US2894864A - Method for inactivating metallic powder - Google Patents

Description

United States Patent lVIETHOD FOR INACTIV ATIN G METALLIC POWDER No Drawing. Application September 3, 1953 Serial No. 378,406

Claims priority, application Sweden September '1-9, 1952 1 Claim. (Cl. 1486.3)

This invention relates to a method for inactivating metallic powder. Metallic powder-oftenpossesses the property of reactingmore. or less rapidly with other substances and this property may be. employed, for example,'in.order to utilize the generation of. heat. resulting from such. a re- .action. For many other purposes it is, however,limportant, to reduce this tendency of reacting with v.other substances. In those cases when metallic powldersform part of pyrotechnic compositions and explosives it is desirable, for example, that the metallic powder be given 'a certain inactivity so that uncontrolled reactions and,

thus, qualitative changes do not take place during storage. No perfect method for such inactivation is known. Trials with treatment with chromate or bi-chromate have only given results of a relatively limited range. In so far as impregnations, for example with fatty acids, have been employed the aim has not been an inactivation but to achieve other properties in the powder, for example to increase its coating capacity in colours and dyes (by so called polishing).

The main object of the invention is to provide a method for inactivating oxidizable metallic powders, especially such as are ingredients in explosives, pyrotechnic compositions, gas or smoke-producing mixtures etc. According to this method the grains of powder are coated by a thick coating or layers of oxide, appropriately of a greater thickness than that formed by oxidation at ordinary air temperature. Such thick oxide layer may, for example, be obtained by heating the metallic powder, while stirring, in air at a raised temperature and the metallic powder is then impregnated with a high-molecular fatty acid, for example by treating it with a solution of such an acid in an organic solvent or a mixture of several organic solvents.

In general it is most advantageous to subject this metallic powder to a pickling or cleaning and washing process before the heating iniair in order to remove the layer of oxide or other coatings which have been formed in the air or other layers with which the powder may be coated.

The temperature to which the powder shall be heated depends to a considerable extent on its nature. Relatively large ranges of temperature may be used and the duration of the heating period will depend on the temperature which is chosen for the heating. For aluminium powder, for instance 400600 C. may be required. It is advisable, however, to carry out the oxidation in the vicinity of the melting point of aluminium, for exam ple between 580 and 600 C. For magnesium and Zinc powders, on the other hand, considerably lower temperatures must be employed, for example a range of temperatures of 300400 C. for Mg, appropriately 380-400 C., and for Zn a range of temperature of ZOO-300 C., appropriately 280300 C. It is advisable in many instances before impregnating the oxidation layer of the grains of the metallic powder to reinforce it by treating the powder 'pens as a result of; the impregnation.

with water, and the simplest way of eifectingthis is to immerse and cool. the metallic powder in water directly after it has been subjected to its oxidation heating. The improvement obtained by doing so appears to be due to a closing of cracks which have arisen in the oxide layer. in the course of oxidation.

The impregnation with high-molecular fatty acids can be carried out in ditferent ways, essentially depending upon the state of aggregation of the fatty acid. Of the fatty acids which may 'be' considered stearic acid has proved suitable, and an impregnation with such an acid is most easily effected by mixing the powder with a solution of this acid in one or more organic solvents, which after the conclusion of the impregnation process are removed .by filtering and/or evaporation. Theimpregnation is carried out during the mixing of the products according to known methods.

It is diflicult to decideto .a full extent what really hap- It isprobable that at least on its surface the oxide layer forms with the fatty acid a layer of metallic soap which is difficult'to packingeffect deep into the oxide layer.

The invention can be applied'to oxidizable metallic powders of all kinds. The method will, however, be exemplified for the metals aluminium, magnesium and zinc which especially can be used in explosive technics and pyrotechnics.

Example 1.An aluminium powder is mixed and suspended and stirred in Z-normal nitric acid for half an hour and is then washed with water and dried. The aluminium powder is then heated in a rotating furnace to 600 C. during forty-eight hours after which it is rapidly cooled down and stirred in cold water for about 10 minutes. The powder is then dried for two hours at C. The powder thus treated is mixed or suspended in a 10% stearic acid solution with equal parts of benzene and alcohol as solvents, said suspension being carried out for about half an hour. After that drying takes place while the solution is sucked off with heating for about one hours time. The powder is stored at 60 C. for about 10 days.

Example 2.-A magnesium powder is pickled with a 0.5% ammonium chloride solution for five minutes and washed with water and dried. The powder is heated in a rotating furnace at 400 C. for two hours and then cooled down rapidly in cold water for five minutes with stirring, and dried for two hours at 80 C. The powder thus oxidized is mixed or suspended in a 10% stearic acid solution with equal parts of benzene and alcohol for about half an hour. Drying is then carried out while the solution is being sucked oil with heating for a period of one hour. The powder is stored at 60 C. for about 3 days.

Example 3 .A zinc powder is pickled with 5% oxalic acid for 15 minutes, washed with water and dried. The powder is heated in a rotating furnace at a temperature between 200300 C. between 3 and 24 hours. The powder is then suspended in a 10% stearic acid solution with equal parts of benzene and alcohol as solvents for about half an hour. Drying is then effected while the solution is being sucked off in heat for one hour. The powder is stored at 60 C. for 5-10 days.

The thickness of the impregnation coating naturally is of certain importance but it has proved possible, while maintaining the protective elfect, to reduce this thickness considerably, which is important with regard to the thermic reactions which it may be desirable to release in explosives and pyrotechnic mixtures. The thickness of Reagent Powder treated Powder not treated H10, 70 C no reaction reacts after about -10 minutes.

H01 conc reaction after 2 momentary reaction.

minutes.

H01 4-11 heat increase 7 C. sharp reaction after after which the 50 seconds. reaction slowly diminishes.

N aOH 4-n do sharp reaction after ca. 15 seconds.

Ba/NO 60 0 no reaction reaction after 1 hour.

The invention is not limited to the examples shown but can be varied in several ways within the scope of the invention. The oxidation can thus be carried out by heating in an atmosphere of refined gas with an addition of oxygen, or other oxidizing gas. The oxidation can also be eifected by means of an oxidizing liquid.

What we claim is:

A method of inactivating aluminum powder for pyro- '7 technics and explosives comprising the steps of oxidizing aluminum powder in which the individual grains are mainly free from any surface layer of oxide, by heating said powder in a gas containing free oxygen to a temperature of 400 to 600 C., during a time of 2 to 3 hours cooling the powder thus treated, suspending the powder in a 10% solution of highmolecular weight fatty acid in anorganic solvent, removing said solvent, drying the powder, and storing the said powder at an elevated temperature not exceeding C., said aluminum powder as a result being provided with superimposed coatings, an inner coating consisting of an aluminum oxide layer and an outer coating consisting of a fatty acid and fatty acid soap layer, said aluminum powder being capable of re- 'sisting attacks from atmospheric humidity and water or salt solutions.

. References Cited in the file of this patent UNITED STATES PATENTS Re. 16,340 Backer May 4, 1926 1,137,986 Kuttner May 4, 1915 1,204,709 Smelling Nov. 4, 1916 1,869,041 Bengston July 26, 1932 1,996,392 Torrene Apr. 2, 1935 2,120,468 Hybinette June 14, 1938 FOREIGN PATENTS 218,619 Great Britain Apr. 29, 1924