CA1096113A - Fluidized bed recovery of sodium carbonate from spent cooking liquor - Google Patents

Abstract

Abstract of the Disclosure This invention relates to the recovery of chemicals from the pulping of cellulosic material and more particularly to improved efficiency in the operation of fluidized bed recovery processes for recovering sodium carbonate from pulp mills by leaching the sodium carbonate recovered from the fluidized bed with an aqueous leachant to reduce the alkali metal chlorides content.

Description

In pulping operations, raw cellulosic fibrous ~aterials, such as wood chips are digested in a cooking liquor containing sodium carbonate together with other adjunct chemicals such as sodium hydroxide, sodium sulfite, and sodium sulfides as may be required for the particular process. Sodium carbonate is commonly used in the preparation oS cooking liquors for most common wood pulping processes such as Kraft, alkaline sulfite, neutral sulfite semi-chemical (NSSC~ and other processes as descri~ed in "Pulping Processes" by Sven A. Rydholm, Interscience Publishers (1965). More recently the so-called ~non-sulfur pulping" method using sodium carbonate and sodium hydroxide has been developed. This techni~ue is described in U. S. Patent 3,954,553 issued May 4, 1976 entitled "Non-sulfur.Pulping Proces.~ for Gorrugat-ing Medium".

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~L~96~3 The spent cooking liquor from all of these processes contain sodium carbonate, sodium salts of organic reaction products from the decomposition of wood in addition to small proportions of alkali metal chlorides which are introduced 1' with the wood or as impurities in the cooking chemicals.
,' Sodium carbonate is also produced by combustion of the organic I, products in the spent liquor. It is important to the economics ~, Il of all of these processes that the sodium carbonate in the j' spent cooking liquor as well as that produced by combustion , of the spent liquor is recovered and processed to reconstitute j cooking liquor for reuse.
In spent liquor recovery processes the spent liquor is concentrated by evaporation of water and the concentrated liquor is burned to yield sodium carbonate which is contaminated ~ with alkali metal chlorides. In conventional burning processes the spent liquor is burned ina furnace at a temperature of 1600F to 2000F to yield a molten smelt of sodium carbonate. The smelt also contains residual quantities of the fusion products of alkali metal chlorides and an~ other inorganic chemicals that may be present. The smelt is dissolved in water together with make up chemicals and processed to reconstitute cooking liquor ~or reuse. Such conventional burning processes are illustrated in U. S. Patents 3,746,612;
3,833,462; 3,740,308; and 3,740,307.
In these processes, the content of alkalimetal chlorides (e.g. ~aCl and KCl) and other chemicals which lower the fusion temperature of the smelt are of no 1~96~3 consequence because the smelting temperature of 1600F to 2000F is more than adequate to form a molten fusion product.
The chloride salt content has been a problem only from the 1 standpoint that it represents a dead load on the recovery ¦ and recycle process for reasons disclosed in the article 'i entitled "Removal of Sodium Chloride from Kraft Recovery ~ Systems" by W. A. Moy, Peter Joyce and G. E. Styan, Pulp i and Paper Magazine of Canada, Vol. 75, ~o. 4 (1974) pp. 88-90.
1~ More recently it has been proposed to burn the 1 concentrated spent cooking liquor in a fluidized bed reaction ' process to recover the sodium carbonate. In this fluidized bed process the spent cooking liquor is introduced into a ¦ reaction zone containing a bed of granules which is main-~ tained in the "fluidized" state by the upward movement of a gas such as air therethrough. The fluidized bed is maintained at elevated temperatures (e.g. 1200 to 1~00F) by combustion of the organic components of the spent liquor with or without supplemental combustion fuels. Under these conditions the water is vaporized from spent cooking liquor and the organic chemicals are thermally oxidized to form a solid residue of sodium carbonate,together with any sodium chloride or potassium chloride,which deposit on the granules and form agglomerates. In this process the granules can be inert materials such as silica or they can be agglomerates of previously recovered sodium ~96113 carbonate. Such fluidized bed processes are disclosed in U. S. Patent 3,309,26~ and Canadian Patent 800,518 and form no part of the invention as such.
7. These prior art recovery processes using a fluidized I bed combustion reaction have been intolerant of high chloride ¦~ content make up chemicals or process water or materials, because the resulting high sodium chloride levels in the fluid bed product markedly dropped the temperature at which I the product became sticky and rendered the bed inoperable.
~ Thus an object of the present invention is to improve the operation of a ftuidized bed recovery process by reducing the chloride level of recycled sodium carbonate and thus of ' the bed product, so as to increase the fusion point of ~ the agglomerates in the fluidized bed and thereby to , permit operation at higher temperatures than would otherwise be possible.
In these fluidized bed processes the presence of , minor porportion9 (e.g. about 1~ to 1.3~a~ by weight) alkali metal I chloride~ such as sodium chloride or potassium chloride ! in the agglomerates has the effect of forming a low melting eutectic with the sodium carbonate which drastically lowers the temperature at which the fluidized bed can be operated due to incipient fusing or melting of the bed particles.
This detracts from the efficiency of the fluidized bed operation. Furthermore, the proportion of sodium chloride 9~ 3 and potassium chloride increases in the sodium carbonate with each recycle thereby further detracting from the efficiency of the process.
l The present invention provides a method for removing 1 such alkali metal chlorides from the agglomerates thereby facilitating the operation of the fluidized recovery process ~ at higher temperatures.
¦l A primary feature of the present invention i5 in ~~ providing a leaching method for removing relatively more ~ sodium and potassium chloride salts than sodium carbonate from the agglomerates from the fluidized bed in the extract stream as compared to the b~lk of the bed, thus depleting the recycled sodium carbonate of chlorides and thereby raising ¦~ the incipient fusion point of the fluidized bed upon sub-i` sequent recycle. As a secondary feature, this process also j; concentrates potassium in the extract stream, which therefore increases the potential value of this stream as a by-produc~.
It is appreciated that leaching processes for various i chlorides and carbonate salts such as in U. S. Patent ¦; 3,079,227 and 3,833,462 have been proposed in the past although such processes have not been applied to the fluidized bed reaction process of the present invention.
Other objects 9 features and advantages will be apparent from the following description and drawing which is a schematic process flow diagram for practicing the present invention~

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: In attaining the advantages of the present invention one feature resides in the process for recovering sodium carbonate from concentrated spent aqueous, wood pulp 1 cooking liquor containing sodium carbonate together with ¦, minor proportions of alkali chlorides and organic digestion ¦~ products wherein the recovered sodium carbonate is dissolved in an aqueous solution together with make up chemicals to reconstitute cooking liquors which are in turn reused 1 and recycled, the improvement comprising the steps of, l introducing said spent liquor into a fluidized bed reaction zone, maintaining said spent liquor in said fluid bed reaction zone at a temperature and for a time sufficient to vaporize : i water, thermally decompose the organic digestion products 15 i and form agglomerates comprising sodium carbonate to-gether with alkali metal chlorides, and then ` leaching said agglomerates with a proportion of an aqueous leachant and at a temperature and for a time sufficient to reduce said alkali chloride content of said agglomerates to maintain the incipient fusion point temperature of said fluidized bed above at least about 1200F.
In recovering sodium carbonate from spent liquor by the fluidized bed processes described above, the fluidized bed reactions are normally carried out at a temperature of about 1300F. At temperatures much above 1300-1350F, ~096~13 S-13569 particularly ~hen sulfur compounds are present, these fluidized beds become sticky and begin to fuse or melt into large masses which terminate fluidization. The temperature at which this fusion begins is called the incipient fusion ~ point. The maximum operable temperature limit is further influenced by the presence of potassiumcarbonate; the potassium ' being deriv~d from the wood. The eutectic fusion point tempera-ture for a mixture o~ sodium and potassium carbonates is 1, approximately 1300~F so this is the limiting fluidized bed ll operating temperature in the absence o~ the alkali chloride salts.
l' It has been observed that the fluidized bed reaction be-f~ comes very slow at temperatures below approximately 1150F
' so that it is desired to operate such fluidized beds above 1 approximately 1200F. The eutectic fusion point for mixtures ' of sodium chloride and either sodium carbonate or sodium ; sulfate is more than 100F below the eutectic for sodium and potassium carbonate. This is the reason it becomes practically impossibl~ to operate such fluidized beds when the sodium chloride content is greater than approximately 1~.
Referring n~w to the drawing, wood chips are charged to a , digester where they are mixed with reconstituted cooking liquor containing sodium carbonate together with sodium hydroxide and/or~
sulfide and sulfites as may be required by the particular cook and digested at elevated temperatures ~e.g. 350 to 380F) at elevated steam pressures (e.g. 120 to 185 psig) to yield wood pulp and spent cooking liquor containing spent cooking chemical and organic by-products by the digestion reaction. The spent liquor typically contains 75~ to 14~ of dissolved solids com~rising inorganic chemical and organic wood digestion products.

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~ej96113 The pulp and spent liquor are transferred from the digester to a pulp washing and separation station where the pulp is filtered from the spent liquor and washed for further processing~ The spent li~uor flows to a concentrator (e.g.
1l a multiple effect evaporator) where it is concentrated by evaporation to a solids content in the range of about 35-50~ i b~ weight.
il From the concentrator the spent liquor concentrate flows lll to an aerated fluidized bed reaction process as in U. S. Patent i~ 3,309,262 or Canadian Patent 800,518 operating at a temperature ¦ of about 1200-1300F where the water is vaporized, the organic ¦ products of digestion are thermally decomposed or burned, I~ and the sodium carbonate and sodium and potassium chlorides - , are deposited as agglomerates.
The agglomerates then pass into the leaching process which is enclosed by the dashed lines on the drawing. This step represents the departure of the present process from - the prior art. ~n this leaching process the agglomerates are leached with an aqueous leachant such as water at a temperature and for a time sufficient to remove sufficient sodium and potassium chlorides from the agglomerates so that when the agglomerates are subsequently dissolved in water and reconstituted as cooking liquor, the incipient fusion point of the subsequent fluidized bed reaction product is at least 1200F and preferrably about 1250F. To achieve this incipient fusion point temperature, it is usually required that the sodium carbonate agglomerates before leaching contain no more total chloride content than about 0.7~0-0.9~ by :
1~39Gli3 weight of the agglomerates (when reported as sodium chloride).
This chloride content can usually be attained by leaching the ~i agglomerates until the total chloride content (reported a~ !
¦~ sodium chloride) is no more than about 0.5~ and preferrably no ~1 more than about D~3~o by weight of the agglomerates before reuse. ¦
¦~, Modification of the leaching step can he made by one skilled ,~ in the art to adapt the present invention's particular ~, process under consideration with mimimum experimentation.
! It is understood that it is necessary to consider the 1I total amounts of chlorides that are being introduced into the ¦~ process from external sources in determining the level to which the agglomerates must be leached to provide the fluidized ¦¦ bed ~usion temperature of about 1200F or higher. This can be ¦l accomplished by a simple material balance calculation for the '~ particular process at hand. The amount of chloride being introduced from external sources (e.g. salt water or make up `, , chemicals) is subjec-t to analysi3 or calculation.
In the usual practice of the present invention the l~' aqueous leachant is water although in some instances the leachant , contains dissolved salts such as sodium carbonate to minimize 1, the dissolution of the sodium carbonate in the leaching process.
i, The proportion of leachant per unit of weight of ii agglomerates is not particularly critical although sufficient , ~ leachant should be used to remove chloride salts to the level ;~ indicated above. The ratio of leachant to agglomerates should be as low as possible so as to achieve the leaching of the chloride " ~

1~96113 salts with a minimu~ loss of the sodium carbonate in the extract, In that the agglomerates usually contain about 99~ by weight or more of sodium carbonate and other active cooking 'i , chemicals and less than about 1~ by weight of chloride salts, it ¦
,, is possible to remove high proportions of chloride salt3 as -compared to the proportions of sodium carbonate by merely allcw-ing the leachant to approach saturation with respect to both sodium car~onate and the chloride salts. In a batch process the proportion of aqueous leachant to agglomerates in the range of '' '~ about 0,1 to 1,0 pound per pound of agglomerate has been 'i~ observed to be satisfactory. Much of this water can be retai~ed , as water of crystallization or occlusion, The proportion jl of saturated extract has been experimentally observed to be in ',, the range of 0.1 to 0.3 pound per pound of agglomerate, lS ~ These proportions are illustrative of practical conditions , and higher or lower proportions can be used if desired. It is , understood that higher proportions of leachant may result in ' greater losses of sodium carbonate due to solubility. Lower !. proportions may have a lower leaching efficiency due to ,I saturation with chloride salts. The proportions can also be different in counter-current and multi-stage extractions, The leaching process can be accomplished in a single- or multi-stage mixing chamber where liquid/solid leaching is carrie~' out in batch, counter-current or co-current fashion, However, ~ it is important to remove the extract 5 containing the sodium chloride from the residual agglomerate. This is most effectively . ~

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~1~96~13 s-l356g accomplished by displacement in a counter-current extraction process. The residual extract at any stage may also be reduced by forming a filter cake or by centrifuging The !' leaching can be conducted in as many stages as in practical 1 to achieve the desired result. It is understood, of course, ll~, that the selection of these process conditions is based upon the economics of leaching efficiency and solubility losses of Ii sodium carbonate. Under ordinary conditions the leachant will ¦ reach saturation with sodium car~onate and approach saturation 1' with respect to chloride salts.
Stationary solid bed leaching e~uipment and moving bed ~- leaching equipment as described in Chapter 13 of Unit Operations i' of Chemical Engineering by Warren L. McCabe and Julian C. Smith, 1 McGraw-Hill Book Company (1956) can be employed Leaching processes and equipment of this type are also disclosed in Section 11 of Chemical Engineers' Handbook edited by J. H. Perry, McGraw-Hill Book Company (1950). The extract can be further separated from the residual agglom^rates by centrifuging.
~' After leaching the leached agglomerates are dissolved in ' water together with appropriate make up chemicals to reconstitute' the cooking liquor which is then recycled for reuse as is conventional in the art.
In this process only the leaching process step enclosed within ~ the dashed lines in the drawing represents a departure from the ' ~`
prior art. All of the other steps are conventional and can be practiced as in the prior art.
In the Examples that follow all parts are parts by weight, all percentages are weight percentages, and all temperatures are in F unless stated other~ise.

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l S-13569 f EXample 1 A sample of agglomerates from a neutral sulfite semi-chemical pulping process using a fluidized bed reaction as in 1 U. S. Patent 3,309,262 is processed according to the present 1, invention The sample, consisting of small spherical particles l approximately 1/2 to 1 mm in diameter, is chemically analyzed to contain about 1.8% of water insolubles (presumably Ca, Mg, or Il Si compounds) and less than 0.1~ drying loss at 230F The 10 ~~ composition of the water soluble portion is determined to be:
I' Component~ by Weiqht ~' ~a2C03 34.2 i K2C 3 4.4 Na2S0~ 60.7 1! Chlorides , (as ~aCl)0.7 Ii 100.O
The assignment of all the `'chloride" to the "sodium" is for convenience in reporting.
!l A leaching experiment is performed to determine the jl distribution of these components between the solid and solution phases. Four portions of lO0 grams each of the above fluidized ., I
!~ bed agglomerates are placed in four 100 ml beakers and placed ; ll in an oven maintained at about 230F until the temperature of i the beaker and its contents has reached the oven temperature.
1 Water at a temperature of approximately 212F (i.e. the boiling point) is added in eleven separate 20 to 50 ml stages (400 ml in all in ll stages) to the first beaker, with slurrying and settling followed by transfer of the supernatan-t extract solution to the ~g6~13 next beaker in the series after each stage.
,; This eleven stage leaching process is accomplished ,~ at approximately equally spaced time intervals of 10 to 15 ~I minutes, with the entire process being accomplished over a period ~! of about 2 hours. The contents of each beaker is stirred, ¦l allowed to settle while in the oven, and then decanted to ~ the next beaker in series before addition of the extract ¦ll solution from a prior beaker. The extract solution from ~, the last beaker is collected in a sequential series of l, sample bottles, so as to perform a counter-current leaching process.
~I The transfer of such extract solutions is continued ~ with 5 more 15 minute stages in 1 hour and 15 minutes after ; l~, the last addition of water. In all approximately 165 ml !; of extract solution is collected, corresponding to about 25, 40, 50, and 50 ml samples, from first to last collscted.
Il The balance o~ the water added is apparently retained by '~ the agglomerates or lost by evaporation. Measurements - of the temperature of the slurry in these beakers varies ,~ from about 210F to about 150F during the course of the `` leaching Some of; the contents of the first beaker dis-solve in the process leaving only about 1/3 or 1/4 the original amount.
' The extract solution samples collected from the last beaker as well as the total contents solids and 1~9~ 3 entrained extract are analyzed for chloride (reported as NaCl) on the basis of total soluble solids, with the i following results:
, Extract Solution Samples ~ NaCl by Weiqht ~i First 25 ml 3.76 ~ext 40 ml 2.57 I ~ext 50 ml 1.98 ¦, Last 50 ml 1.58 I Slurry Remaininq in Beakers ~NaCl by Weiqht ~' ~, First Beaker 0.30 Second Beaker 0.50 ~' Third Beaker 0.56 Fourth Beaker 0.64 I, Thus the sodium chloride content of the slurry - 15 , is progressively reduced by the leaching while that of the extract solution progressively increased. In this Example, the chloride content reported as ~aCl content ` ,, of the material in the first beaker is reduced from an initial value of 0.7% by weight to 0.3~ by weight ~aCl 1 which is less than one half the original value. At the same time the ~aCl was concentrated in the extract , .
',~ solution to 3.76% or to more than 5 times the relative concentration in the initial agglomerates.
This Exampla demonstrates the practice of the present invention in a counter-current leaching process for .

~96113 removing sodium chloride in a relatively concentrated , extract solution from fluidized bed agglomerates~ When these agglomerates are recycled and reused in cooking j li~uor as described above, the introduction ofO.5% ~aCl from ex-5ternal sources can be toIerated while still maintaining the in-cipient fusion point of the fluidized bed above about 1200F.
Example 2 Six hundred and eight grams of the fluidized bed agglomerates used in Example 1 are leached in a single 10I stage batch leaching process with 250 ml of water at a ¦' temperature of about 212F (i.e. the boiling ¦
'point) for about 6 hoursl with occasional stirring. A sample of about 50 ml of the extract solution is analyzed as !,follows: 1, ~ ~ by Weiqht of Soluble Solids Component As Reported Corrected to 100 a2C03 49.8 53~1 K2C03 7.7 8.2 I' ~a2S04 32.3 34.4 i NaCl 4.0 4.3 Total "solubles"
~- ' accounted for: 93.8 100.0 When compared with the initial agglomerate composition, i the percentage of potassium has approximately doubled and the chloride content had increased approximately six fold. This Example further indicates that it takes only ~096~13 about 100 grams of dissolved "solids" in the extract to contain all of the chloride in the original 608 gram sample.
; This indicates that the chloride content of the leached ~ agglomerates can approach zero if required by the large 1, amount of chloride introduced into the process.
i~ The agglomerates obtained from this Example are suitable for use in reconstituting cooking liquors and Il recycling and recovery in a fluidized bed reaction process ¦¦ at a temperature of at least about 1200F ~i.e. the incipient li fusion point is above 1200F) when the proportion of additional, a~l introduced into the process is lesi than about 0.7% by weight of the agglomerates.
! Furthermore, it is apparent that a counter-current ~l leaching process u9ing a lower ratio of leachant to ~i agglomerates will further increase the relative proportion ¦' f the ~aCl to Na 2C 3 in the extract solution, ', Example 3 ' j, This Example illustrates a fluidized bed recovery ,' in conjunction with a non-sulfur ~aOH/Na2CO 3 pulping process !~ as de~cribed in U.S. Patent 3,~54,553 using 15 mole ~ j ~ li ~aOH and 85 mole ~ ~a2C03 as cooking chemicals, having '~ " a loss in washing and recovery of 15~ of the total cooking chemical for each cycle, made up by the lS mole ' addition of ~aOH in the reconstitution of the cooking liquor, In this process the make up ~aOH would introduce sufficient chloride to provide an equilibriu~ concentration ~ -- ;

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1g~96~'13 of 1.5% NaCl in the agglomerates of Na 2CO 3 from the fluidized bed. This would correspond to a make up caustic containing j, approximately 2% NaCl on a weight basis. Without the practice 1' of the present invention the fluidized bed would hav~ an 1, incipient fusion point below about 1200F.
, When the Na~C03 agglomerates which would other,~ise contain ¦ 1,5% ~aCl are leached with water as in Example 1 in the process to reduce its sodium chloride content to 0.3% by ¦~ weight, on the subsequent cycle through the fluidized bed , reactio~, the bed product would then-contain approximately Il 0.480% ~aCl:
! 0.15 x 1~5 = 0.22S
' 1~ 0.85 x 0.3 = 0,255 '', 1.00 x 0.48~ = 0.480 ,, This chloride level is low enough so that the fluidized ~, bed has an incipient fusion point of about 1300F.
Upon subsequence recycles the ~a2C03 agglomerates ' are leached to reduce the sodium chloride level to 0.3%
~, before reconstitution of the cooking li,quor. In the ~ course of this leaching process 0;180% by weight of NaCl i' would be removed in the extract solution.
Additionally when the ratio of sodium carbonate plus potassium carbonate to sodium chloride is 57~5 to 4.0 as in Example 2 then this extract solution ~ill also contain ~096113 approximately 0.180 x 57~5/4.0 = 2.59% of the total . initial agglomerates as sodium or potassium carbonate, This would correspond to an additional loss of less than , approximately 3% of the total chemical which could be made , up either as sodium carbonate or sodium hydroxide.
!~ When the ratio o~ approximately 1 part of carbonate ¦,to 1 part of chloride is achieved, the additional loss ¦iof chemical would be reduced to an essentially negligible ¦~level of approximately 0.2% of the total chemical used.
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Claims (7)

HAVING THUS DESCRIBED THE INVENTION, WHAT IS CLAIMED IS:

1. In the process for recovering sodium carbonate from concentrated spent aqueous, wood pulp cooking liquors containing sodium carbonate together with minor proportions of alkali chloride and organic digestion products wherein the recovered sodium carbonate is dissolved in an aqueous solution together with make up chemicals to reconstitute cooking liquors which are in turn reused and recycled, the improvement comprising the steps of, introducing said liquor into a fluidized bed reaction zone, maintaining said liquor in said fluid bed reaction zone at a temperature and for a time sufficient to thermally decompose the organic digestion products and form agglomerates comprising sodium carbonate together with alkali metal chlorides, and leaching said agglomerates with a proportion of an aqueous leachant and at a temperature and for a time sufficient to reduce said alkali chloride content of said agglomerates to maintain the incipient fusion point temperature of said fluidized bed at at least about 1200°F.

2. The process of claim 1 wherein said agglomerates are leach-ed until the total chloride content is no more than about 0.5%
by weight when reported as sodium chloride.

3. The process of claim 1 wherein said agglomerates are leached until the total chloride content is no more than about 0.3% by weight when reported as sodium chloride.

4. The process of claim 1 wherein said agglomerates are leached to maintain the incipient fusion point temperature of said fluidized bed at a temperature at least about 1250°F.

5. The process of claim 1 wherein the temperature of said leaching is in the range of 70°F to the boiling point of said leachant.

6. The process of claim 5 wherein said temperature is in the range of about 100°F to about 200°F.

7. The process of claim 1 wherein the proportion of leachant to agglomerates is in the range of 0.1 to 1.0 pound of water per pound of agglomerate.