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Effect of NaOH on aragonite precipitation in batch and continuous crystallization in causticizing reaction
Powder Technology 129 (2003) 15 – 21 www.elsevier.com/locate/powtec
Effect of NaOH on aragonite precipitation in batch and continuous crystallization in causticizing reaction Haruo Konno a,*, Yasunori Nanri a, Mitsutaka Kitamura b a
Pulp and Paper Research Laboratory, R&D Division, Nippon Paper Industries Co., Ltd., 5-21-1, Oji, Kita-ku, Tokyo 114-0002, Japan b Department of Chemical Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashihiroshima 739-8527, Japan Received 2 November 2001; received in revised form 8 August 2002; accepted 29 August 2002
Abstract In order to obtain CaCO3 crystals with high aragonite content in the causticizing reaction in the paper industry, the effect of the NaOH addition at various temperatures was investigated using a batch operation. It is apparent that the NaOH addition increased aragonite composition in the crystal at all temperatures investigated. Then, aragonite crystallization was investigated using a continuous operation. In the continuous operation, it appeared that the dominant factor for the aragonite formation is the low molar ratio of Na2CO3 to Ca(OH)2. The lower supersaturation and the higher ratio of [Ca2 +]/[CO23 ] in the presence of NaOH are advantageous to precipitate aragonite in the causticizing reaction. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Crystallization; Aragonite; Causticizing reaction; Continuous operation; Batch operation; Supersaturation
1. Introduction Precipitated calcium carbonate (PCC) is widely used as a filler and coating pigment in the paper industry. There are two reasons for using PCC: it is an inexpensive material and it contributes to a high brightness and good opacity of paper. CaCO3 has three polymorphs: calcite, aragonite and vaterite. The first two are produced commercially. Aragonite has a slightly greater average index of refraction than calcite [1] and a needle-like characteristic shape. Hence, aragonite is expected to have higher performance properties compared to calcite for filler and coating pigment in paper. Although the carbonation process is the most widely adopted process for producing PCC, CaCO3 is the only product in this process. In the causticizing reaction, which is carried out in the chemical recovery process of the kraft pulping method, CaCO3 is a by-product of the NaOH production. There is a commercial significance when this by-product can be produced in the form of aragonite with its high-performance properties [2,3]. The crystallization of aragonite has been investigated in the liquid – liquid system [4 – 7]. However, little information is known about the * Corresponding author. Tel.: +81-3-3911-5106; fax: +81-3-3911-9476. E-mail addresses: [email protected], [email protected] (H. Konno).
crystallization of aragonite in the causticizing reaction, which is a liquid – solid system [8]. In the previous paper [9,10], we reported that various operational factors influenced the aragonite precipitation in the causticizing reaction. It was concluded that aragonite precipitation was favored at 50 jC and in the presence of NaOH at the initial stage of the causticizing reaction [9]. The crystallization of aragonite was accelerated by lowering the feed rate of Na2CO3 solution to Ca(OH)2 suspension and by increasing the stirring rate. The mechanism of the effect of such controlling factors was also shown [10]. Following these works, in this report, the effect of the NaOH presence on the aragonite crystallization at various temperatures with batch operation is investigated. On the basis of this result, aragonite crystallization using a continuous operation, where NaOH is constantly present in the system, is investigated.
2. Experimental 2.1. Crystallization 2.1.1. Materials Reagent grade chemicals and ultra-pure water were used for the causticizing reaction.
0032-5910/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 2 ) 0 0 2 7 5 - 9
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H. Konno et al. / Powder Technology 129 (2003) 15–21
2.1.2. Batch operation A 1000-ml crystallizer was soaked in a water bath, which was regulated at 25, 50 and 75 jC ( F 0.1 jC). The crystallization was carried out by adding 1.61 mol/l Na2CO3 (100 g/l as Na2O) to the Ca(OH)2 suspension containing 1 mol/l NaOH for 2 h using a tube pump with a constant feed rate. The mixture was stirred constantly. The added Na2CO3 is equivalent to a 1.2 molar quantity of Ca(OH)2. The reaction conditions for the batch operation are shown in Table 1. 2.1.3. Continuous operation A 500-ml jacketed crystallizer connected to a water bath which was regulated at 50 and 75 jC ( F 0.1 jC) was used. In Fig. 1, the experimental apparatus for the continuous operation is shown. The crystallizer was constantly fed with 13.2 wt.% Ca(OH)2 suspension and 1.61 mol/l Na2CO3 solution using a tube pump. The product suspension (the mixture of CaCO3 and alkaline solution) was stirred constantly and withdrawn continuously through the outlet at the bottom of the crystallizer using a tube pump to keep the suspension volume constant. The steady state was confirmed by the particle size distribution and alkaline concentration measurement. In this paper, reaction temperature, residence time and the molar ratio of Na2CO3 to Ca(OH)2 were variables. Reaction conditions are shown in Table 2. 2.2. Analysis of solutions For the batch operation, the suspension in the crystallizer was sampled at 0, 15, 30, 60, 90 and 120 min since the addition of Na2CO3 solution was started. Subsequently, the suspension was filtered quickly under reduced pressure. For the continuous operation, the suspension was sampled through the outlet at every residence time to check the steady state and obtain a sample to analyze. The suspension was filtered in a similar manner as above. A sample of filtrate by both operations was taken and neutralized with HCl, then the Ca2 + concentration in the neutralized filtrate was measured with an ICP Atomic Emission Spectrometer [9]. NaOH and Na2CO3 concentrations were measured by HCl titration [9]. The products were washed 5 times with
Table 1 The experimental conditions of the batch operation Reaction temperature (jC) Initial NaOH concentration (mol/l) Ca(OH)2 suspension concentration (wt%) Adding time of Na2CO3 solution (h) Na2CO3 solution concentration (mol/l) Stirring speed (rpm)
25
50 0
75 1
13.2 2 1.61 ( = 100 g/l as Na2O) 400
Fig. 1. Experimental apparatus for the continuous operation.
ethanol to remove NaOH and dried under reduced pressure. The samples, thus, obtained were measured using powder X-ray diffraction (RAD-C; Rigaku) to determine aragonite composition in the crystallized CaCO3. Residual Ca(OH)2 was calculated from the concentration of NaOH produced because the consumption of Ca(OH)2 corresponds to the production of NaOH in the causticizing reaction. The crystals were observed using a SEM (JSM-840A; JEOL) to examine the size and shape of the crystals. Particle size distribution of the sample was measured with a particle size analyzer (Master Sizer-S, Malvern).
3. Results and discussion 3.1. Batch operation 3.1.1. The influence of temperature during crystallization The causticizing reaction was carried out in the presence of 1 mol/l NaOH at 25, 50 and 75 jC. In Fig. 2, the dependence of aragonite composition (%) in the crystallized CaCO3 (Wa) on the molar ratio of Na2CO3 added to Ca(OH)2 (R) during crystallization is shown. The results without NaOH addition are shown to compare with the results of the previous paper [9]. It can be seen in Fig. 2 that the NaOH addition increased Wa at all temperatures investigated. Although no precipitation of aragonite occurred without NaOH at 25 jC, with the addition of NaOH aragonite precipitated at R = 0.6 and Wa increased up to a maximum of 50% at R = 1.2. At 50 and 75 jC, aragonite was predominantly precipitated even at the initial addition of Na2CO3 (R = 0.15, 0.3) in the suspension containing 1 mol/l NaOH, and Wa was increased to around 90%. In particular, at 75 jC, the increase of aragonite precipitation was remarkable. It is thought that the NaOH stimulated the crystal nucleation and growth of aragonite. In general, high temperature tends to favor aragonite formation [4]. This tendency is also seen in the CaCO3 formation in the
H. Konno et al. / Powder Technology 129 (2003) 15–21
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Table 2 The experimental conditions and the results of the continuous operation
Reaction temperature (jC) Molar ratio (Ca(OH)2/Na2CO3) Residence time (min) Wa (%) Residual Ca(OH)2 (%) NaOH (mol/l) Na2CO3 (mol/l) [Ca2 +] ( 10 3 mol/l) [Ca2 +][CO23 ] ( 10 3) [Ca2 +]/[CO23 ] ( 10 3) Average particle size (Am)
Run 1
Run 2
Run 3 (1)
Run 3 (2)
Run 4 (1)
Run 4 (2)
Run 5 (1)
Run 5 (2)
Run 6 (1)
Run 6 (2)
50 1:1.2 30 56 9.6 1.43 0.27 0.37 0.099 1.40 33.2
75 1:1.2 30 28 7.6 1.47 0.24 0.20 0.048 0.84 24.3
50 1:1.2 60 70 7.7 1.46 0.27 0.31 0.083 1.17 38.9
50 1:1.2 60 79 8.1 1.46 0.26 0.29 0.076 1.13 40.2
75 1:1.2 60 28 6.9 1.48 0.26 0.18 0.045 0.69 30.8
75 1:1.2 60 29 7.6 1.47 0.27 0.17 0.047 0.62 28.4
50 1:1 60 94 12.0 1.55 0.13 0.60 0.075 4.74 46.2
50 1:1 60 93 13.5 1.52 0.15 0.50 0.076 3.32 45.7
75 1:1 60 100 6.4 1.65 0.06 0.29 0.018 4.65 9.3
75 1:1 60 98 7.2 1.63 0.10 0.30 0.030 3.03 10.7
causticizing reaction using Ca(OH)2 suspension containing 1 mol/l NaOH. Fig. 3 shows a SEM photograph of precipitated CaCO3 at different temperatures. At 25 jC, aggregated particles consisting of a mixture of needle-like (aragonite) and granular (calcite) crystal are observed. At 50 and 75 jC, only needle-
like crystals are observed. As shown in Fig. 2, Wa at 50 jC was similar to that at 75 jC. However, the needle-like shapes were very different. At 50 jC, the needle-like particles have a size of about 5 Am in length and about 0.2 Am in width, and they are aggregated. On the other hand, at 75 jC the needle-like particles with 5 –15 Am in length are not much aggregated. The increase of the crystal size and the decrease of the aggregation with temperature may be because Ca2 + concentration is decreased with increasing temperature, resulting in lower supersaturation. 3.2. Continuous operation
Fig. 2. Effect of the addition of NaOH on the Ca(OH)2 suspension at different temperatures. (A) NaOH addition. (B) No addition. ( w ) 25 jC, (5) 50 jC, (D) 75 jC. (A) 5: previous data [9].
3.2.1. Aragonite precipitation by the continuous operation At the initial stage of Na2CO3 addition in the batch operation, the presence of NaOH favors the precipitation of aragonite. This means the presence of NaOH is effective on the nucleation of aragonite crystals. In the continuous operation, Ca(OH)2 reacts with Na2CO3, then NaOH and CaCO3 are produced continuously. Therefore, in this operation CaCO3 crystals nucleate and grow in the presence of NaOH. In addition, from the industrial point of view, the continuous operation has an advantage in increased productivity [11]. Hence, aragonite crystallization behavior in the causticizing reaction was investigated with the continuous operation at 50 and 75 jC where the crystals with high aragonite content were obtained in the batch operation. The results of this operation are shown in Table 2. Runs 3, 4, 5 and 6 were carried out twice to check the reproducibility. As shown in Table 2, reproducibility is generally good. In the conditions with constant molar ratio (1:1.2), the effect of residence time and reaction temperature on aragonite precipitation was investigated (Runs 1 – 4). At 50 jC with longer residence time, the precipitates with higher aragonite composition were obtained. However, at 75 jC, aragonite precipitation was not influenced by the residence time. Moreover, Wa at 50 jC was greater than that at 75 jC. This tendency is similar to the results of the batch operation with the absence of NaOH. Furthermore, the molar ratio was varied keeping the constant residence time (60 min). With changing the molar
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H. Konno et al. / Powder Technology 129 (2003) 15–21
It was observed that NaOH concentration in the suspension is similar in each condition at the molar ratio 1:1.2, indicating that the rate of reaction is similar at each condition. NaOH concentration in the molar ratio of 1:1 was a little higher than that of 1:1.2. On the other hand, Na2CO3 concentration in the molar ratio of 1:1 was lower than that of 1:1.2. This is because the addition of excess Na2CO3 solution to Ca(OH)2 decreased the NaOH concentration and increased Na2CO3 concentration of the suspension. In the continuous experiment, a large proportion of calcite is predicted to precipitate at the beginning of the continuous operation at all conditions because the percentage of aragonite was increased as the operation proceeded to the steady state. At 75 jC with molar ratio 1:1, during three residence times, the viscosity of the suspension in the crystallizer was dramatically increased. Since pillar-like shaped particle precipitated at this condition (described below), the viscosity of the suspension increased. Aragonite crystals may nucleate and grow predominantly during three residence times, and only aragonite was precipitated after three residence times.
Fig. 3. SEM photographs of CaCO3 particles obtained at different temperatures in the batch operation. (a) 25 jC, (b) 50 jC, (c) 75 jC.
ratio to 1:1 (Runs 5– 6), Wa became to be more than 90% at both temperatures. At 75 jC, the increase of Wa was intensive and aragonite was almost the only phase to precipitate.
3.2.2. The shape of CaCO3 formed by the continuous operation Fig. 4 shows the shape of the particles formed by the continuous operation. In the conditions with molar ratio 1:1.2 and shorter residence time (30 min), aggregated particles consisting of pillar-like and granular crystal are observed at both temperatures (Fig. 4a: 50 jC, Fig. 4b: 75 jC). Similarly, at a longer residence time (60 min) aggregation of particles is also observed (Fig. 4c: 50 jC, Fig. 4d: 75 jC). At 50 jC, the amount of pillar-like-shaped aragonite crystals increased with increasing residence time, and the crystal size of that shape also increased. The increase of average particle size of aggregation with residence time shown in Table 2 also supported that a longer residence time contributed to crystal growth. At 75 jC, the proportion of the crystal with pillar-like shape is lower than that of granular shape (calcite), which is consistent with XRD measurement. At 75 jC, the crystal size of pillar-like and granular was not influenced by the residence time. The increase of average particle size with residence time may be responsible for the high degree of aggregation. In contrast to the above results, particles obtained at the molar ratio of 1:1 (Runs 5– 6) showed characteristic shape at each temperature unlike those at the 1:1.2 molar ratio. At 50 jC, fine needle-like particles are aggregated to form large particles with a size of about 45 Am (Fig. 4e). While at 75 jC, the needle-like particles 10 – 20 Am in length were observed (Fig. 4f) and they are not aggregated. The crystal size of aragonite increased and the degree of aggregation decreased with temperature. These results can be explained in the similar manner described in Section 3.1.1. The particle shape obtained in Run 6 are
H. Konno et al. / Powder Technology 129 (2003) 15–21
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Fig. 4. SEM photographs of CaCO3 particles obtained in the continuous operation. (a) Ca(OH)2/Na2CO3 = 1:1.2 (50 jC, 30 min). (b) Ca(OH)2/Na2CO3 = 1:1.2 (75 jC, 30 min). (c) Ca(OH)2/Na2CO3 = 1:1.2 (50 jC, 60 min). (d) Ca(OH)2/Na2CO3 = 1:1.2 (75 jC, 60 min). (e) Ca(OH)2/Na2CO3 = 1:1 (50 jC, 60 min). (f) Ca(OH)2/Na2CO3 = 1:1 (75 jC, 60 min).
similar to that of the batch operation at 75 jC (Fig. 3c). In the higher temperature conditions to form aragonite preferentially, the particles with higher aspect ratio (length to width) may be generally obtained [12]. 3.2.3. The effect of [Ca2+] and [CO32 ] The effect of [Ca2 +] and [CO32 ] on the aragonite precipitation was also investigated. This [CO32 ] is equal
to [Na2CO3] in Table 2. Table 2 shows the two indices; [Ca2 +][CO32 ] and [Ca2 +]/[CO32 ] ratio after reaching a steady state in the continuous operation. The influence of [Ca2 +][CO32 ] on the aragonite precipitation was investigated. This [Ca2 +][CO32 ] was correlated to the supersaturation. There is no large difference in the NaOH concentration of runs at each temperature, therefore, the solubility product of CaCO3 was considered to
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H. Konno et al. / Powder Technology 129 (2003) 15–21
Fig. 5. The effect of [Ca2 +][CO 23 ] on the aragonite precipitation at 50 and 75 jC with the continuous operation. ( w ) 50 jC, (o) 75 jC.
Fig. 7. The influence of [CO23 ] on the aragonite crystallization at 50 and 75 jC. ( w ) 50 jC, (o) 75 jC.
be almost constant. Moreover, the preliminary measurements of the solubility products of CaCO3 in the NaOH solution further supported this similarity at the NaOH concentration range produced by this experiment. The relation between [Ca2 +][CO32 ] and Wa is plotted in Fig. 5. Wa was increased as [Ca2 +][CO32 ] was decreased at each temperature, which indicated that aragonite was readily precipitated at lower supersaturation in the causticizing reaction. This result confirmed that the effect of the supersaturation on the aragonite precipitation observed in the batch operation in the previous paper [10]. Similarly, the relation between [Ca2 +]/[CO32 ] ratio and Wa is plotted in Fig. 6. At both temperatures, Wa was increased as the ratio was increased. Moreover, Fig. 7 shows the plots between [CO32 ] and Wa at each temperature. Although the plots were on the different
lines at each temperature, Wa was increased with decreasing [CO32 ] of the suspension. These results indicated that a lower [CO32 ] was required to form aragonite crystals in the causticizing reaction with the continuous operation.
Fig. 6. The effect of [Ca2 +]/[CO 23 ] ratio on the aragonite crystallization at 50 and 75 jC. ( w ) 50 jC, (o) 75 jC.
4. Conclusion Crystallization of aragonite was investigated based on batch and continuous operations. The following results were obtained. (1) In the batch operation, the NaOH addition increased Wa (aragonite composition (%) in the crystallized CaCO3) at all temperatures investigated. In particular, the increase of Wa at 75 jC was remarkable. (2) The size of the crystal increased and the degree of aggregation decreased with temperature. These were due to the lower supersaturation with temperature. (3) In the continuous operation, the molar ratio of Na2CO3 to Ca(OH)2 was the most significant factor to precipitate aragonite. Almost 100% aragonite can be crystallized in the condition with the molar ratio 1:1 at 75 jC. (4) Particles obtained at the molar ratio of 1:1 at each temperature showed different shapes. At 50 jC, fine needleshaped particles were aggregated to form large particles with a size of about 45 Am, while at 75 jC, the needle-like particles 10 – 20 Am in length with less aggregation were observed. (5) Wa was increased as [Ca2+][CO32 ] was decreased at each temperature, which indicated that aragonite was readily precipitated at low supersaturation in the causticizing reaction. Then, high [Ca2+]/[CO32 ] was required to form aragonite in the causticizing reaction in the continuous operation.
H. Konno et al. / Powder Technology 129 (2003) 15–21
References [1] R.W. Hagemeyer, Paper Coating Pigments TAPPI Monograph Series, vol. 38, Tappi Press, Atlanta, Georgia, 1976, p. 40. [2] Y. Nanri, K. Kanai, K. Takahashi, H. Konno, H. Goto, Int. Chem. Recovery Conf. Proc. (2001) 369. [3] US Patent no. 6190633. [4] J.L. Wray, F. Daniels, J. Am. Chem. Soc. 79 (9) (1957) 2031. [5] O. So¨nel, J.W. Mullin, J. Cryst. Growth 60 (1982) 239. [6] M.M. Reddy, G.H. Nancollas, J. Cryst. Growth 35 (1976) 33. [7] M. Kitamura, J. Colloid Interface Sci. 236 (2000) 318.
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[8] C. Merris, Innovative advances in the forest products industries, AIChE Symp. Ser. 94 (319) (1998) 103. [9] H. Konno, Y. Nanri, M. Kitamura, Powder Technol. 123 (1) (2002) 33. [10] M. Kitamura, H. Konno, A. Yasui, H. Masuoka, J. Cryst. Growth 236 (2002) 323. [11] J.W. Mullin, Crystallization, 3rd rev. ed., Butterworth-Heimann, Oxford, 1992, p. 398. [12] A. Kato, K. Jonosono, S. Nagashima, J. Inorg. Mater. Jpn. 245 (1993) 234.
Effect of NaOH on aragonite precipitation in batch and continuous crystallization in causticizing reaction Haruo Konno a,*, Yasunori Nanri a, Mitsutaka Kitamura b a
Pulp and Paper Research Laboratory, R&D Division, Nippon Paper Industries Co., Ltd., 5-21-1, Oji, Kita-ku, Tokyo 114-0002, Japan b Department of Chemical Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashihiroshima 739-8527, Japan Received 2 November 2001; received in revised form 8 August 2002; accepted 29 August 2002
Abstract In order to obtain CaCO3 crystals with high aragonite content in the causticizing reaction in the paper industry, the effect of the NaOH addition at various temperatures was investigated using a batch operation. It is apparent that the NaOH addition increased aragonite composition in the crystal at all temperatures investigated. Then, aragonite crystallization was investigated using a continuous operation. In the continuous operation, it appeared that the dominant factor for the aragonite formation is the low molar ratio of Na2CO3 to Ca(OH)2. The lower supersaturation and the higher ratio of [Ca2 +]/[CO23 ] in the presence of NaOH are advantageous to precipitate aragonite in the causticizing reaction. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Crystallization; Aragonite; Causticizing reaction; Continuous operation; Batch operation; Supersaturation
1. Introduction Precipitated calcium carbonate (PCC) is widely used as a filler and coating pigment in the paper industry. There are two reasons for using PCC: it is an inexpensive material and it contributes to a high brightness and good opacity of paper. CaCO3 has three polymorphs: calcite, aragonite and vaterite. The first two are produced commercially. Aragonite has a slightly greater average index of refraction than calcite [1] and a needle-like characteristic shape. Hence, aragonite is expected to have higher performance properties compared to calcite for filler and coating pigment in paper. Although the carbonation process is the most widely adopted process for producing PCC, CaCO3 is the only product in this process. In the causticizing reaction, which is carried out in the chemical recovery process of the kraft pulping method, CaCO3 is a by-product of the NaOH production. There is a commercial significance when this by-product can be produced in the form of aragonite with its high-performance properties [2,3]. The crystallization of aragonite has been investigated in the liquid – liquid system [4 – 7]. However, little information is known about the * Corresponding author. Tel.: +81-3-3911-5106; fax: +81-3-3911-9476. E-mail addresses: [email protected], [email protected] (H. Konno).
crystallization of aragonite in the causticizing reaction, which is a liquid – solid system [8]. In the previous paper [9,10], we reported that various operational factors influenced the aragonite precipitation in the causticizing reaction. It was concluded that aragonite precipitation was favored at 50 jC and in the presence of NaOH at the initial stage of the causticizing reaction [9]. The crystallization of aragonite was accelerated by lowering the feed rate of Na2CO3 solution to Ca(OH)2 suspension and by increasing the stirring rate. The mechanism of the effect of such controlling factors was also shown [10]. Following these works, in this report, the effect of the NaOH presence on the aragonite crystallization at various temperatures with batch operation is investigated. On the basis of this result, aragonite crystallization using a continuous operation, where NaOH is constantly present in the system, is investigated.
2. Experimental 2.1. Crystallization 2.1.1. Materials Reagent grade chemicals and ultra-pure water were used for the causticizing reaction.
0032-5910/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 ( 0 2 ) 0 0 2 7 5 - 9
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H. Konno et al. / Powder Technology 129 (2003) 15–21
2.1.2. Batch operation A 1000-ml crystallizer was soaked in a water bath, which was regulated at 25, 50 and 75 jC ( F 0.1 jC). The crystallization was carried out by adding 1.61 mol/l Na2CO3 (100 g/l as Na2O) to the Ca(OH)2 suspension containing 1 mol/l NaOH for 2 h using a tube pump with a constant feed rate. The mixture was stirred constantly. The added Na2CO3 is equivalent to a 1.2 molar quantity of Ca(OH)2. The reaction conditions for the batch operation are shown in Table 1. 2.1.3. Continuous operation A 500-ml jacketed crystallizer connected to a water bath which was regulated at 50 and 75 jC ( F 0.1 jC) was used. In Fig. 1, the experimental apparatus for the continuous operation is shown. The crystallizer was constantly fed with 13.2 wt.% Ca(OH)2 suspension and 1.61 mol/l Na2CO3 solution using a tube pump. The product suspension (the mixture of CaCO3 and alkaline solution) was stirred constantly and withdrawn continuously through the outlet at the bottom of the crystallizer using a tube pump to keep the suspension volume constant. The steady state was confirmed by the particle size distribution and alkaline concentration measurement. In this paper, reaction temperature, residence time and the molar ratio of Na2CO3 to Ca(OH)2 were variables. Reaction conditions are shown in Table 2. 2.2. Analysis of solutions For the batch operation, the suspension in the crystallizer was sampled at 0, 15, 30, 60, 90 and 120 min since the addition of Na2CO3 solution was started. Subsequently, the suspension was filtered quickly under reduced pressure. For the continuous operation, the suspension was sampled through the outlet at every residence time to check the steady state and obtain a sample to analyze. The suspension was filtered in a similar manner as above. A sample of filtrate by both operations was taken and neutralized with HCl, then the Ca2 + concentration in the neutralized filtrate was measured with an ICP Atomic Emission Spectrometer [9]. NaOH and Na2CO3 concentrations were measured by HCl titration [9]. The products were washed 5 times with
Table 1 The experimental conditions of the batch operation Reaction temperature (jC) Initial NaOH concentration (mol/l) Ca(OH)2 suspension concentration (wt%) Adding time of Na2CO3 solution (h) Na2CO3 solution concentration (mol/l) Stirring speed (rpm)
25
50 0
75 1
13.2 2 1.61 ( = 100 g/l as Na2O) 400
Fig. 1. Experimental apparatus for the continuous operation.
ethanol to remove NaOH and dried under reduced pressure. The samples, thus, obtained were measured using powder X-ray diffraction (RAD-C; Rigaku) to determine aragonite composition in the crystallized CaCO3. Residual Ca(OH)2 was calculated from the concentration of NaOH produced because the consumption of Ca(OH)2 corresponds to the production of NaOH in the causticizing reaction. The crystals were observed using a SEM (JSM-840A; JEOL) to examine the size and shape of the crystals. Particle size distribution of the sample was measured with a particle size analyzer (Master Sizer-S, Malvern).
3. Results and discussion 3.1. Batch operation 3.1.1. The influence of temperature during crystallization The causticizing reaction was carried out in the presence of 1 mol/l NaOH at 25, 50 and 75 jC. In Fig. 2, the dependence of aragonite composition (%) in the crystallized CaCO3 (Wa) on the molar ratio of Na2CO3 added to Ca(OH)2 (R) during crystallization is shown. The results without NaOH addition are shown to compare with the results of the previous paper [9]. It can be seen in Fig. 2 that the NaOH addition increased Wa at all temperatures investigated. Although no precipitation of aragonite occurred without NaOH at 25 jC, with the addition of NaOH aragonite precipitated at R = 0.6 and Wa increased up to a maximum of 50% at R = 1.2. At 50 and 75 jC, aragonite was predominantly precipitated even at the initial addition of Na2CO3 (R = 0.15, 0.3) in the suspension containing 1 mol/l NaOH, and Wa was increased to around 90%. In particular, at 75 jC, the increase of aragonite precipitation was remarkable. It is thought that the NaOH stimulated the crystal nucleation and growth of aragonite. In general, high temperature tends to favor aragonite formation [4]. This tendency is also seen in the CaCO3 formation in the
H. Konno et al. / Powder Technology 129 (2003) 15–21
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Table 2 The experimental conditions and the results of the continuous operation
Reaction temperature (jC) Molar ratio (Ca(OH)2/Na2CO3) Residence time (min) Wa (%) Residual Ca(OH)2 (%) NaOH (mol/l) Na2CO3 (mol/l) [Ca2 +] ( 10 3 mol/l) [Ca2 +][CO23 ] ( 10 3) [Ca2 +]/[CO23 ] ( 10 3) Average particle size (Am)
Run 1
Run 2
Run 3 (1)
Run 3 (2)
Run 4 (1)
Run 4 (2)
Run 5 (1)
Run 5 (2)
Run 6 (1)
Run 6 (2)
50 1:1.2 30 56 9.6 1.43 0.27 0.37 0.099 1.40 33.2
75 1:1.2 30 28 7.6 1.47 0.24 0.20 0.048 0.84 24.3
50 1:1.2 60 70 7.7 1.46 0.27 0.31 0.083 1.17 38.9
50 1:1.2 60 79 8.1 1.46 0.26 0.29 0.076 1.13 40.2
75 1:1.2 60 28 6.9 1.48 0.26 0.18 0.045 0.69 30.8
75 1:1.2 60 29 7.6 1.47 0.27 0.17 0.047 0.62 28.4
50 1:1 60 94 12.0 1.55 0.13 0.60 0.075 4.74 46.2
50 1:1 60 93 13.5 1.52 0.15 0.50 0.076 3.32 45.7
75 1:1 60 100 6.4 1.65 0.06 0.29 0.018 4.65 9.3
75 1:1 60 98 7.2 1.63 0.10 0.30 0.030 3.03 10.7
causticizing reaction using Ca(OH)2 suspension containing 1 mol/l NaOH. Fig. 3 shows a SEM photograph of precipitated CaCO3 at different temperatures. At 25 jC, aggregated particles consisting of a mixture of needle-like (aragonite) and granular (calcite) crystal are observed. At 50 and 75 jC, only needle-
like crystals are observed. As shown in Fig. 2, Wa at 50 jC was similar to that at 75 jC. However, the needle-like shapes were very different. At 50 jC, the needle-like particles have a size of about 5 Am in length and about 0.2 Am in width, and they are aggregated. On the other hand, at 75 jC the needle-like particles with 5 –15 Am in length are not much aggregated. The increase of the crystal size and the decrease of the aggregation with temperature may be because Ca2 + concentration is decreased with increasing temperature, resulting in lower supersaturation. 3.2. Continuous operation
Fig. 2. Effect of the addition of NaOH on the Ca(OH)2 suspension at different temperatures. (A) NaOH addition. (B) No addition. ( w ) 25 jC, (5) 50 jC, (D) 75 jC. (A) 5: previous data [9].
3.2.1. Aragonite precipitation by the continuous operation At the initial stage of Na2CO3 addition in the batch operation, the presence of NaOH favors the precipitation of aragonite. This means the presence of NaOH is effective on the nucleation of aragonite crystals. In the continuous operation, Ca(OH)2 reacts with Na2CO3, then NaOH and CaCO3 are produced continuously. Therefore, in this operation CaCO3 crystals nucleate and grow in the presence of NaOH. In addition, from the industrial point of view, the continuous operation has an advantage in increased productivity [11]. Hence, aragonite crystallization behavior in the causticizing reaction was investigated with the continuous operation at 50 and 75 jC where the crystals with high aragonite content were obtained in the batch operation. The results of this operation are shown in Table 2. Runs 3, 4, 5 and 6 were carried out twice to check the reproducibility. As shown in Table 2, reproducibility is generally good. In the conditions with constant molar ratio (1:1.2), the effect of residence time and reaction temperature on aragonite precipitation was investigated (Runs 1 – 4). At 50 jC with longer residence time, the precipitates with higher aragonite composition were obtained. However, at 75 jC, aragonite precipitation was not influenced by the residence time. Moreover, Wa at 50 jC was greater than that at 75 jC. This tendency is similar to the results of the batch operation with the absence of NaOH. Furthermore, the molar ratio was varied keeping the constant residence time (60 min). With changing the molar
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It was observed that NaOH concentration in the suspension is similar in each condition at the molar ratio 1:1.2, indicating that the rate of reaction is similar at each condition. NaOH concentration in the molar ratio of 1:1 was a little higher than that of 1:1.2. On the other hand, Na2CO3 concentration in the molar ratio of 1:1 was lower than that of 1:1.2. This is because the addition of excess Na2CO3 solution to Ca(OH)2 decreased the NaOH concentration and increased Na2CO3 concentration of the suspension. In the continuous experiment, a large proportion of calcite is predicted to precipitate at the beginning of the continuous operation at all conditions because the percentage of aragonite was increased as the operation proceeded to the steady state. At 75 jC with molar ratio 1:1, during three residence times, the viscosity of the suspension in the crystallizer was dramatically increased. Since pillar-like shaped particle precipitated at this condition (described below), the viscosity of the suspension increased. Aragonite crystals may nucleate and grow predominantly during three residence times, and only aragonite was precipitated after three residence times.
Fig. 3. SEM photographs of CaCO3 particles obtained at different temperatures in the batch operation. (a) 25 jC, (b) 50 jC, (c) 75 jC.
ratio to 1:1 (Runs 5– 6), Wa became to be more than 90% at both temperatures. At 75 jC, the increase of Wa was intensive and aragonite was almost the only phase to precipitate.
3.2.2. The shape of CaCO3 formed by the continuous operation Fig. 4 shows the shape of the particles formed by the continuous operation. In the conditions with molar ratio 1:1.2 and shorter residence time (30 min), aggregated particles consisting of pillar-like and granular crystal are observed at both temperatures (Fig. 4a: 50 jC, Fig. 4b: 75 jC). Similarly, at a longer residence time (60 min) aggregation of particles is also observed (Fig. 4c: 50 jC, Fig. 4d: 75 jC). At 50 jC, the amount of pillar-like-shaped aragonite crystals increased with increasing residence time, and the crystal size of that shape also increased. The increase of average particle size of aggregation with residence time shown in Table 2 also supported that a longer residence time contributed to crystal growth. At 75 jC, the proportion of the crystal with pillar-like shape is lower than that of granular shape (calcite), which is consistent with XRD measurement. At 75 jC, the crystal size of pillar-like and granular was not influenced by the residence time. The increase of average particle size with residence time may be responsible for the high degree of aggregation. In contrast to the above results, particles obtained at the molar ratio of 1:1 (Runs 5– 6) showed characteristic shape at each temperature unlike those at the 1:1.2 molar ratio. At 50 jC, fine needle-like particles are aggregated to form large particles with a size of about 45 Am (Fig. 4e). While at 75 jC, the needle-like particles 10 – 20 Am in length were observed (Fig. 4f) and they are not aggregated. The crystal size of aragonite increased and the degree of aggregation decreased with temperature. These results can be explained in the similar manner described in Section 3.1.1. The particle shape obtained in Run 6 are
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Fig. 4. SEM photographs of CaCO3 particles obtained in the continuous operation. (a) Ca(OH)2/Na2CO3 = 1:1.2 (50 jC, 30 min). (b) Ca(OH)2/Na2CO3 = 1:1.2 (75 jC, 30 min). (c) Ca(OH)2/Na2CO3 = 1:1.2 (50 jC, 60 min). (d) Ca(OH)2/Na2CO3 = 1:1.2 (75 jC, 60 min). (e) Ca(OH)2/Na2CO3 = 1:1 (50 jC, 60 min). (f) Ca(OH)2/Na2CO3 = 1:1 (75 jC, 60 min).
similar to that of the batch operation at 75 jC (Fig. 3c). In the higher temperature conditions to form aragonite preferentially, the particles with higher aspect ratio (length to width) may be generally obtained [12]. 3.2.3. The effect of [Ca2+] and [CO32 ] The effect of [Ca2 +] and [CO32 ] on the aragonite precipitation was also investigated. This [CO32 ] is equal
to [Na2CO3] in Table 2. Table 2 shows the two indices; [Ca2 +][CO32 ] and [Ca2 +]/[CO32 ] ratio after reaching a steady state in the continuous operation. The influence of [Ca2 +][CO32 ] on the aragonite precipitation was investigated. This [Ca2 +][CO32 ] was correlated to the supersaturation. There is no large difference in the NaOH concentration of runs at each temperature, therefore, the solubility product of CaCO3 was considered to
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Fig. 5. The effect of [Ca2 +][CO 23 ] on the aragonite precipitation at 50 and 75 jC with the continuous operation. ( w ) 50 jC, (o) 75 jC.
Fig. 7. The influence of [CO23 ] on the aragonite crystallization at 50 and 75 jC. ( w ) 50 jC, (o) 75 jC.
be almost constant. Moreover, the preliminary measurements of the solubility products of CaCO3 in the NaOH solution further supported this similarity at the NaOH concentration range produced by this experiment. The relation between [Ca2 +][CO32 ] and Wa is plotted in Fig. 5. Wa was increased as [Ca2 +][CO32 ] was decreased at each temperature, which indicated that aragonite was readily precipitated at lower supersaturation in the causticizing reaction. This result confirmed that the effect of the supersaturation on the aragonite precipitation observed in the batch operation in the previous paper [10]. Similarly, the relation between [Ca2 +]/[CO32 ] ratio and Wa is plotted in Fig. 6. At both temperatures, Wa was increased as the ratio was increased. Moreover, Fig. 7 shows the plots between [CO32 ] and Wa at each temperature. Although the plots were on the different
lines at each temperature, Wa was increased with decreasing [CO32 ] of the suspension. These results indicated that a lower [CO32 ] was required to form aragonite crystals in the causticizing reaction with the continuous operation.
Fig. 6. The effect of [Ca2 +]/[CO 23 ] ratio on the aragonite crystallization at 50 and 75 jC. ( w ) 50 jC, (o) 75 jC.
4. Conclusion Crystallization of aragonite was investigated based on batch and continuous operations. The following results were obtained. (1) In the batch operation, the NaOH addition increased Wa (aragonite composition (%) in the crystallized CaCO3) at all temperatures investigated. In particular, the increase of Wa at 75 jC was remarkable. (2) The size of the crystal increased and the degree of aggregation decreased with temperature. These were due to the lower supersaturation with temperature. (3) In the continuous operation, the molar ratio of Na2CO3 to Ca(OH)2 was the most significant factor to precipitate aragonite. Almost 100% aragonite can be crystallized in the condition with the molar ratio 1:1 at 75 jC. (4) Particles obtained at the molar ratio of 1:1 at each temperature showed different shapes. At 50 jC, fine needleshaped particles were aggregated to form large particles with a size of about 45 Am, while at 75 jC, the needle-like particles 10 – 20 Am in length with less aggregation were observed. (5) Wa was increased as [Ca2+][CO32 ] was decreased at each temperature, which indicated that aragonite was readily precipitated at low supersaturation in the causticizing reaction. Then, high [Ca2+]/[CO32 ] was required to form aragonite in the causticizing reaction in the continuous operation.
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References [1] R.W. Hagemeyer, Paper Coating Pigments TAPPI Monograph Series, vol. 38, Tappi Press, Atlanta, Georgia, 1976, p. 40. [2] Y. Nanri, K. Kanai, K. Takahashi, H. Konno, H. Goto, Int. Chem. Recovery Conf. Proc. (2001) 369. [3] US Patent no. 6190633. [4] J.L. Wray, F. Daniels, J. Am. Chem. Soc. 79 (9) (1957) 2031. [5] O. So¨nel, J.W. Mullin, J. Cryst. Growth 60 (1982) 239. [6] M.M. Reddy, G.H. Nancollas, J. Cryst. Growth 35 (1976) 33. [7] M. Kitamura, J. Colloid Interface Sci. 236 (2000) 318.
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[8] C. Merris, Innovative advances in the forest products industries, AIChE Symp. Ser. 94 (319) (1998) 103. [9] H. Konno, Y. Nanri, M. Kitamura, Powder Technol. 123 (1) (2002) 33. [10] M. Kitamura, H. Konno, A. Yasui, H. Masuoka, J. Cryst. Growth 236 (2002) 323. [11] J.W. Mullin, Crystallization, 3rd rev. ed., Butterworth-Heimann, Oxford, 1992, p. 398. [12] A. Kato, K. Jonosono, S. Nagashima, J. Inorg. Mater. Jpn. 245 (1993) 234.