Field of the invention

The invention relates to a method for preparing cellulose pulp according to the preamble of the appended claim 1. The invention also relates to a device for implementing the aforementioned method in accordance with the preamble of the appended claim 13.

Background of the invention

The purpose of pulping is to remove, from a lignin-containing raw material, the lignin which binds fibers to each other, and to detach the fibers from each other so that they can be utilized in the manufacture of cellulose and/or paper. When wood is used as the raw material, cooking is preceded by chipping of the raw material, i.e. chopping of the wood to the optimal size required by the cooking process. In sulphate cooking, or so-called kraft pulping, the cooking takes place by utilizing as a cooking liquor strongly alkaline white liquor which consists primarily of sodium hydroxide (NaOH) and sodium sulfide (Na ₂ S).

In sulphate cooking, a significant portion of the hemicellulose contained in the cellulose material is partly wasted. This is because in reactions with white liquor the hemicellulose is exposed to an alkaline peeling reaction, depolymerization, wherein hemicellulose dissolves as organic acids to the cooking liquor. The depolymerization is accelerated when the alkali content increases and the temperature rises. Attempts are made to reduce the peeling of hemicellulose by using cooking liquor containing polysulfide. It has, in fact, been observed that polysulfide stabilizes the hemicellulose, which then endures cooking liquor better in pulping conditions. Thus, the yield of the cooking is improved. The polysulfide treatment should be performed as early as possible in the cooking process, i.e. already in the impregnation stage by cooking liquor, so that it would have the greatest effects. It has also been observed that the earlier in the impregnation stage the polysulfide treatment is performed, the more complete the dissolution i.e.

delignification of lignin is in the cooking, and the higher is the viscosity of the pulp having certain Kappa number. Adding anthraquinone to the polysulfide cooking also improves the yield.

Thus, it is a known process to use polysulfide white liquor or orange liquor, as white liquor containing polysulfide is also known, in sulfide cooking. Normal white liquor is transformed into polysulfide white liquor, and this liquor is used similarly to normal white liquor. In the impregnation stage, after the impregnation into the wood chips, and during the increase of temperature to over 100 ⁰ C, the polysulfide reacts with the carbohydrates contained in the chips, wherein they have become less vulnerable to alkaline degradation. Later during the cooking, when the temperature has risen over 140 ⁰ C, the rest of the polysulfide degrades rapidly as a result of thermal degradation.

White liquor containing polysulfide is made of the white liquor produced in a chemical recovery plant by means of various methods, which for example partly oxidize the sodium sulfide contained in the liquor into polysulfide. In these methods oxidizing catalysts are typically used in order to polymerize the hydrogen sulfide contained in the white liquor. This kind of a method is disclosed, for example, in patent publications WO 97/42372 and US 6,866,748.

It is also possible to produce polysulfide directly by adding elementary sulfur to the white liquor. In suitable conditions, the sulfide ions in the white liquor react with elementary sulfur and produce polysulfide.

It is also possible to produce polysulfide from black liquor obtained from the cooking. WO publication 93/22493 discloses a method in which sulfur-containing gases released from black liquor are treated to produce hydrogen sulfide. The produced hydrogen sulfide is led to white liquor, in which formation of polysulfide takes place. In WO publication 99/14423 black liquor is gasified to form sulfur-containing gases, primarily hydrogen sulfide containing gases, which gases are burned. From the acidic hydrogen sulfide containing gas produced as a result of burning, hot aqueous elementary sulfur is formed, which is

mixed into sulfide-containing liquor, such as white or green liquor to produce polysulfide.

A problem with current methods for preparing cellulose that utilize white liquor containing polysulfide is that they require special apparatuses that cause investment and operating costs.

The yield of sulfide cooking is also affected by the hydrogen sulfide concentration of the cooking liquor used in the impregnation stage. The yield is improved when the hydrogen sulfide concentration of the impregnation liquor is increased. Attempts have been made to increase the hydrogen sulfide concentration of the cooking liquor used in the impregnation stage for example by bringing black liquor obtained from the cooking stage to the impregnation, as disclosed in US patent 5,053,108. Publication WO 03/062524 suggests increasing the hydrogen sulfide concentration of the cooking stage by increasing the dry matter content in the cooking stage, whereby black liquor from cooking is evaporated and recirculated to the beginning of the cooking stage. The black liquor thus formed is led from the end of the cooking stage to the impregnation. Furthermore, in SE patent 521678, the sulfide concentration is increased in the impregnation stage by introducing black liquor from pulping to it, the dry matter content of the black liquor being raised by evaporating water from it. All these methods do, of course, bring about some kind of increase in the hydrogen sulfide concentration of the impregnation solution, but the absolute hydrogen sulfide concentration obtained by means of the same is not necessarily very high when compared for example to white liquor.

Brief description of the invention

The purpose of the present invention is to provide an improved method and a device for manufacturing sulfate cellulose, which avoids the above-mentioned prior art problems and by means of which the yield of cooking, especially in the manufacture of softwood pulp, is improved.

To attain this purpose, the method according to the invention is primarily characterized in what will be presented in the characterizing part of the independent claim 1.

The device according to the invention, in turn, is primarily characterized in what will be presented in the characterizing part of the independent claim 13.

The other, dependent claims will present some preferred embodiments of the invention.

The invention is based on the idea that the impregnation liquor used for impregnation is concentrated and its polysulfide content is increased before conveying the impregnation liquor to the impregnation. The concentrating of the impregnation liquor, i.e. evaporating of water therefrom, increases its hydrogen sulfide concentration on a high level. The hydroxyl concentration of impregnation liquor will also remain sufficiently high in spite of the degradation reactions of carbohydrates. In addition, the concentrated impregnation liquor is circulated, wherein the carbohydrates degraded in connection with the hydrolysis at the beginning of the impregnation are returned to the impregnation, which improves the total yield of the process according to the law of mass action. Furthermore, the carbohydrates thus saved have been found to have a catalyzing effect on the cooking.

The concentrating of the impregnation liquor also makes it possible to use a higher impregnation temperature, which in the method according to the invention is 100 to 170 ⁰ C ₁ thus being preferably 100 to 13O ⁰ C at the beginning of the impregnation and 120 to 15O ⁰ C at the end of the impregnation. Thanks to the high temperature of impregnation, the hydrogen sulfide in the impregnation liquor reacts with the lignin and binds chemically to the wood matrix of the chips. As a result, the actual cooking becomes faster and the total cooking yield increases. The high impregnation temperature does not only make the impregnation more efficient, wherein impregnation no longer takes place in the cooking stage, but it also provides that the acids formed in the degradation of

carbohydrates are neutralized already in the impregnation stage and not in the. cooking stage, wherein less alkali is needed for digesting the cellulose in the cooking stage, that is, the cooking is more selective.

The concentration or dewatering of impregnation liquor may be effected by any method suitable for evaporating liquor, for example by expanding heated liquor or evaporating it. When the impregnation liquor is dewatered, the content of sodium sulfide therein rises in relation to the content of sodium hydroxide; in other words, its sulfidity increases. In the concentration of impregnation liquor, free water is evaporated from the impregnation solution, wherein the concentration differences required by the diffusion taking place in the impregnation remain high and the diffusion rate inside the chips increases. By means of the method, the hydrogen sulfide concentration of the impregnation liquor can be doubled when compared to the process of prior art which uses black liquor for impregnation.

The concentrated impregnation liquor is conveyed after dewatering to an impregnation liquor tank, in which its retention time is adjusted to be such that soap, or the like, in the impregnation liquor rises on the surface of the impregnation liquor in the tank, and can be separated from it. In this way it is possible to reduce the COD load in the cooking and washing stages following the impregnation stage.

The polysulfide content of the concentrated impregnation liquor is increased before circulating the impregnation liquor back to the impregnation. The polysulfide oxidates the reactive terminal groups of the carbohydrates to be alkali stable, and improves the yield of the cooking. The increasing of the polysulfide content can be done by adding elementary sulfur or suitable polysulfide compound directly to the concentrated impregnation liquor. The increasing of the polysulfide content can also be done by adding polysulfide forming reagents to the concentrated impregnation liquor. It is, for example, possible to add oxidized black liquor to the impregnation liquor. For this purpose it is possible to use black liquor obtained from the cooking, in which black liquor oxygen has been added. The oxygen causes the oxidation of the

sulfur compounds in the black liquor into sulfide. Furthermore, the oxygen oxidates the lignin structures of the black liquor to quinone compounds which reduce the sulfide in the black liquor as well as the hydrogen sulfide in the impregnation liquor into polysulfide. The oxidation reaction of black liquor can be described in a simple manner by the following equation:

lignin in black liquor + O ₂ -» quinone + ... (1)

The reduction reaction of sulfide by quinone can be described as follows:

quinone + (n + 1 ) S ^2" → S _n S ^2" + 2 n ^e" (2)

The reduction reaction is a catalytic reaction in which the quinones operate as catalysts. Thus, the polysulfide can be produced in a pulping apparatus, and it is not necessary to introduce it from outside the process.

By means of the method according to the invention, conditions of impregnation are achieved, in which the concentrations of dissolved organic and inorganic dry matter in the impregnation liquor are high when compared with methods of prior art. Another advantage of the invention is that by means of the invention, the hydrogen sulfide concentration of the impregnation liquor to be used in the impregnation stage can be raised as high as possible, when compared to the hydrogen sulfide concentration of white liquor, wherein the yield from the cooking stage following the impregnation stage is improved. Furthermore, as a result of the reactions between lignin and sulfide in the impregnation stage, the bulk step in the cooking becomes faster. Moreover, less water is conveyed with the impregnated chips into the cooking stage, which makes the cooking more efficient.

The high hydrogen sulfide concentration of the impregnation liquor also increases the amount of polysulfide in the impregnation liquor, when the polysulfide content of the impregnation liquor is increased by

means of oxidized black liquor obtained from the cooking stage. It is advantageous for reactions catalyzed by quinone compounds formed of black liquor that the sulfide content of impregnation liquor is high. The retention time of impregnation liquor in the impregnation liquor tank ensures a sufficient reaction time for the quinone reactions. The use of the black liquor obtained from the cooking in the formation of polysulfide also brings about other advantages: the reagent necessary for forming the polysulfide, i.e. black liquor is readily available in a pulp mill, and expensive purchased chemicals are not required. In addition, separate, expensive reactors are not required for the production of the polysulfide. It is easy to implement the method into an existing pulping process, and it does not require installation of space occupying process equipment in the pulp mill. The invention especially improves the yield of softwood pulp. In practical tests it has been observed that the yield improves as much as 2 to 5 %.

Brief description of the drawings

In the following, the invention will be described in more detail with refer- ence to the appended Fig. 1 which shows, in a schematic view, a cellulose pulping device according to the invention.

Detailed description of the invention

In this description, the term impregnation liquor refers to the alkaline solution used in the impregnation stage, consisting primarily of white liquor. The effective agents in white liquor are sodium hydroxide (NaOH) and sodium sulfide (Na ₂ S). The term line refers to any pipe, duct or channel suitable for conveying a liquid, a gas, or a suspension.

Figure 1 is a schematic view showing a device for cellulose pulping according to the invention. The chips used as raw material for cellulose pulp are first brought to a gas removal stage. In the gas removal stage, the chips are treated in a gas removal device 2 with a hot gaseous medium, such as steam, to remove gases from within and between the chips. After the gas removal, the chips are transferred by a transfer

device 3 via a line 4 to the impregnation stage. Impregnation liquor from an impregnation liquor tank 6 is also supplied via a line 7 into the line 4.

In the embodiment of Fig. 1 , the impregnation stage is shown in two steps, which is typical particularly for continuous pulping processes. The impregnation is effected in the impregnation stage, in an impregnation vessel 5, in which the first stage 5a and the second stage 5b of the impregnation are typically connected to each other, for example placed on top of each other in the same vessel in such a way that the flow of chips is directed downwards through the stages. The chips are brought to the impregnation from the upper end of the first impregnation stage 5a, together with the impregnation liquor, it travels through said step and further down to the second impregnation stage 5b. In the first impregnation stage 5a, the impregnation is effected downstream, and during it the concentration of hydrogen sulfide in the impregnation liquor inside the chip increases because the hydroxide in the impregnation liquor is quickly consumed at the beginning of the impregnation. The water in the chips is diffused out of the chips into the impregnation liquor, diluting it. This diluted impregnation liquor is conveyed along a line 15 from the end of the first impregnation stage 5a to be concentrated. The first impregnation stage takes 15 to 120 min, preferably 15 to 45 min.

In the second impregnation stage 5b, the chips are impregnated upstream with the impregnation liquor introduced from the impregnation liquor tank 6 via a line 8. The impregnation liquor is heated in a heat exchanger 9 before it is conveyed to the second impregnation stage 5b. The heating of the impregnation liquor increases the temperature of the second impregnation stage 5b, wherein the reactions between sulfide and lignin become faster. If necessary, the second impregnation stage 5b can also be arranged downstream. One aim of the second stage is to increase the alkali concentration inside the chips so that the alkali content inside the chip as well is sufficient for the cooking stage. Another aim of the second impregnation stage is to displace the rest of the water in the chips and

have it to be concentrated together with the impregnation liquor obtained from the second stage 5b. The water-containing impregnation liquor is conveyed from the second stage to be concentrated along the line 15. The second impregnation stage 5b takes 5 to 60 min, preferably 10 to 45 min. The impregnation stage may also consist of a single stage only, wherein the chips are impregnated downstream in the impregnation vessel. In the impregnation stage 5, the temperature is 100 to 17O ⁰ C. Preferably, the temperature is lower in the first impregnation stage 5a than in the second impregnation stage 5b. The temperature is preferably 100 to 135°C in the first impregnation stage 5a and preferably 120 to 150°C in the second impregnation stage 5b.

White liquor is conveyed via a line 18 to the line 15 which thus conveys impregnation liquor to be concentrated from the first impregnation stage 5a and the second impregnation stage 5b. The addition of white liquor is necessary, not only to prevent bubbling of the impregnation liquor to be evaporated and to make dewatering more efficient, but also to achieve the alkali content required for cooking.

In the impregnation stage the cooking liquor is impregnated into the chips and initial delignification takes place. From the impregnation stage, the chips are conveyed together with a part of the impregnation liquor via a line 10 to the cooking stage in which the actual bulk delignification and, if desired, also the residual delignification, that is, the actual cooking takes place. The cooking takes place in a cooking vessel 11 at a temperature from 130 to 190°C, depending on the wood raw material, and it takes 30 to 150 min. The cooking stage may comprise both downstream and upstream stages whose alkali concentrations and temperatures can be controlled. The fibers detached or separated from each other during pulping are transferred via a line 12 to be washed in a washing vessel 13, i.e. to a washing stage. The washing liquid necessary in the washing is conveyed to the washing vessel 13 via a line 19. The purpose of washing is to separate that part of lignin which comes with the fibers, and the cooking liquor from the fibers. The washed fiber pulp is led to further processing via a line 14.

The above-mentioned stages of impregnation and cooking can be performed either step by step in separate vessels, or together in one and the same vessel, step by step. The stages can also be carried out in an elongated vessel on top of each other so that the impregnation stage is at the top. The chips thus move downward from one stage to another in the vessel.

The lignin-containing liquor, or black liquor obtained from the cooking stage and produces in the pulping process, is led to the chemical recovery plant of the pulp mill (not shown in the figure) via a line 20. The purpose of the chemical recovery is to regenerate the chemicals used in the pulping to such a form, for example into white liquor that they can be re-used in the pulping.

The impregnation liquor obtained from the first impregnation stage 5a and the second impregnation stage 5b is conveyed along the line 15 and the circulating liquor obtained from the transfer device 3 is conveyed along a line 21 to be concentrated in a concentration stage, in an evaporating vessel 17. The evaporating vessel 17 can be an expansion tank or a separate evaporator unit with one or several evaporating/dewatering stages. The thermal energy required by the evaporating vessel can be thermal energy brought from the cooking stage or elsewhere from the pulping process, or secondary energy obtained therefrom. Preferably, at least one evaporator unit of an evaporation plant belonging to the chemical recovery plant of the pulp mill is used for evaporating the impregnation liquor, said evaporator unit being in the same heating medium circulation with the other evaporator units of the evaporation plant.

From the evaporating vessel 17 the concentrated impregnation liquor is conveyed along a line 22 to the impregnation liquor tank 6, from which the impregnation liquor is recirculated to the first impregnation stage 5a and the second impregnation stage 5b via the lines 7 and 8. In this way, a circulation of impregnation liquor is provided. Also, the circulating liquor required by the transfer device 3 is returned via the

line 7. The water removed from the impregnation liquor in the concentration is removed as expansion steam or aqueous water via a line 27.

The polysulfide content of the concentrated impregnation liquor having high hydrogen sulfide concentration is increased by adding polysulfide, such as elementary sulfur or suitable polysulfide compound therein after the concentration. The polysulfide can be added into the concentrated impregnation liquor via a line 23. The line 23 is connected to the line 22 leading from the evaporating vessel 17 to the impregnation liquor tank. The polysulfide to be used can also be polysulfide produced by means known methods in a separate reactor (not shown in the figure), such as polysulfide produced of white liquor in the presence of a catalyst. It is also possible to use polysulfide produced by adding elementary sulfur to the white liquor.

According to the most advantageous embodiment of the invention, polysulfide is formed of the black liquor obtained from the cooking stage substantially in the same process with pulping and concentration of impregnation liquor. Thus, the black liquor obtained from the cooking stage is oxidized and the oxidized black liquor thus obtained is conveyed into the concentrated impregnation liquor. This embodiment is illustrated by means of broken line arrows in the Figure 1. The black liquor obtained from the cooking stage is conveyed via a line 24 to the line 22 conveying concentrated impregnation liquor. Oxygen is conveyed via a line 25 into the black liquor, in the line 24, before the addition of the black liquor into the impregnation liquor line 22. The oxygen oxidates the lignin structures of the black liquor into quinone compounds. In the impregnation liquor tank 6, the quinone compounds catalytically reduce hydrogen sulfide in the impregnation liquor into polysulfides. The act of conveying the impregnation liquor to the impregnation liquor tank and its retention time ensure sufficient reaction time for the quinone reactions.

Naturally, it is also possible to increase the polysulfide content of the concentrated impregnation liquor by forming a part of the polysulfide by

means of polysulfide formation and by bringing a part of the polysulfide as a polysulfide addition to the concentrated impregnation liquor. It is also possible to use black liquor obtained from the evaporation plant of the pulp mill in the formation of the polysulfide. The black liquor is oxidized before it is conveyed into the concentrated impregnation liquor.

By the concentration of the impregnation liquor it is possible to increase the hydrogen concentration of the impregnation liquor, which improves, as already said above, the cooking yield. By adding polysulfide, it is possible to further improve the cooking yield, because the high hydrogen sulfide concentration of the impregnation liquor in the impregnation liquor tank enables extensive formation of polysulfide. The polysulfide in the concentrated impregnation liquor degrades partly back into sulfide under the effect of the temperature and other conditions prevailing in the second impregnation stage 5b of the impregnation reactor. Part of the polysulfide is removed into the cooking along with the liquor passed thereto and is further degraded therein. Thus, the polysulfide is not accumulated into the impregnation liquor circulation, and it has no effect on the lignin reactions taking place in the cooking.

The impregnation liquor tank 6 is equipped with an apparatus for separating soap. The retention time of the concentrated impregnation liquor collected into the impregnation liquor tank 6 is adjusted to be such that the soap therein accumulates on the surface of the impregnation liquor and can be discharged therefrom via a duct 26.

The invention also makes it possible to use anthraquinone, which is expensive as such, to increase the yield in the manufacture of sulphate cellulose pulp. Thanks to the invention, the dosage of anthraquinone can be reduced, because the concentrating of the aqueous impregnation liquor concentrates all of the chemicals therein.

Naturally, the above-presented device for preparing fiber pulp also comprises a large number of process devices, such as sieves, pumps

and valves, whose placement and operation in the pulping apparatus are obvious as such for a person skilled in the art and which are, for clarity, not shown in the figure.

The invention is not intended to be limited to the embodiments presented as examples above, but the invention is intended to be applied widely within the scope of the inventive idea as defined in the appended claims. The invention can thus be applied in various types of cooking methods and digesters, such as a continuous cooking method and its various modifications, or batch digesting, particularly displacement batch digesting. The raw material used can be any ligno- cellulose material, such as softwood, hardwood, bagasse or eucalyptus.