FIELD OF INVENTION
The present invention relates to graphite electrodes and nipples and process for preparing the high performance graphite nipples.
BACKGROUND OF THE INVENTION
Graphite electrodes are used worldwide in arc furnaces for steel making. These electrodes are usually used in column of 3 connected by a member called graphite nipple / connecting pin. The graphite nipple is fitted in the threaded female portion i.e. socket of adjoining graphite electrodes and the system of joining two graphite electrodes with graphite nipple is called an electrode joint.
In arc furnaces, the high temperature is needed to melt steel. This is achieved by passing high current through columns of graphite electrode. In this electro-thermal process, an arc is produced between column of electrodes and the charge, which in-turn melts the steel. During operation of arc furnace, graphite electrodes in columns get continuously consumed due to oxidation and sublimation at the lowermost end.
The productivity of arc furnaces is affected due to stoppages etc. During operation, the electrode joint heats up due to resistance of graphite material and joint resistance as well. At heated electrode joint, expansion of nipple and expansion of electrode takes place simultaneously.' Severe loop stresses are generated in electrode socket, if thermal expansion of nipple exceeds thermal expansion of electrodes it may result into failure of electrode joint. A nipple with lower thermal expansion in longitudinal direction along with higher flexural strength is recommended for the best performance of electrode joint.
Therefore, manufacturing of graphite nipple is generally done with super premium needle coke having lower CTE. Extrusion of nipple stock is done with relatively finer particles as compared to what is used for electrode recipe. This is followed by multiple carbonization and impregnation prior to graphitisation up to 2700 °C or more. Structural perfection is achieved in terms of less
porosity by multiple impregnations and followed by carbonization to make nipple stronger than electrode.
However there is limit in reliability of existing process of making high performance graphite nipple, as it is very sensitive to the small variations in quality of raw materials and process change as well.
In present invention, PAN based carbon fiber addition was done in calcined petroleum coke prior to mixing with coal tar pitch followed by extrusion. The extruded nipple samples are carbonized, impregnated with coal tar pitch and again carbonized followed by graphitisation. Addition of carbon fibers showed significant improvement in flexural strength alongwith enough reduction in longitudinal CTE of graphite without affecting other properties like density, resistivity etc. This' method can be suitably used in making of high performance nipple graphite.
PRIOR ART
Griffin et al have reported (US patent No. 4,998,709) addition of Mesophase pitch based carbon fibers in extrusion blend to reduce longitudinal CTE of graphite nipple. These authors have used 8 to 20 weight percent Mesophase pitch based carbon fibers (2 inch in length) having Young's modulus greater than 379 GPa. Although, there is substantial reduction in longitudinal CTE of graphite, the process uses high amount of Mesophase based carbon fibers, and graphitisation cycle is very long. Also fiber addition showed decrease in modulus of rupture of graphite nipple.
Shao et al have reported (US patent No. 6,280,663 Bl) that the improvement in graphite nipple quality can be achieved by addition of Mesophase pitch carbon fibers. Though improvement in graphite nipple can be achieved by addition of smaller amount of Mesophase based carbon fibers as claimed by Shao et al, Mesophase based carbon fibers are costly and also brittle, and difficult to handle during mixing with coke particles.
In US patent No. 6,916,435 B2 Kortovich et al have described the addition of various types of carbon fibers namely Mesophase pitch fibers and PAN based fibers in concentrations of 0.4 to about 5.5 % by weight of total mix components. Even though this work showed significant improvement in longitudinal CTE and Flexural strength, this work is addressed mainly to electrodes and cathodes and not to graphite nipple. This formulation / methodology may not be applicable to nipple stock since the granulometry as well as its formulation are fairly different from those of electrodes
It is quite clear therefore that a specific solution is desired to ensure the manufacturing of high performance graphite nipple possessing lower longitudinal CTE and higher flexural strength. Since carbon fiber is costly material, it is desired to draw maximum benefit of carbon fibers by adding it in minimum quantity but get required enhancement in flexural strength and lowering of CTE.
SUMMARY OF THE INVENTION
According to this invention, a process for the making of high performance graphite nipple comprises of the following steps.
1. 70 to 75 % weight of calcined petroleum coke, 25 to 30 % weight of coal tar pitch and the 0.3 to 1.4 % weight carbon fibers of total blend components are used for extrusion blend.
2. All coke particles of blend have particle size not more than 5 mm and coke fraction which is 30 to 38 % of total blend having 50 to 60 % particles less than 100 microns in size.
3. PAN based carbon fibers, having average diameter 6 to 12 microns and length not more than 10 mm were added in coke particles at the start of mixing.
4. The mixing is done to open the chopped bundles of carbon fibers into filaments, homogeneously dispersed in paste and green body as well.
5. Coal tar pitch is added as liquid or solid to coke particles containing carbon fibers etc. at 60°C above the softening point of coal tar pitch.
6. The paste generated around 170°C and after completion of mixing cycle is cooled to 120 °C and transferred to extrusion pot. Paste is further extruded as per conventional process through suitable die to form stock of green rod i.e. nipple stock.
7. Nipple stock is further carbonized up to 800 to 900°C which is then multiple densified by impregnation with coal tar pitch followed by carbonization up to 800 to 900°C and finally those are graphitised up to 2700 to 2900 °C.
8. Samples are tested to check their properties.
OBJECTIVES OF THE INVENTION
An objective of present invention is to provide process for preparing graphite articles like nipple with reduced longitudinal coefficient of thermal expansion and increased flexural strength by adding 0.3 to 1.4 parts of PAN based carbon fibers to 100 parts of total blend components.
It is an objective of present invention to get maximum benefit of carbon fiber properties at lower amount of carbon fiber addition, by dispersing carbon fiber filament in paste or green body. The main objective of the present invention is to provide a process for making graphite articles like graphite nipple.
It is yet another objective of present invention to provide process for preparing graphite article like nipple with reduced longitudinal coefficient of thermal expansion and increased flexural strength compared to the conventional articles.
Still another objective of present invention is to provide process for preparing graphite article like nipple with reduced longitudinal coefficient of thermal expansion and increased flexural strength with out compromising other properties.
DETAILED DESCRIPTION OF THE INVENTION
As mentioned above, the graphite articles particularly graphite nipple can be produced by mixing all components of the blend i.e. 70 to 75 % by weight of calcined petroleum coke, 25 to 30 % by weight of coal tar pitch and the 0.3 to 1.4 % by weight of carbon fibers of total blend components (including coke, binder, and additives). As stated, coke particles having size less than 5 mm are used in the blend and fraction of coke particles which is 30 to 38 % of total blend have 50 to 60 % particles less than 100 microns in size.
At the start of mixing, PAN based carbon fibers were added to coke particles in specific sequence as chopped bundles. Each filament of carbon fiber is of 6 to 12 microns in diameter, having length not more than 10 mm. Fifty thousands filaments were present in each chopped bundle. Mixing is done to ensure opening of chopped bundle into individual filaments of carbon fibers having dispersion throughout the body.
After mixing of all particulate fractions, a body is formed by extrusion though a die into cylindrical rod termed as green nipple stock. The green nipple stock is water-cooled and further heat treated to up to 800 to 900°C called baking. After this, baked rod is multiple densified by impregnation with coal tar pitch followed by rebaking up to 800 to 900°C. Finally, rods are graphitised up to 2700 to 2900°C. The graphitisation is done in either Acheson or Lengthwise graphitisation (LWG) furnace, where carbon to graphite transition takes place.
Working example-1
A trial was conducted using chopped PAN based carbon fibers of approximate 7 microns diameter and length of approx 6 mm and average tensile strength of 4.1 Gpa and tensile modulus of 230 GPa. The fibers were blended and mixed with coke particles in laboratory mixer, which is followed by binder pitch addition. The coke fractions for control case as well as fiber case' having particles not more than 5 mm and fraction of coke particles, which is 30 to 38 % of total blend, have 50 to 60 % particles less than 100 microns in size are used.
.Total mixing time is not more than 1 hour, which was done around 170° C and conditioned paste extruded to 12 mm diameter green stock by using manual laboratory extrusion press. Green stocks were carbonized i.e. baked up to 900 °C followed by natural cooling to room temperature. The baked samples densified by impregnating with zero Quionoline insoluble (QI) coal tar pitch and again carbonized i.e. rebaked upto 900°C. The rebaked samples were graphitised upto 2800 °C. Table 1 below summarizes the properties of the final graphitised stock. For the purpose of comparison, data for control nipple stock is also included in the table.
Table 1: Graphite Nipple Properties
(Table Removed)
Working example-2
A trial was conducted using chopped PAN based carbon fibers of approximate 7 microns diameter and length of approx 6 mm and average tensile strength of 4.1 Gpa and tensile modulus of 230 GPa. The fibers were blended in Eirich Mixer and mixed with coke particles, which was followed by binder pitch addition. The coke fractions for control case as well as fiber case having particles not more than 5 mm and fraction of coke particles, which is 30 to 38 % of total blend having 50 to 60 % particles less than 100 microns in size are used.
Total mixing time is not more than 1 hour, which was done around 170° C and paste is poured in extrusion cylinder, where vacuum is applied to remove entrapped air/gas. The paste is extruded to 50 mm diameter green stocks (rods) by using semi automatic extrusion press. Green stocks were carbonized i.e. baked up to 900 °C followed by soaking and natural cooling to room, temperature. The baked samples are densified by impregnating with zero Quionoline insoluble
•(QI) coal tar pitch and again carbonized i.e. rebaked upto 900°C. The rebaked samples were graphitised upto 2800°C. Properties of the finally graphitised material were measured which are summarized in Table 2 below:
Table 2: Graphite Nipple Properties
(Table Removed)

-CLAIMS:
What is Claimed:
1. Process for making graphite article particularly graphite nipple for connecting graphite
electrodes used in electric arc furnaces, which comprises of following steps.
(a) Filler coke i.e. calcined petroleum coke fraction contains particles not more than 5 mm. Coke fraction, which has 50 to 60 % particles less than 100 microns in size, is 30 to 38 percent of weight of total blend.
(b) Adding PAN based carbon fibers in coke particles at the start of mixing.
(c) 20 to 25 % by weight coal tar pitch added as solid or liquid to filler coke, which is 70 to 75% by weight of total blend components including additives.
(d) The paste generated after completion of mixing cycle, cooled to approximate 120 °C and further transferred to extrusion pot. Paste is extruded as per conventional process through suitable die to form stock of green rods i.e. nipple stocks.
(e) Nipple stock is further carbonized upto up to 800 to 900°C, which is then multiple densified by impregnation with coal tar pitch followed by carbonization up to 800 to 900°C and finally these are graphitised up to 2700 to 2900 °C.

2. A process of claim (1) wherein said PAN based carbon fibers 0.3 to 1.4 parts of 100 parts of total blend components were added.
3. A process of claim (1) wherein said PAN based carbon fibers added have length not more than 10 mm.
4 A process of claim (1) wherein said PAN based carbon fibers added have Young's modulus of 200 to 250 Gpa.
5. A process of claims (1 to 4) wherein said carbon fibers have average diameter 6 to 12 microns.
.6. A process of claims (1 to 5) wherein said carbon fiber contribute in making of high performance nipple only upon dispersion of carbon fiber filaments in paste or green body and adhesion of carbon fiber with matrix.