Refractory for arc furnace bottom

1. Dc arc furnace bottom electrode

For THE Clecim type OF DC arc furnace, the anode next to the steel rod is an electrode sleeve brick with the bottom dry ramming material outside. Unlike an AC arc furnace, this part of the refractory material (mainly electrode bushing bricks) is corroded very seriously by current concentration in the center, current shock and electromagnetic stirring. The bottom of an AC arc furnace is eroded and scoured more slowly, while a pot shaped pit forms below the DC arc furnace electrode. In order to improve the life of the bottom electrode, the following methods are adopted :(1) the electrode is placed at the bottom of a small movable furnace, which can be replaced by heat quickly to a certain extent; (2) Hot repair. That is, when the bottom electrode loss to a certain extent, out of steel, steel rod butt on the bottom electrode, and then with refractory materials (dry material or hot material, etc.) around the steel rod, so that the bottom electrode life can reach more than 1500 furnaces. In fact, with this repair method, the bottom electrode life can reach more than 3000 furnace.

For GHH and VAI types, dry ramming is used between the pins (sheets). The current is relatively dispersed and the electrode loss is uniform. Under the condition of not repairing, the life of bottom electrode is 300~1000 furnace. In order to improve the service life, the base electrode was repaired. The methods adopted are as follows :(1) after the furnace cools down, remove residual slag, weld extended steel needle, and then tie magnesia dry material in the gap between the welded steel needle; (2) The conductive refractory material can not be used for heating before. This makes the bottom electrode life up to 1000 furnace, it is expected to reach 4000 furnace. Experimental studies have been done in this field for a long time and satisfactory results have been obtained. At present, GHH also adopts a small furnace bottom with movable electrode area, which can be changed by heat quickly (within 8h).

For AN ABB type DC arc furnace, it appears that the whole furnace bottom is conductive and the current is dispersed. However, the base electrode erosion is still uneven. When the erosion of the bottom center is serious, the base is thin and the resistance becomes smaller, the current density increases and becomes hotter, so the erosion is faster and the pit is deeper. Therefore, the base electrode of this structure is repaired frequently. As the ABB type DC arc furnace bottom is the same as the modern ULTRA-high power AC arc furnace, it can heat up at any time and cool down for cold up, so its service life can be maintained for a long time. More than 13,000 furnaces have been recorded.

 

2. Refractory for bottom electrode of DC arc furnace

The life of bottom electrode of DC arc furnace is actually the life of refractory material. So the refractory material for the bottom electrode of dc arc furnace is very important. Although DC arc furnace has been developed for more than 20 years, it is considered that the bottom electrode of DC arc furnace is the key, which determines the service life of the bottom electrode.

For the Clecim base electrode, there are magnesia, magnesia carbonaceous, magnesia aluminium and magnesia chromium. Europe prefers carbonless basic bricks, while Japan considers thermal shock a serious problem and should use magnesium bricks with high carbon content, and has achieved 1311 furnaces. Perhaps to further improve the thermal shock and erosion resistance of cask bricks, magnesia alumina spinel materials and magnesia chrome bricks were used in Europe. Since the Clrcim base electrode relies on thermal repair to maintain its high life, the repair material and construction effect are critical. These materials have also been successfully used in ultra-high power AC arc furnace bottoms. Refractory materials around the bottom electrode of Clecim DC arc furnace are subjected to intense molten steel movement, thermal shock, and scrap loading. In order to reduce erosion, reduce repairing times, improve the life of bottom electrode and improve the operation efficiency of furnace, it is necessary to study the bottom electrode cover brick and repairing material deeply.

The service life of mGO-C brick is only 50 furnaces when the base electrode of small DC arc furnace is used with magnesium carbon sheath brick with carbon content of 18%. The erosion rate of MGO-C sheath brick is 7-10mm/ furnace, while that of magnesium carbon sheath brick with low carbon content is significantly increased, up to 76 furnaces, that is, the erosion rate is 5-7mm/ furnace. The service life of magnesium carbon base electrode sheath brick in domestic small arc furnace is also more than 300 furnaces. The reasons for the serious erosion of magnesium carbon bushing brick are as follows :(1) the design of bottom electrode is unreasonable, the current density is too high, which causes the steel rod to overheat and melt too deeply; (2) The intense central electromagnetic stirring causes the carbon in THE MGO-C brick to dissolve quickly into the steel, which destroys the bonding of the MGO-C brick and accelerates the erosion of the magnesia particles. Therefore, mGO-C bricks with high carbon content have poor erosion resistance and low life.

For a dc arc furnace with conductive electrode at the bottom of a steel needle or sheet, more dry materials are used between the needles or sheets, and some are made into prefabricated parts. Mgo-c materials are now available in Japan to increase longevity. It is impossible to greatly increase the life of the base electrode by improving the performance of the refractory. Therefore, in recent years, foreign countries have carried out hot work. The thermal filler material used is a conductive refractory (conductive steel needle and steel sheet buried). The MGO-FE thermal filler was developed in Europe. The author has carried on the development work, and in the induction furnace bottom test, has obtained the more ideal result. The conductive ramming material developed by the author has been tested in domestic 10T DC arc furnace (GHH), and the hot and cold bottom filling electrode has been successfully obtained. So as long as the bottom electrode is repaired on time, the life of GHH or VAI type bottom electrode can reach thousands of times, even more than ten thousand furnace times.

The conductive refractory for ABB type bottom electrode has been thoroughly studied by the author. The conductive magnesium carbon brick, cold ramming material and hot patching material have been used in ABB type and GHH type DC arc furnace bottom electrode and satisfactory results have been obtained.

Improving the service life of bottom electrode of DC arc furnace is one of the important contents in the development of DC arc furnace. To improve the service life of bottom electrode is to improve the service life of refractory materials. Therefore, it is the most effective method to improve the quality of bottom electrode refractory and strengthen the repair.

 

3. Refractory for ac arc furnace bottom

Because the history of ac arc furnace steelmaking is relatively long, the refractory material used for furnace bottom has also undergone a change. The bottom of the furnace began to be pounded with bituminous tar magnesia or brine magnesia binder. Because the brine magnesite bottom erosion faster, can not meet the use requirements, and although the bitumen tar magnesite durability is better, but construction trouble, and there is environmental pollution. As a result, more magnesia bricks and lower grade magnesia carbon bricks were subsequently used to build the bottom of the furnace. The brick furnace bottom has good durability, but it takes time and labor to build, sometimes the brick will float up and cause the production to stop. In the 1990s, China began to use the furnace bottom dry ramming materials. Its construction is simple, convenient, do not bake, after the construction, can be directly put into use steel. The material is magnesium calcium iron series dry vibration ramming material. By using the low melting point calcium ferrite contained in it, it can be sintered quickly to produce strength when the temperature is above 1200℃. Under the hydraulic pressure of molten steel, it can be rapidly sintered and contracted, with the shrinkage reaching 4%-5% and the volume density increasing to more than 3.0g/cm. It has excellent corrosion resistance and good results. The refractory per ton steel consumption has been reduced from 4-8kg of asphalt magnesia brick to less than 2kg. Some modern eAF dry bottom feeders consume as little as 0.3kg per ton of steel. This mainly corresponds to the steelmaking process technology; In the process of use, the material is sintered layer by layer, sintered and densified near the molten steel, resistant to the erosion and erosion of molten steel, sintering layer below is a transition layer, and then a loose non-sintered layer. This loose layer has low density and low thermal conductivity, which plays an important role in the insulation of arc furnace. After 10 years of promotion and development, the vast majority of China's electric furnace bottom use this kind of ramming material.

The construction and use of dry bottom material are the key factors affecting the effect of operation. The bottom of the furnace shall, more appropriately, be called dry vibrating castable. Because the density of vibration densification of this material is higher than that of ramming by more than 0.2g/cm, the erosion resistance will be significantly improved. Therefore, the dry material should be vibrated, especially for the dc arc furnace bottom, which is not easily repaired by heat. Unfortunately, the vibration construction of this material is not much, and most of them are made densified by simple construction methods such as pounding or trampling, so the potential of this material cannot be fully developed. During the repair, the slag and scrap at the bottom of the furnace must be cleaned up; otherwise, when used to a certain extent, the steel slag under the repair material may melt and float the bottom material under the impact of the steel block. When the steel is discharged, the floating furnace bottom block may block the outlet and affect the discharge. In the process of use, the bottom of the furnace should be observed at any time, when there is a rise, should be repaired in time (generally with the same dry material to fill the pit), to prevent further expansion of the rise and prevent the occurrence of steel leakage phenomenon. It is worth noting that in the use of the first few furnaces, because the sintering layer is thinner, the ability to withstand the impact of scrap steel is weak, should be added light material, and lower the feeding basket, to prevent scrap steel on the bottom of the furnace to have a large impact.