Content of the material
Introduction to Understanding Electric Arc Furnaces (EAF)
Electric arc furnaces are a firebrick-lined U-shaped vertical vessel in which scrap steel is melted by an electric arc instead of the usual fossil fuel. They are sometimes used as mini-steel producers, manufacturing steel products such as structural steel rods and bars.
Many years ago I was engineer in an aluminum smelter here in the Highlands of Scotland, and we had an EAF which used to scare the life out of me. Continually emitting sparks and flames, to say nothing of the fumes, the electric arc furnace transformer was in a nearby switch room and the breaker used to drop out with such a bang!
I am sure we will find out together in this article that technology in EAFs has advanced since the 1970’s, just as the health impacts around electric arc furnaces have improved by the installation of HVAC suction units drawing the gasses and particulates away from the immediate working areas.
This then is an article on understanding electric arc furnaces, and we begin with a brief history of these furnaces.
Step 3: Winding a New Secondary Coil
This part is what gives the transformer a bit more “oomf” to it. Also, it allows us to hook it up to our leads to create an arc.After cleaning out the transformer is done with, and creating an “I” block, place this block into a bench vice and wind your 8 gauge wire around your contraption how many wires high as you can and then repeat for however many layers outward (easier said than done, trust us). Fold the paper over the wiring and tape the sides shut (please do this, it’s an integral part of keeping it all together.) Remove the top of the “I” and pull out your wiring in one piece. Tape your wiring so it retains its shape for extra measure.
Place your new secondary coil back into the shell with the wire ends facing out in the opposite direction of the first coil. Replace the lid of your shell that you cut off and glue on with epoxy* or a very strong adhesive.*Beware, as certain two part epoxies may react with metal and cause possible problems. Make sure the epoxy is safe for metal.
YC: Conceptualization, methodology, model development, model validation and manuscript draft. YC and QL: parametric studies and data post processing. AKS and YC: review and editing. CQZ: project supervision and funding acquisition.
Electric Arc Furnace Working Principle and Operation
Electric arc furnaces work on the principle of the electrode forming an arc between itself and the top layer of metal to be melted.
The operation begins with a delivery of steel scrap to the scrap bay, located at one end of the EAF building. The scrap comes in many forms, from shredded crushed cars to kitchen white goods, steel sections, and plates. These are loaded onto a scrap container known as a basket, which has bottom-opening doors, along with some pig iron (an iron ingot product of the blast furnace) as required for the finished grade of steel.
The basket is loaded to ensure that the topmost layer is of light scrap, then a layer of heavy scrap, alternating until the basket contains the correct mix and weight of scrap.
On its way to the furnace, the basket may pass through a large pre -heat system, which uses electric arc furnace industrial heat in the form of exhaust gasses to heat the scrap, thereby increasing efficiency using waste heat.
Next the basket of scrap is transported on its steel wheeled trolley to the furnace where the furnace roof is retracted and ready for the charge.
As I mentioned earlier I was engineer at an aluminum smelter that had an EAF for melting iron, used to secure the rods to the carbon anode blocks. (See my article on Aluminum Smelting from Bauxite). I found charging to be the worst and most dangerous part of the operation- as any molten metal lying in the furnace gets displaced, it spills out the top and sets fire to any oil or grease in the vicinity- very scary!
Anyway, once the charge is in the furnace, the lid is replaced on the furnace and the electrodes lowered onto the top layer of light scrap. An arc is struck and the electrodes start sinking into the scrap, their rate of advance and voltage are controlled automatically, the oxygen burners are energized, and melting commences.
The electric arc furnace temperature of the molten metal is around 2970F, but alarmingly the temperature of the arc is something in the region of 11000F!
A normal melt of 100T would take just over an hour to complete, using about 400kWh of power.
The formation of slag is an important component of EAF steel production, and is encouraged for several reasons as listed below;
1. Slag forms on the top of the molten steel and acts as a sponge to gather the impurities in the metal.
2. Slag acts as an insulation layer on top of the metal preventing excessive heat loss.
3. Slag helps reduce the wearing away of the refractory.
Slag formation is aided by the injection of magnesite and dolomite in the form of magnesium oxide and lime from calcium oxide. (These can also be added to the scrap charge.) Now the melt has reached its end product, another basket of scrap can be added and once all metal is completely molten, samples can be taken and the molten steel adjusted by addition of more slag producers and injection of oxygen with inclusion of chemicals to burn away the typical impurities of sulphur, aluminum manganese, and phosphorus (their oxides being held in the slag).
This can cause the slag to foam vigorously and spill out the slag overflow.
Once the samples have confirmed, the correct temperature and composition of the molten steel it is then tapped into the pre-heated cradle where alloys may be added along with more lime.
Reference Web: P2Pays – EAF working principles and operation.
Step 7: Improvements
To improve this project, we should have realized that the dimensions of transformers and the amount of wire needed also varies. Our secondary coil could only fit 5 wires high and 3 wires out, while another transformer may fit 6 wires high. The videos make winding the wire seem a lot more simple than how it really went (with blood, sweat and tears). More safety precautions and some sort of case or cover for the EAF could be implemented. Another important factor is that the EAF can only run for about 2-4 minutes tops, whereas any longer would result in the electrical tape and rubber insulation completely melting from the intense heat. Furthermore, the carbon rods end up deteriorating after continued use.
Footnote: Our project failed to work, as we made our tolerances on the “I” block too small, and upon replacing the new secondary coil, we scraped off the insulated coating and exposed the copper wire to the laminated iron shell of the transformer (which is a huge problem for obvious reasons). Another possible issue is due to the sharp corners of the iron core, which also cut into the coating. We will be attempting to redo and replace the secondary coil, and possibly try to file/round the sharp edges. However, the procedure has been fixed for this instructable and should work as intended. We are submitting this for a school project, and plan to complete this and make it functional within the next week or two. We’ll be posting an update of the testing soon (with a video).
Other information:The electrodes (Carbon/Graphite rods) had a resistance of about 3Ω, and our “failed” transformer had a voltage input of about 5V and an output of 0.1V. However, we found some cuts on the insulation of the secondary coil and opted not to test further due to the definite chance of a short circuit from the coils to the iron shell.
If you have any suggestions, criticism, or concerns, please let us know in the comments. Enjoy!