In situ monitoring of used-sand regeneration in foundries by impedance spectroscopy

Abstract

Abstract. This work deals with the impedimetric monitoring of used-sand regeneration in the foundry industry. During the regeneration of used sand, a quartz sand similar to new sand is produced from already used molding and core sand, which especially serves to produce new cores. We explore whether the regeneration progress can be assessed in situ based on measured impedance spectra and their features. The impedances of plate capacitors filled with different typical used-sand mixtures, consisting of quartz sand, coal dust, and bentonite, were repeatedly measured in a frequency range from 500 Hz to 1 MHz. The reproducibility of the measurements proved to be sufficient for practical applications. The mean impedances were plotted in Nyquist diagrams. From these plots, systematic impedance–composition correlations could be determined for two of the three component systems. Conclusions about the regeneration state could be drawn from the impedance curves by introducing various features. These were the median, the mean, and the standard deviation of the frequency-dependent resistance and reactance. With these indicators, it was then possible to establish one-to-one relations between the material composition of the molding materials and the impedance measurement. In field measurements on regenerated used sands, this observation was confirmed, and the regeneration progress was observed on the basis of the impedimetrically determined characteristic data.

Classification of Sand-Binder Mixtures from the Foundry Industry Using Electrical Impedance Spectroscopy and Support Vector Machines

“…We focused on two types of features: statistical features of impedance spectra on the one hand (feature class A, [ 37 ]) and parameters of approximation curves fitted to the impedance–locus curves on the second hand (feature class B, [ 37 ]). The four features belonging to class A used in our studies were the median and the mean of both the measured resistance and the measured reactance of a MUT sample over the entire frequency range (see [ 17 ]). The three features belonging to class B were the slope, the intercept with the vertical axis, and the coefficient of determination of a regression line for the measured impedance–locus curve of a MUT sample in double-logarithmic representation.…”

“…For example, Robinson et al [ 21 ] mention its effect on the permittivity of the measured substance. In our previous EIS analyses of different sand types and mixtures, we were also able to show that the impedance spectra are characteristic of each mixture at the given particle size distribution of the mixtures [ 17 , 22 , 23 ].…”

“…MUT 5 and 6 are mixtures of MUT 1 and different prefabricated typical binder mixtures. MUT 1 and MUT 6 have been used in previous investigations [17], and MUT 4 in [23]. Bentonite is denoted as "B", and other additives are denoted as "A".…”

“…(see [17]). The three features belonging to class B were the slope, the intercept with the vertical axis, and the coefficient of determination of a regression line for the measured impedance-locus curve of a MUT sample in double-logarithmic representation.…”

“…Process optimization in terms of energy consumption and product yield is highly desirable, but it is not at all clear how the continuous process state monitoring required for such a closed-loop system could be realized in the field and at a reasonable cost. We have investigated the merits of EIS as a measuring tool to identify the process moment at which regeneration is complete, and the processed former waste sand is available for reuse [ 17 ].…”