Fertiliser mixing operation:\nIn ammonium nitrate fertiliser production, molten ammonium nitrate is mixed with ground limestone (or dolomite) using which mixer?

Introduction / Context:
Neutralising or conditioning molten ammonium nitrate with finely ground mineral additives produces fertiliser grades with improved handling and reduced caking. The operation demands robust mixing of a paste-like mass at moderate temperatures and high throughput, making selection of the mixer critical for product quality and safety.

Given Data / Assumptions:

• Molten ammonium nitrate is blended with ground limestone.
• Continuous, intensive mixing of a viscous/pasty mixture is needed.

Concept / Approach:
Pug mills feature twin contra-rotating shafts with paddles that fold and knead the mass while conveying it forward, ideal for sticky or pasty mixtures such as fertiliser conditioning, clay tempering, or moist agglomerates. Banbury mixers are batch kneaders for polymers; muller mixers (pan with heavy wheels) are used in foundry sand preparation; mixer-extruders suit some plastics but are less common for fertilisers in this specific service.

Step-by-Step Solution:

Verification / Alternative check:
Fertiliser process descriptions routinely show pug mills for conditioning ammonium nitrate with fillers/limestone before prilling or granulation.

Why Other Options Are Wrong:

• Banbury/planetary: batch, high-shear lab or polymer service.
• Muller: suited to foundry sands; different mechanics.
• Mixer-extruder: not the standard for this fertiliser step.

Common Pitfalls:
Insufficient mixing time or poor residence control in the pug mill can lead to segregation or hot spots; residence time distribution tuning is essential.

Final Answer:
Pug mill