Polarization effects of unconsolidated sulphide-sand-mixtures

Due to the increasing demand for some metals the investigation of abandoned mine waste dumps for the potential reuse of ore mining residues becomes more important. This study examines the suitability of the spectral induced polarization (SIP) method to distinguish different ores in the mine dumps and evaluate the deposited amounts. We investigated SIP characteristics of four sulphides: pyrite (FeS 2), chalcopyrite (CuFeS2), galena (PbS) and sphalerite (ZnS). Laboratory measurements on fully saturated synthetic ore-sand-mixtures (FeS 2, PbS, ZnS) with three different grain radii (between 31 μm and 500 μm), three different ore concentrations (0.5 vol.-%, 2 vol.-%, 6 vol.-%) and with different electrical conductivities of the pore fluid (between 25 mS/m and 400 mS/m, sodium chloride) for pyrite- and chalcopyrite-sand mixtures have been carried out. A new four-point sample holder for precise and reproducible measurements of the unconsolidated mineral-sand mixtures has been developed. The measurements were done at constant temperature of 20∘ C within a frequency range from 1 mHz to 45 kHz using a high-accuracy impedance spectrometer. Several trends for the induced polarization signature have been observed. First, the polarization signal increases with higher ore mineral content. Second, both phase amplitude and peak frequency increase with decreasing grain radius. Third, the frequency of the phase maximum is shifted to higher values for increasing fluid conductivity. Fourth, all ores showed similar phase characteristics, solely sphalerite indicated lower phase magnitude. By fitting the IP spectra on basis of a Cole-Cole model, the parameters chargeability (m) and time constant (τ) have been derived. The following relations were confirmed: 1) chargeability rises with increasing ore mineral concentration. 2) The time constant increases with increasing grain radius. Ore-specific quantities are determined by these relations. The results show that conclusions about the ore mineral content, grain radius and fluid conductivity can be derived from laboratory SIP measurements. The method seems to be suitable to identify sulphide minerals in sandy mixtures but a differentiation between the different types of ore minerals remains a challenging task. Nevertheless, more investigations are required to improve the reliability of these findings towards a field scale application.