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200 g sieved particles of 3.2?4 mm was put into the fluidized bed at a superficial gas velocity of 1.4 m/s. The particles were fluidized and abraded in the bed. All the particles were taken out after an interval of

30 min and weighed. Then, all the particles were put back into the fluidized bed again to continue the abrasion test. 2.5. Measurement of release rate The release rate of coated urea in water was measured. For each sample,

25 g coated particles were put into a beaker containing

500 ml deionized water. Each beaker was covered with a PE film to prevent water evaporation and kept at

25 °C in a constant temperature box. At certain time intervals,

5 mL of solution was sampled for nitrogen content, and

5 mL of water was added into the beaker to maintain the solution volume constant. The solution was agitated before sampling. The nitrogen concentration was determined using alkaline potassium persulfate digestion and UV spectrophotometry according to the China Standard GB 11894-89 method. Potassium persulfate in aqueous solution decomposes to potas- sium hydrogen sulfate and atomic oxygen when the temperature is higher than

60 °C, and potassium hydrogen sulfate disso- ciates to H+ completely under alkaline conditions. The atomic oxygen produced at 120?124 °C converts the compound containing nitrogen to a nitrate. The nitrate concentration can be determined by measuring the absorbency with a UV spectrophotometer. 3. Results and discussion 3.1. Strength of modified sulfur The particle strength of sulfur/modified sulfur was measured. The strength of modified sulfur change with the DCPD/sulfur ratio and different reaction time is shown in Fig. 2. The strength of the modified sulfur increased markedly as the DCPD/sulfur ratio increased, as shown in Fig. 2(a), but the strength change with the reaction time did not show a clear trend, as shown in Fig. 2(b). 3.2. Micro-structure of sulfur and modified sulfur In order to observe the micro-structure of sulfur and modified sulfur, samples were prepared according to the following Fig. 3. Micro-structure of sulfur and modified sulfur DCPD/sulfur ratio: (a), (d): 0%;

(b), (e): 5%;

(c), (f): 10%. Table

2 Coating material composition No. Composition Coating quantities, %

1 S 22.6

2 S 32.1

3 S+2% DCPD (wt) 16.1

4 S+2% DCPD (wt) 18.6

5 S+5% DCPD (wt) 26.5

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

183 (2008) 88C93 procedures. Molten modified sulfur after

2 h reaction was po........