An investigation on mechanism of acid drain in heap leaching structures

Gravelly drainage layer and semi-slotted polyethylene pipes are two main components of drainage system of heap leaching structures. Fluid is locally drained and directed to semi-slotted polyethylene pipes by gravelly drainage layer. Then, it is sent to a pregnant leach solution (PLS) pond by the semi-slotted polyethylene pipes gravitationally. In this paper, initially the existing relations, suggested for the design of drainage system of heap leaching structures, are analyzed. Then, the main differences between fluid drain in landfills and heap leaching structures are presented, and consequently, two new correction factors, named "replacement" and "delay" factors are suggested. The replacement factor illustrates the ratio of mean value of heap layers area to the acid-washed area and the delay factor represents the reduction of heap layers area. The new correction factors are added to Giroud equation and the modified format of the equation is suggested for computing gravelly drainage layer thickness of heap leaching structures. Also, these two correction factors are added to Majdi et al. equation and two new relations are suggested for determination of main and auxiliary drainage semi-slotted polyethylene pipes diameters in these structures. Finally, based on the new relations, adequate drainage systems are designed for two case studies ("Tarom" and "Sarcheshmeh 2" heap leaching structures) and the results are compared with those calculated by the exiting equations.