Fig 2 Two types of the tapping designs
AC power source needs three electrode columns within the furnace for the three electrical phases. These electrodes have increased the arc flare during operation which can impinge on the refractory side walls resulting into hot spots. The hot spots are to be taken care of by the refractory design. AC EAF also needs three holes through the refractory roof and the centre section of the refractory roof between the electrodes is often an area which limits the furnace performance. The DC EAF has a single electrode through the roof with the electric arc passing directly to the liquid steel bath which contacts the bottom anode electrode to complete the electric circuit. DC EAF has a lesser arc flare to the refractory side wall and hence no hot spots. Roof design is simpler with less difficult operating conditions. However, the furnace hearth is to contain the bottom electrode, which complicates the refractory design of the furnace bottom.
The use of supplemental O2 lances and burners for the increase of the melting rate impacts refractory design and performance. O2 directed from the lances or burners can be deflected by scrap or charge materials and can impinge on the refractory lining. This results in localized overheating and accelerated wear of the refractory. Localized oxidizing conditions can also occur because of O2 and this can result in speedy erosion of the refractory lining.
Refractory wear mechanisms in EAF
There are several types of refractory wear mechanisms as described below to which EAF refractories are subjected to during the making of steel. It is necessary to understand properly the wear mechanisms operating in each zone for proper designing and managing of the EAF refractory system.
Corrosion – It is the most important wear mechanism in case of EAF refractories. Corrosion takes place due to the chemical reactions of the metallic oxides (FeO, SiO2, or MnO) in the slag with the refractory materials. Magnesia (MgO) from the refractory lining is soluble in the liquid slag, with saturation levels ranging from 6 % to 14 %, depending on the FeO content and the bath temperature. The chemical corrosion reactions result in the wearing of the lining and the product of the reactions become part of the slag. Corrosion reactions can be minimized by neutralizing FeO with fluxes and controlling the O2 content of the slag. Corrosion can also be minimized by saturating the slag with MgO through external means (e.g. addition of calcined dolomite or calcined magnesite). Another way to control corrosion is to use refractory bricks which contain carbon (C). The C in the refractory deoxidizes corrosive slag at the refractory / slag interface thus minimizing lining corrosion.
Oxidation – In refractory wear by oxidation, C of the refractory lining is oxidized by reacting either with O2 or FeO in the slag. As the C of the refractory lining react (to be followed)