Particles dust clouds

Many finely divided metal powders in suspension in air are potential e] losion hazards, and causes for ignition of such dust clouds are numerous [Hartmann and Greenwald, Min. MetalL, 26, 331 (1945)]. Concentration of the dust in air and its particle size are important fac tors that determine explosibility. Below a lower Umit of concentration, no explosion can result because the heat of combustion is insufficient to propagate it. Above a maximum limiting concentration, an explosion cannot be produced because insufficient oxygen is available. The finer the particles, the more easily is ignition accomplished and the more rapid is the rate of combustion. This is illustrated in Fig. 20-7. 

The standard unit normally used for measuring dust particles is the micron (pm one-thousandth of a millimeter). The smallest particle visible to the unaided eye is between 50 and 100 pm and the most dangerous sizes are between 0.2 and 5 pm. Particles larger than this are usually unable to penetrate the lung defenses and smaller ones settle out too slowly. Some dusts can be both toxic and fibrous (e.g. asbestos) and are therefore harmful even outside these parameters. It may therefore be assumed that dusts which are visible (i.e. between 50 and 100 pm), are quite safe. However, this is not the case, as dust clouds never consist solely of particles of one size. Analysis would show percentages of all sizes, and it is for this reason that special care is needed in measuring dust clouds and concentrations. 

A dust cloud comprising a distribution of particle sizes soon fractionates, e.g. visible matter settles to the ground in a few minutes. Hence the size distribution of airborne particles may differ significantly from that of the source material. (This is particularly relevant to occupational hygiene measurements involving toxic dust emissions.)... 

The finely powdered silicide is a significant dust explosion hazard [1]. The lower explosion limit for a calcium-silicon dust cloud of mean particle size 9.7 pm was measured as 79 g/m3, in good agreement with a calculated value [2], Other dust cloud parameters are presented and related to predictions [3],... 

According to present-day concepts, our solar system was formed from a huge gas-dust cloud several light years across in a side arm of the Milky Way. The particle density of this interstellar material was very low, perhaps 108-1010 particles or molecules per cubic metre, i.e., it formed a vacuum so extreme that it can still not be achieved in the laboratory. The material consisted mainly of hydrogen and helium with traces of other elements. The temperature of the system has been estimated as 15 K. 

The extinction curve of light which has passed through dust clouds tells us which particles are present in the cosmic dust ... 

Pulmonary agents have been absorbed into porous powders (e.g., pumice) and disseminated as dust clouds. The agents are slowly released by the dust particles thereby greatly increasing the persistency of the agents. 

Mists are dust clouds in which the particles happen to be liquid. Should that liquid be combustible, even though it is nowhere near its flash-point, explosion is possible. 

The flammability and explosivity of high-sulfur petroleum coke dust (particle size <75 pm) were examined. Air-dried powder was non-explosive but fire-prone above 400°C. A 5 mm layer became incandescent at 420-470° and a dust cloud ignited at 520-660°C [1]. The fire and explosion hazards of petroleum coke or anthracite, when used in the manufacture of furnace electrodes, may be reduced by heat treatment [2],... 

At the other end of the spectrum, many very fine powders are frequently to difficult handle, and may also give rise to hazardous dust clouds when they are transported. It may therefore be necessary to increase the particle size. Examples of size enlargement processes include granulation for the preparation of fertilisers, and compaction using compressive forces to form the tablets required for the administration of pharmaceuticals. 

An interstellar dust cloud containing aligned particles may be looked upon as a linearly birefringent (and possibly linearly dichroic) medium (van de Hulst, 1957, p. 58) the cloud acts like a retarder. We showed at the end of Section 2.11 that linearly polarized light becomes circularly polarized upon transmission by a retarder. The first clear evidence for this kind of polarization mechanism was reported by Martin (1972), where light from the Crab Nebula was the source of linearly polarized incident light. 

In 2004, the NASA Stardust spacecraft passed through the dust cloud surrounding the nucleus of comet Wild2 and captured more than 10 000 particles ranging from 1 to 300 pm in size (Brownlee et al., 2006). These particles were returned to Earth for study in terrestrial laboratories in 2006. Initial results are described in the December 15, 2006, issue of Science. 

Static Electrification of Dust Particles. Kunkel (l Y) has made an extensive study of the charge and size distribution of particles ranging from 0.5- to 30-micron radius in dust clouds in air and has investigated both calm and turbulent conditions. 

The finely powdered silicide is a significant dust explosion hazard [1]. The lower explosion limit for a calcium—silicon dust cloud of mean particle size 9.7 pm was... 

Mists are dust clouds in which the particles happen to be liquid. Should that liquid be combustible, even though it is nowhere near its flash-point, explosion is possible [1] [2]. Mist explosions attract increasing study [3]. It is possible that many vapour cloud explosions have had a mist component. The editor surmises that, under appropriate circumstances, evaporation of volatile mist by the heat of a vapour (or mist) explosion might generate a larger pressure pulse than simple thermobaric effects on air. Foams are inverse mists and should show similar explosive potential. 

It is reasonable to consider the assumption that life began, somehow, among one of the mixtures of small organic molecules that are produced by abiotic processes. The only natural examples in hand today are the components of meteorites that have fallen to Earth (see Section 5.2.1) and particles returned by the Stardust mission. Spectroscopy has also yielded partial lists of the organic molecules in interstellar space and interplanetary dust clouds. 

This chapter deals with the motion of masses of particles over extended terrain. Thus far we have considered what may be termed transport, in channels and pipes (Chapter 18). The more general problem of dust cloud movement and formation forms the discussion of the present chapter. 

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