Precipitation of BaCO3 in a semi-batch reactor with double-tube gas injection nozzle

Journal of Crystal Growth 102 (1990) 434 440 North Holland

434

PRECIPITATION OF BaCO3 IN A SEMI-BATCH REACTOR WITH DOUBLE-TUBE GAS INJECTION NOZZLE Noriaki KUBOTA, Takahiro SEKIMOTO and Kenji SHIMIZU Department ofApplied Chemistry, Iwate Unii’ersits. 4

Leda.

Moriol~a 020, Japan

Received 2 November 1989; manuscript received in final form 14 January 1990

BaCO1 crystals were produced in a semi-hatch reactor h\ adding CO~gas continuously to the agitated BaS aqueous solution through a double-tube gas injection nozzle. Larger crystals were obtained in comparison with the case where the gas was directly dispersed in the whole reactor as usually done. The gas bubbles, around which the higher supersaturation regions are thought to he localized, were made to contact only with the solution inside the Outer tube of the nozzle. The gas was absorbed there and the adsorbed gas was transferred into the agitated hulk solution through the lower opening of the nozzle and then it reacted with Ba~ ion in the bulk solution. The larger crystals were thought to he obtained because of lower nucleation rate caused h~the limited bubbling region. In addition, the lower pH of the solution in the nozzle was thought to help in lowering the nucleation rate, since CO~ gas was dissolved as HCO1 ion rather than as CO~ ion at such lower pH values.

1. Introduction

2. Experimental

The gas liquid reaction is used for producing crystalline particles of sparingly soluble materials such as BaCO5 or CaCO3. The size distribution of these particles is very difficult to control in industrial processes. Generally speaking, if the nucleation rate is relatively suppressed in comparison with the crystal growth rate, larger crystals must be obtamed. In a gas liquid reaction where the gas is dispersed as tiny bubbles, the higher supersaturation is believed to be localized in the liquid phase in the vicinity of the gas liquid interfaces. Nucleation is easy to occur in such regions because of higher supersaturation. If the gas is dispersed only in a limited part of the reactor, a lower nucleation rate must be realized. For this purpose, we used a specially designed gas injection nozzle, the double-tube gas injection nozzle, to produce BaCO3 crystals by adding CO2 gas into solution. this paper, we show thatBaS the aqueous relatively larger In crystals have been obtained by using this nozzle and give a

BaS is readily soluble in water with hydrolysis, forming Ba(OH)2 and Ba(SH)2 [1]. According to the literature [1,21, if you add CO., gas to the solution, the following reactions will occur as the first steps at higher pH (in the strongly alkaline region).

qualitative explanation to the results. 0022-0248 90 $03.50

1990

CO. 2 Ba

+ OH + + 2CO~

2 H20 C0 BaCO 3.

+

2 H20.

As the reactions proceeds, the pH will be decreased and the following reactions will occur in a weakly alkaline region, evolving H.,S gas. CO. SH 2 +++ 2CO~ Ba

+

H.,O BaCO CO~ 5.

+

2 H2S.

Fig. I shows the reactor fitted with the doubletube gas injection nozzle. of 3. The Pure working CO., gas volume was conthe reactor was 400 cm tinuously added to the agitated (400 rpm) BaS aqueous solution at 40° to produce BaCO 3 crystals. The gas flow rate was kept constant at

Elsevier Science Publishers B.V (North-Holland)

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rr Fig. 2. Details of the double-tube gas injection nozzle: (1) outer tube; (2) gas inlet tube; (3) upper opening; (4) orifice; (5) lid; (6) pH electrode.

Fig. 1. Reactor fitted with the double-tube gas injection nozzle: (1) stirred gland and guide; (2) pH electrode; (3) double-tube gas injection nozzle; (4) lid; (5) glass impeller.

3/s (room temperature and atmo9.4 X 10pressure) cm throughout all the runs, but the spheric Ba2~ initial concentration was changed in the range of 3.61 X 10 to 8.92 x 10 g Ba2~/cm3. During the reaction, the solution was sampled at given intervals and the Ba2 + ion concentration was determined by EDTA titration. The crystals were also sampled and the size and the number of them were measured with an Elzone Particle Counter (Elzone 18OXY, Particle Data Inc., USA). SEM photographs of the crystals were also taken. The pH of the solution was monitored during the reaction. In fig. 2, the double-tube gas injection nozzle is shown in detail. The gas, introduced through the glass inlet tube (2), was bubbled from a tiny orifice (4) (diameter ca. 1 mm). The gas was absorbed into the solution within an outer tube (1) of the nozzle, but the excess of it was allowed to escape from the upper opening (3). The solution in the nozzle naturally moved up and down in a pulsative motion by gas bubbling. By this pulsative motion, the gas-absorbed solution in the noz-

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436

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Precipitation ofBaCO

5 in semi-batch reactor

Table I Typical analytical data of BaS used

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________________________________________________________

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Weight %

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zle was transported and mixed into the agitated bulk solution in the reactor. (This tmxing process was confirmed separately by observing the motion of red color of the phenolphthalein added to the acidic solution in the nozzle, which was set in NaOH alkaline solution the reactor.) pH of the solution within the in outer tube of The the nozzle was measured during the reaction. Besides the double-tube gas injection nozzle, a single-tube gas injection nozzle was used for cornparison. The reactor equipped with this nozzle is shown in fig. 3. The gas bubbles were directly dispersed in the whole reactor in this case. Barium sulfide used, of which typical analytical data are listed in table 1, was supplied from a Japanese company. The CO 2 gas was introduced from a commercially available cylinder. Deionized water was used for making the solution after degassed with nitrogen,

3. Results Fig. 4 shows the time change of pH of the agitated bulk solution during reaction when the 14 12

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double-tube gas injection nozzle was used. The time change of the Ba + 2 ion concentration is also shown in fig. 5. The reaction seems to have re ached an equilibrium for the solutions of lower initial concentration, but not for the solutions of higher concentration. Fig. 6 shows the pH of the solution inside the outer tube of the double-tube gas injection nozzle against time, together with the pH of the bulk solution. The pH inside the nozzle dropped to nearly 6 at an early stage of the run (within 10 mm) and then it remained constant. Fig. 7 shows the size distributions of the crystals obtained at three different reaction times of an experiment for the initial solution concentration of CB. 4.49 x 10 (The distributions are cut ~.

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Crystal size distributions (double-tube gas injection nozzle). 14

off in the smaller size range below 1.5 ~tm because of the detection limit of the Elzone Particle Counter.) Larger crystals gradually increased in number

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but smaller decreased. spread widely ones up to about 7 ~tmThe at thedistribution end of the run. Photographs of the crystals obtained at the end of run (the crystals at 9 = 480 mm in fig. 7) are shown in fig. 8. They were pillar-like crystals having square cross sections. 2~concentration fig. 9, against the pH time and the areInshown for Ba the case when the single-tube injection nozzle was dropped used. Both the pH and thegasBa2~ concentration much earlier than those in the case of the double-tube gas injection nozzle (see figs. 4 and 5). This means that the reaction proceeded much faster. Fig. 10 shows the size distribution and a photograph of the crystals obtained at the end of the run (9 60 mm). The crystal size in this case was smaller than that in the case of the double-tube gas injection nozzle (fig. 7). The crystals were thinner or needle-like rather than pillar-shaped. Thus, relatively large BaCO 3 crystals were obtamed in the semi-batch reactor by using the

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double-tube gas injection nozzle. We think that this is because the nucleation was limited in comparison with the case where the gas was dispersed tube gasbubbles injection as tiny in nozzle. the whole reactor by the single-

3 crystals dissolved at lower pH of around 6. BaCO3 crystals are considered to dissolve by the reaction: 21+2HCO BaCO3+C02+H.,O Ba 3 In relation to this matter, an additional experiment on dissolution of BaCO3 crystals suspended as slurry2~ inconcentration water was done by bubbling is shown againstCO2 timegas. in The fig. 12, Ba together with the pH of the slurry. The Ba2 ion concentration increased with time and the pH decreased to nearly 6. It is clear that the BaCO 3 crystals dissolved at lower pHs around 6. As already shown in fig. 6, the pH of the solution inside the nozzle dropped to nearly 6 at

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4. Discussion

decrease in the last stage of the run (see fig. 11).

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3 crystals into water by CO~gas bubbling.

N. Kubota et al.

/ Precipitation

an early stage of the run. At such lower values of the solution pH, CO2ingas have been dissolved into the the must nozzle as HCO 3 ion [3]. The pH of the bulk agitated solution, on the other hand, was still at higher levels. Therefore, CO2 gas once dissolved as HCO3 in the solution within the nozzle was thought to be transported into the bulk agitated solution by the pulsation motion mentioned earlier and to2~ change CO~ ion, ion toto give BaCO Then it reacts with Ba 3 crystals. This dissolution mechanism might also contribute in suppressing nucleation, since CO~ ion wasexcept never for present in the solution within the nozzle the very early stage of the run. At the same time, this CO 2 dissolution mechanism could be favored to keep a relatively higher reaction rate, since the higher CO~ ion concentration in the bulk agitated solution was brought on by the higher solubility of the HCO3 ion.

ofBaCO

439

5 in semi batch reactor

As shown previously in2ion figs. concentration, 4. 5 and 9 as time the changes of pH and Ba~ reaction proceeded slowly in the case the doubletube gas injection nozzle was used, while it proceeded fast for the case of the single-tube gas injection nozzle. The long reaction time in the former case might have played an important role for obtaining larger crystals by giving longer growth time. We did, therefore, an additional experiment. That is, we did a slow reaction experiment using the single-tube gas injection nozzle (the latter case) by reducing the gas flow rate. 2~concentraTime are changes pH13, andinthe Ba the reaction tion shownof inthefig. which rate is seen to be retarded. Fig. 13 also shows the size distribution of the crystals obtained at the end of the run (0 480 mm). Although some larger crystals are present, the majority are smaller in size. The distribution is clearly different from the corresponding distribution (at 0 480 mm in fig. 7). The nucleation-suppressing effect of the double-tube gas injection nozzle is confirmed. —

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3 were obtained in a semi-batch reactor fitted with a double-tube gas injection nozzle. This was thought to be due to the fact that the higher supersaturation regions around tiny CO 2 bubbles were restricted within the nozzle and hence the nucleation was suppressed. The lower pH of the solution in the nozzle. where the CO2 gas was bubbled, was considered also to help reducing nucleation.

Nomenclature

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5. Conclusions The relatively large crystals of BaCO

0 480mmn

~

~0

C~aI CBa

2~ion concentration (g/cm3) Initial Ba2~concentration (g/cm3) Ba

0

Reaction time (mm)

1000.0

Crystal Size L [pm]

Fig. 13. Long-time experiment for the case of single tube gas injection nozzle: (a) time change of pH and Ba2~ ion concentration; (b) crystal size distribution at the end of run (9 480 mm).

Acknowledgment Part of this study was financially supported by Barium Chemicals Co. Ltd., Tokyo, Japan.

440

N. Kubota et al.

Precipitation of BaCO

References [1[ Encyclopedia of Chemical Technology, Japanese Reprint Edition, Vol. 2, Eds. R.F. Kirk and D.F. Othmer (Maruzen, Tokyo, 1960) p. 320. [2[ S. Uchida, K. Noda and H. Akabori, World Congress III of Chemical Engineering, Tokyo, 1986, 8d-203.

5 in semi batch reactor

[31Y. Abe and T. Hannya, Ippan Suishitu Kagaku (Kyoritsu Shuppan, Tokyo, 1975) [translated from Aquatic Chemistry

written by W Stumn and J.J. Morgan (Wtley. New York. 1970)1