gruyter, Berlim).

[0006] The species whose woods are used in the production of cellulose pulp are classified as softwood (such as pine) or hardwoods (such as eucalyptus), differentiated by their anatomy and chemical composition (Fengel, D. and Wegener, G.; Wood-Chemistry, Ultrastructure, Reactions, 2nd Edition, Walter de Gruyter, Berlin, 1989).

[0007] Softwood contains more lignin and less cellulose than the corresponding hardwood, and its lignin is also less reactive and delignification kinetics slower due to its more resistant structure to chemical attack. The cooking of softwoods thus requires more drastic cooking conditions in terms of H-factor and chemical charge than those used in hardwoods (Fengel, D. and Wegener, G.; Wood-Chemistry, Ultrastructure, Reactions, 2nd Edition, Walter de Gruyter, Berlin, 1989). On the other hand, due to their chemical composition in extractives compounds, during cooking softwoods release components that originate tall oil which, if not removed, make difficult the produced black liquor evaporation process (liquid fraction containing the by-products of the wood cooking process) and potentiate the deposition of pitch in the produced pulp.

[0008] When cooking hardwoods, Kappa Number and typical yields are 15 - 19 and 50 - 55 %, respectively. For softwoods, Kappa Number and typical yields are 25 - 30 and 44 - 48 %, respectively (Shackford, L.; A Comparison of Pulping and Bleaching of Kraft Softwood and Eucalyptus Pulps, 36th International Pulp and Paper Congress and Exhibition, October 2003, São Paulo, Brazil).

[0009] The differences in the cooking processes of softwood and hardwood wood species are addressed by Baptista (Baptista, Cecilia de Melo Correia; Influência das condições de cozimento sobre a estrutura da lenhina e a branqueabilidade da pasta kraft de Pinus pinaster, Universidade da Beira Interior, Tese de Mestrado, 2007), having studied the Kraft cooking process of Pinus pinaster at a plateau temperature of 170 °C. As the document explains, the dissolution of lignin takes place in three distinct phases, the initial phase, the bulk phase, and the residual phase. The initial phase of delignification, slow and controlled by the diffusion of the liquor and reaction products, occurs with an increase in the H-factor up to values of 100, which corresponds to a delignification around 25 %. The next phase, the bulk phase, is characterized by an acceleration of delignification and is controlled by chemical reactions. The delignification progresses until 90 % delignification, achieved with an H-factor of 1400, and then enters in the final, slow, residual phase, achieving 95 % delignification for an H-factor of 5000, and a Kappa Number of 22.

[0010] When comparing these results with the Kraft delignification process of Eucalyptus globulus, namely the delignification curves of each species, it is possible to verify that under the typical conditions of the E. globulus laboratory cooking to reach a Kappa Number 18, involving an H-factor of 480, temperature of 160 °C and an alkali charge of 17 %, the Pinus pinaster cooking is still at the beginning of the bulk phase, with 60 - 70 % delignification and a Kappa Number of 100.

[0011] The dissolution of lignin during kraft cooking does not occur uniformly. The cooking liquor initially penetrates into the fibers through the lumen and diffuses transversely through the cell wall, until reaching the middle lamella and cell corners. Delignification begins on the secondary wall, in the initial phase of cooking, controlled by diffusion. The porosity created by the dissolution of part of the lignin and hemicelluloses in the cell wall, allows the liquor to reach the middle lamella and initiate the bulk phase of delignification. With the partial dissolution of the lignin in the middle lamella, the three-dimensional structure of the wood begins to disintegrate and the fibers to separate. In hardwoods, the lignin in the secondary cell wall is more reactive than in the softwoods and, consequently, the fiber separation point is reached faster (Saka, S. R.; Thomas, J.; Gratzl, J. S.; Abson, D., Topochemistry of Delignification in Douglas-fir Wood with Soda, Soda-Anthraquinone and Kraft Pulping as Determined by SEM-EDXA*, Wood Science and Technology, 16, 139-153, 1982).

[0012] For the reasons described above, the mixed cooking of softwoods and hardwoods raises additional challenges from the point of view of process and quality of the final pulp. To achieve a given Kappa Number with a mixture of softwoods and hardwoods, it will occur: (i) an overcooking of the hardwood if the used process conditions are close to the ones used for softwoods, or (ii) an undercooking of the softwood is observed if the process conditions are suited for hardwoods. Thus, and since an undercooking of the softwood originates a partial non-separation of the fibers in the final pulp, the industrially mixed cooking typically involve softwood as the dominant wood, with a smaller percentage of hardwood, using softwoods typical cooking conditions. Although overcooked, hardwood, due to its chemical composition richer in cellulose, and under suitable cooking conditions, can contribute favorably to the final pulp production yield and for the reduction of the volumetric consumption of wood (m 3< of wood / ton of pulp) in the process.

[0013] With cooking conditions adjusted to softwood wood, Oliveira et al. (Oliveira, R.C.; Foelkel, C.E.B.; Gomide, J.L.; Produção de celulose kraft a partir de misturas de madeiras de Pinus strobus var. chiapensis e Eucalyptus urophylla, de origem híbrida, Revista Árvore, 3(2), 195 - 207, 1979) have conducted Kraft cooking studies of wood chip mixtures of Pinus strobus var. chiapeneis and Eucalyptus urophylla hybrid. They found that pine wood is more difficult to delignify than eucalyptus wood, thus requiring more drastic cooking conditions. Pine wood, due to its lower density and lower yield, when added to a mixture with predominant eucalyptus (mixture with 33 % pine) originates a higher volumetric consumption of wood per ton of final product. There is also a decrease in the screened pulp yield (for example, for an H-factor of 1100, there is a yield of 49 % with eucalyptus and of 46 % with 33 % of pine mixed with eucalyptus) and an increase of Kappa Number (21 with eucalyptus and Kappa Number 33 with 33 % pine mixed with eucalyptus). On the other hand, the results of these authors show that the addition of 33 % of eucalyptus in mixture with pine does not result in a significant change of the screened pulp yield, whose recorded value was around 42 - 43 . In turn, for an H-factor of 1100 the Kappa Number value decreases from 42 to 33 with the addition of 33 % eucalyptus to pine. The work does not characterize the mechanical properties of the pulps obtained.

[0014] In 1984, Madderm and co-authors (Maddern, k.n.; Brumby, P. M.; Mulcahy, J.P., Alkaline pulping of hardwood and hardwood-softwood mixtures, Appita, 37 (9), 723-728, 1984) found that the cooking yield of softwood and hardwood mixtures is lower than the equivalent cooking yield of individual species, namely that of the hardwood. Low softwood incorporations result in a lower total yield due to the hardwood overcooking. The incorporation of 25 % pine in a mixture with eucalyptus leads to a yield decrease to 45 %, compared to the yield obtained by cooking only eucalyptus, 47 %. The Kappa Number increases from 18 to 21. The alkali charge for mixed wood cooking are also higher compared to cooking of individual species. Mechanical properties of the obtained pulps are not shown in this work by Madderm and co-authors.

[0015] Gulsoy et al. (Gulsoy, S.K.; Tufek, S., Effect of Chip Mixing Ratio of Pinus pinaster and Populus tremula on Kraft Pulp and Paper Properties, Ind. Eng. Chem. Res. 52, 6, 2304-2308, 2013) studied pine (softwood) and poplar (hardwood) mixtures with regard to their behavior when subjected to Kraft cooking, with cooking conditions suitable for softwood wood. The authors verified that the incorporation of a hardwood to a softwood allows an increase in the total yield obtained (yield of 49 % and a Kappa Number of 49 with the addition of 25 % of poplar versus a yield of 47 % and a Kappa Number of 60 with only pine) and obtaining a pulp that is easier to refine. In turn, the incorporation of 25 % of pine gives a total yield of 52 % and a Kappa Number of 23 compared to a total yield of 54 % and a Kappa Number of 13 with cooking only poplar. The authors also verified that for high pine incorporations, pulps with high viscosity and rejects content are obtained. Additionally, a reduction in the mechanical properties of the pulp obtained, with regard to tear strength, bursting and tensile index, was shown by the addition of hardwood to the predominant softwood wood mixture. This decrease is clearly more noticeable than the increase in the values of these properties by the addition of softwood to the predominant hardwood mixture.

[0016] Bassa et al. (Bassa, Ana Gabriela; Silva, Marcelo; Bassa, Alexandre; Sacon, Vera; Schmidt, Flavia; Silva, Francides, Mixed Brazilian Eucalyptus and Pinus species - Bleaching evaluation, TAPPI Engineering, Pulping and Environmental Conference, 36, 2008) also recovered some of the works, from the 70s, on cooking mixtures of softwoods and hardwoods. In 1979, Chen and co-authors (Chen, R.; Garceau, J.J.; Kokta, B.V. Hardwood mixed with softwoods in kraft pulping, Tappi, Atlanta, 61(7), 35-38, 1978) studied the cooking of mixtures of Populus tremuloides (aspen) and Picea glauca (Canada pine). They found that adding poplar (a hardwood) to pine (a softwood) in the cooking resulted in an increase in yield of up to 4 %, with less generation of rejects and a better delignification.

[0017] A work also from the 70s, by Hunt and Hatton (Hunt, K. and Hatton, J.V., Increased pulp production by use of hardwoods in softwood kraft mills (1976), Aspen Bibliography, Paper 5001) showed that it is advantageous to add hardwoods to softwoods for their cooking. They found that higher yield results were obtained with mixtures of softwoods with up to 20 % hardwoods than when softwoods were cooked individually, with no decrease in strength properties. A low refining energy is required for the resulting pulps.

[0018] Zanão and co-authors studied the kraft cooking of eucalyptus and pine mixtures with the addition of polysulfides and for a target Kappa Number of 19. The increase in the yield obtained (in the range of 1.0 - 2.2 %) was attributed to the polysulfides.

[0019] Kraft cooking, as previously mentioned, is generally conducted to obtain chemical pulps with low Kappa Number (on the order of 15 - 30) and yields of 45 - 55 %. There have been several studies and industrial practices in which it is intended to increase the cooking yield, interrupting the delignification (higher Kappa Number). This is a common practice in pulp production in Sweden. The separation of fibers, not having occurred completely by reaction and dissolution of lignin, is complemented by mechanical defibration, at the exit of the digester, of the partially delignified chips (Ek, Monica; Gellerstedt, Göran; HenrKIsson, Gunnar, Pulp and Paper Chemistry and Technology, Volume 2, Pulping Chemistry and Technology, 2009, De Gruyter, Berlim). However, these practices essentially involve monospecies.

[0020] Another possibility for increasing the yield of wood cooking is described in patent document US10060075B2, which discloses a cooking method that allows a high yield through Kraft cooking until obtaining a Kappa Number value of no less than 30 and a reject stream and an accept stream. The reject stream is then subjected to mechanical processes in the presence of bleaching agents, such as elemental chlorine, chlorine dioxide, ozone and hypochlorite, before being reunited with the accept stream. Additionally, the described Kraft cooking process also involves the use of chemical additives such as, for example, anthraquinone and polysulfides. However, the application of the technology to mixtures of softwoods and hardwoods is not exemplified.

[0021] The 1971 patent application US3827934A discloses obtaining a high-yield (55 - 80 %) hardwood pulp with a Kappa Number of at least 50, wherein the pulp is obtained by a method involving a modification of the normal chemical process through cooking using a lower alkaline charge (an effective alkali concentration between 6 and 15 %) and less severe temperature conditions, between 110 and 154 °C. The pulps obtained by this chemical process are then subjected to mechanical treatment with a countercurrent, double-disk rotary refiner.

[0022] The work of Hart (Hart, P., Production of High Yield Bleached Hardwood Kraft Pulp: Breaking the Kraft Pulp Yield Barrier, Tappi Journal, 10(9), 2011) also uses a similar method: for hardwood cooking and to increase the yield of the Kraft process (up to 65 %, and Kappa Number up to 60), in order to reduce wood consumption and operating costs, a lower effective alkali charge is used. A higher reject content is observed at the exit of the digester. The latter is subsequently reduced by mechanical action.

[0023] The patent application US4869783A, 1986, describes a Kraft cooking process that has higher yield values than what is characteristic for chemical processes, which involves a first pre-defibration, where wood chips are subjected to a combination of compression and twisting forces to partially separate the fibers, followed by a chemical treatment at high temperatures and pressures to obtain Kappa Number of about 45 to 70. The pulp is then delignified with chlorine, chlorine dioxide or a mixture of the two, lowering the Kappa Number to values between 15 and 25.

[0024] An alternative to increase the efficiency of Kraft cooking concerns the application of a wood impregnation step that is longer in time, as described by de Souza et al. (de Souza, B.; Gustavo, B.; Colodette, J. L.; Gomes, F. J. Borges; Carvalho, D.C., Enhancement of eucalypt pulp yield through extended impregnation cooking; Nordic Pulp & Paper Research Journal 2018, 33(2), 175-185). Wedin and co-authors (Wedin, H.; Lindstrom M.; Ragnar, M., From simple theory to industrial application - extended impregnation kraft cooking, 5th International Colloquium on Eucalyptus Pulp (ICEP), 9-12 Maio, 2011 Porto Seguro, Brazil) also considered this approach, combining a longer impregnation time with a lower cooking temperature to obtain a higher Kappa Number, up to approximately 28. Increasing the impregnation time also made it possible to reduce the reject content. Also Tavast and co-authors (Tavast, D.; Brannvall, E., Increased pulp yield by prolonged impregnation in softwood kraft pulping, Nordic Pulp & Paper Research Journal, 32(1), 2017) have studied the application of increased impregnation time in softwood cooking. It was also found that this increase in the impregnation time and, in addition, the decrease in the temperature of the impregnation phase, allows an easier separation of the fibers at high Kappa Number and a decrease of the obtained reject content.

[0025] Finally, in 2011, Hart and Colson (Hart, P.; Colson, G.; Antonsson S.; Hjort A., Impact of impregnation on high Kappa Number hardwood pulps, BioResources, 6(4), 2011) also verified the impact of increasing the time and decreasing the temperature of the impregnation phase on the cooking of hardwood mixtures. It is verified that increasing the impregnation time results in an easier separation of the fibers, even at high Kappa Number, as well as in a reduction of the obtained reject content. The described cooking is interrupted to obtain high Kappa Number and high reject contents. These are defibrated in a high consistency refiner.

[0026] Thus, there is a need for a Kraft pulp production process that exhibits a high yield and that overcomes the mentioned difficulties and technological challenges of cooking wood mixtures. There is still the need to obtain improved pulps that produce paper products with increased papermaking properties, such as mechanical strength properties.SUMMARY OF INVENTION

[0027] The present invention is directed to a process for the production of a Kraft pulp comprising the steps of: a) selecting a mixture of hardwood and softwoods chips in a minimum proportion of 51 % by weight of hardwood; b) impregnating with a cooking liquor by raising the temperature between 0.5 and 1.5 °C / min to a cooking temperature; c) cooking the mixture resulting from step b) in a pressurized reactor; d) stopping the cooking when between 30 % and 60 % of the lignin has been removed from the softwood; e) reactor decompression and discharge; f) separating the cooking liquor from the partially delignified wood resulting from step d); g) mechanical defibration of the wood resulting from step f) so that an homogeneous pulp suspension is formed.

[0028] A second aspect of the present invention is a paper product comprising the Kraft pulp obtainable by the process described.

[0029] Another aspect of the invention is a process for the production of paper products from the Kraft pulp produced by the process described in this invention, comprising the steps of: a) constituting a suspension of the Kraft pulp fibers in a paper machine; b) removing the water from the suspension; c) drying and formation of the paper product. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1. Delignification kinetic curves of individual species E. globulus (◆) and P. pinaster (■) and of E. globulus / P. pinaster mixtures, 85 / 15 (△), 70 / 30 (○) and 95 / 5 (◊). IEg - initial phase of delignification for cooking E. globulus; IIEg - bulk delignification phase for cooking E. globulus; IPp - initial phase of delignification for cooking P. pinaster; IIPp - bulk phase of delignification for cooking P. pinaster.DETAILED DESCRIPTION AND DESCRIPTION OF PREFERRED EMBODIMENTS

[0031] A process for producing a Kraft pulp is here described, using a mixture of chips from hardwood and softwood, in which the first predominates over the second, with typical hardwood cooking conditions, as described in this patent application and claim 1, which, surprisingly, makes it possible to simultaneously obtain a high yield (>60 %) Kraft cellulosic pulp and with increased mechanical properties compared to pulp produced from wood containing 100 % hardwoods.

[0032] Additionally, the pulp of the invention can be produced on an industrial Kraft pulp production line, normally used for the production of unbleached pulp with yields of 45 - 55 %.

[0033] The process described in this invention includes a selection of hardwood and softwood, with hardwood in a higher proportion. This is followed by impregnation and slow rising (0.5 to 1.5 °C / min) of the mixture with cooking liquor until the cooking temperature is reached. Then, the cooking of the chip mixture can take place in a pressurized reactor, either continuously or discontinuously. The cooking is stopped when at least 30 % of the lignin has been removed from the softwood.

[0034] The decompression and discharge of the reactor occurs sequentially, where the cooking and separation of the black liquor from the resulting wood took place. Finally, the resulting wood is mechanically defibrated until a homogeneous pulp suspension is obtained.

[0035] The cooking interruption point is one of the innovative aspects, together with the other technical characteristics, of this process, since it allows maximizing the pulp production yield. In the present invention, the softwood, more difficult to delignify, was taken as reference.

[0036] This optimum cooking interruption point corresponds to the point where the cooking liquor has completely diffused through the cell wall of the softwood fibers and reached the middle lamella and cell corners, where there is the highest concentration of lignin of the wood structure. At this point, which occurs at the beginning of the bulk phase of delignification, fiber separation is promoted. In softwood wood, this point is reached for higher Kappa Number than in the case of hardwood (Lindstrom, 2011). In softwoods, due to their chemical nature (higher lignin content and lower reactivity), there is a lower extent of depolymerization and lignin dissolution before fiber separation than in the case of hardwoods. At this point, it was found that the softwood chips are completely impregnated, up to 60 % of the lignin has already been removed, and the subsequent process of mechanical fiber separation is facilitated, resulting in a low reject content (in the 85 / 15 mixture between 0.7 and 2.5 %). At this point, hardwood chips are already in a more advanced stage of delignification, and the mechanical separation of their fibers is even easier than for softwood.

[0037] Figure 1 translates these results by the delignification kinetic curves of the individual species and examples mixtures as detailed below.

[0038] Kraft refers to Kraft cooking, or sulfate cooking. This chemical process consists of cooking the wood in a cooking liquor usually consisting of sodium hydroxide and sodium sulfide, at temperatures on the order of 140 to 180 °C, in pressurized reactors.

[0039] Kraft pulp refers to pulp obtained by the Kraft cooking.

[0040] A high yield corresponds to yields > 60 % in the context of a chemical Kraft cooking process.

[0041] A partially delignified pulp refers to a material resulting from a cooking interrupted at a stage prior to the residual stage, usually indicated as the end of a cooking for obtaining Kraft pulps.

[0042] The cooking yield is calculated by the following formula: weight pulp / weight wood × 100 (weights on an absolutely dry basis)

[0043] The cooking Kappa Number is calculated by the following formula:

[0044] H-factor refers to a single variable used in the Kraft cooking process for combining the variables of temperature and time into a single variable representing the cooking severity.

[0045] Sulfidity refers to the percentage of sodium sulfide relative to the active alkali (sum of sodium hydroxide concentration and half of sodium sulfide concentration) of a cooking liquor.

[0046] A cooking liquor refers to an aqueous solution consisting of sodium hydroxide (NaOH) and sodium sulfide (Na 2 S) or to another solution that provides the same technical cooking effect.

[0047] Tissue paper corresponds to papers used for hygienic and sanitary purposes, either in a domestic environment or in public places.

[0048] A containerboard refers to paper commonly used to make corrugated boards.

[0049] The top and bottom layer of a corrugated board is the so-called linerboard. It is usually a two-layer product, a top and a bottom layer. Virgin and recycled fiber are used in the production of this type of paper. In the first option, when the fiber used is predominantly virgin fiber chemically produced by the Kraft method, the product is called kraftliner. When predominantly recycled fibers are used, the product is referred to as testliner. The fluting between two liners is so-called fluted paperboard (fluting, or corrugated medium).

[0050] The cooking delignification is calculated by the following formula: lignin in the pulp × yield / wool lignin × 100

[0051] In a preferred embodiment of the invention, a mixture of hardwood and softwood chips is selected in a proportion of 85 % by weight of hardwood.

[0052] In a preferred embodiment of the invention, the impregnation is done with a cooking liquor having an active alkali by weight of wood of 0.150 - 0.200 gNa 2 O / g wood dry basis and a sulfidity of 28 - 32 %.

[0053] In a preferred embodiment of the invention, the impregnation is done with a ratio of liquor volume to wood weight of 3 - 8 L / kg.

[0054] In a preferred embodiment of the invention, the cooking is done at a cooking temperature of 140 to 160°C.

[0055] In a preferred embodiment of the invention, the process further comprises a step of drying the pulp after the step of wood mechanical defibration.

[0056] In a preferred embodiment of the invention, eucalyptus wood is selected as hardwood.

[0057] In a preferred embodiment of the invention, pine wood is selected as softwood.

[0058] In a preferred embodiment of the invention, the paper product comprising the Kraft pulp obtainable by the process described, is selected from the group consisting of kraftliner paper, testliner paper, corrugated paper, bag paper, shopping bag paper, flexible packaging paper, tissue paper and printing and writing paper.

[0059] In a preferred embodiment of the invention, the process for the production of a paper product comprising the Kraft pulp comprises a prior step of disintegrating the Kraft pulp in its dry form.Examples Example 1Cooking stepDetails of the procedure

[0060] The E. globulus and P. pinaster wood mixtures were cooked in an MK digester with white liquor under the conditions presented in detail in the following section, after analysis of the white liquor (TAPPI - White Liquor ABC Test). After cooking, the partially delignified wood was separated from the black liquor (placed in a thin fabric bag), washed with fresh water to a conductivity below 100 mS / m and centrifuged, thus allowing the determination of the total yield. The black liquor was collected and subsequently analysed for residual alkali charge (TAPPI T 625 cm-14, Analysis of soda and sulfate black liquor), residual sulfide (SCAN-N 31: 94), White, green and black liquor Hydrogen sulphide ion concentration (revised 1994), density, pH, non-volatile total solids content (determination in an oven at 105°C based on the Standard Methods for the examination of water and wastewater", 20th Edition, 2-54 to 2-58), ash (@ 525°C), calorific value (lower and upper, following standard E711-81,Standard Test Method for gross calorific value of refusederived fuel by the bomb calorimeter).

[0061] The partially delignified wood was defibrated in a VA-type defibrator (a refiner commonly used in the production of mechanical pulps). The applied force was 150 kPa, with a spacing between discs of 2 mm*. The defibration was performed using a continuous water stream at the inlet along with the material to be defibrated. The defibrated material (pulp + rejects) was collected in a cloth bag and subsequently centrifuged. Finally, the obtained material was sieved to separate the pulp from the reject and to determine the pulp yield, reject content and Kappa Number (ISO 302:2004, Pulpsdetermination of Kappa Number).

[0062] After determining the Kappa Number of the unbleached pulp obtained, the pulp was refined in the PFI refiner (ISO 5264-2), the shopper degree (°SR) determined (ISO 5267 / 1) and laboratory sheets prepared (ISO 5269-1:2005, Pulps - Preparation of laboratory sheets for physical testing - Part 1: Conventional sheet-former method) with a grammage of 135 g dry / m 2< , to evaluate the papermaking properties.Detail of cooking conditions

[0063] Taking as reference the tests carried out for 100 % E. globulus, the following parameters were kept as constant in the Kraft cooking (MK digester): alkali charge (Active Alkali (in the form of Na 2 O) / Wood Weight = 17 %), Sulfidity (28%), activity (90%), liquor-to-wood ratio (4 Liters of White Liquor / Kg of Wood), maximum cooking temperature (160 °C), heating time to maximum temperature (140 min).

[0064] Cooking trials with mixtures 85 / 15 E. globulus / P. pinaster (dried wood basis) were performed and evaluations of the impact of P. pinaster on papermaking properties were carried out. Table 1. Performed mixed cooking and respective conditions.# Cooking34585 / 1585 / 1585 / 15T max 160 °C Same H factor as 2T max 160 °C Time at T max less than 3T max 160 °C Time at T max between 3 and 4Maximum cooking temperature (°C)160160160Time at T max (min)331521Total time (min)171153159H-factor (heating + plateau)290174210Initial temperature (°C)26.425.227.3Warm-up time to T max. (min)138138138Decompression time and up to 90 °C, opening (min)121212H-factor (heating + plateau + decompression)300184222Kappa Number (KN)476959 Evaluation of the cooking step

[0065] For the cooking of the mixture 85 / 15 where it was possible to reach the target KN 50, 60 and 70, a range of screened pulp yields of 62 - 70 %, reject content of 0.7 - 2.3 % were obtained. These pulp yield results are slightly lower than those obtained in the 100 % E. globulus cooking, for the same KN (table 2).

[0066] The incorporation of 15 % P. pinaster wood has promoted an increase in wood specific consumption in a range of 2.38 - 2.65 m 3< / ton a.d., compared to those obtained for 100 % E. globulus (2.20 - 2.31 m 3< / ton a.d.), related to the apparent density of each species and respective screened pulp yield. Table 2. Results of the cooking step.Wood85 / 15 E. globulus / P. pinaster100 % E. globulus (Reference)Wood basic density (kg / m 3< )543567Kraft cooking conditionsAA / WW** (g Na 2 O / g madeira base seca )0.1700.1700.1700.1700.1700.170Sulfidity (%)282828282828Liquor-to-wood ratio (L / kg)4.04.04.04.04.04.0H-factor (heating + plateau)290210174191177125H-factor (heating + plateau + decompression)300222184215185141T max. (°C)160160160160160160Time at T max (min)33211518208Total time (min)171159153153155143KN506070506070Screened pulp yield(% m / m madeira )62.665.669.668.772.972.2Rejects content (% m / m madeira )0.71.02.30.40.80.9S.C.W*** (m 3< / ton a.d. *)2.652.532.382.312.182.20Mixture delignification (%)79.675.773.680.975.968.8* a.d. - air dry ** AA / WW - Active Alkali / Wood Weight *** S.C.W - Specific Consumption of Wood Papermaking properties evaluation

[0067] Papermaking properties of the unbleached pulps with KN 50, 60 and 70 were evaluated. For this purpose, sheets with a grammage of 135 g dry / m 2< weight were prepared, with an isotropic fiber distribution structure, and tested according to standard methodologies described in table 3. Sheets were also prepared and evaluated after refining the unbleached pulp in the PFI refiner (ISO 5264-2) at 1500 and 3000 revolutions. Table 3. Physical, optical and structural tests and corresponding standards.Opacity,%ISO 2470Drainability, °SRISO 5267 / 1Grammage, g / m 2< ISO 5270Density, kg / m 3< NP EN 20534Bulk, m 3< / kgNP EN 20534Burst index, kPa. m 2< / gNP 687Tensile index, N.m / gISO 1924 / 2Tear index, mN.m 2< / gNP EN 21974Gurley Air Resistance, (100 ml) / sISO 5636 / 5Internal bonds (Scott-bonds),TAPPI UM 403J / m 2< Bendtsen air roughness, ml / minISO 8791

[0068] Table 4 shows the overall results of the effect of incorporating 15 % in weight of P. pinaster on the papermaking properties for Kraftliner paper, where improvements for this type of paper should reside in its resistance properties, namely, burst, tensile and tear indexes. Table 4. Results for papermaking properties.Wood85 / 15 E. globulus / P. pinaster100 % E. globulus (Reference)KI506070506070Type of refining (revolutions or time)PFI*PFI*PFI*Valley (27 min)Valley (28 min)Valley (28 min)Drainability (°SR)292929292929Grammage (g / m 2)< 145.8146.8147.8143.4148.1148.9Thickness (µm)195190195199203216Bulk (cm 3< / g)1.321.301.321.391.371.45Burst Index (kPa.m 2< / g)5.76.15.95.55.54.5Tensile Index (kN.m / Kg)83.786.682.680.483.270.5Tear Index (mN.m 2< / g)11.610.710.710.89.18.2Elongation (%)3.63.73.52.82.92.8T.E.A. Index (J / g)2.01.82.01.51.61.3Tensile stiffness (kN / m)222221862117233024012423Opacity (%)100.199.999.899.999.799.8Capillarity (mm / (10 min))393532453938Gurley air resistance (s / (100mL))597458364136Bendtsen Roughness (mL / min)144128127169142320Bendtsen air permeability (mL / min)542409541433342462Internal bonds (Scott-bond) (J / m 2< )439472415350357335* interpolated values for °SR = 29 ** T.E.A - Tensile Energy Absorption

[0069] A comparative analysis of the structural and strength properties of the pulp resulting from the mixture 85 / 15 (values interpolated to °SR 29) was performed, evaluating the results for the different KN (50, 60 and 70), in comparison with 100 % E. globulus pulps, for the same KN. It was concluded that the incorporation of 15 % in weight P. pinaster promoted an improvement of the strength properties, with an increase in the tear index (7 % for KN 50, 18 % for KN 60 and 27 % for KN 70), an increase in the burst index (4 for KN 50, 11 % for KN 60 and 31 % for KN 70) an increase in the tensile index (4 % for KN 50, 4 % for KN 60 and 17 % for KN 70), an increase in internal bonds (25 % for KN 50, 32 % for KN 60 and 24 % for KN 70) and an increase in elongation (29 % for KN 50, 28 % for KN 60 and 25 % for KN 70).Example 2

[0070] Mixtures (E. globulus / P. pinaster) 90 / 10 were cooked with the cooking steps as described in Example 1. A screened yield higher than 60 % was obtained. An improvement of strength properties such as burst, tensile and tear indexes, internal bonds and elongation also occurred. Table 5. Results of the cooking step.Wood90 / 10 E. globulus / P. pinasterKraft cooking conditionsH-factor (heating + plateau)129H-factor (heating + plateau + decompression)139T max. (°C)160Time at T max (min)9Total time (min)147KN88Screened pulp yield (% m / m madeira )69.3Reject content (% m / m madeira )1.2 Example 3Detail of cooking conditions

[0071] Mixtures of woods were cooked at the proportions 95 / 5 and 70 / 30 E. globulus / P. pinaster (dried wood basis) with the cooking steps and conditions as described in Example 1.

[0072] Table 6 depicts the cooking conditions for the mixture 95 / 5. Table 6. Performed mixed cooking and respective conditions.# Cooking34595 / 595 / 595 / 5Maximum cooking temperature (°C)160160160Time at T max (min)23168Total time (min)161154146H-factor (heating + plateau)230175128Initial temperature (°C)32.024.330.4Warm-up time to T max. (min)138138138Decompression time and up to 90 °C, opening (min)101110H-factor (heating + plateau + decompression)240183138KN476675

[0073] Table 7 depicts the cooking conditions for the 70 / 30 mixture. Table 7. Performed mixed cooking and respective conditions.# Cooking34570 / 3070 / 3070 / 30Maximum cooking temperature (°C)160160160Time at T max (min)473423Total time (min)185172161H-factor (heating + plateau)390305230Initial temperature (°C)31.129.429.2Warm-up time to T max. (min)138138138Decompression time and up to 90 °C, opening (min)161010H-factor (heating + plateau + decompression)400315239KN536570

[0074] For all the trials, screened pulp yields higher than 60 and an increase of the properties of burst, tensile and tear indexes, internal bonds and elongation were obtained, in line with those verified in the previous examples, and as described below.Evaluation of the cooking step

[0075] For the cooking of the mixture 95 / 5 where it was possible to reach the target KN 50, 60 and 70, a range of screened pulp yields of 68 - 71 %, reject content of 0.7 - 1.1 % were obtained (table 8). For the cooking of the mixture 70 / 30 where it was possible to reach the target KN 50, 60 and 70, a range of screened pulp yields of 69 - 70 %, reject content of 0.4 - 0.7 % were obtained (table 8). Table 8. Results of the cooking step.Wood95 / 5 E. globulus / P. pinaster70 / 30 E. globulus / P. pinasterWood basic density (kg / m 3< )559519Kraft cooking conditionsAA / WW** (g Na 2 O / g madeira base seca )0.1700.1700.1700.1700.1700.170Sulfidity (%)282828282828Liquor-to-wood ratio (L / kg)4.04.04.04.04.04.0H-factor (heating + plateau)230175230175230175H-factor (heating + plateau + decompression)240183240183240183T max. (°C)160160160160160160Time at T max (min)231623162316Total time (min)161154161154161154KN506050605060Screened pulp yield(% m / m madeira )68.169.568.169.568.169.5Rejects content (% m / m madeira )0.71.30.71.30.71.3S.C.W*** (m 3< / ton a.d. *)2.362.322.362.322.362.32Mixture delignification (%)80.971.180.971.180.971.1* a.d. - air dry ** AA / WW - Active Alkali / Wood Weight *** S.C.W - Specific Consumption of Wood Papermaking properties evaluation

[0076] Table 9 depicts the general results of the effect of the incorporation of 5 % by weight and 30 % by weight of P. pinaster in the Kraft pulp production on the papermaking properties for Kraftliner paper. Table 9. Results for papermaking properties.Wood95 / 5 E. globulus / P. pinaster70 / 30 E. globulus / P. pinaster100 % E. globulus (Referência)KN506070506070506070Type of refining (revolutions or time)PFIPFIPFIPFIPFIPFIValle y (27 min)Valle y (28 min)Val ley (28 min )Drainability (°SR)292930302930292929Grammage (g / m 2< )144. 7147. 9145. 2146. 5146. 8145. 8143.4148.1148 .9Thickness (µm)185196196183192195199203216Bulk (cm 3< / g)1.281.321.351.251.311.341.391.371.4 5Burst Index (kPa.m 2< / g)6.56.05.67.16.86.35.55.54.5Tensile Index (kN.m / Kg)88.782.884.886.491.789.780.483.270. 5Tear Index (mN.m 2< / g)10.710.610.711.912.511.810.89.18.2Elongation ( % )3.73.73.94.84.23.82.82.92.8T.E.A. Index (J / g)2.12.12.22.72.62.31.51.61.3Tensile stiffness (kN / m)20842045198318252194210323302401242 3Opacity ( % )100. 099.999.999.899.8100. 099.999.799. 8Capillarity (mm / (10 min) )302936262732453938Gurley air resistance (s / (100mL))454841866758364136Bendtsen Roughness (mL / min)12412015388120115169142320Bendtsen air permeability (mL / min)363240311151176254433342462Internal bonds (Scott-bond) (J / m 2< )466513484579604385350357335** T.E.A - Tensile Energy Absorption

[0077] A comparative analysis of the structural and strength properties of the Kraft pulp resulting from the mixture95 / 5 and 70 / 30, evaluating the results for the different KN (50, 60 and 70), in comparison with 100 % E. globulus pulps, for the same KN, was also performed.

[0078] It was concluded that the incorporation of 5 % in weight of P. pinaster promoted an improvement of the strength properties, with an increase in the tear index (16 % for KN 60 and 30 % for KN 70), an increase in the burst index (18 % for KN 50, 9 % for KN 60 and 24 % for KN 70) an increase in the tensile index (10 % for KN 50 and 20 % for KN 70), an increase in internal bonds (33 % for KN 50, 44 % for KN 60 and 44 % for KN 70) and an increase in elongation (32 % for KN 50, 28 % for KN 60 and 39 % for KN 70).

[0079] It was concluded that the incorporation of 30 % in weight of P. pinaster promoted an improvement of the strength properties, with an increase in the tear index (10 % for KN 50, 37 % for KN 60 and 44 ', for KN 70), an increase in the burst index (29 % for KN 50, 24 % for KN 60 and 40 % for KN 70) an increase in the tensile index (7 % for KN 50, 10 % for KN 60 and 27 % for KN 70), an increase in internal bonds (65 % for KN 50, 69 % for KN 60 and 15 % for KN 70) and an increase in elongation (71 % for KN 50, 45 % for KN 60 and 36 % for KN 70).

Claims

1. A process for the production of a Kraft pulp comprising the steps of: a) selecting a mixture of hardwood and softwoods chips in a minimum proportion of 51 % by weight of hardwood; b) impregnating with a cooking liquor by raising the temperature between 0.5 and 1.5 °C / min to a cooking temperature; c) cooking the mixture resulting from step b) in a pressurized reactor; d) stopping the cooking when between 30 % and 60 % of the lignin has been removed from the softwood; e) reactor decompression and discharge; f) separating the cooking liquor from the partially delignified wood resulting from step d); g) mechanical defibration of the wood resulting from step f) so that a homogeneous pulp suspension is formed.

2. The process according to claim 1, wherein in step a) it is selected a mixture of hardwood and softwood chips in a proportion of 85 % of hardwood by weight.

3. The process according to any of the preceding claims, wherein the impregnation of step b) is done with a cooking liquor having an active alkali by weight of wood of 0.150 - 0.200 gNa2O / g wood dry basis and a sulfidity of 28 - 32 %.

4. The process according to any of the preceding claims, wherein the impregnation of step b) is done with a ratio of liquor volume to wood mass of 3 - 8 L / kg.

5. The process according to any of the preceding claims, wherein the cooking of step c) is done at a cooking temperature of 140 to 160°C.

6. The process according to any of the preceding claims, wherein the process further comprises a step of drying the pulp after step g).

7. The process according to any of the preceding claims, wherein in step a) eucalyptus wood is selected as hardwood.

8. The process according to any of the preceding claims, wherein in step a) pine wood is selected as softwood.