Improved pitch products, processes for their preparation and use

Abstract

[0010] The present invention relates to a pitch product comprising a petroleum-derived distillation residue and a coal tar-derived distillation residue, the pitch product being characterized by a concentration of asphaltene content (SARA) of at least 84%. Furthermore, the present invention relates to a pitch binder comprising the pitch product, in particular for use in the manufacture of graphite electrodes for electric arc furnaces and carbon anodes and Soderberg pastes for aluminum production. The present invention also relates to a graphite electrode comprising the pitch binder, as well as a carbon anode comprising the pitch binder. The present invention also provides a process for producing a pitch product comprising a petroleum-derived distillation residue and a coal tar-derived distillation residue, the process comprising a petroleum vacuum distillation process step to obtain the petroleum-derived distillation residue, and a process for producing a graphite electrode or a carbon anode comprising the process for producing a pitch product.

Description

[Technical field]

[0001] Technical Field The present invention relates generally to pitch products comprising petroleum-derived distillation bottoms and coal tar-derived distillation bottoms, and more specifically to pitch products for use in the manufacture of graphite electrodes for electric arc furnaces and carbon anodes for aluminum production.

[0002] Additionally, the present invention relates to processes for producing such pitch products. [Background technology]

[0003] background Graphite electrodes for steel production and pre-baked carbon anodes for the aluminum industry are produced by hot mixing calcined coke and a hydrocarbon carbon precursor and forming the mixture into a green electrode shape that is carbonized in a subsequent baking process. The hydrocarbon binder material provides sufficient mechanical strength for the unbaked (green) electrode shape and is converted to carbon during the baking process. The resulting carbon semi-graphitized electrodes meet the requirements as anodes in electrolytic cells used in the production of aluminum. Electrodes used in electric arc furnaces for steel production are further impregnated with a hydrocarbon-impregnated pitch, carbonized, and subsequently graphitized. For use as electrodes in Soderberg cells, a paste is produced by hot mixing of dry aggregates (calcined coke, anthracite, graphite, etc.) and a hydrocarbon binder, which is formed into briquettes or other preformed shapes, transferred to the Soderberg cell, and subsequently carbonized in the electrolytic cell itself.

[0004] Traditionally, the hydrocarbon pitch binder material in the manufacture of these electric arc furnace graphite electrodes and aluminum production carbon anodes (including prebaked anodes and Soderberg anodes) is coal tar pitch, as it meets the mechanical requirements in the green stage and is converted to electrically conductive carbon with very high coke yields during the carbonization process, thereby avoiding high porosity in the resulting article due to the formation of less volatiles during the carbonization process. Also, the hydrocarbon impregnated pitches used for impregnating graphite electrodes are typically based on coal tar distillation products.

[0005] Coal tar has always been available in abundance as a by-product of the production of metallurgical coke used in steel production, but recently, due to declining demand for virgin iron, less metallurgical coke has been produced and therefore less coal tar is available.

[0006] Another drawback of coal tar pitch binders is that they contain somewhat high amounts of benzo(a)pyrene (about 10,000 ppm in the typical viscosity range), which is classified as a carcinogen. In addition to the B(a)P content, other polyaromatic hydrocarbons are considered to be hazardous to health and the environment.

[0007] Petroleum-derived pitches have been considered in an attempt to produce alternative pitches from petroleum sources that contain less benzo[a]pyrene than coal tar.

[0008] However, petroleum-derived pitch, even when used as a pure binder material in the electrode production process, does not achieve the same quality parameters as coal tar pitch. The first drawback is that it has a lower coke yield than coal tar pitch, and secondly, it does not have any of the major quinoline insoluble components that are considered beneficial for high quality anodes in aluminum production and that are contained in conventional coal tar-based pitches.

[0009] A further drawback of known petroleum-derived pitch materials is their softening points, typically below 100° C., which is too low for the use of hydrocarbon pitch binder materials in the manufacture of graphite electrodes, which have a softening point target of 110-130° C. Such low softening points also limit the use of petroleum pitch in blends with coal tar pitch (e.g., U.S. Pat. No. 5,746,906).

[0010] Additionally, the typical low flash points of existing petroleum-based pitches, below 200°C, raise safety concerns in electrode fabrication processes, which can include hot-mix processes at temperatures up to 200°C.

[0011] In summary, all attempts to produce suitable petroleum pitch on an industrial scale have not provided a reliable alternative to coal tar pitch binder that can supply large quantities of product to the aluminum industry.

[0012] More specifically, neither pure petroleum-derived pitches nor blends of petroleum pitch and coal tar pitch containing amounts of petroleum pitch substantially greater than 10% have been able to meet the requirements for use as a hydrocarbon pitch binder material in the manufacture of carbon and graphite electrodes. Summary of the Invention [Problem to be solved by the invention]

[0013] In view of the above, it is a general object of the present invention to provide an alternative to coal tar pitch binder that meets the requirements for use as a hydrocarbon pitch binder material in the manufacture of carbon and graphite electrodes while allowing for increased security of supply.

[0014] Another object of the present invention is to provide an alternative pitch binder that provides similar coke values ​​and similar processing and performance of graphite electrodes, pre-baked anodes, and pastes used in the Soderberg technology.

[0015] It is a further object of the present invention to provide a more environmentally friendly alternative to coal tar pitch binders. [Means for solving the problem]

[0016] overview In a first aspect of the present invention, there is provided a pitch product comprising a blend of a petroleum-derived distillation residue and a coal tar-derived distillation residue in a ratio of 20:80 to 70:30 by weight, the pitch product being characterized by an asphaltene content (SARA as measured by clay-gel absorption chromatography according to ASTM D2007) of at least 84%.

[0017] In a second aspect of the invention, there is provided a pitch binder comprising the pitch product described throughout this document, said pitch binder being for use in the manufacture of any type of carbon-based formed profile, in particular for use in the manufacture of graphite electrodes for electric arc furnaces and carbon anodes and Soderberg paste for aluminium production.

[0018] In a third aspect of the present invention, there is provided a graphite electrode comprising the pitch binder.

[0019] In a fourth aspect of the present invention, there is provided a carbon anode comprising the pitch binder.

[0020] In a fifth aspect of the present invention, there is provided a process for producing a pitch product, the pitch product comprising a petroleum-derived distillation residue and a coal tar-derived distillation residue, the process comprising a petroleum vacuum distillation process step to obtain the petroleum-derived distillation residue, and blending the petroleum-derived distillation residue and the coal tar-derived distillation residue.

[0021] In a sixth aspect of the present invention, there is provided a process for manufacturing a graphite electrode or a carbon anode comprising the process described above for producing a pitch product. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Detailed Description The pitch products described throughout this document are offered as an alternative to coal tar pitch binders that meet the requirements of the aluminum and / or graphite industries while having the advantages of low benzo(a)pyrene content and ample availability.

[0023] In a first aspect of the present invention, there is provided a pitch product comprising a petroleum-derived distillation residue and a coal tar-derived distillation residue, the pitch product being characterized by an asphaltene content of at least 84%, or at least 86%, or at least 90%, as measured by the SARA clay-gel absorption chromatography method according to ASTM D2007.

[0024] Additionally, the pitch product may have a resin content of less than 10% as measured by the clay-gel absorption chromatography method according to ASTM D2007.

[0025] Surprisingly, the inventors have found that pitch products containing petroleum-derived distillation bottoms and coal tar-derived distillation bottoms and containing similar levels of asphaltene content compared to known coal tar pitch binders as measured by the SARA clay-gel absorption chromatography method according to ASTM D2007 exhibit coke values ​​of at least 45% alkane at a target softening point (e.g., 110-150°C Mettler). Asphaltene content concentrations of at least 84%, or at least 86%, or at least 90%, respectively, can result in increased coke values. The pitch products ensure sufficient mechanical strength of the resulting green coke shapes and low shrinkage upon carbonization. The resulting carbonized shapes of the bound coke exhibit low porosity and air permeability as well as sufficient electrical conductivity.

[0026] As is well known, asphaltenes are solids with high melting points that are insoluble in paraffinic solvents and tend to easily form isotropic coke due to their highly aromatic ring structure and high molecular weight. For example, asphaltenes can be pentane or heptane insoluble (see, for example, EP 0072243 B1).

[0027] SARA analysis is a commonly used method to measure saturates, asphaltenes, resins, and aromatics in heavy crudes, distillates, and feedstocks. SARA methods other than clay-gel (ASTM D-2007) can be TLC / FID according to IP-469 or preparative HPLC (IP-368) followed by IP-143. SARA analysis is available in the industry from companies such as Intertek and Iactroscan.

[0028] Additionally, the amounts of asphaltene content stated throughout this document may include losses as defined in ASTM D-2007.

[0029] In one embodiment of the present invention, the pitch product may have a thixotropic behavior with a high viscosity recovery (DIN91143-2) after 60 seconds of at least 20%, or at least 40%, or at least 60%, or even up to 90%. Such thixotropic behavior indicates good processing and impregnation properties. Without being bound by any theory, it is believed that the surprisingly high recovery rate is due to the pitch structure returning to its original state after the shear force effect due to a lower amount of solid components that disturb the intermolecular arrangement and interactions between larger molecules.

[0030] High thixotropy of the liquid binder pitch is important because a key requirement of the pitch binder for electrode fabrication is its ability to process and wet and impregnate the compressed electrodes formed from coke in the electrode fabrication process. The molten liquid pitch is pumped and mixed with the coke, which applies high shear forces. Typically, such pumping and mixing of the pitch requires a low viscosity of the liquid pitch. In contrast, impregnation and pore filling of the compressed electrodes requires a higher viscosity of the binder pitch to retain the binder pitch in the pores. The viscosity at high shear rates should be low, while the viscosity at low shear rates should be high. Additionally, a high build-up or recovery rate of viscosity after application and release of shear energy is advantageous for this application.

[0031] More specifically, the pitch products of the present invention may have a concentration of asphaltene content (SARA) of at least 84%, or at least 86%, or even at least 90%, and a high viscosity recovery after 60 seconds of at least 25%, or at least 40%, or even at least 60%.

[0032] In one embodiment of the present invention, the pitch product may have a resin content (SARA) of less than 10%, which may contribute to its high coke yield.

[0033] In another embodiment of the invention, the pitch product may have a B(a)P content of less than 8500 ppm, or less than 7000 ppm, or less than 5000 ppm, or even less than 3000 ppm, and / or a 16EPA-PAH Sum (polycyclic aromatic hydrocarbons according to the US Environmental Protection Agency (EPA)) of less than 7% (m / m), or even less than 5%. A sufficiently low B(a)P content and / or 16EPA-PAH Sum obviously results in improved environmental friendliness compared to pure coal tar derived pitch products. The low B(a)P content pitch binder product can also be advantageously used in the manufacture of electrodes for aluminum (prebaked anodes and Söderberg anodes) and electric arc furnaces.

[0034] In further embodiments, the pitch product may have a coke yield of at least 45% alkane or at least 50% alkane or at least 55% alkane at a softening point of 110-150° C. Mettler. Since the pitch product is converted to carbon during the carbonization process, a sufficiently high coke yield allows for the avoidance of high porosity in the resulting profile due to less volatiles being formed during the carbonization process. At the same time, a high coke yield avoids high shrinkage during baking of the green anode, which may result in the risk of cracking and even profiles outside of dimensional tolerances. Furthermore, since the pitch product is effectively a carbon precursor, a sufficiently high coke yield is crucial to achieving the porosity target of the baked anode / electrode.

[0035] In a further embodiment, the pitch product may have a flash point of 200-270°C, preferably 220-245°C, which may allow the pitch product to be processed in accordance with safety requirements that may be required for hot mix processes. More specifically, the pitch product of the present invention may have an asphaltene content of at least 84%, or at least 86%, or even at least 90%, and a flash point of 200-270°C, preferably 220-245°C.

[0036] In one embodiment of the invention, the pitch product may have a Mettler softening point of 110-150° C., which is the target range for making electrodes used in steel and aluminum production, as well as Soderberg paste used in aluminum production.

[0037] According to the present invention, the pitch product may be a blend of petroleum-derived distillation residue and coal tar-derived distillation residue. In one embodiment, the blend may have a ratio of 20:80 to 70:30, preferably 30:70 to 60:40, and even more preferably 40:60 to 50:50. Such a ratio may result in a Mettler softening point of 110 to 140° C., a quinoline insoluble range of 2 to 12%, preferably 2 to 8%, a beta-resin content of 13 to 25%, and a coke yield (value) measured by the Alcan method of at least 45%.

[0038] More specifically, the pitch product of the present invention may have an asphaltene content of at least 84%, or at least 86%, or even at least 90%, and a blend ratio of 30:70 to 60:40, and even more preferably 40:60 to 50:50.

[0039] In certain embodiments of the present invention, the pitch product may have a coke yield of at least 45% alkane, a Mettler softening point of 110-140° C., a quinoline insolubles range of 2-12%, and a beta-resin content of 13-25%. More preferably, the quinoline insolubles range is 2-8%.

[0040] In an embodiment, a pitch product that is a blend of a petroleum-derived distillation bottoms and a coal tar-derived pitch can be optimized with respect to the blending ratio to have an increase in coking value (alkanes) that is at least 0.5 wt.% greater than the calculated weighted average coking values ​​of the petroleum-derived distillation bottoms and the coal tar-derived distillation bottoms, respectively.

[0041] In a preferred embodiment, the petroleum-derived distillation residue obtained by the petroleum vacuum distillation process step may be characterized by an asphaltene content of at least 80%, or at least 84%, or at least 86%, as measured by clay-gel absorption chromatography according to ASTM D2007, and a softening point of at least 110° Mettler or at least 120° Mettler. The petroleum-derived distillation residue may have a coke yield of at least 45% alkane.

[0042] In another embodiment, the pitch product of the present invention may comprise a petroleum-derived distillation residue derived from a feedstock produced by pyrolysis of a petroleum stream. Preferably, the feedstock comprises at least 30 wt.% asphaltene content, less than 10% saturates, and less than 40% resin content as measured by clay-gel absorption chromatography according to ASTM D2007. A feedstock having such a composition allows for a high yield of the product.

[0043] In a second aspect of the invention, there is provided a pitch binder comprising the pitch product as described throughout this document, said pitch binder being for use in the manufacture of any type of carbon-based formed article, such as mouldings, bricks, profiles, filter grids for electrodes, refractory applications, etc., particularly for use in the manufacture of graphite electrodes for electric arc furnaces and carbon anodes and Soderberg paste for aluminium production. The pitch binders described throughout this document can also be used as hydrocarbon-impregnated pitches.

[0044] In a third aspect of the present invention, there is provided a graphite electrode comprising the pitch binder. Furthermore, the pitch binder may also be used as an impregnating pitch in the manufacture of the graphite electrode. Such pitch binders and impregnating pitches may support the integrity of the carbon body by filling the pores of the carbonized electrode.

[0045] In a fourth aspect of the present invention, there is provided a carbon anode comprising the pitch binder.

[0046] In a fifth aspect of the present invention, there is provided a process for producing a pitch product, the pitch product comprising a petroleum derived distillation residue and a coal tar derived distillation residue, the process comprising a petroleum vacuum distillation process step to obtain the petroleum derived distillation residue.

[0047] In a further embodiment of the present invention, there is provided a process for producing a pitch product as described throughout this document, comprising a petroleum vacuum distillation process step and a separate coal tar vacuum distillation process step to obtain a petroleum-derived distillation residue and a coal tar-derived distillation residue, respectively, and blending the respective residues, each of which is produced by separate vacuum distillation of a coal tar and a petroleum-derived feedstock, and is subsequently mixed at a controlled composition.

[0048] The benefit of the process of the present invention is that it may be possible to maintain the amount of asphaltenes as measured by SARA at a similar level compared to known coal tar pitch binders, and other properties of the pitch do not appear to be degraded compared to known coal tar pitch binders.

[0049] Another benefit is that such a process may allow the coke value of the pitch product to be maintained at a high level (eg, at least 40% alkane) at a target softening point (eg, 110-150° C. Mettler).

[0050] Further benefits of the process according to the invention are the highly thixotropic behavior of the resulting products and the high viscosity recovery after 60 seconds of at least 25%.

[0051] Another advantage of the process of the present invention is that it can achieve high final product yields when using a petroleum-derived feedstock produced by pyrolysis of a petroleum stream. Preferably, such feedstock has a composition comprising at least 30 wt.% asphaltenes, less than 10% saturates, and less than 40% resins as measured by the SARA clay-gel absorption chromatography method according to ASTM D2007.

[0052] In contrast to producing pitch products with the process of the present invention, conventional coal tar pitch binders extended with petroleum-based pitches are produced by distillation at ambient pressure and higher temperatures, followed in some cases by air blowing, and either mixing the coal tar and petroleum-based raw materials before distillation or first distilling a single component and then blending the components. The drawbacks of such products resulting from ambient pressure distillation are high mesophase and toluene insoluble content due to high processing temperatures that lead to cracking and mesophase formation. Other drawbacks are low coke yield, high volatile content, and high viscosity that deteriorate the pitch's impregnation and binding properties and processability, as well as a low flash point that causes safety issues in the electrode fabrication process.

[0053] Another drawback of distillation at ambient pressure is that petroleum tar distillation is reactive and the temperatures required for distillation at atmospheric pressure already initiate conversion to solid carbon components in the heating chambers and columns, resulting in excessive fouling rates in pitch production potentially leading to plant reliability issues.

[0054] In one embodiment of the present invention, the petroleum-derived distillation residue obtained from the petroleum vacuum distillation process step may be characterized by an asphaltene content of at least 80%, or at least 84%, or at least 86%, as measured by clay-gel absorption chromatography according to ASTM D2007, and a softening point of at least 110° Mettler or at least 120° Mettler. The petroleum-derived distillation residue may have a coke yield of at least 45% alkane.

[0055] In one embodiment of the present invention, the distillation process step is carried out at a vacuum level of 10-400 mbar, preferably 50-250 mbar, and at a temperature of 200-400°C, preferably 280-370°C.

[0056] The process of the present invention allows for precise control and prevention of potential mesophase formation for low secondary quinoline insolubles content of the pitch.

[0057] Additionally, separate distillation of coal tar and petroleum tar may result in more optimized product properties of the separate distillation residues, and therefore may produce a higher quality pitch product.

[0058] Additionally, the process of the present invention provides a high level of reliability by achieving the required softening point and viscosity of the binder at lower temperatures compared to conventional ambient pressure distillation, thus resulting in better plant reliability. The lower distillation temperatures used in the vacuum distillation process avoid decomposition reactions such as mesophase and coke formation that result in plant fouling and regular shutdowns.

[0059] Additionally, the process of the present invention can result in pitch products of high quality and reliability that exhibit sufficiently high coking values ​​and low 16EPA PAH content at low viscosity for use in the manufacture of electrodes used in steel and aluminum production as well as Soderberg pastes used in aluminum production. The 16EPA PAH levels of the resulting binders are lower than pure coal tar derived products, thus creating a more environmentally friendly material.

[0060] Petroleum-derived distillation bottoms and coal tar-derived distillation bottoms can be blended in a ratio of 20:80 to 70:30, preferably 30:70 to 60:40, more preferably 40:60 to 50:50. This, combined with an optimum viscosity that favorably affects the binding performance of the pitch product, can result in a replacement of coal tar pitch binder with similar or lower toluene insolubles, similar or lower beta-resin content, and similar or lower secondary quinoline insolubles content (mesophase formation).

[0061] The process according to the invention, and more specifically the blend ratio, can result in a Mettler softening point of 110-140° C., a quinoline insoluble range of 2-12%, preferably 2-8%, a beta-resin content of 13-25%, and a coke yield (value) as measured by the Alcan method of at least 45%.

[0062] In addition, the process of the present invention, and more specifically the blending ratio, can result in a blend having a coking value (alkane) that is at least 0.5 wt.% greater than the calculated weighted average coking value of each of the petroleum-derived distillation residue and the coal tar-derived distillation residue.

[0063] In a sixth aspect of the present invention, there is provided a process for manufacturing a graphite electrode or carbon anode, comprising a process for producing a pitch product as described throughout this document. EXAMPLES

[0064]

Table 1

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Table 2

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Table 3

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Table 4

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Table 5

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Table 6

Claims

1. 1. A pitch product comprising a blend of a petroleum derived distillation residue and a coal tar derived distillation residue in a ratio of from 20:80 to 70:30 by weight, said pitch product being further characterized by an asphaltene content of at least 84% as measured by clay-gel absorption chromatography in accordance with ASTM D2007.

2. The pitch product described in claim 1, having thixotropic behavior with a high viscosity recovery after 60 seconds of at least 20% when measured in accordance with DIN 91143-2.

3. 2. The pitch product of claim 1 having an asphaltene content of at least 84% (as measured by clay-gel absorption chromatography according to ASTM D2007) and a high viscosity recovery after 60 seconds of at least 25%.

4. 2. The pitch product of claim 1 having a resin content of less than 10% as measured by clay-gel absorption chromatography according to ASTM D2007.

5. 2. The pitch product of claim 1 having a B(a)P content of less than 8500 ppm in accordance with the U.S. Environmental Protection Agency and / or a 16 EPA-PAH Sum of less than 7% (m / m).

6. 2. The pitch product of claim 1 having a coke yield of at least 45% alkane as measured in accordance with ASTM D4715.

7. 2. The pitch product of claim 1 having a flash point of 200 to 270°C.

8. 8. The pitch product of claim 7 having a flash point of 220 to 245°C.

9. 2. The pitch product of claim 1 having a Mettler softening point of 110 to 150°C.

10. 2. The pitch product of claim 1 which is a blend of petroleum derived distillation residues and coal tar derived distillation residues.

11. 2. The pitch product of claim 1 having a coke yield of at least 45% alkane, a Mettler softening point of 110-140° C., a quinoline insoluble range of 2-12%, and a beta-resin content of 13-25%.

12. The pitch product of claim 1, having an Alcant Coking Value at least 0.5 wt. % greater than the calculated weighted average coking values ​​of said petroleum-derived distillation bottoms and coal tar-derived distillation bottoms, respectively, as measured in accordance with ASTM D4715.

13. 2. The pitch product of claim 1, wherein the petroleum derived distillation residue is characterized by an asphaltene content of at least 80% as measured by clay-gel absorption chromatography according to ASTM D2007, and a softening point of at least 110° Mettler.

14. 2. The pitch product of claim 1, wherein the petroleum derived distillation residue is derived from a feedstock produced by pyrolysis of a petroleum stream, the feedstock comprising at least 30 wt.% asphaltene content, less than 10% saturates, and less than 40% resin content as measured by clay-gel absorption chromatography in accordance with ASTM D2007.

15. Use of the pitch product according to any one of claims 1 to 14 in the manufacture of carbon-based formed articles.

16. 15. Use of a pitch binder comprising the pitch product according to any one of claims 1 to 14 in the manufacture of graphite electrodes for electric arc furnaces and carbon anodes and Soderberg pastes for aluminium production.

17. 17. A graphite electrode comprising the converted pitch binder of claim 16.

18. A carbon anode comprising the converted pitch binder of claim 16.

19. A process for obtaining a pitch product according to any one of claims 1 to 14, comprising a petroleum-derived distillation residue and a coal tar-derived distillation residue, the process comprising: a petroleum vacuum distillation process step for obtaining the petroleum-derived distillation residue; and blending the petroleum-derived distillation residue and the coal tar-derived distillation residue in a mixing ratio of 20:80 to 70:30 by weight.

20. 20. The process of claim 19, wherein the petroleum derived distillation residue is characterized by an asphaltene content of at least 80% as measured by clay-gel absorption chromatography according to ASTM D2007, and a softening point of at least 110° Mettler.

21. 20. The process of claim 19, wherein the petroleum derived distillation residue is derived from a feedstock produced by pyrolysis of a petroleum stream, the feedstock comprising at least 30 wt.% asphaltene content, less than 10% saturates, and less than 40% resin content (as measured by clay-gel absorption chromatography in accordance with ASTM D2007).

22. 20. The process of claim 19, further comprising a coal tar vacuum distillation process step to obtain a coal tar derived distillation residue, and blending the petroleum derived distillation residue with the coal tar derived distillation residue.

23. 20. The process of claim 19, wherein the distillation process step is carried out at a vacuum level of 50-250 mbar and a temperature of 280-370°C.

24. A process for producing a graphite electrode or a carbon anode comprising the process according to any one of claims 19 to 23.

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