KR20010049536A - Oxygen delignification of lignocellulosic material - Google Patents

Abstract

PURPOSE: A preparing method of oxygen delignification of lignocellulose material is provided, which has an improved oxygen delignification process that requires reduced amounts of capital equipment. CONSTITUTION: A method for treating lignocellulosic pulp comprises the steps of: (a) mixing, in a pulp soaking stage, the lignocellulosic pulp with an alkali to create a mixture and allowing the mixture to remain in residence therein for a determined period of time to enable an alkali hydrolysis leaching of colored materials from the mixture and a swelling of pulp fibers, the lignocellulosic pulp containing lignin bonded to cellulose fibers; (b) mixing a feed of the mixture from the pulp soaking stage with both oxygen and steam to form a combined stream; and (c) feeding the combined stream to a pressurized oxygen delignification stage to enable a reaction between the oxygen and the lignin of the lignocellulosic pulp, the swelled fibers facilitating the reaction between the oxygen and the lignin of the lignocellulosic pulp and further removal of colored materials from the lignocellulosic pulp.

Description

Oxygen delignification of lignocellulosic material {OXYGEN DELIGNIFICATION OF LIGNOCELLULOSIC MATERIAL}

FIELD OF THE INVENTION

FIELD OF THE INVENTION The present invention relates to oxygen delignification of pulp and, more particularly, to an improved method of oxygen delignification that requires a reduction in equipment funds.

Prior art

Pulp manufacturers around the world are working to reduce water consumption in order to obtain minimum impact mills (MIM). Conceptually, minimal impact mills are mills that generate a minimum amount of water and air emissions without adversely affecting wood and energy consumption or product quality. MIM is accomplished in a slow stepwise manner that requires many modifications and adjustments of current mill practices. This includes: (1) minimizing leakage; (2) closed water loops in timber storage; (3) closed screen room; (4) efficient brown stock washing; (5) high yield and low energy intensive pulping methods; (6) expanded oxygen delignification; And (7) a partially closed bleach plant through the reuse of some bleach filtrate streams.

Lignocellulosic pulp originating from natural or regenerated fibers contains color-causing compounds that must be removed during the bleaching operation to produce clear, high quality pulp. The removal of such compounds from natural or regenerated fibers is generally carried out in a single step, but more generally in a series of chemical treatments which may comprise at least two of the following compounds: acid bath, ozone, chlorine, hypochlorite, Chlorine dioxide, hydrogen peroxide, peracid, chelants, alkalis, enzymes, etc. Oxygenation is generally applied to pulp early in the bleaching process to remove most of the pulp coloring material such as lignin, extracts, dyes, pigments, inks and the like.

Despite some concerns about the effect on oxygen pulp quality and bleaching of oxygen delignification with chlorine dioxide, the use of oxygen delignification is a global trend. The most recent trend has been with regard to so-called expanded oxygen delignification. Of these, the most prominent technique is the dual stage method. This is said to be more efficient and selective than conventional single step methods. Expanded oxygen delignification is of interest for MIM because it allows less residual lignin to be removed in the bleach plant and terminates pulping at higher kappa numbers. Kappa number test is used to determine the amount of lignin remaining in pulp after cooking. Kappa number is defined as the number of ml of 0.1 N potassium permanganate solution consumed by 1 g of pulp and corrected for the 50% consumption of potassium permanganate initially added. The higher the kappa number, the more lignin is present in the pulp, and the lower the kappa number, the less lignin is present in the pulp.

Since oxygen delignification is more selective than pulping, it is wise to stop cooking at higher kappa numbers and remove as much lignin as possible by oxygen delignification. In this way, process yield and throughput of the mill are increased; Wood consumption is reduced; Causticization and recovery loads are reduced; Pulp quality is maintained.

The improvement in efficiency and selectivity of the dual stage oxygen delignification process is evident over the single stage method. However, since they are site specific, it is difficult to refer to them quantitatively. Mill experience with eucalyptus kraft pulp of kappa 16-18 has shown a 5-10% increase in kappa drop across the process when switching to dual stage oxygen delignification in a single step. This increase is said to be achieved without a penalty of any significant pulp viscosity.

In theory, the bleach plant should produce low volume effluents including metals, chlorides and organics. Thus, the initial stage of the bleaching sequence should produce a filtrate that is easily recycled to the recovery system. That is, the filtrate contains low chloride and should in principle be alkaline. This idea has resulted in a so-called chlorine-free light (ECF) light bleaching method which essentially requires some form of extended oxygen delignification.

Double stage oxygen delignification as currently implemented requires high capital investment. Thus, there is a need for extended oxygen delignification that achieves the purpose of ECF light bleaching at significantly reduced capital costs.

Double stage oxygen delignification can be implemented in a number of ways with moderate consistency. Despite the high capital investment required to install this technology, deligninization rates in the range of 40-50% for hard wood and 50-60% for soft wood are obtained as a result, most of which require subsequent ECF light bleaching and Suitable for the combination of Among them: (1) two pressurization steps at high pressure with and without intermediate washing; (2) two pressurization steps, with and without intermediate washing, wherein the first step is high pressure and the second step is low pressure; (3) two pressurization steps with and without intermediate washing, where the first step is low pressure and the second step is high pressure; (4) two press steps, as in the case of (1) to (3), with a metal removal step through chelation being in between; (5) two pressurization steps as in the case of (1) to (3) with intermediate washing, adding hydrogen peroxide in the second step; Two pressurization steps as in the case of (1) to (3) with oxidative lignin activation between steps (6); And (7) two pressurization steps as in the case of (1) to (3) with weak acid hydrolysis between steps to remove metals and hexene uronic acid and the like.

Approaches (1) to (6) are being implemented commercially, and approach (7) is still under evaluation. All these techniques are described by Barna, J., Salles, DVC, Salvador, E. and Colodette, JL, entitled "Effects of Hydrogen Peroxide Addition in the Second Step of the Double-Stage Oxygen Deligninization Process," 8): 57-66, 1997).

The dual step method is better adapted to oxygen delignification kinetics. Oxygen delignification reactions are well known to occur in two phases. The first phase is rapid and controlled by diffusion. Most of the reaction occurs rapidly by electrophilic attack of oxygen and other free radicals on the remaining lignin structure, including free phenolic units. The second phase is slow and controlled by a type of chemical reaction that includes an electrophilic attack of oxygen and other intermediate species on lignin, as well as a nucleophilic attack of peroxides (organic and inorganic) produced in the first phase of the reaction. . Thus, in the first phase of the reaction, significant delignification occurs and little pulp brightening occurs, while in the second phase both delignification and brightening occur.

The rate of delignification occurring in the first and second phases is a question under investigation and it seems to depend on the type of pulp and the method of conducting oxygen delignification. However, many researchers believe that most of the delignification occurs in the first phase, whereas most of the brightening occurs in the second phase.

Given that the process takes place in two phases, it is reasonable to devise the process to exploit this reaction kinetics. Thus, if the oxygen delignification method is implemented as a dual stage method, it is more suitable for the kinetics. Since the first phase or the reaction is rapid, the first stage of the dual stage process can be carried out for a shorter time than the second stage and the rest of the reaction takes place in a second stage reactor which runs for a longer time.

1 shows a schematic of a prior art dual stage oxygen delignification process without interstage washing. Two high shear mixers 10 and 12 are required, one of which is in front of the first reactor 14; The second one is shown in front of the second reactor 16. Oxidized white liquid (OWL), which is oxygen and a high alkaline liquid, and / or caustic is added before the first reactor 14; If desired, provision may be made for such chemicals to be added between the reactors 14 and 16. The washing step between the two reactors is optional and is generally not required because its beneficial effect is questionable.

A layout of the system including the intermediate washing step 18 is shown in FIG. 2. However, since ultimately hydrogen peroxide needs to be added to the second reactor 16, the presence of the intermediate washing step 18 is preferred to avoid the loss of peroxides in reactions with partially oxidized organic carryovers.

The selectivity of the oxygen delignification first phase is not sensitive to alkali dosage, while so is the efficiency. On the other hand, the selectivity is sensitive to the alkali dose in the second phase, while the efficiency is not. In view of this fact, the first oxygen reactor 14 is preferably operated at high alkalinity in order to maximize efficiency. On the other hand, the second reactor 16 must operate at low alkalinity to maintain process selectivity. In addition, selectivity was shown to be negatively affected by oxygen pressure in the first phase reaction, while efficiency was positively affected. On the other hand, both efficiency and selectivity are positively affected by the oxygen pressure in the second phase. Thus, in order to maintain process selectivity, the first oxygen reactor 14 should operate at low oxygen pressure and the second oxygen reactor 16 at high oxygen pressure. In other words, the alkali dosage should be high in the first reactor 14 and low in the second reactor 16, and vice versa for the oxygen dosage (partial pressure). Therefore, it is reasonable to add the total amount of alkali to the pulp of the first reactor 14 and to use only residual alkali in the second reactor 16. On the other hand, oxygen is divided between the two reactors, so that a larger fraction must be applied to the second reactor 16.

The function of the mixer 12 located between the two stages is to mix the oxygen added in the second stage and to remix the remaining oxygen bubbles which eventually coalesce in the first reactor. Since the alkali is easily mixed with the pulp, there is no need to remix it.

It was also shown that selectivity was negatively affected by temperature in the first phase reaction, while efficiency was positively affected. On the other hand, the temperature in the second phase has a positive effect on the efficiency and not on the selectivity. Therefore, it is preferable to carry out the process with the temperature in the first reactor lower than in the second reactor.

In the prior art processes, the desired delignification rate is achieved using the form of the dual stage oxygen delignification method shown in FIGS. 1 and 2. Compared with conventional single stage oxygen delignification, the dual stage method requires heavy equipment and therefore is more expensive to install. In the case of FIG. 1, additional reactors and mixers are required. In the case of FIG. 2, additional reactors, mixers and washers are required. Thus, the benefits of double stage oxygen delignification are limited by the significant capital investment required for its installation.

The present invention seeks to improve the process for removing colored compounds from lignocellulosic material using alkali and oxygen. In this method, the pulp is first treated with alkali and then further with oxygen. An important feature of the present invention is the treatment of the pulp with alkali prior to oxygen delignification. Alkali dipping of the pulp fibers results in alkali hydrolysis / filtration of the readily available colored material and expansion of the pulp fiber which promotes removal of the colored material in the subsequent oxygen delignification step. As a result, a 5 to 10% improvement in the removal rate of the colored material is achieved compared to the conventional oxygen delignification method. The present invention allows achieving the same delignification rate as the prior art dual stage oxygen delignification process requiring much more capital investment.

In addition, the process of the present invention is particularly efficient for delignifying lignocellulosic material, which typically contains higher lignin content. For example, the pulp having the following kappa number is preferably in accordance with the method of the present invention. Namely: 30 to 40 for softwood kraft pulp, 17 to 25 for hardwood kraft pulp and 15 to 30 for recycled fibers. This specification expresses all compositions as weight percentages unless otherwise indicated.

The present invention includes treating lignocellulosic pulp with alkali in a vessel at atmospheric pressure. This treatment, hereinafter referred to as pulp soaking, is carried out at a pH in the range of about 9 to 14, advantageously in the range of about 11 to 12. Preferred conditions for pulp soaking are as follows: temperature 70-90 ° C., consistency 10-12%, retention time 0.5-1 hour and alkali capacity of 1-2% based on the pulp dry weight. Alkaline capacity can vary substantially depending on the type of lignocellulosic material being treated. The preferred way to control pulp soaking is by monitoring the slurry pH rather than the alkali capacity. Lignin removal during soaking is a function of the initial lignin content of the pulp as measured by kappa number. The higher the initial pulp kappa number, the more efficient the lignin removal during the dipping process, especially when the pulp is washed or compressed after dipping.

After alkali soaking, the pulp can be entered directly into a subsequent oxygen delignification step, washed in a conventional washer or concentrated to a consistency of about 30 to 35% in a wash press.

The soaked pulp is then treated with oxygen in a medium consistency oxygen delignification step, hereinafter referred to as the O-step. This O-step is carried out with 10-12% consistency, 11-12 pH, 60-95 ° C., reaction time of 60-90 minutes, 400-600 kPa pressure and 1.2-2.5% oxygen capacity. Magnesium salts such as magnesium sulfate and the like may be added in amounts of 0.01 to 0.1% by weight (based on dry pulp) and most advantageously about 0.02 to 0.03% by weight magnesium to protect the pulp carbohydrate against alkali induced degradation.

Magnesium salts should be added before alkali addition to facilitate mixing of the salt with the pulp. The capacity of the alkali and magnesium salts will depend on the presence or absence of a washing or pressing step between the pulp soaking step and the O-step.

The method of the present invention can be understood more clearly with reference to FIGS. 3 to 7. 3 shows the process of the present invention in which washing is omitted between the pulp dipping step and the O-step. The method comprises two separate steps, pulp soaking step 5 and O-step 52, without washing or pressing in between. The pulp coming from the Brown Stock Wash operation receives both MgSO ₄ through feed 51 and alkali (NaOH containing oxidized white liquid or non-oxidized white liquid) from source 54 via pump 56. . The pulp / alkali slurry then enters the dense immersion vessel 58 at atmospheric pressure, where it is maintained for the desired residence time.

The pulp slurry is then sent to O-step 52 where additional alkali and magnesium salts can be added as needed by suction of pump 60. The slurry then enters a high shear mixer 62 where medium pressure steam and oxygen are added to the pulp slurry via feeds 64 and 66, respectively. The pulp slurry is then pumped to pressurized reactor 68 where the slurry is maintained for the desired reaction time. After the reaction is finished, the pulp is discharged to the blow tube and pumped into a washer (not shown) after the O-step.

4 shows a second embodiment of the present invention in which the pulp slurry is washed between pulp dipping step 50 and O-step 52. In this case, after dipping, the pulp slurry is pumped by the pump 70 to the washer 72 and then conveyed in a conventional O-step as described above. 5 shows that after immersion in immersion step 50, the pulp is pumped to wash press 80 instead of a normal washer. In this case, the slurry is dewatered to 35-40% consistency and then transferred to standpipe 82 and diluted to 10-14% consistency using the filtrate from the subsequent O-step. In the standpipe, the pulp receives additional alkali and magnesium salts via feed 84 and then enters O-step 52 for treatment as described above.

Another embodiment of the invention is shown in FIGS. 6 and 7. In FIG. 6, it is shown that it comprises two separate steps without washing or pressing in between, first alkali soaking step 100 and then O-step 102. The embodiment shown in FIG. 7 is similar except for the fact that the pulp is washed or compressed between an alkali immersion step and an oxygen delignification step. The main difference between these methods is that the oxygen treatment is carried out at a lower reaction pressure (hydrostatic pressure), i.e. mini-O, compared to the method described above with respect to FIGS. This step is carried out with 10-12% consistency, 11-12 pH, 70-80 ° C., 60-120 min reaction time, 150-300 kPa pressure (overpressure or outlet pressure), 0.5-1.05% oxygen capacity. Magnesium salts such as magnesium sulfate (0.02 to 0.03% magnesium capacity) may be added to protect the pulp carbohydrate against alkali induced degradation. Magnesium salts should be added before alkali addition to facilitate mixing of the salt with the pulp. Note that the amount of alkali and magnesium salts depends on the presence or absence of a washing and pressing step between the pulp dipping step and the O-step.

If the pulp is not washed (ie FIG. 6), the alkali requirement is a minimum amount of only about 10-20% of the total amount required. This additional alkali is needed to replenish the fraction consumed during the soaking step. On the other hand, if the pulp is compressed or washed after immersion (ie, FIG. 7), additional alkali is needed. Alkali capacity of 1 to 1.5% is required. Note that the alkali requirements vary substantially with the type of lignocellulosic material.

After soaking, the pulp slurry is conveyed directly to the O-stage, where additional alkali and magnesium salts are added by suction of the pump; The slurry then enters a high shear mixer or static mixer where medium pressure steam and oxygen are added to the pulp. The pulp is then pumped to the pre-maintenance tube 104 and then to the down-air tower 106 (or directly to the up-air tower) for the desired reaction time. The pre-maintenance tube 104 is generally elevated to 200 kPa, and the down-air tower 106 operates at atmospheric pressure. The rise-air tower is at atmospheric pressure but operates at the outlet pressure of the column; The bottom pressure of the tower into which oxygen is injected depends on the height of the tower. After the end of the reaction, the pulp is pumped into the post-stage washer (FIG. 6). 7 shows the presence of a cleaning system between stages.

The main advantages of the technique described with respect to FIGS. 3 to 7 are: (1) Pulp quality and process efficiency, while they are less capital intensive than the dual stage method because they require one less pulp cleaner, a high shear mixer and a pressurized reactor. Is maintained; (2) they are easy to retrofit single stage oxygen delignification plants to have the advantage of increasing delignification performance by 5-10% without much investment; (3) They may also be applied to the mini-O method (low pressure oxygen delignification), which likewise increases the delignification rate by 5-10% with minimal capital investment.

Thus, the difference between the methods of the present invention with respect to the prior art is a reduction in the overall capital cost investment compared to the dual stage oxygen delignification process. In addition, they can increase delignification by 5-10% when applied in connection with both conventional single step processes and mini-O oxygen delignification methods.

In summary, the present invention treats lignocellulosic pulp with alkali at atmospheric pressure, followed by high or low pressure exposure of the pulp to oxygen in a pressurized vessel. The present invention is applicable to all kinds of fibrous raw materials, including pulp produced by methods such as kraft, soda, sulfite, magnesite, cold soda, NSSC and the like. Such fibers can be obtained from hardwood, softwood, bamboo, bagasse, straw and other non-woody fiber feeds. The method can also be applied to de-inked recycled fibers and certain grades of brown recycled fibers.

1 is a schematic diagram of a prior art dual stage oxygen delignification plant without interstage washing of a pulp stream.

2 is a schematic of a prior art dual stage oxygen delignification plant with an interstage washing of a pulp stream.

3 is a schematic diagram of an oxygen delignification plant embodying the present invention without the step-by-step washing of the pulp stream.

4 is a schematic diagram of an oxygen delignification plant embodying the present invention with an interstage washing of a pulp stream.

5 is a schematic diagram of an oxygen delignification plant embodying the present invention with an interstage pressure wash of a pulp stream.

6 is a schematic diagram of another embodiment of an oxygen delignification plant embodying the present invention.

7 is a schematic diagram of another embodiment of an oxygen delignification plant incorporating the present invention.

Summary of the Invention

The present invention forms a process for delignification of pulp from which one stage of a two stage oxygen delignification plant has been removed. In particular, the pulp soaking step allows the pulp stream to be mixed with the alkali feed and stay for a sufficient period of time in order to alkali hydrolyze the colored material from the pulp and expand the pulp fibers. The pulp stream is then mixed with an oxygen feed and processed through a mixer. The output of the mixer is fed to a pressurized reactor where oxygen reacts with lignin of the pulp fibers. The expanded pulp fibers promote oxygen reaction with lignin and removal of additional coloring material.

In a first preferred form of implementation of the method, no interstage washing step is required. In the second embodiment, the washing step is introduced between the dipping step and the oxygen reaction step to increase the process efficiency, thereby increasing the process efficiency but adding capital expense.

The scope of the present invention and the range of the preferred manner and the preferred method of implementing the method of the present invention in various steps are provided.

The pulp alkali treatment can be carried out in atmospheric vessels and pressurized vessels, preferably in atmospheric vessels. The alkali used may be of various origins, including plain NaOH, oxidized white liquid (OWL), non-oxidized white liquid (WL), and the like. It is most advantageous that the reaction takes place with 0.5 to 5% by weight NaOH. The vessel may be a dedicated vessel or a high density tower already present in most pulp mills. The treatment referred to as pulp soaking (S) is carried out with about 6 to 14 weight percent pulp consistency. A pH above 11 promotes alkali soaking. It is most advantageous that the immersion takes place at a pH of about 11-12. Raising the immersion temperature to 40-95 ° C. accelerates the expansion of lignocellulosic pulp. It is most advantageous for the immersion to take place at 70 to 95 ° C. to accelerate the reaction. Immersion times of 15 to 240 minutes swell the fibers advantageously to promote the reaction between oxygen and lignin. It is most advantageous to complete the dipping with a soaking time of about 20 or 30 minutes to 60 minutes. However, the alkali capacity can vary substantially depending on the type of lignocellulosic material used.

After alkali soaking, the pulp can be directly subjected to subsequent oxygen treatment, washed in a conventional washer or concentrated to a consistency of about 30 to 35% in a wash press. The equipment needed to wash or thicken the pulp is standard and available on the market. This work affects the overall direct and indirect costs of the present invention. When the pulp is washed after soaking, provision may be made for further pulp treatment such as chelation to remove the metal and weak acid hydrolysis to remove the metal prior to subsequent oxygen treatment.

The soaked pulp is then treated with oxygen in three different ways: (1) at low pressure (mini-O), hereinafter referred to as (EO); (2) a conventional MC (medium consistency) single stage oxygen delignification method, referred to below as O; And (3) MC dual stage oxygen method, hereinafter referred to as O / O or OO, depending on whether the pulp is washed between stages. The oxygen used in this treatment may be of purity ranging from 80 to 100%, preferably from 90 to 100%.

Low pressure oxygen treatment or mini-O (EO) may be carried out with a consistency of 8 to 14% by weight, preferably 10 to 12% by weight. Preferred conditions for the other parameters are as follows: pH is 10 to 14, preferably 11 to 12, alkali capacity is 1 to 4%, preferably 1 to 2% and temperature is 50 to 120 ° C., preferably 70-90 ° C., reaction time 30-180 minutes, preferably 60-90 minutes, reaction pressure 100-600 kPa, preferably 150-300 kPa and oxygen capacity 0.2-2%, preferably 0.5-1% . In this step, a magnesium salt such as magnesium sulfate may be added to protect the pulp carbohydrate against alkali induced degradation. Magnesium salts may be added before or with alkali, but are preferably added before alkali to promote mixing of the salt with the pulp. The dose of magnesium salt may range from about 0.01 to 0.1% (as magnesium), preferably from about 0.02 to 0.03%, based on the pulp dry weight. Hydrogen peroxide may be added in the (EO) step at a dose of 0.2-4%, preferably 0.5-1%, to increase delignification. The addition of peroxide is viable only if the pulp is washed after soaking.

O, a conventional single stage oxygen treatment, can be carried out with a consistency of 8 to 15%, preferably 10 to 12%. Preferred conditions for the other parameters are as follows: pH is 10 to 14, preferably 11 to 12, alkali capacity is 1 to 4%, preferably 1 to 2% and temperature is 50 to 140 ° C., preferably 80 to 100 ° C, reaction time is 30 to 180 minutes, preferably 60 to 90 minutes, reaction pressure is 100 to 800 kPa, preferably 400 to 600 kPa and oxygen capacity is 0.5 to 4%, preferably 1 to 2% . In this step, magnesium salts such as magnesium sulfate and the like may be added to protect the pulp carbohydrate against alkali induced degradation. Magnesium salts may be added before or with alkali, but are preferably added before alkali to promote mixing of the salt with the pulp. The dose of magnesium salt may range from about 0.01 to 0.1% (as magnesium), preferably from about 0.02 to 0.03%, based on the pulp dry weight. Hydrogen peroxide may be added in step (O) at a dose of 0.2-4%, preferably 0.5-1%, to increase delignification; The addition of peroxide is viable only if the pulp is washed after soaking.

The dual stage oxygen treatment, OO / O or OO, can be carried out with a consistency of 8 to 14%, preferably 10 to 12%. Preferred conditions for the other parameters are as follows: pH is 10-14, preferably 11-12, alkali capacity is 1-4%, preferably (1.5 / 0.5%), temperature is 50-140 ° C., preferably Preferably (85/95) ° C., reaction time is 30 to 180 minutes, preferably (30/60) minutes, reaction pressure is 100 to 800 kPa, preferably 400 to 600 kPa and oxygen capacity is 0.5 to 4%, preferably (1.5 / 0.5%). In this step, magnesium salts such as magnesium sulfate and the like may be added to protect the pulp carbohydrate against alkali induced degradation. Magnesium salts may be added before or with alkali, but are preferably added before alkali to promote mixing of the salt with the pulp. The dose of magnesium salt may range from about 0.01 to 0.1% (as magnesium), preferably (0.02 / 0.0%), based on the pulp dry weight. Hydrogen peroxide may be added to the dual stage oxygen treatment at a dose of 0.2 to 4%, preferably (0.0 / 0.5%) to increase delignification; The addition of peroxide is viable only if the pulp is washed after soaking.

For purposes of this specification, "/" separates the first step addition or condition from the second step addition or condition. For example, (0.03 / 0% Mg) means adding 0.03% Mg in the first step and adding 0% Mg in the second step.

Experiment

In order to identify and to best understand the present invention, different types of lignocellulosic material were allowed to follow these steps. The comparison between the method of the present invention and the prior art method is based on the results of kappa drop, viscosity drop and brightness increase across various methods. Kappa drop, viscosity drop and brightness increase were calculated by the following equation: kappa drop = (kappa _in -kappa _out ) / kappa _in ; Viscosity drop = (viscosity _in -viscosity _out ) / viscosity _in ; Brightness Increase = (Brightness _in -Brightness _out ). Kappa number, viscosity, and brightness values were determined according to the Technical Association of the Pulp and Paper Industry (Tappi) standard method. All experiments were performed twice, and the results are shown as averages.

Except where noted, the procedures and operating conditions used in the various methods discussed are described below:

Pulp Alkaline Dipping (S): 12% consistency, 85 ° C., 30 minutes, 1.5% alkali. The reaction was carried out in a high shear mixer / reactor made of Hastelloy with temperature and pressure regulators and a device for the injection and removal of gases. Mixing was performed intermittently every minute for 4 seconds at 200 rpm. Although the alkali capacity, temperature and reaction time were varied, these were described in the appropriate examples.

Single O-Step (O): performed with 12% consistency, 95 ° C., 60 minutes, 1.5% oxygen, 1.5% NaOH and 0.03% magnesium itself at 600 kPa. The reaction was carried out with the same equipment and settings described above under Alkali Immersion.

Dual O-Stages (O / O and OO): The first O-Stage is performed with 10% consistency, 85 ° C., 60 minutes, 1.5% oxygen, 1.5% NaOH and 0.03% magnesium itself at 600 kPa. It was. The second step was performed using 1.5% NaOH and 1.5% oxygen at 10% consistency, 95 ° C., 60 minutes, 600 kPa. When a wash was performed between the two steps, the wash was performed through a press wash as described below. Dual O-steps without intermediate washes are referred to as O / O steps and those with intermediate washes are referred to as OO-steps. The reaction was carried out with the same equipment and settings described above under Alkali Immersion.

Low Pressure or Mini-O-Step (EO): This step was performed using 1.0% oxygen, 1.5% NaOH and 0.03% magnesium itself at 10% consistency, 85 ° C., 60 min, 200 kPa. The pressure was reduced to zero at 200 kPa for a 60 minute reaction while simulating a hydraulic tower. The reaction was carried out with the same equipment and settings described above in the alkaline immersion section, except that the pressure was manually reduced from 200 kPa to 0 at 5 minute intervals.

Press Wash: Press wash between stages was performed after the step by diluting the pulp with 4% consistency and then compressing it with about 35% consistency. This corresponds to a cleaning efficiency of about 80%, for example considering an embodiment in which pulp was introduced into the washing step with 10% consistency. Note that cleaning between stages is unmarked, while no washing is generally indicated by a slash mark (/).

The following examples are provided to illustrate the present invention.

Example 1 Optimization of Immersion Time and Temperature

Kraft pulp samples used in this example were obtained from a eucalyptus tree in the laboratory. After pulping, the brown pulp had an initial kappa number of 19.6, a viscosity of 58.7 mPa · s and an ISO brightness of 28.9%. Immersion was performed at 65, 75 and 85 ° C. for a period of 15, 30, 60 and 180 minutes. Other conditions were kept constant as described in the paragraph above. After soaking, the pulp was thoroughly washed and then kappa number, viscosity and brightness were analyzed. The results presented in Table 1 show that the immersion efficiency measured by kappa drop is positively affected by both time and temperature. However, the benefits of soaking decrease slightly after 30 minutes reaction, especially at 85 ° C. Increasing the time from 30 minutes to 240 minutes resulted in an increase in kappa drop of only 1%. Therefore, the time of 30 minutes was considered sufficient at the temperature of 85 degreeC. Since the temperature did not significantly affect the pulp viscosity, practically subsequent O-steps were generally performed at temperatures equal to or above the 85 ° C. value, so the 85 ° C. value was considered most appropriate. Brightness increase was very low before and after the soaking phase and was not accompanied by kappa drop. Similarly, the exposure of pulp to alkali at warm temperatures resulted in a lignin darkening reaction that overshadowed the expected brightness increase due to partial lignin removal.

Example 2: Optimization of Dipping pH

The same kraft pulp samples used in Example 1 were immersed with alkali capacities of 0.5, 1.0, 1.5, 2.5 and 4% NaOH. Other immersion conditions were kept constant as described above. After soaking, the pulp was washed thoroughly with distilled water and the kappa number, viscosity and brightness were measured. The results in Table 2 show that while increasing the immersion pH above 12 has only some advantages for kappa drop, the pulp viscosity is slightly disadvantageous. Therefore, pH 12, equivalent to an alkali dose of 1.5%, was considered most satisfactory for this pulp sample.

Example 3: effect of pulp delignification

Various hardwood kraft pulp samples used in this example were obtained from eucalyptus trees. Pulping conditions were adjusted to produce low, medium and high delignified pulp for hardwood standards. After pulping, the low degree delignification sample had a kappa number of 14.3, a viscosity of 35.7 mPa · s and an ISO brightness of 34.9%; The moderate delignification sample had a kappa number of 16.8, a viscosity of 47.5 mPa · s and 32.3% ISO brightness; The high delignification sample had a kappa number of 19.6, a viscosity of 58.7 mPa · s and an ISO brightness of 28.9%. Immersion was carried out under the conditions described above. After soaking, the pulp was washed thoroughly with distilled water and the kappa number, viscosity and brightness were measured.

The results presented in Table 3 indicate that immersion is more effective at reducing kappa number when applied to pulp with higher initial kappa number. This may be explained by the higher content of potential alkaline hydrolyzable / filterable lignin in pulp with high kappa number. The effect on increasing pulp viscosity and brilliance was mild and was clearly independent of the initial kappa number of the pulp.

Example 4 Effect of Lignocellulosic Material Types

Four types of lignocellulosic material were compared. Industrial hardwood kraft pulp (HWD) with a kappa of 17.1, a viscosity of 43.4 mPa · s and an ISO brightness of 36.8%; Industrial softwood kraft pulp (SWD) with a kappa of 32.2, a viscosity of 42.7 mPa · s and an ISO brightness of 26.4%; Industrial recycled fiber samples (MOW) produced from de-inked lower mixed office waste of kappa of 14.4, viscosity of 11.9 mPa · s and ISO brightness of 55.1%; An industrial recycled fiber sample (RCM) produced from a deinked cupside material of kappa of 69.8 and 42.5% of ISO brightness. Immersion was carried out under fixed conditions as described above. After soaking, the pulp was washed thoroughly with distilled water and the kappa number, viscosity and brightness were measured. The results presented in Table 4 indicate that the performance of the alkali soaking operation depends substantially on the type of lignocellulosic material. The best steep drop was achieved in hardwood kraft pulp, indicating that this material contains the highest content of filterable / hydrolysable lignin promoted by alkali. In addition, the viscosity of the hardwood pulp did not substantially change during the soaking treatment. Despite the higher lignin content, both softwood kraft and RCM pulp were not so easy to soak. This pulp resulted in a lower kappa drop across the immersion compared to the HWD sample. In addition, SWD experienced greater viscosity loss than its HWD counterpart. The RCM and MOW samples lost brightness across the immersion, which means the development of alkali-promoted darkening reactions. In the case of MOW samples, the effect of soaking on kappa drop was very small; The colored material present in the MOW sample is mostly non-ligninous and likely to interfere with alkaline hydrolysis / filtration.

The following examples show the effect of pulp alkali immersion (S) on the overall performance of the subsequent oxygen bleaching step with or without intermediate washing.

Example 5 Effect of Pulp Soaking on the Overall Performance of Various Types of Oxygen Deligninization of Hardwood Kraft Pulp

The hardwood kraft sample used in this example was the same as that described in Example 1. Immersion was performed under fixed conditions as described in the paragraph above. Subsequent oxygen delignification treatment was performed under the following conditions, as described above: O: 10% consistency, 95 ° C., 60 minutes, 600 kPa overpressure, 1.5% NaOH, 1.5% O ₂ , 0.03% Mg; (EO): 10% consistency, 85 ° C., 60 minutes, 200 kPa pressure, 1.5% NaOH, 0.8% O ₂ , (0.03 Mg); O / O: 10% consistency, (85/95 ° C.), (30/60 minutes), 200 kPa overpressure, (1.5 / 0% NaOH), (1.5 / 0.5% O ₂ ), (0.03 / 0% Mg ); OO: 10% consistency, (85/95 ° C.), (30/60 minutes), 600 kPa pressure, (1.5 / 1.0% NaOH), (1.5 / 0.5% O ₂ ) and (0.02 / 0.02% Mg).

The results in Table 5 show that the amount of lignin removal due to soaking is not additive to that obtained in subsequent oxygen treatments regardless of the mode of oxygen application under consideration. However, more than half of the soaking benefits are transferred to the subsequent oxygen delignification step. For example, the soaking itself results in a kappa drop of 8.9% and the conventional O-step 37.5%. If the benefit is additive, 46.4% delignification is expected after S / O treatment. Instead, values of 43.7% were obtained experimentally. This very same trend is seen in S / (EO), S / (OO), S / O / O and S / OO processing. This difference was initially due to the absence of a washing step between dipping and various oxygen treatments. However, the insertion of a washing step between these treatments was still not enough to add to the benefits of the treatment. Similarly, the soaking treatment removes the lignin fraction that would otherwise be removed in the subsequent oxygen treatment. Although no additional benefits are expected, the overall delignification can still be increased by 7.4% using this technique, so there is still an advantage to applying immersion.

Immersion with a single O-stage, S / O and SO results similar to the delignification rate obtained using dual stage oxygen delignification, O / O and OO, without any significant penalty and brightness increase in pulp viscosity Brought it. It is noteworthy that one immersion with a single O-stage requires much less installation capital than the dual stage oxygen delignification method.

Another interesting point shown in Table 5 relates to the S / (EO) and S / (EO) methods, one immersion with low pressure (mini-O) oxygen delignification. Immersion treatment adds an additional 5-7% kappa reduction to the (EO) step. This advantage is significant given the fact that EO (oxygen extraction) operations are designed to achieve lower delignification levels than conventional O-stages. The 5% improvement in the (EO) step, which provides 20-25% delignification, is more pronounced than the 8% improvement in the O-step, which provides 35-40% delignification. Note that this benefit is obtained with a very low capital investment without any penalty for pulp quality.

Example 6 Effect of Pulp Soaking on the Overall Performance of Various Types of Oxygen Deligninization of Softwood Kraft Pulp

The soft kraft pulp used in this example was obtained from Western North American pulp mill and made from Spruce. After pulping, the brown pulp had a kappa number of 32.2, a viscosity of 42.7 mPa · s and an ISO brightness of 26.4%. Immersion was performed at fixed conditions as described in the paragraph above. Subsequent oxygen delignification treatment was performed under the following conditions: O: 10% consistency, 95 ° C., 60 minutes, 600 kPa pressure, 2.0% NaOH, 2.0% O ₂ , 0.03% Mg; (EO): 10% consistency, 85 ° C., 60 minutes, 200 kPa pressure, 2.0% NaOH, 1.0% O ₂ , 0.03% Mg; O / O: 10% consistency, (85/95 ° C.), (30/60 minutes), 600 kPa pressure, (2.0 / 0% NaOH), (1.5 / 0.5% O ₂ ), (0.03 / 0% Mg ); OO: 10% consistency, (85/95 ° C.), (30/60 minutes), 600 kPa pressure, (1.5 / 0% NaOH), (1.5 / 0.5% O ₂ ) and (0.02 / 0.02% Mg).

The results from various oxygen delignifications on softwood pulp samples followed the same trends observed for hardwood treatment. Immersion benefits, however, were less for softwood samples, as exemplified in Table 6. The advantages due to immersion were particularly insufficient when implemented prior to the low pressure oxygen stages S / (EO) and S (EO) methods. Although the benefits of soaking were lower than in softwood samples compared to hardwood samples, the performance of the oxygen delignification step was much higher for softwood samples. However, this is very well documented in literature and worldwide mill scale work.

Example 7 Effect of Soaking on Overall Pulp Bleaching in the D (EOP) D Sequence

Oxygenated hardwood and softwood pulp samples described in Examples 5 and 6, respectively, were further bleached by an ECF (chlorine free) bleaching process using the D (EOP) D sequence. Brightness targets were 90% for HWD samples and 89% for SWD samples. As recommended by the Technical Association of Pulp and Paper Industry (Tappi) as described in the Tappi publication TIS 0606-21 entitled "Recommended Method for Specifying Pulp Bleaching Steps", the designation of D (EOP) D is the first D-step. , An (EOP) step and then another D-step, followed by a sequence comprising three separate steps with a washing or pressing step between the steps. In the (EOP) step, alkali, oxygen and hydrogen peroxide were injected at several minute intervals from each other in the same step. The conditions used in the various bleaching steps were as follows: First D-step: 10% consistency, 75 ° C., 60 minutes, final pH of 3.0 and kappa coefficient of 0.24; (EOP): 10% consistency, 85 ° C., (15/75) min, 200 kPa pressure, 10.5 final pH, 1.4% NaOH, 0.5% O ₂ , 0.5% H ₂ O ₂ , 0.03% Mg; Second D-step: 10% consistency, 75 ° C., 240 minutes, 3.8 final pH and variable amount of ClO ₂ depending on pulp pretreatment and type. Control of pH in the first and second D-steps was achieved through small additions of NaOH or H ₂ SO ₄ at that stage as needed.

The results presented in Table 7 for hardwood pulp show that the S / O or SO process, which is a combination of dipping and conventional oxygen delignification, saves the same amount of chlorine dioxide as the O / O or SO process, which is a dual stage oxygen delignification Shaded lines). ClO ₂ savings are calculated as the difference between the specific treatment group and the control. Compared to a single O-step, the S / O and SO processes saved 1.1 to 1.3 kg ClO ₂ / pulp odt. On the other hand, when compared to low pressure oxygen delignification (EO), the S (EO) and S (EO) processes also resulted in saving about 1.2 to 1.4 kg of chlorine dioxide in ClO ₂ / pulp odt. For softwood pulp samples (Table 7), a similar trend was observed, but the absolute value of chlorine dioxide savings was not the same.

In view of the above problem, it can be concluded that immersion with conventional oxygen delignification yields similar results to double stage oxygen delignification. On the other hand, when applied prior to conventional or low pressure oxygen delignification, such as in S / O, SO, S / (EO) or S (EO) processes, immersion is over dried ton of fully bleached pulp. To save about 1 to 1.5 kg of chlorine dioxide per ClO ₂ ). The washing step between dipping and high or low pressure oxygen steps has advantages; This advantage is insufficient to justify the installation of expensive cleaning systems. If a cleaning installation already exists, cleaning between stages is recommended.

Example 8 Effect of Pulp Soaking on the Strength Properties of Bleached Pulp

As measured by viscosity and brightness reversal, no significant effect of immersion on pulp quality is observed in Table 7. To further confirm this, beatability and strength property tests were performed on hardwood samples described and treated as in Example 5 and bleached as described in Example 7. The beating curves are shown for no dry pulp using a PFI mill at 0, 1500, 3000, 4500 revolutions. The strength properties of the bleached pulp were measured using the Tappi standard method. The values reported in Table 8 are at 40 ° SR.

Since alkaline immersion is performed under alkaline conditions at high temperatures, it could be assumed that the pulp strength properties will be slightly compromised. However, the results in Table 8 show that the strength properties and the pulp viability do not change by immersion treatment. In addition, there was no negative effect on pulp strength properties and viability when oxygen treatment was performed alone or with immersion. Thus, it is concluded that immersion does not negatively affect pulp strength when performed under well optimized conditions.

It is to be understood that the above description is only illustrative of the present invention. Various alternatives and modifications may be invented by those skilled in the art without departing from the invention. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations that fall within the scope of the appended claims.

An important feature of the present invention is the treatment of the pulp with alkali prior to oxygen delignification. Alkali dipping of the pulp fibers results in alkali hydrolysis / filtration of the readily available colored material and expansion of the pulp fiber which promotes removal of the colored material in the subsequent oxygen delignification step. As a result, a 5 to 10% improvement in the removal rate of the colored material is achieved compared to the conventional oxygen delignification method. The present invention allows achieving the same delignification rate as the prior art dual stage oxygen delignification process requiring much more capital investment.

In addition, the process of the present invention is particularly efficient for delignifying lignocellulosic material, which typically contains higher lignin content. For example, the pulp having the following kappa number is preferably in accordance with the method of the present invention. Namely: 30 to 40 for softwood kraft pulp, 17 to 25 for hardwood kraft pulp and 15 to 30 for recycled fiber.

The main advantages of the technique described with respect to FIGS. 3 to 7 are: (1) Pulp quality and process efficiency, while they are less capital intensive than the dual stage method because they require one less pulp cleaner, a high shear mixer and a pressurized reactor. Is maintained; (2) they are easy to retrofit single stage oxygen delignification plants to have the advantage of increasing delignification performance by 5-10% without much investment; (3) They may also be applied to the mini-O method (low pressure oxygen delignification), which likewise increases the delignification rate by 5-10% with minimal capital investment.

Thus, the difference between the methods of the present invention with respect to the prior art is a reduction in the overall capital cost investment compared to the dual stage oxygen delignification process. In addition, they can increase delignification by 5-10% when applied in connection with both conventional single step processes and mini-O oxygen delignification methods. In summary, the present invention treats lignocellulosic pulp with alkali at atmospheric pressure, followed by high or low pressure exposure of the pulp to oxygen in a pressurized vessel. The present invention is applicable to all kinds of fibrous raw materials, including pulp produced by methods such as kraft, soda, sulfite, magnesite, cold soda, NSSC and the like. Such fibers can be obtained from hardwood, softwood, bamboo, bagasse, straw and other non-wood fiber feeds. The method can also be applied to de-inked recycled fibers and certain grades of brown recycled fibers.

Claims (10)

A process for treating lignocellulosic pulp comprising the following steps: a) in the pulp dipping step, the lignocellulosic pulp comprising lignin bound to the cellulose fibers is mixed with an alkali to form a mixture, from which the alkali hydrolysis filtration of the colored material and the expansion of the pulp fibers are possible. Maintaining the mixture therein for a designated period of time in order to; b) mixing the feed of the mixture from both the pulp dipping step with both oxygen and steam to form a combined stream; c) feeding the combined stream to a pressurized oxygen delignification step to enable a reaction between the oxygen and the lignin of the lignocellulosic pulp, wherein the expanded fiber is subjected to the oxygen and the lignocellulosic pulp Promoting said reaction of the liver and further removal of colored material from said lignocellulosic pulp. The method of claim 1 wherein the pulp soaking step is performed at a pH of 11 or greater. The method of claim 1 wherein the alkali added to the pulp dipping step is at least one of NaOH, oxidized white liquid and non-oxidized white liquid. The method of claim 1 wherein the pulp soaking step is performed within a temperature range of about 40-95 ° C. The method of claim 1 wherein the pulp soaking step is performed with about 6 to 14 weight percent pulp consistency. The method of claim 1 wherein the pulp soaking step is performed for a holding time of about 15 to 240 minutes. The method of claim 1 wherein step b) further comprises the step of washing the output of the pulp dipping step. The process of claim 1, wherein step a) further comprises adding magnesium sulfate prior to alkali. The method of claim 8, wherein magnesium sulfate is added in the mixture as a magnesium based on the dry weight of the pulp at a dose ranging from about 0.01 to 0.1 wt%. 10. The method of claim 9, wherein the dipping occurs at a temperature of 70 to 95 ° C.