Possibility of eddy current testing of low-conductive heterogeneous media

Abstract

The possibility of copper ore quality testing for proportion of minerals containing copper in it by the eddy current method has been studied. Copper ore is presented as a low-conductivity heterogeneous medium consisting of two homogeneous ore constituents (enclosing rock and copper mineral). These constituents have significantly different specific electrical conductivity (SEC). For the analysis, the effective medium approximation is used. In this case a heterogeneous ore can be considered homogeneous with an effective SEC, which depends on the quantitative composition of its constituents. To solve this problem, the surface parametric eddy current probe (ECP) of a local type has been developed. Its design is optimized for operation at high frequencies (units – tens of MHz). ECP winding consists of 10 turns with a diameter of 11 mm, which wound up in one layer on a dielectric frame. The ECP signals in the complex plane from samples of copper ore with different content of copper mineral (chalcopyrite) in the resonant operation mode are studied using eddy current flaw detector of Eddycon C type. The operating frequencies are 5, 7 and 9 MHz. It has been established that with increasing frequency in the specified range, signals amplitude has a pronounced tendency to decrease. It is shown that as the frequency increases from 5 to 7 MHz, the phase sensitivity of the developed ECP to the content of chalcopyrite in the ore also decreases. Thus, the phase angle between signals from ore samples with low and high chalcopyrite content at the frequency of 5 MHz is 110°, and at the frequency of 7 MHz – 87.5°. With a further frequency increase to 9 MHz, the phase sensitivity remains practically unchanged, and the amplitude of the signals continues to decrease. This can be explained by a decrease of the ECP sensitivity to SEC changes with a transition to the over-extreme operating point of the complex plane by operating frequency increase. The fundamental possibility of using the eddy current method for the quantitative testing of chalcopyrite in copper ore has been confirmed. Prospects for further research related to the development of an integral-type ECP, in particular, screen ones, whose signals are determined by the content of chalcopyrite in copper ore samples as a whole, are formulated.