Why mineral fertilisers cake in railcars

A wagon of fertiliser arrives at the terminal, the hatches open — and the cargo stands there as one solid block. This is neither rare nor a defect: practically every type of bulk mineral fertiliser — ammonium nitrate, NPK, potash, urea — is prone to caking, even when treated with special anti-caking agents.

What happens in transit

The longer the cargo travels in the wagon, the damper it gets. Hygroscopic fertilisers draw moisture from the air, that moisture redistributes through the mass and binds the particles together. In winter frost is added to this: the moisture freezes slightly and the cargo becomes hard to discharge.

A journey also means a daily temperature swing. By day the steel body heats up, by night it cools faster than the cargo itself; moisture-laden air in the pores of the mass meets the cold wall and gives up condensate. Shipping has known this for a long time as cargo sweat: the West of England P&I Club tells masters to compare the dew point inside the hold with the dew point outside and to ventilate only when the outside air is drier — or, with no instruments, to apply the three-degree rule. A railcar has no ventilation at all, so the moisture has nowhere to go.

Critical humidity: why dry weather is no guarantee

Every fertiliser has a critical relative humidity (CRH) — the threshold above which the product starts drawing water from the air in earnest. For urea it is around 73 % at 30 °C. But caking starts earlier: according to the International Fertiliser Society the risk zone runs roughly ten percentage points below the CRH. The air still counts as normal while bridges between granules are already growing.

For mixtures the threshold collapses. A urea and ammonium nitrate mixture has a CRH of only about 18 % at 30 °C — such products must not share one storage volume, or they will go soft in almost any weather.

A monolith forms in hours, not months

The mechanism is crystal bridging. Moisture is drawn to the contact points between granules by capillary and thermal forces, dissolves salt there, then evaporates and leaves a solid bridge behind. In laboratory tests on 1.6 mm urea granules at 70 % humidity and 30 °C the bridges grew from an almost invisible contact to 0.9–1.25 mm within a few hours of storage.

So a monolith in a wagon does not mean the cargo has been travelling for half a year. A few days with the wrong combination of moisture and temperature are enough.

Why anti-caking agents do not close the case

Treatment reduces the tendency but does not repeal the physics. The International Fertiliser Society names the key factor as not absolute humidity but the difference in moisture between particles and between batches: drier and damper material mixed in one wagon will inevitably exchange moisture. Add the pressure of the column above, the temperature gradient between the core of the mass and the wall, dust and fines — and you get exactly the picture the receiving terminal sees.

Potash adds particle shape to the list: angular crystals and a wide size distribution make it particularly prone to caking, and a crust builds on a stockpile from nothing more than the daily humidity cycle.

What it costs at unloading

With a railcar vibrator in place, a typical wagon takes up to 5 minutes including clean-up instead of 15–20 minutes of manual work. An extreme case from our practice: damp urea left standing in wagons — up to 10 hours of hard manual labour per wagon against about 30 minutes with the VH-500.

What is done about it

The vibrator delivers powerful low-frequency impacts — up to 550 J each — directly to the wagon frame. The hung-up mass and the ice crust break loose and discharge through the hatches. One operator runs the whole process, from the unloader body or from a remote control.

The honest limit

If cargo is frozen solid all the way through, vibration will not discharge it — thawing comes first. And on volumes: at 2–3 wagons a day professional equipment usually does not pay off, unless a single wagon takes 10 hours to clear.