J . Chem. Tech. Biorechnol. 1985, JSA, 382-386

Dissolution Kinetics of Colemanite in Water Saturated by Carbon Dioxide

Mahir Alkan, M. Muhtar Kocakerim and Sabri Colak

Department of Chemistry, Atatiirk University, Erzurum, Turkey

(Manuscript received I November 1984 and accepted 4 March 198s)

The dissolution kinetics of original and calcinated samples of the boron containing mineral colemanite, in COz-saturated water were studied. Effects of particle size, calcination temperature and reaction temperature were evaluated. It was observed that the dissolution is chemically-controlled. The reaction rate decreased with increase in particle size, and increased with increase in the calcination and reaction temperatures. The activation energy for solution of the sample calcinated at 400°C as calculated as 57.7 kJ mol-'.

Keywords: Boron; boron mineral; colemanite; boric acid; carbon dioxide; dissolu- tion kinetics.

1. Introduction

Boron is a rare but very widely distributed element, present in the earth's crust in a proportion of only about 3 partsxlo-'; it occurs in traces in most soils and plants, but is only found in a concentrated form in a few places. It is never found free in nature, but invariably occurs as the oxide BzO3 in combination with the oxides of other elements to form borates of greater or lesser complexity.'

Boron compounds are very widely employed in many branches of industry, for example in medicine for the preparation of disinfectants and drugs, and in the glass industry for the production of optic and chemically stable glass. Boron compounds are also used in the cosmetic, leather, textile, rubber and paint industries. They also find application in the wood-processing industry as a protection against moulds.'

In recent years the production of boron and its compounds has increased greatly, as it can be used in nuclear engineering, as fuel for rocket motors, in hard and refractory alloys, in high-quality steels, in the production of heat resistant polymers and also as catalysts.'

Colemanite has a density of 2.40 g c ~ r - ~ ; its chemical formula is 2Ca0.3B,O3.5H20. Production of boron compounds from borate ores,3 and dissolution kinetics of borate ores in several acid solutions have been investigated.46 Giilensoy and Kocakerim' studied the solubility of colernanite in CO2-containing water, but did not study the kinetics. In the present work the kinetics of dissolution of colemanite in water saturated by COz is studied and some kinetic data are reported.

2. Experimental

2.1. Preparation of materials

The colemanite samples used in the study were hand picked from the Espey Mine, Turkey. After cleaning, the colemanite was sieved to give -40+60, -60+100 and -100 mesh size fractions.

For use in experiments to study the effect of calcination temperature on the reaction rate, some samples were calcinated at 380 and 400°C for 5 h in an ash furnace. Colemanite partly loses

382

Dissolution kinetics of colemanite 383

water of crystallisation from its structure depending on the calcination temperature, and therefore both crystal lattice and chemical composition differ from the original state.

The chemical compositions of the original (uncalcinated) and calcinated colemanite samples used are given in Table 1.

Table 1. Chemical compositions and molecular weights of original and calcinated colemanite samples used in the study

Chemical composition (%)

(“C) CaO B 2 0 3 H 2 0 (g mol-’)

Calcination temperature ~ _ _ _ . ~ weight

Original 27.23 50.47 22.30 412 380 28.48 52.79 18.73 394 400 33.21 61.94 4.79 34 1

2.2. Method Dissolution experiments were carried out in 150cm3 glass flasks, equipped with gas inlet and outlet tubes and a magnetic stirrer. The flask was immersed in a constant temperature bath.

Distilled water (100cm’) was saturated with previously washed and dried C 0 2 at the desired experimental temperature. Then I g of the solid was added to the reaction vessel. During the desired time period, C 0 2 was passed through the reaction mixture at a fixed stirring rate. The contents of the vessel were then filtered. The amounts of Ca2+ and H3B03 in the filtrate were determined volurne t r i~a l ly .~~~ The reacted fraction of mineral was calculated in terms of B 2 0 3 , because during the reaction, the dissolved B203 in the mineral is equivalent to the H3B03 produced.

3. Results and discussion

The experimental data show that the reaction rate is affected by particle size, calcination temperature and reaction temperature. Figure 1 shows the reacted fraction versus time plots for the results obtained.

Time ( s )

Figure 1. Reacted fraction versus time plots of results obtained. 0, experiment 1; A , experiment 2; 0, expcriment 3; 0, experiment 4; A , experiment 5; m, experiment 6; X , experiment 7; +, experiment 8.

384 M. Alkan el a/.

3.1. The effect of particle size The effect of particle size on reaction rate was studied by running experiments using three size fractions. -40+60. -60+100 and -100 mesh. The reaction rate increases as the particle size decreases, as shown.

3.2. The effect of calcination temperature A series of tests were performed at 17°C using -100-mesh colemanite samples which were uncalcinated or had been calcinated at 380 or 400°C. Results indicate that as the calcination temperature increases, the reaction rate increases.

3.3. The effect of reaction temperature In these experiments, - 100-mesh colemanite samples calcinated at 400°C were used. Experi- ments were carried out at 17, 25, 33 and 40°C. As seen in Figure 1, the reaction rate increases with increase in the reaction temperature.

3.4. The mechanism of the dissolving process The following reactions probably occur during the dissolving process in terms of experimental data:

2Ca0. 3B203. nH20+4C02+(1 1 -n)H20+2Ca2++6H3BO3f4HCO; ( 9 HCO;+H~O=H,O++CO:-

Ca2++CO:-=CaC03

HiOf +HCO,=H,O + HzC07

(ii)

(iii)

(W

H2CO?=H20 + COz (v>

where n is a number which can change from 5 to 0 depending on the calcination temperature. At the beginning of the process, the concentration of Ca2+ in solution increased because of

reaction (i) until [Ca”] fCO$-]>Ksp; then the concentration of Ca2+ decreased according to reaction (iii) until it had an almost constant value.

l2W 2400 3600 4800 6000 7200

Time (s)

Figure 2. l-(l-X)’’3 versus time plots. 0. experiment 1; A . experiment 2; 0. experiment 3: 0. experiment 4; A , experiment 5; B. cxperiment 6: X . experiment 7; +. experiment 8.

Dissolution kinetics of colemanite 385

3.5. The reaction rate If a noncatalytic fluid-solid reaction such as:

A(g)+ bB(s)=Products (1) is chemically-controlled, the integrated rate equation of this reaction in terms of the shrinking core model is:9

The time for complete conversion (X=l), t", is

@bR (3) t' =-

Figure 2 illustrates 1-(1 -X)"3 versus time plots from the results obtained. These results indicate that all the reactions are chemically-controlled.

Rate constants, calculated using equations (2) and (3), have been summarised in Table 2. It has been assumed in these calculations that the solid phase consists of spherical particles with an average radius between the radii corresponding to upper and lower mesh size fraction.

The rate constants on lines 5 , 6, 7 and 8 in the same table were used to construct an Arrhenius

bksCgMh

Table 2. Rate constants

Reaction Calcination Particle Rate constant Experiment temperature temperature size k,x 10"

no. ("C) ("C) (mesh) (cm s - I )

17 17 17 17 17 25 33 40

Original Original Original

380 400 400 400 400

-40+60 -60+100

-100 -100 -100 -100 -100 -100

0.47 0.48 0.48 1.35 4.09 1.56

16.51 23.52

25.00 r

N

0 x 20.00 - c L 0)

0 * z

10 20 30 40 50 Temperature ("C)

Figure 3. Soluhility of C 0 2 in water in Erzurum (where atmospheric pressure is approximately 610 tom) as a function of temperature.

386 M. Alkan c? a/.

plot which showed that the activation energy and the pre-exponential factor for this sample were 57.7kJ mol-' and 1.07~10'cm s-I, respectively.

4. Conclusions

Rate constants for different fractions are almost equal as expected. It has been observed that the value of rate constant increases as the calcination temperature

increases. This is probably the result of destroyed crystal lattice. Although the solubility of C 0 2 in water is inversely proportional to temperature as seen in

Figure 3. the reaction rate for -100-mesh colemanite calcinated at 400°C increases with increase in the reaction temperature. The activation energy for this sample is 57.7 kJ mol-' confirming that the reaction is chemically controlled."

Nomenclature

X = Reacted fraction f = Time (s) r" = The time for X=l (s) b = Stochiometric coefficient of solid k , = Rate constant for surface reaction (cm s-I) C, = Concentration of gas (mol ~ r n - ~ M B = Molecular weight of solid (g mol-I) R = Radius of initial particle (cm) @ B = Density of solid (g ~ m - ~ )

References

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Kemp. H. P. The Chemistry of Borares. Part 1. Borax Consolidated Limited. London, S.W.1. 1956. p. 1. Nemodruk. A. A.; Karalova. Z. K. Analvtical Chemisfry of Boron. Academy of Sciences of the U.S.S.R. Vernadskii Institute of Geochcmistry and Analytical Chemistry, translated by Israel Program for Scientific Translations. Jerusalem, 1965, pp. I . 2, 33. Sallay, S. I. U.S. Patent 4 196 177, 1980. Chem. Abs. 93-49645~. Kalacheva. V. G.; Karazhanov, N. A , ; Kim, G. E.; Kats-David, G. G. Khim. Prom-St. (Moskow), 1980. 6, 355-6 (Russ). Chem. Abs. 93-99138f. Yuritsina. G . G.: Kim. G. E.; Kalacheva. V. G. Insf. Khim. Nefr. Prir. Solei. Gurev. USSR, 1978, 3690-78. Chem. Abs. 92-48036t. Guseva. G. P.: Kalacheva, V. G.; Shestaperova. V. N.; Gaferova. N. A,: Shvartz. E. M. USSR Patent 553 815. 1981. Chem. Abs. 94-142029f. Giilensoy, H.; Kocakerim. M. M.. Bulletin offhe Mineral Reseurch and Explorarion Institute of Turkey, 1978. 90, I . Giilcnsoy. H. Kompleksomefrinin Esaslari ve Kompleksometrik Tifrusyonlar. Istanbul Universitesi Yayinlari, Istanbul, 1977. 144. Levenspiel. 0. Chemical Reaction Engineering. John Wiley and Sons, New York. 1972. pp. 367-368. Burkin. A. R. The Chemistry of Hydrometallurgical Processes. E and F. N . Spon Ltd, London, 1966. p. 35.

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