Finishing step control in urea plants:\nUrea prilling should occur just above its melting point with minimal residence time; otherwise, the product quality deteriorates mainly due to ________.

Introduction / Context:
In urea finishing (prilling or granulation), melt temperature and residence time must be carefully controlled. If the urea melt is held too hot for too long before solidification, thermal condensation reactions proceed, forming biuret—an impurity with agronomic drawbacks. Understanding this linkage helps operators set prilling tower conditions and troubleshoot off-spec material.

Given Data / Assumptions:

• Prilling spray occurs just above urea’s melting point (~132–135 °C), ideally with short hold-up.
• Overheating or long residence promotes side reactions.
• We need the primary product-quality risk from such conditions.

Concept / Approach:
The key side reaction is 2 urea → biuret + NH3. Biuret formation increases with melt temperature and time, and higher biuret percentages can harm sensitive crops (e.g., in foliar feeding). Other defects like shape or stickiness can result from different operating issues (droplet formation, humidity), but the textbook risk tied to overly hot/long prilling is biuret growth.

Step-by-Step Solution:
Recognize thermal chemistry: prolonged hot melt → condensation to biuret.Relate to prilling control: keep temperature near melting point; minimize residence.Therefore, biuret formation is the principal deterioration mode.

Verification / Alternative check:
Quality control limits in fertilizer standards specify maximum biuret; operating manuals emphasize minimizing hot-hold times to control biuret content.

Why Other Options Are Wrong:
Low bulk density, non-sphericity, or stickiness relate to atomization, tower dynamics, and moisture—not specifically to thermal condensation; carbamate carryover is upstream, not a prilling melt-time effect.

Common Pitfalls:
Confusing product-shape issues with chemical-spec issues; the latter (biuret) is the critical thermal side product.

Final Answer:
biuret formation