EP0215192A1 - Electrode binder - Google Patents

Abstract

1. The use of an electrode binder of coal tar with a softening point (Kraemer-Sarnow) of from 90 to 105 degrees C, a content of quinoline-insolubles (QI) of from 0.5 to 5% by weight, from 25 to 35% by weight of beta-resins, a content of ash formers of less than 0.2% by weight, preferably less than 0.1% by weight, and an Na content of less than 50 ppm, preferably less than 20 ppm, obtainable by the filtration of a coaltar pitch which is mixed with a 1 - to 2.5-fold quantity of an inorganic or organic filter aid, relative to the QI content of the pitch, by means of filter cartridges with a pore size of from 50 to 100 mu m at a temperature of from 250 to 300 degrees C at a pressure of up to 8 bar and where appropriate by distilling off oils from the filtered pitch in order to fix the softening point, as a binder in the manufacture of anodes with reduced burn-off for the aluminium industry, in particular in the manufacture of previously burned block anodes with an addition of up to 30% by weight of anode residues, relative to the solid-material portion of the anode material, with an Na content of more than 1500 ppm.

Description

The invention relates to an electrode binder made of coal tar for the production of shaped carbon bodies, as are used, for example, in the production of aluminum.

A carbon anode for aluminum extraction should have high strength, low porosity, low electrical resistance and the lowest possible burn-up. These properties are decisively influenced by the binder used. It is characterized by its softening point, the coking residue and the content of insoluble parts in quinoline and toluene, as well as ash and sulfur. Often, the density and the distillation process are also Specification used. Coal tar pitches are used almost exclusively as binders because of their favorable coking behavior. They have the following specification data (Ullmann's Encyclopedia of Industrial Chemistry, 4th edition, volume 22, page 423):

The β-resins in particular influence the baking and binding capacity, and the QI content increases the strength of the binder coke. Ash formers are undesirable because they contribute to the contamination of the aluminum.

The aim of the aluminum smelters is to develop an anode with high conductivity and low erosion in order to reduce operating costs. Only a part of the used carbon is used for the reduction of the aluminum ore, another part is lost through side reactions, eg conversion of carbon dioxide into carbon monoxide, and through "sanding" of the anode. "Sanding" is caused by the faster consumption of the bandage medium coke compared to petroleum coke, the filler coke grain loosening from the composite and falling into the bath.

The influence of ash formers on the reactivity of the anode carbon and thus on the anode consumption has recently been investigated in detail. The effects of sodium and iron were examined in particular (Petersen: Effect of sodium content of pitch on selective oxidation on baked blends of pitch / fines / Light Metals, 1981, pages 471 - 476). It was found that neither the porosity with the only small scattering range nor the iron content with a selective oxidation at 950 ° C. in carbon dioxide correlate with the burnup of a fired, dense carbon molded body produced from pitch and coke powder. In the case of sodium alone, there was a dependence of the burnup and the "sanding" on its concentration in the pitch. The transferability of these results to industrially manufactured anodes cannot be guaranteed due to the different granulometry and the lower proportion of binder, especially since air permeability must also be taken into account. Especially with pre-burned block anodes, which contain Na-rich anode residues, different results can be expected.

Coal tars already contain sodium as raw tars. The Na content is further increased by the usual neutralization of the acidic chloride ions to reduce corrosion in the distillation plants. Bad luck are the ash builders and therefore that too Concentrated sodium so that the electrode binders normally contain about 500 to 1500 ppm sodium.

It is known that the ash formers can be removed from tars and pitches by suitable separation processes such as centrifuging, separating, filtering and promoter-accelerated settling. In these processes, however, the quinoline-insoluble material required for anode strength is also deposited.

It was therefore the task of developing an electrode binder based on coal tar, with which anodes of high strength, low electrical resistance and low air permeability can be produced, which show reduced CO₂ burnup and less severe "sanding".

The object is achieved according to the invention by a coal tar pitch with a softening point (Kraemer-Sarnow) of 90 to 105 ° C., a content of quinoline-insoluble (QI) / of 0.5 to 5% by weight, 25 to 35% by weight of β- Resins, an ash former content of less than 0.2% by weight, preferably less than 0.1% by weight, and a Na content of less than 50 ppm, preferably less than 20 ppm, in solution, obtainable by filtration a coal tar pitch, which is mixed with 1 to 2.5 times the amount of an inorganic or organic filter aid, based on the QI content of the pitch, by means of filter candles with a gap width of 50 to 500 µm at a temperature rature from 250 to 300 ° C under a pressure of up to 8 bar and, if necessary, by distilling off pitch fractions from the filtered pitch to adjust the softening point.

Diatomaceous earth (diatomaceous earth), volcanic ash or the like can be used as inorganic filter aids. Organic filter aids are activated carbon, charcoal, peat coke, brown coal coke and the like. The grain size of these filter aids corresponds to 0.2 to 3 times the gap width of the filter.

The reactivity of the anodes produced with the pitch according to the invention is reduced not only in the case of Söderberg anodes, but even in the case of prebaked block anodes which contain up to 30% by weight of anode residues, based on the solids content, with a sodium content of more than 1500 ppm. This was not to be expected on the basis of the known publications, since the Na content of the block anodes can be influenced only slightly by the binder and, depending on the proportion and degree of purification of the anode residues (“butts”), is approximately 350 (14% butts) or 900 ppm (20% butts). This behavior can not be explained about the catalytic effect of sodium on the CO₂ reactivity of the coke.

It is also surprising that the strength, measured as flexural strength, of the anodes does not differ from that of the conventional anodes, at least in the range of the usual mixing ratios when using the binder according to the invention, although the burning losses are higher with the same binder content and the apparent density is usually lower than with anodes with known binders.

werden bei 200°C mit 7 Gew.-Teilen Diatomeenerde der Körnung 30 - 300 µm vermischt und auf 270°C erhitzt. Das Gemisch wird dann über ein Spaltrohrfilter mit einer Spaltweite von 150 µm im Kreislauf gefahren, bis sich bei einem Druck von 1,5 bar eine Filterschicht aufgebaut hat. Danach wird das Filtratpech entnommen. Der Filtrationszyklus wird bei einem Druck von 7 bar abgebrochen und nach Abreinigen der Filterfläche wiederholt, bis die gesamte Pechmenge filtriert ist. Das Filtratpech ist durch folgende Analysendaten ge­kennzeichnet:
EP (K.-S.) 66°C
QI 1,0 %
TI 25,6 %
Aschebildner 0,06 %
Na 9 ppmThe invention is explained in more detail using example 1. The advantages of the pitch according to the invention result from a comparison with Example 2, in which a common electrode pitch is used.

are mixed at 200 ° C with 7 parts by weight of diatomaceous earth with a grain size of 30-300 µm and heated to 270 ° C. The mixture is then circulated through a canned filter with a gap width of 150 µm until a filter layer has built up at a pressure of 1.5 bar. The filtrate pitch is then removed. The filtration cycle is stopped at a pressure of 7 bar and repeated after cleaning the filter surface until the entire amount of pitch has been filtered. The filtrate pitch is characterized by the following analysis data:
EP (K.-S.) 66 ° C
QI 1.0%
TI 25.6%
Ash formers 0.06%
Na 9 ppm

12% by weight of oils are distilled off from the filtrate pitch at a pressure of 100 mbar. A bad luck with the following characteristics is obtained:
EP (KS) 99 ° C
QI 1.1%
TI 28.8%
β-resins 27.7%
Ash formers 0.07%
Na 10 ppm
Coking residue 52%
(Conradson)

A mixture of 86 parts by weight of petroleum coke and 14 parts by weight of anode residues is mixed with this pitch. Four mixtures with 14, 16, 18 and 20 wt .-% binder are formed into block anodes and burned in a known manner. The burning losses are shown in FIG. 1 and the properties of the anodes in FIGS. 2 to 7.

Example 2 (comparison)

For comparison, anode mixtures with 14, 16, 18 and 20% by weight of a conventional electrode binder made from coal tar are produced from the same solid mixture as in Example 1. The electrode pitch is characterized by the following analysis data:
EP (K.-S.) 93 ° C
QI 12.0%
TI 35.0%
β-resins 23.0%
Ash formers 0.21%
Na 390 ppm
Coking residue
(Conradson) 58.1%

The results of the investigations on the anodes are compared with those from Example 1 in FIGS. 1-7. The measuring points of Example 1 are represented by a + sign and those of Example 2 by a Δ sign. 1 shows the higher burning losses when using the pitch according to the invention as a binder. However, this only affects the apparent density (Fig. 2) if the binder content is over 16%. This point is also the optimum density for the anodes produced with the pitch according to the invention. The optimum for the comparison anodes is 18%. The specific electrical resistance (Fig. 3) is the same for both types of anodes. This also applies approximately to the bending strength (Fig. 4). The influence of the different density is not noticeable here either.

The Na content (FIG. 5) of the anodes with the binder according to the invention is only slightly lower. The reactivity, measured as total loss - burnup and sanding - after 7 h at 960 ° C in CO₂, is however greater by about 3% by weight, based on the anode mass, for the anode with the normal binder pitch.

Another advantage with regard to low anode consumption is the low air permeability (FIG. 7) of the anode at 20 ° C. with the binder according to the invention. The permeability is measured in nPerm (nPm) (1 Pm means a gas flow of 1 cm³ / s through a 1 cm² area of a 1 cm thick test specimen with a pressure loss of 1 dyn / cm² with a viscosity of the gas of 0.1 Pa · s). According to Keller and Fischer ("Development of Anode Quality Criteria by Statistical Evaluation of Operational Results in the Electrolysis", Light Metals, 1982, pages 729 - 740), the dependency of the reduced consumption of anode material is:
ΔNC = 9.3 * Δ AP - 3.7 * Δ RR.
ΔNC is the reduced consumption of anode material in g of carbon per kg of aluminum, ΔAP denotes the difference in air permeability in nPm and Δ RR that of the residual reactivity in% by weight (total carbon loss used, based on the carbon used). The minimum of air permeability is approximately 16% for the binder according to the invention and approximately 18% for the comparative anode.

In summary, it can be stated that anodes which are produced with the pitch according to the invention as a binder have the same strength and the same specific electrical resistance as known anodes, but have a 3% lower burn-off. This lower burn-off is already achieved with about 16% by weight of binder. When accepting a slight increase in electrical resistance and one for block anodes acceptable decrease in the strength of the anodes could also save about 11% of the binder and be replaced by the much cheaper petroleum coke. This unexpected behavior of the binder according to the invention is possibly due to an improved wetting behavior compared to petroleum coke.

Claims (2)

1. Electrode binder from coal tar, characterized by a softening point (Kraemer-Sarnow) of 90 to 105 ° C, a quinoline-insoluble content (QI) of 0.5 to 5% by weight, 25 to 35% by weight ß Resins, an ash-forming content of less than 0.2% by weight, preferably less than 0.1% by weight, and a Na content of less than 50 ppm, preferably less than 20 ppm, obtainable by filtration of a Coal tar pitch, which is mixed with 1 to 2.5 times the amount of an inorganic or organic filter aid, based on the QI content of the pitch, by means of filter candles with a gap width of 50 to 100 µm at a temperature of 250 to 300 ° C under a pressure of up to 8 bar and, if necessary, by distilling off oils from the filtered pitch to adjust the softening point. 2. Use of the electrode binder according to claim 1, as a binder in the manufacture of anodes with reduced erosion for the aluminum industry, in particular in the manufacture of prebaked block anodes with an addition of up to 30% by weight of anode residues, based on the solids content of the anode mass a Na content of more than 1500 ppm.

Images