Salt (sodium chloride, NaCl) is a major dry bulk cargo, with global seaborne trade of approximately 20 to 30 million tonnes per year. The cargo serves three principal end-uses: road de-icing (the largest single use, with strong seasonal demand in cold-climate countries), chemical industry feedstock for chlorine and sodium hydroxide manufacture, and food and water-treatment industries. The IMSBC Code regulates salt shipments under a Group C schedule with handling requirements driven by hygroscopic caking and dust generation.
Schedule structure
The IMSBC Code includes the following salt schedule entries:
- Salt: bulk sodium chloride from solar, mined, or vacuum-evaporated sources. Group C (not liquefiable, no chemical hazard).
- Salt cake (UN 1994 in some classifications): by-product sodium sulphate. Different cargo, separate schedule.
- Salt rock: large lump rock salt from mined sources. Group C.
Most marine salt trade is the general “salt” entry covering road salt and industrial salt grades.
Cargo properties
Salt is supplied in three principal commercial grades reflecting source:
- Solar salt: produced by evaporation of seawater or brine in shallow ponds. Supplied as crystalline material with particle size typically 1 to 10 millimetres. The dominant source globally for road de-icing.
- Mined rock salt: produced from underground halite deposits. Supplied as crushed lump material with particle size 5 to 50 millimetres.
- Vacuum salt: produced by evaporation of mined brine in vacuum evaporators. Supplied as a fine pure white crystalline material for food and pharmaceutical use.
Bulk density of solar and mined rock salt is approximately 1.2 to 1.4 tonnes per cubic metre. Stowage factor is approximately 0.7 to 0.85 cubic metres per tonne.
Cargo handling concerns
Salt is non-toxic, non-flammable, and non-self-heating. The principal handling concerns are:
- Hygroscopic caking: salt absorbs atmospheric moisture and forms hard cakes if exposed to high humidity or rain ingress. Holds must be weather-tight, hatch covers must be intact, and bilge wells must be inspected and dried before loading. Caked salt is difficult to discharge and may require mechanical breakage at the destination port.
- Corrosion of steel: chloride is highly corrosive to steel. Cargo holds carrying salt should have intact paint coatings, and bilge wells should be inspected for corrosion damage. Significant accumulated salt residues from prior cargoes can accelerate hold corrosion.
- Dust generation: salt loading and discharge generate moderate dust loads, particularly for fine grades. Dust suppression is required at most modern terminals.
- Compatibility with subsequent cargoes: salt residues can contaminate subsequent grain, alumina, or food cargoes. Thorough hold cleaning between salt and other cargoes is essential.
Major routes
Salt seaborne trade flows include:
- Mexico to United States and Canada: the dominant North American flow, with Mexican solar salt from Guerrero Negro and other Pacific coast saltworks supplying US East Coast road de-icing markets.
- Australia to Northern Asia: significant exports of solar salt from Western Australia (Dampier, Port Hedland) to Japanese and Chinese chemical plants.
- India to Northeast Asia: solar salt from Gujarat saltworks to Japanese and Chinese markets.
- Egypt to European markets.
- Tunisia to European markets.
- Chile to North American markets.
The seasonal nature of de-icing demand creates significant trade pattern variation: large salt cargoes move from Mexico, the Caribbean, and Mediterranean producers to North American and Northern European markets in autumn and early winter to build pre-season stockpiles.
Loading and discharge
Loading is by shore conveyor and shiploader at major saltworks and mining export terminals. Loading rates of 1,500 to 4,000 tonnes per hour are typical. Hold preparation requires standard cleanliness, intact paint coatings, and weather-tightness.
Discharge is by grab-fitted shore cranes at receiving ports. Caked salt may require mechanical breakage with hand tools or compressed air vibrators before grab discharge can proceed efficiently.