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Ind. Eng. Chem. Res. 1998, 37, 2426-2429
Production of Sodium Perborate Monohydrate by Microwave Heating S. Kocakus¸ ak,* H. J. Ko1 rogˇ lu, O 2 . T. Savas¸ c¸ ı, and R. Tolun TU ¨ BI˙ TAK-MRC, Materials and Chemical Technology Research Institute, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey
In this work, sodium perborate tetrahydrate was dehydrated to perborate a monohydrate by using 350 W, 2450 MHz microwave radiation energy. The dehydration was achieved in 25 min without melting. In this dehydration process, the heating time and the temperature were easily controlled by microwave energy. The product obtained was in powder form containing 15.215.6 wt % active oxygen and 95-97 wt % NaBO3‚H2O. In addition, the product was not contaminated by impurities. Introduction Sodium perborate is marketed as tetrahydrate, trihydrate, monohydrate and as anhydr under the name of sodium oxoborate or sodium perborate. All these forms of sodium perborate except the trihydrate are of industrial importance. The best known form is sodium perborate tetrahydrate (NaBO3‚4H2O); however, importance and popularity of the monohydrate have also been increasing rapidly. The major area of consumption for perborates is the detergent formulations where they act as active oxygen sources. Sodium perborate tetrahydrate was discovered in 1898 by Tanator and produced since 1907 (Elvers, 1991). However, the structure of it was enlightened in 1901 (Elvers, 1991; Hansson, 1901). The formula of its anion is
Sodium perborate tetrahydrate is a white powder having a bulk density of 0.65-0.90 g/cm3 and mean particle size of 0.1-1.0 µm. When stored under cool, dry conditions, its active oxygen loss per year is about 1 wt %. The rate of active oxygen loss, the rate of decomposition, increases as humidity and temperature of the environment increase. When sodium perborate tetrahydrate is stored in an unsuitable environment, as it turns into three hydrate (NaBO3‚3H2O), a problem of cake formation develops (Gelder, 1956). Both tetra- and trihydrate can be stored without decomposition under 15 °C. Sodium perborate monohydrate (NaBO3‚H2O) is also a white powder having a bulk density of 0.5-0.6 g/cm3. When directly crystallized from its solution, its bulk density is 0.4-1.0 g/cm3. It contains 16 wt % active oxygen, and when heated, it decomposes while releasing oxygen. The main advantages of the monohydrate over the tetrahydrate are its higher active oxygen contect and dissolution rate. While 2 g of tetrahydrate can be dissolved in 1 L of water at 15 °C in 6-8 min, a monohydrate under the same conditions dissolves in less than a minute.
In industrial scale, perborate monohydrate is produced by the dehydration of sodium perborate tetrahydrate in fluidized-bed dehydrators at 50-90 °C and generally under air with 40% relative humidity (Kocakus¸ ak et al., 1997). However, due to the rather small temperature differences between the effluent air and the drying mass and the rapid saturation of the drying air, the driving forces for mass and heat transfer is small. In addition, due to overdehydration, abrasion of particles, and collision between them, the risk of ending up with a high powder content product is high. An abrasion resistant perborate monohydrate can be produced in fluidized-bed systems. According to the patent of Interox (Degussa, 1988), to produce high abrasion resistant and high crush strength granules of sodium perborate monohydrate, sodium metaborate and hydrogen peroxide are pulverized together in a fluidized bed with air at 100 °C. The bulk density of this product is claimed to be 0.4-1 g/cm3; however, its dissolution rate is rather low compared with that of other commercial perborates. For the above-mentioned problems of fluidized-bed production, making use of microwave (MW) energy seems to be a good solution. Microwave heating has been used in processing industries since the 1960s. The pioneering applications were made in food processing (McKetta, 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 et al., 1991; Foster, 1988), rubber (Gent and Hindi, 1989), chemicals (Strauss and Faux, 1990), pharmaceuticals (Linders et al., 1988), fuel (Depew et al., 1991; Norton et al., 1991), construction materials (Liu et al., 1990), metallurgy (Liu et al., 1990; Standish and Woroner, 1991; Standish and Huag, 1991), and the treatment of hazardous and hospital wastes (Petersen, 1991; Collins et al., 1991; Morrell et al., 1986). In most applications 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. 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, therefore, eliminating crust formation due
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Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2427 Table 1. Properties of the Sodium Perborate Tetrahydrate Used as Raw Material and Monohydrates Produced products obtained from experiments
property active oxygen content (wt %) water content (mol)/(mol of product) NaBO3‚H2O content (wt %) bulk density (g/cm3) sieve analysis (wt %) +355 +250 +180 +125 +90 +45 -45
experiment 1 experiment 2 experiment 3 MW energy: 350 W MW energy: 350 W MW energy: 350 W typical raw material freq.: 2450 MHz freq.: 2450 MHz freq.: 2450 MHz properties (sodium perborate time: 16 min time: 25 min time: 24 min for a tetrahydrate) final temp.: 52.9 °C final temp.: 62 °C final temp.: 70.7 °C commercial (Etibank supply) raw material: 10 g raw material: 15 g raw material: 20 g monohydrate 10.57 3.86 66.00 0.66-0.70
15.60 1.15 94.46 0.50
15.20 1.30 94.91 0.55
15.25 1.28 95.22 0.53
15.00 1.31 93.75 0.60
20.65 31.69 27.65 17.97 1.40 0.45 0.18
6.60 8.35 8.11 2.79 2.78 61.10 3.10
20.45 27.52 27.93 16.58 2.57 1.94 1.01
9.58 21.63 27.36 15.96 2.24 21.17 2.06
11.58 26.65 27.35 28.90 2.25 2.20 1.07
to surface heating. Also, the heating is fast and can be easily controlled (Lynch, 1977; Perkin, 1979; Wiesehofo¨r, 1989; Standish and Woroner, 1991; Standish and Huag, 1991; Kocakus¸ ak et al., 1994a, 1995, 1996). Experimental Studies In this study an ARC¸ ELI˙ K 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 a Cr-Ni 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 in 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 a rotating tray in 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 showed 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. To simulate industrial continuous processes, the samples were introduced to the microwave oven on a glass-reinforced Teflon sheet. For the experiments the desired amount of perborate tetrahydrate (10-20 g) was weighed on a glass-reinforced Teflon sheet and placed in the oven at ambient temperature. Then, the microwave energy was started. No convective heating was applied throughout the test. The temperature changes in the samples were followed as described above. Oxygen content changes were followed by taking small
Figure 1. Microwave heating of 10-g samples of sodium perborate tetrahydrate with 350-W microwave energy.
samples (0.2 g each) from the initial sample at various time intervals. To produce high-grade sodium perborate monohydrate, the heating regime and time were easily controlled. To determine the properties of the raw material and the products, for bulk densities and particle size analysis, standard procedures were used. Peroxide contents were calculated via the determination of active oxygen content by manganometric titration. Perborate contents were calculated via asidimetric determination of sodium and B2O3 content of the samples. Sodium perborate tetrahydrate produced by EtibankTurkey was used as raw material in the study. Properties of the raw material are shown in Table 1. Results and Discussion Experimental studies were first started with 10 g of sodium perborate tetrahydrate using a 700-W microwave energy. In this case, due to a high level of energy applied, decomposition, active oxygen loss, together with melting and agglomeration of the sample were observed. To eliminate these undesirable effects the experiment was repeated with 350-W microwave energy. The results obtained are shown in Figure 1.The properties of the final product are shown in Table 1. The product obtained had a somewhat lower bulk density value of about 0.50 g/cm3 and was a fluffy powder with a smaller particle size than the raw material. This decrease in the bulk density and the particle size was due to
2428 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
tions which will give way to dehydration to a monohydrate in about 25 min will result in products with good powder properties such as bulk density and particle size and distributions. Shorter dehydration times, due to swelling and decrepitation, result in low bulk densities, and porous products. However, these products are better from a dissolution rate point-of-view. Conclusion
Figure 2. Microwave heating of 15-g samples of sodium perborate tetrahydrate with 350-W microwave energy.
In this study sodium perborate tetrahydrate samples where dehydrated to sodium perborate monohydrate in 25 min by using 350-W and 2450-MHz microwave radiation. The resulting product was in free-flowing powder form containing 15.20-15.60 wt % active oxygen and 95-97 wt % NaBO3‚H2O. A patent on this production process was also granted recently to the authors (Kocakus¸ ak et al., 1994b). Literature Cited
Figure 3. Microwave heating of 20-g samples of sodium perborate tetrahydrate with 350-W Microwave energy.
decrepitation. However, there seemed to be no appreciable decrease in the active oxygen content. Therefore, it can be assumed that no decomposition took place. Figure 1 shows that when the desired active oxygen content is reached, an increase in temperature is observed. To improve the properties of the product, by slowing down the rate of dehydration, thus the rate of decrepitation, further experiments were run by increasing the amount of the starting raw material 50% and 100% but keeping the other parameters of the first experiment the same. The results obtained are shown graphically in Figures 2 and 3. The properties of the products obtained are shown in Table 1 in comparison with a commercial product produced by other techniques. As seen from these results, the dehydration time, thus the rate of it, was decreased and less powdery products of higher bulk densities of about 0.55 g/cm3 were obtained. Product quality was as good as the commercial product. In addition, since they were prepared on a Teflon band which gives way to easy removal of the product without sticking, they were less contaminated. Due to somewhat smaller bulk densities, their dissolution rates were determined to be 25% higher than those of the commercial product. As seen from the related figures, in these last two cases also, as the experiments approached dehydration to the monohydrate, there were noticeable increases in the temperature rates. This phenomenon gave way to easy process control. Experimental results also indicate that MW applica-
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Received for review October 20, 1997 Revised manuscript received February 9, 1998 Accepted February 24, 1998 IE970726C
