RU2800748C2 - Mixed composition containing petroleum coke and pyrolytic carbon for electrodes - Google Patents

Abstract

FIELD: carbon electrodes.

SUBSTANCE: group of inventions relates to a mixed composition for the production of a carbon electrode containing a mixture of petroleum coke and pyrolytic carbon, and a method for its preparation, an electrode composition containing the specified composition and a method for its preparation, a carbon electrode containing the specified composition and suitable for use as an anode in an aluminium electrolytic cell, and a method for producing it, as well as a method for producing aluminium using the specified carbon electrode. The mixed composition contains a mixture of (i) petroleum coke in an amount of 20 to 99 wt.% and (ii) pyrolytic carbon in an amount of 1 to 80 wt.% based on the total weight of the mixed composition. Moreover, the mixed composition contains at least two particle size fractions: (i) granular more than 0.5 mm and (ii) finely dispersed below 0.5 mm, where pyrolytic carbon is present at least in the granular size fraction.

EFFECT: improving the mechanical and chemical properties of the anode to minimize carbon consumption.

16 cl, 20 dwg, 1 tbl, 5 ex

Description

The present invention relates to a mixed composition containing a mixture of petroleum coke and pyrolytic carbon, an electrode composition, and the use of this electrode as an anode in a process for producing aluminum.

Aluminum production is carried out in electrolytic cells or electrolyzers (known as the Hall-Héroult process). The electrolysis of Al ₂ O ₃ takes place in a cryolite molten bath located between the carbon anodes and the molten metal. The aluminum ions in Al ₂ O ₃ are reduced to form molten aluminum. Molten aluminum collects at the bottom of the cell. The oxide ions react with the carbon anode to form carbon dioxide, thus the carbon anode is consumed in the electrolytic reaction.

Carbon consumption during electrolysis requires replacement of pre-baked anodes every 3-4 weeks depending on anode size and current density. Anodes at various stages of consumption are present in all cells due to the limitation of constant replacement and minimal bath disturbance. Approximately three-quarters of the anode is consumed during an anode change. The rest is called cinders.

Various reactions in the cell contribute to the consumption of anode carbon. Those that do not lead to reduction of metals contribute to excess carbon consumption, for example, combustion in air (O ₂ + C → CO ₂ , with O ₂ from ambient air), attack by carboxy groups (CO ₂ + CO, with CO ₂ as a product redox reaction producing aluminum) and selective oxidation (dusting). Dusting occurs as a secondary effect of the CO ₂ attack due to a reactivity imbalance between the various phases of the coke, which allows solid carbon fragments to fall out.

The cost of a carbon anode is 15-20% of the total cost of aluminum electrolysis production. Therefore, the quality of the carbon anode is critical and significantly influences the energy consumption and the environmental impact of aluminum electrolysis. Anode quality is characterized by five main characteristics:

- reactivity, which determines most of the excess carbon consumption per ton of aluminum produced. The lower the reactivity, the lower the excess carbon consumption;

- density, which is the main factor determining the service life of anodes in electrolyzers. The higher the density, the longer the service life. Extending anode life reduces the number of anode replacements required;

- resistance to thermal shock, which determines whether fractures or cracks will occur when the anodes are placed in the cells or during their cycles;

- low electrical resistance, which determines the loss of electricity due to the large electrical currents used in the Hall cell;

- high chemical purity, which determines the quality of the aluminum product due to the direct consumption of the anode in the electrolytic cell, any metallic or other impurities that it contains tend to pass into the aluminum product and can adversely affect its mechanical properties. High chemical purity also has a positive effect on reactivity.

The raw materials used to produce the dry composition in prebaked anode plants are typically petroleum coke, coal tar pitch binder, crushed cigarette butts, green scrap (shaped but not fired anodes) and fired scrap (shaped and fired anodes that are not on the specification). Typically, pre-baked anodes consist of about 65% coke, 15% pitch, and 20% recycled anode cinders.

At the mixing and pressing plant, the incoming calcined coke and recycled material are crushed, screened / sorted to the desired fractions and put together to form a dry aggregate. The coke fractions used for the manufacture of anodes are usually characterized as coarse, intermediate, and finely dispersed. The dry aggregate is preheated and mixed with a binder, typically coal tar pitch, which is also preheated to melt. The preheated mixture is processed to its final shape, which ensures that the structural shape of the densified anode block is retained. The green compact is then heated at an elevated temperature, for example about 1100° C., to form a fired anode before it is usable in an electrolysis cell.

The continued growth in demand for aluminum metal, coupled with the declining and fluctuating quality of aluminum grade coke in terms of both coke density and purity, makes it more challenging for anode plants to supply anodes of consistent quality. Poor quality coke is more reactive, resulting in higher carbon consumption in the smelter.

About 95% of the SO ₂ emissions from the smelter can be attributed to the sulfur contained in the incoming petroleum coke used in anode production. Thus, environmental regulations are aimed at reducing sulfur emissions, while coke suppliers offer materials with a higher sulfur content. Low sulfur coke material is becoming less and less available on the market and the price is constantly rising. In addition, the sulfur levels in many traditional "high sulfur" anode quality green cokes are increasing. Five years ago, green coke of anode quality with a high sulfur content was considered to be coke with a sulfur content of 3-4%. Today, a more typical level is 4-6%

The difference in the price of a barrel of low-sulphur sweet oil and high-sulphur crude oil, the “sweet-sour spread”, results in the need for more refineries to process cheaper, higher-sulphur crude oil. This higher sulfur crude produces coke with higher sulfur and metal impurities (especially vanadium and nickel). Kilns are using more of this coke to meet growing demand from the aluminum industry.

Because aluminum smelters have not significantly changed specifications for coke sulfur content due to smelter environmental constraints, high coke sulfur levels must be offset by blending with lower sulfur coke. As a result, the difference in sulfur content in the coke used in typical anode blends is increasing - where coke with a sulfur content of 1-2% can be blended with coke with a sulfur content of up to 4-6% to produce specification 1 smelter anode coke, 0-3.5%.

With the growth rate of primary aluminum production projected, the industry will have no choice but to use these higher sulfur coke blends. The sulfur content of high sulfur coke used in blends is increasing and is likely to continue to increase.

In addition to environmental regulations, the sulfur content greatly affects the quality of the anode. The high sulfur coke undergoes desulfurization both during calcination and during anode firing. Desulfurization adversely affects the properties of the coke, such as reducing the actual density and increasing the porosity, and additional desulfurization during anode firing can adversely affect the properties of the anode, such as increased reactivity with respect to both air and carboxy groups.

In addition, very low sulfur concentrations in the anode have been shown to adversely affect anode quality and possibly cell performance by increasing the anode's reactivity towards carboxy groups.

It has been shown that the anodic reaction with CO ₂ can be the main contributor to the formation of carbon dust in electrolytic cells. Carbon dust in increased amounts will increase the cell temperature and ultimately reduce the existing efficiency and metal production. Many publications have shown that the reactivity of the anode with CO ₂ increases with increasing sodium concentration.

However, the catalytic behavior of sodium is also known to be counteracted by the presence of sulfur. The reaction of sodium with sulfur has been shown to inhibit the catalytic behavior of sodium and reduce the tendency of sodium to migrate during the roasting process. Coke and anodes with a higher sulfur content will therefore have lower carboxyl group reactivity and less chance of dusting.

It was found that by separating the low sulfur coke into a coarse fraction in the anode plant and then increasing the amount of high sulfur coke used in the fine fraction, the likelihood that sulfur could inhibit the action of sodium increased, thus reducing the total anode CO ₂ reactivity. However, this reduction in anode reactivity with respect to carboxy groups may come at the cost of a lower anode density depending on the types of coke used in the blends. For example, when mixing low sulfur, low vibratory bulk density coke with high sulfur coke, high vibratory bulk density, placing the higher density material in the ball mill circuit can reduce the density of the anode.

In addition to the sulfur content, fluctuations in the density of aluminum grade coke increase. There is a problem of achieving and maintaining high and constant densities of baked anodes based on calcined petroleum cokes from various sources or other solid carbon substitutes with varying material properties. With coke blending equipment available and different grades of coke available, coke blending is one possible way to optimize dry aggregate density. However, this depends on the priority of the respective property of the coke. Until now, the mixing of high porosity and low porosity cokes has rarely been given priority. Commercial barriers such as price and material shortages also place limits on optimal blending.

Various attempts have been made in recent decades to blend or replace petroleum coke with various carbon sources.

Edwards et al. in 1969, an attempt was made to create pyrolytic carbon suitable for making anodes by cracking natural gas and liquefied petroleum gas in a moving load reactor (Edwards, J.H., R.J. Tyler, and P.L. Waters. "The production of electrode carbon from Australian fossil fuels." Institute of Fuel, Australian Membership Conference, Adelaide, November, paper. Vol. 13. 1974). It was found that test electrodes made solely from pyrolytic carbon using standard formulation techniques had unacceptable physical properties and test cell characteristics. The main problem, as it turned out, was that pyrolytic carbon, due to its "onion" structure, did not bind well with pitch and gave a very weak electrode. The low reactivity of the pyrolytic carbon compared to the reactivity of the binder pitch was also considered a potential problem in cell operation. Gardner et al. suggested that the inclusion of 30-40% pyrolytic carbon in the electrode assembly would not adversely affect electrode performance if pyrolytic carbon was used as a finer material in the range of minus 100 mesh (0.15 mm) (Gardner, H. J., P. L. Waters, and A. Watts, "Production of electrode carbon from brown-coal char and gaseous hydrocarbons." Effective Use of Hydrocarbon Resources: Preprints of Papers, National Conference on Chemical Engineering, Adelaide, August 25-26, 1976, The. Institution of Engineers, Australia, 1976). This fraction is crushed finer than the size of the onion layers. Thus, the bulbous structure disappears when crushed.

Due to the availability and low cost of brown coal, Gardener et al. disclosed studies on the use of lignite semi-coke as a substitute for petroleum coke in the production of carbon for aluminum smelting by enriching lignite. Brown coal char is less dense and more reactive than petroleum coke. It can be improved by impregnating the char with pyrolytic carbon derived from petroleum gas. Unfortunately, impregnated coal produced a higher carbon consumption than petroleum coke due to increased mechanical losses and dusting resulting from differences between the reactivity and consumption rate of impregnated coal and coke matrix binder.

EP 1766105 discloses a process for making a carbon electrode suitable for use as an anode in an aluminum electrolytic cell which comprises a mixed aggregate of coke solids, particulate carbonaceous material other than coke nut, and coal tar pitch or petroleum pitch or a combination of these pitches. .

Farr-Wharton et al disclosed in Electrochimica Acta, Vol. 25, pp. 217-221, Pergamon Press Ltd. 1980 that ordered carbons such as graphite and pyrolytic carbon showed high resistance to chemical and electrochemical oxidation, but despite oxidation resistance, the consumption rate was high due to erosion.

The aluminum industry avoids the use of highly isotropic coke, a fine textured coke that exhibits similar properties in all directions, especially coke tail, because they have high coefficients of thermal expansion (CTE) and low open porosity. Anodes made from these materials are more susceptible to thermal cracking when heated rapidly in aluminum electrolyzers. They may also suffer from lower mechanical strength and dusting problems during cell operation. The coke nut typically has a low level of open macroporosity for pitch penetration. This reduces the ability of the pitch to adhere and bind the structure during carbonization.

Edwards et al. disclosed in Light Metals, p-36, 2009 (Edwards, Les, et al. "Use of shot coke as an anode raw material." Essential Readings in Light Metals. Springer, Cham, 2016. 36-41.), that the influence on the anode thermal expansion coefficients can be minimized by adding isotropic coke and coke nuts to the fine fraction. However, this will result in increased reactivity with air. It is expected that when highly isotropic coke is added uniformly to fractions of all sizes, most anode properties are somehow degraded in direct proportion to the levels added.

Due to environmental requirements, aluminum smelters use low sulfur coke as a key strategy to reduce SO ₂ emissions. The negative impact of this action is an increase in the reactivity of the anode in relation to CO ₂ . To minimize negative impacts at the coal plant, the following actions are proposed: (i) increase the dwell time during anode roasting, (ii) direct the higher sulfur coke into the fine fraction, (iii) minimize the proportion of low sulfur coke in the mixture unit, choosing a coke with a very low sulfur content.

As environmental regulations aim to reduce sulfur emissions, while coke suppliers offer materials with higher sulfur content, the search for low sulfur blending material is a constant need.

In addition, the performance of the anode and aluminum product is improved by minimizing other metallic impurities, primarily V and Ni, which are also present in traditional low grade petroleum cokes.

Mixing with a higher density fraction of pyrolytic carbon can increase anode density, which increases life by increasing the mass of carbon available in the anode.

In general, the increase in reserve reserves of carbon anode resources is beneficial for the sustainable development of the aluminum industry.

Since the cost of producing carbon anodes is about 20% of the cost of producing aluminum metal in the Hall process, the overall ongoing goal is to improve the mechanical and chemical properties of the anode to minimize carbon consumption.

The present invention relates to a mixed composition containing a mixture of (i) petroleum coke in an amount of from 20 to 99 wt.% and (ii) pyrolytic carbon in an amount of from 1 to 80 wt.%, based on the total weight of the mixed composition, where the mixed composition contains at least two particle size fractions (i) granular greater than 0.5 mm and (ii) finely dispersed below 0.5 mm, where pyrolytic carbon is present at least in the granular size fraction.

The petroleum coke may be a mixture of various petroleum cokes. The pyrolytic carbon may be a mixture of various pyrolytic carbons.

Typically, the granular size fraction is in the range of 0.5 to 16 mm, preferably 0.5 to 8 mm. Typically, the fine particle size fraction is in the range of 0.005 to 0.5 mm.

A mixed composition may also be referred to by a person skilled in the art as a composition or aggregate.

The mixed composition is preferably a dry (binder-free) mixed composition.

Preferably, calcined petroleum coke (CPC) is used as the petroleum coke (Predel, H. (2000). Petroleum coke. Ullmann's Encyclopedia of Industrial Chemistry). Preferably, the sulfur content of the petroleum coke is in the range of 0 to 10% by weight, more preferably in the range of 0.5 to 8.5% by weight, more preferably in the range of 1.5 to 7.0% by weight of calculation for the total mass of petroleum coke. Petroleum coke is often referred to as petcoke.

The word "pyrolytic carbon" refers to solid carbon obtained by the pyrolysis of light hydrocarbons in the absence of oxygen (see, for example, Muradov, Nazim. "Low to near-zero CO ₂ production of hydrogen from fossil fuels: Status and perspectives." International Journal of Hydrogen Energy 42.20 (2017): 14058-14088). The preferred pyrolytic carbon for the anode is high density solid elemental carbon obtained by deposition on carbon pellets. It is preferred over thermal carbon black produced by thermal/plasma processes or nanostructured carbon grown on metal/oxide catalysts.

Pyrolytic carbon can be obtained by decomposition and precipitation of carbon from gaseous hydrocarbon compounds on suitable underlying substrates (carbon materials, metals, ceramics and mixtures thereof) at temperatures from 1000 to 2500 K and at pressures from 0.5 to 5000 kPa (abs.) (chemical vapor deposition or infiltration). The support may be either porous or non-porous, and may be either a reactor carrier support (a pre-installed part) or a granular and powdered material. The latter can be implemented as a fixed bed, a moving bed, a fluidized bed or a moving stream. The production of pyrolytic carbon is not limited to a particular energy source, but fossil-fuel-powered production reactors, electrically or plasma-driven, are possible.

A wide range of microstructures, e.g., isotropic, lamellar, nuclei on a substrate, and varying amounts of residual hydrogen, can occur in pyrolytic carbon depending on the deposition conditions (temperature, type, concentration and flow rate of the feed gas, surface area of the underlying substrate, etc.). ).

As a rule, the density of pyrolytic carbon is in the range from 1.6 to 2.3 g/cm ³ , preferably from 1.8 to 2.2 (true density in xylene, ISO 8004).

Generally, pyrolytic carbon impurities are: S in the range of 0 to 1%, preferably 0 to 0.5%, more preferably 0 to 0.1%. Fe in the range of 0 to 1000 ppm, preferably 0 to 500 ppm, Ni in the range of 0 to 250 ppm, preferably 0 to 100 ppm, V in the range of 0 to 450 ppm , preferably 0 to 250 ppm, more preferably 0 to 100 ppm. Na in the range of 0 to 200 ppm, preferably 0 to 100 ppm.

As a rule, the particle size of pyrolytic carbon after pyrolysis is at least 5 wt. %>1 mm, preferably 50 wt. %>0.5 mm.

Typically, the crystal size (XRD) of pyrolytic carbon is in the range of 20 to 60 Å, preferably 30 to 50 Å, (XRD, ISO 20203).

Typically, the porosity of the pyrolytic carbon pellet is less than 15%, preferably <10%, most preferably less than 5% (Hg porosimetry, DIN66133).

Typically, the specific surface area of pyrolytic carbon, measured by Hg porosimetry (DIN66133), is in the range of 0.001 to 5 m ² /g, preferably 0.01 to 2 m ² /g.

Preferably the mixed composition contains a mixture in which the content of petroleum coke is in the range from 30 to 98 wt.%, more preferably in the range from 40 to 95 wt.%, more preferably in the range from 50 to 95 wt.%, more preferably in the range 60 to 95 wt.%, more preferably in the range of 70 to 95 wt.%, more preferably in the range of 80 to 95 wt.%, more preferably in the range of 85 to 95 wt.%, even more preferably in the range of 90 to 95% by weight, based on the total weight of the mixed composition.

Preferably the mixed composition contains a mixture: wherein the content of pyrolytic carbon is in the range from 2 to 70 wt.%, more preferably in the range from 5 to 60 wt.%, more preferably in the range from 5 to 50 wt.%, more preferably in the range from 5 to 40 wt%, more preferably in the range of 5 to 30 wt%, more preferably in the range of 5 to 20 wt%, more preferably in the range of 5 to 15 wt%, even more preferably in the range of 5 up to 10 wt.% based on the total weight of the mixed composition.

With regard to total pyrolytic carbon: preferably at least 30 wt.% of the total pyrolytic carbon of the mixed composition is in the granular fraction, more preferably at least 40 wt.%, even more preferably at least 50 wt.%, more preferably at least 60 wt%, more preferably at least 70 wt%, more preferably at least 80 wt%, more preferably at least 90 wt%, even more preferably at least 95 wt%, even more preferably all pyrolytic carbon is in the granular size fraction.

With regard to total pyrolytic carbon: preferably 30 to 100 wt.% of the total pyrolytic carbon of the mixed composition is in the granular fraction, more preferably 40 to 100 wt.%, even more preferably 50 to 100 wt.%, more preferably 60 to 100 wt%, more preferably 70 to 100 wt%, more preferably 80 to 100 wt%, more preferably 90 to 100 wt%, even more preferably 95 to 100 wt%, even more preferably all pyrolytic carbon is in the granular size fraction.

With regard to total pyrolytic carbon: preferably 70 to 0 wt.% of the total pyrolytic carbon of the mixed composition is in the fine size fraction, more preferably 60 to 0 wt.%, even more preferably 50 to 0 wt.%, more preferably 40 up to 0 wt%, more preferably 30 to 0 wt%, more preferably 20 to 0 wt%, more preferably 10 to 0 wt%, even more preferably 5 to 0 wt%, even more preferably none of the total pyrolytic carbon is in the fine sized fraction.

Preferably 30 to 80 wt.%, more preferably 40 to 70 wt.%, even more preferably 50 to 65 wt.% of the particles have a granular particle size, and 20 to 70 wt.%, more preferably 30 to 60 wt.%, even more preferably from 35 to 50 wt.% of the particles have a fine particle size.

Preferably the mixed composition contains at least three particle size fractions (i) coarse over 4 mm, (ii) intermediate 4 to 0.5 mm, and (iii) fine below 0.5 mm, where pyrolytic carbon is present at least in the intermediate and/or large fractions.

With regard to total pyrolytic carbon: preferably at least 30 wt.% of the total pyrolytic carbon of the mixed composition is in an intermediate size fraction, more preferably at least 40 wt.%, even more preferably at least 50 wt.%, more preferably at least at least 60 wt.%, more preferably at least 70 wt.%, more preferably at least 80 wt.%, more preferably at least 90 wt.%, even more preferably at least 95 wt.%, even more preferably of all pyrolytic carbon is in the fraction of intermediate size.

With regard to total pyrolytic carbon: preferably 30 to 100 wt.% of the total pyrolytic carbon of the mixed composition is in the intermediate size fraction, more preferably 40 to 100 wt.%, even more preferably 50 to 100 wt.%, more preferably 60 up to 100 wt%, more preferably 70 to 100 wt%, more preferably 80 to 100 wt%, more preferably 90 to 100 wt%, even more preferably 95 to 100 wt%, even more preferably of all pyrolytic carbon is in the fraction of intermediate size.

With regard to total pyrolytic carbon: preferably 70 to 0 wt.% of the total pyrolytic carbon of the mixed composition is in the fine and/or coarse size fraction, more preferably 60 to 0 wt.%, even more preferably 50 to 0 wt.%, more preferably 40 to 0 wt%, more preferably 30 to 0 wt%, more preferably 20 to 0 wt%, more preferably 10 to 0 wt%, even more preferably 5 to 0 wt% even more preferably, none of the total pyrolytic carbon is in the fine and/or coarse size fraction.

Preferably 30 to 80% by weight, more preferably 40 to 75%, even more preferably 50 to 75% of the particles are of intermediate and/or coarse particle size, and 20 to 70%, more preferably 25 to 60%, even more preferably, 25 to 50% of the particles are fine.

Preferably 10 to 40%, more preferably 20 to 30% of the particles have a coarse particle size, 20 to 50%, more preferably 30 to 40% of the particles have an intermediate particle size, and 25 to 55%, more preferably 30 up to 50%, more preferably 35 to 45% of the particles are fine.

Depending on the production of anodes, anode manufacturers use different fractions and sizes, typical values are shown in the following table.

The mixed composition is preferably obtained by mixing pyrolytic carbon and petroleum coke; preferably by mixing dry pyrolytic carbon and dry petroleum coke.

The mixed composition may be mixed at the roasting plant, for example before or after the firing of the petroleum coke, preferably after firing. Alternatively, the mixed composition may be mixed before or after shipment to the appropriate locations. Alternatively, the blended composition can be blended at the anode plant, either prior to crushing, screening and sizing, or by adding pyrolytic carbon directly to the already crushed, screened and graded petroleum coke aggregate.

In addition, the present invention relates to an electrode composition, preferably an anode composition, containing a mixture of (i) a mixed composition of petroleum coke and pyrolytic carbon as described above, (ii) cinders and/or scrap, and (iii) a binder material.

Typically, from 35 to 95 wt.% of the total weight of the composition of the anode is a mixed composition, preferably from 50 to 80 wt. %. As a rule, from 0 to 40 wt.% of the total weight of the composition of the anode are cinders and/or scrap, preferably from 15 to 30 wt. %. Typically, 5 to 25% by weight of the total weight of the anode composition is binder, preferably 10 to 20% by weight, even more preferably 13 to 18% by weight.

The use of cinders and scrap is known in the art (Belitskus, David. "Effect of carbon recycle materials on properties of bench scale prebaked anodes for aluminum smelting." Metallurgical Transactions B 12.1 (1981): 135-139; Schmidt-Hatting, W., Kooijman, A.A., & Perruchoud, R. (2016) Investigation of the quality of recycled anode butts In Essential Readings in Light Metals (pp.251-266) Springer, Cham.; Schmidt-Hatting, Wolfgang, and A. Kooijman, "Optimization of the anode carbon consumption with respect to butt recycling." Light Metals 1993 (1993): 579-585.

All binders known from the literature can be used (Perruchoud, Raymond C., Markus W. Meier, and Werner K. Fischer. "Survey on worldwide prebaked anode quality." LIGHT METALS-WARRENDALE-PROCEEDINGS. - TMS, 2004). Preferably, coal tar pitch or a combination of pitch binder can be used as the binder.

Preferably, the total sulfur content of the anode composition is from 0 to 5 wt.%, more preferably in the range from 0.5 to 3.5 wt.%, more preferably in the range from 1.0 to 2.5 wt.%. Preferably, the ratio in the mixed composition depends on the desired final sulfur content.

The mixed composition and cinders and/or scrap are preferably preheated, preferably to a temperature (to melt the binder) of 100 and 175°C, and mixed with a binder, typically coal tar pitch, which is also preferably preheated to melt. The preheated anode composition is preferably pressed to its final shape, ensuring that the pressed anode block retains its structural shape. The unfired compact is subsequently preferably heated at an elevated temperature, eg 1000 to 1250° C., to form the fired anode until it is suitable for consumption in the electrolytic cell.

The present invention also relates to a carbon anode suitable for use as an anode in an aluminum cell, which contains a mixed composition containing a mixture of (i) petroleum coke in an amount of from 20 to 99 wt.% and (ii) pyrolytic carbon in an amount of from 1 to 80 wt.% based on the total weight of the mixed composition, where the mixed composition contains at least two particle size fractions (i) granular more than 0.5 mm and (ii) finely divided below 0.5 mm, where pyrolytic carbon is present at least in a granular size fraction.

Preferably, the carbon anode obtained in accordance with the present invention provides one or more of the following performance properties, preferably all mentioned parameters.

The density of the green material is preferably at least up to 1.50 g/cm ³ . The established intervals for CPC anodes are from 1.54 to 1.66 g/cm ³ (ISO 12985-1).

The bulk density is preferably at least up to 1.50 g/cm ³ (ISO 12985-1). The established intervals for CPC anodes are from 1.50 to 1.63 g/cm ³ (ISO 12985-1).

Thermal shock resistance and mechanical resistance are preferably greater than 6 MPa (ISO 12986-1), where 8-14 MPa is a typical range for CPC anodes.

The compressive strength is preferably more than 25 MPa (ISO 18515).

The electrical resistance is preferably below 80 μΩ. 50-60 µΩ is a typical industrial range.

The so-called residual air after the test reaction with air is preferably above 65% by weight, more preferably above 85% by weight, in the case of air reactivity (typically 65-90, ISO 12989-1). The so-called residual CO ₂ after the test reaction with CO ₂ is preferably above 80% by weight, more preferably above 90% by weight for CO ₂ reactivity (where 80-95 are common, ISO 12988-1).

The present invention relates to the use of a carbon anode according to the present invention as an anode in an aluminum cell.

Any of the aforementioned new electrodes or electrodes made by the method of the present invention can be used in a process for producing aluminum by molten salt electrolysis of alumina, which includes the electrolysis of alumina dissolved in a molten salt at an elevated temperature by passing a direct current through the anode to the cathode, located in the specified molten salt, where the specified anode is any of the above-mentioned electrodes.

Although a specific electrode suitable for molten alumina to aluminum salt electrolysis according to the present invention has been described above in order to illustrate how the invention can be used to advantage, it should be understood that the present invention is not limited thereto. That is, the present invention may suitably include, consist of, or essentially consist of the listed elements. Moreover, the present invention illustratively disclosed herein can be suitably practiced in the absence of any element not specifically disclosed herein. Accordingly, any modifications, variations, or equivalent arrangements that may be made by those skilled in the art should be considered to be within the scope of the present invention.

The results obtained for the mixture-composition clearly demonstrate the potential of the mixing approach.

Mixing low sulfur pyrolytic carbon reduces the total sulfur content of aluminum melting anodes while the other parameters mentioned remain within the desired performance ranges and the blend exceeds the expected average of the individual blend components. This advantage is demonstrated by examples.

The examples demonstrate that the disadvantage observed for high sulfur coke is completely eliminated by pyrolytic carbon:

The requirements for pitch and appearance of baked coke mix anodes are typical.

The air reactivity reaches a level suitable for pre-baked anodes.

Fe, Si, Ca, and P impurities are within typical limits.

The S content is logically half that observed in pure HS anodes and meets stringent requirements (less than 1.5%) for SO ₂ emissions.

The high coefficient of thermal expansion (CTE) of high sulfur coke is fully offset by the low coefficient of pyrolytic carbon.

In addition, the disadvantages of pure pyrolytic carbon anodes are also eliminated:

The specific electrical resistance and mechanical characteristics of mixed anodes are within the normal range.

The reactivity of mixed anodes with respect to CO ₂ is close to the typical range.

Thus, the present invention provides reduced sulfur carbon electrodes having characteristics including density, air permeability, compressive strength, elastic modulus, thermal conductivity, thermal conductivity, air reactivity, and carboxy reactivity, which are within acceptable ranges. for aluminum plants.

Examples

Parameters (See "Anode Manufacture, Raw Materials Formulation and Processing Parameters, Kristine L. Hulse, R&D Carbon, Page 10-14")

Density of unfired material

The apparent density of the green material was measured from the geometric dimensions and the weight of the anode immediately after densification (ie the weight of the green anode divided by the calculated volume of the green anode). Changes in this parameter indicate that there are changes in the quality of raw materials, process disturbances, especially in molding temperature and mixing conditions.

Density of fired material

The apparent density of the fired material was measured by dividing the mass of the fired anode by the calculated volume of the fired anode. The high density of fired material reduces the air permeability of the anode, its electrical resistivity and can extend the life of the anode in the cells. Extremely high density can lead to thermal shock problems The apparent density of fired material is controlled (Sadler et al. 1995): by choice of raw material; aggregate granulometry; optimal pitch content; optimal processing to avoid poor compaction during molding (or expansion during firing).

Bending strength

Flexural strength indicates the presence of microcracks in the anode structure. Low flexural strength values usually indicate problems with coke grain stability, molding conditions, or high heating rates during roasting (Fischer and Perruchoud 1992). This mechanical property is important in the handling, installation and reinforcement of the anode block.

Compressive strength

The strength of the anode is mainly dependent on the strength of the coke, the softening temperature of the pitch, and the pitch content (Wilkening and Beilstein 1994). It is important that the anode has sufficient mechanical strength to withstand manipulation during processing and installation of the anode. The stubs must be strong enough to allow the used stubs to be removed from the bath (Sadler et al. 1995).

Specific electrical resistance

The electrical resistivity of carbon anodes should ideally be as low as possible. This is necessary to reduce the energy losses associated with resistive heating of the anodes (Sadler et al. 1995). Resistivity is highly dependent on the underlying coke structure, anode density and pore distribution. Invisible cracks, imperfections, and other imperfections may be the result of mixing or pressing problems, excessive paste moisture, or thermal shock during firing or cooling. The presence of microcracks is observed by high standard deviation values (Fischer and Perruchoud 1987). Very low electrical resistivity values and high levels of thermal conductivity can result from overbaking. This situation can cause burning problems.

CO ₂ reactivity and air reactivity

The reactivity values are important for determining the sensitivity of the anode to excessive carbon consumption and dusting in the electrolysis cell. This is strongly influenced by impurities present in the raw material (Hume 1993) and firing parameters such as temperature and holding time on heating (Fischer et al. 1993).

The present invention

Comparative examples

Sulfur content:

Sources of carbon

pyrolytic carbon

The pyrolytic carbon in example 1 was obtained by decomposition of natural gas and precipitation on the CPC (with a particle size of 0.5-2.5 mm, a sulfur content of 1.1 wt.% and a true density in xylene of 2.09 g/cm ³ ) in a fluidized layer at a temperature of 1100-1300°C and a pressure of 1-2 bar (abs).

Pyrolytic carbon from example 2-5 was obtained in a fixed bed reactor at 1200°C and a pressure of 1.0-1.2 bar (abs.) by decomposition of methane and precipitation on CPC (this type had a particle size of 1-4 mm, sulfur content of 0.95 wt.% and a similar true density in xylene).

HS coke

HS coke is a typical calcined petroleum coke with a sulfur content of 3.1 wt%, impurity levels of 400 ppm V, 800 ppm Si, 700 ppm Fe, 500 ppm Ca, and a true density in xylene of 2, 07 g/cm ³ , total porosity 25.7% (DIN66133),

This material is available from standard vendors and CPC vendors.

Coke With HQ

This coke is used as a “high quality” reference CPC and has a sulfur content of 0.95 wt%, impurity levels of 30 ppm V, 10 ppm Si, 80 ppm Fe, 20 ppm Ca, and true gravity 2.08 g/ ^cm3 . Total porosity 20%.

SRS

The material referred to as CPC denotes the pyrolytic carbon support of Example 2-5, which is coke with 1.1 wt% S and a true density in xylene of 2.09 g/cm ³ . Impurity levels are 180 ppm V, 100 ppm Si, 90 ppm Fe, 50 ppm Ca. Total porosity 23%.

Pitch

Material

The binder used in the examples is coal tar pitch with a typical Mettler softening point (ISO 5940-2) of 113°C. Other important characteristics of the pitch were insolubility in quinoline (ISO 6791) 8.4%, insolubility in toluene (ISO 6376) 28% and true density in helium (ISO 21687) 1.31 g/cm ³ .

Pitch content

Obtaining anodes 1-5, 1C, 2C, 3C

The anodes were fabricated by a multi-stage procedure. The first stage was the sifting of raw materials of pyrolytic carbon and CPC into fractions of 8-4 mm, 4-1 mm, 1-0.5 mm and 0.5-0.25 mm. The second stage was the formation of fine particles (fraction <0.25 mm) in the desired amount. Fraction 8-4 mm was made from pre-baked scrap with the appropriate ratio of pyrolytic carbon and petroleum coke. Formulation-specific quantities of pyrolytic carbon and/or HS CPC or Cox HQ for each fraction were mixed together to form a so-called dry aggregate. This dry aggregate was then heated above the softening point of coal tar pitch (113° C.) and mixed with the coal tar pitch binder. The pasty mixture was transferred to a hydraulic press and molded into a so-called green anode at a pressure of >400 bar. Then the unfired anodes were fired at a temperature of 1100°C. After firing, the adhesive material was removed from the anode block and the anode was cleaned. At least three different amounts of pitch were tested for each dry aggregate composition, and three test anode blocks were made and tested for each of these pitch concentrations.

1. Measurement for examples 1, 1C, 2C

Explanations for Fig. 1-10

The gray areas in the drawings show typical values for anodes used in the aluminum industry.

All data points are the average of the three individual anodes tested.

The following symbols are used to distinguish the examples in the drawings:

1.1 Density (Fig. 1)

The density must be high in order to increase the mass of coal fed to the electrolysis.

Conclusion

The density of the green and baked anodes in Example 1 is in the desired high range. Surprisingly, the density of the 50/50 mixed composition is not a linear interpolation between high density pyrolytic carbon and low density HS coke. Instead, the density of the mixture is shifted towards the desired higher densities.

1.2 Strength (Fig. 2)

The strength must be high in order to improve the thermal shock resistance.

Conclusion

The strength of the 50/50 mixture in example 1 is also a non-linear average for examples 1C and 2C, as expected by a person skilled in the art. This surprising finding is of great importance for the mixing approach of the present invention, since an anode made entirely of pyrolytic carbon (Example 1C) fails due to insufficient mechanical strength.

1.3 Electrical resistance (Fig. 3)

The electrical resistivity must be low in order to reduce the electricity consumption in the electrolysis cell.

Conclusion

The electrical resistivity in example 1C is not a linear average for examples 1C and 2C. Again, it is biased in a more desirable direction with low electrical resistance, which reduces the power consumption of the electrolytic cell.

1.4 CO ₂ reactivity (Fig. 4): Residual CO ₂ reactivity:

The residual reactivity with CO ₂ must be high in order to reduce the amount of carbon that reacts with CO ₂ .

Dust reactivity with respect to CO ₂ :

The reactivity of the dust with respect to CO ₂ must be low in order to avoid carbon particles entering the bath, which will lead to an increase in resistivity.

Loss of reactivity towards CO ₂ :

The loss of reactivity with CO ₂ should be low in order to reduce the amount of carbon that reacts with CO ₂ .

Conclusion:

Surprisingly, an anode made from less contaminated pyrolytic carbon (example 1C) has a higher reactivity with respect to CO ₂ . This is due to the higher degree of dusting. Dusting in this case is caused by a selective reaction of the binder matrix due to a reactivity mismatch between the uncontaminated pyro-C granular structure and the contaminated, more reactive matrix. Surprisingly, however, the 50/50 mixture compensates for this reactivity mismatch, although some reactivity mismatch remains between contaminated HS coke particles, tar matrix, and relatively pure pyrolytic carbon particles.

1.5 Reactivity with air (Fig. 5)

Residual reactivity towards air:

The residual reactivity with air must be high in order to reduce the amount of carbon that reacts with air.

Reactivity of dust in relation to air:

The reactivity of the dust with respect to air must be low so that carbon particles do not enter the bath, which increase the resistivity.

Loss of reactivity towards air:

The loss of reactivity with air must be low to reduce the amount of carbon that reacts with air.

Conclusion

The air reactivity of the mixed anode (Example 1) is in the desired range and fully compensates for the high reactivity of the HS coke (Example 2C).

1.6 Impurities

Conclusion

Impurities depend only on the mixture ratio and are a linear average for both materials, that is, example 1 is the average of examples 1C and 2C, as one skilled in the art would expect. However, it is worth emphasizing that the present invention provides a solution to the problem of sulfur emissions and also reduces the associated pollution. The concentration of sulfur in Example 1, as well as other impurities, is well within government emission limits and industry specifications. The surprising aspect is that two materials that are so different in composition can be used in a mixture as an anode feedstock.

2. Example 2-5, 3C

Explanations

The gray areas in the drawings show typical values for anodes used in the aluminum industry.

All data points are the average of the three individual anodes tested.

The following symbols are used to distinguish the examples in the drawings:

2.1 Density (Fig. 6)

The density must be high in order to increase the mass of coal fed to the electrolysis.

Conclusion

Examples 2-5 show, in comparison to 3C, that the desired high density can be achieved with a high proportion of pyrolytic carbon in the anode. Surprisingly, the density is even above the average industry range, indicating an advantage over the mixture compared to the standard CPC anode.

2.2 Strength (Fig. 7)

The strength must be high in order to improve the thermal shock resistance.

Conclusion

In the case of compressive strength, a reduction would be expected when mixed with pyrolytic carbon. This is due to the layered structure and, as a consequence, the high anisotropy of pyrolytic carbon. However, the strength remains at an acceptable level, which could not be expected given prior art attempts to use pyrolytic carbon in electrodes.

2.3 Electrical resistance (Fig. 8)

The electrical resistivity must be low in order to reduce the electricity consumption in the electrolysis cell.

Conclusion:

Pyrolytic carbon has low electrical resistance. Thus, the characteristics of the anode depend on the amount of pyrolytic carbon in the mixture.

2.4 Reactivity with CO ₂ (Fig. 9)

Residual reactivity towards CO ₂

The residual reactivity with CO ₂ must be high in order to reduce the amount of carbon that reacts with CO ₂ .

Dust reactivity with respect to CO ₂

The reactivity of the dust with respect to CO ₂ must be low in order to avoid carbon particles entering the bath, which will lead to an increase in resistivity.

Loss of reactivity with CO ₂

The loss of reactivity with CO ₂ should be low in order to reduce the amount of carbon that reacts with CO ₂ .

Conclusion

The CO ₂ reactivity in Examples 2-5 is again higher than Control 3C due to more dusting caused by the reactivity mismatch, but is still in the typical anode range.

2.5 Reactivity with air

Residual reactivity with air

The residual reactivity with air must be high in order to reduce the amount of carbon that reacts with air.

Reactivity of dust in relation to air

The reactivity of the dust with respect to air must be low so that carbon particles do not enter the bath, which increase the resistivity.

Loss of reactivity with air

The loss of reactivity with air must be low to reduce the amount of carbon that reacts with air.

Conclusion

Again, the air reactivity is not significantly affected by mixing pyrolytic carbon into the HQ coke.

2.6 Impurities

Conclusion

The impurities again follow the simple mixing rule.

Claims (16)

1. A mixed composition for the production of a carbon electrode containing a mixture of (i) petroleum coke in an amount of from 20 to 99 wt.% and (ii) pyrolytic carbon in an amount of from 1 to 80 wt.%, based on the total weight of the mixed composition, where the mixed the composition contains at least two particle size fractions: (i) granular more than 0.5 mm and (ii) fine below 0.5 mm, where pyrolytic carbon is present at least in the granular size fraction. 2. The mixed composition according to claim 1, wherein the content of said petroleum coke is 70 to 95% by weight and the content of said pyrolytic carbon is 5 to 30% by weight. 3. The mixed composition according to claim 1, wherein at least 30% by weight of the total pyrolytic carbon of the mixed composition is in the granular fraction. 4. The mixed composition according to claim 1, wherein at least 70% by weight of the total pyrolytic carbon of the mixed composition is in the granular fraction. 5. The mixed composition according to claim 1, wherein said mixed composition contains at least three particle size fractions: (i) coarse above 4 mm, (ii) intermediate from 4 to 0.5 mm, and (iii) fine below 0.5 mm, where pyrolytic carbon is present at least in the intermediate and/or coarse fractions. 6. A blended composition according to claim 1, wherein at least 30% by weight of the total pyrolytic carbon of the blended composition is in an intermediate size fraction. 7. The mixed composition according to claim 1, wherein the density of said pyrolytic carbon is in the range of 1.8 to 2.2 g/cm ³ . 8. The mixed composition according to claim 1, wherein the crystal size of said pyrolytic carbon is in the range of 30 to 50 Å. 9. A blended composition according to claim 1 wherein calcined petroleum coke is used and wherein the sulfur content is in the range of 1.5 to 7.0% by weight, based on the total weight of the petroleum coke. 10. The method of obtaining a mixed composition according to claim 1, which includes mixing pyrolytic carbon and petroleum coke. 11. The composition of the electrode containing a mixture of (i) a mixed composition of petroleum coke and pyrolytic carbon according to any one of paragraphs. 1-9, (ii) cinders and/or scrap, and (iii) binder material. 12. The composition of the electrode according to claim 11, where from 35 to 95 wt.%, based on the total mass of the electrode composition, is a mixed composition, from 0 to 40 wt.%, based on the total mass of the electrode composition, are cinders and / or scrap, and from 5 to 25 wt.%, based on the total weight of the composition of the electrode is a binder. 13. A method for producing an electrode composition according to claim 11, which includes preheating said mixed composition and cinders and/or scrap and mixing said preheated mixture with a binder. 14. A carbon electrode suitable for use as an anode in an aluminum cell, which contains a mixed composition containing a mixture of (i) petroleum coke in an amount of from 20 to 99 wt.% and (ii) pyrolytic carbon in an amount of from 1 to 80 wt. % based on the total weight of the mixed composition, where the mixed composition contains at least two particle size fractions: (i) granular more than 0.5 mm and (ii) finely divided below 0.5 mm, where pyrolytic carbon is present at least in the fraction granular size. 15. A method of obtaining a carbon electrode suitable for use as an anode in an aluminum cell, which includes mixing a mixed composition according to any one of paragraphs. 1-9 with cinders and/or scrap and binder material at a temperature of from 100 to 175°C to form a paste and firing said solid at a temperature of from 1000 to 1250°C to form a carbon electrode. 16. A method for producing aluminum, where the carbon electrode according to claim 14 is used as a carbon anode in an aluminum electrolytic cell.