SE501581C2 - Lignocellulosic pulp bleaching - by treating with an acid, complexing agent, a calcium and magnesium cpd. then peroxide bleaching - Google Patents

Abstract

Lignocellulosic pulp (I) is (i) acid treated at pH upto 5 (ii) treated with a complexing agent then (iii) a calcium and a magnesium cpd. added to the pulp at 3.5-9 pH followed by (iv) bleaching with a peroxide contg. cpd. at 7-13 pH. Stages (i) of (iii) may be in any order.

Description

15 20 25 30 35 - takes place at a near neutral pH. It is 501 581 2 subsequent alkaline peroxide bleaching. With this process, it is essential that the treatment with complex automobiles is also not suitable for use in treating pulps which contain large amounts of ions of transition metals.

The invention According to the invention, there has been provided a process in which lignocellulosic pulp is treated under the conditions specified in the claims, whereby the pulp content of negative ions for a subsequent alkaline peroxide bleaching can be effectively reduced while the content of positive ions can be optimized so that a pulp with high strength and brightness can be obtained with peroxide-containing compound. a low consumption of According to the invention is meant a process for bleaching lignocellulosic pulp with a peroxide-containing compound, in that the pulp before said bleaching in any order is treated acidic at a pH up to about 5 and treated with a complexing agent, and that the pulp then dewatered or washed to remove ions of transition metals.

Then a magnesium compound and a calcium compound are added - a weight ratio of added Mg to Ca in the range from about 1: 1 with up to about 1: 4, after which the pulp, without intermediate dewatering or washing, is bleached at a pH in the range from about 7 up to about 13.

The initial acid treatment in combination with treatment with a complexing agent according to the present process is effective from lignocellulosic pulps. By rendering means of releasing all types of metal ions or washing the pulp, the effectively present process can therefore have the dewatered ions thus released removed from the pulp suspension. It has proved possible to very effectively reduce the concentration of the ions of transition metals, in particular manganese, copper and iron, which are negative for the subsequent peroxide bleaching.

In the present process, magnesium ions are reintroduced into the pulp before bleaching, thereby increasing the brightness and pulp strength while lowering the lignin content and the consumption of bleaching agents. By also reintroducing calcium ions into the pulp, a surprising improvement in the final properties of the pulp is achieved, especially with regard to the brightness and the reduction in kappa number. It has been found according to the present process that the amounts of the added magnesium and calcium ions are of decisive importance in achieving this, especially the ratio between and calcium ions of the content of magnesium and calcium ions are of great importance for the synergistic effect of the pulp. Thus, the amounts are of magnesium crucial importance, but also the resulting mass of properties after the alkaline peroxide bleaching.

The present acid pretreatment in combination with complexing agents and subsequent all By the addition of a limited but sufficient dewatering or washing, means that mainly ions are released and removed from the pulp. substantially all of this magnesium can be reintroduced into the pulp, even under the alkaline conditions prevailing in the peroxide bleaching of the present process. As a result, the amount of magnesium invested and the amount of the amount of magnesium invested in the pulp in terms of calcium and the amount of calcium in the pulp during the peroxide bleaching will largely correspond. On the same in peroxide bleaching to largely conform.

With the present method it is possible to effect only one several steps without intermediate pH pre-treating the mass in washing, or in adjustment. By then raising the pH step before dewatering or and peroxide bleaching the pulp in the presence of added magnesium and calcium ions, it is possible to obtain a pulp with high brightness, low lignin content and with substantially preserved pulp strength using a reduced amount of peroxide-containing compound. Depending on i.a. carried out with relatively small amounts of both complexing agents and type pulp, the process can also often be used as a magnesium and calcium compound and with an uncomplicated dewatering before the peroxide bleaching.

The weight ratio between added Mg and Ca should be in the range from about 1: 1 up to about 1: 4, for the alkaline peroxide bleaching to be effective. Suitably the weight ratio of added Mg to Ca is within 1: 3, the range is from 1: 1.2 up to preferably from 1: 1.5 up to 1: 2.6.

The amount ranges from about 150 as weight percent magnesium per weight dry mass. Conveniently charged magnesium compound may be within the ppm up to about 1000 ppm, the amount is in the range from 200 up to 700 ppm and preferably in the range from 250 up to 500 ppm.

As the magnesium-containing charged magnesium compound compound, magnesium sulphate, magnesium chloride, magnesium carbonate or calcium chloride, magnesium nitrate, preferably magnesium sulphate is suitably used. calcium-containing compound or calcium nitrate, calcium sulphate calcium carbonate, preferably calcium sulphate. which contain both useful, such as dolomite. Also compounds magnesium and calcium 1 suitable ratio is that the whole amount of magnesium or calcium ions. It is also within the scope of the invention or part of can be added for e.g. pH control or pulp suspension via the water used Thus, hard water containing magnesium or calcium ions or a combination thereof can be advantageously diluted. ' be used to reintroduce these ions into the mass. Examples of such hard waters are fresh water from limestone bedrock, backwater from paper-masked chalk is used as a filler or process water from sulphite production with magnesium or chines there or calcium as a base. washed calcium compound, added by but peroxide bleaching The mass is dewatered or before magnesium and carried out without intermediate dewatering or washing. The addition can take place after adjusting the pH to within the range from approx. 7 up to approx. 13. The addition can also take place together with the chemical. When the magnesium and calcium compound are added after or in connection with pH adjustment to within the pH range where the subsequent bleaching is to be carried out, the peroxide-containing compound can be added simultaneously. However, the addition of magnesium and calcium compound preferably takes place without intermediate pH adjustment, after which the pH is adjusted to in the range from up to about 13 before the peroxide-containing compound is added and the bleaching is carried out.

In the acid treatment process Ca 7 according to the invention it is carried out at a pH up to about 5, suitably from 1.5 up to 4 and preferably from 2 up to 3.

In the acid treatment, the pH can be adjusted by adding the mass with an acid or an acidic liquid. As the acid, inorganic mineral acids or residual acid from a chlorine dioxide reactor are used, individually or in any mixture.

Suitably an inorganic mineral acid such as sulfuric acid, nitric acid or hydrochloric acid, preferably sulfuric acid is used.

The treatment with complexing agents can be carried out where it is technically appropriate. The treatment with complexing agents can take place in a separate step before or after the acid treatment. Suitably, however, the treatment takes place with complexing agents and the acid treatment combined in one step. the pulp has, among other things, manganese ions in a particularly unfavorable effect on alkaline peroxide bleaching. Compound formers which form strong complexes with different manganese ions are therefore mainly used as complexing agents. Such suitable complexing agents, mainly nitrogen-containing, are nitrogen-containing organic compounds, polycarboxylic acids, nitrogen-containing polyphosphonic acids and nitrogen-containing polyalcohols. Preferred nitrogen-containing polycarboxylic acids are acetic acid (DTPA), nitrilotriacetic acid EDTA. diethylenetriaminepentaphosphonic acid. diethylenetriaminepenta- (EDTA) or especially preferred DTPA ethylenediaminetetraacetic acid (NTA) and or Preferred nitrogen-containing polyphosphonic acids are As complexing agents, other compounds can also be used, such as polycarboxylic acids, preferably 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. These are sodium salts of the above-mentioned complexing agents, since it is furthermore preferred that the use of the bleaching agent is thereby facilitated. 501 581 10 15 20 25 30 35 6 The choice of pH in the treatment with complexing agents is not critical in the present process, since those for the subsequent alkaline magnesium and calcium ions can be optimized in terms of both the peroxide bleach positive amount and position in the pulp. In the process according to the invention, the treatment with complexing agents can thus be carried out at a pH in the range from about 1.5 up to about 13. In this way, the treatment with complexing agents can be combined with the acidic treatment, without the complexing ability being appreciably reduced.

The amount of complexing agent charged depends on the type and amount of transition metal ions in the efficiency of dewatering or washing. The amount is further affected by incoming pulp, as well as the acid treatment and subsequent 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 5 kg / ton dry mass calculated as 100% product. The amount is suitably 0.05 up to 2.5 kg / ton dry mass and preferably in the range from 0.1 up to gene in the range from 1 kg / ton dry mass calculated as 100% product.

Prior to the acid treatment according to the present process, the pulp can be treated acid in one further step. As a result, a particularly low content of ions of transition metals can be obtained. Such a treatment may be suitable for alkaline peroxide bleaching of pulps with unusual or where these high levels of transition metal ions are difficult to release from the pulp. An example of this is organosol masses. in a preferred embodiment of the present process the acid treatment is carried out in the presence of an effective at a pH such chemicals are ozone, acid delignifying chemical, which is up to about 5. Suitable hydrogen peroxide, peracids or salts thereof, chlorine dioxide or chlorine. It salts thereof or chlorine dioxide, it is preferable to use ozone, peracids or because in this case a very strong and selective delignification can be obtained. It is especially preferred to use ozone. Perc acid or salts thereof process, comprises organic and inorganic peracids or salts thereof. As organic peracid, aliphatic peracids, aromatic peracids or salts thereof are used.

Preferably peracetic acid or permyric acid is used. Peroxomonosulfuric acid (Caros acid) or salts of inorganic peracids such as peroxocarbonate, various perborates or peroxosulfates are suitably used as inorganic peracid. It is preferred that or some salt thereof with sodium as cation. use peracetic acid or peroxomonosulfuric acid It is especially preferred to use peracetic acid.

When the present process is carried out by delignification with peracid or salts thereof, the amount may be within up to about 100 calculated as 100% commodity. It is suitable that the amount is in the range from about 1 kg / ton dry mass in the range from 2 up to 45 kg / ton dry mass and preferably in the range from 3 up to 25 kg / ton dry mass calculated as 100% product.

When the present process is carried out by delignification with ozone, the amount may be in the range from about 0.5 up to about 30 kg / ton dry mass, from 1 up to 20, preferably from 1.5 up to 10 kg / ton dry mass. mass, preferably within kg / ton dry mass and When the present process is carried out with delignification with chlorine dioxide, the amount can be stated (SF) according to SF = total active chlorine in kg / ton dry mass / (1) kappa number before delignification according to method A 2.63 kg of active chlorine. I lie within as a betting factor 1 kg chlorine dioxide corresponds to the present process, the betting factor can range from about 0.1 range from 0.5 up to 5 up to 3.

The process up to about 10, suitably within and preferably from 1 with an acidic treatment according to the invention is carried out dewatering or washing steps after it and the treatment with complexing agents. In this way, ions of transition metals can be effectively removed before the peroxide bleaching. The washing liquid can e.g. consist of fresh water or backwater according to the present 501 581 10 15 20 25 30 35 8 water from one or more bleaching steps or extraction steps. washing, or less completely removing By dewatering or is meant methods to more the liquid phase from pulp- to reduce the content of undesirable ions in said suspension. the suspension for Dewatering means an increase in the pulp concentration, e.g. by suction or pressing through filters. Washing refers to combinations and sequences, where the pulp concentration with washing liquid and raising method is selected a washing method which effectively removes those in alternately lowered by dilution by dewatering. In the present consideration of what is process technically and economically appropriate. the pretreatment ions released, under particularly high demands on the efficient removal of transition ions of transition metals, or washed in a further one or more steps before the peroxide bleaching. Thus, the pulp can be dewatered, released ions of transition metal wash can be removed directly by dewatering or after each step.

In the process according to the invention, the acid treatment can be carried out at a temperature of from about 10 up to about 100 ° C, suitably from 25 up to 90 ° C and preferably from 40 up to 80 ° C.

In the process with complexing agents is carried out at a temperature of up to about according to the invention, the treatment of from about 100 ° C, suitably from 25 up to 95 ° C and preferably from 40 up to 90 ° C.

The acid treatment and the treatment with complexing agents can each be carried out for a time of from about 1 up to about 600 minutes, suitably from 5 up to 120 minutes and preferably from 10 up to 60 minutes.

The pulp concentration in the acid treatment and the treatment with complexing agents can be from about 1 up to about 60 welds from 3 up to 35% by weight. % by weight, preferably from 2 up to 40 and preferably The time for the treatment with magnesium and calcium compound after the addition to the pulp may be from about 1 up to about 600 minutes, preferably from 5 up to 120 minutes and preferably from 10 up to 60 min. According to the invention, the addition for the subsequent bleaching takes place suitable combination of pH, temperature and In the process of magnesium and calcium compound at a pulp concentration.

Bleaching with peroxide-containing compound is carried out 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 up to 11.

The peroxide-containing compound includes inorganic peroxide compounds such as hydrogen peroxide, sodium peroxide or peroxosulfuric acid (Caros acid) or organic peroxide compounds such as peracetic acid and permyric acid. It is suitable that the peroxide-containing compound consists of hydrogen peroxide or a mixture of hydrogen peroxide and oxygen, preferably hydrogen peroxide.

With hydrogen peroxide as the peroxide-containing compound, the pulp can be treated at a temperature of up to about 150 ° C, suitably from 45 up to 10 ° C and preferably from 60 up to 90 ° C. The residence time may be from about 1 up to about 2000 minutes, preferably from 30 up to 1500 minutes and preferably from 60 up to 600 minutes. With hydrogen peroxide from about as peroxide-containing compound, the pulp concentration can be from about 1 up to about 70% by weight, suitably from 3 up to 60% by weight from 10 up to 50% by weight.

Treatment with the above and preferably other of the stated peroxide-containing compounds, takes place within normal intervals in terms of temperature, time and pulp concentration for the respective bleaching agents which are known to the person skilled in the art.

In preferred embodiments with hydrogen peroxide as the peroxide-containing compound, the amount of hydrogen peroxide may be in the range of about 100% critical but has been set for economic reasons. Suitably up to 50 from 5 up to 40 kg / ton dry mass calculated as 100% hydrogen peroxide. l up to about 60 kg / ton dry mass calculated as hydrogen peroxide. The upper limit is not the amount of hydrogen peroxide in the range of 2 kg / ton dry mass and preferably In preferred embodiments with a mixture of hydrogen peroxide and oxygen as the peroxide-containing compound, the amount of oxygen may be up to about 15 kg. 02 / ton dry mass; Suitably the amount of oxygen is in the range from 1 up to 10 kg O 2 / ton dry mass, preferably from 2 up to 8 kg O 2 / ton dry mass.

After the pretreatment steps and the subsequent bleaching with peroxide-containing compound, the pulp can be used directly for the production of paper. If desired, the pulp can also be final bleached to higher brightness by treatment in a rium dithionite or several steps, with e.g. hydrogen peroxide, ozone, sodium or chlorine dioxide. The final bleaching may also contain alkaline extraction steps, which may be reinforced with peroxide and / or oxygen.

Lignocellulose-containing pulp refers to pulps containing fibers that have been exposed by chemical or mechanical treatment or recycled fibers. The fibers can be of hardwood or softwood. Chemical pulp refers to pulps digested according to the sulphate, sulphite, soda or organosolv process. By mechanical pulp is meant pulp produced by refining chips in a board refiner (refiner pulp) or by grinding logs in a 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 is chemically digested pulp and preferably chemically digested pulp which has been delignified with oxygen prior to the present process. It is especially preferred that the lignocellulosic pulp consists of sulphate pulp of softwood.

The process according to the invention can be carried out in a yield up to the range from 40 up to 80% the range from 45 up to 65%.

The process pulps of about 90%, suitably within and preferably within according to the invention can be carried out on chemically digested pulps with an initial kappa number in the range from about 3 up to about 100, preferably 4 up to 60 and preferably up to 40. is measured according to standard method SCAN-C 1:77. The invention and its advantages are further illustrated by the following examples, which are merely intended to illustrate the invention without limiting it. In the description, claims and examples given% and parts, refers to% by weight and parts by weight unless otherwise indicated. Furthermore, the pH values stated in the description, the requirements and the examples refer to the pH at the end of each treatment unless otherwise stated.

In the examples below, the kappa number, viscosity and brightness of the pulp were determined according to the SCAN standard methods C 1:77 R, C peroxide and peracetic acid were determined by titration with nat- 16-16: 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 7.7, a brightness of 51.6% ISO and a viscosity of szs dm3 / kg, was treated with ice kg H 2 SO 4 / ton dry pulp to give a pH of 2.0. The acid treatment was carried out at 50 ° C, for 30 minutes and by 10% by weight. After raising the pH to 5-5.5, 2 kg of EDTA / ton of dry pulp was added together with a pulp concentration of magnesium sulphate in an amount of 300 ppm calculated as magnesium and calcium sulphate in an amount of 600 ppm calculated as calcium. The treatment took place at 50 ° C, for 60 minutes and a pulp concentration of 10% by weight. Thereafter, the pulp was bleached with hydrogen peroxide at a temperature of 90 ° C, 240 minutes and a pulp concentration of 10% by weight. a residence time of The addition of hydrogen peroxide was 25 kg / ton dry mass calculated as 100% hydrogen peroxide and the pH was 11. After that, the mass was washed with deionized water with a pH of 5.5. the acid treatment and the peroxide bleaching In this case, the pulp was first dewatered to a pulp concentration of 25% by weight, 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. For comparison, experiments were also performed with the addition of only 900 2), the addition of only 900 (experiment 3) and without the addition of either magnesium or (experiment 4). hydrogen peroxide, is shown in the following table. ppm magnesium (try ppm calcium calcium sulphate Results after bleaching with SÛ1 581 12 TABLE I Pulp properties after H2O2 Test Mg Ca Relative viscosity Brightness ppm ppm Mg = ca (dm3 / kg) 06 Iso) 5 1 300 600 1: 2 750 85 .0 2 900 0 --- 740 82.1 3 0 900 --- 670 82.0 4 0 0 --- 665 77.4 The table shows that pretreatment of pulp according to the present invention gives a hot. strong mass with high light

Claims (10)

10 l5 20 25 30 35 501 581 13 Patent claims A process for bleaching lignocellulosic pulp with a peroxide-containing compound, characterized in that the pulp before said bleaching in any order is treated acidically at a pH up to about 5 and treated with a complexing agent, that the pulp is then dewatered or washed to remove transition metal ions, that a magnesium compound and a calcium compound are then added with a weight ratio of added Mg and Ca in the range of from about 1: 1 up to about 1: 4 after which the mass, without intermediate dewatering or washing, is bleached at a pH in the range from about 7 up to about 13. 2. A method characterized in that the lignocellulosic material of chemically digested pulp. according to claim 1, the mass is constituted 3. A process according to claim 1 or 2, wherein the peroxide-containing compound is hydrogen peroxide or a mixture of hydrogen peroxide and oxygen. 4. A method according to any one of claims, characterized in that the added amount is 150 up to about 1000 ppm calculated as the weight fraction Mg of dry mass. magnesium is in the range from approx Process according to any one of the preceding claims, that the weight ratio between added Mg and Ca is in the range from 1: 1.2 up to 1: a. 4 6. Process according to any one of the preceding claims, characterized in that the acid treatment takes place at a pH in the range from 1.5 up to 4. Process according to any one of the preceding claims, characterized in that the complexing agent is a nitrogen-containing organic compound. Process according to one of the preceding claims, characterized in that the acid treatment and the treatment with a complexing agent take place in one step. requirement, according to any one of the preceding claims, that the acid treatment 9. Procedure 501 581 14 takes place in the presence of a delignifying chemical. 10. A process according to claim 9, characterized in that the delignifying chemical comprises ozone, peracids or salts thereof and chlorine dioxide.