EXPLOSIVE DUST

Ashburton Guardian, Volume 56, Issue 72, 7 January 1936, Page 8

EXPLOSIVE DUST

AN INDUSTRIAL MENACE. CAUSE OF BIG DISASTERS. Wherever dried grains such as wheat, corn, oats or rice arc handled in bulk—for example in elevators, or wherever finely-ground products, such .as starch, flour, sugar, sulphur, coal, or aluminium, are man or handled, all these substances give rise to a very fine dust, which because of its chemical nature, is liable to form an explosive mixture with air. When conditions happen to be favourable, it may suddenly ignite with disastrous results .'says a writer in the "Melbourne Age"). . Experience has shown that in all such operations, particularly those on a large scale, great- care must be exercised in the design of plant and in the general factory operations. The Coin Products Refining Company, of Illinois, had one of the largest plants of its kind in America—it was exceedingly well designed and managed. The starch-dust hazard was known to be real, and strict precautions were taken accordingly. Many engineers devoted their full time to safety problems; there was no smoking and no carrying of matches; sparks from external causes and from electrical discharges were rigorously eliminated; all bearings were regularly oiled, and so forth. And yet, on January 4, 1921, the worst dust explosion on record occurred, blowing the starch packing house to pieces, and killing fortytwo men. An inquiry revealed that a hot bearing was the cause, the oil hole having become clogged with starch. Not to be Ended. This instance serves to emphasise the danger associated with fine dusts. At the time, it also served to bring about a more strict enforcement of the safety clauses in similar industries throughout the country. That that risk can never be wholly eliminated is evident from this instance alone and also from the fact that only a few weeks ago a similar explosion occurred, this time in tlie Glider Soya Beans Products factory in Chicago when about ten people were killed and sixty injured. "The explosion," so the news item .reads, "lifted the six-storey building from the ground, transforming it into a wreck of twisted steel and concrete." In Australia dust explosions are not altogether unknown. Some feu- months ago, one occurred in an empty grain silo, when a youth sought to enter it with a naked light. And some years back there was a rather disastrous coal-dust explosion in Queensland, when several miners lost their lives. The mine, by the way, was quite free from fire-damp at the time. These explosions must not be confused with gaseous explosions, with which everyone is more of less familiar. A comparison, however, is quite apt, since the essential characteristics- are much the same in both cases.

In the first place, the gas or the dust must consist of a substance which will burn or oxidise readily. It must be mixed with air—that is, oxygen—in suitable proportions. A flame of some soit must be introduced to start the action, the heat of which causes the sudden propagation of the flame throughout the whole mass, with rapid expansion, production of pressure, and, in fact, the general characteristics of an explosion. Ideal Dust. The more finely divided the dust, the more dangerous it is likely to be. As the ultimate limit of sub-division is the molecule itself, we can actually regard a gas as a sort of ideal dust. From this point of view gases are in an ideal state for burning or exploding, and hence are invariably more dangerous than dusts although the latter are dangerous enough. All the factors that apply to inflammable gases are, with small modification, applicable to dusts. This even includes the secondary detonations, which in .the motor-car engine give rise to "knocking." This latter unpleasant effect is due to the explosion flame heating and compressing some of the gas before it manages to set it alight, with the result that this part of the gas self-ig-nites or detonates, causing a sudden kick in the otherwise even explosion. All detonations and their harmful effects are entirely eliminated in dust explosions by a simple and perhaps .peculiar device. The factories are built with huge window spaces and glass roofs, so that the initial explosion will blow'the glass to pieces, thus relieving the pressure and preventing the added kick due to a detonation.

In recent years considerable attention has been paid not only to the nature of explosive dusts, but to the conditions under which they are likely to become dangerous. The inflammability depends on the size and the concentration of the particles, their chemical nature, and ease of oxidation, the existence of a layer of absorbed oxygen on the surface of the particles the thermal conductivity and so forth. For example, wheat dust, which is produced by the rubbing together of millions of grains whenever wheat is being handled, particularly when it is being bundled about in a closed space like "an elevator, has been subject to careful analysis. Its chief constituents are 34 per 'cent of starch and 22 per cent, of crude fibre or cellulose, both in a very fine state with the former the more dangerous. Fifty Different Dusts. In England, something like fifty different dust samples have been collected from various factories, including sugar, starch, cocoa, flour, cork, rice, mustard, wheat, elevator dust, sulphur, aluminium and coal. Experiments with these reveal ignition temperatures as low as 500 degrees eentigrato, explosion pressure as high as 8001 b per square inch, and flame velocities of a mile a second, all serving to emphasise , the perilous nature of inflammable dusts. Speaking more generally, explosive

dusts form but a particular example of a number of finely divided .substances which can exist in the air. They include fogs, mists, clouds, smokes, and harmless as well as harmful dusts. They can all be produced on a laboratory scale and as such have been subject to careful investigation, tobacco smoke, by the way, is quite a favourite study.

Of particular importance is the problem of smoke abatement in industrial areas. Apart from the usual settling chambers, dust collectors and so forth the most interesting modern treatment is the electro-static precipitation, of Cottrell, a method which was first suggested just more than 100 years ago. ~ The smoke is passed through a battery of sixty to a hundred pipes, down each of which runs a, high tension wire, charged with anything up to 100,000 volts. The steady, silent discharge from the wire electrifies the particles, which arc immediately attracted to the insides of the tubes, and there they are collected. This electrical cleansing has rapidly come into favour, and in many cases the material recovered pays for the treatment. One such installation is to be found ill sulphuric acid plant at Port Pirie; another at the briquetting plant at Yallourn.