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 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.
 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
 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
 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.
 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
 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 (SO
 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 Na
 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.
 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.
 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.
 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 (CO
 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.
 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 (Na
 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 Na
 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 Na
 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.
 a) gasification, b) leaching, c) dissolving, d) filtration and oxidation, e) causticizing, f) dust cleaning and combustion in steam boiler, g) SO
 The present invention thus provides a method for recovery of pulping chemicals in an alkaline sulphite pulping process and for production of steam.
 The method will here be described in detail with reference to
 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 (H
 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 (Na
 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.
 The chemicals remaining in the reactor after the gasification process in step a), mainly comprises Na
 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 (Na
 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.
 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 (Na
 The solid residue remaining after the leaching process in step b) consists mainly of sodium carbonate (Na
 The leaching liquor used in the leaching process is preferably a portion of the substantially pure sodium carbonate (Na
 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 (Na
 If desired a portion of the pure Na
 The solution that was separated in the second leaching stage of step b), which contains the sodium sulphide (Na
 The hydrogen sulphide (H
 The thus generated sulphur dioxide (SO
 The use of substantially pure Na
 The chemical recovery plant can easily be supplemented with a separate plant for production of H
 The recovery process can, as mentioned above, optionally be completed by mixing (step i) the resulting substantially pure sodium sulphite (Na
 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.
 Chemical charges into the ASAM cooking process in the example according to the present invention are expressed as kg per tonne of dry wood:
Na 260 Na 51 Na 30 NaOH (excluding NaOH 100 from hydrolysis of Na Na 5 Anthraquinone 0.7 Methanol 15 vol-% of total liquid
 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
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
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
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
 Some Advantages of the Invention
 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.
 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.
 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.
 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.