The baking stage
This time we are concerning ourselves with the baking stage of graphite electrode manufacture. The goal of this stage is to solidy the raw materials, eliminate gases, carbonize coal tar pitch to solidify the carbon and enhance overall strength. (reminder: coal tar pitch is the binding material, compare: ABC of graphite electrodes: how are graphite electrodes produced? (3b) – formulation) This is achieved under an off-air environment by heating up green electrodes to 1,000 ℃ -to 1,350 ℃ by thermal conduction.
The baking process most often constitutes the real bottle-neck stage of graphite electrode production. It takes up to 35 days to completion and therefore, many furnaces are used simultaneously besides each other and sometimes span entire fields that remind of football fields.
The two most widely used technologies to bake electrodes are the down draft kiln, and the ring furnace. Furthermore, for the re-baking process, manufacturers use tunnel furnaces and car bottom furnaces (discussed in the next sub chapter). As filling materials for the insulation and heat transfer materials of the furnace are used metallurgical coke and quartz sand; for the heat transfer, natural gas is used.
Down draft kiln & ring furnace
In the down draft kiln, heated air circulates according to the blue arrows in the chart below. The kiln connsists of the firebox, the stack area, the damper, and the chimney.
Layers of sawdust and metallurgical coke are placed on the bottom. The electrodes are stacked on the shelves as shown, the distance between them most be 10 – 20 mm and the distance to the fire wall is 60 – 100 mm to allow for ideal baking results. Thermocouples are used to monitor the temperature of the kiln.
- Advantages: low investment, short process time
- Disadvantage: pollution, low utilization of thermal energy
Ring furnaces come in two forms: with or without lids with either 16, 24, 28 or 32 >>rooms<< as shown in the drawing below. The depth of each room, which remind a little bit of tombs, is 3.6 to 4.8 meters. In the process, ring furnaces are loaded, heated, cooled down, unloaded and maintained after which a new cycle begins.
source: archive GES China
Compared to down draft kilns, ring furnaces are more environmentally friendly since they use desulphurization devices and electrostatic precipitators to avoid pollution.
As explained above, electrodes are put into the chambers. A single fire path alongside the rooms is designed to pass on the heat and raise the temparature in the rooms. The temparatures are increased slowly from 0 to 1,350°C.
In the drawing above you can see that the temperatures in chamber 6 is at its max while the five previous rooms exhibit lower temperatures. The reason is that at an earlier point in time rooms 1 to 5 were heated up to 1,350°C and they were isolated from the fire path Subsequently, the temperatures cooled down.
- Advantages: Environmental friendly，saves energy, high output
- Disadvantages: large investment(20m), huge temperature difference inside furnaces
- The bigger the furnace, the longer the heating time
- The smaller the size of particles, the longer the heating time
- The bigger the bulk density of product, the longer the heating time
- Heating up principles
- RT-350 ℃: raise 6 ℃/hour
- 350-600 ℃: raise 2-3℃/hour
- 600-800 ℃: raise 4℃/hour
- 800-1300 ℃: raise 10℃/hour
- 1200-800 ℃: cool down 50 ℃/hour
- 800-400 ℃: naturally cool down
- 400 ℃: baking process finishes
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sources: Thespruchecrafts.com, https://www.thesprucecrafts.com/what-kind-of-kiln-is-it-2746127, retrieved 21.09.18