Are non-combustible dusts like cement or limestone an ATEX concern?
Abstract
No. Cement, limestone, sand and similar mineral dusts cannot burn, so they cannot form an explosive atmosphere and need no dust zones. In cement plants the ATEX concern is the solid fuel (coal, petroleum coke, biomass) and any combustible additive, not the mineral product.
No, a dust that cannot burn cannot explode, so pure cement, limestone, sand, gypsum or clinker dust creates no explosive atmosphere and no ATEX zone. The ATEX concern in a cement or mineral plant comes from the combustible materials that are handled alongside the mineral: ground coal and petroleum coke for the kiln burner, biomass and other alternative fuels, and any organic additive or contaminant.
Short answer
- The EU workplace directive 1999/92/EC defines an explosive atmosphere as a mixture of air with flammable substances, including dusts. Materials that are already fully oxidised, such as limestone (calcium carbonate), cement clinker and sand, do not burn and fall outside it.
- The Irish Health and Safety Authority describes the outcome of the standard screening test the same way: a dust that does not propagate flame from the ignition source is classed as non-combustible.
- In cement plants, solid fuel grinding and firing (coal or petroleum coke mill, mill filter, pulverised fuel bins and conveying to the burner) and alternative fuel handling are the usual dust explosion areas.
- Mixing a combustible dust with inert mineral dust does reduce its explosibility, but only at high inert contents; it is not a protection you get by chance.
- Non-combustible dust still matters for health (respirable dust, crystalline silica in some minerals) and for ordinary dust protection of equipment, just not for ATEX.
Why a mineral dust cannot explode
A dust explosion is a fast combustion of particles in air. Combustion needs a material that can still react with oxygen and release heat. Limestone, chalk, cement, clinker, sand and most metal oxides are the products of combustion or of high-temperature processing; there is nothing left to burn. However fine the particles and however dense the cloud, no flame can propagate, which is exactly what the screening test checks: disperse the dust as a cloud around an ignition source in a test apparatus and see whether a flame spreads.
That conclusion holds for the pure material. It does not hold when the "mineral" contains a combustible fraction. If in doubt, a go/no-go screening test on a representative sample settles the question.
Where the ATEX hazards are in a cement plant
| Area | Material | Dust explosion hazard? |
|---|---|---|
| Quarry, crusher, raw mill, raw meal silos | Limestone, clay, shale, sand | No, unless combustible matter is present |
| Kiln, clinker cooler, cement mill, packing | Clinker, gypsum, cement | No |
| Solid fuel storage and mill, mill filter, fine fuel bins, dosing and burner feed | Ground coal, lignite, petroleum coke | Yes: assess, classify zones, protect mill and filter |
| Alternative fuel reception and feeding | Biomass, dried sewage sludge, shredded waste | Often yes; test the actual material |
| Fuel gas or oil burners | Natural gas, fuel oil | Gas and vapour zones, a separate assessment |
The fuel line is the classic case. A coal or petroleum coke mill, its filter and the fine fuel bins contain a combustible dust cloud in normal operation, often warm and dry, together with credible ignition sources such as hot drying gas, tramp metal and smouldering deposits. Coal appears on the list of combustible dusts of the US Occupational Safety and Health Administration (OSHA). These parts of the plant need area classification, Ex equipment and explosion protection, as described in What makes a dust explosive? and Dust zones 20, 21 and 22.
Can inert dust make a combustible dust safe?
Yes, but only in large proportions, which is why it is used deliberately in coal mines and not relied on by accident elsewhere. Work by the US National Institute for Occupational Safety and Health (NIOSH) on coal dust mixed with rock dust (finely ground limestone) found:
- little change in the minimum explosible concentration of the coal until the mixture contained more than about 50 % rock dust;
- explosion pressures falling rapidly between about 70 and 80 % rock dust;
- no ignition at about 75 % rock dust in its 20 litre laboratory chamber, in relatively good agreement with large-scale mine tests.
US mining law reflects this. 30 CFR 75.403 requires the combined coal dust, rock dust and other dust in underground coal mines to be at least 80 % incombustible, and more where methane is present. The explanation of the GESTIS-DUST-EX database gives examples of inert contents at which ignition was no longer possible in 1 m³ tests: 65 % by weight of calcium carbonate for one bituminous coal, 50 % of sodium hydrogen carbonate for a sugar. The required share depends on the dust, the inert material and their particle sizes.
For a cement plant this means: a coal mill filter does not become safe because some raw meal dust drifts into it. The fuel dust in the mill and filter is essentially pure fuel.
Example: A cement plant switches from coal to a mix of coal and dried sewage sludge pellets. The limestone crusher and cement mill still need no dust zones. The new sludge receiving hall, its conveyors and the shared fuel mill do: the operator has the sludge fines tested, extends the explosion protection document and checks that the mill's vents and isolation were sized for the dust with the highest Kst.
Common mistakes
- Classifying the whole cement plant as hazardous. Over-classification adds cost and hides the real risk areas.
- Assuming "it is a cement plant, so nothing burns". The fuel mill is a coal or petcoke dust plant inside it.
- Relying on mineral dust contamination for safety. Inerting needs high, controlled inert contents.
- Forgetting new fuels. Biomass and waste-derived fuels change the dust hazard and need testing.
Related on excodex
- What makes a dust explosive? Particle size, concentration and the dust explosion pentagon
- Dust zones 20, 21 and 22 explained with practical examples
- How do I know if my dust is combustible?
- Which dust test results do you need to select Ex equipment and size explosion vents?
Standards and further reading
- Directive 1999/92/EC, Article 2, definition of explosive atmosphere: legislation.gov.uk
- IEC 60079-10-2, classification of areas, explosive dust atmospheres.
- Health and Safety Authority (Ireland), ATEX delivery guide on dust, classification test: hsa.ie
- K. L. Cashdollar, Dust explosions overview, NIOSH (rock dust inerting of coal): stacks.cdc.gov (PDF)
- GESTIS-DUST-EX, explanation of the characteristics (inerting by inert solids): dguv.de (PDF)
- 30 CFR 75.403, maintenance of incombustible content of rock dust: ecfr.gov
- OSHA, combustible dust overview: osha.gov
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.