SE189413C1 - - Google Patents

Description

Inventor: S Stank ° Priority requested from 16 November 1954 (France) In the production of aluminum by small electrolysis of alumina, clay soil, lost in small cryolite, fire anodes of binder-baked carbon powder are used, which hang down in the electrolyte, which is kept at 950 to 980 ° C. The quality of these electrodes has a great influence on the course and results of the electrolysis; they must have high purity. when their combustion residues - metals and silicon - are introduced into the produced aluminum. It Or also of importance, that they amo flawless, ie. free frail cracks, which would stoma the power distribution, and that they are consumed uniformly, without carbon dust being introduced into the electrolysis bath. Finally, they must be able to withstand sudden temperature changes without breaking, e.g. (In they are introduced into the electrolyte malt, their temperature rapidly rising from room temperature to a distance of 9 ° C.

During the course of the electrolysis, the anodes are combusted with the aid of the oxygen contained in the alumina, and their combustion amounts to a considerable proportion of the total production cost of the aluminum. So far, it has only been possible in exceptional cases to come down to a net consumption of less than 470 kg per tonne of aluminum produced. By net consumption is meant in this case the value that is arrived at by beralining on the basis of the waste products at the power lines to prefabricated anodes, resp. ph p, round selection of the disappearing volatile constituents in the electrode mass at continuous anodes of the soderberg type.

The present invention has made it possible to significantly improve the quality of such anodes and to reduce the consumption of electrode material per ton of aluminum produced. The invention consists primarily in using as a binder for the carbon powder a pitch with carefully defined properties according to the design part of the patent claim and further in applying to the carbon powder a continuous grain size distribution with a minimum percentage of coarser grains, with an average diameter exceeding 5 mm. A minimum percentage of finer grains, with an average diameter of less than 0.16 mm according to the cantilevered parts of claims 2 and 3, these percentages not being the same for prefabricated anodes as for continuous soderberg anodes. In addition, the invention relates to certain property health and regulatory measures, as stated in patent claims 2 and 3 and which will be described in the following.

The properties of the part as a barrier agent for the use of the carbon powder have had a significant influence on the quality of the anodes intended for use in connection with aluminum production. This pitch is usually made of coal-fired pitch, but certain types of petroleum pitch can also be used. In accordance with the present invention, the softening temperature of the pitch should exceed 70 ° C and should preferably be between 70 and 85 ° C. The pitch consists of a complicated mixture of the hydrocarbons and its exact composition has not yet been determined. The aforementioned softening temperature Or determined using the Kramer — Sarnow method and Or in accordance with the temperature at which a given amount of mercury (5 grains) passes through a five cm high pitch column which perfectly adheres to the lower spirit of a 6 mm diameter, cla roret Or placed in a vertical layer in a vessel, which is heated p0. such a salt that the temperature rises by 1 degree Celsius per minute.

Becket itself acts as a binder in the raw anode, but during the burning process it loses its more volatile constituents, so that (let Dupl. At 12 h: 2; 12 i: 33; 21 h: 20/01; 21 h: 20 / 05; 40 e: 4; 80 b: 8112 2- - then there is the remaining coke which binds the carbon grains in the fire anode, so that this coked residue of the pitch has a decisive effect on the quality of the finished anode.

In accordance with the invention, a pitch is selected which gives a coke residue in excess of 50%. The coke residue is determined as follows: A pitch sample is placed in a small crucible and slowly heated to a temperature of 460 to 4700 for a heating time of about 20 minutes. The small crucible is then thanked with a tight-fitting lid and inserted into another, larger crucible, the space between the cradles of the hated crucibles being filled with coal to prevent air from entering. The whole is heated in a muffle furnace at 800 ° C for a shelf life of four hours. After cooling, the coke residue is weighed without air entrainment, the weight of which must exceed 50% of the original weight of the pitch sample.

Another significant factor associated with pitch Or the content of resin compounds insoluble in benzene but soluble in anthracene oils. It is, in fact, these constituents of the pitch which, as a concern of the binding properties, which are necessary in order to obtain a good anode. The resins must amount to more than 20 °, / c, of the total weight of the pitch.

To determine the content of benzene-insoluble resins in the pitch, pure, crystallizable benzene (95% distillation between 79.5 and 80.5 degrees Celsius) was used.

The anthracene oils, extracted from coal tar, are complex mixtures; for the solution of benzene-soluble resin anem in quality control of pitch, oils with the following properties are used: Specific gravity at 15 degrees Celsius: 1,085.

Content of oil distilling below 250 ° C: less than 15%.

Water content: less than 0.5%.

The distillation curve for the anthracene oils used should fall between the curves shown in Fig. 1.

Finally, it is important that the coked residue of the benzene insoluble in benzene and in anthracene oils exceeds a content of 89%, as shown in the following Table I: Table I.

Beck test 1 2 3 4 6 7 Alloying temperature, C-degrees 82 ° 79 ° 79 ° 83 ° 76 ° 77 ° 78 ° Coke residue,% 52.11 52.24 58.6 52.847.04 50.2 Insoluble in benzene, ° A , 32.31 31.82 27.38 34.82 28.12 27.21 99.90 Oltisl. in anthracene oils,% 7.89 10.60 6,612,8,5,19 10,57 Resins, 0184. in benzene and soluble in anthracene oils,% 24,42 21,22 20,73 22,72 19,97 22 , 02 19,33 Coke residue of these resins, °; `, Results, obtained with anodes 91.30 excellent 92.6791,47 very much 87.74 less 87.67 less 86.41 badly 84.39 very at the front. of aluminum gottgott gott gott daligt The pre-coked residue of the resins was determined by the examination method for the pitch described above.

It can be seen that pitch sample no. 4, all the other properties of which are excellent, gay-poor anodes, because the coke residues of the resins did not reach a sufficiently high content.

It is most convenient to use a pitch with a very small fraction, which distills at 360 ° C.

A pitch of good quality according to the invention can be used both for the production of pre-used pre-pressed and fired anodes and for the production of coal pulp for soderberg anodes. On the other hand, the grain size distribution chosen for the carbon powder will be somewhat different in these two anode types, such as hammer to be discussed in the following.

Pressed and pre-fired anodes.

For the production of such anodes, very pure coke, such as pitch coke (high temperature coke) or petroleum coke (low temperature coke), is used, freed from more volatile constituents by dry distillation in the absence of wool, and ground into powder. The specific gravity of the powder should be between 2 and 2.05, preferably between 2.02 and 2.05 for petroleum coke, and between 1.94 and 1.97 for pitch coke.

The ground coal Mr ha has a continuous grain size distribution, 'armed means that it will contain cows of all sizes Iran 15 mm down to dust particle size. On the other hand, the app should show the following properties: At least 45% of the material must be retained on a sieve with a 0.83 mm mesh opening, and of these 45 ° if a third of the grains should have grain sizes between 5.5 and 15 mm; 33 to 35% fine grains should pass through a sieve with a 0.16 mm mesh opening, while 25% pass through a sieve with a 0.089 mm mesh opening.

The following particle size distribution, which is given as an illustrative example, to which the invention is not limited, gives satisfactory results: The screen mesh opening in mm 5,543,331,700,830,360,160,089 Transmission,% 711129% 35 °; ,, - --3 The content of pitch to be used with a cooking powder of this grain size distribution, amounts to about 15%. The mixing of baking powder and pitch is carried out according to the procedure set forth in French Pat. No. 992,508. This meant that the cooking powder, which is retained on a sieve of 0.16 mm mesh size, is first introduced into a mixing machine and stirred while heating to a temperature of 140 ° C. Then pitch is introduced, and the whole is mixed again, until all the coke grains become a choice. vatta. In this step, only such grains were added, the sum was returned to the sieve with a 0.16 mm mesh opening, and the mixing was completed.

A raw anode is then obtained by pressing the pulp under a pressure of 400 to 700 kgf / cm 2, and burning the material until the specific gravity of the burning anode is greater than the specific gravity of the coke used to produce the pulp. Experience has shown in practice that in this way an anode is obtained in which the binder coke arising from the stream is burned at the same time as the coke grains which form the main mass of the anode, thus avoiding the release of coke grains and the entry of carbon dust. in the electrolyte.

In the case of anodes manufactured on the above-mentioned Alt, an average anode consumption of 430 kg per tonne of aluminum produced was obtained over a period of six months. After deducting the anode play, ie. the part of the anode that remains in the anode holder after maximum felling was the net consumption: 430 - 40 = 390 kg per tonne of aluminum.

Coal mass for continuous anodes of the soderberg type.

DO the continuous electrode is burned at the temperature of the electrolysis bath, i.e. at about 950 ° C, a pitch coke or petroleum coke must be used, the specific gravity of which is between 1.94 and 2, if it is to be possible to avoid the formation of carbon dust by loosening grains in the electrolysis bath.

The grain size distribution The Mom coke powder shall be characterized by the following properties: At least 15% of the grains must have a grain size between 5.5 and 15 mm; to 60 °, /, of the baking powder shall pass through a sieve of 0,15 mm mesh size, and 40 to 48% of the same shall also be able to pass through a sieve of 0 089 nun mesh size.

The following is an illustrative example of a grain size distribution, which allows to achieve good results: Sieve mesh opening in mm 5.54 3.33 1.71 0.83 0.36 Transmission in% 4 7 4 6 8 Sieve mesh opening mm 0.16 0.089 0,070,00,00,0 Transmission in% 9 13 8 7 9 56% The mixing of the baking powder with the binding pitch is carried out in the same manner as above for the preparation of ready-fired anodes.

The content of pitch in relation to the total weight of the pulp Or about 30 ° / 0; it should be large enough for the mass to be sufficiently liquid to be able to flow out over the entire byre surface of the continuous anode.

This flowability is particularly important, as the current is led to the anode through vertically arranged metal bolts. As the lower part of the anode is consumed during the progress of the electrolysis, the anode material slides down into its surrounding housing, so that the consumption is compensated. It should now be noted, however, that during this downward movement the upper part of the anode is not displaced by the cradle of the lung cavity, without there always being a layer of ram mass, more or less floating, which has stuck to this cradle. As a result, along the cradle of the housing, a relaxation occurs between the part of the anode which has already hardened and the still semi-liquid ram mass. It is important that the mass applied to the upper spirit of the anode be sufficiently flowable to be able to fill the voids caused by said relaxation. If such a "filling" does not take place, the strips or cavities are retained in the anode mass, and it has been found that air enters it and burns the anode material inside the housing, a condition which seems very disturbing during the electrolysis.

In order for the mass to be sufficiently laltiluten in order to avoid this inequality, the mass' displacement # is regulated.

The leveling test is carried out as follows: Samples taken from the mixing machine on the mass are die-cast into cylinders of 50 mm length and 25 mm diameter. These are placed ph a 120 mm Ihng plate strip, which Or slightly inclined and provided with recesses; the samples are placed in the recesses in such a way that the upper end of the cylinders is at a distance of 15 to 20 mm from the upper end of the plate strip. The lower spirits of the samples can be freely pushed downwards. The plate strip with the samples is placed under a slope of 6.5 ° in a drying chamber heated to 200 ° C, used throughout Mlles for two hours. After cooling, the demand of each specimen was met. orig. the length x 100 must be between 60 and 80 ° / ,, `.

During a period of six months, when using continuous soderberg-type anodes, produced from a mass of the type described above, it has been found that the consumption of anode mass was 514 kg per ton of aluminum produced, the demand The proeentual strain - - which corresponds to 440 kg finished fire anode per ton of aluminum.

The present invention thus constituted a very significant technical advance in the manufacture of anodes intended for use in the production of aluminum by melt electrolysis.

Claims (3)

Patent claim: 1. A process for the preparation of blocks pressed and fired or of continuous anodes of carbon powder and pitch for the production of aluminum by melt electrolysis, characterized in that a pitch with the following properties is used as binder for the carbon powder: the softening temperature, determined according to Kramer-Sarnow's method. Jiggers between 70 'and 85 °, the coke residue exceeds 50% of the original weight of the rancid, the content of benzene insoluble in benzene () eh in anthracene oils is at least 20% of the weight of the pitch, and the coke residue in the coking of these resins is at least 89% of the weight of the resins. Process according to claim 1, for the production of anodes in the form of ready-fired, pressed blocks, characterized in that in addition to said pitch, either a petroleum coke powder is used, the specific gravity of which is Jigger in the range 2 to 2.05, preferably in the range 2.02. to 2.05, or a pitch coke powder, the specific gravity of which is between 1,94 and 1,97 and the grain size distribution is such that at least 45% remains on a screen with 0,83 mm mesh size and the content of barley in the area 5, 5 to 15 mm is about 15%, while the content of finer grains, which is passed through a sieve with a 0.16 mm mesh opening, amounts to 33 to 35 °, / o, 25 °, of which passes through a sieve with 0.089 mm mesh opening, and that the raw anode after pressing is fired in such a way that its specific gravity after firing exceeds the specific gravity of the coke used for preparing the pulp. 3. Process according to patent claim 1, Mr production of continuous soderberg anodes, characterized in that in addition to said pitch a coke powder is used, the specific gravity of which Egger is in the range 1.94 to 2 and which mile-hailer approx. 15 °, /, barley of sizes between 5.5 and 15 mm and 50% to 60 finer grains, which are permeated by a sieve with 0.15 mm mesh coupling and of which 40 to 48% alien are permeated by a sieve with 0.089 mm mesh coupling, and that a sufficient mung pitch was added to the coke powder to form, after mixing, a flowable mass, for which a drying strain sample gives a strain of 60 to 80% of the original length of the specimen. Request publications: Patentshrif ter frein Sverige 133 594, 136 746.