ingot and return size, etc.). To reduce the energy consumption


of a shaft melting furnace, the following measures should be in place: • high degree of furnace utilization, such as continuous melting opera- tion, if possible;


• shaft size adapted to the feed stock; • automated charging operation; • installation of a laser in the shaft to monitor the filling level and optimize the starting point of charging;


• installation of a shaft cover; • control of furnace pressure; • evaluation of the operating data; • personnel training. Any interruption of the melting pro-


cess leads to a loss of thermal energy, so a continuous melting operation is the most efficient. Also, when the process is interrupted, molten metal resolidifies. When it is resumed, the lost melting energy must be input again. This double-melting promotes oxida- tion and formation of dross. Optimally using the capacity of the


furnace is not always possible. In situ- ations of low furnace utilization, the furnace’s holding capacity should be used. While the melting operation is being suspended, the required amount of molten metal can be taken from the holding bath. During this time, the fur- nace operates exclusively in the holding mode. Heat loss can be minimized by closing the shaft cover. Only when the bath is 50% empty should a new melt- ing campaign start, which should run over several hours. Material preheating is another de-


cisive factor in optimizing energy use. The wide range of energy consump- tion rates of shaft melting furnaces is largely due to the difference in shaft geometries. An energy-efficient shaft furnace features a cold feeding zone, followed by a warm shaft zone, in which the material is preheated, and a melting zone, in which energy density should be as high as possible. A uni- formly charged shaft with high filling height and high packing density also aids in high thermal energy utilization. Shaft furnaces are charged through


lifting and tilting devices. To ensure the shaft is optimally filled at all times, the shaft cross sections may be scanned by a laser beam at a suitable position below the charging furnace. The laser-based monitoring system detects the filling level directly in the shaft, enabling the charg- ing process to start as early as possible.


38


In a typical diecasting shop, effi-


ciency can be enhanced further with the installation of a roller table as a charging device (Fig. 3), where charg- ing and furnace plant control are co- ordinated in such a way that handling of the returns and ingot material take place automatically. The surface temperature of furnaces


often is used as a measure of radiation loss. However, this neglects heat losses via furnace doors during charging. Melting furnaces designed for charging metal from above into the cold shaft zone suffer virtually no heat losses during charging.


The only measure for the overall


energy performance of a furnace is the overall efficiency, which is ob- tained by multiplying the efficiency of the firing system by the efficiency of the furnace technology. Good shaft melting furnaces achieve efficiencies better than 50%.


MC About the Authors


Klaus Malpohl is development manager and Rudolf Hillen works in development of melting technology for StrikoWestofen GmbH, Wiehl-Bornig, Germany.


For More Information


“Clean Up Your Aluminum Melts,” R. Gallo, MOD- ERN CASTING November 2008.


Ensure Melt Quality Early, Reduce Energy Use


I


n the past, diecasting operations where dominated by mass produced products, but more diecasters are fo-


cusing on products with exacting quality requirements, due to the development of new alloys and casting techniques. Casting defects like oxide inclu-


sions and porosity often are caused by insufficient quality of the molten metal. A common practice is to treat the metal by purging gases while the melt is in the transfer ladle on its way from the melting furnace to the dosing or bale-out furnace, but this requires time, capital investment and energy. By maintaining a clean melt from the beginning, the melt temperature can be reduced since the treatment of the bath is no longer needed. The cleanliness of the melt is in-


fluenced by the melting and holding process and quality of the charged


material. For standard aluminum-silicon alloys, two-chamber melting furnaces can produce melts with a low enough density that they can be cast without any additional treatment. For higher-grade castings, the melt


treatment can commence in the hold- ing bath of the melting furnace by means of porous plugs (Fig. A). The main objectives pursued by inert gas purging are to de-gas, pre-clean and homogenize the melt. However, treat- ment by purging plugs installed in the bottom of the holding chamber is not recommended if the furnace is batch- charged or must be completely emptied often. Additionally, the purging plug may be infiltrated by molten metal with bath temperatures of 1,472F (800C) or above. This impairs the efficiency of purging and increases the consumption of purging gas.


MC


Fig. A. Porous plugs installed in the bottom of the holding chamber improve the quality of the melt by purging gas.


MODERN CASTING / August 2010


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