Coal dust, detonations

Powder Explosions. See under Coal Dust Explosion Hazards from its Uses in Vol 3, C359-R Dust Explosions in Vol 5, D1578-R and Powder-Air Explosions and Detonations in this Vol... 

The theoretical calculation of the detonation temperature as a safety criterion was adopted only in France. In all other countries (and more recently also in France) a practical criterion has been introduced based on an experimental evaluation of the effect of explosion of a sample of the explosive on a mixture of air and methane, or on a suspension of coal-dust in air, under conditions similar to those existing in mines. For this purpose testing galleries were devised, simulating the conditions of mine galleries. 

Cybulski established that penetration of coal-dust into the explosive near the detonator is most dangerous, creating conditions particularly favourable to deflagration. 

A partial misfire may have various causes. For instance, the charge may be contaminated by coal-dust, as described above. The incomplete detonation of hardened cartridges of ammonium nitrate explosive occurs widely. It is obvious that the incomplete detonation of damp explosives is also possible. 

Sulphur dust is more dangerous than coal-dust, because of the low ignition temperature of sulphur suspensions in air. According to Dubnov [53] 100 g charges of the U.S.S.R. explosives Ammonit No. 1 and 8 ignited sulphur dust. The same explosives did not ignite a methane-air mixture when the quantities were 400 and 500-650 g respectively. In sulphur mines explosives of very low detonation temperature should be used. 

The use of sheaths very slightly weakens the strength of an explosive but does not reduce its sensitiveness to detonation. On the other hand it considerably increases its safety for use in mines with methane and sensitive coal-dust. 

Method of testing in the presence of coal-dust. Testing in the presence of coal-dust is carried out in an experimental gallery without a diaphragm. The lay-out of the explosive cartridges and the electric detonator in the mortar is the same as that described above. 

Coal-dust is injected into the experimental gallery by means of a spray gun 500-700 mm long with a 150-220 mm bore, loaded with 50 5g of the explosive under test. The charge is fitted with a No. 8 fulminate-tetryl electric detonator in a paper or copper tube. 6 0.1 kg of coal-dust is spread over the charge. 

Type II—explosives which, when a 1400 g charge is fired in a steel mortar in an experimental gallery, fail to ignite a 9% methane-air mixture or coal-dust. They are used with an inert sheath in rock workings. Their rate of detonation is about 3300 m/sec (charge 30 mm dia.) at a density of 1.1, their transmission distance 6 cm. 

The transmission of the detonation figure for the complete cartridge is 4-7 cm. Pobedit P-8 is tested not only in a steel mortar in a gallery, but also suspended free in methane atmosphere and in the presence of coal-dust. 

Thus the reaction rate sought is not constant and, for example, in very wide tubes (underground vaults) the combustion energy of coal dust may be used in the wave, although in laboratory equipment combustion of dust proves so slow that detonation is out of the question. 

Turning to the detonation of condensed EM we note that in this case the study of the equation of state of a dense gas in which repulsion of molecules is more important than their thermal motion turned out to be non-trivial (see the fundamental work by L. D. Landau and K. P. Stanyukovich).29 Water-filled EM were studied by Yu. B. Khariton.30 At present A. N. Dremin is developing ideas on the specific influence of a shock wave on the kinetics of reaction in an EM.31 For gas-dispersion systems the structure of detonation waves has become the subject of numerous studies related to explosions of coal dust husks in grain elevators, gas suspensions of dust in wood processing, etc.32,33 34 5 Works on gas suspensions have also been published abroad.36,37 In gas suspensions we may expect that the reaction rate is determined by diffusion and depends weakly on the temperature. 

Mixtures of coal dust with air are explosive and their detonation by blasting must be prevented (- Permitted Explosives). 

It is important to prevent any deflagration of permitted explosives. Since the deflagration of an explosive proceeds at a much slower rate than its detonation, it may ignite methane-air and coal dust-air mixtures. This must be prevented by using suitable compositions (-> Permitted Explosives) and application techniques. 

Shotfiring in coal mines constitutes a risk in the presence of firedamp and coal dust. Permitted explosives are special compositions which produce short-lived detonation flames and do not ignite methane-air or coal-dust-air mixtures. 

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