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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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,

Graphite electrode prices on a roller-coaster ride (2)

Four main factors cause graphite electrode price chaos

Of the global graphite production, approximately 70% are being accumulated by China. For meeting its own steel industry’s huge demand, China is already trying to keep graphite inside of its own borders by imposing export duties.

Not least these measures caused the increase of graphite electrode prices, which oscillated between 500 and 600 US-Dollars around the turn of the millenium. Reaching their peak in 2012 at roughly 3.000 US-Dollars per ton, electrode prices quickly plummeted under the level of 2.000 US-Dollars.

graphite electrode prices, JAN 17 - APR 18

However, after graphite electrode prices had hit an all-time low, vast turbulences erupted in spring 2017 culminating in record high graphite electrode prices. Panic purchases by steelworks managers, unreliability of many partners and general upheaval were the consequence. But what caused those turbulences? We found a total of four main factors.

  1. the decrease of available raw materials for graphite.
  2. an increase in demand for steel.
  3. the chinese government introduced bold restrictions.
  4. partners involved acted unpredictably.

As a consequence of all these factors, chaotic market situations emerged, which complicated the situation further. The common denominator? The biggest graphite producer – China!

Supply, demand and governmental intervention

The demand for graphite electrodes skyrocketed in 2017 due to the steel industry’s demand. Yet, the recovering global economy and thereby driven steel industry only faced empty graphite warehouses, because the producers sold most of their stocks after the lowest electrode prices in history. Everyone with stocks, had quickly sold them after the first few demand increases at relatively low electrode price levels.

Just as the West, China is increasingly interested in environmental protection. For solving this tremendous task, the government tightens environmental regulations to a strangeling degree. This caused many graphite electrode producers to halt their production in favor of government inspections, shrinking the total output in spring 2017 to almost zero.

Facing this supply bottleneck, end users paid double or triple of the regular electrode price, causing further electrode price increases. Some suppliers noticed that willingness, disregarded contracts and sold in favor of the highest bid. Consequences were not to be feared, since the contracting parties were to far abroad. By declaring “Force Majeure” some European contractors also ignored contracts hoping for better conditions but causing further insecurity.

And – what does the future bring?

So, what will the future be like? Well, even stricter environmental regulations are a heavy burden for the graphite industry. The latest example happened in northern China from 2017 to 2018, when governmental actions caused the production to stop between October and March.

Furthermore, the most populated nations in the world, China and India, are experiencing the rise of an increasingly wealthy middle class, which yearns to show off their wealth in western means – how about a trendy and sophisticated electric vehicle, which communicates perfectly with the owners smartwatch and smartphone? Due to these and similar shifts, the Swiss UBS predicts a rise of global graphite demand in the next decades by an immense 264% of todays world market volume.

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Sources
1) „Graphit hat Zukunftspotenzial“, In: miningscout.de, 19.01.2018,
https://www.miningscout.de/blog/2018/01/19/graphit-hat-zukunftspotenzial/, accessed: 22.05.2018.
2) UBS-Studie: Diese Rohstoffe profitieren vom Siegeszug der Elektroautos“, In: wallstreet-online.de, 23.10.2017, https://www.wallstreet-online.de/nachricht/9996039-ubs-studie-rohstoffe-profitieren-siegeszug-elektroautos, accessed: 22.05.2018.