RU2132411C1 - Method of preparing electrode paste - Google Patents

Abstract

FIELD: electrode production. SUBSTANCE: nonfired coke is proposed to be used in the form of activated dust with 0.08 mm fraction content at least 65%, weight percentage of nonfired coke in total coke charge ranging from 3 to 15% and specific surface of activated dust 0.08 mm fraction being at least 5500 sq.cm/g. EFFECT: expanded raw material resource. 3 cl, 3 tbl

Description

 The proposed method relates to electrode production, in particular to the production of anode paste for self-baking anodes of aluminum electrolysis cells.

Currently, in the production of the anode mass, one of the types of filler used petroleum coke, which underwent preliminary heat treatment at a temperature of 1150-1250 ^o C.

 Traditionally, heat treatment is carried out in tubular rotary calcining furnaces that operate efficiently on cokes that do not contain “fines” (fines). In this regard, coke "trifle" is not in demand and in certain periods accumulates in oil coke producers in significant quantities.

 The difficulties of calcining coke "little things", due to its intense combustion and dust extraction, require the development of alternative methods of involving this product in production.

 Among them, it is worth noting quite well-studied methods of enlarging petroleum coke by briquetting and tabletting, but the most practical interest is the use of "crude" petroleum coke fines, bypassing the calcination stage.

 The use of non-calcined or partially calcined cokes is known both in our country and abroad.

It is known auth. St. N 704896 (C 01 B 31/02, C 25 B 11/12), according to which, in order to increase the physicomechanical properties of the carbon mass and self-firing electrodes and reduce their cost, it contains, wt.%:
Semi-coke - 20 - 45
Coal tar pitch - 20 - 28
Thermoanthracite - the rest.

According to the patent of Germany N 969036 for the production of electrodes of "crude" petroleum coke, containing, in addition to moisture, 8-20% volatile, mixed with 10-40% liquid or solid organic plasticizer, the mixture is heated to 38-250 ^o C in order to obtain a uniform plastic masses, form electrodes from the said mass and burn them with a gradual increase in temperature from 550 to 1100 ^o C. The electrodes can be graphitized by heating to temperatures above 2500 ^o C. Plasticizer - anthracene oil or coal tar pitch.

 The closest in technical essence and the achieved result is a technical solution of the company Alcan Research and Development Ltd. (Canada, N 843910, class C1A), which consists in the fact that for the preparation of a paste for carbon electrodes particles of partially calcined coke dust are crushed together with larger particles of fully calcined coke in a ratio of 60: 40 to 95: 5. Preferably, the partially calcined coke dust is 5-10% of the weight of the aggregate, the remainder is particles of fully calcined coke> 0.295 mm in size, with some of these particles having a size <0.295 mm. The aggregates are mixed with a binder, for example pitch, in an amount of 28-35%. Pre-fired carbon electrode is obtained by pressing paste with subsequent firing of the obtained workpiece.

The disadvantages of the prototype include the following:
- to use this method is possible only in the production of molded electrodes with their subsequent firing;
- the method uses partially calcined coke dust, which allows to expand the raw material base through the use of under-calcined "fines" of oil, pitch coke of the calcining department, as well as screening of pitch coke, but does not allow the use of non-calcined oil "fines";
- despite the fact that the prototype method allows the introduction of up to 40% partially burnt coke dust into the dust, it is still preferable to use 5-10%.

 According to the authors, the above disadvantages are the result of an insufficient degree of dust grinding (grinding fineness is controlled by the fraction of 0.295 mm) and the lack of special preparation (activation) of dust.

 The objective of the proposed invention is the involvement in the production of electrode mass for self-baking anodes of substandard (by size) coke feed, regardless of the degree of heat treatment, specifically the crude "little things" of petroleum coke.

This object is achieved in that in a method for producing electrode mass for self-burning anodes of aluminum electrolysis cells, including crushing, classification and grinding of coke, dosage of components of a coke charge containing non-calcined coke, preparation of a hydrocarbon binder and its mixing with a coke charge, non-calcined coke are introduced into the charge the activated form of dust with a content of fraction - 0.08 mm of at least 65% and a specific surface area of at least 5500 cm ² / g, wherein the weight fraction of uncalcined coke in coke th charge support in an amount of 3-15% and the content of a hydrocarbon binder in the electrode mass support in an amount determined by the formula:
C _binding = (C _{p St} - 0,1C) ± q,
where C _St - mass fraction of hydrocarbon binder in the electrode mass using a mixture of calcined and non-calcined coke, wt.%;
C _{St. R} - mass fraction of hydrocarbon binder in the ordinary electrode mass using a calcined coke charge, wt.%;
C is the mass fraction of non-calcined coke in the coke charge, wt.%;
q is a correction factor that takes into account the accuracy of dosing of the hydrocarbon binder.

 The technical essence of the proposed solution lies in the fact that the use of the dust fraction obtained by simultaneous grinding of calcined coke and "crude" petroleum coke fines in the proposed ratio and to the proposed degree of grinding, which provides a sufficiently developed specific dust surface, allows one to obtain an electrode mass that meets the requirements of TU.

It should be noted that when grinding in the proposed method, the "crude" fines of petroleum coke are crushed and activated to a greater extent, as a result of which the fraction - 0.08 mm (more precisely, the fraction - 0.05 mm) is close in nature to the α ₁ - binder fraction , therefore, the amount of "crude" fines in each case should be determined by the number of α ₁ fractions in the binder used.

 The authors conducted a series of experiments using dust crushed in conventional ball mills and vibratory mills. The results are shown in table. 1.

 A mixture of coal tar pitch ZSMK and Magnitogorsk MK in the ratio of 50:50 was used as a binder in the test mass, and the fineness of grinding dust (0.08 mm in class) was kept constant.

 The authors in the process of experiments were primarily interested in the upper allowable limit of the addition of a fine fraction of "crude" petroleum coke, therefore, in table. 1 shows data from 5% to 35%.

 An additive in an amount of less than 5% practically does not affect the quality of the anode mass, therefore, mass data with an additive of less than 5% are not given. The lower limit of the additives claimed in the claims and equal to 3%, is determined only from the point of view of economic feasibility.

In the table. 1 for the base of comparison, the indicators of testing the anode mass without the addition of "crude" fines are taken, since the comparison with the parameters of the prototype is not correct due to the fundamental difference in the purpose of the resulting masses, namely: in the prototype for the production of molded electrodes, in the proposed solution - for the formation of self-baking anodes, as well as due to differences in the feedstock.)
In the table. 1 also shows the requirements of the technical specifications for the anode mass.

From the table. 1 shows that
- with an increase in the content of “raw” fines in dust, the specific surface both during grinding in a ball mill and with a vibrating mill increases to a certain limit, then stabilization is observed, and then the specific surface growth decreases, which is probably due to coagulation of fine dust. But the level of specific surface with a vibrating mill is much higher;
- with an increase in the content of "crude" fines, the content of the binder decreases, but at the same time, K _t increases, which positively affects the formation of a self-burning anode;
- the results of technological testing of the mass with 15% of the "crude" fines of a vibrating mill (the upper limit of the recommended range) shows:
in terms of destructibility in the CO ₂ current, the indices are improved in comparison with the control sample and are at the lower limit of the technical requirements;
in terms of electrical resistivity, the indicators are comparable to a control sample and practically fit into the requirements of technical specifications. The overestimated value of the resistivity of the control sample is probably associated with raw materials;
in terms of mechanical strength, a deterioration is observed both in comparison with the control sample and with the requirements of the technical specifications, although with a vibrating mill the indicator of 29.5 MPa is comparable to the technical requirements of at least 30 MPa.

 The experiments showed that the introduction of "crude" fines of petroleum coke makes it possible to obtain an anode mass that practically meets the requirements of TU provided that the dust is prepared with a vibrating grinder, and the maximum permissible content of "crude" fines is 15% (preferably 10%).

This indicator is optimal for the mass using a coal binder with α ₁ - fraction of 8%.

Further tests were aimed at identifying the dependence of the properties of the anode mass on the content of α ₁ - fraction in the binder. The results are presented in table. 2.

The results show a direct relationship between the content of α ₁ fraction in the pitch and the optimal limit of the content of "crude" fines in the mass.

So, for coal tar pitch, the results of technological testing of mass N 2 and N 4 are comparable, although in the mass of N 2 - 12% of the "raw" fines, and in the mass of N 4 -10%. This is due to the fact that in the mass of N 2 the content of the α ₁ fraction is lower than in the mass of N 4.

In oil pitch, the content of α ₁ fraction is insignificant, therefore, the optimum limit of the content of "crude" fines in the base mass can be increased in comparison with the mass based on coal tar pitch (up to 15%).

Experiments have shown that intensive grinding of non-calcined petroleum coke fines together with calcined coke changes the physicochemical properties of the latter. As a softer material, the non-calcined "trifle" is destroyed more intensively, and its surface is activated. In the process of co-milling, the calcined fines are pressed into the grains of more durable calcined coke. As a result, we get dust from the vibrating mill
firstly, compacted by non-calcined trifle, which explains the decrease in its destructibility;
secondly, dust having a surface that is more active from the physicochemical point of view.

Further, the mixture of the mixture with the pitch passes by the following mechanism:
on the active surface of non-calcined fines, selective sorption of heavy pitch components is observed. As a result, pitch layers are enriched with light fractions. This contributes to a greater distillation of light fractions during heating of the mixture and leads to some deterioration (but within TU) of the quality of the mass in terms of mechanical strength and electrical resistivity. At the same time, enrichment of pitch interlayers with light fractions allows reducing the binder content without decreasing K _t .

The indicators claimed in the claims and expressed in absolute terms are justified by the following:
1. "... administered as activated dust ..."
First of all, the “crude” oil fines are crushed with a vibrating mill, which is activated during grinding and, from the physicochemical point of view, approaches the properties of α ₁ , the binder fraction.

2. "... non-calcined coke is introduced ... with a fraction content of 0.08 mm not less than 65% ..."
The experiments showed that it is the fraction - 0.08 mm (more precisely, 0.05 mm in it) in the structuring of the anode that is approaching the α ₁ fraction so far. When the content of this class is less than 65%, a decrease in the quality of the anode mass is observed, which is probably associated with insufficient compaction during the grinding of calcined coke particles.

3. "... the content ... support in the amount of 3-5%"
The lower limit is justified by economic feasibility, since with the introduction of "crude" fines of less than 3%, the quality of the mass practically does not change, and the costs increase due to the redistribution of the vibrating mill.

The upper limit is determined for the mass on the basis of a binder with a minimum content of α ₁ fraction.

 The increase in the content of "raw" coke in the charge above 15% leads to a sharp deterioration in the quality of the anode mass (see table. N 1).

 This is due to the fact that only a certain amount of “raw” coke is pressed into the calcined coke during joint grinding, and its excess during aggregation is coagulated). With the addition of raw coke more than 15%, the number of coagulated particles of raw coke increases, which worsens the conditions for the formation of coke from a binder.

 4. The second claim.

 Mathematical dependence allows you to adjust the binder content depending on the content of "raw" coke. The coefficient 0.1 is determined experimentally.

Moreover, the authors recommend based on the tests to adhere to an indicator of 15-20%, as a total indicator for the content of "raw" coke in the coke charge and the content of α ₁ fraction in the binder.

 The indicator recommended by the authors of 15-20% is not a dogma and may vary for each specific plant, taking into account its suppliers of raw materials and the possibility of redistribution of fine grinding and dosing.

5. "... specific surface area of at least 5500 cm ² / g."

 This limit on the specific surface of the fraction - 0.08 mm corresponds to the content (65%) of this fraction in the dust.

 The grinding fineness is controlled by the fraction - 0.08 mm (not less than 65%). If we take into account that the non-calcined material wears off more easily, then the presence of 65% of the fraction - 0.08 mm in the total dust gives the presence of non-calcined fines - 0.08 mm more than 65%, which corresponds to a specific surface area of at least 5500 cm / g.

 Subject to all conditions expressed in the claims, the task is achieved - the use of substandard (by size) coke raw materials, regardless of the degree of heat treatment.

 Common features of the proposed method and prototype is that in both cases the coking charge partially contains coke with an insufficient degree of heat treatment, which is introduced in the form of dust obtained by co-grinding coke with partial and complete heat treatment, but in the proposed method, the grinding process leads to a finer dust (the content of the fraction is 0.08 mm is not less than 65%) with the simultaneous activation of "raw" coke, which makes it possible to use such an anode mass in a self-baking anode.

 Thus, the proposed technical solution is significantly different from the prototype.

 Therefore, this proposal meets the criteria of the invention of "novelty."

To determine the “prior art”, a search was carried out in patent and scientific literature:
- a known method of improving the quality of the electrode mass through the use of surface-active substances (surfactants). By author St. USSR N 408529 (class C 01 B 31/04) as a surfactant use glycerin tar in an amount up to 5% by weight of pitch;
- a known method of increasing the density and mechanical strength of coke grains by introducing into the dry composition of the electrode mass of coke a certain size. According to the patent NDP N 77932 (class 80 B 8/12) coke is introduced in an amount of 40-60 weight. parts with a grain size of 20-0 mm, and the grain content> 15 mm should be more than or equal to 10% (preferably 15-20%) and grains <0.3 mm - less than or equal to 35%;
- there is a method of adjusting the ductility of the anode mass depending on the concentration in the pitch of components insoluble in benzene. According to the patent NDP N 99171 (class C 25 C 3/06), the required ductility is calculated by the formula
P = 400 - 10 (A + B) ± 5,
where (A + B) is the amount of insoluble in benzene, in%;
- a known method of improving the quality of the mass by changing the process of mixing the filler and binder. In US patent N 2645583 (CL 106-56), the filler is divided into two fractions - coarse and fine, the sizes of the smallest particles of the coarse fraction should correspond to the pore sizes of the largest particles of this fraction (the smallest particles of the coarse fraction have a size of about 0.2 mm ) A large fraction (30-75% by weight of the filler) is separately mixed with a binder, then a small fraction is added and mixing continues until the mixture is completely homogeneous.

None of the known solutions revealed a solution similar to the claimed, namely:
to obtain a high-quality anode paste for self-baking anodes, use "crude" petroleum fines in the form of activated dust with a content of at least 65% of the 0.08 mm fraction obtained by co-grinding with calcined coke, and the proportion of non-calcined coke in the coke charge is 3- fifteen%.

 The proposed technical solution is illustrated by an example.

Example 1
For the manufacture of the anode mass, a mixture of coal tar pitch ZSMK and Magnitogorsk MK was used in a ratio of 50:50 (content of α ₁ - fraction 8%). As a non-calcined additive in the charge, 10% of fines of "crude" petroleum coke ANHK (actual density 1.4 g / cm ³ and mass fraction of volatile 8%) were used. Pitch dosing accuracy (q) = ± 0.2%.

Before grinding, the coke breeze was dried at a temperature of 130 ^o C for 5 hours, after which the coke acquired sufficient flowability and was less compressed during grinding.

The dried fines were crushed on a HERZOG vibration dampener simultaneously with calcined coke (screening). The grinding fineness was controlled by the class of 0.08 mm (65.6% with a specific surface area of 5760 cm ² / g.

 In the manufacture of the experimental anode mass, the composition of grains shown in table. 3.

 According to the formula proposed by the authors, we calculate the content of the binder in the mass, taking into account the fact that in the ordinary mass its content is 33% (see table. N 1).

С _St. = (С _{St. R.} - 0.1С) ± q = (33 - 0.1 • 10) ± q
With _sv = 31.8%
The anode mass is obtained by traditional mixing of the coke charge with the calculated quantity of the c / u pitch.

 Indicators of technological testing of this mass are given in table. 1, column 13.

Claims (3)

 1. Method for the production of electrode mass for self-burning anodes of aluminum electrolytic cells, including crushing, classification and grinding of coke, dosing of coke charge components, preparation of a hydrocarbon binder and its mixing with a coke charge containing non-calcined coke, characterized in that the non-calcined coke is introduced in the form of activated dust with a fraction content of 0.08 mm not less than 65%, while the mass fraction of non-calcined coke in the coke charge is maintained in an amount of 3-15%. 2. The method according to claim 1, characterized in that the content of the hydrocarbon binder in the electrode mass is supported in an amount determined by the formula
C _sv = (C _sv.r - 0.1 C) ± q,
where C _St - the mass fraction of the hydrocarbon binder in the electrode mass using a mixture of calcined and non-calcined coke, wt.%,
C _{St. R} - mass fraction of hydrocarbon binder in the ordinary electrode mass using a calcined coke charge, wt.%;
C is the mass fraction of non-calcined coke in the coke charge, wt.%;
q is a correction factor that takes into account the accuracy of dosing of the hydrocarbon binder. 3. The method according to claim 1, characterized in that the specific surface area of the activated dust fraction is 0.08 mm of non-calcined coke support at least 5500 cm ² / year

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