CA1072262A - Pulp mill process with sodium chloride removal - Google Patents
Pulp mill process with sodium chloride removalInfo
- Publication number
- CA1072262A CA1072262A CA253,550A CA253550A CA1072262A CA 1072262 A CA1072262 A CA 1072262A CA 253550 A CA253550 A CA 253550A CA 1072262 A CA1072262 A CA 1072262A
- Authority
- CA
- Canada
- Prior art keywords
- sodium
- sodium chloride
- white liquor
- liquor
- sodium carbonate
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
- D21C11/0078—Treatment of green or white liquors with other means or other compounds than gases, e.g. in order to separate solid compounds such as sodium chloride and carbonate from these liquors; Further treatment of these compounds
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- Paper (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Sodium chloride is removed from a pulp mill process by having a recovery and regeneration operation by deposition from regenerated white liquor. In the preferred embodiment, a solid mixture of sodium carbonate, sodium sulphate and sodium chloride first is removed from the white liquor by evaporation thereof and is recycled partly to the furnacing step of the recovery and regeneration operation and partly to smelt solution formed in the recovery and regeneration operation, and sodium chloride for removal is deposited by manipulation of the partially concentrated white liquor.
Sodium chloride is removed from a pulp mill process by having a recovery and regeneration operation by deposition from regenerated white liquor. In the preferred embodiment, a solid mixture of sodium carbonate, sodium sulphate and sodium chloride first is removed from the white liquor by evaporation thereof and is recycled partly to the furnacing step of the recovery and regeneration operation and partly to smelt solution formed in the recovery and regeneration operation, and sodium chloride for removal is deposited by manipulation of the partially concentrated white liquor.
Description
~ 62 This invention relates to ~he recovery of sodium chloride from pulp mill operations.
In bleached pulp mills, attempts have been made to eliminate noxious liquid effluents by introducing bleach plant liquid effluents containing all the spent wash water, bleaching chemicals and purification chemicals to the recovery and regeneration system of the mill. The recovery and regeneration system generally includes a furnacing step which burns all the organic matter of the spent pulping li~uor and ~he bleach plant liquid affluents and chemi~al conver~ion of the product~ of the ~urnacing step to active pulping chemicals.
Chlorine-containing chemicals, such as ~hlorine and chlorine dioxide, are used in at least some of the bleaching steps in the bleach pla,nt~and sodium hydroxide usually is used in purification of the pulp. By the eombination of the sodium atoms and chlorine atoms of these ~hemicals~ the bleach plant efflu~ents contain sodium chloride which thereby is introduced into the recovery and r generation operation. Since the chemical con~ersions carried out in the recovery and regeneration operation do not affect the sodium chloride, when bleach plant liquid e~fluents axe introduced to the recovery and regeneration , - operation, sodium chLoride must be removed from the cycle to prevent its build up therein.
The present invention removes sodium chloride in solid form from the regenerated pulping liquor prior to its .
recycle to the puLping step. The regenerated pulping liquor resulting from the recovery and regeneration operation usually contains minor quantities of components other than the acti~e pulping chemicals and sodium chloride.
.: - , . ~ ' .:, ~ :.
. .
~V~z62 One such component is sodium sulphate. This arises in the kraft mill process, wherein sodium sulphide and sodium hydroxide are the active pulping chemicals, from incomplete regeneration of sodium sulphide in the regeneration and oxidation of sodium sulphide in subsequent processing.
Sodium sulphate is also present in soda mill pulping ~iquor, wherein sodium hydroxide is the active pulping ~hemical, and arises from the presence of sulphur in the wood, purchased fuel~ the wash water and as contaminant of make up chemicals added to compensate for uses in the recovery and regeneration cycle.
Another component which usually is present in ; the pulping liquors is sodium carbonate. In the kraft mill : ...
and soda mill recovery and regeneration prosesses, the smelt formed in the furnace contains sodium carbonate which is causticized to form the sodium hydroxide in the pulping .:
liquor. The causticization operation is less than 100~ .
- , efficien* and hence the presence of unregenerated sodium carbonate in the pulping li~uors.
20 : In accordance.with the present invention, pulping ~. .
: liquor containing sodium hydroxide and preferably also sodium sulphide, as the active p.ulpang chemicals, sodium . -;
chloride, sodium carbonate and sodiu~ sulphate is concentrated . ~ . . .
~ by evaporation to deposit sodium sulphate and sodium carbonate i ~ ~ along with some sodium chloride and, after separation of the deposited material from the first evaporation, sodium chloride - in solid form is recovered by manipulation of the partially : concentrated solution.
.~ The deposited material from the first evaporation i5 at least partially recycled to the furnacing step while the remainder of the deposited material, if any, is recycled : .
. ~ 3 .: :
, . , . ' -, . ' ~: :' ' ~ 6~
to a stage after furnacing and before the causticizing step.
The quantity of sodium chloride recycled to the furnacing step in the recycled deposited material is controlled to a level which does not unduly increase the effect of already-present sod}um chloride on the parameters of the furnacing step.
The accompanying drawing is a schematic flow sheet of a bleached kraft pulp mill in accordance with a preferred embodiment of the invention.
n In the drawingf wood chips, or other raw cellulosic fibrous material, are fed by line 10 to a digester 12 wherein the chips are cooked in pulping liquor fed by line 14 and containing sodium sulphide and sodium hydroxide as the active pulping chemicals.
The resulting pulp passes by line 16, usually after intermediate washing, to a bleach plant 18 wherein the pulp is subjected to bleaching op~erations with chlorine dioxide solutions and chlorine fed by line 20, caustic extraction operations with sodium hydroxide solution fed by lin~ 22 and washing operations with water fed by line 24.
The bl~ached pulp is recovered from the bleach plant 18 by line 26.
.
; In the bleach plant 18, it is preferred to employ .
the sequence of D/CEDED wherein D/C represents ~leaching with an aqueous solution of chlorine dioxide and chlorine wherein the chlorine dioxide provides khe majority of the active chlorine of the solution, D represents bleac~ing with an a~ueous solution of chlorine dioxi~e and E represents caustic extraction with aqueous sodium hydroxide solution. Washing is carried out intermediate each step and after the D2 stage.
.
.
~Z~2 The liquid effluents from the bleach plant containing spent chemicals, materials extracted from the pulp in the bleach plant operations, spent wash water and sodium chloride arising from the bleach plant chemicals used. The liquid effluents from the bleach plant 18 pass to the recovery and regeneration operation o the pulp mill, and this is shown schematically by the single line 28 passing to the black liquor resulting from the digester 12 in line 30.
Pre~erably, the liquid effluents from the bleach plant 18 are handled and pass to the pulp mill recovery system in the manner described in.detail in.~.S. Patent No. 4,03~,3720 Irrespective o~ ~he manner of introduction of the bleach plant effluents to the recovery system, the presence of sodium chloride in the cyclic system will result.
Black liquor containing bleach plant effluent , : passes by line 32 to a furnace 34, generally after concentrating ~ the liquor. In t~e furnace, all the organic material ~rom .
the digester and the bleach plant present in the li~uid e~1uents therefrom entering the furnace 34 is burned off and there result~ a smel~ containi~g sodium carbonate, sodium : sulphid~ sodium sulphate and sodium chloride, the sodium sulpha~e resul~ing from incomplete conversion to sodium sulphide.
The smelt exiting ~he furnace 34 is passed by line 36 to a smelt dissolver 38 to which water is fed by line 40 to dissolve the smelt to form green liquor. The green liquor passes by lines 42 and 44 to a causticizer 46 ~or conversion : of the bulk of the sodium carbonate values in the green liquor to sodium hydroxide by reburned lime fed by line 48, .precipitating calcium carbonate being removed by line 50 .
' ' . ' : .. :
~Z~6z The resulting white liquor contains sodium hydroxide, sodium sulphide, sodium chloride, sodium sulphate and sodium carbonate, typically in the following quantities:
NaOH 7.5 to 9.7 wt Na~S 1.8 to 5 wt Na2C3 1.7 to 3.1 wt %
NaCl 1.5 to 5 wt Na2S4 0.1 to 1 wt % .
The sodium carbonate present in the white liquor arises from the inherent inefficiency of the causticiæing step WhilP the sodium sulphate arises from sodium sulphate which :
is not converted to sodium sulphide in the furnace and subsequent oxidation of sodium sulphide.
While recycle of the white liquor resulting from ~ .
the causticizer directly ~o the d:iges~er 12 does not give i : ~ :
rise to any pxactical problems with respect to the sodium ~ :
sulphate a~d sodium carbonate sinl~e the furnacing and cauæticizing operations respectively control the sodium sulphate and sodium car~onate cont~nts of the white liquor, , the sodium chloride concentration would build up with succes-~: sive recycles since the sodium chloride is unafected by the ~; various operationsO Steps,; therefore, are taken to remove -sodium chloride from the system in a quantity substantially equal to that introduced from outside the pulp mill recovery system, mainly from the bleach plant, while the loss of ~ useful chemicaLs is substantially avoided. :
: ~ The white liquor resulting from the causticizer 46 passes by lines 52 and 54 to a first stage evaporator 56 wherein the white liquor is boiled to concentrate the same.
; 30 The first solid phases to precipitate are burkeite, the ..
double salt of sodlum sulphate and sodium carbonate, ...
.. , - . . .: . ". . . ,. .~.
.. .. , , - : . .. : .. : :. . : :
~ 2~2 Na2C03 2Na2S04, and sodium carbonate~ As concentration continues, these phases will continue to precipitate and when the white liquor becomes saturated with sodium chloride, sodium chloride also will coprecipitate. The extent to which coprecipitation of sodium chloride with burkeite and sodium carbonate, and hence contamination of the burkeite and sodium carbonate by sodium chloride, is allowed to occur depends on a number of factors, as discussed in more detail below.
The p~ially ooncentrated white liquor, saturated with respect to sodium chloride, sodium carbonate and sodium sul~
phate, is passed to a second evaporator 58 by line 60 for further concentration by boiling to precipitate a solid phase which is predominantly sodium chloride contaminated by sodium carbonate and sodium sulphate. -~
The concentrated white liquor resulting from the second stage evapora~or 58 passes by line 62 to the pùlping liquor in line 14 after suitable dilution to the required concentration for pulping by diluting liquor in line 64.
While the concentrated white liquor in line 62 ~ontains residual sodium chloride, the latter has a substantially constant value due to the removal of sodium chloride from the system by the second stags evaporation.
- The solid phase separated from the concentrated white li~uor in the second stage evaporator 58 passes by line 66 tD a leacher 68 for purification. In th~ leacher 68, the mixture is leached by water fed by line 70 to dissolve all the sodium carbonate and sodium sulphate along with some ~ ~ -sodium chloride from the solid phase, leaving subs~antially - pure sodium chloride which is removed from the leacher 68 by line 72.
Under steady stage operation conditions, the qua~tity .
of sodium clloride removed from the pulp mill by line 72 is ;~ -~ 7 ~ . , , . '~ ' ' , , .
:
equal to the ~uantity of sodiumi chloride introduced to the recovery and regeneration operation from sources external to the pulp mill.
The aqueous solution of sodium carbonater sodium sulphate and sodium chloride resulting from the leaching operation is recycled by line 74 to the white liquor in line 52. Alternatively, the solution in line 74 may be fed to the green liquor in line 42 or 44, if desired.
The solid mixture of burkeite, sodium carbonate and sodium chloride separated from the evaporator 56 i5 passed at least partially by lines 76 and 78 to the furnace 34 and the remainder of the mixture is passed by lines 76 and 80 to the green liquor in line 42. The solid mixture is shown _.
for convenience as passing directl~ into the furnace 34 by line 78. However, the mixture usually is fed to black liquor, such as in line 30 or 32, be~ore or after concentration thereof.
Recycle of at least part of the solid mixture in line 76 to the furnace 34 by line 78 has the effect of consuming at least part of the ~oclium sulphate so that, under steady state operation, the quantity of sodium sulphate in the mixture is controlled at a substantialIy constant level.
. .
~ - The p~oportion of sodiumi car~onate fed to the fur~
. .
~ nace along withi the sodium sulphate consumed therein is ~ . :
controlled by the recycle of part of the solid mixture in :::
line 76 to the green liquor in line 42. This latter recycle .
has the effect of increasing the sodium sulphate concentration : in the green liquor in line 44 above the level it would be ~ - .
~; without ~uch recycle, so that the relative proportion of sodium sulphate to sodium carbonate in the ~hite liquor in line 52 is increased. ~Ience, the dead load o~ sodium carbonate to i which the furnace 34 is sub~ect is subjected by the recycle in line 78 is controlled.
.
- 8 - ~ :
.
~ 6~
The effect of the presence of sodium chloride in the furnace is a factor determining the quantity of sodium chloride to be present in line 76 and the proportions to be fed to the furnace 34 and to the green liquor in line 42.
The introduction of bleach plant effluent to the recovery and regeneration system introduces sodium chloride to the furnace, as mentioned above. Sodium chloride also may arise from other external sources, such as in those cases where the wood chips are formed from sea-borne logs and where brackish water is used. Sodium chloride present in the recycling concentrated white liquor also cycles through the furnace.
Since the sodium chloride is a dead load material passing through the furnace, as the quantity of sodium chloride passing through the furnace increases, the capacity of the furnaçe ~o accept active materials, such as organic material to be burned and spent pulping chemicals, decxeases 90 that potential pulp production decreases.
Further t sodium chloride vaporizes and sublimes during tha furnacing operation, so that an increase in the sodium chloride load on the furnace increases the load on the gas particulate removal devices, commonly electrostatic ~ ~ precipitators. In cases where the precipitations already `~ operate at their load capacity an increase in the particulates in the flue~gas resulting from increased sodium chloride , leads to added air pollution.
In the pulp mill utilizing the illustrated procedure, a certain load of sodium chloride on the furnace must be -tolerated, typically about 200 lbs NaCl/air dry tow ~ADT) pulp and at this level there is no adverse affect on the mill parameters.
~ ' - g_ ''' :.
.. .
~2~6~
Sodium chloride concentration has little eff~ct on green liquor preparation and white liquor formation, although very high concentrations of sodium chloride result in an increased specific gravity which may inhibit efficient dregs separation from the green liquor.
Hence, in co-precipitating sodium chloride with the burkeite and sodium carbonate and recycling sodium chloride contaminated burkeite in line 76 to the furnace and the green liquor, a balance must be arrived at which allows an adequate feed of sodium sulphate to the furnace 34 to compensate for the sodium sulphate arising from the furnace and æubsequenk operations, while the additional sodium .
chloride load on the furnace is held below a level above which the adverse effects of the presence of sodium chloride become intolerable and the recycle of sodium sulphate in the green liquor in line 44 does not become too high. It is preferred to maintain the codium chloride feed to the furnace :
in line 78 at a low value, prefer,ably less than 100 lbs/ADT, more :~
pre~era~ly below 50 lbs~ADT and hence it is preferred $o use : 20 : the split recycle sh~wn in Figure 1 not only or the purpose of decreasing the load of sodium carbonate on the furnace, but also to decrease the load of sodium chloxide on the ~urnace when the total quantity of sGdium chloride in the ~ ~ :
mixture in line 76 is high. : ~
. ~ The ability to accept sodium chloride in the mixture precipitated in the first stage evaporator 56 allows a large degree of flexibility in the operation-of the first .
~' stage evaporator~ Changing white liquor compositions and pulp mill conditions may cause the saturation point of the white liquor with respect to sodium chloride to vary. By operating the first stage evaporator in such a way that the precipitate always contains sodium chloride irrespective of ' :
'~'' 10 .. ~, , ~ 26~
the expected changes in white liquor concentratiorl and pulp mill compositions with the ability to minimize the effect of such co-precipitated sodium chloride, the necessity for precise control on the first stage evaporation is avoided.
It is possible to separate sodium chloride from the partially concentrated white liquor in line 52 other than by the second stage evaporation mentioned above. Thus~ the partially concentrated white liquor may be cooled to precipi-tate substantially pure sodium chloride from the cooled white liquor. After separa*ion of the precipitated sodium chloride, the cooled white liquor may be recycled to the digestion stage after suitable dilution, or, may be further evaporated to precipitate further sodium chloride in admixture with sodium carbonate and sodium sulphate, the precipitated mixture being .
recycled to the green liquor in line 42 or 44 or to the white liquor in line 52 while the concentrated white liquor is recycled to the digester, after suitable dilution.
,:, The present invention, therefore, provides a pulp ~ ~ mill operation-in which sodium chloride is removed and in which 106ses of useful chemicals are avoided. Modificatiolls .
are possible within the scope of the invention.
: ' ~ ~ .
.
;
.
. , , , .
.. .
;
. . .
.
' ', : ' .
In bleached pulp mills, attempts have been made to eliminate noxious liquid effluents by introducing bleach plant liquid effluents containing all the spent wash water, bleaching chemicals and purification chemicals to the recovery and regeneration system of the mill. The recovery and regeneration system generally includes a furnacing step which burns all the organic matter of the spent pulping li~uor and ~he bleach plant liquid affluents and chemi~al conver~ion of the product~ of the ~urnacing step to active pulping chemicals.
Chlorine-containing chemicals, such as ~hlorine and chlorine dioxide, are used in at least some of the bleaching steps in the bleach pla,nt~and sodium hydroxide usually is used in purification of the pulp. By the eombination of the sodium atoms and chlorine atoms of these ~hemicals~ the bleach plant efflu~ents contain sodium chloride which thereby is introduced into the recovery and r generation operation. Since the chemical con~ersions carried out in the recovery and regeneration operation do not affect the sodium chloride, when bleach plant liquid e~fluents axe introduced to the recovery and regeneration , - operation, sodium chLoride must be removed from the cycle to prevent its build up therein.
The present invention removes sodium chloride in solid form from the regenerated pulping liquor prior to its .
recycle to the puLping step. The regenerated pulping liquor resulting from the recovery and regeneration operation usually contains minor quantities of components other than the acti~e pulping chemicals and sodium chloride.
.: - , . ~ ' .:, ~ :.
. .
~V~z62 One such component is sodium sulphate. This arises in the kraft mill process, wherein sodium sulphide and sodium hydroxide are the active pulping chemicals, from incomplete regeneration of sodium sulphide in the regeneration and oxidation of sodium sulphide in subsequent processing.
Sodium sulphate is also present in soda mill pulping ~iquor, wherein sodium hydroxide is the active pulping ~hemical, and arises from the presence of sulphur in the wood, purchased fuel~ the wash water and as contaminant of make up chemicals added to compensate for uses in the recovery and regeneration cycle.
Another component which usually is present in ; the pulping liquors is sodium carbonate. In the kraft mill : ...
and soda mill recovery and regeneration prosesses, the smelt formed in the furnace contains sodium carbonate which is causticized to form the sodium hydroxide in the pulping .:
liquor. The causticization operation is less than 100~ .
- , efficien* and hence the presence of unregenerated sodium carbonate in the pulping li~uors.
20 : In accordance.with the present invention, pulping ~. .
: liquor containing sodium hydroxide and preferably also sodium sulphide, as the active p.ulpang chemicals, sodium . -;
chloride, sodium carbonate and sodiu~ sulphate is concentrated . ~ . . .
~ by evaporation to deposit sodium sulphate and sodium carbonate i ~ ~ along with some sodium chloride and, after separation of the deposited material from the first evaporation, sodium chloride - in solid form is recovered by manipulation of the partially : concentrated solution.
.~ The deposited material from the first evaporation i5 at least partially recycled to the furnacing step while the remainder of the deposited material, if any, is recycled : .
. ~ 3 .: :
, . , . ' -, . ' ~: :' ' ~ 6~
to a stage after furnacing and before the causticizing step.
The quantity of sodium chloride recycled to the furnacing step in the recycled deposited material is controlled to a level which does not unduly increase the effect of already-present sod}um chloride on the parameters of the furnacing step.
The accompanying drawing is a schematic flow sheet of a bleached kraft pulp mill in accordance with a preferred embodiment of the invention.
n In the drawingf wood chips, or other raw cellulosic fibrous material, are fed by line 10 to a digester 12 wherein the chips are cooked in pulping liquor fed by line 14 and containing sodium sulphide and sodium hydroxide as the active pulping chemicals.
The resulting pulp passes by line 16, usually after intermediate washing, to a bleach plant 18 wherein the pulp is subjected to bleaching op~erations with chlorine dioxide solutions and chlorine fed by line 20, caustic extraction operations with sodium hydroxide solution fed by lin~ 22 and washing operations with water fed by line 24.
The bl~ached pulp is recovered from the bleach plant 18 by line 26.
.
; In the bleach plant 18, it is preferred to employ .
the sequence of D/CEDED wherein D/C represents ~leaching with an aqueous solution of chlorine dioxide and chlorine wherein the chlorine dioxide provides khe majority of the active chlorine of the solution, D represents bleac~ing with an a~ueous solution of chlorine dioxi~e and E represents caustic extraction with aqueous sodium hydroxide solution. Washing is carried out intermediate each step and after the D2 stage.
.
.
~Z~2 The liquid effluents from the bleach plant containing spent chemicals, materials extracted from the pulp in the bleach plant operations, spent wash water and sodium chloride arising from the bleach plant chemicals used. The liquid effluents from the bleach plant 18 pass to the recovery and regeneration operation o the pulp mill, and this is shown schematically by the single line 28 passing to the black liquor resulting from the digester 12 in line 30.
Pre~erably, the liquid effluents from the bleach plant 18 are handled and pass to the pulp mill recovery system in the manner described in.detail in.~.S. Patent No. 4,03~,3720 Irrespective o~ ~he manner of introduction of the bleach plant effluents to the recovery system, the presence of sodium chloride in the cyclic system will result.
Black liquor containing bleach plant effluent , : passes by line 32 to a furnace 34, generally after concentrating ~ the liquor. In t~e furnace, all the organic material ~rom .
the digester and the bleach plant present in the li~uid e~1uents therefrom entering the furnace 34 is burned off and there result~ a smel~ containi~g sodium carbonate, sodium : sulphid~ sodium sulphate and sodium chloride, the sodium sulpha~e resul~ing from incomplete conversion to sodium sulphide.
The smelt exiting ~he furnace 34 is passed by line 36 to a smelt dissolver 38 to which water is fed by line 40 to dissolve the smelt to form green liquor. The green liquor passes by lines 42 and 44 to a causticizer 46 ~or conversion : of the bulk of the sodium carbonate values in the green liquor to sodium hydroxide by reburned lime fed by line 48, .precipitating calcium carbonate being removed by line 50 .
' ' . ' : .. :
~Z~6z The resulting white liquor contains sodium hydroxide, sodium sulphide, sodium chloride, sodium sulphate and sodium carbonate, typically in the following quantities:
NaOH 7.5 to 9.7 wt Na~S 1.8 to 5 wt Na2C3 1.7 to 3.1 wt %
NaCl 1.5 to 5 wt Na2S4 0.1 to 1 wt % .
The sodium carbonate present in the white liquor arises from the inherent inefficiency of the causticiæing step WhilP the sodium sulphate arises from sodium sulphate which :
is not converted to sodium sulphide in the furnace and subsequent oxidation of sodium sulphide.
While recycle of the white liquor resulting from ~ .
the causticizer directly ~o the d:iges~er 12 does not give i : ~ :
rise to any pxactical problems with respect to the sodium ~ :
sulphate a~d sodium carbonate sinl~e the furnacing and cauæticizing operations respectively control the sodium sulphate and sodium car~onate cont~nts of the white liquor, , the sodium chloride concentration would build up with succes-~: sive recycles since the sodium chloride is unafected by the ~; various operationsO Steps,; therefore, are taken to remove -sodium chloride from the system in a quantity substantially equal to that introduced from outside the pulp mill recovery system, mainly from the bleach plant, while the loss of ~ useful chemicaLs is substantially avoided. :
: ~ The white liquor resulting from the causticizer 46 passes by lines 52 and 54 to a first stage evaporator 56 wherein the white liquor is boiled to concentrate the same.
; 30 The first solid phases to precipitate are burkeite, the ..
double salt of sodlum sulphate and sodium carbonate, ...
.. , - . . .: . ". . . ,. .~.
.. .. , , - : . .. : .. : :. . : :
~ 2~2 Na2C03 2Na2S04, and sodium carbonate~ As concentration continues, these phases will continue to precipitate and when the white liquor becomes saturated with sodium chloride, sodium chloride also will coprecipitate. The extent to which coprecipitation of sodium chloride with burkeite and sodium carbonate, and hence contamination of the burkeite and sodium carbonate by sodium chloride, is allowed to occur depends on a number of factors, as discussed in more detail below.
The p~ially ooncentrated white liquor, saturated with respect to sodium chloride, sodium carbonate and sodium sul~
phate, is passed to a second evaporator 58 by line 60 for further concentration by boiling to precipitate a solid phase which is predominantly sodium chloride contaminated by sodium carbonate and sodium sulphate. -~
The concentrated white liquor resulting from the second stage evapora~or 58 passes by line 62 to the pùlping liquor in line 14 after suitable dilution to the required concentration for pulping by diluting liquor in line 64.
While the concentrated white liquor in line 62 ~ontains residual sodium chloride, the latter has a substantially constant value due to the removal of sodium chloride from the system by the second stags evaporation.
- The solid phase separated from the concentrated white li~uor in the second stage evaporator 58 passes by line 66 tD a leacher 68 for purification. In th~ leacher 68, the mixture is leached by water fed by line 70 to dissolve all the sodium carbonate and sodium sulphate along with some ~ ~ -sodium chloride from the solid phase, leaving subs~antially - pure sodium chloride which is removed from the leacher 68 by line 72.
Under steady stage operation conditions, the qua~tity .
of sodium clloride removed from the pulp mill by line 72 is ;~ -~ 7 ~ . , , . '~ ' ' , , .
:
equal to the ~uantity of sodiumi chloride introduced to the recovery and regeneration operation from sources external to the pulp mill.
The aqueous solution of sodium carbonater sodium sulphate and sodium chloride resulting from the leaching operation is recycled by line 74 to the white liquor in line 52. Alternatively, the solution in line 74 may be fed to the green liquor in line 42 or 44, if desired.
The solid mixture of burkeite, sodium carbonate and sodium chloride separated from the evaporator 56 i5 passed at least partially by lines 76 and 78 to the furnace 34 and the remainder of the mixture is passed by lines 76 and 80 to the green liquor in line 42. The solid mixture is shown _.
for convenience as passing directl~ into the furnace 34 by line 78. However, the mixture usually is fed to black liquor, such as in line 30 or 32, be~ore or after concentration thereof.
Recycle of at least part of the solid mixture in line 76 to the furnace 34 by line 78 has the effect of consuming at least part of the ~oclium sulphate so that, under steady state operation, the quantity of sodium sulphate in the mixture is controlled at a substantialIy constant level.
. .
~ - The p~oportion of sodiumi car~onate fed to the fur~
. .
~ nace along withi the sodium sulphate consumed therein is ~ . :
controlled by the recycle of part of the solid mixture in :::
line 76 to the green liquor in line 42. This latter recycle .
has the effect of increasing the sodium sulphate concentration : in the green liquor in line 44 above the level it would be ~ - .
~; without ~uch recycle, so that the relative proportion of sodium sulphate to sodium carbonate in the ~hite liquor in line 52 is increased. ~Ience, the dead load o~ sodium carbonate to i which the furnace 34 is sub~ect is subjected by the recycle in line 78 is controlled.
.
- 8 - ~ :
.
~ 6~
The effect of the presence of sodium chloride in the furnace is a factor determining the quantity of sodium chloride to be present in line 76 and the proportions to be fed to the furnace 34 and to the green liquor in line 42.
The introduction of bleach plant effluent to the recovery and regeneration system introduces sodium chloride to the furnace, as mentioned above. Sodium chloride also may arise from other external sources, such as in those cases where the wood chips are formed from sea-borne logs and where brackish water is used. Sodium chloride present in the recycling concentrated white liquor also cycles through the furnace.
Since the sodium chloride is a dead load material passing through the furnace, as the quantity of sodium chloride passing through the furnace increases, the capacity of the furnaçe ~o accept active materials, such as organic material to be burned and spent pulping chemicals, decxeases 90 that potential pulp production decreases.
Further t sodium chloride vaporizes and sublimes during tha furnacing operation, so that an increase in the sodium chloride load on the furnace increases the load on the gas particulate removal devices, commonly electrostatic ~ ~ precipitators. In cases where the precipitations already `~ operate at their load capacity an increase in the particulates in the flue~gas resulting from increased sodium chloride , leads to added air pollution.
In the pulp mill utilizing the illustrated procedure, a certain load of sodium chloride on the furnace must be -tolerated, typically about 200 lbs NaCl/air dry tow ~ADT) pulp and at this level there is no adverse affect on the mill parameters.
~ ' - g_ ''' :.
.. .
~2~6~
Sodium chloride concentration has little eff~ct on green liquor preparation and white liquor formation, although very high concentrations of sodium chloride result in an increased specific gravity which may inhibit efficient dregs separation from the green liquor.
Hence, in co-precipitating sodium chloride with the burkeite and sodium carbonate and recycling sodium chloride contaminated burkeite in line 76 to the furnace and the green liquor, a balance must be arrived at which allows an adequate feed of sodium sulphate to the furnace 34 to compensate for the sodium sulphate arising from the furnace and æubsequenk operations, while the additional sodium .
chloride load on the furnace is held below a level above which the adverse effects of the presence of sodium chloride become intolerable and the recycle of sodium sulphate in the green liquor in line 44 does not become too high. It is preferred to maintain the codium chloride feed to the furnace :
in line 78 at a low value, prefer,ably less than 100 lbs/ADT, more :~
pre~era~ly below 50 lbs~ADT and hence it is preferred $o use : 20 : the split recycle sh~wn in Figure 1 not only or the purpose of decreasing the load of sodium carbonate on the furnace, but also to decrease the load of sodium chloxide on the ~urnace when the total quantity of sGdium chloride in the ~ ~ :
mixture in line 76 is high. : ~
. ~ The ability to accept sodium chloride in the mixture precipitated in the first stage evaporator 56 allows a large degree of flexibility in the operation-of the first .
~' stage evaporator~ Changing white liquor compositions and pulp mill conditions may cause the saturation point of the white liquor with respect to sodium chloride to vary. By operating the first stage evaporator in such a way that the precipitate always contains sodium chloride irrespective of ' :
'~'' 10 .. ~, , ~ 26~
the expected changes in white liquor concentratiorl and pulp mill compositions with the ability to minimize the effect of such co-precipitated sodium chloride, the necessity for precise control on the first stage evaporation is avoided.
It is possible to separate sodium chloride from the partially concentrated white liquor in line 52 other than by the second stage evaporation mentioned above. Thus~ the partially concentrated white liquor may be cooled to precipi-tate substantially pure sodium chloride from the cooled white liquor. After separa*ion of the precipitated sodium chloride, the cooled white liquor may be recycled to the digestion stage after suitable dilution, or, may be further evaporated to precipitate further sodium chloride in admixture with sodium carbonate and sodium sulphate, the precipitated mixture being .
recycled to the green liquor in line 42 or 44 or to the white liquor in line 52 while the concentrated white liquor is recycled to the digester, after suitable dilution.
,:, The present invention, therefore, provides a pulp ~ ~ mill operation-in which sodium chloride is removed and in which 106ses of useful chemicals are avoided. Modificatiolls .
are possible within the scope of the invention.
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Claims (8)
1. A process of pulping cellulosic fibrous material including the steps of:
contacting said cellulosic fibrous material with a pulping liquor containing sodium hydroxide as an active pulping chemical;
separating pulped material from spent pulping liquor;
subjecting said spent pulping liquor to recovery and regeneration steps comprising furnacing said spent pulping liquor to form a smelt containing sodium carbonate, dissolving said smelt to form an aqueous smelt solution and causticizing said smelt solution to provide a white liquor containing dissolved quantities of sodium hydroxide, sodium carbonate and sodium sulphate, said white liquor also containing sodium chloride introduced to the process as a by-product or con-taminant;
evaporating at least part of said white liquor to deposit therefrom sodium chloride, sodium carbonate and sodium sulphate;
forwarding at least part of the sodium sulphate and sodium carbonate admixed with part of the deposited sodium chloride to said furnacing step;
forwarding any portion of the sodium sulphate and sodium carbonate admixed with part of the deposited sodium chloride not forwarded to said furnacing step to said smelt solution; and recovering part of the deposited sodium chloride.
contacting said cellulosic fibrous material with a pulping liquor containing sodium hydroxide as an active pulping chemical;
separating pulped material from spent pulping liquor;
subjecting said spent pulping liquor to recovery and regeneration steps comprising furnacing said spent pulping liquor to form a smelt containing sodium carbonate, dissolving said smelt to form an aqueous smelt solution and causticizing said smelt solution to provide a white liquor containing dissolved quantities of sodium hydroxide, sodium carbonate and sodium sulphate, said white liquor also containing sodium chloride introduced to the process as a by-product or con-taminant;
evaporating at least part of said white liquor to deposit therefrom sodium chloride, sodium carbonate and sodium sulphate;
forwarding at least part of the sodium sulphate and sodium carbonate admixed with part of the deposited sodium chloride to said furnacing step;
forwarding any portion of the sodium sulphate and sodium carbonate admixed with part of the deposited sodium chloride not forwarded to said furnacing step to said smelt solution; and recovering part of the deposited sodium chloride.
2. A process of pulping cellulosic fibrous material including the steps of:
contacting said cellulosic fibrous material with a pulping liquor containing sodium hydroxide as an active pulping chemical;
separating pulped material from spent pulping liquor;
subjecting said spent pulping liquor to recovery and regeneration steps comprising furnacing said spent pulping liquor to form a smelt containing sodium carbonate, dissolving said smelt to form an aqueous smelt solution and causticizing said smelt solution to provide a white liquor containing dissolved quantities of sodium hydroxide, sodium carbonate and sodium sulphate, said white liquor also containing sodium chloride introduced to the process as a by-product or contaminant;
evaporating at least part of said white liquor to deposit therefrom sodium chloride, sodium carbonate and sodium sulphate;
separating said deposited sodium chloride, sodium carbonate and sodium sulphate from the resulting partially concentrated white liquor;
forwarding at least part of said deposited sodium chloride, sodium carbonate and sodium sulphate to said furnacing step;
forwarding the remainder, if any, of said deposited sodium chloride, sodium carbonate and sodium sulphate to said smelt solution;
manipulating said partially concentrated white liquor to deposit sodium chloride therefrom; and recovering said latter deposited sodium chloride.
contacting said cellulosic fibrous material with a pulping liquor containing sodium hydroxide as an active pulping chemical;
separating pulped material from spent pulping liquor;
subjecting said spent pulping liquor to recovery and regeneration steps comprising furnacing said spent pulping liquor to form a smelt containing sodium carbonate, dissolving said smelt to form an aqueous smelt solution and causticizing said smelt solution to provide a white liquor containing dissolved quantities of sodium hydroxide, sodium carbonate and sodium sulphate, said white liquor also containing sodium chloride introduced to the process as a by-product or contaminant;
evaporating at least part of said white liquor to deposit therefrom sodium chloride, sodium carbonate and sodium sulphate;
separating said deposited sodium chloride, sodium carbonate and sodium sulphate from the resulting partially concentrated white liquor;
forwarding at least part of said deposited sodium chloride, sodium carbonate and sodium sulphate to said furnacing step;
forwarding the remainder, if any, of said deposited sodium chloride, sodium carbonate and sodium sulphate to said smelt solution;
manipulating said partially concentrated white liquor to deposit sodium chloride therefrom; and recovering said latter deposited sodium chloride.
3. The process of claim 2 wherein said pulping liquor also contains sodium sulphide as an active pulping chemical.
4. The process of claim 2 wherein the quantity of sodium chloride recovered is substantially equal to the quantity of sodium chloride introduced to the cycle.
5. The process of claim 2 wherein part only of said deposited sodium chloride, sodium carbonate and sodium sulphate is forwarded to said furnacing step and the remainder of said deposited sodium chloride, sodium carbonate and sodium sulphate is forwarded to said smelt solution.
6. The process of claim 2 wherein said partially concentrated white liquor is manipulated by evaporating said partially concentrated white liquor to cause deposition of a mixture of sodium chloride, sodium carbonate and sodium sulphate, separating the deposited mixture from the resulting concentrated white liquor, leaching said separated deposited mixture to dissolve all the sodium carbonate and sodium sulphate and part of the sodium chloride therefrom and leave solid pure sodium chloride, separating said latter sodium chloride from the resulting aqueous solution as said recovered sodium chloride, and recycling said resulting aqueous solution to said white liquor or to said smelt solution.
7. The process of claim 2 wherein said partially concentrated white liquor is manipulated by cooling hot par-tially concentrated white liquor to precipitate substantially pure sodium chloride and separating said latter sodium chloride from the cooled white liquor as said recovered sodium chloride.
8. The process of claim 2 including evaporating said cooled white liquor to cause precipitation of a mixture of sodium chloride, sodium carbonate and sodium sulphate, separating the precipitated mixture from the resulting con-centrated white liquor and recycling said separated precipitated mixture to said white liquor or to said smelt solution.
Priority Applications (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA253,550A CA1072262A (en) | 1976-05-27 | 1976-05-27 | Pulp mill process with sodium chloride removal |
| JP51154389A JPS5939560B2 (en) | 1976-05-27 | 1976-12-23 | Pulp manufacturing method that removes sodium chloride |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA253,550A CA1072262A (en) | 1976-05-27 | 1976-05-27 | Pulp mill process with sodium chloride removal |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1072262A true CA1072262A (en) | 1980-02-26 |
Family
ID=4106059
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA253,550A Expired CA1072262A (en) | 1976-05-27 | 1976-05-27 | Pulp mill process with sodium chloride removal |
Country Status (2)
| Country | Link |
|---|---|
| JP (1) | JPS5939560B2 (en) |
| CA (1) | CA1072262A (en) |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS5117602B2 (en) * | 1974-03-22 | 1976-06-03 |
-
1976
- 1976-05-27 CA CA253,550A patent/CA1072262A/en not_active Expired
- 1976-12-23 JP JP51154389A patent/JPS5939560B2/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5939560B2 (en) | 1984-09-25 |
| JPS52144404A (en) | 1977-12-01 |
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