Blend composition for an electrode comprising petroleum coke and pyrolytic carbon

Abstract

The present invention relates to a blend composition comprising a mixture of petroleum coke and pyrolytic carbon; an electrode formulation and the use of the electrode as an anode in a process for the production of aluminium.

Description

[0001] The present invention relates to a blend composition comprising a mixture of petroleum coke and pyrolytic carbon; an electrode formulation and the use of the electrode as an anode in a process for the production of aluminium.

[0002] The production of aluminium is carried out in an electrolytic cell or bath (known as the Hall-Héroult process). The electrolysis of Al2O3 takes place in a molten cryolite bath layered between a carbon anode and the molten metal. The aluminium ions within the Al2O3 are reduced to form molten aluminium. The molten aluminium is collected at the bottom of the electrolytic cell. The oxide ions react with the carbon anode to produce carbon dioxide and thus the carbon anode is consumed in the electrolysis reaction.

[0003] The carbon consumption during the electrolysis process requires the pre-baked anodes to be replaced every 3 - 4 weeks, depending on the anode size and the current density. Due to the continuous replacement and the limitation of minimal disruption to the cell, there are anodes in different consumption stages in all cells. When the anode is replaced, approximately three-quarters of the anode is consumed. The remaining part is called the butts.

[0004] The various reactions in the cell lead to the consumption of the anode carbon. Those that do not lead to metal reduction lead to excessive carbon consumption, such as airburn (O2 + C → CO2, where O2 comes from ambient air), carboxy attack (CO2 + C → CO, where CO2 is a product of the redox reaction in the production of aluminium), and selective oxidation (dusting). Dusting exists as a secondary effect of CO2 attack due to the reactivity imbalance between different coke phases, causing the crumbling of solid C.

[0005] The cost of the carbon anode accounts for 15 - 20% of the total cost of aluminium electrolysis production. Therefore, the quality of the carbon anode is crucial and significantly affects the energy consumption and environmental impact of aluminium electrolysis. The quality of the anode is characterized by 5 main properties:

[0006] - Reactivity, which determines the greater part of the excessive carbon consumption for producing one tonne of aluminium. The lower the reactivity, the lower this excessive carbon consumption.

[0007] - Density, which is the main factor determining the anode life in the electrolytic cell. The greater the density, the longer the life. Increasing the anode life reduces the number of anode replacements required.

[0008] - Thermal shock resistance, which determines whether cracking or fissuring occurs when the anode is placed in the electrolytic cell or during its cycling.

[0009] - Low electrical resistivity, which determines the power loss due to the large currents employed in the Hall cell.

[0010] - High chemical purity, which determines the quality of aluminum products. This is because of the direct consumption of the anode in the electrolytic cell, and any metals or other impurities contained are easily transferred to the aluminum products and may adversely affect their mechanical properties. High chemical purity also has a positive effect on reactivity.

[0011] The raw materials used to prepare the dry formulation in a pre-baked anode plant are typically petroleum coke, coal tar pitch binder, crushed butts, green scrap (anodes that have been formed but not baked), and baked scrap (formed and baked anodes that are not qualified). Pre-baked anodes are typically made from approximately 65% coke, 15% pitch, and 20% recycled anode butts.

[0012] In the paste plant, the incoming calcined coke and recycled materials are crushed, screened / graded to obtain a predetermined fraction, and added together to form a dry aggregate. The coke fractions used to manufacture anodes are typically characterized as coarse, medium, and fine fractions. The dry aggregate is preheated and mixed with a binder (usually coal tar pitch) that has been preheated to melt. The preheated mixture is pressed into the final shape, ensuring that the pressed anode block retains its structural form. Subsequently, the green compact is heated at an elevated temperature, such as approximately 1100 °C, to form a baked anode, which is then suitable for consumption in the electrolytic cell.

[0013] The continuous growth in aluminum metal demand, combined with the decline and fluctuation in the quality of aluminum-grade coke, as well as the density and purity of the affected coke, makes it more challenging for anode manufacturing plants to provide anodes of stable quality. Low-quality coke grades have higher reactivity, resulting in higher carbon consumption in the furnace.

[0014] Approximately 95% of the SO2 emissions generated by furnaces can be attributed to the sulfur present in the incoming petroleum coke used in anode preparation. Therefore, environmental regulations aim to reduce sulfur emissions, while coke suppliers provide higher-sulfur materials. Low-sulfur coke materials are becoming less available in the market and their prices are rising steadily. In addition, the sulfur content of many traditional "high-sulfur" anode-grade green coke has increased. Five years ago, high-sulfur anode-grade green coke was considered to have a sulfur content of 3 - 4%. Nowadays, a more typical content is 4 - 6%.

[0015] The price difference between a barrel of low-sulfur desulfurized crude oil and high-sulfur sour crude oil ("sweet-sour spread") has led to more refineries processing cheaper, higher-sulfur crude oils. These higher-sulfur crude oils produce coke with higher sulfur and metal impurity contents (especially vanadium and nickel). Calcining furnaces use more of this coke to meet the growing demand of the aluminum industry.

[0016] Due to the furnace environment limitations, the aluminum furnaces have not significantly changed the coke sulfur specifications. Therefore, it is necessary to blend with lower sulfur coke to offset the high coke sulfur content. As a result, the sulfur content difference of the coke used in the typical anode blends is increasing - where coke with a sulfur content of 1 - 2% can be blended with coke with a sulfur content as high as 4 - 6% to achieve a furnace anode coke specification of 1.0 - 3.5%.

[0017] With the expected growth rate of primary aluminum production, the industry has no other choice but to use these higher sulfur blended cokes. The sulfur content of the high sulfur coke used in the blends is increasing and may continue to increase.

[0018] In addition to environmental regulations, the sulfur content also seriously affects the anode quality. High sulfur coke undergoes desulfurization during calcination and anode baking. Desulfurization will negatively affect the coke properties, such as reducing the true density, increasing the porosity, and the additional desulfurization during anode baking will negatively affect the anode properties, such as increasing the air and carboxyl reactivity.

[0019] Furthermore, it has been shown that extremely low sulfur concentrations in the anode will negatively affect the anode quality and the potential potroom performance by increasing the anode carboxyl reactivity.

[0020] It has been shown that the reaction of the anode with CO2 may be the main factor causing carbon dusting in the electrolytic cell. A high content of carbon dust will increase the electrolytic cell temperature and ultimately reduce the current efficiency and metal production. Many publications indicate that the anode CO2 reactivity increases with the increase in sodium concentration.

[0021] However, it is also known that the catalytic behavior of sodium is offset by the presence of sulfur. It is shown that the reaction of sodium with sulfur inhibits the catalytic behavior of sodium and reduces the tendency of sodium to migrate during baking. Therefore, higher sulfur coke and anodes will have lower carboxyl reactivity and less potential for dusting.

[0022] It is found that by separating the low sulfur coke into the coarse fraction of the anode aggregate and then increasing the high sulfur coke used in the fine fraction, the possibility of sulfur inhibiting the sodium effect increases, thereby reducing the overall anode CO2 reactivity. However, depending on the type of coke used in the blend, this reduction in anode carboxyl reactivity may come at the cost of a lower anode density. For example, if low sulfur coke with a low vibration bulk density is blended with high sulfur coke with a high vibration bulk density, placing the higher density material in the ball mill circuit has the potential to reduce the anode density.

[0023] In addition to the sulfur content, the density fluctuations of aluminum grade coke increase. There are problems in obtaining and maintaining a high and consistent baked anode density based on calcined petroleum coke from different sources or other solid carbon substitutes with varying material properties. Coke blending is a possible method to optimize the dry aggregate density if equipment for coke blending is available and if cokes of different grades can be obtained. However, this depends on the priority given to the properties of the respective cokes. So far, little priority has been given to blending high-porosity and low-porosity coke types. Commercial barriers such as price and material shortages also limit optimal blending.

[0024] Over the past few decades, various attempts have been made to blend or substitute petroleum coke with different carbon sources:

[0025] In 1969, Edwards et al. attempted to prepare pyrolytic carbon suitable for anode manufacture by cracking natural gas and liquefied petroleum gas in a moving-burden 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 prepared only from pyrolytic carbon by standard formulation techniques had unacceptable physical properties and test cell performance. The main problem seemed to be that pyrolytic carbon could not bind well with pitch due to its “onion” structure and produced very weak electrodes. The low reactivity of pyrolytic carbon relative to the pitch binder was also considered a potential problem in cell operation. In Gardner et al., it was hypothesized that if pyrolytic carbon was used as a finer material in the range less than 100 mesh (0.15 mm), incorporation of 30 - 40% pyrolytic carbon in the electrode aggregate would not negatively affect electrode performance (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 was ground finer than the size of the onion-like layers. Thus, the onion-like structure disappeared due to grinding.

[0026] Due to the availability and low cost of brown coal, Gardener et al. disclosed research on using brown coal coke instead of petroleum coke in the preparation of carbon for aluminum furnaces by upgrading brown coal. Brown coal coke is less dense and more reactive than petroleum coke. This could be improved by impregnating the coke with pyrolytic carbon obtained from petroleum gas. Unfortunately, due to the increased mechanical losses and pulverization caused by the difference in reactivity and consumption rate between the impregnated coke and the binder coke matrix, the impregnated coke had a higher carbon consumption than petroleum coke.

[0027] EP 1766105 discloses a method for manufacturing a carbon electrode suitable for use as an anode in an aluminum reduction cell, which includes a blend of granular shot coke, granular carbonaceous materials other than shot coke, and coal tar pitch or petroleum pitch or a combination of these pitches.

[0028] 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) have high tolerance to chemical and electrochemical oxidation, but despite the oxidation resistance, the consumption rate is high due to corrosion.

[0029] The aluminum industry avoids using highly isotropic cokes, cokes with fine-grained textures that exhibit similar properties in all directions, especially shot coke, because they have a high coefficient of thermal expansion (CTE) and low open porosity. Anodes made from these materials are more vulnerable to thermal shock fracture during rapid heating in an aluminum electrolysis cell. During cell operation, they also suffer from lower mechanical strength and dusting problems. Shot coke typically has a low level of open macroporosity for pitch penetration. This reduces the ability of the pitch to interlock and structurally bond together during the carbonization process.

[0030] 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 effect on the anode coefficient of thermal expansion can be minimized by adding isotropic coke and shot coke to the fines fraction. However, this will result in increased air reactivity. It is suspected that when very isotropic coke is uniformly added to all size fractions, most anode properties will be affected in a certain way proportional to the added level.

[0031] According to environmental regulations, aluminum plants use low-sulfur coke as a key lever in a strategy to reduce SO2 emissions. The negative impact of this action is an increase in the CO2 reactivity of the anode. The following measures are recommended in the carbon plant to minimize the negative impact: (i) increasing the anode baking soak time, (ii) directly introducing higher-sulfur coke into the fines fraction, (iii) minimizing the proportion of low-sulfur coke in the blend by selecting coke with very low sulfur.

[0032] Since environmental regulations aim to reduce sulfur emissions, there is an ongoing task of finding low-sulfur blending materials even though coke suppliers offer higher-sulfur materials.

[0033] Additionally, achieve the benefits of anode performance and aluminum products and minimize other metal impurities (most importantly V and Ni) that are also present in traditional low-grade petroleum coke.

[0034] Mixing in a portion of higher-density pyrolytic carbon can increase the anode density, which improves the lifespan by increasing the quality of the available carbon in the anode.

[0035] Generally, expanding the reserve of alternative resources for carbon anodes is beneficial to the sustainable development of the aluminum industry.

[0036] Since the cost of preparing carbon anodes is approximately 20% of the cost of preparing aluminum metal by the Hall process, there is usually an ongoing task of improving the mechanical and chemical properties of the anodes to minimize carbon consumption.

[0037] The present invention relates to a blend composition comprising, as a mixture based on the total weight of the blend composition: (i) 20 - 99% by weight of petroleum coke, and (ii) 1 - 80% by weight of pyrolytic carbon, and the blend composition comprises at least two particle size fractions: (i) particles greater than 0.5 mm and (ii) fines less than 0.5 mm, and the pyrolytic carbon is present at least in the particle size fraction.

[0038] The petroleum coke can be a blend of different petroleum cokes. The pyrolytic carbon can be a mixture of different pyrolytic carbons.

[0039] Generally, the particle size fraction is 0.5 - 16 mm, preferably 0.5 - 8 mm. Generally, the fines size fraction is 0.005 - 0.5 mm.

[0040] Those skilled in the art may also refer to the blend composition as a formulation or aggregate.

[0041] The blend composition is preferably a dry (binder-free) blend composition.

[0042] 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 0 - 10% by weight based on the total weight of the petroleum coke, more preferably 0.5 - 8.5% by weight, even more preferably 1.5 - 7.0% by weight. Petroleum coke is usually abbreviated as petcoke.

[0043] The term "pyrolytic carbon" encompasses solid carbon produced by the pyrolysis of light hydrocarbons in the absence of oxygen (see, for example, Muradov, Nazim. "Low to near-zero CO2 production of hydrogen from fossil fuels: Status and perspectives." International Journal of Hydrogen Energy 42.20 (2017): 14058-14088). The preferred pyrolytic carbon for anodes is high-density solid elemental carbon produced by deposition on carbon particles. This is superior to pyrolytic carbon black produced by thermal / plasma methods or nanostructured carbon grown on metal / oxide catalysts.

[0044] Pyrolytic carbon can be produced by the decomposition of gaseous hydrocarbon compounds and carbon deposition on a suitable underlying substrate (carbon materials, metals, ceramics, and their mixtures) at temperatures of 1000 - 2500 K and pressures of 0.5 - 5000 kPa (absolute pressure) (chemical vapor deposition or infiltration). The substrate can be porous or non-porous and can be a support substrate (pre-installed component) in a reactor or particulate and powdered materials. The latter can be implemented as a fixed bed, moving bed, fluidized bed, or entrained flow. The preparation of pyrolytic carbon is not limited to a specific energy supply, and fossil fuel combustion, electric heating, or plasma-driven preparation reactors are possible.

[0045] A wide range of microstructures can occur in pyrolytic carbon, such as isotropic, laminated, substrate-nucleated, and different residual hydrogen contents, depending on the deposition conditions (temperature, type, concentration, and flow rate of the source gas, surface area of the underlying substrate, etc.).

[0046] The density of pyrolytic carbon is typically 1.6 - 2.3 g / cc, preferably 1.8 - 2.2 g / cc (true density in xylene, ISO 8004).

[0047] The impurities in pyrolytic carbon are typically: 0 - 1%, preferably 0 - 0.5%, more preferably 0 - 0.1% S.

[0048] 0 - 1000 ppm, preferably 0 - 500 ppm Fe, 0 - 250 ppm, preferably 0 - 100 ppm Ni, 0 - 450 ppm, preferably 0 - 250 ppm, more preferably 0 - 100 ppm V. 0 - 200 ppm, preferably 0 - 100 ppm Na.

[0049] The particle size of pyrolytic carbon after pyrolysis is typically at least 5 wt% > 1 mm, preferably 50 wt% > 0.5 mm.

[0050] The crystal size (XRD) of pyrolytic carbon is typically 20 - 60 Å, preferably 30 - 50 Å (XRD, ISO 20203).

[0051] The porosity of pyrolytic carbon particles is typically below 15%, preferably <10%, most preferably below 5% (Hg porosimetry, DIN66133).

[0052] The specific surface area of pyrolytic carbon measured by Hg porosimetry (DIN66133) is typically 0.001 - 5 m2 / g, preferably 0.01 - 2 m2 / g.

[0053] Preferably, the blend composition comprises a content of petroleum coke of 30 - 98 wt%, more preferably 40 - 95 wt%, more preferably 50 - 95 wt%, more preferably 60 - 95 wt%, more preferably 70 - 95 wt%, more preferably 80 - 95 wt%, more preferably 85 - 95 wt%, even more preferably 90 - 95 wt% of the blend composition by total weight.

[0054] Preferably, the blend composition comprises a content of pyrolytic carbon of 2 - 70 wt%, more preferably 5 - 60 wt%, more preferably 5 - 50 wt%, more preferably 5 - 40 wt%, more preferably 5 - 30 wt%, more preferably 5 - 20 wt%, more preferably 5 - 15 wt%, even more preferably 5 - 10 wt% of the blend composition by total weight.

[0055] Based on the total pyrolytic carbon: Preferably, at least 30 wt% of the total pyrolytic carbon of the blend composition is the particulate 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%, and even more preferably all of the pyrolytic carbon is of the particulate size fraction.

[0056] Based on the total pyrolytic carbon: Preferably, 30 - 100 wt% of the total pyrolytic carbon of the blend composition is the particulate fraction, more preferably 40 - 100 wt%, even more preferably 50 - 100 wt%, more preferably 60 - 100 wt%, more preferably 70 - 100 wt%, more preferably 80 - 100 wt%, more preferably 90 - 100 wt%, even more preferably 95 - 100 wt%, and even more preferably all of the pyrolytic carbon is of the particulate size fraction.

[0057] Based on the total pyrolytic carbon: Preferably, 70 - 0 wt% of the total pyrolytic carbon of the blend composition is in the fine powder size fraction, more preferably 60 - 0 wt%, even more preferably 50 - 0 wt%, more preferably 40 - 0 wt%, more preferably 30 - 0 wt%, more preferably 20 - 0 wt%, more preferably 10 - 0 wt%, even more preferably 5 - 0 wt% is in the fine size fraction, and even more preferably all of the pyrolytic carbon is in the fine powder size fraction.

[0058] Preferably, 30 - 80 wt% of the particles, more preferably 40 - 70 wt%, even more preferably 50 - 65 wt% have a granular particle size, and 20 - 70 wt% of the particles, more preferably 30 - 60 wt%, even more preferably 35 - 50 wt% have a fine particle size.

[0059] Preferably, the blend composition comprises at least three particle size fractions: (i) a coarse fraction greater than 4 mm, (ii) an intermediate fraction between 4 mm and 0.5 mm, and (iii) a fine fraction less than 0.5 mm, and the pyrolytic carbon is present at least in the intermediate fraction and / or the coarse fraction.

[0060] Based on the total pyrolytic carbon: Preferably, at least 30 wt% of the total pyrolytic carbon of the blend composition is in the intermediate size 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%, and even more preferably all of the pyrolytic carbon is in the intermediate size fraction.

[0061] Based on the total pyrolytic carbon: Preferably, 30 - 100 wt% of the total pyrolytic carbon of the blend composition is in the intermediate size fraction, more preferably 40 - 100 wt%, even more preferably 50 - 100 wt%, more preferably 60 - 100 wt%, more preferably 70 - 100 wt%, more preferably 80 - 100 wt%, more preferably 90 - 100 wt%, even more preferably 95 - 100 wt%, and even more preferably all of the pyrolytic carbon is in the intermediate size fraction.

[0062] Based on the total pyrolytic carbon: Preferably, 70 - 0 wt% of the total pyrolytic carbon of the blend composition is in the fine and / or coarse size fraction, more preferably 60 - 0 wt%, even more preferably 50 - 0 wt%, more preferably 40 - 0 wt%, more preferably 30 - 0 wt%, more preferably 20 - 0 wt%, more preferably 10 - 0 wt%, even more preferably 5 - 0 wt%, and even more preferably none of the pyrolytic carbon is in the fine and / or coarse size fraction.

[0063] Preferably 30 - 80% by weight, more preferably 40 - 75% by weight, even more preferably 50 - 75% by weight of the particles have an intermediate and / or coarse particle size, and 20 - 70% by weight, more preferably 25 - 60% by weight, even more preferably 25 - 50% of the particles are a fine fraction.

[0064] Preferably 10 - 40%, more preferably 20 - 30% of the particles have a coarse particle size, 20 - 50%, more preferably 30 - 40% of the particles have an intermediate particle size, and 25 - 55%, more preferably 30 - 50%, even more preferably 35 - 45% of the particles are a fine fraction.

[0065] Depending on the anode preparation, anode manufacturers use different fractions and sieving; typical values are summarized in the table below:

[0066] Grade 2 fraction <0.5mm >0.5 mm Grade 3 fraction <0.25mm 0.25 - 4 mm >4 mm Grade 4 fraction <0.25mm 0.25 - 1 mm 1 - 4 mm >4 mm Grade 5 fraction <0.25mm 0.25 - 1 mm 1 - 2 mm 2 - 4 mm >4 mm Grade 6 fraction 0.25 mm 0.25 - 0.5 mm 0.5 - 1 mm 1 - 2 mm 2 - 4 mm 4 - 8 mm

[0067] The blend composition is preferably prepared by mixing pyrolytic carbon and petroleum coke; preferably by mixing dry pyrolytic carbon and dry petroleum coke.

[0068] The blend composition can be mixed in a calcining plant, for example, before or after the calcination of petroleum coke, preferably after calcination. Alternatively, the blend composition can be mixed before or after shipping at the respective port. Alternatively, the blend composition can be mixed in an anode preparation plant before crushing, sieving, and sizing, or by directly adding pyrolytic carbon to an aggregate of petroleum coke that has already been crushed, sieved, and sized.

[0069] Furthermore, the present invention relates to an electrode formulation, preferably an anode formulation, which comprises a blend of (i) a blend composition of petroleum coke and pyrolytic carbon as described above, (ii) spent anodes and / or scrap, and (iii) a binder material.

[0070] Typically, 35 - 95% by weight, preferably 50 - 80% by weight of the total weight of the anode formulation is the blend composition. Typically, 0 - 40% by weight, preferably 15 - 30% by weight of the total weight of the anode formulation is spent anodes and / or scrap. Typically, 5 - 25% by weight, preferably 10 - 20% by weight, even more preferably 13 - 18% by weight of the total weight of the anode formulation is the binder.

[0071] The use of scrap anodes and cullet is known in the prior 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).

[0072] All binders known in 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, a combination of coal tar pitch binder or pitch binder can be used as the binder.

[0073] The total sulfur content of the anode formulation is preferably 0-5 wt%, more preferably 0.5-3.5 wt%, and even more preferably 1.0-2.5 wt%. Preferably, the composition ratio of the blend depends on the desired final sulfur content.

[0074] Preferably, the blend composition, spent anodes and / or cullet are preheated, preferably to a temperature of 100 °C to 175 °C (to melt the binder), and mixed with a binder, typically coal tar pitch (also preferably preheated to melting). Preferably, the preheated anode formulation is pressed into its final shape, ensuring that the compacted anode block retains its structural form. Preferably, the green compact is subsequently heated at an elevated temperature, such as 1000 - 1250 °C, to form a baked anode, which is then suitable for consumption in an electrolytic cell.

[0075] The present invention includes a carbon anode suitable for use as an anode in an aluminum reduction cell, which comprises a blend composition, the blend composition comprising a mixture of (i) petroleum coke in an amount of 20 - 99 wt% and (ii) pyrolytic carbon in an amount of 1 - 80 wt% based on the total weight of the blend composition, and the blend composition comprising at least two particle size fractions (i) particles larger than 0.5 mm and (ii) fines smaller than 0.5 mm, and the pyrolytic carbon is present at least in a particle size fraction.

[0076] Preferably, the carbon anode prepared according to the present invention provides one or more of the following performance properties, preferably all of the parameters mentioned:

[0077] The green density is preferably at least as high as 1.50 g / cm3. The established range for CPC anodes is 1.54 - 1.66 g / cm3 (ISO 12985 - 1).

[0078] The baked density is preferably at least as high as 1.50 g / cm3 (ISO 12985 - 1). The established range for CPC anodes is 1.50 - 1.63 g / cm3 (ISO 12985 - 1).

[0079] The thermal shock and mechanical resistance are preferably higher than 6 MPa (ISO 12986 - 1), while for CPC-based anodes, it is typically 8 - 14 MPa.

[0080] The compressive strength is preferably greater than 25 MPa (ISO 18515).

[0081] The electrical resistance is preferably lower than 80 μΩm. 50 - 60 μΩm is the typical industrial range.

[0082] In the case of air reactivity (typically 65 - 90, ISO 12989 - 1), the so-called air residue after the air test reaction is preferably greater than 65 wt%, more preferably greater than 85 wt%. For CO2 reactivity, the so-called CO2 residue after the CO2 test reaction is preferably greater than 80 wt%, more preferably greater than 90 wt% (where 80 - 95 is conventional, ISO 12988 - 1).

[0083] The present invention includes using the carbon anode of the present invention as the anode in an aluminum reduction cell.

[0084] Any of the new electrodes manufactured by the method of the present invention described above can be used in the method for preparing aluminum by the molten salt electrolysis of alumina, which includes electrolyzing alumina dissolved in the molten salt by passing direct current through the anode to a cathode disposed in the molten salt at an elevated temperature, wherein the anode is any of the above electrodes.

[0085] Although the specific electrodes that can be used for the molten salt electrolysis of alumina to aluminum according to the present invention have been described above to illustrate the manner in which the present invention can be advantageously used, it should be understood that the present invention is not limited thereto. That is, the present invention can suitably include, consist of, or consist essentially of the said elements. In addition, the invention disclosed illustratively herein can be suitably practiced in the absence of any element not specifically disclosed herein. Therefore, any and all modifications, variations, or equivalent arrangements conceivable by those skilled in the art should be considered to be within the scope of the present invention.

[0086] The results obtained with the blend composition well demonstrate the potential of the blending method.

[0087] The blending of low-sulfur pyrolytic carbon reduces the total sulfur content in the aluminum smelting anode, while the other parameters mentioned remain within the required performance range, and the blend exceeds the expected average value of the individual blend components. This advantage was confirmed in the examples.

[0088] The examples confirmed that the weaknesses observed for high-sulfur coke have been completely compensated for by pyrolytic carbon:

[0089] The pitch requirements and baked apparent of the blended coke anodes are typical.

[0090] The air reactivity reaches a level suitable for pre-baked anodes.

[0091] The impurities Fe, Si, Ca, and P are within typical ranges.

[0092] The S content is logically half of that observed for pure HS anodes and complies with the strict regulations related to SO2 emissions (less than 1.5%).

[0093] The high coefficient of thermal expansion (CTE) of high-sulfur coke is completely compensated for by low-sulfur pyrolytic carbon.

[0094] In addition, the disadvantages of pure pyrolytic carbon anodes have also been eliminated:

[0095] The resistivity and mechanical properties of the blend anodes are normal.

[0096] The CO2 reactivity of the blended anodes is close to the typical range.

[0097] Accordingly, the present invention provides a carbon electrode with reduced sulfur, characterized by density, air permeability, compressive strength, elastic modulus, thermal conductivity, thermal diffusivity, air reactivity, and carboxyl reactivity within the acceptable range for an aluminum smelting furnace. Examples:

[0098] Parameters (see "Anode Manufacture, Raw Materials Formulation and Processing Parameters, Kristine L. Hulse, R&D Carbon, pages 10 - 14"):

[0099] Green density:

[0100] Just after pressing, the green apparent density is measured from the geometric dimensions and the anode weight (i.e., the mass of the green anode divided by the calculated volume of the green anode). Variations in this parameter indicate changes in raw material quality, process disturbances, especially in forming temperature and mixing conditions.

[0101] Baked density:

[0102] The baked apparent density is measured by dividing the mass of the baked anode by the calculated volume of the baked anode. A high baked density tends to reduce the air permeability of the anode, its specific resistance, and can extend the anode life in the cell. An extremely high density can cause thermal shock problems. The baked apparent density is controlled by the following (Sadler et al., 1995): raw material selection; aggregate particle size analysis; optimum pitch content; optimum processing to avoid poor pressing during forming (or swelling during baking).

[0103] Flexural strength:

[0104] Flexural strength indicates the presence of microcracks in the anode structure. Low flexural strength values usually indicate problems in coke particle stability, forming conditions, or high heating rates during baking (Fischer and Perruchoud 1992). This mechanical property is important during the handling, setting, and rodding of anode blocks.

[0105] Compressive strength:

[0106] The anode strength mainly depends on coke strength, pitch softening point, and pitch content (Wilkening and Beilstein 1994). It is important that the anode has sufficient mechanical strength to withstand handling during processing and anode setting. There should be sufficient strength in the stub to be able to remove the bath solution from the used stub (Sadler et al., 1995).

[0107] Specific resistance:

[0108] Ideally, the specific resistance of the carbon anode should be as low as possible. This is to reduce the energy loss associated with resistive heating in the anode (Sadler et al., 1995). The basic coke structure, anode density, and pore distribution highly influence the resistivity. Mixing or pressing problems, excessive moisture in the paste, or thermal shock during baking or cooling may result in invisible cracks, defects, and other imperfections. The presence of capillary cracks is observed through high standard deviation values (Fischer and Perruchoud 1987). Over-baking may lead to extremely low resistivity values and high thermal conductivity values. This situation may cause the problem of overheating.

[0109] CO2 reactivity and air reactivity:

[0110] Reactivity values are important for determining the sensitivity of the anode to excessive carbon consumption and pulverization in the electrolytic cell. This is strongly influenced by the impurities present in the raw materials (Hume, 1993) and baking parameters such as temperature and heat soaking time (Fischer et al., 1993).

[0111] The present invention:

[0112]

[0113]

[0114] * = Used as a carrier for pyrolytic carbon Comparative example:

[0115]

[0116] * = Used as a carrier for pyrolytic carbon Sulfur content:

[0117]

[0118]

[0119] Carbon source:

[0120] Pyrolytic carbon:

[0121] The pyrolytic carbon in Example 1 was prepared by decomposing natural gas in a fluidized bed at a temperature of 1100 - 1300 °C and a pressure of 1 - 2 bar (absolute pressure) and depositing it on CPC (particle size 0.5 - 2.5 mm, sulfur content 1.1 wt%, true density in xylene 2.09 g / cm 3 )

[0122] The pyrolytic carbon of Examples 2-5 was prepared by methane decomposition in a fixed-bed reactor at 1200 °C under 1.0-1.2 bar (absolute pressure) and deposited on the CPC (this type has a particle size of 1-4 mm, a sulfur content of 0.95 wt%, and a similar true density in xylene).

[0123] HS coke:

[0124] HS coke is a typical calcined petroleum coke with a sulfur content of 3.1 wt%, an impurity content of 400 ppmV, 800 ppm Si, 700 ppm Fe, 500 ppm Ca, and a true density in xylene of 2.07 g / cm3, and a total porosity of 25.7% (DIN 66133).

[0125] This material is available from standard CPC suppliers and traders.

[0126] Coke C HQ:

[0127] This coke is used as a "high-quality" reference CPC and is characterized by a sulfur content of 0.95 wt%, an impurity content of 30 ppmV, 10 ppm Si, 80 ppm Fe, 20 ppm Ca, and a true density of 2.08 g / cm3. The total porosity is 20%.

[0128] CPC:

[0129] The material called CPC represents the substrate for the pyrolytic carbon of Examples 2-5, which is a coke with 1.1 wt% S and a true density in xylene of 2.09 g / cm3. The impurity content is 180 ppm V, 100 ppm Si, 90 ppm Fe, 50 ppm Ca. The total porosity is 23%.

[0130] Pitch:

[0131] Material:

[0132] 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 are quinoline insoluble matter (ISO 6791) of 8.4%, toluene insoluble matter (ISO 6376) of 28%, and true density in helium (ISO 21687) of 1.31 g / cm 3 .

[0133] Pitch content:

[0134] Example Asphalt 1 15-17％ 2 14-17％ 3 14-15％ 4 14-15％ 5 14-15％ 1C 13-15％ 2C 16-18％ 3C 12-18％

[0135] Preparation of anodes 1-5, 1C, 2C, 3C:

[0136] The anode is prepared in a multi-step process. The first step is to screen the pyrolytic carbon and CPC raw materials into fractions of 8 - 4 mm, 4 - 1 mm, 1 - 0.5 mm, and 0.5 - 0.25 mm. The second step is to produce the required amount of fine powder (fraction < 0.25 mm). The 8 - 4 mm fraction is made from pre-baked cullets and the corresponding proportions of pyrolytic carbon and petroleum coke. Specific formula amounts of pyrolytic carbon and / or HS CPC or coke HQ of each fraction are mixed together to obtain a so-called dry aggregate. Then the dry aggregate is heated above the softening point of coal tar pitch (113 °C) and mixed with a coal tar pitch binder. The paste mixture is transferred to a hydraulic press and formed into a so-called green anode at a pressure of ≥ 400 bar. Subsequently, the green anode is baked at a temperature of 1100 °C. After baking, the adhering material is removed from the anode block and the anode is cleaned. For each dry aggregate formula, at least three different amounts of pitch are tested, and for each of those pitch concentrations, three test anode blocks are manufactured and tested.

[0137] 1. Measurements for Examples 1, 1C, 2C:

[0138] Figures 1 - 10 Explanation of:

[0139] The grey areas in the figure show the typical values of pilot anodes used in the aluminium industry.

[0140] All data points represent the average of three independent anodes tested.

[0141] The following symbols are used to distinguish the examples in the figure:

[0142]

[0143] 1.1 Density ( Figure 1 ):

[0144] The density should be higher to increase the weight of carbon used in electrolysis.

[0145] Conclusion:

[0146] The green and baked densities of the anodes in Example 1 are fully within the required high range. Surprisingly, the density of the 50 / 50 blend composition is not a linear interpolation between the high-density pyrolytic carbon and low-density HS coke. Instead, the density of the mixture is shifted towards the required higher density.

[0147] 1.2 Strength ( Figure 2 ):

[0148] The strength should be higher to improve thermal shock resistance.

[0149] Conclusion:

[0150] As would be expected by those skilled in the art, the strength of the 50 / 50 mixture in Example 1 is also the non-linear average of Examples 1C and 2C. This surprising finding is highly relevant to the blending method according to the present invention, since the anode made entirely of pyrolytic carbon (Example 1C) fails due to insufficient mechanical strength.

[0151] 1.3 Specific resistance ( Figure 3 ):

[0152] The specific resistance should be low to reduce the power consumption in the electrolytic cell.

[0153] Conclusion:

[0154] The specific resistance in Example 1C is not the linear average of Examples 1C and 2C. It also shifts in the more desirable direction of lower resistance, which reduces the energy consumption in the electrolytic cell.

[0155] 1.4 CO2 reactivity ( Figure 4 ):

[0156] CO2 reaction residue:

[0157] The CO2 reaction residue should be high to reduce the amount of carbon reacting with CO2.

[0158] CO2 reactive dust:

[0159] The CO2 reactive dust should be low to avoid carbon particles falling into the bath, which increases the resistivity.

[0160] CO2 reactivity loss:

[0161] The CO2 reactivity loss should be low to reduce the amount of carbon reacting with CO2.

[0162] Conclusion:

[0163] Surprisingly, the anode made of less contaminated pyrolytic carbon (Example 1C) has a higher reactivity to CO2. This is caused by a higher degree of pulverization. In this case, pulverization is caused by the selective reaction of the binder substrate due to the reactivity mismatch between the structure of the uncontaminated pyrolytic carbon particles and the contaminated and more reactive substrate. However, surprisingly, although there is still some reactivity mismatch between the contaminated HS coke particles, the pitch substrate and the relatively pure pyrolytic carbon particles, the 50 / 50 mixture compensates for this reactivity mismatch.

[0164] 1.5 Air reactivity ( Figure 5 ):

[0165] Air reaction residue:

[0166] The air reaction residue should be high to reduce the amount of carbon reacting with air.

[0167] Air-reactive dust:

[0168] The air-reactive dust should be low to avoid carbon particles falling into the bath, which would increase the resistivity. Air-reactive loss:

[0169] The air-reactive loss should be low to reduce the amount of carbon reacting with air.

[0170] Conclusion:

[0171] The air-reactivity of the blend anodes (Example 1) is fully within the desired range and fully compensates for the high reactivity of HS coke (Example 2C).

[0172] 1.6 Impurities:

[0173]

[0174] Conclusion:

[0175] The impurities depend only on the blend ratio and are the linear average of the two materials, i.e., Example 1 represents the average of Examples 1C and 2C, as would be expected by a person skilled in the art. However, it is worth emphasizing that the present invention provides a solution to the sulfur emissions problem and also reduces the associated pollution. The sulfur concentration and other impurities in Example 1 fully comply with government emission limits and industrial specifications. The surprising aspect is that two materials with different compositions can be used together as anode raw materials in the form of a blend.

[0176] 2. Examples 2 - 5, 3C:

[0177] Description:

[0178] The grey areas in the graphs show the typical values of experimental anodes used in the aluminum industry.

[0179] All data points represent the average of three independent anodes tested.

[0180] The following symbols are used to distinguish the examples in the figures:

[0181]

[0182] 2.1 Density ( Figure 6 ):

[0183] The density should be high to increase the weight of carbon used in electrolysis.

[0184] Conclusion:

[0185] Compared with 3C, Examples 2 - 5 show that the target high density can be achieved by a high proportion of pyrolytic carbon in the anode. Surprisingly, the density is even higher than the average industrial range, indicating the performance advantage of this blend compared to standard CPC anodes.

[0186] 2.2 Strength ( Figure 7 ):

[0187] The strength should be relatively high to improve the thermal shock resistance.

[0188] Conclusion:

[0189] In the case of compressive strength, it is expected to decrease when blended with pyrolytic carbon. This is due to the laminated structure and the high anisotropy of the resulting pyrolytic carbon. However, the strength remains at an acceptable level, which cannot be expected from the prior art attempts to use pyrolytic carbon in electrodes.

[0190] 2.3 Resistance ( Figure 8 ):

[0191] The specific resistance should be relatively low to reduce the power consumption in the electrolytic cell.

[0192] Conclusion:

[0193] Pyrolytic carbon has a low resistance. Therefore, the anode performance is proportional to the amount of pyrolytic carbon in the blend.

[0194] 2.4 CO2 Reactivity ( Figure 9 ):

[0195] CO2 Reaction Residue:

[0196] The CO2 reaction residue should be relatively high to reduce the amount of carbon reacting with CO2.

[0197] CO2 Reaction Dust:

[0198] The CO2 reaction dust should be relatively low to avoid carbon particles falling into the bath, which would increase the resistivity. CO2 Reaction Loss:

[0199] The CO2 reaction loss should be relatively low to reduce the amount of carbon reacting with CO2.

[0200] Conclusion:

[0201] Due to the higher pulverization caused by the reactivity mismatch, the CO2 reactivity in Examples 2 - 5 is also higher than that in Reference Example 3C, but it is still within the typical range for anodes.

[0202] 2.5 Air Reactivity ( Figure 10 ):

[0203] Air Reaction Residue:

[0204] The air reaction residue should be relatively high to reduce the amount of carbon reacting with air.

[0205] Air Reaction Dust:

[0206] The air reactivity of the dust should be low to avoid carbon particles falling into the bath, which would increase the resistivity. Air reactivity loss:

[0207] The air reactivity loss should be low to reduce the amount of carbon reacting with air.

[0208] Conclusion:

[0209] By mixing pyrolytic carbon into the HQ coke, the air reactivity is similarly not significantly affected. 2.6 Impurities:

[0210] S V Si Ni Fe Na P Ca 2 0.64％ 28 ppm 39 ppm 71 ppm 178 ppm 56 ppm 7 ppm 25 ppm 3 0.8％ 29 ppm 29 ppm 81 ppm 117 ppm 53 ppm 4 ppm 25 ppm 4 0.75％ 35 ppm 42 ppm 81 ppm 130 ppm 66 ppm 5 ppm 30 ppm 5 0.78％ 31 ppm 34 ppm 88 ppm 159 ppm 113 ppm 4 ppm 31 ppm 3C 0.86％ 28 ppm 31 ppm 86 ppm 128 ppm 59 ppm 3 ppm 30 ppm

[0211] Conclusion:

[0212] The impurities also conform to the simple mixing rule.

Claims

1. An anode composition comprising, as a mixture based on the total weight of the anode composition: (i) 20 - 99% by weight of petroleum coke and (ii) 1 - 80% by weight of pyrolytic carbon, and the anode composition comprises at least two particle size fractions: (i) particles greater than 0.5 mm and (ii) fines less than 0.5 mm, and the pyrolytic carbon is present at least in particle size fractions of 2 - 1 mm, 4 - 2 mm and / or 8 - 4 mm, wherein at least 40% by weight of the total pyrolytic carbon of the anode composition is the particulate fraction, wherein the specific surface area of the pyrolytic carbon is 0.001 - 5 m 2 / g, and wherein the pyrolytic carbon is produced by the decomposition of gaseous hydrocarbon compounds and by deposition on carbon particles.

2. The anode composition according to claim 1, wherein the pyrolytic carbon is present in a particle size fraction of 4 - 2 mm and / or 8 - 4 mm.

3. The anode composition according to claim 1, wherein the pyrolytic carbon is present in a particle size fraction of 4 - 2 mm.

4. The anode composition according to claim 1, wherein the pyrolytic carbon has the following particle sizes: (i) <0.25 mm, 0.5 - 0.25 mm, 0.5 - 1 mm, 2 - 1 mm, 4 - 2 mm and 8 - 4 mm, or (ii) <0.25 mm, 0.5 - 0.25 mm, 0.5 - 1 mm, 2 - 1 mm and 4 - 2 mm, or (iii) 4 - 2 mm, or (iv) 2 - 1 mm.

5. The anode composition according to claim 1, wherein the content of the petroleum coke is 40 - 98 wt% and the content of the pyrolytic carbon is 2 - 60 wt%.

6. The anode composition according to claim 1, wherein the content of the petroleum coke is 50 - 95 wt% and the content of the pyrolytic carbon is 5 - 50 wt%.

7. The anode composition according to claim 1, wherein at least 50 wt% of the total pyrolytic carbon of the anode composition is in particulate fraction.

8. The anode composition according to claim 1, wherein 90 - 100 wt% of the total pyrolytic carbon of the anode composition is in particulate fraction.

9. The anode composition according to claim 1, wherein the anode composition comprises at least three particle size fractions: (i) a coarse fraction greater than 4 mm, (ii) an intermediate fraction of 4 - 0.5 mm, and (iii) a fine fraction less than 0.5 mm, and the pyrolytic carbon is present at least in the intermediate fraction and / or at least in the coarse fraction.

10. The anode composition according to claim 9, wherein 40 - 100 wt% of the total pyrolytic carbon of the anode composition is in the intermediate size fraction, 40 - 0 wt% of the total pyrolytic carbon of the anode composition is in the coarse size fraction, and 20 - 0 wt% of the total pyrolytic carbon of the anode composition is in the fine size fraction.

11. The anode composition according to claim 1, wherein the density of the pyrolytic carbon is 1.8 - 2.2 g / cc.

12. The anode composition according to claim 1, wherein the crystallite size of the pyrolytic carbon is 30 - 50 Å.

13. The anode composition according to claim 1, wherein calcined petroleum coke is used, and wherein the sulfur content is 1.5 - 7.0 wt% based on the total weight of the petroleum coke.

14. The anode composition according to claim 1, wherein the porosity of the pyrolytic carbon particles is less than 15%.

15. A method for preparing the anode composition according to claim 1, the method comprising mixing pyrolytic carbon and petroleum coke.

16. An anode formulation, comprising a mixture of: (i) an anode composition of petroleum coke and pyrolytic carbon according to any one of claims 1 - 14, (ii) anode butts and / or scraps, and (iii) a binder material.

17. The anode formulation according to claim 16, wherein 35-95% by weight of the total weight of the anode formulation is the anode composition, 0-40% by weight of the total weight of the anode formulation is the spent anode and / or scrap, and 5-25% by weight of the total weight of the anode formulation is the binder.

18. A method for preparing the anode formulation according to claim 16, which comprises preheating the anode composition and the spent anode and / or scrap, and mixing the preheated anode composition and the spent anode and / or scrap with the binder.

19. A method for manufacturing aluminum, wherein the anode formulation according to claim 16 is used as a carbon anode in an aluminum reduction cell.

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