Method of producing needle coke for low CTE graphite electrodes
A method of producing low CTE graphite electrodes from needle coke formed from a coal tar distillate material having a relatively high initial boiling point.
1. A method of creating a low coefficient of thermal expansion graphite electrode, comprising:
(a) heating a needle coke precursor comprising at least 80% coal tar distillate having a boiling point of greater than 280° C. under pressure, thereby converting 60-90% of the coal tar distillate into raw coke;
(b) calcining the raw coke to create low coefficient of thermal expansion needle coke having a final production yield of 55% to 95% of the coal tar distillate;
(c) milling the needle coke;
(d) mixing the milled needle coke with coal tar binder pitch to create a mix;
(e) extruding the mix to form a green electrode;
(f) baking the green electrode to create a baked electrode; and
(g) graphitizing the baked electrode to create a low coefficient of thermal expansion graphite electrode.
2. The method of claim 1 wherein the mixing step contains from about 15% by weight to about 35% by weight coal tar binder pitch.
3. The method of claim 2 wherein the crushed raw coke of step (b) is calcined to a temperature of from about 1300° C. to about 1500° C.
4. The method of claim 3 further comprising calcining the coal tar distillate of step (b) at a rate of temperature change of about 300° C. per hour to about 400° C. per hour.
5. The method of claim 4 further comprising holding the temperature of step (b) for from about 20 minutes to about 40 minutes.
6. The method of claim 1 wherein the low coefficient of thermal expansion graphite electrode has a coefficient of thermal expansion of from about 0.005 ppm/° C. to about 0.150 ppm/° C.
7. The method of claim 1 further comprising crushing the raw coke of step (a) prior to the calcining of step (b).
8. The method of claim 1 wherein the coal tar distillate has a modified Conradson carbon value of at least about 1%.
9. The method of claim 8 wherein the pressure is about 50 psig.
10. The method of claim 9 further comprising holding the temperature of step (a) for from about 16 hours to about 25 hours.
11. The method of claim 1 wherein the pressure of step (a) is of from about 20 psig to about 100 psig.
12. The method of claim 1 further comprising heating the coal tar distillate of step (a) at a rate of from about 35° C. per hour to about 65° C. per hour.
13. The method of claim 1 further comprising a needle coke yield wherein the yield of needle coke of step (b) is of from about 80% to about 95% of the raw coke.
14. The method of claim 1 further comprising a final production yield wherein the yield of needle coke of step (b) is of from about 55% to about 85% of the coal tar distillate.
15. The method of claim 1 wherein the coke precursor is substantially free of crushed coal.
16. The method of claim 1 wherein the method does not include a hydrogenation step.