How are graphite electrodes made? – (3eII) rebaking

Why baking one more time?

In this post you are going to learn about the rebaking process for graphite electrodes making and its equipment. Rebaking is necessary since after the first baking process, only 50 % of the pitch material is actually carbonized.

In almost all cases, graphite manufacturer use tunnel furnaces for rebaking. As a good standard, tunnel kilns from Riedhammer GmbH in Germany are used.

Tunnel kiln

In the small graphic below, you can see a typical design of a tunnel furnace with three heating zones for pre-heating, baking and cooling. The graphite electrodes are loaded onto seggar cans sliding on a railway through the furnace. 

The target baking temperature is about 700 ℃ with the following process times:

  • Pre-heating process time: 96 hours
  • Baking process time: 34 hours
  • Cooling down process time: 37 hours

 

riedhammer_tunnel_furnace

source: http://www.riedhammer.de

 

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sources

http://www.riedhammer.de/System/00/01/64/16493/ed_enUS/EMS%20Tunnel%20Kilns%20Sanitaryware.pdf

 

How are graphite electrodes made? (3e) – baking

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.

Baking technologies

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.

downdraft-56a7646c5f9b58b7d0ea0de3.jpg

source: thespucecrafts.com

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.

ring furnace

 

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

 

Baking principles

  • 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

How are graphite electrodes produced (3d) – forming stage

the forming stage follows upon kneading and is reached by means of hot extrusion of the semi-finished electrode. Other processes used for forming graphite products (such as blocks) are explained in later articles.

Extrusion forming of graphite electrodes

After kneading, the paste must be brought into the characteristic electrode cylinder form. Standard sizes range from dia. 200mm to 700mm (8″ – 26″) and lengths from 1500 mm to 2700mm (60″ – 110″). The extrusion process is depicted in diagam 1.

Schematic-of-the-direct-hot-extrusion-process.jpg

Diagram 1: Schematic of the direct, hot extrusion process. McGraw-Hill Concise Encyclopedia of Engineering. © 2002 by The McGraw-Hill Companies, Inc.

  1. The paste is loaded into a thick wall container
  2. The paste is forced through an extrusion die secured in a holder. The extrusion force is applied by a ram with a reusable intermediate dummy block. Pressure is produced hydrostatically.
  3. the paste flow from the extrusion die is in the same direction as the forward motion of the ram.
  4. the extruded paste will be undergoing synchronous cutting

Extrusion force is related to

  • friction between billet length and container
  • material
  • cross sectional area (=diameter) of the final product

The size of the die is generally slightly bigger than that of the final product. The longer the semi finished electrode, the longer needs to be the die. After the material is cut to the desired size, it is cooled at a temperature of 100°C.

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How are graphite electrodes produced? ABC of graphite electrodes (3c) – kneading

this article focuses on the fourth production step for making graphite electrodes >>kneading<<. The small particles whose structure and composition was formulated in step 3, are now intermingled to a paste (remember – those components were the aggregate,- binding,- and filling materials).

There are different kinds of kneaders used

  • Sigma double arms: one pair of blades, discountinuing working mode, blades rotate in different directions with different velocities.
  • Eirich strong kneaders: two pairs of blades with different lengths and rotation directions, enables the material to rotate in four directions
  • pressurized kneaders
  • flaking kneaders for fine grains (up to 0.042 mm)

As heating media, graphite producers use the full range of either electricity, steam or hot oil with the later consisting of Diphenyl Oxide: 26.5% Biphenyl + 73.5% Biphenyl Ether. This is added in interlayer of kneader

Eirich kneaders

Favorized by many graphite companies, Eirich kneaders  allow for rotations in 4 directions and therefore homogeneous properties of the product. Each Eirich kneader has three characteristic components:

media_FD8744_01_1[1251]_eirichmedia_image_croppedthumbnailscheme_100x120

Eirich mixer model 1, Source: Eirich GmbH, website (2018)

kneader_2_source_sigmachina_eirichchina

Eirich mixer model 2, Source: sigmachina – eirichchina

  1. The rotating mixing pan, which delivers the mixture into the area of the mixing tools
  2. One or more mixing tools arranged eccentrically. The direction of rotation and the speed of the mixing tool(s) can be optimally adapted to the different applications.
  3. The bottom/wall scraper, providing additional agitation action. It prevents cakings on the wall and bottom of the pan and facilitates discharge when the mixing cycle is complete.

Kneading principles

Graphite producers follow 3 principles when kneading

  • When kneading temperature is below the standard, the process time should be longer
  • When pitch’s melting point is below the standard, process time will be shorten
  • Process time will be longer if material particles are relatively small.

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Sources:

https://www.eirich.com/en/mixing_principle

ABC of graphite electrodes: how are graphite electrodes produced? (3b) – formulation

this article focuses on the third production step for making graphite electrodes >>formulation<<. Small particles are combined with larger ones to form what we call the “bonestructure and binding material” of graphite electrodes. Our goal is to create a dense grain structure keeping the formation of pores to a minimum.

For each graphite electrode manufacturer this stage is a well kept trade secret. As explained in the raw material section, the most ideal raw material to produce graphite electrodes is needle coke. Therefore, electrodes of the highest quality contain as much of needle coke as possible – 100 %. However, as we will see later, there are also inferior qualities sold and so the formulation may contain petroleum coke as a substitute for needle coke.

There are three components to the structure (please also compare diagram below)

Screenshot (1)
particle structure of graphite electrodes. Source: GES Europe
  • aggregate material (= bone structure)
  • filling material
  • binding material (=pitch)

We call a grain size big with grain diameters is bigger than 1 mm. Medium in the range of 0,8 mm to 0,5 mm. Small grains have diameters of between 0,5 mm and 0,042 mm the last being the technical minimum at the moment.

Aggregate and filling material might be either petroleum or needle coke depending on the targeted quality. The three parameters to compose a recipe are thus: choice of raw material(s), grain sizes of aggregate and filling materials, ratio of aggreate, filling and binding materials.

As the name suggests, pitch is used to bind the different grains together and further increase density of the structure. It is also important for the later forming process. For the production fo graphite electrodes, this will be extrusion. Therefore, a higher content of pitch is desirable. Reason: for extrusion, bigger grain sizes are use for aggregate and filler; therefore more pitch is needed to fill up the pores.

Since the most important aspect about the composition is the different grain sizes to generate a high density, there is almost no limitation to the recipe. One formulation using four different grain sizes (petroleum coke alone, needle coke alone, or a mixture of both) might read as follows:

-> 0,8 mm – 40 %; 0,4 mm  – 20 %; 0,2 mm – 20 %; 0,1 mm – 20 %.

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