Important properties required for needle coke used in the synthetic graphite electrode are the coefficient of thermal expansion (CTE) to be low. CTE depends on the structure and shape of coke as a raw material. This property is almost determined in the formation of raw coke and can be little improved by the operation (and its condition) of a calciner.
6.1.3.4. Uses
PC is roughly divided into two types, amorphous coke and needle coke. The majority of amorphous coke is used as an electrode for aluminum smelting and is also used as the aggregate of the synthetic graphite block produced in the similar process to produce a synthetic graphite electrode in electric arc furnace steelmaking.
6.1.3.4.1 Aggregate of Graphite Electrode for Aluminum Smelting
Aluminum metal is produced by reducing aluminum oxide (Al2O3) with carbon when electrolyzing the molten salt: 2Al2O3 + 3C → 2Al + 3CO2
In reduction of aluminum oxide, carbon is consumed according to the formula above. A source of carbon consumed is the anode used in the aluminum smelting furnace, and PC is used as a raw material (aggregate) of the anode.
The anode has two types, the prebake type that is calcined in advance and the Soederberg type that is calcined using waste heat in reduction, but both anodes use the aggregate (coke) and the binder pitch as a raw material. Demand in anode is increased with the growing aluminum industry, and approximately 10 000 000 t of the aggregate for an anode is used in 2009, and the majority of them use petroleum coke. A range of 100 000–200 000 t of PC is used.
Amorphous coke is also used in the process similar to the production of the synthetic graphite electrode described below, and the synthetic graphite block is also produced.
6.1.3.4.2 Aggregate for Graphite Electrode in Electric Arc Furnace Steelmaking
Since the synthetic graphite electrode in electric arc furnace steelmaking is used at high temperature, needle coke in which the crystal structure of aggregate coke is highly ordered is used. Coal tar is a mixture of polycyclic aromatic hydrocarbons, and carbonization of coal tar is anticipated to yield a hard carbon material with the highly crystalline structure. However, in coal tar there is a QI fraction, an amorphous hydrocarbon, which prevents the growth of crystals [6]. Development of the industrial process for elimination of QI first allows the production of pitch needle coke that can be used as the aggregate of the graphite electrode in electric arc furnace steelmaking (refer to Sections 1 and 3.1.2), thereby creating application of the carboniferous material.
The synthetic graphite electrode for steelmaking is produced by kneading needle coke as the aggregate with a binder pitch followed by molding, baking for carbonization of the binder pitch, impregnation of the pitch for adjustment of density, secondary baking, and graphitization. The synthetic graphite electrode for steelmaking is used in an electric arc furnace, which discharges arcs to melt steel scraps. The synthetic graphite electrode is used under severe condition, and the temperature at the electrode tip exceeds 3000 °C when used. Therefore, importance in quality of needle coke as a main raw material, particularly the low CTE, is emphasized. Pitch needle coke has the well‐developed crystal structure as compared with petroleum needle coke and has the properties in which the CTE of products obtained after graphitization is low.
The Acheson furnace has been used in the graphitization process in which heat treatment exceeding the temperature of 2000 °C is performed to produce the synthetic graphite electrode for steelmaking. The Acheson furnace has been developed in the 1990s and is widely used at present as the graphitization equipment. Adoption of the lengthwise graphitization (LWG) furnace in which the electric current is directly applied to energize a semifinished product (baked product) for graphitization in a short time has been initiated in the 1960s, and at present the LWG furnace stays as mainstream equipment in adoption for the graphitization process of the synthetic graphite electrode.
A baked product exhibits irreversible thermal expansion called the puffing in the graphitization process. A method of decreasing the puffing of needle coke is reported by Mochida et al. as follows [8]:
1 1. Reducing the contaminant atoms sulfur and nitrogen in the coke feed or coke.
2 2. Modifying pore distribution in the coke.
3 3. Changing the heating programs used in calcination and graphitization to control the evolution of contaminant gases.
4 4. Trapping the heteroatoms or modifying the time of their evolution (accelerating or delaying.
Nippon Steel Chemical Co., Ltd. (at present, Nippon Steel & Sumikin Chemical Co., Ltd.) has developed in the 2000s pitch needle coke with reduced puffing, and it is widely used.
In China where production of PC is grown, a similar process to remove the QI fraction is developed, and the production of the aggregate for the synthetic graphite electrode in electric arc furnace steelmaking is grown. An annual production volume of PC including needle PC with the grade lower than the regular grade is estimated to be approximately 500 000 t.
Typical properties of PC used as the aggregate in various applications are given in Table 6.1.3.6.
6.1.3.5. Environmental and Safety Aspects
The CAS number relevant to PC is 94113‐91‐4, and it contains little metal impurities, and the VM is low so that there is no report specific to the environmental impacts and safety of PC.
In the publication in 2010 based on the report of the PreSIEF, PC is exempted by European Chemical Agency (ECHA) from registration in REACH.
Table 6.1.3.6 Physical properties of pitch coke.
| Proximate analysis | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Real | Volatiles | Fixed | |||||||
| Elemental analysis | Metal analysis | ||||||||
| density | matter | carbon |
Moisture
|