US2425711A - Production of metal hydrides - Google Patents

Description

Patented Aug. 19, 1947 UNITED STAT E S PATENT OFFICE 2,425,711 PRODUCTION OF METAL HYDBZIDES Peter P. Alcxanden'Marlilchead, Mass, assignor to Metal Hydrides Incorporated, Beverly, Mass, acorp'oration of Massachusetts Application July 8, 1944, Serial No. 544;047

(ClI'23 -204) 14 Claims.

This invention relates to the production of metal hydrides and has for its object certain improvements in the method of producing metal hydridesv Metal hydrides are formed by bringing hydrogen gas into contact with a hydride-forming metal at its temperature of reaction. The reaction is an exothermic one, resulting in an evolution of considerable heat. -While external heat is a temperature so high that the metal hydride cannot be formed. 7

It is customary toplace the metal to be hydrided in a heated retortwhich isclosed and evacuated to remove air. Hydrogen gas-is-then fed into the retort; the'retort is externally'heated until the metal reaches its temperature of reaction; and the conversion of the metal to its hydride then-begins. The exothermic heat resulting from the reaction tends promptly to elevate the temperature still further and'the application of external heat to the'retortis, therefore, discontinued. To render the retort gastight, itis customary to dispose agasket between the top of the retort and its cover. The'hydrogen initially fed into the retort is consumed very rapidly and a partial vacuumtends to be established within the retort. If the gasket or any part of the retort is notperfectlytight, airis drawn into the retort with the result that partially hydrided metal isoxidized. If the air continues to seep into the retort, the oxidation proceeds until the temperature is so high that hydrogen "previously consumed by the metal, to form metal hydride, is evolved from the partially hydrided metal with the resultant formation of a dangerously explosive mixture inthe-retort.

If the metal to be hydrided is placed in the retort in the form of ingots, for example, overheating by the exothermic heat may not only fuse the ingots together but may fuse them" into a single mass. This result is highly disadvantageous for several reasons. In the firstplace, the metal tends to fuse against the retort wall, which is usually steel, and therefore takes on impurities from the retort. "Second, it is usually desired to have the ingot of metal converte'dinto 'hydrides.

an ingot of metal hydride'of substantially the same size and shape. Fusion of the ingots makes this result impossible. Third, fusion of the ingets into a single mass results in a hard and compact cake which is removedfrom the retort with considerable diiiiculty. Fourth, fusion of the ingots into asingle mass makes it practically impossible for the hydrogen completely to permeate the mass and the efficiency of the hydriding action is, therefore, impaired.

The problem of overheating is met in the production generally of metal hydrides. It is a particularly serious problem, however, when converting the more highly reactive metals to their This is especially true of the alkaline earth metals, such as calcium, barium and stroncordance with the invention, the rate offer-mation of the metal hydride is slowed down by diluting the hydrogen with an inert gas. The-inert gas is used in amount surficient-to inhibit objectionable overheating of the metal bythe exothermic heat ofthe reaction.

Since the hydride-forming metal is confined in a reaction zone,-in a-closed retort, dilution-of the hydrogen with an inert gas is made to result ina lowerconcentration of hydrogen than would be the case if the hydrogen were not diluted. In other words, whereas the reaction zone would normally be filled solely with hydrogenat a-given pressure, it is now filled withan inert gas admixed with a relatively small amount of hydrogen at the same pressure. That relatively small amount of hydrogen is'all the hydrogen available for reaction with the hydride-forming metal until a further it amount is fed to the reaction zone. By limiting the amount of hydrogen thus fed to the reaction zone, the-rate of formation of the metal hydride may be regulated; and by regulating the rate of formation of the metal hydride, the rate of generation of exothermic heatmay be regulated. Therefore, by diluting the hydrogen in the reaction zone, overheating of the metal or metal hydride may be avoided. With little experience an operative can soon hit upon a suitable dilution to prevent a rise in temperature sufiicient to fuse the metal to be hydrided, or to dissociate the metal hydride formed, or to prevent the formation of metal hydride, while at the same time maintaining the reaction zone at a temperature at which the desired hydriding action takes place.

A sufiicient amount of inert gas is fed into the reaction zone to keep it under positive pressure as the reaction goes to completion. Due to the presence of the inert gas, which does not react with the hydride-forming metal, the consumption of the limited amount of hydrogen in the reaction zone by the hydriding reaction does not cause a partial vacuum to be established within the retort. The pressure of the inert gas in the retort is sufficiently high so that when the limited amount of hydrogen in the reaction zone is consumed to produce metal hydride, the drop in pressure is not sufiicient to create a vacuum; in other words, the pressure within the retort does not fall below atmospheric pressure outside of the retort. If, therefore, the retort should develop a leak, air will not seep into the retort.

In a presently preferred practice of the invention, a suitable amount of inert gas is introduced into the retort, and hydrogen gas is then fed automatically into the retort at a predetermined pressure until the reaction has gone to completion. That is, when the pressure within the retort drops below a given level, hydrogen is automatically fed into the retort to restore the pressure. Theamount of inert gas in the retort and the amount of hydrogen progressively fed into the retort may be so correlated that the concentration of hydrogen in the retort does not rise to a point at which the rate of formation of the metal hydride results in an excessive generation of heat. This control may in fact be used to maintain the reaction zone at an optimum temperature for the desired conversion.

Thesexand other advantages of the invention willbe better understood by referring to the accompanying drawing, taken in conjunction with the following description, in which:

Fig. 1 is a diagrammatic representation, in cross-section, of an apparatus that may be employed in the practice of the invention; and

Fig. 2 is a cross-section on the line 22 of Fig. 1.

The apparatus shown comprises an outer retort I 0 supported within a heating furnace II, the retort being held in position by means of two or more supports I 2 resting on the top of the heating furnace. The retort is made preferably of heatresistant steel. It is fitted with a removable cover I 3, a gasket I4 being disposed between the cover and a bent over circumferential portion of the upper end of the outer retort. The removable cover has attached thereto a lower vertical pipe I5 with lateral branch pipes I6 and I! fitted with pressure control valves I8 and I9, respectively. Branch IGis connectable with a source of hydrogen and branch I I with a source of inert gas, such as helium or argon. An upper vertical branch pipe 20, provided with a valve 2 I, connects with the lower vertical pipe I 5 and is connectable with a source of vacuum. A removable inner retort 25 fits within the outer retort.

The heating furnace II consists essentially of a rectangular chamber 26 having a refractory bottom 21, side and end refractory walls 28, and

a refractory top 29 having an opening of a size not much larger than, and adapted to receive, the outer retort. An expanding opening 2-!) is provided at or near a lower corner of one of the side walls of the chamber for the introduction of heating gases into the chamber. A similar opening 3| is preferably provided at or near an opposite corner as a spare or auxiliary means for introducing heating or cooling gases into the chamber. When not in use, opening 3| is suitably closed. A flue opening 32 extends through the same wall as opening 30, preferably at a higher level, so that heating gases passed into the chamber through opening 38 tend to pass almost completely around the outer retort before leaving the chamber through flue opening 32.

The apparatus may be used as follows: A charge of calcium ingots 35, for example, is placed in the bottom of inner retort 25, in a reaction zone. A plurality of the ingots is advantageously stacked on top of one another, as shown, so as to expose as much surface as possible, the ends and sides of the ingots preferably not being in contact with the wall of the inner retort. The inner retort is then placed in outer retort Ill and removable cover I3 is placed on outer retort I0 and locked thereon to provide a sealed joint. Vertical branch 20 is then connected to a source of vacuum. With valve 2| of the vertical branch open, and valves I8 and I9 in lateral branches I6 and I! closed, the outer and inner retorts are placed under vacuum to remove objectionable air.

Heating gases are passed through opening 30 into heating chamber 26 where they circulate almost completely around the retort and then pass through flue opening 32 to the outside atmosphere. As the lower portion of outer retort I 9 is heated, the lower portion of inner retort 25 is also heated. The evacuation of the inner and outer retorts is preferably continued during this initial heating operation because it aids in the removal of objectionable air. On completion of the evacuation step, valve 2| is closed.

Valve I9 in lateral branch I1 is then opened to admit a suitable amount of inert gas, such as helium or argon, into the retorts. Enough inert gas is introduced to create a positive pressure, for example 10 to 15 pounds per square inch, within the retorts. Helium is the more readily available inert gas at the moment but argon is in some respects more desirable because it is heavier and tends to diffuse less slowly from the retorts. A limited amount of hydrogen gas is then passed into the retorts by opening valve I8 in lateral branch I6. The heating gases are passed into the heating chamber until the inner retort and hence calcium ingots 35 reach a temperature at which the hydriding reaction starts.

As soon as the reaction begins, exothermic heat is generated and the temperature of the charge tends to rise still further. Introduction of the heating gases into the heating chamber is then stopped. Hydrogen gas is preferably fed automatically into the retorts at a predetermined pressure until the reaction has gone to completion. This may be accomplished by a suitable pressure control valve. Valve I8 may, for example, be considered as a pressure control valve in lateral branch IE or as a conventional pressure control valve connected to the outlet of a tank of hydrogen gas connected to lateral branch I 6. When the pressure within the retorts drops below a given level, additional amounts of hydrogen are automatically fed into the retorts to restore the pressure. The amount of inert gas maintained in the retorts and the amount of hydrogen progressively fed into the retorts may be so correlated that the concentration of hydrogen in the retorts does not rise to a point at which the heat of formation of the calcium hydride results in an excessive rise in temperature. This control may be used to maintain the reaction zone in the inner retort at an optimum temperature for the desired conversion, and thus avoid overheating together with its objectionable results.

While the method of the invention has been illustrated with calcium, it will be clear that the other alkaline earth metals are applicable, such as barium and strontium. The method may also be practiced with the alkali metals, such as sodium, potassium and lithium. It may also be practiced with other hydride-forming metals. The invention is more particularly applicable to the highly reactive hydride-forming metals, especially in their primary form. While not as important, it will also be clear that the invention is applicable to the production of the so-called impure metal hydrides, such as those made by the magnesium process; for example, by reacting an alkaline earth or an alkali metal oxide with magnesium in the presence of hydrogen gas.

By regulating the amounts of inert gas and hydrogen gas. the reaction zone may be readily maintained at a temperature sufiicientl-y high for the desired hydriding reaction to take place and at the same time sufficiently low to prevent undesired fusion of the hydride-forming metal. Under these optimum operating conditions, the ingots 0f hydride-forming metal are converted into ingots of metal hydride of substantially the same size and shape.

Various other procedures may be employed in the practice of the invention. For example, the pressure in the retort may be raised a few points above atmospheric pressure by injecting a suitable amount of hydrogen or inert gas or both. This prevents the infiltration of air into the retort if the retort, or gaskets, are not quite tight. If inert gas is initially used to raise the pressure in the retort, hydrogen may then be injected to raise the pressure a suitable amount, for example ten to fifteen pounds. The reaction may then be permitted to proceed until the hydrogen is consumed, At that point the reaction stops, until additional hydrogen is admitted. In this way the speed of reaction and therefore the evolution of exothermic heat may be controlled. In other cases it may be desirable to admit a mixture of hydrogen and inert gas to the retort more or less simultaneously. If the procedure is well established, the dilution may be such that the reaction does not proceed too rapidly and the hydrogen is never completely consumed since it is gradually supplied to the retort. The inert gas acts continuously as a diluting medium or a brake.

While the invention is illustrated with the use of helium as the inert gas, it will be clear to those skilled in this art that other inert gases, such as argon, may be employed, either alone or admixed. The monatomic inert gases appear to be particularly useful. It is important only that the gas employed be inert with respect to the ingredients going into the charge and that it function as a diluent and brake to the speed of reaction or con- Version.

In the production of sodium hydride, the sodium combine with the hydrogen at a relatively low temperature; and since sodium hydride dissociates very rapidly at temperatures above 400 C., it is important to prevent the generation of excessive exothermic heat as the hydride-forming reaction takes place. This can be done by the suitable use of inert gas. The invention is likewise applicable, for example, in the treatment of lithium containing silicon and lithium containing boron to produce lithium hydride, and of sodium containing boron to produce sodium hydride, althoughsuch combinations are not considered true alloys.

It will be clear to those skilled in this art that the invention is applicable generally to the production of metal hydrides and that in the appended claims the'hydride-forming metal may be considered initiallypresent as such or in the form of a compound, or an alloy or other combination; but in any event in a form in which the metal itself is or is made available for reaction with the hydrogen to form the desired metal hydride.

This application is a continuation-in-part of my copending applications Serial Nos. 503,793 and 503,794, filed September 25, 1943. In my copending application Serial No. 544,048, filed July 8, 1944, the claims are directed to the method of producing alkaline earth metal hydrides broadly and calcium hydride specifically in accordance with the invention.

I claim;

1. In the method of producing metal hydrides by reacting a hydride-forming metal with hydrogen gas at an elevated temperature, the improvement which comprises confining a charge of hydride-forming metal in a reaction zone, admitting hydrogen gas and a substantial amount of inert gas to the reaction zone, heating the hydride-forming metal in the reaction zone in the presence of the hydrogen gas and inert gas to a temperature sufficiently high to start the hydriding reaction, limiting the amount of hydrogen gas initially admitted to the reaction zone to limit the exothermic heat generated by the reaction to an amount insufiicient to fuse the heated hydride-forming metal, gradually admitting iurther amounts of hydrogen gas to the reaction zone to continue the hydriding reaction in the presence of the inert gas, and regulating the rate of formation of the metal hydride and hence the rate of generation of exothermic heat until the hydriding reaction has gone substantially to completion in the presence of the inert gas by regulating the amounts of hydrogen gas thus admitted to the reaction zone to prevent fusion of the heated hydride-forming metal.

2. Method according to claim 1, in which the reaction zone is evacuated to remove air therefrom before the hydrogen and inert gases are admitted.

3. Method according to claim 1, in which the amount of inert gas admitted is sufficient to place and maintain the reaction zone and charge under substantial positive pressure.

4. Method according to claim 1, in which the charge is heated to a temperature sufficiently high to start the hydriding reaction by the application of externally applied heat, discontinuing the application of externally applied heat, and continuing the hydriding reaction solely with the regulated amount of exothermic heat thus I generated.

5. Method according to claim 1, in which the hydrogen gas is fed into the reaction zone automatically at a predetermined pressure until the hydriding reaction goes to completion.

6. Method according to claim 1, in which the reaction zone is evacuated to remove air therefrom before the hydrogen and inert gases are admitted, and admitting the inert gas in amount sufiicient to place and maintain the reaction zone and charge under substantial positive pressure.

7. Method according to claim 1, in which the reaction zone is evacuated to remove air therefrom before the hydrogen and inert gas are admitted, admitting the inert gas in amount sufiicient to place and maintain the reaction zone and charge under substantial positive pressure,-

heating the charge to a temperature sufficiently high to start the hydriding reaction by the application of externally applied heat, discontinuing the application of externally applied heat, and continuing the hydriding reaction solely with the regulated amount of exothermic heat thus generated.

8. In the method of producing alkali metal hydrides by reacting an alkali metal with hydrogen gas at an elevated temperature, the improvement which comprises confining a charge of alkali metal in a reaction zone, admitting hydrogen gas and a substantial amount of inert gas to the reaction zone, heating the alkali metal in the reaction zone in the presence of the hydrogen gas and inert gas to a temperature suificiently high to start the hydriding reaction, limiting the amount of hydrogen gas initially admitted to the reaction zone to limit the exothermic heat generated by the reaction to an amount insuificient to fuse the heated alkali metal, gradually admitting further amounts of hydrogen gas to the reaction zone to continue the hydriding reaction in the presence of the inert gas, and regulating the rate of formation of the alkali metal hydride and hence the rate of generation of exothermic heat until the hydriding reaction has gone substantially to completion in the presence of the inert gas by regulating the amounts of hydrogen gas thus admitted to the reaction zone to prevent fusion of the heated alkali metal.

9. Method according to claim 8, in which the reaction zone is evacuated to remove air therefrom before the hydrogen and inert gases are admitted.

10; Method according to claim 8, in which the amount of inert gas admitted is sufficient to place and maintain the reaction zone and charge under substantial positive pressure.

11. Method according to claim 8, in which the charge is heated to a temperature sufiiciently high to start the hydriding reaction by the application of externally applied heat, discontinuing the application of externally applied heat, and continuing the hydriding reaction solely with the regulated amount of exothermic heat thus generated.

12. Method according to claim 8, in which the hydrogen gas is fed into the reaction zone automatically at a predetermined pressure until the hydriding reaction goes to completion.

13. Method according to claim 8, in which the reaction zone is evacuated to remove air therefrom before the hydrogen and inert gases are admitted, and admitting the inert gas in amount suificient to place and maintain the reaction zone and charge under substantial positive pressure.

14. Method according to claim 8, in which the reaction zone is evacuated to remove air therefrom before the hydrogen and inert gas are admitted, admitting the inert gas in amount willcient to place and maintain the reaction zone and charge under substantial positive pressure, heating the charge to a temperature sufficiently high to start the hydriding reaction by the application of externally applied heat, discontinuing the application of externally applied heat, and continuing the hydriding reaction solely with the regulated amount of exothermic heat thus generated.

PETER P. ALEXANDER.

REFERENCES CITED The following references are of record in the file of this patent:

I UNITED STATES PATENTS Number Muckenfuss May 8, 1934 OTHER REFERENCES Chemical News, vol. CXXII, 122, No. 3189, by E. Tomkinson, page 241, May 27, 1921.

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