NO742827L - - Google Patents

Description

Procedure for separating sodium chloride from sodium chloride-containing melts obtained by recycling and regeneration of used cooking liquid. . The present invention relates to the removal of dissolved salts, typically sodium chloride, from sulphidic liquids, typically aqueous sodium sulphide-containing solutions of the type that are formed during recovery and regeneration in paper and wood pulp factories. In the production of wood pulp that is to be used for the production of paper, wood or other raw materials containing cellulose fibers are subjected to chemical boiling in a boiling liquid, whereby a mass of the cellulose-containing, fibrous material is formed. In the present invention, this cooking ash will contain or consist partly of sodium sulphide. The mass is then subjected to bleaching and cleaning in a bleaching plant.

The spent cooking liquor from the decomposition is usually subjected to a series of recovery and regeneration steps to recover unused chemicals and to produce fresh cooking liquor.

A widely used cooking process is the so-called Kraft process. Although the present invention will in the following be described in detail with reference to this Kraft process, the method is also applicable to other types of methods where sodium sulphide is used or where aqueous sodium sulphide is produced as an intermediate product. Typical examples of such processes are the hSyutbytte-pretreatment-Kraft process, polysulphide processes, alkafide and sodium-based sulphite processes.

In the usual Kraft process, raw cellulose fiber material, usually wood chips, is boiled by heating in a cooking liquid,

commonly referred to as white lye, and which contains sodium sulphide and sodium hydroxide as the active chemicals, whereby one obtains

prepared a wood pulp and used cooking liquid, usually called black liquor. The black liquor is separated from the wood pulp by washing in a washing machine, after which the wood pulp is sent to a bleaching plant for bleaching and cleaning.

The black liquor is then sent to a recovery and . regeneration system where the black liquor is concentrated, usually by evaporation, after which this concentrated black liquor is burned in a furnace so that a melt containing sodium carbonate and sodium sulphide is obtained. A sodium and sulfur-containing compound, usually sodium sulfate, is usually added to the black liquor before it is fed into the furnace, although such sodium and sulfur-containing compounds may be added at any convenient time, e.g. to white liquor before boiling, thereby supplementing the sodium and sulfur compounds that are lost in the recovery system.

Said melt is then dissolved in water, after which a fresh liquid is obtained which is then clarified to remove undissolved solids, nicely cleaned fresh washes containing dissolved amounts of sodium carbonate and sodium sulphide, are then causticized with slaked lime, whereby the sodium carbonate is converted. to sodium hydroxide and calcium carbonate sludge. The resulting liquid becomes white liquor which can then be recycled to the boiling stage to provide at least part of the boiling liquor*

The actual bleaching usually includes a series of bleaching and cleaning steps along with washing steps. The bleaching step includes using bleaching agents, and in the present invention, at least one of the bleaching steps preferably includes at least one chlorine-containing bleaching agent. Such chlorine-containing bleaches include chlorine, chlorine dioxide and sodium hypochlorite.

The cleaning itself also usually includes a treatment with a sodium hydroxide solution, and this step is usually referred to as a caustic extraction step. In certain cases, the bleaching and the caustic extraction step can be combined, e.g. by using the so-called °oxygen bleaching<n>process. When oxygen bleaching is used, this process 1 in connection with the present invention will preferably be used in combination with one or more bleaching steps where chlorine-containing bleaching chemicals are used.

A particular bleaching process which has been widely used includes a first bleaching of the pulp with an aqueous solution containing chlorine or a mixture of chlorine dioxide and chlorine, an intermediate wash, a caustic extraction using an aqueous sodium hydroxide solution, further washing followed by bleaching with an aqueous solution of chlorine dioxide,

a new washing and a new caustic extraction using sodium hydroxide, a further washing and a final belching with a chlorine dioxide solution and then a final washing. This is often called the so-called CEDED process. The present invention will be described with special reference to this method, although other methods can also be used, e.g. by using an aqueous reading containing approx. 100% chlorine dioxide in the first bleaching stage.

The above-described CEDED process can be carried out by using a so-called "dynamic bleaching process," as described in Canadian Patent No. 783,483. In this method, mass-treating readings are passed successively through mats of fibers, where the fibers are kept relatively stationary in relation to each other The washing steps, apart from washing after the last bleaching step and cleaning, can be omitted.

The used washing solutions from such bleaching plants have usually been discharged into naturally occurring bodies of water such as rivers, lakes and oceans, without any attempt being made to recover chemicals from such solutions, although in certain cases operations have been carried out for to recover solid particles. One of the main reasons why no attempt has been made to recover these chemicals is that the readings are very dilute and of little value. Such bleaching plants will also produce spent bleach washes and spent washes from the caustic extraction. These washes have a repulsive color and are toxic to harmful aquatic and marine organisms and highly polluting as they contain fibers and materials that consume oxygen in the water. It is desirable to avoid such pollution and therefore such liquids and readings are taken out of the facility.

Due to the use of chlorine-containing bleaches and sodium-containing cleaning agents, the used washing water will contain significant amounts of sodium chloride. In addition to this, in cases where the used bleaching liquids and the used wash from the caustic extraction are combined, a déVis combination of the remaining chlorine and sodium values will be obtained so that sodium chloride is formed. In the present invention, the normally flowing liquids, namely used washing water, used readings from the bleaching plant and used wash ash from the caustic reaction, will preferably be mixed so that a single stream is obtained which is often referred to as BPE, and where such a stream of readings are not transported out of the facility.

The amount of chlorine-containing bleaches and the amount of sodium hydroxide used during the caustic extraction should preferably be balanced so that one sodium atom is obtained for each chlorine atom, whereby these chemicals form sodium chloride and a neutral flowing wash is obtained. An equivalence between chlorine and sodium atoms in the bleaching plant is preferred, as this results in an unchanged sodium balance in the plant. In the aforementioned CEDEE process where chlorine or mixtures of chlorine dioxide and chlorine are used where the amount of available chlorine provided by chlorine dioxide is low, then in the first step an amount of sodium hydroxide solution in excess of that required for the extraction must be added to adjust for the chlorine atoms present. If no excess is added,

only from 40 - 50$ of the filtrate from the first chlorination step could be recovered to be able to correspond to the stoichiometric equivalent of sodium atoms present during the extraction. When the available chlorine is provided 1 all essentially by means of chlorine dioxide, typically over approx. 70%, then the quantities of sodium and chlorine atoms will essentially be equivalent, and it is therefore preferred to use this last method.

Sodium chloride present in the liquid from the bleaching plant can also arise from sodium chloride present in the pulp when it is fed into the bleaching plant. Such sodium chloride may be present in cases where the logs are soaked in seawater before they are used for the production of wood chips. The fact that logs are used flattened in soapy water also means that sodium chloride is present in the black liquor from washing the pulp in the washing plant. If brackish water is also used to provide water for the bleaching plant, then sodium chloride will again be present in the bags from this plant.

Alkaline metal salts can also be added to the plant itself from other sources, e.g. from the cellulose fibers themselves. Furthermore, sodium chloride can be encountered as a contaminant in supplementary chemicals, e.g. from a nocturnal wake or formed as a product of a chemical process. Thus, supplementary chemicals can be sodium sulfate from a natural source contaminated with sodium chloride or sodium hydroxide formed by electrolysis of sodium chloride precipitates and contaminated with sodium chloride.

In the present invention, the liquids from the bleaching plant are preferably sent for recovery and regeneration, so that this liquid remains inside the plant. It has previously been proposed in Canadian Patent No. 832,347 and U.S. Pat. patent no. 3,698,995 that one can reduce the pollution problems in connection with bleaching plants by using used washing water to wash the wood pulp in the washing plant. The use of used washing water in this way reduces the total water requirement for the facility. In the present invention, it is preferred to use liquid from the bleaching plant and this washing consists of a mixture of used washing water, preferably provided by a countercurrent washing as described in Canadian patent no. 832,347 and U.S. patent no. 3,798,955, used bleaching chemicals and used washings from the caustic extract ion to wash the pulp in the washing plant. By using this principle, the need for water can be reduced and you also get a facility that is free of leaking liquids.

In a particular way of carrying out such a counter-current washing in a $ CEDED process, where the pulp is washed, after each step, there is a complete counter-current of liquids, namely used bleaching liquid, used liquid from the caustic extraction as well as the washing water, and de-ice liquids flow countercurrently with respect to the pulp through the bleaching plant. In such a process becomes healthy

water brought into contact with the pulp after the last bleaching step, and used washing water from this washing is then mixed with the used chlorine dioxide reading from the last bleaching step. The resulting mixture is split into two streams, and the main amount - is used to wash© the pulp from the last caustic extraction step, while the remaining part is mixed with liquid from the last caustic extraction step, and the mixture is partially mixed with liquid from the washing step after the intermediate bleaching step .

The remaining part of the mixture is used as wash water to wash the pulp after the intermediate bleaching step.

Spent readings from this step are mixed with aqueous readings from the above washing, and the resulting reading representing the combined bags from subsequent stages of the bleaching plant is used partly to wash the pulp from the first caustic extraction step and partly to wash the pulp from the first bleach step. The spent wash liquor from the washing of the pulp from the first caustic extraction step is mixed with the spent caustic extraction wash from the first caustic extraction, and this mixture is divided into two streams, one representing an alkaline liquid, while the other is used as wash water for the pulp from first bleaching step. The used washing water from this stage is mixed with used bleaching chemicals from the first bleaching stage, whereby an acidic wash is obtained, part of which constitutes an outflowing acidic wash from the bleaching plant.

Another part of the aforementioned acidic wash can be used to mix with pulp from the washing step in order to thereby bring the pulp to the consistency required in the first bleaching step. Another part of said acidic wash can be used as the aqueous medium for gaseous chlorine as used in the first bleaching step. In this way, part of said acidic liquid, which is thereby made up of the last two parts, can be recycled in the first bleaching step.

The acidic used bleaching liquid can then be mixed with alkaline caustic oak immediately ionic liquid, thereby obtaining a solution from the bleaching plant which can then be used to wash the mass in the first washing step.

The liquid from the bleaching plant can be fed into the second stage of regeneration and recovery if this is desired.

Furthermore, the liquid from the bleaching plant can be split into two or more streams which can be supplied at different places during recovery and regeneration, e.g. to give nsvakt, washing water*' or to dilute concentrated white liquor.

The amount of sodium chloride in the wash from the bleaching plant

will vary depending on the bleaching sequence used.

In a typical process where a mixture of chlorine dioxide and chlorine is used in the first stage of a DEDED process, the amount of sodium chloride will vary between 54.5 and 72.5 kg per tonnes of pulp, all depending on the amount of chlorine dioxide used. When the total amount of chlorine available in the first stage is provided using 70% chlorine dioxide and 30% chlorine, the amount will typically be approx. 54.5 kg per tons of mass.

By leading the liquids from the bleaching plant to a recovery and regeneration system, the entire system is closed with regard to flowing washes, and sodium chloride cannot thus be removed from the plant by throwing away the liquid from the bleaching plant. Sodium chloride remains unconverted through the recovery steps for black liquor, and will consequently build up in the system by continuous recycling of regenerated white liquor.

In order to prevent such a build-up and at the same time apply the principle with regard to a plant without outflowing waste liquids, it is important to remove sodium chloride from the system. Such removal of sodium chloride must be designed so that other valuable components that can be used or converted into active chemicals are not removed from the system together with the sodium chloride, so that a normal chemical balance and economy is maintained. It is also desirable to remove an amount of sodium chloride from the plant that corresponds to the amount supplied and/or produced inside the plant, typically approx. 54.5 kg per tons of mass.

In a preferred embodiment, the present invention therefore relates to removing sodium chloride from the plant which is supplied with chemicals to the bleaching plant. In its broadest aspect, however, the invention relates to the removal of sodium chloride which is present and which comes from various sources, e.g. in one or more of the ways mentioned above.

In Canadian Patent No. 915,361 and U.S. Pat. patent no. 3,746,612, it is proposed to remove sodium chloride from the recovery and regeneration system by concentrating the white liquor, preferably by evaporation, in order to precipitate and remove sodium chloride from this white liquor. This method is satisfactory and can be used to remove the desired amount of sodium chloride from the plant.

Sodium chloride is also present in the fresh lye before causticization together with sodium sulphide and sodium carbonate. The causticization that is normally carried out on the fresh lye is therefore exposed to the sodium chloride content in the lye, and this can lower the effectiveness of the causticization itself, thereby increasing the amount of unreacted sodium lye present in the white lye, thereby reducing the alkaline content (NaOH + Na2S )

in the white lye compared to a causticization of fresh lye in the absence of sodium chloride. In addition to this, the presence of sodium chloride in the fresh liquor will cause corrosion problems in the apparatus. It is therefore very advantageous if sodium chloride can be removed from the system before causticisation.

However, it is not possible to use the method mentioned in the above-mentioned Canadian Patent No. 915,361 and U.S. Pat. patent no. 3,746,612 to precipitate sodium chloride directly from the fresh liquor, which is due to the fact that significant amounts of sodium carbonate are present in this which would also be precipitated.

The present invention makes it possible to recover sodium chloride from the system after formation of the melt and before causticisation. The present invention thus relates to the removal of sodium chloride from sodium sulphide-containing liquids which are usually essentially free of sodium hydroxide.

In the present method, the components of a melt containing sodium sulphide, sodium carbonate, sodium sulphate and sodium chloride are fractionated, whereby an aqueous solution of alkali sulphide and alkali metal chloride is obtained, as well as a solid mass containing alkali metal carbonate and alkali metal sulphate which is essentially free of alkali metal sulphide and alkali metal chloride. Next, the aqueous reading of the alkali metal sulphide and the alkali metal chloride is treated so that the alkali metal chloride is deposited, after which the precipitated salt is separated from the concentrated reading.

In addition to sodium sulphide, sodium carbonate and sodium chloride, the fresh lye and the smelt from which it is produced will usually contain large amounts of other salts, typically sodium and sulfur compounds, namely sodium sulphate, but also usually smaller amounts of sodium sulphite, sodium thiosulphate and sodium polysulphide, which is due to inefficient operation of the furnace and subsequent oxidation of sodium sulphide.

Some of these sodium and sulfur compounds usually remain in the reading during the processing according to the present invention, typically sodium thiosulfate and sodium polysulfide. These compounds in the white liquor are recycled to the cooking tank, but the concentration does not build up as they are recovered and regenerated when the spent cooking liquor is sent for such recovery and regeneration. The present invention is hereinafter described with special reference to sodium sulfate, as this particular sodium and sulfur compound is the dominant one of said compounds. However, the present invention also relates to other sodium and sulfur compounds as well as other salts with similar solubility characteristics.

The solid mass containing sodium carbonate and sodium sulphate is separated from the melt in the fractionation step, and it can be dissolved in 1 water after which the resulting sulphide-free fresh liquor can be causticised, whereby a sulphide-free white liquor is obtained which can then be mixed with processed sodium sulphide solution during dilution, if this is desired , whereby a cooking liquid is obtained which is suitable for recycling to the cooking stage. Uncausticized quantities of sodium carbonate and sodium sulphate, i.e. unregenerate cooking caustics present in the sulphide-free white liquor, will therefore pass into the black liquor and then be recovered.

The way in which one fractionates and treats the sodium sulphide liquor to remove sodium chloride is critical to the present invention* The invention generally involves separating the main amount of sodium carbonate and sodium sulphate from the other components of the smelt, and using an aqueous sodium sulphide solution with a low content of sodium carbonate and sodium sulphate compared with fresh liquor as the medium from which sodium chloride is secreted. In one embodiment, the solid mass is dissolved in water, whereby an aqueous solution of four components is obtained. Instead of braking the normal amount of water that is usually used to produce fresh liquor and which is determined based on considerations connected with the recausting step, the amount of water used to dissolve the solid mass will preferably be an amount that is just sufficient to to dissolve the smelt.., In this way the amount of water required to produce the fresh liquor is lowered, and consequently the total amount of water to be used can be lowered compared to the amount used for the concentration of white liquor as described in Canadian patent no. 915*361 and U.S. patent no. 3,746,612, where normal quantities of water are used in the formation or production of Hv fresh liquor. As less water is used, it is necessary to remove less water during evaporation to recover the same amount of sodium chloride, which can lower both investment and operating costs.

r The aqueous solution that results from dissolving the melt in an aqueous solution is concentrated so that a mixture of sodium carbonate and sodium sulfate is deposited, and the concentration is continued until the solution is essentially

saturated with sodium chloride. The deposited mixture is removed and the resulting aqueous solution is treated. for depositing or extracting sodium chloride. This can be achieved by concentrating the aqueous solution further, whereby a mixture of sodium chloride, sodium carbonate and sodium sulphate is deposited, which is then removed from the concentrated aqueous solution, and from which pure sodium chloride can be recovered. The concentrations can both be carried out by evaporation, preferably by boiling, under reduced pressure or superatmospheric pressure if this is desired.

In an alternative method for removing sodium chloride after removing the deposited mixture of sodium carbonate and sodium sulfate, the resulting aqueous solution of sodium sulfide can be cooled, after which sodium chloride is deposited in total. substantially in pure form, and the sodium chloride is then removed from the cooled solution. The mother liquor can then be further concentrated, if this is desired, whereby a mixture of sodium chloride, sodium sulphate and sodium carbonate is deposited which is then removed from the resulting concentrated sodium sulphide solution, whereby additional amounts of sodium chloride are removed from said sodium sulphide solution. However, this last concentration step can be omitted if this is desired.

An alternative fractionation process for the melt that can be used includes leaching the melt to remove from it essentially all sodium sulphide and the essentially all amount of sodium chloride, whereby a solid mass consisting essentially of sodium carbonate and sodium sulph* is obtained. The resulting aqueous sodium sulfide solution containing dissolved sodium chloride, usually together with some dissolved sodium carbonate and sodium sulfate, can then be subjected to the above described process for removing sodium chloride. The solid mass can be converted into sulphide-free white liquor as described above.

Own. 1 is a schematic flow diagram for one embodiment according to the present invention integrated in a plant for the production of wood pulp. Fig. 2 is a schematic flow diagram of a modified recirculation arrangement for the embodiment of Fig. 1. Fig. 5 is a schematic flow diagram. of one

another modification of the embodiment in fig. 1.

Fig. 4 is a schematic flow diagram for a

further embodiment of the invention.

Fig. 5 is a schematic flow diagram for a modification of the embodiment of Fig. 4. Fig. 6 is a schematic flow diagram for a third embodiment according to the present invention. Fig. 7 is a schematic flow diagram for a modification of the embodiment of Fig. 6. Fig. 8 is a schematic flow diagram for a

further modification of the embodiment in fig, 6.

Fig. 9 is a schematic flow diagram for a further modification of the embodiment in Fig. 6. Fig. 10 shows a schematic flow diagram for a fourth embodiment according to the present invention* Fig. 11 is a schematic flow diagram for a modification of the embodiment in fig. 10, and

fig. 12 is a schematic flow diagram for a further modification of the embodiment of FIG. 10.

In fig. 1, wood chips or other raw cellulose-fibrous material is introduced via line 10 to a boiler 12, where the wood chips are boiled with a cooking liquid which is supplied through line 14 and which contains sodium sulphide and sodium hydroxide as the active chemicals, and where you consequently use the Power process.

The resulting wood pulp and black liquor are separated, and the pulp is washed 1 a washer 16. The pulp is washed as shown in the figure with an aqueous bleaching liquid supplied through line 18. Alternatively, the pulp can be washed with water or "contaminated condensate<*> , and the bleach liquor can be used elsewhere in the system, as described in more detail below. To avoid the formation of hydrogen sulfide during this washing step where the bleach liquor is used, it is preferred that the bleach liquor has a neutral or slightly alkaline pH, typically around pH 9.

The washed and unbleached pulp is fed via line 20 to a bleaching plant 22 where the pulp is subjected to a series of bleaching and cleaning processes, where one or more chlorine-containing bleaching agents are used. Typically, such bleaching and cleaning will include bleaching with chlorine, chlorine dioxide or mixtures thereof supplied through line 24, and cleaning by caustic extraction using an aqueous sodium hydroxide solution supplied through line 26, typically in a CEDED process as it is described above. The pulp is washed before bleaching, typically after each bleaching or caustic extraction using water supplied through line 28. The used washing water from the bleaching plant together with used chemicals from the bleaching and the caustic extraction make up the liquid from the bleaching plant in line 18.

The washing itself preferably includes a counter-flow of wood pulp and washing water through the bleaching facility*

The quantity of sodium hydroxide used in the caustic extraction of the mass is preferably in a slight excess in relation to the stochlometric requirement of one sodium atom for each chlorine atom in the bleaching alkalis, whereby the wash has an alkaline pH as mentioned above. The confessed

and cleaned pulp with the desired whiteness is recovered from the bleaching plant 22 through line 30 and is led out of the plant for the production of paper.

If desired, the liquid in line 18 can be added directly to the black liquor in line 32, although this method is less preferred as it increases the total water requirement.

The liquid from the bleaching plant in line 18 contains significant amounts of sodium chloride which is transferred to the black liquor in line 32. The black liquor is evaporated in an evaporator 34 before it is fed through line 36 to a furnace 38 of any suitable and known construction. The water recovered from the evaporator 34 through line 40 can be used to provide at least part of the water needed in the system, eg* as part of the water that is fed to the bleaching plant through line 28 after suitable purification if this is necessary.

Sodium sulfate or another source of sodium and sulfur values, e.g. used acid, the black liquor is added to line 36, usually by an addition through line 42 until the vaporized liquid in line 36. The sodium sulfate "which can be added in solid, mushy form or an aqueous solution, is used to supplement the sodium and sulfur values that are lost from the system during the chemical recovery and regeneration.

The black liquor forms in furnace 38 a melt containing sodium sulphide and sodium carbonate and further unreacted components consisting of sodium chloride and sodium sulphate as well as other sodium sulphide oxygen salts.

A melt containing sodium sulphide, sodium carbonate, sodium chloride and sodium sulphate is thus obtained from the furnace. This melt is subjected to different treatments in accordance with varying embodiments of the present invention

for the removal of sodium chloride.

In accordance with the embodiment shown in fig. 1, the melt is dissolved in a dissolution chamber 44 by means of water supplied through line 46. The water preferably consists of "weak wash water" from the washing of the calcium carbonate sludge after the causticization. The water that is led through line 46 to chamber 44 may be partially used bleaching liquid from line 18, especially in cases where fresh water or contaminated condensate is led to sinks 16. Bleaching plant liquid that has been used in this way can first be used to wash calcium* carbonate sludge and consequently provide a so-called weak washing water.

The resulting fresh liquor line 48 will also contain recycled chemicals as described in more detail below. In addition to sodium sulphide*, the fresh liquor will contain dissolved amounts of sodium carbonate, sodium sulphate and sodium chloride from the smelting and/or from the bleaching liquid as used in line 46. The amount of sodium sulphate and other sodium sulfur oxygen salts in the fresh liquor will depend on the efficiency of the furnace and the degree of oxidation of the oxidizable sodium-sulfur salts, usually sodium sulphide after firing in the furnace.

In accordance with this embodiment of the invention, the fresh liquor is evaporated or otherwise concentrated in a first evaporator-crystallizer 50, whereby a mixture of sodium carbonate and sodium sulfate is deposited or precipitated. The evaporation which can be carried out in any suitable way, typically by boiling, under a reduced pressure or at super-atmospheric pressure if this is desired, until a point is reached close to the saturation point of sodium chloride, i.e. to a point where further concentration will result in a precipitation of sodium chloride. The bulk of the sodium carbonate and sodium sulfate at 1 the fresh liquor will be precipitated in this step, usually as anhydrous sodium carbonate mixed with the double salt, burkeite, NagCO^.SWagSO^.

Although other unreacted cooking chemicals which may be present in smaller amounts can be precipitated together with sodium carbonate and sodium sulphate, the method will be described with special reference to these two latter compounds.

In certain cases, the evaporation and precipitation can be carried out to a point where the solution is less than completely saturated with sodium chloride* Furthermore, a certain precipitation of small amounts of sodium chloride in this first-stage pre-evaporator crystallizer 50 can be tolerated.

Said evaporator-crystallizer 50 can be of any suitable shape, e.g. in the form of a single evaporation vessel or may consist of a series of evaporation vessels.

The evaporation of the fresh liquor to obtain deposited sodium carbonate and sodium sulphate can be carried out under varying conditions. Typically, the evaporation will be carried out at temperatures from 50 - 127°C, with reduced pressure or at overpressure, as desired, in order to obtain the fresh liquor at its boiling point at the selected temperature.

The sodium sulphide concentration on the fresh liquor is also a factor that determines the deposition of sodium carbonate and sodium sulphate, and it will usually vary between 5 and 30 weight percent Na2S.

The precipitated mixture of sodium carbonate and sodium sulfate is separated from the moir liquor and led through line 52 to dissolution vessel 54. The water that evaporates from the fresh liquor in evaporation vessel 50 is led out through line 56 and collected for later use.

The mother liquor, which consists of an aqueous sodium sulphide solution containing sodium sulphide, sodium chloride and remaining amounts of the aforementioned sodium carbonate and sodium sulphate, is led from the first-stage evaporator crystallization vessel 50 through a line

58 to the second stage evaporation vessel-crystallization separation vessel 60. Although two separate evaporation vessels/crystallization vessels 50 and 60 are shown, this is only for reasons of expediency

to describe this embodiment of the invention. Two separate evaporations can be carried out in the same apparatus by separating

u*t the solid precipitate from the mother liquor in each step. Alternatively, one or more separate evaporation vessels can be used, if this is desired.

The aqueous sodium sulphide solution saturated with regard to sodium chloride and remaining amounts of sodium sulphate and sodium carbonate, but not saturated with regard to sodium sulphide, is evaporated in the second-stage evaporation vessel/crystallization vessel 60, whereby a mixture of sodium chloride, sodium carbonate and sodium sulphate is precipitated, and this • removed through line 62. Water vapor is removed through line 64. The evaporation of the sodium sulphide solution in this second-stage evaporation vessel can be carried out in any suitable manner, typically by boiling, and if desired under reduced pressure or overpressure.

The second stage evaporation can be carried out within a wide temperature range and wide range with respect to the sodium sulphide concentration. A typical temperature range is from 50 138°C with reduced pressure or superatmospheric pressure, as desired to obtain the sodium sulphide solution at its boiling point at the selected temperature. A typical range of variation for the sodium sulphide concentration is from 20 - 45 weight percent KagS .

; The water that is collected through lines 56 and 64

can be used to provide part of the water that is needed in the system. E.g. the water in lines 56 and 64 can be used to provide at least part of the water that is fed to the bleaching plant through line 28.

The mixture recovered in line 62 essentially consists of sodium chloride contaminated with sodium carbonate and

sodium sulfate, and will later be treated to recover substantially pure sodium chloride. As shown in the drawing, the mixture is fed to a leaching vessel 66, which is supplied with water by means of line 68, thereby dissolving essentially all sodium carbonate and sodium sulfate in the mixture together with sodium chloride, whereby essentially pure sodium chloride is recovered through line 70 after washing if this is desired^ to remove entrained washing. In those cases where such washing is used, the used washing water can be used to provide part of the water which is supplied to the out-sloping vessel through line 68*

The leaching of the recovered mixture can be carried out within a wide temperature range, preferably so that a minimal chloride concentration is obtained in the aqueous phase that occurs during the leaching. Usually the temperatures will last in the range from 18 43°C.

The concentration of the aqueous sodium sulphide solution in the second-stage evaporation vessel 60 as well as the leaching itself is fed in such a way that the amount of sodium chloride that is recovered through line 70 preferably essentially corresponds to the amount of sodium chloride that is normally supplied to the system through line 18 from the bleaching plant , whereby a balanced system is obtained.

Any suitable method can be used to ... get substantially pure sodium chloride separated from the mixture. The precipitate can e.g. is subjected to a fractionation by a physical classification of crystalline solids. Crystals of sodium chloride are large cubic crystals, while crystals of. sodium carbonate are small.and needle-like and crystals of burkeite are small and flat.

The precipitate in line 62 can be suspended with an aqueous solution saturated with sodium chloride, sodium carbonate and sodium sulphate and passed up through a tower, whereby heavier and larger sodium chloride crystals are separated, while the sodium carbonate and burkeite crystals continue out the tower. These latter crystals can then be collected in an appropriate manner later.

The precipitate does not need to be separated from the concentrated sodium sulfide solution that occurs in the second stage evaporation/crystallization vessel 66 for this physical separation, as the sodium sulfide solution may be a saturated solution. The sodium carbonate and sodium sulfate that remain in the saturated sodium sulfide solution after the solid has been separated

phase, can be recycled using line 74 to boiler 12

and from there to furnace 38. Alternatively, the solid phase is separated before diluting the concentrated sodium sulphide solution.

The sodium chloride obtained in this way is usually somewhat contaminated with sodium carbonate and sodium sulfate. In such cases, a mixture may be subjected to a leaching in the same manner as the solid mixture in line 62. Another way of obtaining a physical separation involves passing the suspension of the precipitated compounds through a sieve plate made of sodium carbonate through which the burkeite crystals pass, but holds back the larger sodium chloride crystals. The sodium chloride crystals can then be recovered. Any contamination from sodium carbonate and sodium sulphate can be removed by leaching. The suspension, after sodium chloride has been removed, can then be treated to remove sodium carbonate and burkeite crystals, e.g. by contact with a fine sieve through which the crystals cannot pass.

The aqueous reading of sodium carbonate, sodium sulfate and sodium chloride which is formed in leaching vessel 66, or in a leaching combined S^d a physical separation as described above, can be led through line 72 to the fresh liquor in line 48. This is achieved by the design which is shown in fig. 1, by feeding the solution to melting solution vessel 44

together with water supplied through line 46 to dissolve the melt. As shown in the modification of fig. 2, then the aqueous solution can be added to the fresh liquor after it has been formed, in order to thereby obtain the recycled solution in the fresh liquor. Alternatively, the aqueous solution in line 72 can be led to evaporation vessel 50, where it is combined with the fresh liquor during the first stage of evaporation.

The concentrated sodium sulphide solution which is formed in evaporation vessel/crystallization vessel 60, and which is then led out through line 74, can be partially recycled by means of line 76 to the fresh liquor in line 48, typically by leading this to melt dissolution vessel 44 ceci together with the water supplied through 46 to dissolve the smelt. Alternatively, you can

one as shown by the modification on fig. 2, feed the recycled sodium sulphide solution in line 74 to the first stage evaporation vessel 50 to be combined there with the fresh liquor that is evaporated.

Sn recycle of the sodium sulfide solution Greatly increases the alkali content (ie, HagS + HaOH) in the aqueous sodium sulfide solution which is concentrated in the second stage evaporation/crystallization vessel 60.

The remaining part of the concentrated sodium sulphide solution, which now has a reduced content of sodium chloride and which may possibly contain remaining quantities of unreacted cooking chemicals, is led out through line 78.

The mixture of sodium carbonate and sodium sulphate in line 52 is led to dissolution vessel 54 and is dissolved there in water which is supplied through line 80, whereby a sulphide-free fresh liquor is formed which is led through line 82 to the caust-lacing vessel

84, where all sodium carbonate values are substantially converted

to sodium hydroxide by means of lime which is supplied to causticizing vessel 84 through line 86. The calcium carbonate sludge that precipitates in vessel 84 is separated from the resulting sulphide-free white liquor and is passed through line 88 to a lime kiln 90 for the formation of further quantities of lime. The calcium carbonate sludge is washed with water or BPE to remove entrained amounts of white liquor before the sludge is fed to the lime kiln 90. The used wash water from the latter washing is the weak wash water which is preferably used to dissolve the aael in melt solution vessel 44 together with recycled sodium sulphide solution. The use of this dilute sodium hydroxide solution in the dissolution of the smelt further increases the strong alkali content of the concentrated sodium sulphide solution formed in the other evaporation-crystallization vessel 60.

The sulphide-free white liquor, which consists of an aqueous solution of sodium hydroxide containing sodium sulphate and uncausticized sodium carbonate, is passed through line 92 for mixing with. the sodium sulphide solution in line 78, possibly with a suitable dilution with water or BPB, if this is necessary, in order to thereby provide a desired ratio between sodium sulphide and sodium hydroxide for the formation of white liquor in line 94 which is then recycled to form at least part of , the boiling sink in line 14. The sulphide-free white lye in line 92 can be used in whole or in part in another way. E.g. it can be fed to the bleach plant 22 as part of the caustic extract ion wash supplied through line 26. In those cases where the bleach plant 22 includes at least one oxygen bleaching step, the sulphide-free bleach in line 92 can be used to provide the necessary sodium hydroxide for this step.

The sulphide-free white liquor in line 92 can also be used to increase the content of strong alkali in the sodium sulphide solution in line 58. This can be achieved by feeding the sulphide-free white liquor to the melt dissolution vessel 44, to the fresh liquor 48 or to the first stage evaporator 50. Denae recycling is only carried out in this way that the remaining portion of the sulphide-free lye which mixes with the concentrated sodium sulphide solution in line 78, contains an amount of sodium sulphate corresponding to the

which is in the smelting from furnace 38, thereby avoiding a build-up of sodium sulphate in the system*

The water required for the dilution of the sodium sulphide solution in line 78 or the white liquor in line 94,

can partly be provided by water obtained from the lines 40,

56 and 64, knead at BPE.

A n@rv£or of residual amounts of sodium chloride and unreacted cookware in the recycled white liquor in line. 94, is usually not a disadvantage as sodium sulfate and other sodium and sulfur compounds used in furnace 38 as well as sodium carbonate are circulated in the system to caustic vessel 84. Any amounts of sodium chloride in the white liquor in line 94 circulate through the system, and although it constitutes a certain load, the quantity under stable conditions remains substantially constant, as the quantity removed in the evaporator-crystallization vessel 60 corresponds approximately to the quantity of sodium chloride supplied to the system. So even if the white liquor in line 94 contains a certain amount of non-active chemicals, under stable conditions these will essentially be constant and will thus not impair operation to a significant extent....

In the modification shown in fig. 3, the recirculation of the sodium sulfide solution through line-76 is omitted. This method increases the amount of sodium chloride in the sodium chloride solution compared to the recycling method, and can be used where the desired amount of sodium chloride has been removed from the sodium sulfide solution in another evaporator 60 under the conditions prevailing there.

The pure sodium chloride removed through line 70 can be used in various ways. Typically, d* will be used to regenerate chemicals for the bleaching plant. Thus, the sodium chloride can be used to develop sodium hydroxide and chlorine by electrolysis of said aqueous solution, and said sodium hydroxide can be used in the bleaching plant through line 26 while the chlorine is fed. through line 24. Alternatively, the sodium chloride can be used to develop chlorine dioxide and chlorine by a reaction with sodium chlorate and sulfuric acid, after which the chlorine dioxide and chlorine are fed to the bleaching plant through line 24. Furthermore, the sodium chloride can be electrolyzed as an aqueous feed to sodium chlorate for use in a chlorine dioxide producing reaction involving the reduction of sodium chlorate in an acidic medium.

Fig. 4 shows another embodiment of the invention where a different fractionation method is used for the smelting. In this and other figures, the methods have been shown which include treatment of the smelt to remove sodium chloride and a solid mixture of sodium carbonate and sodium sulphate to

get an aqueous sodium sulphide solution formed with a reduced sodium chloride content. It is understood that these methods can be integrated into a system for the production of wood pulp in the same way as shown in fig. 1.

As shown in fig. 4 is a melt containing sodium sulphide, sodium carbonate, sodium sulphate and sodium chloride

an oven as described above with reference to fig. 1, led through line 110 to a melt leaching vessel 112. In this, the melt comes into contact with water which is supplied through line 114, and soi can be weak washing water as described above with reference to fig. 1. A recycled concentrated sodium sulfide solution in line 116 may also be used for the melt leach.

During the actual leaching, the sodium sulphide and sodium chloride compounds will dissolve from the melt together with some sodium carbonate and sodium sulphate, whereby a solid mass is obtained consisting essentially of a mixture of sodium carbonate and sodium sulphate which can be removed via line 118. The mixture in line 118 is fed to causticization As described above in connection with fig. 1. The melt leaching can be carried out at the same temperatures as the first stage evaporation, typically from 50 138aC.

The aqueous sodium sulfide solution is fed from melt leaching vessel 11 via line 120 to an evaporator/crystallization vessel 122 of any suitable construction.

In this, the aqueous sodium sulphide solution will be concentrated, whereby a mixture of sodium chloride, sodium carbonate and sodium sulphate is precipitated which is then removed through line 124. The concentration of the sodium sulphide solution is usually carried out by boiling, either under reduced pressure or under above atmospheric

pressure, as desired.

The evaporation of the sodium sulphide solution can be carried out under the same conditions as indicated 1 compound with fig. 1-3 with regard to the second stage evaporator 60.

The water that evaporates from the aqueous sodium sulphide solution is removed through line 126 and after condensation the water can be used to provide a portion of it. water required in the system. The resulting concentrated sodium sulphide solution with a lowered content of sodium chloride and essentially free of sodium carbonate and sodium sulphate is removed from the evaporator/load crystallization vessel 122 via line 128, it is then partly recycled in line 116 and partly led via line 130 for the production of white liquor for further recycling to the boiler as described above with reference to fig. 1.

The mixture of sodium chloride, sodium carbonate and sodium sulphate which is separated from the concentrated sodium sulphide solution consists essentially of sodium chloride, eg is fed to a leaching vessel 132 or subjected to any other suitable separation, and where the mixture is contacted with water which is supplied through line 134 which rises in essentially all sodium carbonate and sodium sulfate together with some sodium chloride * and in return you get a solid mass of essentially pure sodium chloride which is recovered through line 136.

The temperature of the water which is supplied through line 134 to leaching chamber 132 can lie in the range indicated above for the leaching carried out in connection with the embodiment in fig. 1-3-The solution of sodium carbonate, sodium sulfate and sodium chloride that occurs in leaching vessel 132 is led through line 138 to melt leaching chamber 112 as part of the aqueous solution that is contacted with the salt or to the sodium sulfide solution in line 120.

The amount of sodium chloride that is recovered via conduit

I36 is preferably the same amount that is supplied to the Invlnntngs system for the black liquor. ■-■ ,

The embodiment of the invention which is described with reference to fig. 4, has the advantage over the design of Hg* 1 - 3. that the aforementioned design only requires an evaporation, while two concentrations must be used in the design shown in fig. 1 and 3" whereby the amount of water required to evaporate is reduced, which lowers both the capital and operating costs of the process compared to what is indicated in fig. 1-3.

In the modification of the method from fig. 4 which is shown in fig. 5, the recycling of the sodium sulphide solution through line 116 has been omitted. This method can be carried out in cases where the desired amount of sodium sulphide can be removed from the sodium sulphide solution in the evaporator 122.

The sodium chloride recovered through conduit 136 may be used in any desired manner as described above in connection with the embodiment of FIG. 1 and 2. w

Fig. 6 shows an alternative method 1 compared to that shown in fig. 1-3 for recovery of sodium chloride.

A melt containing sodium sulfide, sodium carbonate, sodium sulfate, and sodium chloride prepared in a furnace as described above with reference to Fig. 1 is fed via line 210

to a melt dissolution chamber 212. The melt is dissolved by means of water supplied through line 214. The resulting fresh liquor in line 216 also contains recycled chemicals* as described in more detail below. In addition to sodium sulphide, the fresh liquor also contains dissolved amounts of sodium carbonate, sodium sulphate and sodium chloride during the smelting.

In accordance with this embodiment, the precipitate in line 216 is first evaporated in a first stage evaporator 218 to precipitate sodium sulfate and sodium carbonate until the white liquor 1 is substantially saturated with sodium chloride, i.e. up to the point where further concentration would result in a precipitate of sodium chloride.

First-stage evaporation can be carried out under the temperature and sodium sulphide concentration indicated above in connection with the fresh liquor concentration stage in the embodiment of fig. 1 - 3.

The water that occurs during evaporation can be recovered via line 220 and used to provide part of the water in the system.

The concentration of the fresh liquor in the first stage evaporator 218 results in the precipitation of the main amount of sodium sulphate and sodium carbonate from the fresh liquor. The salts that are precipitated in the first stage evaporator 218, i.e. unregenerate cooking chemicals, can be removed through line 222 and sent to causticization as described above with reference to fig. 1.

The sodium sulphide solution that occurs in the first-stage evaporator 218 is led through line 224 to the second-stage crystallization chamber 226, where the sodium sulphide solution is cooled by evaporation, if this is desired, until essentially pure sodium chloride is precipitated from the solution. It is possible to precipitate essentially all sodium chloride by means of cooling, as the solubility of sodium chloride under the prevailing conditions decreases with decreasing temperature, while the solubility of sodium carbonate and burkeite only changes slightly with decreasing temperature in relation to the solubility variation for sodium chloride. The precipitated sodium chloride in the second stage crystallization chamber 226 can be removed from this through line 228 and washed to remove entrained liquid.

The temperature to which the sodium sulphide solution is cooled in order to precipitate sodium chloride depends to some extent on the temperature during fresh liquor evaporation, although temperatures in the range from 27 to 65° C are usually used as the lower limit, and the temperature between the beginning and ending temperature erature. is preferably as wide as possible.

The sodium sulphide concentration in the cooled sodium sulphide solution can vary within wide limits, typically 5 - 28 weight percent NagS. It is preferred to use concentrations in the upper part of this range in order to reduce to a minimum the amount of sodium chloride that must be recycled. The sodium chloride is recovered via line 228 in the form of a solid product which can be used in various ways as indicated above.

The sodium sulphide solution from the second stage crystallization chamber 226 with reduced sodium chloride content is fed through line 230 to the third stage evaporator 232. Although the two separate evaporators 218 and 232 and the intermediate crystallization chamber 226 are indicated in the drawing, this is only for convenience of description of this implementation of the invention. One can carry out two separate evaporations and one

intermediate leather crystallized in the same apparatus, however, so that one then

must separate the solid precipitate from the mother liquor after each step.

If desired, separate vessels can be used.

In the third-stage evaporator 258, the sodium sulphide solution is evaporated or otherwise concentrated in order thereby to precipitate further amounts of sodium chloride and unregenerated cooking chemicals from this. The concentration of the sodium sulphide solution in the third stage evaporator 232 can be carried out in such a way that a part of or essentially all chemicals that are not used during boiling are precipitated in the partially concentrated sodium sulphide solution.

The concentration of the sodium sulphide solution in the third stage ibr vapor 23'2 is preferably carried out by evaporation^typically by boiling as described above. The water that occurs during the evaporation is recovered via line 234, and can be used to provide part of the Yiann that is needed in the system.

The concentration of the sodium sulphide solution in evaporator 232 can be introduced under the conditions indicated above in connection with the second stage evaporation 1 embodiment in fig. 1-3. The solid precipitate from the third-stage evaporator 232 is removed from this through line 236 and led to a dissolution vessel 238. The solid precipitate essentially consists of sodium chloride contaminated with small amounts of sodium carbonate and sodium sulfate. v. Water is supplied to dissolution vessel 238 through line 240

and sodium chloride, sodium carbonate and sodium sulphate are dissolved.

The aqueous solution resulting from the precipitate i

The vessel 238 can be recycled via line 242 to the fresh liquor in line 216. The aqueous solution in line 242 can also be recycled to the melt dissolution vessel 212 to be combined there with the water supplied via line 214 to produce the fresh liquor. Alternatively, it can the aqueous solution in line 242 is added

a dissolution of the melt in water leaving dissolution chamber 212. In any case, the aqueous solution in conduit 242 will be present in the fresh liquor fed to evaporator 218.

In order to maintain a balanced system during continuous operation, it is preferred to remove from the system via line 228 essentially the same amount of sodium chloride that is fed into the system.

The concentrated sodium sulfide solution that is formed

in another evaporator/crystallization chamber 232 in line 244 is partially recycled via line 246 to the fresh liquor in line 216, typically by feeding this to the melt dissolution chamber 212 together

with water supplied via line 214. The remaining portion of the concentrated sodium sulphide solution, which now carried a reduced sodium chloride content, is passed through line 248 to form white liquor as described above with reference to the sodium sulphide solution in line 78. The flow diagram in fig. 7 shows a modification of the method in fig. 6. In this modification, the cooled sodium sulfide solution becomes

which occurs in the second stage of crystallization Bchamber 226 split into two streams, and one of the streams is led through line 250 to the evaporator 232 while the other is led through line 252 to the fresh liquor in line 216. This modification is preferred in cases where there is a significant recycling of sodium sulphide inside the system. In the modification of Fig. 6, the recycled sodium sulphide solution is not subjected to evaporation in the evaporator 232, whereby the evaporation load on this evaporator is lowered. The recycled sodium sulphide solution is recycled directly to the fresh liquor in the modification of fig. 7 rather than it being fed to the melt dissolution chamber as shown in Fig. 6. In addition to this, the solid mixture of sodium car- .

bonate, sodium sulfate and sodium chloride are removed from the sodium sulfide solution in the evaporator 232 and are directly recycled to the melt dissolution chamber 212 through line 236, and dissolved by water supplied via line 214. This modification avoids having a separate dissolution step for this precipitate as it is shown in fig. 6.

A further modification of the embodiment of fig. 6 which is shown in fig. 7, consists of a water supply line 254 so that you can dilute the sodium sulphide solution in line 224, whereby you can modify the concentrations of the components within the areas that ensure a precipitation of pure sodium chloride when cooling in crystal! sealing chamber 226. Furthermore, devices have been provided in line 256 to remove water vapor arising from the sodium sulphide solution during cooling. Usually, the amount of water that occurs during cooling when carrying out such evaporation will be returned to the system, e.g. through line 254, so that there is no loss of water from the sodium sulphide solution during the cooling step in order to precipitate pure sodium chloride.

The modification of fig. 8 incorporates a recirculation of a solid mixture of sodium carbonate, sodium sulfate and sodium chloride through line 236 to melt dissolution chamber 212, provision of a water supply line 2§k and provision of means for removing water vapor through line 256 from

the sodium sulphide solution crystallizes during the cooling in! ngs chamber 226, as described above in connection with fig. 7.

A recirculation of the sodium sulphide solution is completely omitted in the modification of fig. 8. This method can be used where the alkalinity of the sulphide solution in line 224 is sufficient to remove the desired amount of sodium chloride through line 228 during cooling.

In fig. 9, a melt containing sodium sulphide, sodium carbonate, sodium chloride and sodium sulphate is supplied through line 310 to a melt dissolution chamber 312 where the melt is dissolved in water, usually weak wash water supplied through line 314. The resulting fresh liquor in line 316 containing recycled sodium sulphide solution brought to an evaporator 318 where the fresh liquor is evaporated, typically by boiling, whereby a mixture of sodium carbonate and sodium sulfate is precipitated which is removed from the evaporator 318 through line 320 to be led to causticization as described in more detail above in connection with fig. 1.

The evaporation of the fresh liquor in the evaporator 318 is continued up to the point where a saturation of the resulting sodium sulphide solution with respect to sodium chloride is reached, and the evaporated water is removed through line 322.

The concentration of the fresh liquor in the evaporator 318 can be carried out under the same conditions as stated above in

in connection with the fresh liquor evaporation in the evaporator 50 in the design shown on fig. 1-3.

The hot sodium sulphide solution is fed after dilution, if this is desired, possibly with the help of water, supplied via line 324, to a crystallization chamber 328, where the sodium sulphide solution is cooled, whereby essentially pure sodium chloride is precipitated, and this is removed from the second stage crystallization chamber 328 via line 330. The amount of sodium chloride that is removed is preferably substantially the same as that present in the melt in line 310, apart from any amounts of sodium chloride that circulate constantly in the system.

The cooling of the hot sodium sulphide solution to precipitate pure sodium chloride can be carried out in the same way as described for the cooling step in the second stage crystallization chamber 226 in the embodiment of fig. 6-8.,

The sodium chloride which is removed through line 330 is used in various ways as described in detail above in connection with fig. 1. The cooling in expansion chamber 328 can be carried out by sudden evaporation if this is desired, and the water vapor can be removed through line 332. The water removed in this way is typically returned to the system through line 324, so that there is no net loss of water in this crystallization chamber

328.

The sodium sulphide solution with a reduced content of sodium chloride is removed from chamber 328 through line 334, and is partly recycled via line 536 to the fresh liquor in line 316 and partly via line 338 to the white liquor which is recirculated to the boiler.

The embodiment in fig. 9 therefore shows a method where the evaporation is carried out on the sodium sulphide solution after the sodium chloride has been removed in the embodiment in fig. 6, and the modification of this embodiment which is shown in fig. 7 is omitted. The procedure in this case can be used where the remaining part

of sodium chloride in the sodium sulphide solution 1 line 334 is so small that it can be tolerated in the rest of the system, and thereby

it is not necessary to remove further quantities in a new evaporation step, and the solution is therefore kept in one closed circuit in the actual processing of the smelt.

Another embodiment of the invention is shown in fig. 10 and represents a combination of the fractionation method shown in fig. 4 and the sodium chloride separation shown in fig. 7. In this case, a melt containing sodium sulphide, sodium carbonate, sodium chloride and sodium sulphate is fed through line 410 into a leaching chamber 412 where the melt together with recycled solids is contacted with water supplied via line 414, in order to thereby dissolve the present amounts of sodium sulphide and sodium chloride together with part of the sodium carbonate and sodium sulphate. The leaching itself can be carried out under the same conditions as described above in connection with the leaching of the smelt 1 the embodiment in fig. 4 and 5.

The remaining solids, which consist essentially of sodium carbonate and sodium sulfate 1 essentially free of sodium sulfide and sodium chloride, are removed from leaching chamber 412

through line 416 and is led to causticization as described above in connection with fig. 1.

The hot sulphide solution which occurs during the leaching is led through line 418 to a crystallization chamber" 420 where the sodium sulphide solution is cooled to thereby precipitate

essentially pure sodium chloride, and this is removed through line 422..... The actual cooling of the hot sodium sulphide in dip solution to precipitate pure sodium chloride can be carried out in the same way as described for the actual cooling in the second-stage crystallization chamber 226 in the design shown on fig. 6 - 8.

Sn. dilution of the sodium sulphide solution in line 418 can be carried out by supplying water through line 424 whereby suitable conditions are obtained to obtain a precipitation of pure sodium chloride upon cooling.

Evaporation of water can be carried out during the cooling, and. this water is removed through line 426. This water can be returned, e.g. through line 424, so that there is no evaporation loss of water during cooling.

The cooled sodium sulphide solution is removed from crystallization chamber 420 through line 428, and is partly recycled through line 430 to leaching chamber 412.

the remaining part of the sodium sulphide solution is led through line 432 to an evaporator 434 where the sodium sulphide solution becomes

evaporated by boiling to precipitate sodium chloride, sodium carbonate and sodium sulfate from the solution, and the evaporated water is removed through line 436.

The concentration of the cooled sodium sulphide solution in the evaporator 434 can be carried out under the conditions described above in connection with the evaporator 232 in the embodiment shown in fig* 6-8.

The precipitated solids are removed from the evaporator 434 through line 438 and recycled to the leach tank 412 to be mixed with water supplied through line 414. The concentrated sodium sulphide solution resulting from the concentration in the evaporator 434 is sent through line 440 to produce white liquor for recycling.to the boiler

as described above in connection with the embodiment in fig. 1.

Fig. 11 and 12 show modifications of the method in Fig. 10. In the modification of fig. 11, recirculation of the sodium sulphide solution through line 430* has thus been omitted

Deifce can be used in cases where the cooled sodium sulphide solution 1 line 418 is such that the desired amount of pure sodium chloride can be removed. In the modification of fig. 12, there is a recycling of the sodium sulphide solution through line 430, but a concentration in the evaporator 434 has been omitted. This method can be used where the remaining amount of sodium chloride in sodium sulphide in the dip solution in line 44Q is so small that it can be tolerated in the system* Examples

The invention is further illustrated by means of. the following examples.

Example 1

The relative solubilities of the components in the system NagS-NaCl-NagCO^-NagSO^ were examined at different temperatures eg sulphide concentrations. Small amounts of sodium hydroxide were present in the fluids used. The following table indicates the results obtained.

Example II

Based on the known dissolution data from Example I

and known solubility data for the system XføCl-HagCO^-Na^O^

a mass balance was calculated for a plant that produced approx. 5O0 tonnes of pulp per 24 hours a day and where the method shown on Fig. 1 and 2 was used.

10,700 kg per hour of a melt that contained. ,1820 kg sodium sulphide, 1600 kg sodium chloride, 7300 kg sodium carbonate

and 285 kg of sodium sulphate was contacted with 43,500 kg of water per hour pleasure 46) and 5000 kg per hour of a recycled stream (line 72) seam contained 870 kg of sodium chloride, 610 kg of sodium carbonate, 45 kg of sodium sulfate and 4,050 kg of water, whereby one obtained. produced 61,000 kg fresh liquor per hour (line 48 j containing 1,910 kg sodium sulphide, 2,430 kg sodium chloride, 8,250 kg sodium carbonate, 332 kg sodium sulphate and 48,000 kg water. 20,800 kg per

hour of a recycled solution (line 76) containing 7,300 kg sodium sulphide, 590 kg sodium chloride, 192 kg sodium carbonate, 32 kg. sodium sulfate and 12,400 kg of water were added to the fresh liquor.

The resulting solution was boiled at 113°C (evaporator

50) to evaporate 33,000 kg of water per hour and by which one

precipitated 7,900 kg per hour of a solid mixture containing 7,100 kg of sodium carbonate and 275 kg of sodium sulphate, this mixture was removed through line (52). •

The resulting sodium sulphide solution of 40,000 kg

. per hour containing 19,200 kg of sodium sulphide (eh concentration of 22.7$) 3,050 sodium chloride, 835 kg of sodium carbonate, §§.kg of sodium sulphate and 26,800 kg of water was boiled (evaporator 60) at 130°Ea; to evaporate 12,100 kg of water and for to have 2,920 kg deposited per hour

of a solid mixture consisting of 2,260 kg of sodium chloride, 620 kg of sodium carbonate and 48 kg of sodium sulphate. The solid mixture was

then removed (wire62). 25,300 leg per hour of the concentrated sodium sulphide solution containing 9,130 kg of sodium sulphide (36.3$), 730 kg of sodium chloride, 230 kg of sodium carbonate, 40 kg of sodium sulphate and 15,000 kg of water was removed (line 74) and split into a recycled stream of 19,850 kg per hour. hour which was mixed* with fresh liquor

(line 76) and 5,000 kg per hour of the solution containing I. 830 kg of sodium sulphide, 146 kg of sodium chloride, 45 kg of sodium carbonate, 0.7 kg of sodium sulphate and 3,000 kg of water and this was sent on for the production of white liquor (line 78).

The solids formed during second evaporation were leached at 35°C with 3,920 liters of water per hour (leaching chamber 66), whereby 5,450 kg per hour of a recycled water solution (line 72), and in return you got 1,400 kg per hour of pure sodium chloride. Example III

A mass balance was calculated on the basis of approx. 500 tonnes of wood pulp per day using the method shown in Fig. 3, where the recycling of sodium sulphide is omitted. 10,700 kg per hour of a melt containing 7,300 kg of sodium carbonate, 1,820 kg of sodium sulphide, 1,600 kg of sodium chloride and 255 kg of sodium sulphate was dissolved in 56,500 kg of water, and the resulting fresh liquor was mixed with 4,900 kg per hour of a recycled leaching solution containing 640 kg of sodium carbonate, 795 kg of sodium chloride, 61 kg of sodium sulphate and 3,410 kg of water" The aqueous solution was boiled (evaporator 50) to evaporate 48,500 liters of water per hour, and sodium bicarbonate monohydrate and sodium sulfate are precipitated, a total of 9,250 kg per hour which was removed through line f 2, and which contained 7*700 kg of sodium carbonate, 275 kg of sodium sulphate and 1,300 kg of water. The resulting sodium sulphide solution of 16,600 kg per hour containing 620 kg of sodium carbonate, 1,820 kg of sodium sulphide (11$), 2,340 kg of sodium chloride, 66.5 kg of sodium sulphate and II. 500 kg of water was boiled (in evaporator 60) at 10p°C to evaporate 8,000 liters per hour of water, whereby 2,900 kg were precipitated per hour of a solid* and in return 5,200 kg per* hour of a sodium sulphide solution was sent on as white liquor (line 74) and which contained 26 kg of sodium carbonate, 1,820 kg of sodium sulphide (35%), 123 kg of sodium chloride, 5* 9 kg of sodium sulphate and 3• 230 liters of water.

The deposited solids consisted of 640 kg of sodium carbonate, 2,200 kg of sodium chloride and 61 kg of sodium sulphate and this was leached at 40°C (leaching vessel 66) with 3,420 liters of water per hour, whereby 4,950 kg per hour of a recycled liquid (line 72) as well as 1,430 kg of pure sodium chloride per hour. Example IV A mass balance was calculated based on 500 tonnes of mass per day using the method shown in fig. 4, and the solubility data shown in Example Xt; as well as known solubility data for the system HagCO^-WagSo^-NaCl.. 10,700 kg per hour of a melt containing 1,820 kg of sodium sulphide, 1*700 kg of sodium chloride. 7*300 kg sodium carbonate and 255 kg sodium sulfate were leached (in leaching chamber 112)

at 113°C with 11,050 liters of water, and the resulting solution was mixed with 5,370 kg per hour of a recycled solution (line 138) containing 870 kg of sodium chloride, 590 kg of sodium carbonate, 47 kg of sodium sulphate and 3,900 liters of water as well as 20,000 liters per hour of a recycled sodium sulphide solution (line 1161 containing 7,200 kg of sodium sulphide, 580 kg of sodium chloride, 182 kg of sodium carbonate, 32 kg of sodium sulphate and 11,900 liters of water, whereby 40,000 liters per hour of a sulphide solution (in line 120) containing 9,200 kg of sodium sulphide were formed , 5*050

kg of sodium chloride, 835 kg of sodium carbonate, 88 kg of sodium sulphate and 26*800 liters of water. The fresh cloth was boiled at 130°C (in evaporator 122) to evaporate 12,100 liters of water, whereby 2,920 kg were precipitated per hour of solids containing 2,760 kg sodium chloride, 620 kg sodium carbonate and 48 kg sodium sulfate, and these solids were then removed (through line 124).

The resulting concentrated sodium sulphide solution of 25,300 kg per hour containing 9,130 kg of sodium sulphide, 730 kg of sodium chloride, 230 kg of sodium carbonate, 40 kg of sodium sulphate and 15,000 liters of water, was split in a recycled stream (in line 116) of 19,850 kg per hour and another stream of 5,000 kg per hour which was exported as white liquor (in line 130) and which contained 1,820

kg sodium sulphide, 146 kg sodium chloride, 45 kg sodium carbonate,

8 kg of sodium sulphate and 3,000 liters of water.

The solids which were removed during the sodium sulphide concentration were leached (1 leaching chamber 132) in 3,900 liters of water per hour at 35°C, whereby 5,400 liters per hour were obtained. hour of a recycled strSm (in line 138), and in return you got 1,400 kg per hour of pure sodium chloride.

Example V

The procedure from fig. 5.and calculated a mass balance on the basis of a plant that produces approx. 500 tonnes of pulp per day and night.

10,700 kg per* hour of a melt containing 7*300 kg of sodium carbonate, 1,820 kg of sodium sulphide, 1,700 kg of sodium chloride and 255 kg of sodium sulphate was mixed with 4,900 kg per hour. hour of a recycled charge (line 138) containing 640 kg of sodium carbonate, 795 kg of sodium chloride, 61 kg of sodium sulphate and 3,410 kg of water fb*r leaching with 9,500 kg per hour of water at 100°C, whereby 16,700 kg per hour of an aqueous sodium sulphide solution (in line 120) containing 660 kg of sodium carbonate, 1,910 kg of sodium sulphide, 2,330 kg of sodium chloride, 66 kg of sodium sulphate and 12,600 kg of water, and in return 9,250 kg of solids were obtained for recausting (in line 118) and these substances contained 7,700 kg of sodium carbonate, 275 kg of sodium sulphate and 1,300 liters of crystal water.

The sodium sulphide solution was boiled (in evaporator 122) and 8*000 liters were evaporated per hour of water and precipitated 2,900 kg of solids per hour. The resulting concentrated sodium sulphide solution of 5*200 kg per hour containing 26 kg of sodium carbonate, 1,910 kg of sodium sulphide, 123 kg of sodium chloride, 5.9 kg of sodium sulphate and 3,230 kg of water quickly came out as white liquor (in line 128).

The solids deposited during the concentration of the sodium sulphide solution were leached (in a leaching chamber

132) at 40°C with 3,400 liters of water per hour, which resulted in 4,860 liters per hour of a recycled aqueous reading (in line 138) and 1,420 kg per hour of pure sodium chloride.

Example VI

Based on the solubility data provided in Example

I, ea mass6bala33.se calculated on the basis of a plant that produced approx. 500 tonnes of pulp per day, and where the method shown in fig. 7, the following example was performed.

10,700 kg per hour of a melt containing 1,820 kg of sodium sulphide, 1,600 kg of sodium chloride, 7,300 kg of sodium carbonate and 255 kg of sodium sulphate was mixed with 450 kg per hour of recycled solids (line 236) containing 357 kg of sodium chloride, 85 kg of sodium carbonate and 5.5 kg of sodium sulphate, after which the mixture was dissolved in 48,500 liters of water per hour, which resulted in 60,500 liters per hour of a fresh liquor containing 1,820 leg of sodium sulphide, 1,890 kg of sodium chloride, 7,700 kg of sodium carbonate, 285 kg of sodium sulphate and 48,500 liters of water per hour.

The fresh cloth was mixed with 108,000 liters per Ume of a recycled sodium sulfide solution (line 252) containing

25,000 kg of sodium sulphide, 6,900 kg of sodium chloride, 1,820 kg of sodium carbonate, 188 kg of sodium sulphate and 8,200 liters of water, after which the mixture was boiled at 100°C (in evaporator 218) and 49,500 kg were evaporated per hour of water and precipitated 7,800 kg of solids per hour* These solids consisted of 7*550 kg of sodium carbonate and 275 kg of sodium sulphate and were removed from the resulting sodium sulphide solution (through line 222) for causticisation.

The resulting sodium sulphide solution (in line 224) of 118*100 kg per hour containing 26.5p0 sodium sulfate (22.4?0, 8,800 kg sodium chloride, 1*940 kg, sodium carbonate, 200 kg sodium sulfate and 81,000 Ilter of water was added to 6,900 liters of water per hour (through line 254). The solution was cooled (in crystallization chamber 226) to 50°C, and 1,495 kg per hour of pure sodium chloride were crystallized (which was removed through line 228) and 125*000 kg per hour of a sodium sulphide solution containing 26,500 kg of sodium sulphide, 7*400 kg of sodium chloride were formed , 1,940 kg of sodium carbonate, 200 kg of sodium sulphate and 88,100 liters of water.

The latter sodium sulphide solution was divided into a recycled stream of 108,000 kg per . hour (in line 246) and a current of 8,450 kg per hour (in line 250) containing 1*910 kg

sodium sulfate, 510 kg sodium chloride, 130 kg sodium carbonate, 13 kg sodium sulfate and 6,000 kg water. This solution was boiled in evaporator 232 at 130°C and 3,050 liters of water were evaporated

per hour and fikle precipitated a recycled solid mixture (in line 236), and the result was 5,000 kg per hour of one concentrated sodium sulphide solution which was discharged as white liquor (through

iedning 248) and which contained 1,910 kg of sodium sulphide ($36), 144 kg of sodium chloride, 45 kg of sodium carbonate, 8 kg of sodium sulphate and 2*970 liters of water.

Example VII One mass balance was calculated based on the procedure in fig. 8 where there is no recirculation of sulphide solution, as well as a plant that produced approx. 500 tonnes of mass

per* day.

In 56.@0O liters of water per hour, 10,700 were dissolved

kg per hour of a melt containing 7,600 kg of sodium carbonate, 1,810 kg of sodium sulphide, 1*700 kg of sodium chloride and 255 kg of sodium sulphate. 3*840 kg per hour of recycled solids (in line 236) containing 1,280 kg of sodium carbonate, 2.45Q kg of sodium chloride and 104 kg of sodium sulfate was also dissolved in the water.

The fresh cloth was boiled (in evaporator 218) and 3,800 liters of water per day were obtained. hour and precipitated 9*200 kg per hour of solids containing 7*600 kg of sodium carbonate, 275 kg of sodium sulphate and 1300 liters of crystal heat, and these substances were removed from 24,100 liters per hour of sodium sulphide solution (through line 222) and was taken out for causticisation. The sodium sulphide solution which contained 1,340 kg

sodium carbonate, 1,920 kg of sodium sulphide, 4,000 kg of sodium chloride, 107 kg of sodium sulphate and 14,850 liters of water were then cooled (in

crystallization chamber 226) to 50°C while evaporating 3,050 liters of water per hour, which resulted in a crystallization of 1,930 kg per* hour of pure sodium chloride which was removed through line 228. The resulting sodium sulphide solution

of 19*600 kg per hour containing 1-540 kg of sodium carbonate, 1,920 kg of sodium sulfide (9.1$), 2,570 kg of sodium chloride, 107 kg of sodium sulfate and 13,900 kg of water was boiled (in evaporator 232) for

to have 10*350 liters of water evaporated per hour and 3,800 kg were deposited per hour of solids. These solids were removed for recycling through line 236. 5,200 kg per hour of concentrated sodium sulphide solution containing 26 kg of sodium carbonate, 1,910 kg of sodium sulphide,

123 kg of sodium chloride, 5.9 kg of sodium sulfate and 3*230 liters of water were sent on for the production of white liquor (through line 243).

' Example VIII

On the basis of the procedure on page 9, a mass balance was calculated based on a plant of 500 tonnes of mass per day and night.

In 56,000 liters of water, 10,700 kg were dissolved per hour of a melt containing 7,600 kg of sodium carbonate, 1,820 kg of sodium sulphide, 1,900 kg of sodium chloride and 255 kg of sodium sulphate. 73,500 kg per hour of a recycled sodium sulphide double solution (in line 336) containing 1,180 kg sodium carbonate, 16,200 kg sodium sulphide, 4,270 kg sodium chloride, 178 kg sodium sulphate and 51,500 kg yann was then added to the reading. The fresh cloth was boiled

(in evaporator 318) and one fiddle evaporated 50,400 kg of water per hour

and had 7,700 kg precipitated per hour of solids removed (through line 320) for causticization, which contained 7,450

kg of sodium carbonate and 262 kg of sodium sulphate.

81,000 kg per hour of concentrated sodium sulphide solution containing 1,360 kg sodium carbonate, 18,100 kg sodium sulphide, 6,200 kg sodium chloride, 198 kg sodium sulphate eg 57,500

kg of water was cooled to 50°C (in crystallization chamber 328), whereby 1,420 kg per hour of pure sodium chloride (which was removed through line 330).

The cooled sodium sulphide solution (in line 334) was split into a recycled stream of 73,500 kg per hour (in line 336) and in a current of 7,750 kg per hour which was carried on to form a white liquor (in line 338) and this contained 132 kg of sodium carbonate, 1,820 kg of sodium sulphide, 475 kg of sodium chloride, 20 kg of sodium sulphate and 5,800 liters of water.

Claims (61)

1. Process for separating sodium chloride from a mixture containing sodium sulphide, sodium carbonate, sodium sulphate and sodium chloride, characterized by fractionating said mixture to thereby produce an aqueous solution of said sodium sulphide and sodium chloride as well as a solid mixture of sodium carbonate and sodium sulfate essentially free for sodium sulphide and sodium chloride, after which solid sodium chloride is removed from the aforementioned aqueous solution. 2. Process mfite according to claim 1, characterized in that said mixture is fractionated to produce said solid mixture of sodium carbonate and sodium sulfate substantially free of said sodium sulfide and sodium chloride as well as an aqueous solution containing sodium sulfide, sodium chloride, sodium carbonate and sodium sulfate as an aqueous solution . 3. Method according to claim 2, characterized in that said sodium chloride is removed from said aqueous sodium sulphide accumulation by concentrating this so that you get precipitated sodium chloride, sodium carbonate and sodium sulphate, after which you remove the precipitated sodium chloride, sodium carbonate and sodium sulphate from the concentrated sodium sulphide solution I recover substantially pure sodium chloride from said precipitated sodium chloride, sodium carbonate and sodium sulfate. 4. Method according to claim 3, characterized in that. said sodium chloride, sodium carbonate and sodium sulphate are precipitated from said sodium sulphide solution as a mixture, and by said essentially pure sodium chloride being recovered by leaching the latter mixture after it has been separated from the concentrated sodium sulphide solution, thereby obtaining an aqueous solution of sodium carbonate, sodium sulphate and sodium chloride as well as a solid mass of substantially pure sodium chloride. 5. Procedure according to claim 3, grade! seeing that said concentration of said aqueous sodium sulphide solution is carried out by boiling at temperatures from approx. 49 to approx. 138°C. 6. Method according to claim 5, characterized in that said aqueous sodium sulphide solution has a sodium sulphide concentration of from approx. 20 to approx. 45$. 7. Procedure according to claim 4, grade! cert v. é d that said leaching is carried out at temperatures from approx. 18 to approx. 43°C. 8. Procedure according to claim 3"characterized by that. said fractionation is carried out by dissolving said mixture in an aqueous medium, whereby an aqueous solution of the components in said mixture, concentrates said aqueous solution to thereby precipitate a mixture of sodium carbonate and sodium sulfate while avoiding precipitation of the main amount of sodium chloride, after which said precipitated mixture is removed from the resulting aqueous sodium sulfate in double solution. 9. Method according to claim 8, characterized in that said concentration of the aqueous solution of . said components in said mixture are carried out by boiling at temperatures from approx. 40 to approx. 127°C 10. Method according to claim 9, characterized in that said aqueous solution has a sodium sulphide concentration of from approx. 5 to approx. 30$. 11. Method according to claim 8, characterized in that part of said concentrated sodium sulphide solution is recycled to said aqueous solution of said components of said mixture. 12. Method according to claim 8, character! characterized in that said sodium chloride, sodium carbonate and sodium sulphate precipitate from said sodium sulphide solution as a mixture and in that substantially pure sodium chloride is obtained by leaching the latter mixture after separating the mixture from said con concentrated sodium sulphide solution, whereby an aqueous solution of sodium carbonate, sodium sulphate and sodium chloride is obtained as well as a solid mass of essentially pure sodium chloride, after which said aqueous solution of sodium carbonate, sodium sulphate and sodium chloride is recycled to said solution of the components of said mixture. 13. Method according to claim 3, characterized by the fact that said fractionation is carried out by leaching said mixture with an aqueous medium whereby you obtain said aqueous sodium sulphide solution containing said sodium sulphide, said sodium chloride, sodium sulphate and sodium carbonate and back obtains an iast mixture of sodium carbonate and sodium sulphate in all Essentially free of sodium sulphide bg sodium chloride. 14. Method according to claim 13" characterized in that said leaching is carried out at temperatures from approx. 49 more about. 127°C 15. Method according to claim 13, characterized in that a part of said concentrated sodium sulphide solution is recycled to said aqueous sodium sulphide solution which occurs during said leaching. 16. Method according to claim 13* characterized in that said sodium chloride, sodium carbonate and sodium sulfate are precipitated from said sodium sulfide solution as a mixture and in that said essentially pure sodium chloride is obtained by leaching said mixture after it has been separated from said concentrated sodium sulfide in dip solution , whereby an aqueous solution of sodium carbonate, sodium sulfate and sodium chloride and a solid mass of substantially pure sodium chloride is obtained, and where the aforementioned Handy solution of sodium carbonate sodium* is recycled sulphate and sodium chloride to said aqueous sodium sulphide solution resulting from said leaching. 17. Method according to claim 2, characterized in that said sodium chloride is removed from said aqueous sodium sulphide solution by concentrating this so that sodium chloride, sodium carbonate and sodium sulphate are precipitated, after which the resulting suspension is subjected to a physical separation on the basis of crystal sizes in order to thereby separate from the concentrated aqueous sodium sulphide solution a solid mass be consisting essentially of precipitated sodium chloride and part of said precipitated sodium carbonate and sodium sulphate. 18. Method according to claim 17, characterized in that said separated solid mass containing sodium chloride, sodium sulfate and sodium carbonate is treated to recover in total substantially pure sodium chloride. 19. Method according to claim 17, characterized in that said fractionation is carried out by dissolving said mixture in an aqueous medium to thereby obtain an aqueous solution of the components in said mixture, concentrating said aqueous solution to thereby precipitate a mixture of sodium carbonate and sodium sulfate while avoiding a precipitation of the main amount of sodium chloride, after which said precipitate is removed mixture from the resulting aqueous sodium sulfide solution. 20. Method according to claim 17, characterized in that said fractionation is carried out by leaching said mixture with an aqueous medium to thereby obtain said aqueous sodium sulphide solution containing said sodium sulphide, said sodium chloride, sodium sulphate and sodium carbonate and a solid mixture of sodium carbonate and sodium sulphate 1 all essential free of sodium sulphide and sodium chloride. 21* Method according to claim 1, characterized by fractionating said mixture in order to thereby obtain said solid mixture of sodium carbonate and sodium sulfate substantially free of sodium sulphide and sodium chloride as well as a warm, aqueous solution containing sodium sulphide and sodium chloride. sodium carbonate and sodium sulphate, after which said sodium chloride is removed from said aqueous sodium sulphide solution by cooling said hot aqueous solution to thereby precipitate essentially pure sodium chloride, after which this is removed from the mother liquor. 22. Method according to claim 21, characterized in that said aqueous solution is cooled to temperatures in the range from o.a. 27 to 66°C. 23. Method according to claim 22, characterized in that said hot aqueous solution has a sodium sulphide concentration in the range from approx. 5 to etc. 28#. 24. Method according to claim 21, characterized in that said mother liquor is concentrated to thereby precipitate a mixture of sodium chloride, sodium sulfate and sodium carbonate, after which the mixture is removed from the resulting concentrated sodium sulfide solution. 25. Process according to claim 24, characterized in that said concentration is carried out by boiling at temperature trips from approx. 49 to approx. 138°C... 26* Method according to claim 21, characterized in that said fractionation is achieved by dissolving said mixture in an aqueous medium to thereby obtain an aqueous solution of the components in said mixture, boiling said aqueous solution to thereby precipitate a mixture of sodium carbonate and sodium sulph that while avoiding a precipitation of the main amount of sodium chloride, after which said precipitate is removed mixture from the resulting hot aqueous sodium sulfide solution. 27. Method according to claim 26, characterized in that said boiling is carried out at temperatures from approx. 49 more about. 127°C. 28* Method according to claim 27, character! serted by that. said aqueous solution of said components of said mixture has a sodium sulphide concentration of from approx. 5 to approx. 30#. 29. Process according to Jcrav 26, characterized in that part of said mother liquor is recycled to said hot aqueous sodium sulphide solution. ,., ,. :J 30. Method according to claim 29" characterized in that the remaining part of the mother liquor is concentrated in order to precipitate a mixture of sodium chloride, sodium chloride, sodium sulphate and sodium carbonate, after which the latter mixture is removed from the resulting concentrated sodium sulphide solution and the removed mixture is recycled to the aforementioned mixture of components. 31. Method according to claim 30, characterized in that the concentration is carried out by boiling at temperatures from approx. 49 to approx. 138°C. 32. Method according to claim 31, characterized in that said mother liquor has a sodium sulphide concentration of approx. 20 to approx. 45#. 33. Method according to claim 21, man aft! cert in that said fractionation is achieved by dissolving said mixture in an aqueous medium to thereby obtain an aqueous solution of the components in said mixture, after which said aqueous solution is boiled in order to precipitate from this a mixture of sodium carbonate and sodium sulphate at the same time, while avoiding precipitation of the main amount of sodium chloride, whereupon said precipitated mixture is removed from the resulting hot aqueous sodium sulphide solution, after which said mother liquor is concentrated in order to precipitate from this a mixture of sodium chloride, sodium sulphate and sodium carbonate , removes the latter mixture from the resulting concentrated sodium sulfide solution and recycles the removed mixture into said mixture of components. 34. Method according to claim 21, characterized in that said fractionation is achieved by leaching said mixture with a hot aqueous medium in order to obtain said hot aqueous sodium sulphide solution containing said sodium sulphide, said sodium chloride, sodium sulphate and sodium carbonate at the same time that you get back a solid mixture of sodium carbonate and sodium sulphate essentially free of sodium sulphide and sodium chloride. 35. Method according to claim 34* characterized in that said leaching is carried out at temperatures from approx. 49 to approx. 127°C. 36. Method according to claim 35, characterized in that part of said mother liquor is recycled to . said warm aqueous sodium sulphide solution. 37. Method according to claim 36, characterized in that the remaining part of the mother liquor is concentrated in order to precipitate from this a mixture of sodium chloride, sodium sulfate and sodium carbonate, after which the latter mixture is removed from the resulting concentrated sodium sulfide solution and the removed mixture is recycled to the aforementioned mixture of components. 38. Procedure According to claim 37. characterized in that said concentration is carried out by boiling at temperatures from approx. 49 to approx. l38°C. 39. Method according to claim 38, characterized in that said mother liquor has a sodium sulphide concentration of from approx. 20 to approx. 45$. 40. Method according to claim 21, characterized in that said fractionation is achieved by leaching said mixture with a warm aqueous medium in order to thereby obtain said hot, aqueous sodium sulphide solution containing said sodium sulphide, said sodium chloride, sodium sulphate and sodium carbonate while obtaining a solid mixture of sodium carbonate and sodium sulphate substantially free of sodium sulphide and sodium chloride, after which the aforementioned mother liquor is concentrated to precipitate from this a mixture of sodium chloride, sodium sulfate and sodium carbonate, after which the latter mixture is removed from the resulting concentrated sodium sulfide solution and recycles the removed mixture into said mixture of components. 41. Process for the production of wood pulp, X characterized by boiling cellulose-containing fibrous material with a cooking liquid containing sodium hydroxide and sodium sulphide as the active cooking chemicals in order to thereby produce a wood pulp and used cooking liquid, after which said wood pulp is subjected to a series of steps for bleaching and cleaning where used at least one chlorine-containing bleaching chemical in at least one of the mentioned bleaching steps as well as aqueous solutions containing sodium hydroxide in the said cleaning step, takes out a sodium chloride-containing aqueous liquid from the said series of bleaching and cleaning steps into the said used cooking liquid, processes said used cooking liquid containing said used liquid from said bleach* <p> g purification step into a mass containing sodium sulphide, sodium chloride, sodium carbonate and sodium sulphate, fractionates said mass in order to obtain an aqueous solution of said sodium sulphide, said sodium chloride, sodium carbonate and sodium sulphate as well as a solid mixture of sodium carbonate and sodium sulphate essentially free of sodium sulfide and sodium chloride, after which one removes sodium chloride in solid form from said aqueous solution, processes said solid mixture into an aqueous solution and causticizes the sodium carbonate content in this to produce a sulfide-free white liquor, mixes at least a part of this sulphide-free white liquor with at least a part of said aqueous sodium sulphide solution after having f Jer-. nine; said sodium chloride in solid form, and uses the resulting white liquor as part of said cooking wash. 42. Method according to claim 41, characterized in that a part of the aqueous sodium sulphide solution, after removing said sodium chloride in solid form, is recycled to said aqueous solution of said sodium sulphide, said sodium chloride, sodium carbonate and sodium sulphate. 43. Procedure according to claim 41, character! see that a part of said sulphide-free white liquor is added to said aqueous sodium sulphide solution from which said sodium chloride has been removed' 44. Method according to claim 41, character! characterized in that said fractionation is achieved by dissolving said mass 1 in an aqueous medium to thereby obtain an aqueous solution of the components in said mass, concentrates said latter aqueous solution in order to thereby precipitate from it a mixture of sodium carbonate and sodium sulphate while avoiding precipitation of the main amount of sodium chloride" and removes said precipitated mixture from said resulting aqueous sodium sulphide solution as said solid mixture. 45. Method according to claim 44, characterized in that said sodium chloride is removed from said aqueous sodium sulphide solution by precipitating a mixture of said sodium chloride, sodium carbonate and sodium sulphate from said aqueous sodium sulphide solution, the latter mixture is leached after separating it from the resulting sodium sulphide solution thereby obtaining an aqueous solution of sodium carbonate, sodium sulphate and sodium chloride as well as a solid mass of essentially pure sodium chloride. 46. Method according to claim 45" characterized in that said aqueous solution of sodium carbonate, sodium sulfate and sodium chloride is recycled to said aqueous solution of components in said mass* 47. Procedure According to requirement 44, grade! certed by the said sodium chloride being removed from the said aqueous sodium sulphide solution, whereby essentially pure sodium chloride is precipitated from this, which is then removed from the mother liquor* 48. Procedure according to claim 47" character! served by recycling a part of said mother liquor to said aqueous solution of the components in said mass. 49. Method according to claim 48, characterized in that a mixture of sodium carbonate, sodium chloride and sodium sulphate is precipitated from the remaining part of said mother liquor, the precipitated mixture is separated from the resulting sodium sulphide solution and the separated part is recycled. 50. Method according to claim 41, characterized in that said fractionation is achieved by leaching said mass with an aqueous medium in order to thereby obtain said aqueous sodium sulphide solution containing said sodium sulphide, said sodium chloride, sodium sulph and sodium carbonate while obtaining a solid mixture of sodium carbonate and sodium sulphate essentially free of sodium sulphide and sodium chloride. 51. Method according to claim 50, characterized in that said sodium chloride is removed from said aqueous sodium sulphide solution by precipitating a mixture of said sodium chloride, sodium carbonate and sodium sulphate from said con concentrated sodium sulphide solution, after which the latter mixture is leached after separating it from said concentrated sodium sulph in immersion solution, whereby an aqueous reading of sodium carbonate, sodium sulphate and sodium chloride is obtained as well as a solid mass of essentially pure sodium chloride" 52. Method according to claim 51, characterized in that said aqueous solution of sodium carbonate. sodium sulfate and sodium chloride are recycled as part of said aqueous medium for leaching said mass. 53* Method according to claim 50, characterized in that said sodium chloride is removed from said aqueous sodium sulphide solution by precipitating essentially pure sodium chloride from this, after which you remove the precipitated sodium chloride from the mother liquor. 54. Method according to claim 53, characterized in that a part of said mother liquor is circulated to said aqueous sodium sulphide solution. 55* Method according to claim 54, characterized in that a mixture of sodium carbonate, sodium chloride and sodium sulfate is precipitated from the remaining part of the mother liquor, dSn separates the precipitated mixture from this resulting concentrated sodium sulphide solution and recycles the separated mixture to said mass. 56. Method according to claim 4l, characterized in that said wood pulp is washed after it has been produced and before it is transferred to said series of bleaching and cleaning steps And said sodium chloride-containing aqueous washes are used in said washing. 57" Method according to claim 41, characterized in that said series of bleaching and cleaning steps includes a first step of bleaching with chlorine, chlorine dioxide or a mixture of these* a first step of caustic extraction using sodium hydroxide solution, a second step of bleaching with chlorine dioxide , a second-stage caustic extraction with sodium hydroxide solution and a third-stage bleaching with chlorine dioxide, and where the amount of sodium hydroxide used tSsv&ssJ amount of chlorine compounds used in said bleaching step, and where the washes after bleaching and caustic extraction are mixed, thereby obtaining a aqueous liquid containing sodium chloride with essentially neutral pH. 58. Method according to claim 57, characterized by e d . that one separates the said wood pulp I mentioned used cooking sink at the same time as washing the said wood pulp after it has been produced and before it is taken to the said series of bleaching* and cleaning steps, washing the said wood pulp after each mentioned bleaching and caustic extraction, and where the water used for washing in the latter washing step is passed in countercurrent with the wood pulp through a series of bleaching and cleaning steps, mixes used washing water from the latter washing with used chemicals from said bleaching and said caustic extraction whereby an aqueous liquid containing sodium chloride is obtained and used in said washing of said pulp before this is taken to said series of bleaching and cleaning steps, whereby said liquid is transferred to said used cooking liquid. 59» Method according to claim 57, characterized in that said bleaching and caustic extraction steps are carried out by percolating an aqueous solution of treating chemicals through a mass of wood fibers while keeping the fibers relatively stationary with respect to each other except the movement due to the passage of said aqueous solution through said wood mass" 60. Method according to claim 44, character! cert v e d that said causticization?> is carried out by contacting said aqueous solution prepared from said solid mixture of sodium carbonate and sodium sulfate with lime to convert essentially all sodium carbonate into sodium hydroxide while simultaneously obtaining precipitated calcium carbonate, after which said calcium carbonate is separated, washed this calcium carbonate essentially free of entrained sodium hydroxide compounds, regenerates the lime from said washed calcium carbonate and uses used washing water from the latter washing as part of said aqueous medium for dissolving said mass. 61. Method according to claim 50, characterized by that said causticisation is carried out by contacting said aqueous solution prepared from said solid mixture of sodium carbonate and sodium sulphate, with lime to convert essentially all sodium carbonate, separates said calcium carbonate, washes said calcium carbonate essentially free of entrained sodium hydroxide, regenerates lime from said washed calcium carbonate and uses the used washing water from the latter washing as a part ay the aqueous medium, which is used for leaching the said solid mass..