Acheson Process

The Acheson process is the oldest and still most important graphitization process. The carbon articles are placed with their long axes perpendicular to the direction of the electrical current in the furnace bed and are surrounded with a resistive bed of granular coke in which most of the Joule is produced. A constant furnace resistance is essential for uniform product quality (avoidance of local temperature peaks). The way in which the furnace is loaded is therefore critical.

In Acheson furnaces articles with very different shapes can be graphitized, making the process very versatile. The capacities of currently operated furnaces range up to net loads of 100 t or more. A typical Acheson furnace is 12 to 15 m long and 3 to 3.5 m wide and is loaded with 35 to 55 t of carbon articles. After a heating time of 3 days final temperatures of 2800 to 3000°C are attained. A further 8 to 10 days are required for cooling the furnace, so that a production cycle takes about 2 weeks.

In the Acheson process only ca. 30% of the energy of ca. 3 to 10 kWh/kg supplied is utilized for graphitization. Other disadvantages are the high cost of loading, the poor time and space utilization yield and the high emission of noxious gases (mainly SO₂), whose controlled disposal is exceptionally difficult due to the size of the older furnaces and the high operating temperatures. New plants have thus been designed to operate continuously or semicontinuously (transportable individual furnaces between stationary mounted electrodes, the electrodes being run down from above in coupled U-shaped units which move in unison). This results in more rational production and easier disposal of noxious gases.

Castner Process

The Castner process operates without intermediate resistive materials. The heat is thus exclusively produced in the carbon articles themselves, which are directly spanned between two electrodes at least one of which is movable to allow for changes in dimensions.

Castner furnaces are smaller than Acheson furnaces. Heating up and cooling down phases are significantly shorter than in the Acheson process. Also here there is a switch to continuous operation.

The Castner process is characterized by more efficient energy utilization and by more uniform graphitization compared with the Acheson process. Disadvantageous is the more complex furnace construction and restrictions concerning the dimensions of the carbon articles.

Other Graphitization Processes

The other graphitization processes transfer the heat indirectly to the carbon articles, which makes continuous operation easier.

Heating can be carried out by induction in which the furnace is surrounded by an induction coil and the dimensions of the article and the coil have to be precisely correlated with one another. Alternatively heating takes place by radiation, which is more flexible.

Both processes are limited to smaller carbon articles and are utilized for the production of specialty products.

Purification Graphitization

Very pure graphite sorts are attainable by modifying the above-described processes. Purification can be achieved purely thermally or thermochemically. The purely thermal processes entail increasing the firing temperatures to ca. 3000°C and firing times and using carbon black to absorb the escaping volatile impurities. In thermochemical processes impurities are converted into volatile halogen compounds by reaction with: elemental chlorine or fluorine, halocarbon compounds, solid halides.