PatentDe  


Dokumentenidentifikation EP1092059 19.07.2007
EP-Veröffentlichungsnummer 0001092059
Titel PARTIELLE SELBSTKONSTIFIZIERUNG ALKOHOLISCHER LAUGE ZUM HOLZAUFSCHLUSS
Anmelder U.S. Borax Inc., Valencia, Calif., US
Erfinder BAIR, Charles M., Thousand Oaks, CA 91360, US;
MCBROOM, Robert B., Santa Clarita, CA 91351, US;
TRAN, Honghi N., Toronto, Ontario M6G 3C8, CA
Vertreter derzeit kein Vertreter bestellt
DE-Aktenzeichen 69936259
Vertragsstaaten AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LI, LU, MC, NL, PT, SE
Sprache des Dokument EN
EP-Anmeldetag 28.05.1999
EP-Aktenzeichen 999553068
WO-Anmeldetag 28.05.1999
PCT-Aktenzeichen PCT/US99/12067
WO-Veröffentlichungsnummer 1999063151
WO-Veröffentlichungsdatum 09.12.1999
EP-Offenlegungsdatum 18.04.2001
EP date of grant 06.06.2007
Veröffentlichungstag im Patentblatt 19.07.2007
IPC-Hauptklasse D21C 11/00(2006.01)A, F, I, 20051017, B, H, EP
IPC-Nebenklasse D21C 11/04(2006.01)A, L, I, 20051017, B, H, EP   

Beschreibung[en]

This invention relates to the use of borates in the recausticization of alkali-containing liquors such as are produced in the chemical and semi-chemical pulping of wood and in the oxidation and bleaching of wood pulp, and more particularly to the partial autocausticization of such liquors by the addition of limited amounts of borate. It also relates to a process for complete recausticization of such liquors by a combination of partial autocausticization using borate with conventional lime recausticization.

BACKGROUND OF THE INVENTION

There are a variety of processes which utilize alkali-based chemicals such as sodium hydroxide in the pulping, bleaching or oxidation of wood materials. These processes include chemical and semi-chemical methods for breaking down wood chips or other wood-based starting materials into wood fiber for the production of paper, cardboard and similar cellulose-based products. Other processes which use such alkali-based chemicals include the oxidizing and bleaching of wood pulp for paper production.

In a typical chemical-based wood pulping process, such as the kraft process, wood chips are treated with an aqueous solution of mainly sodium hydroxide (caustic soda) to separate out lignin and other organic constituents which bind the cellulose fibers together in order that the wood can be broken down into individual fibers for various uses such as paper making. In the kraft process this solution also contains sodium sulfide. The sodium hydroxide reacts and combines chemically with lignin forming an organic-based solution referred to as black liquor or spent liquor. The black liquor is separated from the fiber and burned in a recovery boiler to recover heat from the organics. In the process of burning, the black liquor is converted into smelt, a molten phase in which the sodium-organic complex has been converted to sodium carbonate. In the kraft process sodium sulfide is also formed. In order to regenerate sodium for reuse in the pulping process, sodium carbonate must be converted back to sodium hydroxide or "recausticized". The molten smelt is typically dispersed with steam as it is poured into an aqueous solution, such as recycled dilute white liquor, weak wash or water, in which it dissolves to form a sodium carbonate solution referred to as green liquor due to the dark green appearance caused by the presence of an insoluble residue known as dregs. In some operations, such as in the soda process, the smelt is cooled and solidified prior to dissolution. The green liquor is sent to a causticizer where sodium carbonate is converted back to sodium hydroxide, thus producing "white liquor" for reuse in the wood pulping process. This process, known as causticization, is accomplished by the reaction of sodium carbonate with calcium hydroxide, also known as hydrated or slaked lime, in the green liquor. In the process of regenerating sodium hydroxide, the calcium hydroxide is converted to calcium carbonate, as a precipitate (also known as lime mud), which is then converted back to calcium hydroxide in a separate lime recovery circuit, also known as a lime recovery cycle, so that it can be reused in the causticizer again. In the lime recovery circuit calcium carbonate is burned in a kiln to drive off carbon dioxide as a gas, converting the calcium carbonate to calcium oxide, which is then hydrated with water in the green liquor to reform calcium hydroxide which can be reused in the causticization step.

An alternative method of recausticization which does not require the use of lime and the associated lime recovery process was developed in the 1970's by Jan Janson, a researcher in Finland (US Patent No. 4,116,759). Janson proposed that sodium carbonate in the smelt could be causticized automatically ("autocausticized") in the recovery boiler by the addition of borate to the wood pulping circuit, thus eliminating the need for subsequent recausticization by calcium hydroxide and the accompanying lime recovery circuit.

The chemical reactions proposed by Janson for the autocausticization process were:

  1. (1) Cooking or bleaching (delignification):



            Na2HBO3LignOH ↔ LignONa + NaH2BO3



  2. (2) Combustion:



            2 LignONa + x.O2 → Na2CO3 + y.CO2 + zH2O



  3. (3) Autocausticization:



            2 NaH2BO3 + Na2CO3 → 2 Na2HBO3 + CO2 + H2O



In autocausticizing, sodium metaborate acts like a catalyst, in that it will react with sodium carbonate in the smelt to produce a more basic disodium borate and carbon dioxide. When the disodium borate is dissolved in water, it is hydrolyzed to regenerate sodium hydroxide and the original sodium metaborate, hence eliminating the need for lime and the lime kiln and associated lime recovery cycle.

In a typical kraft process, sodium hydroxide is recovered for reuse in the process using the traditional lime recausticization methods described above. Autocausticization offers several potential benefits over recausticization with lime. These include elimination of the capital costs associated with the lime recovery circuit, reduction of energy costs by elimination of the need to burn the calcium carbonate to release carbon dioxide and elimination of other operating costs associated with the lime recovery circuit. Alternatively, in some operations, where sodium is not being recovered and reused in the process, autocausticization offers significant potential cost savings due to reduced chemical requirements, since borate is not used up in the process, but is instead returned to the start of the process for reuse along with the regenerated sodium hydroxide. However, Janson teaches in the '759 patent that it is essential to keep the sodium to boron molar ratio equal to or less than 2 (Na/B≤2) in order to ensure complete causticization.

Large scale trials (Janson, Jan and Bengt Arhippainen, "Mill Scale Development of the Borate-Based Kraft Pulping Process", International Conference on Recovery of Pulping Chemicals, Vancouver, British Columbia, Canada, September 22-25, 1981) were conducted in the early 1980's to investigate the commercial applicability of autocausticization using borate. However, operating difficulties were encountered and the process was never adopted on a commercial basis. Such difficulties are largely related to changes in the physical properties of the black liquor due to the presence of high levels of borate, such as large increases in the dissolved solids content and viscosity, leading to difficulties with spraying and droplet size in the recovery boiler, reduced evaporation rate and the transporting of the liquor from the digestor to the recovery boiler. Also, a reduction in the heating value of the black liquor may require the addition of supplemental fuel in the recovery boiler.

Despite the potential benefits offered by autocausticization, it has not been adopted commercially in view of the problems associated with the process. It is an object of this invention to provide an improved causticization process which will provide some of the significant benefits of autocausticization, while minimizing the difficulties associated with it.

DESCRIPTION OF THE INVENTION

According to the present invention there is provided a method for regenerating sodium hydroxide in a wood pulping process which method comprises:

  1. (a) partially autocausticizing a sodium carbonate-containing smelt by reaction with borate using a sodium to boron molar ratio of at least 3:1, thereby producing a reacted portion and an unreacted portion of said smelt;
  2. (b) dissolving said partially autocausticized smelt in aqueous solution, thereby regenerating sodium hydroxide from said reacted portion of said smelt and forming dissolved sodium carbonate from said unreacted portion of said smelt; and
  3. (c) reacting said dissolved sodium carbonate with lime, thereby regenerating additional sodium hydroxide.

Preferably the borate used according to the invention is an inorganic borate compound, preferably boric acid, boric oxide or sodium borate. Preferred sodium borates are sodium metaborate, sodium tetraborate or a hydrated compound thereof, more preferably sodium tetraborate pentahydrate.

The autocausticizing according to the invention is suitably carried out at a boron to carbonate molar ratio of less than about 2:1 and a sodium to boron molar ratio of greater than about 3:1. The boron to carbonate molar ratio preferably is in the range of from 0.01:1 to 1:1, most preferably 0.02:1 to 0.8:1 and most preferably 0.05:1 to 0.4:1. The sodium to boron molar ratio preferably is in the range of from 4:1 to 400:1, more preferably 5:1 to 200:1, more preferably 10:1 to 100:1.

The said borate is suitably introduced in the method of the invention by addition to a process liquor selected from green liquor and black liquor.

The method according to the invention is suitably used when the wood pulping process is a wood pulp beaching process or a kraft process.

One particular embodiment according to the invention comprises:

  1. (a) partially autocausticizing a sodium carbonate-containing smelt by reaction with borate, thereby producing a borate-containing partially autocausticized smelt which includes a reacted portion and an unreacted portion;
  2. (b) dissolving in aqueous solutions said borate-containing partially autocausticized smelt, including said reacted and unreacted portions thereof to form a borate-containing liquor, thereby regenerating sodium hydroxide and forming dissolved sodium carbonate from said reacted and unreacted portions, respectively, in said borate-containing liquor; and
  3. (c) introducing lime into said borate-containing liquor so as to react said dissolved sodium carbonate with said lime, thereby regenerating additional sodium hydroxide from said borate-containing liquor.

The invention further comprises a partially causticized aqueous liquor composition suitable for use in a wood pulping process according to the invention comprising sodium carbonate, sodium hydroxide, sodium borate, lime, calcium carbonate and water, and having a boron to carbonate molar ratio of less than about 2:1, preferably in the range of from 0.01:1 to 1:1 and a sodium to boron molar ratio of greater than about 3:1, preferably in the range of from 4:1 to 400:1.

According to one embodiment, the invention provides an improved method for causticizing sodium carbonate-containing smelt resulting from the combustion of black liquors, wherein a limited amount of borate is added such that only a portion of the sodium carbonate is autocausticized. The method of this invention provides reduced borate deadload in the circuit, resulting in improved recovery boiler operating conditions such as reduced black liquor viscosities and higher reaction efficiencies compared with full autocausticization at higher ratios of sodium to bom. This invention further provides a method for recausticization of sodium carbonate-containing smelts, wherein partial autocausticization is used in combination with lime recausticization to achieve improved conversion of sodium carbonate back to sodium hydroxide upon hydration of the smelt and lime recausticizing.

The method according to the invention may be used for causticizing sodium carbonate-containing smelt at sodium to boron molar ratios exceeding 3:1, wherein the amount of borate used is less than the stoichiometric requirement for complete autocausticization of all of the alkali carbonate present in the black liquor. It has been found that the autocausticization reaction can proceed with unexpectedly high efficiencies under these conditions. Further, it has been observed that this process of partial autocausticization occurs at a rate which exceeds 100% stoichiometric efficiency at low levels of borate addition, based on the autocausticization reactions proposed in the '579 patent.

According to Janson's proposed reactions two moles of boron are consumed per mole of sodium carbonate recovered, as shown in equation (3) above. At boron addition levels equivalent to about 52% of the stoichiometric requirements for full autocausticization of sodium carbonate, and at a sodium to boron molar ratio of about 2.9:1, the observed reaction efficiencies averaged about 86%. However, in tests at low levels of borate addition, equivalent to 5% and 10% of full autocausticization requirements, the conversion of sodium carbonate to sodium hydroxide was determined to be 9-17% and 15-17%, respectively, which is significantly above the theoretical 100% reaction efficiency. The sodium to boron molar ratios in these tests were about 20:1 and 11:1, respectively. This suggests that under these conditions of low borate addition and high sodium to boron molar ratios the autocausticization reaction may lead to the formation of a different borate composition than was proposed in the '579 patent. In particular, the reaction product is believed to be Na3BO3 (trisodium borate), rather than Na2HBO3 (disodium borate, also written as Na4B2O3) which was proposed by Janson. As a result, a higher level of autocausticization is achieved for a given level of borate used.

Partial autocausticization may occur to some extent in both the gas phase and the smelt. The reaction between borate and sodium carbonate can take place at temperatures as low as about 600°C and can be carried out at temperatures up to at least 925°C. Temperatures in the lower furnace of a recovery boiler where partial autocausticization would be expected to occur can range from as low as about 700 to 850°C in portions of the smelt to as high as 1100-1200°C in the gas phase or char bed. Thus a broad temperature range in which partial autocausticization may be carried out is between about 600° and about 1200°C. Typically the partial autocausticization reaction will occur automatically in the recovery boiler following the combustion of the organic-based black liquor. The combustion reaction leading to the formation of sodium carbonate is shown in equation (2) above. In order to achieve maximum reaction efficiency, the method of this invention requires that the molar ratio of boron (B) to carbonate (CO3) in the smelt, produced from combustion of the black liquor, be kept below 2:1, the stoichiometric requirement for full autocausticization according to equation (3), above. Preferably the boron to carbonate molar ratio is in the range of from about 0.01:1 to 1:1, more preferably in the range of from about 0.02:1 to 0.8:1, and most preferably in the range of from about 0.05:1 to 0.4:1. In addition, the molar ratio of sodium to boron should be above 3:1, preferably in the range of from about 4:1 to 400:1, more preferably in the range of from about 5:1 to 200:1 and most preferably in the range of from about 10:1 to 100:1. Under these conditions, the partial autocausticization reaction efficiency has been found to increase with increases in the sodium to boron ratio. However, very low boron to carbonate molar ratios, such as below about 0.01: 1, and very high sodium to boron molar ratios, such as above about 400:1, would require very low levels of borate addition, such that minimal autocausticization of sodium carbonate would be achieved, despite the high reaction efficiency.

The borate requirements for partial autocausticization can be provided in a variety of inorganic borate forms including boric acid, boric oxide, and sodium borates such as sodium tetraborate and sodium metaborate and the various hydrated forms thereof. The preferred way of adding the borate into the process is to mix it into the spent (black) liquor or green liquor. It appears that an important factor in promoting the autocausticization reaction at high molar ratios of sodium to boron is the avoidance of excess sodium hydroxide in the reaction mixture prior to reaction, to avoid premature conversion of the borate reactants into autocausticization reaction products. However, the presence of such sodium hydroxide levels prior to recausticization would not be expected in normal wood pulping operations.

Another embodiment of the present invention is recausticization of a sodium carbonate-containing smelt by successive causticization steps including partial autocausticization of sodium carbonate, followed by a lime causticization step in which additional sodium carbonate is converted back to sodium hydroxide. Such an approach will avoid or minimize many of the problems associated with full autocausticizing such as high dissolved solids, high viscosity and low heating value of the black liquor by avoiding the need for high levels of borate in the recirculating liquor, while providing many of the benefits of autocausticizing. It will reduce the lime recovery circuit energy requirements or reduce the lime consumption in plants which do not recover lime. The process will therefore provide increased operating capacity in plants which are limited by the throughput capacity of an existing lime recovery circuit. The complete recausticization process begins with partial autocausticization of a sodium carbonate-containing smelt with borate, such as in a recovery boiler as described above, thereby producing a reacted portion and an unreacted portion of the smelt. Following partial autocausticization, the smelt is dissolved in water or aqueous liquor to make up an aqueous solution referred to as green liquor, thereby regenerating sodium hydroxide from the reacted portion of the smelt and dissolving the residual sodium carbonate from the unreacted portion. Lime, in the form of calcium oxide or calcium hydroxide, is added to the green liquor, containing the residual sodium carbonate. The lime converts dissolved sodium carbonate to sodium hydroxide and in the process the lime is converted to calcium carbonate. The resulting calcium carbonate mud can then be sent to a conventional lime recovery circuit for conversion back to calcium oxide if desired. The sodium hydroxide-containing solution, which is now referred to as white liquor, is ready for reuse in the delignification process or related pulping circuit.

EXAMPLES:

The following examples illustrate the method of this invention.

EXAMPLE 1

Varying amounts of sodium metaborate dihydrate, NaBO2•2H2O, were reacted with 5 gram samples of sodium carbonate at 850°C in order to simulate autocausticizing. Four different levels of the sodium metaborate were tested (see Table 1). The amounts of sodium metaborate used were 0.5, 1.0, 2.5, and 5.0 grams. A repeat of the 5.0 g level was also run. These mixtures corresponded to 5%, 10%, 26%, and 52% autocausticizing, respectively, based on the stoichiometry proposed by Janson (equation 3). The molar ratios of sodium to boron were 20, 10.6, 4.8 and 2.9, respectively. Table 1 STARTING CONDITIONS Experiment g Na2CO3 g NaBO2·2H2O Na/B Molar Ratio % Theoretical Autocausticizing 1 5.0 0.5 20 5 2 5.0 1.0 10.6 10 3 5.0 2.5 4.8 26 4 5.0 5.0 2.9 52 5 5.0 5.0 2.9 52

The sodium metaborate samples were each heated in tared platinum crucibles to about 200°C for about an hour to drive off most of the water of crystallization. Then the 5 gram samples of sodium carbonate (anhydrous) were added to each crucible, and the furnace temperature was increased to 850°C. The furnace was held at 850°C for 30 minutes, and then the samples were removed to a dessicator filled with nitrogen to minimize exposure to carbon dioxide in the atmosphere. The samples were analyzed for carbonate and hydroxide in order to determine the extent of the reaction. Table 2 RESULTS % Autocausticization based on: % Efficiency based on: Experiment %CO3 2- % OH- %CO3 2- % OH- 1 17 9 340 180 2 17 15 170 150 3 31 31 119 119 4 47 53 90 102 5 45 50 87 98

The results (Table 2) showed that the carbonate level was lowered and the hydroxide content increased as a function of the amount of borate added, indicating that a reaction had taken place, even at high molar ratios of sodium to boron. In fact it was observed that the reaction efficiency increased, substantially exceeding 100%, as the molar ratios of sodium to boron increased. The data suggests that reaction under these conditions may lead to formation of Na3BO3 (trisodium borate), a different borate composition than was proposed by Janson. The benefit to a mill would be that less borate would be required for a given amount of caustic regenerated, thus reducing chemical costs as well the high amount of NaBO2 deadload which must be carried throughout the pulping/chemical recovery circuit.

EXAMPLE 2

Five gram samples of black liquor were combined with varying amounts of sodium tetraborate (Na2B4O7) and heated at 850°C for 30 minutes. It was estimated that the combustion of black liquor would produce 280 grams Na2CO3 per kilogram of wet black liquor. The stoichiometric requirement of sodium tetraborate would therefore be 106 grams Na2B4O7 per kg wet black liquor based on the following assumed reactions:



        Na2B4O7 + 5Na2CO3→ 4Na3BO3 + 5CO2



        4Na3BO3 + 5H2O → Na2B4O7 + 10NaOH



The amounts of borate used corresponded to 12, 19 and 48% (partial) autocausticization, 100% (full) autocausticization and 201% autocausticization (double the assumed stoichiometric requirement). The results are shown in Table 3. TABLE 3 Experiment Molar Ratio Na/B Theoretical % Autocaust. Actual % Autocaust. based on: % Efficiency based on: OH CO3 OH CO3 1 21 12 12 41 106 351 2 14 19 22 48 115 256 3 5.7 48 36 53 76 110 4 3 100 59 74 58 73 5 1.7 201 80 96 40 48

As in example 1, these results show that partial autocausticization is feasible and that the reaction does take place at sodium to boron ratios greater than 3:1, and can be carried out with greater efficiency under these conditions.

There is the undesirable possibility that if lime and borate react in the lime recausticization stage, insoluble calcium borates could form and precipitate out of solution, thereby being removed from the solution along with the calcium carbonate mud. This would result in undesirable losses of borate and could lead to problems in the operation of the lime kiln. In order to address this problem several tests were run to determine if calcium borate would form during lime recausticization in a simulated kraft process green liquor. The results showed that almost all of the borate reports to the filtrate and that which was found with the solids was likely present as adhering liquor. There was no evidence that calcium borates were formed under the conditions of lime recausticization.

While the descriptions herein relate largely to the chemical recovery process of causticization as applied in the kraft process, the method of this invention is also applicable to other alkali-based processes such as other forms of chemical and semi-chemical pulping of wood and processes which generally require the use of sodium hydroxide in the pulping, bleaching or oxidation of cellulose-based materials. Accordingly, various modifications and changes of the invention can be made and, to the extent that such variations incorporate the spirit of this invention, they are intended to be included within the scope of the appended claims.


Anspruch[de]
Verfahren zur Regenerierung von Natriumhydroxid in einem Holzaufschlussprozess, wobei dieses Verfahren umfasst: (a) das partielle Autokaustifizieren einer Natriumcarbonat-haltigen Schwarzlaugenschmelze durch Reaktion mit Borat unter Verwendung eines molaren Verhältnisses von Natrium zu Bor von mindestens 3:1, wodurch ein umgesetzter Teil und ein nicht umgesetzter Teil dieser Schwarzlaugenschmelze erzeugt wird; (b) das Auflösen dieser partiell autokaustifizierten Schwarzlaugenschmelze in wässriger Lösung, wodurch Natriumhydroxid aus dem umgesetzten Teil dieser Schwarzlaugenschmelze regeneriert wird, und sich gelöstes Natriumcarbonat aus dem nicht umgesetzten Teil dieser Schwarzlaugenschmelze bildet; und (c) die Reaktion von diesem gelösten Natriumcarbonat mit Calciumoxid, wodurch weiteres Natriumhydroxid regeneriert wird. Verfahren gemäß Anspruch 1, wobei dieses Borat eine anorganische Boratverbindung ist, vorzugsweise Borsäure, Boroxid oder Natriumborat. Verfahren gemäß Anspruch 2, wobei diese anorganische Borverbindung ein Natriumborat ist, vorzugsweise Natriummetaborat, Natriumtetraborat oder eine Hydratverbindung von diesen, weiter bevorzugt Natriumtetraborat-Pentahydrat. Verfahren gemäß einem der Ansprüche 1 bis 3, wobei dieses Autokaustifizieren bei einem molaren Verhältnis von Bor zu Carbonat von weniger als etwa 2:1 und einem molaren Verhältnis von Natrium zu Bor von mehr als etwa 3:1 durchgeführt wird. Verfahren gemäß Anspruch 4, wobei das molare Verhältnis von Bor zu Carbonat im Bereich von 0,01:1 bis 1:1, vorzugsweise von 0,02:1 bis 0,8:1 und weiter bevorzugt von 0,05:1 bis 0,4:1 liegt. Verfahren gemäß Anspruch 4 oder 5, wobei das molare Verhältnis von Natrium zu Bor im Bereich von 4:1 bis 400:1, vorzugsweise von 5:1 bis 200:1, weiter bevorzugt von 10:1 1 bis 100:1 1 liegt. Verfahren gemäß einem der vorhergehenden Ansprüche, wobei das Borat in diesen Prozess eingeführt wird, indem es zu einer Prozessflüssigkeit zugegeben wird, die ausgewählt ist aus Grünlauge und Schwarzlauge. Verfahren gemäß einem der vorhergehenden Ansprüche, wobei dieser Holzaufschlussprozess ein Holzpulpenbleichprozess oder ein Kraft-Prozess ist. Partiell kaustifizierte, wässrige Flüssigkeitszusammensetzung, die zur Verwendung in einem Holzaufschlussprozess gemäß einem der vorhergehenden Ansprüche geeignet ist, die Natriumcarbonat, Natriumhydroxid, Natriumborat, Cacliumoxid, Calciumcarbonat und Wasser umfasst, und ein molares Verhältnis von Bor zu Carbonat von weniger als etwa 2:1, vorzugsweise im Bereich von 0,01:1 bis 1:1, und ein molares Verhältnis von Natrium zu Bor von mehr als etwa 3:1, vorzugsweise im Bereich von 4:1 bis 400:1, aufweist.
Anspruch[en]
A method for regenerating sodium hydroxide in a wood pulping process which method comprises: (a) partially autocausticizing a sodium carbonate-containing smelt by reaction with borate using a sodium to boron molar ratio of at least 3:1, thereby producing a reacted portion and an unreacted portion of said smelt; (b) dissolving said partially autocausticized smelt in aqueous solution, thereby regenerating sodium hydroxide from said reacted portion of said smelt and forming dissolved sodium carbonate from said unreacted portion of said smelt; and (c) reacting said dissolved sodium carbonate with lime, thereby regenerating additional sodium hydroxide. A method according to claim 1 wherein said borate is an inorganic borate compound, preferably boric acid, boric oxide or sodium borate. A method according to claim 2 wherein said inorganic boron compound is a sodium borate, preferably sodium metaborate, sodium tetraborate or a hydrated compound thereof, more preferably sodium tetraborate pentahydrate. A method according to any one of claims 1 to 3 wherein said autocausticizing is carried out at a boron to carbonate molar ratio of less than about 2:1 and a sodium to boron molar ratio of greater than about 3:1. A method according to claim 4 wherein the boron to carbonate molar ratio is in the range of from 0.01:1 to 1:1, preferably 0.02:1 to 0.8:1 and more preferably 0.05:1 to 0.4:1. A method according to claim 4 or 5 wherein the sodium to boron molar ratio is in the range of from 4:1 to 400:1, preferably 5:1 1 to 200:1, more preferably 10:1 to 100:1. A method according to any one of the preceding claims wherein said borate is introduced into said process by addition to a process liquor selected from green liquor and black liquor. A method according to any one of the preceding claims wherein said wood pulping process is a wood pulp bleaching process or a kraft process. A partially causticized aqueous liquor composition suitable for use in a wood pulping process according to any.one of the preceding claims, comprising sodium carbonate, sodium hydroxide, sodium borate, lime, calcium carbonate and water, and having a boron to carbonate molar ratio of less than about 2:1, preferably in the range of from 0.01:1 1 to 1:1 and a sodium to boron molar ratio of greater than about 3:1, preferably in the range of from 4:1 to 400:1.
Anspruch[fr]
Méthode de régénération d'hydroxyde de sodium dans un procédé de réduction du bois en pâte, laquelle méthode comprend : (a) l'autocaustification partielle d'un salin contenant du carbonate de sodium par réaction avec un borate, en utilisant un rapport molaire du sodium au bore d'au moins 3/1, produisant ainsi une partie ayant réagi et une partie n'ayant pas réagi audit salin ; (b) la dissolution dudit salin partiellement autocaustifié dans une solution aqueuse, régénérant ainsi l'hydroxyde de sodium issu de ladite partie ayant réagi audit salin et formant du carbonate de sodium dissous à partir de ladite partie n'ayant pas réagi audit salin ; et (c) la réaction dudit carbonate de sodium dissous avec de la chaux, régénérant ainsi davantage d'hydroxyde de sodium. Méthode selon la revendication 1, dans laquelle ledit borate est un composé de borate inorganique, de préférence de l'acide borique, de l'oxyde borique ou du borate de sodium. Méthode selon la revendication 2, dans laquelle ledit composé de bore inorganique est un borate de sodium, de préférence du métaborate de sodium, du tétraborate de sodium ou un composé hydraté de celui-ci, mieux encore du tétraborate de sodium pentahydraté. Méthode selon l'une quelconque des revendications 1 à 3, dans laquelle ladite autocaustification est effectuée à un rapport molaire du bore au carbonate inférieur à environ 2/1 et à un rapport molaire du sodium au bore supérieur à environ 3/1. Méthode selon la revendication 4, dans laquelle le rapport molaire du bore au carbonate est dans la plage de 0,01/1 à 1/1, de préférence de 0,02/1 à 0,8/1 et, mieux encore, de 0,05/1 à 0,4/1. Méthode selon la revendication 4 ou 5, dans laquelle le rapport molaire du sodium au bore est dans la plage de 4/1 à 400/1, de préférence de 5/1 à 200/1, mieux encore de 10/1 à 100/1. Méthode selon l'une quelconque des revendications précédentes, dans laquelle ledit borate est introduit dans ledit procédé par addition à une liqueur de traitement choisie parmi une liqueur verte et une liqueur noire. Méthode selon l'une quelconque des revendications précédentes, dans laquelle ledit procédé de réduction de bois en pâte est un procédé de blanchiment de pâte de bois ou un procédé kraft. Composition de liqueur aqueuse partiellement caustifiée adaptée pour être utilisée dans un procédé de réduction de bois en pâte selon l'une quelconque des revendications précédentes, comprenant du carbonate de sodium, de l'hydroxyde de sodium, du borate de sodium, de la chaux, du carbonate de calcium et de l'eau, et ayant un rapport molaire du bore au carbonate inférieur à environ 2/1, de préférence dans la plage de 0,01/1 à 1/1 et un rapport molaire du sodium au bore supérieur à environ 3/1, de préférence dans la plage de 4/1 à 400/1.






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