Method for extinguishing a metal fire and fire extinguishing agent therefor

Abstract: An efficient fire extinguishing agent is described which is suitable for extinguishing a fire of a burning metal such as a magnesium powder and which may be used, for example, as a filling for a fire extinguisher and which is stable for long storage periods without loss of flow­ ability and ejectability from the extinguisher. The fire extinguishing agent is a powdery blend of (a) 95 to 70% by weight of a high-purity boron oxide powder having a specified high B₂O₃ purity and a low water content and (b) 5 to 30% by weight of an inorganic powder of generally spherical particle configuration. Glass beads of a specified particle diameter and silica·alumina-based hollow microspheres serve as the inorganic powder.

Description: BACKGROUND OF THE INVENTION The present invention relates to an efficient method for extinguishing a burning metal fire and a fire extinguishing agent suitable therefor. More particularly, the invention relates to a method for extinguishing a burning metal fire which cannot be extinguished by pouring water, or rather gains headway by pouring water, and is difficult to extinguish by sprinkling a conventional fire extinguishing agent, as well as to a fire extinguishing agent suitable therefor. As is known, certain metals are combustible in air and are extremely dangerous when the metal takes fire in view of the difficulty of extinguishing the fire. Examples of such dangerous metals include magnesium, aluminium, zinc, titanium, zirconium, iron, rare-earth metals, e.g. neodymium, and the like in a powdery form as well as alkali metals such as sodium, potassium and the like irrespective of their form. The metals of the former group are combustible, especially, in a fine powdery form and, once the powder takes fire, the metal burns violently, sometimes causing a serious explosion. When the metal powder is burning and heated at high temperatures, the metal readily reacts with water to produce explosive hydrogen gas. Therefore, pouring water on a burning metal powder can never be a means of extinguishing the fire and must be strictly avoided in order not to cause an explosion of the hydrogen gas and the so-called steam explosion by which the metal powder is scattered around, thereby badly spreading the fire. Conventional fire extinguishing agents other than water, such as carbon dioxide gas and Halons as well as powdery fire extinguishing agents, i.e. so-called dry chemicals, are also almost ineffective against metal powder fires. One barely effective means of extinguishing a burning metal powder fire is to sprinkle dry sand or a special powdery chemical or a dry powder such as sodium chloride, sodium carbonate and the like, by which the fire may be suppressed to some extent if not completely extinguished. The use of such a dry powder is not advantageous in practice because a quite large amount of the powder must be applied and the metal powder heated at high temperatures remains for some time in the core portion of the powder pile in the form of a red-hot ember, which must be kept as such sometimes for 30 to 60 minutes or even longer, involving a danger of burning up again depending on the conditions. In addition, it is in practice a difficult matter to stock a large amount of sand in an absolutely dry condition. Alkali metals such as sodium and potassium are still more dangerous than the metal powders of the above-mentioned class. These alkali metals, even at room temperature or not in a powdery form, readily and violently react with water and evolve a large quantity of heat,thereby causing melting of the metal and producing hydrogen gas which spontaneously takes fire, sometimes explosively. Therefore, these alkali metals must be strictly kept away from contact with water. Other known fire extinguishing agents are almost ineffective against alkali metal fires. Like the powders of the former group of metals, a barely effective means for extinguishing an alkali metal fire is to completely cover up the burning site of the alkali metal with a large volume of dry sand or dry powder as mentioned above; this effectively suffocates the fire but takes a rather long time. The present inventor previously had the view that such a metal fire may be efficiently extinguished by sprinkling on it a powder of high-purity boron oxide almost free from water and he conducted extensive experiments by using a boron oxide powder or a blend of a boron oxide powder and a mineral powder such as talc, clay, mica and the like, thereby obtaining a promising result (EP-A-0323350, published on 05.07.89). However a problem with such a powder or powder blend is that coalition or caking of the particles takes place during storage of the powder, thus making subsequent sprinkling of the powder difficult. When, for example, a fire extinguisher is filled with the powder and used to eject the powder under a gaseous pressure by opening the valve after storage for a length of time, the ejectability of the powder gradually decreases during storage so as to leave a considerable portion of the powder unejected in the fire extinguisher as a consequence of the decreased flowability of the powder due to caking. SUMMARY OF THE INVENTION The present invention accordingly has as an object to provide a novel efficient method for extinguishing a fire of a metal, such as magnesium, aluminum, zinc, titanium, zirconium, iron and rare-earth metals, e.g. neodymium, in particular in a powdery form, as well as alkali metals, e.g. sodium and potassium, and also a fire extinguishing agent suitable therefor, without the above-described problems and disadvantages of the prior art methods and fire extinguishing agents. Thus, the fire extinguishing agent of the invention is a powdery blend comprising: (a) a powder of boron oxide having a particle diameter in the range from 5 to 1000 µm, of which the content of B₂O₃ is at least 90% by weight and the content of water does not exceed 2% by weight; and (b) inorganic particles having a generally spherical particle configuration, which are either (b-1) glass beads having a particle diameter in the range from 5 to 200 µm and rendered hydrophobic on the surface; or (b-2) hollow microspheres of silica·alumina having a particle diameter in the range from 50 to 600 µm. In particular, it is preferable that the water content in the boron oxide powder as the component (a) is 0.5% by weight or less. The blending proportion of the components (a) and (b) is in the range from 95:5 to 70:30 by weight. The method of the present invention for extinguishing a metal fire accordingly comprises sprinkling the above-defined powdery fire extinguishing agent over and to cover the burning site of the metal. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As is described above, the fire extinguishing agent according to the invention is a binary blend of a boron oxide powder as the component (a) and an inorganic powder of generally spherical particles as the component (b), which latter serves to prevent the boron oxide powder as the principal ingredient from coalition or caking and to enhance the flowability of the powder. The boron oxide powder as the principal ingredient in the fire extinguishing agent should be sufficiently pure to contain at least 90% by weight of B₂O₃ and no more than 2% by weight, or preferably than 0.5% by weight, of water. The reagent-grade boron oxide available on the market contains about 85% by weight of B₂O₃ and about 10% by weight of water forming boric acid with the boron oxide. Boron oxide powders of such a grade cannot be used as the component (a) in the fire extinguishing agent of the present invention. Further, commercially available boron oxide of analytical grade contains about 97% by weight of B₂O₃ and about 2% by weight of water and can be used as the component (a) of the fire extinguishing agent but is not the preferred grade. A boron oxide powder which is wholly satisfactory for this purpose can be obtained by a heat treatment of the above-mentioned analytical-grade boron oxide powder, for example at 160°C for about 2 hours, so as to decrease the water content therein to 0.5% by weight or lower. The boron oxide powder should have a particle diameter in the range from 5 to 1000 µm. In particular, a boron oxide powder having a particle diameter in the range from 5 to 200 µm is suitable as a filling of fire extinguishers of the cartridge type or stored-pressure type while a powder having a particle diameter in the range from 200 to 1000 µm is suitable for sprinkling using shovels, buckets and the like. A fine boron oxide powder having a particle diameter smaller than 5 µm cannot be used in the fire extinguishing agent of the present invention because too fine boron oxide particles are readily blown and scattered away by the violently rising flame and may as a result not effectively cover up the burning site. When the boron oxide powder is too coarse on the other hand, an unduly long time would be taken before the boron oxide particles are melted to form an air-shielding layer, in addition to the problem that a somewhat larger amount of the powder must be sprinkled to form a covering powder layer having a sufficiently high suffocating effect. The component (b) of the fire extinguishing agent to be blended with the above-described boron oxide powder is an inorganic powder of generally spherical particles which can be either (b-1) glass beads or (b-2) hollow microspheres of silica·alumina. These spherical particles of the inorganic powder should be surface-treated with a suitable water-repellent agent such as a silicone oil so as to be rendered hydrophobic or water-repellent, since otherwise the particles may absorb moisture and lose flowability when they are kept standing in the atmospheric air. Typically, the spherical glass beads suitable for use in the fire extinguishing agent according to the present invention have a particle diameter in the range from 5 to 200 µm and an apparent density of 2.5 g/cm³. Chemically, the glass of the beads preferably contains of the order of 72% of SiO₂, 14% of Al₂O₃, 13.5% of Na₂O and K₂O as a total, 9% of CaO and 3.5% of MgO. When properly surface-treated with a silicone oil to be rendered hydrophobic, the glass beads have an angle of repose in the range from 24° to 28°. Hollow glass spheres can also be used as a substitute for the above-described glass beads. The spherical glass beads are surface-treated to impart hydrophobicity or water-repellency. The surface treatment can be performed, for example, by dipping the glass beads in a suitable organosilicon compound, including organochlorosilane compounds, e.g. methyl chlorosilanes and derivatives thereof, methyl hydrogen polysiloxanes or derivatives thereof and the like, as an organic solution and drying the beads in air. Instead of the hydrophobic treatment of the boron oxide powder, glass beads and/or hollow microspheres of silica·alumina, a similar improvement for the flowability of the powder blend can be obtained by blending the powder blend with a small amount, e.g. 1 to 2% by weight, of a finely-divided silica powder having an average particle diameter of 90 to 130 µm and rendered hydrophobic on the surface. The blending proportion of the boron oxide powder and the spherical glass beads should be in the range from 95:5 to 70:30 by weight. When the powder blend forming the fire extinguishing agent of the present invention is sprinkled over a burning metal, the air-shielding crust layer formed after extinguishing of the fire may have a high mechanical strength as compared with a layer formed from boron oxide alone. While the particles of the boron oxide powder have a polyhedral particle configuration with inherently poor flowability which is even further decreased when the powder is kept in a vessel for a long period of time, the stability in the flowability of the powder can be greatly improved by blending the boron oxide powder with glass beads having a spherical particle configuration and a very small angle of repose as is mentioned above. Alternatively, hollow microspheres of silica·alumina can be used as the component (b) in the fire extinguishing agent according to the present invention in place of the glass beads; these microspheres can be prepared by subjecting naturally occurring and refined volcanic glass particles to a heat treatment with rapid temperature increase to cause softening of the particles and vaporization and expansion of the structural water. Typically, the silica·alumina-eased hollow microspheres have an apparent density of 0.15 to 0.20 g/cm³ and a particle diameter in the range from 50 to 600 µm. Chemically, the silica·alumina-based hollow microspheres are preferably composed of quantities of the order of 76% of SiO₂, 14% of Al₂O₃ and 10% of other oxides and have a melting point of about 1200 °C. The silica·alumina-based hollow microspheres have an angle of repose in the range from 30° to 32°. The blending proportion of the boron oxide powder and the silica·alumina-based hollow microspheres should be in the range from 95:5 to 70:30 by weight in order to prevent the powder blend from coalition or caking and to improve the flowability of the powder blend. In the following, the fire extinguishing method and the fire extinguishing agent of the invention are described in more detail by way of examples. Example 1. A boron oxide powder of a polyhedral particle configuration having an average particle diameter of 60 µm, apparent density of 1.15 g/cm³ and angle of repose of 43.2° and containing 98% by weight of B₂O₃ and 0.5% by weight of water was blended with (1) glass beads which had been subjected to a hydrophobic treatment and which had an average particle diameter of 45 µm, apparent density of 1.40 g/cm³ and angle of repose of 24.6°, (2) silica·alumina-based hollow microspheres having an average particle diameter of 200 µm, apparent density of 0.18 g/cm³ and angle of repose of 31.0° or (3) a combination of these two kinds of inorganic powders in blending proportions of (a) 85:15, (b) 90:10 and (c) 85:10:5, respectively, by weight. These powder blends are referred to as the blends A, B and C respectively, hereinafter. Table 1 below shows the overall apparent density and angle of repose of these three blends A, B and C. For native purpose, Table 1 also shows the corresponding values of further powdery blends D and E which were, respectively, a 90:10 by weight blend of the boron oxide powder and talc of an irregular particle configuration having an average particle diameter of 22 µm and a 93:7 by weight blend of the boron oxide powder and mica of a flaky particle configuration having an average flake diameter of 30 µm. Generally speaking, powdery fire extinguishing agents are imparted with higher flowability when the apparent density thereof is smaller. Further, a smaller angle of repose means a smaller tendency toward caking or coalition along with an increase in the flowability. As to the particle configuration, the tendency of a powder toward caking is smaller when the particles have a configuration closer to a true sphere. As is clear from the data for the blends D and E in Table 1, the angle of repose of a boron oxide powder was almost unchanged or rather slightly increased by the adiixture of an inorganic powder having an irregular or flaky particle configuration. The apparent density of the powder blend is also about the same as the boron oxide powder per se. Therefore, only little improvement could be obtained in the flowability of these comparative powder blends, which also exhibited a tendency toward caking in the lapse of time for storage. In contrast thereto, the powder blend according to the invention had a remarkably decreased angle of repose with improved flowability as a consequence of the adiixture of the boron oxide powder with an inorganic powder of a spherical particle configuration, along with disappearance of caking. In particular, the overall apparent density of the powder blend could be remarkably decreased by using the silica·alumina-based hollow microspheres as the inorganic powder, also contributing to the improvement of the flowability of the powder blend. Example 2. The powder blends A, B, C and D prepared in Example 1 as well as the boron oxide powder as such were used as a filling of a portable fire extinguisher. Thus, a portable fire extinguisher was filled with 5.0 kg of one of the powders or powder blends and pressurized with nitrogen gas to a pressure of 9.5 kg/cm² and the thus powder-filled extinguishers were stored at room temperature for up to 12 months. Immediately after filling and periodically during the storage period, the valves of the extinguishers were opened one by one to eject the filling powder by the nitrogen gas pressure, so as to determine the amount of the powder or powder blend left unejected in the extinguisher, from which the amount of the ejected powder or powder blend was determined. The results are shown in Table 2 below. The boron oxide powder used in the above-described tests contained at least 98% by weight of B₂O₃ and less than 0.5% by weight of water and had a particle size distribution in the range from 5 to 200 µm. The glass beads and the silica·alumina hollow microspheres were respectively those described before as a typical product. As is clear from the above-given results of the test, the fire extinguishing agent according to the present invention is very stable in respect of ejectability from the fire extinguisher as compared with the boron oxide powder alone or a blend of the boron oxide powder with talc, over a long period of storage. Example 3. A 20 g heap of magnesium powder on the centre portion of a stainless steel-made dish of 30 cm diameter was set on fire using a gas torch. When the fire had spread all over the surface of the powder heap, the powder was shuffled so that the powder burnt violently, raising white bright flames with evolution of intense heat. Then, the fire extinguishing test was conducted by sprinkling one of the powders A, B, C and D and the boron oxide powder used in the preceding examples in amounts of 19g, 15g, 19g, 22g and 18g, respectively. The effectiveness in fire extinguishing was good in each of the tests using these five kinds of powders, with efficient suppression of the flames and without smoking and embers left after extinguishing, except that a small noise was heard during the sprinkling of the powders C and D. Following are the remarks on the surface condition after extinguishing. Powder A: a hard crust layer formed, complete melting indicated at the high-temperature portion Powder B: a hard crust layer formed, granular appearance at the high-temperature portion Powder C: a very hard crust layer formed, granular appearance at the high-temperature portion Powder D: a somewhat brittle and granular crust layer formed at the high-temperature portion Boron oxide powder: a strong, glassy crust layer formed at the high-temperature portion As is apparent from the above-given results of the fire extinguishing tests, the admixture of the inorganic powder of spherical particles with the boron oxide powder does not cause any decrease in the fire extinguishing effect of the powder. Although the above-described tests were conducted by using a magnesium powder, substantially the same good results of fire extinguishing can be obtained even in extinguishing a fire on a powder of zinc, titanium, zirconium, iron, rare-earths and other metals.

Claims: A powdery fire extinguishing agent which is a blend comprising:- (a) from 95% to 70% by weight of a powder of boron oxide having a particle diameter in the range from 5 to 1000 µm, of which the content of B₂O₃ is at least 90% by weight and the content of water does not exceed 2% by weight; and (b) from 5% to 30% by weight of an inorganic powder of particles having a generally spherical particle configuration, which are either (b-1) glass beads having a particle diameter in the range from 5 to 200 µm and rendered hydrophobic on the surface, or (b-2) hollow microspheres of silica·alumina having a particle diameter in the range from 50 to 600 µm. A fire extinguishing agent as claimed in claim 1, wherein the water content of the boron oxide powder does not exceed 0.5% by weight. A fire extinguishing agent as claimed in either of the preceding claims, wherein the glass beads have been rendered hydrophobic by dipping in an organic solution of an organosilicon compound and drying in air. A fire extinguishing agent as claimed in either of claims 1 and 2, wherein the powder blend also contains a small amount of a finely-divided silica powder having an average particle diameter of 90 to 130 µm and rendered hydrophobic on its surface. A method for extinguishing a fire of a burning metal which comprises sprinkling over the burning metal a powdery fire extinguishing agent which is a blend comprising: (a) from 95% to 70% by weight of a powder of boron oxide having a particle diameter in the range from 5 to 1000 µm, of which the content of B₂O₃ is at least 90% by weight and the content of water does not exceed 2% by weight; and (b) from 5% to 30% by weight of an inorganic powder of particles having a generally spherical particle configuration, which are either (b1) glass beads having a particle diameter in the range from 5 to 200 µm and rendered hydrophobic on the surface, or (b-2) hollow microspheres of silica alumina having a particle diameter in the range from 50 to 600 µm.

Inventors: YAMAGUCHI HISAYOSHI, YAMAGUCHI, HISAYOSHI
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY  LIMITED
Application year: 1990
Main classification: A62D1
IPC codes: A62D1, A62C3