SE500605C2 - Delignification of softwood pulps to give high brightness - using peracid, complexant and peroxide, with retention of pulp viscosity and strength - Google Patents

Abstract

Lignocellulose-contg. pulp is delignified and bleached using a peracid (or a salt of a peracid). The pulp is then treated with a complexant, followed by a Cl-free bleaching agent comprising one or more of a peroxide, ozone or sodium dithionite. The pulp to be treated with peracid is a chemically digested pulp. Peracetic acid is pref., when delignification is performed at pH 3-10. The peroxide is H2O2 or a mixt. of this and O2. The pulp is washed (after treatment with complexant) at a pH of at least 4. The complexant is a nitrogenous cpd., esp. DTPA or EDTA. Complexation is effected at pH 2.5-11. In addn., delignification with peracid may be preceded by oxygenation.

Description

lO (TI ÜÜ 6Ü5 2 and with a reasonable number of bleaching agents, it has become necessary to examine the reasonable consumption of even of powerful and thus difficult-to-control bleaching agents with a high delignifying and / or bleaching ability.

The invention According to the invention, the lignocellulosic pulp has been provided with a process in which it is delignified and bleached under the conditions stated in the claims, whereby a good delignifying and bleaching effect is obtained even before the chlorine-free bleaching.

The process involves delignification and bleaching of lignocellulosic pulp by delignifying the pulp with a peracid or salts thereof, treating with complexing agents and then bleaching with chlorine-free bleach.

Through the present process it has become possible to delignify the pulp before a chlorine-free bleaching, so that the subsequent treatment with complexing agents can be used to optimize the conditions for the subsequent chlorine-free bleaching, taking into account the advantageous and disadvantageous metal ions. Thus, it is known that ions of alkaline earth metals, especially in their original positions in the pulp, have a beneficial effect on the selectivity in bleaching and the consumption of chlorine-free bleaching agents such as peroxide-containing compounds and ozone.

Peracid or salts thereof according to the present process, include organic and inorganic peracids or salts thereof. used aliphatic thereof. By Peracetic acid or permyric acid Suitable peracetic acids, aromatic peracids or salts.

As inorganic peracid, peroxomonosulfuric acid (Caros acid) or salts of inorganic peracids such as peroxocarbonate, various perborates or peroxosulfates are suitably used. Sodium is suitably used as the cation in the salts, as cheap is included in the chemical balance in the pulp mill. That something salty about it. Such salts are normal and sodium naturally, it is preferred to use peracetic acid or peroxomonosulfuric acid or it is especially preferred to use l5._ bÛÛ 605 3 because it is advantageous from the manufacture- In addition, peracetic acid, nings and user point of view. peracetic acid limited corrosive. Any wastewater containing e.g. The degradation products of peracetic acid can be easily recycled to chemicals.

Peracetic acid according to the present process, can be prepared by reaction between so-called such equilibrium peracetic acid to acetic acid and hydrogen peroxide, equilibrium peracetic acid, by distillation of distant hydrogen peroxide, acetic acid and sulfuric acid or by reaction between acetic anhydride and hydrogen peroxide directly in the bleaching step, so-called in-situ peracetic acid. A typical equilibrium peracetic acid contains about 42% peracetic acid and about 6% hydrogen peroxide, i.e. the weight ratio of peracetic acid to hydrogen peroxide in this case is about 7: 1. In the present process, it is advantageous to use equilibrium peracetic acid. The amount of peracid or salts thereof, should be in the range from about 1 up to about 100 kg / ton dry as 100%. It is suitable that the amount 2 up to 45 mass and preferably in the range from 3 up to 25 mass calculated is in the range from kg / ton dry kg / ton dry mass calculated as 100% product.

Delignification with peracid or salts thereof is suitably carried out at a pH in the range from about 2.5 up to about 12. In preferred embodiments where the delignification is carried out with peracetic acid or peroxomonosulfuric acid, the pH is suitably in the range from 3 up to and preferably in the range from 5 up to 7.5.

Delignification with other of the above-mentioned peracids or salts thereof, takes place within normal pH ranges for the respective bleaching agents which are known to the person skilled in the art. The pulp has, among other things, manganese ions in a particularly unfavorable effect on bleaching with chlorine-free bleaching agents, such as oxygen, ozone and alkaline peroxide compounds. Compound formers which form strong complexes with different manganese ions are therefore mainly used as complexing agents. Such suitable inorganic compounds, nitrogen-containing poly complexing agents are nitrogen-containing, mainly nitrogen-containing polycarboxylic acids, 4 phosphonic acids and nitrogen-containing polyalcohols. Preferred nitrogen-containing polycarboxylic acids are diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA) or nitrilotriacetic acid (NTA) and especially preferred DTPA or EDTA. Preferred nitrogen-containing polyphosphonic acids are diethylenetriamine pentaphosphonic acid. Other compounds can also be used as complexing agents, such as polycarboxylic acids, suitably oxalic acid, citric acid or tartaric acid, or phosphonic acids. Furthermore, organic complexes which are formed during pulp treatment with e.g. chlorine-free bleach.

The choice of pH in the treatment with complexing agents is of crucial importance for the removal of the undesirable trace metal ions while simultaneously preserving the desired ions of alkaline earth metals. Appropriate pH range is a function of e.g. the type and amount of trace metal ions in incoming pulp. In the process according to the invention the treatment with complexing agents is carried out at a range from about 2.5 up to gene, the pH should be within about 11, suitably from 3.5 up to 10 and preferably from 4.5 up to 9.

The choice of temperature during the treatment with complex builders is of great importance for the removal of the undesirable Thus the content of rising temperature during the treatment with This kappa number reduction increases. to 90 ° C, the temperature increases markedly. worth the trace metal ions. decreases manganese ions with results in and at surprisingly also viscosity complexing agents. that the brightness an increase in temperature from e.g.

The treatment with complexing agents should be carried out from 26 up to about 100 ° C, 40 up to 95 ° C and preferably from 55 at a temperature of suitably from up to 90 ° C.

The amount of complexing agent charged is and the amount depending on the type of trace metal ions is further affected by the type of complexing agent and the conditions of the treatment with complexing agent, such as temperature, residence time and pH. The amount of complexing agent invested should, however, be up to about 10 kg / ton. The amount is suitably within the range from dry mass calculated as 100% product. about 0.1 incoming mass. The amount of ROH (HC * d / V \ J '~ J \ J gene in the range from 0.3 up to 5 and preferably in the range from 0.5 up to 1.8 kg / ton dry mass calculated as 100% product .

In preferred embodiments where both the delignification with peracid and the treatment with complexing agents are carried out at near neutral pH, the need for pH from bleaching and during washing is minimized. This adjustment. In this way, the leaching treatment steps can also be used internally, resulting in a small total sewage volume, which enables a significantly higher degree of closure of the pulp mill.

Chlorine-free bleach or ozone in any sequence As the chlorine-free bleach consists of peroxide-containing compound and ozone, it is advantageous to bleach the compound before ozone. As chlorine-free bleach, it may also comprise peroxide-containing premix or mixture. with the peroxide-containing rium dithionite can be used. The peroxide-containing compound is suitably composed of inorganic peroxide compounds such as hydrogen peroxide or peroxomonosulfuric acid (Caros acid). It is preferred that the peroxide-containing compound be hydrogen peroxide or a mixture of hydrogen peroxide and oxygen.

With hydrogen peroxide as the chlorine-free bleach, the pulp can be treated at a pH of from about 7 up to about 13, preferably at a pH of from 8 up to 12 and preferably at a pH of from 9.5 up to 11.5. Bleaching with other of the above-mentioned chlorine-free bleaches takes place within normal pH ranges for the respective bleaches which are known to the person skilled in the art.

The process according to the invention is suitably carried out with a washing step after the complex image treatment. The washing effectively removes the complexed trace metal ions that are negative for the subsequent chlorine-free bleaching, mainly ions of manganese but also ions of e.g. copper and iron. In order to preserve the trace metal ions in the pulp which are advantageous for the subsequent chlorine-free bleaching, mainly magnesium and calcium ions, the pH at the wash 'The pH of the wash should suitably be in the range of at least about 4. the range from 5 up to about 11, preferably from 6 up to 10.

The washing liquid may consist of fresh water, optionally (fl ___] Ü 6 {} 5 6 with the addition of a pH-regulating chemical or waste water from one or more bleaching steps or extraction steps, in such a way that a suitable pH at the wash is obtained. Types of possible content are low in undesirable metal ions such as manganese, iron and copper.

By washing methods to more or less completely continue to consist of other purified wastewater, provided that after the complex image treatment, is meant to remove the liquid phase from the pulp suspension in order to reduce the content of e.g. dissolved trace metal ions in said suspension. The washing methods can mean an increase in the pulp concentration, e.g. by suction or pressing. The washing methods can also involve a reduction of the pulp concentration, e.g. by diluting with washing liquid. Washing also refers to combinations and sequences in which the pulp concentration is increased and decreased alternately several times a washing method, once or twice. is technically and economically suitable.

The efficiency of the wash can be stated as a proportion of the liquid phase removed of the liquid phase that was present in the pulp suspension before washing. The total washing efficiency is calculated as the sum of the efficiency over each washing step. after a treatment step from e.g. 10 to 25% pulp concentration a washing efficiency of 66.7%. After a subsequent washing dilution to 3% and then a total washing efficiency of 96.9% is dewatered with the washing efficiency at 75%, suitably. Thus, a dewatering of the pulp suspension gives steps, where the pulp first to 25% is obtained with respect to soluble contaminants. the present process should be at least about 100% and preferably 100%. It is preferred that the washing efficiency be in the range from look up to iooæ. when using in the range from 90 up to within the range from 92 up to especially the present process, the conditions for the chlorine-free bleaching are optimized, so that a high brightness, kappa number reduction and viscosity are achieved at 10 r / each 3 \ JU CIJD 7 minimal consumption of chlorine-free bleach. This becomes possible without the use of auxiliary chemicals such as stabilizers and protectors in the chlorine-free bleaching. Residual and alkali can be used directly in the bleaching step, in the peracid step or running bleaching chemicals such as hydrogen peroxide to the advantage in any other suitable step, so that an optimal combination of process technology and production cost is achieved.

Lignocellulose-containing pulp refers to pulps containing fibers or mechanically exposed by chemical treatment or recycled fibers. The fibers can be of hardwood or softwood. By chemical pulp sulphate- By mechanical pulp is meant pulp produced by refining is meant pulps digested according to the sulphite, soda or organosolv process. of chips in slab refiner (refiner pulp) or by grinding logs in grinding mill (stone grinding pulp). By lignocellulosic pulp is also meant pulps produced by modifications or combinations of the above-mentioned methods or processes. Examples of such masses are thermomechanical, chememechanical or chemothermomechanical masses. Suitably the lignocellulosic pulp consists of chemically digested pulp and preferably of sulphate pulp. It is particularly preferred that the lignocellulosic composition consists of sulphate pulp of softwood. The process according to the invention can be carried out in a yield up to the range from 30 up to 80% the range from 45 up to 65%.

The implementation of pulps by about 90%, suitably within and preferably within the present process, can take place in any position in the bleaching sequence, e.g. immediately after the production of the pulp. When the method according to the invention is applied to chemically digested pulp, it is preferably delignified in an oxygen step before the delignification with peracid. The process according to the invention can be carried out on chemically digested masses with an initial kappa number in the range from about 2 up to about 100, suitably 5 up to 60 from 10 up to 40. The kappa number is measured according to standard method SCAN-C 1:77. and preferably in the process according to the invention the delignification with peracid should be carried out at a temperature of from about up to about 100 ° C, suitably from 30 up to 90 ° C and preferably from 50 up to 80 ° C. The delignification with peracid should be carried out for a time of from about 1 up to about 960 minutes, suitably from 10 up to 270 minutes and preferably from 30 up to 150 the delignification with peracid may be from about 1 up to about min. The pulp concentration at 70% by weight, suitably from 3 up to 50% and preferably from 10 up to 30% by weight.

In the complexing process, it is carried out for a period of from about 1 up to about 15 up to 240 minutes and preferably from 35 up to 120 minutes. The mass concentration according to the invention should be the treatment 96O min, suitably from the nen in the treatment with complexing agents may be from about 1 up to about 60% by weight, suitably from 2.55 up to 40% and preferably from 5.5 up to 25% by weight.

With hydrogen peroxide as the chlorine-free bleach, the pulp should be treated at a temperature of from about 30 to 60 hours of from about 5 up to about 960 minutes, preferably from 60 preferably from 190 up to 360 minutes. With hydrogen peroxide as the chlorine-free bleach, the pulp concentration may be from about suitably from 3 up to 50 up to about 100 ° C, preferably up to 90 ° C, and for up to 420 minutes, 1 up to about 70% by weight,% by weight and preferably from 10 up to 30% by weight. Treatment with other of the above-mentioned chlorine-free bleaches takes place within normal intervals of the respective bleaches which are known to the person skilled in the art. In preferred embodiments with chlorine-free bleach, hydrogen peroxide should be in the range of about hydrogen peroxide as the amount of pulp charged in the bleaching step up to about 60 kg / ton dry mass calculated as 100% hydrogen peroxide. The upper limit is not critical but has been set for economic reasons.

Suitably the amount of hydrogen peroxide ranges from 6 up to 50 kg / ton dry up to 40 kg / ton dry mass calculated as 100% hydrogen peroxide. mass and preferably from 13 In preferred embodiments with ozone as the chlorine-free 50 ° 605 bleach, the amount of ozone may be in the range of 0.5 up to about 30, preferably from 1.5 up to 5 kg / ton dry mass.

After complexing agent and subsequent chlorine-free directly for lower requirements for brightness. the range from about kg / ton dry pulp, preferably within up to 15 kg / ton dry pulp and the delignification with peracid, treatment with bleaching, the pulp can be used preparation also lightness, by treatment in of paper with final bleach to one or more final bleaching with the mass can desired higher step. Suitable chlorine-free bleaching extraction steps also take place, which can be reinforced with peroxide as stated above, possibly with intermediate and / or oxygen gas. This completely eliminates the formation and emissions of AOX. It is also possible to final bleach with chlorine-containing bleaches such as chlorine dioxide and still have a very limited formation and emission of AOX, since the pulp content of lignin after the present process is greatly reduced.

The invention and its advantages, which are to be limited in more detail by the following examples, are only intended to illustrate the invention without it. The percentages and parts given in the description, the requirements and the examples are by weight percentages and parts by weight unless otherwise indicated.

Furthermore, the pH values stated in the description, the claims and the examples refer to the pH at the end of the respective treatment unless otherwise stated.

In the examples below, the kappa number, viscosity and brightness of the pulp according to the SCAN standard methods C 1:77 R, C peroxide and peracetic acid were determined by titration with nat -16: 62 and C 11-75: R, respectively. The consumption of hydrogen thiosulphate resp. potassium permanganate and sodium thiosulfate.

Example 1 Oxygen delignified softwood sulphate pulp with a kappa number of 12.4, a brightness of 38.4% ISO and a viscosity of 1100 dm3 / kg, delignified with peracetic acid (PAA), treated with EDTA and bleached with hydrogen peroxide, to show effect of pH in the treatment with complexing agents.

The amount of peracetic acid charged was 22.4 kg / ton dry mass l0 l5 CD U1 Síl fi 6 calculated as 100% product. during the delignification, the temperature was 70 ° C, the treatment time was 60 minutes and the pulp concentration was 10% by weight and the pH was between 5 and 5.5. After delignification, the pulp was treated with 2 kg of EDTA / ton of dry pulp at varying pH, a temperature holding time of 60 minutes and weight percent. Thereafter, the pulp was bleached with hydrogen peroxide at a temperature of 90 ° C, a pulp concentration of 90 ° C, a residence time of 240 minutes and a pulp concentration of 10% by weight. The addition of hydrogen peroxide was 25 kg / ton dry mass calculated as 100% hydrogen peroxide and the pH was 10.5-11. After each step, the mass was washed with deionized water at a pH of 6.0. In this case, the pulp was first dewatered to a 25% pulp concentration, after which it was diluted to a pulp concentration of 3% by weight. After a few minutes, the pulp was dewatered to a pulp concentration of 25% by weight. The total washing efficiency was thus about 97%. The results after bleaching with hydrogen peroxide are shown in the following table.

TABLE I pH at Pulp properties after H2 O2 treatment with Kappatal viscosity Brightness complexing agent (am3 / kg) (æ Iso) 1.5 4.2 900 71 2.7 3.4 920 76 4.8 3.0 940 81, 4 2, 9 945 83 7.9 3.0 940 81, 5 4.0 890 75 12.3 4.5 840 65 The table shows that complex image processing of pulp according to the present invention at a pH in the range from about 2.5 up to about 11 greater brightness increase and kappa number reduction than treatment gives a significant at a pH outside this range.

Example 2 Oxygen-delignified sulphate pulp of kappa number of 12.4, a brightness of 49.8% ISO and a viscosity of hardwood with a * OG 605 hydrogen peroxide, to show complexing agents and more particularly the importance of treatment with complexing agents in a separate step The conditions for the delignification with peracetic acid and the bleaching with hydrogen peroxide were in accordance with Example 1. The conditions for the treatment with EDTA were apart from comparison, the pulp was treated according to Example 1, that the pH was consistently 5.8 In the absence of complexing agents at pH 6.0, a temperature of 90 ° C and a residence time of 60 minutes (Experiment 2). the mass was prepared with peracetic acid in the presence of EDTA at pH 5.1, (Experiment 3), the mass according to Example 1. The results followed by bleaching with hydrogen peroxide After each step were washed after bleaching with hydrogen peroxide, as shown in the following: Chart.

TABLE II __ Pulp properties after H2O2 Experiments Kappatal viscosity Brightness (dm3 / kg) (% Iso) l 3.8 1063 87.2 2 4.7 1013 77.3 3 6.6 .93l 80.6 The table shows that treatment of pulp according to the present invention with complexing agents in a separate step, gives a very high brightness increase and kappa number reduction while the highest viscosity of the pulp is obtained.

Example 3 The oxygen delignified sulphate pulpwood used in Example 2 was treated to show the effect of the initial delignification with the san according to the present process, peracetic acid on the pulp properties. The conditions of the delignification with peracetic acid, the treatment with EDTA, as well as were in accordance with the example the mass with EDTA and bleaching delignification with per- at the bleaching with hydrogen peroxide 2. For comparison, treated with hydrogen peroxide without acetic acid (Experiment 2). Following accordance with Example 1. The results after bleaching with hydrogen prior to each step, the pulp was washed in peroxide, as shown in the following table. 12 TABLE III Pulp properties after H2O2 Experiments Kappatal viscosity Brightness (dm3 / kg) (% Iso) 3.8 1063 87.2 7.5 1109 82.5 The table shows that delignification with peracetic acid before complex image treatment and bleaching with hydrogen peroxide, gives a pulp with considerably higher brightness and lower lignin content, while the difference in the viscosity of the pulp is comparatively small.

Example 4 The oxygen delignified softwood sulphate pulp used in Example 1 was treated in accordance with the invention, followed by ozone and hydrogen peroxide. The sequence was peracetic acid - complexing treatment - hydrogen peroxide - ozone PAA - X - P - Z - P. The conditions of the delignification with peracetic acid, EDTA, as with example 2. bleaching with - hydrogen peroxide, i.e. the treatment with in the bleaching with hydrogen peroxide was in accordance With comparison, i.e. X - the mass at the mass without P - Z - P a temperature lignification with peracetic acid, (experiment 2). In the ozone step, ° C was bleached, a contact time of tration of turn of 2 minutes and a mass concentration 37% by weight. The consumption of ozone was 2.6 kg / ton dry mass and the pH was 2.1. In the second hydrogen peroxide step, the pulp was bleached at a temperature of 70 ° C, holding time of weight percent. The addition of hydrogen peroxide was a top and a pulp concentration of 10 kg / ton dry hydrogen peroxide and the pH was 11.0. After 60 minutes of pulp counted as 100% each step, the pulp was washed in the Results according to the following table. according to example 1. the second hydrogen peroxide step, is shown in TABLE IV Pulp properties after H2O2 Experiment v Capacity viscosity Brightness (dm3 / kg) (æ Iso) 0.4 750 90.3 0.9 800 86.9 500, 605 13 The table shows that treatment of pulp according to the present invention followed by bleaching with ozone and hydrogen peroxide, enables a completely chlorine-free bleaching to over 90% ISO and removal of practically all lignin in the pulp while maintaining sufficient pulp strength.

Claims (11)

10 15 20 25 30 35 509 605 14 Patent claims A process for the delignification and bleaching of ligno- that the pulp is delignified with a peracid or salts thereof, varcellular pulp, characterized therefrom, after being treated with complexing agents and then bleached with chlorine-free bleach comprising peroxide-containing compound and ozone in any sequence or mixture. 2. A method according to claim 1, characterized in that the lignocellulosic mass consists of chemically digested mass. 3. A process according to claim 1 or 2, characterized in that the peracid consists of distilled equilibrium peracetic acid. 4. A process according to claim 3, characterized in that the delignification with peracetic acid is carried out at a pH in the range from 3 up to 10. Process according to Claim 4, characterized in that the delignification with peracetic acid is carried out at a pH in the range from 5 up to 7.5. Process according to any one of the preceding claims, characterized in that the peroxide-containing compound consists of hydrogen peroxide or a mixture of hydrogen peroxide and oxygen. Method according to one of the preceding claims, characterized in that the pulp is washed after the complex image treatment at a pH of at least about 4. Process according to one of the preceding claims, characterized in that the complexing agent is a nitrogen-containing organic compound. 9. A process according to claim 8, characterized in that the nitrogen-containing organic compound is diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid (EDTA). Process according to any one of the preceding claims, characterized in that the treatment with a complexing agent is carried out at a pH in the range from about 2.5 up to about 11. 11. ll. Process according to one of the preceding claims, characterized in that the delignification with peracid is preceded by an oxygen step. 15