Steel Mini Mill. A short introduction

Steel Making Process

Steel is an iron alloy that has specific additives that give it increased strength, resistant to corrosion and other properties (depending on the metals added in it).

Steel gives a better life and more durability and has replaced nearly all kinds of iron works. An ancient art, steel manufacturing goes back to ancient Rome, China and even India.

Steel production, however, was done on a limited basis as the process of refining iron ore and the tight quality control required was difficult.

This changed with the Bessemer process, where concentrated oxygen (99% purity) replaced normal air. Since air is nearly 78% nitrogen and is chemically neutral in the process, it would take a lot of time and effort to refine the ore.

Introduction of pure oxygen through the Bessemer process largely increased the efficiency and reduced the time required, making steel a commercial viability. Overtime, other processes have been introduced, which are much more efficient.

Steel Mini Mill & Electric Arc Furnace

Once a secondary process, minimills mostly consists of recycling scrap metal to create steel. Modern steel making industry has changed this, with many industries using it exclusively.

The most common method of steel making in minimills is using and Electric Arc Furnace (EAF). EAFs consist of a large vat known as the furnace, which is lined with corrosion resistant chemicals.

Scrap iron placed inside the furnace in a layered fashion (bottom layer of small pieces, middle of large and then the top of smaller scrap pieces).

How Graphite Electrodes are used

Three thick graphite rods are lowered into the furnace and are electrically charged. When the graphite cores touch the metal, they create an electric arc which heats the scrap.

As the top scrap melts, the graphite rods are lowered and the electric potential increased. After a certain depth, the rods are slightly raised, allowing the liquid metal to pool around it and conduct the heat and electricity more efficiently.

The process is highly controlled and to decrease the time for smelting, the furnace can be with heated through a gas/fuel fire or even charged with pre melted material from the last batch.

Once the scrap is melted and impurities float on top, the furnace is tilted and tapped, allowing for the metal to flow down into a ladle furnace, where further chemicals can be added according to customer requirements.

Since the electric arc furnace is a batch process that can be started up or shut down with ease, it has become a very popular choice as it allows steel manufacturers to follow the market demand and cut down on production without incurring process losses.

Sources

https://books.google.de/books?id=FAud8CE5stsC&pg=PA361&redir_esc=y#v=onepage&q&f=false

https://books.google.de/books?id=FAud8CE5stsC&pg=PA361&redir_esc=y#v=onepage&q&f=false

http://geo.msu.edu/extra/geogmich/minmills.html#:~:text=In%20a%20mini%2Dmills%2C%20electricity,melt%20(primarily)%20scrap%20steel.&text=Minimills%20make%20steel%20from%20scrap,into%20the%20semi%2Dfinished%20forms.

https://www.britannica.com/technology/steel/Electric-arc-steelmaking

http://www.cim.mcgill.ca/ (link below)

Click to access TM18-4_as_published.pdf

 

 

 

 

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.

Do you have any QUESTIONS until here? Please feel free to ask.

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,