|Title of Invention||
A METHOD FOR GENERATING SODIUM HYDROXIDE IN A WOOD PULPING PROCESS .
|Abstract||A method is provided for the use of borates in the recausticization of alkali-containing liquors such as those produced in the chemical and semi-chemical pulping of wood and in the oxidation and bleaching of wood pulp. Sodium hydroxide is regenerated from sodium carbonate-containing smelts in such pulp processes by autocausticization through the addition of limited amounts of borate. Autocausticization may be carried out at boron to carbonate molar ratios below 2:1 and at sodium to boron molar ratios exceeding 3:1, resulting in high reaction efficiency. Partial autocausticization using borate may also be combined with conventional lime recausticization for the regeneration of sodium hydroxide from sodium carbonate-containing smelts.|
|Full Text||PARTIAL AUTOCAUSTICIZATION OF ALKALI LIQUORS FOR WOOD PULPING
This application claims the benefit of U.S. Provisional Application No. 60/087,779 filed
June 1, 1998. 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) Cooking or bleaching (delignification):
Na2HBO3LignOH ? LignONa + NaH2BO3
2 LignONa + x.O2 ? Na2CO3 + y.CO2 + zH2O
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 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.
BRIEF DESCRIPTION OF THE INVENTION
This 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 boron. 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.
DETAILED DESCRIPTION OF THE INVENTION
According to this invention an improved process is provided 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 Na4O5) 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
(C03) 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.
The following examples illustrate the method of this invention.
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.
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.
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.
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:
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.
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.
What is claimed is:
1. An improved method for regenerating sodium hydroxide in a wood pulping process
, wherein a sodium carbonate-containing smelt is autocausticized by reaction with borate, the
improvement comprising adding a limited amount of borate sufficient to provide 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, thereby regenerating sodium hydroxide by autocausticizing said sodium carbonate-
containing smelt at a high reaction efficiency rate.
2. The method according to claim l wherein said borate is added in the form of an inorganic
3. The method according to claim 2 wherein said inorganic borate compound is selected
from the group consisting of boric acid, boric oxide and sodium borate.
4. The method according to claim 2 wherein said inorganic boron compound is a sodium
5. The method according to claim 4 wherein said sodium borate is selected from the group
consisting of sodium metaborate, sodium tetraborate and the hydrated compounds thereof.
6. The method according to claim 4 wherein said sodium borate is selected from the group
consisting of sodium metaborate and the hydrated compounds thereof.
7. The method according to claim 4 wherein said sodium borate is selected from the group
consisting of sodium tetraborate and the hydrated compounds thereof.
8. The method according to claim 4 wherein said sodium borate is sodium tetraborate
9. The method according to claim 1 wherein the boron to carbonate molar ratio is in the
range of from about 0.01:1 to 1:1.
10. The method according to claim 1 wherein the boron to carbonate molar ratio is in the
range of from about 0.02:1 to 0.8:1.
11. The method according to claim 1 wherein the boron to carbonate molar ratio is in the
range of from about 0.05:1 to 0.4:1.
12. The method according to claim 1 wherein the sodium to boron molar ratio is in the range
of from about.4.1 to 400:1.
13. The method according to claim 1 wherein the sodium to boron molar ratio is in the range
of from about 5:1 to 200:1.
14. The method according to claim 1 wherein the sodium to boron molar ratio is in the range
of from about 10:1 to 100:1.
15. The method according to claim 1 wherein said borate is added in the form of sodium
borate, the boron to carbonate molar ratio is in the range of from about 0.02:1 to 0.8:1 and the
sodium to boron molar ratio is in the range of from about 5:1 to 200:1.
16. The method according to claim 1 wherein said borate is introduced into said process by
addition to a process liquor selected from the group consisting of green liquor and black liquor.
17. The method according to claim 1 wherein said borate is added in the form of sodium
tetraborate pentahydrate, the boron to carbonate molar ratio is in the range of from about 0.05:1
to 0.4:1, the sodium to boron molar ratio is in the range of from about 10:1 to 100:1 and said
borate is introduced into said process by addition to a process liquor selected from the group
consisting of green liquor and black liquor.
18. A method for regenerating sodium hydroxide from a sodium carbonate-containing smelt
in a wood pulp bleaching process comprising autocausticizing said smelt by reaction with borate,
said borate being added in a limited amount sufficient to provide 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 for regenerating sodium hydroxide in a wood pulping process comprising the
(a) partially autocausticizing a sodium carbonate-containing smelt by reaction with
borate, 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
20. The method according to claim 19 wherein said autocausticization reaction is carried out
by adding a limited amount of borate sufficient to provide 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.
21. The method according to claim 19 wherein said wood pulping process is a wood pulp
22 A partially causticized aqueous liquor composition suitable for use in a wood pulping
process 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 and a
sodium to boron molar ratio of greater than about 3:1.
23. A partially causticized aqueous liquor composition according to claim 22 wherein the
molar ratio of boron to carbonate is in the range of from about 0.01:1 to 1:1 and the molar ratio of sodium to boron is in the range of from about 4:1 to 400:1.
A method is provided for the use of borates in the recausticization of alkali-containing liquors such as those produced in the chemical and semi-chemical pulping of wood and in the
oxidation and bleaching of wood pulp. Sodium hydroxide is regenerated from sodium
carbonate-containing smelts in such pulp processes by autocausticization through the addition of limited amounts of borate. Autocausticization may be carried out at boron to carbonate molar ratios below 2:1 and at sodium to boron molar ratios exceeding 3:1, resulting in high reaction efficiency. Partial autocausticization using borate may also be combined with conventional lime recausticization for the regeneration of sodium hydroxide from sodium carbonate-containing smelts.
|Indian Patent Application Number||IN/PCT/2000/565/KOL|
|PG Journal Number||31/2009|
|Date of Filing||27-Nov-2000|
|Name of Patentee||U. S. BORAX INC.|
|Applicant Address||26877 TOURNEY ROAD, VALENCIA, CA|
|PCT International Classification Number||D21C 11/04|
|PCT International Application Number||PCT/US1999/12067|
|PCT International Filing date||1999-05-28|