Safety in fertilizer finishing: Why are ammonium nitrate prills or granules commonly coated with limestone powder or similar inert materials?

Introduction:
Ammonium nitrate (AN) is a powerful oxidizer and, under certain conditions, can detonate, especially when contaminated or confined. Finishing steps such as coating with limestone (calcium carbonate) improve the product’s safety and handling characteristics in agriculture.

Given Data / Assumptions:

• AN is hygroscopic and prone to caking.
• Thermal and impact sensitivity are significant safety concerns.
• Limestone coating is inert and compatible with fertilizer use.

Concept / Approach:
Coating AN with finely divided limestone dilutes the reactive surface, reduces dust, decreases hygroscopicity, and lowers the risk of transition to explosive behavior by absorbing heat and impurities on contact. The coating also helps prevent prill coalescence and reduces sensitivity to shock and friction, improving storage safety.

Step-by-Step Solution:
Recognize AN’s inherent oxidizing hazard and sensitivity.Note that an inert mineral coating reduces risk and improves stability.Select the purpose most aligned with safety: avoiding the risk of explosion.

Verification / Alternative check:
Safety codes and fertilizer practice documents recommend inert coatings and controlled additives to render AN safer (e.g., CAN grades contain limestone/dolomite to moderate sensitivity).

Why Other Options Are Wrong:

• Increasing nitrogen content: Coating dilutes N fraction slightly; it does not increase it.
• Cutting production cost: Coating adds a step and material; cost is not the driver.
• Adding extra nutrient: Limestone supplies Ca but the intention here is safety, not primary nutrition.
• Cosmetic color: Incidental; the core driver is safety.

Common Pitfalls:
Assuming coatings are only for caking control; while that is helpful, the principal motivation is risk mitigation for storage and transport.

Final Answer:
avoid the risk of explosion.