US2294532A - Method and means for extinguishing burning molten magnesium and the like - Google Patents

Description

UNITED STATES PATENT OFFICE METHOD AND MEANS FOR EXTINGUISHING BURtTING MOLTEN MAGNESIUM AND THE Joseph J. Fahey and Michael Fleischer, Washington, D. C., and William W. ltubey, Westmolfeland Hills, Md;

No Drawing. Original application May 2, 1942,

Serial No. 441,539. Divided and this application July 15, 1942, Serial No. 451,210

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 G. 757) 14 Claims.

The invention described herein may be made and used by and for the Government of the United States for governmental purposes without the payment to us of any royalty therefor.

air to penetrate to the magnesium under conditions tending to maintain or reestablish combuslilOrl. f

Recent attempts to improve upon extinguishers This invention relates to extinguishing of burnof magnesium (as distinguished from the use of mg magnesium bombs and the like and aims water, which is a'combustion accelerator, and not generally to improve the same. The present apan extinguisher) have for the most part been plication is a division of our application Ser. No. confined to trial of mixtures of the above-men- 44l,539, filed May 2, 1942. tioned materials, and to proposals to use other Attempts in the prior art to develop a method in materials of like properties. and means of extinguishing burning magnesium The present invention, unlike these prior eflorts have been only partially successful. is based on the discovery of a method and means Thus attempts to blanket and smother the making it possible for the first time with nonburning molten magnesium with materials incombustibles to effectively exclude oxygen from fusible in such application, actually permit the 15 burning magnesium and similar combustible light combustion to continue until the magnesium is metals and alloys (of which magnesium may be burned out. This applies to the use of sand taken as representative), and this to such de- (S102), generally considered the best method gree that the combustion is actually stopped in available heretofore, and to the use of mineral mid-course and a substantial portion of the magtalc, clay minerals such as kaolinite, and to the nesium preserved in the unoxidized state, and use of lime and the like, which confine the flames thus prevented from adding to the heat and somewhat but permit air to penetrate to the illumination of combustion. burning magnesium According to the present invention, this is ac- Moreover, many of such materials contain apcomplished by forming and maintaining or repreciable quantities of free or loosely bound wanewing a filmof highly viscous lassy noncomter, as in the case of clays, kaolinite, talc, lime, bustible material on the surface of the burning greensand, etc., which when liberated by the molten magnesium, which eflectively isolates the heat of the burning magnesium, increase the magnesium from the oxygen necessary to its comcombustion and cause local or minor explosions bustion, said film having a viscosity, at the temdisplacing the blanketing material and scatterperature of burning molten magnesium bombs ing theme. (considered as ranging from 1000 to 1500" C.)

Attempts to mother the bombs with bitumithousands of times the viscosity of thick honey nous or resinous materials, either singly or mixed at room temperature, the preferred viscosity bewith filler materials, suiier from the disadvaning of the order of from several thousand to miltage that the materials themselves are combuslions of poises. tible, and evolve clouds of acrid smoke militat- In accordance with this invention the film may ing against their use in closed or poorly ventibe established by the use, in proper form, of a lated places, and greatly hindering the efiorts of natural or artificial inorganic substance or mixapplying personnel in all events. ture of substances wholly or largely composed of The household standby, common salt (NaCl) non-combustible materials which become molten which melts at 800 C., has been suggested as a smothering agent, but has been shown by tests to be relatively ineflective. The salt, when piled on the burning magnesium bomb, decrepitates so violently that the covering over the magnesium is likely to blow off or crack open, allowing air to reach the magnesium. Moreover, the salt crystallizes readily when cooled to the temperature of 800 C., which is above the kindling temperature of magnesium, and thus additionally permits at magnesium bomb combustion temperatures (1500 C. and below) exist at such temperatures in the described highly viscous glassy state; and solidify amorphously when cooled below the temperatures at which they become molten. Such film-producers are herein termed vitrescibles.

The present invention has shown that naturally occurring jl trescible minerals, including all of the feldspars, garnets, and pyroxenes which become melted at 1500 C., orlbelow; properly sized particles of rocks composed predominantly of these vitrescible minerals in conjunction with quartz, mica, feldspathoids, clays, and other materials in lesser amounts; and artificial vitrescible materials-such as properly sized particles of inorganic glasses (either in vitreous or devitrified condition), dehydrated zeolites, and dehydrated artificial borates-having the indicated melting point, are all suitable for the practice of this invention, melts at 1110 C. and has viscosities of from about one hundred million poises at 1150 C. to about one hundred twenty thousand poises at 1425" 0.; orthoclase (potash-feldspar) melts at 1170 C., and has viscosities even higher than albite, remaining at about ten million poises at 1400 C. and at about four million poises at ilar rock ingredients, is not detrimental to the,

Albite (soda-feldspar) for example,

practice of the invention, and indeed, tests made with anorthosite (a rock composed largely of plagioclase feldspar containing approximately mica and clinozoisite) showed that when applied with a particle size in the range of 10 mesh to 200 mesh, approximately (see Standard Screen Scale, Handbook of Chem. and Phys. 14th edlt., Chem. Rub. Pub. 00., p. 891) this material extinguished burning molten magnesium substantially as satisfactorily as pure i'eldspars.

The presence of constituents containing loosely bound water of hydration is detrimental if excessive, as the freeing of the water at high temperature tends to support the magnesium combustion, and also tends to cause local minor explosions which disrupt the continuity of the film. For this reason prudence dictates that such constituents, if employed, be dehydrated, or that the proportion of them, if present, he kept sufiiciently low to not interfere with the action of the extinguishing film.

Naturally occurring vitrescible minerals falling within the above specifications for extinguishers according to this invention, all of which are indicated by our research to be suitable as extinguishers of burning molten magnesium include, for example, the potash feldspars, soda feldspars, plagioclase feldspars-which are solid solutions containing variable proportions of albite and anorthite-and pyroxenes such as spodumene; all of which are natural minerals rich in alkali aluminum silicate ingredients and have'the physical characteristics above specified. Andradite, acmite, and jadeite' may also be employed.

Naturally occurring vitrescible containing rocks falling within the above specifications for extinguishers and indicated by our researches to be suitable as such extinguishers, comprise, for example, those having as principal constituents, one or more of the vitrescible minerals abovementioned, including: syenites and trachytesrocks composed largely of alkali-rich feldspars with minor amounts of other minerals such as hornblende-nepheline syenites and phonolitesrocks composed largely of nepheline and alkalirich feldspars with minor amounts of other minerals--mofizonites and latitesrocks composed largely of plagioclase feldspars and potash feldspars with minor amounts of other minerals anorthosltes-rocks composed largely of plagioclase feldspars with minor amounts of other minerals-granites, rhyolites, pegmatites, and aplites-rocks composed largely of alkali-rich feldspars and quartz with minor amounts of other 76 minerals-and vitrescible-containing rocks which may be grouped as varieties of these and similar to them as well as natural glasses such as tufi's and volcanic ash.

Artificial vitrescible compositions may, of course, be made in accordance with the above extinguisher specifications, as by fusing together appropriate constituents or by dehydration of naturally occurring or artificially prepared hydrated compounds of such vitrescible materials. Such artificially prepared vltrescibles, as in the case of natural ones, may be employed in the crystalline or the vitreous (glassy) state; the former is preferable, as the latent heat of fusion of the crystalline material is extracted from the environment, thereby tending to assist in cooling of the burning" material. Anhydrous residues of artificially dehydrated zeolites and dehydrated borates may be employed, though probably not as cheaply as the natural minerals and rocks falling in the specified group; and similarly glass cullet and other cheap forms of glass, properly particle sized, are within the scope of this invention.- T

Our research shows that for best extinguishing effect in the case of magnesium fires, particularly when burning on combustible surfaces, a suitable grading of the particle sizes of the vitrescible materials is dictated.

When particulate material according to this invention is applied to molten magnesium burning on a combustible surface such as wood floor, the combustion of the magnesium ceases, but charring and destructive distillation of the wood continues during cooling of the hot magnesium. We have discovered that if a substantial amount of material finer than 200 mesh is present, the gases formed by the destructive distillation are unable to percolate through the covering material, pressure is built up, and finally the gases are released explosively with removal of the covering material, and frequently this is accompanied by ignition of the gases and even of the exposed magnesium if the latter has not cooled below its ignition temperature.

We have discovered that this difiiculty can be obviated by employing the vitrescible material in such form that interstices arev provided for the escape of the products of destructive distillation, as by grading the particulate material to largely eliminate fines passing a 200 mesh screen. Our research has shown that the vitrescible material, whether of natural feldspar, low melting rock, glass cullet, or other composition, is best prepared for use by crushing or grinding and screening to obtain a particulate vitrescible extinguisher substantially free from particles finer than 200 mesh, and composed chiefly of particles ranging in size from 10 to 200 mesh. It is not necessary, for the purposes of this invention that the range of sizes include any very small particles, as our research has shown that a commercially screened body of vitrescible-including particles substantially all larger than 20 mesh, and for the most part smaller than 10 mesh is a very satisfactory extinguisher, and that the inclusion of smaller sizes, particularly those larger than 200 mesh (making available very roughly graded material) is not deleterious, so long as there is a sufiicient quantity of larger particles present to engage with one another and prevent undue slumping of the material, a characteristic of fine powders particularly when agitated by eiiluent gases. It is therefore deemed that the term "comminuted" as di ti uished from powdered, or from merely crushed, most aptly defines the appropriate genus of sizes.

In' extinguishing a burning magnesium bomb in accordance with this invention it is only necessary to wait the prescribed time to enable burning out of the thermite core (and detonation of any possible explosive charge) and to then deposit on the burning bomb in any suitable manner, as with a long-handled shovel and thereafter from a bucket, a suflicient quantity of the particulate vitresoible material to completely cover it. Experience has demonstrated that the contents of a 12 quart bucket of to 200 mesh vitrescible material, weighing about forty pounds, is more than ample to completely extinguish a two'pound or four pound magnesium incendiary bomb in from 15 to 30 seconds, though the hot molten magnesium continues to cause charring and destructive distillation of underlying wood flooring or other combustible material for a few minutes thereafter.

After once covering the bomb with vitresoibles, which requires only a few seconds, no further action need be taken with regard to the bomb itself, and the householder or other user is free to undertake other activities such as the extinguishing of subsidiary fires set by thermite sparks. At a later convenient time the solidified residual magnesium may be removed and the vitresoible material may be shovelled up for reuse, care being taken to gather only material free from unoxidized magnesium.

Due to the fact that this method actually causes the burning of the magnesium to stop in mid-course, damage to the underlying combustible flooring is minimized, and the flooring will usually char to a depth of not more than a quarter of an inch, whereas with sand treatment, under which the bomb burns itself out, there is grave danger of actual burning through of the floor, and starting of additional fires in the underlying regions. If the bomb is burning on a floor having open cracks through which air or molten magnesium may penetrate it is of course desirable to lightly cover the burning bomb with vitresoible extinguisher, pour additional vitresoible material adjacent it, and after shovelling the bomb onto the adjacent vitresoible, to then cover it over completely. This same practice will of course still further minimize damage to flooring and the like.

The vitresoible materials discovered by this invention to be highly satisfactory as magnesium incendiary extinguishers are extremely cheap and geographically widely distributed. The cost of quarrying and grading the natural materials is slight and the materials are in most cases susceptible of shipment in open gondola cars and of bulk storage in the open, making for cheapness of distribution. Feldspars that are low grade so far as ceramic use is concerned, and the dumps of quarries heretofore engaged in supplying such trade, are suitable magnesium extinguishers, adding to the cheapness of the extinguisher.

The melting of the vitresoible extinguisher is confined to the surface in immediate contact with the burning molten magnesium, as shown by the fact that no substantial melting in the body of the vitresoible material is noticeable, while examination of the solidified pool of molten magnesium which has been extinguished by the method, shows, as confirmed by the polarizing microscope, that the magnesium surface is coated with an extremely thin film of vitreous composition. Occasionally, the film is seen to be raised in bubbles, some broken open with bared magnesium exposed, or with the underlying magnesium coated over with a renewed film.

This observed mode of action of our vitresoible extinguisher enables one to theorize as to the cooperative action of the several features embodied in our composition, and while we are not to be bound by such theory, we deem it proper to set it forth as an explanation of the observed phenomena. According to this theory, where the vitresoibles come in direct contact with the burning molten magnesium, they melt to form the film of highly viscous material (thousands of times as viscous as thick honey at room temperature), and if efiluent gases or vapors are generated under the film, it is blown up into a bubble. The generation of gases diminishes and ceases as the molten magnesium cools, so that some of the bubbles formed will not be ruptured. When others are ruptured, as by gas from wood distillation, the magnesium underlying them may have already fallen below its kindling temperature, in which event it is likely that no further coating will occur; otherwise, further particles of the vitresoible falling into the space resulting from the ruptured bubble become fused and maintain or renew the ruptured film over the underlying magnesium, thus effectively reestablishing its isolation from atmospheric oxygen. It is believed the inclusion, with heavier particles, of particles between 20 and 200 mesh assists in assuring such reestablishment; in any event their inclusion, as above noted, is not deleterious.

From the foregoing description it will be clear that the present invention is not limited to the specific embodiments disclosed to illustrate the same. For example, the segregation of an appropriate quantity of vitresoibles to constitute an extinguisher of the class described may be accomplished in any suitable way, segregation in a bucket being representative of segregation in any other form of package or container, as will be obvious to those skilled in the art. In the present divisional application, however, we claim only the form of the invention in which the method depends on the employment of, and the extinguisher consists essentially of, properly prepared vitrescible material in crystalline state, which melts from the crystalline state when applied to the burning molten metal, with consequent extraction from the environment of the heat of fusion of the crystals, but which solidifies amorphously when cooled.

Comminuted amorphous materials, due to the nature of the fracture of glassy bodies, present sharp thin bevelled edges. The crystalline vitresoible materials, on the other hand, when comminuted, for the most part break with good cleavage forming surfaces presenting only blunt angles between them. Thus the crystalline vitrescibles may be handled with the bare hands without serious dang-er of cutting or puncturing the skin, which obviously is not the case if comrninuted natural or artificial glass is employed. For the same reasons the crystalline vitresoibles are less productive of damage to carpet pile and the like if accidentally scattered thereon in the home, and less likely to produce internal injury if taken into the mouth or stomach of children or pets. Moreover the crystalline materials melt sharply at definite temperatures and retain their shape and discrete particulate structure at temperatures even immediately below their melting points. Amorphous materials such as glass, on

the contrary, do not melt sharply but merely undergo a progressive softening over a considerable temperature range. The practical consequences of these facts when such materials are employed as extinguishers of magnesium fires show a greater desirability of the crystalline materials for the following reasons:

The crystalline material which is in direct contact with the burning molten magnesium melts to form an extremely thin film of glassy material over the molten metal. This extremely thin film so far as can be determined completely excludes oxygen from the burning magnesium. The crystalline material immediately above this thin film retains its discrete particulate form, with the result that heat and gasses from combustion of underlying material easily escape through the interstices in the overspread. In addition, the discrete particulate form I being retained, the hotter particles of crystalline material lying close to the fihn present a large free radiation surface in comparison with their volume, and thus they themselves cool rapidly.

When amorphous /materials are applied to the burning magnesium, on the other hand, the material progressively softens/ depth before the actual fluid layer becomes es biished, excludin the oxygen from the burning metal/ /As a re suit, a considerable depth of the amorphous material adjacent the molten magnesium assumes a softened and more or less coalescing condition. In effect, a much thicker layer of more or less coalescent material is formed than when the crystalline material is used. The extra thickness of the coalesced material is disadvantageous for several reasons: (1) it acts as a heat insulator causing much greater heat retention by the molten metal, thereby increasing the duration and extent of the damage to underlying flooring and the generation of gasses therefrom, and increasing the length of time required for solidifying and cooling of the extinguished magnesium; (2)

' it prevents the gasses from the charring wood from escaping, thereby reducing the removal of heat thereby and increasing the danger of a violent release of built-up gas pressure which might cause the'flre to break out again; (3) the coa-' lesced material itself has less free radiation surface in comparison with its heat retaining volume and less interstices through which the heat-may escape and thus itself constitutes a. very hot, poorly conductive heat-retaining body, still further prolonging the time necessary for cooling of the mass. Still other advantages of crystalline, as compared 'to initially glassy material, probably exist; in all events, as above stated, the crystalline form is preferred.

We claim as our invention:

1. The method of extinguishing burning molten magnesium which consists in applying directly to the surface of the burning metal a quantity of comminuted non-combustible inorganic vitrescible crystalline material which melts from its crystalline form in the temperature range of burning molten magnesium and forms on the surface of the magnesium a non-combustible oxygen excluding glaze-like film, said film having when molten within said temperature range a viscosity within the range of several thousand to millions of poises.

2. A method according to claim 1 in which the crystalline material applied directly to the surface of the burning metal consists of comminuted material selected from the naturally occur- 75 ring vitrescible minerals having said physical characteristics.

3. A method according to claim 1 in which the crystalline material applied directly to the surface of the burning metal consists of comminuted material selected from the natural rocks consisting largely of vitrescible minerals having said physical characteristics. r

4. A method according to claim 1 in which the crystalline material applied directly to the surface of the burning metal consists of comminuted artificially prepared vitrescible crystalline material having the said physical characteristics.

5. A method according to claim 1 in which the crystalline material applied directly to the surface of the burning metal consists of comminuted mineral material selected from the natural minerals rich in alkali aluminum silicate ingredients and having said physical characters cs.

6. A method according to claim 1 in which the crystalline material applied directly to the surface of the burning metal consists of comminuted material selected from the natural rocks rich in alkali aluminum silicate ingredients and having said physical characteristics.

7. A new manufacture packaged for use on and constituting a fire-extinguisher for direct application to the surface of burning molten magnesium and the like, which extinguishes the molten metal fire in mid-course, consisting essentially Of non-combustible vitrescible inorganic crystalline material prepared in the form of graded, small sized particles so distributed in size as to provide interstices for the escape of gases incidental to the fire to be extinguished when applied thereto, said vitrescible material being of a character that melts from its crystalline form when applied to the burning molten metal and forms on the surface of the metal non-combustible oxygen-excluding glaze w ti has a viscosity within the range of several thousand to millions of poises when in molten state and at the temperature of the burning metal and which solidifies amorphously when cooled.

8. A manufacture according to claim 7 in which the crystalline material consists of material selected from the naturally occurring vitrescible minerals having the said physical characteristics, and is preparedin particles for the most pgzt lying within the range of'200 to 10 mesh s s. 9.. A manufacture according to claim 7 in which the crystalline material consists ofmaterial selected from the naturally occurring rocks consisting largely of vitrescible minerals having said physical characteristics, and is prepared in particles for the most part lying within the range of 200 to 10 mesh sizes.

10. A manufacture according to claim 7 in which the crystalline material consists of artificially prepared material having the said physical characteristics, and is prepared in particles for the most part lying within the range of 200 to 10 mesh sizes.

11 A manufacture according to claim 7 in which the crystalline material consists of material selected from the natural minerals rich in alkali aluminum silicate ingredients and having said physical characteristics, and is prepared in particles for the most part lying within the range of 200 to 10 mesh sizes.

12.'A manufacture according to claim 7 in which the crystalline material consists of material selected from the naturally occurring rocks rich in alkali aluminum silicate ingredients and 14. A manufacture accordingto claim 7 in having said physical characteristics, and is prewhich the crystalline material consists of compared in particles for the most part lying within minuted feldspar, which has said physical charthe range of 200 to 10 mesh sizes. acteristics.

13. A manufacture according to claim 7 in 5 JOSEPH J. FAHEY. which the crystalline material is selected from MICHAEL FLEISCHER. the group consisting of the potash feldspars, soda WILLIAM W. RUiBEY.

feldspars, plagioclase feldspars, andradlte, Spodumene, acmite, jadeite and rocks composed largely thereof.