METHOD FOR MAKING A CALCINED NEEDLE COKE

TECHNICAL FIELD
[0001] The present disclosure, in general, relates to coal tar pitch based needle coke manufacturing process, in which pitch is derived from coal tar produced as a by-product in the coke plant. In particular, the present disclosure related to a method for making calcined needle coke.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Graphite electrodes are used in the steel industry to melt the metals in electric arc furnaces. The heat needed to melt the substrate metal is generated by passing a current through electrodes and forming an arc between electrodes and the metal. Typically, a current of more than 1,00,000 amperes is often used for forming the arc.
[0004] Electrodes are generally manufactured using needle coke and binder pitch. Needle coke has anisotropic microstructure. For creating graphite electrodes, the needle coke must have low electrical resistivity and low coefficient of thermal expansion (CTE). The CTE is determined by blending the needle coke and the binder pitch, extruding the blend to form an electrode and graphitizing the electrode to 3000 °C. The CTE value is then measured on graphitized electrodes.
[0005] Needle coke properties are determined by the raw materials and operating parameters of the coking process. Needle coke grade depends on CTE value, premium grade needle coke has CTE less than 0.4*10-6/ °C. The CTE value of the graphitized electrode is measured in the longitudinal direction using either a dilatometer or the capacitance method. The extruded electrodes are graphitized at a very high temperature of about 3000 °C for removing volatizing impurities and getting the graphitic crystalline structure. The volatile impurities can expand the electrodes resulting in electrode breakage.
[0006] For needle coke, the raw materials should be highly aromatic, provide good coking yield, and be very low in ash and infusible solids. In general, petroleum-derived feedstocks and coal tar derived pitch are used for needle coke production. Typically, fluid catalytic cracking (FCC) decant oil is used as a raw material for petroleum needle coke. The problem of FCC decant oil is the presence of FCC catalyst as ash particles. The ash increases the CTE value, thereby necessitating the removal of the catalyst from the FCC decant oil.
[0007] A second approach is to use coal-based raw materials for needle coke production. In this process, coal tar is derived from the coking process used to produce metallurgical coke from coal. The coal tar is obtained as overhead product and contains infusible carbonaceous solids (called quinolone insoluble ‘QI’) formed by gas-based carbonization and because of coal carryover. Despite these solids, coal tar is the desirable starting material for producing needle coke because coal tar is highly aromatic and has high carbon yield ranging from 20% to about 35% as determined by Modified Conradson Carbon (MCC) test. However, to obtain the low CTE coke from coal tar, QI must be removed.
[0008] Approaches in which solids have been removed from coal tar for the preparation of needle coke include Japanese Patent No. JP19850263700, Misao et al., in which a process is described to produce needle coke by removing QI from the coal tar and or coal tar pitch to produce QI free coal tar and pitch which is used in a delayed coker.
[0009] US Patent No. 8,007,658 B2, Miller et al., describes a process for producing low CTE needle coke from coal tar. In this patent (Miller), the QI from coal tar is removed, then passed through an adsorption zone of activated carbon to reduce the nitrogen content of the coal tar. The coking of the coal tar to produce coke is done at a temperature of 450 °C to 500 °C and a pressure of about ambient to 100 psig. The coke produced is calcined at a temperature of 1400 °C. The CTE of the coke produced is 2 X 10-7/°C.
[0010] Further, in German Patent No. DE3347352, Masayoshi et al., a process is described for producing needle coke in which hydrogenation is used to remove solid components from the coal tar.
[0011] Unfortunately, coal tar-based needle coke produced by the prior art includes steps which are costly and having a low yield of coke. Furthermore, many prior art processes require significant energy input as high temperatures are often necessary for the catalyst to remove a substantial amount of solids from the raw materials. In some of the prior art they have used coal tar distillate oils having more than 280 °C boiling point but their yield is low.
[0012] Therefore, it is desired to have a process for producing needle coke for low CTE graphite electrodes, which does not require the use of a petroleum-derived feedstock. Indeed, it would be desirable to have a process for converting coal tar pitch and coal tar distillate mixture into needle coke which results in a high coke yield.

OBJECTS OF THE DISCLOSURE
[0013] In view of the foregoing limitations inherent in the state of the art, some of the objects of the present disclosure, which at least one embodiment herein satisfy, are listed hereinbelow.
[0014] It is a general object of the present disclosure to provide a method for converting the coal tar pitch and coal tar distillate into needle coke.
[0015] It is an object of the present disclosure to provide a method for creating a raw material suitable for graphite electrodes.
[0016] It is another object of the present disclosure to provide a method in which coke yield is increased by using coal tar distillate having a relatively high boiling point.
[0017] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.

SUMMARY
[0018] This summary is provided to introduce concepts related to a method for making calcined needle coke. The concepts are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
[0019] In an embodiment, the present disclosure relates to a method for making calcined needle coke. The method includes carbonizing a needle coke precursor comprising a coal tar pitch, a coal tar distillate and aluminum trichloride (AlCl3) at 230-260 °C to form a polymerized pitch, where the coal tar distillate having boiling point greater than 280 °C; washing the polymerized pitch with HCl to form a modified pitch; carbonizing the modified pitch at 450-500 °C; calcining the modified pitch at 950-1050 °C to get the calcined needle coke.
[0020] In an aspect, the coal tar pitch and the coal tar distillate are taken in 7:3 ratio.
[0021] In an aspect, the AlCl3 is taken as approx. 20 wt.% of the coal tar pitch and the distillate mixture.
[0022] In an aspect, the heating rate of the polymerized pitch is 3 °C/min to 230-260 °C.
[0023] In an aspect, the polymerized pitch is kept for 6 hours between 230-260 °C.
[0024] In an aspect, the modified pitch is carbonized to 450-500 °C at 3 °C/min heating rate.
[0025] In an aspect, the modified pitch is carbonized for 18 hours in an inert atmosphere to form a green coke.
[0026] In an aspect, the green coke is calcined at 5 °C/min to at least 1000 °C.
[0027] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
[0028] The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and methods that are consistent with the subject matter as claimed herein, wherein:
[0029] FIG. 1 illustrates a process flow diagram for the production of a needle coke in accordance with an embodiment of the present disclosure;
[0030] FIG. 2 illustrates an SEM image of the coke produced from carbonization of CTP & AO mixture with AlCl3;
[0031] FIG. 3 illustrates the SEM image of the coke produced from carbonization of CTP & AO mixture;
[0032] FIG. 4 illustrates the SEM image of the coke produced from carbonization of CTP; and
[0033] FIG. 5 illustrates a method for making a calcined needle coke in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION
[0034] The detailed description of various exemplary embodiments of the disclosure is described herein with reference to the accompanying drawings. It should be noted that the embodiments are described herein in such details as to clearly communicate the disclosure. However, the amount of details provided herein is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0035] It is also to be understood that various arrangements may be devised that, although not explicitly described or shown herein, embody the principles of the present disclosure. Moreover, all statements herein reciting principles, aspects, and embodiments of the present disclosure, as well as specific examples, are intended to encompass equivalents thereof.
[0036] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises”, “comprising”, “includes”, “consisting” and/or “including” when used herein, specify the presence of stated features, integers, steps, operations, elements and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components and/or groups thereof.
[0037] It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may, in fact, be executed concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved.
[0038] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, e.g., those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.
[0039] Embodiments explained herein pertain to a method of the creation of low coefficient of thermal expansion CTE needle coke, using Coal Tar Pitch (CTP) and coal tar distillate as raw materials obtained through the distillation of the coal tar. Coal tar is derived from the destructive distillation of coal in a coking process used to produce metallurgical coke from coal. The coal tar thus produced is distilled in the distillation column which produces at least two products, the overhead product is coal tar distillate and the bottom product is coal tar pitch. The low boiling point distillate having 1-2 aromatic ring are generally separated to provide the chemicals such as benzene and naphthalene, while higher boiling distillate has limited use. The higher boiling point distillate contains a very high proportion of polycondensed aromatics. But the higher boiling distillate would require very high pressure to convert them into coke and it would also give very less yield if used directly for needle coke production. Therefore, in the method proposed herein, the higher boiling distillate is mixed along with coal tar pitch in the hot condition and then AlCl3 is added in the mixture. This mixture is carbonized slowly to get the modified pitch. The modified pitch is then washed with HCl to remove the remaining AlCl3. This pitch is then carbonized to get the green coke and then calcined to get the calcined needle coke, which is used for electrode manufacturing.
[0040] Coal tar is distilled in a distillation column to get the coal tar distillate as top product and coal tar pitch as bottom product. The coal tar is heated to a temperature of 400 °C and kept at that temperature for 30 minutes. Coal tar pitch is generated as the bottom product which is left out after distilling off the coal tar distillate. The coal tar distillate is taken in a distillation column and heated to a temperature of 150 °C to distill out the light oils, to a temperature of 220 °C to separate the naphthalene oils, to a temperature of 280 °C to separate the creosote oil. The oils bottom product of coal tar distillate at about 280 °C is anthracene oils (AO), which is used for needle coke making.
[0041] As per FIG. 1, the coal tar pitch (CTP) and anthracene oil (AO) is mixed in a different ratio about 1:1 to about 9:1, more preferably about 7:3 w/w respectively in a vessel 101. For ensuring proper mixing, the CTP is first heated to a temperature of about 80 °C to melt it and convert it to the liquid phase. The AO is then added to it and stirred continuously for a period of 10-15 minutes. Aluminum trichloride (AlCl3) in the fraction of 10-25 wt% of CTP and AO mixture, more preferably 20 wt% of that mixture is added to the hot mixture and stirred continuously for 15 minutes to ensure proper mixing to form a polymerized pitch. The mixture is then taken in an alumina boat and is heated in a tube furnace (carbonization reactor 102) at a rate of 3 °C/min in an inert atmosphere to a temperature of 250 °C for a period of 6 hours to form modified pitch. The modified pitch is then acid washed in a vessel 103 to remove the alumina which is formed during the reaction. The modified pitch and HydroChloride (HCl) is taken in 1:5 ratio. The acid solution is prepared by mixing 5% vol. of HCl in water. The pitch is then washed with water twice to neutralize it. The modified pitch is dried in a hot air oven to a temperature of 80 °C for a period of 4 hours.
[0042] The modified pitch is crushed and taken in an alumina boat. It is heated in the tube furnace (104) at a rate of about 1 °C/min to 5 °C/min, more preferably to about 3 °C/min to a temperature of about 450 °C to about 525 °C for a period of 6 – 18 hours to produce green coke. The green coke produced is heated at a rate of 5 °C/min to a temperature about 1000 °C or more for a period of 1 hour to produce calcined needle coke in the vessel (105). The green and calcined coke is characterized by proximate, ultimate analysis, SEM, density etc. Properties of the pitch are presented in Table 1:

Ash, % VM+IM, % FC, % C, % H, % N, % S, % O, % QI, % TI, % Density, g/cc
0.3 49.8 49.9 92.1 3.98 1.29 0.81 1.82 8.34 12.23 1.19
Table 1: Properties of coal tar pitch

[0043] Then, the calcined coke is mixed with the pitch in a ratio of 4:1 respectively and extruded in the form of a pencil of diameter 10 mm and length 40 mm. The pencil is carbonized in the tube furnace where it is heated at a rate of 3 °C/min to a temperature of 1000 °C for a period of 1 hour. The carbonized pencil is used for the measurement of coefficient of thermal expansion (CTE) in a dilatometer where it is heated to a temperature of 1000 °C and the curve between the relative change in length vs temperature is obtained. The CTE value is calculated from the slope of the curve after the correction of the baseline. Properties of calcined coke for three different cases are presented in Table 2:
Calcined coke from the pitch Calcined coke from a mixture of pitch & AO Calcined coke from a mixture of pitch, AO & AlCl3
C, % 98.12 98.4 98.18
Density, g/cc 1.83 1.85 1.89
Ash, % 1.2 1.14 1.68
Table 2: Properties of calcined coke for three different cases
[0044] The graphite electrode has a coefficient of thermal expansion ranges from 0.0778 ppm/ °C to 0.0898 ppm/ °C. The calcined needle coke yield ranges from 51% to 58% of the modified pitch.
EXAMPLE 1
[0045] A coal tar distillate with an initial boiling point of 280 °C along with coal tar pitch produced by distilling coal tar at 420 °C for 30 minutes is used as feedstock for needle coke production. The coal tar pitch and the coal tar distillate is mixed in 7:3 w/w ratio and heated to about 80 °C and stirred for about 15 minutes and then AlCl3 (20wt.% of mixture) is added and stirred for about 15 minutes. The mixed pitch is carbonized at about 250 °C in an inert atmosphere at 3 °C/min for about 6 hours in a tube furnace to produce modified pitch.
[0046] The modified pitch is heated at a rate of 3oC/min to 450 °C and held at that temperature for 18 hours to produce the green coke. A very high yield of 60% of green coke is obtained. The green coke is then crushed and calcined by heating in an inert atmosphere in a tube furnace at a heating rate of 5 °C/min to a temperature about 1000 °C and then held at that temperature for 1 hour. The yield of calcined coke is 97% so overall yield based on the original mixture is 58%. The calcined coke yield is 7% higher in the case of CTP and coal tar distillate mixture in comparison to coke produced from only CTP. The SEM results of the coke formed at these conditions show prominent needle-like shape in its structures.
[0047] The calcined coke is mixed with the pitch in a ratio of 4:1 respectively and extruded in the form of a pencil of diameter 10 mm and length 40 mm. The pencil is carbonized in the tube furnace where it is heated at a rate of 3 °C/min to a temperature of 1000 °C for a period of 1 hour. The carbonized pencil is used for the measurement of coefficient of thermal expansion (CTE) in a dilatometer where it is heated to a temperature of 1000 °C. The CTE value was obtained at 8.98*10-8/°C. The SEM image show needle-like structure which is presented in FIG. 2.
EXAMPLE 2
[0048] The mixture of coal tar pitch and anthracene oil is mixed in 7:3 ratio but AlCl3 was not added in this case. The mixture of pitch and anthracene is heated in the tube furnace at a rate of 3 °C/min to a temperature of 450 °C for 18 hours. The yield of the coke from the mixture is 36%. The green coke is heated in a tube furnace to a temperature of 1000 °C at a rate of 5 °C/min for a period of 1 hour to carry out the calcination. The yield of the calcined coke from the mixture is 33.7 %. The calcined needle coke hence formed is crushed and mixed with a pitch in the ratio of 4:1 respectively. The mixture is extruded in the form of a pencil. The pencil is carbonized in the tube furnace at a rate of 3 °C/min to a temperature of 1000 °C for 1 hour. The CTE value was reported as 7.8*10-8 /°C. The SEM image is presented in FIG. 3.
EXAMPLE 3
[0049] To illustrate the advantages of the inventive process, a coal tar pitch is coke in the same apparatus using the same coking conditions. The CTP is heated in the tube furnace at a rate of 3 °C/min to a temperature of 450 °C for 18 hours. The yield of the green coke from the pitch is 52%. The green coke is heated in a tube furnace to a temperature of 1000 °C at a rate of 5 °C/min for a period of 1 hour to carry out the calcination. The yield of the calcined coke from the pitch is 51%. The coke yield is less in this case. The calcined needle coke hence formed is crushed and mixed with a pitch in the ratio of 4:1 respectively. The mixture is extruded in the form of a pencil. The pencil is carbonized in the tube furnace at a rate of 3 °C/min to a temperature of 1000 °C for 1 hour. The CTE value was reported as 7.78*10-8 / °C. The SEM image is presented in FIG. 4.
[0050] FIG. 5 illustrates example method 500 of making calcined needle coke. The order in which the method 500 is described is not intended to be construed as a limitation, and any number of the described method blocks may be combined in any order to implement the method 500, or an alternative method.
[0051] At block 502, the method includes carbonizing a needle coke precursor comprising a coal tar pitch, a coal tar distillate and aluminum trichloride (AlCl3) at 230-260 oC to form a polymerized pitch. In an aspect, the coal tar distillate having boiling point greater than 280 oC.
[0052] At block 504, the method includes washing the polymerized pitch with Hydrochloride (HCl) to form a modified pitch.
[0053] At block 506, the method includes carbonizing the modified pitch at 450-500 °C.
[0054] At block 508, the method includes calcining the modified pitch at 950-1050 °C to get the calcined needle coke.
[0055] Thus, with such proposed method of making a calcined needle coke, with less amount of coal tar pitch, more amount of calcined needle coke is obtained, as the coal tar distillate is also utilized in making of the calcined needle coke.
[0056] Also, to increase the needle coke yield, oils derived from coal tar having more than 280 °C boiling point is also added in the coal tar pitch and carbonized to get the green coke and then calcined to get the calcined coke. The calcined coke is starting material for graphite electrodes which exhibits a reduced coefficient of thermal expansion.
[0057] In addition, the inventive method proposed herein for producing needle coke provides a low CTE and high needle coke yield from the mixture of coal tar pitch and coal tar distillate without the use of hydrogen, thermal energy, and extremely high pressure. The initial boiling point of the coal tar distillate used in the inventive process is greater than about 280 °C. In addition, it has been found highly advantageous that the inventive process produces a relatively high yield of coke from the mixture of coal tar pitch and coal tar distillate.
[0058] Coal tar is distilled at about 400 to 420 °C to get the coal tar distillate and pitch. The coal tar pitch and coal tar distillate are mixed and heated about 80-90 °C and then aluminum trichloride (AlCl3) is mixed to increase the polymerization of the coal tar distillate with a pitch. The AlCl3 has a molecular weight of 133.34 g/mol and 98% purity. This mixture is then heated to about 250 °C for about 6 hours in an inert atmosphere at 3 °C/min or lower heating rate in a tube furnace to produce modified pitch. This modified pitch is then washed with hydrochloric acid (HCl) to remove the AlCl3 from the pitch and thus producing a pitch with a low amount of AlCl3. This pitch is then carbonized at about 450 °C to 500 °C for 12-18 hours at a heating rate of 3 °C/min in an inert atmosphere in a tube furnace to produce green coke. The green coke is then characterized by density, SEM, proximate and ultimate analysis. The green coke is further calcined at about 1000 °C or more in an inert atmosphere for 1-2 hours at a heating rate of 5 oC/min to produce calcined needle coke. The calcined coke is characterized by density, SEM, proximate, ultimate and CTE.
[0059] Furthermore, those skilled in the art can appreciate that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[0060] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[0061] While the foregoing describes various embodiments of the present disclosure, other and further embodiments of the present disclosure may be devised without departing from the basic scope thereof. The scope of the present disclosure is determined by the claims that follow. The present disclosure is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

WE CLAIM:

1. A method of making a calcined needle coke, comprising:
carbonizing a needle coke precursor comprising a coal tar pitch, a coal tar distillate and aluminum trichloride (AlCl3) at 230-260 °C to form a polymerized pitch, wherein the coal tar distillate having boiling point greater than 280 °C;
washing the polymerized pitch with Hydrochloride (HCl) to form a modified pitch;
carbonizing the modified pitch at 450-500 °C; and
calcining the modified pitch at 950-1050 °C to get the calcined needle coke.
2. The method as claimed in claim 1, wherein the coal tar pitch and the coal tar distillate are taken in 7:3 ratio.
3. The method as claimed in claim 1, wherein the AlCl3 is taken as approx. 20 wt.% of the coal tar pitch and the distillate mixture.
4. The method as claimed in claim 1, wherein the heating rate of the polymerized pitch is 3 °C/min to 230-260 °C.
5. The method as claimed in claim 1, wherein the polymerized pitch is kept for 6 hours between 230-260 °C.
6. The method as claimed in claim 1, wherein the modified pitch is carbonized to 450-500 °C at 3 °C/min heating rate.
7. The method as claimed in claim 1, wherein the modified pitch is carbonized for 18 hours in an inert atmosphere to form a green coke.
8. The method as claimed in claim 7, wherein the green coke is calcined at 5 °C/min to at least 1000 °C.