US2109302A - Carbon electrode - Google Patents

Description

Patented Feb. 22, 1938 UNITED STATES PATENT OFFICE CARBON ELECTRODE No Drawing. Application October 31, 1933, Se-

rial No. 696,113. In Germany November 5,

4 Claims.

This invention relates to a process for the manuiacture of carbon electrodes.

Carbon electrodes are at present usually manufactured by adding a binding agent to coke 5 cl a low ash content, shaping the mixture by pressing and subsequently baking the mouldings. In this manufacture, the nature of the binding agent plays an important part. It is generally considered desirable to employ a binding agent which combines a fair adhesive capacity at elevated temperatures with a high coking power. The hitherto usual binding agents, such as tar, hard or soft pitch, irrespective as to whether they are employed individually or in admixture are however not fully satisfactory in this respect.

It is known from the work of Franz Fischer and others, (compare Brennstoifchemie 1925, Vol.6, pages 33 and 39and idem 1930, vol. 11, page 65) that by extracting carbonaceous material with benzene under pressure an extract is obtained which can be separated by means of petroleum ether into a constituent soluble in said ether (hereinaiter termed "oil bitumen) and an insoluble portion (hereinafter termed "solid bitumen). It is also known that the aforesaid bitumens are largelymesponsible for the baking effect in the cokiflcation'of coal.

It has now been ascertained that both oil bitumen and solid bitumen are contained in tar, soit pitch and hard pitch and that the ratio of these constituents in said substances has a considerable influence on their behaviour as binding agents in the production of carbon electrodes. It has moreover been ascertained that useful properties of a binding agent, that is to say, it exhibits above all, good adhesive properties at elevated temperatures and a high coking power. On the other hand oil bitumen possesses only an inadequate adhesive capacity and also a very low coking power under the conditions obtaining during the manufacture of electrode carbons. Finally, the residue which is insoluble in solvents (hereinafter termed "solid residue"), while having the highest coking power possesses no adhesive power at the baking temperatures.

The present invention is based on the foregoing considerations and provides a process for the production of carbon electrodes, in which the aforesaid constituents of tar, pitch and other carbonaceous materials, namely solid bitumen, oil bitumen and the insoluble residue, are employed as binding agents in predetermined arbitrary proportions. By virtue oi. the present insolid bitumen more particularly embodies the vention it is thus possible, particularly when taking into account the fact that the specific properties of oil bitumen and of the solid residue are blended in solid bitumen, to vary the properties of the binding agent in a systematic manner by combining the aforesaid three constituents in proportions which have been predetermined according to specific demands, and thus to attain properties in the binding agent and in the finished electrodes which are far superior to those obtained by employing commercial pitches and tars as binding agents. This is so for the reason that the ratio oLthe constituents in the latter is solely governed by their origin or method of production and that it is thus merely a matter of chance, to what extent that ratio also answers the requirements of a binding medium. It may be said that the proportion of the constituents found in the commercially available pitches and tars invariably is far from oorresponding to the optimum proportions in this respect.

According to the present invention the bind ing agent employed for making carbon electrodes contains solid bitumen as the main and base constituent. By adding oil bitumen in small amounts of the order of 5-10 percent the softening point of the solid bitumen can be lowered when necessary without however perceptibly impairing its adhesive capacity. On the other hand the coking power of the solid bitum'en can be increased to a moderate extent by adding insoluble residue without substantially altering its binding power.

The properties of the individual constituents diilfer to a lesser degree, also according to their origin, i. e. depending upon whether they are derived from tar, soft or hard pitch. Thus for example, oil bitumen extracted from soft pitch is more viscous and has a higher coking power than oil bitumen from tar. By virtue of this fact it is possible to combine constituents of difi'erent origin in order to produce the particular properties of the binding agent desired in individual cases.

The invention is not restricted to the use of solid bitumens or oil bitumens derived from the aforesaid substances tar and pitch, but includes the use of bitumens derived from any materials containing the same. Moreover theisolation of the individual constituents, may be eflected by extraction with the aid of solvents, such as benzine, benzene and tetrahydronaphthalene. 0ccasionally it is even possible to produce the desired mixing ratio directly. by carrying out the isolating process in a suitable manner.

The process of the present invention has an extremely favourable effect upon the production and quality of carbon electrodes. Since the basic constituent of the binding agent of the present invention, 1. e. the solid bitumen is of pulverulent consistency at ordinary temperature and will remain solid even on addition of the small amounts or oil bitumen which may be employed for lowering its softening point, it is possible to produce a completely homogeneous mixture with the powdered coke in a very short time, at ordinary temperature and by means of simple. mechanical apparatus, instead of employing kneading machines operating at elevated temperatures as was hitherto necessary. This circumstance is particularly advantageous in the production of continuous (Soderberg) electrodes. During the shaping by pressing, the further advantage arises that the finished mixture can be pressed immediately after heating to the softening temperature. Moreover when employing the binding agent of the present invention the firing process can be carried out in a more simple manner and in a shorter space of time without cracking becoming evident and it is possible to remove the sagger after firing without any diiliculty. At the same time a considerable saving in binding agent results with the same binding power being obtained.

The consistency of the electrode carbons themselves is also modified in an advantageous manner. By virtue of the high coking power of the binding agent the degree of porosity is substantially reduced, thereby obtaining greater durability n chemical and mechanical respects and also reducing the wear during use. Moreover the electrical resistance of the fired carbons is favourably influenced.

The following examples serve to illustrate the influence of the alteration of the proportions of the individual constituents of the binding agent in accordance with the invention, on the properties of carbons produced under otherwise identical conditions, it being remarked that the density of a standard electrode carbon manufactured in the hitherto usual manner with the addition of 20 percent of a tar-pitch mixture was taken as 100 after pressing and amounted after firing to 87 percent.

Example I Solid bitumen obtained by extraction from tar by known methods in an amount of 32 percent by weight of the tar has a coking capacity which is 61 percent higher than that of the original tar. The softening point is fairly close to 73 C.

Electrode carbons prepared by admixing 15 Percent of said solid bitumen exhibit after pressing, a density of 97 and after firing a density of 90. Although on pressing, the density of the standard carbon (100) was not attained, the density after firing is higher than that of the comparison carbon (87).

Emmple II A binding agent consisting of 90 percent of solid bitumen from tar as set forth in Example I, and 10 percent of oil bitumen from hard pitch (sofng point 16 C.) wasprepared by fusing the ture and the said agent had a softening point of 64 C. Carbons prepared with the addition of 15 percent oi'the aforesaid binding agent exhibited after being pressed, a density of 98 whilst after firing the density amounted to 90.

Example IV A binding agent consisting of 90 percent of solid bitumen from hard pitch and 10 percent of oil bitumen from tar i a softening point of 79 C. Carbons prepared with the addition of 15 percent of this binding agent exhibited a density of 98 after pressing and 91 after firing.

Example V A binding agent was prepared by fusing together 95 percent of solidbitumen from tar and percent of insoluble hard pitch residues, which binding agent had a coking power which is 67 percent higher than that of the tar and has a softening point of 78 C. The density of carbons formed with the aid thereof amounted to 97 after pressing and was found to he 91 after firing.

I claim:-

1. As a binding agent for the manufacture of carbon electrodes a bituminous composition at least about 90 percent by weight of which is solid bitumen.

2. A binding agent for the manufacture of carbon electrodes consisting of at least about 90 percent of solid bitumen, the remainder being oil bitumen.

3. An unbaked carbon electrode comprising powdered coke and a bonding medium, the latter substantially consisting of at least about 90 percent by weight or solid bitumen, the remainder being oil bitumen.

4. An unbaked carbon electrode comprising powdered coke and a bituminous bonding medium, the latter consisting of at least about 90% by weight of solid bitumen.

EBERHARD NEUKIRCH.