Method and arrangement for oxidizing white liquor

Abstract

A method and arrangement for oxidizing white liquor (1) is disclosed. The method may comprise feeding oxygen-containing gas (2) into a reactor (3) comprising the white liquor, wherein the oxygen-containing gas is fed into the reactor through a plurality of elements (4) formed at least partially of a porous material.

Description

[0001] METHOD AND ARRANGEMENT FOR OXIDIZING WHITE LIQUOR

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to a method and arrangement for oxidi zing white liquor .

[0004] BACKGROUND

[0005] White liquor is a strongly alkaline aqueous solution containing mainly sodium hydroxide (NaOH) and sodium sulfide (Na2S ) . It is used in Kraft pulping processes in cooking, in which lignin and hemicellulose are separated from cellulose fibers for the production of pulp . In order to make kraft pulping economically feasible , the cooking chemicals have to be recovered, which is accomplished by the chemical recovery process . After cooking of the pulp, the resulting black liquor containing the cooking chemicals and lignin is converted back to white liquor in the chemical recovery process .

[0006] A portion of the white liquor derived from the chemical recovery process may be diverted from the chemical recovery process and used for other purposes , e . g . in the bleaching of pulp for adj usting pH, as it is a source of NaOH, or for maintaining the NaOH inventory of the pulping process . For such purposes , Na2S present in the white liquor may not be desirable (unlike in cooking) , because in the bleaching it may cause unwanted reactions with cellulosic fibers that are being bleached . To this end, the Na2S in the portion of the white liquor that is diverted from the chemical recovery process for other purposes may need to be oxidized into Na2SO4 , which is inert towards cellulosic fibers .

[0007] With effective oxidation, the white liquor used in bleaching will not cause unwanted side effects , such as degradation of cellulose fibers , or consumption of H2O2 to unwanted side reactions . It i s typically des irable to have the oxidation proceed to completion, because Na2S20s produced as an intermediate in the reactions involved is highly corrosive .

[0008] I f the white liquor cannot be oxidized efficiently for whatever reason, it may not be available for said other purposes , and NaOH may have to be purchased from outside the chemical recovery process , which is costly and may increase the NaOH balance of the pulping process .

[0009] SUMMARY

[0010] A method for oxidi zing white liquor is disclosed . The method may comprise feeding oxygen-con- taining gas into a reactor comprising the white liquor, wherein the oxygen-containing gas is fed into the reactor through a plurality of elements formed at least partially of a porous material .

[0011] BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The accompanying drawings , which are included to provide a further understanding of the embodiments and constitute a part of this specification, illustrate various embodiments . In the drawings :

[0013] Figure 1A illustrates an embodiment of a method and an arrangement for oxidi zing white liquor ;

[0014] Figure IB illustrates another embodiment of a method and an arrangement for oxidi zing white liquor ;

[0015] Figure 2A illustrates schematically another embodiment of the method and arrangement ;

[0016] Figure 2B illustrates schematically the embodiment of the method and arrangement of Fig . 2A in top view;

[0017] Figure 3A illustrates an exemplary embodiment of an element in side view;

[0018] Figure 3B shows a cross-section of the exemplary embodiment of the element of Fig . 3A; Figure 3C shows a cross-section of a hori zontal branch and the arrangement of elements thereon ;

[0019] Figure 4A illustrates an embodiment of a radial flow impeller for stirring white liquor in a reactor ; and

[0020] Figure 4B further illustrates a cross-section of one of the rotor blades of the radial f low impel ler shown in Fig . 4A .

[0021] DETAILED DESCRIPTION

[0022] A method for oxidi zing white liquor is disclosed .

[0023] The method may comprise feeding oxygen-con- taining gas into a reactor comprising the white liquor . The oxygen-containing gas may be fed into the reactor through a plurality of elements formed at least partially of a porous material .

[0024] An arrangement for oxidi zing white liquor is disclosed .

[0025] The arrangement may comprise a reactor for oxidi zing the white liquor, and a plurality of elements formed at least partially of a porous material configured to feed an oxy- gen-containing gas through the elements to the reactor .

[0026] The oxidation of the white liquor may be done in order to oxidi ze Na2S at least partially into Na2SO4 . The oxidation may be at least theoretically considered to proceed according to the following equations :

[0027] 1 ) Na2S + H2O NaSH + NaOH [ sodium sulfide is dissociated in the white liquor into sodium hydrosulfide ]

[0028] 2 ) 2NaSH + 2O2 Na2S20s + H2O + heat [ oxidation reaction into sodium thiosulfate ]

[0029] 3 ) Na2S20s + O2 + 2NaOH 2Na2SOs + H2O + heat [ oxidation further into sodium sulfite ]

[0030] 4 ) 2Na2SOs + O2 2Na2SO4 + heat [ oxidation further into sodium sulfate ] 5) Na2S + 202 Na2SO4 + 3 heat [overall reaction for complete oxidation]

[0031] The actual reactions occurring during the oxidation are likely to be more complex and may include e.g. Na2S2 as an intermediate.

[0032] Na2S20s as an intermediate product can be highly corrosive, which is undesirable. Therefore it may be beneficial for the oxidation to proceed to Na2SO4 in a manner as complete as possible.

[0033] As indicated by the equations, the oxidation reaction is exothermic, i.e. it produces significant amounts of thermal energy. As a rise in temperature will slow down the oxidation reactions, the reactor may be equipped with cooling equipment.

[0034] The temperature of the white liquor in the reactor may be maintained e.g. at a temperature in the range of about 100 - 120°C. The cooling equipment may be configured to maintain the temperature of the white liquor in the reactor e.g. at a temperature in the range of about 100 - 120°C.

[0035] The efficiency of the reaction may be calculated as the proportion of Na2S originally present in the white liquor that is oxidized into Na2SO4, for example as the percentage of the moles of the Na2S that is oxidized into Na2SO4 of the moles of the Na2S originally present in the white liquor.

[0036] The white liquor may be oxidized to such an extent that it comprises e.g. at most 1.5 g / 1, or at most 1 g / 1 of Na2S. The concentration of the Na2S may be measured e.g. by the standard SFS-EN ISO-10304. Such a quality is well suited e.g. for adjusting pH in peroxide bleaching stages. If the concentration of the Na2S is higher, the white liquor may still be used e.g. for adjusting pH in oxygen delignification stages.

[0037] In order for the oxidizing reaction to proceed efficiently, the reaction should preferably be in a stoichiometric balance. With the method and arrangement, it may be possible to feed the oxygen-containing gas into the white liquor as small bubbles, which may be retained in the white liquor for providing oxygen to the reactions efficiently. The coalescence of the gas bubbles may be reduced .

[0038] The oxidation efficiency of the reactor may thereby be increased, in some situations even doubled.

[0039] The elements may be capable of withstanding the relatively harsh conditions in the reactor, including high pH, temperature and pressure.

[0040] Any embodiments described in this specification may be considered to be disclosed both in the context of the method and the arrangement.

[0041] The elements may be formed at least partially of a porous metal, or a porous ceramic material, porous plastic fiber material, porous glass material, and / or porous fiberglass material. The porous metal may be e.g. sintered metal. The metal may be e.g. stainless steel. Examples of suitable metal / steel materials may include e.g. ASTM 312 steel, duplex stainless steel or Hastelloy (an alloy containing mainly nickel as well as chromium, molybdenum, iron, and cobalt) .

[0042] The porous material of the elements may allow the oxygen-containing gas to pass through them and to form gas bubbles of a desirable diameter (or diameter range) . It may be desirable for the bubbles to have a relatively small diameter, such that the oxygen can easily react with the white liquor, and the bubbles may avoid coalescing with each other.

[0043] The porous material may have capillary tubes (through pores) , e.g. a large number of interconnected capillary channels, that may allow the oxygen-containing gas to pass through the porous material and form gas bubbles at the interface of the porous material and the white liquor. The pore structure of the elements may be uniform or substantially uniform across the element (s) .

[0044] The shape of the elements is not particularly limited. The elements may be e.g. cylindrical; conical; disc-shaped; spherical; hemispherical; in the shape of a spherical cap / dome; or in the shape of a plate or a sheet (for example, a square sheet) .

[0045] The elements may be formed e.g. from a metal powder by sintering. Sintering may be understood as referring to compacting and forming a solid mass of the material by pressure and / or heat without melting the material to the point of liquefaction. In the sintering, the porosity of the material of the element may be adjusted to be such that it is suitable for feeding the oxygen-containing gas into the reactor comprising the white liquor and has or provides a desired nominal filtration accuracy.

[0046] Each element may be attached, e.g. sintered or welded, to a connector. The connector may be attached to piping for directing the oxygen-containing gas to the elements, such that the oxygen-containing gas may be directed through the connector to the elements and distributed through the pores of the elements to the white liquor. The connector (s) may be formed e.g. of a suitable metal .

[0047] The elements may be capable of withstanding pressure, high pH (e.g. a pH in the range of about 13 - 13.5) , high temperature (e.g. a temperature in the range of about 100 - 130 °C) and / or corrosion. The elements may be washable. For example, there may be a need to wash off precipitated matter that may be formed on the element and that may clog pores of the element.

[0048] The elements, for example sintered metal elements, may have a nominal filtration accuracy of at least 1 pm, or at least 2 pm. The elements may have a nominal filtration accuracy in the range of 2 - 120 pm, or in the range of 40 - 120 pm. Small values of the nominal filtration accuracy may provide smaller gas bubbles , which may improve oxidation efficiency, but they may in some situations be more prone to clogging .

[0049] The nominal filtration accuracy refers to the ability of the element to retain particles of a determined si ze . The nominal filtration accuracy may be determined by filtering stainless steel particles with the element using air or water as the flow medium, the stainless steel particles having a predetermined particle si ze or particle size distribution, and determining the proportion of the particles that are removed by the filtering .

[0050] An element having a nominal filtration accuracy of 2 pm may be understood as referring to an element that is capable of removing 95 % of stainless steel particles having a particle si ze of 10 pm using water as a flow medium or 95 % of stainless steel particles having a particle si ze of 0 . 7 pm using air as a flow medium .

[0051] An element having a nominal filtration accuracy of 40 pm may be understood as referring to an element that is capable of removing 95 % of stainless steel particles having a particle si ze of 45 pm using water as a flow medium or 95 % of stainless steel particles having a particle si ze of 2 . 5 pm using air as a flow medium .

[0052] An element having a nominal filtration accuracy of 120 pm may be understood as referring to an element that is capable of removing 95 % of stainless steel particles having a particle s i ze of 110 pm us ing water as a flow medium .

[0053] The elements , or the porous material , may be capable of forming, or configured to form, gas bubbles of an average diameter in the range of 1 - 10 pm, or in the range of 1 - 2 pm .

[0054] The elements may have about 3 - 105- 3 - I O7pores / m2. In other words , the dens ity of the pores on the surfaces of the elements may be about 3 -IO5- 3 -IO7pores / m2.

[0055] The elements formed at least partially of the porous material (i.e. the plurality of the elements) may have a total surface area of at least 0.5 m2, or at least 0.7 m2. The total surface area of the elements may be considered to be the sum of the surface areas of each of the elements (of the plurality of the elements) . If the total surface area is too small, a larger portion of the bubbles that the elements generate may coalesce with each other.

[0056] The plurality of elements may comprise e.g. at least 20, or at least 50 elements, or at least 100 elements, or at least 150 elements, or at least 200 elements. However, the number of the elements may depend e.g. on the size and shape of the reactor; the size and shape of the elements; and / or other factors.

[0057] The plurality of the elements may be arranged such that they are distributed substantially evenly along (or across) the area of a horizontal cross-section of the reactor. However, the arrangement of the elements may depend e.g. on the size and shape of the reactor; the size and shape of the elements; and / or other factors .

[0058] At least a part of the gas bubbles generated when feeding the oxygen-containing gas through the plurality of elements may be fed downwards in the reactor. This may increase the residence time of the bubbles in the white liquor. This may improve the oxidation efficiency .

[0059] The elements, or at least a part of the elements, may be arranged such that they direct at least a part of the oxygen-containing gas as gas bubbles downwards, thereby increasing their residence time in the reactor. This may be done e.g. by disposing individual elements such that at least a part of a porous surface of individual elements is at least partially at an angle in the range of 0-90°, e.g. at an angle in the range of 0-60° or at an angle of about 45°, with respect to the horizontal plane. At least e.g. 20 % , or 30 % , or 50 % (v / v) of the oxygen-containing gas may be directed downwards as gas bubbles, thereby increasing their residence time in the reactor.

[0060] The oxygen-containing gas may be fed to the reactor e.g. at a rate of at least 100 g Cy / s (i.e. at least 100 g of oxygen per second) , or at a rate in the range of 100 - 400 g Cy / s. The required rate may depend e.g. on the composition of the white liquor. For example, white liquor obtainable from a pulping process that uses softwood as a starting material may require a higher rate than white liquor obtainable from a pulping process that uses hardwood or a mixture of hardwood and softwood .

[0061] The oxygen-containing gas may be fed to the reactor e.g. at a pressure of about 3 - 20 bar, for example about 10 - 12 bar.

[0062] The oxygen-containing gas may form gas bubbles of a desired average diameter when fed from the plurality of the elements into the white liquor. Small bubbles may promote the efficiency of the reaction, as the interface between the oxygen-containing gas and the white liquor may be increased; however, at the same time a sufficient amount of the oxygen-containing gas should be provided to the reactor. Small bubbles may also have a higher tendency to coalesce as compared to larger bubbles. For example, the oxygen-containing gas may form gas bubbles of an average diameter in the range of 1 - 10 pm, or in the range of 1 - 2 pm.

[0063] The oxygen-containing gas may be e.g. oxygen, such as pure oxygen, air, or any mixture or combination thereof. The oxygen-containing gas may comprise e.g. 21 - 100 % (v / v) , or 90 - 100 % (v / v) , or 95 - 100 % (v / v) of oxygen. The white liquor in the reactor may be stirred to improve oxidation efficiency . The stirring may promote the solubil ity of the oxygen from the oxygen-con- taining gas and / or prevent the gas bubbles generated at the elements from coalescing .

[0064] Thus the arrangement may further comprise an agitation device for stirring the white liquor .

[0065] The white liquor in the reactor may be stirred using an agitator device provided with a radial flow impeller . The arrangement may thus comprise an agitator device provided with a radial flow impeller . In some embodiments , the agitator device may be provided with at least one , or at least two radial flow impellers , for example one , two or three radial flow impellers .

[0066] The radial flow impeller ( or each radial flow impeller, or at least one of the radial flow impellers ) may comprise a rotor provided with a disc-shaped body, and several rotor blades mounted to the rotor and extending radially from the rotor . A cross-section of each of the rotor blades may be concave and may be open in the direction of rotation of the rotor .

[0067] Such an agitation device may be particularly well suited for the method and arrangement as it may be capable of retaining the gas bubbles in the white liquor relatively long . Such an agitation device may also significantly improve the efficiency of the oxidation reaction .

[0068] In such a rotor, the rotor blades may be vertically asymmetrical with respect to a plane of the disc-shaped body . Not to be bound by theory, it may be that such a rotor may cause a vortex of the white liquor moving downwards from the plane of the disc-shaped body, which may improve the residence time of gas bubbles generated at the elements .

[0069] The radial flow impeller may have a diameter or at least 1 / 3 of the width of the reactor . The reactor may have a volume of at least 10 m3. The reactor may have a volume of about 10 -30 m3, or e . g . about 20 -25 m3.

[0070] The reactor may be e . g . essentially cylindrical , for example vertically cylindrical . However, other shapes may be contemplated . The reactor may have a ratio of the height of the reactor to the width of the reactor of at least 2 : 1 . Such a reactor may be beneficial in the sense that it may improve the residence time of the gas bubbles in the white liquor and thereby improve the efficiency of the reaction, for example in embodiments in which the oxygen-containing gas leaving the reactor is not collected and circulated back to the reactor .

[0071] A pulping plant comprising the arrangement according to one or more embodiments described in this specification is also disclosed .

[0072] Use of the arrangement according to one or more embodiments described in this specification for oxidi zing white liquor is also disclosed .

[0073] EXAMPLES

[0074] Reference will now be made in detail to various embodiments , an example of which is il lustrated in the accompanying drawings .

[0075] The description below discloses some embodiments in such a detail that a person skilled in the art is able to utili ze the embodiments based on the disclosure . Not all steps or features of the embodiments are discussed in detail , as many of the steps or features will be obvious for the person skilled in the art based on this specification .

[0076] For reasons of simplicity, item numbers will be maintained in the following exemplary embodiments in the case of repeating components .

[0077] Figure 1A illustrates a method and an arrangement for oxidi zing white liquor 1 . Oxygen-containing gas 2 is fed into a reactor 3 comprising the white liquor 1 . The reactor 3 is depicted as essentially cylindrical , although the shape of the reactor 3 is not particularly limited, and various other shapes may be contemplated . The reactor 3 may further comprise an inlet and an outlet for the white liquor 1 , although these are not shown in Fig . 1A for clarity . The arrangement is shown in thi s Fig . when in use , i . e . when the reactor contains white liquor 1 .

[0078] The arrangement shown in Fig . 1 further comprises an inlet 6 for the oxygen-containing gas 2 and distribution piping 7 for the oxygen-containing gas 2 . The oxygen-containing gas 2 may be fed through the inlet 6 to the distribution piping 7 and from the distribution piping 7 to a plurality of elements 4 . The oxygen-con- taining gas 2 is fed into the reactor 3 through the plurality of elements 4 . The arrangement may comprise e . g . at least 20 , or at least 100 , or at least 150 of the elements 4 depending e . g . on the geometry of the reactor 3 and of the elements 4 .

[0079] The elements 4 are formed at least partially of a porous material . The elements 4 , illustrated schematically in this Fig . , may be cylindrical or essentially cylindrical , but various other shapes (e . g . those described in this specification) may also be contemplated . The elements 4 may be any elements described in this specification and may be formed at least partially of any porous material described in this specification . In this embodiment , at least a part of the elements 4 are arranged such that they are configured to direct at least a part of the oxygen-containing gas 2 as gas bubbles downwards . This may increase the residence time of the gas bubbles in the reactor .

[0080] The arrangement further comprises an agitation device 8 for agitating the white liquor 1 . The agitation device 8 may assist in distributing the oxygen-contain- ing gas 2 throughout the white liquor 1 and thereby achieving an efficient reaction. The agitation device 8 depicted in this embodiment comprises two radial flow impellers 5, although in other embodiments, it could comprise e.g. one radial flow impeller or three or more radial flow impellers. The radial flow impellers 5 are mounted on a shaft 9. In the embodiment in which the agitation device 8 comprises at least two radial flow impellers 5, the radial flow impellers 5 may be disposed concentrically at a distance of each other. Thus one of the radial flow impellers 5 is disposed above the other. The agitation device 8 further comprises a motor 10 configured to rotate the shaft 9 and the impellers 5.

[0081] The radial flow impeller 5, or the radial flow impellers 5, may be disposed above the plurality of the elements 4.

[0082] Various measurements and geometries of the reactor 3 and of the radial flow impeller (s) 5 may be contemplated. A skilled person is capable of selecting suitable measurements and geometries, but some examples of possible measurements and geometries are described in this context.

[0083] The reactor 3 may have a width wr. The radial flow impeller 5 may have a width w . The width w of the radial flow impeller 5 may be at least 1 / 3 of the width of the reactor wr.

[0084] The oxidation reaction of the white liquor 1 is exothermic, and if the temperature rises, the reaction may slow down. Therefore the reactor 3 may require cooling. In the embodiment shown in Fig. 1A, the reactor 3 comprises cooling equipment, such as a cooling jacket 11. Cooling liquid 25 may be fed into the cooling jacket 11. However, additionally or alternatively, the cooling equipment could include e.g. one or more heat exchangers .

[0085] Figure IB illustrates another embodiment of a method and an arrangement for oxidizing white liquor 1. Other aspects of the embodiment shown in Fig. IB are generally similar to those of the embodiment shown in Fig. 1A, except that the elements 4 are in the shape of sheets or e.g. in the shape of a grid. The elements 4 are also larger than in the embodiment of Fig. 1A. Assuming that the size of the reactor 3 is similar as in Fig. 1A, the number of the elements 4 may thus be smaller than in the embodiment of Fig. 1A.

[0086] The plurality of the elements 4 are arranged at the bottom of the reactor 3 in this embodiment. In this embodiment, the elements 4 are arranged such that they are configured to direct the oxygen-containing gas 2 as gas bubbles upwards or mainly upwards, although other configurations could also be contemplated. The radial flow impeller 5 that is disposed closer to the plurality of the elements 4 is also disposed higher than in the embodiment of Fig. 1A.

[0087] The embodiment of Fig. IB also does not include any cooling equipment, although e.g. a cooling jacket 11 as shown in Fig. 1A, a heat exchanger, or other suitable cooling equipment could be included.

[0088] Figure 2A illustrates schematically another embodiment of the method and arrangement. In this Fig., only a part of the reactor 3 containing the white liquor 1 is shown. Other aspects of the embodiment of the arrangement shown in Fig. 2A may otherwise be generally similar e.g. to those of the embodiment shown in Fig. 1A.

[0089] The oxygen-containing gas 2 may be fed into a plurality of elements 4 via an inlet 6 and distribution piping 7. The distribution piping 7 having horizontal branches 12 and the plurality of elements 4 are shown in side view. Individual elements 4 in the horizontal branches 12 of the distribution piping 7 are depicted as lines. The horizontal branches 12 may be at the same level with each other (i.e. at the same level in the vertical direction) . This may improve the distribution of gas bubbles from the elements 4 and / or reduce the coalescence of the gas bubbles.

[0090] A horizontal branch 12 of the distribution piping is shown in this side view as extending slightly upwards. As shown in this horizontal branch 12, individual elements 4 are arranged at an angle smaller than 90°, e.g. at an angle of about 45°, with respect to the horizontal plane 13, e.g. the horizontal plane of the distribution piping 7 and / or of the branch 12. In such an arrangement, the elements 4 are configured to direct at least a part of the oxygen-containing gas 2 as gas bubbles downwards. This may increase the residence time of the gas bubbles in the reactor 3. The elements 4 in the horizontal parts of the distribution piping 7 depicted only as lines may also be arranged in such a manner, i.e. all of the individual elements of the plurality of the elements 4 may be arranged in such a manner. In this embodiment, the elements 4 may be e.g. elements such as those described in Figs. 3A and 3B. Thus at least a part of the porous surface of the cylindrical elements 4 is arranged at such an angle with respect to the horizontal plane 13, such that the elements 4 are configured to direct at least a part of the oxygen-containing gas 2 as gas bubbles downwards. However, in other embodiments, the elements 4 could be arranged e.g. horizontally.

[0091] The radial flow impeller 5 mounted on the shaft 9 is depicted in Fig. 2A only schematically. It could be e.g. a radial flow impeller as shown in Figures 4A and 4B, but other types of radial flow impellers may also be contemplated.

[0092] Figure 2B illustrates schematically the embodiment of the method and arrangement of Fig. 2A in a top view of the bottom of the reactor 3 shown in Fig. 2A. The reactor 3 is not shown for clarity. The inlet 6 is connected to the distribution piping 7. In this exemplary embodiment, the distribution piping 7 comprises a plurality, specifically six, horizontal branches 12 arranged at a distance of each other. However, the number of the horizontal branches 12 is not particularly limited and may depend e.g. on the size(s) and shape (s) of the elements 4 and the reactor 3. For example, the distribution piping 7 could comprise e.g. three, four, five, six or more horizontal branches 12. In other embodiments, the distribution piping 7 could be e.g. in the form of a grid. The plurality of the elements 4 are depicted schematically as lines. The elements 4 are arranged along the horizontal branches 12 of the distribution piping 7 at a distance of each other. The elements 4 may be considered to be distributed substantially evenly along the area of a horizontal cross-section of the reactor 3.

[0093] Figure 3A illustrates an exemplary embodiment of an element 4 in side view. The element 4 comprises a part 14 formed of a porous material, through which the oxygen-containing gas 2 may be fed into the white liquor. The part 14 is essentially cylindrical or cylindrical, although a skilled person may easily envisage various other shapes. The top of the part 14 (and thus the top of the element 4) may be closed. The part formed of the porous material 14 may be formed e.g. of sintered stainless steel or other sintered metal. However, the porous material may be any other porous material described in this specification. The pore structure of the element 4, or of the part 14 formed of the porous material, may be considered to be uniform or substantially uniform across the entire element 4 or the part 14 thereof. However, as a skilled person will understand, at a microscopical level, there may be local variations in the pore structure of the element 4 or the part 14 thereof .

[0094] The element 4 further comprises a connector 15 through which the oxygen-containing gas 2 may be fed and distributed to the part 14 formed of the porous material. The part 14 formed of the porous material is attached to the connector 15. The element 4 may further comprise a fitting 16. The fitting 16 may allow the element 4 to be attached to a distribution piping, for example the distribution piping 7 shown e.g. in any one of Figs. 1A to 2B. The oxygen-containing gas 2 may thus be fed through the connector 15 into the part 14 and therethrough into the white liquor.

[0095] Figure 3B is a cross-section II-II of Fig. 3A. The part 14 formed of the porous material is tubular, and a channel 17 for the oxygen-containing gas 2 extends along at least a portion of the length of the part 14 (but not through the top of the part 14) . The connector 15 and the channel 17 are in fluid connection, such that the oxygen-containing gas 2 fed through the connector 15 may flow to the channel 17 and through the pores (not shown) of the part formed of the porous material 14 to the white liquor.

[0096] A plurality of elements 4 as shown in Figs. 3A and 3B could be included e.g. in the arrangements shown in any one of Figs. 1A to 2B.

[0097] Figure 3C shows a cross-section of a horizontal branch 12, for example of a horizontal branch 12 shown in Figs. 2A and 2B. It shows schematically four elements 4 attached to the horizontal branch 12 via their connectors 15 and fittings 16. The individual elements 4 are arranged at an angle smaller than 90°, e.g. at an angle of about 45°, with respect to the horizontal plane 13, e.g. the horizontal plane of the horizontal branch 12. In such an arrangement, the elements 4 are configured to direct at least a part of the oxygen-containing gas 2 as gas bubbles downwards. This may increase the residence time of the gas bubbles in the reactor. In this embodiment, the elements 4 may be elements such as those described in Figs. 3A and 3B . Thus at least a part of the porous surface of the part 14 formed of the porous material is arranged at such an angle with respect to the hori zontal plane 13 , such that the elements 4 are configured to direct at least a part of the oxygencontaining gas 2 as gas bubbles downwards . However, in other embodiments , the elements 4 could be arranged e . g . hori zontally and / or vertically .

[0098] Figure 4A illustrates an embodiment of a radial flow impeller 5 for stirring the white liquor in the reactor . The radial flow impeller 5 comprises a rotor 18 provided with a disc-shaped body 19 . The radial flow impeller 5 further comprises several rotor blades 20 mounted to the rotor 18 and extending radially from the rotor 18 . The cross-section of each of the rotor blades 20 is concave and is open in direction of rotation of the rotor, as shown in Figure 4B . The direction of the rotation of the rotor is illustrated with an arrow in the figure .

[0099] The radial flow impeller 5 as shown in Figs . 4A and 4B could be included e . g . in the arrangements shown in any one of Figs . 1A to 2B . The rotor further has a hub 23 through which it may be mounted to a shaft , such as the shaft 9 shown in Figs . 1A or IB .

[0100] Mounting of the rotor blades 20 to the discshaped body 19 may be asymmetrical when seen in longi tudinal direction of the rotor blades 20 .

[0101] Fig . 4B further illustrates a cross-section of one of the rotor blades 20 of the radial f low impeller 5 shown in Fig . 4A . All of the rotor blades 20 of the radial flow impeller 5 shown in Fig . 4A may be identical to the one shown in Fig . 4B .

[0102] The cross-section of the rotor blade 20 is concave and i s open in direction of rotation of the rotor 18 . The direction of the rotation of the rotor 18 is illustrated with an arrow in the figure . The rotor blade 20 is vertically asymmetrical with respect to a plane 24 of the disc-shaped body, the plane 24 being illustrated with a dotted line . The upper edge 21 of the rotor blade 20 extends further in the direction of the rotation of the rotor 18 than the lower edge 22 of the rotor blade 20 . In other words , the cross-section of each of the rotor blades 20 extends a greater portion on a vertical upper side of the disc-shaped body 19 . The rotor blades 20 have an elliptical cross-sectional shape (with no discontinuities in the curved blade shape geometry) .

[0103] Not to be bound by theory, it may be that such a rotor 18 may cause a vortex of the white liquor moving downwards from the plane 24 of the disc-shaped body, which may improve the residence time of gas bubbles generated at the elements 4 in the reactor 3 as compared e . g . to a Rushton turbine .

[0104] EXAMPLE 1

[0105] A reactor with a volume of 14 m3for oxidi zing white liquor was adapted by adding a distribution piping containing 6 branches . The reactor and the distribution piping were similar to those shown in Figs . 1A, 2A and 2B . Altogether 279 elements formed of porous sintered stainless steel (ASTM 312 ) were attached to the branches , such that the total surface area of the porous elements was 0 . 84 m2. The elements had a nominal filtration accuracy of 100 pm .

[0106] Oxygen was fed into the reactor containing white liquor at a rate of approx . 180 g / s .

[0107] A solution containing approx . 0 .2 gNa2S / L was obtained . The coefficient of utili zation of O2 in the reaction was calculated to be at least approx . 80 % .

[0108] The reactor originally contained a Rushton turbine as an agitator device . The Rushton turbine was however replaced with an agitator device as depicted in Figs . 4A and 4B . It was noted that the Rushton turbine , in particular without the elements , provided a very poor efficiency of oxidation ; the replacement of the agitator device significantly improved the efficiency . The combination of the elements and the agitator device provided the highest efficiency of oxidation .

[0109] It is obvious to a person skil led in the art that with the advancement of technology, the basic idea may be implemented in various ways . The embodiments are thus not limited to the examples described above ; instead they may vary within the scope of the claims .

[0110] The embodiments described hereinbefore may be used in any combination with each other . Several of the embodiments may be combined together to form a further embodiment . A method, a product , or a use disclosed herein, may comprise at least one of the embodiments described hereinbefore . It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments . The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages . It will further be understood that reference to ' an ' item refers to one or more of those items . The term "comprising" is used in this specification to mean including the feature ( s ) or act ( s ) followed thereafter, without excluding the presence of one or more additional features or acts .

Claims

CLAIMS1. A method for oxidizing white liquor (1) , wherein the method comprises feeding oxygen-containing gas (2) into a reactor (3) comprising the white liquor, wherein the oxygen-containing gas is fed into the reactor through a plurality of elements (4) formed at least partially of a porous material.

2. The method according to claim 1, wherein the elements are formed at least partially of a porous metal, such as sintered metal, or a porous ceramic material, a porous plastic fiber material, a porous glass material, and / or a porous fiberglass material.

3. The method according to claim 1 or 2, wherein the elements have a nominal filtration accuracy of at least 1 pm, or a nominal filtration accuracy in the range of 2 - 120 pm, or in the range of 40 - 120 pm.

4. The method according to any one of claims 13, wherein the elements have about 3 -105- 3 -IO7pores / m2.

5. The method according to any one of claims 1- 4, wherein the elements formed at least partially of the porous material have a total surface area of at least 0.5 m2, or at least at least 0.7 m2.

6. The method according to any one of claims 1- 5, wherein the plurality of elements comprises at least 20, or at least 50 elements, or at least 100 elements, or at least 150 elements, or at least 200 elements .

7. The method according to any one of claims 1- 6, wherein the plurality of elements are arranged such that they are distributed substantially evenly along the area of a horizontal cross-section of the reactor.

8. The method according to any one of claims 1- 7, wherein the elements are arranged such that they direct at least a part of the oxygen-containing gas asgas bubbles downwards, thereby increasing their residence time in the reactor.

9. The method according to any one of claims 1 - 8, wherein the oxygen-containing gas is fed to the reactor at a rate of at least 100 g / s, or at a rate in the range of 100 - 400 g / s.

10. The method according to any one of claims 1 - 9, wherein the white liquor in the reactor is stirred using an agitator device (8) provided with a radial flow impeller (5) , wherein the radial flow impeller comprises : a rotor (18) provided with a disc-shaped body (19) ; and several rotor blades (20) mounted to the rotor and extending radially from the rotor; wherein a cross-section of each of the rotor blades is concave and is open in the direction of rotation of the rotor.

11. The method according to claim 10, wherein the rotor blades (20) are vertically asymmetrical with respect to a plane (24) of the disc-shaped body (19) .

12. An arrangement for oxidizing white liquor, the arrangement comprising a reactor for oxidizing the white liquor, and a plurality of elements formed at least partially of a porous material configured to feed an oxy- gen-containing gas through the elements to the reactor.

13. The arrangement according to claim 12, wherein the elements are formed at least partially of a porous metal, such as sintered metal, or a porous ceramic material, a porous plastic fiber material, a porous glass material, and / or a porous fiberglass material .

14. The arrangement according to claim 12 or 13, wherein the elements have a nominal filtration accuracy of at least 1 pm, or a nominal filtration accuracyin the range of 2 - 120 gm, or in the range of 40 - 120 gm.

15. The arrangement according to any one of claims 12 - 14, wherein the plurality of elements comprises at least 50 elements, or at least 100 elements, or at least 200 elements.

16. The arrangement according to any one of claims 12 - 15, wherein the plurality of elements are arranged such that they are distributed substantially evenly along the area of a horizontal cross-section of the reactor.

17. The arrangement according to any one of claims 12 - 16, wherein the reactor has a volume of at least 10 m3.

18. The arrangement according to any one of claims 12 - 17, wherein the elements are arranged such that they are configured to direct at least a part of the oxygen-containing gas as gas bubbles downwards, thereby increasing their residence time in the reactor.

19. The arrangement according to any one of claims 12 - 18, wherein the arrangement comprises an agitator device (8) provided with a radial flow impeller (5) for stirring the white liquor (1) in the reactor (3) , wherein the radial flow impeller comprises: a rotor (18) provided with a disc-shaped body (19) ; and several rotor blades (20) mounted to the rotor and extending radially from the rotor; wherein the cross-section of each of the rotor blades is concave and is open in direction of rotation of the rotor.

20. The arrangement according to claim 19, wherein the rotor blades (20) are vertically asymmetrical with respect to a plane (24) of the disc-shaped body (19) .

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