904
INDUSTRIAL AND ENGINEERING CHEMISTRY
should be installed where overheating might occur unnoticed. Smoking and the careless use of matches, common sources of dust ignition, need no explanation. There is no definite method of eliminating this hazard, which can be regulated only to the extent that employees are willing to comply with the prohibiting regulations. A knowledge of the danger should be sufficient to impress upon an employee the necessity of refraining from smoking and the striking of matches in dusty atmospheres. So many explosions have been caused by the use of open lights that it is necessary to place special emphasis on this common cause of dust explosions. The open flame as a means of light in industrial plants has been generally eliminated by the use of electric lights, but in many cases when the power goes off or the plant is shut down for repairs lanterns are used. Blow torches and metal cutting or welding flames, sometimes used in making repairs, create a hazard where there is a chance of dust being thrown into suspension. This hazard must be regulated by placing restrictions on their use and by carefulness on the part of the employees. Spontaneous combustion may be an indirect cause of an explosion by starting a fire which may ignite a dust cloud. This hazard can be eliminated by keeping the plant free from dust accumulations and by preventing the storage of material subject to spontaneous combustion in or near a dusty section of the plant. A number of other causes of dust explosions, including fires which occur in unusual ways, a t unexpected times and places, due to unusual circumstances, and the so-called unpreventable fires, may be listed as miscellaneous. Dust explosions may occur after lightning strikes a plant or from sparks from a passing locomotive or a fire in a nearby plant. Fires may occur a t points where steam pipes pass too close to combustible material. The really unpreventable fires and explosions, however, constitute a relatively small percentage. Attention to the adoption and enforcement of fire-protection measures now generally recommended will materially reduce the losses from dust explosions. Plants Producing Dusty Material
I n plants where the dust or powdered material is actually produced or manufactured, still other precautions are necessary in order properly to safeguard the employees and the property from the explosion hazard. Plant cleanliness is of the utmost importance. Dust-collecting and dust-removing equipment of the most approved type is an absolute necessity in practically all such plants. Strict attention also must be given to the elimination of the various sources of ignition. The sources of ignition existing in the plants handling dust in package form and in the plants handling loose material are present in the dust-manufacturing plants, and in addition there is the hazard incident to the use of high-speed grinding equipment. Many of the fires and explosions which have occurred during the manufacturing process are reported to have started in the grinding equipment, usually from metallic sparks. Practically all products coming to the grinders contain foreign material, which may strike sparks while passing through the grinders and ignite the dust in suspension within the machine, It is a t this point that probably the greatest difficulty is experienced in obtaining protection from dust explosions. It is, of course, impossible to control the dust within the machine, because the producing of dust is its function. The entrance of foreign material into the grinder may be partially controlled by means of screens and separators, but a small percentage of foreign material of about the same size and weight as the material being ground will slip through. Evidently, neither of the two methods of controlling the dust-explosion hazard mentioned-preventing the
1’01. 17. No. 9
formation of dust clouds and eliminating all sources of ignition-can be effectively used in the grinding equipment. In plants where grinding constitutes a major part of the process it may therefore be necessary to introduce an inert gas into the grinding machines to prevent the formation of an explosive mixture of dust and air. Tests have shown that for nearly all of the dusts now generally considered explosive it is impossible to produce an explosion or propagate 8 flame through a dusty atmosphere when the oxygen of the air in which the dust is carried in suspension has been reduced to 12 per cent. To do this it is necessary to replace part of the 21 per cent of oxygen found in air with an inert gas, such as nitrogen or carbon dioxide. I n a few cases a reduction of the oxygen content to less than 12 per cent may be necessary, as tests have indicated that a reduction to 8.5 per cent of oxygen is necessary to prevent explosions of sulfur dust. Where the use of inert gas is considered essential for the prevention of dust explosions, a thorough study of the subject should be made to determine the conditions under which the dust will explode and the amount of inert gas that would be needed to prevent the formation of an explosive mixture. Summary of Methods for Reducing Dust Hazard
All the methods suggested for reducing the dust explosion hazard in dusty industries fall into three general recommendations : (1) PFevent the formation of dust clouds or the accumulation of dust in sufficient quantity to form a cloud, either by maintaining general plant cleanliness or by installing dust-collecting equipment. (2) Eliminate all sources of ignition. (3) Maintain an atmosphere of reduced oxygen content incapable of supporting a dust explosion where it is impossible either to prevent the formation of dust clouds or to eliminate the sources of ignition effectively.
The heavy losses already caused in various lines of industry by dust explosions should be an incentive to the owners and operators of dusty plants to adopt promptly all possible precautions for the elimination of the dust explosion hazard. If the owners or operators wish to have samples of the dust in their plants tested to determine its explosibility the Bureau of Chemistry will make such tests for them. Samples should be forwarded to the Dust Explosion Laboratory, Bureau of Chemistry, U. S. Department of Agriculture, Washington, D. C.
Centennial of the Stearin Industry In 1825 Chevreul, with Gay-Lussac, took out the first patent for the industrial application of the results of his memorable discovenes. From this patent a new industry arose, that of stearin candles, which marked a very important step in the progress of illuminating technic. The work of Chevreul has been extraordinarily productive of practical results. The soap industry owes to his discoveries the impulse which drew it away from empirism and modernized its methods; the glycerol industry found in the “used liquors” a raw material without which it would not have been able t o exist; and the oil works industry owes its progress to the development of the applications of fatty substances. Even the manufacture of paint is indebted to the investigations made by Chevreul in 1850 for a large part of its progress during the last century. The SocibtC de Chimie Industrielle thought that it should not allow the hundredth anniversary of the first application of these works of Chevreul to pass without notice. A celebration will be held in Paris the 11th of October, a t the meeting of the Cinquieme Congress de Chimie Industrielle. The President of the Republic, the Government, the Academy of Sciences, and the National Museum of Natural History will take part in this ceremony, which will be held in the place where Chevreul made his remarkable discoveries. This celebration will close the Congress de Chimie Industrielle, which meets in Paris beginning October 4, 1925.
