Effect of metal proximity on remote electromagnetic detection of molten metal level in pyrometallurgical furnaces

The production of metals such as nickel, copper, and platinum group metals (PGMs) is typically done by smelting concentrated ores in pyrometallurgical furnaces. The process of smelting involves generating high temperatures by arcing or chemical reactions and separating the concentrate into the product, being molten metal or matte and the waste, being slag. The density of the metal is generally higher than that of slag and therefore, slag tends to float on top of the metal. It is desirable to determine the position of the metal and slag interface to maximize production efficiency. Reliable and continuous level measurements are desired by smelting operators to determine when to commence tapping (removal of molten metal from the furnace) and monitor the tapping operation. In addition, metal level control is important for the long-term structural integrity of the furnace. Today, common methods of estimating metal level in furnaces include mass and energy balance calculations and sounding bar measurements from above the furnace. To overcome the shortcomings of the current methods, and to improve the utility and accuracy of metal/slag level measurement, a continuous sensing, eddy current based system, embedded in the furnace sidewall, is being studied. The eddy current drive and pickup coil sensor system developed in this study was able to measure the signal in a laboratory setting at a liftoff distance of 305 mm from the simulated metal/slag interface. However, while the electromagnetic fields pass easily through refractory brick, they may be affected by the presence of conducting material in the vicinity of the probe. The effect of such material, such as copper cooling plates in an actual furnace, on the signal response was examined. It was observed that the cooling plate next to the coil configuration increased the signal by 28 % whereas, when the copper plate was placed behind the coils, the signal was reduced by 74 %. The purpose of this paper is to describe prior art in the field of metal level measurement in pyrometallurgical furnaces, outline the developed eddy current based system and to present the experimental results of laboratory testing.