Sulfur-coated urea (SCU), a type of inorganic coated slow-release fertilizer, utilizes sulfur as the primary coating material to encapsulate granular urea, achieving controlled nitrogen release. Its production process involves the following key steps:
1. Raw Material Preparation

• Urea Granules: Particles with a diameter of 1.00-5.60 mm (≥90% compliance) are selected and preheated in a fluidized bed to 55-75°C to soften the surface for enhanced coating adhesion.
• Sulfur: High-purity sulfur (≥99.8%) is melted at 130-155°C under 6.9-10.5 MPa pressure for spray coating.
• Sealants: Molten paraffin, polyethylene wax, or heavy oil is used to seal sulfur membrane pores, preventing initial nitrogen burst release.
• Conditioning Agents: Diatomaceous earth or talc (1.8% by weight) is added to improve particle flowability and adjust membrane porosity.

2. Sulfur Coating Process

• Spray Coating: In a rotating drum or pan coater, molten sulfur is sprayed onto preheated urea granules, forming a 50-150 μm thick sulfur membrane.
• Sealing Treatment: A 3% paraffin-coal tar mixture or polyethylene wax is applied at the drum exit to seal membrane cracks.

3. Post-Treatment

• Conditioning Agent Application: Diatomaceous earth is coated to reduce particle stickiness and improve handling.
• Screening: Double screening ensures particle uniformity (≥90% within 1.00-4.75 mm or 3.35-5.60 mm ranges).

4. Quality Control

• Nutrient Content: Total nitrogen (32-37% for SCU, varying by type) and sulfur (15-30%) levels are strictly controlled.
• Release Performance:
  • 1-day nitrogen release: ≤40% (Type I), ≤30% (Type II).
  • 7-day nitrogen release: ≤60% (Type I), ≤45% (Type II).
• Physical Properties: Moisture content ≤1%, biuret ≤1.2%, and particle hardness ≥30 N.

5. Packaging & Storage

• Products are sealed in moisture-proof bags to prevent caking and degradation.

Technical Advancements

• Condensation Coating: A centrifugal process reduces coating time to 1 second, achieving sulfur layers of 20-50%.
• Multi-Layer Membranes: Combining sulfur, waxes, and polymers optimizes release rates.

This process ensures SCU provides sustained nitrogen supply (80-120 days) while supplementing sulfur, aligning with crop demand cycles and enhancing fertilizer efficiency by 30% compared to conventional urea.