High carboxy (CO2) reactivity residue.2 2. High air reactivity residue.
3 3. Low specific electric resistivity.
4 4. Low thermal conductivity.
With the capacity increase of aluminum smelters, larger anodes have come into use. Cracking of the anodes due to thermal shock has been observed occasionally in these cases. Good thermal shock resistance of the anodes is therefore desirable. Improvements in thermal shock behavior will occur if CTE, the Youngs modulus, and the flexural strength are decreased and the thermal conductivity is increased.
6.1.2.5.3 Needle Coke
In the production of steel by the EAF process, the demand for graphitic electrodes is expected to be about 1.5 × 106 t/a in 2014. Charging stock for the electrodes is needle coke (about 85%) and pitch binder for backing and impregnation. About 35% of the total amount of steel produced (1400 × 106 t/a in 2014) is through EAF.
Needle coke is the highest grade product of calcinated petroleum coke. Only few companies in the world manufacture needle coke using proprietary technology. These companies are in the range of capacities and qualities: ConocoPhillips (plants in Lake Charles and Killingholme‐Immingham), Seadrift/GTI (plant in Port Lavaca), Jx Holdings, C‐Chem, Mitsubishi, Petrocoke Japan, and Indian Oil.
The production of needle coke requires specific feedstocks, coking, and calcination conditions. The hardware employed is similar to that of conventional delayed cokers.
The feeds for needle coke production are decant oil and coal tar [42–44].
Because of the massive thermal and mechanical stresses in the arc furnaces, the highest demands are placed on the electrode properties. Moreover, subsequent modified graphitization due to the baking of the electrodes results in additional stricter quality requirements.
The price of needle coke is about four times in comparison to anode coke grade, which fluctuates between 600 and 900 $/t in 2012.
The graphite electrodes consumption worldwide is about 50% for Asia, 30% for Europe, and 20% for America [36].
One reason for improved needle coke quality is the development to use graphite electrodes with higher diameter = 60–80 mm. This requires a higher amount of coke grains greater than a ½ in. – target is >20% [34].
Modern lengthwise graphitization is considerably more economic and only needs a third of the time required by conventional Acheson graphitization, but the more drastic reaction conditions demand superior qualities from the needle coke. For this reason needle coke specifications have greatly improved over the last 15 years. Room for further improvement seems only marginal.
For needle coke the chemical properties (low contents of S, Ni, V, Na, Ca, Si) and the following physical characteristics must be optimal:
1 1. High real density and high vibrated bulk density.
2 2. High grain stability and high proportion of coarse grain.
3 3. Low CTE.
In addition the microcrystalline structure is important because all physical parameters depend on it. A high level of anisotropy, regular micropores between the graphite lattice planes, a high crystallite size especially after graphitization, and crystal areas, which contrary to graphite have to be offset against each other, are indispensable. In order to obtain these needle coke qualities, three manufacturing areas have to be optimized:
1 1. Feed selection for delayed coking (low sulfur and low metal residues with high contents of 3‐, 4‐, and 5‐ring aromatics and low asphalt content).
2 2. Delayed coking conditions (longer cycle, higher coke drum pressure, higher recycle ratio, and optimum furnace coil out temperature (COT) ramp; but a too high COT ramp is negative!).
3 3. Calcination conditions (heating rate and final calcination temperature).
Table 6.1.2.6 Typical range of needle coke types.
| Property | Ultra‐premium needle coke | Super premium needle coke | Intermediate premium needle coke |
|---|---|---|---|
| CTE (20–200 °C) (10−6/°C) | 0.25 | 0.35 | 0.6 |
| Sulfur content (wt%) | <0.2 | 0.2–0.4 | 0.4–0.6 |
| Vibrated bulk density (g/cm3) | 0.85–0.90 | 0.83–0.90 | 0.83–0.90 |
| V/Ni content (mg/kg) | <10 | <10 | <10 |
| Real density (g/cm3) | 2.14–2.15 | 2.13–2.15 | 2.13–2.14 |
| H content (wt%) | <0.03 | 0.03–0.05 | 0.03–0.05 |
The quality analysis of needle coke is special because two parameters are specified that can only be analyzed after graphitization: the CTE and the puffing factor, i.e. the irreversible lengthwise expansion during graphitization (requirement: <2%). The quality comparison has been more complicated up to now by the fact that the preparation, graphitization, and measurement of CTE were proprietary company methods. Standardization however is nearly complete (last step: proposal of CTE–DIN method, an English translation exists as ISO draft) [45].
The quality requirements of electrodes depend on the electrode diameter and also on the type of arc furnace (DC or AC operation). That is why different types of needle coke are available. Some properties of the three needle coke qualities “ultra premium,” “super premium,” and “intermediate premium” are listed in Table 6.1.2.6. The CTE values refer to the measuring temperature range of 20–200 °C or 20–60/80 °C after preparation and graphitization to receive small electrodes according to the DIN/ASTM method. The tests to produce “ultra‐premium” needle coke with CTE levels of 0.25 × 10−6/°C had success, thus introducing the third needle coke quality “ultra‐premium needle coke” into the market.
An additional challenge in the use of electrodes in the arc furnace process is the design and production of the nipples (screw connections between the individual electrodes). Through pilot tests, test runs, and guaranteed production consistency for needle coke are the basis for delivery agreements. Continuous improvements are necessary.
6.1.2.6. Environmental and Safety Aspects
6.1.2.6.1 Green Coke
Although green coke contains polycyclic aromatics,
