Effect of Temperature and Boron Contents on the Autocausticizing

The autocausticizing reactions of sodium carbonate/borate mixtures have been ... These are subsequently removed from the liquor cycle and have their o...
0 downloads 0 Views 134KB Size
Download PDF
Ind. Eng. Chem. Res. 2004, 43, 6285-6291

6285

Effect of Temperature and Boron Contents on the Autocausticizing Reactions in Sodium Carbonate/Borate Mixtures Daniel K. Lindberg* and Rainer V. Backman Åbo Akademi University, Process Chemistry Centre, FI-20500 Åbo, Finland

The autocausticizing reactions of sodium carbonate/borate mixtures have been studied by simultaneous TG/DTA. The main reaction occurs in a molten state of the salts, where the carbonate and borate react to form CO2 and more sodium-rich borates. The extent of decarbonation of Na2CO3 is dependent on the amount of added boron, the temperature, and the composition of the surrounding gas atmosphere. At an overall molar ratio of B/Na < 1 and at temperatures of 900-1000 °C, the borate composition of the melt is intermediate to NaBO2 and Na3BO3. The decarbonation reaction is favored by high temperatures and low CO2 contents in the gas phase. The composition of the borate components in the melt shifts toward the Na3BO3 composition at high temperatures, low CO2 partial pressures, and low boron additions. If the surrounding gas contains CO2, the salt melt will be recarbonated as the temperature is lowered and the melt crystallizes. Introduction

borate) (eq 2) does not occur to a great extent according to Janson.3

The causticizing of green liquor is an important part of the regeneration of pulping chemicals in the kraft pulping process. The normal green liquor causticizing process demands the addition of lime, which is recovered in the lime cycle through calcination of lime mud in the lime kiln. If the demand of lime for the production of NaOH in the pulping chemicals is higher than the capacity of the lime mill, the recausticizing plant becomes the bottleneck of the plant operation. Studies have been made on the possibilities for different types of autocausticizing reactions, where different compounds in the chemical recovery cycle decarbonate the smelt already in the recovery boiler, thus reducing the need for lime addition to the recausticizing plant.1 Decarbonation of the smelt in the recovery boiler can also be achieved by adding different metal oxides, which are insoluble in alkaline solutions. These are subsequently removed from the liquor cycle and have their own recovery cycle. This concept is called direct causticizing and TiO2 or Fe2O3 have been proposed for this type of causticizing. The decarbonation of the smelt by alkali borates was suggested at the end of the 1970s.1-8 It was suggested that the sodium carbonate in the smelt would react with the sodium borates, releasing CO2 and forming more sodium-rich borates. The formed sodium borates would then react with water in the recausticizing plant, forming NaOH and the original sodium borate. The proposed main decarbonation reaction according to Janson1-8 is

5 Na2CO3(s,l) + Na2B4O7(s,l) ) 4 Na3BO3(s,l) + CO2(g) (5)

Na2CO3(s,l) + 2 NaBO2(s,l) ) Na4B2O5(s,l) + CO2(g) (1)

Na3BO3(s,l) + H2O(l) ) 2 NaOH(aq) + NaBO2(aq) (6)

A possible reaction between Na4B2O5 (sodium pyroborate) and Na2CO3 forming Na3BO3 (sodium ortho-

This concept would mean a more efficient autocausticizing, as 2 mol of NaOH is formed per added mole of NaBO2, instead of 1 mol of NaOH per mole of NaBO2 in the Janson concept. This reduces the need for borate addition and decreases the operational problems caused by the borate addition.

* To whom correspondence should be addressed. Tel: (358) 2-2154761. Fax: (358) 2-2154962. E-Mail: daniel.lindberg@ abo.fi.

Na2CO3(s,l) + Na4B2O5(s,l) ) 2 Na3BO3 (s,l) + CO2(g) (2) The Na4B2O5 would dissolve in the green liquor forming NaOH and regenerating NaBO2 (sodium metaborate)

Na4B2O5(s,l) + H2O(l) ) 2 NaOH(aq) + 2 NaBO2(aq) (3) However, the full-scale implementation of this concept was not successful due to the presence of large amounts of borate in the liquor cycle, creating different types of operational problems. A recent study9 showed that the decarbonation reaction could proceed to produce an even more sodium-rich borate, Na3BO3. This would mean that the autocausticizing reaction could be more effective than suggested by Janson, assuming NaBO2 is the borate formed in the green liquor tank. The proposed reactions were

Na2CO3(s,l) + NaBO2(s,l) ) Na3BO3(s,l) + CO2(g) (4)

10.1021/ie040016o CCC: $27.50 © 2004 American Chemical Society Published on Web 08/24/2004

6286

Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004

Previous studies and new experiments show that the decarbonation of sodium carbonate by borates is more complex than suggested in the autocausticizing concepts by Janson1-8 and Tran.9 Physical and chemical conditions, like temperature, partial pressure of CO2, and borate contents, have significant effects on the extent of the decarbonation reactions in sodium carbonate/ borate mixtures. Objectives. The objective of this study is to assess the extent of decarbonation in a boron-containing smelt, and shed light on which borate compounds and components will be present in the smelt at varying physical and chemical conditions. This is done by critically reviewing the experimental results from previous studies and by presenting new data on decarbonation reactions of alkali carbonate- and borate-containing melts. Previous Studies. Previous studies on the decarbonation of sodium carbonate by the addition of borates have partly concerned glass science, where the focus has been on the manufacturing of sodium-rich borate glasses.10-15 These studies have shown that when the starting material for these glasses has been Na2CO3 and B2O3 (or various sodium borates), a large part of the CO2 that was expected to be degassed during the manufacturing in fact is retained in the glass, most likely as Na2CO3. Some studies have concerned the indirect production of NaOH3,9,16-18 and the work by Flood et al.19 concerned a general acid/base theory for oxide melts. The experiments in most of these studies were carried out by melting mixtures of sodium carbonate and sodium borates or boron oxide, and measuring directly or indirectly the degree of decarbonation. The experiments were performed at varying temperatures, from 700 to 1200 °C (most of them around 1000 °C), and in varying gas atmospheres, in air, N2, or CO2. Some experiments were performed to determine the rate of the decarbonation reactions.17,18 The decarbonation was measured by thermogravimetric techniques and Janson,3 Kasper et al.,11 and Karki et al.15 also compared the thermogravimetric results with chemical analysis of the carbonate contents in the glass or melt. Volatilization of sodium caused by the breakdown of the alkali salts can occur at high temperatures, but it has not been measured or taken into account in any of the thermogravimetric studies. Mixture Composition. In experiments with highboron mixtures (B/Na > 1) all of the Na2CO3 reacts and releases CO2 at temperatures above 900 °C. In experiments with mixtures having B/Na < 1, part of the Na2CO3 remains unreacted in the melt or the glass. More Na2CO3 remains unreacted as the concentration of boron in the mixture is reduced (Figures 1 and 2). Pure Na2CO3 does not dissociate at temperatures below its melting point (858 °C). The molar ratio of oxygen-to-boron of the non-carbonate components is a measure of the borate composition in the melt,19 and is directly related to the potential formation of NaOH in the green liquor (O/B ) 2 corresponds to NaBO2, O/B ) 2.5 corresponds to Na4B2O5, and O/B ) 3 corresponds to Na3BO3). The O/B ratio at B/Na < 1 is between 2 and 3, except for the experiments done by Tran,9 where the O/B ratio is above 3 for some compositions. The O/B ratio increases with decreasing concentration of boron (Figures 3 and 4). According to the concept by Janson1-8 the O/B ratio does

Figure 1. Amount of Na2CO3 in the mixture as a function of B/Na ratio. The amount of Na2CO3 is calculated as Na2CO3/(Na2CO3 + Na2O + B2O3) × 100% on a molar basis. The theoretical compositions for the autocausticizing concepts by Janson1-8 and Tran9 are plotted as comparison. Experiments have been performed in air or N2 at 1 bar pressure and at temperatures of 925-1050 °C, and the results are either from this study or recalculated from previous studies.

Figure 2. Amount of Na2CO3 in the mixture as a function of B/Na ratio. The amount of Na2CO3 is calculated as Na2CO3/(Na2CO3 + Na2O + B2O3) × 100% on a molar basis. The theoretical compositions for the autocausticizing concepts by Janson1-8 and Tran9 are plotted as comparison. Experiments have been performed in CO2 at 1 bar pressure and at 1000 and 1200 °C, except for three experiments from this study, which have been performed in 15% CO2/85% N2. The results are either from this study or recalculated from previous studies.

not exceed 2.5 if the most sodium-rich borate formed is Na4B2O5, and according to the concept by Tran9 the O/B ratio could go up to 3. The results from the experiments suggest that the decarbonation can exceed reaction 1, but only Tran’s experiments9 suggest that a O/B ratio of 3 or higher can be reached. In CO2 atmospheres, the O/B ratio is lower than in air or N2 and is below 2.5 in experiments with B/Na > 0.2. The experiments suggest that the theoretical borate components in the melt or the carbonate/borate mixture would not consist entirely of Na3BO3, but could also contain NaBO2 or Na4B2O5. Abdullaev et al.20,21 have identified a sodium borate, which is even more sodium-rich than Na3BO3, the

Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004 6287

Figure 3. Chemical composition of the non-carbonate part of the melt as a function of the B/Na ratio. The calculated O/B molar ratio corresponds to a composition of the stoichiometric borate compounds NaBO2, Na4B2O5, and Na3BO3. Experiments have been performed in air or N2 at 1 bar pressure and at temperatures of 925-1050 °C, and the results are either from this study or recalculated from previous studies.

longer melting times and higher melting temperatures lead to less polymerized glasses and less carbonate retention in the glasses. Decreasing B/Na ratios also give less polymerized glasses, but higher carbonate retention in glasses. Experiments done in air or N2 show a larger scatter for the amount of unreacted Na2CO3 (Figure 1) and O/B ratios (Figure 3) of the non-carbonate components in the melts and glasses compared to composition of the melts or glasses in experiments done in CO2 (Figures 2 and 4). The amount of Na2CO3 decreases and the O/B ratio of the non-carbonate components increases in the melts and glasses as a function of added boron for experiments in CO2. The amount of Na2CO3 is slightly lower and the O/B ratio is slightly higher at 1200 °C than at 1000 °C (Figure 4). This indicates that reaction 3 is more efficient at higher temperatures. The results from experiments done in air or N2 show larger scatter and no clear temperature effect can be seen when different studies are compared to each other. Volatilization of Na2CO3 is more likely to occur in air or N2 than in CO2rich atmospheres, and the volatilization of Na2CO3 in the experiments in air and N2 can be a source of error for the melt and glass compositions. Pure NaBO2 melts at 968 °C and does not volatilize at temperatures below 1000 °C, and the possible volatilization of Na3BO3 and Na4B2O5 is to a large extent unknown. Janson3 suggested that Na4B2O5 might decompose to NaBO2, B2O3, and gaseous Na and O2 at temperatures above 10001100 °C. However no measurements have been made on the composition of the volatilized components. The effect of the starting material has not been explicitly studied, but at B/Na < 1 there should be no difference if the boron source is B2O3, NaBO2, or an intermediate between these two from an equilibrium perspective, if the surrounding gas atmosphere is controlled. Reaction 7 will always go to completion in air at temperatures over 900 °C, and in CO2 at over 1000 °C, as can be seen in Figures 1 and 2, where the Na2CO3 content in the mixture is zero.

B2O3(l) + Na2CO3(l) ) 2 NaBO2(l) + CO2(g) (7) Figure 4. Chemical composition of the non-carbonate part of the melt as a function of the B/Na ratio. The calculated O/B molar ratio corresponds to a composition of the stoichiometric borate compounds NaBO2, Na4B2O5, and Na3BO3. Experiments have been performed in CO2 at 1 bar pressure and at 1000 and 1200 °C, except for three experiments from this study, which have been performed in 15% CO2/85% N2. The results are either from this study or recalculated from previous studies.

compound Na5BO4, but the formation of this compound in autocausticizing reactions has not been suggested. Kamitsos and co-workers22-24 studied the speciation of the borates in sodium borate glasses with high sodium contents by Raman and IR spectroscopy. They also studied the carbonate retention in the glasses when Na2CO3 was used as a starting material. Their results showed that as the sodium content in the glass increases, various polymerized borate units break down to the smaller anions pyroborate (B2O54-) and orthoborate (BO33-). The pyroborate units break down to orthoborate units as B/Na in the glass approaches the orthoborate composition (B/Na ) 0.333). Kamitsos et al.22-24 also showed that at B/Na < 0.5, carbonate retention occurs in the glass. They also showed that

If the gas atmosphere is not controlled, the starting material will have an effect, as the amount of CO2 released depends on the mixture composition. The starting material may also have a kinetic effect, as more bonds in highly polymerized boron sources will need to be broken compared to more depolymerized sources, to create the highly depolymerized Na3BO3. Generally, higher sodium content in the boron source will give a less polymerized starting material. Effect of PCO2. The composition of the gas atmosphere in the experiments was shown to be important for the decarbonation of Na2CO3. The effect of higher CO2 partial pressure is to stabilize Na2CO3 in the melt (Figure 2). In air all Na2CO3 has reacted at temperatures of around 1000 °C in mixtures with B/Na ≈ 0.5 (corresponding to the sodium pyroborate composition). At 1 bar CO2, all Na2CO3 has reacted in mixtures with B/Na ≈ 1 (corresponding with the sodium metaborate composition). In both cases, lower boron concentrations lead to stabilization of Na2CO3. The O/B ratio of the noncarbonate components is higher in air than in 100% CO2. This suggests that possible formation of Na3BO3 will be inhibited by higher partial pressures of CO2.

6288

Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004

Figure 5. Amount of Na2CO3 in the mixture with varying B/Na ratios as a function of temperature. The amount of Na2CO3 is calculated as Na2CO3/(Na2CO3 + Na2O + B2O3) × 100% on a molar basis. The data are taken from Carrie`re et al.12

Effect of Temperature. Carrie`re et al.12 investigated the effect of temperature on reaction 4. The experiments were performed at temperatures between 700 and 950 °C. The results show that higher temperatures lead to more decarbonation of the mixture (Figure 5). At low temperatures the decarbonation is more dramatically affected by temperature changes than at high temperatures. The effect of temperature may be a reflection of a high portion of solids in the mixture at low temperatures, indicating that decarbonation of the mixture is favored in the molten phase due to either kinetic or equilibrium constraints. The melting range was reported to be between 600 and 740 °C. Tran et al.18 showed that the decarbonation of a mixture of Na2CO3 and NaBO2 proceeded further at 900 °C than at 750 °C. Effect of Time. Yusuf & Cameron17 investigated the rate of the decarbonation reaction with mixtures containing Na2CO3 and NaBO2 at temperatures varying from 600 to over 1000 °C. Their experiments at high temperatures showed that reaction 4 is rapid. At lower temperatures (600-850 °C) the decarbonation reaction is considerably slower, and the amount of CO2 generated suggests a reaction stoichiometry closer to reaction 1 than reaction 4. It was also shown that the decarbonation reactions are reversible and the reaction is slowed significantly if the generated CO2 is not removed. Tran et al.18 showed that reaction 4 did not reach completion after several hours at 750 or 900 °C. Experimental Work. New experiments were made in order to investigate whether the decarbonation reaction is reversible in CO2 atmospheres, and to measure the decarbonation in mixed N2/CO2 gas atmospheres. Decarbonation experiments were also done in N2 and CO2 to compare the results with previous studies. Experimental Setup. A simultaneous DTA/TGA analyzer was used to investigate the decarbonation of Na2CO3 by the addition of NaBO2‚2H2O. Mixtures with B/Na ratios of 1:3 were used for the experiments and the weight of the samples was between 5 and 7 mg. The samples were placed in a platinum cup and were heated to 950 or 1000 °C at a heating rate of 10 or 20 °C/min, then kept at the maximum temperature for a half hour or 1 h, cooled to 200 °C, and reheated to the maximum temperature with the same heating rate. A gas flow was

Figure 6. DTA/TGA experiments on sodium carbonate/borate mixtures in different gas atmospheres. Weight loss of the samples (full lines) and the temperature profiles in the experiments (dotted and dashed lines) are plotted as a function of time. The two horizontal lines represent a H2O-free melt and a H2O- and CO2free melt. Details of the experimental results are explained in the text.

introduced into the thermobalance during the experiments, and the gas composition was 100% N2, 100% CO2, or a 15% CO2/85% N2 mixture on a volume basis. Experiments with B/Na ) 0.31 and 0.2 were run in mixtures of 15% CO2. The weight changes of the samples as a function of temperature and reaction temperatures (differential temperature signal) were measured in the experiments to quantify the degree of decarbonation of the sample and identify the temperature range for the decarbonation reaction. Results The initial weight loss in the samples for all experiments was due to dehydration of the sodium metaborate, occurring at low temperatures (Figure 6). At temperatures between 700 and 800 °C reaction peaks coincide with a significant weight loss of the samples, indicating the start of the decarbonation reaction. The decarbonation in N2 starts at 715 °C and as 950 °C is reached the O/B ratio of the borates is 2.90 (Figure 3), which corresponds to a mixture containing 10.5 mol % Na2CO3, 10.5 mol % Na4B2O5, and 79 mol % Na3BO3. As the temperature is kept constant for 1 h at 950 °C, a small weight loss still occurs and it exceeds the maximum weight loss if only H2O and CO2 escape from the sample. This suggests that some volatilization of either the carbonate or the borates occurs at these conditions. As the temperature is lowered, the weight of the sample remains constant. The mixture composition is based on the thermogravimetric results assuming that only CO2 and H2O are lost. The composition is therefore calculated as the experiment has reached the peak temperature, to avoid uncertainties due to possible volatilization of other components, like sodium and boron, during the time the peak temperature is kept constant. In 15% CO2 the decarbonation starts at 765 °C and as 1000 °C is reached the O/B ratio is 2.68 (Figure 4), which corresponds to a mixture containing 32 mol %

Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004 6289

Na2CO3, 32 mol % Na4B2O5, and 36 mol % Na3BO3. As the temperature is lowered, the sample gains weight, which indicates a recarbonation of the mixture at lower temperature. Experiments with B/Na ) 0.31 and 0.20 show similar results. When B/Na ) 0.20, the O/B ratio is 2.81, indicating that the borate composition is closer to Na3BO3 when the boron addition is lower. In 100% CO2 the decarbonation starts at 795 °C and as 1000 °C is reached the O/B ratio is 2.26 (Figure 4), corresponding to a mixture containing 50 mol % Na2CO3, 32 mol % NaBO2, and 18 mol % Na4B2O5. The sample is also recarbonated as the temperature is lowered. With B/Na ) 1:3 the experiments show that the weight loss is the highest in 100% N2 and the lowest in 100% CO2. This indicates that the CO2 in the gas-phase inhibits the autocausticizing reactions. Na2CO3 is also stabilized in high partial pressures of CO2. Recarbonation of the mixtures when the temperature is lowered only occurs in experiments with CO2 in the purge gas. This suggests that the autocausticizing reactions are equilibrium reactions and reversible. In 100% N2 all the released CO2 is flushed away and recarbonation of the salt mixture is impossible. Reaction peaks from the differential thermal analysis of the mixtures coincide with the initial decarbonation and recarbonation and no additional reaction peaks have been observed (Figure 7). This suggests that the initial melting of the mixtures coincides with the initial decarbonation. We have no evidence for a possible autocaustizicing reaction in a solid mixture, as suggested by Yusuf & Cameron.17 The autocausticizing reaction in N2 agrees quite well with the concept of Tran.9 However, in 15% CO2 the decarbonation is less complete and in 100% CO2, the reaction 1 according to Janson3 does not go to completion, leaving unreacted NaBO2 in the mixture. Discussion Effect of Other Smelt Components. The effect of other smelt components on the decarbonation reaction has not been thoroughly investigated. Flood et al.19 did some experiments on the effect of Na2SO4 on the reactions in a carbonate/borate melt. The decarbonation of Na2CO3 was not affected or decreased only slightly in melts with high sulfate contents at 1000 °C. Tran et al.18 showed that the addition of NaCl or KCl to a Na2CO3 and NaBO2 mixture has insignificant effects on the reaction rates, but the addition of KCl causes the decarbonation reaction to begin at lower temperatures. This is probably due to the lowering of the first melting temperature of the mixture. The occurrence of NaOH in the smelt could be favorable for the formation of Na3BO3. Tran18 showed that the dehydration reaction 8 is more rapid and more complete than the decarbonation reaction 4.

2 NaOH(s,l) + NaBO2(s,l) ) Na3BO3(s,l) + H2O(g) (8) The effect of potassium on the decarbonation reactions in the smelt could be important, as Flood et al.19 and Lim et al.25 showed that the decarbonation of K2CO3 by the addition of borates does not occur to the same extent as in the corresponding sodium system. Relatively high potassium contents in the black liquor could

Figure 7. DTA curves showing the reaction peaks in the experiments in varying gas atmospheres as a function of temperature for mixtures having a B/Na ratio of 1:3. The bold lines show the weight change and the thin line shows the temperature difference compared to a standard.

therefore reduce the autocausticizing of the smelt. No studies have been published on the effect of mixed sodium and potassium salts on the decarbonation of carbonates by the addition of borates. In alkali-rich potassium and lithium carbonate/borate glasses, the decarbonation is at a maxiumum in pure Li systems and at a minimum in pure K systems, with a linear trend at the intermediate alkali compositions.26 Claes et al.27 showed that dehydration of alkali hydroxide melts by the addition of B2O3 occurs to a greater extent in potassium-rich melts than in sodium-rich melts, and the minimum dehydration occurred at 1:1 molar ratio of Na and K. The effect of Na2S on the decarbonation of a boratecontaining smelt is not known. Heterogeneous Reactions. The effect of borate additions on the smelt behavior is more complicated in a recovery boiler than under laboratory conditions. The temperature in the lower furnace is highly variable and other gaseous and condensed compounds affect the overall chemistry. Tran et al.18 suggested that the char in the lower furnace might accelerate the decarbonation

6290

Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004

reaction and the formation of Na3BO3. Their suggested overall reaction was

Na2CO3(s,l) + NaBO2(s,l) + C(s) ) Na3BO3(s,l) + 2 CO(g) (9) The partial pressure of CO2 in the lower furnace (∼0.1 bar) is considerably higher than that in air, which may have an inhibiting effect on the decarbonation reactions. However, the partial pressure of CO2 near the smelt bed could locally be reduced if the smelt contained char. The presence of char could therefore indirectly enhance the autocausticizing reaction by reducing the partial pressure of CO2. It has also been suggested that the formation of Na3BO3 could take different paths than the reaction between NaBO2 and Na2CO3 in the smelt.18 In a boiler, Na3BO3 could also be formed if gaseous sodium compounds are released and react with NaBO2 in the smelt. Volatilization of Sodium. The extent of decarbonation in carbonate/borate mixtures, measured in previous studies, is mainly based on thermogravimetric measurements, where the volatilization of sodium was considered negligible or was not taken into account. Thermogravimetric techniques are not able to distinguish between volatilization of Na and the formation of CO2 from reactions between carbonates and borates. This means there is a risk of overestimating the extent of decarbonation if volatilization of Na occurs simultaneously with the decarbonation reactions. Janson3 suggested that Na4B2O5 might decompose to NaBO2 and B2O3 and gaseous Na and O2 at temperatures above 1000-1100 °C, based on thermogravimetric measurements. However, the decomposition or volatilization mechanism of Na4B2O5 and Na3BO3 is unknown. Decomposition and volatilization of Na2CO3 can also occur at temperatures around 1000 °C. Karki et al.15 tried to minimize the possible volatilization of sodium by shortening the melting times of their mixtures. However, this could also affect the completion of the decarbonation, leading to an underestimation of the decarbonation. Conclusions This study has shown that the partial pressure of CO2 and the temperature are important factors for the decarbonation of sodium carbonate/sodium metaborate mixtures. In N2 the autocausticizing reaction proposed by Tran9 goes close to completion. However, as the partial pressure of CO2 rises, the autocausticizing reaction is inhibited and in 100% CO2 even the autocaustizicing reaction proposed by Janson3 does not go to completion. High partial pressures of CO2 stabilize the solid or liquid Na2CO3, and reduce the amount of released gaseous CO2 (or formed Na3BO3). Higher carbonate contents in the mixture give a borate composition with higher O/B ratios, i.e., closer to the orthoborate composition compared to pyroborate and metaborate. Previous studies have also shown that higher temperatures give a more complete decarbonation of the mixture. The decarbonation reactions occur mainly at temperatures over 700 °C and the reactions are reversible in gas atmospheres containing CO2, as temperature is lowered. The borate composition in the molten phase is generally intermediate to NaBO2 and Na3BO3 at B/Na < 1. The main borate species in a melt with carbonate present are BO33- and B2O54- anions and possibly some polymerized borate units with a

stoichiometric composition close to metaborate. Formation of Na3BO3 is enhanced by low CO2 contents in the gas phase, high temperatures, and low boron additions. In a furnace environment, decarbonation reactions could be accelerated by reacting with the char. Formation of Na3BO3 could be enhanced by the reaction of other borates with volatile sodium compounds. NaOH in the smelt could also enhance the formation of Na3BO3. The effect of potassium is uncertain. Additions of potassium may lower the melting point of the inorganics, which lowers the initial reaction temperature, but the addition of potassium might also inhibit the decarbonation reaction. Acknowledgment The financial support by US Borax and the Nordic Energy Research Program is gratefully acknowledged. Thanks to Professor Mikko Hupa and Dr. Saied Kochesfahani for comments on an early version of the paper and thanks to Peter Backman for performing the thermal analysis tests. Literature Cited (1) Janson, J. The use of unconventional alkali in cooking and bleaching. Part 1. A new approach to liquor generation and alkalinity. Pap. Puu 1977, 59 (6-7), 425. (2) Janson, J. The use of unconventional alkali in cooking and bleaching. Part 4. Kraft cooking with the use of borate. Pap. Puu 1978, 60 (5), 349. (3) Janson, J. The use of unconventional alkali in cooking and bleaching. Part 5. Autocausticizing reactions. Pap. Puu 1979, 61 (1), 20. (4) Janson, J. The use of unconventional alkali in cooking and bleaching. Part 6. Autocausticizing of sulphur-containing model mixtures and spent liquors. Pap. Puu 1979, 61 (2), 98. (5) Janson, J. Autocausticizing alkali and its use in pulping and bleaching. Pap. Puu 1979, 61 (8), 495. (6) Janson, J. Pulping processes based on autocausticizable borate. Sven. Papperstidn. 1980, 14 (83), 392. (7) Janson, J.; Pekkala, O. The use of unconventional alkali in cooking and bleaching. Part 2. Alkali cooking of wood with the use of borate. Pap. Puu 1977, 59 (9), 546. (8) Janson, J.; Pekkala, O. The use of unconventional alkali in cooking and bleaching. Part 3. Oxygen-alkali cooking and bleaching with the use of borate. Pap. Puu 1978, 60 (2), 89. (9) Tran, H.; Mao, X.; Cameron, J.; Bair, C. M. Autocausticizing recovery boiler smelt with sodium borate. Pulp Pap. Can. 1999, 100 (8), 35. (10) Iwase, M.; Watanabe, H.; Nakayama, N.; Hori, H. Solubilities of CO2 in candidate glasses for nuclear waste immobilisation. Part 1. Systems, Cs2O+SiO2, Cs2O+B2O3, Rb2O+SiO2 and Na2O+B2O3. Glass Technol. 1994, 35 (1), 41. (11) Kasper, J. E.; Feller, S.; Sumcad, G. L. New sodium borate glasses. J. Am. Ceram. Soc. 1984, 67 (4), C71. (12) Carrie`re, E.; Guiter, H.; Thubert, F. Action de l’anhydride borique sur le carbonate de sodium. Bull. Soc. Chim. Fr. 1949, 16 (5), 796. (13) Pelton, A. D. Thermodynamic calculations of chemical solubilities of gases in oxide melts and glasses. Glass Sci. Technol. 1999, 72 (7), 214. (14) Kawahara, T.; Yamagata, K.; Sano, N. The CO2 solubilities of high basic melts. Steel Res. 1986, 57 (4), 160. (15) Karki, A.; Feller, S.; Lim, H. P.; Stark, J.; Sanchez, C.; Shibata, M. The density of sodium-borate glasses related to atomic arrangements. J. Non-Cryst. Solids 1987, 92 (1), 11. (16) Ritchie, C. F.; Black, L. G. Method of producing caustic borate products. U.S. Patent 2,146,093, 1939.

Ind. Eng. Chem. Res., Vol. 43, No. 20, 2004 6291 (17) Yusuf, Z.; Cameron, J. Decarbonization of sodium carbonate with sodium metaborate. In Proceedings of the International Chemical Recovery Conference: Changing Recovery Technology to Meet the Challenges of the Pulp and Paper Industry, Whistler, BC, Canada, June 11-14, 2001; Oral Presentations, 361. (18) Tran, H.; Mao, X.; Lesmana, N.; Kochesfahani, S. H.; Bair, C. M.; McBroom, R. Effects of partial borate autocausticizing on kraft recovery operations. Pulp Pap. Can. 2002, 103 (12), 74. (19) Flood, H.; Fo¨rland, T.; Roald, B. The acidic and basic properties of oxides. III. Relative acid-base strenghts of some polyacids. Acta Chem. Scand. 1947, 1, 790. (20) Abdullaev, G. K.; Mamedov, K. S.; Buludov, N. T. Sodium oxide-cadmium oxide-boron oxide system. Zh. Neorg. Khim. 1982, 27 (11), 2948. (21) Abdullaev, G. K.; Rza-Zade, P. F.; Mamedov, K. S. Sodium oxide-aluminum oxide-boron oxide system. Zh. Neorg. Khim. 1983, 28 (1), 208. (22) Kamitsos, E. I.; Karakassides, M. A. Effect of melt temperature on glass structure. Phys. Chem. Glasses 1989, 30 (6), 235. (23) Kamitsos, E. I.; Karakassides, M. A. Structural studies of binary and pseudo binary sodium borate glasses of high sodium content. Phys. Chem. Glasses 1989, 30 (1), 19.

(24) Kamitsos, E. I.; Karakassides, M. A.; Patsis, A. P. Spectroscopic study of carbonate retention in high-basicity borate glasses. J. Non-Cryst. Solids 1989, 111 (2-3), 252. (25) Lim, H. P.; Karki, A.; Feller, S.; Kasper, J. E.; Sumcad, G. The density of potassium borate glasses related to atomic arrangements. J. Non-Cryst. Solids 1987, 91 (3), 324. (26) Zhang, H.; Koritala, S.; Farooqui, K.; Boekenhauer, R.; Bain, D.; Kambeyanda, S.; Feller, S. A model for carbon dioxide retention in alkali borate and borosilicate systems. Phys. Chem. Glasses 1991, 32 (5), 185. (27) Claes, P.; Mernier, F.; Wery, L.; Glibert, J. Composition dependence of the oxoacidobasic properties of molten hydroxides. Electrochim. Acta 1999, 44 3999.

Received for review January 5, 2004 Revised manuscript received May 17, 2004 Accepted May 21, 2004 IE040016O