TML Rheological Testing: Determination of the Transportable Moisture Limit for Bulk Mineral Cargoes

TML rheological testing determines the Transportable Moisture Limit of fine, moisture-sensitive mineral cargoes and characterizes how their flow behavior changes with water content. By combining standardized flow moisture point methods with modern rheometry, the test provides the critical data shippers need to prevent cargo liquefaction at sea. This article explains the principles, sample types, tested parameters, procedures, and practical applications of TML rheological testing under the IMSBC Code framework.

Technical Overview

TML stands for Transportable Moisture Limit, the maximum moisture content at which a solid bulk cargo may be safely loaded and carried by sea without the risk of liquefaction. The concept is defined in the International Maritime Solid Bulk Cargoes (IMSBC) Code, which classifies moisture-sensitive cargoes as Group A materials. Under the Code, the certified TML is taken as 90 percent of the Flow Moisture Point (FMP), the moisture content at which a test specimen first develops flow behavior under dynamic loading in a standardized apparatus.

Liquefaction is fundamentally a rheological phenomenon. Fine mineral cargoes normally behave as frictional, granular solids whose shear strength is provided by interparticle contacts. When the moisture content is sufficiently high, cyclic forces generated by ship rolling, pitching, and slamming cause pore water pressure to rise, effective stress to collapse, and the material to transform into a viscous slurry capable of shifting inside the hold. This shift can produce a sudden list and, in severe cases, the capsizing and loss of the vessel. TML rheological testing quantifies this transition by tracking parameters such as yield stress and viscosity as a function of moisture content, providing both a regulatory certification value and a scientific description of the cargo's flow behavior.

Testing Samples

TML rheological testing applies to any fine-grained bulk material that may liquefy during maritime transport. Typical sample categories submitted for testing include:

• Iron ore fines and iron ore concentrates, including sinter feed and pellet feed
• Nickel laterite ore and nickel sulfide concentrates, historically associated with liquefaction incidents
• Copper, lead, zinc, and other base metal sulfide concentrates shipped as filter cake
• Bauxite and other alumina-bearing ores classified as Group A cargoes
• Mineral sands such as ilmenite, rutile, and zircon concentrates
• Fine coal, coal slurry, and coal-water mixtures
• Iron and steel plant residues such as mill scale, filter dust, and sludges
• Fly ash, limestone fines, and other fine industrial minerals shipped in bulk

Sample Requirements

Representative sampling is essential because the FMP depends strongly on mineralogy and particle size distribution. Samples are typically taken from stockpiles, conveyor belts, or cargo holds using recognized sampling procedures, with a combined mass of approximately 10 to 20 kilograms sufficient for a full determination. All samples must be sealed in airtight containers immediately after sampling to prevent moisture loss or gain during transport to the laboratory.

Sample Condition Assessment

Before testing, the laboratory records cargo description, origin, nominal particle size, and visual condition. Materials that have already arrived wet, slumped, or with free-standing water on the surface are flagged immediately, since visible moisture migration in the hold is itself a warning sign that the cargo may be at or above its TML.

Testing Items

A complete TML rheological test program typically includes the following determinations:

• Moisture content of the as-received sample, determined by oven drying to constant mass
• Flow Moisture Point (FMP), the moisture content at which flow behavior first appears under dynamic test conditions
• Transportable Moisture Limit (TML), calculated as 90 percent of the FMP
• Apparent viscosity and flow curve behavior across a range of shear rates
• Yield stress, determined by model fitting (Bingham or Herschel-Bulkley) or by direct vane measurement
• Shear-thinning or shear-thickening behavior and degree of thixotropy
• Moisture sweep profiles showing the decline of yield strength as water content increases
• Supporting index properties such as particle size distribution and specific gravity where required

Moisture Content Determination

Moisture content is the baseline measurement for every determination and the value that appears on the shipper's declaration. It is established gravimetrically by drying representative sub-samples at standardized temperatures, following procedures equivalent to ASTM D2216 or the iron ore moisture standard ISO 3087.

Flow Moisture Point and TML

The FMP is bracketed experimentally by testing a series of specimens at incrementally increasing moisture contents until flow is observed under the standardized dynamic loading of the chosen apparatus. The TML is then calculated as 90 percent of the interpolated FMP, providing a margin that accounts for sample heterogeneity, testing uncertainty, and moisture redistribution within the hold during voyage.

Rheological Characterization

Beyond the certification value, rheological characterization builds a moisture-yield stress curve for the cargo. As water content rises toward the FMP, the yield stress drops sharply and approaches zero, marking the loss of the load-bearing granular skeleton. This curve allows engineers to assess how far a cargo sits from liquefaction under realistic moisture variability, rather than relying on a single pass-fail number.

Testing Methods

The IMSBC Code recognizes three reference procedures for determining the FMP and TML, and modern laboratories supplement these with instrumented rheometry for deeper insight into flow behavior.

Sample Preparation

Regardless of the method, the received material is homogenized, split into test portions, and adjusted to a sequence of target moisture contents by careful drying or spraying and mixing. Each moistened portion is allowed to equilibrate, typically overnight, so that water is fully distributed within the pore structure before testing.

Flow Table Method

The flow table method, described in the IMSBC Code and in ISO 12742 for copper, lead, zinc, and nickel sulfide concentrates, compacts a specimen into a truncated conical mold on a mechanically driven flow table. The table is then subjected to a standardized sequence of drops, and the specimen is examined for plastic deformation or flow. Testing at progressively higher moisture contents brackets the FMP, from which the TML is derived. The method is best suited to fine to medium-grained concentrates and ores.

Penetration Test Method

The penetration method uses a cone penetrometer to measure the depth of penetration into a prepared specimen after standardized compaction and dynamic conditioning. Deep, rapid penetration indicates soft, near-flowing behavior. The FMP is identified from the relationship between penetration depth and moisture content, and this method is particularly suitable for very fine materials and filter cakes where surface flow is difficult to judge visually.

Proctor/Fagerberg Method

The Proctor/Fagerberg method applies controlled compaction energy to specimens at a series of moisture contents and interprets the resulting compaction behavior to locate the flow point. It is designed for coarser or less cohesive cargoes where mold slump on a flow table is not a reliable indicator, and it provides a robust alternative for certain ores and industrial minerals.

Rheometer-Based Protocols

Instrumented rheometry uses rotational rheometers equipped with vane or helical impeller geometries that minimize wall slip in dense, particle-laden pastes. Stress-controlled ramps and steady shear steps generate flow curves from which yield stress and plastic viscosity are extracted, while moisture sweeps map the strength loss curve approaching the FMP. Cyclic loading routines can simulate the repeated stress reversals experienced in a seaway. These data complement the certified reference method results and support engineering assessment of margins, drainage behavior, and stabilization measures.

Applications

TML rheological testing results are used across the mining, shipping, and regulatory communities:

• Shipper documentation for Group A cargoes, including the TML certificate and moisture declaration required before loading
• Pre-shipment inspection and cargo surveying by independent inspection agencies and P&I club surveyors
• Port state control verification and acceptance decisions at load and discharge ports
• Concentrator and dewatering process optimization, including thickener, filter press, and disc filter performance checks
• Stockpile management, covering drainage, tarping, and re-testing schedules during wet-season storage
• Assessment of new or unscheduled cargoes that lack a complete IMSBC individual schedule
• Incident investigation, expert witness work, and resolution of trade disputes following suspected liquefaction events
• Research into liquefaction mitigation, such as binder or lime addition and controlled moisture targets

FAQ

What is the difference between FMP and TML?

The Flow Moisture Point is the experimentally determined moisture content at which the cargo first develops flow under the dynamic loading of the test apparatus. The TML is a regulatory safety value set at 90 percent of the FMP, and cargo may not be loaded for sea transport when its actual moisture content exceeds this limit.

Which cargoes require TML certification?

Any cargo classified as Group A under the IMSBC Code, meaning materials containing a proportion of fine particles and moisture sufficient to present a liquefaction hazard, requires a TML certificate and a moisture declaration prior to loading. Common examples include iron ore fines, nickel ore, and base metal concentrates.

Is TML a fixed property of the material?

No. The FMP and TML depend on mineralogy, particle size distribution, and the degree of compaction. When the source ore, beneficiation route, or product specification changes significantly, the TML must be re-established. Best practice also calls for periodic re-testing even for consistent products to confirm the certified value remains valid.

How much sample is needed and how long does testing take?

A full determination normally requires approximately 10 to 20 kilograms of representative material, sealed against moisture change. Because testing involves multiple moisture increments and equilibration periods, typical turnaround is three to seven working days, with expedited programs available when loading schedules demand faster results.

Can rheometer testing replace the standardized methods?

For official certification and shipping documents, the recognized IMSBC reference methods, flow table, penetration, or Proctor/Fagerberg, remain the basis of the declared TML. Rheometry is used as a powerful complement that quantifies yield stress, viscosity, and the strength-loss curve, providing engineering insight and an early-warning picture of how close a cargo is to liquefaction.

What happens if cargo moisture exceeds the TML?

The cargo must not be loaded. Loading moisture-sensitive cargo above its TML exposes the ship to liquefaction, cargo shift, structural damage, and potential capsizing, and it exposes the shipper to criminal and civil liability under international maritime regulations.