Sulfur-coated urea (SCU) and polyurethane-coated urea (PCU) exhibit significant differences in their manufacturing processes, primarily reflected in five key stages:

1. Raw Material Handling

• Sulfur-Coated Urea
  • Sulfur Treatment: Melted at 150°C and atomized via high-pressure nozzles to coat urea granules preheated to 65°C.
  • Urea Pretreatment: Surface slightly melted to enhance sulfur adhesion.
  • Additives: Sealants (e.g., paraffin, coal tar) and conditioners (e.g., diatomaceous earth) are applied to close membrane pores and prevent caking.
• Polyurethane-Coated Urea
  • Polyurethane Synthesis: Prepolymer generated by reacting isocyanate with polyol, with precise control of temperature and catalyst dosage.
  • Urea Pretreatment: Heated and shaped in a rotary drum to create a micro-melted surface for polyurethane adhesion.
  • No Additives: Relies on polyurethane’s inherent chemical structure to form a dense membrane.

2. Coating Technology

• Sulfur-Coated Urea
  • Thermal Coating: Sulfur is spray-coated under high pressure, forming multi-layer microcrystalline polymer wraps.
  • Sieve Control: Dual sieving (raw material inlet and pre-product outlet) ensures particle uniformity.
• Polyurethane-Coated Urea
  • Three-Stage Rotary Drum: Integrates heating/shaping, polyurethane spraying, and pre-cooling functions.
  • Continuous Spraying: Polyurethane forms a film on rolling granules, with thickness adjusted via spray volume.

3. Curing Conditions

• Sulfur-Coated Urea
  • Ambient Cooling: Sulfur coating solidifies at room temperature.
  • Sealant Application: Paraffin is sprayed at 104°C to seal membrane cracks.
• Polyurethane-Coated Urea
  • Temperature-Controlled Curing: Cured in a 70°C oven for 24 hours to ensure full crosslinking of urethane linkages.
  • Humidity Control: Moisture exclusion during curing to prevent reaction interference.

4. Post-Processing

• Sulfur-Coated Urea
  • Fluidized Bed Cooling: Cooled to 38–40°C, with diatomaceous earth added to prevent caking.
  • Sieve Finishing: Removes oversized/undersized particles to meet size standards.
• Polyurethane-Coated Urea
  • Rotary Drum Cooling: Post-drum cooling via cold air exchange, followed by discharge.
  • Recycle System: Substandard intermediates are returned to the drum for re-coating, improving yield.

5. Process Characteristics Comparison

Conclusion

The SCU process centers on sulfur melting and spraying, relying on sealants and sieving for membrane integrity, making it suitable for cost-sensitive field crops. The PCU process, through chemical polymerization, emphasizes continuous production and membrane flexibility, catering to high-value crops and precision agriculture. Their differences in equipment, reaction conditions, and post-processing define their respective applications in agricultural settings.