US2228923A - Process of preparing boron carbides - Google Patents

Description

Jn- 14, 1941- w. KAUFMANN ETAL 2,228,923

PROCESS OF PREPARING BORON CARBIDES Filed July 25, 1958 2 sheets-sheet 1 Jan. 14, 1941. W KAUFMANN HAL 2,222,923

PROCESS OF PREPARING BORON CARBIDLS Filed July 25, 1958 2 Sheets-Sheet 2 INVENTORS 2 ATTOFIN EY Patented Jan. 14, 1941 UNITED STATES PROCESS OF PREPARING BORON CARBIDES Waldemar Kaufmann and Alwin Krauss, Frankfort-on-the-Main-Griesheim, Germany, assignors, by mesne assignments, to Walther H. Duisberg, New York, N. Y.

Application July 23, 1938, Serial No. 220,994 In Germany January 30, 1936 3 claim.

This invention relates to a process of preparing boron carbide by heating boric acid and carbon. 'I'his application is a continuation-impart of U. S. patent application Serial No. 122,270, filed 5 January 25, 1937, in the name of Waldemar Kaufmann and Alwin Krauss.

In that specification there is described a proc.. ess of preparing boron carbide which comprises causing a mixture of commercial hydrous boric acid (HaBOa) and carbon to react by the action of an electric arc.

A known process of preparing boron carbide consists in heating a mixture of dehydrated boric acid (B203) and petroleum coke in an electric resistance furnace. Mixtures with hydrous boric acid cannot be used for this purpose because, owing to the str'ong formation of foam attending the dehydration of the boric acid, the material lying round the centrally situated heating resistance is removed from its position; moreover the steam formed in the interior which has to flow through the whole material would react undesirably with the carbon. The dehydration of commercial hydrous boric acid (H3BO3) on a large scale is diflicult because ceramic material as well as metals are attacked by the fused boric acid.

New we have found that boron carbides may directly be prepared from mixtures of commercial hydrous boric acid and carbon such as petroleum coke or the like by heating the mixture not in an electric resistance furnace but by means of an electric arc. In preparing the mix- I ture of boric acid and carbon the proportion of 35 boron to carbon in the finished product may be varied within large limits. Especially B4G and BeC may readily be obtained in a good yield.

For instance on a relatively thin layer of a mixture of commercial hydrous boric acid and petroleum coke in any suitable container of heat resisting material an electric arc is generated between at least two carbon electrodes introduced from above. In this case the expression relatively thin relates to the thickness of the layer in comparison with the dimensions of the container and the electrodes. Depending upon the dimensions of the container the layers of the parent mixture introduced have a thickness of some centimeters, for instance about 3 to 25 centimeters. Further quantities of the mixture are then carefully added in the form of further layers lying one above the other until the electric arc is covered. A layer of boron carbide fused on the surface is soon formed round the arc and especially below it, while water vigorously evaporates. The steam may readily escape through the relatively thin layer of the reacting mass covering the arc and may likewise flow off through the hollow space formed round the electric arc. Fresh material may slowly be added continuously or in the form of further thin layers carefully placed one above the other as the dehydration of the layers around the hollow space and especially of those covering it from above and the formation of finished boron carbide are proceeding by the heat of the electric arc. The radiation of the arc directly transforms the mixture surrounding it into boron carbide, with evolution of carbon monoxide. The boron carbide rst accumulates in the form of a liquid below the electrodes. Thereby and by pushing forward the dehydrated portions of the reacting material the quantity of the fused boron carbide may continuously be increased so that, by raising the electrodes and continuously charg- 5 ing fresh starting mixture a block of boron carbide is obtained which solidies from below and increases upwards. It is immaterial whether small quantities of incompletely dehydrated mixture come into the molten mass because the Water contained therein instantly evaporates and can readily escape along the electrodes together with the carbon monoxide formed.

Of course, the process may be varied. For instance a three-phase process with three electrodes or a single-phase process with one electrode and the molten mass as second electrode may be applied. In the latter case first only one electrode may be placed in the mixture and a second current-carrying agent, for instance a carbon plate, is supplying the current from below.V After a part of the boron carbide has been fused this molten mass serves as the second electrode.

It is possible, for instance, to introduce a current-carrying agent into the mixture from below through an opening in the wall of the container and to place above the layers one or more electrodes. By the action of the electric arc boron carbide is first formed between the various current conductors and this boron carbide, in the liquid as well as in the solidied state, may in the further course of the reaction act as conductor to the electric arc.

In the following two modes of operating the process which have proved to involve a special industrial value are described.

The accompanying drawings diagrammatically illustrate the preparation of boron carbide according to the invention, Figs. l to 5 being 50 vertical cross-sections through the electric arc.

Figs. l, 2 and 3 are cross-sections through an arrangement of two electrodes introduced from above,

Figs, 4 and 5 are cross-sections through a de- 55 vice with one electrode introduced from above, the second current conductor being placed below.

According to Fig. l a layer 2 of the starting material of a thickness of about l5 centimeters is introduced intera container 5 of heat resisting 60 material, for instance of metal or a ceramic' mass. A suitable mixture of starting material consists of 100 parts by weight of commercial hydrous boric acid (HsBOa) and 30 parts by weight of petroleum coke. This mixture, when used in the process described in the following passage, yields a product with a content of boron of '78 per cent. The invention, however, is not limited to this proportion, but other mixing proportions may likewise be applied. By the increase of the content of boric acid in the mixture necessary for the preparation of products especially rich in boron the losses by evaporation of the boric acid also increase so much that it is advisable to recover these quantities with the aid of suitable separators.

Directly above the layer 2 of the starting material an electric arc 3 is generated in the usual manner between two electrodes l introduced from above in an inclined position with a tension of to 100 volts. For this purpose the electrodes I are brought in contact for a moment and then separated from each other.

As it may become necessary to generate the electric arc 3 also during the reaction the electrodes l must be supported in such a manner that they may readily be moved towards each other and again separated from each other. It is, however, also necessary to arrange the electrodes in such a manner that they may readily be lifted and lowered.

By the action of the electric arc 3 the surface of the mixture 2 situated below begins to melt, while vigorously forming steam. As soon as the formation of steam ceases the electric arc 3 is carefully covered with a further layer of the mixture. If the arc is extinguished by this layer it must again be generated in the above described manner. Further steam is formed and the material agglomerates and in a short time a hollow space 4 is formed around the electric arc 3 which hollow space increases in proportion with the reaction of the surrounding mass caused by the heat. The boron carbide formed melts and accumulates in the form of a thin layer 6 `below the electric arc 3. The electrodes l and therewith the arc 3 are then somewhat lifted and the layer covering the arc is increased by the addition of further mixture. This new layer is soon dehydrated by means of the hot material situated below and `by the flame -of carbon monoxide. The steam escapes upwards into the air without having an opportunity of reacting with the boron carbide formed. As by the radiation of the electric arc only a thin layer of the boron carbide is maintained in the molten state the mass solidies below this layer so as to form a solid block 5 of boron carbide. By lifting the electric arc and adding new material new layers are permanently formed and the block 5 increases thereby. The block is surrounded by a zone 8. Near the block of boron carbide it consists of non-molten boron carbide, towards the outside it becomes more and more spoiled with the crude materials. The outer layer 2 consists of more or less strongly dehydrated pure starting mixture. The hard block of boron carbide may readily be separated from the outer layer 8 with which it is only loosely associated; this is done, for instance, with a brush of steel.

In the case of large containers with carbon Velectrodes of, for instance, a diameter of 25 centimeters and a length of several meters the high weight of these electrodes, when they are arranged in an inclined manner, necessitates complicated regulating devices. The irregular burning off of the thick coals also aggrava-tes the adjustment of the desired distance. Therefore it is necessary to apply an arrangement for regulating the distance of the electrodes determining the amperage, for instance an arrangement according to Figs. 4 and 5 of the drawings.

A circular stick of coal l movably suspended, for instance, by a rope or cable and a coal-plate l horizontally placed thereunder serve as electrodes. The circular stick of coal I is set on the coal-plate and a mixture of boric acid and petroleum coke is piled around it. After the current has been switched in the electric arc is generated by lifting the round coal. By the radiation of the electric arc a hollow space 4 is soon formed in in the surrounding mixture. The solid boron carbide 5 formed in the same man-ner as described above settles on the coal-plate 'I in the form of a thin, continuously increasing layer. As the boron carbide conducts electricity the current is not interrupted thereby. A short time after the beginning of the reaction the electric arc, therefore, burns between the circular stick of coall and the block of boron carbide 5 or the molten Vpart 6 covering its surface. The operation ,is performed. as described above. A difference only consists in the following: the dehydrated material introduced cannot as easily as in the foregoing case reach the reaction Zone by itself so that from time to time the material round Ythe coal must be pushed down, preferably with a wooden stick. Small quantities of incompletely dehydrated boric acid which at this moment fall into the reaction zone do not disturb as they are at once dehydrated.

There may, of course, also be worked with a three-phase current either by bringing two sticks of coal l from above towards one coal-plate 'l which may, for instance, form the bottom of the container, or by using three sticks of coal l and applying the ycoal-plate 'l as zero in star connection.

We claim:

1. A process of preparing boron carbide from a mixture of commercial hydrous boric acid and carbon which comprises introducing into a container of heat-resisting material a small amount of a mixture forming a relatively thin layer on thebottom of said container, generating an electric arc on said layer whereby the mixture begins to -m'elt forming steam, adding further, thin layers lying one above the other around the electric arc, as soon as the formation of steam ceases from each layer, heating each layer in the same manner as said first layer, continuing to add said. thin layers until a hollow space is formed around the electric arc substantially consisting of molten boron carbide, and then raising the electric arc and repeating the above procedure until a solid block of boron carbide is formed 'below the thin layer of molten boron carbide surrounding the arc.

2. A process in accordance with` claim 1 in which each charge forms a 'layer of a thickness of about -15 centimeters.

3. A process in accordance with claim l in which each charge is a mixture of parts by weight of commercial yhydrous boric acid and. of 30 parts by weight of petroleum coke, and in which each layer has a thickness of about l5 centimeters.

WALDEMAR KAUFMANN. ALWIN KRAUSS.

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