THE DUSTS, THEIR EFFECTS
The main mission of RAM® Environnement is to fight against dust emanations which can be often dangerous. But in order to bring an efficient answer to the problem of dust, it’s necesary to know perfectly the various types of dusts, their characteristics and their effects on humain beings.
The various types of dust
In the mining and quarrying industry, dust can proceed from various sources: alluvial rocks, volcanic rocks, metamorphic rocks, chalky rocks or basalt. It may also come from iron or manganese ore, bauxite, coal, phosphates etc.
This dust may be classified into various categories: sporadic or fugitive, semi-permanent or permanent.
Sporadic or fugitive:These emissions are provoked by blasting in mines, unloading of tipper trucks and movement of fine particles of stored products into free air.
Semi-permanent: These emissions are mostly produced by drilling operations and movement of vehicles along roads, tracks and assigned routes.
Permanent: Covers all the emissions produced by grinding, crushing, handling, sifting and riddling installations.
In the food-processing industry, dust comes from products such as wheat, barley, maize, peas, manioc or sugar. Handling and storage operations can lead to formation of thick dust clouds in areas which are very often confined.
These dust types are particularly inflammable, and therefore dangerous.
The natural tendency of dust to free itself and reduce to slime is directly linked to:
- The nature of the material (dust is one of the finest parts of the material, whether it is an ore or a mineral)
- Moisture content, which will be more or less depending on whether the material naturally repels or attracts water.
- Shape, which may favour movement through the air and/or reduce sedimentation speed.
This table shows a number of important elements used in defining dust, in graphic form:
Analogy : lower than 1 micron, the particles are similar to smoke and exhaust emissions.
Appearance : ultra-microscopic or microscopic for particles lower than 10 microns. Only particles higher than 10 microns are visible to the naked eye.
Sedimentation : the speed of descent allows the risk of environmental pollution to be assessed according to the dimensions of the particles in question.
Wind action is very important. These effects lead dust particles to fly away or migrate to areas around installations.
This graph shows the suspension of a particle of mineral at a height of 15 metres, the average height of a standard storage situation, with laminar winds of 10 and 30 km/h. It will be noticed that according to their particular sedimentation speed, a 100-micron dust particle will touch down 150 metres away from its take-off point for a 10-km/h wind and 400 metres away for a 30-km/h wind.