OXYGEN DELIGNIFICATION OF PULP IN THE PRESENCE OF CARBONATES AS ALKALI

Abstract: A process for oxygen delignification of chemical pulp comprising an oxygen treatment unit operation stage in which the pulp is treated while suspended in an aqueous solution with oxygen in the presence of alkali, whereby the pulp consistency is less than 20%; more than 50 percentage by weight of the alkali consist of one or more carbonates; and the pH of the aqueous solution during the process is adapted to provide for the essential absence of bicarbonate in said solution during the treatment. Furthermore a process for the bleaching of chemical pulp comprising a peroxide bleaching unit operation stage and an oxygen delignification process of said kind.

Description: OXYGEN DELIGNIFICATION OF PULP IN THE PRESENCE OF CARBONATES AS ALKALI The present invention relates to a process for pulp deligniflcation and/or pulp bleaching, in which chemical pulp is treated while suspended in an aqueous solution with oxygen in the presence of alkali. Background of the invention TCF (= Totally Chlorine Free) deligniflcation and bleaching processes have become more common over time. These processes are distinguished by the fact that essentially no chlorine-containing chemicals are added during the bleaching sequences. A prerequisite for successful TCF delignification/bleaching is a lower lignin content in the pulp fed into the bleaching department of the pulp mill, i.e. a lower Kappa number, than previous bleaching processes, as the chlorine free bleaching chemicals are less specific with regard to the removal or modification of the coloured components in chemical pulp than, for instance, chlorine or chlorine dioxide. In other words, TCF bleaching chemicals have a tendency to affect also the cellulosic fibres. Previously, these lower Kappa numbers were accomplished in the cooking section of the pulping process. Lately, however, the trend in the art has been to use one or more oxygen deligniflcation stages to achieve the required lower Kappa numbers. Conventional oxygen deligniflcation processes are, however, associated with a few problems. As the degree of deligniflcation increases, the relative speed of deligniflcation decreases and the carbohydrate degradation increases. Alkali, usually sodium hydroxide, is used in the oxygen deligniflcation processes in order to dissolve the oxygen treated coloured components in the chemical pulp. Sodium carbonate is also mentioned in prior art documents as a plausible alkali. A higher alkali charge will provide for improved dissolution of the coloured components, i.e. lower Kappa numbers, but unfortunately also to hydrolysis of the fi- bres, causing loss of pulp viscosity. The problem of fibre degradation in oxygen deligniflcation processes has been touched upon in prior art documents. SE 393 138 discloses an oxygen deligniflcation process in which sodium bicarbonate is used instead of sodium carbonate. This is indicated to give higher viscosity and carbohydrate yield at a given lignin content than when sodium carbonate is used. An important prerequisite for this is, however, that the partial pressure of carbon dioxide during the deligniflcation process is controlled within specified limits, else either the deligniflcation speed or the selectivity will fall outside of acceptable limits. FR 2 670 513 relates to an oxygen deligniflcation process in which a mix of sodium hydroxide and sodium carbonate is used as alkali. It is indicated that the sodium carbon- ate charge should be half the charge, at the most, of the sodium hydroxide. By this process any need for protective agents, such as magnesium salts, is said to be eliminated. However, this process has to be run in two different oxygen treatment unit operation stages. SE 450 394 discloses an oxygen deligniflcation process aimed at the problem of depolymerisation of carbohydrates. This process consists of a number of unit operation stages, in which the pulp is treated with, apart from oxygen, nitrogen dioxide and optionally nitric acid. SE 219 677 discloses a process for pulp purification by oxygen deligniflcation, which is suggested to provide for suppression of fibre degradation. Firstly, the pulp is imbibed by an aqueous 0.5-4.0% solution of sodium carbonate or a 0.1-2.0% solution of sodium hydroxide. It is indicated that when sodium carbonate and sodium hydroxide are used in similar concentrations, the extent of removal of lignin, resins and pentosan from sulphate pulp is similar. Secondly, i.e. after the imbibment, the pulp is compressed to remove excess liquid to provide wet pulp with a consistency of about 20-66.7%. Thirdly, the compressed pulp is dis- integrated to make it porous, after which, fourthly, the disintegrated pulp is treated with oxygen-containing gas under pressure and at an elevated temperature below 100°C. US-A-4,115,186 discloses a process for bleaching cellulose pulp with molecular oxygen, which process comprises four treatments, that are carried out one after another. SE-B-364,322 discloses a process for bleacing of wood pulp. The process com- prises two treatments, that are carried out one after another. SE 9604217-1 discloses a method and apparatus for delignifying chemical pulp. The method comprises at least two separate treatments. Thus, all prior art processes to overcome the basic problem of fibre degradation in oxygen deligniflcation processes are either complicated by a multitude of process stages, by critical parameter limits, by the use of hazardous substances such as nitrogen dioxide or nitric acid, or by the use of energy consuming and fibre damaging operations such as compression and disintegration of pulp. Accordingly, the problem to be solved is thus to provide a process for oxygen deligniflcation of chemical pulp in the presence of alkali, which is less or, preferably, not at all burdened by the inconveniences associated with prior art. This problem has been solved by means of the present invention, which involves a process for pulp deligniflcation as defined by the appended claims. More particularly, the present invention relates to a process for oxygen deligniflcation of chemical pulp, which process comprises an oxygen treatment unit operation stage in which chemical pulp is treated while suspended in an aqueous solution with oxygen in the presence of alkali, whereby the pulp consistency is less than 20%, preferably from about 3 to 15%, in particular from about 10 to 12%; the alkali is preferably used, as added to the solution, in an amount of from about 5 percent by weight, preferably from about 5 to 50 percent by weight, and in particular from about 10 to 30 per by weight, calculated on the weight of said material, whereby more than about 50 percent by weight, preferably from about 90 to 100 percent by weight of said alkali consist of one or more carbonates; and the pH of the aqueous solution is adapted to provide for the essential absence of bicarbonate in said solution during the process. Depending upon various product and/or process properties, such as the type of pulp under treatment, desired Kappa number, acceptable treatment period etc., the artisan may chose a suitable set of parameter values within the ranges indicated above, if desired (or even required) based on some trial-and-error experimentation. In this context the term "unit operation stage" indicates an essentially self-contained process, in which basically all, or at least the chemical treatments, that are required to achieve the desired effect, are performed substantially contemporaneously. The term "oxygen" is intended to comprise elementary oxygen, molecular oxygen, air, hydrogen peroxide, perborate, ozone, and any other oxygen carrying means or oxygen carrying compound conventionally used for the purpose of pulp deligniflcation; elementary oxygen and molecular oxygen, in particular molecular oxygen, is however preferred. The term "alkali" is intended to comprise hydroxides and carbonates of Li, Na, K, Rb, Cs, Ca, NH , and calcium oxide. Oxygen deligniflcation according to present invention to given Kappa numbers provides for better pulp yield than oxygen deligniflcation according to prior art, even when performed at higher temperature. When performed at similar deligniflcation temperatures, the process according to the present invention usually also results in a better final viscosity than obtained with prior art. Apart from solving the problems indicated above, it has also surprisingly been found that it is possible to run the present oxygen deligniflcation process at comparatively high temperatures, and thereby achieve higher deligniflcation speed or rate, with essentially no corresponding diminution of the pulp yield or the pulp viscosity. Carbonates are advantageous, in the present context, in that they have a big buffer capacity, providing for a comparably low and even alkali profile during the oxygen deligniflcation process. The use of carbonates as alkali is furthermore advantageous in that carbonates are available in large amounts in the pulp mill. Conventional green liquor may be used in the present unit operation stage, either as such or after some suitable pre-treatment. Previously, the sodium carbonate in the green liquor has been converted into sodium hydroxide in a caus- ticizing section. The present invention provides for an option to eliminate the need for such conversion, and thus also for such conversion itself. Various recovery methods can be used in order to provide the carbonate to be used in the present oxygen deligniflcation process. Exemplary of such recovery methods are those according to the so-called black liquor gasification technology, a recent recovery technology in which the recovered chemicals are provided in separate streams, in particular one stream of sulphur-rich liquor and one stream of sulphur- lean liquor, from which sodium carbonate easily can be obtained with greater ease, and in purer form, than from conventional recovery methods, thus making black liquor gasification well suited for the purpose of the present invention. Black liquor gasification is described by e.g. Dahlquist, E., Jacobs, R. in "Development of a Dry Black Liquor Gasification Process", 1992 International Chemical Recovery Conference Proceedings, CPPA and TAPPI, June 7- 11, Seattle, WA, USA, p.457, and by Stigsson, L.L., Hesseborn, B., in "Gasification of Black Liquor" 1995 International Chemical Recovery Conference Proceedings, CPPA and TAPPI, April 24-27, Toronto, Ontario, Canada, p.B 277 The carbonate(s) may be any suitable alkaline carbonate, but is/are preferably chosen among Li, Na, K, Rb, Cs, Ca, and ammonium carbonates, in particular Na and K carbonates, and combinations thereof. In a preferred embodiment essentially all of the alkali consists of one or more carbonates. It is in particular preferred to use carbonates in the absence of sodium and potassium hydroxides, as tests have shown better results regarding final viscosity as well as pulp yield when using only carbonate, than when using carbonate in combination with such a hydroxide. Apart from other alkali, the carbonate(s) may be used in combination with one or more additional deligniflcation or stabilising chemicals, such as for instance magnesium salts, which is common in the art. The oxygen treatment unit operation stage may according to one embodiment be an OP-stage, i.e. basically an oxygen stage supplied with a small charge of hydrogen peroxide, or a PO-stage, i.e. basically a hydrogen peroxide stage supplied with a small charge of oxygen. The oxygen deligniflcation process is preferably carried out at a temperature of about 80 - 160°C, in particular 90 - 130°C. During the process, the pressure is preferably about 0.3 - 2 MPa, suitably 0,5 - 0,8 MPa. The pH of the aqueous solution during the process is preferably above about 9, specifically from about above 10. The present invention also relates to a process for the bleaching of chemical pulp which comprises the present process for oxygen deligniflcation of chemical pulp, i.e. a process comprising an oxygen treatment unit operation stage in which chemical pulp is treated while suspended in an aqueous solution with oxygen in the presence of alkali, whereby the pulp consistency is less than about 20%, preferably from about 3 to 15%, in particular from about 10 to 12%), whereby more than about 50 percentage by weight, preferably from about 90 to 100 percent by weight of the alkali consist of one or more carbonates; and the pH of the aqueous solution is adapted to provide for the essential absence of bicarbonate in said solution during the process. The delignification/bleaching process according to the present invention may comprise a peroxide bleaching unit operation stage, in which the pulp is preferably treated while suspended in an aqueous solution containing alkali, whereby more than 50 percentage by weight of the alkali consist of one or more carbonates. Said bleaching process may comprise one or more additional bleach- ing/delignification stages, such as for instance peroxide treatment stages (commonly denoted P-stages), conventional oxygen treatment stages (commonly denoted O-stages), boronhydrid treatment stages (B-stages), sequestering agent treatment stages (commonly denoted Q- stages), further alkali extraction stages (commonly denoted E-stages), even chlorine treatment stages such as hypochlorite treatment stages (commonly denoted H-stages), chlorine dioxide treatment stages (commonly denoted D-stages), and chlorination stages (commonly denoted C-stages), although such chlorine treatment stages are absent in the preferred embodiments of the present invention. It should be noted that the bleaching/delignification process according the present invention may also comprise a number of oxygen treatment unit operation stages according to the invention, which unit operation stages may be arranged consecutively or in parallel, as well as being positioned separately at various locations in the bleaching/delignification process without any direct connections between said unit operation stages. The process may also comprise a combination of a number of consecutive and/or parallel unit operation stages and separate unit operation stages. Oxygen deligniflcation according to the present invention may be applied in combination with various, per se known, pulp mill processes, e.g. sulphate processes, such as ITC or EMCC processes (ITC = Isothermal Cooking; EMCC = Extended Modified Continu- ous Cooking) and polysulphide processes, and sulphur-free processes, such as soda-AQ processes, as well as sulphite processes. ITC is a modified sulphate cooking process, in which the alkali charge is divided and the alkali profile levelled out; an ITC-process comprises one counter-current and one concurrent zone and is carried out at low cooking temperatures, usu- ally about 160°C, and with long cooking times. ITC-processes are described in US-A- 5,470,437 and US-A-5, 567,280. Polysulphide processes are described in, for instance, US-A- 3,874,991 (Kleppe). Soda-AQ processes are essentially sulphur- free cooking processes carried out at about 160-170°C with charges of NaOH and anthraquinone (= AQ); this type of process is advantageous in that it is odourless. Soda-AQ processes are described by e.g. Gratzl, J. S. "The reaction mechanisms of AQ in alkaline pulping", Eucepa Symp. June 1980 Helsinki paper 12, 27pp. In a preferred embodiment of the present invention no elementary halogen or gaseous halogen compound is added during the pulp mill process. In a specific embodiment, the process is an ECF (ECF = Elementary Chlorine Free) process, in which essentially no elementary chlorine is added during the process, and in another embodiment the process is a TCF process, in which no or essentially no chlorine compound is added during the process. The present invention will be further described below with reference to a few examples, the purpose of which are merely to illustrate the invention, and which should not be regarded as limiting the scope of the present invention; this scope is defined solely by the ap- pended claims. Examples Example I: Sulphate pulp from isothermal cooking of softwood was subjected to oxygen deligniflcation according to the present invention, using carbonate and carbonate- containing compositions as alkali, as well as using sodium hydroxide alkali in a reference experiment. The pulp had an initial Kappa number of 22.8, a brightness of 27.6%> ISO and an initial viscosity of 1090 ml/g. All experiments followed the following, general procedure: Moist or wet pulp corresponding to 25 g dry substance was slushed in about 2 1 of water in a pulper at 3000 rpm. The pulp was dewatered on a wire cloth screen, and the filtrate was brought through the filter cake a second time in order to minimise the loss of fines. Then the pulp was weighed in order to calculate the contents of liquid, after which the pulp was shredded by hand. The shredded pulp was transferred to a polyethylene bag, and alkali, water, and an amount of an aqueous solution magnesium sulphate corresponding to 0.1 % Mg 2+ , based on dry pulp, were added to the pulp in the bag. The alkali was added as 1 M solutions of NaOH and Na 2 CO 3 , respec- tively. After having sealed the bag by means of welding, the bag was subjected to shaking for about one minute in a shaker apparatus in order to distribute the alkali, water, and the magnesium sulphate solution all through the pulp. The complete quantity of obtained pulp suspension, having a pulp consistency of 12%, was then brought into a steel autoclave lined with polytetrafluoroethylene (TEFLON*). After having sealed the autoclave, oxygen was introduced from a gas bottle at a positive pressure of 0.7 MPa into the autoclave through a valve in the autoclave lid. The deligniflcation reaction was started by submerging the autoclave into a heated glycol bath, in which the autoclave was rotated for different periods of time according to each individual experiment. Quenching of the deligniflcation reaction was carried out by submerging the autoclave into cold water. The lid valve was opened, thus releasing the autoclave from the oxygen pressure, whereupon the lid was removed to unseal the autoclave, and the pulp was washed on a wire cloth screen according to a feedback procedure, in which the pulp is slushed in about 1 1 of deionised water, the pulp is dewatered on a wire, the filtrate recirculated through the dewatered pulp in order to minimise the loss of fines, whereafter 2 doses of 0.5 1 water is passed through the pulp, the filtrate from which is discarded; this feedback procedure is repeated three times. Details and results of experiments # 0-3 are set forth in Table I below. Table I Experiments Accordingly, oxygen deligniflcation to essentially similar Kappa numbers, about 12, using the process according to the present invention produced better pulp yield than oxygen deligniflcation according to prior art, even when performed at higher temperature. When performed at similar deligniflcation temperatures, the process according to the present invention also resulted in a higher final viscosity than obtained with prior art. Example II: The experiments in this Example were carried out as described in Example I, except that the pulp had an initial Kappa number of 26, the pulp suspension had a consistency of 10%, and that the O-stage was followed by a Q-stage in which the pulp suspension was treated with 0,27% DTPA at 90°C for 60 minutes, and thereafter in an OP-stage with oxygen at 0.7 MPa, 0.1% Mg, and 1.5% hydrogenperoxid. Details and results of experiments # 4-5 are set forth in Table II below. Table II Experiments Accordingly, deligniflcation sequences to essentially similar Kappa numbers, about 6, using the process according to the present invention produced better pulp yield than deligniflcation according to prior art. Example III: The experiments in this Example were carried out as described in Example I, except that in experiment #8 potassium carbonate was used as alkali. Details and results of experiments # 6-8 are set forth in Table III below. Table III Experiments Accordingly, oxygen deligniflcation to essentially similar Kappa numbers, about 10, using the process according to the present invention produced better pulp yield than oxygen deligniflcation according to prior art. Example IV: The experiments in this Example were carried out as described in Example I, except that soda-AQ pulp, having an initial Kappa number of 20, a brightness of 33.8%o ISO and an initial viscosity of 902 ml/g was used in experiments #9-10, and liner pulp, having an initial Kappa number of 97.4 and a brightness of 11.39% ISO was used in experiments #11-12. Details and results of experiments # 9-12 are set forth in Table IV below. Table IV Experiments Accordingly, oxygen deligniflcation to essentially similar Kappa numbers using the process according to the present invention produced higher pulp yield than oxygen deligniflcation according to prior art, irrespective of pulp type.

Claims: Claims 1. A process for oxygen deligniflcation of chemical pulp, which process comprises an oxygen treatment unit operation stage in which chemical pulp is treated while suspended in an aqueous solution with oxygen, preferably molecular oxygen, in the presence of alkali, characterised in that the pulp consistency is less than 20%; more than 50 percentage by weight of the alkali consist of one or more carbonates; and the pH of the aqueous solution during the process is adapted to provide for the essential absence of bicarbonate in said solution during the treatment. 2. A process according to claim 1 , characterised in that essentially all of the alkali consists sodium carbonate, potassium carbonate, or a combination thereof. 3. A process according to claim 1, characterised in that essentially no elementary halogen or gaseous halogen compound is added to the chemical pulp during the process. 4. A process according to claim 1, characterised in that the chemical pulp is treated with oxygen at a temperature of about 90-130°C. 5. A process according to claim 1, characterised in that the pressure during the oxygen treatment is from about 0.5 to about 0.8 MPa. 6. A process according to claim 1, characterised in that the pH of the aqueous solution during the process is above about 10. 7. A process according to claim 1 , characterised in that the treated pulp originates from a soda process, such as a soda-AQ process, or a sulphate process, such as an ITC-process or a polysulphide process, or a sulphite process. 8. A process according to claim 1, characterised in that at least part of the alkali originates from green liquor and/or black liquor, preferably from an alkali recovery process, such as an alkali recovery process based on black liquor gasification technology. 9. A process for the bleaching of chemical pulp, which process comprises a peroxide bleaching unit operation stage, characterised in that it comprises an oxygen deligniflcation process according to any preceding claim. 10. A process according to claim 9, characterised in that the pulp is treated in the peroxide deligniflcation unit operation stage while suspended in an aqueous solution containing alkali, whereby more than 50 percentage by weight of the alkali consist of one or more carbonates. AMENDED CLAIMS [received by the International Bureau on 04 July 2000 (04.07.00); original claim 5 replaced by new claim and remaining claims unchanged (1 page)] 1. A process for oxygen deligniflcation of chemical pulp, which process comprises an oxygen treatment unit operation stage in which chemical pulp is treated while suspended in an aqueous solution with oxygen, preferabh' molecular oxygen, in the presence of alkali, characterised in that the pulp consistency is less than 20° o: more than 50 percentage by weight of the alkali consist of one or more carbonates: and the pH of the aqueous solution during the process is adapted to provide for the essential absence of bicarbonate in said solution during the treatment. 2. A process according to claim 1. characterised in that essentially all of the alkali consists sodium carbonate, potassium carbonate, or a combination thereof. 3. A process according to claim 1. characterised in that essentially no elementary halogen or gaseous halogen compound is added to the chemical pulp during the process. 4. A process according to claim 1. characterised in that the chemical pulp is treated with oxygen at a temperature of about 90 - 130°C. 5. A process according to claim 1, characterised in that the pressure during the oxygen treatment is from about 0.3 to about 2 MPa. 6. A process according to claim 1 , characterised in that the pH of the aqueous solution during the process is above about 10. 7. A process according to claim 1, characterised in that the treated pulp originates from a soda process, such as a soda-AQ process, or a sulphate process, such as an ITC-process or a polysulphide process, or a sulphite process. 8. A process according to claim 1 , characterised in that at least part of the alkali originates from green liquor and/or black liquor, preferably from an alkali recovery process, such as an alkali recover}' process based on black liquor gasification technology. 9. A process for the bleaching of chemical pulp, which process comprises a peroxide bleaching unit operation stage, characterised in that it comprises an oxygen deligniflcation process according to any preceding claim. 10. A process according to claim 9, characterised in that the pulp is treated in the peroxide deligniflcation unit operation stage while suspended in an aqueous solution containing alkali, whereby more than 50 percentage by weight of the alkali consist of one or more carbonates.

Inventors: BERTHOLD FREDRIK, LINDSTROEM MIKAEL, TORMUND DISA, BERTHOLD, FREDRIK, LINDSTROEM, MIKAEL, TORMUND, DISA
Applicants: BERTHOLD FREDRIK, LINDSTROEM MIKAEL, SVENSKA TRAEFORSKNINGSINST, TORMUND DISA, BERTHOLD, FREDRIK, LINDSTROEM, MIKAEL, STFI, TORMUND, DISA
Application year: 2000
Main classification: D21C9
IPC codes: D21C9