1. A graphite electrode and/or nipple comprising: 0.01 wt% to 1.00 wt% of carbon nanotubes.

2. A graphite electrode and/or nipple comprising: 96 wt% to 99.9% of graphite; 0.01 to 1.00 wt% of carbon nanotubes; and balance being conventional impurities.

3. The graphite electrode and/or nipple as claimed in claim 1 or 2, wherein the carbon nanotubes are in the range of 0.025 wt% - 0.75 wt%.

4. The graphite electrode and/or nipple as claimed in claim 1 or 2, wherein the carbon nanotubes are selected from the group consisting of single-walled CNT, multi-walled CNT and a mixture thereof.

5. A process for preparation of a graphite electrode and/or nipple comprising: mixing 0.01wt% to 1 wt% of CNT with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; and graphitizing the green rod at a temperature in the range of 2500oC to 3300oC.

6. The process as claimed in claim 5, wherein the carbon nanotubes are in the range of 0.025 wt% - 0.75 wt%.

7. The process as claimed in claim 5, wherein the carbon nanotubes are selected from the group consisting of single-walled CNT, multi-walled CNT and a mixture thereof.

8. The process as claimed in claim 5, wherein the coke is selected from calcined petroleum coke, pitch coke or a mixture thereof. 15

9. The process as claimed in claim 5, wherein the shaping of the paste is done by a process selected from the group consisting of extrusion, injection, vibro-pressing and molding.

10. The process as claimed in claim 5, wherein the green rod is baked at a temperature in the range of 650oC to 1100oC before graphitizing.

11. The process as claimed in claim 5, wherein the green rod is baked at a temperature in the range of 650oC to 1100oC to obtain a baked rod; and the baked rod is impregnated with molten pitch before graphitizing.

12. The process as claimed in claim 11, wherein before graphitizing the baked rod after being impregnated is rebaked at a temperature in the range of 650oC to 1100oC.

13. The process as claimed in claim 12, wherein the steps of impregnating the baked rod and rebaking the baked rod after being impregnated are repeated at least one time.

14. A graphite electrode and/or nipple prepared by the process comprising: mixing 0.01wt% to 1 wt% of CNT with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; and graphitizing the green rod at a temperature in the range of 2500oC to 3300oC.

15. The graphite electrode and/or nipple as claimed in claim 14, wherein the carbon nanotubes are in the range of 0.025 wt% to 0.75 wt%.

16. The graphite electrode and/or nipple as claimed in claim 14, wherein the carbon nanotubes are selected from the group consisting of single-walled CNT, multi-walled CNT and a mixture thereof. 16

17. The graphite electrode and/or nipple as claimed in claim 14, wherein the coke is selected from calcined petroleum coke, pitch coke or mixture thereof.

18. The graphite electrode and/or nipple as claimed in claim 14, wherein the shaping of the paste is done by a process selected from the group consisting of extrusion, injection, vibro-pressing and molding.

19. The graphite electrode and/or nipple as claimed in claim 14, wherein the green rod is baked at a temperature in the range of 650oC to 1100oC before graphitizing.

20. The graphite electrode and/or nipple as claimed in claim 14, wherein the green rod is baked at a temperature in the range of 650oC to 1100oC to obtain a baked rod; and the baked rod is impregnated with molten pitch before graphitizing.

21. The graphite electrode and/or nipple as claimed in claim 20, wherein before graphitizing the baked rod after being impregnated is rebaked at a temperature in the range of 650oC to 1100oC to obtain a rebaked rod.

22. The graphite electrode and/or nipple as claimed in claim 21, wherein the steps of impregnating the baked rod and rebaking the baked rod after being impregnated are repeated at least one time.

TECHNICAL FIELD
The present disclosure relates to graphite electrodes and/or nipples containing small quantity of carbon nanotubes. The present disclosure also relates to a process for preparation of graphite electrodes and/or nipples comprising small quantity of carbon nanotubes (CNTs). The present disclosure also relates to graphite electrodes and/or nipples prepared by the process of the present disclosure. The graphite electrodes and/or nipples of the present disclosure have significantly high flexural strength, low coefficient of thermal expansion and low electrical resistivity. The process of the present disclosure is economically significant to justify the use of CNT for improved levels of properties.
BACKGROUND
Graphite electrodes, connected by pins called graphite nipples, are globally used in the steel industry. High temperatures (>1300 deg C) are required to melt iron scraps in electric arc furnaces. This is achieved by passing a high-ampere electric current through graphite electrodes, which causes an arc discharge between the graphite electrode and the steel scrap. In typical furnace operations, a series of electrodes are attached together with the help of graphite nipples (such as connecting pins) to form a vertical column of a rigid electrode system. As the bottom electrode column is consumed in the process, the column is maintained in length by attaching additional electrodes from the top.
The graphite electrodes and/or nipples (also called “connecting pins” ) are made-up of graphite and utilized as an assembly in electric arc furnaces (EAF) to manufacture steel. EAFs are mostly using three columns of electrodes fed with electric current of high intensity (up to 100 kAmps, sometimes more) in order to produce an electric arc which melts the steel scrap introduced in the furnace. Electrodes are cylindrical rods with diameters ranging from 75 mm (sometimes less) to 800 mm (sometimes more), and lengths ranging from 1 meter (sometimes less) to 3 meters (sometimes more); the usual density is 1.65 to 1.75 kg per liter, and the corresponding weight ranges from 7.5 kg (or less) to 2.5 MT (or more).
A threaded socket is machined in each of both end faces of the cylinder; usually, a threaded “nipple” (most of the time bi-conical shape, but sometimes cylindrical) is screwed by the electrode manufacturer in one of the two sockets. The socket and the nipple are machined in such a way that only half of the nipple can penetrate in the socket; when utilized on EAF, the half part of the nipple which remains out of the socket is screwed into the socket
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of another electrode in order to mechanically assemble and electrically connect the two electrodes.
Most commonly used graphite electrodes have threaded sockets at both ends, and graphite nipples are small threaded plugs, that are screwed between two electrodes. Thus, a nipple-electrode joint can be seen as a male-female coupling or a plug-socket joint. Actually, one nipple is plugged into two electrode sockets from either end. The nipple essentially has a smaller cross-section than that of the electrodes it connects. And hence, it must have a higher mechanical strength than the rest of the column. Besides that, low electric resistivity (ER) and low coefficient of thermal expansion (CTE) are essential characteristics of a good electrode and nipple. The lower the electrical resistivity, the higher will be the current carrying capacity of the electrodes. Similarly, the lower the coefficient of thermal expansion, the lesser will be the expansion/contraction of the nipple upon heating/cooling, and it will hold the electrodes firmly.
The problem of providing good conducting electrodes and matching nipples for such electrodes has been a continuous concern of graphite electrode companies. As stated above, higher flexural strength (FS) and lower CTE are desirable features of a good electrode/nipple. But it is very difficult to improve both the properties simultaneously. Usually, any modification to increase FS also increases CTE, and any modification to decrease CTE also decreases FS. This invention provides a solution to increase FS and decrease CTE simultaneously, by the novel use of CNT in graphite electrode/ nipple recipe. The process disclosed here offers a bundle of additional benefits besides increasing FS and lowering CTE.
Several incidents can happen when using electrodes on EAF, which altogether reduces the productivity and the production of the furnace, and increase steel production cost. Failure of the Electrode -Nipple joint, accounts for a major percentage of such incidents.
The idea of manufacturing a composite material containing graphite as a matrix and carbon fibers (CF) as a reinforcing agent has been in practice for many years. Such materials are usually referred to as carbon-carbon composites. However, the use of CF in graphite electrodes for electric arc furnace (EAF) is very limited. As mentioned earlier, high flexural strength (FS) and low coefficient of thermal expansion (CTE) are desirable features of graphite nipples to perform well in EAF.
US Patent No. 4,998,709 discloses the use of 13% mesophase pitch-based CF in the conventional nipple process to reduce the CTE from 4.3 to 1.3 ×10-7/°C; but this decreased
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the rupture modulus from 1340 to 1156 psi, indicating lower FS. Besides that, it also increased the electrical resistivity, which is undesirable.
US Patent No. 6,280,663 discloses (i) the use of 3.2% mesophase pitch-based CF to increase the FS from 22.3 to 24.9 MPa (in the example-1 therein) and (ii) use of 1.5% CF to increase the flexural strength from 3011 to 3369 psi (in the example-2 therein). Although the patent reports significant decrease in CTE, it is important to note that CTE was measured in a very low temperature range (30 to 110 deg C), while the standard temperature range for CTE measurement for EAF electrodes is 25 to 525 deg C.
US Patent No. 6,916,435 presents a comparative performance of graphite electrode mixed with mesophase-pitch-based CF and PAN-based CF. According to this patent, 4% pitch-based CF was found most effective in flexural strength improvement (from 1511 to 1926 psi), while CTE also reduced (from 0.29 to 0.12 ×10-6/°C). However, considering the high cost of CF this may not be economically viable solution.
Indian Patent Application No. 982/DEL/2009 discloses the use of PAN-based CF in nipple mix. It was found that 1.1% CF addition improved the FS by 31.5%, and reduced the CTE by 4.4%. The process disclosed may still not be sufficiently viable economically as the amount of carbon fiber used is still quite high.
A research paper by Rajeev et al. (recently published in Applied Nanoscience, 2014) reports the strengthening of semicoke based carbon composites by using 1-10 wt% CNT. In this work, coal tar pitch was converted into semicoke and used as a matrix. CNT was mixed to the semicoke and molded into rectangular test pieces, which were finally graphitized to see FS enhancement. However, this formulation is not applicable to graphite electrode/nipple production. Graphite electrode for electric arc furnace cannot be made with semicoke alone, as such electrode will not provide the required flexural strength and other important characteristics.
SUMMARY
The graphite electrodes and/or nipples prepared by the process of the present disclosure overcome drawbacks like high porosity, low flexural strength, high coefficient of thermal expansion and high electrical resistivity of the electrodes and/or nipples as compared to the electrodes and/or nipples disclosed in the prior art documents.
An aspect of the present disclosure provides a graphite electrode and/or nipple
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comprising: 0.01 wt% - 1 wt% of carbon nanotubes.
Another aspect of the present disclosure provides a graphite electrode and/or nipple comprising of 96 – 99.99 wt% of graphite; 0.01 to 1 wt% of carbon nanotubes; and balance being conventional impurities.
Another aspect of the present disclosure provides a process for preparation of a graphite electrode and/or nipple comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; and graphitizing the green rod at a temperature in the range of 2500oC to 3300oC.
Another aspect of the present disclosure provides a graphite electrode and/or nipple prepared by the process comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; and graphitizing the green rod at a temperature in the range of 2500oC to 3300oC.
Additional aspect of the present disclosure provides a graphite electrode and/or nipple having low porosity, low electrical resistivity (ER), significantly high flexural strength (FS), and low coefficient of thermal expansion (CTE), by the addition of an extremely low amount of CNT.
These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the subject matter, nor is it intended to be used to limit the scope of the subject matter.
DETAILED DESCRIPTION
The present disclosure provides a graphite electrode/nipple comprising 0.01 wt% to 1 wt% carbon nanotubes.
The present disclosure also provides a graphite electrode and/or nipple comprising: 96 wt% to 99.9 wt% of graphite; 0.01 wt% to 0.75 wt% of carbon nanotubes; and balance being conventional impurities.
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The conventional impurities may be present in the graphite electrode and/or nipple, the non-limiting examples of which may be iron oxide, silicon oxide, oxides of alkaline earth metals like magnesium, strontium and calcium etc., oxides of transition metal or the respective metals or mixtures thereof. The impurities will depend upon the nature, composition and origin of the coke used for the preparation of the graphite electrode and/or nipples of the present disclosure.
The present disclosure further provides a process for preparation of a graphite electrode and/or nipple comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; and graphitizing the green rod at a temperature in the range of 2500oC to 3300oC.
An embodiment of the present disclosure provides a a process for preparation of a graphite electrode and/or nipple comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; baking the green rod at a temperature in the range of 650oC to 1100oC to obtain a baked rod; and graphitizing the baked rod at a temperature in the range of 2500oC to 3300oC.
Yet another embodiment of the present disclosure provides a process for preparation of a graphite electrode and/or nipple comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; baking the green rod at a temperature in the range of 650oC to 1100oC to obtain a baked rod; impregnating the baked rod with a molten pitch to obtain a impregnated rod; and graphitizing the impregnated rod at a temperature in the range of 2500oC to 3300oC.
Still another embodiment of the present disclosure provides a process for preparation of a graphite electrode and/or nipple comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; baking the green rod at a
7
temperature in the range of 650oC to 1100oC to obtain a baked rod; impregnating the baked rod with a molten pitch to obtain a impregnated rod; rebaking the impregnated rod at a temperature in the range of 650oC to 1100oC to obtain a rebaked rod; and graphitizing the rebaked rod at a temperature in the range of 2500oC to 3300oC.
In an embodiment of the present disclosure the steps of impregnating the baked rod and rebaking the baked rod after being impregnated are repeated at least one time.
The present disclosure also provides a graphite electrode and/or nipple prepared by the process comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; and graphitizing the green rod at a temperature in the range of 2500oC to 3300oC.
An embodiment of the present disclosure provides a graphite electrode and/or nipple prepared by the process comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; baking the green rod at a tempertaure in the range of 650oC to 1100oC to obtain a baked rod; and graphitizing the baked rod at a temperature in the range of 2500oC to 3300oC.
Another embodiment of the present disclosure provides a graphite electrode and/or nipple prepared by the process comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; baking the green rod at a temperature in the range of 650oC to 1100oC to obtain a baked rod; impregnating the baked rod with a molten pitch to obtain a impregnated rod; and graphitizing the impregnated rod at a temperature in the range of 2500oC to 3300oC.
Further an embodiment of the present disclosure provides a graphite electrode and/or nipple prepared by the process comprising: mixing 0.01wt% to 1 wt% of carbon nanotubes (CNT) with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture; cooling the carbonaceous mixture to obtain a paste with shapeable consistency; shaping the paste to obtain a green rod; baking the green rod at a temperature in
8
the range of 650oC to 1100oC to obtain a baked rod; impregnating the baked rod with a molten pitch to obtain a impregnated rod; rebaking the impregnated rod at a temperature in the range of 650oC to 1100oC to obtain a rebaked rod; and graphitizing the rebaked rod at a temperature in the range of 2500oC to 3300oC.
In an embodiment of the present disclosure the steps of impregnating the baked rod and rebaking the baked rod after being impregnated are repeated at least one time.
In an embodiment of the present disclosure the paste is being shaped by different processes. The paste can be shaped into a green rod by a process selected from extrusion, injection, vibro-pressing and molding.
The paste in the process of the present disclosure is cooled to obtain a shapeable consistency of the paste so that the paste can retain the shape given to it after shaping process.
The graphitized rod obtained after the graphitization step of the present disclosure is subjected to machining to obtain the final dimensions of graphite electrode and/or nipple.
In an exemplary non-limiting embodiment of the process of the present disclosure the coke is crushed and sieved in various fractions of different grain sizes; said fractions are mixed in pre-determined proportions along with CNT known as recipe, and fed into a mixer together with binder pitch, at around 160 deg C; some other additives such as anti-puffing agents (iron oxide or other material), lubricant (extrusion oil or other material), etc. can also be added at that stage. At this temperature of 160 deg C, pitch is liquid and binds the grains of coke to make a plastic paste, which is cooled down to around 120 deg C and then extruded in form of a cylinder through an extrusion press. The product achieved at this stage is called green electrode; usually, it is then baked at around 800 deg C to give a baked electrode; during baking, the pitch is transformed into pitch coke by losing most of its volatile fractions; this process creates some porosity inside the baked electrode which is less dense than the green electrode.
Usually, the porosity of the baked electrode is filled up by pitch through an impregnation process; the impregnated electrode is rebaked; in some cases, the same operations (impregnation and re-baking) may be repeated number of times, preferably two times.
During the next step, the electrode is heated to 3000 deg C, in a graphitization furnace in order to improve its various properties like resistivity (besides several other improvements
9
of various properties); this step gives a graphitized electrode which is then machined (lateral surface and both end faces of the cylinder, plus one threaded socket in each end face).
Nipples (connecting pins) are manufactured in a similar way, but since their diameter is approximately half of the electrode diameter, they need to compensate that smaller section by a significantly higher mechanical strength which is achieved by using finer coke fractions at the green stage and performing several impregnation and re-baking cycles before graphitization.
The process of the present disclosure is a method to increase flexural strength and decrease coefficient of thermal expansion simultaneously. According to the process of the present disclosure the graphite nipple has been made by a typical nipple extrusion method, using conventional raw materials blended with 0.01–1 wt% carbon nano tubes (CNT).
Carbon nanotubes used in the present disclosure can be single-walled, multi-walled, or a suitable mixture of the two. Preferably, the nanotubes may be multiwalled. A CNT is a single- or multi-graphene layers rolled into a tubular shape, having diameter in nanometer scale but length in micrometer scale. Due to this extremely high aspect ratio (length/diameter ratio = 1000), CNT possesses extraordinary mechanical, electrical and thermal properties. CNT is a member of recently-discovered new allotrope of carbon – called fullerene, which exists in between the two historically-known allotropes – graphite and diamond. Therefore, CNT inherits many good properties of diamond as well as graphite. For instance, CNT possesses a hardness and thermal capacity close to diamond, but electrical conductivity and thermal stability comparable to graphite. It exhibits extraordinary mechanical properties, viz. Young’s modulus > 1000 GPa and tensile strength > 100 GPa.
The amount of carbon nanotubes used in the present disclosure are in the range of 0.01 wt% to 1.0 wt%, preferably 0.025 wt% to 0.75 wt%, more preferably 0.15 wt%.
The coke used in the present disclosure for the preparation of the graphite electrodes and/or nipples may be selected from the group consisting of calcined petroleum coke (CPC), pitch coke or a mixture thereof, preferably a mixture of CPC and pitch coke is used. In the mixture of CPC and pitch coke the CPC is present in the range of 60wt% to 80 wt% and pitch coke is present in the range of 20 wt% to 40 wt%. The pitch coke is a calcined form of coke obtained from coal tar pitch and it is also known as calcined pitch needle coke.
The phrase “wt%” or “weight%” as used in the present disclosure refers to the weight of the reactant with respect to the total weight of the composition.
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Without wishing to be bound by theory, it is believed that addition of CNT brings about certain vital changes by bonding with binder pitch in a way leading to significant improvement of important properties namely, ER, FS and CTE. Even in the context of their use in graphite electrodes/nipples, CNT performs chemical bonding with various aromatic compounds present in the binder pitch, during the baking process and thus helps in holding many long-chain carbon molecules inside the rod, thus facilitating better carbonization, higher density, and lesser porosity.
The graphite nipple of the present disclosure is utilized to assemble and connect two graphite electrodes.
The present disclosure provides a graphite electrode and/or nipple having significantly better flexural strength, low coefficient of thermal expansion, and low electrical resistivity, by the use of an extremely low amount of CNT.
Additional advantage of using CNT in this process is that it reduces the binder pitch requirement by 1-10%.
Another important significance of the present process is that CNT performs chemical bonding with various aromatic compounds present in the binder pitch, and restricts the outflow of binder pitch during the baking process.
Generally it has been noticed in the electrode/nipple processing without use of CNT that there is an outflow of binder pitch from the rods in the baking stage, resulting in loss of carbon and a process inconvenience due to the liquid pitch bonding with packing grains, which need to be thereafter scraped out. However with the use of CNT, this inconvenience is significantly reduced, thereby bringing above economic advantage.
Still another economic advantage of the present process is that the use of CNT leads to less porosity at baked stage. Owing to the less porosity, the impregnation pitch requirement reduces by 1-10%.
EXAMPLE
The example given below is just for illustration of the present disclosure and should not be construed to limit the scope of the present invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the subject matter.
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Example-1
In an example of graphite electrode/nipple production, 6 g CNT was blended with 3000 g CPC; 900 g molten binder pitch (160 deg C) was added to it and mixed in an Eirich mixer for 30 minutes to obtain a carbonaceous mixture; allowing this mixture to cool to obtain a paste with extrudable consistency; this paste with extrudable consistency was extruded at 120 deg C temperature and 15 MPa pressure to obtain a green rod; the extruded green rod was heated at 900 deg C to obtain a baked rod; then for impregnation molten pitch was injected into the baked rod at 240 deg C temperature and 1.2 MPa pressure to obtain an impregnated rod; this impregnated rod was again heated at 900 deg C to obtain a rebaked rod; and the rebaked rod was graphitized at 2800 deg C to obtain the final graphite rod for electrode/nipple machining.
Apparent density was measured by weighing the samples on digital balance (Mettler Toledo, Model PB1502-S/FACT, Switzerland) and determining volume by standard water-displacement method (after 24 h soaking in water). Electrical resistivity (ER) was measured by standard two-point probe method using digital micro-ohm meter (Motwane, Model LR-2065, India). Flexural strength was measured on universal testing machine (FSA, Model-M50, India) by 4-point standard method. Coefficient of thermal expansion was determined by dilatometer (Unitherm, Model-1101, USA) measured in a temperature range of 25–525 deg C.
In similar manner, graphite electrodes and/or nipples were prepared by varying CNT wt% and were characterized by using methods described in Example-1. The characteristics of such electrodes and/or nipples are shown below in Table 1.
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TABLE-1
CNT
Apparent Density (AD)
Porosity
Electrical Resistivity (ER)
Percent Change in Resistivity
Flexural Strength (FS)
Percent Change in FS
Coeff. of Thermal Expansion (CTE)
Percent Change in CTE
%
g/cc
%
μΩ.cm
%
DaN/cm2
%
X10-6/°C
%
0
1.592
29.6
699
0.0
113
0.0
1.04
0.0
0.025
1.641
27.4
654
-6.4
116
2.7
0.95
-8.7
0.075
1.712
24.2
600
-14.2
119
5.3
0.91
-12.5
0.150
1.737
23.1
569
-18.6
140
23.9
0.81
-22.1
0.225
1.735
23.2
653
-6.6
126
11.5
0.88
-15.4
0.300
1.733
23.3
657
-6.0
122
8.0
0.90
-13.5
0.375
1.717
24.0
661
-5.4
117
3.5
0.91
-12.5
0.563
1.712
24.2
679
-2.9
115
1.8
0.95
-8.7
0.750
1.691
25.2
692
-1.0
114
0.9
0.98
-5.8
0.938
1.650
27.0
710
1.6
113
0.0
1.01
-2.9
It is evident from the Table that use of 0.15 wt% CNT decreases the electrical resistivity by 19%, increases the flexural strength by 24 %, and decreases the longitudinal CTE by 22% as compared to the reference (zero CNT) case.
Here it is worth mentioning that, nowadays, MWCNT and CF are commercially available in the same price range, but the CNT requirement is about one-tenth of CF requirement to achieve the same FS level in a graphite electrode/nipple. For example, the highest FS reported in US Patent No. 6916435B2 is 1926 psi = 132.8 DaN/cm2, which was achieved with 4% CF addition; while FS of the same order (140 DaN/cm2) is achieved in the present disclosure with only 0.15% CNT addition. Yet, for one-to-one comparison of the electrode properties with the same amount of CNT and CF, a test sample was made with 0.15% CF; but no change in FS could be noticed with respect to the reference (zero CNT) case.
ADVANTAGES:
1. The electrodes and/or nipples of the present disclosure have comparatively enhanced density by 1-10% due to the addition of calculated amount of CNT.
2. The electrodes and/or nipples of the present disclosure have comparatively lesser
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electrical resistivity by 1-20% due to the addition of CNT.
3. The electrodes and/or nipples of the present disclosure have comparatively increased flexural strength by 1-30%.
4. The electrodes and/or nipples of the present disclosure have comparatively reduced longitudinal coefficient of thermal expansion (CTE) by 1- 30%.
5. The electrodes and/or nipples of the present disclosure have comparatively reduced binder pitch requirement by 1-10%.
6. The electrodes and/or nipples of the present disclosure have comparatively reduced impregnation pitch requirement by 1-10%.
Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein.
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WE CLAIM:
1. A graphite electrode and/or nipple comprising: 0.01 wt% to 1.00 wt% of carbon nanotubes.
2. A graphite electrode and/or nipple comprising:
96 wt% to 99.9% of graphite;
0.01 to 1.00 wt% of carbon nanotubes; and
balance being conventional impurities.
3. The graphite electrode and/or nipple as claimed in claim 1 or 2, wherein the carbon nanotubes are in the range of 0.025 wt% - 0.75 wt%.
4. The graphite electrode and/or nipple as claimed in claim 1 or 2, wherein the carbon nanotubes are selected from the group consisting of single-walled CNT, multi-walled CNT and a mixture thereof.
5. A process for preparation of a graphite electrode and/or nipple comprising:
mixing 0.01wt% to 1 wt% of CNT with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture;
cooling the carbonaceous mixture to obtain a paste with shapeable consistency;
shaping the paste to obtain a green rod; and
graphitizing the green rod at a temperature in the range of 2500oC to 3300oC.
6. The process as claimed in claim 5, wherein the carbon nanotubes are in the range of 0.025 wt% - 0.75 wt%.
7. The process as claimed in claim 5, wherein the carbon nanotubes are selected from the group consisting of single-walled CNT, multi-walled CNT and a mixture thereof.
8. The process as claimed in claim 5, wherein the coke is selected from calcined petroleum coke, pitch coke or a mixture thereof.
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9. The process as claimed in claim 5, wherein the shaping of the paste is done by a process selected from the group consisting of extrusion, injection, vibro-pressing and molding.
10. The process as claimed in claim 5, wherein the green rod is baked at a temperature in the range of 650oC to 1100oC before graphitizing.
11. The process as claimed in claim 5, wherein the green rod is baked at a temperature in the range of 650oC to 1100oC to obtain a baked rod; and the baked rod is impregnated with molten pitch before graphitizing.
12. The process as claimed in claim 11, wherein before graphitizing the baked rod after being impregnated is rebaked at a temperature in the range of 650oC to 1100oC.
13. The process as claimed in claim 12, wherein the steps of impregnating the baked rod and rebaking the baked rod after being impregnated are repeated at least one time.
14. A graphite electrode and/or nipple prepared by the process comprising:
mixing 0.01wt% to 1 wt% of CNT with 71wt% to 84 wt% of coke and 16 wt% to 29 wt% of molten pitch to obtain a carbonaceous mixture;
cooling the carbonaceous mixture to obtain a paste with shapeable consistency;
shaping the paste to obtain a green rod; and
graphitizing the green rod at a temperature in the range of 2500oC to 3300oC.
15. The graphite electrode and/or nipple as claimed in claim 14, wherein the carbon nanotubes are in the range of 0.025 wt% to 0.75 wt%.
16. The graphite electrode and/or nipple as claimed in claim 14, wherein the carbon nanotubes are selected from the group consisting of single-walled CNT, multi-walled CNT and a mixture thereof.
16
17. The graphite electrode and/or nipple as claimed in claim 14, wherein the coke is selected from calcined petroleum coke, pitch coke or mixture thereof.
18. The graphite electrode and/or nipple as claimed in claim 14, wherein the shaping of the paste is done by a process selected from the group consisting of extrusion, injection, vibro-pressing and molding.
19. The graphite electrode and/or nipple as claimed in claim 14, wherein the green rod is baked at a temperature in the range of 650oC to 1100oC before graphitizing.
20. The graphite electrode and/or nipple as claimed in claim 14, wherein the green rod is baked at a temperature in the range of 650oC to 1100oC to obtain a baked rod; and the baked rod is impregnated with molten pitch before graphitizing.
21. The graphite electrode and/or nipple as claimed in claim 20, wherein before graphitizing the baked rod after being impregnated is rebaked at a temperature in the range of 650oC to 1100oC to obtain a rebaked rod.
22. The graphite electrode and/or nipple as claimed in claim 21, wherein the steps of impregnating the baked rod and rebaking the baked rod after being impregnated are repeated at least one time.