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
  • D21C9/00—After-treatment of cellulose pulp, e.g. of wood pulp, or cotton linters ; Treatment of dilute or dewatered pulp or process improvement taking place after obtaining the raw cellulosic material and not provided for elsewhere
  • D21C9/10—Bleaching ; Apparatus therefor
  • D21C9/1015—Bleaching ; Apparatus therefor with use of means other than pressure, temperature
• • 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

Definitions

• the invention relates to a method for the treatment of spent liquor generated in kraft pulp production, in which method spent liquor is treated following kraft pulp production at the black liquor evaporation plant or prior to evaporation. Particularly, by means of said method the formation of scale on heat exchanger surfaces is avoided.
• cellulose-containing material like wood is treated at elevated temperature with alkaline cooking liquor containing sodium hydroxide and sodium hydrogen sulfide.
• Fresh, inorganic cooking liquor is called white liquor
• spent liquor containing material dissolved from the starting material, e.g. wood is called black liquor.
• white liquor is regenerated from the chemicals contained in the black liquor.
• the properties of the black liquor entering the evaporation plant in a modified cooking process differ from those of a traditional cook, and may complicate the evaporation of black liquor. This is particularly severe if it is desired to treat the cellulose-containing material entering the kraft process, preferably as wood chips, with a solution containing large amounts of sodium carbonate and only minor amounts of sodium hydroxide and sodium hydrogen sulfide, if any. Most preferably, such a solution is a mixture of the molten chemicals leaving the recovery boiler and weak white liquor, referred to as green liquor.
• the black liquor described above exiting a modified cooking process and bound for the evaporation plant, contains large amounts of calcium bound to the dissolved material.
• the dissolved material consists of lignin and other components of wood soluble in the cooking liquor.
• the dissolved material degrades due to the alkalinity and raising temperature of the cooking liquor, whereby the bound calcium is liberated, reacting with, e.g. the materials extracted from the raw material thereby forming calcium soaps, and with the carbonate in the cooking liquor forming calcium carbonate.
• the solid soaps formed from the extracted material are separated from the black liquor prior to evaporation, and as evaporation proceeds, in order to minimize the disadvantages caused.
• the bulk of the calcium carbonate formed has already crystallized in the digester, and the solubility of dissolved calcium carbonate increases with temperature.
• the solids content of the black liquor rises as temperature increases, and there is no significant crystallization of the calcium carbonate entering the evaporation plant in the black liquor as evaporation proceeds; thus, normally no problems are caused by precipitation on the evaporator heat transfer surfaces that would limit the plant's capacity of evaporating water from the black liquor.
• a separate deactivation unit must be provided adjacent to the evaporation plant, raising investment and running costs.
• the system further comprises a unit for the addition of solid calcium carbonate to provide the seeding material as required. As the spent liquor is heated by steam in a heat exchanger, it is likely that the scaling problem is carried over from the evaporation plant to said heat exchanger.
• black liquor from a modified kraft cooking process is treated according to claim 1 without diverting the liquor outside the evaporating plant, whereby the calcium bound to the wood components dissolved in the black liquor is liberated in a controlled manner, not negatively affecting the evaporation process.
• the liberation of calcium bound to the material dissolved in the black liquor is dependent on the solids content and alkalinity of the black liquor, and on temperature
• the liquor stream flows from one evaporation unit to the next in an expedient order. If this is not possible, the black liquor is treated in separate processing equipment prior to the evaporation plant in order to liberate calcium and form calcium carbonate.
• Figure 1 represents a solubility chart showing the dependency of the liberation of bound calcium on the solids content and alkalinity and the temperature of a black liquor
• Figure 2 shows the sequence of black liquor flow in a six-stage evaporation plant according to the prior art
• Figure 3 represents a solubility chart showing how the critical temperature of calcium liberation is reached in an evaporation plant according to Figure 2;
• Figure 4 shows the flow sequence in an evaporation plant according to the invention
• Figure 5 shows, by means of a solubility chart, how calcium is liberated when a flow sequence according to Figure 4 is used;
• Figure 6 represents an arrangement to deactivate calcium before the black liquor is led to the evaporation plant.
• Figure 7 shows, by means of a solubility chart, how calcium is liberated when deactivation at the cooking unit, according to Figure 6, is used.
• the black liquor is directed from stage 4 to stage 2, and on to stage 3, from where it is directed to stage 1.
• stage 4 the solids content of the liquor is preferably from about 30 to about 35 %, and its temperature from about 110°C to about 120 °C; in stage 3, the solids content of the liquor is preferably from about 35 to about 45 %, and its temperature from about 90°C to about 120 °C.
• Figure 5 shows how calcium is not yet liberated in stages 2 and 3, only in stage 1.
• the solids content of the black liquor is so high that the sodium sulfate and sodium carbonate therein starts to form a solid phase generally referred to as double salt. Double salt crystallizes in a much larger amount than calcium carbonate, which thus is prevented from forming a precipitate of its own on the heat transfer surfaces; instead it is included in the extensive crystallization of double salt.
• the double salt on the other hand, is easily dissolved by conveying into the unit, for example, black liquor having a solids content below the solubility limit of the double salt. Though the calcium carbonate does not dissolve into such black liquor, which is typically saturated with calcium carbonate, the calcium carbonate is removed from the evaporator heat transfer surfaces as the surrounding double salt dissolves.
• stage sequence may vary from that disclosed above, there may for example be several evaporation stages following the crossing of the calcium carbonate solubility threshold.
• An advantage of the embodiment disclosed above is that there are very moderate costs associated therewith when an existing evaporation plant is updated.
• a new evaporating plant may be constructed at no extra cost relative to an arrangement according to the prior art.
• the black liquor temperature is raised above the critical value for a sufficiently long period in order for the calcium to be liberated and form calcium carbonate before the black liquor is led to the evaporation plant.
• This treatment is referred to as calcium deactivation.
• FIG. 6 shows an arrangement of tanks in a batch displacement kraft cooking plant as disclosed in, for example Finnish Patent 92224.
• Warm black liquor from black liquor tank (a) is led to heat exchanger (b), where its temperature is raised close to the critical temperature for calcium liberation, using in the heat exchanger hot black liquor from pressurized holding tank (c).
• heat exchanger (b) the state of the black liquor is changed in order to cross the critical limit for calcium liberation, by raising the temperature with straight steam heating, and/or by raising the solids content by addition of black liquor higher in solids (d), and/or regulating alkalinity by adding pH-lowering solution from tank (e); said solution may be for example water from a tall oil digestion plant.
• the black liquor is kept in pressurized holding tank (c) until calcium carbonate has been formed. From the holding tank, the black liquor is directed to the evaporation plant via heat exchanger (b).
• Figure 7 shows a solubility chart for an arrangement according to Figure 6.
• the black liquor leaving tank (a) is in a state corresponding to point (1).
• the liquor After passing heat exchanger (b), the liquor has moved into a state corresponding to point (2); subsequently the liquor is taken across the solubility limit to point (3) (by raising the temperature) or (4) (by raising both solids content and temperature).

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• Life Sciences & Earth Sciences (AREA)
• Engineering & Computer Science (AREA)
• Wood Science & Technology (AREA)
• Paper (AREA)

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Publications (1)

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Cited By (5)

Citations (6)

• 1997
  • 1997-07-11 FI FI972954A patent/FI972954A/sv unknown
• 1998
  • 1998-07-09 WO PCT/FI1998/000580 patent/WO1999002771A1/sv active Application Filing

Patent Citations (6)

Non-Patent Citations (1)

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