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Master's Theses Graduate College
12-2003
Study of Trisodium Borate Formation and Its Reaction with Green Liquor in Partial Autocausticizing
Pasupathy Rajan Subbaiyan
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Recommended Citation Subbaiyan, Pasupathy Rajan, "Study of Trisodium Borate Formation and Its Reaction with Green Liquor in Partial Autocausticizing" (2003). Master's Theses. 4957. https://scholarworks.wmich.edu/masters_theses/4957
This Masters Thesis-Open Access is brought to you for free and open access by the Graduate College at ScholarWorks at WMU. It has been accepted for inclusion in Master's Theses by an authorized administrator of ScholarWorks at WMU. For more information, please contact [email protected]. STUDY OF TRISODIUM BORA TE FORMATION AND ITS REACTION WITH GREEN LIQUOR IN PARTIAL AUTOCAUSTICIZING
by
Pasupathy Rajan Subbaiyan
A Thesis Submitted to the Faculty of The Graduate College in partial fulfillment of the requirements forthe Degree of Master of Science Department of Paper Engineering, Chemical Engineering and Imaging
WesternMichigan University Kalamazoo, Michigan December 2003 Copyright by Pasupathy Rajan Subbaiyan 2003 ACKNOWLEDGMENTS
I would like to thank my Thesis Advisor Dr. John Cameron for his valuable advice and ideas, which brought some cohesiveness to this topic. His help in coalescing my ideas into something substantive has been invaluable. Secondly, I would like to thank Murugavel Anbalagan and Biljana Bujanovic, who took time to discuss with me their ideas in the experimental set-up and for helping me to carry out some experiments. I also thank the members of my thesis committee, Dr David Peterson and Dr Said AbuBakr for taking time to review my work.
Lastly, I would like to thank the U.S. Borax Inc. and the Department of Energy forproviding necessary financialsupport to complete this project.
Pasupathy Rajan Subbaiyan
ii STUDY OF TRISODIUM BORA TE FORMATION AND ITS REACTION WITH GREEN LIQUOR IN PARTIAL AUTOCAUSTICIZING
Pasupathy Rajan Subbaiyan,_M.S.
WesternMichigan University, 2003
This thesis investigates a new development of autocausticizing the green liquor with trisodium borate. Sodium metaborate (NaBO2) reacts with the sodium
° carbonate (Na2CO3) in the char bed at 850 C in the recovery furnace to form trisodium borate (Na3BO3) which in tum, when dissolved in water produces sodium hydroxide (NaOH) regenerating sodium metaborate (NaBO2). The main objective of this research is to study the formation of trisodium borate from the reaction of sodium metaborate with sodium carbonate at different molar ratios of carbonate to borate and to determine the effect of the process parameters such as products concentration and temperature on the reaction of trisodium borate with green liquor producing sodium hydroxide. The results show evidence of trisodium borate formation due to decarbonization of sodium carbonate by sodium metaborate and with increase in sodium carbonate to sodium metaborate molar ratios, the decarbonization of sodium carbonate lowers, where more carbon dioxide (CO2) is released, which needs to be effectively purged out to improve decarbonization. The reaction of trisodium borate with green liquor (Na2S, H2O and Na2CO3) was fast and about 85% of trisodium borate formed reacts with water to form sodium hydroxide. TABLE OF CONTENTS
ACKNOWLEDGMENTS...... n
LIST OF TABLES...... V
LIST OF FIGURES ...... ·...... Vl
CHAPTER
1. INTRODUCTION
Overview of Original Kraft Chemical Recovery Process...... 1
Recovery Boiler...... 3
Recausticizing Process...... 4
Slaking and Causticizing...... 5
Calcining...... 6
Overall Chemistry...... 8
2. AUTOCAUSTICIZING OF SMELT WITH SODIUM BORATES
Autocausticizing...... 9
Decarbonization of Sodium Carbonate with Sodium Metaborate...... 11
Conventional Causticizing AfterPartial Borate Autocausticizing...... 12
Chemistry of Autocausticizing Reactions...... 12
3. OBJECTIVES AND EXPERIMENTAL ...... 15
4. METHODOLOGY
Experimental Reagents...... 1 7
Preparation of Trisodium Borate...... 17
Determination of Reaction Rate and Equilibrium...... 18
Causticizing...... 19
iii Table of Contents - Continued
CHAPTER
5. RESULTS AND DISCUSSION
Decarbonization of Sodium Carbonate with Sodium Metaborate...... 20
Reaction ofNa3BO3 with Green Liquor...... 26
Causticizing Efficiency Results...... 29
Cost Savings...... 31
CONCLUSIONS...... 32
FINANCIAL SUPPORT AND FACILITIES...... 33
REFFERENCES...... 34
APPENDICES
A. Decarbonization and Weight Loss Details...... 36
B. Amount ofNaOH Produced after the Reaction ofNa3BO3 with H2O...... 37
C. Split up of Decarbonization Results...... 38
D. Effectof Temperature on Hydrolysis ofTrisodium Borate...... 39
E. Effectof Concentration on Hydrolysis of Trisodium Borate...... 42
F. Partial Autocausticizing and Causticizing Details...... 44
iv LIST OF TABLES
1. Different Carbonate to Borate Molar Ratio and CO2 Evolved forEach Condition...... 20
2. Percentage ofNaOH Produced at DifferentInitial Na3B03 Concentration...... 28
3. Conditions for Causticizing Experiments...... 30
V LIST OF FIGURES
1. Typical KraftRecovery Cycle...... 7
2. Percentage ofDecarbonization ofNa2CO3 by NaBO2 ...... 21
3. Theoretical and Actual Weight Loss Due to Decarbonization...... 23
4. Percentage of Trisodium Borate Reacted with Water to Form NaOH...... 24
5. Summary of Autocausticizing Results...... -...... 25
6. Reaction Rate ofNa3BO3 with Water at DifferentTemperatures ...... 27
7. Effectof Concentration on the Rate and Equilibrium for the Reaction of Trisodium Borate with Water...... 29
8. Effect of Time on Causticizing Efficiency at 35% Autocausticizing Level...... 30
vi CHAPTER!
INTRODUCTION
Overview of Original Kraft Chemical RecoveryProcess
Kraft pulping is a modification of the soda process, which employs the use of sodium hydroxide (NaOH). Originally, the soda process could not compete with the sulfiteprocess due to the cost associated with sodium hydroxide (NaOH) and the need for chemical recovery. Around 1879, 111 C.F. Dhal in Germany was looking for alternativesto reduce costs associated with the soda process. The makeup chemical used in the soda process was sodium carbonate (Na2CO3), and he replaced it with the cheaper sodium sulfate (Na2SO4). Upon heating sodium sulfate with carbon at high temperature, one forms sodium sulfide (Na2S) (Reaction 1). Mixing sodium sulfide with water provides a mixture of sodium hydroxide (NaOH) and sodium hydrosulfide (NaSH) (Reaction 2).
With this innovation, the Kraft process was born.The first mill was in Sweden in 1890 and the first mill in the United States was up and running in Roanoke Rapids, NC
(Champion) in 1909.
Na2SO4 � Na2S (sodium sulfide)...... (1)
Na2S + H2O NaOH + NaSH ...... (2)
1 Kraft pulps initially had two distinct disadvantages121, a dark pulp and chemical recovery was required. However, the pulp is stronger and has better dimensional stability.
Chemical recovery systems rapidly became quite advanced and most advanced mills are now shooting for98-99% chemical recovery.
The Kraft recovery cycle is a closed cycle process designed to destroy toxic waste from pulping, to co-generate steam and power and more importantly to recycle pulping chemicals. A series of process and different types of equipment are used to convert the weak black liquor into re-generated pulping liquor and in the process generates steam and power.
The Kraft recovery cycle is a series process beginning with application of pulping liquor, (white liquor) to wood chips. Pulping results in the dissolution of 40 to 60 % of the weight of the wood. The liquor containing the spent pulping chemicals and the dissolved organic matter is separated from the pulp by washing and concentrated by evaporation. The concentrated liquor is then burned in the recovery boiler. A black liquor boiler has two main functions. On one hand, it generates steam from the heat energy liberated from the combustion of the organic constituents in the black liquor as it is burned in the boiler. On the other hand, the spent chemicals from the pulp digesting process (sulfur and sodium) are separated from the components and recovered. The recovery boiler thus serves as a chemical reactor. The pulping chemicals are recovered from the bottom of the furnace as a molten smelt which is then dissolved in water to form green liquor. The resultant green liquor is treated with lime and causticized, which is the last step in the regeneration of cooking liquor. A substantial amount of equipments are
2 required forthis recycling process who's capital cost representing a significant fractionof the cost of a bleached Kraftpulp mill.
RecoveryBoiler
The major chemical components of the black liquor solids are carbon, oxygen, hydrogen, sodium and sulfur. The burning of black liquor generates mainly carbon dioxide and water in the form of flue gas (Reaction 3). One could think that, in an ideal recovery boiler process, all the diverse sulfur and sodium compounds would be transformed to sodium sulfide (Na2S) and sodium carbonate (Na2C03). Sodium carbonate and sodium sulfide flowsout as molten smelt. There are a number of chemicals present in the recovery process in minor composition. 13•41
Black Liquor+ 02 --+ Na2C03+ Na2S+ Flue Gas ...... (3)
There are three zones present inside the chemical recovery boiler. The reduction zone is present at the bottom of the furnace, the drying zone where the liquor is fired, and the oxidation zone in the turbulent upper section. The reduction of oxidized sulfur compounds at the furnace hearth is an endothermic reaction that removes useful thermal energy from the system:
3 Recausticizing Process
The function of the recausticizing plant is to convert sodium carbonate into active sodium hydroxide and remove various impurities introduced from the furnace and limekiln. The process begins the dissolution of molten smelt in weak liquor (weak wash) to form green liquor (so-called because of green color). The green liquor is then clarified to remove "dregs" (insoluble substances), and is reacted with lime (CaO) to form white liquor. The white liquor is clarified to remove precipitated lime mud (CaC03), and is then ready to be used for cooking. Auxiliary operations include washing of both the dregs and lime mud forsoda recovery and calcining or reburning of lime mud to regenerate lime.
The causticizing reaction can be explained in two steps. The lime first reacts with water to form calcium hydroxide (Reaction 4), which in turn reacts with sodium carbonate (present in the green liquor) to form sodium hydroxide (Reaction 5). 151
CaO + H20 - Ca(OH)z + Heat ...... ( 4)
Ca(OH)2 + Na2C03 - 2NaOH +CaC03 ...... ( 5)
Both sodium carbonate and sodium hydroxide are soluble, while calcium hydroxide (slaked lime) and calcium carbonate (lµne mud) are limited in solubility and essentially take part in the reaction as solids: Since calcium carbonate is more insoluble than calcium hydroxide, the reaction is driven to the right. The equilibrium for this reaction may be written as hydroxide ion squared over carbonate ion (Equation 6). 151
4 K= -- ...... (6)
The most significant feature of this equilibrium equation is that the numerator
(hydroxide ion) is squared, while the denominator (carbonate ion) is not. Because of this, the numerator increases faster than the denominator as the initial liquor concentration increases, which reduce the degree of completion for this reaction or causticizing efficiency(Equation 7).
NaOH Causticizing Efficiency (CE) = as Na20 ...... (7)
Sometimes the causticizing efficiency is corrected by subtracting from both numerator and denominator the sodium hydroxide already present in the green liquor.
Slaking and Causticizing
Green liquor and reburned lime are fed continuously at a controlled rate to the slaker, where high temperature and violent agitation promote rapid conversion of quick lime into milk-of-lime. Proper slaking is impo�ant to the subsequent operations of causticizing and lime mud settling. The slurry is channeled from the mixing section where unreacted particles (grit) settle to the bottom and are raked out for disposal as landfill. A significant portion of the reaction also takes place in the slaker. If lime and green liquor are fed at relatively high temperature to the slaker, considerable steam 5 generation can be expected due to the exothermic reactions. A high temperature is helpful in accelerating the reaction rate and ensuring a good causticizing efficiency.1 51
Liquor continuously overflows from the slaker into a series of agitated tanks with about two hours of total retention, sufficient time for the causticizing reaction to be carried to completion.
Calcining
The CaCO3 is insoluble in white liquor and is removed as lime mud and washed to remove entrained white liquor and the weak wash liquor is sent to the dissolving tank.
The washed mud is then calcined in a limekiln to regenerate lime. Calcining is a high temperature endothermic reaction and therefore uses a large amount of heat energy. The calcining reaction is as follows
CaCO3 + Heat - CaO + CO2 ...... ( 8)
The resulting lime is used for causticizing and the calcium cycle is complete. A typical chemical recovery cycle is summarized in Figure 1.
6 PULPING
� Na,S
WASHING
Cao
EVAPORATION CAUSTICIZING
BURNING
Figure 1. Typical KraftRecovery Cycle
Overall Chemistry
The inorganic chemistry of the kraft cycle is not very complex. There are two
intersecting loops, a sodium loop and a calcium loop. The sodium loop consists of the
reaction during pulping, black liquor burningand _pausticizing. The calcium loop consists
of causticizing and calcining reactions. The key chemical reactions can be summarized as
follows.15 1
7 Pulping:
NaOH + Na2S + Wood - Pulp+ Liquor Solids...... (9)
Reductive Burning:
Liquor Solids+ 02 - Na2C03 + Na2S + CO2 +H20 ...... (10)
Causticizing:
Na2C03 + H20 + CaO - CaC03 + 2NaOH...... (11)
Calcining:
CaC03 - CaO + C02 ...... (12)
8 CHAPTER2
AUTOCAUSTICIZING OF SMELT WITH SODIUM BORATES
Autocausticizing
Autocausticizing processes are non-conventional causticizing processes based on the use of amphoteric salts to release carbon dioxide (CO2) from sodium carbonate
(Na2CO3) in the kraftfurnace and generate sodium hydroxide (NaOH) in the green liquor.
The important part of autocausticizing is the borate based autocausticizing, which can supply either part or all of the sodium hydroxide requirements in the kraft pulping process. When autocausticizing supplies all the hydroxide available from sodium carbonate (Na2CO3), the calcining/causticizing process is eliminated, and when autocausticizing supplies part of the hydroxide, the material and energy requirements of calcining and causticizing are reduced.
6 7 8 In 1970s, Janson 1 • • 1 suggested the use of sodium borates for autocausticizing process. He proposed that by reacting sodium carbonate (Na2CO3) in the molten smelt with sodium metaborate (NaBO2) to form disodium borate (N<14B2O5) as shown in
Reaction 13. The disodium borate subsequently is hydrolyzed to form sodium hydroxide
(NaOH) and to regenerate sodium metaborate (NaBO2) in the dissolving tank as shown in
Reaction 14.
9 This process is attractive because it produces NaOH directly from the green liquor, thus potentially eliminating the recausticizing cycle. There were a host of other compounds such as alumina (AhO3), silica (SiO2), and disodium phosphate (Nc4P2O7), which may be used as autocausticizing agents.
Two basic differences between borates and other compounds are
1) Borates are water soluble while others are not (except forNc4P2O 7).
2) The decarbonization of Na2CO3 (Reaction 13) and the hydrolysis of the resulting
higher Na/B ratio borates (Reaction 14) occur rapidly in the borate case because they
happen in solution and are not the result of solid-solution interactions. Also, it does
not react with sulfide in the smelt.
A review of various autocausticizing processes conducted by Grace 191 suggested that full scale implementation of borate process is technically difficult and economically
unattractive, mainly because of the high borate "deadload" imparted into the liquor cycle
which resulted in rising the viscosity of black liquor arid thereby reducing its heating
value. The high inorganic load reduces the organic concentration in the black liquor.
Partial autocausticizing may be an attractive alternative for the mills that required
incremental causticizing and limekiln capacities. However, Janson suggested that
Reaction 13 would be severely hindered if the Na:B molar ratio of the reactants is greater
than 1.5:1 (or 1.5) and that it would not even occur if Na/B is greater than 3. This means
that partial autocausticizing is technically not feasible because Na/B is likely to be greater
than 3. By the end of 1980's, this development became dormant.
10 1101 In 1990, US Borax Inc., in search of new applications for borate products, began to re-examine technologies that involve the use of borates in the pulp and paper industry. Previous research work was reviewed and several research groups conducted numerous experiments based on the previous research work.
It was found out that in air, sodium borates can react with molten sodium carbonate at any Na/B value, and that the reaction product is likely to be trisodium borate, 3Na2O.B2O3 (or Na3BO3). These findings imply that partial autocausticizing with borate may now be technically feasible. The borate based autocausticizing processes can supply either part or entire hydroxide required for the kraftpulping processes.
Decarbonization of Sodium Carbonate with Sodium Metaborate 1101 Recently, Tran, Mao, Cameron and Bair found that only one mole of sodium metaborate (NaBO2) is required for the generation of two moles of sodium hydroxide
(NaOH). Their simultaneous differential and thermogravimetric (DTA/TGA) studies of the reaction between sodium metaborate (NaBO2) and sodium carbonate (Na2CO3) indicated that the decarbonization of sodium carbonate occurred in accordance to
Reaction 15, when trisodium borate (Na3BO3) was formed. On dissolving in water, the trisodium borate reacts with the water to form sodium hydroxide and regenerate the metaborate, as shown by Reaction 16. Their experimental results shows thatthe actual autocausticizing reactions are considerably more effective than proposed by Janson.
11 Na3BO3 + H2O 2 NaOH + NaBO2 ...... (16)
The acceptance of borate-based autocausticizing depends largely on the careful investigation of the solid and molten phase reactions and the determination of the equilibrium and the rate controlling process variables ofthe hydrolysis ofNa3BO3.
111 121 Cameron and Zaki • found that the reaction between sodium metaborate
(NaBO2) and sodium carbonate (Na2CO3) begins in the solid phase at temperatures between 600 and 850°C. In the solid phase, approximately two moles of sodium carbonate are required to generate one mole of carbon dioxide. Once the salts melt, the reaction is extremely rapid with one mole of sodium metaborate reacting with one mole of sodium carbonate to generate one mole of carbon dioxide.
Conventional Causticizing After Partial Borate Autocausticizing
With partial borate autocausticizing, only part of the sodium carbonate reacts with
sodium metaborate in the recovery furnace and the remaining sodium carbonate is
converted into sodium hydroxide through the conventional lime cycle. Mill trials have
reported an increase in the causticizing efficiencywith partial borate autocausticizing.
Chemistryof Autocausticizing Reactions
The chemical parameters of interest in dete:rriiningthe equilibrium ofthe reaction
of trisodium borate (Na3BO3) in green liquor are the total titrable alkali (TTA), active
alkali (AA), effective alkali (EA), the sulfidity (S), Causticizing efficiency (CE), and
151 activity definedby the followingequations.
12 TTA =NaOH + Na2S + Na2CO3 as Na2O ...... : ...... (17)
= AA NaOH + Na2S as Na2O ...... (18)
= EA NaOH + 0.5 (Na2S) as Na2O ...... (19)
Na2S Sulfidity (S) = ----- as Na2O ...... : ...... (20) NaOH + Na2S
NaOH Causticizing Efficiency(CE) = as Na2O ...... (21)
= Activity as Na2O ...... (22)
From the literature of boron compounds 1141 the Equilibrium constant K for the reaction of Na3BO3 in green liquor can be assumed as follows.
From Reaction 16,
2 NaBO2 [OH] K =---- ...... (23) [NaBO3)
13 The Equilibrium of the Reaction 16 will be studied initially from the above
Equation 23 and the exact equilibrium constant will be determined fromthe experimental data. This can be estimated fromthe solubility limits ofNaB02 and Na3B03.
14 CHAPTER3
OBJECTIVES AND EXPERIMENTAL
This proposal utilizes a new development of autocausticizing the green liquor with trisodium borate, thus potentially eliminating the need for slakers, causticizers and the limekiln. It is widely believed that sodium metaborate (NaBO2) can react with the sodium carbonate (Na2CO3) in the char bed at 850°C in the recovery furnace to form trisodium borate (Na3BO3) which in turn, when dissolved in water produces sodium hydroxide (NaOH)regenerating sodium metaborate (NaBO2).
The main objectives of this research are to study the following:
1. The formation of trisodium borate (Na3BO3) from the reaction of sodium
metaborate (NaBO2) with sodium carbonate (Na2CO3) at different molar ratios of
carbonate to borate.
2. The effect of temperature and active alkali concentration forthe reaction between
3. The effect of borate autocausticizing on partial conventional causticizing with
slaked lime.
The experimental part forthis research work is composed of three phases.
1. The first phase involves the study of fonIUltion of trisodium Borate (Na3BO3) at
different molar ratios of sodium metaborate (NaBO2) to sodium carbonate
15 2. The second phase is to study the reaction rate of trisodium Borate (Na3B03) with
green liquor (Na2S, H20 and Na2C03) and the effectof active alkali concentration
and temperature on this reaction rate.
3. The third phase is to convert the remaining sodium carbonate (Na2C03) after
partial autocausticizing (second phase) to sodium hydroxide (NaOH) using lime
and compare the final results with the conventional causticizing results.
16 CHAPTER4
METHODOLOGY
Experimental Reagents
Synthetic trisodium borate salt and green liquor, representing borate autocausticizing liquor, was prepared using reagent grade sodium carbonate, sodium borate, and sodium sulfideand commercial burntlime.
Preparation of Trisodium Borate
Sodium carbonate salt and sodium metaborate salt was mixed together at different molar ratios of carbonate to borate in a tubular alumina reactor and the reactor was then placed in a muffle furnace. The reaction temperature was increased either continually or in discrete increments till it reaches the desired temperature. It is the heated constantly at
850 to 900°C for 6 hrs to allow the reaction to proceed. Nitrogen was passed through the reactor to remove carbon dioxide generated and to prevent the establishment of dynamic equilibrium or reverse reaction. The degree of the reaction was determined by the amount of weight lost due to the evolution of carbon dioxide. The resultant salt was then cooled and ground to small particles. The ground salt was used for the determination of equilibrium and rate controlling variables such as�temperature, and the concentration of active alkali.
NaBO2 + Na2CO3 - Na3BO3 + CO2 ...... (15)
17 Determination of Reaction Rate and Equilibrium
The liquor with known concentration of sodium sulfide was prepared in a round bottom flask. It was then heated to the specific temperature using a heater with a temperature controller, which also has a magnetic stirrer fitted into it. The temperature of the reaction was monitored using thermocouples. The flask was also fitted with a vapor condenser to condense the vapor formed during the reaction. Once the temperature reached the desired value, previously identified amount of Na3B03+Na2C03 salt required for the experimental run was added to the system. Small amount of sample from the system was withdrawn under vacuum from the reactor, filtered inline, and immediately immersed in an ice bath to arrest the reaction instantly. The borate white liquor was analyzed using a Metrohm 685 Dosimat ion analysis instrument by analytical procedure developed by U.S Borax Incorporated, which is a variant of ABC titration (TAPPI
standard method for analyzing the white liquor). The final concentration was controlled by manipulating the amount of Na3B03+Na2C03 salt added to the system. This auto titrator determines the concentration of hydroxyl, carbonate, sulfur and borate ions. The
effect of temperature and active alkali concentration on the rate of Reaction 16 and
equilibrium was determined by measuring the ionic concentrations at different time
intervals.
Na3B03 + H20 2 NaOH + NaBOz ...... : ...... (16)
18 Causticizing
The amount of unconverted sodium carbonate is calculated in moles after partial autocausticizing and the equal moles of burnt lime (CaO) were used to convert the
remaining carbonate to hydroxide. For the causticizing experiment that starts at a pre definedautocausticizing level, sodium carbonate, metaborate and sodium hydroxide were
dissolved in deionized water and heated to 95°C. Since the slaking reaction between burnt lime (CaO) and water is highly exothermic, the addition of the burnt lime raised the
reactor temperature to the target temperature and the reactor was then held at this
temperature through the course of the reaction. Small samples were periodically
withdrawn under vacuum from the reactor, filtered inline, and analyzed, using a duo
titrator ion analyzer developed by U.S. Borax Inc. (variant of the TAPPI standard ABC
titration method).
Na2C03 + H20 + CaO - CaC03 + 2NaOH ...... (11)
19 CHAPTERS
RESULTS AND DISSCUSSION
Decarbonization of Sodium Carbonate with Sodium Metaborate
The decarbonization of sodium carbonate with sodium metaborate was carried at five different carbonate to borate molar ratios or autocausticizing levels as shown in
Table 1. Each experiment was duplicated once to confirm the results. From the amount
CO2 of released during the decarbonization, it is evident that sodium metaborate is not
100% effective in decarbonizing sodium carbonate. The decarbonization is higher at lower carbonate to borate molar ratios and it reduces almost linearly with increase in carbonate to borate ratio.
Table 1. Different Carbonate to Borate Molar Ratio and CO2 Evolved for Each Condition
Autocausticizing Moles of Moles of Moles of Moles ofH2O Moles of
Level Na2CO3 NaBO2 CO2 evolved Evaporated Na3BO3 Salt Formed
Based on CO2 Loss
15 I 0.15 0.136 0.3 0.136 15 1 0.15 0.139 0.3 0.139 25 1 0.25 0.205 0.5 0.205 25 1 0.25 0.205 0.5 0.205 35 1 0.35 0.259 0.7 0.259 35 1 0.35 0.250 0.7 0.250 45 0.8 0.36 0.246 0.72 0.246 45 0.8 0.36 ,0.247 0.72 0.247 55 0.8 0.44 0.293 0.88 0.293 55 0.8 0.44 0.293 0.88 0.293
20 The decarbonization efficiency of sodium metaborate is less than the expected
value (100%). This may be due to the formation of intermediate borate salt in the beginning of the reaction that hinders further decarbonization of the unreacted sodium carbonate, which lowers the decarbonization of sodium carbonate by sodium metaborate.
Thus at higher carbonate to borate molar ratio, more intermediate borate salts are formed in the beginningof the reaction, which reduces the decarbonization of sodium carbonate.
The decarbonization of sodium carbonate by sodium meta-borate is over 90% at
15% autocausticizing level (calculated as the amount of sodium metaborate in moles per
one mole of sodium carbonate), but reduces with increase in autocausticizing level as
shown in the followingFigure 2.
100
90
80 y = -6.367x + 95.387 R2 = 0.9279 '0 .! 70 ca QI 60 .!ca 50
QI 40
30
20
10
0 0.15 0.25 0.35 0.45 0.55 Ratio of Metaborate to Carbonate
Figure 2. Percentage of Decarbonization ofNa2CO3 by NaBO2
21 The decarbonization values are based on the weight loss due to the evolution of carbon dioxide. Assuming the reaction goes to completion, the cumulative carbon dioxide release appears to support the stoichiometry of one mole of metaborate is required to release one mole of carbon dioxide at low metaborate to carbonate ratios and lees so at higher ratios.
Although CO2 release is close to the stoichiometry of Reaction 15, it does not reach completion at higher concentrations of NaB02•
With purge tube located above the reactor bed show that the decarbonizing reactions are highly reversible and if the carbon dioxide is not removed, reactions can be much slower. This could be the potential reason for lower decarbonization percentage at higher autocausticizing level, where more CO2 is released. However, in the recovery furnace, the reaction between carbon and carbon dioxide should serve as a natural sink for the carbon dioxide and this reversibility may not limit the reaction. Figure 3 supports the above arguments because the actual weight loss values are close to the theoretical value at low metaborate to carbonate ratio and starts deviating from the theoretical line with increase in metaborate to carbonate ratio (more CO2 is released) indicating the purging is inefficient. This problem can be over come in the recovery furnace as explained above.
22 16
14
12
vr 10
8 ·; 6
4
2
0 0.15 0.25 0.35 0.45 0.55 Brate to Carbonate Rtio
Figure 3. Theoretical and Actual Weight Loss Due to Decarbonization
The trisodium borate salt was allowed to cool at room temperature beforereacting
· it with water. Sodium borates, particularly Na3BO3 can expect to be carbonated if they are exposed to CO2. This is because the Na2O component of the compounds might react with CO2 to formNa 2CO3 leading to arguments on purging of CO2.
The partially decarbonized sodium carbonate salt was then reacted with water to determine the amount of trisodium borate formed out of the decarbonized salt. The causticizing efficiency method was used to detepnine the actual amount of sodium
carbonate converted to sodium hydroxide fro� the original amount of sodium carbonate
present in the system. From the stoichiometry of Reaction 16, the actual amount of
trisodium borate formed was determined from the NaOH formed and the corresponding
23 percentage out of the decarbonized sodium carbonate salt was determined based on weight loss during decarbonization.
Na3B03 + H20 ------+ 2 NaOH + NaB02 ...... (16)
From Figure 4 data, we can conclude that about 85 % of trisodium borate salt
(calculated based on weight loss during decarbonization reaction) reacted with water to produce sodium hydroxide at room temperature and there is no trend based on initial metaborate to carbonate ratio.
95
90
85
80
GI 75 GI ,;e. 70
65
60
55
50 0.15 0.25 0.35 0.45 0.55 Initial Metaborate to Clrbonate Rtio
Figure 4. Percentage of Trisodium Borate Reacted with Water to Form NaOH
24 The occurrence of Reaction 15 at different autocausticizing level assuming the reaction reaches completion, suggests that partial autocausticizing is technically feasible since it requires proportionally less borate.
Figure 5 shows the split up of the decarbonized salt and unreacted sodium carbonate for different molar concentrations of sodium metaborate. The decarbonization of sodium carbonate by sodium metaborate to formtrisodium borate is summarized in the following. graph, which is a combination of weight loss calculations and hydrolysis results of the resultant salt. See Appendix A, B, and C for the experimental conditions.
100%
90%
80% Carbonate
70% □ Unconvred Trisodium
Cl) 60% Borate
50%
40% convred to NaOH 30%
20%
10%
0% 0.15 0.25 0.35 - 0.45 0.55 Initial Metaborate to Carbonate Rtio
Figure 5. Summary of Autocausticizing Results
25 Reaction ofNa�BO� with Green Liquor
The reaction of trisodium borate-sodium carbonate (Na3B03+Na2C03) salt with green liquor (H20 and Na2S) was instantaneous even at room temperature. The experiment results showed that trisodium borate formed during decarbonization sodium carbonate (Na2C03) reacts with water readily to produce NaOH. The reaction occurs very fast and the effect of temperature on the equilibrium of the reaction is negligible (Figure
6). At the conditions shown in Figure 6, increase in rate of reaction before equilibrium results from higher temperature. The reaction attains equilibrium so fast that the temperature is not very critical even at 25 °C. In actual mill conditions, this reaction takes place above 100 °C, it can be expected that temperature may not be critical to the rate of reaction and equilibrium. See Appendix D for experimental conditions foreach run.
Na3B03 + H20 2 NaOH + NaB02 ...... (16)
t
26 30.00
25.00
20.00 ♦ ♦ ...
RI 15.00 �:- : ♦ 90 Degrees RI
10.00 ■ 70 Degrees
... 50 Degrees
5.00
0.00 2 4 6 8 10 12 14 16 Time in min
Figure 6. Reaction Rate ofNa3B03 with Water at Different Temperatures
The other critical parameter on the rate and equilibrium of Reaction 16 is concentration of products (Na, OH, C03 ions). The experimental results shows that active alkali concentration does not have any major effect on reaction rate and equilibrium, although the reaction attains equilibrium faster with low concentration. This little effect can be negligible because the reaction takes placr instantaneously. Industries normally operate at an active alkali range of 100-130 g7L as Na20. The change in concentration of products is only due to the change in concentration of NaOH as Reaction 16 proceeds
towards right side with sodium carbonate and sodium sulfide (0% sulfidity) remaining
constant throughout the reaction. As shown in Figure 7, the difference in rate and 27 equilibrium of Reaction 16 at 40-110 g/L as Na2O range is fairly small. This suggests the effect of concentration may not be very critical on rate and equilibrium of Reaction 16 even at the industrial operating conditions. See Appendix E for experimental conditions for each run. A detailed calculation of theoretical amount of NaOH expected from the amount of Na3BO3 added to the system at different levels of concentration is shown in
Table 2. The amount of NaOH formed is same for two different concentrations but the third experiment with lowest initial Na3BO3 concentration produced a higher percentage ofNaOH. These results indicate that very lower concentration ofNa3BO3 produces more
NaOH, but under the industrial operating condition (70-110 g/L of Na2CO3 + NaOH as
Na2O) the concentration does not have any effecton the final equilibrium of Reaction 16.
Table 2. Percentage ofNa OH Produced at Different Initial Na3BO3 Concentration
Concentration Final equilibrium Na2COJ + Na38O3 Amount of NaJBO3 Expected NaOH Actual NaOH %NaOH
Na2CO3 + NaOH Na2CO3 + NaOH added in gms present in the salt g/L g/L Formed
as Na2O g/L as Na2O g/L in g/L
40 38.985 16.54 15.28 9.56 13.36 1.39
72 72.445 33.09 30.56 19.12 22.73 1.19
110 108.835 49.5 45.72 28.6 33.65 1.18
28 120.00 �------
80.00 - • d"� ♦ 40g/L as Sodium Oxide �z 60.00 + 1/J � J: ('Q 0 :::! • ■ 72g/L as Sodium Oxide t'Q Cl z 40.00 ... 110 g/L as Sodium Oxide
20.00 +------
0.00 +--��-�--�--�-�--�--- 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Time in min
Figure 7. Effectof Concentration on the Rate and Equilibrium for the Reaction of
Trisodium Borate with Water
Causticizing Efficiency Results
In Figure 8 below, borate based autocausticizing was simulated with equivalent amount of sodium metaborate, and sodium hydroxide and reducing the sodium carbonate : and burnt lime. This simulated borate autocausticizing results was compared to the actual autocausticizing by producing the sodium hydroxide through trisodium borate, which was formed by decarbonizing sodium carbonate. The causticizing results from trisodium borate had a lower initial causticizing efficiency, because the decarbonization reaction
29 does not go to completion due to improper purging of CO2 evolved and only 85 % of the trisodium borate salt formed reacts with water, resulting in lower than expected sodium hydroxide concentration.
Table 3. Conditions for Causticizing Experiments
Run Name TTA Sulfidity Stoichiometry Borate to Carbonate
g/L as Na2O % % Ratio
Metaborate 120 30 100 0.35
Trisodium Borate 186 30 100 0.35
100
90
80 o' V 70 I�
'ij 60 1;� -+- Trisodium Borate 50 1(/ ---- Meta-Borate and NaOH .N ·u 40
30 if/
20
10
0 0 5 10 15 20 25 30 35 40 45 :50 55 60 65 70 Time in Min ,.
Figure 8. Effectof Time on Causticizing Efficiency at 35 % Autocausticizing Level
30 The experimental results show that the unconverted trisodium borate salt present in the system does not affect the final causticizing efficiency even though the TTA is very high compared to the other curve contradicting the literature. The literature on causticizing efficiency says that higher TTA reduces final causticizing efficiency, but that is not to be seen on the trisodium borate curve and Na3BO3 gives better causticizing efficiency. This result has to be studied further in detail to confirm the effect of Na3BO3 and TTA on the causticizing efficiency.
Cost Savings
Dr. John Cameron, my graduate advisor helped me calculating the cost savings for borate autocausticizing. The basis forthese calculations is as follows:
1. Energy required in limekiln is 7 million Btu per ton of CaO.
2. 450 to 600 lb of lime is required per ton of pulp.
3. Kiln Energy varies between 1.5 to 2 million Btu/ton of pulp.
4. About 53 million tons of kraft and soda pulp produced annually in the
U.S.
About 90 trillion Btu's of energy are used in limekilns. The lime usage can be reduced from 10 to 30 % with partial autocausticizing. Since there would be increased energy cost in evaporation of the black liquor and in operation of the recovery furnace, it was assumed that the use of borate would reduce the total limekiln energy by 10%.
Therefore the annual energy savings is then 90 x 1012 x 0.1= 9.0 trillion Btu's per year for full market penetration. Assuming 50 % market penetration results in energy savings of
4.5 trillion Btu's per year.
31 CONCLUSIONS
1. NaB02 reacts readily with molten Na2C03 in air at differentautocausticizing level
(NaB02/Na2C03 molar ratios) and over 85% of trisodium borate (Na3B03)
formedreacts with water to formNaOH.
2. Trisodium borate (Na3B03) is relatively stable once formed and the reaction
product of Reaction 15 can be more, if CO2 is removed effectively which will
increase the kinetics of reaction. In the recovery furnace, most of the CO2 will
converted to CO and therefore the decarbonization reaction should proceed as an
irreversible reaction within the furnace.
3. The trisodium borate (Na3B03) produces two moles ofNaOH and the Reaction 16
is quite fast even at room temperature. Temperature and concentration have little
or no effecton the rate and equilibrium of Reaction 16.
4. The trisodium borate salt contradicts the literature on the rate and equilibrium of
the causticizing reaction after partial autocausticizing at higher TT A
concentration and thereforeit needs to be studied further in detail.
5. The results confirm the previous findings and will help optimizing the
autocausticizing process in the industry.
32 FINANCIAL SUPPORT AND FACILITIES
This thesis represents a part of the project, "Use of Borate Autocausticizing to
Supplement Lime Kiln and Causticizing Capacities", which has been accepted by the
Department of Energy. This project is jointly funded by U.S. Borax Inc. and the
Department of Energy. In accordance with that, the financial support for the necessary chemicals and laboratory supplies were provided.
The facilities available at the Department of Paper & Printing Science &
Engineering and those at the department of chemistry were used during the experimental work on this thesis.
33 REFFERENCES
1. Thomas M. Grace, Alkaline Pulping, Pulp and Paper Manufacture, Volume-5, 5-
7, 473-476.
2. Omini Rosen, KraftRecovery- Short Course, TAPPI Press, Orlando, FL, 2000.
3. KraftRecovery Operations- Short Course, TAPPI Press, Orlando, FL, 1993.
4. Kraft Pulping - Short Courses, Atlanta, GA , T APPI Press, 1994
5. Gary A.Smook, Handbook for Pulp and Paper Technologists, Second Edition,
1992.
6. Janson, J. Paperi Ja Puu, " The conventional Use of Unconventional Alkali in
Cooking and Bleaching- Part 5. Autocausticizing Reactions", 61(1) 20-30(1979).
7. Janson, J. Paperi Ja Puu, " The conventional Use of Unconventional Alkali in
Cooking and Bleaching- Part 6. Autocausticizing of Sulfur-Containing Model
Mixtures and Spent Liquor", 61,20-30( 1979).
8. Janson, J- Pulping Processes Based on Aotocausticizing Borate. Svensk
Paperstidn. 63 (1980): 392-395
9. Clay, D.T., Grace, T.M.: Measurement of high-solids black liquor to high solids
as an investment. Tappi 64 (1981); 12, 49-52
10. Tran, Mao, Cameron, and Bair, "Autocausticization of smelt with Sodium
Borates", Pulp and Paper Canada, 100, No.:9 (1999).
,. 11. Z.Yusuf and J.Cameron, "The reaction between Sodium Carbonate and Sodium
Metaborate in both Solid and Liquid States", International Chemical Recovery
Conference, Whistler, British Columbia, Canada (June 11-14,2001).
34 12. Z.Yusuf and I.Cameron, " Autocausticizing Sodium Carbonate with Borate",
2000 AIChE Annual Meeting, Los Angels, CA (Nov 12-17, 2000)
13. Joseph M. Genco, Proserfina D. Bennett, Haixuanzou, "Kraft Pulping with Mill
Produced Autocausticizing White Liquor- Effects of Sulfidity"- Pulp and Paper
Development Center, University ofMaine.(2001)
14. Boran, Mettallo- Boran Compounds and· Borances- Edited by Roy M.
Adams,173-176 (1964).
15. Janson, J. Paperi Ja Puu, " The conventional Use of Unconventional Alkali in
Cooking and Bleaching- Part 1. A New Approach to Liquor Generation and
Alkalinity", 59(6-7) 425-430 (1977).
16. Carriere. E., Guiter, H. and Thubert, "The Action of Boric Anhydride on Sodium
Carbonate", Bull. Soc. Chim. France,796. (1949)
35 Appendix A. Decarbonization and Weight Loss Details
Experimental Conditions:
Temperature: 925°C
Decarbonizing Time: 6.00 Hrs
Before Decarbonization After Decarbonization Run Total No Autocausticizinq Amount of Amount of Amount of Weight loss CO2t H2O Level Na8O2.2H2O Na2CO3 present in qms in qms % oms oms gms 1 15 15.278 106.009 5.4 11.38 5.98 2 15 15.2474 106.005 5.4 11.51 6.11 3 25 25.4125 106.007 9 18.00 9.00 4 25 25.413 106.006 9 18.02 9.02 5 35 35.6335 106.004 12.6 23.98 11.38 6 35 35.6335 106.002 12.6 23.59 10.99 7 45 36.666 84.8 12.96 23.80 10.84 8 45 36.666 84.8 12.96 23.84 10.88 9 55 44.814 84.8 15.84 28.74 12.90 10 55 44.81 84.8 15.84 28.72 12.88
36 Appendix B. Amount ofNaOH Produced from the Reaction ofNa;!BO;! with H20
Note: Calculation of actual Na3B03 reacted with water is also included
Na2C Metaborate/Carbonate NaOH NaBO2 03 Na38O3 Reacted Ratio moles moles moles (0.5*moles NaOH)
0.15 0.181 0.19 0.71 0.09 0.15 0.169 0.21 0.73 0.08 0.25 0.343 0.30 0.76 0.17 0.25 0.363 0.33 0.82 0.18 0.35 0.374 0.42 0.65 0.19 0.35 0.38 0.42 0.68 0.19 0.45 0.32 0.44 0.44 0.16 0.45 0.305 0.45 0.41 0.15 0.55 0.24 0.63 0.26 0.12 0.55 0.38 0.47 0.40 0.19
37 Appendix C. Split up of Decarbonization Results
Metaborate/ %Na3B03 Reacted Carbonate % Decarbonization %Na3803 Formed with water to Ratio based on Weight Loss Based on CE after Hydrolysis form NaOH 0.15 91.66 72.18 78.74 0.25 82 72.55 88.47 0.35 72.71 63.08 86.75 0.45 68.47 59.34 86.66 0.55 66.59 58.98 88.57
38 Appendix D. Effect of Temperature on Hydrolysis of Trisodium Borate
Appendix D1. Run No.1
Parameters Maintained Temperature = 70°C TTA = 100 g/L as Na2O Sulfidity= 30% Autocausticizing Level= 35% Amount of Chemicals Added Na2S.9H2O = 34.837 gms Na3BO3 + Na2CO3 = 33.02 gms
Sample Time Mean AA OH No. in min g/L C03 g/L B02 g/L S g/L g/L as as as as as Na20 Na20 Na20 Na20 Na20 1 1 13.63 33.97 14.65 40.98 47.59 2 2 15.43 39.50 16.60 39.26 54.93 3 3 17.00 41.93 15.99 37.88 58.93 4 5 15.42 42.00 16.26 39.19 57.41 5 10 18.56 46.09 13.79 32.61 64.65 6 15 18.48 45.04 14.71 34.33 63.52
39 Appendix D6. Run No. 2
Parameters Maintained Temperature = 50°C TTA = 100 g/L as Na2O Sulfidity= 30% Autocausticizing Level = 35% Amount of chemicals added Na2S.9H2O = 34.837 gms Na3BO3 + Na2CO3 = 33.02 gms Water = 250 mL
Sample Time Mean AA
No. in min OH g/L C03g/L 802 g/L S g/L g/L as as as as as Na20 Na20 Na20 Na20 Na20 1 1 13.94 35.49 10.23 35.56 49.43 2 2 16.28 42.18 12.97 36.30 58.45 3 3 16.19 41.31 14.64 36.92 57.50 4 5 18.06 45.48 14.67 34.39 63.53 5 10 17.49 44.97 15.29 35.60 62.46 6 15 18.17 46.69 14.61 34.70 64.86
40 Appendix D�. Run No. 3
Parameters Maintained Temperature = 90°C TTA = 100 g/L as Na2O Sulfidity= 30% Autocausticizing Level = 35% Amount of chemicals added Na2S.9H2O = 34.837 gms Na3BO3 + Na2CO3 = 33.495 gms Water = 250 mL
Sample Time Mean AA
No. in min OH g/L C03 g/L B02 g/L S g/L g/L as as as as as Na20 Na20 Na20 Na20 Na20 1 1 15.20 34.11 10.24 35.07 49.30 2 2 15.97 36.64 14.24 39.10 52.61 3 3 18.24 44.24 12.04 35.98 62.48 4 5 19.19 45.52 12.70 36.07 64.71 5 10 14.58 35.84 12.19 34.88 50.42 6 15 20.12 46.31 12.01 34.42 66.43
41 Appendix E. Effect of Concentration on Hydrolysis of Trisodium Borate
Appendix E1• Run No. 1
Parameters Maintained Room Temperature TT A = 40 g/L as Na2O Autocausticizing Level = 25% Amount of Chemicals Added Na3BO3 + Na2CO3 = 16.5379 gms Water = 250 mL
Sample Time AA No. in min 2/L as Na20 1 1 27.55 2 2 31.97 3 3 34.80 4 5 35.70 5 10 39.42 6 15 38.55
Appendix E2. Run No. 2
Parameters Maintained Room Temperature TTA = 72 g/L as Na2O Autocausticizing Level = 25% Amount of Chemicals Added Na3BO3 + Na2CO3 = 33.0899 gms Water = 250 mL Sample Time AA No. in min 2/L as Na20- 1 1 52.09 2 2 61.53 3 3 64.37 4 5 73.06 5 10 73.11 6 15 71.78 42 Appendix EJ: Run No. 3
Parameters Maintained Room Temperature AA = 110 g/L as Na2O Autocausticizing Level = 25% Amount of Chemicals Added Na3BO3 + Na2CO3 = 49.5 gms Water = 250 mL
Sample Time AA No. min 2/Las Na20 1 1 76.94 2 2 91.44 3 3 97.06 4 5 106.45 5 10 108.44 6 15 109.23
43 Appendix F. PartialAutocausticizing and Causticizing Details
Appendix. F 1
Experimental Run at 30% Sulfidityat 120 g/L TTA with Borate
Starting Concentration Run at 100% Stoichiometry, 95°C Na2CO3 = 54.6 g/L as Na2O BurntLime = 45.9634 gms Na2S =36 g/L as Na2O Amount of Water Added = 800 mL NaBO2 = 14.7 g/L as Na2O NaOH = 29.4g/L as Na2O
Sample Time Mean Causticizin2 No. in min OH2/L C03 2/L Efficiency % 0 29.40 54.60 35 1 5 41.86 38.06 52.3774 2 10 55.48 20.22 73.2893 3 15 65.29 17.62 78.748 4 20 61.37 14.17 81.2417 5 25 63.41 13.37 82.5866 6 30 66.29 12.83 83.7841 7 35 65.37 11.54 84.9954 8 40 68.42 12.38 84.6782 9 45 65.62 10.92 85.733 10 50 70.53 12.86 84.5785 11 55 67.22 11.67 85.2073 12 60 72.88 12.16 85.7008 13 65 69.79 12.60 84.7069 14 70 63.44 10.31 86.0203
44 Appendix.F�
Experimental Run at 30% Sulfiditywith Trisodium Borate at 35% Autocausticizing Level.
Starting Concentration forTrisodium Borate Salt Preparation
Na2CO3= 106.005 gms Na2BO2= 35.6335 gms
Hydrolysis of Trisodium Borate+ Sodium Carbonate Amount of Water Added = 400 mL After Hydrolysis Results Na2CO3 = 101.2 g/L as Na2O NaBO2 = 32.54 g/L as Na2O NaOH = 29.0lg/L as Na2O Run at 100% Stoichiometry, 95°C at 30% Sulfidity BurntLime = 42.3 gms at 85.83% Activity Na2S = 56.75 g/L as Na2O Total Quantity = 390 mL NaBO2 = 14. 7 g/L as Na2O
Sample Time Mean Causticizint in No. min OH2/L C03 2/L Efficiency % 0 29.01 101.20 22.2794 1 5 59.10 62.31 48.678 2 10 81.82 39.30 67.5528 3 15 92.93 30.53 75.2713 4 20 95.16 25.26 79.0234 5 25 99.31 22.59 81.4684 6 30 102.13 20.53 83.2627 7 35 103.65 18.91 84.5708 8 40 104.56 18.25 85.1396 9 45 105.21 18.35 85.1489 10 50 104.80 18.13 ,'85.2518 11 55 105.21 17.52 85.7248 12 60 106.30 17.12 86.1287 13 65 106.80 17.50 85.9212 14 70 107.90 17.10 86.3200
45