Ind. Eng. Chem. Res. 1996, 35, 159-163
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Drying of Wet Borax Pentahydrate by Microwave Heating S. Kocakus¸ ak,* H. J. Ko1 roglu, T. Go1 zmen, O. T. Savas¸ cı, and R. Tolun Chemical Engineering Department, TUBITAK-MRC, P.O. Box 21, 41470 Gebze-Kocaeli, Turkey
In this study, drying of borax pentahydrate [Na2B4O7‚5H2O] (BPH) by microwave energy is investigated. Using 100-700 W and 2450 MHz microwave energy, 5-10% moisture containing BPH placed on a Teflon-lined belt is dried at 60-100 °C selectively, without any loss of water of crystallization. In this process the microwave oven is first set to 70 °C and then wet BPH which is previously heated to 70 °C is placed in it. The sample first heats up to 80-100 °C by the effect of microwave energy, and then its temperature sharply drops, resulting in complete removal of moisture. This process gives way to no decomposition and disintegration of hydrated crystalline molecule and there are no contaminants involved. Therefore, dry borax pentahydrate dried this way retains its original purity and physical form. Since there is no need to heat the drying system and the air and to keep the relative humidity of the air constant, microwave drying is more economical than the conventional process. In addition, a sharp decrease of temperature right after complete drying gives way to easy control of the process by simply regulating the temperature. Introduction Turkey has about 60% of the world’s known boron reserves (Turkish Republic State Planning Organization, 1977). The majority of these boron minerals is tincal (borax decahydrate; Na2B4O7‚10H2O). A large amount of mined boron minerals are sold as raw materials. A tincal concentration plant with 400 000 tons annual capacity is being operated in Kı`rka by Etibank. Refined borax pentahydrate (Na2B4O7‚5H2O) is also produced with 160 000 tons/year annual capacity at the same site. Since borax pentahydrate has a better market potential than borax decahydrate, its production has been gaining an increasing importance. Crystallization and drying of borax pentahydrate have been among the major problems, causing capacity losses. These problems have not been solved in a satisfactory manner as yet. Therefore investigations, involving modern techniques, still continue (Kocakus¸ ak et al., 1993 a,b). The structure of borax pentahydrate is in the Na2B4O5‚(OH)4‚3H2O form. Two of the water molecules of the borax pentahydrate structure are chemically bounded hydroxyl groups (Nies and Campbell, 1964; Kemp, 1956) as shown in Figure 1. The remaining three molecules of water are outside the ion structure like water of crystallization. Dehydration of water of crystallization which is outside the ion structure is very easy. Although referred to as borax pentahydrate, its crystals actually contain not 5 but 4.67 mol of water. Therefore the correct structure of borax pentahydrate is in Na2B4O7‚4.67H2O or Na2O‚2B2O3‚4.67H2O form. This structure has been confirmed by X-ray single crystal analysis which showed that two of the water of crystallization sites are only partially filled (Powell, 1991). Commercial pure borax pentahydrate contains 4.75 mol rather than 5 mol of water because of apparent vacancies in the crystal structure (Smith, 1985). Singlecrystal X-ray structure determinations have shown that the borate ions in the pentahydrate and decahydrate are identical (Giacovazzo, 1973; Adams, 1964). Borate ion in anhydrous borax is polymeric in nature and is formed via oxygen bridging of triborate and pentaborate groups (Krogh, 1974). Borax pentahydrate has a vapor pressure of 17.7 kPa (133 mmHg) at equilibrium at 59 °C. Borax pentahy0888-5885/96/2635-0159$12.00/0
Figure 1. Structure of tetraborate ion (Nies and Campbell, 1964).
drate converts to dihydrate easily at 0.26 kPa (2 mmHg) pressure and 88 °C temperature (Thompson and Welch, 1980; Menzel et al., 1935; Gmelin Handbuch, 1969). If dehydration process takes place around 140 °C, decrepitation of pentahydrate occurs. After decrepitation, products are in powder form. The powdered product has 0.05 g/cm3 bulk density with 90% void volume (Rheese and Hammer, 1969). Dehydration of borax pentahydrate depends on the vapor pressure of the water of the medium, so it depends on relative humidity. If the vapor pressure of the medium is rather lower than the equilibrium pressure of dehydration, formation of dihydrate starts. In industrial applications borax pentahydrate is dried in rotary or fluidized bed dryers by heating with air at 70 °C which is prepared by heating air at ambient temperature with 40% relative humidity. However these types of dryers have some drawbacks and limitations. In rotary dryers, mixing is not good and product and exit gas temperatures are very close to each other; therefore the air becomes saturated quickly. As a result, driving forces of mass and heat transfer are not sufficient enough. On the other hand, higher temperatures to increase drying rates cause dusting of the product due to decrepitation and thus product yield is decreased. In continuous systems, due to difference in particle size and distribution, residence times of particles in a rotary dryer differ from each other and some particles can leave the dryer without drying. For this reason design of the sections in between the feed and product sections is very important. In a fluidized bed, mixing is complete during drying. However the temperatures of the product and exit gases are the same. This situation is a problem from the point of view of heat transfer. Possibilities of powder formation and © 1996 American Chemical Society
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dusting of the product are quite high as a result of excessive dehydration and attrition by collison of particles in the fluidized bed (Werther, 1992). In spite of these facts, an alternative better process which dries borax pentahydrate without dusting has not yet been applied in production scale (Smith and Mc Broom, 1992). To solve the above-mentioned heat transfer, dusting, and powder formation problems, microwave heating has been considered as an alternative approach and successful results have been obtained (Menon and Mujumbar, 1987; Kocakus¸ ak, 1993a). Microwave heating differs from other conventional heating methods. Microwave heating is volumetric and starts from the center of the material and spreads to the surface (Mc Ketta, 1985; Ayappa et al., 1991). Microwave energy is especially suitable for preheating, drying, and dehydration. Inner sections of the material are quite hotter than the surface, so that evaporation of water and diffusion from inner section to outer sections are much easier. No crust formation occurs during the drying and water escapes easily. Heating of the material depends on its physical and chemical structure. Ionic conduction and dipole rotation in the electromagnetic field cause an increase in temperature during the microwave heating. For this reason homogeneous heating of the material can be obtained. In the case of heterogeneous materials only the desired component, such as water, is heated selectively in drying. Due to rapid vapor generation within material, most of the moisture is vaporized before leaving the sample. If the sample is initially very wet, the pressure inside the sample rises very rapidly; liquid may be removed from the sample under the influence of a total pressure gradient. The higher the initial moisture the greater is the influence of the pressure gradient on the total mass removal. Thus there is in effect a sort of pumping action. This leads to very rapid drying without the need to overheat the atmosphere (Lyons et al. , 1972; Menon and Mujumdar, 1987). Also, in this case, only the material is heated in the process and there is no necessity to heat the furnace, the transportation band, and the ambient air. During heating it is not necessary to have isolation and cooling (Schiffmann, 1987). During microwave heating, materials having polar and smaller molecules absorb microwave energy more than others. Water is both polar and has a small molecule, so it is heated quickly and easily to vaporize. Microwave drying is controlled easily by simply interrupting microwave energy which results in immediate, precise, and efficient control. Microwave heating is quite fast so that dryer sizes and necessary plant areas are rather small (Schiffmann, 1987). Selective heating is possible for heterogeneous materials in drying processes by microwave heating. Depending on the dielectric constant of the material, only water has been affected and the main mass and transport system is not heated. Conduction of hot water molecules increase the temperature of the system, but they are rather lower than conventional systems. Excessive heating and dusting of the product can be avoided. As a result better quality products has been obtained (Kocakus¸ ak et al., 1993 a,b; Mc Ketta, 1985). Microwave heating applications have possibilities for working with preheated air, infrared systems, and vacuum systems also, so that a combined process becomes more economical from the viewpoint of operating and investment costs (Schiffmann, 1987).
Table 1. Particle Size Distribution of Borax Pentahydrate Crystals Used in Experiments screen size (µm)
weight (%)
screen size (µm)
weight (%)
+355 -350 +250 -250 +180 -180 +125
44.77 16.53 14.58 9.96
-125 +90 -90 +45 -45
4.02 4.85 5.29
Experimental Section In this study an ARC¸ ELIK ARMD-580 microwave oven was used. It has a frequency of 2450 MHz, a microwave output power of 700 W, five microwave energy steps, a convection heat power of 1400 W, a thermostat which can be adjusted between 40 and 250 °C, and turbocirculation. Microwave energy creates a magnetic field in the oven. For this reason it is difficult to measure the temperature inside the oven by means of conventional laboratory methods. Therefore, drying was interrupted and temperatures were measured by a thermocouple for each measurement. The probe of the thermocouple was inserted into a specified section of the sample with a specified inclination. For temperature measurements, a Testo-Term 900 type precision thermometer with CrNi probe and accuracy of ( 0.1 °C was used. The thermocouple was calibrated with boiling water at the beginning of each experiment. The magnetic field intensities show differences of various parts in the microwave oven. The temperature distribution by dielectric loss or the increase in temperature is measured as follows: the test tubes filled with water were placed on rotating tray of the oven. After the waterfilled tubes were heated for 30 s, the temperature of the water in each tube was measured from bottom to top using a thermocouple. The heating profile in the oven shows that the circle which has a 10 cm diameter has an almost uniform magnetic field 1-2 cm above the plate. Our result of determining the temperature profile is almost in line with literature (Ayappa et al., 1991) but it is practically more convenient. The crystalline borax pentahydrate produced by Etibank which was enriched in powder by adding fines (below 90 µm) to achieve about 10% of the overall mixture was used in this study. The particle size distribution of the crystalline BPH prepared by this way (48% Na2B4O7‚5H2O) is shown in Table 1. BPH is wetted with saturated BPH solution at 70 °C to obtain samples containing 5-7% moisture. A saturated solution is used on purpose to avoid dissolution of fine particles. Preliminary heating of the wetted BPH to 70 °C was carried out in closed containers to achieve heating under saturated atmosphere. Drying experiments were carried out at five different microwave energy levels, with 100 g samples, spread on a Teflonlined sheet to form a 2-3 mm thick layer. Whether the drying is complete or not is determined by chemical analysis of the product via determining its Na2O and B2O3 content. Determination of sodium oxide content was carried out by addition as an excess of solution of hydrochloric acid and back-titration with a standard volumetric solution of sodium hydroxide using methyl red indicator for end-point determination (ISO 1916 1972). Subsequent titration of the boric oxide (% B2O3) was carried out with a standard volumetric solution of sodium hydroxide in the presence of certain polyhydroxy compounds such as mannitol using phenolphthalein indicator for end point determination (ISO 1915 1972, ISO 1916 1972).
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 161 Table 2. Change of Average Particle Size of Microwave (MW) Dried BPH MW power used (W) 700 500 350 210 105
average particle size (µm) beginning end 260 255 260 264 260
specific surface area (cm2/g) beginning end
330 310 320 316 312
123 116 123 121 123
96 103 100 100 98
Table 3. Properties of Microwave-Dried BPH Crystal MW power (W)
Na2B4O7‚5H2Oa (%)
form
bulk density of product (g/cm3)
700 500 350 210 105
99.12 101.16 100.16 100.63 100.00
crystalline crystalline crystalline crystalline crystalline
0.93 1.00 1.00 1.02 1.02
Figure 2. Microscopic photograph of the original crystalline borax pentahydrate to which previously powder was added.
a BPH contains actually 4.75 mol of crystallization, therefore the calculated values of BPH were found above 100%.
Particle size and distribution of the samples were determined by a conventional sieve test. Specific gravities were determined by pycnometry using kerosine. Specific surfaces were calculated from particle size and specific gravity, assuming BPH crystals are in cubic form. The average particle size for a mixture of particles is calculated by the volume-surface mean diameter, which is related to the specific surface area (Mc Cabe et al., 1985). Calculation of average particle size is based on differential analysis. Average particle size is calculated by using the equation
Ds )
1 n
∑ i)1
(xi/Dpi)
Figure 3. Microwave-dried borax pentahydrate (the distance between lines in background is 100 µm; both Figures 2 and 3 are in the same magnification). Table 4. Microwave-Drying Results of BPH Crystals
where subscripts i are individual increments, Ds ) volume-surface mean diameter, n ) number of increments, Dpi) average particle diameter, and xi ) Mass fraction in a given increment. The surface area of the particles in each fraction is calculated and the results for all fractions are added to give the specific surface (total surface area of a mass of particles) (Mc Cabe et al., 1985). Results and Discussion The properties of the dried BPH are given in Tables 2 and 3. As seen from these tables, the average particle size of BPH increases after microwave drying. Detailed sieve analysis indicates that the 10% fines almost disappears by granulation to larger particle sizes. The microscopic photograph of the original crystalline borax pentahydrate to which previously the powder was added is shown in Figure 2. The microwave-dried borax pentahydrate is shown in Figure 3. In Figure 3, the edges of the crystals are stuck to each other. At the same time, fines are lowered in weight in the sieve analysis. Specific surface area decreases which is in line with the observed particle size increase. This could be attributed to sedimentation of the solute in saturated solution trapped between crystal particles and sticking of particles to each other by bridge formation. In the last experiment at a power level of 105 W, drying was much more difficult in comparison with
MW power (W)
B2O3 in dried product (%)
residence time (s)
700 500 350 210 105
47.54 48.52 48.05 48.27 47.97
28 43 62 110 150
temp temp at achieved (°C) end (°C) 94 93.5 92 90 65
84 87 87 87 60
other microwave level experiments. Preliminary sintering of the product which could be disintegrated easily was observed. The temperature did not rise much, because at the beginning microwave energy was not sufficient; therefore, the material temperature in the oven fell to below 65 °C. The saturated solution trapped between crystal particles created sediments of pentahydrate and decahydrates of borax, and some sections of mass became harder by bridge formation between particles. During further drying with low energy near the end point of drying borax decahydrate crystals were dehydrated by microwave energy and the system reached the desired pentahydrate composition (48% B2O3); some dusting was observed by disintegration of borax decahydrate. Another interesting result seen from Table 3 is that, unlike in the case of conventional drying, microwave drying does not affect the bulk density and the fine crystalline structure of the BPH dried. Results concerning drying parameters are given in Table 4 and Figures 4 and 5. From Table 4 it is seen that almost complete drying of BPH is advanced without
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proximately 22 kW-hton. If energy conversion losses to microwave is considered, this figure is about 30 kWh/ton. Conclusion
Figure 4. Drying of BPH for different energy levels.
Borax pentahydrate can be dried by microwave energy without dehydration. Results of particle size analysis show that average particle size increases 2030% and dusting does not occur. Fine dusty particles added to the wet BPH at the beginning decrease and in some cases they disappear by agglomerating to larger particles. In addition the product is not contaminated by addition of impurities due to drying. Drying times are very short and the temperatures are relatively low, less than 1 min and 70-80 °C, respectively. Process control can easily be achieved by simply regulating the temperature of the product and the humidity of the outlet gas. As microwave power decreases, the drying period increases. Below 200 W drying time is considered too long; therefore 700-200 W microwave power is found to be suitable for drying. An evaluation of consumed microwave energy and evaporated amount of water shows that, to dry 5% moisture of BPH, approximately 22 kW-h/ton microwave energy is necessary. If energy conversion losses to microwave are considered, this figure increases to about 30 kW-h/ton as electrical energy. Literature Cited
Figure 5. Microwave drying of BPH, power-residence time plot.
any loss of 5 mol of water of crystallization. Mass balance determinations showed good correlation between the amount of evaporated water and the chemical composition of the dried mass. From Figure 4 it is seen that the temperature of the mass rises quickly to 90-100 °C and then drops back suddenly to near 80 °C or lower, indicating that drying is complete. This phenomenon can be used for controlling the drying operation in an easy manner. In Figure 5 microwave energy level applied and the related microwave power are plotted against the drying time (residence time) which is assumed to be the peak points in Figure 4. From this figure approximate calculations for energy consumption for each level can be carried out. The average consumption of microwave energy to evaporate water from wet borax pentahydrate (5% H2O) to dryness calculated from Figure 5 is ap-
Adams, R. M. Boron, Metallo-Boron Compounds and Boranes; Interscience Publishers, Wiley: New York, 1964. Ayappa, K. G.; Davis, H. T.; Davies, E. A.; Gordon, J. Analysis of Microwave Heating of Materials with Temperature-Dependent Properties. AIChE J. 1991, 313-322. Giacovazzo, C.; Menchetti, S.; Scordari, F.; Crystal Structure of Tincalconite, Am. Mineral. 1973, 58 (5-6), 523-530. In Kirk Othmer (1992) and in Smith (1985). Gmelin Handbuch. Der Inorganischen Chemie, Na; 8. Auflage, System No. 21; 1969. ISO 1915 1972. International Standard (ISO) 1915; Boric Oxide for Industrial Use-Determination of Sodium Oxide Content, Volumetric Method; 1972. ISO 1916 1972. International Standard (ISO) 1916; Disodium Tetraborates for Industrial Use-Determination of Sodium Oxide and Boric Oxide Contents and Loss on Ignition; 1972. Kemp, P. H. The Chemistry of Borates; Borax Consolidated Limited; W. S. Cowell Ltd.: Butter Market Ipswich, London, 1956; Part I. Kocakus¸ ak, S.; Ko¨roglu, H. J.; Tolun, R.; Ekinci, E. Drying of Wet Borax Pentahydrate by Microwave Heating. Turkish Patent No. 26667, 1993a. Kocakus¸ ak, S.; Tolun, R.; Ekinci, E.; Ko¨roglu, H. J.; Akc¸ ay, K.; C¸ olak, O. N.; Drying of Wet Boric-Acid by Microwave Heating, Turkish Patent No. 26174; 1993b. Krogh-Moe, J. Crystal Structure of Sodium Diborate Na2O.2B2O3. Acta Crystallogr. 1974, B30, 578. Lyons, D. W.; Hatcher, J. D.; Sunderland, J. E. Drying of a Porous Medium with Internal Heat Generation. Int. J. Heat Mass Transfer 1972, 15, 897-905. In Schiffmann (1987). Mc Cabe, W. L.; Smith, J. C.; Harriot, P. Unit Operations of Chemical Engineering. Properties and Handling of Particulate Solids; Mc Graw-Hill Book Company: Singapore, 1985; pp 749766. Mc Ketta, J. Encyclopedia of Chemical Processing and Design; Marcel Dekker: New York, 1985; Vol. 30, pp 202-228. Menzel, H.; Schultz, H.; Sieg, L.; Voight, M. Boric Acids and Alkali Salts of Boric Acids (IX) the System Sodium Tetraborate-Water. Z. Anorg. Allg. Chem. 1935, 224, 1-2; Chem. Abstr. 1935, 29, 7848. Menon, A. S.; Mujumdar, A. S. Drying of Solids; Principles, Classifications and Selection of Dryers. Handbook of Industrial Drying; Marcel Dekker: New York, 1987.
Ind. Eng. Chem. Res., Vol. 35, No. 1, 1996 163 Nies, N. P.; Campbell, G. M. Inorganic Boron-Oxygen Chemistry. In Boron, Metallo-Boron Compounds and Boranes; Adams, R. M., Ed.; Interscience Publishers and John Wiley and Sons, Inc.: New York, 1964; Chapter 3. Powell, D. R.; Gaines, D. F.; Zerella, P. J.; Smith, R. A. Refinrmed of the Structure of Tincalconite. Acta Crystallogr. 1991, C47 (11), 2279-2782. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley and Sons, Inc.: New York, 1992; Vol. 4, pp 365-413. Rheese, R. C.; Hammer, H. N. American Potash and Chemical Corp. US Pat. 3 454 357, July 8, 1969 Schiffmann, R. F. Microwave and Dielectric Drying. In Handbook of Industrial Drying; Marcel Dekker: New York, 1987; pp 327355. Smith, R. A. Boric Oxide, Boric and Borates. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH Verlagsgesellschaft mbH: Weinheim, Federal Republic of Germany, 1985; Vol. A4, pp 263-280. Smith, R. A.; Mc Broom, R. B. Boron Compounds, Oxides, Acid, Borates. In Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley and Sons, Inc.: New York, 1992; Vol. 4, pp 365413.
Thompson, R.; Welch, A. J. E. Boron, Boron-Oxygen Compounds. In Mellor’s In Comprehensive Treatise on Inorganic Theoretical Chemistry; Mellor, Ed.; Longman: New York, London, 1980; Vol. V, Part A. Turkish Republic State Planning Organization. The Fourth Development Plan For Five Years, Selected Committee Report on Boron Minerals, May 1977; Publication No. DPT 1566, OIK. 254, Ankara, Turkey. Werther, J. Fluidized-Bed Reactors. In Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH Verlagsgesellshaft mbH: Weinheim, Federal Republic of Germany, 1992; Vol. B4, pp 239-274.
Received for review February 17, 1995 Revised manuscript received July 6, 1995 Accepted August 22, 1995X IE950119U
X Abstract published in Advance ACS Abstracts, November 15, 1995.
