This article describes the masonry characteristics of refractory materials at the bottom of the conductive furnace and the matters needing attention in the normal use of conductive refractory bricks for DC electric furnaces.
Key words: steelmaking; conductive magnesia carbon brick; masonry characteristics; precautions
Conductive magnesia-carbon refractories for steelmaking electric furnaces were first developed by Radex in Austria, and have achieved good results in use. The magnesia-carbon brick produced by Radex Company has been specially treated (model PMK9412VT), and the resistivity (1.8~2.0) has little change with temperature. Cold starting is possible due to good electrical conductivity at low temperatures. At the same time, the company has also developed a conductive ramming material (with a residual carbon content of 15%, and a resistivity of the order of 10-3Ω·m) and a magnesium-carbon high-temperature castable (after drying for 30 minutes, a resistivity of 2.5×10-4Ω·m ).
To ensure electrical conductivity, flake graphite is used between the horizontal joints of the furnace bottom working lining and the conductive magnesia carbon bricks of the permanent lining. Its working lining and permanent lining are all fired bricks. The refractory materials used for repairing include cold ramming materials and high temperature repairing materials. The first furnace service reached 2250 times. The consumption of repair materials is: cold ramming material 0.19kg/t, high temperature repair material 0.20kg/t. Table 1 shows the physical and chemical properties of a furnace bottom refractory material.
| Type | Working lining fired bricks | Permanent lining fired bricks | Ramming material | Repair material | |
| Chemical components/% | MgO | 76 | 86 | 74 | 46 |
| CaO | 76 | 86 | 74 | 20 | |
| fixed carbon | 20 | 10 | 16 | 19 | |
| Resistivity/Ω.m | finished product | 0.00005 | 0.0002 | ||
| 200℃×5h | 0.0046 | ||||
| 1400℃×2h,burial | 0.00004 | 0.00017 | 0.00018 | 0.0008 | |
| 1000℃ thermal conductivity/W.(m.℃)-1 | 19.2 | 11.0 | 6.0 | ||
What are the characteristics of refractory masonry at the bottom of the conductive furnace?
The lower part of the bottom electrode is vertically built with two layers of conductive magnesia carbon bricks with a total thickness of 775mm. The brick joints between the two layers of bricks are staggered and have a certain angle. Knotted and tamped with insulating material in between, the temperature measuring points No. 1~4 are symmetrically installed at the depth of 150mm in the lower conductive brick, and another 8 temperature measuring points are evenly assembled on the circumference of the steel-copper composite board to realize the close monitoring of the anode temperature (figure 1). The permissible temperature of No. 1~4 is 600℃, and the bottom of the furnace is forced to cool by axial flow fans.
Figure 1 Bottom electrode structure
What points should be paid attention to in the normal use of conductive refractory bricks for DC electric furnaces?
The carbon content of the anode conductive brick of the DC electric furnace is between 10% and 20%, and the carbon content of the conductive sand carbon is about 5%. After a special heat treatment process, the fixed carbon in it is guaranteed to conduct electricity. In actual operation, the following points should be done , to protect the furnace bottom:
(1) After hot repair, at least one furnace of steel should be smelted before the furnace can be shut down to prevent the conductive brick from being exposed at high temperature and oxidized and decarburized.
(2) Normal smelting must adopt the operation of retaining steel. When the furnace is planned to be shut down, 10t of scrap steel should be installed to make it have good contact with the remaining molten steel.
(3) Pay attention to the amount of carbon and avoid peroxidation of molten steel as much as possible.