SE512137C2 - Process for bleaching lignocellulosic pulp - Google Patents

Abstract

The present invention relates to a process for bleaching of lignocellulose-containing pulp, in which the pulp is treated at a pH between about 1 and about 7 in the presence of a magnesium compound, whereupon the pulp is washed and subsequently bleached with a peroxide-containing compound. The initial treatment removes from the pulp those trace metal ions that have a negative effect on the subsequent bleaching with a peroxide-containing compound. Owing to the presence of magnesium ions in dissolved form during the initial treatment, the magnesium ions are retained in those positions in the pulp where they have a particularly positive effect on the effectiveness of the bleaching stage.

Description

lO 15 20 25 30 35 512.137.

The invention According to the invention, a process has been provided in which lignocellulosic pulp is treated under the conditions stated in the claims, whereby the content of trace metal ions is selectively changed so that the subsequent bleaching with a chlorine-free bleach is made more efficient.

According to the invention, there is provided a process for bleaching lignocellulosic pulp, by treating the pulp at a pH in the range from about 1 up to about 7 in containing alkaline earth metal, after which the pulp is bleached with a chlorine-free bleach. effective in removing metal ions from lignocellulosic pulps. At the same time, the presence of a compound is known. An initial acid treatment is a process for the ions of alkaline earth metals, especially in their original positions, the selectivity of chlorine-free bleaching agents such as peroxide-containing compounds, ozone and oxygen. By being present in the pulp, it has a positive effect on bleaching and the consumption of spirit process removes undesirable metal ions from the pulp suspension, while maintaining positions. The latter is achieved by the presence of a compound containing alkaline earth metal in the treatment liquid, at such a pH and temperature the alkaline earth metal ions are generally in their original state that the compound is in loose form on contact with the mass. An advantage of the present procedure is further that the number of treatment steps is reduced compared with previous procedures. in the case of the alkaline initial treatment, by compound is meant earth metal, compounds containing magnesium, calcium or barium or a mixture of such compounds. Suitably the alkaline earth metal is magnesium or calcium, preferably magnesium. It is most preferred that the compound containing alkaline earth metal be a mixture of a compound containing magnesium and a compound containing calcium. in the case of the initial treatment containing the alkaline earth metal, the compound in dissolved form should be 5'12 _1357. _, _¿1__ 3 at the contact with the mass to achieve a good effect.

The way to achieve this varies i.a. with the mass a suitable up to about 7 in suitable temperature and concentration of the compound containing alkaline earth metal, said compound can be obtained to type and By the range from about 1 properties. choice of pH in combination with present in dissolved form.

The magnesium-containing compound used is suitably magnesium sulphate, magnesium chloride, magnesium carbonate or magnesium nitrate, preferably magnesium sulphate. As the calcium-containing compound, calcium chloride, calcium nitrate, calcium sulphate, preferably calcium chloride are suitably used.

In the process according to the invention, the initial treatment is carried out at a pH of from about 1 up to about 7, preferably from 1.5 up to or calcium carbonate, preferably 6 and preferably from 2 up to 5. It is also possible to carry out the initial treatment. at a pH of from 3.1 up to 4. at the initial treatment in the presence of earth metal, the pH can be adjusted with an acid or an acidic liquid. As of alkaline by adding acid to the pulp, a mixture of chlorine dioxide-reactive mineral acids or residual acid is used, individually or in any mixture. Suitably inorganic mineral acid such as sulfuric acid, nitric acid or hydrochloric acid, preferably sulfuric acid.

The amount of compound containing alkaline earth metal charged in the initial treatment can range from about 100 up to about 4000 parts by weight of alkaline earth metal per part by weight of dry mass.

Suitably the amount of alkaline earth metal is 3000 ppm and preferably to 2000 ppm.

With magnesium as ppm, calculated as containing in the range from 200 up to charged compound in the range from 500 up alkaline earth metal the amount of magnesium compound charged and other conditions at the initial, the treatment is selected so that the mass content of magnesium before bleaching with chlorine-free bleach amounts to at least about 50 percent of the original content. The amount of magnesium compound charged and other conditions is suitably selected so that the content of magnesium in the mass before bleaching with chlorine-free bleach is in the range from 100 up to 300 percent of the initial content, preferably - unfortunately in the range from 130 up to 200 percent.

Chlorine-free bleach includes peroxide-containing compound, oxygen or ozone in any sequence or mixture.

Sodium dithionite can also be used as chlorine-free bleach.

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

Bleaching with chlorine-free bleach is suitably carried out at a pH higher than the pH of the initial treatment in the presence of a compound containing alkaline earth metal. In this way, the trace metal ions that are positive for the bleaching are maintained even in the bleaching step itself. 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. 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 bleaching with chlorine-free bleach suitably takes place in the presence of an additional compound containing alkaline earth metal. The compound containing alkaline earth metal present in the bleaching may be the same or a different compound than that used in the initial treatment. The alkaline earth metal is suitably calcium, magnesium or barium or a mixture of such compounds. In this way a further improvement is achieved in terms of selectivity in the bleaching and consumption of the chlorine-free bleaching agent. It is especially preferred that the bleaching be in the presence of a calcium compound. The calcium compound can be introduced into the pulp suspension outside the bleach tower, e.g. by mixing in the line up to the bleaching tower 10 15 20 25 30 35 512 137 »5 net. The calcium compound can also be introduced into the pulp suspension in the bleaching tower, before bleaching begins.

The amount of compound containing alkaline earth metal in the bleaching can range from about 100 up to about 4000 ppm, calculated as the weight fraction of alkaline earth metal per weight fraction of dry mass. Suitably the amount of compound containing alkaline earth metal charged is in the range from 300 up to 3000 ppm and preferably in the range from 600 up to 2000 ppm.

When a calcium compound is present chlorine-free is suitably of calcium chloride, calcium nitrate, calcium sulphate or calcium carbonate.

It is at this bleaching, this is preferred to use calcium chloride. It is further suitable that the calcium compound is added at a combination of pH, temperature and concentration of the calcium compound, so that said compound is in active form upon contact with the pulp. It is suitable here for the range from about 7 up to about 11. A positive effect of the calcium compound is obtained that the pH is within, however, even if said compound is precipitated in the pulp suspension, e.g. as calcium carbonate.

The amount of calcium charged and other conditions when adding calcium, is chosen so that the pulp content of calcium before bleaching with chlorine-free bleach amounts to at least about 25 percent of the calcium content of the pulp before the initial treatment. In this case, the pulp can be treated with calcium both during the initial treatment and during the chlorine-free bleaching. The amount of calcium compound charged and other conditions are selected before bleaching with chlorine-free bleach, the choice is from 70 up to 150 percent of the original content, preferably in the range from 90 up to 120 percent.

It is located or part of the amount of suitable so that the mass content of calcium within the scope of the invention that whole alkaline earth metals can be added to the pulp suspension via the water used for e.g. pH control, dilution. Thus, hard water containing mainly magnesium or calcium ions or a combination thereof, can be advantageously used in washing or the initial treatment, or to effect the bleaching step. The gene of the corresponding mass concentration suitable for dilution in the washing step is thereby possible to reduce batch-alkaline precipitation that the combination of pH, temperature and concentrated alkaline earth metal can be selected so that said earth metal is in loose form on contact such hard water is fresh water from limestone bedrock, backwater from paper machines where lime or chalk is used as a filler or earth metal, subject to the mass. Examples of process water from sulphite production with magnesium or calcium as base.

The process according to the invention is suitably carried out with a washing step after the initial treatment and before the bleaching with chlorine-free bleach. effectively removes the trace metal ions that are Negative for the subsequent chlorine-free bleaching, e.g. ions of manganese, copper and iron. In order to preserve the trace metal ions that are positive for the subsequent chlorine-free bleaching, mainly magnesium and calcium ions, it is preferred that the pH at the wash is equal to or higher than the pH at the initial treatment. However, the pH of the wash should be in the range of from about 3 up to about 10, preferably in the range of from 5 up to 10. It is especially preferred that the pH of the wash is at least about 2 pH units higher than the pH of the initial treatment. .

The washing liquid may consist of fresh water, possibly with the addition of a pH-regulating chemical. The washing liquid from a bleaching step or extraction step, in such a way that a suitable pH during the washing is obtained. of possibly may also consist of waste water or more The washing liquid may further consist of other types of purified waste water, provided that the content is low of non-manganese, iron and copper.

By washing between the initial bleaching with chlorine-free bleach, completely reducing the treatment and meant methods that more liquid phase from the content of dissolved The washing methods can or less remove the pulp suspension of said trace metal ions in suspension. desirable metal ions such as increasing the pulp concentration, e.g. by suction or pressing through filters. The washing methods can also involve a reduction of the pulp concentration, e.g. by diluting with washing liquid. Washing also refers to the combined pulp concentration alternately several times. In the present case, which effectively removes the trace metal ions released during the initial treatment, taking into account the mixture.

The efficiency of the wash can be stated liquid phase of the liquid phase that was present in the pulp suspension before washing. at least about 80 percent, preferably in the range of 90 up to 100% and preferably to 100%.

The implementation of the initial treatment in the presence of alkaline earth metal, may be the presence of a bleaching agent. Bleaching agents active in the pH range suitable for the initial treatment, are i.a. chlorine dioxide, ozone and acidic peroxide-containing compounds. is i.a. organic ions and sequences, where elevation and or process is selected a washing method is lowered, one of which is process engineering and economical as removed removed Washing efficiency can be in the range of 95 up of a compound also occur in which are Acid peroxide-containing compounds compounds such as peracetic acid and inorganic compounds such as hydrogen peroxide and peroxosulfuric acid (Caros acid). The initial treatment gene in the presence of ozone or peroxosulfuric acid, preferably takes place in the presence of ozone, since these bleaches only to a lesser extent affect the mass content of ions of alkaline earth metals.

The process according to the invention can also be carried out with an extraction step before the bleaching with chlorine-free bleach. This is especially suitable when the initial treatment takes place at or higher than the pH of the initial treatment. The pH of the extraction step is suitably carried out at a pH which is equal, suitably in the range from about 7 up to about 11 and preferably in the range from 8 up to 10.

Regardless of the number of steps or type of treatment after in the presence of a bleach. The extraction 10 15 20 25 30 35 i w Hm. 512 137 '8 the initial treatment, it is appropriate that the pH in the subsequent steps is at least as high as the pH in the previous step in order to preserve the mass content of alkaline earth metal ions. In a sequence where the pulp is first treated at a pH of about 2 in the presence of a magnesium compound and then washed and bleached with hydrogen peroxide, the pH of the washing step should be at least about 4 and the pH of the bleaching step at least about 8.

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. By chemical pulp according to sulphate, sulphite- By mechanical pulp is meant pulp produced by refining means pulp digested 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 or processes. Examples of combinations of the above-mentioned methods or such masses by modifications are thermomechanical, chememechanical or chemothermomechanical masses. Suitably the lignocellulosic pulp consists of chemically digested pulp and preferably of sulphate pulp of softwood.

The process according to the invention can be carried out on pulps with a yield of up to about 90%, suitably in the range from 40 up to 80% and preferably in the range from 45 up to 65%. the present method. The implementation of can take place in any position in the bleaching sequence, e.g. immediately after the production of the pulp. When the process according to the invention is applied to chemically digested pulp, it is preferably delignified in an oxygen step before the present process.

The process according to the invention can be carried out on chemically digested pulps in a range from about 3 up to about 100, suitably 4 up to 60 and preferably from 5 up to 40. The coating number is measured according to standard method SCAN-C 1:77. in the process of the invention, the initial treatment is carried out at a temperature of from about 10 up to an initial kappatai within 10 to about 100 ° C, preferably from 25 up to 90 ° C and preferably up to 80 ° C. ° C. carried out for a period of minutes, preferably from 1 up to 120 minutes and preferably from 10 up to 60 minutes. The pulp concentration in the initial treatment can be from about 1 up to about 60% by weight, suitably from 2 up to 40% and preferably from 3 up to 35% by weight. as a chlorine-free bleach, the pulp is treated at a temperature of from about 30 up to about 100 ° C, preferably from 60 up to 90 ° C, from about 10 .mu.m. a time of 30 up to about 960 minutes, preferably from 60 up to 360 minutes. With hydrogen peroxide as the chlorine-free bleach, the pulp concentration may be 1 up to about 70% by weight, preferably from 3 up to 60% by weight and preferably from 10 up to 50% by weight. Treatment with other of the above-mentioned chlorine-free bleaches takes place within normal ranges from about temperature, time and pulp concentration for the respective bleaches which are known to those skilled in the art. with preferred chlorine-free bleach, the range is from about 1 calculated as 100% critical but has been set for economic reasons. Suitably the amount of hydrogen peroxide is in the range from 2 up to 50 kg / ton from 5 up to 40 kg / ton dry mass calculated as 100% hydrogen peroxide.

After that treatment in alkaline earth metal and subsequent chlorine-free bleaching, the pulp can be used directly for the production of paper with lower requirements for lightness. The mass can also be final bleached to the desired higher brightness, by treatment in steps. Suitably, the final bleaching also takes place with such chlorine-free bleaching agents as stated above, possibly with intermediate alkalis with peroxide and / or oxygen. As a result, the formation and embodiments with hydrogen peroxide as the amount within up to about 60 kg / ton dry mass of hydrogen peroxide. hydrogen peroxide The upper limit is not dry mass and preferably the initial presence of an extraction step, which may be amplified emissions of AOX completely eliminated. one or more of the invention and its advantages are further illustrated by the following examples, which are merely intended to illustrate the invention without limiting 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, in the description, the requirements and pH at nothing else are stated. the pH values given in the examples, the end of the respective treatment unless in the following examples the kappa number of the pulp, and brightness were determined according to SCAN standard methods and the hydrogen peroxide consumption by means of iodometric titration. Example 1 Oxygen delignified sulphate pulp of viscosity softwood with a kappa number of 18, a brightness of 34.2% ISO and a viscosity of 1000 dm3 / kg, was treated 2.1 +/- 0.1 in the presence of various alkaline earth metals. The pH of the sulphate pulp after oxygen delignification was about 11, which was adjusted by adding 15 kg of sulfuric acid / ton of dry pulp. The amount of alkaline weight fraction charged at a pile of earth metal was 1000 ppm calculated as the respective mass. At the initial 50 ° C, the treatment time 30 minutes and the pulp concentration at those conditions as well as the compound containing alkaline earth-alkaline earth metal After the initial a pH to remove the metal ions adversely affecting it was subsequently bleached by 90 ° C, a residence time of alkaline earth metal per dry treatment, the initial temperature was 10% by weight. the treatment was selected species metal, so that present was loose in the treatment, the pulp was washed at the pulp suspension. of 6-7, chlorine-free Then the mass with jande bleaching. hydrogen peroxide at a temperature of 240 minutes and a pulp concentration of 10% by weight. 25 kg / ton dry mass calculated 10.5. alkaline earth- The addition of hydrogen peroxide was as 100% hydrogen peroxide and the pH was approx. For comparison, the pulp was treated without the addition of metal, followed by the specified conditions bleaching with (experiment 4). As a further comparison, the pulp was treated with magnesium as in Experiment 1, intermediate washing steps (Experiment 5). The results of the hydrogen peroxide experiments below but without 10 15 l 25 20 25 30 512 137-1 are shown in the following table.

TABLE I Alkaline Mass Properties After Peroxide Bleaching Test Soil Capacity Viscosity Brightness Metal (dm3 / kg) (95 Iso) 1 Mg 9.2 830 63.4 2 Ca 10.5 800 53.0 3 Ba 11.4 880 51.0 4 --- * 12.8 840 45.5 5 Mg ** 12.2 880 41.0 * No alkaline earth metal added ** No washing after the treatment with magnesium The table shows that treatment of pulp according to the present invention at a pH of approx. 2.1 in the presence of a compound containing alkaline earth metal, gives a significantly greater increase in brightness and kappa number reduction than treatment in the absence of such a compound.

Example 2 The oxygen delignified softwood sulphate pulp used in Example 1 was treated in the presence of a magnesium compound, at a pH in the range of 1.5 up to 11 to show the pH of the initial treatment.

The amount of magnesium charged was 1000 ppm magnesium per impact calculated as the weight percentage of dry mass. The conditions at the initial one, apart from the subsequent bleaching with hydrogen peroxide, were also the same 1. After the initial treatment, the pulp was washed at a pH which was about 2 pH units higher than at the initial treatment. The results of the experiments are shown in the following table. same as in of pH. the treatment was example 1, the variation The conditions as in example -J 10 15 20 25 30 512 137 12 TABLE II pH at the Mass properties after peroxide bleaching Attempt initial Capacity _Viscosity Lightness treatment (dm3 / kg) (ä, Hyssop l 1.5 9.1 795 64.0 2 2.0 9.2 835 63.2 3 3.3 9.6 820 61.0 4 4.4 9.7 sis i 60.2 5 6.1 9.9 825 59.3 6 6.8 10.2 840 57.5 7 10.8 12.1 880 43.6 The table shows that treatment of pulp according to the present invention at a pH in the range from about 1 up to about 7 in the presence of a magnesium compound, gives a significantly greater increase in brightness and kappa number reduction than treatment at a pH above this range. to show the effect of pH in the subsequent washing step, during the initial treatment the pH was adjusted by adding sulfuric acid.The amount of magnesium charged was 1000 ppm calculated as weight percentage amount e- sium per dry mass. The conditions of the initial treatment as well as of the bleaching with hydrogen peroxide were the same as in Example 1. After the initial treatment, the pulp was washed at a pH in the range from 3 up to about 11, by adding sodium hydroxide to deionized water. The results of the experiments are shown in the following table. 10 15 20 25 30 35 512 91-57 e. 13 TABLE III pH i Pulp properties after peroxide bleaching Try washing- Capacity viscosity Lightness step (anë / kg) (æ Iso) 1 3.0 ll, 9,890 52.9 2 4, 3 10.2 830 58.9 3 4.8 10.2 840 59.4 4 6.9 9.0 835 63.4 5 8.2 9.0 830 64.0 6 10.8 9.0 800 59 The table shows that treatment of pulp according to the present invention with an intermediate wash at a pH in the range from about a significant increase in brightness and kappa number reduction.

Example 4 The coniferous wood was treated to 12% by weight of magnesium per dry mass.

The amount of magnesium charged in the presence at a pH of 2.1 +/- 0.1, for when bleaching with hydrogen peroxide up to about 10, gives an example 1 using oxygen delignified sulphate masses of a magnesium to show the effect of pH in the range from 7 up The conditions were 1000 ppm calculated which at the initial treatment was the same as in Example 1. The conditions during the bleaching with hydrogen peroxide were also the same as in Example 1, except for the variation pH. After the initial treatment, the pulp was washed at a pH of 6-7.

The results of the experiments are shown in the following table.

TABLE IV pH in Pulp properties after peroxide bleaching Try bleaching - Capacity viscosity Brightness step (am3 / kgL (a. Iso) l 7.1 12.0 900 50.2 2, 8.2 9.9 830 58.0 3 10.3 9.2 825 63.1 4 ll, 9 9.1 780 62.5 The table shows that treatment of pulp according to the present invention with hydrogen peroxide as chlorine-free bleach, gives a very high increase in brightness and coat - number reduction.

Example 5 The oxygen delignified softwood pulpwood pulp used in Example 1 was treated in the presence of a magnesium compound at a pH of 2.1 +/- 0.1, to show the effect of a calcium compound in bleaching with hydrogen peroxide (Experiment 2) .

The amount of magnesium charged was 1000 ppm calculated as the weight percentage of dry pulp. Calcium was added after washing 1000 ppm The conditions magnesium per in an amount of calculated by weight calcium per dry mass. the initial treatment was the same as in Example 1. The conditions for the bleaching with hydrogen peroxide were also the same as in Example 1, except for the presence of a calcium compound. After the initial treatment, the pulp was washed at a pH of 6-7. For comparison, the pulp was bleached even without the presence of the bleaching step (Experiment 1). following table. a calcium compound in The results of the experiments are shown in TABLE V Pulp properties after peroxide bleaching Experiments Capacity viscosity Lightness Residual H2 O2 (dm3 / kg) (a Iso) (kg / ton) 9.2 830 63.1 0 8.9 850 68.2 3, The table shows that treatment of pulp according to the present invention with a calcium compound present in bleaching with a significantly higher brightness at a lower consumption of hydrogen peroxide than bleaching hydrogen peroxide, gives without calcium present. Example 6 The oxygen delignified softwood pulpwood pulp used in Example 1 was treated in the presence of a magnesium compound at a pH of 2.1 +/- 0.1, washed and bleached with hydrogen peroxide, to show the effect of a selective metal oxide. profile in the mass before bleaching. The amount of magnesium charged was as a percentage by weight of magnesium per dry mass. 1000 ppm 1000 ppm Calcium was added after washing in an amount of 512 137. as a percentage by weight of calcium per dry mass (Experiment 4).

The conditions at the initial treatment were calculated the same as in Example 1. The conditions at the bleaching with hydrogen peroxide apart from the presence of an experiment 4. After the initial treatment, the pulp was washed at a pH of about 3 (Experiment 1) and 6-, respectively. 7 (experiments without magnesium in the initial treatment and calcium in the bleaching step (experiments 1 and 2). Some metal ions before (experiments were the same as in example 1, calcium compound in 2-4). For comparison, the pulp was treated with the content of the sulphate pulp of 0) and after the initial treatment, as well as the brightness after bleaching with hydrogen peroxide are shown in the following table.

TABLE VI Experiments Metal content before bleaching, ppm Brightness after Mg Ca Mn Fe Cu _ bleaching (% ISO) 0 300 1800 68 8.3 1.5 ---- 1 30 220 6.5 5.8 <0.5 45, 5 2 100 640 20 7.0 0.9 45.7 3 550 690 18 6.3 0.7 63.2 4 550 1700 18 6.3 0.7 68.2 The table shows that the present process with a treatment in presence of a compound containing alkaline earth metal followed by bleaching with brightness hydrogen peroxide than bleaching without calcium present. hydrogen peroxide, gives a significantly higher at a lower consumption of

Claims (10)

.aiiii _L ._. íi ._..- f "'" .l 10 15 20 25 30 35 512 157 16 Patent claim A process for bleaching lignocellulosic pulp, characterized in that the pulp is treated at a pH in the range from about 1 up to about 7 in the presence of a magnesium compound, that the pulp is washed wherein the pH of the washing liquid is equal to or higher than the pH at the treatment in the presence of 'a magnesium compound' and then 'bleached with a peroxide-containing compound. 2. A method according to claim 1, characterized in that the lignocellulosic mass consists of chemically digested mass. 3. A method according to claim 1, characterized in that the amount of magnesium compound charged is in the range from about 500 up to about 4000 ppm, calculated as weight percent magnesium compound per weight percent dry mass. 4. A process according to claim 1 or 2, characterized in that a calcium compound is present in the treatment of pulp in the presence of a magnesium compound. 5. A method according to any one of the preceding claims, characterized in that a bleaching agent is present in the treatment of pulp in the presence of a magnesium compound. Process according to any one of the preceding claims, characterized in that the treatment in the presence of a magnesium compound takes place at a pH in the range from 1.5 up to 6. Process according to any one of the preceding claims, characterized in that the content of magnesium after the treatment in the presence of a magnesium compound and washing, and before bleaching with a peroxide-containing compound is in the range from 100% up to 300% of the original content. Process according to 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 bleaching. with the peroxide-containing compound occurs at a pH equal to or higher than the pH in the previous step. 512 137 17 10. A process according to any one of the preceding claims, characterized in that the bleaching with the peroxide-containing compound takes place in the presence of a calcium compound.