When particle size changes the shipping classification

A phosphate rock case study in liquefaction risk, IMSBC classification and why particle size matters.

For some solid bulk cargoes, particle size can change how the cargo behaves during a voyage, whether it presents a liquefaction risk and, ultimately, how it must be classified and managed under the IMSBC Code.

That is exactly what happened with phosphate rock fines. A new dedicated IMSBC schedule now recognises the liquefaction risk presented by the fine, uncalcined material and applies the controls needed to reduce that risk.

When fines change the risk

Cargo liquefaction is one of the most serious hazards associated with the carriage of solid bulk cargoes. The recent tragic loss of life on a bulk carrier in the Bay of Bengal highlights the significant risks associated with bulk cargo liquefaction.

A cargo containing sufficient moisture can appear stable when loaded but lose strength under the compaction, vibration and motion of a voyage. If the cargo becomes unstable and shifts, the consequences for vessel stability can be catastrophic.

The IMSBC Code recognises that many fine-particle cargoes, when they contain sufficient moisture, may be liable to flow. Particle-size distribution, moisture condition and mineralogy can all materially affect how a solid bulk cargo behaves during transport.

For PHOSPHATE ROCK FINES (uncalcined) , particle size was central to the technical case.

PHOSPHATE ROCK FINES (uncalcined) : a new Group A schedule

Prior to Amendment 08-25 to the IMSBC Code, there were two existing individual schedules for Phosphate Rock ore in the IMSBC Code - once each for the calcined and uncalcined form.

In 2023, a submission for a new individual schedule for PHOSPHATE ROCK FINES (uncalcined) was submitted by Australia to the Sub-Committee on Carriage of Cargoes and Containers (CCC) 9. The proposal related to a crushed and washed form of phosphate rock with a higher proportion of finer particles (D50 of 0.3 to 0.35 mm) than the currently shipped cargoes, as testing confirmed that this cargo is liable to liquefy if carried with a moisture content exceeding its TML. 

Consequently, it meets the criteria for a Group A cargo, irrespective of any intention to ship it at a moisture content below its TML. A new group A individual schedule was therefore proposed for this cargo and was formally adopted into the IMSBC Code 2025 Edition (Amendment 08-25). This could be applied voluntarily from 1 January 2026, with an official entry into force on 1 January 2027.   

For the newly scheduled PHOSPHATE ROCK FINES (uncalcined), the fine particle size materially changes the cargo's liquefaction behavior. As a result, its hazard classification and carriage controls differ from those of the previously approved, larger PHOSPHATE ROCK (uncalcined) cargo (Group C).

Why the classification matters operationally

The distinction between Group A and Group C has significant practical consequences.

For PHOSPHATE ROCK FINES (uncalcined) under the new Group A schedule, the cargo may liquefy when its moisture content exceeds its TML.

For any Group A cargo, including Group A and B cargoes, the IMSBC Code requires the shipper to establish approved procedures for sampling, testing and controlling moisture content to ensure the cargo remains below its TML. In practice, shippers typically document and have these procedures approved as a Moisture Management Plan (MMP).

Specific requirements also cover TML determination, moisture-content testing and certification, and precautions for loading during precipitation.

These requirements reduce a serious safety risk. They also have operational and commercial consequences.

Testing takes time. Moisture has to be understood and managed. Rainfall can affect loading. Sampling, certification, and the MMP must be in place. These requirements can affect port planning, loading windows and the overall logistics of getting a cargo onto a vessel.

So getting the classification right matters.

A cargo with Group A properties needs the controls appropriate to that risk. Equally, the controls applied should reflect the actual characteristics and behaviour of the cargo being shipped.

The classification determines the controls, and those controls have real operational consequences.

The principle that particle size can affect IMSBC treatment is also reflected elsewhere in the Code. For example, particle-size distribution forms part of the criteria distinguishing BAUXITE and BAUXITE FINES and also  IRON ORE from IRON ORE FINES, together with mineralogical criteria, as outlined in Skuld's guidance on the IMSBC iron ore schedules.

Building the technical case

The new phosphate rock fines schedule is also an example of how the IMSBC Code can evolve when an existing schedule does not adequately address the behaviour of a cargo.

Proposals for a new cargo to be listed in the IMSBC Code require comprehensive technical evidence detailing the cargo's characteristics and hazards to justify the proposed classification and carriage requirements. This data is first scrutinised by the relevant national competent authority, which for Australia is AMSA, before being submitted to the CCC for further evaluation within the formal IMO technical review process. 

Dr Maria Davoren led the technical and scientific work for the client supporting the case for the new PHOSPHATE ROCK FINES (uncalcined) schedule.

We will cover that process in more detail in a future Insight, with a practical look at what is involved in developing the technical case for a new IMSBC schedule, including a proposed new DIRECT SHIPPING ORE SPODUMENE schedule.

The takeaway

For phosphate rock fines, particle size was material to the cargo's liquefaction behaviour. The technical evidence supported creating a dedicated Group A schedule, bringing necessary controls that have significant safety, operational, and commercial consequences. The underlying lesson is that cargo classification must accurately reflect the material being shipped; sometimes that means finding the right existing IMSBC schedule, and sometimes, as with these fines, evidence dictates that a new schedule is required.