Tests of Hand Extinguishers on Zinc-Dust Fires
The Bureau of Mines Makes Some Important Investigations Along This Line—Description of the Results Obtained from Tests, in Which Various Types of Extinguishers Were Used
FIRES in zinc dust have sometimes given particular trouble owing to their resistance to extinguishers and the liability of explosions. Similar problems, notably with aluminum dust’, have previously been subjects of investigation by the Bureau of Mines.
Description of Tests
The zinc-dust used in the tests was commercial material of which 99 per cent, would pass through a 200-mesh screen; its composition was as follows2: Zinc, 90.6 per cent.; cadmium, 0.48; lead, 1.86; iron, 0.002; and oxygen (by difference) 7.06 per cent. Small lots of dust, roughly one to two pounds, were heated in a ladle over a forge until ignited. The burning dust was dumped in a heap on an asbestos board and the effects observed when water, sodium bicarbonated solution, carbon tetrachloride, silicon tetrachloride, and a frothy mixture were applied. A test of the frothy mixture in extinguishing fire in 300 pounds of zinc dust was then made. The results obtained with the different extinguishers are described below.
The danger of fire and explosions from liberated hydrogen when water comes in contact with zinc dust has long been recognized. Schwartz’ stated in 1904 that zinc dust is very susceptible to moisture, as water oxidizes it at ordinary temperature, liberating hydrogen; the decomposition is accompanied by sufficient heat to ignite the hydrogen and may also set fire to stocks of zinc dust—zinc commences to burn at 360° C.
Since the rapidity of decomposition of water by zinc increases with the temperature, water leaves much to be desired as an extinguisher. The writers found that when water was first poured on burning zinc dust, the fire was aggravated, and flames puffed out. scattering burning metal. It was possible to extinguish the fire by using sufficient water to thoroughly soak and cool the zinc; but when used in smaller amount, the scattering action, the evolution of explosive hydrogen, and the difficulty of adequate cooling a large bulk of zinc dust for complete extinguishing, make water an unsatisfactory as well as dangerous extinguisher. Water in large quantity, as from a fire hose, would undoubtedly lie most effective. but the tests described in this paper deal only with the small amounts of liquid contained in hand extinguishers.
Saturated Sodium Bicarbonate Solution
A solution of this salt in water is effective as a fire extinguisher in many instances, because inert carbon dioxide gas, which tends to smother fire, is evolved when the solution is heated. The smothering effect is thus added to the cooling effect of the water. However, no difference could be observed in the effect of the solution over plain water in tests on the 1 to 2 pound lots of burning zinc dust.
- Leighton, A., The inflammability of aluminum dust, Bureau of Mines Tech. Paper, 152, 1919.
- Analysis by W. A. Selvig, assistant analytical chemist, Bureau of Mines.
- Schwartz, Dr. von, translation by Salter, C. T. C., fire and explosion risks, Chas. Griffin & Co., London, 1904, 375 pp.
The usual soda and acid type of extinguisher contains free sulphuric acid in addition to bicarbonate and water. It is not advisable to use acid mixture on zinc, because any excess of acid or insufficient mixing would leave free acid that acts vigorously on zinc, producing hydrogen.
When the commercial material is applied in small quantities to burning zinc, it aggravates the fire almost as the water did, at the same time dense, obsuring clouds of zinc chloride and oxide were evolved. Within doors the smoke might quickly accumulate in densities which completely stop vision; in fact, the high obscuring properties of smoke from zinc dust-carbon tetrachloride mixtures were utilized during the war for smoke screens.*
Two proprietary carbon tetrachloride fire extinguishers were also tested. One produced similar results to the commercial carbon tetrachloride, while the other contained a combustible liquid (to prevent freezing) that remained after carbon tetrachloride had been volatilized, and burned with a smoky flame. When large quantities of carbon tetrachloride liquid were applied it was possible to extinguish the fires.
This was tried but was no better than the foregoing. Moreover, it fumed so badly, owing to its vapors reacting with atmospheric moisture, that there was nothing to commend it.
Frothy mixture (the preparation “foamite firefoam” was used) consists of a solution of aluminum sulphate and a solution of sodium bicarbonate and licorice extract, the latter for producing a stable foam. The two solutions are mixed when the extinguisher is inverted for use, and react forming carbon dioxide gas that fills the foam bubbles. When a frothy mixture was applied, the zinc dust was covered without disturbance at any stage of the process. The foam prevented access of air, soon extinguishing fire in two pounds of zinc dust. Frothy mixture proved most effective and satisfactory of any of the extinguishers tested, so a large fire test was made, 300 pounds of zinc dust being piled on the ground, making a heap from 24 to 30 inches in diameter at the base by about 12 inches high. Fire was started over a large portion of the surface and to a depth of about two inches with small “candles” of zinc dust-sodium chlorate mixture*. “Starters” for the candles consisted of iron dust-potassium permanganate, which ignited on contact with glowing punk. The fire at first made dense smoke, mostly from the candles, and then settled to a slow quiet burning with little evolution of smoke. Fire was visible on the surface, but there were no flames.
‘Richter, G. A., Combustion smokes, Jour. Ind. Eng. Chem., vol13, Mar., 1921, pp. 343-5.
When the fires was well under way, about five minutes after the start, a frothy fire extinguisher of 2 1/2 gallon capacity, was played upon it. No disturbance or scattering of dust occurred, and the pile was soon enveloped. Gases containing steam and zinc smoke bubbled out through the foam, opening the covering, so another 2 1/2-gallon extinguisher was used. ‘This made a covering about 4 inches deep over the pile, but gases continued to evolve. The rate of evolution rapidly subsided, in 15 minutes the foam was removed and the fire examined. Zinc was still burning, so the foam was replaced and allowed to stand another 15 minutes. As little evolution of gas was seen, the fire was again examined. The zinc was still found to be burning, so the foam was replaced and a third extinguisher added. The fire was left undisturbed until the following morning— 17 hours later—when examination showed that the fire had been extinguished.
Frothy mixture thus presents the following advantages over other extinguishers for zinc dust fires: There is little evolution of combustible, explosive gas. No irritating or poisonous gases or vapors are evolved. Application of the froth does not aggravate or scatter the fire, but smothers it quietly.
From the standpoint of thermochemistry, a fire may be extinguished by cooling until combustion no longer takes place; excluding air by means of a less reactive or non-reactive substance; or by both processes. Combustion is here taken to mean the chemical combination of a material with oxygen of the air, heat and light being produced. An ideal fire extinguisher would itself be non-combustible, non-reactive with the material to be extinguished and of high cooling power when brought into contact with the burning matter.
Comparing the extinguishers tested in the light of the above statements, all are non-combustible inair, but all tend to react with zinc with production of heat. Thus, 18 grams of water may combine with 65 grams of zinc to produce 81 grams of zinc oxide and 2 grams of hydrogen, while 17 large calories of heat are liberated by the process. Of carbon tetrachloride 76 grams may combine with 65 grams of zinc to produce 136 grams of zinc chloride and 6 grams of free carbon; 83 large calories are liberated Silicon tetrachloride may react similarly to carbon tetrachloride, but only 33 large calories are evolved Carbon dioxide may be reduced to carbon monoxide, 17 calories being evolved, or to free carbon with evolution of 36.5 calories. Zinc (65 grams) burning in air to oxide liberates 85 calories.
Chemical reactions are more prone to be self-supporting when heat is evolved and also depending upon the quantity of heat. Since the largest amount of heat is produced by reaction of zinc with carbon tetrachloride, fears were entertained that if a single mass of hot zinc metal (such a mass being less readily cooled than dust) were brought into contact with carbon tetrachloride liquid, vigorous reaction even approaching actual combustion might occur. If this were true, fire might sometimes be promoted instead of extinguished. A test of 2 pounds of molten zinc dumped from a red-hot ladle into a quart of carbon tetrachloride gave, however, no violent reaction. Dense zinc chloride smoke was evolved for a half-minute, and in two minutes all of the liquid had evaporated and the metal was comparatively cool; considerable carbon was deposited upon it.
The relative values of water, carbon tetrachloride, and silicon tetrachloride as cooling agents are shown below:
Water boils at a much higher temperature than carbon tetrachloride or silicon tetrachloride, one pound of water in liquid form has five times as much cooling power as the others, moreover the evaporation of water absorbs 11 times as much heat as an equal weight of carbon tetrachloride and 14 times that of silicon tetrachloride. Hence, water is greatly superior; in fact no better cooling agent is known.
Foamite itself has a negligible cooling effect; furthermore, it is a non-conductor of heat preventing dissipation of heat from the material it envelops.
1 his is sometimes of advantage in stopping spread of fire, but it also tends to retain high temperature in the enveloped material so that fire may rekindle if air is readmitted. However, heat is slowly evolved through foamite so that danger of rekindling eventually passes.
In spite of the disadvantage of foamite in low cooling power and in preventing rapid self-cooling, it appears best for extinguishing incipient fires in zinc dust when hand extinguishers are used. A fire should be watched after application of the foam, till all danger of rekindling has passed.
Physiological Action of Vapors and Reaction Products
Water is of course non-poisonous, and hydrogen resulting from its action on zinc has no physiological effect. It is absolutely safe so far as poisonous effects on fire fighters are concerned.
Carbon dioxide gas, which fills the bubbles of firefoam, may be breathed with impunity when present in air to the extent of 1 or 2 per cent., but 10 per cent, is dangerous. Zinc can reduce carbon dioxide to carbon monoxide which is intensely poisonous, 20 parts of carbon monoxide in 10,000 parts of air may cause unconsciousness if breathed for 30 minutes. But the danger from carbon monoxide that arises from burning carbonaceous matter is much greater than that from carbon dioxide gas used in fire extinguishers. The latter is very remote, but still should be recognized.
Carbon tetrachloride is decomposed by fires in the presence of air with production of phosgene, hydrogen chloride and chlorine’, very irritating and poisonous gases. Eighty-six volumes of phosgene in a million volumes of air kill dogs after 20 minutes exposure.’ Vapor of the carbon tetrachloride is a dangerous anaesthetic, but a good deal must be Breathed to produce ill effects. Description of a zincdust fire received by the Bureau of Mines states “Two men poured possibly eight or ten 1 1/4-quart fire-guns filled with carbon tetrachloride into the machine, after which their nostrils, throat, and lungs gave them much trouble for the remainder of the day.” Fire fighters should avoid inhaling gases or vapors from carbon tetrachloride in fires. Gas masks, such as the new “Fireman’s” mask, will remove all the poisonous fire gases from air which may he breathed. A description of the Fireman’s gas mask will be published as a separate report.
’Fieldner, A. C., Katz, S. H., Kinney, S. P., and Longfellow, E. S., Poisonous gases from carbon tetrachloride fire extinguishers, Jour. Franklin Ins., vol. 190, Oct., 1920, pp543-65.
6Chemical Warfare Service.
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Silicon tetrachloride vapor is poinous and irritating. but as it has no advantages over the other materials as an extinguisher for zinc fires, it need not be considered further here.
Zinc burns in air. forming zinc oxide that may float as a white or bluish smoke, but which is not especially dangerous to breathe. Zinc chloride smoke from the action of the chloride containing extinguishers on burning zinc is irritating to nostrils, throat and lungs, though not severe.
From the above statements, it is seen that no especial physiological hazards to firefighters come from burning zinc or its compounds with fire extinguishing materials. The other hazards that occur are common to fires in general.
Small tes s were made with water, sodium carbonate solution, carbon tetrachloride, silicon tetrachloride, and a frothy mixture as extinguishers for zinc-dust fires. The frothy mixture extinguished the fire most quietly and effectively, so it was tested on a larger fire in 300 pounds of zinc dust. Three 2J4-gallon foam extinguishers smothered the fire, but danger of re-kindling had passed only after elapse of several hours, due to retention of heat in the metal by the non-conducting foam blanket.
Thermal data show that water, carbon tetrachloride, and carbon dioxide react with zinc with liberation of considerable heat, carbon tetrachloride producing most. However, no violent reaction occurred when red hot molten zinc was poured into carbon tetrachloride.
Athough some fire extinguishing materials may be injurious or may be decomposed in fires to produce toxic gases or vapors, no especially poisonous gases or fumes ate evolved from burning zinc itself or from the action of extinguisher material on the zinc.
The zinc dust used in these experiments was supplied by the Grasselli Chemical Co., Cleveland, Ohio, and the foamite extinguishers by the Foamite Firefoam Sales Co.. Pittsburgh, Pa,—Reports of Investigations, U. S. Bureau of Mines.