The Electric Arc Furnace – History & UHP Concept (part 2)

In this post, we will be talking about the history of EAF-steelmaking & the almost mystical concept of Ultra High Power Arc Furnaces (or short UHP).

History

The first successful and operational furnace was invented by James Burgess Readman in Edinburgh, Scotland in 1888 and patented in 1889. This was specifically for the creation of phosphorus

Though the arc furnace came out in the 1900s, the major development has waited its time till the end of the WWII. During the latest decades, the development has escalated and brought revolutionary changes to the steel industry.

While EAFs were widely used in World War II for production of alloy steels, it was only later that electric steelmaking began to expand.

The low capital cost for a mini-mill—around US$140–200 per ton of annual installed capacity, compared with US$1,000 per ton of annual installed capacity for an integrated steel mill—allowed mills to be quickly established in war-ravaged Europe This fact

also allowed them to successfully compete with the big United States steelmakers, such as Bethlehem Steel and U.S. Steel, for low-cost, carbon steel “long products” (structural steel, rod and bar, wire, and fasteners) in the U.S. market.

Need for Ultra-High Power Arc Furnces

Since the concept of the UHP arc furnace(ultrahigh power arc furnace) emerged in the mid-60s, people have tried to build the bigger furnace with higher power and capacity.

The application of advanced technologies like dynamic compensation system and water-cooling system boosts the development vigorously.

There are several essential elements that caused the evolution:

  • The increasing quantity of scrap steel;
  • The rapid development of the entire steel industry.

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Sources

https://en.wikipedia.org/wiki/Electric_arc_furnace

Degner, M. et ali, Steel Institute VDeh (2008), Steel Manual, Düsseldorf Verlag Stahleisen GmbH.

httpswww.shutterstock.comdeimage-vectormetallurgy-iron-steel-production-electric-arc-1391332520

The Electric Arc Furnace – Advantages & Deficiencies

This is our series on EAF-steelmaking for new in the job managers.

The 5-part series summarizes several articles from wikipedia and the not-so-plentyful literature about the topic.

There is a lot of material covered. After reading the posts, you will be expert in the following topics:

  • Advantages vs Disadvantages of EAF-use (part 1)
  • history and UHP concept (part 2)
  • Operation mode (part 3)
  • EAF vs other furnaces (part 4)
  • Construction (part 5)

If you want to know even more about the topic of Electric arc furnaces, I can also recommend articles written by Matteo Sporchia (on LinkedIN).

Introduction

Industrial arc furnaces range in size from small units of approximately one ton capacity (used in foundries for producing cast iron products) up to about 400 ton units used for secondary steelmaking) (average is 80 to 120 MT).

Arc furnaces used in research laboratories and by dentists may have a capacity of only a few dozen grams. Industrial electric arc furnace temperatures can be up to 1,800 °C (3,272 °F), while laboratory units can exceed 3,000 °C (5,432 °F).

The electric arc furnaces are the most widely used steel furnace around the world. It can be used to smelt high-quality steel and other special steels.

Advantages

  • capabable of producing the full range of steel grades
  • not dependent on a particular type of charge (scrap, sponge, iron, pig iron, hot metal),
  • low capital outlay,
  • melting process can be programmed and automated
  • high efficiency and flexibility

The use of EAFs allows steel to be made from a 100% scrap metal feedstock. This greatly reduces the energy required to make steel when compared with primary steelmaking from ores.

Another benefit is flexibility: while blast furnaces cannot vary their production by much and can remain in operation for years at a time, EAFs can be rapidly started and stopped, allowing the steel mill to vary production according to demand.

Although steelmaking arc furnaces generally use scrap steel as their primary feedstock, if hot metal from a blast furnace or direct-reduced iron is available economically, these can also be used as furnace feed.

As EAFs require large amounts of electrical power, many companies schedule their operations to take advantage of off-peak electricity pricing.

A typical steelmaking arc furnace is the source of steel for a mini-mill, which may make bars or strip product. Mini-mills can be sited relatively near to the markets for steel products, and the transport requirements are less than for an integrated mill, which would commonly be sited near a harbour for access to shipping.

The supply and the price of electricity become stable, which makes it possible to generalize the arc furnace;

  • The arc furnace tends to be larger and more powerful;
  • Less investment, quick to construct and fast cost recovery;
  • The temperature and the component of the molten steel can be controlled with accuracy. The arc furnace can also smelt various kinds of different steels.Compared with others, the arc furnace also has several obvious advantages:
  • The arc can heat the furnace and the steel up to 4000-6000℃directly and smelt special steels that contain refractory elements like W and Mo.
  • The arc furnace could remove the toxic gases and the inclusions while deoxidizing and desulfurating.
  • High flexibility. The arc furnace is capable of engaging production continuously or intermittently.
  • Currently, owing to the development of related technologies, the electric arc furnaces could be well -integrated with traditional steel-making processes. But there are also several deficiencies

Deficiencies

  • The arc can only generate point-like heating sources, which will cause uneven heat distribution in the furnace.
  • The arc will react with the furnace gases and vapor and release large quantities of H 2 and N 2.

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Sources

https://en.wikipedia.org/wiki/Electric_arc_furnace

Degner, M. et ali, Steel Institute VDeh (2008), Steel Manual, Düsseldorf Verlag Stahleisen GmbH.

httpswww.shutterstock.comdeimage-vectormetallurgy-iron-steel-production-electric-arc-1391332520

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

 

Do you have questions? Please don’t hesistate to get in touch with us.

Please SHARE this article.

sources

Click to access 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

 

I hope you enjoyed reading this article on baking green electrodes. Please feel free to SHARE the article.

 

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.

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