The ABC of graphite electrodes: How are graphite electrodes produced? (3a)

this is part 3 of our mini series into graphite electrodes. It covers the basics of graphite electrode manufacture. Since we want to delve deeper into each phase, it will be necessary to talk about the production in several articles. This one gives you an overview of the production stages.

Introduction into graphite electrode production

Here is a concise overview of the production stages

a) Calcination of petroleum coker & recrystallization

b) form ulation

c) kneading

d) forming

e) baking

f) (impregnation)

g) graphitization

h) machining

You may have noticed that I put the phase f) into brackets. The reasons is that some graphite electrode qualities do not undergo this step. More on this in later postings. Also, technically we can only speak of >>graphitized<< electrodes after phase g) is complete. This is when a change in molecular structure has happened from carbon to graphite.

a) Calcination of petroleum coke & recrystallization

The purpose of this phase is decomposition/purification of petroleum coke. To know more about petroleum coke, please have a look back on our article on the raw materials of graphite electrodes.

Essentially, we try to remove organic materials and moisture (=volatiles or volatile matter) inside the petroleum coke. Before the calcination process, we typically refer to the coke as >>green coke<< or sometimes >>raw coke<<. For this process, electrode producers use a rotary kiln (diagram below).

rotary kiln as a means to calcination

Source: Yongchang Cai, Mathematical problems in engineering, 2017.

In the kiln there are several zones inside. The petroleum coke enters the kiln in a pot (inner pot) that subsequently moves from the right to the left end on the trajectory of entry to exit in the rotary tunnel/corridor. In the first zone, called the drying or preheating zone, the pot is heated temperature is about 800°C to 900°C. The volatile matter starts evaporating at around 300°C.

The calcination zone, which makes up about half of the furnaces length, is 1,200°C to 1,350°C. There is also a cooling zone (or a second kiln). It is not shown in the drawing above but just imagine a third zone with a temperature range of 800°C to 900°C. The whole process takes about 30 to 50 minutes to completion. The released volatile gases in the kiln go to a waste heat recovery boiler to produce steam and ultimately refire the kiln.

Recrystallization

The thermal treatment of petroleum coke has yet another benefit besides a mere purification: we somewhat alter the directionality and density of the petroleum coke which is important later for a superior electrical and mechanical properties.

 

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Sources: 

Yongchang, Cai, Modeling for the Calcination Process of Industry
Rotary Kiln Using ANFIS Coupled with a Novel Hybrid Clustering Algorithm, (2017)in: Mathematical problems in engineering 2017: Mathematical Problems in Engineering Volume 2017, Article ID 1067351, 8 pages https://doi.org/10.1155/2017/1067351,