2862
Ind. Eng. Chem. Res. 1997, 36, 2862-2865
Production of Sodium Perborate Monohydrate by Fluidized-Bed Dehydration S. Kocakus¸ ak,* H. J. Ko1 rogˇ lu, K. Akc¸ ay, O 2 . T. Savas¸ c¸ ı`, and R. Tolun Materials and Chemical Technology Research Institute, TU ¨ BI˙ TAK-MRC, P.O. Box 21, 41470 Gebze, Kocaeli, Turkey
Sodium perborate monohydrate is used in the formulations of concentrated detergents and some medical, disinfectant, and cleaning preparations. Sodium perborate monohydrate (NaBO3‚H2O or NaBO2‚H2O2) contains 16% active oxygen. It does not have a specific melting point. When heated, it decomposes by releasing oxygen and water. Sodium perborate monohydrate is preferred to tetrahydrate due to its higher oxygen content and faster dissolution rate. In addition, in storage, monohydrate is more stable than tetrahydrate. Sodium perborate monohydrate is produced commercially either by dehydration of tetrahydrate or by crystallization of monohydrate. In this work sodium perborate tetrahydrate is dehydrated in a fluidized-bed dehydrator at 100180 °C with about 0.5 h retention time. The product obtained was pure and had the desired bulk density and active oxygen content. Its dissolution rate was high, and it had good properties of storage and resistance to dusting. During dehydration when dry air was used, dehydration temperatures of 145-150 °C and a retention time of 0.5 h were found to be suitable for the production of monohydrate with an active oxygen content of 15.6% and bulk density of 0.5-0.6 g/cm3. Introduction Sodium perborate is available as tetrahydrate, trihydrate monohydrate, and anhydrous. Except trihydrate, all of these sodium perborates have commercial importance. The most widely known sodium perborate is the tetrahydrate (NaBO3‚4H2O). However, monohydrate (NaBO3‚H2O) has also been gaining importance. Sodium perborate tetrahydrate and monohydrate are used in bleaches, cosmetics, medicine formulations, and detergents as active oxygen sources. For example, detergent formulations can contain about 30% sodium perborate tetrahydrate or about 15% sodium perborate monohydrate. Due to its faster dissolution rate, higher active oxygen content, and longer storage life, monohydrate is preferred to tetrahydrate. Sodium perborate tetrahydrate was first synthesized in 1898. However, its molecular structure was clarified in 1901 when it was prepared by adding hydrogen peroxide to the mixture of sodium hydroxide and borax solution (Elvers et al., 1991). Now it is known that its anion formula contains two peroxide bridges represented as Sodium perborate tetrahydrate is a white
powder whose bulk density is 0.65-0.90 g/cm3 and particle size is 0.1-1.0 mm. When stored in a cold, high relative humidity atmosphere, yearly active oxygen loss rate increases as temperature and relative humidity increase. When stored under unsuitable conditions, cake formation occurs by conversion of tetrahydrate to trihydrate (Honig, 1970). Both tetrahydrate and trihydrate can be stored without decomposition under 15 °C. Thermogravimetric analysis has shown that dehydration and then decomposition of sodium perborate tetrahydrate take place in three stages as 20-150, 150-165, and 165-400 °C. Conversion of tetrahydrate to monohydrate takes place at the first stage at 20-150 °C. S0888-5885(96)00744-0 CCC: $14.00
Sodium perborate monohydrate is commercially produced in fluidized-bed reactors. According to Interox patent (Brichard and Colery, 1976, 1978), sodium metaborate and hydrogen peroxide are pulverized in an air fluidized bed at about 100 °C to produce granular sodium perborate monohydrate. The bulk density of the product is 0.4-1.0 g/cm3. However, the dissolution rate of this product is somewhat slower than those of the products produced by other methods. In other production methods, as a source of boron, sodium tetraborate, technical-grade borax decahydrate, borax pentahydrate, and kernite and tincal solutions and as an active oxygen source, hydrogen peroxide solution is used. When sodium perborate tetrahydrate crystallizes out from the mixture of these solutions, it is centrifuged out and obtained as 3-10% water containing wet cake. This cake is dried by air at about 100 °C (Honig, 1970) in either rotary or fluidized-bed driers. The dried perborate is cooled down to 25 °C while being kept in the fluidized bed. Attention must be paid to keep the temperature of the perborate being dried at 60 °C. Otherwise, melting of the perborate being dried takes place. Sodium perborate monohydrate can be produced by dehydrating tetrahydrate in fluidized bed by hot air at 180-210 °C (Denaeyer and Kegelart, 1968). To obtain abrasion-resistant products, it is necessary either to operate under vacuum (Degua and Cuer, 1985) or to operate at 100-180 °C under 10-40% relative humidity (Elvers et al., 1991). Alternatively, operations under 40-80% relative humidity while keeping effluent air temperature at a minimum 60 °C are also reported (Dillenburg et al., 1972). In general, sodium perborate tetrahydrate is converted to monohydrate without decomposition when dehydrated by air under 40% relative humidity at 5090 °C. However, in this case, since the temperatures of the drying mass and the effluent air are close to each other and the drying air reaches its saturation point in a very short time, the driving forces for mass and heat transfer are not effective enough. In addition, inthe © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2863
Figure 1. Schematic diagram of the experimental system.
fluidized bed, the possibility of overdehydration and dusting of the product due to abrasion is quite high. In any case, in industry there has not been any better dehydration method as yet which uses low enough temperatures to prevent active oxygen losses. In recent years there have been attempts to produce monohydrate from tetrahydrate by microwave dehydration, which also looks promising (Kocakus¸ ak et al., 1994). Experimental Studies In this work perborate monohydrate is produced from perborate tetrahydrate by fluidized-bed dehydration. 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. As seen from Figure 1, the fluidized-bed system used consisted of a pressurized air controller, an electrical air heater, an energy controller, a fluidized bed (a glass
Table 1. Specifications of Etibank Sodium Perborate Tetrahydrate H2O B2O3 Na2SO4 CaO SiO2 MnO Fe (ppm)
Chemical Composition (%) 22.10 heavy metal 23.45 Ash 0.028 active O2 17.65 Cl 0.020 Ph sol. 0.032 water insol. 3.0 stability
Powder Properties bulk density (g/cm3)
0.001 44.74 10.57 0.027 10.6 0.043 17.65
0.66-0.77
Sieve Analysis mm +0.35 +0.25 +0.18 +0.125 +0.09 +0.045 -0.045 wt % 20.65 31.69 27.65 17.97 1.40 0.45 0.18
tube of 14 cm diameter and 40 cm length), an effluent air cooler, and a dust collector. Pressure variations in the fluidized bed were measured by a suitable vacuum indicator, fluidized-bed bottom, bed, and top temperatures, and the temperature and relative humidity of the effluent air were measured continuously by suitable thermocouples and a relative humidity probe.
2864 Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 Table 2. Dehydration Parameters run no. dehydration parameter
1
2
3
4
raw material load, g influent air flow rate, m3/h influent air temperature, °C influent air relative humidity, % effluent air temperature, °C retention time, min
500 ( 0.01 5 ( 0.5
500 ( 0.01 5 ( 0.5
500 ( 0.01 5 ( 0.5
500 ( 0.01 5 ( 0.5
120 ( 3
150 ( 3
165 ( 3
180 ( 3
1 ( 0.5
1 ( 0.5
40 ( 2 120
1 ( 0.5
1 ( 0.5
40 ( 2
40 ( 2
40 ( 2
80
60
55
Table 3. Properties of the Sodium Perborate Monohydrate Produced by Fluidized Bed Dehydration run no.
a
property
1
2
3
4
mean particle size, µm bulk density, g/cm3 active oxygen content, wt % Na2O content, wt % dissolution rate, g/L‚min dusting, wt %
300 0.50 15.70 32.40 0.061 37.84
314 0.51 15.64 32.30 0.125 14.47
370 0.51 15.50 32.98 0.152 33.72
370 0.51 15.34 32.99 0.141 17.42
theoretical 0.4-1.0 16.0 32.1 32.13a
Dusting test result of a commercial sodium perborate monohydrate.
As the raw material, sodium perborate tetrahydrate produced at the Bandı`rma facilities of Etibank was used. Specifications of this tetrahydrate are shown in Table 1. Raw material and product quality tests were carried out in line with the relevant ISO standards: ISO 1915 1972 (1972); ISO 1916 1972 (1972); and Turkish standard TS 1822 (1975). The state owned mining company which produces boron compounds also is the biggest producer in the country. Dehydration parameters of the experiments are shown on Table 2. As seen from this table, in dehydration experiments, for each run 500 g of tetrahydrate was used, which resulted in a 10 cm unfluidized bed height. In all experiments the influent air flow rate was kept constant as 5 m3/h. The influent air relative humidity was about 1%. In experiments carried out at higher temperatures to reduce retention times, dry air was also used. Influent air temperatures were 120, 150, 165, and 180 °C. Results and Discussion Properties of the monohydrates obtained after each run are shown on Table 3. As seen from this table and Table 2, as the temperature of the influent air increased and the relative humidity of it decreased, retention times were reduced. However, as the retention time decreased due to higher temperatures, some active oxygen losses took place. Active oxygen contents of the products obtained were determined to be between 15.5% and 15.7%, which were somewhat lower than the theoretical value of 16%. In addition, due to faster dehydration, as the influent air temperature increased and thus the retention time decreased, the mean particle size of the product increased. As seen from Table 3, when compared with a commercial monohydrate, the product obtained has similar, if not better, dusting properties (Schaller and Simmersbach, 1970). This property can be further improved by pulverizing silicate or perborate solution on the monohydrate granules while in the fluidized bed. Industrial applications
along this line are reported in the literature (Bertsch et al., 1988). The products obtained by fluidized-bed dehydration were in granular form containing 15-16% active oxygen and 96-99% NaBO3‚H2O. Since the product granules were porous in nature, their dissolution rates were fast (Table 3). Product purity was satisfactory, which mainly depends on the purity of the raw material and the materials of construction of the equipment used. Conclusions As can be seen from the experimental results, sodium perborate monohydrate can successfully be produced by fluidized-bed dehydration. The product obtained has similar and/or better properties, such as a higher rate of dissolution, when compared with the commercially available ones. Literature Cited Bertsch, F. B.; Mu¨ller, K.; Liesr, T. Kontinuierliches verfahren zur herstellung von natriumperporat-granulaten. European Patent Ep 0 328 768 A1, 1988. Brichard, J. V.; Colery, J. C. Verfahren zur herstellung von granulatfo¨rmigen natriumperborat-monohydrat und das dabei erhaltane produkt. DP 2 650 225, 1976. Brichard, J. V.; Colery, J. C. Verfahren zur herstellung von granulatfo¨rmigen natriumperborat-monohydrat und das dabei erhaltane produkt. DP 2 813 326, 1978. Degua, J.; Cuer, J. P. Procede de fabrication de perborate de sodium monohydrate a partir de perborate de sodium tetrahydrate. European Patent Ep 0 155 894 A1, 1985. Denaeyer, J. L.; Kegelart, W. Verfahren zur herstellung von natriumperborat monohydrat. DP 1 930 286, 1968. Dillenburg, H.; Honig, H.; Fuchs, P. Verfahren zur herstellung on abriebfesten natriumperborat-monohydrat. DP 2 258 319, 1972. Elvers, B.; Hawkins, S.; Schulz, G. Peroxo Compounds, Inorganic. Ullmann’s Encyclopedia of Industrial Chemistry, VCH: Weinheim, Germany, 1991; Vol. A19, pp 177-197. Honig, H. Verfahren zum Entwa¨ssern von Natriumperborattetrahydrat. DP 1 801 470, 1970. ISO 1915 1972. Boric Oxide for Industrial UsesDetermination of Sodium Oxide Content; International Standard (ISO) 1915; Volumetric Method, 1972.
Ind. Eng. Chem. Res., Vol. 36, No. 7, 1997 2865 ISO 1916 1972. Disodium Tetraborates for Industrial UsesDetermination of Sodium Oxide and Boric Oxide Contents and Loss on Ignition; International Standard (ISO) 1916; 1972. Kocakus¸ ak, S.; Ko¨rogˇlu, H. J.; Ekinci, E.; Akc¸ ay, A.; Tolun, R. Production of sodium perborate monohydrate using microwave heating. Turkish Patent 27023, 1994. Schaller, A.; Simmersbach, E. Verfahren zur herstellung von perborat-monohydrat. DP 2 040 507, 1970. TSE 1822 1975. Hydrated Sodium Perborates for Industrial UsesDetermination of Sodium Oxide, Boric Oxide and Available
Oxygen Contents; Turkish Standard (TS) 1822; Volumetric Method; 1975.
Received for review November 25, 1996 Revised manuscript received May 6, 1997 Accepted May 12, 1997X IE960744E X Abstract published in Advance ACS Abstracts, June 15, 1997.
