Development of ultrasonic thickness measurements of solidification front inside metallurgical reactors

The corrosive molten salts used for aluminum electrolysis attack the walls of the electrochemical reactors, reducing their lifetime and increasing production costs. Fortunately, a ledge is formed on those walls by the solidification of the molten salts used as the electrolytic bath, due to the heat losses through the sides of the reactors. This ledge is essential, protecting the sidewalls from the corrosive effect of the bath. Its thickness must however be controlled to avoid a reduction of the efficiency caused by a partial reduction of the electrolysis surface. One challenge is to measure the ledge thickness inside operating cells. The originality of this work resides in the development of an ultrasonic device to provide a non-intrusive ledge thickness measurement. Experimental results are obtained on a specially developed setup using the proposed ultrasonic device and are compared to ledge thicknesses obtained with an intrusive probe. The ultrasonic thickness measurement concept is validated up to 950 degrees C with a satisfactory accuracy inside 6 mm of the intrusive measurement.