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Table 4. Ladle Holding Time for Melt Temperature to Drop from 3000F (1649C) to 2,420F (1,326.7C)


Lining


chemical energy, there is a future potential for increasing arc efficiency by utilizing more energy efficient long arcs (higher voltage and lower cur- rent) with a foamy slag, to decrease the heat losses by blanketing the arc. In an industrial trial, chemical


energy from oxygen combustion of natural gas was introduced in a 4 ton EAF through installation of an oxyfuel burner through the door. Effective combustion of natural gas provides energy to the solid charge during the melting period. Te electrical energy consumption was decreased from 480- 500 kWh/ton without oxyfuel burners to 400-420 kWh/ton with burners. Direct injection of oxygen by a lance to the solid charge and melted steel can reduce electrical energy con- sumption by decreasing scrap melting time and direct generation of chemical energy from oxidation reactions in the melt. Te introduction of coherent jet has decreased electrical energy con- sumption 10% and also reduces melt down time 13%. Scrap preheating systems, oxyfuel


burners and postcombustion of CO require additional capital investment. By comparison, the addition of a material such as SiC, which produces exothermic reactions during the oxy- gen blow, does not require any capital investment (Fig. 3). Because the heat of oxidation


reaction is generated within the liquid steel, heat transfer efficiency from exothermic reactions should be nearly 100%. Tis expected efficiency is much higher than the typical 40% efficiency for post-combustion of CO above the bath. In the study, the amount of exothermic heat generated during oxygen boiling was increased by adding SiC with the solid charge. Te energy and operational effects of adding enough SiC with the scrap charge to represent 0.4% to 0.6% of


delays while liquid metal is in the furnace; addition of chemical energy


3


Results and Conclusions Major opportunities for


energy savings were identi- fied as: improvement in scheduling and decreasing


Alumina castable


Low density magnesia board


Alumina porous castable


Preheat, F (C)


1,290(699) No preheat


1,290(699)


the charge weight was investigated in a 20-ton acid-lined EAF. Te addi- tion of SiC reduced electrical energy consumption by 7.1% and increased productivity by nearly 5%. Effective ladle design, preheat


practices and use are important for steel casting production. Te tap temperature of the liquid steel typi- cally is superheated 250F to 500F (121C to 260C) above the steel’s liquidus to compensate for heat losses during tapping and holding in small ladles with large surface area to volume ratios. In spite of the relatively short time the steel is in contact with the ladle lining, the huge thermal gradients in the lining drive high values of heat flux through the refractory surface. Initial information about heat losses during steel ladling was taken from a survey of steel casting facilities and from industrial measurements at seven plants.


The influence of the thermal


properties of different ceramic materials typically used for steel ladle linings on heat losses during use was analyzed. From this work, a new type of ladle lining was devel- oped at Missouri University S&T. It was based on porous ceramics with the potential to significantly decrease heat losses and save consid- erable ladle preheat energy. Te data collected through the


survey and trials was analyzed to determine the factors that were most important to energy losses in the ladle. One of the most important factors was found to be the ladle capacity.


for melting steel; and improvement in ladle practice. CFD modeling, and industrial and laboratory trials deter- mined the effects of these changes in reducing electrical power consump- tion. Tis data will be used in the future for development of a spread-


Measured Time, min


7 9


18 Modeled Time, min


Open top 5 7


12


Isolated top 10 13


30


Te tap temperature was found to be significantly lower for higher capacity ladles. A computational fluid dynamics (CFD) model was used to study the effects of ladle size and validate the industrial measurements. Te temperature of the liquid steel


at tap typically varies between 2,950F (1,621 C) and 3,200F (1,760C) at steel casting facilities. Tese tempera- tures are close to the softening tem- perature of the complex Al, Ca, Si, and Mg oxide compounds which are often used for ceramic linings. Also, the high rate of chemical reactions between the lining and components of the liquid steel and slag takes place at these tem- peratures. As a rule, ladles are not fully soaked even when used multiple times and are therefore used under unsteady state heat transfer conditions. Even in cases where the lining is preheated prior to tap, a significant part of the heat energy from the liquid steel accu- mulates inside the lining during the first 5-30 minutes after tap. Foundry ladle operations require


special ceramic lining materials. A specially designed low density porous alumina castable was introduced. It has very low thermal conductivity and was determined to improve energy effi- ciency in the ladle (Table 4).


ONLINE RESOURCE


Visit www.moderncasting.com to read the paper on which this article was based.


sheet type model to allow metalcasters to calculate energy usage and melt temperature losses. Tis article is based on a research paper, “Increasing Melting Energy Efficiency in Steel Foundries,” presented at the 2012 AFS Metalcasting Congress.


February 2013 MODERN CASTING | 49


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