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1335 kg/m3 in density were used in the experiments, which were produced by the Ningxia Petrochemical Company of Available online at www.sciencedirect.com Powder Technology

183 (2008) 88C93 www.elsevier.com/locate/powtec ? Corresponding author. Tel.: +86

10 62788993;

fax: +86

10 62772051. E-mail address: wangtj@mail.tsinghua.edu.cn (T.-J. Wang). 0032-5910/$ - see front matter ?

2007 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2007.11.022 PetroChina Co. Ltd. Dicyclopentadiene was a commercial product with purity 94.0% from the Hangzhou Yangli Petrochemical Co. Ltd. Sulfur was a commercial product with purity 99.9%. 2.2. Sulfur modification A known amount of DCPD and commercial sulfur were mixed uniformly in a beaker at

145 °C controlled with an oil bath. After a specified time of reaction, the modified sulfur was obtained. The reaction time is in the range of 1C6h. Experiments showed that the viscosity of the molten modified sulfur significantly increased with the DCPD fraction and the reaction time. Bordoloi and Pearce proposed a viscosity expression for modified sulfur with DCPD at

140 °C [8], g ? 19:46 exp 11:33X1:78 t ? ? ;

X ?

0 ? 0:4 ? ? ?1? where η is the viscosity of molten modified sulfur, cP;

X is the mole fraction of DCPD;

t is the reaction time, h. Considering that too high a viscosity is not suitable for the spraying coating process, therefore, based on the exploratory experiments, the DCPD fraction was set in the range of 0C10% sulfur by weight. 2.3. Fluidized bed coater and coating procedures A fluidized bed coater, shown in the Fig. 1, was used for the coating of the urea particles. The fluidized bed was made of an organic glass column

150 mm in diameter and was widened in the upper section at

300 mm to restrict particle entrainment. A spraying nozzle of an air-atomized nozzle was centrally set above the fluidized bed. Urea particles of 1kg in weight were put in the fluidized bed, and the bed was fluidized at a superficial gas velocity of 2.0 m/s. The molten coating material under a pressure of 0.2 MPa was atomized with compressed air, and sprayed onto the urea particles in the fluidized bed for coating. The operation para- meters in the coating process were listed in Table 1. After spraying a set quantity of coating material, the particles were taken out for analysis. 2.4. Measurement of strength and abrasion resistance In order to examine the properties of modified sulfur as a coating material, the compressive strength of the particles prepared with the modified sulfur was measured. Modified Fig. 1. Schematic diagram of the coating process in a fluidized bed. Table

1 Operation parameters of coating process in a fluidized bed Superficial gas velocity, m/s Air pressure, MPa Air flow rate, L/min Molten sulfur flow rate, mL/min Molten sulfur temperature, °C Temperature in coater, °C

2 0.2

25 30

145 60 Fig. 2. Compressive strength of sulfur/modified sulfur granules versus DCPD/S ratio and reaction time.

89 Y.-H. Liu et al. / Powder Technology

183 (2008) 88C93 sulfur in the molten state was dropped into a series of cylindrical molds, forming column particles

2 mm in diameter and

3 mm in height. The compressive strength of the particles was measured with a particle strength meter (KQ-3, Kunming, China). For each sample, an average value of

20 particle measurements was taken. The abrasion resistance of sulfur or modified sulfur coated urea particles was examined in the fluidized bed by checking the weight loss versus abrasion time. For comparison, the abrasion resistance of urea particles is also measured. The collisions between particles and particle collision with the wall in the fluidized bed were used to simulate the abrasion environment in transportation, stockpiling, and fertilization.