Classifications

• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1005—Pretreatment of the pulp, e.g. degassing the pulp
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1026—Other features in bleaching processes
  • D21C9/1042—Use of chelating agents
• • D—TEXTILES; PAPER
  • D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
  • D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/16—Bleaching ; Apparatus therefor with per compounds

Definitions

• the present invention relates to a method for bleaching lignocellulose-containing pulp with a peroxide-containing compound, in which the pulp, prior to the bleaching and in optional order, is subjected to acid treatment at a pH of up to about 5 and treated with a complexing agent in order to release transition metal ions from their positions in the pulp.
• acid treatment a magnesium compound and a calcium compound are added to the pulp at a pH of from about 3.5 to about 8 in order to reintroduce an optimum amount of magnesium and calcium ions into the pulp.
• the pulp is thereafter bleached with a peroxide-containing compound at a pH of from about 7 to about 13.
• the pulp is bleached in one or more stages.
• mechanical pulp has been bleached with peroxide-containing compounds in alkaline environment, the intention being to remove chromophoric groups but preserve the lignin content.
• it has become increasingly common to treat also chemical pulp as early as in the first bleaching stages with peroxide-containing compounds in alkaline environment, thereby to remove chromophoric groups as well as lignin. Unless the pulp is pretreated, however, such bleaching is less effective than bleaching with chlorine-containing bleaching agents.
• the prior art teaches the bleaching of chemical as well as mechanical pulp with peroxide-containing compounds in alkaline environment and in the presence of magnesium salts. Most of the known bleaching processes do not comprise any acid pretreatment, which preserves the content of desirable as well as undesirable ions in the pulp.
• EP-A-0 402 335 discloses the pretreatment of chemical pulp with a complexing agent in order to render more effective subsequent alkaline peroxide bleaching. In this process, it is important that the treatment with a complexing agent takes place at an almost neutral pH. Moreover, this process is not suitable for treating pulp containing large amounts of transition metal ions.
• EP-A-0 511 695 discloses acid treatment of a chemical pulp, thereafter adding magnesium and subsequently treating the pulp with alkaline peroxide bleaching.
• the invention relates to a method in which lignocellulose-containing pulp is treated under the conditions recited in the appended claims, whereby its content of ions having an adverse effect on subsequent alkaline peroxide bleaching is effectively reduced, while at the same time its content of favourable ions is optimised, thereby to obtain a pulp of high strength and brightness with a low consumption of the peroxide-containing compound.
• the invention provides a method for bleaching lignocellulose-containing pulp with a peroxide-containing compound, in which the pulp, prior to the bleaching and in optional order, is subjected to acid treatment at a pH of up to about 5 and treated with a complexing agent, then adding to the pulp, after the acid treatment, a magnesium compound and a calcium compound at a pH of from about 3.5 to about 8 in order to reintroduce magnesium and calcium ions into the pulp, whereupon the pulp is bleached at a pH of from about 7 to about 13.
• the initial acid treatment combined with the treatment with a complexing agent in accordance with the invention effectively releases all types of metal ions in lignocellulose-containing pulp.
• the thus-released ions can be effectively removed from the pulp suspension. It has therefore been found that the present method effectively reduces the concentration of those ions of transition metals, in particular manganese, copper and iron, that have an adverse effect on the subsequent peroxide bleaching.
• magnesium ions are reintroduced into the pulp prior to bleaching, thereby increasing the brightness and the strength of the pulp while at the same time reducing the lignin content and the consumption of the bleaching agent.
• calcium ions are reintroduced into the pulp, there is achieved a surprising improvement of the final properties of the pulp, especially with respect to brightness and reduction of the kappa number.
• the content and the positions of magnesium as well as calcium ions have to be optimised to attain the synergistic effect enabling effective peroxide bleaching.
• the magnesium and calcium ions have to be added at a pH of from about 3.5 to about 8 in order that the pulp should obtain a sufficient content of these ions.
• the present invention it is possible to obtain a pulp of high brightness, low lignin content and a on the whole preserved pulp strength while consuming a reduced amount of the peroxide-containing compound.
• a pulp of high brightness, low lignin content and a on the whole preserved pulp strength while consuming a reduced amount of the peroxide-containing compound.
• it is often possible to carry out the method while using a comparatively small amount of complexing agent and a simple washing stage before the peroxide bleaching.
• the acid treatment and the addition of magnesium and calcium compound are carried out in this order and essentially within different pH ranges.
• the treatment with the complexing agent may be performed whenever suitable from the point of view of process technique.
• the treatment with a complexing agent may be carried out in a separate stage before or after the acid treatment or after the addition of magnesium and calcium compounds.
• the treatment with a complexing agent is combined with the acid treatment or the addition of magnesium and calcium compounds.
• the acid treatment and the treatment with a complexing agent suitably are performed in a single stage before the magnesium and calcium compounds have been added.
• it is preferred that the treatment with a complexing agent and the addition of magnesium and calcium compounds take place in a single stage after the acid treatment.
• the pulp is suitably dewatered or washed and is then bleached with a peroxide-containing compound at an alkaline pH.
• the weight ratio between added Mg and Ca can be from about 1:1 to about 1:4 in order to render effective the alkaline peroxide bleaching.
• the weight ratio of Mg to Ca is from 1:1.2 to 1:3, preferably from 1:1.5 to 1:2.6.
• the amount of magnesium compound charged, as well as the amount of calcium compound charged may be up to about 4000 ppm, calculated as alkaline earth metal by weight of dry pulp.
• the amount of both the magnesium compound and the calcium compound is in the range 100-3000 ppm, preferably in the range of 500-2000 ppm.
• the amount of magnesium compound charged, as well as the other conditions is so chosen that the content of magnesium in the pulp before the peroxide bleaching amounts to at least about 50% of the content of magnesium before carrying out of the present method (the original content).
• the amount of magnesium compound charged, as well as the other conditions is so chosen that the content of magnesium in the pulp before the peroxide bleaching is in the range of 100-300% of the original content, preferably in the range of 130-200%.
• the amount of calcium compound charged, as well as the other conditions, is so chosen that the content of calcium in the pulp before the peroxide bleaching amounts to at least about 25% of the original content.
• the amount of calcium compound charged, as well as the other conditions is so chosen that the content of calcium in the pulp before the peroxide bleaching is in the range of 50-150% of the original content, preferably in the range of 65-100%.
• the magnesium-containing compound used is magnesium sulphate, magnesium chloride, magnesium carbonate or magnesium nitrate, preferably magnesium sulphate.
• the calcium-containing compound used is calcium chloride, calcium nitrate, calcium sulphate or calcium carbonate, preferably calcium sulphate. Also compounds containing magnesium and calcium in a suitable ratio may be used, such as dolomite.
• the magnesium and calcium compounds should be in dissolved form when brought into contact with the pulp. This can be obtained in various ways, depending on the type and properties of the pulp, among other things. By combining a suitable pH of from about 3.5 to about 8 with suitable temperatures and concentrations of the magnesium and calcium compounds, these compounds can be brought into dissolved form.
• magnesium or calcium ions can be added to the pulp suspension via the water used e.g. for pH adjustment or dilution.
• hard water containing magnesium or calcium ions, or a combination thereof may advantageously be used for reintroducing these ions into the pulp. Examples of such hard water are fresh water from limestone bedrock, white water from paper-making machines using lime or chalk as filler, and process water from sulphite production using magnesium or calcium as base.
• the present method preferably comprise a dewatering or washing stage after the treatment with the complexing agent and the addition of magnesium and calcium compounds.
• undesirable metal ions can be effectively removed before the peroxide bleaching.
• the pulp may also be dewatered or washed before and/or after the complexing agent treatment.
• undesirable metal ions are meant not only transition metal ions but also any excess of magnesium and calcium ions.
• the pH in the washing liquid must be at least about 4.
• the pH of the washing liquid is in the range of 5-13, preferably 6-12.
• the acid treatment is performed at a pH of up to about 5, suitably a pH of from 1.5 to 4, and preferably a pH of from 2 to 3.
• pH can be adjusted by adding an acid or an acid liquid to the pulp.
• inorganic mineral acids or residual acid from a chlorine dioxide reactor either separately or in optional mixture.
• an inorganic mineral acid such as sulphuric acid, nitric acid or hydrochloric acid, preferably sulphuric acid.
• Manganese ions in the pulp have an especially adverse effect on alkaline peroxide bleaching. Compounds forming strong complexes with different manganese ions are therefore primarily used as complexing agents. Suitable complexing agents from this point of view are nitrogen-containing organic compounds, primarily nitrogen-containing polycarboxylic acids, nitrogen-containing polyphosphonic acids, and nitrogen-containing polyalcohols. Preferred nitrogen-containing polycarboxylic acids are diethylenetriamine pentaacetic acid (DTPA), ethylenediamine tetraacetic acid (EDTA) and nitrilo-triacetic acid (NTA), DTPA and EDTA being especially preferred. Diethylenetriamine pentaphosphonic acid is a preferred nitrogen-containing polyphosphonic acid.
• DTPA diethylenetriamine pentaacetic acid
• EDTA ethylenediamine tetraacetic acid
• NTA nitrilo-triacetic acid
• Diethylenetriamine pentaphosphonic acid is a preferred nitrogen-containing poly
• complexing agents such as polycarboxylic acids, suitably oxalic acid, citric acid or tartaric acid, or phosphonic acids.
• organic acids as are formed in the treatment of the pulp with chlorine-free bleaching agents, for example, may also be used as complexing agents.
• the sodium salts of the above complexing agents are preferably used, making the bleach plant a more closed system.
• the choice of pH in the treatment with a complexing agent is not critical in the present method, since the magnesium and calcuium ions having a favourable effect on the subsequent alkaline peroxide bleaching can be optimised both with respect to amount and position in the pulp.
• the treatment with a complexing agent can thus be performed at a pH of from about 1.5 to about 13.
• the treatment with a complexing agent can be combined with the acid treatment, without there being any appreciable reduction of the complexing ability.
• the amount of complexing agent charged depends on the type and the amount of transition metal ions present in the incoming pulp, as well as on the effectiveness of the acid treatment and the subsequent dewatering or washing stage. Furthermore, the amount also depends on the type of complexing agent and on the conditions prevailing in the treatment with the agent, such as temperature, residence time, and pH. However, the amount of complexing agent charged can be up to about 5 kg/tonne of dry pulp, based on a 100% product. Suitably, this amount is from 0.05 kg to 2.5 kg/tonne of dry pulp, preferably from 0.1 kg to 1 kg/tonne of dry pulp, based on a 100% product. It is especially preferred that the amount of complexing agent is in the range of 0.2-0.6 kg/tonne of dry pulp, based on a 100% product.
• the pH in the pulp suspension before the addition of magnesium and calcium compounds is from about 3.5 to about 8, suitably in the range of 4-7.5, and preferably in the range of 4.5-7. It is especially preferred that the pH is in the range of 5-6.5.
• the pulp Before the acid treatment in the present method, the pulp can be subjected to acid treatment in an additional stage, thereby to obtain a particularly low content of transition metal ions.
• additional treatment may be suitable in alkaline peroxide bleaching of pulp in which the content of transition metal ions is unusually high or in which these ions are difficult to release.
• One such instance is organosolv pulp.
• the acid treatment is performed in the presence of a delignifying chemical which is effective at a pH of up to about 5.
• Suitable delignifying chemicals are ozone, acid hydrogen peroxide, per-acids, such as Caro's acid or peracetic acid, and salts thereof, chlorine dioxide and chlorine.
• ozone, per-acids or salts thereof, or chlorine dioxide are especially preferred.
• chlorine dioxide When chlorine dioxide is used as a delignifying chemical in the present method, it is possible to bleach the pulp to full brightness with an initial chlorine dioxide stage followed by a peroxide stage and still reducing the produced and discharged amount of chlorine containing compounds to an extremely low level.
• the present method optimizes the conditions for an effective peroxide bleaching, whereby a reduced adding of chlorine dioxide can be balanced with an additional, and economically reasonable, adding of peroxide.
• a reduced adding of chlorine dioxide means that the content of chlorine containing compounds in the pulp, which has been bleached, decreases and the possibilities to close the plants increases essentially.
• the present method it is possible to decrease the total amount of chlorine containing compounds in the waste water, from all stages, to less than 5 kg/ton pulp, based on the element chlorine.
• the total amount of chlorine containing compounds in the waste water from all stages will be decreased suitably to less than 3,5 kg/ton pulp and preferably to less than 2 kg/ton pulp, based on elementary chlorine.
• the amount may be from about 0.5 kg to about 30 kg/tonne of dry pulp, suitably from 1 kg to 20 kg/ tonne of dry pulp, and preferably from 1.5 kg to 10 kg/ tonne of dry pulp.
• CF total amount of active chlorine in kg/tonne of dry pulp / (1) kappa number prior to delignification according to the present method.
• 1 kg of chlorine dioxide is equivalent to 2.63 kg of active chlorine.
• the charging factor may be from about 0.1 to about 10, suitably from 0.5 to 5, and preferably from 1 to 3.
• the present method can include a dewatering or washing stage after the acid treatment and the treatment with a complexing agent.
• the pulp may also be dewatered or washed before and/or after the complexing agent treatment.
• transition metal ions can be effectively removed prior to the peroxide bleaching.
• the washing liquid may consist of fresh water, optionally with an addition of a pH-adjusting chemical, or white water from one or more bleaching stages or extraction stages, giving a suitable pH in the washing stage.
• the acid treatment, treatment with complexing agent and the addition of magnesium and calcium can be carried out at a temperature of from about 10°C to about 100°C, suitably from 25°C to 90°C, and preferably from 40°C to 80°C, and for a time of from about 1 min to about 600 min, suitably from 5 min to 120 min, and preferably from 10 min to 60 min.
• the pulp concentration may be from about 1% by weight to about 60% by weight and preferably from 3% by weight to 35% by weight.
• the bleaching with a peroxide-containing compound is performed at a pH of from about 7 to about 13, suitably a pH of from 8 to 12, and preferably a pH of from 9 to 11.
• the peroxide-containing compound comprises inorganic peroxide compounds, such as hydrogen peroxide, sodium peroxide and peroxo sulphuric acid (Caro's acid), and organic peroxide compounds, such as peracetic acid and performic acid.
• the peroxide-containing compound suitably consists of hydrogen peroxide or a mixture of hydrogen peroxide and oxygen, and preferably is hydrogen peroxide.
• the pulp may be used as such for making paper. If so desired, the pulp may also be finally bleached to a higher brightness in one or more stages, e.g. by means of hydrogen peroxide, ozone, sodium dithionite or chlorine dioxide.
• Final bleaching can also include alkaline extraction stages which may be fortified by peroxide and/or oxygen.
• lignocellulose-containing pulp pulp containing fibres that have been exposed by known chemical or mechanical treatment, or recycled fibres.
• the lignocellulose-containing pulp consists of chemically digested pulp, preferably chemically digested pulp that has been delignified with oxygen prior to the present method. It is especially preferred that the lignocellulose-containing pulp consists of sulphate pulp of softwood.
• the kappa number, the viscosity and the brightness of the pulp were determined according to the SCAN Standard Methods C 1:77 R, C 15-16:62 and C 11-75:R, respectively.
• the consumption of hydrogen peroxide and peracetic acid were established by titration with sodium thiosulphate, and potassium permanganate and sodium thiosulphate, respectively.
• Example 1 Oxygen-delignified sulphate pulp of softwood having a kappa number of 15.7, a brightness of 37.9% ISO and a viscosity of 990 dm3/kg was treated with 15 kg H2SO4/tonne dry pulp, giving a pH of 2.
• the acid treatment (A) was performed at 50°C, a part concentration of 10% by weight and for 30 min. After raising the pH to 5, EDTA (Q), magnesium sulphate (Mg) and calcium sulphate (Ca) were added (test 1). The conditions were so chosen that magnesium and calcium were dissolved in the pulp suspension. The treatment took place at 50°C, a pulp concentration of 10% by weight and for 60 min. The amount of EDTA added was 2 kg/tonne dry pulp. The amount of magnesium and calcium compounds added was, respectively, 1000 ppm and 1500 ppm, based on alkaline-earth metal per dry pulp.
• the pulp was bleached with hydrogen peroxide (P) at a temperature of 90°C, a residence time of 240 min and a pulp concentration of 10% by weight.
• the addition of hydrogen peroxide was 25 kg/tonne dry pulp, based on 100% hydrogen peroxide, the pH was 10.5-11.
• the pulp was washed with deionised water having a pH of 5.5.
• the pulp was first dewatered to a pulp concentration of 25% by weight, and then 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.
• Example 2 Oxygen-delignified sulphate pulp of softwood having a kappa number of 8.2, a brightness of 45% ISO and a viscosity of 820 dm3/kg was treated in accordance with the present method (test 1).
• the acid treatment, the addition of EDTA, magnesium sulphate and calcium sulphate in a single stage and the bleaching with hydrogen peroxide were performed as in Example 1. After each stage, the pulp was washed as in Example 1.
• the complexing agent and the magnesium sulphate (test 2), as well as only the complexing agent (test 3) were added after the acid treatment.
• the conditions in tests 2 and 3 are otherwise in keeping with those indicated above.
• Example 3 Oxygen-delignified sulphate pulp of softwood having a kappa number of 14.8, a brightness of 36.4% ISO and a viscosity of 1000 dm3/kg was delignified with chlorine dioxide (D), treated with EDTA in the presence of magnesium and calcium, and bleached with hydrogen peroxide.
• chlorine dioxide was added in an amount equivalent to 30 kg active chlorine/tonne dry pulp, i.e. the batching factor was 2.
• Delignification was performed at a temperature of 50°C, a pulp concentration of 10% by weight and for 50 min, pH at the end being 3.9.
• Example 1 The addition of EDTA, magnesium sulphate and calcium sulphate in a single stage and the bleaching with hydrogen peroxide were as in Example 1. After each stage, the pulp was washed as in Example 1. For comparative purposes, the pulp was treated without calcium (in test 2), without calcium or magnesium (in test 3) and without complexing agent, calcium or magnesium (in test 4). The results obtained after the bleaching with hydrogen peroxide appear from the Table below.
• Example 4 The sulphate pulp used in Example 3 was delignified with ozone (Z), treated with EDTA in the presence of magnesium and calcium at different pH values, and bleached with hydrogen peroxide. Delignification with ozone was performed at a pH of 2.1, the temperature of 25°C, a pulp concentration of about 40% by weight and for 2-3 min. The amount of ozone charged was 7-8 kg/tonne dry pulp. The combined treatment with EDTA and addition of magnesium and calcium sulphate was performed as in Example 1, except that pH was varied. The bleaching with hydrogen peroxide was performed as in Example 1. After each stage, the pulp was washed as in Example 1. The results obtained after the bleaching with hydrogen peroxide appear from the Table below.
• Example 5 The sulphate pulp used in Example 3 was delignified with ozone (Z), treated with magnesium and calcium, and bleached with hydrogen peroxide (P).
• the complexing agent was added after Z in the same stage as in Mg and Ca (Q2, test 1), as well as before Z (Q1, test 2), in order to illustrate the effect of the point of addition of the complexing agent. Delignification with ozone and the combined treatment with EDTA and addition of magnesium and calcium sulphate were in test 1 performed as in Example 1, as were the bleaching with hydrogen peroxide. In Q1, 15 kg H2SO4 was added per tonne dry pulp, giving a pH of 2.0. Otherwise, Q1 was performed in keeping with Q2, except that the residence time was 30 min.
• Example 6 Oxygen-delignified sulphate pulp of softwood having a kappa number of 7.7, a brightness of 51.6% ISO and a viscosity of 825 dm3/kg was treated with 15 kg H2SO4/tonne dry pulp, giving a pH of 2.0.
• the acid treatment was performed at 50°C for 30 min and with a pulp concentration of 10% by weight.
• 2 kg EDTA/tonne dry pulp was added together with magnesium sulphate in an amount of 300 ppm, based on magnesium, and calcium sulphate in an amount of 600 ppm, based on calcium. The treatment took place at 50°C, for 60 min and with a pulp concentration of 10% by weight.
• the pulp was bleached with hydrogen peroxide at a temperature of 90°C, for a residence time of 240 min and with a pulp concentration of 10% by weight.
• the addition of hydrogen peroxide was 25 kg/tonne dry pulp, based on 100% hydrogen peroxide, and pH was 11.
• the pulp was washed with deionised water having a pH of 5.5.
• the pulp was first dewatered to a pulp concentration of 25% by weight, and was then 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 pulp was bleached with different combinations of D and P to illustrate the effect of the batching factor on the total amount of the produced chlorine containing compounds at a constant final brightness of 88% ISO.
• the delignification with chlorine dioxide were as in Example 1, apart from the batching factor which was from 0.67 up to 2.
• the combinated addition of EDTA, magnesium sulphate and calcium sulphate were as in Example 1.
• the bleaching with hydrogen peroxide were as in Example 1, apart from that the temperature was 105°C and that the amount of hydrogen peroxide was varied.
• the pulp was washed as in Example 1.
• the amount of chlorine containing compounds from the sequence was measured in the total volume of waste water.
• the amounts of chlorate and chloride were measured with ion-chromatography according to the standardized method.
• the amount AOX was measured according to SCAN-W 9:89.
• the total amount of the elementary chlorine was measured in each test with titration after reduction with SO2 at room temperature.
• the same pulp was bleached and delignified to approximately the same brightness as in the sequence D-(EOP)-D-(EP)-D (Test 6).
• the conditions in each stage were conventional.
• the amounts of chlorine dioxide and hydrogen peroxide in test 6 are total in the sequence.
• the partial pressure of oxygen in the EOP-stage was 0.22. The results obtained after the bleaching with hydrogen peroxide appear from the Table below.
• Example 8 Oxygen-delignified sulphate pulp of softwood having a kappa number of 9.6, a brightness of 40.5% ISO and a viscosity of 890 dm3/kg was delignified with chlorine dioxide (D) in the presence of EDTA (Q), and bleached with hydrogen peroxide (P) and oxygen (O) in the presence of magnesium and kalcium, i.e. in the sequence (D+Q) (P+O+Mg+Ca). The pulp was bleached with various combinations of D and P to illustrate the effect on the batching factor on the total amount chlorine containing compounds produced at a constant final brightness of about 90% ISO.
• D chlorine dioxide
• Q EDTA
• O hydrogen peroxide
• O oxygen
• the pulp was bleached with various combinations of D and P to illustrate the effect on the batching factor on the total amount chlorine containing compounds produced at a constant final brightness of about 90% ISO.
• the delignification with chlorine dioxide was performed at a pH of 1.8-2.0, and a temperature of 45°C for 25 min.
• the batching factor was varied from 1.0 to 2.5.
• the added amount of EDTA was 1.5 kg/ton dry pulp.
• magnesium sulpate was added in an amount of 300 ppm calculated as magnesium and calcium sulphate was added in an amount of 600 ppm calculated as calcium.
• sodium hydroxide and hydrogen peroxide were added and the pulp concentration was adjusted to 10% by weight.
• the used autoclave was pressurized with oxygen to 0.22 MPa measured at room temperature. After raising the temperature to 110°C, the pulp was bleached for 240 min. pH became 11-11.5 after bleaching.
• Example 6 After each stage, the pulp was washed as in Example 6. The amount of chlorine containing compounds from the sequence were measured in the total volume of waste water. The amounts of chlorate and chloride were measured with ion-chromatography according to the standardized method. The amount AOX was measured according to SCAN-W 9:89. The total amount of the elementary chlorine was measured in each test through titration after reduction with SO2 at room temperature. As comparison, the same pulp was bleached and delignified to approximately the same brightness as in the sequence D-(EOP)-D-(EP)-D (Test 5). The conditions in each stage were conventional. The amounts of chlorine dioxide and hydrogen peroxide in test 5 are total in the sequence. The partial pressure of oxygen in the EOP-stage was 0.22.

Landscapes

• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)
• Polysaccharides And Polysaccharide Derivatives (AREA)

Applications Claiming Priority (4)

Publications (4)

Family

ID=26661708

Family Applications (1)

Country Status (7)

Cited By (15)

Citations (5)

• 1994
  • 1994-04-05 EP EP94200924A patent/EP0622491B2/fr not_active Expired - Lifetime
  • 1994-04-05 DE DE69420306T patent/DE69420306T2/de not_active Expired - Lifetime
  • 1994-04-05 AT AT94200924T patent/ATE184060T1/de not_active IP Right Cessation
  • 1994-04-15 FI FI941764A patent/FI941764A/fi unknown
  • 1994-04-15 CA CA002121387A patent/CA2121387C/fr not_active Expired - Fee Related
  • 1994-04-18 BR BR9401526A patent/BR9401526A/pt not_active Application Discontinuation
  • 1994-04-19 NO NO941421A patent/NO302767B1/no unknown

Patent Citations (5)

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events