US1351865A - Process for the manufacture of magnesium powder - Google Patents

Description

D. S. NICOL.

PROCESS FOR THE MANUFACTURE OF MAGNESIUM POWDER.

APPLICATWON FILED JULY 23.1911.

1,35 1,865. PatentedSept. 7, 1920.

UNITED STATES PATENT OFFICE.

DAVID S. NICOL, OF MONTREAL, QUEBEC, CANADA, ASSIGNOR TO SHAWINIGAN ELECTED-METALS COMPANY, LIMITED, 01' MONTREAL, QUEBEC, CANADA.

PROCESS FOR THE MANUFACTURE OF MAGNESIUM POWDER.

Specification of Letters Patent.

Patented Sept. 7, 1920.

Application filed July 23, 1917. Serial N0. 182,285.

-Province of Quebec and Dominion of Canada, have invented certain new and useful Improvements in Processes for the Mannfacture of Magnesium Powder, of which the following is a full, clear, and exact description. a

This invention relates to improvements in the process of powdering magnesium, and the object of the invention is to provide a method by which powdered magnesium may be produced more rapidly and more cheaply than heretofore.

A further object is to rovide a process in which the dangers of re and explosion in the existing processes are for practical purposes eliminated.

A still further object is to provide a process which will produce a free flowing powder, that is to say, a powder which does not tend tocling in masses or lumps when poured.

Another object is to provide a powder which will have much better keeping properties, that is to say, will be less readily oxidizable by ordinary atmospheric action than the powders now manufactured.

Still another object is to provide a process of manufacture in which the size of the powder grains can be regulated to a certain extent, so that the loss incident to the production of smaller or larger grains than desired is less than at present.

Before proceeding with a detailed description of old and new processes, it appears advisable to state that so-called magnesium powder is for the most part comparatively coarse, so that the individual grains can be readily distinguished with the naked eye. The size of the grain of course varies with the purpose for which the powder is intended, so that some grades may be very fine, resembling flour. It is well known that magnesium has a great aflinity for both oxygen and nitrogen, and when ignited, burns rapidly. This burning is so rapid with finely powdered magnesium that it may very properly be described as explosion. These qlialities of magnesium, together with others which need not be enlarged upon, make the operation of pulverizing it both difiicult and dangerous.

At the present time, the method used is to cast the magnesium into bars or ingots, which are reduced to powder by sawing, turning or milling. The cuttings produced are classified by screening, those too large for use being ground in some formof mill to reduce them to the proper size.v This method requires a large and costly plant, and a very considerable outlay for labor to attend the large number of machines. The process is very slow and tedious, owing both to its nature and to the constant necessity of starting new cuts, replacing pieces of metal which are too short to be further treated, and re-casting the remnants into suitable shape. The large number of machines in motion and the difiiculty of collecting the fine powder makes the plant an extremely dangerous one, as friction may at any time ignite the fine'powder. Another of the disadvantages of this method .is the difliculty of melting magnesium without considerable loss, so that the constant melting and re-melting of the metal to utilize the stubs of bars multiplies this loss. The product resulting from this method is far from satisfactory. Even after careful screening, the grains are not of uniform size, owing to their very irregular shape. When examined microscopically, the grains are found to be in the form of chips 'or shavingspresenting knife edges which cause the grains to cling together, and also present a large amount of surface for oxidation. These characteristics of the powder render it unstable and diflicult to deal with in certain applications.

According to the present invention, the process, in place of being an intermittent one, is continuous, and eliminates .to a large extent re-melting, so that a certain loss is overcome in this way. The individual grains of the powder produced are approximately spherical in form and of high metallic luster, so that they present the minimum of surface for oxidation, and that surface is clean and smooth, and therefore more capable of resisting oxidation. The shape of the grains and the smoothness of the surface make a powder which is very mobile. The apparatus required is inexpensive and the output larger than with the old method. The result is a much superior product obtainable at greatly reduced cost.

The process consists essentially in melting the magnesium and discharging it under pressure in a fine stream into a jet of cold and therefore safe to use.

gas, so that the stream of molten metal is broken up into minute particles and instantly chilled. By regulating the .discharge of metal and the velocity of the gas jet, the size of the powder produced can also be regulated within limits.

In greater detail, the process is as follows :The metal is melted in a. closed vessel, and in order to maintain the metal in clean workable condition, a flux of lithium chlorid is employed. This flux melts and floats on top of the molten metal, and forms a protective screen between the metal and the atmosphere. The flux is necessary as all attempts to melt the magnesium in a neutral atmosphere alone have been unsatisfactory. The magnesium is therefore melted under the flux of lithium chlorid in a closed vessel, and a gas pressure maintained in the vessel above the flux suflicient to expel the metal from the vessel. The gas used is preferably hydrogen. The

metal is allowed to escape from a point near the bottom of the vessel through a suitable valve and nozzle. The nozzle has preferably a small bore, so that the metal emerges in a'fine stream or jet, which may be readily broken up into small particles by a jet of gas of suitable velocity.

Molten magnesium at a bright red heat will not combine with hydrogen, but willcombine with nitrogen to form magnesium nitrid. This action takes an appreciably longer time than the combination with oxygen or carbon dioxid or carbon monoxid.

Asair is composed of oxygen, nitrogen andtraces of carbon dioxid, it is obviously impossible to use an air jet to break up the stream of molten metal. Theoretically, hydrogen would be the ideal gas, but in practice, its use is attended with so much danger that it cannot be considered. Carbon dioxid combines too readily with metal to be practicable, and is moreover somewhat dangerous. Nitrogen, although it combines with the molten metal, does so more reluctantly than the other gases mentioned, and has the advantage of not combining with the cold metal. It further has the advantage of being non-poisonous and non-inflammable, Since the period of contact between the gas jet and the hot metal is so brief, ithas been found entirely practicable to use nitrogen for the disintegrating jet. When the stream of metal is small and the jet of nitrogen, of sufiicient I volume and velocity, the metal stream is to combine with nitrogen ceases, and the tom into fine particles and chilled so quickly that the amount of nitrid formed is so small as to be negligible. When once the metal is chilled to solid state, its ability product therefore remains unchanged. The jet of nitrogen blows the metal dust or powder 1nto a suitable receptacle, where the powder may be separated from the nitrogen and removed for ading in any suitable screening device. and compressed for re-use. By a careful regulation of the volume and velocity of the metal and nitrogen jets, the size of the powder produced may be regulated suiiiciently to insure the formation of practically no powder of unmarketable size. If powder of undesirable size is produced, it may be remelted. The metal being molten, and therefore liquid, at the. time it is broken up by the nitrogen jet, possesses the characteristics of cohesion and surface tension common to all liquids, in a sufiicient degree to cause the particles of metal to assume spherical or approximately spherical form before they are chilled.

In melting the magnesium and maintaining-it in molten state, care must be taken not to overheat the metal, that is to say, not to raise it above the temperature which will keep it nicely fluid. If too high temperature is maintained, not only the metal suffers directly but the flux is decomposed and forms a slag or dross which settles to the bottom of the crucible and necessitates a thorough cleaning out of the apparatus. The column of molten metal above the discharge outlet may not be sufiicient to discharge the metal by gravity at the rate desired, and therefore it may be necessary to resort to pressure means. This is most conveniently accomplished by introducing gas under pressure into the crucible above the metal, so that the gas ressure will tend to drive the metal out. or this purpose, hydrogen is most satisfactory, as it does not combine with the magnesium, and as only small quantities are used, there is little or no danger of explosion. It will be understood, however, that the invention is not limited to the use of hydrogen, or any gas above the metal nor to the use of nitrogen as a disintegrating medium.

The process in short therefore consists in melting magnesium in a closed vessel, protected by a flux, and discharging the molten metal ina fine stream into a jet of gas of suitable velocity, which tears the stream of molten metal into the multitude of fine particles, and subsequently separating the metal and gas and possibly compressing the gas for re-use.

Obviously the process is not limited to any particular apparatus, but the apparatus hereinafter described has been found suitable.

In the drawings Figure 1 is a vertical sectional view of the apparatus as a whole.

Fig. 2 is an enlarged sectional view of the crucible outlet.

Fig. 3 is a front elevation of the he nitrogen is collected Fig. 4 is a section on the line 44, Fig. 1. Fig. 5 is a fragmentary vertical sectional view, showing the clean-out of the. crucible.

Fig. 6 is an enlarged fragmentary sectional detail view of the clean-out.

Referring more particularly to the drawings, 11 designates a crucible of suitable shape and material having lugs 12, by means of which it is suspended in a suitable furnace 13, the lugs resting on metal bearers 14 in the furnace walls. The crucible is provided with a cover 15 carrying a charging chute 16, which projects through the top of the furnace and is provided with a fluid tight cover 17. It is essential that the joints of the crucible be suiiiciently'tight to hold hydrogen gas under low pressures. At the bottom of the crucible, a central raised portion 18 is provided, into which the outlet pipe 19 is screwed. This outlet pipe projects some distance above the raised portion 18, so that any dross which may collect in the bottom of the crucible will not enter the pipe. The pipe is provided near its lower end with a valve 20, controlling the discharge of molten metal from the crucible. The lower end of the pipe is provided with a hollow nut 21 holding a nozzle 22, preferably of refractory material, such as graphite. It is preferred to use refractory material at this point rather than metal, as the part is quite small and it is essential to the continued efficiency of the machine that the bore of the nozzle remain constant. Metal A drain or clean-out is provided for the crucible and comprises a pipe 23 of suitably large bore, screwed at its upper end into the lowest part of the crucible, and at its lower end into a casing 24, within which a valve 25 is pivotally mounted. This valve comprises a metal cup 26 containing a plug 27 of refractory material, such as graphite, which bears against the end of the pipe 23. The valve is held closed by a screw 28, operating in a nut 29 removably trunnioned in the casing 24. A handle 30 is provided to hold the valve up out of the path of material fiowing through the drain.

The bottom supporting plate 31 of the furnace carries a depending casing 32 which may be termed the spraying or disintegrating chamber. This chamber is in the form of an inverted pyramid having the part 33 toward the apex removable. This part forms a cup for the collection of material not carried out of the chamber. One side of the casing 32 is provided with a large opening giving access to an inclined laterally projecting chamber 34, which may be termed the collecting chamber. Opposite the collecting chamber, an adjustable plate 35 is provided carrying a gas pipe 36 having a nozzle 37, shown in detail in Fig. 3. This nozzle has an arcuate orifice 38 subtending an angle of 180 or more. It will thus be seen that a jet issuing from this nozzle is trough-shaped. The nozzle is connected to the pipe 36 at such an angle that the jet issuing therefrom follows approximately the longitudinal axis of the collecting chamber 34 or a path slightly above the axis. At the lower end of the collecting chamber, a sump 39 is provided for the c0llection of metal powder, which may be re- 'moved through a slide 40 at the bottom of the sump. At the top of the collecting chamber is a separating chamber 41' containing screens 42 adapted to separate fine metal dust from the gas used. Above the screens a pipe 43 is provided through which the gas maybe drawn off for re-use. A cover 44 is provided for the separating chamber to permit cleaning of the screens 41.

The collecting chamber and the gas pipe occupy opposite sides of the pyramidal casing 32, and the remaining sides are occupied one by a housing 45 containing a suitable illuminating element 46 and the other by a suitable peep-hole 47; The peep-hole is preferably provided with a transparent panel 48 carried a suflicient distance from the chamber to prevent it from becoming clouded. The open ends of the housing 45 and the mounting of the transparent panel 48 are provided with shutters 49 to prevent accumulation of metal powder therein. It will be noted that the peep-hole and light are directly in line with one another, and are in line with the point where the stream of molten metal issuing from the crucible meets the jet of gas from the nozzle 37. 50 indicates an opening near the top of the crucible for the introduction of the hydrogen thereinto.

The advantages of this invention will be apparent from the foregoing description, although they will be better appreciated by one skilled in the art than by those not familiar with the handling of magnesium. The apparatus required is simple and inexpensive. The gases used in the process are now obtainable in large quantities at comparatively low cost, so that the cost of operation on this score is quite reasonable. The process may be continuous by merely supplying to the melting pot at stated intervals metal to replace that discharged, and collecting the gas used for disintegration. This operation is comparatively simple, so that one attendant may have charge of a number of machines. The size of the metal particles may be regulated to such an extent that the formation of waste or unmarketable product is practically eliminated. The particles being smooth'and approximately spherical in shape may be graded for size much more accurately than is possible with the irregular sharp-edged particles produced by the present machining processes. Whether or not the particles are spherical or even approximately so, the fact remains that they are smooth and free from sharp fore the powder is more moblle than the machine powder, that is, it will pour more readily and regularly from a receptacle,and has no tendency to cling together in masses. The smooth surface and high metallic luster obtained by this rocess present much less surface for oxidation, and also a surface better calculated to resist oxidation, so that the product has much more stabilit than the machined product. The output 0 tained per unit of capital, or operating cost or space occupied is much greater than can be obtained with the existing processes.

Having thus described my invention, what I claim is 1. A process for the production of magnesium powder, which comprises melting the magnesium in a closed vessel under an inert flux, maintaining hydrogen gas at super-atmospheric pressure in the vessel above .the flux to expel molten metal from the lower part of the ,vessel, and directing a jet of gas againstthe stream of molten metal,

said gas jet being of a volume, velocity and temperature suflicient to disintegrate the edges; therestream into fine particles and chill the particles before chemical combinations of the metal and gas can form.

2. A process of producing magnesium powder, which comprises directing a stream of comparatively cold nitrogen gas against a stream ofmolten magnesium, the gas having volume and velocity sufiicient to tear the stream of molten metal into minute particles and the temperature of the gas being such that the metal particles will chill and solidify too rapidly for the'formation of magnesium-nitrogen compounds.

3. A process for the production of magnesium powder which comprises meltin the magnesium in an atmosphere of a gas w 'ich will not combine with the magnesium at high temperatures and high pressures, and discharging the molten magnesium into a nitrogen atmosphere and into a nitrogen jet by which the molten metallic stream is disintegrated into fine particles.

4. A process for the production of magnesium powder, which com rises melting the magnesium in an atmosp ere of h drogen under cover of an inert flux, disc arging the molten magnesium into an atmosphere of nitrogen, and subjecting the discharged metal to a jet of gas whereby the mass is disintegrated into fine particles.

In witness whereof, I have hereuntoset my hand.

- DAVID S. NICOL.

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