DE1020318B - Process for the production of boron carbide - Google Patents

Description

Method of Making Boron Carbide The invention relates to a process for synthetic production which can be carried out easily and at low cost by Bo, rearbid in an electric arc furnace through which the total production of boron carbide with a significant reduction in the consumption of electrical energy for the unit of measure of the boron carbide produced can be increased considerably.

In on. As is known, a mixture of boron oxide and coke is used under reducing conditions in one equipped with elevating and lowering electrodes electric arc furnace, with the lower electrode ends from the Reaction mixture are surrounded; so it covers the arc.

In a known method of this type, initially from a small. Part of the coal required to reduce the boron oxide is an electricity bridge but soon disappears, whereupon a sump of liquid boron carbide forms through which the current flows.

In contrast, according to the invention, a permanent current bridge made of carbon with a considerable cross section is provided, which does not take part in the reaction. As a result, no carbide sump is formed at all, rather solid carbide blocks that are separate and remain separate are created in the area of the individual electrodes from the start. From each electrode to the opposite end of the bridge there is an arc of light , under the influence of which only a temporary local melting of the reaction mixture located there takes place with continuous immediate solidification. As the electrodes are gradually raised, the ends of which are kept permanently covered by refilling with the reaction mixture, the gradually growing separate carbide blocks forming under the electrodes are connected at the lower ends by the current bridge. The prerequisite for the blocks not uniting is that the electrode ends are spaced sufficiently. In the case of two electrodes without an excessively large furnace cross-section, this is achieved by keeping the furnace oval with electrode attachment points located approximately in the focal points of the oval.

Instead of boron oxide, returned goods can also be used in the process from a previous furnace run or from a mixture of such return material with Boroxv d go out.

The drawing shows an electric arc furnace that is used to carry out the method according to the invention can serve.

Fig. 1 is a front view of the furnace, Fig. 2 is a cross section through the furnace at the level of line a-a in FIG. 1.

The z. B. made of sheet steel furnace shell 1 has an oval cross-section and narrows towards the top. A flange 2 is welded to its upper edge, with which a downward bar 3 is welded. At the bottom is on Shell 1 attached a wide flange 4, with which a downward bar 5 is connected. The outer edge of the flange 4 is circular, while the Inner edge is oval according to the furnace cross-section. Between the outside of the Shell 1 and the flange 4 steel reinforcing ribs 6 are attached.

On the outside of the jacket 1 are near its upper end Hook 7 welded into the chain hook of a hoist, e.g. B. a traveling crane, can be hooked to lift the jacket 1 from the melt product after this has cooled down sufficiently.

Two perforated tubes 8 connected to hoses 9 are under the Flange 2, 3 arranged so that the jacket l can be sprinkled with water. Two further perforated tubes 16 connected to hoses 17 enclose the furnace jacket below and serve to reinforce the water sprinkling emanating from the pipes 8 of the coat.

The furnace rests on a circular support plate 10 one on rails 11 running chassis. On the sheet steel support plate 10 is a container 12, also made of sheet steel, welded to it with an oval bottom, the same shape as the lower end of the furnace shell 1, but smaller in size has so. that this can be slipped over the container 12. Between, the flange 4 of the shell and the support plate 10 becomes a seal 13 appropriate. The furnace is about 2.70 m high and can hold about 6800 kg of reaction mixture and from it pick up empty boron carbide.

For the synthesis of boron carbide in the arc furnace described a mixture of boric anhydride (B203) and coke is used, which is preferred Return material from an earlier oven run is added. With this return item, e # the mixture that has not been completely converted in an earlier furnace cycle, which partly consists of a boron compound that contains less oxygen than 1320, partly consists of boron carbide with more carbon than B4 C.

Generally a furnace mix is used which contains more boron, than is expressed by the equation 2B ,, 03 + 7C = B4C + 6C0, namely, because a significant part of the boron is lost during the furnace cycle. Both stoichiometric ratios represented by this equation would be a Boron carbide with an excess of all carbon in the form of graphite is to be expected. The boron content of the Mischulig can be between your stoichiometric ratio and 65% boron excess kept «-erden. In fact, since there is ini mixture ini generally some boric acid, which is richer than B203, and some boron carbide less boron than the B4 C (partially converted material from an earlier furnace run) corresponds to, are, ratios between the stoichiometric ratio of Boron to free carbon of the coke and 651 / o over your stoichiometric ratio used.

The amount of free carbon: in coke is determined and during manufacture the initial reaction mixture will be invoiced. Preferably it is well annealed Coke is used, especially since its content of all free carbon is precisely determinable. Of the various types of coke that can be used, e.g. B. petroleum coke, metallurgical coke and pitch coke, the latter is preferred because it is more uniform in nature and at About 90% or more free carbon has a low ash content (; below 10 / 0-1 Has.

Boric anhydride can easily be made by being made from Boric acid (H3 B 03) drives out water by heating. Otherwise it is vitreous boric anhydride in powdered form an easy to handle material.

Contains all boron carbides, with the exception of the purest carbide some free carbon in the form of full graphite. A content of all free carbon of about 5% makes this z. B. not useless as a grinding agent. Also elementary Boron can be found in solid solution in boron carbide (B "C). For certain purposes such boron carbide is preferably used. -With the help of the grant Method, a boron carbide can be produced which corresponds to the formula B6, 6C. Such a boron-rich or even more boron-rich carbide is considered technical in specialist circles Called boron. B6, @C contains about 85.6% boron.

To manufacture the high-quality boron product, it is sufficient to add the B0 ,; add up to 65% excess boron. Large quantities of boric acid for the purpose of manufacture If boron carbide is particularly rich in boron, the product contains something glassy B, 0.,. but that can be washed out with water.

The procedure according to the invention is carried out in detail as follows: Vitreous boric anhydride is first mixed with the coke and this mixture is then intimately mixed with the returned material. Boric anhydride and / or coke can be used in a coarser or finer state. However, the finer the starting material, the faster and more complete the reaction. In practice, vitreous boric anhydride is used, which goes through a screen of about 12 mm mesh size. Of course, this material also contains much finer particles. The coke can be used in a grain size of about 12 minutes and below. Satisfactory results are obtained when using 50% of recycled material, but the method can also only be performed with recycle material, if the elements in appropriate conditions are present in ihni.

Before the jacket 1 of the furnace is pushed over the lower container 12 a carbon pad is made in the latter, e.g. B. by pulping a mixture of fine coal and pitch. The top surface of this carbon pad obtains a z @ straight concave shape, in such a way that the outer edge extends to the Upper edge of the container 12 is enough. \ oh you put the jacket 1 over the container 12 has been. the space between the container and the llailtel is filled with coke. Then fine Alumiiliuilioxyd is filled in the container 12, in one Amount, dali it approximately evenly about 10 cni over the top of the container is enough (assuming a coat 1 full about 2.70 in height). This Aluniiniunioxy-d still counts on the document.

Now the hurried mass (about 35 kg) is full of pieces of graphite or carbon introduced approximately in the focal points of the oval of the jacket 1, in the area at During operation of the furnace, the electrodes 14 are set. Between these carbon or graphite nets The current bridge 15 is made with the same material, for the purpose of which: a channel formed in your return goods and / or roll goods is used.

The graphite bz, v. Coal pieces existing bridge 15 forms between the attachment points of the electrodes a continuous current path, preferably from about 330 cnl 'cross section. for an oven of the specified size.

The aluminum oxide layer under the graphite or carbon bridge is used as an insulating layer to prevent a short circuit.

All of the electrodes 14 will. hurried voltage of 100 to 150, preferably 120 volts. Then be after there. Fahrge, tell has been moved so far on the rails 11. that the jacket 1 comes into the correct position zti the electrodes 14, these brought by lowering into finite contact with the `treilibrücke 15, through which the electric current flows during the whole process.

The electrodes 14 are controlled by a control device (not shown), automatically raised and lowered as required.

If the electrodes 14 touch the graphite bridge, then flows through the Electrodes and the bridge a current of about "_'500 jnipere or, dear. A regulator adjusts the electrodes 14 so that the arc (at 120 volts) is iii with a current operated by about 2500 amps.

The reaction mixture is now distributed over the entire surface of the furnace so abandoned that the arc from an approximately 5 cigg high layer of the reaction mixture is covered. If part of the Geinisches rushes directly into the area of the arcing and converts here to boron carbide and carbon oxide. The conditions are reducing the common blanket keeps the air from the in the area of the arcing Reaction zone away. The height of the mixture ceiling on the electrodes should be at least 2.5 cm from the bottom of the electrodes. She shouldn't be in any Trap thicker than about 3 inches to prevent the gases from forming during the reaction can pull off well.

You now only need to close the oven with a mixture "from time to" charge. Aside from the electrodes, the mixture should be around the inside of the Mantle 1 are flared around. For example], -, poor it in the area of the mantle about 10 cm higher than: in the electrodes, at the ends of the long - @. axis of the oval jacket cross-section even up to 30 cm.

As the mixture is refilled, the electrodes rise gradually according to the extent to which the Borca.rbidblocks are in place in the area of the arcs. The reaction leads to the formation of red-hot boron carbide. If local forms a certain liquid phase, it usually only exists for a moment. The temperature at which the formation of boron carbide occurs is about 2350 °.

When the oven cycle is over and the electrodes are lifted up, so it is advisable to place the furnace on the chassis away from the electrodes 14 extend, but the water sprinkling of the jacket 1 is maintained as long as until the contents of the jacket 1 have cooled down to such an extent that there is no risk of burning of the furnace jacket is longer. After the contents of the oven have cooled down enough, that the carbide no longer oxidizes on contact with air, the furnace jacket is of the content lifted off. The two boron carbide blocks are carefully protected from the surrounding, largely separated into a solid mass melted return material, which is used for Use is collected in a subsequent operation. Example 1 After manufacture the carbon base in the container 12 and after placing the jacket l on the Plate 10 of carriage 11 was coke between the wall of the container 12 and the shell l filled and given into the container 12 so much aluminum oxide that this the The edge of the container protruded about 10 cm. Then 34 kg of lump graphite were placed under the attachment points brought the electrodes and made the current bridge in the manner indicated above. Then the furnace was started with a voltage of 120 volts and continuously, the reaction mixture supplied to the extent that around the electrodes 14 around and a mixed blanket about 5 cm thick was maintained between them. The mixture consisted of 41.5% returned material, 41.5% glassy boric anhydride and 17% 90% carbon coke. The return goods had the following composition: 19.07% boron, 40.83% carbon, the remainder mainly oxygen.

The material obtained in this oven cycle was sorted according to appearance and analyzed. Most of it was spongy and surrendered. in the analysis 24.44% carbon and 69.27% boron. The remainder was mostly oxygen. This Boron carbide is useful as a lapping powder and for other purposes. If you want the oxygen content To remove the reaction material, it can be finely chopped up and washed with water wash, which reduces the oxygen to less than 1%. Another Part of the product, which consisted of solid pieces, was composed of 71.77% Boron, 23.93% carbon and -1.31 / o oxygen combined. The oxygen content of this Boron carbide can in any case be reduced to less than 111% in the manner indicated above will.

Another part of the product was on the borderline between good Boron carbide and returned goods. This reaction material consisted of 61.11% boron, 28.55% carbon and 10.34% oxygen. It is used in a mixture with boron carbide.

The rest of the goods were returned goods that were processed in further kiln aisles became.

Of course, the composition changes from zone to zone Blocks. A sample of spongy material yielded 75.63% boron, 22.81% carbon, which is corresponded to a boron-carbon atomic ratio of 3.68: 1. Example 2 Another The furnace cycle was carried out exactly as in Example 1, except that 52.5% were returned, 31.5% glassy boric acid reagent and 17% coke were used. The returned goods existed nel - # - n oxygen from 35.951 / o carbon and 24.11% boron.

As a result of this oven run, the solid and spongy parts together had an average content of. 67.60% boron and 25.67% carbon. The rest was mostly oxygen. The sandy qualities of the reaction mixture contained 59.59% boron and 27.86% carbon. The rest was mostly oxygen. The return material resulting from the furnace passage (to be distinguished from the returned material) resulted in 24.31% boron and 33.63% carbon. The rest was mostly oxygen. If the boron carbide, containing about 67.61 / o boron and 25.6704 carbon, was washed out, the final boron carbide obtained was a product of the composition: 74.44% boron, 24.0611 / o carbon and 0.26% metal 03

It has been found to be useful at voltages below 150 volts to work., but should not work at voltages below 100 volts, otherwise the yield would be too low.

In addition to the voltage and the. Boron content of the starting mixture of the stream: back cross-section, the shape of which can vary within wide limits. Its size: results as a function of the electrode diameter, which can vary depending on the size of the furnace. If the electrode has a square, triangular or any other cross-section, its effective diameter can be determined by considering the electrode cross-section as such, although it does not have a circular cross-section, and with the aid of the relationship F between the radius and the area of a circle = zr2 a diameter is calculated which corresponds to the effective diameter of the electrode. If D is the effective diameter of the electrodes in cm and C is the cross-sectional area of the current bridge in cm'- ', then the ratio of C: D should be at least 2 and at most 12. The cross section of the current bridge is thus determined by the relationship 2 <C, <12.

In practicing the invention, use an oval jacket with two electrodes is preferred for single-phase current. At DrI iphasenstroin with three electrodes could be the cross section of the coat triangular with rounded corners or circular.

The arc furnace described consumes in the synthesis of boron carbide about 0.04 kg of electrode material for each kg of boron carbide produced. This is just about 5.5% of the consumption of resistance coal in a resistance furnace. The described Arc furnace only needs about 591/9 of the amount used in the resistance furnace Energy for the synthesis of boron carbide. Moreover, the yield is: the one described Arc furnace in the unit of time much larger than that of the well-known resistance furnace.

It is known that boron easily reacts with nitrogen to form boron nitride (BN) connects. In addition, boron carbide reacts at or near its formation temperature of 2350 ° with nitrogen in the sense of partial conversion into boron nitride. It is therefore Surprisingly, d'ass the inventive method a yield of high Boron carbide supplies in large quantities, although, apart from the mixture blanket, hardly any the exclusion of: air from the reaction zone is provided. The emerging columnar blocks that fill only a small part of the furnace cross-section, lie after converting a certain amount of the feed mixture into boron carbide with their hot upper ends much higher than when the same amount in the liquid state would have been transformed and collected as a swamp on the furnace floor. This special process of forming columnar blocks inside a large one Oven, very surprisingly, delivers particularly good results.

Claims (1)

PATEN TA NS Pßi1CHE-1. Process for the production of boron carbide, in which a mixture of boron oxide and coke is reacted under reducing conditions in an electric arc furnace equipped with electrodes that can be raised and lowered. the lower electrode ends are surrounded by the reaction mixture and the latter covers the arc, characterized in that the oxide-coke mixture, which is continuously abandoned in accordance with the progressive formation of carbide, is converted into an arc that occurs between each individual electrode and the part directly below it made of carbon. Acting as a current bridge layer is ignited and in which the arc runs after a certain start-up time between the electrodes and the continuously increasing in height, not interconnected carbide blocks, the current generating the arc from one electrode over the arc through the one below it located carbide block to the carbon bridge and flows from this through the second carbide block and the second arc to the other electrode. 2. Experience according to claim 1, characterized in that the numerical ratio between the cross-sectional area C of the current bridge and the electrode diameter satisfies the condition2 <# <12 and the process is carried out with a voltage of 100 to 150 volts applied between the electrodes. 3. Process according to claim 1, characterized in that the process is carried out in such a way that there is continuously unreacted starting mixture above the carbide formation zone in a layer thickness of about 2.5 to 7.5 cm 1, characterized in that when two electrodes are used in a horizontal cross-section, it has an oval shape with a longitudinal axis running through the attachment points of the two electrodes and the electrodes lie approximately in the focal points of the oval 285 837.