Method for recovery of pulping chemicals in an alkaline sulphite pulping process and for production of steam

Abstract

A method of recovery of alkaline sulphite pulping chemicals and for production of steam is disclosed. The method comprises gasification of evaporated spent cooking liquor at conditions resulting in a hydrogen sulphide containing gas and a solid residue. The gas is combusted in a steam boiler, where the hydrogen sulphide is converted into sulphur dioxide and steam is produced. The solid residue is recovered in a leaching process, preferably a two-stage leaching process, where process-foreign substances are removed and the rest of the contents is divided into substantially pure sodium carbonate and a mixture of sodium carbonate, sodium sulphate and sodium sulphide. The substantially pure sodium carbonate is used for absorption of sulphur dioxide from the steam boiler. The mixture of sodium carbonate, sodium sulphate and sodium sulphide is causticized and the resulting sodium hydroxide containing solution can optionally be mixed with the substantially pure sodium carbonate after the absorption of sulphur dioxide in order to produce a fresh cooking liquor.

Description

[0001] The present invention relates to a method for recovery of alkaline sulphite pulping chemicals and for production of steam.

BACKGROUND OF THE INVENTION

[0002] The sulphate pulping process has hitherto been dominating in the production of chemical pulps as well as semi-chemical pulps due to both its ability of disclosing a wide variety of lignocellulosic raw materials to a pulp of good quality and to its well-developed chemical recovery system, which is tested in large scale and has a high energy efficiency. Since the sixties, different alkaline sulphite processes have been proposed for production of pulps having the same or even superior strength qualities as compared to corresponding sulphate pulps. Increased process flexibility has also been noted concerning the adaptation to different technical paper qualities and to the pulp yield. It has also been noted that a proper choice of cooking conditions will result in substantially improved bleaching properties of the unbleached pulp. These alkaline sulphite processes, which are interesting from a fiber point of view, have however been commercially impeded by the fact that there has not been available any process for recovery of the pulping chemicals, that has, in combination with the pulping process, been able to compete in energy and economical aspects with the well-trimmed recovery system of the sulphate process.

[0003] Even though the interest for alkaline sulphite processes got a start at first during the sixties and later, pulping in neutral or alkaline sulphite liquors has been studied and patented much earlier, the first patent issued already in 1880 (Cross, C. F., British Patent No 4,984, 1880). The basic concept was the use of concentrated sodium sulphite liquor to which was added small amounts of other chemicals such as sodium hydroxide, carbonate, bi-carbonate, sulphide or bi-sulphite. To achieve a defibration within a reasonable time, rather high temperatures (170-190° C.) were required, also at a high concentrations of pulping chemicals and this could easily lead to severe degradation of the fiber material. The pulping technology, of that time, was thus substantially limited to semi-chemical pulps, for which much milder conditions could be used. Although several different methods for chemical recovery have been proposed during the years none has been technically successful (Rydholm, S. A., Pulping Processes, Interscience Publishers 1965, s. 422). The field of semi-chemical pulps has been to a high extent limited to so-called NSSC (Neutral Sulphite Semi, Chemical) (yield about 80-85%) from hardwood for production of fluting and, in cases with applicable chemical recovery, as so-called cross-recovery, i.e. use of the spent liquor as make-up in a neighboring sulphate plant.

[0004] The use of anthraquinone as an accelerator of the delignification and as a protection of the carbohydrates was proposed in 1972-1973. Its usefulness also for neutral and alkaline sulphite pulping is disclosed in the Japanese patent No JP 112,903 (1976) (Nomura, Y., Wakai, M. and Sato, H) and in a modified version in the Canadian patent CA 1 079 906 (1980). By the addition of anthraquinone, cooking time was considerably reduced and thus the field of chemical pulp became available under reasonable conditions: A few alternative methods, working at different pH-ranges were proposed (Ingruber, O. V., Pulp and Paper Manufacture, Volume 4, Sulphite Science & Technology, Joint Textbook Committee of the Paper Industry, TAPPI/CPPA 1985, s.41). However, commercial introduction was impeded by the absence of a competitive chemical recovery system. The at that time most approved chemical recovery systems in sulphite processes were based on a processing smelt from a soda recovery boiler for production of sulphite from the sulphide in the smelt. The complexity of the process and the inherent energy losses were a too heavy drawback of the process. This type of chemical recovery in the soda recovery boiler therefore has a commercial application only in occasional cases due to the drawbacks discussed above.

[0005] An interesting version of alkaline pulping is the MSSAQ (Mini Sulphide Sulphite Ahthraquinone) process (Olm, Teder, Wikén, Swedish patent application No 8405061-6; Olm, Teder, Svensk Papperstidning, No 16-1986 89 s.20-22, 25-26; Dahlbom Olm, Teder, Tappi Journal, Vol 73, No 3, March 1990, s 257), which allows the presence of sulphide in the pulping liquor. The sulphide interacts with the anthraquinone to speed up the cooking process thus providing for increased degrees of delignification, especially if a final stronger alkaline delignification stage is added to the process. However, each amount of anthraquinone charged to the process has a corresponding optimal sulphide charge (at a higher anthraquinone charge the optimal amount of sulphide is lower). However, the positive effect of the sulphide is relatively soon is lost at smaller or larger than optimal sulphide charges.

[0006] A further development of alkaline sulphite processes has been to, in addition to a so-called redox-catalyst such as anthraquinone, to add a low boiling point organic solvent, such as methanol (R. Patt och 0. Kordsachia, EP-A1-205778, Sulfitaufschlussverfahren zur Herstellung von Zellstoff aus lignozellulosehaltigen Materialen mit Rückgewinnung der Aufschlusschemikalien). A process called ASAM (Alkaline Sulphite Anthraquinone Methanol) has accordingly been developed and tested in a pilot plant in Germany (Ahonen und Lehner, “Umweltsverträgliche Holzaufschlussverfahren”. Schriftenserie “Nachwachsende Riksstoffe” 13 und 8, Bundesministerium für Ernährung, Landwirtschaft und Forsten, Bonn 1997).

[0007] A chemical recovery system for the ASAM process has also been developed (M. Bobik, D. Chybin, A. Glasner och K. Taferner, WO-A1-9423124, Process for converting sodium sulphate). This system involves combustion in a conventional recovery boiler and a multi-stage carbonization using a part of the purified flue gas to drive off the sulphide as hydrogen sulfide, leaving sodium carbonate in the solution. The hydrogen sulphide is combusted to sulphur dioxide (SO2) and used for production of sodium sulphite (Na2SO3) from the more or less pure sodium carbonate. There is no intention of keeping any substantial amount of sulphide in the fresh cooking liquor produced, but small amounts of sulphate and thiosulphate might remain and are not considered to interfere with alkaline pulping processes. This recovery process has been standard-forming to the contemporary version of the above-mentioned ASAM process (Ahonen und Lehner, “Umweltsverträgliche Holzaufschlussverfahren”). Recovery of methanol is also included into the system, however without mentioning how it is supposed to co-operate with the pulping technology. The system further involves causticizing of the separated sodium carbonate according to a normal sulphate process type, in order to produce sodium hydroxide (NaOH) required for the cooking.

[0008] Pyrolysis of the spent liquor has also been discussed in EP-A1-205778 (R. Patt and O. Kordsachia, Sulfitaufschlussverfahren zur Herstellung von Zellstoff aus lignozellulosehaltigen Materialen mit Rückgewinnung der Aufschlusschemikalien) where the intention is to produce a more or less pure sodium carbonate Na2CO3, where carbon is a contaminant, and a pyrolytic gas, containing sulphur in the form of hydrogen sulphide. The gas is combusted and sulphur dioxide is absorbed. The document EP-A1-205778 also refers to E. Horntvedt, Tappi 53(1.1): 2147 (1970), which discloses a pyrolysis that, contrary to the process of EP-A1-205778, was commercialized, but at the time of the publication of EP-A1-205778 still was fighting against serious drawbacks concerning disturbing amounts of carbon in the pyrolytic residue and a poor energy balance.

[0009] Further development of gasification technology for sulphite pulping processes has been impeded by the weak position of the traditional sulphite industry and the doubts regarding commercial development of new sulphite pulping techniques. The gasification technology development has therefore been directed to chemical recovery in sulphate pulping processes. Gasification technology has thus been of use in smaller reactors, working in parallel to soda recovery boilers, to give a slight increase in the chemical recovery capacity in sulphate processes having inadequate capacity in the recovery boiler. A more general application of gasification technology in sulphate processes, as an alternative to soda recovery boiler technology, has however not been achieved.

[0010] Document SE-B-462106 (A. Andersson and B. Warnqvist) discloses a process for recovery of energy and process chemicals, primarily intended for sulphate processes. In this process, spent liquor is thermally decomposed under an elevated pressure (10-50 bar) and an oxygen supply that is insufficient for complete combustion. The decomposition temperature is below the temperature that gives a melt (i.e. 700-850° C.). The gas formed during the thermal decomposition is led to a first scrubber where hydrogen sulphide and other at sulphur containing compounds are absorbed in a sodium hydroxide solution. Thereafter the gas is led to a second scrubber for washing the gas with water. The thus washed and cooled gas, which is still under high pressure (16-20 bar), is subsequently led to a gas turbine, where energy is produced. The exhaust gas from the gas turbine is finally combusted in a steam boiler, where steam is generated. The sodium carbonate-containing solid residue of the pyrolysis is dissolved in water and the remaining solid phase is separated, while the liquid sodium carbonate containing phase is lead to a causticizing plant.

[0011] Document SE-C2-503455 (L. Stigsson and J.-E. Kignell) discloses a method for preparation of a sulphite containing cooking liquor comprising chemical recovery of a sulphite process spent liquor wherein the spent liquor is decomposed into a hot gas and a melt, in a reactor working at a high temperature. The aim of this method is mainly to concentrate hydrogen sulphide by a sorption/desorption operation. Chemical recovery of sulphite process spent liquor according to this process has turned out to be complicated and expensive.

[0012] Another development of the gasification technology is to use a fluidized bed reactor for the gasification process (E. Dahlquist, R. Jacobs, “Development of a Dry Black Liquor Gasification Process”, 1992 International Chemical Recovery Conference, Proceedings TAPPI/CPPA), which advantageously gives an opportunity to a more exact control of the temperature and the reaction process. The main problem related to the use of a fluidized bed reactor for gasification of sulphate process spent liquors is to prevent absorption of carbon dioxide (CO2) during the absorption of hydrogen sulphide (H2S) from the pyrolysis gas. Gasification of sulphite process spent liquors in a fluidized bed reactor according to the present invention will not be afflicted with this problem.

[0013] Document U.S. Pat. No. 3,711,593, (P. E. Schick and W. H. Flood) discloses a process for regeneration of chemicals from a sulphite pulping process. In this process the reaction is performed in two-stage or multi-stage fluidized bed treatment. The reason for this is that at the very temperature needed to obtain a complete removal of sulphur a considerable part of the carbon remains as a solid carbon residue. The second stage is operated at a higher temperature to allow combustion of remaining carbon to yield a substantially pure sodium carbonate. This process is not applicable in cases where a sodium sulphide-containing residue is desired.

[0014] Gasification technology in connection with alkaline sulphite processes has been suffering from the difficulty to avoid remaining sulphide in the non-volatile residue at reasonable pyrolysis conditions. A solution of the sodium salts of the residue cannot be directly used for absorption of sulphur dioxide, for production of sulphite, since sulphide will react with sulphite and cause a significant loss of active chemicals. In processes where an essentially sulphide free cooking liquor is required, which has normally been the case, an additional separation by leaching regarding the different solubility of sodium sulphide (Na2S) and sodium carbonate (Na2CO3) or an evaporation of hydrogen sulphide (H2S) by the use of a carbon dioxide containing flue gas would be required. Such processes would thus be unreasonably complicated and expensive, also in comparison with the rather complicated and expensive soda recovery boiler method of the above mentioned WO-A1-9423124. Document U.S. Pat. No. 5,507,912 (P. P. H Lownertz) describes a method where dissolved smelt from a soda recovery boiler is divided into a sulphide-rich and a sulphide-poor flow. This method is based upon the recovery of a smelt from a soda recovery boiler. It does further not yield a totally sulphide free cooking liquor for use in sulphite pulp production and it does not include any means for leaching of soluble process-foreign substances, such as K or Cl.

[0015] As the pulp production systems get more and more closed, the need for controlled ejection of process-foreign substances, which particularly enter the process with the wood raw material is accentuated. No satisfying gasification process that is able to deal with this problem has so far been proposed. Document SE-B-448007 (S. Santen, R. Bernhard and S.-E. Malmeblad) describes removal of NaCl, which is sparingly soluble in a concentrated NaOH-solution that can be exclusively produced according to this method. The spent liquor is subjected to a low temperature pyrolysis to a Na2CO3—C-blend, which is further subjected to pyrolysis in a reactor at 600-800° C. Since the primary aim of this process is, by use of external heating (e.g. a plasma generator), to convert the components of the pulping spent liquor to a blend mainly consisting of sodium sulphide, sodium hydroxide, monatomic sodium, hydrogen and carbon monoxide (at 1000-1300° C. in the reactor), this method hardly has any practical use in this context, especially since the externally heating energy required has turned out to be prohibitively large. It is emphasized that the removal of NaCl is far from covering the concept of process-foreign substances, which includes both normally water soluble and non-soluble compounds.

[0016] In summary it can be noted that the alkaline sulphite processes up till now have been lacking a pulping chemical recovery system that is able to compete with those of the sulphate process (P. Axegård, B. Backlund, B. Warnqvist, “The Eco-cyclic Pulp Mill—With Focus on Closure, Energy-Efficiency and Chemical Recovery Development”, Pre-print, 2001 Int. Chemical Recovery Conf., Whistler, B.C., Canada). Recovery systems based on a traditional soda recovery boiler have turned out to be too complicated and expensive. A gasification process thus seems to be the best starting point for further development. However there is a problem to completely expel the sulphur and directly produce a sufficiently pure Na2CO3 for the sulphite production without using impossible temperatures, which would result in large amounts of carbon in the residue. By a subsequent leaching stage it would be possible though to isolate the sodium carbonate, but in that case it would be necessary to expel H2S from the sulphide containing part or to accept an amount of sulphide in the cooking liquor. The former leads to complicated and expensive systems. The latter has surprisingly turned out to be feasible and even desirable, but only if the sulphide content is adjusted to the cooking process requirements. The existing gasification processes have suffered from the problem of achieving a good energy balance due to the reduction heat required for the pyrolysis of sulphite to sulphide, the take out of hot pyrolysis residue (solid or as a melt) for subsequent dissolution in water and/or extensive dust-cleaning of the pyrolysis gases in a scrubber before final combustion. The existing gasification processes have so far also lacked a settled removal of process-foreign substances, soluble as well as non-soluble. It would thus be desirable to develop a chemical recovery process, which involves a good energy balance and means for handling the presence of sulphide in the solid phase of a gasification reactor.

SHORT DESCRIPTION OF THE INVENTION

[0017] The method of the present invention provides a simple, energy effective and flexible method of recovery of alkaline sulphite pulping chemicals, which method will render alkaline sulphite processes competitive in production of high quality pulps also in comparison to a modern sulphate pulping process. The method comprises gasification of the evaporated spent cooking liquor, resulting in a hydrogen sulphide containing gas and a solid residue. The gas is combusted in a steam boiler, where the hydrogen sulphide is converted into sulphur dioxide and steam is produced. The solid residue is recovered in a leaching process, where process-foreign substances are removed and the rest of the contents is divided into substantially pure sodium carbonate and a mixture of sodium carbonate, sodium sulphate and sodium sulphide. The substantially pure sodium carbonate is used for absorption of sulphur dioxide from the steam boiler and the mixture of sodium carbonate, sodium sulphate and sodium sulphide is causticized and the resulting sodium hydroxide containing solution can optionally be mixed with the substantially pure sodium carbonate after the absorption of sulphur dioxide in order to produce a fresh cooking liquor.

SHORT DESCRIPTION OF THE DRAWING

[0018] FIG. 1 shows the alkaline sulphite chemical recovery process schematically in a block diagram. The reference letters a)-k) in the diagram represent the process steps of the present method and correspond to the process steps as described in the claims. The process steps are:

[0019] a) gasification, b) leaching, c) dissolving, d) filtration and oxidation, e) causticizing, f) dust cleaning and combustion in steam boiler, g) SO2-absorption, h) CO2-desorption, i) preparation of fresh cooking liquor, j) production of H2SO4 and k) separate causticizing FIG. 1 also includes the Total Dry Substance (TS) of evaporated spent liquor that is fed into the gasification stage and the amounts of certain chemicals in the different stages of the process expressed in kg/ADt (Air Dry tonne) chips charged in the cook. The steam boiler effect is given in GJ/ADt.

DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention thus provides a method for recovery of pulping chemicals in an alkaline sulphite pulping process and for production of steam.

[0021] The method will here be described in detail with reference to FIG. 1, which illustrates an example of the process adapted to a standard ASAM-process, with the exception that sulphide is accepted and desired in the fresh cooking liquor. The recovery process of the present invention can of course also be adapted to other alkaline sulphite processes.

[0022] In the recovery method of the present invention, evaporated spent cooking liquor is fed into a gasification reactor (step a), in which the evaporated liquor is decomposed into a hydrogen sulphide (H2S)-containing gas and a solid residue. The application of gasification technology provides for a simple regeneration of cooking liquor from spent liquor in one stage. The choice of gasification technology is a response to the need of achieving a simple and robust system that can be scaled to the very large capacities, as required by the pulp mills of today.

[0023] At a low temperature, 700° C., the equilibrium of sulphur gives that most of the sulphur will turn into the gas phase. When gasifying of conventional kraft black liquor, it has in practice been found that the amount of sulphur transferred to the gas phase is in the region of 50-75%. In the current ASAM-case the spent liquor has an initial sulphur content about twice as high as for kraft black liquor, which in the least should lead to an amount of sulphur transferred to the gas phase of at least the same size. The energy balance of the gasification stage is crucial to the competitive strength of the process. The solid residue contains sodium carbonate (Na2CO3) and a controlled concentration of sodium sulphide (Na2S). It has been discovered that the allowance of Na2S in the solid residue is essential to the construction of this system, and it is necessary to be able to control the sulphur content, so as to achieve a level adequate to the digestion process. The gasification process is adjusted so that the content of sulphur remaining in the solid residue is in accordance with the cooking process claims, by a proper selection of temperature and air supply, so that the amount of sulphide in the solid residue corresponds with the amount desired in the fresh cooking liquor. The temperature must be below the melting point of the salts in the non-volatile residue.

[0024] The evaporated spent liquor fed into a gasification reactor has a dry content of 60-85%. In order to achieve a proper decomposition, the gasification is carried out at a temperature of 650-750° C., preferably 700-750° C. and at a pressure of at most 5 bar, preferably at most 2 bar. Air is added to the gasification reaction in an amount of 25-75%, preferably 30-50% of the total amount required for complete combustion of the evaporated spent liquor. Since each reactor is individual and requires different conditions, these parameters have to be experimentally determined in each case. The gasification reaction is advantageously carried out in a fluidized bed reactor, but other types of gasification equipment could also be appropriate.

[0025] The chemicals remaining in the reactor after the gasification process in step a), mainly comprises Na2CO3 and Na2S and are taken out from the gasification reactor as a solid phase. This solid residue is transferred via an optional heat exchanger to a series of treatment stages (steps b-d) where non-process elements are separated and the remaining quantity of sodium salts is divided into two separate cooking liquor lines, one of which is a substantially pure sodium carbonate (Na2CO3) stream and the other a mixture of sodium sulphide (Na2S), sodium sulphate (Na2SO4) and sodium carbonate (Na2CO3). The pure sodium carbonate (Na2CO3) stream is used for absorption of SO2 and the mixed stream containing all sodium salts is led to causticizing. This measure gives a possibility to allow sulphide in the solid phase after gasification, since the sulphide is bypassed from the absorption of SO2. Moreover, since SO2 in not absorbed in NaOH but in Na2CO3, the causticizing need will be much lower than in a conventional process. Division into two cooking liquor lines also gives the opportunity to both conventional cooking where all chemicals are simultaneously charged, and specialized cooking processes where chemicals are charged in two or more steps.

[0026] The solid residue is led into to a leaching stage (step b), where any soluble process-foreign substances (such as K and Cl) and sodium sulphate (Na2SO4) and a portion of the sodium carbonate (Na2CO3) present and the sodium sulphide (Na2S) are dissolved and separated. The solid residue, mainly consisting of sodium carbonate (Na2CO3), remaining after the leaching process is then led to a dissolving stage (step c), where it is dissolved in water, after which it is filtered and oxidized (step d).

[0027] The leaching process of step b), which constitutes a central section of the recovery process, involves at least two leaching stages b′) and b″), wherein the solid residue is leached, preferably in a counter-current process. A counter-current leaching process involves a transport direction of the leaching liquor that is opposite to that of the solid residue. Each leaching stage is carried out in a separate vessel. In some cases, e g when a higher purity of the fractions is desired, it may be advantageous to involve more than two leaching stages.

[0028] In the first leaching stage b′), soluble process-foreign substances such as K and Cl are dissolved and separated. In a second leaching stage b″), sodium sulphide (Na2S), sodium sulphate (Na2SO4) and a portion of the sodium carbonate (Na2CO3) present are dissolved and separated. This separated stream will be further treated in a causticizing stage e).

[0029] The solid residue remaining after the leaching process in step b) consists mainly of sodium carbonate (Na2CO3) and is now substantially free from Na2S, but any non-soluble process-foreign substances are still present. The residue is dissolved in water in a dissolving stage (step c), and the resulting solution is filtered (step d) to remove non-soluble process-foreign substances and any possible remaining carbon. The solution is then treated in an oxidizing stage (step d) to remove even small amounts of disturbing Na2S remnants in order to give a substantially pure sodium carbonate (Na2CO3) solution. If desired the filtration can be carried out after the oxidizing stage. The resulting pure Na2CO3 solution will subsequently be used for absorption of SO2 from the steam boiler flue gas and for production of sulphite as required.

[0030] The leaching liquor used in the leaching process is preferably a portion of the substantially pure sodium carbonate (Na2CO3) stream, taken from the filtrate stream after the filter of step d). Water is fed into the dissolving tank c) and is led through the dissolving stage of step c) and the filtration stage of step d). After the filter, the stream is divided and one part is led to the oxidizing stage of step d) and the other is led to the leaching stages b″) and b′). The leaching liquor taken out after the filtration stage of step d), is first fed into the second leaching stage b′). After having passed the second leaching stage b″), the leaching liquor is divided into two streams. One of those streams is led to the first leaching stage b′) and the other to a causticizing step e). The stream that is led to the first leaching stage b′) amounts to less than 5%, preferably 1-2% of the total amount of the leaching liquor taken out from the second leaching stage b″).

[0031] The solid residue that is transferred to the first leaching stage b′) is thus leached with a solution, which represents a portion of the saturated liquid phase of leaching stage b″). The separated stream taken out from leaching stage b′), containing the soluble process-foreign substances (such as K and Cl) consequently also contains sodium carbonate (Na2CO3), sodium sulphate (Na2SO4) and Na2S, but since the stream is so small the chemical loss is of little consequence. The remains of the solid residue are then transferred to the second leaching stage b″), where sodium sulphide (Na2S) and sodium sulphate (Na2SO4) are dissolved in the filtrate from step d). The saturated leaching liquor taken out from the second leaching stage b″) contains 20-60%, preferably 40-59% of the total amount of the sodium salts that were fed into the second leaching stage. Both leaching stages are carried out at a temperature of 20-100° C., preferably 50-70° C. The conditions of the leaching process should advantageously be adjusted so that the amount of pure Na2CO3 resulting from the filtration/oxidation stage (step d) will be sufficient for the absorption of SO2 from the steam boiler.

[0032] If desired a portion of the pure Na2CO3-solution can optionally be utilized in a separate causticizing stage for production of additional NaOH, for use e.g. in a bleaching plant. In that case the amount of pure Na2CO3-solution resulting from the filtration/oxidation stage must be increased in relation to the amount of NaOH desired.

[0033] The solution that was separated in the second leaching stage of step b), which contains the sodium sulphide (Na2S), sodium sulphate (Na2SO4) and the portion of sodium carbonate (Na2CO3) that is not required for absorption of SO2, is led to a causticizing stage (step e), where it is causticized. The resulting sodium hydroxide (NaOH)-containing solution may be use for preparation of fresh cooking liquor. Since the absorption of SO2 utilizes the pure sodium carbonate resulting from the filtration/oxidation stage, no NaOH will be required for that purpose and it will thus be sufficient if the causticizing stage produces the amount of NaOH required for production of fresh cooking liquor. The causticized liquor, containing mainly Na2S, Na2CO3 and NaOH, is then optionally mixed with the Na2SO3 solution that has passed through the SO2-absorption stage, so as to form the required fresh cooking liquor. The causticized liquor or a part of it may also be charged into the cooking process in a later stage if that would be desired.

[0034] The hydrogen sulphide (H2S)-containing gas from the gasification reactor in step a) is led to a dust cleaning stage (step f). In the dust cleaning stage all possible particles are separated e g in a cyclone or a ceramic filter. The resulting clean gas is then combusted in a steam boiler (step f), for production of steam. Since the gas does not contain any substantial amount of chemicals it would be possible to choose steam conditions considerably higher, than in a conventional recovery boiler. The gas has preferably not been subjected to any cooling before it is led to final combustion, but if desired the gasification air may be heated by heat exchange with the gas before combustion. During the combustion, the hydrogen sulphide (H2S) is converted to sulphur dioxide (SO2). The steam boiler of step f) has a working pressure of 60-120 bar and a super-heating temperature of 450-560° C.

[0035] The thus generated sulphur dioxide (SO2) is led to an absorption stage (step g) where it is absorbed in the substantially pure sodium carbonate (Na2CO3) solution that results from the oxidizing stage of step d). In order to yield a high quality cooking liquor, the resulting solution is then led through an additional carbon dioxide-desorption stage (step h), where carbon dioxide corresponding to the bicarbonate formed, is removed. The result will be a substantially pure sodium sulphite (Na2SO3) solution, which may be mixed with a desired portion of NaOH.

[0036] The use of substantially pure Na2CO3 for absorption of SO2 from the steam boiler flue gas leads to that the causticizing need of the present process will be much lower than in a sulphate process. With a, in this case presumed, NaOH charge of 10% based upon the wood, the causticizing need will amount to only ⅔ of a sulphate process causticizing need.

[0037] The chemical recovery plant can easily be supplemented with a separate plant for production of H2SO4 from SO2-rich flue gas from the combustion in the steam boiler, in order to support the bleaching plant with H2SO4. By including the production of sulphuric acid (step j) and/or the separate causticizing (step k) to the pulping chemical recovery process, a closed alkaline sulphite pulping process can be achieved.

[0038] The recovery process can, as mentioned above, optionally be completed by mixing (step i) the resulting substantially pure sodium sulphite (Na2SO3) solution with a predetermined amount of the sodium hydroxide (NaOH)-containing solution resulting from the causticizing stage (step d) thus giving a fresh sulphite cooking liquor containing a controlled amount of sodium sulphide (Na2S). The predetermined amount is adjusted in relation to the desired amount of sodium sulphite in the fresh cooking liquor.

[0039] Comparison of the energy balances for a modern sulphate process, an ASAM (Alkaline Sulphite Anthraquinone Methanol)-process utilizing the chemical recovery method according to the present invention and a conventional ASAM-process. The examples relate to plants where the lime sludge reburning kiln uses bark as fuel. The surplus of falling bark is burned in a bark boiler and the steam excess, if any, is used for production of electricity by means of condensing power.

[0040] Chemical charges into the ASAM cooking process in the example according to the present invention are expressed as kg per tonne of dry wood: 1 Na2SO3 260 Na2S 51 Na2CO3 30 NaOH (excluding NaOH 100 from hydrolysis of Na2S) Na2SO4 (as ballast) 5 Anthraquinone 0.7 Methanol 15 vol-% of total liquid

[0041] A comparison of the energy balances shows that an ASAM-process with chemical recovery according to the present invention is much more energy efficient than the conventional ASAM process, and even slightly better than the sulphate process 2 Steam consumption GJ/ADt ASAM with chemical Conventional Sulphate recovery according to ASAM- process the present invention process Soot blowing 1.0 0.0 Chemical recovery 4.2 5.2 Fibre line 3.5 3.6 Pulp dryer 2.2 2.2 Misc.process 0.4 0.4 consumption Sum process 11.3 11.4 ˜15 Condensing turbine/ 5.5 6.4 ˜0 steam surplus Back pressure turbine 3.1 2.9 ˜3 Total consumption 19.8 20.7 ˜18

[0042] 3 Steam production GJ/ADt ASAM with chemical Conventional Sulphate recovery according to ASAM- process the present invention process Recovery boiler 17.7 18.2 ˜16 Bark boiler 1.5 1.9 ˜1.5 Recycled secondary 0.6 0.6 0.6 heat Total production 19.8 20.7 ˜18

[0043] 4 Power balance kWh/ADt ASAM with chemical Conventional Sulphate recovery according to ASAM- process the present invention process Mill consumption 712 689 ˜750 Power surplus, sold 656 711 ˜0 Total 1368 1401 ˜750 Back pressure 831 771 ˜750 generation Condensing power 537 630 ˜0 generation Total power 1368 1401 750 generation

[0044] Some Advantages of the Invention

[0045] A difficult problem related to the closing of pulp mills where also the bleaching plant is to be included into the chemical recovery, is normally related to the Na/S balance. In the system of the present invention, this problem is easily solved, due to its good capacity of internal generation of the bleaching chemicals. This leads accordingly to excellent opportunities of closing the bleaching plant together with the rest of the pulp mill.

[0046] In comparison to a sulphate process the recovery method of the present invention achieves a number of savings. The gasification and combustion of the gas in a separate boiler gives two large energy savings. Firstly, the need for soot blowing steam will vanish since the combusted gas is clean. Conventional recovery boilers use 5-10% of the steam produced for soot blowing. Secondly, the energy loss related to the smelt in a conventional recovery boiler of a sulphate process is omitted. This is due to that the non-gaseous chemicals taken out of the gasification reactor are not in the form of a smelt, but in the form of a solid residue, and thus contain less heat.

[0047] The gas boiler of the present recovery method will give about 7% or 1.2 GJ/ADt more useful energy than can be obtained from a conventional recovery boiler. Due to the higher steam data that can be used for the gas boiler, in comparison to a conventional recovery boiler, the electricity production will increase even more.

[0048] Overall the chemical recovery method of the present invention leads to a more favorable steam balance than in the sulphate process. The decreased causticizing need also results in a decreased need of lime and fuel. The saving amounts to 80 kg lime/ADt.

Claims

1. A method for recovery of pulping chemicals in an alkaline sulphite pulping process and for production of steam, wherein

a) an evaporated spent liquor is fed into a gasification reactor in which said liquor is decomposed into a hydrogen sulphide (H2S)-containing gas and a solid residue containing sodium carbonate (Na2CO3) and a controlled concentration of sodium sulphide (Na2S);

b) the solid residue from the gasification reactor in step a) is leached in a counter-current leaching process which involves at least two stages wherein

in a first leaching stage b′) soluble process-foreign elements such as K and Cl are dissolved and separated, and

in a second leaching stage b″) sodium sulphide (Na2S), sodium sulphate (Na2SO4) and a portion of the sodium carbonate (Na2CO3) present are dissolved and separated;

c) the remaining solid residue from the leaching process in step b) is dissolved in water;

d) the solution resulting from step c) is filtered to remove non-soluble process-foreign elements and any possible carbon residue and is treated in an oxidizing stage to give a substantially pure sodium carbonate (Na2CO3) solution;

e) the solution containing the sodium sulphide (Na2S), sodium sulphate (Na2SO4) and sodium carbonate (Na2CO3) that was separated in the second leaching stage of step b) is causticized, resulting in a sodium hydroxide (NaOH)-containing solution;

f) the hydrogen sulphide (H2S)-containing gas from the gasification reactor in step a) is led through a dust collector and is combusted in a steam boiler, producing steam;

g) the sulphur dioxide (SO2) generated in the steam boiler in step f) is absorbed in the substantially pure sodium carbonate (Na2CO3) solution resulting from the oxidizing stage in step d), and

h) carbon dioxide is removed from the solution of step g) in a desorption stage to form a substantially pure sodium sulphite (Na2SO3) solution.

2. A method according to claim 1, wherein the leaching liquor of the first leaching stage b′) is a part of the saturated leaching liquor that is separated from the second leaching stage b″), and the leaching liquor of the second stage b″) is a part of the sodium carbonate (Na2CO3)-containing solution from step c).

3. A method according to claim 2, wherein the proportion of the resulting leaching liquor from the second leaching stage of step b) taken out for use as leaching liquor of the first leaching stage is less than 5%.

4. A method according to claim 3, wherein the proportion taken out is 1-2%.

5. A method according to claim 1, further comprising a step i) for mixing the substantially pure sodium sulphite (Na2SO3) solution resulting from step h) with a predetermined amount of the sodium hydroxide (NaOH)-containing solution resulting from the causticizing step d) thus giving a fresh sulphite cooking liquor containing a controlled amount of sodium sulphide (Na2S).

6. A method according to claim 1, wherein the evaporated spent liquor in step a) is gasified at a temperature of 650-750° C.

7. A method according to claim 6, wherein the evaporated spent liquor in step a) is gasified at a temperature of 700-750° C.

8. A method according to claim 1, wherein air is added to the gasification reactor of step a) in an amount of 25-75% of the total amount required for complete combustion of the evaporated spent liquor.

9. A method according to claim 8, wherein the amount of air added to the gasification reactor is 30-50% of the total amount required for complete combustion of the evaporated spent liquor.

10. A method according to claim 1, wherein said evaporated spent liquor is gasified in step a) at a pressure of at most 5 bar.

11. A method according to claim 10, wherein said liquor is gasified at a pressure of at most 2 bar.

12. A method according to claim 1, wherein the gasification reactor of step a) is a fluidized bed reactor.

13. A method according to claim 1 wherein the gas from the gasification reactor in step a) is lead to the dust collector of step f) without intermediate cooling.

14. A method according to claim 1, wherein the solution containing sodium sulphide (Na2S), sodium sulphate (Na2SO4) and sodium carbonate (Na2CO3) that is separated in the second leaching stage of step b) contains 20-60% of the total amount of the sodium salts that were fed into the second leaching stage.

15. A method according to claim 14, wherein the solution containing sodium sulphide (Na2S), sodium sulphate (Na2SO4) and sodium carbonate (Na2CO3) that is separated in the second leaching stage of step b) contains 40-50% of the total amount of the sodium salts that were fed to the second leaching stage.

16. A method according to claim 1, wherein the temperature of the second leaching stage of step b) is 20-100° C.

17. A method according to claim 16, wherein the temperature is 50-70° C.

18. A method according to claim 1, wherein the steam boiler of step f) has a working pressure of 60-120 bars and a super-heating temperature of 450-560° C.

19. A method according to claim 1, further comprising a step j) for production of sulphuric acid (H2SO4) from a part of the sulphur dioxide (SO2)-containing gas from the steam boiler of step f).

20. A method according to claim 1, further comprising a causticizing step k) for production of sodium hydroxide (NaOH) from a part of the substantially pure sodium carbonate (Na2CO3) solution from step d).

21. A method according to claim 19, wherein the sulphuric acid (H2SO4) produced in step j) and/or the sodium hydroxide (NaOH) produced in step k) and/or the sodium carbonate (Na2CO3) from step d) are (is) used for bleaching of pulp, resulting in a closed alkaline sulphite pulping and bleaching process.