Refractories in the ductwork are alumina bricks (50 % Al2O3 to 70 % Al2O3) or fire clay/alumina guniting mix (40 % Al2O3 to 60 % Al2O3). Both of these materials have the needed combination of the thermal shock resistance and slag resistance for withstanding the operating conditions.
Refractories for DC EAFs
DC electric furnaces have special refractory requirements due to the fact that the return electrode is usually installed in the bottom of the furnace (some DC furnaces use an alternative arrangement with two graphite electrodes). In the case of a current conducting bottom, the refractory lining at the centre of the furnace bottom acts as the anode. A copper plate is usually connected below the conductive refractory and the return copper bus bar is connected to the plate. In this case special requirements for the refractory are low electrical resistance (preferably less than 0.5 milli-ohms per metre), low thermal conductivity, and high wear resistance.
In the case of DC EAFs, the hearth refractory design is to incorporate a bottom electrode. Operating conditions for the bottom electrode refractories are harsh. Localized high temperatures and intense turbulence are common at the surface of the bottom electrode. These conditions need refractories with high temperature stability and strength. For DC EAFs using pin or fin bottom electrodes, the dry vibratable magnesite monolithic refractory containing 5 % to 10 % C can be used. However, it is preferable if the monolithic refractory is more temperature resistant and with higher MgO content. The monolithic materials can be installed either hot or cold. Below the working lining a three layer Mag-C brick is installed. The residual C content of the bricks ranges from 10 % to 14 %.
The billet return electrode configuration employs from one to four large steel billets (around 250 mm in diameter) depending on the size of the furnace. The billets are embedded in the bottom refractory. The billets are surrounded with a basic refractory brick. For the remainder of the hearth, a special magnesite ramming mix is used. Magnesite ramming mix is used to maintain the brick area around the electrode.
The pin type of return electrode uses multiple metal pins of 250 mm to 500 mm in diameter to provide the return path for the electrical flow. These pins actually penetrate the refractory down to the bottom of the furnace where they are attached to a metal plate. Dry magnesite ramming mix is used for the entire hearth lining. This mix is rammed between the metallic pins. Alternatively Mag-C brick can be used in the area around the anode. This helps to improve the furnace bottom life but is more costly. The steel fin return electrode uses steel fins arranged in a ring in the furnace bottom to form several sectors. Each sector consists of a horizontal ground plate and several welded steel fins which protrude upwards through the refractory. (To be followed)#graphiteelectrode #الکترود گرافیت #Grafitelektrot #Графитовыйэлектрод