NO743078L - - Google Patents

Description

Process for the separation of sodium sulfate and sodium chloride from mixtures used for the production of sulfate cellulose.

The invention relates to methods for the separation of mixtures of sodium chloride and sodium sulfate, where the sodium chloride is recovered in substantially pure form and the sodium sulfate is used in the process.

Sodium sulphate is used for paper manufacture, in particular

for the production of kraft paper by the sulphate method as a source of sodium and sulfur to replace consumed compounds in the system. The sulphate process to which the present invention is particularly applicable comprises boiling wood chips or other cellulose-containing fiber materials with a white liquor containing sodium sulphide and sodium hydroxide as the active digestion chemicals,

during the formation of a pulp or paper pulp which is separated from the used cooking liquid, the black liquor.

The pulp then usually goes to bHåce and cleaning steps in specific facilities to produce a pulp with the desired brightness. The black liquor undergoes recovery and regeneration to produce fresh white liquor which is then recycled to the cooking step as at least part of the white liquor which is used to cook more chips.

The recovery and regeneration processes generally comprise an initial concentration of the black liquor followed by combustion in a furnace which, after solidification, leaves a solid mass, a melt, containing sodium sulphide and sodium carbonate. The smelt is dissolved in water to form an aqueous solution, the green liquor, which, after clarification to remove undissolved solids, is causticized with slaked lime, which leads to the conversion of the carbonate to hydroxide with simultaneous precipitation of calcium carbonate. The calcium carbonate is recovered and is usually used for the production of new slaked lime. The aqueous solution obtained after separation of the calcium carbonate Æra liquid is the recycled white liquor.

When replacement chemicals are required, different sources of sodium sulfate can be used. Typically, the sodium sulphate can be produced during the production of chlorine dioxide and chlorine which is used for the bleaching operations in the bleaching plant.

A typical process for producing chlorine dioxide consists in the reduction of sodium chlorate with chloride ions in an acidic medium containing sulfuric acid. Sodium sulphate is recovered as a by-product from the process. In such a process, the basic reactions include the production of chlorine dioxide, chlorine and water according to the equation:

The quantity of replacement chemicals included in the recycling system has been decreasing with the increasing demands on the recycling system, while the need for chlorine dioxide and chlorine has remained at the same level. The amount of sodium sulfate formed during chlorine dioxide production can be lowered while keeping the chlorine dioxide yield at the same level, by using mixtures of hydrochloric acid and sulfuric acid to maintain acidity. The hydrochloric acid can simultaneously provide part or all of the amount of chlorine reducing agent,

as the remainder of the reducing agent may optionally be sodium chloride.

The amount of sodium sulfate produced depends on the relative concentrations of hydrochloric and sulfuric acids. Up to the point where the hydrochloric acid provides half of the acid requirement, the sodium sulphate can be extracted in essentially pure form, however, when the amount of hydrochloric acid exceeds half of the acid requirement, the sodium sulphate can only be extracted in a mixture with sodium chloride.

The plant's need for sodium sulphate and chlorine dioxide can thus be so relaxed that the chlorine dioxide plant has to produce

sere mixtures of sodium sulphate and sodium chloride, if the stoichiometry is to be maintained.

It is not considered beneficial to introduce sodium chloride

in the recycling plant and the regeneration system since this compound is not changed during the recycling processes and is thus built up as dodiast. Thus, when the amount of pure sodium sulphate that can be recovered from the chlorine dioxide plant has been greater than the stocbmetric need, the excess has had to be removed with the consequent loss of sodium and sulphur.

According to one embodiment of the invention, sodium sulfate which is contaminated with sodium chloride and which is produced during the production of chlorine dioxide, can be used as a substitute chemical in the pulp production process.

The mixture of sodium sulfate and sodium chloride is not limited to what is produced when sodium chlorate is reduced with hydrogen chloride in an acidic medium, in a mixture of hydrogen chloride and sulfuric acid.

It is known to produce chlorine dioxide by reduction

of sodium chlorate with a reducing agent in an acidic environment in the form of sulfuric acid, where the reducing agent is a compound which in situ also reduces the chlorine that is formed together with the chlorine dioxide.

This gives essentially pure chlorine dioxide. E.g. the reducing agent may be sulfur dioxide, according to the Mathieson process, or methanol as in the Solvay process. Hydrochloric acid can be used as part of the acid and part of the reducing agent included in the process. Sodium sulphate recovered from these operations can be mixed with sodium chloride. Such mixtures can be used according to the invention.

In Canadian Patent 915,361 and U.S. Pat. patent 3,746,612 describes the removal of sodium chloride from boiler systems by white liquor evaporation.

In parallel application serial no. 29,585/73 (Canada) the application of the process according to Canadian patent 915,361 and U.S. patent is described. patent 3,746,612 on white liquor containing sodium sulfate. The process described in this parallel application can be used for the separation of sodium chloride added during the recovery and regeneration process from sodium sulfate according to the invention.

The mixture of sodium sulfate and sodium chloride can be added at any point. E.g. the mixture can be added to the white liquor after the causticisation step. The white liquor may possibly already contain unregenerate digestion chemicals, generally sodium sulfate, which is due to inefficient combustion and uncausticized sodium carbonate, as well as sodium chloride from the sources described in the above-mentioned application. The mixture can also be added to the green liquor or mixed with the smelt to produce the green liquor, which means that these compounds are found in the white liquor after causticification. It is preferred to add the mixture after combustion to reduce the sodium chloride load on the furnace.

According to the method described in the above-mentioned application, the white matter is concentrated, typically by evaporation and e.g. by boiling under reduced pressure, for deposition of unregenerate digestion chemicals and sodium chloride and where essentially pure sodium chloride is separated from the deposited compounds.

The unregenerated digestion chemicals are returned

to the oven.

Alternatively, the mixture of sodium sulphate and sodium chloride can be added to the black liquor, although this method is less favorable in that it increases the amount of sodium chloride to which the furnace is exposed. The sodium chloride passes the furnace and later processes that treat the white liquor, while the sodium sulfate is used in the furnace with the exception of a smaller amount found in the smelt, the latter amount depending on the efficiency of the incinerator. The white liquor concentration again enables recovery and removal of the sodium chloride, while unregenerate sodium sulphate is recycled to the furnace.

The burning of the black liquor as previously mentioned produces a solid melt which dissolves in an aqueous liquid to form green liquor. In the parallel application serial no.($.20) a method is described for the recovery and removal of sodium chloride from the smelter or from the green liquor and for recirculation of sodium sulfate to the furnace. This method can be used to separate the components sodium sulphate and sodium chloride in the mixture which is formed during the production of chlorine dioxide as previously mentioned.

The mixture of sodium sulphate and sodium chloride can be added directly to the green liquor or can be mixed with the melt that forms the green liquor, which means that there will be sodium sulphate and sodium chloride in it. The root liquor may contain additional amounts of sodium sulfate due to inefficient furnace operation and sodium chloride from sources mentioned in the aforementioned parallel application series no. 77,413/73.

Alternatively, the mixture can be added to the black liquor or to the white liquor, which leads to the presence of sodium chloride and possibly unregenerate sodium sulfate in the smelt and in the green liquor.

However, the latter methods are less favorable

since they increase the amount of sodium chloride that the furnace has to work with.

The burning of the black liquor also produces exhaust gases that contain entrained amounts of solids - which mainly consist of sodium sulphate and some sodium carbonate. The solids can also contain sodium chloride, when the black liquor contains sodium chloride, e.g. by the processes described in the parallel application series no. 25.985/73 and (J.20) mentioned earlier, or originating from the addition of sodium chloride prior to the furnace run,

according to the first edition of the invention.

The mixture of sodium sulphate, sodium chloride and sodium carbonate is separated from the furnace gases as a deposit or soot precipitation. Mixtures of these compounds are also present in the ash funnel, which contains substances that are removed from the boiler pipes in the furnace roof during cleaning processes. If these substances were to be added directly to the furnace, it would produce ever-increasing amounts of sodium chloride in the system.

According to another version of the invention, the soot deposit and ash compounds are added to the green liquor or white liquor, pure sodium chloride is separated from and sodium sulfate is recycled to the furnace according to the method described in parallel applications series no. 25,985/73 and 37,413/73. On

in this way, the sodium and sulfur content of the sodium sulfate and the sodium content of sodium carbonates from the waste gases in the furnace can be recycled to the furnace without causing a build-up of sodium chloride in the recovery system.

Any suitable technique can be used for the removal of pure sodium chloride, and suitable methods are described exhaustively in said parallel application series No. 25,985/73

and 37,413/73. On the attached block diagrams, a special technique is described for each of the two embodiments according to the invention, but it must be understood that the alternatives described in the above-mentioned parallel applications can also be used.

The invention is now described in connection with the attached block diagrams where: Fig. 1 schematically shows a block diagram of an embodiment according to the invention, and

fig. 2 schematically shows another embodiment according to

to the invention.

Reference is first made to fig. 1 where wood chips or other cellulose-containing fiber materials are fed through line 10 to a boiler 12 where the wood chips are boiled with a digestion liquid that enters through line 14 and contains sodium sulphide and sodium hydroxide as active digestion chemicals, consequently the sulphate process is used.

The resulting pulp and black liquor or waste liquor are separated and the pulp is washed in brown pulp washer 16. The pulp is washed on the embodiment shown in this connection with effluent from the bleaching plant, fed through line 18. Alternatively, the pulp can be washed with water or "contaminated condensate", and bleach - the waste water can be used elsewhere in the system. To avoid the formation of hydrogen sulphide during this washing step, when effluent from the bleaching plant is used, it is preferred to set the effluent from the bleaching plant to a neutral or slightly alkaline pH, typically around pH 9.

The washed and unbleached pulp is fed through a line

20 to a bleaching plant 22 where the pulp undergoes a series of bleaching and cleaning steps. In general, the bleaching and cleaning includes bleaching with chlorine, chlorine dioxide or mixtures of these, supplied through line 24, and cleaning by lye extraction with aqueous sodium hydroxide from line 26, typically according to the so-called CEDED, C/DEDED or DEDED process where C denotes bleaching with chlorine , C/D designations bleaching with mixtures of chlorine and chlorine dioxide, D denotes bleaching with chlorine dioxide and E denotes lye extraction. The pulp is washed during the bleaching steps, typically after each bleaching or lye extraction, with water supplied through line 28. The used washing water from the bleaching-washing steps together with used chemicals from the bleaching and lye extraction steps form the bleaching effluent liquid that passes through line 18. The washing processes can consist of counter-current washing of pulp and washing water and preferably the amount of sodium hydroxide used for the lye extraction of the pulp is somewhat greater than the stoichiometric need for one sodium atom per chlorine atom in the bleaching chemicals, so that the bleaching effluent from line 18 has an alkaline pH as mentioned above. The bleached and cleaned pulp with the desired lightness is extracted from the bleaching plant 22 through line JØ and sold as such or fed to another plant for conversion into paper or other pulp products.

If desired, the bleach waste liquid from line 18 can be added directly to the black liquor in line 32, although this method is less favorable since the total water requirement increases.

The effluent from the bleaching plant through line 18 contains considerable amounts of sodium chloride which is transferred to the black liquor in line 32. It is an advantage to proceed in this way since the harmful components in the bleaching effluent liquid through line 18 and diluted chemicals in this liquid are retained in the plant. If desired, however, the bleaching effluent liquid can be sent out as waste water to waterways and other aqueous liquids used for washing the pulp in the brown pulp washer 16. The sodium chloride content in the bleaching effluent in the latter case will then be taken out of the system and not added to the black liquor.

The chlorine dioxide used for the bleaching plant 22 is produced in a chlorine dioxide generator 34 by reducing sodium chlorate through line 36 with hydrogen chloride from line 38,

in the presence of hydrochloric acid which is provided by the aforementioned hydrochloric acid and sulfuric acid through line 40.

The production of chlorine dioxide can take place in any suitable way, typically it takes place at the boiling point of the reaction medium, and the generator 34 is kept under reduced pressure.

When the generator 34 is operated in this manner, the water vapor developed from the aqueous reaction medium dilutes the chlorine dioxide, and chlorine and a gaseous mixture of steam, chlorine dioxide and chlorine exit the reactor 34.

The gaseous mixture leaving the generator is usually fed to a separation plant (not shown) for condensation of the steam and recovery of an aqueous solution of chlorine dioxide containing some dissolved chlorine. The latter aqueous* solution is then used in the bleaching plant 22 for the bleaching steps labeled D. The chlorine that is separated from the gas mixture can be used for the bleaching steps labeled C in the bleaching plant 22.

Alternatively, the separated chlorine can be used for the production of hydrogen chloride for feeding through line 38 as such, or as hydrochloric acid. In this case, the chlorine demand in the bleaching plant 22 can be met by chlorine produced on site in a lye/chlorine cell where an aqueous sodium hydroxide solution for supply to the bleaching plant 22 through line 26 is produced by electrolysis of an aqueous sodium chloride solution.

For the sake of simplicity, it is shown in the drawing that the chlorine dioxide and the chlorine produced in the chlorine dioxide generator 34 are fed directly to the bleaching plant 22 through line 24. However, it will be understood from the above that other operations can be carried out between the generator 34 and the bleaching plant 22.

In addition, the process in the generator 34 at the boiling point of the reaction medium will result in the deposition of sodium salt as a product in the generator 34. In the embodiment shown, the generator 34 is operated with hydrochloric acid which fulfills more than half of the acid requirement, so that a mixture of sodium chloride and sodium sulfate is deposited in the generator 34 and removed through wire 42..

The composition of the sodium sulfate as it exists from line 42 depends on the acidity of the reaction mixture in the generator 34, and can be present as sodium bisulfate if the reaction is carried out at a high acidity, typically above 9N, sodium sesquisulfate at acidity levels between 5 and 7N and neutral sodium sulfate at acidity levels below 4N.

The sodium chlorate solution that goes to the generator

34 through line 36 can be provided by electrolysis of an aqueous sodium chloride solution, the by-product hydrogen can then be used for hydrogen chloride added through line 38.

The black liquor in line 32 is evaporated in an evaporation plant 44 because it passes through lines 46 and 48 to the furnace 50,

which may be constructed in any known manner. The water that is condensed from the evaporator 44 through line 52 can be used as part of the water demand in the system, e.g. as part of the water that is fed to the bleaching plant 22 through line 28, possibly after suitable treatment.

Sodium sulfate is added to the wort either in solid form or as a slurry or as an aqueous solution, directly or indirectly. The sodium sulphate is used as a substitute for part of the fixed sodium and sulfur values in the system.

The sodium sulphate requirement is met by sodium sulphate which is extracted from the chlorine dioxide generator 34 in line 42. The mixture of sodium sulphate and sodium chloride which is added at any suitable place, e.g. to the black liquor after concentration, through line 54. The mixture in line 42 can be supplied to the system in other places, which will be discussed in more detail later. Alternatively, the mixture in line 42 can be added to the system partly through line 54 and partly elsewhere.

The black liquor forms in furnace 40 a melt which contains

where sodium sulphide and sodium carbonate and also unreacted components such as sodium chloride, which partly comes from the sodium chloride content in the mixture through line 42, as well as sodium sulphate and other sodium sulphur-oxygen salts.

The waste gases from the furnace go through line 56 and

has entrained solids which are typically sodium chloride and sodium sulfate together with small amounts of sodium carbonate. This exhaust gas sludge is precipitated in the stbvcatcher 58 which can be of any suitable type.

The melt is dissolved in the melt dissolver 60 with water, usually weak washing water from washing the calcium carbonate sludge, through line 62, and forms the mother liquor in line 64. The mixture of sodium chloride and sodium sulfate in line 42 can be supplied to the mother liquor in line 64 through line 66.

The materials that are precipitated in the dust collector 58, i.e. the flue gas, mainly consist of sodium chloride and sodium sulfate and can be fed through lines 68 and 70 to the root liquor 64. The mixture of sodium chloride and sodium sulfate through line 70 to the root liquor can comprise only part of the material through line 68. The amount of sodium sulphate contained in the lye depends on the amount introduced through line 66

and 70, the combustion effect and the degree of oxidation for oxidizable sodium-sulphur salts, typically sodium sulphide, after combustion.

The water that is fed through line 62 to the smelter 60 can at least partially be effluent liquid from the bleaching plant through line 18, particularly when fresh water or contaminated condensate is fed to the brown mass washer 16. In this way, the root liquor can also contain dissolved amounts of sodium chloride coming from the bleaching effluent 18, either the entire bleach waste liquid from line 18 is used for washing the pulp in the brown pulp washer 16, the whole bleach waste liquid is used for the production of the green liquor from the smelter, the bleach waste liquid is partly used in the brown pulp washer 16 and partly for the production of the green liquor, or the bleach waste liquid is added directly to the black liquor in line 32, and from sodium chloride introduced with the mixtures through lines 42 and 68'fi

If desired, the smelt can be fractionated to remove the sodium sulphide content and thereby produce a sulphide-poor green liquor, essentially free of sodium sulphide.

After clarification, the ground liquor is fed to the causticization plant 72 of ordinary construction. where the sodium carbonate in the green liquor is largely converted to sodium hydroxide by means of lime supplied through line 74 from the lime kiln 76. The calcium carbonate that precipitates from the resulting white liquor is separated and returned after washing to remove entrained white liquor (not shown) to the kiln 76 through line 78. The aqueous solution that is the result of this washing is termed "weak washing water" which can be used as mentioned above for the production of lye.

The white liquor originating from the causticization step in line 80 can be added to the mixture of sodium chloride and sodium sulfate from line 42 through line 82 and/or can be added to the mixture of sodium chloride and sodium sulfate from line 68 through line 84, so that the white liquor does not only contain the active dissolving agents sodium sulfide and sodium hydroxide, but also sodium chloride, and unreacted digestion chemicals mainly such as sodium sulfate and uncausticized sodium carbonate and sodium carbonate from the compounds through line 58 when the flue gas precipitates are fed through line 84 to the white liquor. Additional minor amounts of other unreacted digestion chemicals in the form of sodium-sulfur compounds may also be present.

The white liquor in line 80 is evaporated in a first-stage evaporator 86 to precipitate sodium sulfate, sodium carbonate and other precipitable dissolved substances, until the white liquor is essentially saturated with sodium chloride, i.e. up to a point where further concentration will lead to the precipitation of sodium chloride. Most of the sodium sulfate and sodium carbonate are precipitated in this step, usually as anhydrous sodium carbonate and dobbeSsalted burkeite, Na2C0^.2Na2S0^. Although other unreacted digestion chemical potassium that is present in smaller quantities can be precipitated together with sodium carbonate and sodium sulphate, the process is described particularly in connection with the latter two substances. - In general, the white liquor is concentrated in the first stage evaporator 70 to approx.

26 - 30 weight percent sodium hydroxide plus sodium sulfide.

Some unreacted digestion chemicals such as sodium polysulphide and sodium thiosulphate can remain dissolved in the white liquor.

In certain cases, the precipitation can be stopped at a point where the solution is less than saturated with sodium chloride.

Furthermore, co-precipitation of smaller amounts of sodium chloride in the first stage evaporator 86 can be tolerated.

The concentration of the white liquor in the first stage evaporator 86 is advantageously carried out by evaporation, as described above, typically by boiling, if desired under reduced pressure, although other concentration methods can be used if desired. First stage evaporator 86 can be of any shape, e.g. a single evaporation vessel or may consist of a number of interconnected evaporation vessels. Water originating from the evaporation can be extracted through line 88 and used as part of the system's water demand.

The concentration of white liquor in the first stage evaporator

86 precipitates most of the sodium sulfate and sodium carbonate from the white liquor. The salts that are precipitated from the white liquor, i.e. unregenerated digestion chemicals, are removed in the first stage evaporator 86 from the mother liquor through line 90 and recycled to the black liquor in front of the kiln. Preferably, the mixture is recycled to the black liquor through line 46. Unregenerate digestion chemicals which are taken out through line 90 can also be supplied in solid or aqueous form to the digester 12 or the digestion liquid 14.

By recycling the compounds in this way, sodium, sulfur and carbonate are recovered from the precipitate in the first stage evaporator 86,

and reused in the system.

The partially concentrated white liquor from the first stage evaporator 86 is fed through line 92 to a second stage evaporator 94.

Two separate vaporizers 86 and 94 are shown, but this is only

a selected embodiment which simply describes the method of the invention. Two separate evaporation steps, or other concentration methods can be carried out in the same apparatus, with subsequent separation of solid precipitation from the mother liquor after each step. Alternatively, two or more separate evaporator systems can be used if desired.

The semi-concentrated white lye, saturated with sodium chloride and residual amounts of sodium sulfate and sodium carbonate,

but unsaturated with active digestion chemicals sodium sulfide and sodium hydroxide, are evaporated or otherwise concentrated according to the process described in said Canadian Patent No. 915,361 or U.S. patent no. 3,746,612, for precipitation of non-clumping chemicals, including sodium chloride. The concentration of the white liquor in the second-stage evaporator 94 can be carried out to precipitate part of or practically the entire amount of non-absorbing components in the partly concentrated white liquor, and up to a concentration of NaOH + Na2S of approx. 36 - 42 percent by weight.

The concentration of the partially concentrated white liquor in the second-stage evaporator 94 preferably takes place by evaporation, typically by boiling as described above, although other concentration methods can be used. The water coming from the evaporator • is extracted through line 96 and can be used as part of the system's water circulation. For example, the water from lines 88 and 96 can be used as part of the raw water fed to the bleaching plant through line 28. Most of the water which is evaporated from the white liquor is evaporated in the first stage evaporator 86 and less than 20% is evaporated in the second stage evaporator 94.

The solid precipitate from the second stage evaporator 94 is taken

out through line 98 and fed to a 'dissolving plant 100. The solid precipitate from line 98 consists mainly of sodium chloride contaminated with smaller amounts of unreacted dissolved components, mainly sodium carbonate and sodium sulphate, in the form of anhydrous sodium carbonate and Burkeite. Sodium chloride usually makes up over 80% of this mixture.

The concentrated white liquor from the second stage evaporator 94 has a particularly reduced sodium chloride content and is fed through line 102 where it forms at least part of the digestion liquid which is fed through line 14 to the boiler 12.

The concentrated white liquor through line 102 can be diluted

with water or BPE, if desired, for which it is supplied to the digester 12. The water used for dilution of the concentrated white liquor may be provided at least in part by water from lines 52, 88

and 96.

In addition to the active digestion chemicals, the recycled and concentrated white liquor may contain residual quantities of unreacted digestion chemicals such as sodium sulphate, sodium carbonate, sodium polysulphide and sodium thiosulphate. These substances in the recycled white liquor are not a disadvantage, since the unregenerated digestion chemicals pass the digester and undergo recovery and regeneration steps. The recycled white liquor may also contain sodium chloride which, while the sodium chloride in the recycled white liquor is a dodlast, under steady state conditions remains substantially constant since the amount of sodium chloride withdrawn from the white liquor through line 98 is approximately equal to the amount of sodium chloride introduced into the system from sources other than the recycled white liquor, thus the bleach waste liquid from line 18, line 42 and line 68.

The water is fed to the dissolving plant 100 through line 104, and the water dissolves sodium carbonate, sodium sulfate, other contaminants and some sodium chloride, forming an aqueous solution of these substances, and leaves essentially pure sodium chloride which is taken out through line 106, possibly after washing f or to remove entrained liquid. In such washing, used washing water can form a part of the Mannet which is fed to the dissolution plant through line 104. It is possible to separate the solid mixture in other ways than by leaching, for the extraction of pure sodium chloride. E.g. a hydraulic separation method can be used which uses physical classification based on crystal size. The sodium chloride crystals are large cubic crystals, the sodium carbonate has small and needle-like crystals

and the burkeite crystals are small and flat.

The white liquor through line 80 thus undergoes a concentration to produce a white liquor through line 102 which has a reduced sodium chloride content and a reduced content of unregenerate digestion chemicals. In addition, essentially pure sodium chloride is recovered through line 106 from the compounds deposited from the white liquor concentration, and most of the unregenerate digestion chemicals that precipitate from the white liquor concentration step are recycled before the burning step through line 90.

By using the white liquor concentration method in this way, you can achieve separation of contaminating sodium chloride in a mixture with sodium sulphate in lines 42 and 68, so that the sodium sulphate can be used as a replacement chemical.

Although mixtures of sodium chloride and sodium sulfate are shown through both lines 68 and 42, supply of the mixtures to the system from one or the other of these sources may be omitted.

The water solution that comes out of the dissolving plant 100 through line 108 can be recycled to the white liquor in line 80. The recycled solution through line 108 is therefore kept within the white liquor concentration and the amount of sodium chloride in the recaustification system is reduced.

Alternatively, the aqueous solution from wire

108 is recirculated after possible filtration to the root liquor in line 64. The aqueous solution in line 108 can be recirculated

is fed to the melt dissolver 60 and combined with the water introduced through line 62 to produce the root liquor. Optionally, the aqueous solution can be fed through line 108 to a dissolution of the melt in water that leaves the melt dissolver 60. In both cases, the aqueous solution is present through line 108 in the root liquor that is fed to the causticization plant 72.

It is an advantage to recirculate the aqueous solution in line 108 to a point downstream of the incinerator, due to the sodium chloride content of the solution, which makes it desirable to limit the amount of sodium chloride with which the furnace is charged. The amount of chemical in the recycled aqueous solution through conduit 108 is essentially constant under steady state conditions.

The sodium chloride exiting through line 106 can be used in various ways, e.g. for the production of chlorine dioxide, the production of sodium chlorate or the production of lye extraction ionic liquid as explained above.

Reference is now made to fig. 2 which illustrates an alternative method for recovering sodium chloride. Many steps are the same as in the embodiment of fig. 1 and in many cases the modifications discussed in connection with fig. 1 is also used here. Wood chips are fed through line 210 to the boiler 212, where the chips are boiled with digestion liquid supplied through line 214, containing sodium sulphide and sodium hydroxide as active digestion chemicals, thus according to the sulphate process.

The resulting pulp and black liquor are separated and the pulp is washed in fen brown pulp see washer 216. The pulp is washed with ..wastewater from the bleaching plant fed through line 218. To avoid the formation of hydrogen sulphide during this washing step with bleaching waste water, it is an advantage if the wastewater through line 218 has neutral or slightly alkaline pH, typically around pH 9.

The washed and unbleached pulp is fed through line 220 to the bleaching plant 222 where it undergoes a series of bleaching

and purification step comprising bleaching with chlorine, chlorine dioxide or mixtures thereof, supplied through line 224, and purification by lye extraction with an aqueous sodium hydroxide solution

through wire 226, typically after a CEDED sequence. The pulp is washed during the bleaching operations, typically after each bleaching or lye extraction, with water fed through line 228. The used washing water from the bleaching-washing operations together with used chemicals from the bleaching and lye extraction steps make up the bleaching effluent through line 218.

The bleached and cleaned pulp with the desired degree of lightness is extracted from the bleaching plant 222 through line 230 and can be fed to the paper machine or other processes.

The bleach effluent through line 128 contains significant amounts of sodium chloride which is transferred to the black liquor through line 232.

The chlorine dioxide used for the bleaching plant 222 is produced in a chlorine dioxide generator 234 by reduction in an aqueous medium of sodium chlorate supplied through line 236, with hydrogen chloride through line 238, in the presence of hydrogen chloride in the form of the above and sulfuric acid through line 240.

The production of chlorine dioxide, the separation of chlorine dioxide and chlorine which is produced in the generator 234, the chlorine production position for delivery to the bleaching plant 222 and the method for supplying the bleaching compounds to the bleaching plant 222 have been described earlier in connection with the generator 34 in fig. 1. The considerations made in this connection also apply to generator 234.

In the same way as generator 34 in fig. 1, it is advantageous to run the generator 234 approximately at the reaction medium's boiling point, so that a mixture of sodium chloride and sodium sulfate is deposited from the reaction medium. This mixture is taken out from the generator 234 through line 242.

The black liquor is evaporated in an evaporator 244 before it is fed out through lines 246 and 248 to the furnace 250, which may have any desired construction. The water extracted from the evaporator 244 through line 252 can be used as part of the system's water circulation.

The sodium sulphate is added to the black liquor in furnace 250 as a solid, slurry or solution, either directly or indirectly. The sodium sulfate is used as a replacement for part of the sodium and sulfur that is lost during the chemical recovery and

regeneration.

As described above in connection with fig. 1, the sodium sulfate requirement is covered by sodium sulfate which is withdrawn from generator 234 through line 242. The mixture of sodium sulfate and sodium chloride can be added at any suitable point, e.g. to the black liquor after concentration through line 254.

The black liquor forms in the furnace 250 a melt that retains sodium sulphide and sodium carbonate and unreacted components, in

essentially sodium chloride and sodium sulfate. One thus obtains from the furnace operation a melt containing sodium sulphide, sodium carbonate, sodium chloride and sodium sulphate.

Flue gases from the furnace leave this through line 256 and contain entrained solids, mainly sodium sulfate and sodium chloride. The stove is deposited in the stove collector 258 which can be of any type.

The melt is dissolved in the melt dissolver 260 - in water which is supplied through line 262. The water is preferably "weak wash water" from washing the calcium carbonate sol after causticification for reasons described in the parallel application series no. 37.413/73.

The green liquor formed in line 264 also contains recycled chemicals which are described further below. The mixture of sodium chloride and sodium sulfate in line 242 can be supplied to the alkaline solution through line 266. The substances that precipitate in the sodium chloride collector 258, i.e. a mixture of mainly sodium chloride and sodium sulfate, can be fed through lines 268 and 270

to the root liquor in line 264. In addition to the sodium sulphide, the root liquor thus also contains dissolved sodium carbonate, sodium sulphate and sodium chloride from the smelting, from the bleaching water and from the mixtures in lines 242 and 268.

The root liquor is evaporated or concentrated in another way

in a first evaporator crystallizer 272, depositing a mixture of sodium carbonate and sodium sulfate. This evaporation, which can be carried out in any way, typically by boiling, possibly under reduced pressure, continues until the saturation point for sodium chloride has been reached or around this, i.e. up to a point where further concentration will lead to the precipitation of sodium chloride . Most of the sodium carbonate and sodium

the sultate is precipitated at this point, usually as an anhydrous sodium carbonate mixed with doubly salted burkeite, ^200^. 2 Na 2 SO 3 .

First stage evaporator crystallizer 272 may be of any shape, e.g. a single evaporation container or can consist of a number of interconnected evaporation containers.

The precipitated solid mixture of sodium carbonate and sodium sulfate is separated from the mother liquor and fed through line 274 to the dissolving plant 276. The water evaporated from the root liquor in the evaporator 272 passes through line 278 and is collected.

The mother liquor consisting of an aqueous sodium sulfide solution passes from the first stage evaporator crystallizer 272 through line 280 to the second stage evaporator crystallizer 282. Although two separate evaporator crystallizer plants 272 and 282 are shown, this is only an example which easily describes this embodiment of the invention. Two separate evaporation operations can be carried out in the same apparatus, with separation of the solid precipitate from the mother liquor after each step. Optionally, two or more separate evaporators can be used if desired.

The aqueous sodium sulphide solution, saturated with sodium chloride and residual amounts of sodium sulphate and sodium carbonate, but unsaturated with sodium sulphide, is evaporated in the second stage evaporator-crystallizer 282 and deposits a mixture of sodium chloride, sodium carbonate and sodium sulphate which is removed through line 284. Water vapor is removed through line 286. The evaporation of the sodium sulfide solution in the second stage evaporator-crystallizer 282 can take place in any manner, typically by boiling, and if desired under reduced pressure. The water through lines 278 and 286 can be used as part of the water demand in the system.

The mixture drawn off through line 284 contains predominantly sodium chloride contaminated with sodium carbonate

nat and sodium sulfate and treated to extract essentially pure sodium chloride. As shown, the mixture is fed to a dissolving plant 288 with water supplied through line 290 to dissolve substantially all of the sodium carbonate and sodium sulfate in the mixture along with a portion of the sodium chloride, leaving essentially pure sodium chloride which is withdrawn through line 292 after any washing to remove entrained mother liquor. In the case of sliM washing, the washing water can be used as part of the water

the dissolution plant through line 290.

Preferably, the concentration of the aqueous sodium sulphide solution in the second stage evaporator-crystallizer 282, and the heating operation is carried out in such a way that the amount of sodium chloride extracted through line 292 is approximately the same as the amount of sodium chloride introduced into the system so that a balance can be achieved .

The aqueous solution of sodium carbonate, sodium sulfate and sodium chloride that is formed in the dissolving plant 288 is fed through line 294 to the lye in line 264. This can be achieved by feeding the solution to the melt dissolver 260 together with water through line 2#2 for dissolving the smelt. Optionally, the aqueous solution can be added to the green liquor after it has been prepared.

The concentrated sodium sulphide solution produced in the evaporator crystallizer 282 in line 296 is partially re-circulated through line 298 to the mother liquor in line 264, typically the former is used as at least part of the water supplied through line 262 for dissolving the smelt . This recirculation of the sodium sulfide solution increases the alkalinity of the sodium sulfide solution which is concentrated in the second stage evaporator crystallizer 282.

The rest of the concentrated sodium sulphide solution, which has reduced sodium kifro.Bide content and may contain residual amounts of unregenerate digestion chemicals, is present

in line 300.

The mixture of sodium carbonate and sodium sulfate in line 274, which is fed to the dissolver 276, is dissolved in water supplied through line 302, forming sulphide-free lye which passes through line 304 to the caustication plant 306, where essentially all sodium carbonate is converted into sodium hydroxide by means of lime that is added to the causticisation plant 306. through line 308. The calcium carbonate sol that is precipitated in the caustication plant 306 is separated from the sulphide-free white liquor and fed through line 310 to the lime kiln

312 for burning more lime.

The calcium carbonate sludge is washed with water to remove entrained white liquor for the carbonate goes to the lime kiln 312.

It used the washing water from washing the lime sludge

are mentioned "weak washing waters" which are used with advantage for

ing of the melt in the melt dissolver 260 together with recirculated sodium sulphide solution. Use of this dilute sodium hydroxide solution to dissolve the melt further increases the alkalinity of the sodium sulfide solution which is concentrated in the second stage evaporator crystallizer 282.

The sulphide-free white liquor consisting of an aqueous solution of sodium hydroxide containing sodium sulphate and uncalcified sodium carbonate passes through line 314 and is mixed with the sodium sulphide solution in line 300, with suitable dilution with water or BPE, if necessary, to create the desired ratio of sodium sulphide to sodium hydroxide forming white liquor in line 316, sodium sulphide and sodium hydroxide, which are recycled as part of the digestion liquid in line 214.

Mixtures of sodium chloride and sodium sulfate through lines 242 and 268 can be supplied to the white liquor in line 316,

e.g. through line 318. However, in order to make the amount of natMum chloride to which the furnace 250 is exposed as small as possible,

is it an advantage that these mixtures are added to the lye in line 264 or mixed with the smelt for the formation of the lye.

By thus using the concentration technique as described in connection with fig. 2, sodium chloride can be removed from mixtures of sodium chloride and sodium sulfate through lines 242 and 268 and the sodium sulfate can be used as a replacement chemical without a build-up of sodium chloride in the system.

Without recirculation of sodium sulfide solution through line 298, the sodium sulfide solution concentrated in second stage evaporator crystallizer 282 has a total alkalinity, i.e., Na 2 S + NaOH content, of only about 20 - 30% of the alkalinity in the white liquor. Thus, the yield of sodium chloride that can be obtained by evaporating the sodium chloride solution is only approx. 20 - 30% of the yield of sodium chloride which can be obtained by evaporating the white liquor, according to said Canadian patent no. 915,361 and U.S. Patent No. 3,746,612. In some cases, therefore, the amount of sodium chloride that can be extracted by concentrating the sodium sulphide solution is insufficient to correspond to the preferred version of the invention, which consists in being able to remove from the system the same amount of sodium chloride that is introduced into the system. •

If the former situation exists, one or more operations can be carried out as described in detail

in the parallel application No. 37,413/73, to increase the total alkalinity (Na2S + NaOH) in the sodium sulphide solution through line 280, and concentrate this for precipitation of sodium chloride.

The pure sodium chloride that is precipitated through line 292 can be used in many different ways. E.g. can the sodium chloride be used for the regeneration of bleaching chemicals. For example, the sodium chloride can be used for the production of sodium hydroxide by electrolysis of an aqueous solution of the former, the sodium hydroxide being used in the bleaching plant in line 226. Furthermore, the sodium chloride can be electrolyzed as an aqueous solution to sodium chlorate for use in the chlorine dioxide generator 234.

Modifications can of course be made without thereby going outside the scope of the invention.

Claims (14)

1. Process for digesting cellulose fiber material where the cellulose material is brought into contact with a digestion liquid containing sodium sulphide and sodium hydroxide, digested cellulose material is separated from used digestion liquid, this used digestion liquid undergoes recovery and regeneration steps for the production of a white liquor, said white liquor is recycled as at least a part of said digestion liquid and sodium sulfate is added to the used white liquor to replace sodium and sulfur losses in the process, characterized by producing chlorine dioxide by reaction between sodium chlorate and hydrochloric acid in the presence of sulfuric acid, recovering a mixture of sodium chloride and sodium sulfate from this reaction, feeding said mixture to recycling - and the leg generation steps, for introducing replacement sodium sulfate and sodium chloride from the mixture into the white liquor, concentrating the white liquor to precipitate solid sodium chloride therefrom, and separating said solid sodium chloride from the concentrated white liquor. 2. Method as stated in claim 1, characterized in that the chlorine dioxide is produced in a mixture with chlorine by reducing the sodium chlorate in an acidic medium according to the equation: which hydrochloric acid supplies the entire stoichiometric need for chloride ions and more than half of the total stoichiometric acid demand and the rest of the stoichiometric acid demand are supplied of the sulfuric acid. 3. Method as stated in claim 2, characterized in that said acidic reaction mixture is kept essentially at the boiling point for evaporation of water from the reaction medium, the chlorine dioxide and chlorine are mixed with the evaporated water and said mixture of sodium chloride and sodium sulfate is precipitated from the reaction medium by this evaporation. 4. Method as set forth in claim 1, characterized in that the recovery and regeneration steps comprise concentration of said used digestion liquid, that the concentrated used digestion liquid is burned to a solid melt which dissolves in an aqueous liquid to produce a green liquor which is causticized and forms said white liquor . 5. Method as stated in claim 4, characterized in that the mixture of sodium chloride and sodium sulphate is added to the white liquor for this to be concentrated, so that the white liquor contains the mixture's sodium chloride and sodium sulphate. 6. Method as stated in claim 4, characterized in that the mixture of sodium chloride and sodium sulphate is added to the green liquor so that the white liquor contains the mixture's sodium chloride and sodium sulphate. 7. Process as stated in claim 5 or 6, characterized in that the white liquor is concentrated to precipitate sodium chloride, sodium sulfate and uncalcified sodium carbonate and that sodium sulfate and uncalcified sodium carbonate free of sodium chloride is extracted from the precipitated substances, whereby sodium chloride is separated and extracted from the precipitated compounds such as the aforementioned separated solid sodium chloride and the recovered sodium sulfate and uncalcified sodium carbonate are recycled to the concentrated black liquor. 8. Method as stated in claim 7, characterized in that the white liquor concentration is continued until the white liquor is practically saturated with sodium chloride and so that a mixture of sodium sulfate and unreaerated digestion chemicals is precipitated, the precipitated mixture of sodium carbonate and sodium sulfate is separated out as said recovered mixture of sodium sulfate and uncalcified sodium carbonate free of sodium chloride, and that the concentrated white liquor is further concentrated to precipitate a mixture of sodium carbonate, sodium chloride and sodium sulfate, which is separated from the resulting concentrated white liquor. 9. Procedure as specified in claim 8. characterized in that the separated mixture of sodium carbonate and sodium sulfate is fed to the digestion liquid used for boiling the cellulose material, the separated mixture being fed to the concentrated black liquor. 10. Method as stated in claim 8, characterized in that the separated mixture of sodium carbonate and sodium sulfate is added directly to said concentrated black liquor. 11. Method as stated in claim 8, characterized in that the separated mixture of sodium chloride, sodium sulfate and sodium carbonate is treated for the recovery of essentially pure sodium chloride in the form of said sodium chloride extracted from the precipitated substances during the white liquor concentration. 12. Method as set forth in claim 11, characterized in that essentially pure sodium chloride is extracted from the latter mixture by leaching with water to form an aqueous solution of sodium carbonate, sodium sulfate and sodium chloride which leaves a solid mass which is essentially pure sodium chloride. 13. Method as stated in claim 12, characterized in that the latter aqueous solution is recycled to the white liquor before it is concentrated or during concentration until saturation of the white liquor with sodium chloride. 14. Method as specified in claim 8, characterized in that the concentration and further concentration of the white liquor takes place by evaporation. 15. Method as stated in claim 14, characterized in that the evaporation takes place by boiling the white liquor. 16. Method as stated in claim 14, characterized in that the evaporation takes place by boiling the white liquor under reduced pressure. l!z5. Method as stated in claim 4, characterized in that sodium chloride is provided in said black liquor, that a flue gas is formed which contains entrained solids during the furnace firing, at least part of these solids in the flue gas are deposited as a solid mass containing sodium sulfate and sodium chloride and that these solid components are added to the green liquor or white liquor to concentrate them. 1?. Process for digestion of cellulose-containing fibres, whereby the cellulose fibers are brought into contact with a digestion liquid containing sodium sulphide and sodium hydroxide, the digested cellulose mass is separated from the used digestion liquid, the black liquor, as sodium chloride is present in the black liquor, which is sent to be burned to produce a solid melt and flue gases containing entrained solid components, of which at least a significant part is precipitated into a mass containing sodium sulfate and sodium chloride, that a green liquor is produced from the smelting and this green liquor is causticized into white liquor which is recycled in the form of at least part of the digestion liquid, characterized in that said precipitated solid components are added to the green liquor or white liquor, the white liquor is concentrated to precipitate solid sodium chloride and the sodium chloride is separated from the concentrated white liquor. 19. Process as set forth in claim 18, characterized in that the white liquor, in addition to the active digestion chemicals, contains dissolved sodium chloride from the black liquor and from the added solid components mentioned above, sodium sulfate from said solid components and from unregenerate compounds in the smelting, as well as uncausticized sodium carbonate, and that the white liquor is concentrated to precipitate sodium chloride, sodium sulfate and uncalcified sodium carbonate, which sodium sulfate and uncalcified sodium carbonate free from sodium chloride is extracted from the precipitated compounds and sodium chloride is separated and extracted from the precipitated compounds and that the extracted sodium sulphate and uncalcified sodium carbonate are recycled to the black liquor. 20. Procedure as specified in claim 19, charac- served by continuing the concentration of the white liquor until the white liquor is practically saturated with sodium chloride and a mixture of sodium sulfate and unregenerate digestion chemicals is precipitated, the precipitated mixture of sodium carbonate and sodium sulfate as said recovered mixture of sodium sulfate and uncalcified sodium carbonate free of sodium chloride, that the concentrated white liquor is further concentrated to precipitate a mixture of sodium chloride, sodium carbonate and sodium sulfate which is separated from the resulting concentrated white liquor. 21. Method as set forth in claim 20, characterized in that the separated mixture of sodium carbonate and sodium sulfate is added to the digestion liquid used for boiling the cellulose fiber material, while the concentrated black liquor is added to the separated mixture. 22. Method as stated in claim 20, characterized in that the separated mixture of sodium carbonate and sodium sulfate is added directly to the concentrated black liquor. 23. Method as set forth in claim 20, characterized in that the separated mixture of sodium chloride, sodium sulfate and sodium carbonate is treated to extract essentially sodium chloride. 24. Method as stated in claim 23, character i-, characterized in that the sodium chloride is recovered from the mixture by leaching the mixture with water to form an aqueous solution of sodium carbonate, sodium sulfate and sodium chloride which leaves a solid mass of essentially pure sodium chloride. 25. Procedure as specified in claim 24, charac- .sert by recycling said aqueous solution to the white liquor for concentration or during concentration of the white liquor to saturation with sodium chloride. 26. Method as stated in claim 20, characterized in that the concentration and further concentration of the white liquor takes place by evaporation. 27. Method as stated in claim 26, characterized in that the evaporation is carried out by boiling the white liquor. 28. Method as stated in claim 26, characterized in that the evaporation is carried out by boiling the white liquor under reduced pressure. 29. Method as stated in claim 15, characterized in that the pulp undergoes a series of bleaching and cleaning steps using at least one chlorine-containing bleaching chemical in at least one bleaching step and aqueous solutions containing sodium hydroxide in said cleaning step and that a sodium chloride-containing waste water is fed out after the series of bleaching steps and purification step for said black liquor so that sodium chloride is introduced into the black liquor. 30. Method as stated in claim 29, characterized in that the series of bleaching and cleaning steps consists of a first bleaching with chlorine, chlorine dioxide or a mixture of these, a first lye extraction with sodium hydroxide solution, a second bleaching step with chlorine dioxide, a second lye extraction with sodium hydroxide solution and a third bleaching with chlorine dioxide, whereby the amount of sodium hydroxide used is equivalent to the amount of chlorine used for the bleaching steps and the used bleach and lye extraction liquids are mixed to form a sodium chloride-containing wastewater of substantially neutral pH. 31. Method as stated in claim 30, characterized in that the pulp and the black liquor are separated while the pulp is washed after production and before it is fed to the aforementioned series of bleaching and cleaning steps, that the pulp is washed after each bleaching and liquor extraction step while the washing water used for the latter washing processes are fed in counterflow with the pulp through the series of bleaching and cleaning steps, that the used washing water from these washing steps is mixed with used bleaching and lye extraction chemicals and forms a sodium chloride-containing waste water that is used to wash the pulp for this fed series of bleaching and cleaning steps, whereby the waste liquor is introduced into the black liquor. 32. Method as stated in claim 30, characterized in that the chlorine dioxide is produced by reducing sodium chlorate in an acidic environment according to the equation: whereby, the hydrochloric acid provides all the chloride ions and more than half of the total stoichiometric acid requirement, the rest of the stoichiometric acid requirement being provided by sulfuric acid. 33. Method as stated in claim 32, characterized in that the acidic reaction medium is kept essentially at the boiling point for evaporation of water from the medium, as chlorine dioxide and chlorine are mixed with the evaporated water and a mixture of sodium chloride and sodium sulfate is precipitated from the reaction medium during the evaporation, in that the chlorine dioxide used for bleaching is separated from the mixture of chlorine and evaporated water and the mixture of sodium chloride and sodium sulphate is fed to said black liquor, green liquor or white liquor for evaporation. 3U-. Process for the production of cellulosic pulp using a digestion liquid containing sodium sulphide and sodium hydroxide, whereby the digested material is separated from the black liquor, the black liquor undergoes a series of steps comprising concentration of the black liquor, burning of the concentrated black liquor into a solid melt containing sodium sulphide, sodium sulfate and sodium carbonate, the melt is dissolved in an aqueous liquid into a green liquor, chlorine dioxide is produced by a reaction between sodium chlorate and hydrochloric acid in the presence of sulfuric acid, while a mixture of sodium chloride and sodium sulfate is extracted by this reaction, introducing said mixture into the said series of steps whereby the green liquor contains the sodium chloride in the mixture, concentrates the mother liquor to precipitate a mixture of sodium carbonate and sodium sulfate while avoiding the precipitation of significant amounts of sodium chloride, the precipitated mixture is separated from .the resulting aqueous sodium sulfide solution which is concentrated to precipitate sodium chloride, which sodium chloride is recovered from the concentrated sodium sulfide solution, the separated mixture of sodium carbonate and sodium sulfate is converted into an aqueous solution and the sodium carbonate is causticized to form a sulfide-free white liquor which is mixed with the concentrated aqueous sodium sulfide solution as the resulting white liquor containing the sodium sulphate content in the separated mixture of sodium carbonate and sodium sulphate is used as at least part of the digestion water. 35. Process as stated in claim 3 i-, icharacterized in that the chlorine dioxide is produced in a mixture with chlorine by reduction of sodium chlorate in an acidic environment according to the equation where the hydrochloric acid supplies all the chloride ions and more than half of the stoichiometric amount of acid, the rest of the stoichiometric acid requirement being met by sulfuric acid. 36. Method as stated in claim 35, characterized in that said acidic reaction medium is kept essentially at the boiling point for evaporation of water from the reaction medium, chlorine dioxide and chlorine are mixed with the evaporated water and the mixture of sodium chloride and sodium sulfate is precipitated from the reaction medium during evaporation. 37. Method as set forth in claim 34, characterized in that the mixture of sodium chloride and sodium sulfate from the production of chlorine dioxide is added to the lye for which it is concentrated so that the lye contains the former's content of sodium chloride and sodium sulfate. 38. Method as stated in claim 34, characterized in that the sodium chloride is precipitated in a mixture with sodium carbonate and sodium sulfate and this mixture is leached after separation from the concentrated sodium sulfide solution, for the production of an aqueous solution of sodium carbonate, sodium sulfate and sodium chloride, and a solid mass of substantial pure sodium chloride. 39. Method as stated in claim 34, characterized in that part of the concentrated sodium sulphide solution is used as part of the aqueous solution which dissolves the smelt. 4o; Method as stated in claim 38, characterized in that the aqueous solution of sodium carbonate, sodium sulfate and sodium chloride is recycled to the green liquor. 41. Method as stated in claim 34, characterized by providing sodium chloride in the black liquor, precipitates at least a significant part of the solid components in the rbykgaæ after firing as a solid mass containing sodium sulfate and sodium chloride and adds these solid components to the green liquor for concentration. 42. Method as stated in claim 34, characterized in that the causticisation is carried out by bringing the aqueous solution prepared from the solid mixture of sodium carbonate and sodium sulphate into contact with lime to convert the sodium carbonate into sodium hydroxide while precipitating calcium carbonate which is separated and washed free of entrained sodium hydroxide and is regenerated as burnt lime, and that the washing water when washing the calcium carbonate sludge is used as part of the aqueous medium that dissolves the separated smoke components. 43. Method as set forth in claim 42, characterized in that the used wash water and part of the concentrated sodium sulphide constitute said aqueous medium which dissolves the solid precipitated components from the flue gases. 44.. Method as stated in claim 34, characterized in that the concentration of the green liquor and the sodium sulphide solution takes place by evaporation. 45. Method as stated in claim 44, characterized in that the evaporation is carried out by boiling. 46. Method as stated in claim 44, characterized in that the evaporation by boiling under reduced pressure. 47. Process for the production of pulp of cellulose-fibrous material using a digestion liquid containing sodium sulphide and sodium hydroxide, the produced digested pulp is separated from the black liquor containing sodium chloride, which black liquor is burned and forms a solid melt containing sodium chloride, sodium sulphide, sodium sulphate and sodium carbonate and a flue gas containing entrained solids, at least a part of which is precipitated from the flue gas as solids containing sodium sulfate and sodium chloride, the melt is dissolved in water to form a caustic liquor, said solids from the flue gases are added to the caustic liquor, the caustic liquor is concentrated to precipitate a mixture of sodium carbonate and sodium sulfate while avoiding precipitation of significant amounts of sodium chloride, the precipitated mixture is separated from the formed aqueous sodium sulfide solution which is concentrated to precipitate sodium chloride, the precipitated sodium chloride is recovered from the concentrated sodium sulphide solution and the separated mixture of sodium carbonate and sodium sulphate is formed into an aqueous solution and the content of sodium carbonate is causticised under the shaking of a sulphide-free white liquor which is mixed with said concentrated aqueous sodium sulphide solution, and the resulting white liquor containing the sodium sulphate content of said separated mixture of sodium carbonate and sodium sulphate is used as at least part of the digestion liquid. 48. Method as stated in claim 47, characterized in that the sodium chloride is precipitated in a mixture with sodium carbonate and sodium sulfate and that the mixture is leached after separation from the concentrated sodium sulfide solution, forming an aqueous solution of sodium carbonate, sodium sulfate and sodium chloride and a solid mass of substantially pure sodium chloride. 49. Method as stated in claim 47, characterized in that part of the concentrated sodium sulphide solution is used as part of the aqueous liquid which dissolves the smelt. 50. Method as stated in claim 47, characterized in that the aqueous solution of sodium carbonate, sodium sulfate and sodium chloride is recycled to the green liquor. 51. Method as stated in claim 47, characterized in that the cellulose-containing pulp undergoes a series of bleaching and cleaning steps with at least one chlorine-containing bleaching agent in at least one of the bleaching steps and aqueous sodium hydroxide-containing solutions in the cleaning steps and that a sodium chloride-containing, aqueous waste water is fed from the series of bleaching and cleaning steps and into the black liquor whereby the black liquor will contain sodium chloride. 52. Method as stated in claim 51, characterized in that the series of bleaching and cleaning steps comprises a first bleaching with chlorine, chlorine dioxide or a mixture of these, a first lye extraction with sodium hydroxide solution, a second bleaching step with chlorine dioxide, a second lye extraction step with sodium hydroxide solution and a third bleaching step with chlorine dioxide, whereby the amount of sodium hydroxide is equivalent to the amount of chlorine used for the bleaching steps and that the used bleach and lye extraction ionic liquids are mixed and form an aqueous sodium chloride-containing sewage liquid with essentially neutral pH. 53" Method as specified in claim 52, characterized in that said pulp and black liquor are separated during washing of the pulp after production and for the pulp the said series of bleaching and cleaning steps is applied, the pulp being washed after each bleaching and lye extraction step and the washing water used for the washing processes are fed in counterflow with the pulp through the series of bleaching and cleaning steps, the used washing water after these washing steps is mixed with used bleaching and lye extraction chemicals to form an aqueous sodium chloride-containing waste water that is used for washing the pulp because it undergoes the series of bleaching and cleaning steps, so that the waste water is introduced into the black liquor. 54. Method as stated in claim 47, characterized in that the causticisation is carried out by bringing the aqueous solution formed from the aforementioned mixture of sodium carbonate and sodium sulphate into contact with lime to convert the sodium carbonate into sodium hydroxide and precipitate calcium carbonate which is separated from the mixture and washed in it essentially free of sodium hydroxide and regenerated as burnt lime, and the used washing water from the washing of the calcium carbonate sludge is used as part of the aqueous medium which dissolves the separated, solid flue gas components. 55. Method as stated in claim 54, characterized in that the used washing water and part of the concentrated sodium sulphide form said aqueous medium which dissolves the precipitated flue gas constituents. 56. Method as stated in claim 47, characterized in that the concentration of the green liquor and the sodium sulphide solution is carried out by evaporation. 57. Method as specified in claim 56, characterized in that the evaporation is carried out by boiling. 58. Method as stated in claim 56, characterized in that the evaporation is carried out by boiling under reduced pressure.