HK1070044A - Method for producing and purifying sodium hydride - Google Patents

Description

Process for the preparation and purification of sodium hydride

The invention relates to a method for producing the purest fine-grained sodium hydride and to a method for purifying sodium hydride containing impurities.

Sodium hydride is a salt which forms colorless crystals in the pure form, but is only commercially available in the form of a grey substance due to its doping with sodium. It is extremely sensitive to moisture and burns in dry air at 230 c to form sodium oxide. After slowly releasing hydrogen at temperatures above 300 ℃, the hydrogen rapidly decomposes to elements from 420 ℃ without first melting.

Sodium hydride is frequently used in organic synthetic chemistry due to its basicity to produce a carbanion or deprotonation because it reacts rapidly under mild conditions without the formation of by-products other than hydrogen.

Sodium hydride is also a strong reducing agent, complexed with alkoxides and metal salts and in molten sodium hydroxide at high temperatures, which is mainly used for the preparation of finely powdered metals and for their surface treatment.

Other important fields of application are mixed metal hydrides, such as NaBH, also used in organic synthesis₄Or NaAlH₄In the preparation process of (1). In particular NaAlH₄Has been highlighted in promising emerging fields, such as the field of hydrogen storage (seeEtc., appl. phys., a 72, 221-.

Since its solubility in inert organic solvents is low due to the salt-like character of sodium hydride, the particle size and the size of the surface are decisive for its use in organic syntheses and in particular in the preparation of mixed metal hydrides, which necessitates an additional activation process in the unfavorable case of large particles with correspondingly small surfaces.

The sodium hydride thus obtained is then dispersed in mineral oil or pressed together with NaOH into bricks for sale and shows a grey colour due to the doping with sodium metal (R ö mpp, chemical specialist dictionary, volume 4, 1995, p 2928).

The above-mentioned production processes have the disadvantage that they are relatively expensive, and that they require additional, partly expensive, activation, purification and/or comminution of the sodium hydride for many fields of application. Since the handling of sodium hydride is also not without problems due to its high reactivity, its use is often limited to laboratory scale only.

DE 3313889C 2 describes a method and a device for removing toxic and special waste. In order to remove biological residues, in particular cellulose and glucose, they are heated in an induction furnace together with sodium hydroxide to their decomposition temperature, and then sodium hydride and CO are formed. However, the sodium hydride formed remains dissolved in the sodium hydroxide melt in solid form under the prevailing conditions there, so that it is obtainable analogously to the current production process.

In particular with a view to the field of use that is of interest, such as the hydrogen storage already mentioned, the object of the present invention is therefore to provide a process for preparing sodium hydride which is cost-effective with the lowest possible equipment outlay and provides sodium hydride in pure, finely divided form.

It has surprisingly been found that this object is achieved according to the invention by introducing a carbon-containing compound into a melt containing sodium hydroxide or a mixture of sodium hydroxide and one or more other alkali metal hydroxides, heating the melt to a temperature of 420 ℃ above the decomposition temperature of sodium hydride in the absence of oxygen and water, and then precipitating the reaction product outside the reaction medium by cooling to a temperature of 420 ℃ or less.

The sodium hydride formed first dissolves in the melt, but then decomposes into sodium and hydrogen owing to the prevailing temperature there. It is assumed that the gaseous hydrogen formed in the reaction for the formation of sodium hydride and during its decomposition subsequently entrains sodium as it escapes from the melt, so that this sodium is recombined again by cooling at another point outside the reaction medium and in this way forms sodium hydride in its purest form in the form of a white powder having a particle size of < 20 μm.

Although it is known from the prior art that sodium hydride decomposes rapidly at temperatures above 420 ℃, it has surprisingly been observed that in the process according to the invention hydrogen and sodium are recombined again to form fine-grained sodium hydride of high purity, after cooling to a temperature of 420 ℃ or less, preferably at 150 ℃ and 300 ℃.

As the carbon-containing compound which may be in a solid state as well as in a liquid or gaseous state, it is preferable to use industrial waste such as polyethylene, polypropylene, polyester, waste oil, waste rubber, asphalt, tar, oil residue, and cellulose or a mixture thereof.

The process thus has the advantage that low-cost industrial waste, which otherwise has to be disposed of expensively, is converted into industrially usable material.

It is particularly advantageous to heat the sodium hydroxide-containing melt to a temperature of 650-900 ℃. The maximum yield is achieved here when the temperature of the melt is close to the boiling temperature of sodium (881 ℃), since it is then no longer necessary to carry the sodium away with the hydrogen escaping from the melt, but the sodium can escape from the melt as a gas itself.

In a particularly preferred process, hydrogen is introduced into the sodium hydroxide melt. This has the advantage on the one hand that the flushing with hydrogen is continued, which can provide an oxygen-free and water-free atmosphere. On the other hand it causes efficient expulsion of liquid sodium at melt temperatures below the boiling temperature of sodium. In addition, the hydrogen atmosphere facilitates the recombination of sodium and hydrogen into sodium hydride. Recombination into sodium hydride, while possible in principle, when using an inert gas instead of hydrogen, will become difficult because the impact coefficient between the two particles causing the reaction, i.e. the number of effective collisions, depends in particular on the particle density of the respective particle in the respective volume. This coefficient is of course clearly higher in view of the hydrogen in the hydrogen atmosphere than in the inert atmosphere in which only a proportion of hydrogen is present.

In order to separate the individual products formed in the reaction, it is advantageous to remove a mixture of hydrogen and gaseous or entrained liquid sodium from the reaction space. This makes it possible not only to precipitate the sodium hydride which is recombined in the course of the cooling in a targeted manner, but also to separate off any sodium carbonate which is likewise formed as product and which can be carried off with a gas stream before the precipitation of the sodium hydride, for example by means of a cyclone, in order to obtain as pure sodium hydride as possible.

The sodium hydride produced in this way is obtained in the form of a white, highly pure, very fine powder with a particle size of < 20 μm, and is highly reactive without additional activation.

The hydrogen produced contains no impurities and can be used as required.

The previously described process features that use is made of the fact that sodium hydride dissociates on heating and, as determined for the first time, recombines on cooling under the conditions of the invention, and can also be transferred to the purification of commercially available sodium hydride containing impurities.

Here, instead of using the carbon-containing compound, the sodium hydride containing impurities is introduced directly in the absence of oxygen and water into a melt which is heated to a temperature above the decomposition temperature of the sodium hydride of 420 ℃ and which contains an alkali metal hydroxide or a mixture of alkali metal hydroxides, and is then precipitated again outside the melt medium at a temperature of 420 ℃ or less, preferably at 150 ℃ and 300 ℃.

In this case, it is not absolutely necessary for the melt to contain sodium hydroxide, since sodium hydride is already used.

Here again, the sodium hydride introduced is first dissolved in the melt and then decomposes as a result of the prevailing temperature. It is assumed that the gaseous hydrogen formed here escapes from the melt and entrains sodium. On cooling the reaction mixture outside the melt medium, recombination takes place and sodium hydride is precipitated as a solid, highly pure, finely divided powder.

The temperature of the melt is preferably 650-900 ℃. A significant yield increase can be observed in the purification of the sodium hydride containing impurities, the closer the temperature of the melt is to the boiling temperature of sodium or above that temperature, the more significant. This can be explained as follows: the hydrogen which carries sodium away from the melt is only derived from the decomposition of sodium hydride containing impurities and is therefore only very difficult to carry sodium away over the entire range.

For the same reason, a preferred process further comprises the uninterrupted introduction of hydrogen through the alkali metal hydroxide melt. This is not only advantageous for carrying sodium away from the melt, but is also particularly advantageous for increasing the degree of recombination by increasing the density of hydrogen in the gas volume.

It is advantageous to remove the hydrogen gas containing sodium so that the precipitation of sodium hydride by cooling takes place purposefully outside the reaction space and in this way the separation of sodium hydride from the other reaction products is achieved.

An apparatus for carrying out the process according to the invention is illustrated below, without restricting the possibilities of implementation to this apparatus.

FIG. 1 shows a schematic diagram of an apparatus for preparing sodium hydride according to the above-described process. Wherein is represented as:

1 reactor

2 supply of material

3 measuring instrument

4 Cooling device

5 introduction of Hydrogen gas

6 internal carbonate precipitation

7 external carbonate precipitation

8 precipitation of sodium hydride

9 hydrogen gas removal

The sodium hydride-forming reaction is carried out in a heatable reactor 1, which, in order to avoid loss of hydrogen gas due to its great diffusion rate, is preferably composed of mild steel, in which at least sodium hydroxide or a mixture of sodium hydroxide and one or more other alkali metal hydroxides is present. In order to maintain an oxygen-free and water-free atmosphere, the apparatus is preferably flushed completely with hydrogen gas prior to the introduction of sodium hydroxide. The reactor is heated, for example, but not absolutely necessary, electrically so that the temperature in the sodium hydroxide melt formed is 650-. Via a metering device 2, a defined amount of a solid, liquid or gaseous carbon-containing compound or a mixture of the same is introduced into the melt by means of a measuring device 3, for example a flow meter.

In order to avoid that the reaction already takes place at the metering device due to the high temperature in the reactor, which could cause clogging of the feed inlet, it is an option to cool this area with a cooling device 4.

After the introduction of the carbon-containing compound, the following reaction is carried out in the melt:

"C" here denotes in the very general sense the carbon of the carbon-containing compound.

The heat of reaction released during the reaction advantageously allows the temperature of the melt to be maintained for a longer period of time without additional heating.

In a particularly advantageous embodiment, hydrogen is continuously introduced into the melt by means of a compressor pump 5. Here, the compressor pump 5 is preferably installed separately from the material supply 2. This allows, as already explained above, both the removal of liquid or gaseous sodium from the melt and the recombination to sodium hydride.

In order to prevent the sodium carbonate formed during the reaction from being carried away from the melt by the gas stream and to introduce impurities during the precipitation of the sodium hydride, a first internal device 6 is preferably already present in the reactor, in order to retain the sodium carbonate, for example in the form of a demister.

The gas stream then passes, together with sodium and possibly sodium carbonate which is partially removed from the reactor despite the demister, to an optionally heatable external sodium carbonate precipitation device 7 which is installed after the reactor and in which the sodium carbonate is separated off. The precipitation device may be referred to as a cyclone, for example.

Then, a device 8 for precipitating sodium hydride is connected, which may also consist of a cyclone provided with cooling means. The cooling device serves to recombine the sodium and hydrogen gas to form sodium hydride, which, when converted into the solid phase, precipitates as a highly pure white fine-grained powder and is separated off.

The remaining hydrogen likewise contains no impurities and can be completely or partially reintroduced into the melt or supplied to other applications via the hydrogen discharge 9.

The results of the reaction of various raw materials for the preparation of sodium hydride in the apparatus as described above are illustrated below in table 1, but the present invention is not limited thereto.

Examples

General implementation method

To a NaOH melt heated to a temperature of 670-. A hydrogen gas stream is introduced into the melt, which is discharged together with the gaseous reaction products. A demister arranged in the reactor traps sodium carbonate which is formed as a reaction product in the melt. The hydrogen formed together with sodium as decomposition product of the sodium hydride is first introduced together with the hydrogen stream introduced outside the reactor into a cyclone heated to a temperature of 420 ℃ and 530 ℃ and the sodium carbonate which is undesirably removed is separated off. The remaining gas stream was passed through a second cyclone in which the recombination of the sodium hydride and its precipitation were carried out at a temperature of 150 ℃ and 300 ℃. A portion of the remaining hydrogen is reintroduced into the melt and another portion is collected for other applications.

Table 1 reaction of various raw materials for NaH preparation

EXAMPLES production of a melt of raw Material materials NaH

Through the temperature (DEG C) weight (kg) (g) (percentage of theoretical value)

1 propane gas 150l 6706.845995

2 propane gas 147l 7766.845196

284l 8726.887196 of 3 propane gas

4 Paraffin oil 0.42l 8736.8528 > 99

88.3g 8736.8155 > 99 of 5 rubber

(isoprene)

6 scrap rubber 529g 8736.888695

7 waste rubber/oil 567g 8726.892595

(weight ratio 1: 1)

8 carbon 78.3g 8716.8155 > 99

Each reaction is based on the following reaction equation:

propane gas:

paraffin oil:

isoprene:

carbon:

Claims (11)

1. a process for preparing sodium hydride, wherein a carbon-containing compound is introduced into a melt which is heated to a temperature of 420 ℃ above the decomposition temperature of sodium hydride, contains sodium hydroxide or a mixture of sodium hydroxide and one or more alkali metal hydroxides, in the absence of oxygen and water, and the reaction product is subsequently precipitated outside the reaction medium at a temperature of 420 ℃ or less.

2. A method according to claim 1, characterized in that the industrial waste or a mixture thereof is used as the carbon-containing compound which can be solid, liquid or gaseous.

3. Method according to claim 1 or 2, characterized in that the melt is heated to a temperature of 650-900 ℃.

4. A process as claimed in claim 1 to 3, characterized in that a hydrogen stream is introduced into the sodium hydroxide melt.

5. A process as claimed in claim 1 to 4, characterized in that the sodium hydride formed is removed in the form of its decomposition products together with the hydrogen formed and/or introduced and is precipitated again after subsequent recombination by cooling.

6. Sodium hydride obtainable by the process according to claims 1 to 5.

7. Sodium hydride according to claim 6, characterized in that it is present as a white powder having a particle size < 20 μm.

8. A process for purifying sodium hydride containing impurities, characterized in that the sodium hydride containing impurities is introduced under oxygen-free and anhydrous conditions into a melt containing alkali metal hydroxide or a mixture of alkali metal hydroxides, which is heated to a temperature of 420 ℃ above the decomposition temperature of the sodium hydride, and is subsequently precipitated outside the melt medium at a temperature of 420 ℃ or less.

9. The process as claimed in claim 8, characterized in that the melt is heated to a temperature of 650-.

10. The process as claimed in claim 8 or 9, characterized in that a hydrogen stream is introduced into the alkali metal hydroxide melt without interruption.

11. A process according to claims 8 to 10, characterized in that sodium hydride in the form of its decomposition products, optionally together with the hydrogen introduced, is removed and, after subsequent recombination by cooling, is precipitated again.