NO180495B - Process of delignification and bleaching of chemically suspended lignocellulosic material - Google Patents

Abstract

The present invention relates to a process for delignification and bleaching of chemically digested lignocellulose-containing pulp, wherein the pulp is acid treated at a pH of between about 1 and about 6, whereupon a water-soluble mixture of a magnesium compound and a calcium compound is added at a pH of between about 1 and about 7 before the pulp is treated with a chlorine-free bleaching agent. The chlorine-free bleaching agent includes hydrogen peroxide. After the treatment according to the invention, the pulp may be finally bleached to the desired brightness, suitably with a chlorine-free bleaching agent, such as ozone, to completely avoid formation and discharge of AOX.

Description

The present invention relates to a method for delignification and bleaching of chemically digested lignocellulosic pulp, as stated in the preamble of claim 1, where the pulp is treated at a pH in the range 1-6, after which a water-soluble compound containing alkaline earth metal is added at a pH in the range 1 - 7, before the pulp is treated with a chlorine-free bleaching agent. The initial acid treatment removes the pulp's trace metals, while the subsequent addition of alkaline earth metal ions in aqueous solution returns the ions to the positions in the pulp where they have a particularly positive effect on the preservation of the cellulose chains and thus the viscosity, as well as a positive effect on the bleach consumption in the subsequent bleaching step. After said treatment, the mass is finally bleached to the desired lightness with a chlorine-free bleaching agent consisting of hydrogen peroxide, to completely avoid the formation and release of AOX.

When producing chemical pulp with high lightness, the wood chips are first boiled to expose the cellulose fibres. Part of the lignin that holds the fibers together is thereby broken down and modified so that the lignin can be removed in the subsequent wash. In order to reach a sufficient brightness, however, additional lignin is required to be removed together with brightness-reducing (chromophoric) groups. This is often carried out by delignification with oxygen gas, followed by bleaching in several stages.

For environmental reasons, it is becoming increasingly common for chemical pulps to be treated with chlorine-free bleaches already in the first bleaching step. The major advantage is that emissions of environmentally hazardous organochlorine compounds are drastically reduced by combining the effect of a smaller amount of chlorine-containing bleach with a lower content of lignin, which is the organic component that primarily reacts with chlorine.

Treatment with chlorine-free bleaches, such as hydrogen peroxide, peracetic acid or ozone already in pre-bleaching is certainly known, but delignif i does not see any and the consumption of bleach is less effective than with chlorine-containing bleach, if the pulp is not pre-treated. Thus, a hydrogen peroxide treatment in an alkaline environment is disturbed by the presence of ions of certain metals in the mass, such as Mn, Cu and Fe. With these, metal ions cause splitting of hydrogen peroxide and thus reduce the effectiveness of the peroxide treatment and increase peroxide consumption. According to CA 1,206,704, this can be counteracted through a pre-treatment of the mass with an acid, such as sulfuric acid or nitric acid, whereby the concentration of all metal ions is reduced. In this way, however, metal ions that are not advantageous for peroxide treatment, such as Mg, which ions stabilize peroxides and increase the peroxides' selectivity, also disappear.

It is known from CA 575,636 to add magnesium sulphate to stabilize alkaline peroxide solutions, but the addition takes place directly to the bleaching liquid and in an alkaline environment insoluble magnesium hydroxide precipitates. According to US 4,222,819, it is also known to add magnesium ions to acidic peroxide solutions, but also in this case the addition takes place directly to the bleaching liquid. None of these known methods enables diffusion of the magnesium ions into the mass to such an extent that a mass with high lightness and strength is obtained.

According to the invention, a method has been provided where lignocellulosic pulp is treated with the conditions given in the requirements, whereby the metal ions harmful to a subsequent bleaching are effectively removed and the profile of alkaline earth metals is reset before the pulp is bleached in a chlorine-free bleaching step.

The invention thus relates to a method which is characterized by what is stated in claim 1's characterization part, namely; the mass is acid treated at a pH in the range 1-6, after which a compound containing alkaline earth metal is added at a pH in the range 1 - 7 and an amount of 0.01 - 10 kg/tonne dry mass, considered as alkaline earth metal, and that the mass is then treated with a chlorine-free bleach; further features appear in requirements 2-10.

Acidic treatment with bleaching and/or delignifying chemicals is an effective method for removing metal ions from lignocellulosic masses. It is also known that ions of alkaline earth metals, especially in their original position in the mass, have a positive effect on the selectivity of the delignification and the stability and consumption of chlorine-free bleaching agents such as peroxides, ozone and oxygen gas. By means of the present method, the problem of providing a suitable trace metal profile, which is favorable for the subsequent chlorine-free bleaching, is solved in an economical way by removing the unwanted metal ions while added ions of alkaline earth metals largely repeat the positions in the vicinity of the cellulose chains which were previously occupied by ions of alkaline earth metals. This is achieved by the addition of the compound containing alkaline earth metals occurring at such a pH and such a temperature that the compound is dissolved in water, which enables the diffusion necessary to achieve the intended effect. A further advantage of the present method is that adjustment of the pH between the acidic treatment and the addition of alkaline earth metals is very limited or can be completely excluded, which is process-technically and economically advantageous.

Chlorine-free bleaches include inorganic peroxide compounds, such as hydrogen peroxide and sodium peroxide, organic peroxide compounds, such as peracetic acid, as well as ozone, oxygen gas and sodium dithionite. It is convenient to use hydrogen peroxide (P), oxygen (O) and ozone (Z) in any order or mixture, and it is preferred to use hydrogen peroxide or mixtures of hydrogen peroxide and oxygen gas (PO). The sequence P-Z or (PO)-Z is particularly preferred. When treating with chlorine-free bleach in an alkaline environment, the pH is adjusted appropriately by adding alkali or an alkali-containing liquid to the mass, for example sodium carbonate, sodium bicarbonate, sodium hydroxide, oxidized white liquor or magnesium hydroxide slurry. production of sulphite pulp with magnesium as a base, so-called magnefit pulp.

The acidic treatment is suitably carried out with an acid. As acid, inorganic acids can be used, suitable sulfuric acid, nitric acid, hydrochloric acid or residual acid from a chlorine dioxide reactor, individually or in an optional mixture. It is preferred to use sulfuric acid.

By compounds containing alkaline earth metals is meant water-soluble chemicals containing magnesium or calcium or a mixture of these chemicals. Appropriately, magnesium-containing compounds such as magnesium sulfate or magnesium chloride, or calcium-containing compounds such as calcium chloride or calcium oxide are used. It is preferred to use magnesium sulfate or magnesium chloride and it is particularly preferred to use magnesium sulfate. The combination of temperature and pH when adding the compound containing alkaline earth metals is always chosen so that the compound is present in an aqueous solution in contact with the mass.

According to the present invention, the acidic treatment is carried out at a pH in the range of approx. 1 - 6, suitably 1.5 - 5, preferably 2 - 4. It is particularly preferred that the acidic treatment is carried out at a pH in the range 2 - 3. With magnesium as alkaline earth metal in the compound containing alkaline earth metal, the addition takes place at a pH in the range 1- 7, suitably within the range of 2-6, preferably within the range of 2-4. It is particularly preferred that the addition of magnesium takes place at a pH in the range 2-3. With hydrogen peroxide as a chlorine-free bleaching agent, the mass is suitably treated at a pH in the range 8 - 12, preferably within a pft range of 10 - 12. Treatment with the other chlorine-free bleaching agents mentioned above takes place within the normal pH range for the respective bleaching agents, which would be known to the person skilled in the art.

The treatment according to the invention is preferably carried out with a washing step between the acidic treatment and the addition of alkaline earth metal ions, so that the trace metals which have a negative effect on the treatment with chlorine-free bleach are removed from the pulp suspension.

The performance of the acid treatment, the compound containing alkaline earth metal and chlorine-free bleaching agent can take place in any position in the bleaching sequence, for example directly after digesting the mass or after an oxygen gas step. The method according to the invention is preferably used on pulp which has been delignified with an oxygen gas step before the treatment.

It is also included within the framework of the invention that the pulp during the acidic treatment is also exposed to bleaching and/or delignifying treatment. Bleaching and/or delignifying chemicals that are effective within the pH range that is suitable for the acid treatment are, among other things, chlorine dioxide, ozone, peracetic acid and/or acidic peroxide-containing compound. A combination of acidic treatment and bleaching and/or delignifying treatment conveniently occurs in an ozone step.

By lignocellulosic pulp is meant chemical pulps of bar and/or loess that have been digested according to the sulphite, sulphate, soda or organosolv method or modifications and/or combinations of these. Appropriately, bar and/or løde is used which has been sealed according to the sulphate method and preferably sulphate mass of løde.

The treatment according to the invention can be used on lignocellulosic pulps with an initial kappa number within the range 5 - 40, suitably 7 - 32 and preferably 10 - 20. The kappa number is determined in this connection according to the standard SCAN-C 1:77.

In the method according to the invention, the acid treatment is carried out at a temperature in the range 10 - 95°C, suitably 20 - 80°C and preferably in the range 40 - 80°C, for a period of 1 - 120 min., suitably 10 - 120 min . and preferably 20 - 40 min., and that the addition of the compound containing alkaline earth metal takes place at a temperature in the range 10 - 90°C, preferably 40 - 80°C for a period of 1 - 180 min., suitably 20 - 180 min. and preferably 30 - 120 min. With hydrogen peroxide as chlorine-free bleaching agent, the mass is treated at a temperature in the range 30 - 100°C, preferably 60 - 90°C for a time within the range 30 - 300 min., suitably 60 - 240 min. The mass concentration during the acid treatment and with the addition of alkaline earth metal ions can be 3-35% by weight, preferably 3-15% by weight. With hydrogen peroxide as chlorine-free bleaching agent, the mass concentration can be 3-50% by weight, suitably 3-35% by weight and preferably 10-20% by weight. Treatment with the above-mentioned chlorine-free bleaching agents takes place within normal intervals with regard to temperature, time and mass concentration of the respective bleaching agents, which will be known to the person skilled in the art.

The quantity of charged compound containing alkaline earth metal lies within the range of 0.01 - 10 kg/tonne of dry mass, considered as alkaline earth metal, suitably within the range of 0.5-5 kg/tonne of dry mass known as alkaline earth metal and preferably within the range of 2-4 kg/tonne dry mass, considered alkaline earth metal.

In preferred embodiments with hydrogen peroxide as chlorine-free bleaching agent, hydrogen peroxide is used in area 2

- 50 kg/tonne dry mass, calculated as 100% hydrogen peroxide. The upper limit is not critical, but is set from economic points of view. Appropriately, hydrogen peroxide is used in an amount

of 3 - 30 kg/tonne dry mass and preferably 4-20 kg/tonne dry mass, calculated as 100% hydrogen peroxide.

After the acid treatment, compound containing alkaline earth metal and chlorine-free bleach, the pulp can be used directly for the production of paper with low lightness requirements. The pulp can also be finally bleached to the desired higher lightness by processing in one or more stages. Appropriately, the final bleaching also takes place with such chlorine-free bleaching agents as indicated above, possibly with an intermediate alkaline extraction step, which may be reinforced with peroxide and/or oxygen gas. In this way, the formation and emission of AOX is completely eliminated. It is suitable to final bleach with ozone in one or more stages. It is also entirely possible to use chlorine dioxide and/or hypochlorite in one or more final bleaching stages without large amounts of AOX being formed, this because the content of lignin through the treatment according to the invention is reduced to a sufficiently low level before the chlorine-containing bleach is used.

Application of the method according to the invention further implies that the lightness and kappa number of the resulting mass will be higher, respectively lower than in a method where the compound containing alkaline earth metal is not added or is added at a higher pH. In a bleaching process for chemical pulps, both high lightness and a low kappa number are sought, which later implies a low content of undissolved lignin. At the same time, the consumption of chlorine-free bleach must be as low as possible, which means lower treatment costs. With the present method, these purposes are fulfilled, which is evident from the following examples. Furthermore, the pulp's strength, measured as viscosity, is sufficient, which means that the pulp contains a cellulose chain of sufficient length to give a strong product.

The invention and its advantages are explained in more detail in the following examples. In the description, requirements and examples, the given percentages and parts are calculated as % by weight and parts by weight, unless otherwise stated.

Example 1

Softwood pulp with a kappa number of 17, a lightness of 35% ISO and a viscosity of 970 dm<3>/kg was treated with sulfuric acid at a pH of 2.0. The mass was treated at a temperature of 60°C for 30 min. and with a mass concentration of 10% by weight. After washing the mass with water, magnesium was added in the form of an aqueous solution containing MgSO4, so that the concentration of magnesium in the mass was at least 500 ppm. During the experiment, the pH was varied in connection with this addition between 2.3 and 11.5 by adding sulfuric acid. The mass is then bleached with hydrogen peroxide at a temperature of 90°C, a residence time of 180 min. and a mass concentration of 15% by weight. The final pH was 11.5 and the addition of hydrogen peroxide was 15 kg/ton of dry mass, calculated as 100% hydrogen peroxide. For comparison, magnesium was added directly in the hydrogen peroxide step, according to known technique, under the conditions given above. As a further comparison, the pulp was also treated only with sulfuric acid and hydrogen peroxide under the conditions given above. The kappa number, viscosity and lightness of the pulp were determined according to SCAN standard methods, as well as the hydrogen peroxide consumption by means of iodometric titration. The result of the experiment is shown in the following figures.

From the table it appears that the treatment according to the present invention with MgS04 at a pH within the range of 2 - 6 is essential so that the brightness increase and the kappa number reduction will be maximal and for the viscosity change and the consumption of hydrogen peroxide to be minimal. The importance of the magnesium ions for the lightness of the search is further evident from the comparison at pH 2.3, where in the last experiment only acid treatment preceded the peroxide treatment.

Example 2

Oxygen gas delignified sulphate pulp of softwood with a kappa number of 13.7, a lightness of 37.1% ISO and a viscosity of 1057 dm<3>/kg, in a first step treated with 15 kg of sulfuric acid/tonne of dry pulp at a pH of 1 ,9. The mass was treated at a temperature of 50°C for 60 min. and with a mass concentration of 10% by weight. After washing the mass with water, 0.1 and 1.5 kg of magnesium/tonne of dry mass are added in a second step, in the form of an aqueous solution containing MgSO4. The magnesium was added at a pH of 4.1, a temperature of 50°C for 60 min. and with a mass concentration of 3.5% by weight. The mass was then bleached with hydrogen peroxide at a temperature of 90°C, a residence time of 240 min. at a mass concentration of 10% by weight. The final pH was 11.5, the addition of hydrogen peroxide was 20 kg/tonne dry mass, calculated as 100% hydrogen peroxide. For comparison, the pulp was also treated only with sulfuric acid and hydrogen peroxide under the conditions given above. The kappa number, viscosity and lightness of the pulps were determined according to SCAN standard methods. The results after bleaching with hydrogen peroxide appear in the following table.

From the table it appears that acid treatment followed by the addition of dissolved magnesium and bleaching with hydrogen peroxide according to the present invention affects the pulp positively with regard to kappa number, viscosity and lightness.

Example 3

The oxygen gas delignified sulphate pulp of softwood, which was used in example 2, was treated in the sequences D - EOP - Z - P (experiment 1) and D - EOP - Z - Mg - P (experiment 2), where the conditions in the respective steps were alike in the two sequences. D and EOP denote a conventional chlorine dioxide step and a conventional alkaline extraction step, respectively, which were enhanced with hydrogen peroxide and oxygen gas. Z indicates an ozone level where the pH was 2.3. Mg indicates the addition of 1 kg of magnesium/tonne of dry mass, in the form of an aqueous solution containing MgSO4. Magnesium was added at pH 4.1 at a temperature of 50°C for 30 min. and with a mass concentration of 3% by weight. P indicates a hydrogen peroxide where the mass is treated at a temperature of 80°C for 120 min. The final pH was 11.5 and hydrogen peroxide was added in an amount of 5 kg/ton of dry mass, calculated as 100% hydrogen peroxide. Pulp viscosity and lightness were determined according to standard SCAN methods. The results after bleaching with hydrogen peroxide appear in the following table.

From the table it appears that the addition of dissolved magnesium after an initial acid treatment with ozone and bleaching with hydrogen peroxide according to the present invention affects the mass positively with regard to viscosity and lightness.

Example 4

The oxygen gas delignified sulphate mass of softwood, which was used in example 2, was treated in the sequences Trinnl - Px - Z - P2 (experiment 1) and Trinnl - Px - Z - Mg - P2 (experiment 2), where the conditions in the respective steps were the same in both sequences. Trinnl indicates treatment with EDTA at pH 5. Z indicates an ozone step where the pH was 2.3 and the mass concentration was 10% by weight. Mg indicates the addition of 1 kg of magnesium/tonne of dry mass in the form of an aqueous solution containing MgSO4. Magnesium was added at a pH of 4.1, a temperature of 50°C for 30 min. and with a mass concentration of approx. 3% by weight. P2 denotes a hydrogen peroxide step, where the mass was treated at a temperature of 80°C for 120 min. The final pH was 11.5 and the addition of hydrogen peroxide was 5 kg/ton of dry mass, calculated as 100% hydrogen peroxide. As a comparison, the pulp was also treated in the sequences Trinnl - Px - Z - (PMg) (experiment 3). (PMg) indicates the addition of magnesium directly in the second alkaline hydrogen peroxide step, according to the known technique, the conditions given above. Pulp viscosity and lightness were determined according to SCAN standard methods and hydrogen peroxide consumption was determined by iodometric titration. The results after the second hydrogen peroxide step appear in the following table.

From the table it appears that the addition of magnesium within the indicated pH interval, before bleaching with hydrogen peroxide, positively affects the pulp with regard to viscosity and lightness and reduces the consumption of hydrogen peroxide.

Claims (11)

1. Method for delignification and bleaching of chemically bound lignocellulosic material, characterized in that the pulp is subjected to a bleaching and delignifying treatment at a pH in the range 1-6 with bleaching chemicals selected from chlorine dioxide, ozone, peracetic acid or an acidic peroxide-containing compound, after which the pulp is washed , after which a water-soluble magnesium compound is added at a pH in the range 1 - 7 and in an amount of 0.01 - 10 kg/ton of dry mass, counted as magnesium, and that the mass is then bleached and delignified with a chlorine-free bleaching agent consisting of hydrogen peroxide with a pH in the range 8 - 12. 2. Method according to claim 1, characterized in that the mass is bleached and delignified with a chlorine-free bleaching agent consisting of hydrogen peroxide with a pH in the range 10 - 12. 3. Method according to claim 1, characterized in that the chemically dissolved mass is a sulphate mass. 4. Method according to claim 1, characterized in that the water-soluble magnesium compound is added at a pH in the range 2-6. 5. Method according to claim 1, characterized in that the water-soluble magnesium compound is a mixture of magnesium-containing and calcium-containing compounds. 6. Method according to claim 1 or 5, characterized in that the water-soluble compounds containing magnesium are made up of magnesium sulphate or magnesium chloride. 7. Method according to claim 5, characterized in that the water-soluble compound containing calcium consists of calcium chloride or calcium oxide. 8. Method according to claim 1, characterized in that the mass is washed after the acidic treatment. 9. Method according to claim 1, characterized in that the bleaching, before adding the water-soluble magnesium compound, is carried out at a pH in the range 1.5 - 6. 10. Method according to claim 1, characterized in that the mass, after treatment, is finally bleached with ozone in one or more stages. 11. Method according to claims 1-10, characterized in that the acidic treatment is carried out at a temperature in the range 10 - 95°C during 1 - 120 min., that a water-soluble magnesium compound is added at a temperature in the range 10 - 95°C in the course of 1 - 180 min., and an amount of 0.5 - 5 kg/tonne of dry mass, calculated as magnesium, and where the treated mass has a concentration in the range of 3-35% by weight.