has increased to about 30% [31, 32]. Therefore, the world aluminum consumption is about 70 × 106 t/a.
The green coke qualities containing relatively low levels of metals like vanadium (typically <200 ppm) and low to moderate levels of sulfur (0.5–2%) are not available in the required amount to make anode‐grade coke. The cause of this is that refineries operate with high sulfur and high metal crudes due to attractive price differential and the demand of anode‐grade coke has increased. Therefore the smelters are increasingly having to adapt to higher sulfur and metal levels [33].
Recent test by companies with “inert” anodes (reduction without carbon) has not been successful due to process economics and aluminum purity [31].
Regular calcinate in the fraction of 0.2–1.0 mm is used for titanium dioxide production. More than 70% of the worldwide TiO2 production is produced using the chloride process. This process needs only about a third compared with the process that is using rutile as reduction medium.
1.2 × 106 t/a regular calcinate is used for titanium dioxide production. The separation of this fraction improves the regular calcinate quality for anodes as the smaller sized material is removed. Otherwise this fraction has a better price than anode qualities.
3–4% of calcined petroleum coke, which is called needle coke, is used to produce graphite electrodes.
The use of calcined petroleum coke as recarburizer in the steel industry is about 2%.
The rest of the calcined petroleum coke is the basic for other specialty markets (2–7%).
6.1.2.4.2.2 Needle Coke
Petroleum coke is also used for the production of graphite electrodes (Table 6.1.2.5 and Figure 6.1.2.10). The quantity employed for this purpose has been stagnant for several years up to 2005. This high‐grade quality calcinate is called needle coke. Although the proportion of electrosteel production in overall steel production is steadily increasing, a series of optimizing steps have continuously reduced the electrode consumption per ton of steel. Whereas in 1985, 7 kg electrode material was needed per ton of steel, modern furnaces can manage with less than 2 kg/t. For this reason the consumption of electrodes worldwide was stagnated around 1.100.000 t/a in 2005. By using all process improvements, the needle coke demand has only marginally increased with the growing of electrosteel production. The demand of needle coke in 2014 is 1.5 × 106 t/a [34–37].
Needle coke qualities are used as part components for cathode blocks, sidewall blocks, and ramming pastes for the production of aluminum. A further application of this quality coke is in steel carburization.
A series of special industrial uses of carbon material, which are negligible in terms of quantity, utilize the thermal and chemical resistance, the thermal and electrical conductivity, and graphitization of petroleum coke. The amount required for this purpose is normally covered by carbon dust separated during the production of carbon anodes and electrodes.
6.1.2.5. Quality Aspects
The quality of coke is chiefly influenced by the feed material used in the delayed coking units. Of second and third importance are the coking and the calcining conditions. Structure, density, and VCM contents can vary significantly as well as sulfur, metal, and ash content.
Over the last years an international standardization of testing methods for carbon materials has been developed. ISO, EN, DIN, or ASTM specifications or drafts exist for nearly all parameters. We are closer to standardization even in the case of coefficient of thermal expansion (CTE) measurement, which for a long time has relied on test methods of electrode manufacturers. The situation that national standards (ASTM, DIN) are incorporated in the international ISO specifications makes itself positively noticeable.
6.1.2.5.1 Green Coke
Compared with coal, petroleum coke has:
Higher energy content
Lower ash residue
Lower VCM content
The quality of normal green coke mainly used as fuel is defined by the sulfur and metals content. Additional parameters are the water content (for the quantity calculation) and the ash and VCM content. In the case of green coke used for higher grade products, the level of metals and the grain size distribution are additional quality criteria.
6.1.2.5.2 Regular Calcinate
The quality of calcined petroleum coke produced in Europe is superior to that produced in other continents. Due to regulatory requirements and improvements in efficiency, regular calcinate in Europe has undergone a quality upgrade toward needle coke: the sulfur contents, reactivity results, and CTE figure are lower, and the vibrated bulk density is higher. Some non‐European manufacturers are already producing similar qualities. Typical European quality characteristics are listed below. The figures in brackets refer to quality requirements outside Europe [38–41].
| Sulfur content (wt%) | Maximum 1.7 (3) |
| Ni/V content (ppm) | Maximum 120–150 (200) |
| Na/Ca/Si content (ppm) | Maximum 100 |
| Vibrated bulk density (1–2 mm) (g/cm3) | 0.80–0.85 (0.75–0.80) |
| Air reactivity (1–1.4 mm) (wt%/min) | Maximum 0.15 |
| CO2 reactivity (1–1.4 mm) (wt%) | Maximum 12 |
| CTE factor (carbon anodes) (10−6/°C) | 3.5–4.5 (5.5) |
| Hydrogen content (wt%) | 0.05–0.09 |
| Real density (g/cm3) | 2.06–2.10 |
| Ash content (wt%) | 0.05–0.20 |
| Crystallite size Lc (nm) | 2.5–3.2 |
| Specific electric resistivity (μΩ m) | 460–520 |
| Grain stability (wt%) | Minimum 70 |
| Dust forming factor (wt%) | Maximum 0.02 |
| Water/oil content (wt%) (from antidust additive) | Maximum 0.3 |
Except for the SO2 emissions, the sulfur content has only marginal side effects on aluminum electrolysis.
The goal in aluminum electrolysis is to have minimum consumption of anodes with a good electricity yield. Good production results are 95% electricity yield with an anode consumption of <410 kg per ton aluminum. These figures can be reached if the anodes meet
