A METHOD OF REFINING CELLULOSE PULPS

Abstract: A method of refining cellulose pulps by bleaching or extraction is disclosed, in which the pulps at a consistency of 10 - 65% and in a finely divided form are treated with chemicals at a temperature of 10 to 250`C. The finely divided pulp is fed, impregnated with chemicals and at a consistency of 30 to 65% continuously into a processing apparatus in which an excess system pressure of 5 to 400 kPa prevails and which includes an atmosphere substantially consisting of steam having an oxygen content of less than 1% by volume and a temperature of 100 to 150`C. The pulp is moved rapidly, but without alteration of its fibre characteristics, continuously through the processing apparatus under a substantially turbulent flow and with substantially unaltered solids content. The chemicals charged to the pulp are caused to react so completely with the pulp during its rapid movement through the processing apparatus that they are substantially consumed when the movement is terminated. The pulp is then discharged from the processing apparatus and dried, optionally after further chemical treatment steps.

Description: DE SCRIPT IOg Technical field The present invention relates to refining of cellulose material, such as chemical pulps, e.g. sulphate pulpsj soda pulps and sulphite pulpsj semi-chemical pulps, chemi-mechanical pulps and mechanical pulps such as groundwood pulps produced at normal pressure or excess pressure, refiner mechanical pulps and thermo-mechanical pulps. The term "refining" is used here primarily in reference to bleaching and extraction. Background Art Bleaching of chemical, semi-chemical and mechanical pulps with bleaching chemicals such as chlorine, chlorine dioxide, hypochlorite or lignin-preserving chemicals such as peroxy- compounds and dithionite is well-known in cellulose technology and usually takes place by the chemicals being mixed into the pulp, after which the bleaching reaction - generally at a pulp consistency below 2.0 % - is carried out for several hours at temperatures which seldom exceed 85 C. Befining cellulose pulps, with the intention of removing hemi-cellulose and/or extractive substances such as resin, fatty acids and unsaponifiable substan¬ ces, takes place by admixing of alkali such as sodium hydroxide in the pulp, subsequent to which the refining chemicals are allowed to react with the pulp for some hours at temperatures generally below 85 C. However, higher temperatures than 85°C can also be used in refining cellulose pulps, e.g. in hot alkali refining for producing dissolving pulp. It is known from Svensk Papperstidning No. 15 (1977) pages 480 - 482. that in experiments with peroxide bleaching of unbleached sulphate pulp at 110 C in a digester, a very rapid and complete reaction between the peroxide and the pulp has been established, in¬ dicatingi-dat other reaction mechanisns were imrαlyed at hi .temperatures- than at lower temperatures. I spite of. this the authors-have, however, for reasons of economy proposed a two-stage bleaching sequence con¬ sisting of an oxygen stage followed by a peroxide stage at 7 QO C ' in~- stead of a single-stage peroxide bleaching at above 100 0 to obtain sufficient brightness increase when blea.chlng sulfate pulp, even if the selectivity will be lower than in single-stage high-temperature peroxide bleaching. The experiments were carriedout wLlh.a -shortest b-Leach- ing time of five minutes and with the pulp in a stationary bed and at a pulp concentration of 30 • The Swedish Patent Specification 341 519 describes a method for simultaneously bleaching and drying mechanical pulp to enable obtaining rapid drying of the pulp while retaining brightness. Ehe finely divided pulp, impregnated with hydrogen peroxide at a consistency of 20 - 50 , is dried in an air stream under normal pressure and with a residence time of 2 seconds to 10 minutes at a temperature of 260 - 538 C to a solids content of 65 - 95 • This method has several disadvantages, however. The consumption of expensive bleaching chemicals and energy will be great, and the content of fibre knots too high. Furthermore, the method does not work in the presence of sulphur compounds in the drying gas, and bleaching with reducing bleachers, e.g. dithionite, cannot be carried out, since these decompose due to the oxygen content of the drying air. Disclosure of invention The object of the present invention is to provide a method of refining cellulose pulps by means of bleaching or extraction, which gives the advantage of short processing time in combination with low chemical consumption and low energy consumption, while at the same time the finished product is given good quality characteristics. Accordingly, the present invention relates to a method of refining by bleaching or extraction of cellulose pulps, in which the pulps in a finely divided form at a consistency of 10 - 65 are treated with chemicals at a temperature of 10 - 250 and dried, which is characterized in that the finely divided pulp, impregnated with chemicals, at a consistency of 30 - 65 is continuously fed into a processing apparatus which has an excess system pressure of 5 - 400 kPa and an atmosphere substantially consisting of steam with an oxygen content of less than 1 by volume and a temperature of 100 - 150 C, in that the pulp is rapidly moved, without alteration of the fibre characteristics by mechanical working, through the processing apparatus under substantially turbulent flow and substantially unaltered dry content, and in such a way that added chemicals have substantially completely reacted when the conveying through the processing apparatus is terminated, the pulp subsequently being discharged from the processing apparatus and dried after possible further chemical treatment steps. Best Mode of CarryingOut the Invention -in accordance with the invention, the moisture content of the pulp during passage through the processing apparatus should preferably not be altered by more than at most 8 and especially by less than 6 percentage units. It is especially suitable in accordance with the invention that before entering into, the processing apparatus, the pulp is dewatered to a pulp consistency of greater than 30 , preferably 45 or more, to recover the excess of bleaching solution and to carry out the division into finely divided form in a disc refiner. In the case where the chemical refining operation includes bleaching, the desired steam atmosphere is provided by supplying steam, which preferably has an excess pressure of 100 - 200 kPa. JFurthermore, in accordance with the invention, the steam atmosphere shall only contain at most 1 by volume of oxygen for the heat transfer to function in a satisfactory v/ay. It is advantageous, in accordance with the inven- tion, if the steam is separated from the pulp after the latter - has passed through the processing apparatus, the steam being re¬ cycled to the processing apparatus. A suitable temperature for the steam supplied is 100 - 150 C. Pulp and steam must be supplied to the processing apparatus in such a way that a substantially turbulent flow of pulp is obtained. This can be achieved mechanically or pneumatically. A suitable conveying speed for pneumatic conveying is more than 10 m/sec. The temperature in the processing apparatus is kept at 100 - 150 C, preferably at 105 - 120 C, and the excess pressure at 5 - 400 kPa, preferably 50 - 300 kPa. Suitable bleaching chemicals for use in accordance with the invention are chlorine dioxide, hypochlorite, peroxy . compounds, peroxides and dithionite. Especially suitable for use in accordance with the present invention are hypochlorite and lignin-preserving bleaching agents such as peroxides and dithionite. » In bleaching in accordance with the invention it has been found to be particularly advantageous if the pulp, at a low pulp concentration, e.g. 4 %, is first impregnated with complex- ing agents such as ethylenediaminetetraacetic acid (EDTA), OMPI diethylenetriaminepentaacetic acid (JKTPA) etc. before entry into the processing apparatus, the pulp subsequently being dewatered to a concentration of over 10 , suitably 15 - 35 , to remove heavy metals in the best way. Such dewatering can suitably be done in a drum filter, a centrifuge or a press. After possible shredding into centimeter-large pieces," the pulp is subsequently impregnated with a bleaching chemical solution, which furthermore can contain alkali, e.g. NaOΞ, and pH and peroxide stabilizers, e.g. sodium silicate} and protecting agents, e.g. magnesium sulphate. Impregnation can be carried out by spraying the bleach ing solution onto the flakes or by mechanical mixing, e.g. in a mixer. After this, the pulp is further dewatered once again to a high 'consistency, suitably over 30 , preferably 45 - 65 , so that the excess of bleaching chemicals is removed and can be recovered. This dewatering is suitably performed in a press. The dewatered pulp is then to advantage subjected to further disinte¬ gration, e.g. in a disc refiner or a spike roller shredder, by which means it is given flake form to be easily accessible for the temperature increase in the processing apparatus. The process in accordance with the present invention is also very suitable in extracting hemicellulose and extractive substances from cellulose pulp, e.g. for producing dissolving pulp etc. In this case the chemicals are alkali, e.g. sodium hydroxide or magnesium hydroxide. The procedure is here mainly the same as with the use of bleaching chemicals, i.e. the pulp is treated with an alkali solution at an excess pressure of 50 - 300 kPa, preferably 100 - 200 kPa, in a steam atmosphere provided by the supply of steam separated from the pulp after the passage of the pulp through the processing apparatus, with the difference that the pulp is washed after steam separation and before drying, the dissolved-out hemicellulose and extractive substances being separated (extraction). After treating with chemicals in accordance with the invention, the treated pulp, which has a solids content of at least 40 if it has been subjected to bleaching,* or at least 30 if it has been subjected to extraction, can be taken directly to paper manufacturing or other further processing, if such is \JΛ suitable, whereafter it is finally dried. The pulp can also be dried before the paper manufacturing, which is the most usual case in practice. Such drying is especially suitable if carried out as flash drying, i.e. when the pulp is suspended in a turbulent gas stream with a temperature of 1 10 - 500°C. The transfer of heat from the drying medium to the pulp is thereby facilitated. It is especially suitable for the drying medium to consist of superheated steam at an excess pressure of 20 - 400 kPa, in which case very good heat economy can be achieved by using the excess steam thus obtained for other heating purposes, e.g. as heat source in the chemical process in accordance with the invention. A suitable drying apparatus according to this method, a so-called counter-pressure drier, is described in the Swedish Patent Specification 393 855. In this drying apparatus, the pulp is dried in the form of flakes, which flow through vertical excess pressure towers at a rate of 21 m/s. The pulp flakes and steam are given the high speed by means of fans. The conveying or carrier steam is heated indirectly b pressurized steam pipes, the tempera¬ ture of which is kept considerably higher than that of the carrier steam. The carrier steam heats the moist pulp instantanteously, &ich leads to a rapid evaporation of the moisture in the pulp. A dry pulp is obtained by this drying process after 10 - 20 seconds. During the drying operation, the pulp can also be treated with pH-regulating substances, e.g. S0_, which is supplied in gaseous form, or calcium oxide in powder form. After passing through the drying unit, the excess steam is recovered by having the dried pulp pass through a hydrocyclone. The invention is illustrated by the following working examples. Example 1 Washed birch pulp produced chemi-mechanic lly by partial delignification with bisulphite and defibration in a disc refiner, and having a brightness of 66 SCAU was mixed in a mixer with hot water and 0.2 diethylenetriaminepentaacetic acid (DTPA), calculated on the dry weight of the pulp, so that the pulp consistency was 4 % and the temperature 62°C. The pulp was dewatered to 35 solids content after 30 minutes. The dewatered pulp was shredded to _OMPI_ centimeter-large pieces and mixed in a mixer with a solution containing 18 g/l hydrogen peroxide, 25 g/l sodium silicate,- 9 g/l sodium hydroxide and 0.2 g/l magnesium sulphate. After the admixing of the bleaching solution, the pulp was dewatered in a press to 50 % pulp consistency for removing the excess of chemical The dewatered pulp contained 3.0 % hydrogen peroxide, 5.0 % sodiu silicate, 1.5 sodium hydroxide and 0.04 magnesium sulphate, calculated on the dry weight of the pulp. The pulp obtained was divided into individual fibres and fibre bundles by treatment in a disc refiner and was thereafter continuously fed via a sluice feeder into a processing apparatus in the form of a modified flas drier, in which the carrier was saturated steam at an excess press of 70 kPa and a temperature of 115°C- The steam, which consisted of saturated excess steam from a counter-pressure drier, was introduced into the flash drier in such a way that a turbulent flow was aehaeved, a fan being used for the further transport of the pulp. The conveying speed of the pulp through the processing apparatus was about 10 m/s, and the pulp passed the unit in 8 seconds. The solids content of the pulp on exiting from the modified flash drier was 45 * Before leaving the apparatus, stea was separated from the pulp in a hydrocyclone, and used for the steaming of the wood material supplied. The chemically processed pulp was ed out via a rotary valve feeder, washed with water and analyzed. The analysis result obtained is shown in Table 1 • The water used for washing only had traces of peroxide. Example 2 Example 1 was repeated with the addition that after pass¬ ing through the modified flash drier, the pulp was continuously introduced at substantially constant solids content without wash¬ ing into a drying unit of the counter-pressure type, in which the drying medium was super-heated steam at an excess pressure of 300 kPa and a temperature of 150 C, to dry the pulp. The pipes used for heating the carrier steam were supplied with saturated steam at 160 C, which resulted in that the carrier steam was rapidly super-heated and that a rapid transfer of moisture from the pulp to the carrier steam was obtained. Pulp and steam were thereafter taken to a cyclone for separating steam from the pulp. The solids content of the dried pulp was 91.2 and it had a pH of 7.7. The pulp was analyzed and the results obtained are shown in Table 1. For the sake of comparison, a sample of the impregnated and finely divided pulp used in Example 1 was processed in the counter-pressure drier used in Example 2, with carrier steam at a temperature of 150 C and an excess pressure of 300 kPa, it thus being simultaneously subjected to bleaching and drying while the solids content was changed from 50 to 91.5 • The pulp was analyzed and the analysis results obtained are shown in Table 1 • For still further comparison, a sample of the impregnated and finely divided pulp used in Example 1 was processed in a conventional flash drier described in the above-mentioned Swedish Patent Specification 341 519. The drying air temperature was 450 C and it was heated with the aid of an oil burner. The drying air temperature was 120 C at the end of the drying operation. The pulp was analyzed and the analysis results are shown in Table 1 belows Treatment Starting Example 1 Example 2 Simultan- Simultah - pulp Bleaching Bleaching eous bleach- eous blea i- followed ing and dry- ing and by drying ing in a drying in a i β>jJQ. CSi-tύfiUJϊ.rc conventional drier flash drier Solids content 50 45.0 91.2 91.5 91.1 Brightness according to SCAN % 66.0 85.3 85.5_ 73.0 72.5 Brightness gain, % 19.3 19.5 7.0 6.5 Fibre bundles number per 100 g pulp 60 250. 280 1050 pH 5.6 8.2 7.7 7.8 6.2 Table 1 OMPI ^Z£a^ It will be apparent from Table 1 that quite surprisingly it has been found possible in accordance with the invention to bleach chemi-mechanical pulp under an extremely short time to a very high brightness, and thereafter to dry the pulp without intermediate treatment to a solids content of about 91 while maintaining an acceptable number o ' f fibre bundles. As will be seen from the two right-hand columns, the bleaching result will. be considerably worse if bleaching and drying are done simultaneously, in which case the poor bleaching result can possibly be explained by the bleaching solution evaporating before it has had time to have any substantial bleaching effect. The brightness gains obtained show that only about one third of the ' optimum bleaching ef ect was obtained while simultaneousl bleaching and drying in accordance with the known technique. The method in accordance with the invention was furthermore very economical with energy. Example 5 Washed birch pulp was produced chemi-mechanically by delignification with bisulphite and defibration in a disc refiner The pulp had a brightness of 66 SCAN " and was treated in the way set forth in Example 1, and with the same batches of chemicals before entering the modified flash drier. In the latter, the saturated carrier steam temperature was kept at 105 C, correspond ing to an excess pressure of 20 kPa. The passage time through the drier was 7 seconds. Before departing from the drier, steam was separated from the pulp in a cyclone, and used for steaming the supplied wood material. The chemically treated pulp was fed out via a rotary valve feeder and taken to a storage tank where it was stored for 15 minutes at a dry content of 47 • The temperatu of the pulp at the end of the residence time was measured and found to be 90 C. An analysis of the pulp at the end of the stor¬ ing time showed a hydrogen peroxide content of 0.1 % with a pulp brightness of 84.9 SCAU. The example shows that in accordance with the invention a milder chemical treatment at a lower temperature can be combined with a short after-treatment, e.g. a residence period in a storage tank, for completing the bleach¬ ing process, and reach a higher brightness value even so. Example 4 Example 3 was repeated with the difference that the pulp in the storage tank was diluted to a concentration of 4 with a hot aqueous solution containing sodium dithionite so that the temperature became 76 C and the amount of dithionite charged was 0.4 % t based on. the weight of dry pulp. The residence time in the storage tank was regulated to 10 minutes. An analysis of the pulp processed in this way gave a brightness of 88.3 % SCAN, which is an extremely high brightness for chemi-mechanical pulps, and can be compared to the brightness for fully bleached chemical pulps. Example 5 Example 2 was repeated with the difference that gaseous sulphur dioxide was added to the pulp before its entry in the counter-pressure drier, and in an amount corresponding to 0.3 counted on the dry weight of the pulp. The solids content of the pulp after .passing through the drying unit was 91.8 , its brightness 82.2 % SCAN and its pH 7.0. By the addition of sulphur dioxide it is thus possible, in accordance with the invention, to bleach and dry the pulp as well as adjust the pH to a desired level. Exem el 6 Example 2 was repeated with the difference that the initial pulp consisted of groundwood pulp from spruce wood, having a brightness of 62 SCAN, and that mixing in a mixer with bleaching solution containing hydrogen peroxide, alkali, pH- stabilizers and protectors was excluded. After treatment in the disc refiner the pulp was taken via the sluice feeder into the modified flash drier, the pulp being sprayed immediately after the sluice feeder with a solution containing sodium dithionite and ethylene-diaminetetraaeetic acid (complexing agent) in an amount such that the pulp contained 0.8 % sodium dithionite and 0.15 of the complexing agent, counted on the dry weight of the pulp. Processing conditions were otherwise the same as in Example 2. The processed pulp had a solids content of 91.9 and a bright¬ ness of 73 SCAKT, which is a very high value in using dithionite as a bleaching agent. OMPI For the sake of comparison, an experiment was made with simultaneous bleaching and drying in a conventional flash drier, in accordance with the Patent Specification 341 519 mentioned above, the conditions being the same as in the corresponding comparative experiment in Example 2, with the difference that mixing with bleaching solution in the mixer was replaced by additing dithionite solution when shredding the pulp, so that the pulp contained 0.8 sodium dithionite and 0.15 % ethylene- diaminetetraaeetic acid calculated on its dry weight. The pulp thus treated had a solids content of 91.5 while its brightness was only 63 SCABT. The results show that bleaching and drying in accordance with the present invention completely surprisingly give a very good bleaching effect, while simultaneosu bleaching and drying in conventional flash driers only give minor bright- ness improvement. A possible explanation may be that the dithio¬ nite decomposes in a conventional flash drier, due to the presence of oxygen in the drying air. With bleaching in a steam atmosphere under excess pressure, in accordance with the inven¬ tion, there is no oxygen present in the carrier steam to an extent such that bleaching is disturbed. In the technical literature, the maximum brightness improvement with dithionite bleaching for a period of 60 minutes and 4 pulp consistency is given to be about 10 - 11 units. The treatment according to the invention resulted in a brightness improvement of 11 units, which thus shows that maximum brightness improvement had been achieved. Example 7 Thermo-mechanical pulp produced from 50 % spruce and 50 % aspen, with a brightness of 56.1 SCAIT, was mixed as in Example with 0.2 DTPA and hot water in a mixer so that the pulp consistency was 4 and the temperature 62°C, the pulp then being dewatered to 35 % dry content. The dewatered pulp was mixed in a mixer with a bleaching solution containing 12 g/l hydrogen peroxi 20 g/l sodium silicate, 6 g/l sodium hydroxide and 0.1 g/l magnes sulphate, and was thickened in a press to 50 solids content. Th pulp thus dewatered contained 2.0 % hydrogen peroxide, 4.0 % sodi silicate, 1.0 sodium hydroxide and 0.02 magnesium sulphate, calculated on the dry weight of the pulp. The pulp thus impregnat with bleaching chemicals was taken through a disc refiner and was afterwards ed into a modified flash drier containing a bleaching compartment and a drying compartment with steam separa¬ tion between the compartments and after the drying compartment. The carrier steam temperature in the bleaching compartment was 114 C with an excess pressure of 64 kPa, and it consisted of saturated excess steam, coming partly from the bleaching compart¬ ment and partly from the drying compartment, and introduced via a fan in the bleaching compartment so that a turbulent flow was obtained. The residence time of the pulp in the bleaching compartment was 9 seconds and in the drying compartment 12 seconds, and it was dried to a solids content of 90.5 %, The brightness of the bleached and dried pulp' was 79.2 SCAN, which is a very high brightness for ther o-mechanical pulp. Usual tower bleaching of the pulp would have required a charge of 3 o hydrogen peroxide and a bleaching time of 2 hours. Example 8 A sulphite pulp from spruce wood, which was bleached in one step with chlorine dioxide and neutralized with sodium • z. hydroxide had a viscosity of 1150 dm /kg according to SCAN, an extractive content of 0.42 according to SCAN and a brightness of 69 SCAN. The pulp had a solids content of 30 and was mixed with a diluted solution of sodium hypochlorite and sodium hydroxide to 10 pulp consistency and was dewatered to a solids content of 52 %. The dewatered pulp contained 0.7 % sodium hypochlorite, calculated as active chlorine, and 0.5 % sodium hydroxide counted on the dry weight of pulp. The pulp was shredded to flakes in a disc refiner and introduced into a modified flash drier containing a bleaching compartment and a drying compartment. On entering the bleaching compartment, the carrier steam tempera¬ ture was 120 C, corresponding to an excess pressure of 100 kPa. The residence time in the bleaching compartment was 8 seconds and in the drying compartment 12 seconds. The processed pulp had a solids content of 90.1 , a .viscosity of 1105 dm 5 Λg» an extract content of 0.42 and a brightness of 89.5 » It is apparent from the example that in accordance with the invention it is possible to bleach sulphite pulp in a very short time without notable degradation of the carbohydrates, in comparison with conventional tower bleaching, which would have required a residence time of several hours. Example 9 A semJrbleached pine sulphate pulp with a brightness of •z 76 SCAN and a viscosity of 945 dm /kg was mixed with DTPA, hydrogen peroxide, sodium hydroxide and water such that the pulp consistency was 8 %, the suspension then being thickened to a solids content of 45 » The dewatered pulp contained 0.8 hydrogen peroxide, 0.2 % DTPA and 0.6 % sodium hydroxide. The pulp was shredded to flakes in a disc refiner and was introduced into a modified flash drier containing a bleaching compartment and a drying compartment. The carrier steam temperature on entry into the bleaching department was 120 C, corresponding to an excess pressure of 100 kPa. The residence time in the bleaching compartment was 9 seconds and in the drying compartment 12 seconds. The treated pulp had a dry content of 91.3 % t a viscosit of 922 dm /kg and a brightness of 85 SCAN. The viscosity was thus surprisingly high, considering that the brightness was improved by as much as 9 units. The example shows that in accordance with the invention it is possible to bleach sulphate pulp without notable degradation of the carbohydrates in a very short time in comparison with conventional tower bleaching, which would have required a residence time of several hours. Example 10 Screened spruce sulphite pulp with a brightness of 62 •z SCAET, a viscosity of 1140 dm /kg and an extractive content of 1.88 SCAN was charged with sodium hydroxide and water so that the pulp consistency was 10 and was afterwards thickened to a dry content of 42 $£. The pulp contained 2 % sodium hydroxide. It was introduced into a modified flash drier in which the carrier was saturated steam with a temperature of 115 C, corresponding to an excess pressure of 69 kPa. The residence time in the modified drier was 12 seconds. After separating steam in a cyclone> the pulp was fed out via a rotary valve feeder to a storage tank, where it was diluted with hot water. The solids content of the pulp on exiting from the drier was 39.5 , its OMPl ^ - viscosity 1055 dm /kg and its extractive content was 0.38 SCAN. The example shows that with processing in accordance with the invention it is possible to effectively deresinate sulphite pulp in a very short time. Deresinating conventionally in a tower requires a time of at least one hour. Example 11 In this working example, bleaching under excess pressure in a closed processing apparatus, internally provided with a screw conveyor, was tried out. The turbulent flow of the pulp through the processing apparatus was provided mechanically in this ease. The screw conveyor was placed at the bottom of the processing apparatus, which was disposed horizontally in this case, the conveyor conveying the pulp at a rate of about 1 m/s through the apparatus to a pressure cyclone directly connected to the discharge end, this cyclone being in turn provided with a screw eeder for controlling the residence time of the pulp in the cyclone. In a thermo-mechanical pulp mill, spruce pulp with a solids content of 33 was taken out directly from the disc refiner in which the pulp had been defibrated, and was taken to a chemical mixer under the same excess pressure as the disc refiner, which was 150 kPa in this case. The following, calculated in percent by weight of dry pulp, were mixed into the pulp: 3 H 2 0 ≥ , 5 # Na 2 Si0 , 1.5 % NaOH, 0.02 % MgSO^ • 7 HgOg and 0.2 DTPA. After the bleaching chemicals had been mixed in, the pulp had a dry content of 32 % and a temperature of 110°C. The pulp mixed with bleaching chemicals was then transported by means of a high consistency pump for bleaching in the processing apparatus described above. The total residence time in the apparatus was calculated to about 6 seconds. The pulp was then taken from the apparatus to the pressure cyclone for separating steam, and the pulp separated from steam was allowed to fall down into the screw discharger. The time for passing through the cyclone and the screw discharger was estimated to be about 3 seconds. The pulp coming from the screw discharger had a tempera¬ ture of 96°C, a pulp consistency of 32 and contained 0.06 residual peroxide. After diluting with cold water to 4 : % pulp OMPI - consistency the pH of the pulp suspension thus obtained was measured and found to be 8.1 • The diluted pulp was then dewatered to a pulp concentration of about 30 in a centrifuge and was dried to a solids content of about 92.4 • The brightness of the pulp thus obtained was measured and found to be 74.3 % ISO, which must be regarded as surprisingly high in consideration of the short bleaching time, and the relatively simple bleaching installation which had been used. Example 12 The same bleaching apparatus as in Example 11 was used in this example, with the exception that the bleaching chemicals were not mixed in a special chemical mixer, but were supplied, in the thermo-mechanical pulp mill, to the disc refiner in which the chips were defibrated to pulp. The excess pressure in the disc refiner was 120 kPa on this occasion, and corresponded to a temperature of 123 C. The bleaching chemicals were added at different places along the radius of the grinding discs. 0.15 % DTPA and 3 Ξ O- were thus added close to the centre of the grinding discs, while 1.0 NaOH was added at a point halfway along the radius of the discs, and 3.0 % Na„Si0„ at a point about 5 cm from the outer edge of the disc. The chemical charges given relate to percent by weight of dry pulp. The pulp obtained on defibration in the disc refiner- was blown to a pressure cyclone connected to the processing apparatus, which was provided internally with a screw conveyor. During the passage through the pressure cyclone and the processin apparatus, the excess pressure was reduced from 120 kPa o 50 kPa and thereby the temperature was also reduced from 123 C'to 111°C. After a residence time of 4 seconds in the processing apparatus, the pulp was blown to a second cyclone for separating steam from the pulp. After passing through this cyclone, the temperature in the pulp was measured and found to be 95 C, and its content of residual peroxide to 0.14 • The pulp consistency was 36 %. After diluting with cold water to a pulp consistency of 4 , the pH of the pulp suspension was measured and found to be 8.2. The diluted pulp suspension was then dewatered to a pulp consistency of about 30 % in a centrifuge, and dried to a solids content of 91.8 %. " SURE OMPI WIPO The brightness of the pulp thus obtained was measured and found to be 74.6 $> ISO. In manufacturing thermo-mechanical pulp it is thus possible to already add the bleaching chemicals in the disc refiner, and in applying the method in accordance with the inven¬ tion to still obtain a surprisingly bright pulp in a short time, and with simple bleaching apparatus. The above examples show by way of conclusion that the process in accordance with the invention, applied to bleaching, enables bleaching of both mechanical and chemical pulp during the transport of the pulp together with steam under excess pressure and possible subsequent drying. Apart from the bleach¬ ing effect, excess steam is also obtained hereby, which can be utilized for different purposes, giving the process good heating economy. The bleaching chemical cost is also low with the method according to the invention. Applied to extraction, the possibility is obtained of carrying out an effective extraction reaction during a very short time, while the pulp is being conveyed by steam at excess pressure. In both types of reaction, the rapid reaction time further results in that the investment cost for apparatus will be low, and the need of buildings for setting up apparatus will be minimum. The use of high concentration in the method in accordance with the invention furthermore results in environmental advantages.

Claims: CLAIMS 1. A method of refining cellulose pulps by bleach ng or extraction, the pulps in a concentration of 10 - 65 and in a finely divided form being treated with chemicals at a temperature of 10 - 250°C, and dried, characterized in that the finely divided pulp, impregnated with chemicals and at a consistency of 30 - 65 ι is continuously fed into a processing apparatus having an excess system pressure of 5 - 400 kPa and an atmosphere substantially consisting of steam with an oxygen content of less than 1 by volume and a temperature of 100 - 150°Cj in that the pulp is rapidly moved, without alteration of the fibre characteristics by mechanical working, through the processing apparatus under a substantially turbulent flow and with substantially unaltered dry content; and in that charged chemicals are caused to react so completely with the pulp during its rapid movement through the processing apparatus that the chemicals are substantially consumed when the movement, is termina¬ ted, the pulp being then discharged from the processing apparatus and dried after possible further chemical treatment steps. 2. A method as claimed in claim 1, characterized in that the dry solids content of the pulp during passage through the processing apparatus is altered by at most 8 percentage units, preferably at most 6 percentage units. 3. A method as claimed in claims 1 - 2, characterized in that the pulp, before entering the processing apparatus, is subjected to dewatering to a pulp consistency of over 45 % for removing chemicals, and is transformed into flake form. 4. A method as claimed in claim 1 , characterized in that the pulp is bleached with bleaching chemicals at an excess pressure of 100 - 200 kPa, in that the steam atmosphere is provided by supplying steam; and in that steam is removed from the pulp after it has been discharged from the processing appara¬ tus, the steam then being recycled to said apparatus. 5. A method as claimed in claim 4j characterized in that the cellulose pulp is a high-yield pulp, and in that the bleach- ing chemical is a peroxide or a dithionite. 6. A method as claimed in claims 4 - 5, characterized in that before entry into the processing apparatus the pulp is ' UR£ϊ JOMPΓ wipc impregnated with a complexing agent, dewatered to a consistency of over 10 , preferably 15 - 35 %, impregnated with bleaching solution, pressed and transformed into flake form. 7. A method as claimed in claims 1 - 3» characterized in that the pulp is treated with an alkali solution at an excess pressure of 50 - 300 kPa, preferably 100 - 200 kPa, in that the steam atmosphere is provided by supplying steam, and in that steam is separated from the pulp after the latter has been dis¬ charged from the processing apparatus, said steam being returned to said apparatus, the processed pulp being subsequently washed and dried after possible bleaching. 8. A method as claimed in claim 7, characterized in that a sulphite pulp is extracted with sodium hydroxide. . 9. A method as claimed in claims 1 - 8, characterized in that the drying is carried out in a flash drier. 10. A method as claimed in claim 9, characterized in that the drying in the flash drier is carried out with superheated steam (so-called counter-pressure drying). 11. A method as claimed in claim 10, characterized in that excess steam from drying is recycled to the treatment with chemicals. OMPI ' W,PO ,κ\

Inventors: LINDAHL J, SVENSSON G
Applicants: MODO CHEMETICS AB
Application year: 1979
Main classification: D21C9
IPC codes: D21C9