Ind. Eng. Chem. Res. 1995,34,881-885
881
GENERAL RESEARCH Production of Anhydrous Borax Using Microwave Heating Sidika Kocakugak,*Jiilide H. Ktjroglu, Ekrem Ekinci, and Ragit Tolun Chemical Engineering Department, TUBITM-MRC, P.O. Box 21 41470 Gebze, Kocaeli, Turkey
Dehydration studies of borax penta- and decahydrate minerals were carried out using a microwave (MW) heating system. A 650 W microwave oven (2450 MHz) as well as a 1600 W convective heating unit was used. It was possible to produce 97.5% NazB407 anhydrous borax products. Preheating affected both the quality and dehydration time of the end products. The dehydration was found to be very fast up to 2 mol of water after which its speed dropped progressively. Overall microwave heating produced cleaner products and was much faster than the conventional heating system.
Introduction Turkey has about 60% of the world reserves of boron (Turkish Republic State Planning Organisation, 1977). The majority of these reserves are in tincal (borax decahydrate) form. Most of the mined boron mineral is exported as raw material without processing. About 400 000 metric tons/year of the raw mineral is concentrated and refined as borax deca- and pentahydrates at the &rka Plant of Etibank. Due to marketing difficulties, borax decahydrate production has recently been discontinued or partially operated. One of the main reasons for the lack of demand for borax decahydrate is the extra weight associated with the water, which increases transportation and processing costs. For these reasons, calcination of decahydrate up to anhydrous borax is receiving considerable interest in Turkey (Kocakugak et al., 1992). The borate ion in the borax structure has the chemical formula B40~(0H)4~and is shown in Figure 1(Adams, 1964; Morimoto, 1956). Borax decahydrate and pentahydrate are Naz(B405(OH)4).8H20 and Na~(B405(OH)4).3HzO,respectively. In both of these hydrates, 2 molecules of water are structurally incorporated in the borate ion as hydroxyl groups (Farmer, 1982). The remaining water molecules are outside the ionic structure. Removal of the water molecules of crystallization is relatively easy whereas the removal of water molecules in ionic structure is difficult. Direct heating experiments in an oven and in thermogravimetric analysis (TGA) instruments have shown that the removal of water to the dihydrate state is fast, but the removal of the structurally bonded water is slow and difficult. The removal of water from the tetraborate ion results in ionic condensation giving the monohydrate. The reaction is as follows:
-
(B405(0H)4)2-
(B406(OH>,>2+ HzO
The loss of the last water molecule proceeds according to the following reaction, which has the highest activation energy of all the dehydration steps:
-
(B406(OH),)2-
(B407)'-
+ H,O
The product formed is anhydrous tetraborate. In the hydrated borate, anions up to dihydrate level, OH 0888-588519512634-0881$09.00/0
groups are attached to the boron atoms. Removal of one molecule of water causes a condensation reaction between two hydrated borate anions (Edward et al., 1960). In previous studies, preliminary laboratory tests and TGA of the dehydration of borax deca- and pentahydrates were performed. The effects of temperature and dehydration time were investigated. The dehydration of pentahydrate samples were done in an oven with no air circulation on a flat glass plate (Kocakugak et al., 1992). Dehydration, which is a slow process at room temperature, becomes a fast process at 80-100 "C (Kocakugaket al., 1992;Edward and Ross, 1960;Menzel and Schultz, 1940; McIntosh and Matthews, 1949; Edward and Schultz, 1960). The dihydrate molecule is formed after long dehydration times and high temperatures. Current industrial practice for the removal of the last two molecules from borax melting furnaces (with refractory-lined surfaces) operates at about 700800 "C (Kirk-Othmer, 1978). Molten borax damages the refractory walls and absorbs some inorganic impurities from the surfaces (Turner and Turner, 1924). In order to overcome these difficulties and use some of the benefits relevant to its structure, microwave (MW) dehydration studies were undertaken. Microwave heating has been used in processing industries since the 1960s. The pioneering applications were made in food processing (Mc Ketta, 1985). Recent applications of microwave technology extend to a variety of industries such as asphalt (Nemeth and Smith, 1990), paper and pulp (Sairem Brochure, 1993; Law and Valade, 1991), plastics (Allen and Miller, 1991; Foster, 1988),rubber (Gent and Hinde, 19891, chemical (Strauss and Faux, 19901, pharmaceuticals (Linders et al., 1988), fuel (Depew et al., 1991; Norton et al., 19911, construction materials (Liu et al., 1990), metallurgy (Liu et al., 1990; Standish and Woroner, 1991; Standish and Haug, 19911,and the treatment of hazardous and hospital wastes (Petersen, 1991; Collins et al., 1991; Morrell et al., 1986). In a microwave system, in most applications the heating is volumetric and the process starts from the center and works its way to the surface of the materials due to the mechanism of molecular level heating. Microwave technology offers considerable environmental advantages as in the case of its use in asphalt reclamation (Morrell et al., 1986; Osepchuk, 1984).
0 1995 American Chemical Society
882 Ind. Eng. Chem. Res., Vol. 34, No. 3, 1995
1 1
2-
Ho,
2n-
2 n-
-
____)
OH
n
Figure 1. Structure of tetraborate ion (Adams, 1964;Morimoto, 1956).
MW heating applications are particularly well suited during the initial and final stages of dehydration operations. Heating from the center provides a favorable environment for evaporation and diffusion toward the surface eliminating crust formation due to surface heating. Also, the heating is fast and can be easily controlled (Lynch, 1977; Perkin, 1979; Wiesehofer and Westermeier, 1989; Standish and Woroner, 1991; Standish and Huag, 1991). MW heating is based on dipole rotation and ionic conduction in a magnetic field, and therefore can be directed sharply to the target material avoiding problems related to conventional systems. Other advantages of microwave heating include uniform and selective heating of heterogeneous materials. It can be used in combination with other forms of heating.
Experimental Section The oven used in this study was a Thorn EM1 Domestic Appliances Limited-Tricity convection type microwave oven. The oven operates at 2450 MHz with a 650 W MW heating capacity and has five stages of heating adjustment. It has 1600 W convective heating power adjustable to temperatures between 20-250 "C and turbo circulation. Initially, the samples were introduced to the microwave oven in a glass beaker which was covered with a glass lid to avoid loss of powder due to decrepitation. In later experiments, to simulate industrial continuous processes, the samples were introduced on a glass reinforced Teflon belt surface. In order to produce high grade anhydrous borax, the heating regime and time were controlled. Due to the short decrepitation time, there is intensive powder formation. For this reason, after the B203 percentage of borax reached 60% (dihydrate stage) the produced powder was compacted and the granules were further heated using microwave energy to convert the samples to anhydrate. MW dehydration of the compacted dihydrate did not result in decrepitation. In order to prevent condensation of the dehydrated water, dry hot air circulation was employed in the MW oven. This practice also increases the energy efficiency by removing water molecules which would otherwise absorb MW energy. Borax penta- and decahydrate produced by Etibank were used in the study. A microscope photograph of the crystalline borax pentahydrate product produced by Etibank is shown in Figure 2. Decahydrate, pentahydrate, and dihydrate powders in powder and granule forms were all treated with MW to produce anhydrate.
Results and Discussions MW heating is observed to quickly remove 3 mol of water from pentahydrate to yield dihydrate. Dehydra-
Figure 2. Crystalline borax pentahydrate. Table 1. Preliminary Microwave Heating of 20 g of Pentahydrate Powder, Microwave Power 650 W, Microwave Frequency 2450 MHz MW heating Na2B407 content H20 H2O in product time ( m i d of product (%) evaporated (%) (moVmol borax)
0 15 30 45 60
69.10 85.57 94.51 95.51 95.80
0.00 19.24 26.88 27.68 27.87
5.00 1.87 0.65 0.52 0.49
tion from dihydrate to monohydrate is slow and dehydration of the last mole of water is considerably slower. However, the dehydration rate of the last mole of water is much faster than the conventional methods of dehydration such as direct heating in fixed or fluidized beds. The fast dehydration rates are due to MW heating mechanism which diffuses and accelerates the polar molecules even at the center of the heated particles. The decreasing rate of dehydration at the di- and monohydrate levels is mostly due to the decrease in water content which absorbs the M W energy. Especially during the dehydration of 3 mol of water from the pentahydrate, the diffusion of water is fast. At 200-250 "C, 98% Na2B407 containing borax compound absorbs extensive MW energy and starts to melt locally and form mobile Na+ and B 4 0 ~ ~ions, - due to ionic conduction and microwave absorption. Electric arcs which formed caused chemical decompositionof the glass reinforced Teflon belt surface. At the beginning of arc formation, the experiments were terminated in order to avoid damage to the MW furnace. In the first experiment, borax pentahydrate was heated from room temperature (20-25 "C)to 250 "Cin 1h. The results of this experiment are shown in Table 1. In these experiments, the MW oven was not preheated. The resulting product was 96% NazB407 after 1h of MW heating. The product is a puffy powder with a bulk density of about 0.02 g/cm3. This product needs
Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995 883 Table 2. Microwave Heating of 20 g Pentahydrate Powder, Microwave Power 650 W, Microwave Frequency 2450 MHz with Preheating to 180 "C
MW heating NazB407 content H2O H2O in product time (min) of product (%I evaporated (%) (moVmol borax) 3 92.42 25.24 0.916 6 94.04 26.53 0.708 9 95.11 27.34 0.575 15 95.89 27.94 0.480 28.25 0.429 20 96.31 30 96.89 28.68 0.359 60 96.96 28.74 0.350
s:
Table 3. Dissolution Rates of Anhydrous Borax borax type MW powder MW granule conventional glassy
Figure 3. MW dehydrated 96% anhydrous borax product obtained at room temperature.
Figure 4. Anhydrous borax product obtained by MW dehydration by 180 "C preheating.
to be compacted before it can be used by industry. A microscope photograph of the product is show in Figure 3. In a second experiment, in order to investigate the effect of preheating, the furnace was first heated to 180 "C and the samples were then introduced into the MW oven. 95.8%Na2B407 content was reached after 15 min. After 60 min of heating, a Na2B407 of 97% content was reached. Therefore, preheating affected both process
dissolution rate (g/min) 0.125 0.024 0.00015
time and product quality. The results of the MW heating with preheating are shown in Table 2 and Figures 4 and 5. The MW dehydrated products were either spongy or powdery and have better solubility than products obtained from direct heating methods. Dissolution of anhydrous borax products, obtained by microwave heating of powder and granular form and conventional glassy products, were investigated by dissolving 0.3 g of sample in 1L of distilled water. The conductometric measurements were made and plotted for the uniform sized particles (250-500 pm) in dilute solutions prepared at 100 rpm mixing rate at room temperature. The dissolution rates are shown in Table 3. In conventional anhydrous borax production technologies, the dehydration of borax is carried out at a temperature range of 750-1000 "Cwhere the resultant borax is in glassy form, which is then crushed and sieved. Molten borax at higher temperature is a viscous, sticky, and corrosive material. For this reason, there are usually material problems in production plants. Even in production units manufactured from special steel and refractory materials, high levels of extracted impurities are detected. Maximum allowable impurities for technical anhydrous borax and detected amounts for microwave anhydrous borax produced in this study are shown in Table 4.
69.10
73.65
< CI
0
78.84
a
z 84.82
91.79 I 1
'
0
Figure 5. MW
5
10
15
20
25
30
'
'
1
35
1
1
r
'
l
40
l
l
l
100.00
884 Ind. Eng. Chem. Res., Vol. 34,No. 3, 1995 Table 4. Impurities of Microwave-Produced and Conventional Glassy Anhydrous Borax impurities so42-
Si02 A203
CaO MgO Fez03 c1a
conventional glassy borax (%) 0.41 0.21 0.14 0.03 0.15 0.02 -0.0300
MW-produced bor& (%) 0.0300 0.0060 0.0100 0.0300 0.0050 I0.0005
Depends on raw material.
Figure 6. Anhydrous borax obtained by MW dehydration by 200 "C preheating.
used. The powdered pentahydrate (69.1% Na2B407) sample was spread as a 2-5 mm layer on the surface of the sheet, and after 20 min of MW heating in a 250 "C preheated environment, no sticking of the particles was observed. Their forms were not altered and no decrepitation was observed. After 20 min of heating, the Na2B407 content of the sample was 93.45% and 0.91 mol of HaO/mol of borax was left in the product (monohydrate). In the following experiment, granulated borax pentahydrate (69.1% Na2B407) was M W heated in a 250 "C preheated oven on top of glass reinforced Teflon belt (20 x 20) cm2,using 650 W. After 20 min of MW energy application, 95.7% Na2B407 was obtained and it had 112 mol of water. The product showed no powder formation or disintegration; however, some local melting was observed at some regions of the particles. The previously obtained borax dihydrate powders (85%Na2B407) which had been heated in 15 min from room temperature to 200 "C were compacted to granular form and spread over the Teflon sheet. Granules containing 85% Na2B407 were heated for 20 min in a 250 "C preheated oven, using similar conditions as previous experiments, and 97.5% Na2B407 was obtained. Further heating of the samples by MW power caused melting of the borax granules which damaged the glass reinforced Teflon belt. For higher grade (more than 98% Na2B407), use of a belt material that can resist temperatures higher than 250 "C is required.
Conclusion In microwave heating, electric and magnetic field energy is converted to kinetic energy which may be regarded as heat. Thus the heat does not come from external sources and as a result no impurities are introduced during microwave processing. However, in conventional processes heat is supplied by electrical conduction and/or is produced by burning of fuels as a consequence of which residual impurities, such as carbon and ash, go into the product during processing. Due to process characteristics of MW heating the products are inherently cleaner. These properties may provide the required quality of the dehydrated products for special ceramics and clean frit production. In order to explore the benefits of preheating granular pentahydrate, the oven was first preheated to 200 "C and a granulated pentahydrate sample was heated using 650 W MW power. After 20 min the experiment was stopped due to arc formation. Melting was observed in powder and compacted samples under similar experimental conditions. Contrary to the powder sample, puffiness and volume increase was not observed for compacted and granulated samples at the dihydrate level (Figure 6). A possible explanation is that the granulated and compacted samples contain less captured air and therefore utilize MW energy more efficiently. The product in the beaker was observed to form in two zones. The outer zone had 0.663 mol of HzO/mol of borax and was 94.07% Na2B407, and the inner zone had 0.045 mol of HsO/mol of borax and was 99.59% Na2B407. The outer zone had a granular porous structure, and the inner zone was melted (glassy). The higher Na2B407 and much lower water content of the inner zone indicates the central heating pattern of the sample under MW energy. In order to simulate the continuous heating experiments, glass reinforced Teflon sheet (belt fabric) was
In this study, powdered borax pentahydrate and dihydrate and the granulated borax pentahydrate and dihydrate were dehydrated by microwave energy. Resulting products were either powder or granulated anhydrous 98% Na2B407-containing borax. The reinforced Teflon belt can be used in continuous production up to 250 "C. Further heating of samples of 98% Na2B407-containing borax by MW energy damages the glass reinforced Teflon sheet. Borax pentahydrate samples were dehydrated for 60 min to anhydrous borax by using 650 W and 2450 MHz MW radiation. A patent on this production process in also granted recently to the authors (Kocakugak et al., 1993).
Acknowledgment The authors would like to express thanks to Hande Guqlu for her kind assistance in proofreading of this paper.
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Received for review March 10, 1994 Revised manuscript received October 14, 1994 Accepted October 31, 1994@ IE9401420
* Abstract published in Advance ACS Abstracts, February 1, 1995.
