Blend composition comprising petroleum coke and pyrolytic carbon for electrodes

Abstract

A blend composition contains a mixture of petroleum coke and pyrolytic carbon. An electrode recipe, and the use of this electrode as an anode in the process of producing aluminum are described.

Claims

1: A blend composition, comprising: a mixture of (i) petroleum coke in a content of 20 to 99 weight-%; and (ii) pyrolytic carbon in a content of 1 to 80 weight-%, in view of the total weight of the blend composition, wherein the blend composition contains at least two particle size fractions: (i) granular above 0.5 mm and (ii) fines below 0.5 mm, and wherein pyrolytic carbon is at least present in the granular size fraction.

2: The blend composition of claim 1, wherein the content of said petroleum coke is of 70 to 95 weight-% and the content of said pyrolytic carbon is of 5 to 30 weight-%, in view of the total weight of the blend composition.

3: The blend composition of claim 1, wherein at least 30 weight-% of the total pyrolytic carbon of the blend composition is in the granular fraction.

4: The blend composition of claim 1, wherein at least 70 weight-% of the total pyrolytic carbon of the blend composition is in the granular fraction.

5: The blend composition of claim 1, wherein said blend composition contains at least three particle size fractions: (i) coarse above 4 mm, (ii) intermediate between 4 and 0.5 mm, and (iii) fines below 0.5 mm, and wherein pyrolytic carbon is at least present in the intermediate fractions and/or in the coarse fractions.

6: The blend composition of claim 5, wherein at least 30 weight-% of the total pyrolytic carbon of the blend composition is in the intermediate size fraction.

7: The blend composition of claim 1, wherein a density of said pyrolytic carbon is in the range of 1.8 to 2.2 g/cc.

8: The blend composition of claim 1, wherein a crystallite size of said pyrolytic carbon is in the range of 30 to 50 Å.

9: The blend composition of claim 1, wherein the petroleum coke comprises calcined petroleum coke and wherein a sulfur content is in the range 1.5 to 7.0 weight % in view of a total weight of the petroleum coke.

10: A method of making the blend composition of claim 1, the method comprising: mixing the pyrolytic carbon and the petroleum coke.

11: An electrode recipe, comprising: a mixture of the blend composition of petroleum coke and pyrolytic carbon according to claim 1, (ii) butts and/or scrap, and (iii) a binder material.

12: The electrode recipe of claim 11, wherein 35 to 95 weight-% of the total weight of the electrode recipe is the blend composition, 0 to 40 weight-% of the total weight of the electrode recipe are the butts and/or scraps, and 5 to 25 weight-% of the total weight of the electrode recipe is the binder.

13: A method of making the electrode recipe of claim 11, the method comprising: preheating said blend composition and the butts and/or scraps, and mixing said preheated mixture with a binder.

14: A carbon electrode, suitable as an anode in an aluminum reduction cell, which comprises a blend composition comprising a mixture of (i) petroleum coke in a content of 20 to 99 weight-% and (ii) pyrolytic carbon in a content of 1 to 80 weight-%, in view of the total weight of the blend composition, wherein the blend composition contains at least two particle size fractions: (i) granular above 0.5 mm and (ii) tines below 0.5 mm, and wherein pyrolytic carbon is at least present in the granular size fraction.

15: A method of making a carbon electrode, suitable as an anode in an aluminum reduction cell, the method comprising: mixing the blend composition according to claim 1 with butts and/or scrap and a binder material at an elevated temperature to form a paste, and baking said solid body at an elevated temperature to form the carbon electrode.

16: A method of making aluminum, comprising: electrolyzing aluminum oxide in an aluminum reduction cell comprising the carbon electrode according to claim 14 as a carbon anode.

Description

EXAMPLES

[0093] Parameters (see “Anode Manufacture, Raw Materials Formulation and Processing Parameters, Kristine L. Hulse, R&D Carbon, Page 10-14):

Green Density:

[0094] The green apparent density is measured from the geometrical dimensions and anode weight just after compaction (i.e. mass of green anode divided by calculated green anode volume). Variations in this parameter are an indication that there are raw material quality changes, process disturbances, particularly in the forming temperature and mixing conditions.

Baked Density:

[0095] The baked apparent density is measured from the baked anode mass divided by the calculated baked anode volume. A high baked density tends to reduce anode air permeability, its specific electric resistance and can extend anode life in the cells. Extremely high density can lead to thermal shock problems. Baked apparent density is controlled by (Sadler et al. 1995): raw material selection; aggregate granulometry; optimal pitch content; optimal processing to avoid poor compaction during forming (or expansion during baking).

Flexural Strength:

[0096] Flexural strength indicates the presence of micro cracks in the anode structure. Low flexural strength values usually indicate problems in the coke grain stability, forming conditions or high heat-up rates during baking (Fischer and Perruchoud 1992). This mechanical property is important during handling, setting and rodding of the anode block.

Compressive Strength:

[0097] Anode strength is mainly dependent 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 enough strength in the butts to enable the removal of bath from the used butts (Sadler et al. 1995).

Specific EI. Resistance:

[0098] The specific electrical resistivity of the carbon anodes ideally should be as low as possible. This is to reduce energy losses associated with resistive heating in the anodes (Sadler et al. 1995). Resistivity is highly influenced by the basic coke structure, anode density and pore distribution. Invisible cracks, defects, and other flaws can be the result of mixing or pressing problems, excessive moisture in the paste, or thermal shock during baking or cooling. The presence of hairline cracks is observed through high standard deviation values (Fischer and Perruchoud 1987). Very low values of electrical resistivity and high thermal conductivity levels may be a result of over baking. This situation can cause airburn problems.

CO2 Reactivity and Air Reactivity:

[0099] The reactivity values are important for determining the susceptibility of an anode to excess carbon consumption and dusting in the electrolysis 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).

The Invention:

[0100]

TABLE-US-00002 0.5- 8-4 mm 4-2 mm 2-1 mm 1-0.5 mm 0.25 mm <0.25 mm Pyrolytic Pyrolytic Pyrolytic Pyrolytic Pyrolytic Pyrolytic carbon/ carbon/ carbon/ carbon/ carbon/ carbon/ residual residual residual residual residual residual coke [%] coke [%] coke [%] coke [%] coke [%] coke [%] Example carbon PC | rest PC | rest PC | rest PC | rest PC | rest PC | rest 1 35% 7 | 13 9 | 17 4 | 8  4 | 8  10 | 20 Pyrolytic carbon, 15% CPC*, 50% HS coke 2 25% 0 | 14 7 | 7  7 | 7  7 | 7  7 | 7  14 | 16 Pyrolytic carbon, 25% CPC*, 50% Coke CHQ 3 7% 0 | 14 2 | 12 0 | 14 0 | 14 0 | 14  0 | 30 Pyrolytic carbon, 43% CPC*, 50% Coke CHQ 4 12.6 0 | 14 4 | 10 0 | 14 0 | 14 0 | 14  0 | 30 Pyrolytic carbon, 37.4% CPC*, 50% Coke CHQ 13 5 12.6 0 | 14 0 | 14 4 | 10 0 | 14 0 | 14  0 | 30 Pyrolytic carbon, 37.4% CPC*, 50% Coke CHQ *= used as carrier of the pyrolytic carbon

Comparative Examples

[0101]

TABLE-US-00003 1-0.5 0.5- <0.25 8-4 mm 4-2 mm 2-1 mm mm 0.25 mm mm Pyrolytic Pyrolytic Pyrolytic Pyrolytic Pyrolytic Pyrolytic carbon/ carbon/ carbon/ carbon/ carbon/ carbon/ residual residual residual residual residual residual coke [%] coke [%] coke [%] coke [%] coke [%] coke [%] Example carbon PC | rest PC | rest PC | rest PC | rest PC | rest PC | rest 10 70% 14 | 6 18 | 8  8 | 4  8 | 4  21 | 9  pyrolytic, 30% CPC* 20 100% HS  0 | 20  0 | 26 0 | 12 0 | 12  0 | 30 coke 3C 100%  0 | 14 0 | 14 0 | 14 0 | 14 0 | 14  0 | 14 Coke CHQ *= used as carrier of the pyrolytic carbon

Sulfur Content:

[0102]

TABLE-US-00004 Example Sulfur [%] 1 1.27 2 0.63 3 0.8 4 0.71 5 0.78  1C 0.08  2C 2.39  3C 0.95

Carbon Sources:

Pyrolytic Carbon:

[0103] The pyrolytic carbon in example 1 was produced by decomposition of natural gas and deposition on CPC (having a particle size of 0.5-2.5 mm, a sulfur content of 1.1 wt.-% and a real density in xylene of 2.09 g/cm3) in a fluidized bed at temperatures from 1100-1300° C. and at pressures from 1-2 bar(abs).

[0104] The pyrolytic carbon from example 2-5 was produced in a fixed bed reactor at 1200° C. at 1.0-1.2 bar(abs) by decomposition of methane and by deposition on a CPC (this type had a particle size of 1-4 mm, a sulfur content of 0.95 wt.-% and a similar real density in xylene).

HS Coke:

[0105] The 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 real density in xylene of 2.07 g/cm3, a total porosity of 25.7% (DIN66133),

[0106] This material is available by standard CPC suppliers and traders.

Coke C HQ:

[0107] This coke is used as “high quality” reference CPC and is characterized by a sulfur content of 0.95 wt.-%, impurity levels of 30 ppm V, 10 ppm Si, 80 ppm Fe, 20 ppm Ca and a real density of 2.08 g/cm3. The total porosity is 20%.

CPC:

[0108] The material referred as CPC indicates the substrate for the pyrolytic carbon of example 2-5, which is the coke with 1.1 wt.-% S and a real density in xylene of 2.09 g/cm3. Impurity levels amount to 180 ppm V, 100 ppm Si, 90 ppm Fe, 50 ppm Ca. The total porosity is 23%.

Pitch:

Material:

[0109] The binder used in the examples was a coal tar pitch a typical Mettler softening point (ISO 5940-2) of 113° C. The other important characteristics of the pitch were the quinoline insoluble (ISO 6791) 8.4% toluene insoluble (ISO 6376) 28% and a real density in helium (ISO 21687) of 1.31 g/cm3.

Pitch Content:

[0110]

TABLE-US-00005 Example pitch 1 15-17% 2 14-17% 3 14-15% 4 14-15% 5 14-15%  1C 13-15%  2C 16-18%  3C 12-18%

Production of Anodes 1-5, 10, 2C, 3C:

[0111] The anodes were produced in a multi-step procedure. The first step was the sieving of the pyrolytic carbon and CPC raw materials to fractions of 8-4 mm, 4-1 mm, 1-0.5 mm and 0.5-0.25 mm. The second step was the generation of fines (fraction <0.25 mm) in the desired quantity. The fraction 8-4 mm was made of pre-baked scrap with the respective ratio of pyrolytic carbon and petroleum coke. The recipe-specific amounts of pyrolytic carbon and/or HS CPC or Coke HQ for each fraction were mixed together to obtain the so-called dry-aggregate. This dry aggregate was then heated above softening point of the coal tar pitch (113° C.) and mixed with the coal tar pitch binder. The pastelike mixture was transferred to a hydraulic press and formed to the so-called green anode at pressures 400 bar. The green anodes were subsequently baked at temperatures of 1100° C. After baking, sticking material was removed from the anode block and the anode cleaned. At least three different pitch amounts were tested for each dry aggregate recipe and for each of those pitch concentrations three test anodes blocks were manufactured and tested.

1. Measurement to Example 1, 10, 2C

Comments to the FIGS. 1 to 10:

[0112] The grey zones in the graphics show the typical values for pilot anodes used in the Al industry. All data points represent an average of three individual anodes tested.

[0113] The following symbols are used to discriminate between the examples in the figures:

[0114] MIX 50/50%

[0115] Pyro-C

[0116] HS CPC

1.1 Density (FIG. 1):

[0117] The density should be high to increase the weight of carbon delivered to the electrolysis.

Conclusion:

[0118] The green and baked density of the anodes in example 1 are well in the desired high range. Surprisingly, the density of the 50/50 blend composition is not a linear interpolation between the high density pyrolytic carbon and the low-density HS coke. Instead, the mixture density is shifted toward the desired higher densities.

1.2 Strength (FIG. 2):

[0119] The strength should be high to improve the thermal shock resistance.

Conclusion:

[0120] The strength of the 50/50 mix in example 1 is also a nonlinear average of example 1C and 2C, as a skilled person in art would expect. This surprising finding is of high relevance for the blend approach according to this invention, as an anode completely made of pyrolytic carbon (example 1C) fails due to the insufficient mechanical strength.

1.3 Electrical Resistance (FIG. 3):

[0121] The specific electrical resistance should be low to reduce the electrical consumption in the electrolysis cell.

Conclusion:

[0122] The specific electrical resistance in example 1C is not a linear average of examples 1C and 2C. Again, it is shifted towards the more desired direction of low electrical resistance, which reduces the energy consumption in the electrolytic cell.

1.4 CO2 Reactivity (FIG. 4):

CO2 Reactivity Residue:

[0123] The CO2 Reactivity Residue should be high to reduce the quantity of carbon that reacts with CO2.

CO2 Reactivity Dust:

[0124] The CO2 Reactivity Dust should be low to avoid carbon particles falling into the bath which will increase the resistivity.

CO2 Reactivity Loss:

[0125] The CO2 Reactivity Loss should be low to reduce the quantity of carbon that reacts with CO2

Conclusion:

[0126] Surprisingly, the anode made of the less contaminated pyrolytic carbon (example 1C) has a higher reactivity towards CO2. This results from higher degree of dusting. Dusting is in this case induced by a selective reaction of the binder matrix due to a reactivity mismatch between uncontaminated pyro-C granular structure and a contaminated, more reactive matrix. However, it is surprising that the 50/50 mixture compensates for this reactivity mismatch although some reactivity mismatch remains—between the contaminated HS coke particles, pitch matrix and relatively pure pyrolytic carbon particles.

1.5 Air Reactivity (FIG. 5):

Air Reactivity Residue:

[0127] The Air Reactivity Residue should be high to reduce the quantity of carbon that reacts with air.

Air Reactivity Dust:

[0128] The Air Reactivity Dust should be low to avoid carbon particles falling into the bath which will increase the resistivity.

Air Reactivity Loss:

[0129] The Air Reactivity Loss should be low to reduce the quantity of carbon that reacts with Air.

Conclusion:

[0130] The air reactivity of the blend anode (example 1) is well in the required range and fully compensates the high reactivity of the HS coke (example 2C).

1.6 Impurities:

[0131]

TABLE-US-00006 S V Si Ni Fe Na P Ca 1 1.27% 189 267  96 348 108  8 178 ppm ppm ppm ppm ppm ppm ppm 1C 0.09%  33  56  23  97 118  4  41 ppm ppm ppm ppm ppm ppm ppm 2C 2.39% 339 516 161 588 101 12 320 ppm ppm ppm ppm ppm ppm ppm

Conclusion:

[0132] The impurities only depend on the blend ratio and are a linear average of both materials, i.e. example 1 represents the average of example 1C and 2C, as a person skilled in art would expect. However, it is worth emphasizing that this invention provides a solution for the sulfur emission issue and also reduces the relevant contaminations. The sulfur concentration in example 1 and also the other impurities are well in line with governmental emission restrictions and industry specifications. The surprising aspect is that two materials which are that different in composition can be used in a blend together as anode raw material.

2. Example 2-5, 3C

Comments:

[0133] The grey zones in the graphics show the typical values for pilot anodes used in the Al industry. All data points represent an average of three individual anodes tested.

[0134] The following symbols are used to discriminate between the examples in the figures:

[0135] d 100% C3000

[0136] 50% Pyro-C

[0137] 4-2 mm Pyro-C 50%

[0138] 4-2 mm Pyro-C 90%

[0139] 2-1 mm Pyro-C/New CPC

2.1 Density (FIG. 6):

[0140] The density should be high to increase the weight of carbon delivered to the electrolysis.

Conclusion:

[0141] The example 2-5 show in comparison to 3C that the targeted high density can be achieved with high fraction of pyrolytic carbon in the anode. Surprisingly, the density is even above the average industry range, indicating a performance advantage of the blend compared to the standard CPC anode.

2.2 Strength (FIG. 7):

[0142] The strength should be high to improve the thermal shock resistance.

Conclusion:

[0143] In case of the compressive strength, one would expect a reduction upon blending with pyrolytic carbon. This is due to the laminated structure and the resulting high anisotropy of the pyrolytic carbon. However, the strength remains in an acceptable level, which was not expected according to state of the art attempts of using pyrolytic carbon in electrodes.

2.3 Electrical Resistance (FIG. 8):

[0144] The specific electrical resistance should be low to reduce the electrical consumption in the electrolysis cell.

Conclusion:

[0145] The pyrolytic carbon has a low electrical resistance. Thus, the anode performance scales with the amount of pyrolytic carbon in the blend.

2.4 CO2 Reactivity (FIG. 9):

CO2 Reactivity Residue:

[0146] The CO2 Reactivity Residue should be high to reduce the quantity of carbon that reacts with CO2.

CO2 Reactivity Dust:

[0147] The CO2 Reactivity Dust should be low to avoid carbon particles falling into the bath which will increase the resistivity.

CO2 Reactivity Loss:

[0148] The CO2 Reactivity Loss should be low to reduce the quantity of carbon that reacts with CO2.

Conclusion:

[0149] The CO2 reactivity in examples 2-5 is again higher than in the reference example 3C due to the higher dusting, created by the reactivity mismatch, but it is still in the typical range for anodes.

2.5 Air Reactivity:

Air Reactivity Residue:

[0150] The Air Reactivity Residue should be high to reduce the quantity of carbon that reacts with air.

Air Reactivity Dust:

[0151] The Air Reactivity Dust should be low to avoid carbon particles falling into the bath which will increase the resistivity.

Air Reactivity Loss:

[0152] The Air Reactivity Loss should be low to reduce the quantity of carbon that reacts with Air.

Conclusion:

[0153] The air reactivity is again not significantly affected by blending in pyrolytic carbon into HQ coke.

2.6 Impurities:

[0154]

TABLE-US-00007 S V Si Ni Fe Na P Ca 2 0.64% 28 39 71 178  56 7 25 ppm ppm ppm ppm ppm ppm ppm 3  0.8% 29 29 81 117  53 4 25 ppm ppm ppm ppm ppm ppm ppm 4 0.75% 35 42 81 130  66 5 30 ppm ppm ppm ppm ppm ppm ppm 5 0.78% 31 34 88 159 113 4 31 ppm ppm ppm ppm ppm ppm ppm 3C 0.86% 28 31 86 128  59 3 30 ppm ppm ppm ppm ppm ppm ppm

Conclusion:

[0155] The impurities are again in line with a simple mixing rule.