It is well known that gold heap leaching operations present low operating costs. There some economical considerations in the design that the project metallurgist must evaluate in order to obtain optimum conditions. Basically, gold heap leaching operations are appropriate for low grade gold ores and the costs must be maintained in their low level so that the operation may be attractive during the feasibility study and during the life of mine. The main consideration in the project is the metallurgical performance and the design must evaluate the best leach pad design. In this way, economical factors are especially important for the leach pad due to it covers a large areas ranging from 50,000 m2 to 1,500,000 m2.
The designer must evaluate the topography, the earthworks required to build the leach pad, the liner or geomembranes, the type of materials needed for the construction and the drainage system. It is important to evaluate every aspect independently and later to try to establish a relationship among them. A critical aspect is the slope of the pad and this value may be obtained by considering the flow rate through a porous heap material under constant irrigation. Basically, the designer must be focused on the steady state of the system, which is influenced by the fine material present in the ore and the irrigation rate. Other important aspect is related to the solution flow, which should laminar under ideal conditions. In both case Darcey’s Law is a useful tool in the design.
Also, the hydraulic head of the system have an impact on the solution distribution and percolation through the leach pad. It has been noted in several gold heap leaching operations that the hydraulic head is influenced by the cyanide solution irrigation rate, height of phreatic surface at drainage pipe, height of phreatic area around the pad, the location of the drainage points, the pad slope and the permeability of the material in the heap. It is common to evaluate different hydraulic head and study its variability. According to this, the average hydraulic head is also influenced by the slope of the leach pad, cyanide irrigation rate, length of drainage system and the permeability of the material near the phreatic area. It the design must be compared with a standard, the leach pad designer may consider that a leach pad with slopes of 12o to 14o reports permeability from 10-9 to 10-12 cm/s.