Ind. Eng. Chem. Res. 1997, 36, 4847-4851
4847
Dissolution of Thermally Dehydrated Ulexite in Sulfuric Acid Solution A. Ku 1 nku 1l Department of Chemical Engineering, Engineering Faculty, Inonu University, Malatya, Turkey
M. Tunc¸ Department of Chemistry, Faculty of Science, Yuzuncu Yil University, Van, Turkey
S. Yapıcı, H. Ers¸ ahan,* and M. M. Kocakerim Department of Chemical Engineering, Engineering Faculty, Ataturk University, 25240 Erzurum, Turkey
The present study concerns a work on the investigation of the effect of the dehydration temperature on the dissolution process of a boron mineral, ulexite, in sulfuric acid solution. In the experiments, the dehydration temperature and dissolution temperature were chosen as the parameters. First, the dehydration of ulexite was performed isothermally in an oven, and DTA and X-ray analysis were carried out to observe the structural changes in the mineral. The effect of the dehydration temperature on the dissolution rate of ulexite in sulfuric acid solutions was then investigated, using a batch reactor made of glass. The kinetic parameters of the dissolution process, frequency factor and activation energy, were evaluated using dissolution data. 1. Introduction Boron compounds are commonly used in a very wide range of industrial applications. The production of boron compounds has substantially increased recently due to increasing demand for these compounds in nuclear technology, in rocket engines as fuel, in the production of heat-resistant materials such as refractors and ceramics, in high quality steel, in heat-resistant polymers, in catalysts etc. (Nemodruk and Karalova, 1965; Doon and Lower, 1971). A huge portion of the world’s commercially recoverable boron reserves is in the form of hydrated borate minerals such as ulexite, pandermite, tincal, and colemanite. An important portion of the world’s ulexite reserves was discovered in the Bigadic region in Turkey, and it has been predicted that about 54% of the known world’s boron reserves are in Turkey (MTA, 1982; Karayazici et al., 1980). Ulexite is a commercially important boron mineral, having a chemical formula of Na2O‚2CaO‚5B2O3‚16H2O. It has a triclinic crystal structure, usually in rounded masses of fine transparent white fibrous crystals (cotton balls) and in parallel fibrous aggregates. The distinguishing characteristics are soft “cotton ball” habit, low specific gravity (1900-2000 kg m-3), insoluble in cold water, slightly soluble in hot water, and fused easily. It is an evaporative boron mineral and named after G. L. Ulex, a 19th-century German chemist who discovered the mineral (Hamilton et al., 1987). Commercially, the most-used compounds of boron are boric acid, boron oxides, and sodium perborate. Ulexite and colemanite have been used for the production of these compounds as raw materials. When substances containing hydrate water are heated to given temperatures, they lose their weight, giving off H2O, known as dehydration. Dehydration processes can be applied for technological and economical reasons: to decrease the weight of a material for reducing the transportation cost in the case where it includes hydrated water in large amounts, it may be a necessary step of the chemical process, or to obtain a porous solid for increasing the reaction rate of a solid-fluid reaction. The purpose of applying thermal dehydration to ulexite S0888-5885(97)00213-3 CCC: $14.00
in the present study is to see whether it is possible to obtain a high extraction rate of boric acid and to investigate the kinetic parameters with acidic solutions from dehydrated mineral compared to original mineral. The effect of thermal dehydration of some boron minerals on the dissolution in aqueous water (Gulensoy and Savci, 1976; Gulensoy and Kocakerim, 1977), the dissolution kinetics of thermally dehydrated ulexite in carbon dioxide saturated water (Kocakerim et al., 1993), and the thermal decomposition kinetics of ulexite (Stoch and Waclawska, 1990; Tunc et al., 1997) have been investigated. Some researchers have studied the dissolution of some boron minerals in acidic solutions: in hydrochloric acid (Imamutdinova, 1963; Zdonovski and Imamutdinova, 1964), nitric acid (Imamutdinova and Birkchura, 1967), oxalic acid (Kalacheva et al., 1980), and magnesium borates in sulfuric acid (Kononova and Nozhko, 1981). In these works, it was determined that the film layer of the formed products on the mineral surface affects the reaction rate, and this effect differs for every acid solution (Imamutdinova, 1967). There are some studies about the production of boron minerals, especially boric acid and borax, mostly patented studies: for example, the production of boric acid from ulexite using the mixture of sodium carbonate-bicarbonate solutions (Novak et al., 1982) and the production of boric acid and borax using hydrochloric acid solutions (Dihu et al., 1981). The optimum dissolution conditions of ulexite in CO2-saturated water have been sought (Yapici et al., 1992). No study could be spotted by us about dissolving dehydrated ulexite in sulfuric acid solution. The rapidly growing demand for various boron products requires the treatment of calcium borate ores including ulexite, available in huge amounts in Turkey, in more efficient ways. So increasing the dissolution rate of dehydrated ulexite in sulfuric acid solutions might be an efficient way of treatment of the mineral. The aim of the present study is to investigate the effect of thermal dehydration of ulexite on the dissolution process and to investigate the kinetic parameters in sulfuric acid solution. © 1997 American Chemical Society
4848 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
2. Experimental Section The ulexite ore used in the study was provided from Eskı´s¸ ehı´r. After cleaning the ore from visible impurities, it was crushed, ground, and then sieved to obtain a particle size fraction of -18 to +25 mesh by ASTM sieves. The chemical analysis of ulexite, carried out by classical methods (Scott, 1963), gave the composition 13.98% CaO, 42.14% B2O3, 7.91% Na2O, 35.82% H2O, and 0.15% undissolved part. The dehydration of ulexite was performed isothermally, using a statical method, in an oven made by Heraus, equipped with a temperature controller. After putting 10 g of the sample in a ceramic crucible, the ore was subjected to a given temperature for 5 h and its mass loss was then measured, since it was determined that the mass loss reaches an equilibrium in 5 h, by checking the weight loss of the material in the crucible. To observe the structural change of ulexite, DTA and X-ray analysis were performed. For DTA analysis, a given amount of sample was put into a platinum crucible, and its temperature change was recorded at a constant heating rate of 0.17 K s-1 at a N2 gas flow rate of 0.83 mL s-1 in a Schimadzu Model 50 DTA-TG system. X-ray analysis was carried out with a scan speed of 0.067 deg s-1 and a step sampling of 0.1 deg. The dissolution process of the mineral was carried out in a 250-mL jacketed spherical glass reactor equipped with a mechanical stirrer, a thermostat to control the reaction temperature within (0.5 °C, and a back-cooler to avoid the loss of solution by evaporation. The dissolution process was carried out using 2 M sulfuric acid solution, a solid-to-liquid ratio of 0.01, and a stirring speed of 450 rpm at different temperatures of 12, 18, 25, 30, and 40 °C. As soon as the process finished, the contents were filtered, and B2O3 in solution was analyzed. The analysis of the dissolved mineral in the solution was performed volumetrically. Since the aqueous solutions of boric acid have weak acidic character, it cannot be determined directly by titration with a basic solution. For this reason, the addition of mannitol into the solution gives a character of strong acid to boric acid, thus giving a facility of direct analysis of boric acid by a basic solution, such as sodium hydroxide, as follows:
Figure 1. Mass loss of ulexite with dehydration temperature.
Figure 2. DTA diagram of ulexite.
2CH2OH(CHOH)4CH2OH + H3BO3 ) (CH2OH(CHOH)4CH2)2BO3H + 2H2O (CH2OH(CHOH)4CH2)2BO3H + NaOH ) (CH2OH(CHOH)4CH2)2BO3Na + H2O 3. Results and Discussion 3.1. Dehydration Process. During the thermal dehydration process, ulexite loses some part of its hydrate water content depending upon the dehydration temperature. The mass loss of ulexite by thermal dehydration is given in Figure 1. This figure shows that the mineral started to lose its hydrate water content at about 50 °C and lost most of its water content between the temperatures of 100 and 250 °C and that the mass loss continued gradually up to 650 °C. A DTA diagram of the mineral and X-ray diffractograms of the sample dehydrated at different temperatures were taken to get a clear picture of the dehydration process. The DTA diagram of the sample, given in Figure 2, confirms the findings of mass loss measurements. As seen from the
Figure 3. X-ray diagrams of ulexite for original and dehydrated samples at various temperatures.
figure, the mass loss started as low as about room temperature, and after 100 °C, structural changes take place in the mineral; it first gives a small endothermic peak at about 130 °C and then a more pronounced peak at about 170 °C, showing that ulexite obtains a stable form after about 200 °C. X-ray diffractograms of the original sample and dehydrated ones given in Figure 3 show decreasing peaks with increasing dehydration temperature, and almost no clear peak is observed at 170 °C. This behavior can be interpreted as meaning that the crystal structure of the mineral converts to the amorphous structure. This change can also be seen in electron-
Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4849
Figure 5. Effect of dehydration temperature on dissolution.
Figure 4. Electron microscope photographs of ulexite samples dehydrated at various temperatures.
microscope photographs of the dehydrated mineral at various temperatures in Figure 4. These photographs shows that while the mineral maintains its crystal structure up to 160 °C, the sintering, which is a mechanism by which solid particles coalesce when heated to temperatures below their melting point, is observed above this temperature, especially more clearly at 650 °C. The sintering process influences two features of the particle structure, surface area and porosity. These two properties strongly influence the dissolution process. 3.2. Dissolution Process. The aim of the dissolution process performed is to observe the effect of dehydration on the reaction rate. The dissolution experiments carried out at 30 °C showed the effect of dehydration temperature on the dissolution rate, as seen in Figure 5; the dissolution rate increased with increasing dehydration temperature up to 160 °C, and the process rate decreases with increasing dehydration temperature above this temperature. Even the dissolution rates of ulexite dehydrated at 400 and 650 °C are lower than the original one. This behavior can be explained by the sintering of the dehydrated ulexite beginning at about 170 °C. The unheated ulexite had some degree of porosity, and this may increase with heating up to 170 °C, and then the porosity may diminish with a further increase in temperature, decreasing the dissolution rate. It can be concluded that the porosity of ulexite increases by dehydration up to about 170 °C, increasing the dissolution rate, and then the shrinkage of the porous structure by sintering after this temperature slows down the dissolution process due to the slowing down of intraparticle mass transfer caused by sintering, which is confirmed by the electronmicroscope photographs given in Figure 4. The effect of the dehydration temperature on the kinetic parameters was investigated using the original ore and the ores dehydrated at 160 and 650 °C. The results are given in Figures 6-8 in X (conversion fraction) vs time. The kinetic analysis performed by
Figure 6. Dissolution fraction vs time for original ulexite at various temperatures.
Figure 7. Dissolution fraction vs time for ulexite dehydrated at 160 °C at various temperatures.
taking into consideration the heterogeneous fluid-solid reaction models (Levenspiel, 1972) showed that the process did not fit any of these models. Further analysis taking into consideration that the reactions between a fluid and porous solids can fit pseudohomogeneous kinetic models (Doraiswamy and Sharma, 1984) showed
4850 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997
Figure 8. Dissolution fraction vs time for ulexite dehydrated at 650 °C at various temperatures. Figure 11. ln k vs 1/T plot for the dissolution of ulexit dehydrated at 160 °C. Table 1. Calculated Kinetic Parameters of the Dissolution Process dehydrated at param EA/kJ k0/s-1
Figure 9. ln(1 - x) vs time plot for original ulexite at various temperatures.
Figure 10. ln(1 - x) vs time plot for ulexite dehydrated at160 °C at various temperatures.
that the process can be expressed in the form of a firstorder homogeneous kinetic model; the plots of ln(1 X) vs time gave straight lines for all three samples with correlation coefficients between 0.9960 and 1, whether or not if it is thermally treated as shown in Figures 9 and 10 for the original sample and the sample calcined 160 °C, respectively. Similar pseudohomogeneous behaviors for the heterogeneous reaction systems have been reported in the literature (Hulbert and Huff, 1970;
mol-1
orig
160 °C
650 °C
46.00 3.115 × 105
57.30 4.419 × 107
41.06 2.616 × 104
Ozmetin et al., 1996; Amer and Ibrahim, 1996). From the straight lines of the plots of ln k vs 1/T, given in Figure 11 for the dissolution of the sample at 160 °C, on the basis of the assumption that the rate constant k can be expressed in the form of the Arrhenius law k ) k0 exp(-EA/RT), the activation energies and Arrhenius constants were evaluated for the dissolution processes. Table 1, in which the calculated values of the activation energies and Arrhenius constants are given, shows that the activation energy and Arrhenius constant have the highest value for the leaching of the sample dehydrated at 160 °C and that these values have the lowest value for the sample dehydrated at 650 °C. The increase in the kinetic parameters for the sample dehydrated at 160 °C corresponds to a rise in the rate constant, therefore in the dissolution rate, between 20% and 84%, depending upon the working temperature, with reference to the dissolution rate of the unheated sample. The works on the dissolution of borates in mineral acids were generally recorded by Russian groups, including Imamutdinova and Zdonovski, as mentioned in the Introduction. These groups used a cubic crystal of borates for the investigation of different borates in different mineral acids and recorded that the dissolution rate for all borates and mineral acids become constant after 1-2 min and that the dissolution process was controlled by diffusion through the product layer around the borate mineral. But Kononova and Nozhko (1981) reported that the film mechanism could not satisfactorily explain the process of acid dissolution of borates and that the formation of a product film occurred as a result of salting out of the least soluble reaction product. The fact that the character of the heterogeneous reaction kinetics depends strictly upon the reaction conditions and reactor geometry means that it is not reliable to compare the results of the present study, which has very different geometry, and the above-mentioned ones.
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4. Conclusions The experiments showed that the mineral started to lose its hydrate water content at about 50 °C and lost most of its water content between the temperatures of 100 and 250 °C and that the mass loss continued gradually up to 650 °C. The DTA diagram of the sample confirmed the findings of mass loss measurements, and an endothermic peak at about 130 °C and then a more pronounced peak at about 170 °C were observed, showing that ulexite gets a stable form after about 200 °C. X-ray diffractograms of the original sample and dehydrated ones showed decreasing peaks with increasing dehydration temperature, and almost no clear peak is observed at 170 °C, showing the crystal structure of the mineral gradually converts to an amorphous structure with increasing temperature. This result was confirmed by electron microscope photographs in which the mineral maintained its crystal structure up to 160 °C; the sintering is observed above this temperature, especially more clearly at 650 °C. The dissolution experiments, carried out at 30 °C, to observe the effect of dehydration temperature on the dissolution rate showed that the dissolution rate increased with increasing dehydration temperature up to 160 °C, and the process rate decreases with increasing temperature above this temperature. This behavior was attributed to the sintering of the dehydrated ulexite above this temperature. The kinetic analysis using data obtained for the dissolution of the original, dehydrated at 160 and 650 °C, samples at different temperatures proved that the process fit the first-order pseudohomogeneous kinetic model. The activation energies and Arrhenius constants were determined to have the highest value for the dissolution of the sample dehydrated at 160 °C the lowest value for the sample dehydrated at 650 °C. Nomenclature E ) activation energy, kJ mol-1 k ) rate constant, s-1 k0 ) frequency factor, s-1 R ) universal gas constant, 8314 kJ mol-1 K-1 T ) temperature, K X ) conversion fraction
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Received for review March 13, 1997 Revised manuscript received July 14, 1997 Accepted July 14, 1997X IE9702136
X Abstract published in Advance ACS Abstracts, September 1, 1997.
