CA2101005A1 - Green liquor crystallization - Google Patents
Green liquor crystallizationInfo
- Publication number
- CA2101005A1 CA2101005A1 CA002101005A CA2101005A CA2101005A1 CA 2101005 A1 CA2101005 A1 CA 2101005A1 CA 002101005 A CA002101005 A CA 002101005A CA 2101005 A CA2101005 A CA 2101005A CA 2101005 A1 CA2101005 A1 CA 2101005A1
- Authority
- CA
- Canada
- Prior art keywords
- liquor
- stream
- produce
- sulfur
- recited
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Abandoned
Links
Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/0042—Fractionating or concentration of spent liquors by special methods
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/0064—Aspects concerning the production and the treatment of green and white liquors, e.g. causticizing green liquor
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C11/00—Regeneration of pulp liquors or effluent waste waters
- D21C11/04—Regeneration of pulp liquors or effluent waste waters of alkali lye
Landscapes
- Paper (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
During the production of cellulose pulp, the concentration and temperature of the green liquor in the chemical recovery system, are controlled so as to produce pure monohydrate or pure decahydrate sodium carbonate crystals. The crystals are removed from the green liquor by a centrifuging or screening operation to produce a first stream of green liquor having high sulfidity. The sodium carbonate crystals can be causticized to produce a treatment liquor containing essentially sulfur-free sodium hydroxide.
The green liquor may be causticized, and may be further treated with sulfur compounds obtained by the heat treatment of black liquor, so as to increase its sulfidity to over 50%. The sodium hydroxide can either be used in later stages of pulping, in the bleach plant, or for scrubbing the sulfur containing gases from heat treatment of black liquor so as to produce sodium hydrosulfide. In general, a feasible way of producing liquor streams having vastly different sulfidity in a cellulose pulping process and chemical recovery system is provided.
During the production of cellulose pulp, the concentration and temperature of the green liquor in the chemical recovery system, are controlled so as to produce pure monohydrate or pure decahydrate sodium carbonate crystals. The crystals are removed from the green liquor by a centrifuging or screening operation to produce a first stream of green liquor having high sulfidity. The sodium carbonate crystals can be causticized to produce a treatment liquor containing essentially sulfur-free sodium hydroxide.
The green liquor may be causticized, and may be further treated with sulfur compounds obtained by the heat treatment of black liquor, so as to increase its sulfidity to over 50%. The sodium hydroxide can either be used in later stages of pulping, in the bleach plant, or for scrubbing the sulfur containing gases from heat treatment of black liquor so as to produce sodium hydrosulfide. In general, a feasible way of producing liquor streams having vastly different sulfidity in a cellulose pulping process and chemical recovery system is provided.
Description
2 ~
_REEN LIOUOR CRYSTALLI ZATI ON
BACKGROUND AND SUMM~R~ OF THE INVF.NTION
As descrlbed in the parent application, it is highly desirable -n procedures for producing cellulose pulp to produce two or more liquor streams having vastly different sulfidity during chemical recovery. According to the present invention, a method is provided which allows effective and versatile production of such streams, including the production a stream that may be controlled so that it is essentially sulfur free.
Part of the process according to the present invention involves the treatment of "black liquor".
The term "black liquor" in the present specification and claims is understood in a broad sense as including liquor with dissolved lignin from the cooking of cellulose to produce cellulose pulp, typically in a sulfate process, a sulfide process, or another process containing sulfur as part of the treatment chemical for the pulp. The liquor treating the pulp i8 referred to as "white liquor".
In the present specification and claims, the term "white li~uor" unless otherwise specified, is intended to be broad and cover all liquors typically used in a pulp treatment process in which some -, - , ' ~ ' "' ' 2 ~
sulfur is utilized. The white liquor may be for the sulfate or sulfide process, and at various points during the cook or treatment may be an essentially pure stream of NaOH (that is substantially free of sulfur, although at some point in the cook a white liquor containing sulfur will be utilized).
In the practice of most aspects of the present invention, the black liquor is subjected to heat treatment. It is known that the lignin in black liquor is split by heat treatment under pressure for a predetermined time, whereby gases containing organic sulfur compounds (typically dimethyl sulfide and methyl mercaptan) are generated, as shcw.l in U.S. patent 4,929,307 (the disclosure of which is hereby incorporated by reference herein). The heat treatment may be carried out between the digester and recovery boiler of a typical pulp mill, but it is not limited within that area. Typically the heat treatment is carried out at a temperature of about 170 350C. As much as 30 to 70% of the sulfur in the black liquor may be removed during heat treatment. One purpos0 of the heat treatment i 6 to decrease the sulfur content of the black liquor entering a recovery boiler or like recovery process in order to minimize corrosion problems. Also, the viscosity of the black liquor is reduced by this heat treatment. The sulfur containing gases that are produced from the heat treatment are used advantageously according to the invention.
In one of the most basic aspects of the present invention, in the recovery system in which the black liquor is burned to produce a smelt, and then the smelt is dissolved to produce green liquor, the ' : : :: ~,: . : , ::, . . ,, :, : , 2 ~ 0 ~
concentration and temperature are controlled in such a way as to cause the sodium carbonate within the green liquor to crystallize into either monohydrate or decahydrate crystals. The crystals are then removed from the green liquor, the resulting green liquor having high sulfidity. The removed sodium carbonate crystals can be used to produce a li~uor containing substantially sulfur free sodium hydroxide, which can be used in a wide variety of different manners within the pulp mill.
According to one aspect o~ the present inventi~ , a method of recovering and utilizing chemicals during the production of cellulose pulp using chemicals containing sulfur is provided. The method comprises the following steps: (a) Heat treating black liquor which results from the production of cellulose pulp, to drive off as gases some of the sulfur containing compounds therefrom.
(b) Burning the heat treated black liquor to produce a smelt. (c) Dissolving the smelt with liquid to produce green liquor. (d) Adjusting the concentration of the ~reen liquor, and controlling the temperature, to crystallize sodium carbonate into crystals. (e) Removing the sodium carbonate crystals from the green liguor, to produce a first stream of green liguor having high sulfidity. And, (f) causticizing the sodium carbonate crystals to produce a liquor containing sodium hydroxide.
The first green liquor stream may be causticized to produce a first white li~uor having high sulfidity, and the sulfidity of that white liquor can be increased even more by contacting it :, : .', ' ;' , ; ' -2 1 ~
with the sulfur containing gases produced in step (a) to increase its sulfidity to more than 50%.
Step (e) may be practiced by centrifugal separation, or by a wide variety of filtering techni~ues such as vacuum filtering, disc filtering, or sidehill screening. Step (d) may be practiced to control the temperature so that it is between about 5 20C so as to produce pure decahydrate crystals, or so that it is between about 35-90C, to produce monohydrate crystals. Steps (a)-(f~ in general may be practiced to produce, as the sodium hydroxide containing liquor, a substantially sulfur free sodium hydroxide li~uor.
The stream of sodium hydroxide li~uor can be used in a number of different places in a pulp mill. For example it may be used in a plant for bleaching the cellulose pulp, or it may be added to the latter stages of cooking. Alternatively, it can be used to scrub the gases produced in step (a), which results in a liquor containing sodium hydrosulfide. The sodium hydrosulfide containing liquor may be used as buffer storage for sodium and sulfux, or as a feed stream to a polysulfide generating system.
Alternatively, the gases from step (a) may be used to make sulfuric acid, which is then used in bleaching or acid prehydrolysis, or other uses in a pulp mill where pH adjustment is desired. Also, the white liquor stream, produced by causticizing the green li~uor, can be split in two, with the sulfur containing gases from step (a) contacting only one of the streams, which then is used in an early stage of cooking, while the other, lower sulfidity, white . .
2 ~
liquor stream is used in a later staye of cooking (e.g. in the wash recirculation loop).
According to another aspect of the present invention, a method of producing chemical pulp from cellulose comminuted fibrous material utilizing white liquor, in a pulping process includiny fluids containing sulfur, is provided. This method comprises the following steps: (a) Intentionally removing sulfur from a fluid containing sulfur. (b) Forming a first stream containing sulfur compounds and sodium carbonate. (c) By controlling the solids concentration of the first stream, and the temperature, producing sodium carbonate crystals in the first stream. (d) Separating out the sodium carbonate crystals from the first stream, and then causticizing the first stream to produce white liquor. (e) Including by using removed sulfur from step (a), raising the sulfidity of the first stream. (f) Making at least a second stream of white liquo~ having relatively low sulfur content, by using the separated out sodium carbonate crystals (the first stream having sulfur content substantiall~ higher than the sulfur content of the second stream). (g) Chemical pulping the cellulose material utilizing the first and second streams of white liquor, added at diferent places in the pulping process. And, (h) washing the chemical pulp produced.
According to still another aspect of the presen-t invention, a method of recovering chemicals from black liquor produced during pulping of cellulose with sulfur containing treatment liquids is provided which comprises the following steps:
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(a) Burning black liquor to produce a smelt. (b) Dissolving the smelt with liquid to produce green liquor. ~c) Adjusting the concentration of the green liquor, and controlling its temperature, to crystallize sodium carbonate into either monohydrate or decahydrate crystals. (d) Removing the sodium carbonate crystals from the green liquor, to produce a first stream of green liquor having high sulfidity. And, (e~ causticizing the sodium carbonate crystals to produce a stream of liquid containing substantially sulfur free sodium hydroxide.
It i5 the primary object of the present invention to effectively recover chemicals during the production of cellulose pu:Lp including by causing the green liquor in a recovery loop to produce pure sodium carbonate crystals, and then to ultimately produce liquor streams having vastly different sulfidity. This and other objects will become clear from an inspection of the detailed description of the invention and from the appended claims.
BRIEE DESCRIPTION OF TEE DRAWINGS
FIGURE 1 is a flow schematic illustrating an exemplary method for recovering and utilizing chemicals during the production of cellulose pulp according to the present invention;
FIGURE 2 is a detail view showing the digester and similar components of FIGURE 1 in practicing a modified method according to the present invenkion;
2 ~ 03 FIGURE 3 is a detail view of the heat treatment portions of FIGURE 1 illustrating components used in the practice of another modifled method according to the invention; and FIGURE 4 i5 a view like that of FIGURE 3 showing the practice of still another modification of the method according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
A cellulose pulp producing and chemical recovery exemplary system~according to the present invention is shown generally by re~erence numeral 10 in FIGURE 1. In the conventional digester 11 (which may be a continuous or a batch digester), cooking of cellulose comminuted fibrous material takes place to produce cellulose pulp. The cooking process includes utilization of sulfur containing chemicals, and may comprise a sulfate process, sulfide process, or the like. During cooking, a lignin rich liquid is produced, known as "black liquor", which is either continuously or periodically discharged from the digester 11 in line 12, and is typically subjected to one or more evaporation stages 13, 14 before being fed to a conventional recovery boiler 16. Also, according to the present invention, the black liquor is subjected to heat treatment under pressure for a predetermined period of time in order to reduce the sulfur content of, and the viscosity ;~
of, the black liquor. The heat treatment sta~e, shown at 15 in FIGURE 1, is as basically described in U.S. patent 4,929,307 in which the black liquor "
~, '~ .' '' '' :
is subjected to heat treatment at a temperature of about 170-350C under pressure for a predetermined period of time. By varying the time, temperature, etc., as much as 30 to 70% of the sulfur can be removed from the black liquor before it is fed to the recovery boiler 16.
In the recovery boiler 16, the black liquor is burned, the lignin therein having substantial heat value. The burning of the black liquor produces a smelt which contains the chemicals which are desirably recovered. As is conventional, the smelt is dissolved in wP~r or another liquid in smelt tank 17. According to the invention, the concentration of the liquor produced in the smelt tank 17, known as "green liquor", is controlled, as is the temperature, so as to produce essentially pure sodium carbonate crystals in the green liquor.
As illustrated in FIGURE 1, the green liquor may be clarified in clarifier 18, and the crystals may be formed in a crystallizer l9. The crystallizer 19 can inherently provide temperature control by controlling the degree of evaporation, or othar crystallization function, or temperature control can be provided externally, as indicated schematically at 20 in FIGURE 1, by heating or cooling the green liquor.
When it is desired to produce essentially pure decahydrate sodium carbonate crystals, the concentration and temperature are controlled so that the level of sodium carbonate in the green li~uor is not too high for subsequent effective causticization of the green liquor, and so that the temperature is about 5-20C. When it is desirable to produce . . .
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monohydroate sodium carbonate crystals the temperature is controlled so that it is about 35-90C. The concentration of the green liquor is controlled by the amount of liquid added to the smelt tank 17 to produce the green liquor from the smelt, and/or by evaporation of the green liquor after its production in the smelt tank 17 (e.g. in - the crystallizer 19).
After the production of the sodium carbonate crystals in the green liguor, the crystals are separated. Separation is preerably performed in a centrifugal separator, such as a hydrocyclone, illustrated at 21 in FIGURE 1. The crystals, discharged into line 22 from the bottom of the hydrocyclone 21, are causticized with lime (CaO) in causticizer 23, while the green li~uor is discharged in line 24. Instead of a centrifugal separator 21, various filtration mechanisms may be utilized, such as vacuum drum filters, sidehill screens, and disc filters.
In the causticizer 23, the reactions involving reburned or fresh lime and sodium carbonate are as follows:
CaO 2 Ca(H)2 Na2CO3 ~ Ca(H)2 ---> -2 NaO~ -t CaC03 Preferably the green liquor in line 24 is also causticized in causticizer 25, in a conventional manner at conventional concentration levels, with the lirne mud from both causticizers 23 and 25 being fed to a lime kiln 26. The product stream from the causticizer 23, in line 28, is a sodium hydroxide stream, essentially sulfur-free, while the white liquor in line 30 from causticizer 25 has high ' . ' ~
sulfidity, i.e. a higher sulfidity than in con~entional recovery processes.
The liquors in lines 28, 30 can be used for a wide variety of different purposes. Typically, however, the white liquor in line 30 is used in the digester 11 to cook the cellulose, the relatively high sulfidity white liquor in line 30 being used at early stages of the cook (including perhaps in impregnation). The essentially sulfur free sodium hydroxide in line 28 may either be used as a "white liquor" in the later stages of cook (including the wash recirc~lation line~ in digester 11, as indicated by line 32, or may be used in a bleach plant 33, as indicated by line 34.
The gases produced during the heat treatment 15 are typically fed to the sulfur and fuel gas recovery block 36 in which the fuel gas may be separated into line 37, which may be used as a fuel gas in the lime kiln 26 being introduced into the kiln 26 in line 38. The sulfur containing gases (typically dimethyl sulide and methyl mercaptan) may be treated to produce hydrogen sulfide or the like, and which may be introduced, as shown schematically by line 40 in FIGURE 1, into the white liquor in line 30 (absorbed therein) to increase the sulfidity of the white liguor in line 30 even more (e.g. to over S0%, typically about 60-90%). The processing in b]ock 36 may be as described in the parent application (the disclosure of which is hereby incorporated by reference herein).
FIGURE 2 illustrates a modification of the process of FIGURE 1. In the modification of FIGURE
2, components comparable to those in FIGURE 1 are . :
: ' '. ' 't. ~ -i' , ' ' ,, . ' ,; ,:, ~ i ' ~ ,,,, ~j,, , , ~, ~ " ~, ", " "
, illustrated by the same reference numeral only preceded by a "1".
In FIGURE 2, the white liquor in line 130 is split into two substantially equal streams, in 42 and 43. The hydrogen sulfide, or like sulfur containing chemicals, are absorbed into the white liquor in stream 42, which is then used in the early stage of the digester 111, or in the impregnation vessel (shown in dotted line at 44 in FIGURE 2~.
The lower sulfidity ~hite liquor in line 43 is used at a later stage in the digester 111, e.g. it may be added -- as indicated by dotted line ~5 -- to the wash recirculation Ioop 46 associated with the digester 111.
FIGURE 3 illustrates an alternative use of the materials produced from the heat treatment of the black liquor. In FIGURE 3 components comparable to those in the FI~URE 1 embodiment are shown by the same reference numeral only preceded by a "2".
The gas is produced in the heat treatment stage 215 in FIGURE 3 passed to the sulfur and fuel gas ;;~`
recovery system 236, typically with hydrogen sulfide produced and discharged in line 240. The hydrogen sulfide in line 240 may then be passed to a conventional device 48 for converting the hydrogen sulfide to sulfuric acid. The sulfuric acid, in line 49, may then be used in the bleaching processes in the pulp mill (e.g. bleach plant 33), and/or for acid prehydrolysis in the pulping process. Also, the sulfuric acid can be used outside the mill if a positive sulfur imbalance exists in the mill.
FIGURE 4 illustrates another modification. In the modification of FIGURE 4, structures comparable :, : : , ...................... ..
., , . .. .. . .
~ : ' ', '" , ' ;' :
to those in FIGURE 1 are shown by the same reference numeral only preceded by a "3".
The gases produced in line 50 from heat treatment 315 are fed to a scrubber 51. Also fed to the scrubber 51 is the essentially sulfur free sodium hydrox~.de produced in causticizer 323. The scrubber 51 thus produces, in line 52, a liquid containing sodium hydrosulfide. That liquid containing sodium hydrosulfide may be passed to buffer storaga 53 (for sodium and sulfur), or may be used as a feed stream for a polysulfide generation system 54, the polysulfide produced being used during cooking and/or other processes in the pulp mill.
Example As one exemplary practice of the method described with respect to FIGUR~ 1 through 4, a kraft pulping operation requires 102 tons per day of Na2S and 238 tons per day of sodium hydroxide in order to meet cooking liquor requirements. 26.4 tons of sulfur are removed in the off gas generated during heat treatment in stage 15. The black li~uor exiting the heat treatment stage 15 is burned in ..
recovery boiler 1~, and the resulting chemicals are added to water in smelt tank 17 to produce a green liquor having a ~ulfidity of approximately 15%. The solids content and temperature of the green liquor from smelt tank 17 is adjusted to a level so as to precipitate crystalline sodium carbonate in the form of Na2CO3'l0H2o or Na2C3 H2 The solids Gontent adjustment is made during the smelt dissolution process in tank 17 by controlling the amount of water added, and during evaporation processes (e.g.
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) r;~
in 19) following smelt dissolution, or both, and the degree of sodium carbonate crystallization and subsequent removal is controlled by the solids content of the green liquor, the temperature to which the green liquor is cooled, and the concentration of other compounds such as sodium sulfide and sodium chloride in the green liquor. If the temperature is controlled so that the green liquor is between about 5-20C, the decahydrate form of sodium carbonate crystals is produced, whereas if the temperature is controlled so that it is between about 35-90C, the monohydrate form is produced.
The amount of sodium carbonate crystallized will be approximately 189 tons per day as Na2C03. The concentration of the green liquor is also adjusted to yield good causticizing efficiency. The sodium carbonate crystals are dissolved in an aqueous solution to the proper concentration and causticized sepa~ately in causticizer 23. The sodium carbonate will yield a solution containing approximately 174 tons per day of essentially sulfur free sodium hydroxide, which when causticized in causticizer 25 will yield a white liquor of approximately 25%
sulfidity.
The white liquor in line 30, 130 is divided into two substantially equal streams, 42, 43. One of the streams, in line 43, has a sulfidity of about 25%, and is used in the later stages of cook, e.g.
in digester 111. To the other stream, in line 42, the gas stream containing primarily hydrogen sulfide (and perhaps sulfur free fuel gases~ contacts the white liquor, increasing the sulfidity thereof substantially, to over 50% (in this example ' ' ' ',i' - ,' I ,, ,, "'~
2 ~
approximately 74%). This high sulfidity white li~uor is used in the early stages of cook in digester 111 (or impregnation vessel 44).
It will thus be seen that according to the present invention various methods have been provided for the effective recovery of chemicals, and treatment of pulp, during cellulose pulping processing. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which ~i scope is to be accorded the broadest interpretation , of the appended claims so as to encompass all equivalent processes and methocls.
:.
",, '
_REEN LIOUOR CRYSTALLI ZATI ON
BACKGROUND AND SUMM~R~ OF THE INVF.NTION
As descrlbed in the parent application, it is highly desirable -n procedures for producing cellulose pulp to produce two or more liquor streams having vastly different sulfidity during chemical recovery. According to the present invention, a method is provided which allows effective and versatile production of such streams, including the production a stream that may be controlled so that it is essentially sulfur free.
Part of the process according to the present invention involves the treatment of "black liquor".
The term "black liquor" in the present specification and claims is understood in a broad sense as including liquor with dissolved lignin from the cooking of cellulose to produce cellulose pulp, typically in a sulfate process, a sulfide process, or another process containing sulfur as part of the treatment chemical for the pulp. The liquor treating the pulp i8 referred to as "white liquor".
In the present specification and claims, the term "white li~uor" unless otherwise specified, is intended to be broad and cover all liquors typically used in a pulp treatment process in which some -, - , ' ~ ' "' ' 2 ~
sulfur is utilized. The white liquor may be for the sulfate or sulfide process, and at various points during the cook or treatment may be an essentially pure stream of NaOH (that is substantially free of sulfur, although at some point in the cook a white liquor containing sulfur will be utilized).
In the practice of most aspects of the present invention, the black liquor is subjected to heat treatment. It is known that the lignin in black liquor is split by heat treatment under pressure for a predetermined time, whereby gases containing organic sulfur compounds (typically dimethyl sulfide and methyl mercaptan) are generated, as shcw.l in U.S. patent 4,929,307 (the disclosure of which is hereby incorporated by reference herein). The heat treatment may be carried out between the digester and recovery boiler of a typical pulp mill, but it is not limited within that area. Typically the heat treatment is carried out at a temperature of about 170 350C. As much as 30 to 70% of the sulfur in the black liquor may be removed during heat treatment. One purpos0 of the heat treatment i 6 to decrease the sulfur content of the black liquor entering a recovery boiler or like recovery process in order to minimize corrosion problems. Also, the viscosity of the black liquor is reduced by this heat treatment. The sulfur containing gases that are produced from the heat treatment are used advantageously according to the invention.
In one of the most basic aspects of the present invention, in the recovery system in which the black liquor is burned to produce a smelt, and then the smelt is dissolved to produce green liquor, the ' : : :: ~,: . : , ::, . . ,, :, : , 2 ~ 0 ~
concentration and temperature are controlled in such a way as to cause the sodium carbonate within the green liquor to crystallize into either monohydrate or decahydrate crystals. The crystals are then removed from the green liquor, the resulting green liquor having high sulfidity. The removed sodium carbonate crystals can be used to produce a li~uor containing substantially sulfur free sodium hydroxide, which can be used in a wide variety of different manners within the pulp mill.
According to one aspect o~ the present inventi~ , a method of recovering and utilizing chemicals during the production of cellulose pulp using chemicals containing sulfur is provided. The method comprises the following steps: (a) Heat treating black liquor which results from the production of cellulose pulp, to drive off as gases some of the sulfur containing compounds therefrom.
(b) Burning the heat treated black liquor to produce a smelt. (c) Dissolving the smelt with liquid to produce green liquor. (d) Adjusting the concentration of the ~reen liquor, and controlling the temperature, to crystallize sodium carbonate into crystals. (e) Removing the sodium carbonate crystals from the green liguor, to produce a first stream of green liguor having high sulfidity. And, (f) causticizing the sodium carbonate crystals to produce a liquor containing sodium hydroxide.
The first green liquor stream may be causticized to produce a first white li~uor having high sulfidity, and the sulfidity of that white liquor can be increased even more by contacting it :, : .', ' ;' , ; ' -2 1 ~
with the sulfur containing gases produced in step (a) to increase its sulfidity to more than 50%.
Step (e) may be practiced by centrifugal separation, or by a wide variety of filtering techni~ues such as vacuum filtering, disc filtering, or sidehill screening. Step (d) may be practiced to control the temperature so that it is between about 5 20C so as to produce pure decahydrate crystals, or so that it is between about 35-90C, to produce monohydrate crystals. Steps (a)-(f~ in general may be practiced to produce, as the sodium hydroxide containing liquor, a substantially sulfur free sodium hydroxide li~uor.
The stream of sodium hydroxide li~uor can be used in a number of different places in a pulp mill. For example it may be used in a plant for bleaching the cellulose pulp, or it may be added to the latter stages of cooking. Alternatively, it can be used to scrub the gases produced in step (a), which results in a liquor containing sodium hydrosulfide. The sodium hydrosulfide containing liquor may be used as buffer storage for sodium and sulfux, or as a feed stream to a polysulfide generating system.
Alternatively, the gases from step (a) may be used to make sulfuric acid, which is then used in bleaching or acid prehydrolysis, or other uses in a pulp mill where pH adjustment is desired. Also, the white liquor stream, produced by causticizing the green li~uor, can be split in two, with the sulfur containing gases from step (a) contacting only one of the streams, which then is used in an early stage of cooking, while the other, lower sulfidity, white . .
2 ~
liquor stream is used in a later staye of cooking (e.g. in the wash recirculation loop).
According to another aspect of the present invention, a method of producing chemical pulp from cellulose comminuted fibrous material utilizing white liquor, in a pulping process includiny fluids containing sulfur, is provided. This method comprises the following steps: (a) Intentionally removing sulfur from a fluid containing sulfur. (b) Forming a first stream containing sulfur compounds and sodium carbonate. (c) By controlling the solids concentration of the first stream, and the temperature, producing sodium carbonate crystals in the first stream. (d) Separating out the sodium carbonate crystals from the first stream, and then causticizing the first stream to produce white liquor. (e) Including by using removed sulfur from step (a), raising the sulfidity of the first stream. (f) Making at least a second stream of white liquo~ having relatively low sulfur content, by using the separated out sodium carbonate crystals (the first stream having sulfur content substantiall~ higher than the sulfur content of the second stream). (g) Chemical pulping the cellulose material utilizing the first and second streams of white liquor, added at diferent places in the pulping process. And, (h) washing the chemical pulp produced.
According to still another aspect of the presen-t invention, a method of recovering chemicals from black liquor produced during pulping of cellulose with sulfur containing treatment liquids is provided which comprises the following steps:
' ', ~ ': ' ;
, ~
(a) Burning black liquor to produce a smelt. (b) Dissolving the smelt with liquid to produce green liquor. ~c) Adjusting the concentration of the green liquor, and controlling its temperature, to crystallize sodium carbonate into either monohydrate or decahydrate crystals. (d) Removing the sodium carbonate crystals from the green liquor, to produce a first stream of green liquor having high sulfidity. And, (e~ causticizing the sodium carbonate crystals to produce a stream of liquid containing substantially sulfur free sodium hydroxide.
It i5 the primary object of the present invention to effectively recover chemicals during the production of cellulose pu:Lp including by causing the green liquor in a recovery loop to produce pure sodium carbonate crystals, and then to ultimately produce liquor streams having vastly different sulfidity. This and other objects will become clear from an inspection of the detailed description of the invention and from the appended claims.
BRIEE DESCRIPTION OF TEE DRAWINGS
FIGURE 1 is a flow schematic illustrating an exemplary method for recovering and utilizing chemicals during the production of cellulose pulp according to the present invention;
FIGURE 2 is a detail view showing the digester and similar components of FIGURE 1 in practicing a modified method according to the present invenkion;
2 ~ 03 FIGURE 3 is a detail view of the heat treatment portions of FIGURE 1 illustrating components used in the practice of another modifled method according to the invention; and FIGURE 4 i5 a view like that of FIGURE 3 showing the practice of still another modification of the method according to the invention.
DETAILED DESCRIPTION OF THE DRAWINGS
A cellulose pulp producing and chemical recovery exemplary system~according to the present invention is shown generally by re~erence numeral 10 in FIGURE 1. In the conventional digester 11 (which may be a continuous or a batch digester), cooking of cellulose comminuted fibrous material takes place to produce cellulose pulp. The cooking process includes utilization of sulfur containing chemicals, and may comprise a sulfate process, sulfide process, or the like. During cooking, a lignin rich liquid is produced, known as "black liquor", which is either continuously or periodically discharged from the digester 11 in line 12, and is typically subjected to one or more evaporation stages 13, 14 before being fed to a conventional recovery boiler 16. Also, according to the present invention, the black liquor is subjected to heat treatment under pressure for a predetermined period of time in order to reduce the sulfur content of, and the viscosity ;~
of, the black liquor. The heat treatment sta~e, shown at 15 in FIGURE 1, is as basically described in U.S. patent 4,929,307 in which the black liquor "
~, '~ .' '' '' :
is subjected to heat treatment at a temperature of about 170-350C under pressure for a predetermined period of time. By varying the time, temperature, etc., as much as 30 to 70% of the sulfur can be removed from the black liquor before it is fed to the recovery boiler 16.
In the recovery boiler 16, the black liquor is burned, the lignin therein having substantial heat value. The burning of the black liquor produces a smelt which contains the chemicals which are desirably recovered. As is conventional, the smelt is dissolved in wP~r or another liquid in smelt tank 17. According to the invention, the concentration of the liquor produced in the smelt tank 17, known as "green liquor", is controlled, as is the temperature, so as to produce essentially pure sodium carbonate crystals in the green liquor.
As illustrated in FIGURE 1, the green liquor may be clarified in clarifier 18, and the crystals may be formed in a crystallizer l9. The crystallizer 19 can inherently provide temperature control by controlling the degree of evaporation, or othar crystallization function, or temperature control can be provided externally, as indicated schematically at 20 in FIGURE 1, by heating or cooling the green liquor.
When it is desired to produce essentially pure decahydrate sodium carbonate crystals, the concentration and temperature are controlled so that the level of sodium carbonate in the green li~uor is not too high for subsequent effective causticization of the green liquor, and so that the temperature is about 5-20C. When it is desirable to produce . . .
:' ' ' . `.. ':.: ,' ., ,, ~ :: , : : ~^. ' :.
.. , ~, .
1. ' ~:: ' . ~ "
2 1 0 ~
monohydroate sodium carbonate crystals the temperature is controlled so that it is about 35-90C. The concentration of the green liquor is controlled by the amount of liquid added to the smelt tank 17 to produce the green liquor from the smelt, and/or by evaporation of the green liquor after its production in the smelt tank 17 (e.g. in - the crystallizer 19).
After the production of the sodium carbonate crystals in the green liguor, the crystals are separated. Separation is preerably performed in a centrifugal separator, such as a hydrocyclone, illustrated at 21 in FIGURE 1. The crystals, discharged into line 22 from the bottom of the hydrocyclone 21, are causticized with lime (CaO) in causticizer 23, while the green li~uor is discharged in line 24. Instead of a centrifugal separator 21, various filtration mechanisms may be utilized, such as vacuum drum filters, sidehill screens, and disc filters.
In the causticizer 23, the reactions involving reburned or fresh lime and sodium carbonate are as follows:
CaO 2 Ca(H)2 Na2CO3 ~ Ca(H)2 ---> -2 NaO~ -t CaC03 Preferably the green liquor in line 24 is also causticized in causticizer 25, in a conventional manner at conventional concentration levels, with the lirne mud from both causticizers 23 and 25 being fed to a lime kiln 26. The product stream from the causticizer 23, in line 28, is a sodium hydroxide stream, essentially sulfur-free, while the white liquor in line 30 from causticizer 25 has high ' . ' ~
sulfidity, i.e. a higher sulfidity than in con~entional recovery processes.
The liquors in lines 28, 30 can be used for a wide variety of different purposes. Typically, however, the white liquor in line 30 is used in the digester 11 to cook the cellulose, the relatively high sulfidity white liquor in line 30 being used at early stages of the cook (including perhaps in impregnation). The essentially sulfur free sodium hydroxide in line 28 may either be used as a "white liquor" in the later stages of cook (including the wash recirc~lation line~ in digester 11, as indicated by line 32, or may be used in a bleach plant 33, as indicated by line 34.
The gases produced during the heat treatment 15 are typically fed to the sulfur and fuel gas recovery block 36 in which the fuel gas may be separated into line 37, which may be used as a fuel gas in the lime kiln 26 being introduced into the kiln 26 in line 38. The sulfur containing gases (typically dimethyl sulide and methyl mercaptan) may be treated to produce hydrogen sulfide or the like, and which may be introduced, as shown schematically by line 40 in FIGURE 1, into the white liquor in line 30 (absorbed therein) to increase the sulfidity of the white liguor in line 30 even more (e.g. to over S0%, typically about 60-90%). The processing in b]ock 36 may be as described in the parent application (the disclosure of which is hereby incorporated by reference herein).
FIGURE 2 illustrates a modification of the process of FIGURE 1. In the modification of FIGURE
2, components comparable to those in FIGURE 1 are . :
: ' '. ' 't. ~ -i' , ' ' ,, . ' ,; ,:, ~ i ' ~ ,,,, ~j,, , , ~, ~ " ~, ", " "
, illustrated by the same reference numeral only preceded by a "1".
In FIGURE 2, the white liquor in line 130 is split into two substantially equal streams, in 42 and 43. The hydrogen sulfide, or like sulfur containing chemicals, are absorbed into the white liquor in stream 42, which is then used in the early stage of the digester 111, or in the impregnation vessel (shown in dotted line at 44 in FIGURE 2~.
The lower sulfidity ~hite liquor in line 43 is used at a later stage in the digester 111, e.g. it may be added -- as indicated by dotted line ~5 -- to the wash recirculation Ioop 46 associated with the digester 111.
FIGURE 3 illustrates an alternative use of the materials produced from the heat treatment of the black liquor. In FIGURE 3 components comparable to those in the FI~URE 1 embodiment are shown by the same reference numeral only preceded by a "2".
The gas is produced in the heat treatment stage 215 in FIGURE 3 passed to the sulfur and fuel gas ;;~`
recovery system 236, typically with hydrogen sulfide produced and discharged in line 240. The hydrogen sulfide in line 240 may then be passed to a conventional device 48 for converting the hydrogen sulfide to sulfuric acid. The sulfuric acid, in line 49, may then be used in the bleaching processes in the pulp mill (e.g. bleach plant 33), and/or for acid prehydrolysis in the pulping process. Also, the sulfuric acid can be used outside the mill if a positive sulfur imbalance exists in the mill.
FIGURE 4 illustrates another modification. In the modification of FIGURE 4, structures comparable :, : : , ...................... ..
., , . .. .. . .
~ : ' ', '" , ' ;' :
to those in FIGURE 1 are shown by the same reference numeral only preceded by a "3".
The gases produced in line 50 from heat treatment 315 are fed to a scrubber 51. Also fed to the scrubber 51 is the essentially sulfur free sodium hydrox~.de produced in causticizer 323. The scrubber 51 thus produces, in line 52, a liquid containing sodium hydrosulfide. That liquid containing sodium hydrosulfide may be passed to buffer storaga 53 (for sodium and sulfur), or may be used as a feed stream for a polysulfide generation system 54, the polysulfide produced being used during cooking and/or other processes in the pulp mill.
Example As one exemplary practice of the method described with respect to FIGUR~ 1 through 4, a kraft pulping operation requires 102 tons per day of Na2S and 238 tons per day of sodium hydroxide in order to meet cooking liquor requirements. 26.4 tons of sulfur are removed in the off gas generated during heat treatment in stage 15. The black li~uor exiting the heat treatment stage 15 is burned in ..
recovery boiler 1~, and the resulting chemicals are added to water in smelt tank 17 to produce a green liquor having a ~ulfidity of approximately 15%. The solids content and temperature of the green liquor from smelt tank 17 is adjusted to a level so as to precipitate crystalline sodium carbonate in the form of Na2CO3'l0H2o or Na2C3 H2 The solids Gontent adjustment is made during the smelt dissolution process in tank 17 by controlling the amount of water added, and during evaporation processes (e.g.
, .
.; , : :;:..... -. ,,:
. . , : . ~ . :.. ,. ::, ' '; ' '~
) r;~
in 19) following smelt dissolution, or both, and the degree of sodium carbonate crystallization and subsequent removal is controlled by the solids content of the green liquor, the temperature to which the green liquor is cooled, and the concentration of other compounds such as sodium sulfide and sodium chloride in the green liquor. If the temperature is controlled so that the green liquor is between about 5-20C, the decahydrate form of sodium carbonate crystals is produced, whereas if the temperature is controlled so that it is between about 35-90C, the monohydrate form is produced.
The amount of sodium carbonate crystallized will be approximately 189 tons per day as Na2C03. The concentration of the green liquor is also adjusted to yield good causticizing efficiency. The sodium carbonate crystals are dissolved in an aqueous solution to the proper concentration and causticized sepa~ately in causticizer 23. The sodium carbonate will yield a solution containing approximately 174 tons per day of essentially sulfur free sodium hydroxide, which when causticized in causticizer 25 will yield a white liquor of approximately 25%
sulfidity.
The white liquor in line 30, 130 is divided into two substantially equal streams, 42, 43. One of the streams, in line 43, has a sulfidity of about 25%, and is used in the later stages of cook, e.g.
in digester 111. To the other stream, in line 42, the gas stream containing primarily hydrogen sulfide (and perhaps sulfur free fuel gases~ contacts the white liquor, increasing the sulfidity thereof substantially, to over 50% (in this example ' ' ' ',i' - ,' I ,, ,, "'~
2 ~
approximately 74%). This high sulfidity white li~uor is used in the early stages of cook in digester 111 (or impregnation vessel 44).
It will thus be seen that according to the present invention various methods have been provided for the effective recovery of chemicals, and treatment of pulp, during cellulose pulping processing. While the invention has been herein shown and described in what is presently conceived to be the most practical and preferred embodiment thereof, it will be apparent to those of ordinary skill in the art that many modifications may be made thereof within the scope of the invention, which ~i scope is to be accorded the broadest interpretation , of the appended claims so as to encompass all equivalent processes and methocls.
:.
",, '
Claims (22)
1. A method of recovering and utilizing chemicals during the production of cellulose pulp using chemicals containing sulfur, comprising the steps of:
(a) heat treating black liquor which results from the production of cellulose pulp, to drive off as gases some of the sulfur containing compounds therefrom;
(b) burning the heat treated black liquor to produce a smelt;
(c) dissolving the smelt with liquid to produce green liquor;
(d) adjusting the concentration of the green liquor, and controlling the temperature, to crystallize sodium carbonate into crystals;
(e) removing the sodium carbonate crystals from the green liquor, to produce a first stream of green liquor having high sulfidity; and (f) causticizing the sodium carbonate crystals to produce a liquor containing sodium hydroxide.
(a) heat treating black liquor which results from the production of cellulose pulp, to drive off as gases some of the sulfur containing compounds therefrom;
(b) burning the heat treated black liquor to produce a smelt;
(c) dissolving the smelt with liquid to produce green liquor;
(d) adjusting the concentration of the green liquor, and controlling the temperature, to crystallize sodium carbonate into crystals;
(e) removing the sodium carbonate crystals from the green liquor, to produce a first stream of green liquor having high sulfidity; and (f) causticizing the sodium carbonate crystals to produce a liquor containing sodium hydroxide.
2. A method as recited in claim 1 comprising the further step (g) of causticizing the first green liquor stream to produce a first white liquor having high sulfidity.
3. A method as recited in claim 2 comprising the further step (h) of contacting the first white liquor with sulfur compound containing gases produced from the practice of step (a), to increase its sulfidity.
4. A method as recited in claim 3 wherein the first stream of green liquor has a sulfidity of about 15%, step (g) is practiced to raise the sulfidity of the first white liquor to about 25%, and step (h) is practiced so as to raise the sulfidity of the first white liquor to more than 50%.
5. A method as recited in claim 1 wherein step (e) is practiced by centrifugal separation, vacuum filtering, disc filtering, or sidehill screening.
6. A method as recited in claim 1 wherein step (d) is practiced to control the temperature so that it is between about 5-20 degrees C so as to produce decahydrate crystals.
7. A method as recited in claim 1 wherein step (d) is practiced to control the temperature so that it is between about 35-90 degrees C to produce monohydrate crystals.
8. A method as recited in claim 2 comprising the further step of using the first white liquor to treat cellulose fibrous material in a slurry in an early stage of pulping.
9. A method as recited in claim 8 wherein steps (a)-(f) are practiced so as to produce as the sodium hydroxide containing liquor a liquor containing essentially sulfur free sodium hydroxide.
10. A method as recited in claim 9 comprising the further step of using the stream of substantially sulfur free sodium hydroxide to treat the cellulose slurry at a later stage of pulping.
11. A method as recited in claim 9 comprising the further step of using the stream of substantially sulfur free sodium hydroxide in a plant for bleaching cellulose pulp.
12. A method as recited in claim 1 wherein step (d) is practiced by controlling the solids concentration by adjusting the amount of liquid added in step (c).
13. A method as recited in claim 1 wherein step (d) is practiced by controlling the solids concentration by controlled evaporation of the green liquor after step (c).
14. A method as recited in claim 2 comprising the further step of splitting the white liquor into two different streams, and contacting one of the streams, but not the other, with sulfur components removed from the black liquor during the practice of step (a).
15. A method as recited in claim 2 wherein step (d) is practiced to maintain the solids concentration at such a level that the sodium carbonate concentration in the green liquor is not too high for effective causticization.
16. A method as recited in claim 1 wherein steps (a)-(f) are practiced so as to produce as the sodium hydroxide containing liquor, a liquor containing essentially sulfur free sodium hydroxide.
17. A method as recited in claim 16 comprising the further step of using the stream of substantially sulfur free sodium hydroxide in a plant for bleaching cellulose pulp.
18. A method as recited in claim 1 comprising the further step of scrubbing the gases from step (a) with the sodium hydroxide containing liquor of step (f), to produce a liquor containing sodium hydrosulfide.
19. A method as recited in claim 1 wherein the gases in step (a) are used to make sulfuric acid; and wherein the sulfuric acid is used in the bleaching or acid prehydrolysis of cellulose pulp.
20. A method of producing chemical pulp from cellulosic comminuted fibrous material utilizing white liquor, in a pulping process including fluids containing sulfur, comprising the steps of:
(a) intentionally removing sulfur from a fluid containing sulfur;
(b) forming a first stream containing sulfur compounds and sodium carbonate;
(c) by controlling the solids concentration of the first stream, and the temperature, producing sodium carbonate crystals in the first stream;
(d) separating out the sodium carbonate crystals from the first stream, and then causticizing the first stream to produce white liquor;
(e) including by using removed sulfur from step (a), raising the sulfidity of the first stream;
(f) making at least a second stream of white liquor having relatively low sulfur content by using the separated-out sodium carbonate crystals, the first stream having sulfur content substantially higher than the sulfur content of the second stream;
(g) chemical pulping the cellulose material utilizing the first and second streams of white liquor, added at different places in the pulping process; and (h) washing the chemical pulp produced.
(a) intentionally removing sulfur from a fluid containing sulfur;
(b) forming a first stream containing sulfur compounds and sodium carbonate;
(c) by controlling the solids concentration of the first stream, and the temperature, producing sodium carbonate crystals in the first stream;
(d) separating out the sodium carbonate crystals from the first stream, and then causticizing the first stream to produce white liquor;
(e) including by using removed sulfur from step (a), raising the sulfidity of the first stream;
(f) making at least a second stream of white liquor having relatively low sulfur content by using the separated-out sodium carbonate crystals, the first stream having sulfur content substantially higher than the sulfur content of the second stream;
(g) chemical pulping the cellulose material utilizing the first and second streams of white liquor, added at different places in the pulping process; and (h) washing the chemical pulp produced.
21. A method as recited in claim 20 wherein steps (a)-(f) are practiced so that the second stream of white liquor is substantially sulfur free sodium hydroxide.
22. A method of recovering chemicals from black liquor produced during pulping of cellulose with sulfur containing treatment liquids, comprising the steps of:
(a) burning black liquor to produce a smelt;
(b) dissolving the smelt with liquid to produce green liquor;
(c) adjusting the concentration of the green liquor, and controlling its temperature, to crystallize sodium carbonate into either monohydrate or decahydrate crystals;
(d) removing the sodium carbonate crystals from the green liquor, to produce a first stream of green liquor having high sulfidity; and (e) causticizing the sodium carbonate crystals to produce a stream of liquid containing substantially sulfur free sodium hydroxide.
(a) burning black liquor to produce a smelt;
(b) dissolving the smelt with liquid to produce green liquor;
(c) adjusting the concentration of the green liquor, and controlling its temperature, to crystallize sodium carbonate into either monohydrate or decahydrate crystals;
(d) removing the sodium carbonate crystals from the green liquor, to produce a first stream of green liquor having high sulfidity; and (e) causticizing the sodium carbonate crystals to produce a stream of liquid containing substantially sulfur free sodium hydroxide.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US91885592A | 1992-07-27 | 1992-07-27 | |
| US07/918,855 | 1992-07-27 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2101005A1 true CA2101005A1 (en) | 1994-01-28 |
Family
ID=25441076
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002101005A Abandoned CA2101005A1 (en) | 1992-07-27 | 1993-07-21 | Green liquor crystallization |
Country Status (4)
| Country | Link |
|---|---|
| JP (1) | JP2986137B2 (en) |
| CA (1) | CA2101005A1 (en) |
| FI (1) | FI933297A7 (en) |
| SE (1) | SE9302497L (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2018046482A1 (en) * | 2016-09-08 | 2018-03-15 | Suncarbon Ab | Method and system for treating spent pulping liquor |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FI55065C (en) * | 1971-07-30 | 1979-05-10 | St Regis Paper Co | FOERFARANDE FOER TILLVARATAGANDE AV TALLOLJETVAOLAR UR AVLUTLOESNINGAR |
| FI53728C (en) * | 1974-03-12 | 1978-07-10 | Ahlstroem Oy | FOERFARANDE FOER AOTERVINNING AV KEMIKALIER UR AVLUTAR FRAON SULFATCELLULOSAKOK OCH AVFALLSVATTEN FRAON BLEKNING |
| JPS51116205A (en) * | 1975-04-02 | 1976-10-13 | Ahlstroem Oy | Method of recovering natrium chloride discharged from related process by cellulose manufacture |
| FI52230C (en) * | 1975-08-26 | 1977-07-11 | Keskuslaboratorio | Process for the production of dissolving pulp. |
-
1993
- 1993-07-21 CA CA002101005A patent/CA2101005A1/en not_active Abandoned
- 1993-07-21 JP JP5179555A patent/JP2986137B2/en not_active Expired - Lifetime
- 1993-07-22 FI FI933297A patent/FI933297A7/en unknown
- 1993-07-26 SE SE9302497A patent/SE9302497L/en not_active Application Discontinuation
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| WO2018046482A1 (en) * | 2016-09-08 | 2018-03-15 | Suncarbon Ab | Method and system for treating spent pulping liquor |
Also Published As
| Publication number | Publication date |
|---|---|
| SE9302497D0 (en) | 1993-07-26 |
| JP2986137B2 (en) | 1999-12-06 |
| JPH06173184A (en) | 1994-06-21 |
| FI933297A0 (en) | 1993-07-22 |
| FI933297A7 (en) | 1994-01-28 |
| SE9302497L (en) | 1994-01-28 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| EEER | Examination request | ||
| FZDE | Discontinued |