CA1070457A - Removal of sodium chloride from pulp mill operations - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Sodium chloride is removed from pulp mill systems in which the smelt from the spent pulping liquor recovery operations contains sodium carbonate and sodium chloride, and optionally other components, depending on the pulp mill system. The smelt is made up into a hot aqueous solution thereof, and is evaporatively cooled to crystallize hydrated sodium carbonate, while avoiding deposition of sodium chloride and until the cooled solution is saturated with respect to sod-ium chloride. After separation of the crystallized sodium carbonate, and other crystallized salts, for example, sodium sulfide and sodium sulfate if a Kraft mill system is employed, the mother liquor is evaporated with heating to deposit sodium chloride in a substantially pure form. The sodium carbonate may be converted into an active pulping chemical for recycle, typically into sodium hydroxide by recausticization.

Description

10';'04~ ;J

The present invention is directed to the removal of sodium chloride, from pulping operations, more particu-larly from chemical wood pulping operations including bleaching procedures for the pulp, with enhanced efficiency of pulp mill operations.
,', Generally in the production of pulp suitable for formation into paper, wood or other raw cellulosic fibrous materials, is subjected to chemical digestion in a pulping liquor to form a pulp of the cellulosic fibrous material.
The pulp thereafter is subjected to brightening and purifi-i cation operations in a bleach plant.
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The spent pulping liquor usually is subjected to ~, a series of recovery and regeneration operations to recover :, . . . ........... . ..... .. ..
j pulping chemicals and to provide fresh pulping liquor. Gen-erally, the pulping liquor contains alkali metal salts, usually sodium salts, although potassium or lithium salts could be ~, employed.

Two well-known pulping processes are the Kraft and soda processes. While the present invention will ~e described hereinafter with particular reference to Kraft and soda mills, the process is applicable also to other pulp mill operations involving the use of alkali metal salts as pulping chemicals and which utilize spent chemical recovery and pulping chemical regeneration operations. Many such .,~
operations involve the use of sulphur-containing chemicals, including the Krat process, high yield pretreatment Kraft process, alkaline sulfite process, neutral sulfite process, i - 2 -,~'`

107045~ -acid sul~ite process, bisulfite process, acid bisulfite process, polysulfide process, alkafide process. other pulping operations which may be employed are the soda-oxygen proccss and the holopulping process. The present invention also is applicable to cross-recovery operations used in mixed systems.
In the conventional Kraft process, raw cellulosic fibrous material, generally wood chips, is digested, by heating, in a pulping liquor, known as white liquor and containing sodium sulphide and sodium hydroxide as the active pulping chemicals, to provide a pulp and spent pulping liquor, -~ known as black liquor. The black liquor is separated from the pulp by washing in a brown stock washer and the pulp then is passed to the bleach plant for brightening and purification operations.
The black liquor then is passed to the recovery and regeneration system in which the black liquor first is con-centrated, usually by evaporation, and the concentrated black liquor is burned in a furnace to yield a smelt containing primarily sodium carbonate and sodium sulphide. A sodium-and sulphur-containing compound, generally sodium sulphate, is added to the black liquor generally prior to feed of the concentrated black li~uor to the ~urnace, although such sodium-and sulphur-containing compound may be added at any other convenient point, such as to the white liquor prior to the digestion :~.
~ step, to make up sodium and sulphur values lost from the .~
; recovery system.
The smelt is dissolved in water to yield a raw green liquor which then is clarified to remove undissolved solids.

,i 107045';' The clarified gre~n liquor is causticized with slaked lime whereby the sodium carbonate is converted to sodiùm hydroxide-The resulting liquor is white liquor which then may be re-cycled to the digestion step to provide at least part of the pulping liquor.
As mentioned above another pulping process to which the present invention may be applied is the soda process.
In this process, the pulping liquor consists predominantly of aqueous sodium hydroxide solution. Spent pulping liquor is subiected to a recovery and regeneration procedure as in the Kraft process. The'smelt which results from furnacing in the soda process contains primarily sodium carbonate which, after formation into an aqueous solution, is recausticized to regenerate sodium hydroxide solution for recycle to provide at least part of the pulping liquor.
Bleach plant operations generally involve a se~uence of brightening and purification steps, together with washing ' steps. The brightening steps generally involve the use of bleaching agents. At least one of the brightening steps usually invol~es the use pf at least one chlorine-containing bleaching agent. Such chlorine-containing bleach-ng I agents include chlorine, chlorine dioxide, chlorine monoxide and sodium hypochlorite.
The purification steps generally involve treatment with sodium hydroxide solution, and usually is known as a ~, caustic extraction step. In some instances, the bleaching and caustic extraction steps may be combined, for example, using the so-called "oxygen bleaching" operation.
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,~, 10~0457 A particular bleaching operation which has been employed involves an initial bleaching of the pulp with an aqueous solution containing chlorine or a mixture of chlorine dioxide and chlorine, an intermediate washing, a caustic extraction using aqueous sodium hydroxide solution, a further washing, a bleaching with an aqueous solution of chlorine dioxide, another washing, a further caustic extraction using aqueous sodium hydroxide, an additional washing, a final bleaching with chlorine dioxide solution and a final washing.
This is the so-called OE DED operation. The present invention will be described with particular reference to this procedure, although other procedures may be employed, such as, the use of an aqueous solution containing approximately 100% chlorine dioxide in the first bleaching step.
; The above-described CEDED operation may be carried out using the so-called "Dynamic Bleaching" process outlined in Canadian Patent 783,483. In this process, pulping treating solutions are passed successively through a mat of fibers in which the fibers are maintained relatively stationary with respect to each other. Washing steps, except for a washing after the last step of the bleaching and purification sequence, may be omitted.
The spent wash waters from bleach plants generally have been discharged to water ~odies, such as streams, rivers, .
lak~s and oceans, without any attempt to recover chemicals therefrom, although in some instances solid particle recovery 107()457 operations have ~en made. One of the main reasons that no attempt has been made to recover these chemicals is because they are very dilute and of small value. The bleach plant also produces spent bleaching liquor effluents and spent caustic extraction effluents. These effluents have objec-tionable colour and are toxic and harmful to aquatic and marine biota and polluting since they contain fibers and materials consuming oxygen present in the water. It is desira~le to avoid such environmental pollution, and hence avoid the discharge of these effluents from the mill.
Due to the use of chlorine-containing bleaching chemicals and sodium-containing purification agents, the spent wash water contains substantial quantities of sodium chloride. In addition, when the spent bleaching liquor effluents and the spent caustic extraction liquor effluents are mixed at least part of the residual chlorine and soda values combine to form sodium chloride. The normally dis-chaxged effluents, namely, the spent wash water, the spent bleaching chemicals and the spent caustic extraction liquor may be mixed to provide a bleach plant effluent stream, known as BPE.
The quantity of the chlorine containing bleaching agents and the quantity of sodium hydroxide used as caustic extraction li~uor preferably are balanced to provide about one atom of sodium for each atom of chlorine, whereby these chemica~
form sodium chloride. In practice, the quantlty of sodium hydroxide solution is in a slight stoichiometric excess to en-sure the conversion of all the chlorine values to sodium chloride The equivalence of sodium and chlorine atoms in the ~leach plant ~070457 effluent also is preferred so that the overall sodium inventory of the mill thereby remains unchanged. In the CEDED sequence when chlorine or mixtures of chlorine dioxide and chlorine in which the proportions of available chlorine provided by chlorine dioxide is low, is used in the first stage an amount of sodium hydroxide solution in excess of that required for extraction must be added to match the chlorine atoms present. If no excess is added, only about 40 to 50% of the first chlorination stage filtrate can be recovered to match the stoichiometric equivalent of sodium atoms used in the extraction. Where, however, the available chlorine is provided predominantly by chlorine dioxide, - typically above about 70%, the quantities of sodium and chlorine atoms are substantially equivalent and hence it is preferred to employ the latter sequence.
Sodium chloride present in the bleach plant effluent also may arise from sodium chloride present in the pulp when it is introduced to the bleach plant. Such sodium chloride may be present where the logs are floated in sea water prior to formation of wood chips therefrom. In addition, if brackish water is used to provide the bleach plant wash water, sodium chloride again is present in the bleach plant effluent.
Sodium salts ~ay be introduced to the pulp mill system, from other sources, such as from the cellulosic fibrous material itself.
In the present invention, the bleach plant effluent preferably is added to the spent pulping liquor recovery and regeneration operation, and in this way this effluent is retained within the mill. It has been proposed previously in Canadian Patent No. 832,347 and U.S. Patent No. 3,698,995 to 1070~57 reduce the environmental problems of bleach plant effluent by utilizing the spent wash waters to wash the pulp in the brown stock washer The use of the spent wash water in this manner reduces the overall water requirement of the mill. In the present invention, it is preferred to use the bleach plant effluent, consisting of a mixture of spent wash waters, pre-ferably provided by countercurrent washing as described in Canadian Patent No. 832,347 and U.S. Patent No. 3,698,995, spent bleaching chemicals and spent caustic extraction liquor, to wash the pulp in the brown stock washer and provide thereby an "effluen.-free" pulp mill and a lowering of water requirement.
The bleach plant effluent may be introduced at other stages of the recovery and regeneration operations.
Further, the bleach plant effluent may be split into two or more streams which are introduced at different locations of the recovery and regeneration operations, for example, to wash calcium carbonate mud to provide the "weak wash" water or to dilute concentrated white liquor.
The quantity of sodium chloride present in the bleach plant effluent varies depending on the bleaching se-quence which is employed. In a typical procedure where a mixture of chlorine dioxide and chlorine are utilized in the first stage of a CEDED sequence the quantity of sodium chloride may vary between about 120 and 160 lbs/ton of pulp depending on the proportion of chlorine dioxide used. Typi-cally, when the total available chlorine in the first stage - is provided 70% by chlorine dioxide and 30% by chlorine, the guantity is about 120 lbs/ton of pulp.
The introduction of the bleach plant effluent to 10'7(~45';' the pulping liquor recovery and regeneration operations closes the whole system and sodium chloride is not purged by way of discarded bleach plant effluent. The sodium chloride remains unconverted by the black liquor recovery steps and hence would build up in the system. In order to prevent such build up and at the same time utilize the concepts of an effluent-free pulp mill, it is essential to remove sodium chloride from the system. Such removal of sodium chloride should be such that the other valuable components utilizable as or convertible into pulping chemicals are not removed from and thereby lost to the system along with the sodium chloride. In addition, it is preferred to remove a quantity of sodium chloride from the mill equivalent to the amount introduced to and/or pro-duced within the mill, typically about 120 lbs/ton pulp.

In instances where bleach plant effluent is not ` introduced to the recovery and regeneration operations, but i8 discharged in the normal way, sodium chloride may still be present in the mill recovery and regeneration ':
cycle which is required to be removed to prevent build up.
Such sodium chloride may arise from sea-borne logs which are pulped and washed, thereby removing some sodium chloride from the pulp and introducing it to the b~ck liquor, prior to passage to the bleach plant. Further, where wash water , i for washing the pulp prior to passage to the bleach plant is contaminated with sodium chloride, at least part of this sodium chloride is introduced to the black li~uor.
- ~he present invention is concerned broadly, there-fore, with the remo~al of sodium chloride from pulp mill _ g 1~70~5~
recovery and regeneration systems irrespective of the source of such sodium chloride, and is preferably concerned with the removal of sodim chloride from effluent-free pulp mills.
The process described and claimed in Canadian Patent No. 915,361 and U.S. Patent No. 3,746,612 makes it feasible to remove from the mill on a continuous basis an amount o~
sodium chloride equivalent to the amount of chloride intro-duced to the recovery and regeneration system. This is achieved by concentrating, preferahly by evaporation, the white liquor prior to recycle to the digestion stage in order to precipitate and remc>ve sodium chloride from the white liquor.
The white liquor evaporation process is outlined in the above-mentioned Canadian Patent No. 915,361 and U.S.
Patent No. 3,746,612, while effective in removing sodium chloride from the system in the desired quantities, neverthe- ?
less involves consid~rable energy to evaporate the required amount of water to achieve precipitation of the requisite quantity of sodium chloride and further involves considerable capital investments in special equipment in view of the high temperature and corrosive nature of the solutions. Further, in the process of Canadian Patent No. 915,361 and U.S. Patent 3,746,612, some sodium chloride remains in the recycled white liquor and cycles through the system as a dead load.
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In accordance with the present invention, an alter-native sodium chloride separation procedure is provided which, in its preferred embodiments, enables sodium chloride to be - --removed from the system at a lower cost.

45, In accordance with the present invention, a smelt containing sodium carbonate and sodium chloride is dissolved in an aqueous material to form hot aqueous solution thereof, the hot aqueous solution is cooled to crystallize hydrated ~odium carbonate therefrom while avoiding precipitation of sodium chloride and until the cooled solution is substantially saturated with respect to sodium chloride, the crystallized hydrated sodium carbonate is recovered from the resulting ~queous solution substantially saturated with sodium chloride, the sodium chloride solution is evaporated with heating to crystallize sodium chloride therefrom, and the ctystallized sodium chloride is removed from the resulting mother liquor.
The hot aqueous solution which is cooled by the procedure of the invention therefore contains dissolved ; quantities of the material of the smelt, including sodium carbonate and sodium chloride. In some instances, the 8melt may be substantially sodium chloride-free but the hot aqueous solution nevertheless contains dissolved quantities of sodium chloride as a result of additions of a sodium chloride_containing aqueous solution, such as bleach plant effluent, to the aqueous solution of the smelt. Such proce-dures are within the scope of this invention.
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Further, it may be desired, under certain circum-- stances to form a hot a~ueous solution for treatment in accor-dance with the present invention from only part of a sodium chloride-containing smelt and such procedures are within the scope of this invention.
The hydrated sodium carbonate which is crystallized and recovered in this way then may be recausticized, after making up into an aqueous solution thereof, or otherwise 107()457 reconverted into pulping chemicals. Since the sodium carbon-ate crystallization substantially avoids the crystallization of sodium chloride, little or no sodium chloride is recycled with the pulping liquor as a dead load.

The mother liquor remaining after removal of sodium chloride usually still contains dissolved quantities of sodium chloride and sodium carbonate, and this mother liquor may be recycled to the aqueous solution formed from the materials of the smelt.
The quantity of sodium chloride recovered by the process of the invention preferably is substantially the same quantity as is originally present in the smelt, and this may be achieved by judicious choice of conditions.
Chemicals other than sodium carbonate and sodium chloride may be present in the smelt, depending on the pulp mill system which is used. For example, where the Kraft process is used, the smelt also may contain sodium sulfide and small quantities of various sodium and sulfur compounds including sodium sulfate, sodium sulfite, sodium thiosulfate and sodium polysulfide. Sodium sulfate is present in the most significant amount. In one embodiment of the invention as applied to the Kraft process, at least the sodium sulfate and optionally the sodium dulfide are crystallized along with t~e sodium carbonate.
~he cooling of the hot aqueous solution may be carried out in any convenient manner to a temperature which wi~l result in crystallization of hydrated sodium carbonate, ; optionally together with any other crystallizable salts, with the exception of sodium chloride. The form of the sodium ~; 30 carbonate which is crystallized depends on a number of ~0'70457 factors, including the composition of the cooled solution and the temperature to which the hot aqueous solution is cooled to cause crystallization. The sodium carbonate is crystallized in a hydrated form, which may be the septahydrate or decahydrate. Preferably the de~ahydrate is crystallized The decahydrate form is crystallized at tempera-tures below about 25C. Crystallization of the sodium carbonate in a hydrated form in the invention removes water from the system, thereby decreasing the quantity of water which must subsequently be evaporated to remove water to deposit sodium chloride.
During the cooling step, water preferably is evapora-ted from the aqueous so~ution to increase the concentration of the dissolved salts and this evaporation may constitute the sole form of cooling the hot aqueous solution. The cooling effect of the evaporation, carried out under vacuum, may be enhanced in any desired manner.
The smelt may be fractionated during its formation into the hot aqueous solution to dissolve substantially all the sodium chloride therefrom along with part of the sodium carbonate and part or all of other salts present in the smelt, depending on the conditions of the fractionation. The un- -dissol~ed portion of the smelt is substantially free from sodium chloride and may be ~orwarded directly to the recaus-ticization stage. Fractionation of the smelt in this way decreases the energy required to deposit the sodium carbonate from the hot aqueous solution.
The invention is described further, by way of illustra-tion, ~ith re~erence to the accompanying drawings, in which:

107l)457 Figure 1 is a schematic flow sheet illustrating one embodiment of the invention, as applied to a soda pulp mill; and Figure 2 is a schematic flow sheet illustrating a further embodiment of the invention, as applied to a Kraft pulp mill.

Referring first to Figure 1, wood chips, or other raw cellulosic fibrous material, are fed by line 10 to a digester 12 wherein the wood chips are digested with a pulping liquor fed by line 14 and containing sodium hydroxide as the active pulping chemicalin the soda process.

The resultin~ pulp and sp~nt pulping liquor are separated and the pulp is washed in a brown stoc~ washer 16.
- The pulp is washed, in the embodiment illustrated, with aqueous bleach plant effluent fed by line 18. Alternatively, the pulp may be washed with water or ~'contaminated condensate", and the bleach plant effluent may be utilized elsewhere in the system, as described in more detail below, or discharged as desired.
The washed and unbleached pulp is fed by line 20 to a bleach plant 22 wherein the pulp is subjected to a series of bleaching and puriflcation processes involving the use of one or more chlorine-containing bleaching agents. Generally, the bleaching and purification processes involve bleaching with chlorine, chlorine dioxide or mixtures thereof fed ~y line 24 and purification by caustic extraction, using aqueous eodium hydroxide solution fed by line 26, typically in a OEDED
DCEDED or DEDED sequence. The pulp is washed during the bleach plant operations, typically after each bleaching or caustic extraction operation, by water fed by line 28. The spent wash water from the bleach plant washing operations together with the spent chemicals from the bleaching and - caustic extraction steps provide the bleach plant effluent in line 18.
Preferably, the washing operations in~olve counter-current flow of pulp and wash water through the bleach plant.
; Prefe~ably, the quantity of sodium hydroxide used in the caustic extraction of the pulp corresponds to the stolchiometric requirement of one sodium atom for each chlorine atom in the bleaching chemicals, whereby the bleach plant effluent in ~ e 18 has a neutral p~. The bleached and purified pulp of required brightness is recovered from the bleach plant 22 by line 30 and is sold as such, or passed to paper making procedures.
If desired, the bleach plant effluent in lire18 may be added directly to the spent pulping liquor in line 32, although this procedure is less preferred, since the overall water requirement thereby is increased.
The bleach plant effluent in line18 contains con-siderable quantities of sodium chloride which are transferred to the spent pulping liquor in line 32. The spent pulpingliquor is evaporated in an evaporator 34 prior to passage by line 36 to a furnace 38 of any convenient construction. The water recovered from the evaporator 34 by line 40 may be used to provide at least part of the water requirement of the system, for example, as at least part of the water fed to the bleach plant in line 28, after suitable cleaning, if required.

The spent pulping liquor forms in the furnace 38 a smelt containing sodium carbonate possibly along with some sodi~-sulfide and ~dium sulphate and addi~onally unreacted o~x~ents consistingsubstantially of sodium chloride. Typically, the smelt contains about 70 to about 90~ by weight of sodium carbonate, about 1~
to about 25% by weight of sodium chloride, ~ to about 3% by weiS~t o~ sodium sulfate and 0 to about 4~ by weight of sodium sulfide.
Thus, there is obtained from the furnacing operation a smelt containing sodium carbonate and sodium chloride. In accordance with this embodiment of the present invention, the smelt is dissolved in a smelt dissolver 42 in water fed by line 44. The water ~iay ~e constituted by BPE, evaporator 14S, condensate or ~weak wash" water, if available after dissolv-ing hydrated sodium carbonatein ~ssolver 54. The water passed by line 44 to the smelt dissolver 42 may be partially bleach plant effluent fxom line 18, especially in the case where fresh water or contaminated condensate is fed to the brown stock washer 16.
The resulting hot aqueous solution in li~ 46 also contains-recycled chemicals as described in more detail ~elow.
If desired, all the smelt need not be dissolved in the aqueous material fed by line 44, but instead the smelt may be leached with the aqueous material fed by line 44 to dissolve substantially all the sodium chloride values and part of the sodium carbonate values of the smelt, leaving substantially pure sodium carbonate. Operation in this manner reduces the quantity of water which requires evaporation.
' lhe res~ ~ng substantially pure ~um ca~x~abe ~y beused to form pulping liquor along with the sodium carbonate later separated ' from the hot aqueous solution resulting from the leaching.
s The hot aqueous solution is cooled in a crystal-lizer 48 to crystallize hydrated sodium carbonate, preferably sodium carbonate decahydrate, while avoiding substantially the crystallization of sodium chloride. The cooling prefer-~ ably is achieved by evaporation o~ water from the hot solu-i' - tion under vacuum, the evaporated water being removed by line 50.
The hot aqueous solution in line 46, which generally has a very high temperature, typically a~out 180F to 21~F, may be suhjected to aninitial cooling, with or without evapora-~ tion o~ water, to reduce the cooling load to be borne by the ¦ 3~ sodium carbonate crystallizer, prior to further e~aporative _ I7 -107~}45~

cc>olin~ to cause cr~s.allization of the sodium carbonate.
More than one such initial cooling steps may be used, i desired. Th~ initial cooling may be omitted, if desired.
In a typical operation, the hot aqueous solution at an initial temperature of about 200F is cooled to abcut 120F with some evaporation of water and then the solution is evaporatively cooled to below about 70F, preferably to about 60F, causing crystallization of sodium carbonate deca-hydrate. Since the sodium carbonate is crystallized in the form of the decahydrate, thereby more water i5 removed from the solution.
The evaporative cooling and crystallization o~-sodium carbonate are continued-until the mother liquor-i8 substantially saturated with respect to sodium chloride.
The resulting slurry preferably is diluted with recycled mother liquor to improve the workability thereof and sodium carbonate, substantially free from sodium chloride, is separated from the mother liquor by line 52. While the crystallization of sodium carbonate and its separation from the mother liquor are illustrated as occurring in the same vessel in Figure 1, this is for ease of illustration and normally separate vessels are used.
The sodium carbonate is passed by line 52 to a dis-solving tank 54 wherein it is dissolved in water, or any other suitable aqueous medium, preferably "weak wash" water, fed by line 56 to form an aqueous sodium carbonate solution which i5 passed to a causticizer 58 by line 60. The sodium car~onate in the aqueous solution is converted substantially comple.ely to sodium hydroxide by the action of lime fed by line 62 from a lime kiln 64. The calcium car~onate mud precipitated in the 1~70~57 caustlcizer 58, after suitable washing to remove entrained alkali values, is returned to the lime kiln 64 by line 66 for reconversion to lime. The weak wash water resulting from the calcium carbonate mud washing may, in part be used as the aqueous material fed by line 44 to dissolve the smelt.
The aqueous sodium hydroxide solution resulting from the recausticization process is recycled by line 68 to provide at least part of the pulping liquor fed by line 14 to the digester 12.
The mother li~uor resulting from separation of the sodium carbonate, containing dissolved quantities of sodium chloride and sodium carbonate, and saturated at least with respect to sodium carbonate, is forwarded by line 70 to an evaporator 72 wherein the mother liquor is heated and water is evaporated therefrom to deposit sodium chloride, while avoiding subs~antially the crystallization of sodium carbonate.
Generally, the evaporation of water in the evaporator 72 is achieved by boiling the solution under a reduced pressure, preferably until the solution is substantially saturated with respect to sodium carbonate. The water evaporated in the evaporator 72 is removed by line 74.

The temperature to which the mother liquor is heated to achieve the desired evaporation and crystallization of sodi~m chloride depends on the temperature at which the sodium carbonate crystallization occurred in the crystallizer 48 and the relative concentrations of sodium carbonate and sodium chloride in the mother liquor. The temperature must be higher than a temperature at which sodium carbonate will crystallize, if substantially pure sodium chloride is to be recovered, 5 ,~

and temperatur~s as hi~h as 220F may ~e used. ~t these temperatures, the temperature to ~hich the hot aqueous solution is cooled to crystallize hydra~ed sodium carbonate must be quite low, typically about 32 to 50F. Tempera.ures in the range of about 100 to 140~ have proved to be the desixable range. In the typical procedure outlined above wherein the hot aqueous solution ~as cooled to about 60F to achieve crystallization of hydrated sodium carbonate, the mother liquor may be heated with boiling to about 120F to achieve crystallization of sodium chloride.
The sodium chloride which is crystallized is removed by line 76 in substantially pure form. Preferably, the quantity of sodium chloride removed in this manner is substantially the same as the quan.ity of sodium chloride present in the hot aqueous solution which is subsequently cooled to crystallize hydrated sodium carbonate, less any sodium chloride present in the recycle stream in line 78.
The sodium chloride removed by line 76 may be put to a ~ariety of uses. Typically, the sodium chloride is utilized to regenerate bleach plant chemicals. For example, the 80aium chloride may be used to generate sodium hydroxide by electrolysis of an aqueous solution thereof, the sodium hydroxide ~eing used in the bleach plant in line26. Alter-natively, the sodium chloride may be used to generate chlorine di~xide and chlorine ~y reaction wi~h sodium chlorate and sulphuric acid, the chlorine dioxide and chlorine being fed to the bleach plant by line 24. Further, the sodium chloride may be electrolyzed as an aqueous solution to sodium chlorate for use in a chlorine dioxide producing reaction involving reduction of the sodium chlorate in an acid medium.

1(~70457 ~ evaporation Ol tAe mother liquor from the separation of sodium carbonate and the separation of the sodium chloride crystallized therefrom is illustrated as occurring in a single vessel 72. In pr~ctice, usually t~o separate vessels are used, the slurry resulting from the evaporation in one vessel being passed to a second vessel for separation of .he solid sodium chloride.
The mother liquor resulting from the sodium chloride separation usually will still contain quantities of sodium chlor;de and sodium carbonate and is recycled by line 78 to the smelt dissolver 42 along with the aqueous material in line 44, or, alternatively, is recycled to hot a~ueous solu-tion in line 46.
The mother liquor in iine 78 may contain small quantitites of sodium hydroxide, which would not be removed from the system by the illustrated procedure. The sodium hydroxide may arise from weak wash water from the washinq of the calcium carbonate mud where such water is used to pro-vide at least part of the aqueous material fed by line 44 to the smelt dissolver 42. The sodium hydroxide also may arise from small quantities of sodium oxide values in the smelt.
The mother liquor in line 78 may be carbonated using carbon dioxide,or in any other conve~ient manner, to convert i``4 such sodium hydroxide values to sodium car~onate, prior to passage of the mother liquor in line 78 to the hot aqueous solution in line 46. In this way, any build up of sodium hydroxide values in the recycled mother liquor in line 78 is avoided.
I~ desired, followin~ the separation of the sodium

- 2~ -107(~5, chloride ~ erefrom, the resulting mother liquor ma~ ~e cooled again to precipitate further quanti.ies of hydrated sodium carbonate.
It will be apparent, therefore, that in the process of the embodiment of Figure 1, the sodium carbonate and sodium chlori~e in the smelt are separated one from another into substantially pure product streams which contain o~uanti-ties of the hydrated sodium car~onate and sodium chloride which are substantially equal to the quantities of these materials p~esent-in the smelt.
Turning now to consideration of Figure 2, there is illustrated the application of the present invention to a Kraft mill operation. A smelt, consisting predominan.ly of sodium sulfide, sodium carbonate and sodium chloride, and also containing minor quantities of sodium sulfate and other sodium and sulfur-compounds due to inef~iciences of the furnace, typically containing about 7 to about 22% by weight of sodium sulfidel about 60 to about 78% by ~Jeight of sodium carbonate, about 10 to about 25% by weight of sodium chloride and about 1 to about 6% by weight of sodium sulfate, is passe~ by line 110 to a smelt dissolver 112.
As mentioned previously, in the Kraft mill process sodium hydroxide and sodium sulfide are the active pulping chem.icals and sodium sulphate, or o~er source o~ soda and sulfur values, is added to the bl~ck liguor to provide ma~e up for soda and sul~ur losses.

The smelt is dissolved in the smelt ~issolver 112 in an aqueous material ~ed by line 114, which may ~e water or,in part, ~eak wash water, to form a hot aqueous solu.ion, commonly known as green liquor, in line 116. The ~reen li~uor 1~7Q457 in line lL6 ~ay al~o contain recycled liq~id, as ~ill beco o more a~parent hereinafter.
The hot green liquor, typically having a tempera-ture of about 200F, is cooled in a precooler 118, with evaporation of some water therefrom, if desired. The water evaporated in the precooler is removed by line 120. Typi-cally, the green liquor is cooled to about 100F in the precooler 118.
The precooled green liquor is passed by line 122 to an evaporative cooler 124 wherein the precooled green liquor is subjected to evaporative cooling. Water is evapor2-ted from the precooled green li~uor, to cause cooling of the liquor and thereby bring about crystallization of a mixture of ~odium carbonate, sodium sulfide and sodium sulfate, together with minor quantities of other precipitated sodium and sulfu~
salts which may be present while avoiding crystallization of sodium chloride. The water evaporated is removed by line 126. The evaporative cooling i~ carried out until the mother liquor is substantial~y saturated with res-2~ pect to sodium chloride.
~ue to the rapid increase in solubility of sodium sulfide with increasing temperature, coo~ing of the green liquor to a temperature of less than about ~F, prefer-a~ly about ~0 F, is necessary to ensure crystallization of sodium carbonate, sodium sulfide and sodium sulfate. At these te~peratures the sodium carbonate is crystallized predominan~y as the decahydrate and sodium sulfide is crystallized pre-dominantly as the nonahydrate, thereby removing further water from the solution.
In the illustrated embcdiment, the hot green li~uor is subjected to a two-stage evaporative cooling to achieve crys-1070~57 tallization f a mixture of sodium carbonate, sodium sul-fide and sodium sulfate in the second stage. If desired, the cooling of the green liquor to achieve the desired precipita-tion may be carried out in a single step, and may be carried out in any convenient manner other than evaporative cooling, if desired. Further, the cooling achieved by evaporation may be enhanced by suitable additional cooling.
It may be desired, depending on the initial concen- -tration of the hot green liquor, to crystallize only sodium carbonate and sodium sulfate, leaving the sodium sulfide in the aqueous phase along with the sodium chloride. This particular embodiment will be discussed in more detail below.
The slurry resulting from the evaporative cooling is diluted with recycle mother liquor in line 128 and the diluted slurry passes by line 130 to a separator 132 of any con-venient construction. In the separator 132, the mixture of sodium -carbonate, sodium sulfide and sodium sulfate is separated from the mother liquor and is passed by line 134 to a recaus-ticizer wherein the sodium carbonate is converted to sodium hydroxide, the resulting white liquor being subsequently used as pulping liquor. The sodium sulfate passes to the furnace in the subsequently formed black li~uor.
, The quantity of sodium sulfide removed by line 134 is ~ substantially the same as that contained in the smelt in line ,-~, 110.
The quantity of sodium carbonate and sodium sulfate removed in line 134 may be less than the quantity present in I

10704S, the smelt, and, in the case of sodium sulfate, formed in the green liquor and, hence, a further separation of sodium carbonate and sodium sulfate may be necessary. In the illus-trated embodiment, this is carried out by passing the mother liquor from the separator 132 by lines 136 and 138 to an evaporator 140, with`a portion of the mother liquor being recycled by line ~28 as diluent for the slurry formed in the evaporative cooler 124.
The mother liquor is heated and evaporated in the evaporator 140 to cause the crystallization of sodium carbcn-ate together with the dou~le salt of sodium carbonate and sodium sulfate known as burkeite water vapor is removed by line 141. Typica~ly, the mother liquor may be evaporated at a temperature of about 220F, while avoiding substantially the precipitation of sodium chloride, the evap-oration being continued until the solution i5 substantially saturated with sodium chloride.
The slurry of burkeite and aqueous solution is passed by line 142 to athickener 144 of any convenient con-struction wherein the sodium carbonzte and burkeite is separated from the mother liquor and removed by line 146.
The sodium carbonate and ~urkeite separated in this way may be passed to the recausticizer along with the solids in line 134. Alternatively, a separator may be employed instead of the thickener 144. The thickener underflow is forwarded to the smelt dissolver 112, to the precooler 118 or to any other convenient location.
The mother liquor from the ~urkeite separation is passed hy line 148 to an evaporator 150 wherein water is evaporated to deposit substantially pure sodium chloride, which, 1a7~)45 ,' after separation from the resulting mother liquor, is removed by line 152. The quantity of sodium chloride removed by line 152 preferably is substantially that contained in the smelt in line 110. The sodium chloride removed in this way may be utilized in any convenient manner, typically to form bleaching chemicals.
The evaporation in evaporator lS0 results in water vapor which is removed by line 154 and may be carried out at any convenient temperature. The temperature chosen depends on the initial concentration of the dissolved components of the mother liquor and the quantity of sodium chloride required ~ to ~e removed. The evaporation of the solution in the evaporator lS0 to precipitate sodium chloride generally is carried out until the solution is su~stantially saturated with respect to sodium carbonate and/or sodium sulfide and/or 60dium sulfate, and typically is carried out at a temperature of about 120 F.
The mother liquor resulting from the sodium chloride crysta~iza~n contains residual quantitites of sodium carbonate, sodium sulfide, sodium sulfate and sodium chloride and is recycled by line 156 to a car~onator 158. The mother liquor is contacted with carbon dioxide fed ~y line 160, in order to convert any sodium hydroxide present in the mother liquor in line 156 to sodium car~onate, so that a ~uild up of sodium hydroxide upon repeated recycle is prevented. As mentioned above in connection with the embodiment of Figure 1, such sodium hydroxide may arise rom wea~ wash water and/or from materials in the smelt.
The carbonated mother li~uor is forwarded by line 107~)~5~
162 to the hot green liquor in line116. This may ~e achie-~ed by feeding ,he mother liquor in line 162 along with the aqueous material in line 114 to dissolve the smelt. Alternatively, the mother liquor may be added to the hot green liquor in line 116 after formation thereof.
By recycling the mother liquor from the sodium chlo-ride crystallization in this way, residual quantities of material, which, in a continuously operating procedure, pre-ferably will be substantially constant, loss of chemicals is 2voided.
It may be desirable, in certain instances, to cool the mother liquor in line 156 once more to crysta`llize fur-ther quantities of sodium c~rbonate and/or sodium sulfide and/or sodium sulfate, which may be forwarded to the recaus-ticizer along with the material in line 134.
In the embodiment discussed above, wherein the pre-cipitation of ~odium sulfide is a~oided when the green liquor is cooled, following evaporation to precipitate sodium chloride, the mother liquor resulting after separation of the sodium chloride consists mainly of sodium sulfide solution, which may be evaporated to crystallize therefrom the remaining quantities of sodium carbonate, sodium sulfate, and sodium chloride. The sodium sulfide solution then may be added to the sodium hydroxide solution resulting from the recaustici-- zation to provide white liquor for the recycle as pulping liquor. Alternatively, the sodium sulfide solution ~ay be recycled as the pulping liquor, while part o~ the sodium hydro~ide ~olution resu7ting from the recausticization is forwarded to the bleach plant for use therein as caustic ~xtraction liquor, 10'704S, or in o~ygen-~leaching processes. As another alternative, the sodium sulfide solu~ion may be used in imprecnatlng wood chips. The crystallized solids may be recycled to the hot green liquor.
The invention is illustrated by the following e~amples:

Example 1 A mass balance for the em~odiment of Figure 1 pro-ducing 500 tons/day of pulp was calculated from ~nown solu-bility data for the system Na2C03-~aCl-H20. 25,209 lbs/hr.
of a smelt having a con~titution of 87.5% Na2C03 and 12.5%
NaCl is dissolved in 66,774 lbs/hr. of water to provide 91,667 lbs/hr. of green liquor havlng a temperature of about 200 F.
The green liquor is mixed with 43,372 lbs/hr. of recycle mother liquor containing sodium carbonate in a concentration of 12.4% and sodium chloride in a concentration of 18.4% and having a temperature of 120F, to provide a combined liquid stream which is passed to an evaporative precooler wherein 6,034 lbs of water are removed while the temperature is cooled to about 120F, resulting in 129,005 l~s/hr. of precooled liquor.
The precooled liquor then is evaporatively cooled ; to 59F resulting in crystallization of sodium carbonate decahydrate. 11,36a lbs/hr. of ~m are evaporated from the precooled liq~or in this cooling step, and 52,085 l~s/hr.
of recycle mother liquor, containing 9.24% sodium carbonate and 19.10% sodium chloride, is recycled to the evaporat~ve cooler to dilute the slurry of sodium carbonate decahydrate formed therein.

- 28 _ 107045, ' 169,722 lbs/hr. of slurry is forwarded to a fi3ter wherein 59,~93 lbs/hr. of sodium carbonate decahydrate are recovered, leaving 110,229 lbs/hr. of filtrate, which is split into the mother liquor recycle stream to dilute the slurry in the evaporative cooler and into 58,144 lbs/hr. feed stream to an evaporator. In the evaporator, the feed stream, containing 9.25% sodium carbonate and 19.10% sodium chloride, is heated under a reduced pressure to evaporate 11,929 lbs/hr.
of water from the solution at 120F, resulting in crystalli-zation of sodium chloride from the s~lution.
46,215 lbs/hr. of slurry from the evaporator is passed to a filter whereon 3,15g lbs/hr. of sodium chloride are recovered, resulting in 43,372 lbs/hr. of filtrate stream containing 12.4% sodium carbonate and 18.4% sodium chloride.
After contact with carbon dioxide to convert any sodium hydroxide values to sodium carbonate, the filtrate stream is recycled to the green liquor.
Example 2 A mass balance ~or the ~d~nt of Figure 2 utilized in a p~p ~ill producing 500 ~s/day of pulp was ~lcula~ed ~rom expe~ntal solubilit~ data for the system Na2S-~Ta2C~3-~aCl-H2O. 95,750 lbs/hr of green li~uor containing 4.2% sodium sulfide, 17.6%
sodium car~onate, 3.3% sodium chloride and 0.6S% sodium sul-fate and ha~ing a temperature of 20~F is mixed with 28,4Q8 lbs/hr. of recycled mother liquor from sodium chloride removal containing 10.0% sodium sulfide, 6~2% sodium carbonate, 12.0%
sodium chloride and 0 65% sodium sulfate and 7,755 lbs/hr. of ~urkeite ~hickener underflow at 225F containing 2,713 lbs/hr.
of burkeite and 5,043 lbs/hr of liquor containing 7.3% sodium 107045 ,~
sulfide, 4.6% sodiu~ carbonate, lG.8% sodium chloride and 0.5% sodium sulfate, and the mixture of the three streams is precooled to 120F with evaporation of 6,397 lbs/hr. of water.
The resulting 125,516 lbs/hr. of precooled liquor containing 5.8% sodium sulfide, 15.6% sodium carbonate, 5.9%
sodium chloride and 2.2% sodium sulfate then is evaporatively cooled to 50F, with evaporation of ~,70S lbs/hr. of water.
The resulting slurry containing a solid phase consisting of a mixture of sodium carbonate, sodium sulfide and sodium sul~ate is diluted with 56,695 lbs/hr. of recycled filtrate from the solid phase separation containing 6.0% sodium sulfide, 5.3% sodium carbonate, 13.8% sodium chloride and 4.1% sodium sulfate.
The 169!506 lbs/hr of slurry is passed to a filter whereon 59,327 lbs/hr. of solid is separated from the liquid phase. The separated solids consists of 6.8 % sodium sulide, 28.4% sodium carbonate and 1.1 % sodium sulfate. The 110,179 ; lbs/hr. of filtrate from this solid phase separation is divided into two streams, one of which is recycled to dilute the slurry in the evaporative cooler. The other stream, in a quantity- of 53,484 l~s/hr. and containing 6.0% sodium sulfid~, ; 5.3% sodium carbonate, 13.~/~ sodium chloride and 4.1% sodium sulfateris passed to an evaporator wherein ~,814 ~bs/hr.
of water is evaporated at a temperature of 225F, resulting in crystallization ofbur~eite. The solid phase bur~eite is recovered in the form of a thickener under~low which contains 738 lbs/hr. of sodium carbonate and 1,974 l~s/hr. of sodium ,~ .

1070'~57 sulfate, as solid phase and the thickener underflow is recycled to the green liquor. Thc ~other liquor from this separation, in a quantity of 38,914 lbs/hr. and containing 7.3% sodium sulfide, 4.6% sodium car~onate, 16.8% sodium chloride and 0.5% sodium sulfate, then is evaporated at 120F to crystal-lize sodium chloride therefrom. 31,533 lbs/hr. of slurry from the evaporator is passed to a filter whereon 3,125 ; lbs/hr. of sodium chloride are recovered, resulting in 28,408 l~s/hr. of filtrate stream containing 10.0h sodium sulfide, 6.2% sodium carbonate, 12.0% sodium chloride and 0.68% sodium sulfate.
After contact with carbon dioxide to convert any -sodium hydroxide values to sodium carbonate, the filtrate stream is recycled to the green liquor.
. ' ' .

It will be seen that the process of the present invention provides in its preferred embodiments, a salt removal process in which the need for high temperature-resistant equipment is avoided, and the quantity of water requiring evaporation is decreased, and in which a aead load of sodium chloride does not recycle in the white liquor. In these respects, the present invention represents an improve-ment on the white 11quor evaporation procedures outlined in the aforementioned Canadian Patent ~o. 915,361 and its equiva-lent U.S. Patent No. 3,746,612.

Claims (35)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a pulp mill process which comprises digesting cellulosic fibrous material with a pulping liquor containing at least one active pulping chemical, separating the pulped material from spent pulping liquor, subjecting said spent pulping liquor to combustion in a furnacing step and recover-ing a solid mass containing sodium carbonate, the improvement which comprises dissolving at least part of said solid mass in an aqueous material to form a hot aqueous solution thereof, providing sodium chloride in said hot aqueous solution of said solid mass, cooling said hot aqueous solution to crys-tallize hydrated sodium carbonate therefrom substantially to the saturation of the cooled solution with sodium chloride, whereby said crystallized hydrated sodium carbonate is sub-stantially free from contamination by sodium chloride, separating said crystallized hydrated sodium carbonate from the resulting aqueous solution substantially saturated with sodium chloride, evaporating said sodium chloride solution to deposit sodium chloride therefrom, and separating said deposited sodium chloride from the resulting mother liquor.

2. The process of claim 1 including recycling said resulting mother liquor to said hot aqueous solution of said solid mass.

3. The process of claim 1 wherein said separated sodium carbonate is formed into an active pulping chemical and an aqueous solution thereof is recycled to provide at least part of said pulping liquor.

4. The process of claim 3 wherein said active pulping chemical is sodium hydroxide and is formed from said sodium carbonate by forming said separated sodium carbonate into an aqueous solution thereof and causticizing substantially the sodium carbonate values of said aqueous solution to sodium hydroxide.

5. The process of claim 4 wherein said sodium hydroxide is the sole pulping chemical in said pulping liquor.

6. The process of claim 4 wherein sodium hydroxide and sodium sulfide are the active pulping chemicals in said pulping liquor and said solid mass contains sodium sulfide, and includ-ing crystallizing sodium sulfide from said hot aqueous solution of said solid mass together with said hydrated sodium carbonate.

7. The process of claim 6 wherein said solid mass addi-tionally contains sodium sulfate and including crystallizing at least part of said sodium sulfate from said hot aqueous solu-tion of said solid mass together with said hydrated sodium car-bonate and sodium sulfide.

8. The process of claim 7 including the steps of after said separation of said crystallized sodium carbonate, sodium sulfide and sodium sulfate and before said evaporation of said sodium chloride solution, crystallizing additional quantities of sodium carbonate and sodium sulfate and removing said addi-tionally-crystallized sodium carbonate and sodium sulfate from the resulting mother liquor.

9. The process of claim 1 wherein sodium hydroxide and sodium sulfide are the active pulping chemicals in said pulping liquor and said solid mass contains sodium sulfide, and includ-ing crystallizing hydrated sodium carbonate from said hot aqueous solution of said solid mass while avoiding precipitation of sodium sulfide and sodium chloride, after separation of said crystallized hydrated sodium carbonate, evaporating the result-ing sodium sulfide solution to deposit substantially all the sodium chloride and sodium carbonate values therefrom, separat-ing the deposited mixture of sodium chloride and sodium carbon-ate from the resulting concentrated sodium sulfide solution, re-covering substantially pure sodium chloride from said deposited mixture and recycling tile remainder of said deposited mixture to said hot aqueous solution.

10. The process of claim 1 wherein said hot aqueous solu-tion is cooled while water is evaporated from the solution,

11. The process of claim 5 wherein said hot aqueous solu-tion is cooled from a temperature of about 180° to about 210°F
to a temperature below about 68°F while water is evaporated from said solution and sodium carbonate decahydrate is crystallized in a quantity substantially equivalent to the quantity of sodium carbonate present in the solid mass.

12. The process of claim 11 wherein said hot aqueous solu-tion is cooled initially from a temperature of about 100°F to about 120°F and subsequently is cooled to a temperature of about 60°F, resulting in the crystallization of sodium carbonate de-cahydrate from the cooled solution.

13. The process of claim 12 wherein said initial cooling and said subsequent cooling are achieved by boiling said aqueous solution under a reduced pressure.

14. The process of claim 1 wherein said sodium chloride solution is evaporated to deposit substantially pure sodium chloride therefrom by boiling said solution under a reduced pressure at a temperature greater than the temperature to which said hot aqueous solution is cooled to crystallize said sodium carbonate.

15. The process of claim 11 wherein said sodium chloride solution is evaporated by boiling said solution under a reduced pressure at a temperature of about 100° to 140°F to deposit sub-stantially pure sodium chloride therefrom in a quantity substan-tially equivalent to the quantity of sodium chloride provided in said hot aqueous solution less any sodium chloride recycled thereto in mother liquor from said sodium chloride crystalliza-tion.

16. The process of claim 12 wherein said sodium chloride solution is evaporated to deposit substantially pure sodium chloride therefrom by boiling said solution under a reduced pressure at a temperature of about 120°F.

17. The process of claim 15 wherein said evaporation is continued until the mother liquor from said sodium chloride de-position is substantially saturated with respect to sodium car-bonate.

18. The process of claim 7 wherein said hot aqueous solu-tion is cooled from a temperature of about 180° to 210°F to a temperature below about 55°F while water is evaporated from said solution, and sodium carbonate decahydrate and sodium sul-fide nonahydrate are crystallized from the hot aqueous solution in a quantity substantially equivalent to the quantity of sodium carbonate and sodium sulfide present in the solid mass.

19. The process of claim 18 wherein said hot aqueous solu-tion is cooled initially from a temperature of about 200°F to about 100°F and subsequently is cooled to a temperature of about 50°F, resulting in the crystallization of sodium carbonate de-cahydrate and sodium sulfide nonahydrate from the cooled solution.

20. The process of claim 19 wherein said initial cooling and said subsequent cooling are achieved by boiling said aqueous solution under a reduced pressure.

21. The process of claim 16 wherein said sodium chloride solution is evaporated by boiling said solution under a reduced pressure at a temperature of about 100° to 140°F to deposit substantially pure sodium chloride therefrom in a quantity sub-tantially equivalent to the quantity of sodium chloride provided in said hot aqueous solution less any sodium chloride recycled thereto in mother liquor from said sodium chloride crystalliza-tion.

22. The process of claim 19 wherein said sodium chloride solution is evaporated to deposit substantially pure sodium chloride therefrom by boiling said solution under a reduced pressure at a temperature of about 120°F.

23. The process of claim 21 wherein said evaporation is continued until the mother liquor from said sodium chloride deposition is substantially saturated with respect to sodium carbonate.

24. The process of claim 1 wherein said solid mass addi-tionally contains sodium chloride and said latter sodium chloride constitutes the source of said sodium chloride pro-vided in said hot aqueous solution of said solid mass.

25. The process of claim 24 wherein said solid mass contains about 70 to about 90% by weight of sodium carbonate, about 10 to about 25% by weight of sodium chloride, 0 to about 3% by weight of sodium sulfate and 0 to about 4% by weight of sodium sulfide.

26. The process of claim 24 wherein said solid mass contains about 7 to about 22% by weight of sodium sulfide, about 60 to about 78% by weight of sodium carbonate, about 10 to about 25% by weight of sodium chloride and about 1 to about 6% by weight of sodium sulfate.

27. The process of claim 24 wherein said hot aqueous solution is formed by contacting said solid mass with an aqueous material to dissolve substantially all said sodium chloride therefrom together with part of said sodium carbonate therefrom, and leaving a further solid mass containing sodium carbonate and being substantially free from sodium chloride.

28. The process of claim 27 including utilizing the sodium carbonate content of said substantially sodium chloride-free solid mass to form a sodium hydroxide solution for recycle as part of said pulping liquor.

29. The process of claim 2 wherein said recycling mother liquor contains sodium hydroxide values and including carbonating said sodium hydroxide values to sodium carbonate prior to said recycle.

30. The process of claim 1 wherein said cellulosic fibrous material is wood.

31. The process of claim 30 including subjecting said pulp to a series of bleaching and purification steps using at least one chlorine-containing bleaching chemical in at least one of said bleaching steps and aqueous sodium hydroxide-containing solutions in said purification steps, discharging a sodium chloride-containing aqueous effluent from said series of bleach-ing and purification steps into said spent pulping liquor where-by said solid mass and said hot aqueous solution contain sodium chloride.

32. The process of claim 31 including washing said pulp after formation thereof and before passage to said series of bleaching and purification steps and utilizing said sodium chloride-containing aqueous effluent in said washing.

33. The process of claim 31 wherein said series of bleach-ing and purification steps involves a first stage bleaching with chlorine, chlorine dioxide or a mixture thereof, a first stage caustic extraction using sodium hydroxide solution, a second stage bleaching with chlorine dioxide, a second stage caustic extraction with sodium hydroxide solution and a third stage bleaching with chlorine dioxide, the quantity of sodium hydrox-ide used being equivalent to the quantity of chlorine used in said bleaching steps, the spent bleaching and caustic extraction being mixed to provide a sodium chloride containing aqueous effluent of substantially neutral pH.

34. The process of claim 33 including separating said pulp and spent pulping liquor while washing said pulp after formation thereof and before passage to said series of bleaching and puri-fication operations, washing said pulp after each said bleaching and caustic extraction, the wash water utilized in said latter washing passing countercurrently to the pulp through said series of bleaching and purification steps, mixing the spent wash water from the latter washing with the spent bleaching and caustic extraction chemicals to provide the sodium chloride-containing aqueous effluent and utilizing said latter effluent in said washing of said pulp prior to passage to said series of bleaching and purification steps, whereby said latter effluent is intro-duced into said spent pulping liquor.

35. The process of claim 33 wherein said bleaching and caustic extraction steps are carried out by percolating an aqueous solution of the treating chemical through a mass of the pulp fibers while maintaining the fibers relatively station-ary with respect to each other except for movement caused by passage of said aqueous solution through said pulp mass.