Salt

Every salt handling problem traces back to water or to steel.

Caking and the critical humidity

Salt holds its condition below its critical relative humidity and takes up moisture above it.

The damage happens in the cycle: moisture dissolves crystal surfaces, drying recrystallises them, and the bridges that form between grains are solid.

• Dehumidified silo venting air rather than ambient breathing.
• Short storage times for fine grades, which cake fastest.
• Anti-caking agents where the product specification allows them.

Corrosion

Carbon steel in contact with damp salt corrodes quickly, and the rust is both a maintenance problem and a contamination source in food grades.

Stainless grades, polymer liners and non-metallic chutes are chosen for this reason, and fasteners and supports deserve the same attention as the contact surface.

Abrasion

Crystalline salt wears bends, screw flights and valve seats, particularly in dilute phase conveying at high velocity.

Lower velocity and protected impact zones extend life more than simply specifying a harder alloy.

Food and de-icing grades differ

Food-grade handling adds hygienic design, allergen-free cleaning and traceability, while de-icing salt is coarse, wet and handled outdoors with the corrosion problem multiplied.

What to establish before buying equipment

• Grade, particle size and anti-caking treatment.
• Humidity range inside the storage, not outside the building.
• Material of construction for every contact part including supports.
• Conveying velocity and the resulting wear expectation.

Salt is the textbook hygroscopic material: it takes water from the air and then sets hard.

Above a certain relative humidity it begins to absorb, and the dissolved surface recrystallises into a solid bridge between particles when it dries again. A silo that cycles through humid and dry air builds a crust layer by layer.

• Seal the silo and condition the vent air; this is the single most effective measure.
• Expect strong time consolidation and size the outlet for the stood condition.
• Anticaking additives help and do not replace a dry environment.

Corrosion and caking drive the material and the geometry.

Salt attacks ordinary steel, and damp salt attacks it quickly. Stainless or lined equipment is the norm, and so are geometries without ledges where a crust can start.

• Corrosion-resistant materials throughout, including fasteners and supports.
• Smooth, ledge-free transitions; a crust always starts where material can rest.
• Dry conveying air, specified with a dew point rather than described as dry.

Salt is not combustible and not toxic, so the issues are corrosion, housekeeping and, in food plants, hygiene.

• Dust capture at transfer points; salt dust corrodes the building as well as the equipment.
• Eye protection where handled open.
• Food-grade materials and cleanable design where it is a food ingredient.

The long-term hazard is structural: salt-laden dust that settles on steelwork quietly corrodes it for years.