Dust extinction moisture (DEM) is the moisture content at which a bulk material effectively stops generating dust. It is determined under Australian Standard AS 4156.6 using a rotating drum test: the DEM is the moisture level at which the sample’s Dust Number falls to 10, equivalent to just 0.01% of the sample mass becoming airborne dust.
DEM turns “is this coal dusty?” from an argument into a number. It tells an operation exactly how much moisture a specific product needs to travel, transfer and stockpile without dusting — and, just as importantly, how much added water is wasted beyond that point. This guide explains where the concept comes from, how the test works, why hitting DEM with the minimum water matters commercially, and how wetting agents shift the curve.
Where does the DEM concept come from?
DEM is an Australian invention, born from a practical coal-industry question: how much water is enough? The rotating drum method behind it was developed from Rio Tinto work examining dust emissions as a function of moisture content, building on test development by Farrugia and colleagues in 1989, and was formalised by Standards Australia as AS 4156.6-2000, Coal preparation — Determination of dust/moisture relationship for coal. The standard has been reconfirmed twice — in 2013 and again in 2023 — and remains current.
Two definitions carry the whole concept:
- Dust Number — a gravimetric measure of dustiness: the mass of dust collected from the sample during the drum test, scaled so that a Dust Number of 10 corresponds to a dust yield of 0.01% of sample mass.
- Dust extinction moisture (DEM) — the moisture content at which the Dust Number falls to 10. Below the DEM, dustiness climbs steeply; at or above it, the material is considered to emit no significant dust.
The standard was written for higher rank coals in the −6.3 mm size fraction, but testing laboratories such as TUNRA Bulk Solids at the University of Newcastle routinely adapt the procedure to other bulk materials — iron ore included — and to lump fractions, adjusting sample mass for bulk density. The purpose, in the standard’s own framing, is optimisation: enough moisture to extinguish dust, and no more, which conserves water and caps the total moisture of the product.
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How does the DEM test work?
The AS 4156.6 procedure is deliberately simple, which is why it has survived 25 years largely intact:
- Prepare the sample. A nominal 1 kg of −6.3 mm material is prepared and its initial total moisture measured.
- Adjust moisture in increments. Sub-samples are conditioned to a series of moisture contents spanning the expected curve — from visibly dusty through to visibly damp.
- Tumble each sub-sample in the rotating drum. The drum — fitted with internal lifters and rotated at a set speed for a fixed period, with air drawn through — repeatedly lifts and drops the material, liberating whatever dust that moisture level allows. The airborne dust is captured on a collector and weighed.
- Calculate the Dust Number at each moisture. The collected mass, scaled to the sample, gives a Dust Number per moisture content.
- Plot and read off the DEM. Dust Numbers are plotted against moisture on a log-linear chart, producing a steep descending curve. The moisture content where the curve crosses Dust Number 10 is the DEM. The worked example in the standard lands at a DEM of 8.8% — typical of the mid-to-high single digits most Australian coals return.
Two practical notes for anyone commissioning tests. First, reproducibility is good but not perfect — revision work on the standard reports variation of roughly ±0.2% on moisture and several per cent (relative) on Dust Number, so treat a DEM as a tight band, not a razor edge. Second, the standard includes a reagent-testing provision (more on this below): the same procedure can compare the material treated with a dust suppression reagent against water alone, which is how chemistry claims get separated from marketing.
Why not just add more water? The DEM sweet spot
If moisture extinguishes dust, drenching everything looks like the easy answer. It isn’t, because above the DEM every additional percentage point of moisture is pure cost:
- You’re freighting water. Moisture is dead weight in every wagon and every vessel. On export coal, each extra per cent of total moisture displaces payable product tonnes and can trigger contract moisture penalties — paying twice for the same water.
- Energy and quality penalties. Moisture suppresses the effective calorific value a buyer receives, and downstream users pay to drive it off. For some products, wet fines also mean handling and blending headaches at the receiving end.
- Handling problems arrive fast. Over-wet fines bridge in bins, hang up in transfer chutes, blind screens and cling to wagon bodies. The operational pain of too-wet material is as real as the compliance pain of too-dry.
- Shipping has a hard ceiling. Bulk cargoes are also governed by the transportable moisture limit (TML) regime for liquefaction risk — a reminder that moisture in bulk materials is managed within a window, not maximised.
So the operating target is a band: at or just above DEM, and well below the point where handling, quality and freight penalties bite. That is the genuine engineering challenge — holding a moving product, with weather, blending and transit drying all shifting its moisture, inside a window a percentage point or two wide. Which is exactly why the smarter question is not “how do we add more water?” but “how do we make the water we add count for more?”
Where DEM is used in practice
DEM began as a coal preparation tool and has become a reference point across the bulk materials chain:
- Train load-outs and rail transport. Coal chain dust management programs in NSW and Queensland use DEM as the benchmark for whether loaded coal will dust in transit — product moving at or above its DEM, combined with wagon profiling and veneering, is the accepted control package for rail corridor dust. (Editor: cite the current PWCS/ARTC and Aurizon coal dust management plan references here before publish.)
- Transfer chutes and conveyors. Facility designers use DEM data to specify dust controls at transfer points — a material running below DEM at a drop point will dust regardless of enclosure quality, so moisture conditioning and transfer design get engineered together. Testing houses report demand for DEM work growing precisely because “dust-free” chute, load-out and stockpile design depends on it.
- Stockpiles and ship loading. DEM informs how much moisture a product needs as it is stacked, reclaimed and loaded — and when a surface veneer is the better tool than bulk moisture, which is the stockpile equivalent of the rail decision.
- Exposure compliance. Keeping product above DEM through handling directly reduces the fine particle load workers breathe at bins, chutes and load points — the same fractions assessed against the respirable coal dust and silica limits.
The common thread: DEM converts dust control from a visual judgement (“looks dusty today”) into a specification that can sit in a contract, a licence condition or a design basis. Once it is in those documents, hitting it reliably — at lowest total moisture — becomes a daily operational task.
Reaching dust extinction with less water
Plain water is an inefficient dust suppressant for one physical reason: coal and many mineral fines are hydrophobic. Water’s high surface tension stops it spreading across and penetrating fine particles, so a meaningful share of added water sits as droplets and films that evaporate or drain without ever wetting the dust-generating fraction. The material’s measured DEM reflects that inefficiency — it is the moisture needed when water is doing the job badly.
This is where the standard’s reagent-testing provision earns its place. AS 4156.6 allows the drum test to be run on material treated with a dust suppression reagent, producing a treated dust/moisture curve to set against the water-only curve. A surfactant-based wetting agent such as GRT: Activate reduces water’s surface tension and “super-activates” it, so each litre wets more fines, faster. On the curve, that shows up as dust extinction behaviour achieved at lower added moisture — the practical effect operators see as the same dust result from substantially less water at the spray bar, or a far better dust result from the same water.
The operational wins stack in order of value:
- Product moisture stays lower for the same dust performance — fewer freight and quality penalties on every tonne.
- Water demand drops at the CHPP, load-out and transfer sprays — a licensed-allocation saving that compounds daily.
- Compliance margin widens at the points where people and monitors are: bins, chutes, load points and the rail corridor.
For materials and duties where added moisture must stay near zero — underground roadways, hardstands, or product that cannot tolerate wetting — the toolset shifts from wetting agents to binders and veneers, which we cover in the haul road guide and our stockpile veneering article. The DEM curve tells you which problem you have; the chemistry follows from that.
If you’re specifying a trial: ask the laboratory to run AS 4156.6 with and without the candidate reagent at matched moisture steps. The two curves, side by side, are the entire business case on one chart.
Frequently asked questions
What is dust extinction moisture? The moisture content at which a bulk material effectively stops emitting dust — defined under AS 4156.6 as the moisture at which the rotating drum test’s Dust Number falls to 10, equal to 0.01% of sample mass collected as dust.
What is a Dust Number? A gravimetric dustiness score from the AS 4156.6 drum test: the mass of airborne dust collected from a tumbled sample, scaled against sample mass. Plotted against moisture content, Dust Numbers form the curve from which the DEM is read.
What is a typical DEM for coal? It varies by coal, which is the point of testing — the worked example in the standard itself shows a DEM of 8.8%. Fines content, particle size distribution and surface chemistry all shift the curve, so each product needs its own number.
Does DEM apply to materials other than coal? The standard was written for higher rank coal fines, but laboratories routinely adapt the method to iron ore and other bulk materials, adjusting sample mass for bulk density. The dust/moisture relationship it measures is universal to fine bulk solids.
Is more moisture always better for dust control? No. Above the DEM, extra moisture adds freight cost, quality and calorific penalties, and handling problems in bins and chutes — while shipping moisture limits cap it anyway. The target is a band at or just above DEM, not maximum wetness.
How do wetting agents change the DEM? Surfactant wetting agents cut water’s surface tension so it wets hydrophobic fines it would otherwise bead off. Tested under the standard’s reagent provision, treated material reaches dust extinction behaviour at lower added moisture than water alone — the same control from less water.
Sources
- Standards Australia — AS 4156.6-2000 Coal preparation: Determination of dust/moisture relationship for coal (reconfirmed 2013, 2023) — definitions, scope, dust number 10 example
- Australasian Mine Safety Journal — Settling the dust: how much is too much? — Rio Tinto origins, test specifications, DEM = 8.8% worked example, EN 15051 comparison
- Revision of AS 4156.6 (ACPS conference paper, ResearchGate) — dust number definition (0.01% yield), reproducibility figures, Farrugia et al. lineage
- TUNRA Bulk Solids — Dust (environmental) testing services and new DEM testing equipment commissioning — adaptation beyond coal, demand growth, design applications
- NIOSH NMAM Chapter AD — Quantification of airborne dusts from powders — AS 4156.6 apparatus parameters and international context
