TECHNICAL FIELD

[001] The present invention generally relates to a pitch product comprising a petroleum- derived distillation residue and a coal tar-derived distillation residue, more specifically a pitch product for use in the manufacturing of graphite electrodes for electric arc furnaces and carbon anodes for aluminum production.

[002] In addition, the present invention relates to a process for producing such pitch product.

BACKGROUND

Graphite electrodes for steel manufacturing and prebaked carbon anodes for the aluminum industry are produced by hot-mixing calcined coke and a hydrocarbon carbon precursor and forming the mixture into the green electrode shapes that are carbonized in a subsequent baking process. The hydrocarbon binder material provides enough mechanic strength to the unbaked (green) electrode shape and converts to carbon during the baking process. The resulting carbon semi-graphitized electrodes meet the requirements as anode in the electrolysis cell used in the production of aluminum. Electrodes used in the electric arc furnaces for steel production are further impregnated with a hydrocarbon impregnation pitch, carbonized and subsequently graphitized. For application as electrodes in Soederberg cells a paste is produced by hot mixing of the dry aggregate (calcined coke, anthracite, graphite etc.) and hydrocarbon binder, formed as a briquets or other preformed shapes and transferred into the Soederberg cell where it is subsequently carbonized in the electrolysis cell itself.

[003] Traditionally, the hydrocarbon pitch binder material in the manufacturing of these graphite electrodes for electric arc furnaces and carbon anodes for aluminum production (including prebaked anodes and Soederberg anodes), is coal tar pitch, because it meets the mechanical requirements in the green stage and is converted into electrically conductive carbon during the carbonization process at very high coke yield, thereby avoiding a high porosity in the resulting article due to fewer volatiles formed during the carbonization process. Also, the hydrocarbon impregnation pitch used for impregnation of graphite electrodes is typically based on coal tar distillation products. [004] As by-product of the production of metallurgical coke used in the steel production, coal tar has always been sufficiently available. Recently however, due to the lower demand for virgin iron, less metallurgical coke is produced and hence less coal tar is available.

[005] Another drawback of coal tar pitch binder is the rather high amount of (ca. 10 000 ppm in the typical viscosity range) of benzo(a)pyrene, which is classified as carcinogenic. On top of the B(a)P content other polyaromatic hydrocarbons are considered relevant as hazardous to health and environment.

[006] In an attempt to produce an alternative pitch from petroleum sources which contain less benzo[a]pyrene than coal tar, petroleum-derived pitch has been considered.

[007] However, petroleum-derived pitch does not attain the same quality parameters as coal tar pitch if used as a pure binder material in the electrode production process. A first drawback is that it has a lower coke yield than coal tar pitch, and secondly, it does not have any primary quinoline insoluble constituents. These primary quinoline insoluble constituents are considered as beneficial to the anode quality in aluminum production and are contained in conventional coal tar-based pitch.

[008] A further drawback of known petroleum-derived pitch materials is typically the softening points below 100°C being too low for use in hydrocarbon pitch binder material in the manufacturing of graphite electrodes in which the softening points target is 110-130°C. These low softening points also limit the use of petroleum pitches in blends with coal tar pitch (e.g. US 5,746,906).

[009] In addition, typical low flashpoints below 200°C of existing petroleum-based pitches give rise to safety concerns in the electrode fabrication process that can contain hot mixing processes at temperatures up to 200°C.

[0010] In summary, none of the attempts to produce suitable petroleum pitch on an industrial scale have provided an alternative for coal tar pitch binder able to reliably serve the aluminum industry with high product volumes.

[0011] More specifically, neither pure petroleum-derived pitch, neither blends of petroleum pitch with coal tar pitch with substantially amount of petroleum pitch above 10 %, have so far not been able to meet the necessary requirements for use as hydrocarbon pitch binder material in the manufacturing of carbon and graphite electrodes.

[0012] Given the above, it is a general objective of the present invention to provide an alternative for coal tar pitch binder that allows an increased security of supply and meets the necessary requirements for use as hydrocarbon pitch binder material in the manufacturing of carbon and graphite electrodes.

[0013] Another objective of the present invention is to provide an alternative pitch binder resulting in similar coke values, and similar processing and performance of graphite electrodes, prebaked anodes and pastes used in Soederberg technology.

[0014] A further objective of the present invention is to provide an alternative for coal tar pitch binder that is more environmentally friendly.

SUMMARY

[0015] In a first aspect in accordance with the present invention, a pitch product is provided comprising a blend of petroleum-derived distillation residue and a coal tar- derived distillation residue in a mixing ratio between 20:80 and 70:30 by weight, said pitch product characterized by a concentration of at least 84% asphaltenes (SARA as measured by Clay-Gel Absorption Chromatographic Method according to ASTM D2007).

[0016] In a second aspect of the present invention, a pitch binder comprising the pitch product as described throughout this text is provided, said pitch binder for use in the manufacturing of any type of carbon-based formed shapes and in particular for use in manufacturing of graphite electrodes for electric arc furnaces, and carbon anodes and Soederberg paste for aluminum production.

[0017] In a third aspect in accordance with the present invention, a graphite electrode is provided comprising said pitch binder.

[0018] In a fourth aspect in accordance with the present invention, a carbon anode is provided comprising said pitch binder. [0019] In a fifth aspect in accordance with the present invention, a process for producing a pitch product is provided, said pitch product comprising a petroleum-derived distillation residue and a coal tar-derived distillation residue, said process comprising a petroleum vacuum distillation process step for obtaining said petroleum-derived distillation residue, and blending the petroleum-derived distillation residue and the coal tar-derived distillation residue.

[0020] In a sixth aspect according to the present invention, a process for manufacturing a graphite electrode or a carbon anode is provided comprising said process for producing a pitch product.

DETAILED DESCRIPTION

[0021 ] The pitch product as described throughout this text is provided as an alternative for coal tar pitch binder, meeting the requirements of the aluminum industry and/or graphite industry and having the advantages of low benzo(a)pyrene content and ample availability.

[0022] In a first aspect in accordance with the present invention, a pitch product is provided comprising petroleum-derived distillation residue and a coal tar-derived distillation residue, said pitch product characterized by a concentration of at least 84 % asphaltenes or at least 86 %, or at least 90%, as measured by the SARA method Clay-Gel Absorption Chromatographic Method according to ASTM D2007.

[0023] Further the pitch product may have a resin content below 10% as measured by Clay-Gel Absorption Chromatographic Method according to ASTM D2007.

[0024] The inventors surprisingly found that a pitch product comprising petroleum- derived distillation residue and a coal tar-derived distillation residue, and containing an amount of asphaltenes at a similar level compared to the known coal tar pitch binders as measured by the SARA method Clay-Gel Absorption Chromatographic Method according to ASTM D2007 shows a coke value of at least 45% ALCAN at the target softening point (e.g. 110-150°C Mettler). A concentration of at least 84 % asphaltenes, or at least 86 %, or at least 90% may result in respectively increasing coke values. The pitch product ensures a sufficient mechanical strength of the resulting green coke shapes, and low shrinkage during carbonization. The resulting carbonized shapes of bound coke show low porosity and air permeability and in addition a sufficient electrical conductivity.

[0025] As is well known, asphaltenes are solids which are insoluble in paraffinic solvents and have high melting points, and tend to form isotropic coke readily because of their highly aromatic ring structure and high molecular weight. For example, asphaltenes may be pentane or heptane insolubles (cfr eg. EP0072243B1 ).

[0026] SARA analysis is a commonly used method for measuring saturates, asphaltenes, resins, aromatics in heavy crude oil, distillates and feedstocks. SARA analysis methods other than Clay-Gel (ASTM D-2007) may be TLC/FID following IP-469, or IP-143 followed by preparative HPLC (IP-368). SARA analysis is being made available in the industry by for example Intertek or lactroscan.

[0027] Further, the amounts of asphaltenes as mentioned throughout this text may include loss as defined in ASTM D-2007.

[0028] In an embodiment in accordance with the present invention, the pitch product may have a thixotropic behavior with high viscosity recovery after 60 seconds of at least 20% (DIN91143-2), or at least 40%, or at least 60%, or up to 90%. Such thixotropic behavior indicates good processing and impregnation characteristics. Without being bound by any theory, the surprising high recovery rate could be due to a lower amount of solid constituents that disturb the intermolecular arrangement and interactions between the larger molecules and therefore the re-setting of the pitch structure after the shear force impact.

[0029] A high thixotropy of a liquid binder pitch is important because an important requirement for a pitch binder for electrode fabrication is the ability to process and to 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 and by that high shear forces are applied. Usually, this pumping and mixing of the pitch requires low viscosity of the liquid pitch. In contrast, the impregnation and pore filling of the compressed electrodes requires higher pitch viscosity to retain the binder pitch in the pores. The viscosity at high shear rate should be low whereas the viscosity at low shear rates should be high. In addition, a high built-up or recovery rate of the viscosity after applying and releasing of the shear energy is advantageous in the application.

[0030] More specifically, the pitch product of the present invention may have a concentration of at least 84% asphaltenes (SARA) 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%.

[0031] In an embodiment in accordance with the present invention, the pitch product may have a resin content below 10% (SARA), which may contribute to its high coke yield.

[0032] In another embodiment in accordance with the present invention, the pitch product may have a B(a)P content of less than 8500 ppm , or less than 7000, or less than 5000, or even less than 3000 ppm, and/or a 16 EPA-PAH Sum (Polycyclic Aromatic Hydrocarbons according to 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 16 EPA-PAH Sum results obviously in an improved environmental friendliness compared to pure coal tar derived pitch products. A low B(a)P containing pitch binder product can also be utilized with advantage in the manufacturing of electrodes for aluminum (prebaked anodes and Soederberg anodes) and electric arc furnaces.

[0033] In a further embodiment, the pitch product may have a coke yield of at least 45% Alcan, or at least 50% Alcan, or at least 55% Alcan at softening points between 110-150°C Mettler. As the pitch product is converted into carbon during the carbonization process, a sufficiently high coke yield allows avoiding a high porosity in the resulting shape due to fewer volatiles formed during the carbonization process. At the same time a high coke yield avoids a high shrinkage during baking of the green anode that would result in the risk of cracks as well as shapes being outside of the acceptable tolerances of the dimensions. Further, as the pitch product is in fact a carbon precursor, sufficiently high coke yield is critical to reach porosity targets of the baked anode/electrode.

[0034] In a further embodiment, the pitch product may have a flashpoint between 200 and 270°C, preferably between 220 and 245°C, allowing to process the pitch product according to safety requirements as may be required in hot mixing processes.

More specifically, the pitch product of the present invention may have a concentration of at least 84% asphaltenes or at least 86%, or even at least 90%, and a flashpoint between 200 and 270°C, preferably between 220 and 245°C.

[0035] In an embodiment of the present invention, the pitch product may have a softening point between 110 and 150°C Mettler, being the target range in manufacturing electrodes used in the steel and aluminum production as well as Soederberg pastes used for aluminum production.

[0036] In accordance with the present invention, the pitch product may be a blend of petroleum-derived distillation residue and coal tar-derived distillation residue. In an embodiment, said blend may have a mixing ratio between 20:80 and 70:30, preferably between 30:70 and 60:40, and even more preferably between 40:60 and 50:50. Such mixing ratio may result in softening points between 110 and 140°C Mettler, a quinoline insoluble range of 2-12 %, preferably of 2-8 %, a beta-resin content of 13-25 % and a coke yield (value) measured with the Alcan method of at least 45 %.

[0037] More specifically, the pitch product of the present invention may have a concentration of at least 84% asphaltenes or at least 86%, or even at least 90%, and a mixing ration between 30:70 and 60:40, and even more preferably between 40:60 and 50:50.

[0038] In a particular embodiment of the present invention, the pitch product may have a coke yield of at least 45 % Alcan, a softening point between 110 and 140 °C Mettler, a quinoline insolubles range of 2-12%, and a beta-resin content of 13-25%. More preferably, the quinoline insolubles are in the range of 2-8 %. [0039] In an embodiment, the pitch product being a blend of petroleum-derived distillation residue and coal tar-derived pitch, can be optimized in respect to blending ratio to have a gain in coking value (Alcan) that is at least 0,5 wt.% above the respective calculated weighted mean coking values of the petroleum-derived distillation residue and coal tar-derived distillation residue.

[0040] In a preferred embodiment, the petroleum-derived distillation residue obtained by the petroleum vacuum distillation process step may be characterized by a concentration of at least 80% asphaltenes, or at least 84%, or at least 86% as measured by Clay-Gel Absorption Chromatographic Method according to ASTM D2007 and a softening point of at least 110° Mettler, or at least 120° Metller. Said petroleum-derived distillation residue may have a coke yield of at least 45% alcan.

[0041] In another embodiment, a pitch product according to the present invention may comprise a petroleum-derived distillation residue derived from raw materials produced by the pyrolysis of petroleum streams. Preferably said raw materials including at least 30 wt.% asphaltenes, less than 10 % saturates, and less than 40 % resins, as measured by Clay-Gel Absorption Chromatographic Method according to ASTM D2007. Raw materials with such composition allow a high yield of the said product.

[0042] In a second aspect of the present invention, a pitch binder comprising the pitch product as described throughout this text is provided, said pitch binder for use in the manufacturing of any type of carbon-based formed articles, such as for example forms, bricks, shapes, filter grids for electrode and refractory applications, and in particular for use in manufacturing of graphite electrodes for electric arc furnaces, and carbon anodes and Soederberg paste for aluminum production. The pitch binder as described throughout this text can also be used as hydrocarbon impregnation pitch.

[0043] In third aspect in accordance with the present invention, a graphite electrode is provided comprising said pitch binder. Further, the pitch binder may also be used in graphite electrode manufacturing as impregnation pitch. Such pitch binder and impregnation pitch may support the integrity of the carbon body by pore filling of the carbonized electrodes.

[0044] In a fourth aspect in accordance with the present invention, a carbon anode is provided comprising said pitch binder.

[0045] In a fifth aspect in accordance with the present invention, a process for producing a pitch product is provided, said pitch product comprising a petroleum- derived distillation residue and a coal tar-derived distillation residue, said process comprising a petroleum vacuum distillation process step for obtaining said petroleum-derived distillation residue.

[0046] In a further embodiment in accordance with the present invention, a process for producing the pitch product as described throughout this text is provided, comprising a petroleum vacuum distillation process step and a separate coal tar vacuum distillation process step for obtaining respectively a petroleum-derived distillation residue and coal tar-derived distillation residue, and blending the respective residues. The respective distillation residues are produced by separate vacuum distillation of coal tar and petroleum-derived feedstock and subsequently mixed in tailored compositions.

[0047] A benefit of a process in accordance with the present invention is that it may allow keeping the amount of asphaltenes measured by SARA at a similar level compared to the known coal tar pitch binders. In addition, other pitch properties may not be degraded compared to known coal tar pitch binders.

[0048] An additional benefit is that such process may result in that the coke value of the pitch product can be kept at a high level (e.g. at least 40 % ALCAN) at the target softening point (e.g. 110-150°C Mettler).

[0049] An additional benefit of a process in accordance with the present invention is the high thixotropic behavior and a high viscosity recovery after 60 seconds of at least 25% of the resulting product. [0050] In additional benefit of a process in accordance with the present invention is that in case of using raw materials produced by the pyrolysis of petroleum streams as petroleum-derived feedstock, a petroleum-derived distillation residue may be achieved that results in high final product yield. Preferably, these raw materials have a composition with an amount of at least 30 wt.% asphaltenes, less than 10% saturates, and less than 40% resins in the raw materials as measured by the SARA method Clay-Gel Absorption Chromatographic Method according to ASTM D2007.

[0051] In contrast to producing a pitch product with a process in accordance with the present invention, conventional coal tar pitch binder extended with petroleum-based pitches are produced by distillation at ambient pressure and higher temperatures, in some cases followed by air blowing, and either by mixing the coal tar and petroleum- based raw material before distillation or blending the components after distilling the single components first. Drawbacks of these products resulting from ambient pressure distillation are a high mesophase and toluene insoluble content due to the high processing temperatures leading to cracking and mesophase formation. Other drawbacks are the low coke yields, high volatile content, and high viscosities that deteriorate the impregnation and binder properties and processing of the pitch as well as low flash points giving rise to safety issues in the electrode fabrication processes.

[0052] Additional drawbacks of distillation at ambient pressure is that, as the distillation of petroleum tars is reactive, the temperatures being necessary for distillation at atmospheric pressure already initiate the conversion to solid carbon components in the heating chamber and columns, potentially leading to an excessive fouling rate during pitch production resulting in reliability issues of the plant.

[0053] In an embodiment of the present invention, the petroleum-derived distillation residue obtained from the petroleum vacuum distillation process step may be characterized by a concentration of at least 80% asphaltenes, or at least 84%, or at least 86% as measured by Clay-Gel Absorption Chromatographic Method according to ASTM D2007 and a softening point of at least 110° Mettler, or at least 120° Metller. Said petroleum-derived distillation residue may have a coke yield of at least 45% alcan. [0054] In an embodiment of the present invention, the distillation process steps are performed at vacuum levels between 10 and 400mbar, preferably 50 and 250 mbar, and at temperatures between 200 and 400°C, preferably between 280 and 370 °C.

[0055] A process in accordance with the present invention allows a strict control and prevention of potential mesophase formation for low secondary quinoline insoluble amounts in the pitch.

[0056] Further, separate distillation of coal tars and petroleum tars may result in more optimized product characteristics of the separate distillation residues, yielding in higher quality pitch product.

[0057] In addition, a process in accordance with the present invention gives a high level of reliability by reaching the required softening point and viscosity of the binder at lower temperatures compared to conventional ambient pressure distillation and hence leads to better plant reliability. The lower distillation temperatures used in the vacuum distillation process avoid degradation reactions like mesophase and coke formation, leading to fouling of the plant and regular shutdowns.

[0058] Further, the process of the present invention may result in a pitch product with high quality and reliability showing sufficiently high coking value and low 16 EPA PAH content at low viscosity for use in manufacturing electrodes used in the steel and aluminum production as well as Soederberg pastes used for aluminum production. 16 EPA PAH level of the resulting binder is lower than of the pure coal tar-derived products giving rise to more environmentally friendly materials.

[0059] The petroleum-derived distillation residue and coal tar-derived distillation residue may be blended in a mixing ratio between 20:80 and 70:30, or preferably between 30:70 and 60:40, and more preferably between 40:60 and 50:50. This may result in an alternative for 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) combined with an optimal viscosity which positively impacts the binding performance of the pitch product. [0060] The process in accordance with the present invention, and more specifically the mixing ratio may result in softening points between 110 and 140°C Mettler, a quinoline insoluble range of 2-12 %, preferably of 2-8 %, a beta-resin content of 13- 25 % and a coke yield (value) measured with the Alcan method of at least 45 %.

[0061] In addition, the process in accordance with the present invention, and more specifically the mixing ratio, may result in that the blend may have a coking value (Alcan) being at least 0,5 wt.% above the respective calculated weighted mean coking values of the petroleum-derived distillation residue and coal tar-derived distillation residue.

[0062] In a sixth aspect according to the present invention, a process for manufacturing a graphite electrode or a carbon anode is provided comprising the process for producing a pitch product as described throughout this text.

[0063] TABLE 1 : Below table illustrates a pitch product formulation in accordance with an embodiment of the present invention:

[0064] TABLE 2: Below table represents 3 specific examples of a pitch product in accordance with the present invention:

[0065] TABLE 3: below table represents a preferred embodiment of the present invention, showing two pitch products in accordance with the present invention containing the coal tar- derived distillation residue (CTP) and the petroleum-derived distillation residue (PP) as shown in a mixing ratio of 20:80 and 70:30.

[0066] TABLE 4: Below table provides an overview of analytical procedures of the product parameters as used in this text: