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
  • D21C11/04—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
  • D21C11/12—Combustion of pulp liquors
  • D21C11/122—Treatment, e.g. dissolution, of the smelt
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P40/00—Technologies relating to the processing of minerals
  • Y02P40/40—Production or processing of lime, e.g. limestone regeneration of lime in pulp and sugar mills

Definitions

• the present invention relates to a method of reducing the silicon, phosphor and aluminium contents of cooking liquors in the recovery of chemicals in an alkali delig- nification process of cellulose-containing material.
• silica Si0 2
• Si0 2 silica
• sulphate and soda processes chemicals are recovered by evaporating water from the waste liquor, i.e., black liquor, and then by combusting the organic substance contained in the liquor in a recovery boiler for releasing the chemicals. From the recovery boiler, chemicals are discharged as melt, which is dissolved in water to form green liquor.
• the green liquor contains sodium carbonate which is causticized, by means of cal- cium oxide, to sodium hydroxide needed in pulp produc ⁇ tion.
• the white liquor thereby obtained is circulated to the pulp production system, and the calcium carbonate (lime mud) formed is regenerated in the lime kiln.
• silica dissolved in alkaline black liquor causes problems in all stages of the chemical circulation.
• the silicates of black liquor reach their solubility limit and precipitate as different compounds onto the heat transfer surfaces of the evapora ⁇ tors, thereby impairing the operation of the equipment.
• silicates form deposits on the heat transfer surfaces and increase the viscosity of the chemical melt.
• silicates precipi- tate as calcium-hydrosilicates among the lime mud.
• the lime mud rich in silicate infiltrates poorly, leaving moisture in the lime mud.
• This kind of lime mud is not worth combusting, but it has to be taken to a landfill area instead.
• the amount of white liquor thereby obtained decreases, and also valuable cooking chemicals end up in the landfill area, along with the lime mud.
• silica may be sep ⁇ arated from green liquor by means of carbon dioxide.
• the silicate content of green liquor may be about 10 to 20 g Si0 /1, sometimes even more, which is much more than in corresponding weak black liquor, whereby a better yield may be expected. Separation of silica precipitate from green liquor is not so problematic than separation from black liquor, because green liquor lacks organic substance.
• An object of the present invention is to provide a method which is simpler and more efficient in comparison with prior art methods for essential reducing the silicon, phosphor, and aluminium contents of cooking liquors. Special attention has been paid to making the above men ⁇ tioned impurities into a readily separable form.
• the melt obtained from black liquor combustion and containing sodium carbonate is so treated that the sodium carbonate is recovered in solid form, whereas silicon, phosphor and/or aluminium is separated as a solution con ⁇ taining dissolved sodium silicates, sodium phosphates and/or sodium aluminates; and that
• the solid sodium carbonate is dissolved, for forming a solution which has a low silicon, phosphor, and/or alu ⁇ minium content.
• sodium silicates, sodium phosphates and sodium aluminates dissolve almost limitless in water, whereas solubility of sodium carbonate in water is limited.
• sodium carbonate brought into a crystal state, it may be separated as pure sodium carbon ⁇ ate from the mother liquor rich in sodium silicate, sodium phosphate, and/or sodium aluminate. After separ- ation, crystal sodium carbonate is dissolved, and the solution thereby obtained may be treated in a conven ⁇ tional manner in the causticizing plant, for producing sodium hydroxide solution for pulping and other needs.
• melt containing sodium carbonate is dissolved either in water or weak white liquor from washing of lime mud.
• the sodium carbonate may be separated from this kind of liquor by evaporating liquor as long as crystallization takes place. Evaporation may be a conventional multi-effect process. After evaporation, crystals are separated by processes known per se, such as filtering, from the mother liquor, which contains soluble sodium silicates, sodium phosphates and sodium aluminates.
• Sodium carbonate may be separated from readily soluble silicates, phosphates and aluminates also so that readily soluble compounds are leached from the recovery boiler melt to a small amount of liquid, whereby the sodium carbonate, which does not dissolve so easily, stays main ⁇ ly solid.
• Leaching may be effected, for example, by applying the melt dissolving method developed by Ebara, in connection with a NSSC recovery process (US patents 4,212,702 and 4,141,785; Teder, A., Nordisck Cellulosa, 1984, No. 2, pp. 12-14).
• soda melt may be cooled, if necessary, and grind it thereafter to a desired particle size for leaching.
• silicate constitutes an especially difficult problem in those pulp mills where pulp is pro ⁇ substituted from annual plants.
• the deligni- fication process is typically a soda process, using sodium hydroxide as a cooking chemical.
• the present invention is not limited to a soda process, but it may be correspondingly applied also to processes util ⁇ izing sulphur containing cooking liquors, such as a sul- phate process.
• the sodium sulphide which dissolves more readily than the sodium carbonate, ends up in the mother liquor containing sodium silicates.
• the silicates, phosphates and/or aluminates may, however, be separated from the mother liquor by precipitating them, e.g., with lime or carbon dioxide, thereby obtain ⁇ ing a solution containing sodium sulphide, to be returned to the chemical circulation of the mill.
• the advantageousness of the method of the present inven ⁇ tion is still more prominent because its "waste", i.e. sodium silicate solution has commercial value.
• the value may be even higher than that of the replacement chemical of the alkali discharged in separat ⁇ ing the silicon (all sodium carbonate of the melt is not obtained in solid form; but part of it is wasted in solute form) .
• the situation may be advantageous to those mills which, besides soda cooking, produce high pulps or waste paper pulps.
• Peroxide bleaching of waste paper pulps namely, utilizes large amounts of sodium silicate as a stabilizer to prevent decomposition of peroxide.
• Fl patent 84190 further discloses a method of recovering alkalis from a used wash fluid. For this reason, earlier described alkali losses could be replaced with alkali received from the recovery process. Silicon may be effi ⁇ ciently removed from a silicate-containing solution poor in carbonate, for example, with calcium oxide, because a causticizing reaction, otherwise obstructing use of cal- cium oxide for removing silicon from green liquor, does not take place. Thus, the advantageousness of the inven ⁇ tion is not solely bound up with further use of sodium silicate.
• inorganic impurities may be removed from the mother liquor preferably by "causticizing" with calcium oxide.
• a small amount of lime mud is obtained, in which the content of silicon, phosphor, and aluminium is very high.
• the amount of lime to be discharged from the cycle may be decreased, and it is possible to produce more valuable lime mud for soil improvement.
• Fig. 1 shows a test result, representing the silica content of clean green liquor, depending of the evaporation degree
• Fig. 2 illustrates a preferred embodiment for imple ⁇ menting the method of the invention
• Fig. 3 illustrates a second preferred embodiment for implementing the method of the invention
• Fig. 4 illustrates a third preferred embodiment for implementing the method of the invention.
• the silica content of synthetic green liquor which contained 48 g Si0 2 /1, decreased during the advance of crystallizing evaporation in accordance with Fig. 1, which shows the dependency of the Si0 2 content of the obtained solution on the concen- tration degree of evaporation, in a solution of separated sodium carbonate crystals.
• the result was received by analyzing the sodium and sili ⁇ con contents of the crystals separated in the laboratory, and by calculating, on the basis of these, what the sil ⁇ ica content would be in the purified green liquor other ⁇ wise corresponding to that of the initial situation, if all sodium carbonate could be separated.
• the results indicate that, by evaporating the green liquor to a vol- ume of about one seventh of the original, an end product is received, which is green liquor having a silica con- tent of about 5 g/1. This value may be still further improved by a more efficient crystals washing.
• silica was leached out of a ground melt of sodium silicate and sodium carbonate to a small amount of water having a room temperature. It was found that optimization of the leaching temperature and treating time, it was possible to achieve as low silicon contents as with crystallizing evaporation.
• Fig. 2 illustrates a process where sodium carbonate is separated from green liquor by crystallizing evaporation.
• Black liquor is introduced via nozzles 2 into a waste liquor boiler 1, where it is combusted with air 3.
• the inorganic substance contained by black liquor which inorganic substance is mainly sodium carbonate, or sodium carbonate and sodium sulphide, depending on the cooking process, remains as melt at the bottom of the boiler.
• the melt is fed, in a manner known per se, into a dissolving means where it is dissolved in water or weak liquor from conduit 5 in order to form green liquor.
• green liquor contains insoluble impurities, it is normally clarified or filtered in order to remove the green liquor dregs.
• Filter 6 may preferably be a filter disclosed in international patent application PCT/FI94/- 00485.
• the filtered green liquor is conveyed via conduit 7 to an evaporating means 8.
• Evaporation is most preferably accomplished in several effects and by using falling film evaporators, in which green liquor flows along the outer surface of the evapor ⁇ ating means, i.e., tube or plate.
• the evapor ⁇ ating means i.e., tube or plate.
• the green liquor containing crystals is discharged from the liquor circulation system of the evaporators via conduit 9 to the crystals separation process, which is in this case effected by filters 10.
• the filtrate containing silicate, phosphate and/or alu inate is taken via conduit 11 for potential further use.
• the sodium carbonate crys ⁇ tals 12 are dissolved in a mixing tank 14 into a liquid, which is in this case condensate 15 taken from condenser 16 of the evaporator.
• green liquor 17 is received, which is purer with regard to silicon, phosphor and/or aluminium than the original green liquor from the dis ⁇ solving means 4.
• This purer green liquor may be caustici- zed in a manner known per se for use in delignification.
• Alkali may be wasted along with filtrate 11. This alkali is replaced with an alkali addition 13 into the mixing tank 14.
• Fig. 3 illustrates a second manner of separating solid sodium carbonate. This manner is based on leaching of more easily dissolving compounds, i.e., sodium silicates, sodium phosphates and sodium aluminates into a minor amount of water or into a weak white liquor so that the sodium carbonate, which is the hardest to dissolve, remains solid.
• more easily dissolving compounds i.e., sodium silicates, sodium phosphates and sodium aluminates into a minor amount of water or into a weak white liquor so that the sodium carbonate, which is the hardest to dissolve, remains solid.
• the melt is conveyed from the bottom of recovery boiler 1 into a hopper 21, where is becomes into contact with slurry 22, which contains solid sodium carbonate in a solution of sodium silicate, sodium phosphate and sodium aluminate. Thereby, the melt dissolves and solidifies into small particles.
• the smallest particles pass through a screen 23 and enter a dissolving tank 24, where sodium silicates, sodium phosphates and sodium aluminates are dissolved from particles.
• the coarsest particles pass on to a second dissolving tank 25, where they are dissolved and whereto water 29 is added for dissolving.
• the sol- ution overflows from this tank 25 into the first dissolv ⁇ ing tank 24, wherefrom the lime mud containing solid sodium carbonates is taken via conduit 27 into a filter 28 for separating solid crystals from the mother liquor.
• the sodium carbonate crystals are dissolved into water in tank 30, wherefrom the sodium carbonate solution 31 is taken to be causticized in a manner known per se.
• the filtrate in conduit 32 from filter 28 may be treated in order to remove silicates, phosphates and aluminates.
• Calcium oxide or lime milk is added to it from conduit 33 in order to precipitate silicates, phosphates, and alum ⁇ inates in a clarifier 34.
• the clarified silicon, phosphor and/or aluminium free alkali solution 35 is returned to the chemical circulation via a conduit 31, and the dregs in conduit 36 containing silicon, phosphor and/or alumin ⁇ ium is taken to further use.
• the solidified salt is disintegrated with a crusher 43. It is leached in such an amount of water in a dissolving means 44, that sodium silicates, sodium phosphates and sodium aluminates are dissolved whereas the sodium car ⁇ bonate remains mainly solid.
• the sodium carbonate is separated with a filter 45.
• the filtrate in conduit 46, containing silicates, phosphates, and aluminates, is removed for further use, whereas the sodium carbonate is dissolved into water from conduit 48 in the dissolving means.
• the alkali lost with the filtrate is replaced with an alkali addition 49.
• the sodium carbonate solution is conveyed via conduit 50 into a causticizing process, as described earlier.

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• Paper (AREA)
• Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
• Removal Of Specific Substances (AREA)
• Silicon Compounds (AREA)

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Citations (2)

• 1995
  • 1995-02-16 FI FI950692A patent/FI98225C/sv not_active IP Right Cessation
  • 1995-10-09 CN CN95197648A patent/CN1088130C/zh not_active Expired - Fee Related
  • 1995-10-09 WO PCT/FI1995/000556 patent/WO1996025554A1/en active Application Filing
  • 1995-10-09 AU AU36552/95A patent/AU3655295A/en not_active Abandoned
  • 1995-10-10 ZA ZA958530A patent/ZA958530B/xx unknown

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