Background of the invention TCF (= Totally Chlorine Free) delignification and bleaching processes have be- come 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 sec- tion of the pulping process. Lately, however, the trend in the art has been to use one or more oxygen delignification stages to achieve the required lower Kappa numbers.

Conventional oxygen delignification processes are, however, associated with a few problems. As the degree of delignification increases, the relative speed of delignification decreases and the carbohydrate degradation increases. Alkali, usually sodium hydroxide, is used in the oxygen delignification 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 col- oured 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 delignifica- tion processes has been touched upon in prior art documents.

SE 393 138 discloses an oxygen delignification process in which sodium bicar- bonate is used instead of sodium carbonate. This is indicated to give higher viscosity and car- bohydrate 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 delig- nification process is controlled within specified limits, else either the delignification speed or the selectivity will fall outside of acceptable limits.

FR 2 670 513 relates to an oxygen delignification process in which a mix of so- dium 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 delignification 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 delignification, 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 hy- droxide. 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 oxy- gen-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 delignification 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 com- pression and disintegration of pulp.

Accordingly, the problem to be solved is thus to provide a process for oxygen delignification 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 delignification as defined by the appended claims. More particularly, the present invention relates to a process for oxygen delignification of chemical pulp, which pro- cess 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 pa- rameter 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-con- tained 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 per- oxide, perborate, ozone, and any other oxygen carrying means or oxygen carrying compound conventionally used for the purpose of pulp delignification; 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, NH4, and calcium oxide.

Oxygen delignification according to present invention to given Kappa numbers provides for better pulp yield than oxygen delignification according to prior art, even when performed at higher temperature.

When performed at similar delignification 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 delignification process at comparatively high temperatures, and thereby achieve higher delignification 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 delignification 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 delignification process. Ex- emplary of such recovery methods are those according to the so-called black liquor gasifica- tion 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 car- bonates, and combinations thereof. In a preferred embodiment essentially all of the alkali con- sists 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 delignification 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 perox- ide, or a PO-stage, i. e. basically a hydrogen peroxide stage supplied with a small charge of oxygen.

The oxygen delignification 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 delignification 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 0-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 pres- ent invention may also comprise a number of oxygen treatment unit operation stages accord- ing 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 delignification 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 proc- esses, 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 car- ried 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 delignification 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 % Mg2+, based on dry pulp, were added to the pulp in the bag. The alkali was added as 1 M solutions of NaOH and Na2CO3, 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 magne- sium sulphate solution all through the pulp. The complete quantity of obtained pulp suspen- sion, having a pulp consistency of 12%, was then brought into a steel autoclave lined with polytetrafluoroethylene (TEFLONt). After having sealed the autoclave, oxygen was intro- duced from a gas bottle at a positive pressure of 0.7 MPa into the autoclave through a valve in the autoclave lid. The delignification 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 delignification reaction was carried out by submerging the autoclave into cold water. The lid valve was opened, thus releasing the auto- clave 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 feed- back procedure is repeated three times. Details and results of experiments # 0-3 are set forth in Table I below.

Table I Experiments ITC pulp # 0 # 1 # 2 # 3 (control) Added amount of chemicals, based on 2.5 wt-% 10 wt-% 1.25 wt-% 10 wt-% dry pulp NaOH Na2CO3 NaOH Na2CO3 2.5 wt-% Na2CO3 Added amount of 1 M Na2CO3 solu-23.58 5. 90 23.58 tion, ml Added amount of 1 M NaOH solution, 15.63 7. 83 ml Delignification temperature, °C 100 100 100 130 Treatment time in autoclave, min. 60 90 90 25 Final pH (at 25°C) 11. 9 10. 1 10. 1 10. 0 Final brightness, % ISO 38. 8 38. 9 39. 9 39. 1 Degree of delignification, % 47. 8 44. 7 50. 0 46. 9 Final Kappa number 11.9 12.6 11.4 12.1 Final viscosity, ml/g 912 941 924 907 Pulp yield, based on pulp weight-% 97.3 98.2 97. 6 97.6

Accordingly, oxygen delignification to essentially similar Kappa numbers, about 12, using the process according to the present invention produced better pulp yield than oxy- gen delignification according to prior art, even when performed at higher temperature. When performed at similar delignification temperatures, the process according to the present inven- tion 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 0-stage was followed by a Q-stage in which the pulp sus- pension 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 experi- ments # 4-5 are set forth in Table II below.

Table II Experiments

#4 #5 ITC pulp (control) Added amount of alkali in 0-stage, based 2 wt-% NaOH 10 wt-% Na2CO3 on dry pulp Added amount of alkali in OP-stage, based 2 wt-% NaOH 10 wt-% Na2C03 on dry pulp Delignification temperature, °C 100 100 Final brightness, % ISO 75. 8 72. 3 Final Kappa number 5.9 6.3 Final viscosity, ml/g 920 915 Pulp yield, based on pulp weight-% 95.4 96.5 Accordingly, delignification sequences to essentially similar Kappa numbers, about 6, using the process according to the present invention produced better pulp yield than delignification 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 #6 #7 #8 ITC pulp (control) Added amount of chemicals, based on 2.5wt-% NaOH 10wt-% 10wt-% K2C03 dry pulp Na2CO3 Added amount of 1 M Na2CO3 or 23.58 18.09 K2CO3 solution, ml Added amount of 1 M NaOH solution, 15.63 ml Delignification temperature, °C 100 100 100 Treatment time in autoclave, min. 120 160 160 Final pH (at 25°C) 11. 3 9. 9 9. 8 Final brightness, % ISO 43. 5 44. 0 43. 3 Degree of delignification, % 57.5 57.9 56.1 Final Kappa number 9.7 9.6 10.0 Final viscosity, ml/g 878 895 904 Pulp yield, based on pulp weight-% 96.1 96.8 96.8

Accordingly, oxygen delignification to essentially similar Kappa numbers, about 10, using the process according to the present invention produced better pulp yield than oxy- gen delignification 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% 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 17.39% ISO was used in experi- ments #11-12. Details and results of experiments # 9-12 are set forth in Table IV below.

Table IV Experiments #9 #10 #11 #12 (control) (control) Pulp Soda-AQ Soda-AQ Liner Liner Added amount of chemicals, 2 wt-% 10 wt-% 6.5 wt-% 30 wt-% based on dry pulp NaOH Na2CO3 NaOH Na2CO3 Added amount of 1 M Na2CO3 23. 58 70.75 solution, ml Added amount of 1 M NaOH so-12.50 40. 63 lution, ml Delignificationtemperature, °C 100 100 110 110 Treatment time in autoclave, min. 50 60 75 120 FinalpH 11. 7 10. 1 11. 8 9. 7 Final brightness, % ISO 41. 7 40. 6 21. 2 20. 6 Degree of delignification, % 34.9 Final Kappa number 9. 8 10. 5 61.1 63.4 Final viscosity, ml/g 823 838 Pulp yield, based on pulp weight-98.2 99.2 90.1 91.4 %

Accordingly, oxygen delignification to essentially similar Kappa numbers using the process according to the present invention produced higher pulp yield than oxygen delig- nification according to prior art, irrespective of pulp type.