rmw,I will try and reply to your question in a simple way.Sorry if you find it rather long.
> Arc furnace is a big time energy guzzler.
> Also it has no restrictions on the kind of inputs charged.
> However, the scrap density and quality is always monitored.
> Light scrap like machine shop generated turnings and borings are popular.
> Shredded automobile scrap (properly pressed and baled) is added,but the percentage addition is restricted as this is a major source of tramp elements.
> Heavy melting scrap
> Lastly melting shop and plant rejects and returns.
The scrap should permit the graphite electrodes to strike an arc and penetrate. Along with the charge limestone is added to form a slag. It is in this slag metal interface that the refining takes place. The first step is to create an oxidising slag. This is done by blowing oxygen into the molten bath and adding limestone .Elements like carbon and Phosphorous are removed. Then comes the reducing slag when coke and limestone,magnesite and calcium fluoride (fluospar) are added to the bath. this reduces the oxygen level, and removes the Sulphur .
Lastly after the reducing slag has been removed a refining slag is prepared . Any carbon pickup ,manganese and silicon additions are done now. If the chemistry calls for any further additions of Ni,Cr or Mo these are done. Finally aluminium is added to kill the bath.
One of the greatest advantages of arc furnace melting is the ability to refine and control,carbon, Sulpur and Phosphorous contents,win important elements from slag without any loss and produce a metal low in gases. This makes it largely popular over the induction furnaces where refining is not possible.
Yes you can still have tramp elements which have not got blown away during oxidation,but their presence is reduced..
Graphite electrode consumption and refractory costs are the two other factors in addition to the power costs which affect the viability.
You may refer to the link below for any further reading.