Subsurface Characterization of Heap Leaching Pads using Geophysical Surveys; Case Study: Sarcheshmeh Copper Mine

Summary There are several important factors to be determined in heap leaching operations such as the depth of pregnant leach solution (PLS) within the heap, saturated and unsaturated zones, and the quality of the solution in different parts of the heap. In this paper, integrated geophysical surveys including electrical resistivity, induced polarization (IP) and ground penetrating radar (GPR) methods were used to investigate the internal structure of heap No. 3 at Sarcheshmeh copper mine. The main purpose was to determine acid saturated zones and investigate the preferential flow paths within the heap. To do this, acid sprinkling on pad No. 903 of heap No. 3 in Sarcheshmeh copper mine was stopped for ten days before doing geophysical surveys. Geophysical field data were collected along five parallel lines. Due to the small area of pad No. 903, spreading of the electrode array for resistivity and IP measurements was limited, thus, that maximum depth of 20 m was investigated. The depth of investigation in the GPR method was also reduced as a result of radar wave attenuation in high conductive subsurface material. PLS saturated zones of the heap, dry zones and solution paths were finally determined based on the results of field data interpretation and the results obtained from laboratory resistivity measurements carried out before making geophysical surveys.
Introduction Considering the advantages of the hydrometallurgical methods, copper production industry is growingly using these methods. In heap leaching process, low-grade copper ores are exposed to sulfuric acid percolating down through the heap and solving the copper. The PLS is then collected at the bottom of the heap to be processed for the extraction of the metals. Knowledge of the saturated and unsaturated zones within theses heaps is one of the most important problems to be understood. Geophysical surveys, considering their great advantages such as being non-destructive, efficient, rapid and cost effective, are a very good alternative compared to the expensive and local drilling operations. Electrical methods are especially useful in such application because there is an electrical contrast between saturated and nonsaturated zones. Therefore, in this research, electrical resistivity, IP and GPR methods have been used on a test part of heap No. 3 at Sarcheshmeh copper mine to investigate subsurface solution behavior.
Methodology and Approaches Application of the heap leaching technology to extract low-grade copper ores was decided by the National Iranian Copper Industries Company (NICICO) in the 1990s. There are now six heap leaching utilities constructed at Sarcheshmeh copper mine and the copper is being leached with dilute sulfuric acid. In order to provide favorable electrical measurements, acid sprinkling on pad No. 903 of heap No. 3 was stopped for ten days before acquiring the electrical data, comprising of resistivity, IP and GPR data, along five parallel profiles on the heap surface. 2D resistivity and frequency-domain IP surveys employing the Wenner-Schlumberger electrode array were carried out using ASTRA-100 transmitter and Mary-24 receiver. The electrode spacing of 5 m was selected for these surveys. Frequency values of 0.61 and 9.8 Hz were applied for the IP data acquisition and as a result, the percent frequency effect (PFE) was calculated. In order to have a good GPR penetration, the 50 MHz LOZA GPR system with the antenna length of 3 m was used in the GPR surveys.
Results and 1. The electrical resistivity values were obtained in the range of 2-13 Ω-m, in which the higher values were likely related to low moisture and dry zones. The acid saturated zones were determined as the areas having the lower electrical resistivity values in the range.
2. Low resistivity values extending to the deeper parts of the pad revealed the preferential flow paths of the leaching solutions
3. PFE values were in the range of 0.5-14 % in which the higher IP values were the result of mineral concentration and saturation in the area.
4. Due to the small area of pad No. 903, spreading of the electrodes used for resistivity and IP measurements was limited, and as a result, maximum depth of 20 m was obtained.
5. Maximum GPR depth of investigation of about 30 m was obtained as a result of radar wave attenuation in high conductive material within the heap.
6. Among the performed geophysical methods, the electrical resistivity method was determined as a superior one in comparison to the IP and GPR methods used for monitoring of the heap leaching in the subsurface.