Fertiliser handling: caking, hygroscopic behaviour and equipment

Fertiliser leaves the plant as round, dry, free-flowing granules or prills, and it can arrive at the farm as a single lump the size of a truck body. Nothing mechanical happened in between; the product simply spent time in air that was slightly too damp, under a pile that was slightly too high, for slightly too long. Urea, ammonium-nitrate-based products, NPK compounds, potash and phosphates all do this to different degrees, and they all corrode the equipment that handles them while they do it. What follows is the physics, then the handling decisions.

Which products, and what do they share?

Products

Urea, ammonium-nitrate-based grades, NPK compounds and blends, potash, phosphates, ammonium sulphate — granules or prills of a few millimetres.

Hygroscopicity

All are soluble salts. Urea and ammonium-nitrate-based products are the most sensitive to humid air, potash and phosphates less so; a blend is worse than either component.

Bulk density

Indicatively 0.7–1.2 t/m³ depending on product and granule size; measure the grade actually stored.

Strength

Granules are friable; every drop, screw and elevator head breaks some into fines, and fines cake faster.

Corrosion

Damp product attacks carbon steel, galvanised steel and many alloys; chlorides and ammonium salts are the usual culprits.

Temperature

Warm product stored without cooling migrates moisture and cakes; some products also change crystal form with temperature.

Safety

Ammonium-nitrate-based products carry storage and handling rules of their own, which sit above anything in this note.

What is critical relative humidity, and why does it matter so much?

Every soluble salt has a relative humidity above which it absorbs water from the air and below which it does not: its critical relative humidity, or CRH. Below the CRH a fertiliser stays dry indefinitely. Above it, the surface of each granule dissolves into a film of saturated solution and the product becomes damp, soft and sticky. The CRH falls as temperature rises, so warm humid weather is the worst case, and it falls further for mixtures: a blend of two products has a lower CRH than either alone, which is why a bulk blend of robust components can cake in a store where each component was fine. The CRH of a given product is a measured property; treat published figures as indicative, and the note on measuring material properties as the method.

How does a granule become a lump?

1. Moisture cycling — humidity rises above the CRH and the granule surfaces dissolve; humidity falls and the solution re-crystallises where granules touch, leaving a salt bridge. Each cycle grows the bridges.
2. Pressure — under the weight of a pile the granules press together, contact areas grow and bridges form faster; the bottom of a high pile cakes first.
3. Time — bridges strengthen with age, and moisture migration through a pile continues long after the air outside has dried.
4. Fines — broken granules fill the gaps between whole ones and multiply the contact points.
5. Temperature swings — solubility changes with temperature, so a day-night cycle in an uninsulated store dissolves and re-crystallises salt even at constant humidity.

How is caking prevented in practice?

• Conditioned storage — dehumidification or at least a closed building, doors shut, and the humidity actually logged.
• Cool product in — warm product from production is cooled before storage; warm product in a cold store drives moisture into the pile.
• Anti-caking treatment — coatings applied at production reduce surface dissolution and bridge growth; a supplement, not a substitute.
• First in, first out — a store or silo that discharges in sequence, so nothing sits for months; the pillar note on flow patterns explains why funnel flow leaves the oldest product at the walls.
• Limit pile height and storage time — both are set by product, season and experience; time is usually the cheaper one to change.
• Enclosed transfers — humid air, rain and dust all enter at open transfers; the transfer point note describes the sealing.
• Keep incompatible products apart — blends and cross-contamination lower the CRH.

Which equipment handles fertiliser well?

The equipment has two jobs: move the product without breaking it, and survive it. Gentle handling keeps fines and dust down, which slows caking and protects the people in the store; survival is a materials and geometry question.

Washable design matters more than any single alloy. Fertiliser residue left on a surface is hygroscopic: it draws moisture from the air, stays wet and corrodes the steel underneath while everything around it looks dry. Equipment that can be hosed down, drains completely and has no pockets where product collects will outlast equipment made of a better material with worse geometry.

What to check on an existing fertiliser store

• Humidity and temperature — whether the store's air is logged, and how often it is above the CRH of what is inside.
• Product temperature at intake — warm product into storage is the beginning of most caking.
• Pile age and height — which piles have been there longest, and whether the reclaim actually takes them first.
• Lumps at reclaim — how many, from where, and whether a lump breaker or screen is dealing with them.
• Fines — where they are made: drop heights, elevator heads, screws.
• Corrosion — idler frames, belt structure, elevator casings, cable trays and electrical enclosures; worst where residue sits.
• Residue — chutes, elevator boots, screw troughs, screen decks; whether the wash-down routine exists and is followed.
• Belt condition — cover softening or cracking from anti-caking coatings, and tracking on a damp belt.

A fertiliser store is a piece of process equipment, not a shed, and it repays being treated as one. Our storage and flow control work covers conditioned stores, silos, reclaim and the enclosed transfers between them; the fertiliser duties in the Flow Finder list the equipment types involved.