CA1109611A - Potassium-based pulp mill process - Google Patents
Potassium-based pulp mill processInfo
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- CA1109611A CA1109611A CA324,477A CA324477A CA1109611A CA 1109611 A CA1109611 A CA 1109611A CA 324477 A CA324477 A CA 324477A CA 1109611 A CA1109611 A CA 1109611A
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- potassium
- liquor
- pulping
- smelt
- white liquor
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Abstract
ABSTRACT OF THE DISCLOSURE
Potassium chloride build up in the recovery system of a potassium-base pulping process using potassium hydroxide and preferably potassium sulphide as pulping chemicals is avoided by evaporative deposition of solid potassium chloride from potassium chloride-containing solutions formed after furnacing of spent pulping liquor and either before or after causticization of potassium carbonate therein.
Potassium chloride build up in the recovery system of a potassium-base pulping process using potassium hydroxide and preferably potassium sulphide as pulping chemicals is avoided by evaporative deposition of solid potassium chloride from potassium chloride-containing solutions formed after furnacing of spent pulping liquor and either before or after causticization of potassium carbonate therein.
Description
~1~96Pl , POTASSIUM-BASED PULP MILL PROCESS
FIELD OF INVENTION
The p~esent invention is directed to the production of cellulosic ~ibrous material pulp using potassium based pulping chemicals, to, the recovery of pulping chemicals and to the removal of potassium chloride contaminants.
BACKGROUND TO THE INVE:NTION
Generally in the pxoduction of pulp suitable for formation into paper, wood or other raw cellulosic fibrous material is subjected to chemical digestion in a pulping liquor to form a pulp of the cellulosic fibrous material.
The pulp thereafter is subjected to bleaching and purifi-cation operations ina bleach plant.
The spent pulping liquor usually is subjected to a series of recovery and regeneration operations to recover pulping chemicals and to provide fresh pulping liquor.
Sodium-based chemicals conventionally have been used as the pulping liquor, such as, sodium hydroxidej possibly along with other sodium salts.
One widely used pulping process is the kraft process wherein raw cellulosic fibrous material, generally wood chips, is digested in white liquor containing sodium .
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hydroxide and sodium sulphide as the active pulping chemicals to provide a pulp and 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 bleaching 5 and purification operations.
The black liquor then is passed to the recovery and regeneration systém in which the black liquor is evaporated and the concentrated black liquor is burned in a furnace to yield a smelt containing sodium carbonate and 10 sodium sulphide. Sodium sulphate is usually added to the black liquor 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 is clarified to remove undissolved solids and the green liquor is causticized with reburned lime to convert the sodium carbonate to sodium hydroxide.
The resulting white liquor then is at least partially re-cycled to the pulping step to provide at least part of the pulping liquor.
Conventional bleach plant operations generally ; involve a sequence of bleaching and purification steps, ~ogether with washing steps. The bleaching steps usually involve the use of at least one chlorine-containing bleaching agent, such as, chlorine, chlorine dioxide, chlorine 25 monoxide and sodium hypochlorite. The purification steps conventionally involve treatment with sodium hydroxide solution.
In some instances, the bleaching and extraction steps may be combined, for example, using an oxygen bleaching .
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operation. Where such oxygen bleaching is used, it usually is supplemented by further bleaching operations using chlorine-based bleaching chemicals.
The aqueous effluents of bleach plants including 5 spent bleaching chemicals, spent extraction chemicals and spent wash water conventionally have been discharged to convenient water bodies but the effluents are environment-ally hazardous, possessing objectionable colour and high toxicity towards a~uatic and marine biota, and containing 10 fibres and oxygen-consuming materials. To avoid the dis-charge of such materials and thereby eliminate the harmful and polluting effects of the effluents, it has been suggested in U.S`. Patents No. 3,698,995 and No. 4,039,372 to forward ~ the e~fluents as a single stream or in a plurality of streams - -15 to the chemical recovery system after utilizing at least part thereof in washing the unbleached pulp in the brown stock washer. When this procedure is adopted, the organic contaminants of the bleach plant effluents are consumed in the recovery furnace.
As a result of the use of chlorine-containing bleaching chemicals and sodium-containing purification agents, the aqueous effluents from conventional bleach plant operations contain sodium chloride which then enters the recovery and regeneration cycle when the effluent handling 25 process disclosed in the above patents is used.
Sodium chloride passes without change through the recovery and regeneration system and hence constitutes a dead load which, in continuous operation, may build up to intolerable levels. Prior suggestions have been made to -- ,. ,., : ................. :,, : . : . - : :: ~: . :
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combat this problem by precipitation of the sodium chloride " from white liquor, as described in detail in U.S. Patents Nos. 3,746,612 and 3,950,217, or by separation from smelt components, as described in detail in U.S. Patents Nos.
5 3,986,923, 3,909,344, 3,954,552 and 3,945,880.
It has previously been suggested to utilize potassium-based chemicals in plaee of sodium-based chemieals in pulping proeesses and bleach plant extractions. The use of potassium-based pulping chemicals as ccmpared with sodiunr based chemieals lS advantageous, in that a greater pulp yield is attained with consequently increased pulp pro- ;
duetion and decreased wood costs. The rate of pulping using potassium-based ehemieals is faster than for , sodium-based ehemicals, thereby inereasing the overall mill pulping eapaeity. While the inereased eost of potassium sulphate make-up as eompared with sodium sulphate make-up partly offset these benefits, make-up chemieal requirement ean be minimized.
The use of potassium-based ehemieals in eonjunetion with a bleaeh plant effluent disposal proeedure by way of the pulping llquor reeovery and regeneration cyele would result in an analogous build up of potassium ehloride to that . . .
deseribed above for the sodium-based proeess.
SUMMARY OF INVENTION
:6 The present invention, in its broadest aspect, is direeted to the separation of potassium ehloride from a potassium-based pulp mill spent chemical recovery and re-generation proeess by the concentration of liquors formed in that proeess. The source of the potassium-chloride - .~
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contamination -s not significant to the broadest aspects of the invention, but in the preferred embodiment of the invention the potassium chloride arises mainly from the forwarding of bleach plant effluents to the recovery and Xegeneration system, thereby providing a liquid effluent-free closed cycle potassium-based pulp mill.
It has been discovered that the solubility charac-teristics of thesystems KOH-K2CO3-K2SO4-KCl-H20, KOH-K2S-K2cO3 K2SO4 Cl 2 2 2 3 2 4 2somewhat different from the corresponding sodium salt systems and these differences permit potassium chloride removal from the recovery and regeneration cycle to be effected in a si~plified manner, as compared to the prior processes ., utilized in soda-based pulping systems and disclosed in lS the above-recited patents.
The simplicity of the potassium chloride removal processes adds to the benefits of utilization of potassium-based chemicals outlined above. The simplicity arises from the pronounced tendency for potassium carbonate to remain in the aqueous phase upon concentration of either the green liquor or white liquor, in complete contrast to the tendency of sodium carbonate to precipitate from green liquor or white liquor in the sodium-based systems discussed above.
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The evaporative procedures used in this invention enable an additional economic saving to be achieved, as com-pared with the sodium-based processes mentioned above. Thus, less evaporation is necessary, with consequent heat savings, .
6 ~ 6~1 usually in the form of steam, in the case of the potassium-based process to achieve the required purge of potassium chloride as compared with the sodium-based process to achieve the required purge of sodium chloride. For example, 5 a 3.5 molal active alkali solution (OH ~S ~ requires about 65% evaporation to crystallize the requisite potassium chloride as compared with about 80~ for sodium chloride crystallization ~rom sodium-based white liquor.
The present invention is primarily concerned with the pulping of cellulosic fibrous material using pulping liquors containing potassium hydroxide, and also preferably potassium sulphide, as at least the major proportion of the pulping chemicals. However, sodium ions may be present in the system, arising from a variety of sources, such as, deliberate inclusion of sodium-based pulping chemicals, ; such as, sodium hydroxide and sodium sulpbide, in the pulping liquor and/or from the cellulosic fibrous material which is . ~ .
pulped.
When such sodium ionic species are present in significant amounts, the present invention is concerned generally with smelt solutions and white liquors formed in the regeneration cycle having a potassium molar ratio, i.e., (K/K+Na), ~reater than about 0.5, preferably greater than about 0.6. The potassium molar ratio present in these liquors influences solid phases which can be precipitated therefrom.
Generally, the solid phases differ depending on the potassium molar ratio, different phases oeing experienced in the molar ratio ranges of about 0.5 to about 0.7, greater _! .
, ~IDS6~1 than a~out 0.7 to about 0.9 and greater than about 0.9 to 1.0, in the case of white liquor and the molar ratio ranges of about 0.5 to a~out 0.8, greater than 0.8 to about 0.9 and greater than about 0.9 to 1.0 in the case of smelt solution.
s The build up of excess sodium values within the system, such as may occur when some sodium-containing wood - . species are pulped and a wholly potassium-based pulping liquor is used, may be prevented ~y purging the excess sodium values in the form of solid sodium carbonate.
BRIEF DESCRIPTION OF DR~WINGS
:~ ~ Figure 1 is a schematic flow sheet of a liquid effluent-free potassium based Kraft pulp mill wherein potassium chloride is removed;
Figure 2 is a schematic flo~ sheet of a white liquor evaporation process for use as an alternative to the process of Figure 1 for removal of potassium chloride;
Figure 3 is a schematic flow sheet of a green - liquor evaporation process for the removal of potassium chloride in an effluent-free potassium-based Kraft pulp 2Q mill;
Figure 4 is a schematic flow sheet of an alterna-tive green liquor evaporation process to that illustrated in Figure 3;
Figure 5 is a schematic flow sheet of a smelt . 25 leaching and white liquor evaporation process for the removal of potassium chloride in an effluent-free potassium-:~ based Kraft pulp mill;
Figure 6 is a schematic flow sheet of a smeltleaching and potassium sulphide solution evaporation process ~ .
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S6~1 for the removal.of potassium chloride as an alternative to the process of Figure 5;
Figure 7 is a schematic flow sheet of a modifica-tion of the process of Figure 2 wherein a sodium purge also is effected;
Figure 8 is a schematic flow sheet of an alterna-tive sodium purge process from that of Figure 7; and Figures 9 to 14 are graphical representations of the solubility characteristics of a multi-ionic aqueous 10 system. . .... . .. . . . . ..
DESCRIPTION OF PREFERRED EMBODIMENTS
.Referring to the drawings, Figure 1 is a schematic flow sheet of a liquid effluent-free or "closed cycle" pulp mill which includes a digester 10 to which wood chips are - 15 fed by line 12 and pulping liquor by line 14. The pulping liquor contains potassium hydroxide and potassium sulphide : as the active pulping chemicals. The pulp passes by line . 16 to brown stock washer 18 for separation from black liquor and for washing by bleach plant effluents are possibly by additional w~sh watex fed by line 20.
The washed and unbleached pulp from the brown stock washers 18 passes by line 22 to a bleach plant 24 wherein the pulp is subjected to a series of bleaching, ; caustic extraction and washing operations to result in bleached pulp of the required brightness and purity in line 26.
The actual sequence of operations which is effec-ted on the pulp in the bleach plant 24 is not significant, although it is preferred to effect an initial bleaching - ~ ;: :; .: ~ , . :
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~` 9 ~ 6~1 with an aqueous solution of chlorine dioxide and chlorine in which chlorine dioxide provides about 20 to about 95 of the available chlorine of the solution or with an a~ueous solution of chlorine dioxide and chlorine followed, without an intermediate wash, by an aqueous solution-of chlorine, the overall quantity of chlorine dioxide used being about 20 to about 95~ of the available chlorine of - the solution.
After the initial bleaching step, the pulp may be washed and extracted with potassium hydroxide solution.
Further washing, bleaching with chlorine dioxide solution, washing, extraction with potassium hydroxide, washing, bleaching with chlorine dioxide solution and final washing completes thls preferred sequence of operations.
The chlorine dioxide and chlorine feeds to the ~. , bleach plant 24 are indicated by line 28, the potassium .
. hydroxide solution feeds are indicated by line 30 and the wash water feeds are indicated by line 32. The filtrates from the various operations are passed countercurrently to 20. pulp flow within the bleach plant 24, to form an alkaline (El filtrate1 effluent in line 34 and an acid (D/C filtrate) effluent in line 36, as described in more detail in U.S.
Patent No. 4,039,372. Following a procedure analogous to .. ~ that described in U.S. Patent No. 4,039,372, part of the D/C filtrate is neutralized with potassium hydroxide solution fed by line 38 and then is fed by line 40 for use in the ~ brown stock washer 18 after washing by part of the E
filtrate fed by line 42. The remainder of the D/C and El filtrates are used elsewhere, as described in more detail .
below and entex the recovery system by those routes.
The black liquor and the bleach plant effluents and water used as wash water pass by line 44 to black liquor evaporators 46 wherein the dilute black liquor is concen-trated, the concentrated black liquor passing by line 48 toa recovery furnace 50. In the recovery furnace 50, the organic materials in the concentrated blac~ liquor are .
burned off. Since all the bleach plant effluents in lines 34 and 36 enter the recovery system, the organic materials `; lO contained therein are removed by the furnacing. The bleach plant effluents also introduce potassium chloride to the recovery system so that the liquid smelt exiting the recovery furnace by line 52 contains not only potassium sulphide and potassium carbonate but also potassium chloride.
; 15 Losses of potassium and sulphur values from the system are made up by introducing potassium sulphate to the furnace by line 54, typically by introduction to the dilute black liquor in line 44. The furnacing operation generally is less than 100% efficient in converting 20 potassium sulphate to potassium sulphide, so that the smelt in line 52 also usually contains potassium sulphate, the proportion depending on the efficiency of the furnace.
Potassium sulphate in the liquors formed from the smelt may also arise from oxidation of potassium sulphide therein.
The smelt in line 52 is forwarded to a smelt dissolving tank 55 for dissolving in an aqueous medium fed by line 56 to form green liquor thereby.
The green liquor preferably has the following composition:
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Sulphide 0.25 to 0.75 molality Carbonate 1.2 to 1.8 molality Chloride 0.3 to 1.0 molality Sulphate 0.01 to 0.1 molality Potassium 3.2 to 6.2 molality After filtration or settling to remove un-dissolved dregs, the green liquor is forwarded by line 58 to a recausticizer 60 wherein the potassium carbonate is converted to potassium hydroxide by reaction with reburned 10 lime fed by line 62. The lime mud (calcium carbonate) precipitated in the recausticization is remo~ed by line 64 for kilning to reform reburned lime for feed by line 62.
The remainder of the D/C filtrate in line 66 may be used for kiln gas scrubbing and/or lime mud washing, as ` ~ 15 described in U.S. Patent No. 4,039,372, and hence enters the recovery system in this way. The weak wash water from lime mud washing usually is u~ed as at least part of the aqueous medium in line 56 to prevent loss of the chemicals contained in the weak wash water.
In the event that the furnace 54 is so inefficient that more potassium sulphate is produced than can be dissolved in the green liquor, loss of the excess undissolved potassium sulphate with the dregs may be avoided by leaching the dregs to dissolve the potassium sulphate, such as, by ;~ 25 use of the aqueous medium in line 56 and/or D/C filtrate in line 66, before or after use thereof in lime kiln scrubbing - and/or lime mud washing.
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'-The white liquor resulting from the recausticiza-tion in line 68 is an aqueous solution of potassium hydroxide, potassium sulphide, potassium sulphate, potassium chloride and potassium carbonate, the potassium carbonate arising from incomplete causticization of all the potassium values of the green liquor, and preferably has the following composi-tion:
hydroxide 2.2 to 2.8 molality sulphide 0.25 to 0.75 molality carbonate 0.15 to 0.4 molality chloride 0.30 to 1.0 molality sulphate 0.01 to 0.1 molality potassium 3.2 to 6.2 molality It has been found that the solubility characteris-tics of this system are quite different from the corres-ponding sodium system and these differences enable potassium chloride to be separated by a simpler procedure than the procedures for the corresponding sodium system. In particular, it has been found as a result of extensive investigation of the solubility characteristics of the system that potassium carbonate is very much more soluble in the potassium based white liquor at elevated tempera-tures than sodium carbonate and burkeite are in the so~ium based white liquor. In addition, potassium sulphate is less soluble in the potassium based white liquor than sodium sulphate and burkeite are in the sodium based white liquor.
As a result of the solubility of potassium carbonate in the system, potassium sulphate and potassium chloride can be precipitated from the white liquor by con-.
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13 ~ 6~ -centration thereof, usually by boiling, without contamination by potassium carbonate. This result contrasts markedly with the sodium based systems, described in U.S. Patents Nos.
3,746,612 and 3,950,217, wherein sodium carbonate is always 5 precipitated. In the two-stage evaporative procedure described in U.S. Patent No. 3,950,217, for example, sodium sulphate and sodium carbonate are coprecipitated in the first stage, with sodium carbonate normally being the dominant phase. Since it is required to recycle this 10 precipitate to the furnace for consumption of the sodium sulphate and the sodium carbonate xepresents a dead load on the furnace which decreases furnace capacity, split ; recycle procedures have to be adopted to increase the relative proportion of sodium sulphate in the precipitate 15 to decrease thereby the dead load effect of sodium carbonate _ on the furnace. Such operations are not required in this ; invention and relatively uncontaminated potassium sulphate can be recycled to the furnace.
In Figure 1 there is illustrated one procedure for the separation of potassium chloride wherein the white liquor in line 68 is evaporated by boiling in a white liquor evaporator 70 to precipitate a mixture of potassium chloride and potassium sulphate whic~ is removed from the evaporator 70 by line 72. Generally, the evapora-tion of the white liquor is effected in such a way that thequantity of potassium chloride precipitated and removed corresponds to the quantity entering the system, usually about 90 to about300 lbs. KCl/ton of pulp.
The evaporation of the white liquor in the . ~' ' .
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evaporator 70 may be effected over a wide range of tempera-tures, the lower limit of which is dictated by carbonate solubility while the upper limit is dictated by the atmospheric pressure boiling point of the white liquor, although superatmospherlc pressure may be adopted to in-crease the boiling temperature further. Generally, the evaporation of the white liquor in the evaporator 70 is ~:
effected by boiling at a temperature of about 40 to about 120C, preferably about 40~ to about 70C.
The concentrated white liquor resulting from the one-stage evaporation is removed from the evaporator 70 by line 74 and is diluted to the required concentration for pulping by the remainder of the ~1 fîltrate in line 76, so that in this way the remainder of the El filtrate enters 15 the recovery system. The diluted white liquor then passes ~ ' by line 78 for feed to the digester 14, after supplementa-tion, if required, by an additional external source of pulping liquor by line 80.
The solid phase removed from the evaporator 70 by .
line 72 preferably is treated to remove potassium sulphate therefrom to avoid losses of this expensive chemical with the potassium chloride. As illustrated in Figure 1, the solid mixture in line 72 is leached at an elevated temperature in a leacher 82 by leach liquor fed by line 84 to dissolve all the potassium chloride and some of the potassium sulphate, leaving substantially pure solid potassium sulphate, which is recycled by line 86 to the furnace 50, with the potassium sulphate feed in line 54, or by any other convenient route.
While it is preferred to forward substantially pure potassium ~ . .
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sulphate by line 86 to the furnace 50, some potassium chloride contamination of the potassium sulphate may be tolerated.
The hot leach liquor from the leacher 82 is S passed by line 88 to a cooler 90 wherein the hot liquor is cooled to crystallize potassium chloride which is recovered by line 92, the cooled mother liquor being recycled, after heating, to the leacher 82 by line 84.
The solid potassium chloride in line 92 usually -is contaminated with a small quantity of potassium sulphate, which represents a loss from the system. The relative solubilities of potassium sulphate and potassium chloride do not permit their complete separation by crystallization techniques. The loss of potassium sulphate values in this way may be avoided by using the procedures outlined below with respect to Figures 5 and 6 described in detall below.
The proportion of potassium sulphate in the mixture by line 72 depends on the efficiency of the furnace 50 in converting potassium sulphate to potassium sulphide.
As the efficiency of the furnace increases, the ability to separate pure potassium sulphate from the potassium chloride decreases until, in the limiting condition, the proportion of potassium sulphate decreases to that which is inseparable from potassium chloride by crystallization techniques. Under typical operating conditions, this limitation arises at about 98% efficiency.
The hot leach of the solid mixture generally is effected at a temperature of about 70 to about 100C, pre-ferably about 80 to about 100C. The hot leach liquor '` ' - .
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generally is cooled to a temperature of about 20to about ~C, preferably about 20 to about 40C.
The potassium chloride removed by line 92 may be ; purified to remove the contaminating potassium sulphate by 5 precipitating the sulphate as the barium salt. The potassium ; chloride may be used, if desired, for the formation of potassium hydroxide by electrolysis for use elsewhere in the system.
Turning now to the embodiment of Figure 2, there is &hown therein a two-stage evaporative white liquor pro-... . .
cedure for the separation of potassium chloride, in place ~` of the single evaporative process of Figure 1, followed by - leaching. The other portions of the mill remain the same and are not illustrated in Figure 2.
lSAs shown therein, the white liquor in line 68 is --forwarded to a first white liquor evaporator 100 wherein the white liquor is subjected to a first evaporation by boiling to precipitate substantially pure potassium sulphate, which is removed from the ~irst evaporator 100 by line 102, for passage to the furnace S0 in analogous manner to that described above with respect to the potassium sulphate in line 86 in Figure 1. The evaporation of the white liquor in the first evaporator 100 is effected until the white liquor becomes substantially saturated with potassium chloride.
The partially concentrated white liquor is passed .by line 104 to a second white liquor evaporator 106 wherein ~",!,the white liquor is boiled further to precipitate potassium chloride which is removed by line 108. The concentrated ~ , , .r ~.. ",. .
white liquor in line 74 then is diluted and recycled as described in connection with Figure 1. The potassium chloride usually is contaminated by small quantities of potassium sulphate which are thereby lost from the system, since it cannot be separated by simple crystallization techniques. Further purification of the potassium chloride may be effected by precipitation of the sulphate as barium sulphate.
In place of two separate evaporators 100 and 106, a single evaporator may be used wherein two evaporation steps are effected respectively to crystallize potassium sulphate and potassium chloride.
The white liquor may be boiled over a wide range of temperatures to effect the crystallization in the evapora-tors 100 and 106. Generally, temperatures of about 40to about 120C are used in each of the evaporators, preferably about 40to about 70C. ~
; The embodiment shown in Figure 3 is directed to removal of potassium chloride from green liquor in place Of the white liquor evaporative procedures of Figures 1 and ; 2. In this embodiment, the green liquor in line 58 is .
concentrated by boiling in a green liquor evaporator 110 to precipitate a mixture of potassium sulphate and potassium ;~ chloride, which is removed from the evaporator 110 by line 112.
The precipitation of potassium sulphate and - potassium chloride from the green liquor wherein the pre-:, dominant dissolved salt is potassium carbonate without contamination by potassium carbonate is markedly different :;.
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from the sm21t manipulation procedures used in sodium-based mills, as outlined, for example, in U.S. Patents Nos.
3,986,923, 3,909,344 and 3,954,552, wherein sodium carbonate ; must first be removed before recovery of sodium chloride 5 can be effected.
There is a limiting condition on the operability of this embodiment below which potassium carbonate contamina-tion occurs, such precipitation occurring when the chloride to carbonate molar ratio falls below about 0.3 in the green 10 liquor. However, the ratio generally is well in excess of this value and hence carbonate contamination is unlikely to occur.
The precipitation of the mixture of potassium salts may be effected over a wide range of temperatures, 15 the lower limit being dictated by potassium carbonate solu-bility and the upper limit being dictated by the atmos-pheric pressure boiling point of the green liquor, although superatmospheric pressure may be utilized, if desired, to achieve higher boiling temperatures. Usually, the 20 evaporation of the green liquor is effected by boiling at a temperature of about 40 to about l20C, preferably about 4Qo to about 70 C
The concentrated green liquor resulting from the eVaporation step is removed from the evaporator 110 by line .
114, is diluted to the required concentration by the El - filtrate in line 76 and forwarded to the recausticizer 60 by line 116. The white liquor resulting from the recaustici-zer is recycled by line 68 to the pulping liquor feed in line 14.
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19 :, The mixture of salts in line 112 contains a variable proportion of potassium sulphate which depends on the efficiency of the furnace, as discussed above in connection with the mixture in line 72 of Figure 1.
5 Separable potassium sulphate may be removed from the -mixture in analogous manner to that described above with respect to Figure 1 by hot leaching the mixture in a leacher 118 with aqueous medium fed by line 120 to dissolve all the potassium chloride values and some of the potassium sulphate to leave substantially pure potassium sulphate for -forwarding by line 122 to the furnace 50.
The hot leach liquor is forwarded by line 124 to a cooler 126 wherein the leach liquor is cooled to precipitate potassium chloride for recovery by line 128. As in the 15 case of the Figure 1 embodiment, the solid potassium -- ,~
chloride recovered is contaminated with small quantities of potassium sulphate. The mother liquor from the potassium chloride crystallization is recycled, after heating, to the leacher 118 by line 120.
The leaching and cooling steps effected in leacher 118 and cooler 126 respectively may be carried out under the same temperature conditions as recited above for the leacher 82 and cooler 90 in the embodiment of Figure 1, and reference may be had thereto for the temperature conditions.
~` 25 Figure 4 illustrates a modification of the embodi-ment of Figure 3 wherein two separate e~ap~rative steps ` are effected for separation of potassium sulphate and potassium chloride. The procedure is analogous to the white liquor process described above with respect to Figure 2.
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Green~liquor in line 58 is forwarded to a first green liquor evaporator 130 wherein the green liquor is boiled to deposit potassium sulphate, which is removed by line 132.
The partially concentrated green liquor is forwarded by 5 line 134 to a second green liquor evaporator 136 wherein - .. .
the partially concentrated green liquor is boiled to deposit potassium chloride, which is removed by line 138. The con-centrated green liquor is forwarded by line 114 for further processing as described above with reference to Figure 3.
1~ The concentration of the green liquor in the two evaporators 130 and 136 is effected at the same temperatures as the two evaporators in the two-stage white liquor process of Figure 2 and reference may be had thereto for the res-pective temperatures.
Turning now to the embodiment of Figure 5, there is illustrated therein a potassium chloride removal system wherein potassium sulphate losses with the potassium chloride are substantially eliminated. In place of the smelt being dissolved in an aqueous medium to form green 2a liquor, as is the case in the embodiments of Figures 1 to 4, the smelt in line 52, which first may be solidified, if ,.. .
desired, is subjected to leaching in a leacher 140 by an aqueous medium fed by line 142 to dissolve only potassium sulphide, potassium carbonate and potassium chloride from the smelt and leave the potassium sulphate with the smelt dregs in the solid phase.
The smelt leaching step may be effected over a wide temperature range, generally about 30 to about 110C, , . . .
,,. ~ ~ .
6~ -although the lGwer end of the temperature range is pre~erred to inhibit potassium sulphate solubility. The preferred temperature range is about 30Oto about 70C.
The aqueous solution of potassium sulphide, potassium carbonate and potassium chloride resulting from the smelt leaching is forwarded by line 144 to a re-causticizer 146 wherein the potassium carbonate values are mainly converted to potassium hydro~ide. The white liquor formed in the recausticizer 146 contains potassium hydroxide, potassium sulphide, potassium chloride and potassium carbonate, the latter arising from inefficien-cies in the recausticization process. The white liquor is vlrtually free from potassium sulphate as a result of the initial smelt leaching step.
lS The white liquor is passed by line 148 to a white liquor evaporator 150 wherein the white liquor is concentrated by boiling to precipitate potassium chloride in substantially pure form, the potassium chloride being removed by line 152. The evaporation of the white liquor ~; 20 may be effected over a wide temperature range, generally about 40 to about 120C. It is preferred to operate at the upper end of this range to inhibit crystallization of any potassium sulphate present and the preferred tempera- ;:
ture range is about 70to about 110C.
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~; 25 Any potassium sulphate contamination in the . ~ . . .
~ potassium chloride may be removed by precipitation as ~ .
barium sulphate and, as in the other cases, represents a loss of chemicals from the system. Utilization of the smelt leaching procecure, however, minimizes such losses, "'~;' :: "
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. 22 as compared with the procedures described above with respect : to Figures 1 to 4.
: The solids remaining from the smelt leaching are forwarded by line 154 to a dregs leacher 156 wherein 5 the potassium sulphate is dissolved by wash water fed by line 158 to form a potassium sulphate solution and leave dregs solids for recovery by line 160.
: The potassium sulphate solution in line 162 resulting from the dregs washing is recycled to the furnace, such as, by dilution of the concentrated white liquor in line 164 to form diluted white liquor in line 166 for re-cycle to the digester. Alternatively, the potassium sulphate solution may be forwarded directly to the black liquor evaporators 46.
Figure 6 illustrates a modification of the embodi- -ment of Figure S which also utilizes the smelt leaching operation and the same reference numerals are used to ` denote the same items. In the embodiment of Figure 6, the '! aqueous solution in line 144 containing potassium sulphide, ,;
20 potassium carbonate and potassium chloride is concentrated `,.~! by boiling in an evaporator 170 to precipitate potassium chloride which is removed by line 172.
r'~' The temperature of operation of the evaporator 170 may vary widely, generally about 40 to about120C, 25 preferably towards the upper end of this range to inhibit the tendency for any potassium sulphate present to precipi-~` tate, and in the range of about70 to about llOoc.
. .; .
~. The concentrated liquor in line 174 and formed in ``~ the evaporator 170 may be diluted with potassium sulphate ,',,,~ ' .
,, " . ~-, .,:, ?. ., ,:
23 11~611 solution in line l62, or El filtrate, before passage by line 176 to the recausticizer 146 from which the white liquor in line 148 is removed for recycle to the digester 10. Alternatively, the potassium sulphate solution in 5 line 162 may be forwarded directly to the black liquor -~
evapOrators 46, or to the white liquor in line 148.
The processes for potassium chloride removal described above with reference to Figures 1 to 6 have assumed that sodium does not enter the system, or is present 1~ in such minor quantities that it has no effect on the system. The embodiments of Figures 7 and 8 are directed to processes for the removal of potassium chloride when significant concentrations of sodium salts are present to - the extent that a purge of sodium values is required.
The relative solubility characteristics of sodium -salts in the presence of large concentrations of potassium ; salts,i.e.at high K/K+Na molar ratios, have been determined.
Assuming that the màjor source of the sodium is the wood which is pulped and this corresponds to about one-quarter to about one-half of the potassium content thereof, then the green liquor and white liquor compositions preferably are as follows:
Green liquor:
. . ,;. .
~ sulphide 0.25 to 0.75 molality i 25 carbonate 1.2 to 1.8 molality -,~
chloride 0.3 to 1.0 molality sulphate 0.01 to 0.1 molality K + Na 3.2 to 6.2 molality K/K+Na 0.50 to 0.75 molar ratio .:', ` 24 ~ 6~
White liquor:
hydroxide 2.2 to 2.8 molality sulphide 0.25 to 0.75 molality carbonate 0.15 to 0.4 molality chloride 0.30 to 1.0 molality sulphate 0.01 to 0.1 molality K + Na 3.2 to 0.2 molality K/K + Na 0.50 to 0.75 molar ratio It is possible to effect recovery of potassium chloride over a wide range of K/K+Na molar ratios but simple procedures involving precipitation of potassium sulphate and potassium chloride without the precipitation of potassium carbonate, while maintaining the desired purge ~` of potassium chloride,can only be effected under limited : ''`
; 15 conditions, the ranges depending on the temperature and _ .~ carbonate concentrations.
Referring now to Figure 7, there is shown therein -~ a two-stage white liquor evaporation process similar to that i.9 ; described above with respect to Figure 2, except that in `l 2Q this instance, sodium carbonate coprecipitates with potassium chloride in the second white liquor evaporator 106. The sodium carbonate present in the potassium chloride removed by line 108 represents a purge of sodium values from the i :.
` system. If desired, the potassium chloride and sodium carbonate may be separated by leaching. The conditions of operation of the two evaporators 100 and 106 are as described above with respect to the embodiment of Figure 2, f`;`` and reference may be had thereto for appropriate conditions.
, .- --.
~ In Figure 8, a three-stage white liquor evaporation ..
, ~ .... .
, . ~
, ~
2s 1~96~1 process is illustrated whereby potassium chloride and sodium carbonate are independently separated from the white l~quor.
In this process, the second stage evaporator 106 is opera-ted to deposit potassium chloride, with a small degree of 5 contamination by potassium sulphate.
The concentrated white liquor from the second stage white liquor evaporator 106 is passed by line 180 to a third stage white liquor evaporator 182 wherein the white liquor is evapcrated to deposit sodîum carbonate which is removed by line 184. The concentrated white liquor in line 74 is then recycled as described above.
The first stage evaporation is e~fected as described above in connection with Figure 2 and reference may be had thereto for details thereof. The second stage . ~, .
- 15 evaporation is effected at a low temperature, generally about 40 to about 60C, in order to favour potassium i .
chloride crystallization. The deposition of potassium chloride is effected until substantial saturation of the i,..;:
white liquor by sodium carbonate is reached.
In the third stage evaporator 182, the white liquor is boiled at a high temperature, generally about ~; 90 to 110C, to cause crystallization of sodium carbonate but to keep the potassium chloride in solution. The sodium ~'!" carbonate remove by line 184 may be contaminated with traces ., .
25 of potassium sulphate.
.~
The amount of sodium carbonate which may be ~ ~ removed by the third stage evaporation in the process of - ~igure 8 exceeds the expected sodium inputs to the mill and hence need be operated only intermittently, if :~ , -: . - - .: , , . , . , , ~ . . ., -- . . .
26 ~S~l desired, with a two-sta~e operation being effected for the remainder of the time.
The processes of Figures 7 to 8 are applicable to white liquor in line 68 having a K/K~Na ratio of about 5 0.6 to about 0.7. Attempts to control sodium levels at higher ratios, for example, about 0.7 to about 0.9, produce a carbonate phase which is KNaCO3, which would represent a loss of potassium from the system when the sodium purge is effected, which may be considered undesirable. However, 10 since the quantities of sodium required to be removed are small, the potassium losses resulting from the removal of KNaCO3 also are small, and hence may be tolerable by some mills.
It is also possible to limit sodium build-up by 15 removal from green liquor but at a typical R/K~Na ratio of about 0.8, the sodium is removed as KNaCO3, which may be considered undesirable, as noted above.
EXAMPLES
Example 1 The solubility characteristics of an aqueous ~; system containing the ionic species K , Na , OH , S , Cl , ~ CO3 , and SO4 were studied at various potassium molar ratios ;; of K/K+Na up to 1.0 (i.e. the case where sodium is absent).
Figures 9 to 11 are graphical representations of ;~1 2S part of this study, illustrating, respectively, the solu-`, bility of chloride at 100C whe~ the system is saturated with CO3 and SO4 , the solubility of carbonate at 100C when the system is saturated with Cl and SO4 and the solu-bility of sulphate at 100C when the system is saturated ~,.~" .
' with Cl and CO3 .
Figures 12 to 14 are also graphical representa-tions of part of this study showing the stable solid phase transitions upon variation of potassium molar ratio and total alkali concentration (S ~ OH ) at 50C, 75C and 100C
respectively.
This solubility data was used to determine mass balances for the various pulp mill procedures outlined above , . __ _ . ............. . .
; in Figures 1 to 8. The determinations are outlined in the ; io following Examples II to IX.
: Example II
:
Utilizing the developed solubility data, a mass balance was calculated for the white liquor evaporation : process of Figure 1, omitting the cooling crystallization : 15 of KCl from leach liquor and mother liquor recycle. In --!
this process, the evaporation temperature in evaporator 70 ~-: was assumed to be 100C while the leach temperature in . leacher 82 was assumed to be 30C.
: The results are reproduced in the following ~: 20 Table I:
,~ ' .' .
,. .
.
. . .
- -28 ~56~1 TABLE I
Stream Water Solution Composition-m~lality Solid Phaæs No. lb/ton lb.l/ton pulp OH 3 4 K Cl K2S04 .
5 68 6000 2.5 0.5 0.65 0.30.05 74 1680 9.01 1.8 1.04 1.08. ~.02 72 2.18 0.27 10 88 450 0 0 4.. 84 0 0.09 .~ 86 0.0 0.23 . Example III
A mass balance was calculated for the procedure of Example II in which the cooling crystallization of KCl from .
15 mother liquor is effected and mother liquor recycle occurs.
~: The tempexature in evaporator 70 was assumed to be 100C, that in leacher 82 was assumea to De 100C and that in-cool~r --~: 90 was assumed to be 30C. -The results are reproduced in the following '~i: : 20 Table II:
;.~`3: .
~' .', ~ , .
' .
-, ~'''' .
.., ", ...
-: : : . .::: .::: : : . . : : ,: : . ::: :,: . ; ,, : -TABLE II
Stream Water Solution ~sition Solid Phases : No. lb/ton - m~lality lb.-mol/ton of pulp . . . . . QH . S Cl CO3 . SO4 KCl K2SO4 68 6000 2.5 0.5 0.65 0.3 0.05 -` 74 1680 9.01 1.80 1.04 1.08 0.02 ,~ 72 2.18 0.27 -~ 84 876 . O O 4.84 O 0.09 - . 10 86 o 0.22 88 876 O O 7.33 O 0.148 -. - 92 . 2.18 0.05::
:, .: . ...... ...
~; ' 2 ~ , Example IV
. ,. . ~
... . A mass balance was calculated for the two-stage :.
; 15 white liquor evaporation process of Figure 2. The tempera- _ :
ture of each white liquor evaporation stage was assumed to be .~' - 50C. The results are reproduced in the following Table III:
I TABLE III
S~m Water Solution Cbmposition Solid Phases.
.~ 20 No.lb/bon - molality lb.-m~Vbon o~ pulp OH 3 4 KCl. K2SO4 .
68 6000 2.50.5 0.65 0.3 0,05 , 102 0.234 1042778 5.4 1.08 1.40 0.65 0.025 :~ 108 2.130.036 742148 6.97 1.39 0.82 0.~4 0.015 '; ' ' ' ' ~ .
- ., , - :: ~ . .
3n 1~}963Ll Example V
A mass balance was calculated for the process of Figure 3, with the exception that the leach liquor cooling and mother liquor recycle were omitted. The green Iiquor evapora-tion temperature was assumed to be 100C while the leachingtemperature was assumed to be 30C. The results are repro-duced in the following Table IV:
TABLE IV
-;
Stream Water Solution ~osition Solid Phases 10 No. lb/ton - mDlality lb.-m~l/ton of pulp OH S Cl Co3 SO4 KCl K2S04 ;:. . . ___ __. _ . _ . . . . __ .. . . .
~j 58 6000 0.1 0.5 0.65 1.5 0.05 ,'~ 111 4440 , 114 1560 0.38 1.90 1.12 5.69 0.029~."~
- 15 112 2.140.25 124 44Z 0 04.84 0 0.09 ;~ .
122 0.00.21 - . .
i~ Example VI
;: .
,. ........................................... .
For the process of Figure 4, a mass balance was calculated with the evaporatlon temperatures in the two evaporators being assumed to be 50C. The results are reproduced in the following Table V:
, ~ .
.~ ~ .
..... .
TABLE V
Stream Water Solution ~x~osition Solid Phases No. lb/ton - molality Ib.-mol/ton of pulp OH S Cl Co3 so4 KCl K2S04 - - - ,~
58 6000 0.11 0.53 0.68 1.58 0.053 132 0.20 1342778 0.22 1.08 1.40 3.24 0.037 138 2.15 0.06 114 2110 0.2g 1.47 0.86 4.4 0.021 . .
Exam~le VII
:A mass balance was calculated for the embodiment of Figure 5. A temperature of 50C was assumed for the smelt leaching, a temperature of 100C was assumed for the _ dregs leaching and a temperature of 50 C was assumed for ~", the white liquor evapoxation. The results are reproduced in the following Table VI:
TABLE VI
Stream Water Solution C~osition Solid Phases No. lb/ton - m~lality lb.-m~l/ton of pulp OH S o3 SO4 KClK2S04 , 1442640 0.23 1.141.48 3.41 0.041 154 0.19 162 140 0 0 0 0 1.36 1482640 5~68 1.141.48 0.68 0.041 1642148 6.97 1.390.82 0.84 0.015 152 2.130.08 .
.. ~ , 32 ~ 6~
Example VII r A mass balance for the embodiment of Figure 6 . was calculated, with the smelt leach temperature being . .:
assumed at 50C, the dregs leach temperature at 100C and the green liquor evaporation temperature at 100C. The results are reproduced in the following Table VII:
TABLE VII
S~tre~m H O Solution C~osition Solid Phases !~r~ Nb. lb~ton - m~lality i?~'~ I0 OH S Cl CD3 SO4 KCl K2S04 , . - - .
-. 142 2640 ,, .
1442640 0.23 1.14 1.48 3.41 0.041 154 0.19 15 162 140 j 1742154 0.38 1.90 1.10 5.69 0.029 . :-.
~,~ 172 . 2.15 0.062 -:
, : ~ Example IX
Based on the experimentally-attained solubility data, a mass balance was calculated for the potassium chloride ~ and sodium carbonate removal process of Figure 8, with the : first two white liquor evaporations being assumed to be effected at 50C and the third white liquor evaporation .25 being assumed to be effected at 100C. The results are reproduced.in the following Table VIII:
~ ~ .
~ .
.
:, , , -~
: - . . ~ . : ,: ,.. ,:. , .: . :-- . , :: , .: :. :::. . , -. ~ . : . ,. :
TABLE VIII
Stream H20 Solution G~sit~on Solid Phases No. lb/ton - molality lb,mol/ton of pulp r OH S Cl Co3 so4 KCl Na2003 K2S04 68 6000 2.5 0.5 0.65 0.30 0.005 0.7 102 0.24 104~676 5.60 1.12 1.46 0.67 0.022 0.698 108 2.10 0.03 1802100 7.15 1.43 0.85 0.86 0.015 0.68 ..... .
~81 438 184 0.01 ,. . .
74 1662 9.01 1.80 1.07 0.63 0.018 0.72 0.76 15 *R is the molar ratio K/X+Na The quantity of sodium carbonate removed by this procedure represents the maximum sodium recovery under the assumed operating conditions and greatly exceeds the expected sodium input from wood sources, indicating that the third stage evaporation need be operated only inter-mittently.
SUMMARY
The present invention, therefore, provides a potassium based pulp mill procedure which is able to achieve potassium chloride recovery and further is able to prevent the build up of sodium values within the system. Modifica-tions are possible within the scope of the invention.
.
.. . .: : .: : ,
FIELD OF INVENTION
The p~esent invention is directed to the production of cellulosic ~ibrous material pulp using potassium based pulping chemicals, to, the recovery of pulping chemicals and to the removal of potassium chloride contaminants.
BACKGROUND TO THE INVE:NTION
Generally in the pxoduction of pulp suitable for formation into paper, wood or other raw cellulosic fibrous material is subjected to chemical digestion in a pulping liquor to form a pulp of the cellulosic fibrous material.
The pulp thereafter is subjected to bleaching and purifi-cation operations ina bleach plant.
The spent pulping liquor usually is subjected to a series of recovery and regeneration operations to recover pulping chemicals and to provide fresh pulping liquor.
Sodium-based chemicals conventionally have been used as the pulping liquor, such as, sodium hydroxidej possibly along with other sodium salts.
One widely used pulping process is the kraft process wherein raw cellulosic fibrous material, generally wood chips, is digested in white liquor containing sodium .
: . : . , , , " . , .; ; . . . , ~ .~
1~$~
hydroxide and sodium sulphide as the active pulping chemicals to provide a pulp and 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 bleaching 5 and purification operations.
The black liquor then is passed to the recovery and regeneration systém in which the black liquor is evaporated and the concentrated black liquor is burned in a furnace to yield a smelt containing sodium carbonate and 10 sodium sulphide. Sodium sulphate is usually added to the black liquor 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 is clarified to remove undissolved solids and the green liquor is causticized with reburned lime to convert the sodium carbonate to sodium hydroxide.
The resulting white liquor then is at least partially re-cycled to the pulping step to provide at least part of the pulping liquor.
Conventional bleach plant operations generally ; involve a sequence of bleaching and purification steps, ~ogether with washing steps. The bleaching steps usually involve the use of at least one chlorine-containing bleaching agent, such as, chlorine, chlorine dioxide, chlorine 25 monoxide and sodium hypochlorite. The purification steps conventionally involve treatment with sodium hydroxide solution.
In some instances, the bleaching and extraction steps may be combined, for example, using an oxygen bleaching .
3 l~S6~
operation. Where such oxygen bleaching is used, it usually is supplemented by further bleaching operations using chlorine-based bleaching chemicals.
The aqueous effluents of bleach plants including 5 spent bleaching chemicals, spent extraction chemicals and spent wash water conventionally have been discharged to convenient water bodies but the effluents are environment-ally hazardous, possessing objectionable colour and high toxicity towards a~uatic and marine biota, and containing 10 fibres and oxygen-consuming materials. To avoid the dis-charge of such materials and thereby eliminate the harmful and polluting effects of the effluents, it has been suggested in U.S`. Patents No. 3,698,995 and No. 4,039,372 to forward ~ the e~fluents as a single stream or in a plurality of streams - -15 to the chemical recovery system after utilizing at least part thereof in washing the unbleached pulp in the brown stock washer. When this procedure is adopted, the organic contaminants of the bleach plant effluents are consumed in the recovery furnace.
As a result of the use of chlorine-containing bleaching chemicals and sodium-containing purification agents, the aqueous effluents from conventional bleach plant operations contain sodium chloride which then enters the recovery and regeneration cycle when the effluent handling 25 process disclosed in the above patents is used.
Sodium chloride passes without change through the recovery and regeneration system and hence constitutes a dead load which, in continuous operation, may build up to intolerable levels. Prior suggestions have been made to -- ,. ,., : ................. :,, : . : . - : :: ~: . :
:, : ,.
. . .: :;: : ~ .... , :
combat this problem by precipitation of the sodium chloride " from white liquor, as described in detail in U.S. Patents Nos. 3,746,612 and 3,950,217, or by separation from smelt components, as described in detail in U.S. Patents Nos.
5 3,986,923, 3,909,344, 3,954,552 and 3,945,880.
It has previously been suggested to utilize potassium-based chemicals in plaee of sodium-based chemieals in pulping proeesses and bleach plant extractions. The use of potassium-based pulping chemicals as ccmpared with sodiunr based chemieals lS advantageous, in that a greater pulp yield is attained with consequently increased pulp pro- ;
duetion and decreased wood costs. The rate of pulping using potassium-based ehemieals is faster than for , sodium-based ehemicals, thereby inereasing the overall mill pulping eapaeity. While the inereased eost of potassium sulphate make-up as eompared with sodium sulphate make-up partly offset these benefits, make-up chemieal requirement ean be minimized.
The use of potassium-based ehemieals in eonjunetion with a bleaeh plant effluent disposal proeedure by way of the pulping llquor reeovery and regeneration cyele would result in an analogous build up of potassium ehloride to that . . .
deseribed above for the sodium-based proeess.
SUMMARY OF INVENTION
:6 The present invention, in its broadest aspect, is direeted to the separation of potassium ehloride from a potassium-based pulp mill spent chemical recovery and re-generation proeess by the concentration of liquors formed in that proeess. The source of the potassium-chloride - .~
;
contamination -s not significant to the broadest aspects of the invention, but in the preferred embodiment of the invention the potassium chloride arises mainly from the forwarding of bleach plant effluents to the recovery and Xegeneration system, thereby providing a liquid effluent-free closed cycle potassium-based pulp mill.
It has been discovered that the solubility charac-teristics of thesystems KOH-K2CO3-K2SO4-KCl-H20, KOH-K2S-K2cO3 K2SO4 Cl 2 2 2 3 2 4 2somewhat different from the corresponding sodium salt systems and these differences permit potassium chloride removal from the recovery and regeneration cycle to be effected in a si~plified manner, as compared to the prior processes ., utilized in soda-based pulping systems and disclosed in lS the above-recited patents.
The simplicity of the potassium chloride removal processes adds to the benefits of utilization of potassium-based chemicals outlined above. The simplicity arises from the pronounced tendency for potassium carbonate to remain in the aqueous phase upon concentration of either the green liquor or white liquor, in complete contrast to the tendency of sodium carbonate to precipitate from green liquor or white liquor in the sodium-based systems discussed above.
:
The evaporative procedures used in this invention enable an additional economic saving to be achieved, as com-pared with the sodium-based processes mentioned above. Thus, less evaporation is necessary, with consequent heat savings, .
6 ~ 6~1 usually in the form of steam, in the case of the potassium-based process to achieve the required purge of potassium chloride as compared with the sodium-based process to achieve the required purge of sodium chloride. For example, 5 a 3.5 molal active alkali solution (OH ~S ~ requires about 65% evaporation to crystallize the requisite potassium chloride as compared with about 80~ for sodium chloride crystallization ~rom sodium-based white liquor.
The present invention is primarily concerned with the pulping of cellulosic fibrous material using pulping liquors containing potassium hydroxide, and also preferably potassium sulphide, as at least the major proportion of the pulping chemicals. However, sodium ions may be present in the system, arising from a variety of sources, such as, deliberate inclusion of sodium-based pulping chemicals, ; such as, sodium hydroxide and sodium sulpbide, in the pulping liquor and/or from the cellulosic fibrous material which is . ~ .
pulped.
When such sodium ionic species are present in significant amounts, the present invention is concerned generally with smelt solutions and white liquors formed in the regeneration cycle having a potassium molar ratio, i.e., (K/K+Na), ~reater than about 0.5, preferably greater than about 0.6. The potassium molar ratio present in these liquors influences solid phases which can be precipitated therefrom.
Generally, the solid phases differ depending on the potassium molar ratio, different phases oeing experienced in the molar ratio ranges of about 0.5 to about 0.7, greater _! .
, ~IDS6~1 than a~out 0.7 to about 0.9 and greater than about 0.9 to 1.0, in the case of white liquor and the molar ratio ranges of about 0.5 to a~out 0.8, greater than 0.8 to about 0.9 and greater than about 0.9 to 1.0 in the case of smelt solution.
s The build up of excess sodium values within the system, such as may occur when some sodium-containing wood - . species are pulped and a wholly potassium-based pulping liquor is used, may be prevented ~y purging the excess sodium values in the form of solid sodium carbonate.
BRIEF DESCRIPTION OF DR~WINGS
:~ ~ Figure 1 is a schematic flow sheet of a liquid effluent-free potassium based Kraft pulp mill wherein potassium chloride is removed;
Figure 2 is a schematic flo~ sheet of a white liquor evaporation process for use as an alternative to the process of Figure 1 for removal of potassium chloride;
Figure 3 is a schematic flow sheet of a green - liquor evaporation process for the removal of potassium chloride in an effluent-free potassium-based Kraft pulp 2Q mill;
Figure 4 is a schematic flow sheet of an alterna-tive green liquor evaporation process to that illustrated in Figure 3;
Figure 5 is a schematic flow sheet of a smelt . 25 leaching and white liquor evaporation process for the removal of potassium chloride in an effluent-free potassium-:~ based Kraft pulp mill;
Figure 6 is a schematic flow sheet of a smeltleaching and potassium sulphide solution evaporation process ~ .
:
.. ... . . . . . . ..
S6~1 for the removal.of potassium chloride as an alternative to the process of Figure 5;
Figure 7 is a schematic flow sheet of a modifica-tion of the process of Figure 2 wherein a sodium purge also is effected;
Figure 8 is a schematic flow sheet of an alterna-tive sodium purge process from that of Figure 7; and Figures 9 to 14 are graphical representations of the solubility characteristics of a multi-ionic aqueous 10 system. . .... . .. . . . . ..
DESCRIPTION OF PREFERRED EMBODIMENTS
.Referring to the drawings, Figure 1 is a schematic flow sheet of a liquid effluent-free or "closed cycle" pulp mill which includes a digester 10 to which wood chips are - 15 fed by line 12 and pulping liquor by line 14. The pulping liquor contains potassium hydroxide and potassium sulphide : as the active pulping chemicals. The pulp passes by line . 16 to brown stock washer 18 for separation from black liquor and for washing by bleach plant effluents are possibly by additional w~sh watex fed by line 20.
The washed and unbleached pulp from the brown stock washers 18 passes by line 22 to a bleach plant 24 wherein the pulp is subjected to a series of bleaching, ; caustic extraction and washing operations to result in bleached pulp of the required brightness and purity in line 26.
The actual sequence of operations which is effec-ted on the pulp in the bleach plant 24 is not significant, although it is preferred to effect an initial bleaching - ~ ;: :; .: ~ , . :
. - . , ,, .~. . -.: .. .. :~ . : : .
~` 9 ~ 6~1 with an aqueous solution of chlorine dioxide and chlorine in which chlorine dioxide provides about 20 to about 95 of the available chlorine of the solution or with an a~ueous solution of chlorine dioxide and chlorine followed, without an intermediate wash, by an aqueous solution-of chlorine, the overall quantity of chlorine dioxide used being about 20 to about 95~ of the available chlorine of - the solution.
After the initial bleaching step, the pulp may be washed and extracted with potassium hydroxide solution.
Further washing, bleaching with chlorine dioxide solution, washing, extraction with potassium hydroxide, washing, bleaching with chlorine dioxide solution and final washing completes thls preferred sequence of operations.
The chlorine dioxide and chlorine feeds to the ~. , bleach plant 24 are indicated by line 28, the potassium .
. hydroxide solution feeds are indicated by line 30 and the wash water feeds are indicated by line 32. The filtrates from the various operations are passed countercurrently to 20. pulp flow within the bleach plant 24, to form an alkaline (El filtrate1 effluent in line 34 and an acid (D/C filtrate) effluent in line 36, as described in more detail in U.S.
Patent No. 4,039,372. Following a procedure analogous to .. ~ that described in U.S. Patent No. 4,039,372, part of the D/C filtrate is neutralized with potassium hydroxide solution fed by line 38 and then is fed by line 40 for use in the ~ brown stock washer 18 after washing by part of the E
filtrate fed by line 42. The remainder of the D/C and El filtrates are used elsewhere, as described in more detail .
below and entex the recovery system by those routes.
The black liquor and the bleach plant effluents and water used as wash water pass by line 44 to black liquor evaporators 46 wherein the dilute black liquor is concen-trated, the concentrated black liquor passing by line 48 toa recovery furnace 50. In the recovery furnace 50, the organic materials in the concentrated blac~ liquor are .
burned off. Since all the bleach plant effluents in lines 34 and 36 enter the recovery system, the organic materials `; lO contained therein are removed by the furnacing. The bleach plant effluents also introduce potassium chloride to the recovery system so that the liquid smelt exiting the recovery furnace by line 52 contains not only potassium sulphide and potassium carbonate but also potassium chloride.
; 15 Losses of potassium and sulphur values from the system are made up by introducing potassium sulphate to the furnace by line 54, typically by introduction to the dilute black liquor in line 44. The furnacing operation generally is less than 100% efficient in converting 20 potassium sulphate to potassium sulphide, so that the smelt in line 52 also usually contains potassium sulphate, the proportion depending on the efficiency of the furnace.
Potassium sulphate in the liquors formed from the smelt may also arise from oxidation of potassium sulphide therein.
The smelt in line 52 is forwarded to a smelt dissolving tank 55 for dissolving in an aqueous medium fed by line 56 to form green liquor thereby.
The green liquor preferably has the following composition:
.
... , .. ,,,, . . .. , . . . . . ., . -.: ,............. . : : . : : : ..
$~
Sulphide 0.25 to 0.75 molality Carbonate 1.2 to 1.8 molality Chloride 0.3 to 1.0 molality Sulphate 0.01 to 0.1 molality Potassium 3.2 to 6.2 molality After filtration or settling to remove un-dissolved dregs, the green liquor is forwarded by line 58 to a recausticizer 60 wherein the potassium carbonate is converted to potassium hydroxide by reaction with reburned 10 lime fed by line 62. The lime mud (calcium carbonate) precipitated in the recausticization is remo~ed by line 64 for kilning to reform reburned lime for feed by line 62.
The remainder of the D/C filtrate in line 66 may be used for kiln gas scrubbing and/or lime mud washing, as ` ~ 15 described in U.S. Patent No. 4,039,372, and hence enters the recovery system in this way. The weak wash water from lime mud washing usually is u~ed as at least part of the aqueous medium in line 56 to prevent loss of the chemicals contained in the weak wash water.
In the event that the furnace 54 is so inefficient that more potassium sulphate is produced than can be dissolved in the green liquor, loss of the excess undissolved potassium sulphate with the dregs may be avoided by leaching the dregs to dissolve the potassium sulphate, such as, by ;~ 25 use of the aqueous medium in line 56 and/or D/C filtrate in line 66, before or after use thereof in lime kiln scrubbing - and/or lime mud washing.
~ .
'-The white liquor resulting from the recausticiza-tion in line 68 is an aqueous solution of potassium hydroxide, potassium sulphide, potassium sulphate, potassium chloride and potassium carbonate, the potassium carbonate arising from incomplete causticization of all the potassium values of the green liquor, and preferably has the following composi-tion:
hydroxide 2.2 to 2.8 molality sulphide 0.25 to 0.75 molality carbonate 0.15 to 0.4 molality chloride 0.30 to 1.0 molality sulphate 0.01 to 0.1 molality potassium 3.2 to 6.2 molality It has been found that the solubility characteris-tics of this system are quite different from the corres-ponding sodium system and these differences enable potassium chloride to be separated by a simpler procedure than the procedures for the corresponding sodium system. In particular, it has been found as a result of extensive investigation of the solubility characteristics of the system that potassium carbonate is very much more soluble in the potassium based white liquor at elevated tempera-tures than sodium carbonate and burkeite are in the so~ium based white liquor. In addition, potassium sulphate is less soluble in the potassium based white liquor than sodium sulphate and burkeite are in the sodium based white liquor.
As a result of the solubility of potassium carbonate in the system, potassium sulphate and potassium chloride can be precipitated from the white liquor by con-.
- : -~ ,; ' i . . ` ::::, . ~ ' :''.:. ' :
13 ~ 6~ -centration thereof, usually by boiling, without contamination by potassium carbonate. This result contrasts markedly with the sodium based systems, described in U.S. Patents Nos.
3,746,612 and 3,950,217, wherein sodium carbonate is always 5 precipitated. In the two-stage evaporative procedure described in U.S. Patent No. 3,950,217, for example, sodium sulphate and sodium carbonate are coprecipitated in the first stage, with sodium carbonate normally being the dominant phase. Since it is required to recycle this 10 precipitate to the furnace for consumption of the sodium sulphate and the sodium carbonate xepresents a dead load on the furnace which decreases furnace capacity, split ; recycle procedures have to be adopted to increase the relative proportion of sodium sulphate in the precipitate 15 to decrease thereby the dead load effect of sodium carbonate _ on the furnace. Such operations are not required in this ; invention and relatively uncontaminated potassium sulphate can be recycled to the furnace.
In Figure 1 there is illustrated one procedure for the separation of potassium chloride wherein the white liquor in line 68 is evaporated by boiling in a white liquor evaporator 70 to precipitate a mixture of potassium chloride and potassium sulphate whic~ is removed from the evaporator 70 by line 72. Generally, the evapora-tion of the white liquor is effected in such a way that thequantity of potassium chloride precipitated and removed corresponds to the quantity entering the system, usually about 90 to about300 lbs. KCl/ton of pulp.
The evaporation of the white liquor in the . ~' ' .
..~.
il~g6~
evaporator 70 may be effected over a wide range of tempera-tures, the lower limit of which is dictated by carbonate solubility while the upper limit is dictated by the atmospheric pressure boiling point of the white liquor, although superatmospherlc pressure may be adopted to in-crease the boiling temperature further. Generally, the evaporation of the white liquor in the evaporator 70 is ~:
effected by boiling at a temperature of about 40 to about 120C, preferably about 40~ to about 70C.
The concentrated white liquor resulting from the one-stage evaporation is removed from the evaporator 70 by line 74 and is diluted to the required concentration for pulping by the remainder of the ~1 fîltrate in line 76, so that in this way the remainder of the El filtrate enters 15 the recovery system. The diluted white liquor then passes ~ ' by line 78 for feed to the digester 14, after supplementa-tion, if required, by an additional external source of pulping liquor by line 80.
The solid phase removed from the evaporator 70 by .
line 72 preferably is treated to remove potassium sulphate therefrom to avoid losses of this expensive chemical with the potassium chloride. As illustrated in Figure 1, the solid mixture in line 72 is leached at an elevated temperature in a leacher 82 by leach liquor fed by line 84 to dissolve all the potassium chloride and some of the potassium sulphate, leaving substantially pure solid potassium sulphate, which is recycled by line 86 to the furnace 50, with the potassium sulphate feed in line 54, or by any other convenient route.
While it is preferred to forward substantially pure potassium ~ . .
:, . . . ::
~¢~96~
sulphate by line 86 to the furnace 50, some potassium chloride contamination of the potassium sulphate may be tolerated.
The hot leach liquor from the leacher 82 is S passed by line 88 to a cooler 90 wherein the hot liquor is cooled to crystallize potassium chloride which is recovered by line 92, the cooled mother liquor being recycled, after heating, to the leacher 82 by line 84.
The solid potassium chloride in line 92 usually -is contaminated with a small quantity of potassium sulphate, which represents a loss from the system. The relative solubilities of potassium sulphate and potassium chloride do not permit their complete separation by crystallization techniques. The loss of potassium sulphate values in this way may be avoided by using the procedures outlined below with respect to Figures 5 and 6 described in detall below.
The proportion of potassium sulphate in the mixture by line 72 depends on the efficiency of the furnace 50 in converting potassium sulphate to potassium sulphide.
As the efficiency of the furnace increases, the ability to separate pure potassium sulphate from the potassium chloride decreases until, in the limiting condition, the proportion of potassium sulphate decreases to that which is inseparable from potassium chloride by crystallization techniques. Under typical operating conditions, this limitation arises at about 98% efficiency.
The hot leach of the solid mixture generally is effected at a temperature of about 70 to about 100C, pre-ferably about 80 to about 100C. The hot leach liquor '` ' - .
16 ~ 6 ~ ~
generally is cooled to a temperature of about 20to about ~C, preferably about 20 to about 40C.
The potassium chloride removed by line 92 may be ; purified to remove the contaminating potassium sulphate by 5 precipitating the sulphate as the barium salt. The potassium ; chloride may be used, if desired, for the formation of potassium hydroxide by electrolysis for use elsewhere in the system.
Turning now to the embodiment of Figure 2, there is &hown therein a two-stage evaporative white liquor pro-... . .
cedure for the separation of potassium chloride, in place ~` of the single evaporative process of Figure 1, followed by - leaching. The other portions of the mill remain the same and are not illustrated in Figure 2.
lSAs shown therein, the white liquor in line 68 is --forwarded to a first white liquor evaporator 100 wherein the white liquor is subjected to a first evaporation by boiling to precipitate substantially pure potassium sulphate, which is removed from the ~irst evaporator 100 by line 102, for passage to the furnace S0 in analogous manner to that described above with respect to the potassium sulphate in line 86 in Figure 1. The evaporation of the white liquor in the first evaporator 100 is effected until the white liquor becomes substantially saturated with potassium chloride.
The partially concentrated white liquor is passed .by line 104 to a second white liquor evaporator 106 wherein ~",!,the white liquor is boiled further to precipitate potassium chloride which is removed by line 108. The concentrated ~ , , .r ~.. ",. .
white liquor in line 74 then is diluted and recycled as described in connection with Figure 1. The potassium chloride usually is contaminated by small quantities of potassium sulphate which are thereby lost from the system, since it cannot be separated by simple crystallization techniques. Further purification of the potassium chloride may be effected by precipitation of the sulphate as barium sulphate.
In place of two separate evaporators 100 and 106, a single evaporator may be used wherein two evaporation steps are effected respectively to crystallize potassium sulphate and potassium chloride.
The white liquor may be boiled over a wide range of temperatures to effect the crystallization in the evapora-tors 100 and 106. Generally, temperatures of about 40to about 120C are used in each of the evaporators, preferably about 40to about 70C. ~
; The embodiment shown in Figure 3 is directed to removal of potassium chloride from green liquor in place Of the white liquor evaporative procedures of Figures 1 and ; 2. In this embodiment, the green liquor in line 58 is .
concentrated by boiling in a green liquor evaporator 110 to precipitate a mixture of potassium sulphate and potassium ;~ chloride, which is removed from the evaporator 110 by line 112.
The precipitation of potassium sulphate and - potassium chloride from the green liquor wherein the pre-:, dominant dissolved salt is potassium carbonate without contamination by potassium carbonate is markedly different :;.
.. . .
.~
~396~
from the sm21t manipulation procedures used in sodium-based mills, as outlined, for example, in U.S. Patents Nos.
3,986,923, 3,909,344 and 3,954,552, wherein sodium carbonate ; must first be removed before recovery of sodium chloride 5 can be effected.
There is a limiting condition on the operability of this embodiment below which potassium carbonate contamina-tion occurs, such precipitation occurring when the chloride to carbonate molar ratio falls below about 0.3 in the green 10 liquor. However, the ratio generally is well in excess of this value and hence carbonate contamination is unlikely to occur.
The precipitation of the mixture of potassium salts may be effected over a wide range of temperatures, 15 the lower limit being dictated by potassium carbonate solu-bility and the upper limit being dictated by the atmos-pheric pressure boiling point of the green liquor, although superatmospheric pressure may be utilized, if desired, to achieve higher boiling temperatures. Usually, the 20 evaporation of the green liquor is effected by boiling at a temperature of about 40 to about l20C, preferably about 4Qo to about 70 C
The concentrated green liquor resulting from the eVaporation step is removed from the evaporator 110 by line .
114, is diluted to the required concentration by the El - filtrate in line 76 and forwarded to the recausticizer 60 by line 116. The white liquor resulting from the recaustici-zer is recycled by line 68 to the pulping liquor feed in line 14.
~ .
.: .
.
.;. .
... ~ .. .. .. . . .
.. , ~ , . : ,, -- : ,:: : , -6~
19 :, The mixture of salts in line 112 contains a variable proportion of potassium sulphate which depends on the efficiency of the furnace, as discussed above in connection with the mixture in line 72 of Figure 1.
5 Separable potassium sulphate may be removed from the -mixture in analogous manner to that described above with respect to Figure 1 by hot leaching the mixture in a leacher 118 with aqueous medium fed by line 120 to dissolve all the potassium chloride values and some of the potassium sulphate to leave substantially pure potassium sulphate for -forwarding by line 122 to the furnace 50.
The hot leach liquor is forwarded by line 124 to a cooler 126 wherein the leach liquor is cooled to precipitate potassium chloride for recovery by line 128. As in the 15 case of the Figure 1 embodiment, the solid potassium -- ,~
chloride recovered is contaminated with small quantities of potassium sulphate. The mother liquor from the potassium chloride crystallization is recycled, after heating, to the leacher 118 by line 120.
The leaching and cooling steps effected in leacher 118 and cooler 126 respectively may be carried out under the same temperature conditions as recited above for the leacher 82 and cooler 90 in the embodiment of Figure 1, and reference may be had thereto for the temperature conditions.
~` 25 Figure 4 illustrates a modification of the embodi-ment of Figure 3 wherein two separate e~ap~rative steps ` are effected for separation of potassium sulphate and potassium chloride. The procedure is analogous to the white liquor process described above with respect to Figure 2.
' ''' .
Green~liquor in line 58 is forwarded to a first green liquor evaporator 130 wherein the green liquor is boiled to deposit potassium sulphate, which is removed by line 132.
The partially concentrated green liquor is forwarded by 5 line 134 to a second green liquor evaporator 136 wherein - .. .
the partially concentrated green liquor is boiled to deposit potassium chloride, which is removed by line 138. The con-centrated green liquor is forwarded by line 114 for further processing as described above with reference to Figure 3.
1~ The concentration of the green liquor in the two evaporators 130 and 136 is effected at the same temperatures as the two evaporators in the two-stage white liquor process of Figure 2 and reference may be had thereto for the res-pective temperatures.
Turning now to the embodiment of Figure 5, there is illustrated therein a potassium chloride removal system wherein potassium sulphate losses with the potassium chloride are substantially eliminated. In place of the smelt being dissolved in an aqueous medium to form green 2a liquor, as is the case in the embodiments of Figures 1 to 4, the smelt in line 52, which first may be solidified, if ,.. .
desired, is subjected to leaching in a leacher 140 by an aqueous medium fed by line 142 to dissolve only potassium sulphide, potassium carbonate and potassium chloride from the smelt and leave the potassium sulphate with the smelt dregs in the solid phase.
The smelt leaching step may be effected over a wide temperature range, generally about 30 to about 110C, , . . .
,,. ~ ~ .
6~ -although the lGwer end of the temperature range is pre~erred to inhibit potassium sulphate solubility. The preferred temperature range is about 30Oto about 70C.
The aqueous solution of potassium sulphide, potassium carbonate and potassium chloride resulting from the smelt leaching is forwarded by line 144 to a re-causticizer 146 wherein the potassium carbonate values are mainly converted to potassium hydro~ide. The white liquor formed in the recausticizer 146 contains potassium hydroxide, potassium sulphide, potassium chloride and potassium carbonate, the latter arising from inefficien-cies in the recausticization process. The white liquor is vlrtually free from potassium sulphate as a result of the initial smelt leaching step.
lS The white liquor is passed by line 148 to a white liquor evaporator 150 wherein the white liquor is concentrated by boiling to precipitate potassium chloride in substantially pure form, the potassium chloride being removed by line 152. The evaporation of the white liquor ~; 20 may be effected over a wide temperature range, generally about 40 to about 120C. It is preferred to operate at the upper end of this range to inhibit crystallization of any potassium sulphate present and the preferred tempera- ;:
ture range is about 70to about 110C.
.~
~; 25 Any potassium sulphate contamination in the . ~ . . .
~ potassium chloride may be removed by precipitation as ~ .
barium sulphate and, as in the other cases, represents a loss of chemicals from the system. Utilization of the smelt leaching procecure, however, minimizes such losses, "'~;' :: "
, . , - : , . - , :. . .:
~ $~
. 22 as compared with the procedures described above with respect : to Figures 1 to 4.
: The solids remaining from the smelt leaching are forwarded by line 154 to a dregs leacher 156 wherein 5 the potassium sulphate is dissolved by wash water fed by line 158 to form a potassium sulphate solution and leave dregs solids for recovery by line 160.
: The potassium sulphate solution in line 162 resulting from the dregs washing is recycled to the furnace, such as, by dilution of the concentrated white liquor in line 164 to form diluted white liquor in line 166 for re-cycle to the digester. Alternatively, the potassium sulphate solution may be forwarded directly to the black liquor evaporators 46.
Figure 6 illustrates a modification of the embodi- -ment of Figure S which also utilizes the smelt leaching operation and the same reference numerals are used to ` denote the same items. In the embodiment of Figure 6, the '! aqueous solution in line 144 containing potassium sulphide, ,;
20 potassium carbonate and potassium chloride is concentrated `,.~! by boiling in an evaporator 170 to precipitate potassium chloride which is removed by line 172.
r'~' The temperature of operation of the evaporator 170 may vary widely, generally about 40 to about120C, 25 preferably towards the upper end of this range to inhibit the tendency for any potassium sulphate present to precipi-~` tate, and in the range of about70 to about llOoc.
. .; .
~. The concentrated liquor in line 174 and formed in ``~ the evaporator 170 may be diluted with potassium sulphate ,',,,~ ' .
,, " . ~-, .,:, ?. ., ,:
23 11~611 solution in line l62, or El filtrate, before passage by line 176 to the recausticizer 146 from which the white liquor in line 148 is removed for recycle to the digester 10. Alternatively, the potassium sulphate solution in 5 line 162 may be forwarded directly to the black liquor -~
evapOrators 46, or to the white liquor in line 148.
The processes for potassium chloride removal described above with reference to Figures 1 to 6 have assumed that sodium does not enter the system, or is present 1~ in such minor quantities that it has no effect on the system. The embodiments of Figures 7 and 8 are directed to processes for the removal of potassium chloride when significant concentrations of sodium salts are present to - the extent that a purge of sodium values is required.
The relative solubility characteristics of sodium -salts in the presence of large concentrations of potassium ; salts,i.e.at high K/K+Na molar ratios, have been determined.
Assuming that the màjor source of the sodium is the wood which is pulped and this corresponds to about one-quarter to about one-half of the potassium content thereof, then the green liquor and white liquor compositions preferably are as follows:
Green liquor:
. . ,;. .
~ sulphide 0.25 to 0.75 molality i 25 carbonate 1.2 to 1.8 molality -,~
chloride 0.3 to 1.0 molality sulphate 0.01 to 0.1 molality K + Na 3.2 to 6.2 molality K/K+Na 0.50 to 0.75 molar ratio .:', ` 24 ~ 6~
White liquor:
hydroxide 2.2 to 2.8 molality sulphide 0.25 to 0.75 molality carbonate 0.15 to 0.4 molality chloride 0.30 to 1.0 molality sulphate 0.01 to 0.1 molality K + Na 3.2 to 0.2 molality K/K + Na 0.50 to 0.75 molar ratio It is possible to effect recovery of potassium chloride over a wide range of K/K+Na molar ratios but simple procedures involving precipitation of potassium sulphate and potassium chloride without the precipitation of potassium carbonate, while maintaining the desired purge ~` of potassium chloride,can only be effected under limited : ''`
; 15 conditions, the ranges depending on the temperature and _ .~ carbonate concentrations.
Referring now to Figure 7, there is shown therein -~ a two-stage white liquor evaporation process similar to that i.9 ; described above with respect to Figure 2, except that in `l 2Q this instance, sodium carbonate coprecipitates with potassium chloride in the second white liquor evaporator 106. The sodium carbonate present in the potassium chloride removed by line 108 represents a purge of sodium values from the i :.
` system. If desired, the potassium chloride and sodium carbonate may be separated by leaching. The conditions of operation of the two evaporators 100 and 106 are as described above with respect to the embodiment of Figure 2, f`;`` and reference may be had thereto for appropriate conditions.
, .- --.
~ In Figure 8, a three-stage white liquor evaporation ..
, ~ .... .
, . ~
, ~
2s 1~96~1 process is illustrated whereby potassium chloride and sodium carbonate are independently separated from the white l~quor.
In this process, the second stage evaporator 106 is opera-ted to deposit potassium chloride, with a small degree of 5 contamination by potassium sulphate.
The concentrated white liquor from the second stage white liquor evaporator 106 is passed by line 180 to a third stage white liquor evaporator 182 wherein the white liquor is evapcrated to deposit sodîum carbonate which is removed by line 184. The concentrated white liquor in line 74 is then recycled as described above.
The first stage evaporation is e~fected as described above in connection with Figure 2 and reference may be had thereto for details thereof. The second stage . ~, .
- 15 evaporation is effected at a low temperature, generally about 40 to about 60C, in order to favour potassium i .
chloride crystallization. The deposition of potassium chloride is effected until substantial saturation of the i,..;:
white liquor by sodium carbonate is reached.
In the third stage evaporator 182, the white liquor is boiled at a high temperature, generally about ~; 90 to 110C, to cause crystallization of sodium carbonate but to keep the potassium chloride in solution. The sodium ~'!" carbonate remove by line 184 may be contaminated with traces ., .
25 of potassium sulphate.
.~
The amount of sodium carbonate which may be ~ ~ removed by the third stage evaporation in the process of - ~igure 8 exceeds the expected sodium inputs to the mill and hence need be operated only intermittently, if :~ , -: . - - .: , , . , . , , ~ . . ., -- . . .
26 ~S~l desired, with a two-sta~e operation being effected for the remainder of the time.
The processes of Figures 7 to 8 are applicable to white liquor in line 68 having a K/K~Na ratio of about 5 0.6 to about 0.7. Attempts to control sodium levels at higher ratios, for example, about 0.7 to about 0.9, produce a carbonate phase which is KNaCO3, which would represent a loss of potassium from the system when the sodium purge is effected, which may be considered undesirable. However, 10 since the quantities of sodium required to be removed are small, the potassium losses resulting from the removal of KNaCO3 also are small, and hence may be tolerable by some mills.
It is also possible to limit sodium build-up by 15 removal from green liquor but at a typical R/K~Na ratio of about 0.8, the sodium is removed as KNaCO3, which may be considered undesirable, as noted above.
EXAMPLES
Example 1 The solubility characteristics of an aqueous ~; system containing the ionic species K , Na , OH , S , Cl , ~ CO3 , and SO4 were studied at various potassium molar ratios ;; of K/K+Na up to 1.0 (i.e. the case where sodium is absent).
Figures 9 to 11 are graphical representations of ;~1 2S part of this study, illustrating, respectively, the solu-`, bility of chloride at 100C whe~ the system is saturated with CO3 and SO4 , the solubility of carbonate at 100C when the system is saturated with Cl and SO4 and the solu-bility of sulphate at 100C when the system is saturated ~,.~" .
' with Cl and CO3 .
Figures 12 to 14 are also graphical representa-tions of part of this study showing the stable solid phase transitions upon variation of potassium molar ratio and total alkali concentration (S ~ OH ) at 50C, 75C and 100C
respectively.
This solubility data was used to determine mass balances for the various pulp mill procedures outlined above , . __ _ . ............. . .
; in Figures 1 to 8. The determinations are outlined in the ; io following Examples II to IX.
: Example II
:
Utilizing the developed solubility data, a mass balance was calculated for the white liquor evaporation : process of Figure 1, omitting the cooling crystallization : 15 of KCl from leach liquor and mother liquor recycle. In --!
this process, the evaporation temperature in evaporator 70 ~-: was assumed to be 100C while the leach temperature in . leacher 82 was assumed to be 30C.
: The results are reproduced in the following ~: 20 Table I:
,~ ' .' .
,. .
.
. . .
- -28 ~56~1 TABLE I
Stream Water Solution Composition-m~lality Solid Phaæs No. lb/ton lb.l/ton pulp OH 3 4 K Cl K2S04 .
5 68 6000 2.5 0.5 0.65 0.30.05 74 1680 9.01 1.8 1.04 1.08. ~.02 72 2.18 0.27 10 88 450 0 0 4.. 84 0 0.09 .~ 86 0.0 0.23 . Example III
A mass balance was calculated for the procedure of Example II in which the cooling crystallization of KCl from .
15 mother liquor is effected and mother liquor recycle occurs.
~: The tempexature in evaporator 70 was assumed to be 100C, that in leacher 82 was assumea to De 100C and that in-cool~r --~: 90 was assumed to be 30C. -The results are reproduced in the following '~i: : 20 Table II:
;.~`3: .
~' .', ~ , .
' .
-, ~'''' .
.., ", ...
-: : : . .::: .::: : : . . : : ,: : . ::: :,: . ; ,, : -TABLE II
Stream Water Solution ~sition Solid Phases : No. lb/ton - m~lality lb.-mol/ton of pulp . . . . . QH . S Cl CO3 . SO4 KCl K2SO4 68 6000 2.5 0.5 0.65 0.3 0.05 -` 74 1680 9.01 1.80 1.04 1.08 0.02 ,~ 72 2.18 0.27 -~ 84 876 . O O 4.84 O 0.09 - . 10 86 o 0.22 88 876 O O 7.33 O 0.148 -. - 92 . 2.18 0.05::
:, .: . ...... ...
~; ' 2 ~ , Example IV
. ,. . ~
... . A mass balance was calculated for the two-stage :.
; 15 white liquor evaporation process of Figure 2. The tempera- _ :
ture of each white liquor evaporation stage was assumed to be .~' - 50C. The results are reproduced in the following Table III:
I TABLE III
S~m Water Solution Cbmposition Solid Phases.
.~ 20 No.lb/bon - molality lb.-m~Vbon o~ pulp OH 3 4 KCl. K2SO4 .
68 6000 2.50.5 0.65 0.3 0,05 , 102 0.234 1042778 5.4 1.08 1.40 0.65 0.025 :~ 108 2.130.036 742148 6.97 1.39 0.82 0.~4 0.015 '; ' ' ' ' ~ .
- ., , - :: ~ . .
3n 1~}963Ll Example V
A mass balance was calculated for the process of Figure 3, with the exception that the leach liquor cooling and mother liquor recycle were omitted. The green Iiquor evapora-tion temperature was assumed to be 100C while the leachingtemperature was assumed to be 30C. The results are repro-duced in the following Table IV:
TABLE IV
-;
Stream Water Solution ~osition Solid Phases 10 No. lb/ton - mDlality lb.-m~l/ton of pulp OH S Cl Co3 SO4 KCl K2S04 ;:. . . ___ __. _ . _ . . . . __ .. . . .
~j 58 6000 0.1 0.5 0.65 1.5 0.05 ,'~ 111 4440 , 114 1560 0.38 1.90 1.12 5.69 0.029~."~
- 15 112 2.140.25 124 44Z 0 04.84 0 0.09 ;~ .
122 0.00.21 - . .
i~ Example VI
;: .
,. ........................................... .
For the process of Figure 4, a mass balance was calculated with the evaporatlon temperatures in the two evaporators being assumed to be 50C. The results are reproduced in the following Table V:
, ~ .
.~ ~ .
..... .
TABLE V
Stream Water Solution ~x~osition Solid Phases No. lb/ton - molality Ib.-mol/ton of pulp OH S Cl Co3 so4 KCl K2S04 - - - ,~
58 6000 0.11 0.53 0.68 1.58 0.053 132 0.20 1342778 0.22 1.08 1.40 3.24 0.037 138 2.15 0.06 114 2110 0.2g 1.47 0.86 4.4 0.021 . .
Exam~le VII
:A mass balance was calculated for the embodiment of Figure 5. A temperature of 50C was assumed for the smelt leaching, a temperature of 100C was assumed for the _ dregs leaching and a temperature of 50 C was assumed for ~", the white liquor evapoxation. The results are reproduced in the following Table VI:
TABLE VI
Stream Water Solution C~osition Solid Phases No. lb/ton - m~lality lb.-m~l/ton of pulp OH S o3 SO4 KClK2S04 , 1442640 0.23 1.141.48 3.41 0.041 154 0.19 162 140 0 0 0 0 1.36 1482640 5~68 1.141.48 0.68 0.041 1642148 6.97 1.390.82 0.84 0.015 152 2.130.08 .
.. ~ , 32 ~ 6~
Example VII r A mass balance for the embodiment of Figure 6 . was calculated, with the smelt leach temperature being . .:
assumed at 50C, the dregs leach temperature at 100C and the green liquor evaporation temperature at 100C. The results are reproduced in the following Table VII:
TABLE VII
S~tre~m H O Solution C~osition Solid Phases !~r~ Nb. lb~ton - m~lality i?~'~ I0 OH S Cl CD3 SO4 KCl K2S04 , . - - .
-. 142 2640 ,, .
1442640 0.23 1.14 1.48 3.41 0.041 154 0.19 15 162 140 j 1742154 0.38 1.90 1.10 5.69 0.029 . :-.
~,~ 172 . 2.15 0.062 -:
, : ~ Example IX
Based on the experimentally-attained solubility data, a mass balance was calculated for the potassium chloride ~ and sodium carbonate removal process of Figure 8, with the : first two white liquor evaporations being assumed to be effected at 50C and the third white liquor evaporation .25 being assumed to be effected at 100C. The results are reproduced.in the following Table VIII:
~ ~ .
~ .
.
:, , , -~
: - . . ~ . : ,: ,.. ,:. , .: . :-- . , :: , .: :. :::. . , -. ~ . : . ,. :
TABLE VIII
Stream H20 Solution G~sit~on Solid Phases No. lb/ton - molality lb,mol/ton of pulp r OH S Cl Co3 so4 KCl Na2003 K2S04 68 6000 2.5 0.5 0.65 0.30 0.005 0.7 102 0.24 104~676 5.60 1.12 1.46 0.67 0.022 0.698 108 2.10 0.03 1802100 7.15 1.43 0.85 0.86 0.015 0.68 ..... .
~81 438 184 0.01 ,. . .
74 1662 9.01 1.80 1.07 0.63 0.018 0.72 0.76 15 *R is the molar ratio K/X+Na The quantity of sodium carbonate removed by this procedure represents the maximum sodium recovery under the assumed operating conditions and greatly exceeds the expected sodium input from wood sources, indicating that the third stage evaporation need be operated only inter-mittently.
SUMMARY
The present invention, therefore, provides a potassium based pulp mill procedure which is able to achieve potassium chloride recovery and further is able to prevent the build up of sodium values within the system. Modifica-tions are possible within the scope of the invention.
.
.. . .: : .: : ,
Claims (27)
1. In a method for pulping cellulosic fibrous material comprising a pulping and regeneration cycle which includes the steps of: (a) contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide, as a pulping chemical; (b) separating spent pulping liquor from the pulp; (c) furnacing said spent pulping liquor to provide a smelt containing potassium carbonate; (d) dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced to the cycle as a contaminant or by-product; (e) causticizing said smelt to form a white liquor which contains potassium hydroxide and potassium chloride; and (f) using at least part of the white liquor to provide at least part of the pulping liquor, the improvement which comprises evaporating a potassium chloride-containing solution formed in said pulping and regeneration cycle following the furnacing step to deposit potassium chloride therefrom and separating the deposited potassium chloride from the concentrated solution.
2. In a method for pulping cellulosic fibrous material comprising a pulping and regeneration cycle which includes the steps of: (a) contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical; (b) separating spent pulping liquor from the pulp: (c) furnacing said spent pulping liquor to provide a smelt containing potassium carbonate and potassium sulphate; (d) dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced to the cycle, (e) causti-cizing said smelt solution to form a white liquor which contains potassium hydroxide, potassium sulphate and potassium chloride; and (f) using at least part of the white liquor to provide at least part of the pulping liquor, the improvement which comprises:
evaporating said white liquor by boiling at a temperature of about 40° to about 120°C to deposit therefrom a mixture of potassium chloride and potassium sulphate, separating said deposited mixture from the evaporated white liquor, hot leaching said separated mixture at a tempera-ture of about 70° to about 120°C to dissolve substantially all the potassium chloride therefrom and leave the potassium sulphate, forwarding at least part of said potassium sulphate to said furnacing step, and cooling the hot leach liquor to a temperature of about 20° to about 50°C to deposite potassium chloride therefrom.
evaporating said white liquor by boiling at a temperature of about 40° to about 120°C to deposit therefrom a mixture of potassium chloride and potassium sulphate, separating said deposited mixture from the evaporated white liquor, hot leaching said separated mixture at a tempera-ture of about 70° to about 120°C to dissolve substantially all the potassium chloride therefrom and leave the potassium sulphate, forwarding at least part of said potassium sulphate to said furnacing step, and cooling the hot leach liquor to a temperature of about 20° to about 50°C to deposite potassium chloride therefrom.
3. The process of claim 2 wherein said white liquor is evaporated at a temperature of about 40° to about 70°C, said hot leaching is effected at a temperature of about 80° to about 100°C, and said cooling is effected to a temperature of about 20° to about 40°C.
4. In a method for pulping cellulosic fibrous material comprising a pulping and regeneration cycle which includes the steps of: (a) contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical; (b) separating spent pulping liquor from the pulp; (c) furnacing said spent pulping liquor to provide a smelt containing potassium carbonate and potassium sulphate; (d) dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced to the cycle, (e) causti-cizing said smelt solution to form a white liquor which contains potassium hydroxide, potassium sulphate and potassium chloride; and (f) using at least part of the white liquor to provide at least part of the pulping liquor, the improvement which comprises:
evaporating said white liquor by boiling at a temperature of about 40° to about 120°C to deposit potassium sulphate, therefrom separating said deposited potassium sulphate from the evaporated white liquor, forwarding at least part of said potassium sulphate to said furnacing step, further evaporating said evaporated white liquor by boiling at a temperature of about 40° to about 120°C
to deposit potassium chloride therefrom, and separating said deposited potassium chloride from the further evaporated white liquor.
evaporating said white liquor by boiling at a temperature of about 40° to about 120°C to deposit potassium sulphate, therefrom separating said deposited potassium sulphate from the evaporated white liquor, forwarding at least part of said potassium sulphate to said furnacing step, further evaporating said evaporated white liquor by boiling at a temperature of about 40° to about 120°C
to deposit potassium chloride therefrom, and separating said deposited potassium chloride from the further evaporated white liquor.
5. The process of claim 4 wherein said evaporation of white liquor and said evaporation of evaporated white liquor are effected at a temperature of about 40° to about 70°C.
6. A method for pulping cellulosic fibrous material, comprising:
contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical, separating spent pulping liquor from the pulp, furnacing said spent pulping liquor to provide a smelt containing potassium carbonate, potassium sulphate and potassium chloride, leaching said smelt at a temperature of about 30° to about 110°C to dissolve soluble components therefrom to form a smelt solution and leave the potassium sulphate substantially undissolved, separating the undissolved potassium sulphate from said smelt solution, forwarding at least part of said separated potassium sulphate to said furnacing step, causticizing said smelt solution to form a white liquor which contains potassium hydroxide and potassium chloride, evaporating said white liquor by boiling at a temperature of about 40° to about 120°C to deposit potassium chloride therefrom, and separating said deposited potassium chloride from said white liquor.
contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical, separating spent pulping liquor from the pulp, furnacing said spent pulping liquor to provide a smelt containing potassium carbonate, potassium sulphate and potassium chloride, leaching said smelt at a temperature of about 30° to about 110°C to dissolve soluble components therefrom to form a smelt solution and leave the potassium sulphate substantially undissolved, separating the undissolved potassium sulphate from said smelt solution, forwarding at least part of said separated potassium sulphate to said furnacing step, causticizing said smelt solution to form a white liquor which contains potassium hydroxide and potassium chloride, evaporating said white liquor by boiling at a temperature of about 40° to about 120°C to deposit potassium chloride therefrom, and separating said deposited potassium chloride from said white liquor.
7. The process of claim 6 wherein said smelt leaching is effected at a temperature of about 30° to about 70°C
and said white liquor evaporation is effected at a temper-ature of about 70° to about 110°C.
and said white liquor evaporation is effected at a temper-ature of about 70° to about 110°C.
8. The method of claim 6, wherein said potassium sulphate is forwarded to said furnacing step by dissolving the same in an aqueous medium, diluting the evaporated white liquor with the resulting solution and recycling the diluted white liquor to said contacting step for use as at least part of said pulping liquor therein.
9. The method of claim 2 or 4 wherein the potassium molar ratio (K/K+Na) of said white liquor is at least about 0.5.
10. The method of claim 2 or 4 wherein the potassium molar ratio (K/K+Na) of said white liquor is at least about 0.6.
11. The method of claim 2 or 4 wherein the potassium molar ratio (K/K+Na) of said white liquor is about 0.9 to 1Ø
12. In a method for pulping cellulosic fibrous material comprising a pulping and regeneration cycle which includes the steps of (a) contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical; (b) separating spent pulping liquor from the pulp; (c) furnacing said spent pulping liquor to provide a smelt containing potassium carbonate and potassium sulphate; (d) dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced to the cycle, (e) causti-cizing said smelt solution to form a white liquor which conains potassium hydroxide, potassium sulphate and potassium chloride; and (f) using at least part of the white liquor to provide at least part of the pulping liquor, wherein sodium ionic species are introduced to the cycle as a contaminant or by-product whereby said white liquor con-tains dissolved sodium ionic species and has a potassium molar ratio (K/K+Na) of about 0.6 to about 0.7, the improve-ment which comprises evaporating said white liquor by boiling at a temperature of about 40° to about 120°C to deposit potassium sulphate therefrom, separating the deposited potassium sulphate from the evaporated white liquor forwarding at least part of said separated potassium sulphate to said furnacing step, further evaporating said evaporated white liquor by boiling at a temperature of about 40° to about 120°C
to deposit therefrom a mixture of potassium chloride and sodium carbonate, and separating the deposited mixture from the result-ing concentrated white liquor.
to deposit therefrom a mixture of potassium chloride and sodium carbonate, and separating the deposited mixture from the result-ing concentrated white liquor.
13. The method of claim 12 wherein said white liquor evaporation and further evaporation are effected at a temperature of about 40° to about 70°C.
14. In a method for pulping cellulosic fibrous material comprising a pulping and regeneration cycle which includes the steps of: (a) contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical; (b) separating spent pulping liquor from the pulp; (c) furnacing said spent pulping liquor to provide a smelt containing potassium carbonate and potassium sulphate; (d) dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced to the cycle, (e) causticizing said smelt solution to form a white liquor which contains potassium hydroxide, potassium sulphate and potassium chloride; and (f) using at least part of the white liquor to provide at least part of the pulping liquor, wherein sodium ionic species are introduced to the cycle as a contaminant or by-product whereby said white liquor con-tains dissolved sodium ionic species and has a potassium molar ratio (K/K+Na) of about 0.6 to about 0.7, the improve-ment which comprises evaporating said white liquor by boiling at a temperature of about 40° to about 120°C to deposit potassium sulphate therefrom, separating said deposited potassium sulphate from the evaporated white liquor, forwarding at least part of said separated potassium sulphate to said furnacing step, further evaporating the evaporated white liquor by boiling at a temperature of about 40° to about 60°C
to deposit potassium chloride therefrom, separating said deposited potassium chloride from the further evaporated white liquor, further evaporating the latter white liquor by boiling at a temperature of about 90° to about 110°C
to deposit sodium carbonate therefrom, and separating the deposited sodium carbonate from the resulting concentrated white liquor.
to deposit potassium chloride therefrom, separating said deposited potassium chloride from the further evaporated white liquor, further evaporating the latter white liquor by boiling at a temperature of about 90° to about 110°C
to deposit sodium carbonate therefrom, and separating the deposited sodium carbonate from the resulting concentrated white liquor.
15. The method of claim 14 wherein said white liquor evaporation is effected at a temperature of about 40°
to about 70°C.
to about 70°C.
16. The method of claim 2 or 4, wherein said pulping liquor also contains potassium sulphide as another pulping chemical and is substantially free from sodium-based pulping chemicals.
17. The method of claim 6, 12 or 14, wherein said pulping liquor also contains potassium sulphide as another pulping chemical and is substantially free from sodium-based pulping chemicals.
18. A method of pulping cellulosic fibrous material, comprising:
contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical, separating spent pulping liquor from the pulp, furnacing said spent pulping-liquor to provide a smelt containing potassium carbonate and potassium sulphate, dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced as a contaminant or by-product, evaporating said smelt solution by boiling at a temperature of about 40° to about 120°C to deposit a mixture of potassium chloride and potassium sulphate, separating said mixture from the evaporated smelt solution, hot leaching said separated mixture at a tem-perature of about 70° to about 120°C to dissolve substan-tially all the potassium chloride therefrom and leave the potassium sulphate, forwarding at least part of said potassium sulphate to said furnacing step, cooling the hot leach liquor to a temperature of about 20° to about 50°C to deposit potassium chloride therefrom, causticizing said evaporated smelt solution to form a white liquor containing potassium hydroxide, and recycling at least part of said white liquor to said contacting step to provide at least part of said pulping liquor therein.
contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical, separating spent pulping liquor from the pulp, furnacing said spent pulping-liquor to provide a smelt containing potassium carbonate and potassium sulphate, dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced as a contaminant or by-product, evaporating said smelt solution by boiling at a temperature of about 40° to about 120°C to deposit a mixture of potassium chloride and potassium sulphate, separating said mixture from the evaporated smelt solution, hot leaching said separated mixture at a tem-perature of about 70° to about 120°C to dissolve substan-tially all the potassium chloride therefrom and leave the potassium sulphate, forwarding at least part of said potassium sulphate to said furnacing step, cooling the hot leach liquor to a temperature of about 20° to about 50°C to deposit potassium chloride therefrom, causticizing said evaporated smelt solution to form a white liquor containing potassium hydroxide, and recycling at least part of said white liquor to said contacting step to provide at least part of said pulping liquor therein.
19. The method of claim 18 wherein said smelt solution is evaporated at a temperature of about 40° to about 70°C, said hot leaching is effected at a temperature of about 80° to about 100°C, and said cooling is effected to a temperature of about 20° to about 40°C.
20. A method of pulping cellulosic fibrous material, comprising:
contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical, separating spent pulping liquor from the pulp, furnacing said spent pulping liquor to provide a smelt containing potassium carbonate and potassium sulphate, dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced as a contaminant or by-product, evaporating said smelt solution by boiling at a temperature of about 40° to about 120°C to deposit potassium sulphate therefrom, separating said deposited potassium sulphate from the evaporated smelt solution, forwarding at least part of said potassium sulphate to said furnacing step, further evaporating said evaporated smelt solution by boiling at a temperature of about 40° to about 120°C
to deposit potassium chloride therefrom, separating said deposited potassium chloride.
from said further evaporated smelt solution, causticizing said further evaporated smelt solution to form a white liquor containing potassium hydroxide, and recycling at least part of said white liquor to said contacting step to provide at least part of said pulping liquor therein.
contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical, separating spent pulping liquor from the pulp, furnacing said spent pulping liquor to provide a smelt containing potassium carbonate and potassium sulphate, dissolving said smelt to form an aqueous smelt solution also containing potassium chloride introduced as a contaminant or by-product, evaporating said smelt solution by boiling at a temperature of about 40° to about 120°C to deposit potassium sulphate therefrom, separating said deposited potassium sulphate from the evaporated smelt solution, forwarding at least part of said potassium sulphate to said furnacing step, further evaporating said evaporated smelt solution by boiling at a temperature of about 40° to about 120°C
to deposit potassium chloride therefrom, separating said deposited potassium chloride.
from said further evaporated smelt solution, causticizing said further evaporated smelt solution to form a white liquor containing potassium hydroxide, and recycling at least part of said white liquor to said contacting step to provide at least part of said pulping liquor therein.
21. The method of claim 20 wherein said smelt solution evaporation and further evaporation are effected at a temperature of about 40° to about 70°C.
22. A method of pulping cellulosic fibrous material, comprising:
contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical, separating spent pulping liquor from the pulp, furnacing said spent pulping liquor to provide a smelt containing potassium carbonate, potassium sulphate and potassium chloride, leaching said smelt at a temperature of about 30° to about 110°C to dissolve soluble components there-from to form a smelt solution and leave the potassium sulphate substantially undissolved, separating the undissolved potassium sulphate from said smelt solution, forwarding at least part of said separated potassium sulphate to said furnacing step, evaporating said smelt solution by boiling at a temperature of about 40° to about 120°C to deposit potassium chloride therefrom, separating said deposited potassium chloride from the evaporated smelt solution, causticizing said evaporated smelt solution to form a white liquor containing potassium hydroxide, and recycling at least part of said white liquor to provide at least part of said pulping liquor.
contacting the cellulosic fibrous material with a pulping liquor containing potassium hydroxide as a pulping chemical, separating spent pulping liquor from the pulp, furnacing said spent pulping liquor to provide a smelt containing potassium carbonate, potassium sulphate and potassium chloride, leaching said smelt at a temperature of about 30° to about 110°C to dissolve soluble components there-from to form a smelt solution and leave the potassium sulphate substantially undissolved, separating the undissolved potassium sulphate from said smelt solution, forwarding at least part of said separated potassium sulphate to said furnacing step, evaporating said smelt solution by boiling at a temperature of about 40° to about 120°C to deposit potassium chloride therefrom, separating said deposited potassium chloride from the evaporated smelt solution, causticizing said evaporated smelt solution to form a white liquor containing potassium hydroxide, and recycling at least part of said white liquor to provide at least part of said pulping liquor.
23. The method of claim 22 wherein said smelt leaching is effected at a temperature of about 30° to about 70°C
and said smelt solution evaporation is effected at a temperature of about 70° to about 110°C.
and said smelt solution evaporation is effected at a temperature of about 70° to about 110°C.
24. The method of claim 22 wherein said potassium sulphate is forwarded to said furnacing step by dissolving the same in an aqueous medium and diluting said evaporated smelt solution prior to causticizing the same.
25. The method of claim 18, 20 or 22 wherein said pulping liquor also contains potassium sulphide as another pulping chemical and is substantially free from sodium-based pulping chemicals.
26. The method of claim 18 or 20 wherein the potassium molar ratio (K/K+Na) of said white liquor is at least about 0.5.
27. The method of claim 18 or 20 wherein the potassium molar ratio (K/K+Na) of said white liquor is at least about 0.9 to 1Ø
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB19234/78 | 1978-05-12 | ||
| GB1923478 | 1978-05-12 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1109611A true CA1109611A (en) | 1981-09-29 |
Family
ID=10125951
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA324,477A Expired CA1109611A (en) | 1978-05-12 | 1979-03-29 | Potassium-based pulp mill process |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1109611A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5746886A (en) * | 1993-01-25 | 1998-05-05 | Kvaerner Pulping Ab | Method for the recovery of energy and chemicals from cellulose spent liquor containing potassium using a gasifier |
-
1979
- 1979-03-29 CA CA324,477A patent/CA1109611A/en not_active Expired
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5746886A (en) * | 1993-01-25 | 1998-05-05 | Kvaerner Pulping Ab | Method for the recovery of energy and chemicals from cellulose spent liquor containing potassium using a gasifier |
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