NO175375B - Process for the preparation of binder for carbon masses - Google Patents

Abstract

A binder for carbon masses with reduced combustion is obtained by adding approx. 0.1% by weight of B0 to the binder starting material of a coal tar distillation residue. The effect of the inhibitor compared with addition to the finished binder thereby gets an approx. three times increase.

Description

The invention relates to a method for producing a binder with improved properties, based on coal tar, for carbon masses.

Carbon masses for the production of carbon moldings such as anodes, electrodes and linings consist of a mixture of solid carbon, such as anthracite, coke, petroleum coke, carbon black, etc., and a carbon or petro-derived bituminous binder. Preferably, the masses contain petroleum coke and coal tar pitch. They are predominantly used for the production of pre-fired or self-calcining (Soderberg) anodes for aluminum electrolysis. The consumption of anode material is, as a result of the side reactions (oxidation) and the anode breakdown (flaking), higher than what would be necessary for the reduction of the aluminum oxide.

It is known that certain impurities in the coke and in the binder, such as sodium, calcium and vanadium compounds, accelerate the oxidation and that additions of boron or phosphorus compounds show inhibitory effects (Light Metals, 1982, pp. 713-725).

In US patent document 4188279 it is proposed to produce carbon molded bodies from carbon masses with 15-35% by weight binder

and 0.1-3% by weight boron compounds respectively 0.5-5% by weight phosphorus compounds as oxidation inhibitor..Thereby the inhibitor is added to the homogenized mixture or mixed into the binder component before the addition of the carbon component.

It is also possible to add all three components at the same time and to homogenize, as described in the example.' The off-fire is thereby reduced by 15% by adding 8% by weight of boric acid, based on the coal tar pitch binder.

It has now been shown that the addition of boric acid in the indicated quantities leads to the formation of VB and TB which crystallize out in the aluminum melt and, in the further processing of the aluminum into foils, leads to the formation of holes in the foils.

Phosphorus compounds, such as P2O5, are likewise less suitable inhibitors as they reduce the current yield in the electrolytic process.

It was therefore a task to reduce the amount of inhibitors in the binder or in the carbon masses below

at the same time improving their business.

The task is solved according to the invention by means of a method for producing a binder for carbon masses based on coal tar with additions of boron compounds as an oxidation inhibitor, and the method is characterized by the fact that such an amount of boron oxide is added to the binder base substance that the content of the finished binder does not exceed 0, 3% by weight, and that the binder base substance is then further processed into the binder in the usual way.

As binder base substance, decanted coal tar or distillation residues of this tar with a lower softening point than the binder are used. After adding the boron oxide (B203), the basic substance is processed into the desired binder in one or more stages by vacuum and/or carrier gas distillation.

It is preferred to use a coal-tar pitch as the base substance, so that the processing in one step, for example in a mixer retort or in a thin-layer evaporator, can be carried out under gentle conditions.

In order not to reduce the storage resistance of the binder, the distillation should be carried out at sump temperatures below 380°C. This applies in particular to the production of binders for Soderberg pulps. In the last distillation step, the maximum sump temperature should not be below 350°C, otherwise optimal homogenization will not be achieved.

The boron oxide content in the binder lies within the range from 0.03 to 0.3% by weight, preferably at approx. 0.1% by weight. Within this range, the addition of B203 has no negative effect on the binder's wetting behavior or on the coking properties of pitch/coke mixtures.

The invention is described in more detail using example 1 and comparative examples 2 and 3.

Example 1

1000 parts by weight of a coal tar standard pitch which is characterized by the analytical values listed in Table 1 were melted under inert gas, heated to 180°C and then mixed with 1 part by weight of B2O3 for 2 hours with stirring. The mixture was then distilled in a stirrer retort under a pressure off

100 mbar up to a sump temperature of 360°C. A binder with the analysis data listed in Table 2 was then obtained with a yield of 92%. 22 parts by weight of this binder are mixed with 78 parts by weight of petroleum coke with a defined granulometry and pressed into moulds. The moldings that have been fired at 960°C are oxidized at 960°C in a CO 2 stream to measure the CO 2 reactivity.

The C02 reactivity is a measure of the anode consumption in aluminum electrolysis. The overall burn-off is composed of the direct burn-off and the dust formation, the latter also depending on the granulometry of the petroleum coke. In addition, the density, specific electrical resistance and flexural strength of the specimens were determined. The results are reproduced in Table 3.

Example 2 (Comparison)

Example 2 corresponds to Example 1, but the boron oxide was however added to the binder cooled to 200°C only after the distillation. The binder yield was thereby not changed. The properties of the binder are shown in Table 2, and the measurement results for the test specimens are shown in Table 3.

Example 3 (Comparison)

Example 3 corresponds to Example 2, but without B203 addition. The analysis values for the binder are shown in Table 2, and the examination results for the test specimens are shown in Table 3.

It appears from Tables 2 and 3 that the addition of the boron oxide before or after the distillation causes neither a change in the analysis data for the binder nor a change in the electrical or mechanical properties of test specimens produced from it within the measuring accuracy. In contrast, it appears that the C02 reactivity is clearly dependent

gig of the B203 addition.

Thus, the total fire is reduced by approx. 7% by weight with a B203 addition after the distillation and with approx. 22% by weight when added according to the invention before the distillation. Both the direct fire and the formation of dust show similar results.

It has thus been demonstrated that by means of the present addition of boron oxide to the binder base substance instead, as is known from the state of the art, to the finished binder or to the carbon mass, its inhibitory effect can be increased several times. It is thereby possible, with low inhibitor amounts that are tolerable for the aluminum industry, to significantly reduce anode consumption.

Claims (5)

1. Method for the production of a binder for carbon masses based on coal tar with additions of boron compounds as an oxidation inhibitor, characterized in that such an amount of boron oxide is added to the binder base substance that the content of the finished binder does not exceed 0.3% by weight, and that the binder base substance is then processed further to the binder in the usual way. 2. Method according to claim 1, characterized in that a distillation residue from the coal tar, in particular a coal tar normal pitch, is used as the base substance. 3. Method according to claim 1, characterized in that the boron oxide is added in such an amount that the finished binder contains 0.03-0.3% by weight, preferably approx. 0.1% by weight, boron oxide. 4. Method according to claims 1-3, characterized in that the boron oxide is mixed with the liquid base substance and that the mixture is further processed into the carbon binder by vacuum and/or carrier gas distillation in one or more stages. 5. Method according to claim 4, characterized in that the maximum sump temperature at the last distillation stage lies in the range from 350 to 380°C.