EP0431337B1 - Procédé pour éliminer la silice de la liqueur résiduelle - Google Patents

Procédé pour éliminer la silice de la liqueur résiduelle Download PDF

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Publication number
EP0431337B1
EP0431337B1 EP19900121370 EP90121370A EP0431337B1 EP 0431337 B1 EP0431337 B1 EP 0431337B1 EP 19900121370 EP19900121370 EP 19900121370 EP 90121370 A EP90121370 A EP 90121370A EP 0431337 B1 EP0431337 B1 EP 0431337B1
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EP
European Patent Office
Prior art keywords
stage
waste liquor
value
crystallization
zone
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
Application number
EP19900121370
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German (de)
English (en)
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EP0431337A1 (fr
Inventor
Alfred Dipl-Ing. Glasner
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Waagner Biro AG
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Waagner Biro AG
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Publication of EP0431337A1 publication Critical patent/EP0431337A1/fr
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    • 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
    • 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
    • D21C11/0085—Introduction of auxiliary substances into the regenerating system in order to improve the performance of certain steps of the latter, the presence of these substances being confined to the regeneration cycle
    • 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
    • D21C5/00—Other processes for obtaining cellulose, e.g. cooking cotton linters ; Processes characterised by the choice of cellulose-containing starting materials

Definitions

  • the invention relates to a process for the silicification of the waste liquor, preferably thickened black liquor, obtained during pulping, in particular of annual plants, by lowering the pH by means of gases containing CO2, such as e.g. Flue gases or exhaust gases from lime kilns or evaporation plants.
  • gases containing CO2 such as e.g. Flue gases or exhaust gases from lime kilns or evaporation plants.
  • the waste liquor is a problematic waste product in the alkaline decomposition of pulp from wood and especially from annual plants, using the sulfate, soda or alkaline sulfite process, and contains a number of organic and inorganic compounds that are disposed of by thickening and combustion and at the same time enable recovery of the digestion chemicals.
  • the silicic acid or its salts contained in the waste liquor interfere with the combustion (DE-B-1,119,645, DE-A 31 07 447, DE-A 32 08 200) and also with the recovery of the chemicals, so that theoretically it has already been proposed many times to decalcify the waste liquor.
  • the pebble deposits are contaminated with organic, in particular lignin compounds, so that the separated fine pellet powder is obtained in a gelatinous consistency, which can practically only be separated from the waste liquor by centrifugation.
  • the object of the invention is to avoid centrifugation and to obtain the silica in a washable state, ie coarse-grained.
  • the parameters according to the invention slow lowering of the pH in the precipitation and crystallization zone of the stages and the back-swinging of the pH in the individual stages
  • the gel-like silicic acid resulted in precipitation in pure crystalline form takes place, which can be washed easily.
  • These settling mechanisms are mainly inhibited by avoiding local acidification.
  • FIGS. 1 and 2 The invention is shown for example and schematically in the connected FIGS. 1 and 2.
  • Fig. 1 shows a circuit diagram and Fig. 2 shows the dependence of the solubility of the silica on the pH value with 70-degree black liquor.
  • the waste liquor which is collected in a store 1, flows via line 2 at approximately pH 12 into the first stage 3.
  • This stage 3 is recycled from the store 1 carbonated lye with little dissolved and undissolved SiO2 with the purpose of reducing the number of nuclei, since these are dissolved by the higher pH, and in the mixture the dissolved portion of silica to values less than 4g / l SiO2 (pH 11 - 11.5).
  • a pH of around 10.5 and 0.5 g / l result in a pH of around 10.5 for stages 3 'and 3 "with a dissolved silica content of 2 g / l , with this setting the same amount of silica is precipitated in both stages and thus locally high crystallization rates are avoided.
  • stage 3 a small part of the decalcified waste liquor is removed, fed via a circulating pump 4 to a gassing point 5, where CO2 is introduced either with positive pressure or with negative pressure; the partial flow is oversaturated, so that the pH value drops approximately to 11 to 11.5 and this results in a pre-neutralization.
  • This pre-neutralization is further supported by the partial return of the decilified waste liquor with the pH 10.2 via line 13 'from the overflow of the thickener 14 into the first Step 3.
  • the flow is very turbulent, so that the gas mixes intimately with the waste liquor, whereby a uniform CO2 uptake or carbonization takes place while avoiding local acidification.
  • the pH value at the outlet 6 from the fumigation stage 5 is raised again from saturation to 11 to 11.5 when mixed with the non-fumigated part of the container content.
  • Stage 3 ' is practically identical to stage 3, the pH in the container being kept just above the crystallization value (saturation) and in 5' the silica being precipitated in part by lowering the pH below the solubility equilibrium.
  • This leveling to the saturation value brings a homogenization of the waste liquor while avoiding local acidification, so that the 3rd stage 3 '' is designed for crystallization under optimal conditions with regard to low SiO2 content with few germs but large crystals.
  • the CO2 integration at the fumigation point 5 '' takes place by regulating the required amount of CO2 gas.
  • the number of germs is reduced by oscillating the pH value, so that the finest germs are dissolved again when the pH is raised, so that the dissolved silica, in the subsequent lowering, again largely rests on the existing germs to enlarge them crystallized. Larger germs have a higher sedimentation rate and can thus be more easily separated in a thickener 14.
  • Part of the thickened SiO2 crystals is returned via line 15 to the crystallization or precipitation stage 3 'or 3' ', so that they have the opportunity to grow even further.
  • the other part falls on the belt filter 12 and is obtained according to arrow 8 as a relatively pure but basic silica and can already be washed on the belt filter to avoid chemical losses.
  • the separated decilified liquid (arrow 13) with the dissolved organic impurities is then further thickened or evaporated and then burned.
  • the outgassing gas, often in foam form, in the individual stages 3, 3 'and 3' ' is collected in line 10. After the foam has been destroyed, the alkali is returned to stage 3 and the exhaust gas is removed.
  • the foam problem can also be combated by spraying the foam with carbonized waste liquor, in particular recycling from a subsequent stage, and mechanical separators have also proven successful.
  • the resulting exhaust gas is discharged via line 10, while the CO2-containing reaction gas is fed via line 11.
  • This CO2-containing gas can be flue gas or exhaust gas from evaporation vapors or exhaust gas from lime kilns, the exhaust gas from lime kilns in particular being very CO2-rich. Since the ratio of the circulated amount of lye, ie the lye which is passed through the fumigation point 5, 5 ', 5'', is greater than 1:20, there is a vortex zone in the fumigation apparatus and a relatively quiet zone in the reaction vessel of the stage within each stage , in which the excess gas is degassed and finally crystallization takes place.
  • the dissolving reaction of the silica when the pH is raised occurs after 2 Na2CO3 + SiO2 + H2O ⁇ Na2SiO3 + 2 NaHCO3
  • the precipitation reaction of the silica on the lines or to enlarge the crystals takes place after Na2CO3 + 3CO2 + Na2SiO3 + 4H2O ⁇ 4NaHCO3 + SiO 2
  • the formation of the fine-grained crystals is also facilitated by the fact that the temperature of the waste liquor remains constant at least in stages and is in particular kept at about 70.degree.
  • the average residence time in the individual stages should not be less than half an hour, but is normally 2 to 4 hours. This measure enables fine-grained crystallization without a gel separating from other salts or settling out of the waste liquor or mixed crystals being formed.
  • the return of already formed SiO2 crystals and germs in a sedimentation tank or a hydrocyclone enables the offer of more surface to achieve a lower crystallization rate, whereby larger and more uniform silicic acid crystals are formed, which sediment more easily.
  • the waste liquor can also be used Optimization of the silicification (about 6 g silica / l waste liquor in the 2nd stage 3 ') are concentrated before it is fed to the silicification.
  • this process step is insignificant for the purity of the products of the silicification itself, but has an effect on the degree of silicification.
  • the residence time in the individual containers is also dependent on the cellulose-containing material, which determines the composition and in particular the silica content of the waste liquor.
  • the great advantage of the system is that a band filter 12 is sufficient for the deposition of SiO2, and no energy-intensive centrifuges are required, which simplifies the control of the system and reduces the energy consumption.
  • the belt filter 12 can also be designed as a washing filter without changing anything in the process diagram of the silicification. The use of a washing filter depends on the degree of recovery of the chemicals that occur in the exhaust gases or ashes after the black liquor combustion.
  • the individual stages are operated with a decreasing pH value, a drop in the pH value of approximately 0.5 units from stage to stage being recognized as optimal for the invention.
  • a fluctuation in the pH value less than 0.2 pH units is to be aimed for, which can be adjusted by mixing in the CO2 at the mixing points 5.5 ', 5''.
  • the pH of the waste water which has been decilified to approximately 0.5 g / l can be raised again after the crystallization or precipitation stage 3 ′′, so that there are no excretions to be feared in the feed line 13 to the combustion stage or thickening stage, not shown. This can be done by stripping the CO2 present as bicarbonate, as well as by adding NaOH in the final stage of the evaporation plant.
  • the pH value is raised by alkali supply so that the silica is dissolved. Then, in the stages of the reactor, the pH is lowered again to 10.5 by means of a CO2 solution.
  • the alkalization serves to homogenize the acid and dissolve the crystallization nuclei, so that by carefully lowering the crystallization values, the number of nuclei is reduced to a high degree of purity (avoidance of mixed crystals) to cause the silica to crystallize.
  • This enables an evenly filterable and washable filter cake with a grain size of 3 to 20 ⁇ m. If the filter cake is gel-like due to lignin precipitation, the belt filter sticks together, so that centrifuges are required as a separating unit, which increases the mechanical and energy expenditure.
  • the carbonic acid supplied serves to lower the pH
  • the carbonic acid also serves to precipitate the silica, so that the weight ratio of silica to carbonic acid in the gassing reactors 5' , 5 '' is greater than 5: 1. It is essential that in the individual Stages by the small pH-value reduction the number of crystallization nuclei and these are stimulated by the oscillation of the pH during the stay in the step to grow, so that the grown crystals can be filtered and grow with great purity, some of them the silica is dissolved from the excess Na2CO3 and the other part is excited by the further CO2 supply for renewed nucleation.

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Claims (5)

  1. Procédé pour éliminer la silice de la liqueur résiduelle usée produite de la digestion de cellulose, preférablement de liqueurs noires épaissées et particulièrement celle de plantes annuelles par l'abaissement de la valeur pH à l'aide de gaz contenant de l'anhydride carbonique comme par ex. des fumées ou gaz d'échappement provenant de fours à chaux ou d'installations à évaporation, est caractérisé par l'emploi de plusieurs étages, à savoir trois étages, pour la précipitation de l'acide silicique et aux différents étages le liquide et maintenu à une température constante, particulièrement à 70°C pendant une durée de 30 minutes au minimum mais preférablement pendant 2 à 4 heures et qu'une partie du flux de la liqueur usée est mise en contact repétitivement avec l'anhydride carbonique et transférée ensuite dans une zone de dégazement et/ou de cristallisation où le mélange de la liqueur usée résiduelle ou fraiche provenant de l'étage précédant assure le relèvement de la valeur pH à un niveau justement un peu au dessus de la valeur de décristallisation dans une zone en face de la zone des turbulences ce qui garantit la dissolution de petits cristaux et que le dernier étage est conçu comme étage de cristallisation où la proportion des poids de l'acide silicique d'une valeur pH de 10,2 au débit du gaz carbonique admis dans la zone des turbulences de l'étage de cristallisation est supérieure à 5 : 1.
  2. Procédé suivant la revendication 1 caractérisé en ce qu'une proportion des volumes de la liqueur résiduelle contenue dans la zone des turbulences à celui de la liqeur usée contenue dans la zone de dégazement et de cristallisation est supérieure à 1 : 20.
  3. Procédé suivant la revendication 1 caractérisé en ce qu'une quantité d'anhydride carbonique admis au 3ème étage identiqe à celle necessaire pour la précipitation de l'acide silicique et l'abaissement de la valeur pH de cet étage de 0,5 unités pH.
  4. Procédé suivant la revendication 1 caractérisé en ce qu'une partie de l'acide silicique précipité est reconduit dans un étage 3'en amont et/ou l'étage de précipitation 3''.
  5. Procédé suivant la revendication 1 caractérisé en ce que le trop-plein d'un bassin à précipitation 14 après avoir passé l'étage de précipitation 3'' et les particules fines de l'acide silicique précipité sont réconduits dans le premier étage où ils sont dissouts.
EP19900121370 1989-12-01 1990-11-08 Procédé pour éliminer la silice de la liqueur résiduelle Expired - Lifetime EP0431337B1 (fr)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
AT2741/89 1989-12-01
AT274189A AT393284B (de) 1989-12-01 1989-12-01 Verfahren zur entkieselung der ablauge

Publications (2)

Publication Number Publication Date
EP0431337A1 EP0431337A1 (fr) 1991-06-12
EP0431337B1 true EP0431337B1 (fr) 1994-04-06

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EP19900121370 Expired - Lifetime EP0431337B1 (fr) 1989-12-01 1990-11-08 Procédé pour éliminer la silice de la liqueur résiduelle

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EP (1) EP0431337B1 (fr)
AT (1) AT393284B (fr)
DE (1) DE59005263D1 (fr)
DK (1) DK0431337T3 (fr)

Families Citing this family (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
AT401070B (de) * 1993-12-23 1996-06-25 Austrian Energy & Environment Verfahren zur gewinnung grobkörniger reiner kieselsäure

Family Cites Families (4)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE1119645B (de) * 1957-01-31 1961-12-14 Escher Wyss Gmbh Verfahren zur Entfernung von Kieselsaeure aus alkalischen Ablaugen der Zellstoffindustrie
DE3003090A1 (de) * 1980-01-29 1981-08-13 Debendra Kumar Ph.D. Athinä Misra Verfahren zur entkieselung alkalischer ablaugen
DE3107447C2 (de) * 1981-02-27 1985-08-29 Kraftanlagen Ag, 6900 Heidelberg Verfahren zum Entfernen von Kieselsäure aus Schwarzlauge
DE3208200A1 (de) * 1982-03-06 1983-09-08 Metallgesellschaft Ag, 6000 Frankfurt Verfahren zur kontinuierlichen entfernung von kieselsaeure aus zellstoffablaugen

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Publication number Publication date
DK0431337T3 (da) 1994-07-18
ATA274189A (de) 1991-02-15
EP0431337A1 (fr) 1991-06-12
AT393284B (de) 1991-09-25
DE59005263D1 (de) 1994-05-11

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