Do You Know This Secret of Graphite Electrodes?

There is at least one well-kept secret of graphite producers’: The differences between RP, HP or UHP.

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The Electric Arc Furnace – Part 3 (Operation)

This is the third of the series of EAF for steelmaking. It focuses on the operation part of EAF production.

Operation

A mid-sized modern steelmaking furnace would have a transformer rated about 60,000,000 volt-amperes (60 MVA), with a secondary voltage between 400 and 900 volts and a secondary current in excess of 44,000 amperes. In a modern shop such a furnace would be expected to produce a quantity of 80 metric tonnes of liquid steel in approximately 50 minutes from charging with cold scrap to tapping the furnace.

In comparison, basic oxygen furnaces can have a capacity of 150–300 tonnes per batch, or “heat”, and can produce a heat in 30–40 minutes. Enormous variations exist in furnace design details and operation, depending on the end product and local conditions, as well as ongoing research to improve furnace efficiency. The largest scrap-only furnace (in terms of tapping weight and transformer rating) is a DC furnace operated by Tokyo Steel in Japan, with a tap weight of 420 metric tonnes and fed by eight 32MVA transformers for 256MVA total power.

To produce a ton of steel in an electric arc furnace requires approximately 400 kilowatt-hours per short ton or about 440 kWh per metric tonne; the theoretical minimum amount of energy required to melt a tonne of scrap steel is 300 kWh (melting point 1520 °C/2768 °F).

Therefore, a 300-tonne, 300 MVA EAF will require approximately 132 MWh of energy to melt the steel, and a “power-on time” (the time that steel is being melted with an arc) of approximately 37 minutes. Electric arc steelmaking is only economical where there is plentiful electricity, with a well-developed electrical grid.

In many locations, mills operate during off-peak hours when utilities have surplus power generating capacity and the price of electricity is less.

The scrap is loaded into large buckets called baskets, with “clamshell” doors for a base. Care is taken to layer the scrap in the basket to ensure good furnace operation; heavy melt is placed on top of a light layer of protective shred, on top of which is placed more shred.

These layers should be present in the furnace after charging. After loading, the basket may pass to a scrap pre-heater, which uses hot furnace off-gases to heat the scrap and recover energy, increasing plant efficiency.

The scrap basket is then taken to the melt shop, the roof is swung off the furnace, and the furnace is charged with scrap from the basket. Charging is one of the more dangerous operations for the EAF operators.

A lot of potential energy is released by the tonnes of falling metal; any liquid metal in the furnace is often displaced upwards and outwards by the solid scrap, and the grease and dust on the scrap is ignited if the furnace is hot, resulting in a fireball erupting.

In some twin-shell furnaces, the scrap is charged into the second shell while the first is being melted down, and pre-heated with off-gas from the active shell. Other operations are continuous charging—pre-heating scrap on a conveyor belt, which then discharges the scrap into the furnace proper, or charging the scrap from a shaft set above the furnace, with off-gases directed through the shaft. Other furnaces can be charged with hot (molten) metal from other operations.

After charging, the roof is swung back over the furnace and meltdown commences. The electrodes are lowered onto the scrap, an arc is struck and the electrodes are then set to bore into the layer of shred at the top of the furnace.

Lower voltages are selected for this first part of the operation to protect the roof and walls from excessive heat and damage from the arcs. Once the electrodes have reached the heavy melt at the base of the furnace and the arcs are shielded by the scrap, the voltage can be increased and the electrodes raised slightly, lengthening the arcs and increasing power to the melt.

This enables a molten pool to form more rapidly, reducing tap-to-tap times. Oxygen is blown into the scrap, combusting or cutting the steel, and extra chemical heat is provided by wall-mounted oxygen-fuel burners. Both processes accelerate scrap meltdown. Supersonic nozzles enable oxygen jets to penetrate foaming slag and reach the liquid bath.

Once the scrap has completely melted down and a flat bath is reached, another bucket of scrap can be charged into the furnace and melted down, although EAF development is moving towards single-charge designs.

After the second charge is completely melted, refining operations take place to check and correct the steel chemistry and superheat the melt above its freezing temperature in preparation for tapping. More slag formers are introduced and more oxygen is blown into the bath, burning out impurities such as

Removal of carbon takes place after these elements have burnt out first, as they have a greater affinity for oxygen. Metals that have a poorer affinity for oxygen than iron, such as nickel and copper, cannot be removed through oxidation and must be controlled through scrap chemistry alone, such as introducing the direct reduced iron and pig iron mentioned earlier.

A foaming slag is maintained throughout, and often overflows the furnace to pour out of the slag door into the slag pit. Temperature sampling and chemical sampling take place via automatic lances. Oxygen and carbon can be automatically measured via special probes that dip into the steel, but for all other elements, a “chill” sample—a small, solidified sample of the steel—is analysed on an arc-emission spectrometer.

Once the temperature and chemistry are correct, the steel is tapped out into a preheated ladle through tilting the furnace. For plain-carbon steel furnaces, as soon as slag is detected during tapping the furnace is rapidly tilted back towards the deslagging side, minimising slag carryover into the ladle.

For some special steel grades, including stainless steel, the slag is poured into the ladle as well, to be treated at the ladle furnace to recover valuable alloying elements. During tapping some alloy additions are introduced into the metal stream, and more lime is added on top of the ladle to begin building a new slag layer.

Often, a few tonnes of liquid steel and slag is left in the furnace in order to form a “hot heel”, which helps preheat the next charge of scrap and accelerate its meltdown. During and after tapping, the furnace is “turned around”.

The slag door is cleaned of solidified slag, the visible refractories are inspected and water-cooled components checked for leaks, and electrodes are inspected for damage or lengthened through the addition of new segments; the taphole is filled with sand at the completion of tapping.

For a 90-tonne, medium-power furnace, the whole process will usually take about 60–70 minutes from the tapping of one heat to the tapping of the next (the tap-to-tap time).

The furnace is completely emptied of steel and slag on a regular basis so that an inspection of the refractories can be made and larger repairs made if necessary. As the refractories are often made from calcined carbonates, they are extremely susceptible to hydration from water, so any suspected leaks from water-cooled components are treated extremely seriously, beyond the immediate concern of potential steam explosions.

Excessive refractory wear can lead to breakouts, where the liquid metal and slag penetrate the refractory and furnace shell and escape into the surrounding areas.

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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 – 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

 

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sources

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