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Production of Anhydrous, Crystalline Borax in a Fluidized Bed S. Kocakus¸ ak,* K. Akc¸ ay, T. Ayok, H. J. Ko1 rogˇ lu, O 2 . T. Savas¸ c¸ ı, and R. Tolun Chemical Engineering Department, Tu¨ bı˘ tak-MRC, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey
Industrial production of anhydrous borax is via fusion of borax deca- or pentahydrate at 8001000 °C. The glassy melt obtained thus is then cooled to solidify, crushed, ground, and then sieved to be classified according to its particle size and distribution. Since molten borax is highly corrosive to refractories and steel, among all these operations, the melting of borax is the most critical and the most costly. Our study gives way to production of commercial quality anhydrous and crystalline borax by eliminating melting and other above-mentioned operations. For this purpose, borax pentahydrate is dehydrated in a fluidized bed with gradually increasing the bed temperature up to 550 °C. During this process, as the bed temperature is increased gradually, particular attention is paid to keep the dehydration rate below a certain value to secure the bulk density of the product at levels greater than 0.3 g/cm3. Introduction All important borax compounds contain water of crystallization. The most important of these compounds are borax pentahydrate (Na2B4O7‚5H2O) and borax decahydrate (Na2B4O7‚10H2O), which are used in many areas such as perborate production and detergent formulations. The most suitable borax compound for glass and frit production is anhydrous borax (Na2B4O7). Anhydrous borax is produced in refractory-lined, jacketed furnaces similar to the ones utilized to melt glass. In these furnaces borax deca- or pentahydrate is fused at 800-1000 °C. Since molten borax is highly corrosive to refractories and steel, among all the operations involved, the melting of borax is the most critical and the most costly, which results in difficulties in operation and somewhat contaminated products. The molten borax is either cooled to an amorphous, glasslike solid on chilled rolls or poured into molds in which it is cooled slowly to crystallize. After solidification it is ground and sieved to obtain a powderlike product with desired particle size and distribution. To eliminate this time- and energy-consuming process, various studies have been carried out which have been reported mostly in the patent literature. These attempts have mostly been in dehydrating borax pentahydrate by a fluidized bed via a number of stages (Dra¨gilaˆ, 1973; Campbell et al., 1977; Kawachi et al., 1992). From all these studies good results were obtained with claims of considerable energy savings of more than 50% and other advantages. However, relatively lower bulk densities of the products obtained were the major problem. Therefore, in some studies compaction of the anhydrous borax obtained by a fluidized bed was considered to increase bulk densities from the range of 0.05 g/cm3 to at least 0.3-0.7 g/cm3. Besides dehydration by a fluidized bed, there have also been attempts to dehydrate borax pentahydrate by other techniques such as microwave dehydration (Kocakus¸ ak et al., 1995). From the above information, it is seen that various attempts have been made to obtain anhydrous borax via processes other than fusion. Among these, a fluidized bed still retains its attraction at least from the point of view of its less energy consumption. In the meantime fluidized-bed drying and calcination processes have proven themselves both technically and economically, which resulted in their wide-spread usage. 0888-5885/96/2635-1424$12.00/0
Since, like glassy anhydrous borax, crystalline borax is also sufficiently stable in humid air and can easily be ground, in our study, production of crystalline anhydrous borax was aimed at producing a real alternative to the glassy one. Experimental Studies In our experiments, borax pentahydrate obtained from the Etibank-Kırka production side (Etibor 46) was used. General properties of Etibor-46 are shown on Table 1. As seen from the density, bulk density, and mean particle size values, according to Geldart classification (Geldart, 1986), Etibor-46 is in line with the easily fluidizable sand type and thus is quite suitable for fluidization. The air required for fluidization is obtained from the central pressurized air system of the laboratory. It is used after decompression, and its relative humidity at room temperature was around 10% depending on the climatic conditions. Other chemicals used for analytical and test procedures were analytical grade and were obtained from various suppliers. Quality and property tests concerning the raw material and the products obtained were carried out in line with their known procedures and standards. As seen from Figure 1, the setup used for fluidizedbed dehydration consisted of a cylindrical fluidized bed which had a diameter of 15 cm and a height of 40 cm, an air feed system, with a 4500-W electrical air heater containing a variable voltage transformer, a cooler for effluent gas, and a cyclone, and a bag filter to collect particles from the effluent gas. The temperature of the inlet air was controlled by a suitable temperature controlling system, and velocity, pressure, temperature, and moisture content of the influent air, pressure variation in the fluidized bed, the temperature at the inlet, at the middle, and at the top of the fluidized bed, and the temperature and the moisture content of the effluent gas were continuously measured and recorded. Each experiment was carried out with 0.5 kg of borax pentahydrate. The air flow rate was kept constant at 8 m3/h, providing a superficial velocity of 13 cm/s which was higher than the minimum fluidization velocity even at room temperature. Fine particles having lower © 1996 American Chemical Society
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Figure 1. Schematic diagram of the experimental system. Table 1. Properties of Dehydrated Borax in a Fluidized-Bed Reactor
feed BPH (Etibor 46)
stage I (130-150 °C)
stage II (180-230 °C)
stage III (280-320 °C)
second group studies: humidity controlled, gradual temp. increase (100-400 °C)
48.2 1.88 1.1
59.5 2.09 0.45
64.0 2.18 0.27
67.0 2.20 0.20
68.5 2.26 0.32
69.0 2.30 0.47
7.26 13.59 18.77 23.89 20.72 12.94 2.45 0.38
0.54 11.36 18.17 24.90 24.63 16.55 2.16 1.69
420 amorphous + crystalline
308 crystalline
first group studies stagewise experiments product properties B2O3 content (%) density (g/cm3) bulk density (g/cm3) particle size (µm) -1000 + 710 -710 + 500 -500 + 355 -355 + 250 -250 + 180 -180 + 125 -125 + 90 -90 + 45 -45 mean particle size (µm) amorphous/crystalline
41.75 16.35 15.20 11.30 5.15 4.91 5.34 270 crystalline
44.04 18.03 13.81 14.51 7.17 1.95 0.49 460 amorphous
50.05 18.02 13.30 7.61 1.15 8.45 1.42 500 amorphous
terminal velocities than the interstitial gas velocity were transferred and collected in the cyclone. Depending on the final temperature of the test, up to 30% of the raw or partially dehydrated material was collected in the cyclone. Assuming that these side products should be
60.29 15.68 11.80 7.59 0.94 3.44 0.26 610 amorphous
third group studies: humidity controlled, gradual temp. increase (100-550 °C)
recovered by recycling, products remaining in the bed alone were examined. Dehydration experiments were carried out in three groups. In the first and second group of studies, stagewise and continuous dehydration experiments
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Figure 2. Stagewise dehydration of borax pentahydrate (130-150, 180-230, and 280-320 °C).
were carried out. In the last group experiments, making use of the information collected in the first two group experiments, an attempt is made to produce anhydrous crystalline borax by fluidized-bed dehydration. Results and Discussion Final results of first group stagewise studies are shown in Figure 2 and Table 1. As seen from this figure and table in the first stage, at 130-150 °C, dihydrate was obtained. Then, at 180230 °C, a monohydrate level was achieved. At 280320 °C, a product with water content less than monohydrate was obtained. Finally, in light of previous experiments, at 400 and 550 °C anhydrous and crystalline borax were obtained. However, before reaching this stage of experimental results, a number of experiments were carried out with different heating rates, as a results of which different stages were reached faster and the time spent at each stage was shortened. In these cases bulk densities of the products dropped down considerably to values as low as 0.15 g/cm3 at the end of the first stage. Excessive increases in heating rates resulted in decrepitation or excessive powder formation, in some cases to such an extent that fluidization was not possible due to complete filling up of the dehydrator. Therefore, the results given in Figure 2 and Table 1 can be considered as the minimum for problem-free production with suitable end products. From the above results it can also be concluded that better operations could be possible by controlling the moisture content of the effluent. Therefore, a second group of experiments were carried out by controlling the heating rate in line with the moisture content of the effluent air. In these experiments attention was paid to keep the relative humidity of the effluent air below 15% (at 25 °C) throughout the experiment, except the humidity during the first 30 min which is attributed to surface water and decahydrate impurities in the feed. The results obtained from the experiment where the final temperature reached 400 °C are given in Figure 3 and Table 1. From Table 1 and Figure 3 it is seen that anhydrous borax with a suitable bulk density and particle size distribution can be obtained in a fluidized bed by
controlled heating in line with the controlled relative humidity in the effluent air. The product obtained at 400 °C contained about 68.5% B2O3 (Na2B4O7: 99.0%), which was the minimum content of the commercial glassy anhydrous borax (Foster et al., 1968; Pistor et al., 1974). The product was also of a convenient bulk density, being greater than 0.3 g/cm3 (Kawachi et al., 1992). Polarized light microscopic and X-ray diffraction studies of the product indicated that the product obtained is partially crystalline. Therefore, the third group of experiments where the final temperatures were up to 550 °C were carried out. The results obtained are shown in Figure 4 and Table 1. It is rather interesting to see that with increasing the final temperature from 400 to 550 °C, as a result of the shrinking of the particles from 0.42 to 0.30 mm, the bulk density increased from 0.32 to 0.47 g/cm3. The shrinkage of the particles was presumably due to softening of the glassy material just before crystallization, together with the mechanical effects of fluidization as pressing and rounding (Jenckel, 1935; Stoch, 1987). Polarized light microscopic investigations revealed that this product was totally crystalline of anisotropic nature. The X-ray diffraction pattern showed that the crystals were triclinic in structure, which is in line with the literature (ASTM29-1179). An entirely crystalline product was also an indication of the completeness of dehydroxylation. Results obtained from the above experiments are in agreement with the information given in the literature. Borax pentahydrate [Na2(B4O5(OH)4)‚3H2O] can release the 3 mol of water which is out of the hydroxyl groups in its ionic structure easily and rapidly. Microwave dehydration studies showed that, if this stage of dehydration is not carried out slowly enough, undesirable results such as decrepitation, swelling, melting, sticking, etc., can take place. From dihydrate to approximately monohydrate, water loss is achieved quite rapidly, which is followed by a rather slow and difficult release of water to complete dehydration which requires high temperatures (Smith and Mc Broom, 1992; Adams, 1964; Hartung et al., 1983).
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Figure 3. Partially crystalline anhydrous borax from borax pentahydrate, by the gradual increase of temperature up to 400 °C.
Figure 4. Completely dehydrated and crystalline borax, by the gradual increase of the fluidized-bed temperature up to 550 °C.
DTA TGA studies of Hartung et al. revealed that, when dehydrated, glassy and amorphous dihydrate gives way to formation of crystalline anhydrous borax at 500-600 °C with loss of some water (Hartung et al., 1983). Recent studies carried out by Stoch and Waclawska on the thermal decomposition of hydrated borates with DTA TGA revealed that the removal of final OH groups from the amorphous glassy material is finished just before the endothermal effects (at 520 °C) of the rearrangement of the structure preceding its exothermal effect of crystallization at 575 °C. They detected also the first appearance of crystals, at 420 °C, by X-ray diffraction and scanning electron microscopy (Stoch, 1994; Waclawska, 1995; Stoch and Waclawska, 1995). The dehydration studies revealed that the first 3 mol of water can easily be removed. However, care must be taken for easy operation and desirable product bulk density. If the heating rate is faster than the water removal rate, borax dissolves in its hydrate water; thus, the product in the fluidized bed melts and becomes
sticky. If the heating rate is too high, decrepitation and excessive swelling occur which results in a very low bulk density product (0.02 g/cm3). Therefore, the first part of the operation up to 130 °C was carried out with utmost care with limited moisture in the effluent gas. A very slow rate of increase in temperatures such as 1 °C/min up to 130 °C and 2 °C/min from 150 °C up to 300 °C resulted in a product of convenient bulk density. Conclusions In order to obtain totally anhydrous and crystalline borax of a convenient bulk density by dehydration in a fluidized bed, particular attention is paid to keep the dehydration rate below a certain value. To this purpose, the dehydration up to 200 °C was carried out at the slowest rate, increasing the temperature about 1 °C/ min and keeping the bed temperature at 130-150 °C for a while, until the relative humidity in the effluent air comes down to a reasonable level, e.g., about 1020%. From 150 °C up to about 300 °C swelling of the
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particles, which causes a regular decrease in the bulk density of the product, still required attention. The rate of increase of the bed temperature after 300 °C until 400 °C, more than about 2 °C/min, did not indicate any appreciable increase in the relative humidity at the effluent air. At about 400 °C the beginning of crystallization could be observed in the polarized light microscope and a decrease in the mean particle size together with an increase of the bulk density indicated also the beginning of shrinkage. The relative humidity in the effluent air was practically nil after 400 °C, indicating the end of dehydration. Obtaining completely dehydrated anhydrous crystalline borax required, however, the complete transformation of the amorphous glassy matrix into crystals, at the final temperature of the fluidized bed of 550 °C. Higher bulk densities were also achieved at this stage as a result of the decrease in the particle size. The shrinkage was presumably due to the softening of the glassy material at this crystallization stage, together with the mechanical action of fluidization on the particles as pressing and rounding. Literature Cited Adams, R. M. Boron, Metallo-Boron Compounds and Boranes; Wiley-Interscience: New York, 1964. ASTM, 29-1179 JCPDS-ICDD. Natl. Bur. Stand. (U.S.) 1979, 25, Section 16 64. Campbell, G. W.; David, H.; Wilkins, G.; Muench, J. T. Fluid Bed Dehydration of Borax. U.S. Patent 4 041 132, Aug 9, 1977. Dra¨gilaˆ, V. Procedure for the Production of Anhydrous Borax. Rumanian Patent No. 52766, Dec 5, 1973. Foster, D. S.; Hilton, C. L. Boron Oxides Boric Acid and Borates. Encyclopedia of Industrial Chemical Analysis; WileyInterscience: New York, 1968; Vol. 7, pp 368-384.
Geldart, D. Characteristics of Fluidized Systems. In Gas Fluidization Technology; Geldart, D., Ed.; John Wiley and Sons: New York, 1986; p 3. Hartung, E.; Heide, K.; Naumann, R.; Jost, K. H.; Hilme, W. Untersuchungen zur thermischen Zersetzug von Boraten. J. Therm. Anal. 1983, 26, 277. Jenckel, E. Transformationspunkt von Na2B4O7sGlas, tw: 428°, ber. aus der Viscositat. Z. Elektrochem. 1935, 41, 211/5. Kawachi, S.; Kato, M. Nippon Denki Glass Co. Borax Anhydride. Japan Patent 4243914, Sept 1, 1992. Kocakus¸ ak, S.; Ko¨rogˇlu, H. J.; Ekinci, E.; Tolun, R. Production of Anhydrous Borax using Microwave Heating. Ind. Eng. Chem. Res. 1995, 34 (3), 881-885. Pistor, H.; et al. Bor Verbindungen Anorganishe. Ullmanns Enzyklopoedie der technischen Chemie, 4th ed.; Verlag-Chemie: Weinhein, Germany, 1974; Vol. 8, p 672. Smith, R. A.; Mc Broom, R. B. Boron Compounds. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; John Wiley and Sons: New York, 1992; Vol. 4, pp 365-413. Stoch, L. Intraframework Thermal Reactions of Solids. J. Therm. Anal. 1987, 32, 1651. Stoch, L. Structural Chemistry of Thermal Processes Accompanying Flash Calcination. In Proceedings of the NATO Advanced Research Workshop on “Flash Reaction Processes”; Davis, T. W., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994. Stoch, L.; Waclawska, I. Thermal Decomposition of Hydrated Borates. Thermochim. Acta 1993, 215, 273. Waclawska, I. Thermal Decomposition of Borax. J. Therm. Anal. 1995, 43, 261.
Received for review July 18, 1995 Revised manuscript received November 27, 1995 Accepted December 29, 1995X IE9504462 X Abstract published in Advance ACS Abstracts, February 15, 1996.
