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Crystallization of aragonite in the causticizing reaction
Powder Technology 123 Ž2002. 33–39 www.elsevier.comrlocaterpowtec
Crystallization of aragonite in the causticizing reaction Haruo Konno a,) , Yasunori Nanri a , Mitsutaka Kitamura b a
Pulp and Paper Research Laboratory, Nippon Paper Industries Co., Ltd., R & D DiÕ. 5-21-1, Oji, Kita-ku, Tokyo 114-0002, Japan b Department of Chemical Engineering, Hiroshima UniÕersity, 1-4-1, Kagamiyama, Higashihiroshima City, 739-8527, Japan Received 14 February 2001; received in revised form 8 June 2001; accepted 18 June 2001
Abstract Aragonite, a polymorph of CaCO 3 , is used in the paper industry as filler and pigment. To determine the reaction conditions used during causticization, the solubility of CaŽOH. 2 and CaCO 3 in NaOH solution and the crystallization behavior of aragonite were investigated. The solubility of CaCO 3 in a NaOH solution reached a maximum value with NaOH concentration. Decrease in the solubility after that value was due to the conversion of CaCO 3 to CaŽOH. 2 . In 1 molrl of NaOH solution containing a concentration of Na 2 CO 3 greater than 0.02 molrl, CaCO 3 appeared to be in a stable phase as compared to CaŽOH. 2 . In the crystallization experiments, it was determined that reaction temperature strongly influenced the size, shape, and polymorph of CaCO 3. The highest precipitation of aragonite occurred at 50 8C. At 50 8C, the percentage of aragonite in crystallized CaCO 3 increased as the NaOH concentration in the mother liquor was increased. This result indicates that aragonite was predominantly precipitated in the presence of NaOH. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Crystallization; Aragonite; Calcite; Polymorphs; Causticizing reaction; Solubility
1. Introduction Not until recently but nowadays, CaCO 3 is used extensively as filler and pigment in the paper industry because of its low cost and high brightness. Previously, the papermaking process was primarily acidic, which degrades CaCO 3 . CaCO 3 has become extensively used, as the papermaking process has been carried out from neutral to alkaline conditions. In addition, the need for high opacity in paper by using CaCO 3 as filler and pigment has been increasing w1,2x. CaCO 3 has three polymorphs: calcite, aragonite and vaterite. Among them aragonite has needlelike and columnar-shaped crystal habits, and improves the opacity of paper when it is used as filler and pigment. This type of CaCO 3 has already been commercially produced w2x. In the chemical recovery process of the kraft pulping method, CaCO 3 is produced as a by-product in the causti-
) Corresponding author. Tel.: q81-3-3911-5240; fax: q81-3-39119476. E-mail address: [email protected] ŽH. Konno..
cization step. A flowchart of the kraft pulping method is shown in Fig. 1. In this process, two reactions take place simultaneously, as shown in Eqs. Ž1. and Ž2.. The second reaction is usually called the causticizing reaction.
CaO q H 2 O ™ Ca Ž OH . 2
Ž 1.
Ca Ž OH . 2 q Na 2 CO 3 ° CaCO 3 q 2NaOH
Ž 2.
Hitherto, the research on the causticizing reaction has been focused on the production of NaOH w3–5x. There are very few reports on the crystallization of CaCO 3 in the causticizing reaction w6x, although there is a patent w7x on the process. The causticizing reaction proceeds in very high alkaline solutions and the NaOH concentration in the mother liquor further increases as the reaction proceeds, which makes it difficult to study. The present work is aimed at understanding the mechanism for the crystallization process, especially aragonite, in the causticizing reaction. In this paper, to obtain the fundamental knowledge of this system, the solubility of CaŽOH. 2 and CaCO 3 in the NaOH solution was determined. Then, the effect of the operating conditions Žreaction tempera-
0032-5910r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 1 . 0 0 4 2 4 - 7
H. Konno et al.r Powder Technology 123 (2002) 33–39
34
Na 2 CO 3 is equivalent to a 1.2-molar quantity of CaŽOH. 2 . The detailed reaction conditions are shown in Table 1, and the standard condition was set at a temperature of 50 8C with a CaŽOH. 2 suspension concentration of 13.2% Ž10% as CaO.. 2.4. Analysis of solutions
Fig. 1. Flowchart of kraft pulping process.
ture, CaŽOH. 2 suspension concentration, and initial NaOH concentration. on aragonite precipitation was examined.
2. Experimental 2.1. Materials Solvents for the solubility measurement were prepared from reagent grade, NaOH and Na 2 CO 3 . CaŽOH. 2 and calcite were ultra-pure grade. Aragonite was prepared by the homogeneous precipitation technique of the CaŽNO 3 . 2 – ŽNH 4 . 2 CO system w8x. For the causticizing reaction, all chemicals used were reagent grade. In all experiments, ultra-pure water was used. 2.2. Solubility measurement A three-neck 300-ml round bottom flask was soaked in a water bath that was regulated at 25, 50 and 75 8C Ž"0.1 8C.. Pure NaOH solution Ž0–3 molrl. or NaOH Ž1 molrl. solution containing various Na 2 CO 3 concentrations Ž0.01– 0.04 molrl. was poured into the flask. Excess CaŽOH. 2 and CaCO 3 were added to the solvents, and then this solution was stirred slowly. N2 gas was passed through the flask to prevent the generation of CaCO 3 when the solubility of CaŽOH. 2 was measured. Samples were taken with a syringe, equipped with a 0.45-mm membrane filter, over a period of 1–6 h, and then the filtrate was neutralized with HCl. The Ca2q concentration in the neutralized filtrate was measured with an ICP Atomic Emission Spectrometer ŽSPS-1700VR; Seiko Instruments.. 2.3. Crystallization A 1000-ml crystallizer was soaked in a water bath, which was regulated at 25, 50 and 75 8C Ž"0.1 8C.. The crystallization reaction of CaCO 3 was carried out by adding 1.61 molrl of Na 2 CO 3 Ž100 grl as Na 2 O. to the CaŽOH. 2 suspension for 2 h using a tube pump with a constant feed rate. The mixture was stirred constantly. The added
After the addition of Na 2 CO 3 solution was started, the suspension in the crystallizer was sampled at 0, 15, 30, 60, 90 and 120 min. Subsequently, the suspension was filtered quickly under reduced pressure. A sample of filtrate was taken and neutralized with HCl, then the Ca2q concentration in the neutralized filtrate was measured with an ICP Atomic Emission Spectrometer. The NaOH and Na 2 CO 3 concentrations were measured by HCl titration with a potentiometer Žmodified Whinkler methods. w9x. For the NaOH concentration measurement, excess 20% BaCl 2 solution was added to an aliquot of filtrate to remove carbonate ions by precipitating BaCO 3 . The solution without BaCO 3 was titrated with HCl. For the Na 2 CO 3 concentration ŽCO 32y concentration. measurement, another aliquot of filtrate was directly titrated with HCl to a second end-point. The difference between the two titration volumes represents the Na 2 CO 3 concentration in the solution. The products were washed five times with ethanol to remove NaOH and dried for 4 h under reduced pressure. The samples thus obtained were measured using powder X-ray diffraction ŽRAD-C; Rigaku. to determine the percentage of aragonite in the crystallized CaCO 3 . The crystals were observed using a SEM ŽJSM-5300; JEOL. to examine the size and shape of the crystals.
3. Results and discussion 3.1. Solubility measurement 3.1.1. The solubility of Ca(OH)2 and CaCO3 in the NaOH solution Many reports on the equilibrium of the causticizing reaction at higher temperatures have been shown w4,5x. However, little investigation has been reported on the
Table 1 Reaction condition CaŽOH. 2 suspension concentration Žwt.%. Reaction temperature Ž8C. Adding time of Na 2 CO 3 solution Žh. Na 2 CO 3 solution concentration Žmolrl. Stirring speed Žrpm.
6.6 25 2 1.61 400
13.2
26.4
50
75
Ž s100 grl as Na 2 O.
H. Konno et al.r Powder Technology 123 (2002) 33–39
solubility of CaŽOH. 2 and CaCO 3 at high NaOH concentrations, which simulate the causticizing reaction conditions at temperatures under 90 8C. As shown in Fig. 2, the solubility of CaŽOH. 2 was observed to decrease as the NaOH concentration increased
35
at 25, 50 and 75 8C. The reason for this is the higher NaOH concentration led to a higher OHy concentration, resulting in the prevention of the dissociation of CaŽOH. 2 . Furthermore, the solubility of CaŽOH. 2 decreases with temperature. In the case of CaCO 3 , the solubility of calcite
Fig. 2. Solubility of CaŽOH. 2 , calcite and aragonite in NaOH solution at 25, 50 and 75 8C; `: CaŽOH. 2 , I: calcite, B: aragonite.
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H. Konno et al.r Powder Technology 123 (2002) 33–39
and aragonite increased with NaOH concentration; however, at concentrations of NaOH greater than 1 molrl it decreased at 25 8C. At this concentration region, the solubility of CaCO 3 became greater than that of CaŽOH. 2 . Powder X-ray diffraction ŽXRD. measurement showed that at 1 molrl of NaOH no change of calcium carbonate occurred after the solubility experiment; however, at greater than 2 molrl of NaOH the presence of CaŽOH. 2 in the solids was confirmed. The solid phase of calcite and aragonite is unstable and easily converts to CaŽOH. 2 at 2 molrl of NaOH, both of which explain these results. At 50 8C the solubility of calcite in the NaOH solution peaked at 1 molrl, while that of aragonite peaked at 0.5 molrl of NaOH. This may mean that aragonite is more unstable than calcite and easily converts to CaŽOH. 2 . At 75 8C the solubility of both crystals peaked at 0.5 molrl of NaOH, and decreased at a concentration greater than 0.5 molrl of NaOH. It is expected that both calcite and aragonite are unstable at the higher NaOH concentration and the solubility of calcite and aragonite is much higher than that of CaŽOH. 2 at 75 8C. 3.1.2. The influence of Na2 CO3 As carbonate ions exist in a practical causticizing reaction, it is predicted that the carbonate ion influences the solubility of CaCO 3 in the NaOH solution. The solubility of calcite and aragonite was measured in 1 molrl of NaOH containing Na 2 CO 3 at concentrations ranging from 0.01 to 0.04 molrl at 25, 50 and 75 8C ŽFig. 3.. The solubility of calcite and aragonite decreased as the concentration of Na 2 CO 3 increased. The solubility of calcite at 75 8C and aragonite at 50 8C and 75 8C in the NaOH solution without Na 2 CO 3 cannot be obtained due to the conversion to CaŽOH. 2 as described above. These results show that calcite and aragonite are in a stable solid phase in a 1-molrl NaOH solution containing greater than 0.02 molrl Na 2 CO 3 . This was confirmed by XRD analy-
Fig. 3. Solubility of calcite and aragonite in 1 molrl of NaOH solution containing various concentration of Na 2 CO 3 ; I: calcite at 25 8C, B: aragonite at 25 8C, e: calcite at 50 8C, l: aragonite at 50 8C, `: calcite at 75 8C, v: aragonite at 75 8C.
Fig. 4. Percentage change of aragonite in crystallized CaCO 3 at different temperatures; e: 25 8C, I: 50 8C, n: 75 8C.
sis. As seen in Fig. 3, the solubility of aragonite was higher than that of calcite in the NaOH solution with Na 2 CO 3 , indicating that aragonite is in a metastable phase during the causticizing reaction conditions. Moreover, it is apparent that temperature hardly influences the solubility of calcite and aragonite. 3.2. Crystallization 3.2.1. The influence of temperature during crystallization To investigate the effect of temperature on aragonite precipitation, the crystallization was carried out at 25, 50 and 75 8C. In this experimental series, the standard conditions were applied. In Fig. 4 the change of aragonite composition Ž%. in crystallized CaCO 3 during crystallization is shown. The percentage of aragonite in crystallized CaCO 3 and the mole ratio of Na 2 CO 3 added to initial CaŽOH. 2 were expressed as Wa and R, respectively. In this system, no precipitation of vaterite or basic calcium carbonate w10x was observed. It is shown that aragonite cannot be obtained at 25 8C, however, at 50 8C Wa increased up to a maximum of 70–80%. At 75 8C, the maximum Wa was only 40–50%. It is interesting that calcite precipitates predominately at the initial addition of Na 2 CO 3 Ž R s 0.15.. Then, Wa increased as the addition of Na 2 CO 3 proceeded, and finally Wa became constant at an R value greater than 0.6. It was observed that no transformation of aragonite to calcite occurred during the addition of Na 2 CO 3 . Fig. 5 shows a SEM photograph of precipitated CaCO 3 at different temperatures. The largest proportion of particles with a needle-like shape is seen at 50 8C. These results further support the XRD analysis described previously. In the photograph at 25 8C, large aggregated particles Žcalcite. that consist of fine granular particles are seen. On the other hand, at 50 8C, a majority of particles have a needle-like shape Žaragonite. and a size of about 5 mm. These particles are not strongly aggregated. At 75 8C, mixed particles consisting of needle-like shapes of 10–15
H. Konno et al.r Powder Technology 123 (2002) 33–39
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Fig. 5. SEM photographs of CaCO 3 particles obtained at different temperatures; ŽA. 25 8C, ŽB. 50 8C, ŽC. 75 8C. Fig. 6. Time course changes of wCa2q x, wCO 32y x and wNaOHx at different temperature; e: 25 8C, I: 50 8C, n: 75 8C.
mm length Žaragonite. and granular shapes of 1-mm diameter Žcalcite. are observed. The sizes of the crystals increased as the temperature increased. The results mentioned above indicate the precipitation of aragonite is strongly influenced by temperature. The concentrations of Ca2q, CO 32y and NaOH in the mother liquor were also measured. The results are shown in Fig. 6. The Ca2q concentration during the course of crystallization decreased as the reaction progressed. The dependence of Ca2q concentration on NaOH concentration during the course of crystallization is plotted and compared with the solubility of CaŽOH. 2 in the NaOH solution at 50 8C in Fig. 7. It appears that the Ca2q concentration in the solution is concurrent with CaŽOH. 2 solubility. As the solubility of CaŽOH. 2 decreases with temperature, the results indicate that the supersaturation ratio, which is the driving force of the nucleation, decreases with temperature. This may cause the increase in the crystal size with temperature.
It can be seen that the concentration of CO 32y increased as the temperature decreased. This may be because the NaOH concentration increases with temperature.
Fig. 7. Relationship between the solubility of CaŽOH. 2 Žl. and Ca2q concentration Že. in the causticizing reaction at 50 8C.
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H. Konno et al.r Powder Technology 123 (2002) 33–39
Fig. 8. Percentage change of aragonite in crystallized CaCO 3 at different CaŽOH. 2 suspension concentrations; e: 6.6%, I: 13.2%, n: 26.4%.
3.2.2. The effect of Ca(OH)2 suspension concentration on aragonite precipitation In order to understand the effect of the CaŽOH. 2 suspension concentration on aragonite precipitation, the crystallization was carried out under the conditions at 50 8C. This is the temperature where the greatest precipitation of aragonite crystals occurred. In Fig. 8, the change of Wa in crystallized CaCO 3 during the crystallization is shown at suspension concentrations of 6.6%, 13.2% and 26.4%. It can be seen that Wa increased dramatically with the suspension concentration. In all conditions calcite was precipitated predominately at R s 0.15. Then Wa increased as the addition of Na 2 CO 3 proceeded. The relation of Wa and the NaOH concentration in the mother liquor are plotted in Fig. 9. In this figure, the results are shown at the point at which an equivalent mole of Na 2 CO 3 to CaŽOH. 2 Ž R s 1.0. was added. It is clear that the relationship between Wa and the NaOH concentration was expressed as one curve for any CaŽOH. 2 concentration. This indicates that the crystallization of aragonite strongly de-
Fig. 10. Effect of the addition of NaOH on the CaŽOH. 2 suspension; I: no addition, e: NaOH addition.
pends on the NaOH concentration in the mother liquor of this system. 3.2.3. The influence of initial NaOH concentration on aragonite precipitation From the results in Section 3.2.2, aragonite was readily precipitated in the region of 0.5–1 molrl of NaOH. To observe the effect of NaOH on the precipitation of aragonite, the causticizing reaction was started with CaŽOH. 2 suspended in 1 molrl of NaOH instead of water. Fig. 10 shows the change in Wa during the course of crystallization under the standard conditions. Using a CaŽOH. 2 suspension with 1 molrl of NaOH Wa was higher at R s 0.15 compared to the condition without 1 molrl of NaOH; subsequently, Wa was finally more than 90% at R s 1.2. It is obvious that aragonite was predominately precipitated when NaOH was present in the mother liquor. When the suspension concentration is high ŽFig. 8., the NaOH concentration produced by the reaction may enter the region in which aragonite can be predominantly precipitated at the initial stage of Na 2 CO 3 addition.
4. Conclusion The solubility of CaŽOH. 2 and CaCO 3 in the NaOH solution and the relationship between the crystallization of aragonite and operating conditions were investigated. The following results were obtained:
Fig. 9. Effect of produced NaOH concentration on the crystallization of aragonite at different CaŽOH. 2 suspension concentrations; e: 6.6%, I: 13.2%, n: 26.4%.
1. In the solubility experiment, the higher concentrations of NaOH allow CaCO 3 to be converted into CaŽOH. 2 beyond its peak of the solubility. 2. At 1 molrl of NaOH concentration, calcite and aragonite became a more stable phase compared to CaŽOH. 2 in the presence of CO 32y greater than 0.02 molrl.
H. Konno et al.r Powder Technology 123 (2002) 33–39
3. Solubility of calcite and aragonite was not influenced by temperature under the causticizing reaction conditions. 4. With the addition of Na 2 CO 3 , calcite was precipitated initially ŽR s 0.15., then Wa increased and finally became constant at R ) 0.6 at 50 and 75 8C. 5. Reaction temperature strongly influenced the precipitation behavior of aragonite, and the highest precipitation of aragonite occurred at 50 8C. 6. Precipitation of aragonite increased as the CaŽOH. 2 suspension was increased. Aragonite was predominantly precipitated in the presence of NaOH. 7. Aragonite with more than 90% purity can be obtained using a CaŽOH. 2 suspension containing 1 molrl of NaOH.
39
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x
J.D. Passaretti et al., Tappi J. 76 Ž12. Ž1993. 135. K. Tanaka, J. Soc. Inorg. Mater. Jpn. 193 Ž1984. 31. L.F. Goodwin, J. Soc. Chem. Ind. 45 Ž1926. 360T. C.M. Daily, J.M. Genco, Tappi Proceedings, AIChE Forest Products Division Session 181 Ž1988.. H. Theliander, Nord. Pulp Pap. Res. J. 2 Ž1992. 81. C. Merris, Innovative advances in the forest products industries, AIChE Symp. Ser. 94 Ž319. Ž1998. 103. Japanese Laid-Open Patent Application ŽKokai. No. Hei 10-226974. A. Kato, K. Jonosono, S. Nagashima, J. Soc. Inorg. Mater. Jpn. 245 Ž1993. 234. J.C. Olsen, O.G. Direnga, Ind. Eng. Chem. 33 Ž2. Ž1941. 204. H. Yamada, N. Hara, J. Soc. Inorg. Mater. Jpn. 196 Ž1985. 12.
Crystallization of aragonite in the causticizing reaction Haruo Konno a,) , Yasunori Nanri a , Mitsutaka Kitamura b a
Pulp and Paper Research Laboratory, Nippon Paper Industries Co., Ltd., R & D DiÕ. 5-21-1, Oji, Kita-ku, Tokyo 114-0002, Japan b Department of Chemical Engineering, Hiroshima UniÕersity, 1-4-1, Kagamiyama, Higashihiroshima City, 739-8527, Japan Received 14 February 2001; received in revised form 8 June 2001; accepted 18 June 2001
Abstract Aragonite, a polymorph of CaCO 3 , is used in the paper industry as filler and pigment. To determine the reaction conditions used during causticization, the solubility of CaŽOH. 2 and CaCO 3 in NaOH solution and the crystallization behavior of aragonite were investigated. The solubility of CaCO 3 in a NaOH solution reached a maximum value with NaOH concentration. Decrease in the solubility after that value was due to the conversion of CaCO 3 to CaŽOH. 2 . In 1 molrl of NaOH solution containing a concentration of Na 2 CO 3 greater than 0.02 molrl, CaCO 3 appeared to be in a stable phase as compared to CaŽOH. 2 . In the crystallization experiments, it was determined that reaction temperature strongly influenced the size, shape, and polymorph of CaCO 3. The highest precipitation of aragonite occurred at 50 8C. At 50 8C, the percentage of aragonite in crystallized CaCO 3 increased as the NaOH concentration in the mother liquor was increased. This result indicates that aragonite was predominantly precipitated in the presence of NaOH. q 2002 Elsevier Science B.V. All rights reserved. Keywords: Crystallization; Aragonite; Calcite; Polymorphs; Causticizing reaction; Solubility
1. Introduction Not until recently but nowadays, CaCO 3 is used extensively as filler and pigment in the paper industry because of its low cost and high brightness. Previously, the papermaking process was primarily acidic, which degrades CaCO 3 . CaCO 3 has become extensively used, as the papermaking process has been carried out from neutral to alkaline conditions. In addition, the need for high opacity in paper by using CaCO 3 as filler and pigment has been increasing w1,2x. CaCO 3 has three polymorphs: calcite, aragonite and vaterite. Among them aragonite has needlelike and columnar-shaped crystal habits, and improves the opacity of paper when it is used as filler and pigment. This type of CaCO 3 has already been commercially produced w2x. In the chemical recovery process of the kraft pulping method, CaCO 3 is produced as a by-product in the causti-
) Corresponding author. Tel.: q81-3-3911-5240; fax: q81-3-39119476. E-mail address: [email protected] ŽH. Konno..
cization step. A flowchart of the kraft pulping method is shown in Fig. 1. In this process, two reactions take place simultaneously, as shown in Eqs. Ž1. and Ž2.. The second reaction is usually called the causticizing reaction.
CaO q H 2 O ™ Ca Ž OH . 2
Ž 1.
Ca Ž OH . 2 q Na 2 CO 3 ° CaCO 3 q 2NaOH
Ž 2.
Hitherto, the research on the causticizing reaction has been focused on the production of NaOH w3–5x. There are very few reports on the crystallization of CaCO 3 in the causticizing reaction w6x, although there is a patent w7x on the process. The causticizing reaction proceeds in very high alkaline solutions and the NaOH concentration in the mother liquor further increases as the reaction proceeds, which makes it difficult to study. The present work is aimed at understanding the mechanism for the crystallization process, especially aragonite, in the causticizing reaction. In this paper, to obtain the fundamental knowledge of this system, the solubility of CaŽOH. 2 and CaCO 3 in the NaOH solution was determined. Then, the effect of the operating conditions Žreaction tempera-
0032-5910r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 1 . 0 0 4 2 4 - 7
H. Konno et al.r Powder Technology 123 (2002) 33–39
34
Na 2 CO 3 is equivalent to a 1.2-molar quantity of CaŽOH. 2 . The detailed reaction conditions are shown in Table 1, and the standard condition was set at a temperature of 50 8C with a CaŽOH. 2 suspension concentration of 13.2% Ž10% as CaO.. 2.4. Analysis of solutions
Fig. 1. Flowchart of kraft pulping process.
ture, CaŽOH. 2 suspension concentration, and initial NaOH concentration. on aragonite precipitation was examined.
2. Experimental 2.1. Materials Solvents for the solubility measurement were prepared from reagent grade, NaOH and Na 2 CO 3 . CaŽOH. 2 and calcite were ultra-pure grade. Aragonite was prepared by the homogeneous precipitation technique of the CaŽNO 3 . 2 – ŽNH 4 . 2 CO system w8x. For the causticizing reaction, all chemicals used were reagent grade. In all experiments, ultra-pure water was used. 2.2. Solubility measurement A three-neck 300-ml round bottom flask was soaked in a water bath that was regulated at 25, 50 and 75 8C Ž"0.1 8C.. Pure NaOH solution Ž0–3 molrl. or NaOH Ž1 molrl. solution containing various Na 2 CO 3 concentrations Ž0.01– 0.04 molrl. was poured into the flask. Excess CaŽOH. 2 and CaCO 3 were added to the solvents, and then this solution was stirred slowly. N2 gas was passed through the flask to prevent the generation of CaCO 3 when the solubility of CaŽOH. 2 was measured. Samples were taken with a syringe, equipped with a 0.45-mm membrane filter, over a period of 1–6 h, and then the filtrate was neutralized with HCl. The Ca2q concentration in the neutralized filtrate was measured with an ICP Atomic Emission Spectrometer ŽSPS-1700VR; Seiko Instruments.. 2.3. Crystallization A 1000-ml crystallizer was soaked in a water bath, which was regulated at 25, 50 and 75 8C Ž"0.1 8C.. The crystallization reaction of CaCO 3 was carried out by adding 1.61 molrl of Na 2 CO 3 Ž100 grl as Na 2 O. to the CaŽOH. 2 suspension for 2 h using a tube pump with a constant feed rate. The mixture was stirred constantly. The added
After the addition of Na 2 CO 3 solution was started, the suspension in the crystallizer was sampled at 0, 15, 30, 60, 90 and 120 min. Subsequently, the suspension was filtered quickly under reduced pressure. A sample of filtrate was taken and neutralized with HCl, then the Ca2q concentration in the neutralized filtrate was measured with an ICP Atomic Emission Spectrometer. The NaOH and Na 2 CO 3 concentrations were measured by HCl titration with a potentiometer Žmodified Whinkler methods. w9x. For the NaOH concentration measurement, excess 20% BaCl 2 solution was added to an aliquot of filtrate to remove carbonate ions by precipitating BaCO 3 . The solution without BaCO 3 was titrated with HCl. For the Na 2 CO 3 concentration ŽCO 32y concentration. measurement, another aliquot of filtrate was directly titrated with HCl to a second end-point. The difference between the two titration volumes represents the Na 2 CO 3 concentration in the solution. The products were washed five times with ethanol to remove NaOH and dried for 4 h under reduced pressure. The samples thus obtained were measured using powder X-ray diffraction ŽRAD-C; Rigaku. to determine the percentage of aragonite in the crystallized CaCO 3 . The crystals were observed using a SEM ŽJSM-5300; JEOL. to examine the size and shape of the crystals.
3. Results and discussion 3.1. Solubility measurement 3.1.1. The solubility of Ca(OH)2 and CaCO3 in the NaOH solution Many reports on the equilibrium of the causticizing reaction at higher temperatures have been shown w4,5x. However, little investigation has been reported on the
Table 1 Reaction condition CaŽOH. 2 suspension concentration Žwt.%. Reaction temperature Ž8C. Adding time of Na 2 CO 3 solution Žh. Na 2 CO 3 solution concentration Žmolrl. Stirring speed Žrpm.
6.6 25 2 1.61 400
13.2
26.4
50
75
Ž s100 grl as Na 2 O.
H. Konno et al.r Powder Technology 123 (2002) 33–39
solubility of CaŽOH. 2 and CaCO 3 at high NaOH concentrations, which simulate the causticizing reaction conditions at temperatures under 90 8C. As shown in Fig. 2, the solubility of CaŽOH. 2 was observed to decrease as the NaOH concentration increased
35
at 25, 50 and 75 8C. The reason for this is the higher NaOH concentration led to a higher OHy concentration, resulting in the prevention of the dissociation of CaŽOH. 2 . Furthermore, the solubility of CaŽOH. 2 decreases with temperature. In the case of CaCO 3 , the solubility of calcite
Fig. 2. Solubility of CaŽOH. 2 , calcite and aragonite in NaOH solution at 25, 50 and 75 8C; `: CaŽOH. 2 , I: calcite, B: aragonite.
36
H. Konno et al.r Powder Technology 123 (2002) 33–39
and aragonite increased with NaOH concentration; however, at concentrations of NaOH greater than 1 molrl it decreased at 25 8C. At this concentration region, the solubility of CaCO 3 became greater than that of CaŽOH. 2 . Powder X-ray diffraction ŽXRD. measurement showed that at 1 molrl of NaOH no change of calcium carbonate occurred after the solubility experiment; however, at greater than 2 molrl of NaOH the presence of CaŽOH. 2 in the solids was confirmed. The solid phase of calcite and aragonite is unstable and easily converts to CaŽOH. 2 at 2 molrl of NaOH, both of which explain these results. At 50 8C the solubility of calcite in the NaOH solution peaked at 1 molrl, while that of aragonite peaked at 0.5 molrl of NaOH. This may mean that aragonite is more unstable than calcite and easily converts to CaŽOH. 2 . At 75 8C the solubility of both crystals peaked at 0.5 molrl of NaOH, and decreased at a concentration greater than 0.5 molrl of NaOH. It is expected that both calcite and aragonite are unstable at the higher NaOH concentration and the solubility of calcite and aragonite is much higher than that of CaŽOH. 2 at 75 8C. 3.1.2. The influence of Na2 CO3 As carbonate ions exist in a practical causticizing reaction, it is predicted that the carbonate ion influences the solubility of CaCO 3 in the NaOH solution. The solubility of calcite and aragonite was measured in 1 molrl of NaOH containing Na 2 CO 3 at concentrations ranging from 0.01 to 0.04 molrl at 25, 50 and 75 8C ŽFig. 3.. The solubility of calcite and aragonite decreased as the concentration of Na 2 CO 3 increased. The solubility of calcite at 75 8C and aragonite at 50 8C and 75 8C in the NaOH solution without Na 2 CO 3 cannot be obtained due to the conversion to CaŽOH. 2 as described above. These results show that calcite and aragonite are in a stable solid phase in a 1-molrl NaOH solution containing greater than 0.02 molrl Na 2 CO 3 . This was confirmed by XRD analy-
Fig. 3. Solubility of calcite and aragonite in 1 molrl of NaOH solution containing various concentration of Na 2 CO 3 ; I: calcite at 25 8C, B: aragonite at 25 8C, e: calcite at 50 8C, l: aragonite at 50 8C, `: calcite at 75 8C, v: aragonite at 75 8C.
Fig. 4. Percentage change of aragonite in crystallized CaCO 3 at different temperatures; e: 25 8C, I: 50 8C, n: 75 8C.
sis. As seen in Fig. 3, the solubility of aragonite was higher than that of calcite in the NaOH solution with Na 2 CO 3 , indicating that aragonite is in a metastable phase during the causticizing reaction conditions. Moreover, it is apparent that temperature hardly influences the solubility of calcite and aragonite. 3.2. Crystallization 3.2.1. The influence of temperature during crystallization To investigate the effect of temperature on aragonite precipitation, the crystallization was carried out at 25, 50 and 75 8C. In this experimental series, the standard conditions were applied. In Fig. 4 the change of aragonite composition Ž%. in crystallized CaCO 3 during crystallization is shown. The percentage of aragonite in crystallized CaCO 3 and the mole ratio of Na 2 CO 3 added to initial CaŽOH. 2 were expressed as Wa and R, respectively. In this system, no precipitation of vaterite or basic calcium carbonate w10x was observed. It is shown that aragonite cannot be obtained at 25 8C, however, at 50 8C Wa increased up to a maximum of 70–80%. At 75 8C, the maximum Wa was only 40–50%. It is interesting that calcite precipitates predominately at the initial addition of Na 2 CO 3 Ž R s 0.15.. Then, Wa increased as the addition of Na 2 CO 3 proceeded, and finally Wa became constant at an R value greater than 0.6. It was observed that no transformation of aragonite to calcite occurred during the addition of Na 2 CO 3 . Fig. 5 shows a SEM photograph of precipitated CaCO 3 at different temperatures. The largest proportion of particles with a needle-like shape is seen at 50 8C. These results further support the XRD analysis described previously. In the photograph at 25 8C, large aggregated particles Žcalcite. that consist of fine granular particles are seen. On the other hand, at 50 8C, a majority of particles have a needle-like shape Žaragonite. and a size of about 5 mm. These particles are not strongly aggregated. At 75 8C, mixed particles consisting of needle-like shapes of 10–15
H. Konno et al.r Powder Technology 123 (2002) 33–39
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Fig. 5. SEM photographs of CaCO 3 particles obtained at different temperatures; ŽA. 25 8C, ŽB. 50 8C, ŽC. 75 8C. Fig. 6. Time course changes of wCa2q x, wCO 32y x and wNaOHx at different temperature; e: 25 8C, I: 50 8C, n: 75 8C.
mm length Žaragonite. and granular shapes of 1-mm diameter Žcalcite. are observed. The sizes of the crystals increased as the temperature increased. The results mentioned above indicate the precipitation of aragonite is strongly influenced by temperature. The concentrations of Ca2q, CO 32y and NaOH in the mother liquor were also measured. The results are shown in Fig. 6. The Ca2q concentration during the course of crystallization decreased as the reaction progressed. The dependence of Ca2q concentration on NaOH concentration during the course of crystallization is plotted and compared with the solubility of CaŽOH. 2 in the NaOH solution at 50 8C in Fig. 7. It appears that the Ca2q concentration in the solution is concurrent with CaŽOH. 2 solubility. As the solubility of CaŽOH. 2 decreases with temperature, the results indicate that the supersaturation ratio, which is the driving force of the nucleation, decreases with temperature. This may cause the increase in the crystal size with temperature.
It can be seen that the concentration of CO 32y increased as the temperature decreased. This may be because the NaOH concentration increases with temperature.
Fig. 7. Relationship between the solubility of CaŽOH. 2 Žl. and Ca2q concentration Že. in the causticizing reaction at 50 8C.
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Fig. 8. Percentage change of aragonite in crystallized CaCO 3 at different CaŽOH. 2 suspension concentrations; e: 6.6%, I: 13.2%, n: 26.4%.
3.2.2. The effect of Ca(OH)2 suspension concentration on aragonite precipitation In order to understand the effect of the CaŽOH. 2 suspension concentration on aragonite precipitation, the crystallization was carried out under the conditions at 50 8C. This is the temperature where the greatest precipitation of aragonite crystals occurred. In Fig. 8, the change of Wa in crystallized CaCO 3 during the crystallization is shown at suspension concentrations of 6.6%, 13.2% and 26.4%. It can be seen that Wa increased dramatically with the suspension concentration. In all conditions calcite was precipitated predominately at R s 0.15. Then Wa increased as the addition of Na 2 CO 3 proceeded. The relation of Wa and the NaOH concentration in the mother liquor are plotted in Fig. 9. In this figure, the results are shown at the point at which an equivalent mole of Na 2 CO 3 to CaŽOH. 2 Ž R s 1.0. was added. It is clear that the relationship between Wa and the NaOH concentration was expressed as one curve for any CaŽOH. 2 concentration. This indicates that the crystallization of aragonite strongly de-
Fig. 10. Effect of the addition of NaOH on the CaŽOH. 2 suspension; I: no addition, e: NaOH addition.
pends on the NaOH concentration in the mother liquor of this system. 3.2.3. The influence of initial NaOH concentration on aragonite precipitation From the results in Section 3.2.2, aragonite was readily precipitated in the region of 0.5–1 molrl of NaOH. To observe the effect of NaOH on the precipitation of aragonite, the causticizing reaction was started with CaŽOH. 2 suspended in 1 molrl of NaOH instead of water. Fig. 10 shows the change in Wa during the course of crystallization under the standard conditions. Using a CaŽOH. 2 suspension with 1 molrl of NaOH Wa was higher at R s 0.15 compared to the condition without 1 molrl of NaOH; subsequently, Wa was finally more than 90% at R s 1.2. It is obvious that aragonite was predominately precipitated when NaOH was present in the mother liquor. When the suspension concentration is high ŽFig. 8., the NaOH concentration produced by the reaction may enter the region in which aragonite can be predominantly precipitated at the initial stage of Na 2 CO 3 addition.
4. Conclusion The solubility of CaŽOH. 2 and CaCO 3 in the NaOH solution and the relationship between the crystallization of aragonite and operating conditions were investigated. The following results were obtained:
Fig. 9. Effect of produced NaOH concentration on the crystallization of aragonite at different CaŽOH. 2 suspension concentrations; e: 6.6%, I: 13.2%, n: 26.4%.
1. In the solubility experiment, the higher concentrations of NaOH allow CaCO 3 to be converted into CaŽOH. 2 beyond its peak of the solubility. 2. At 1 molrl of NaOH concentration, calcite and aragonite became a more stable phase compared to CaŽOH. 2 in the presence of CO 32y greater than 0.02 molrl.
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3. Solubility of calcite and aragonite was not influenced by temperature under the causticizing reaction conditions. 4. With the addition of Na 2 CO 3 , calcite was precipitated initially ŽR s 0.15., then Wa increased and finally became constant at R ) 0.6 at 50 and 75 8C. 5. Reaction temperature strongly influenced the precipitation behavior of aragonite, and the highest precipitation of aragonite occurred at 50 8C. 6. Precipitation of aragonite increased as the CaŽOH. 2 suspension was increased. Aragonite was predominantly precipitated in the presence of NaOH. 7. Aragonite with more than 90% purity can be obtained using a CaŽOH. 2 suspension containing 1 molrl of NaOH.
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