Permeability, Solution Delivery, and Solution Recovery: Critical Factors in Dump and Heap Leaching of Gold

Introduction The choice of using a dump or heap leaching process or a fine grinding-agitation leaching process is based on the grade and tonnage of the ore body. Low grade ore and small tonnages are best treated by a low-cost method, such as dump or heap leaching. Permeability, however, is the critical factor in a successful leaching operation, regardless of tonnage or grade. Without permeability in the leach bed, no percolation, no dissolution, no extraction, and no success are possible. Solution delivery to the ore is also essential. Solute-to-mineral contact is a requisite. The solute must be effectively strong and not be depleted in active dissolvers, such as acid, ferric iron, cyanide, and oxygen. It must also be moving across the mineral to be dissolved. No pockets of stagnant solution can be allowed. A third factor is recovery of the now-dissolved metal in the pregnant liquor. All such liquor should be gathered at a reasonable metal concentration to make further pumping and processing worthwhile. Control and recovery of pregnant solutions are probably the most ignored factors of solution mining. This stage in recovery is historically where many operations either fail or become viable. Permeability A permeable heap is normally achieved by taking into account heap building/construction and agglomeration/pelletizing. Even though heap building is critical for good permeability, it will not be discussed here. It has been described in detail by others. Instead, this analysis will concentrate on agglomeration and pelletizing. Differences between agglomeration and pelletizing are emphasized. Agglomeration is referred to when small particles are being attached to larger particles -the ore presents a complete size distribution. On the other hand, pelletizing is a process where all particles are fine (less than 850)µm or 20 mesh), as with mill tailings. The difference is important from two points of view: • Mechanical devices used to achieve the desired attaching action; and • Binder dosages. Mechanical Devices Due to the absence of large particles that fines can become attached to, pelletizing should be done in a centrifugal-action-based device, such as an inclined rotating plate, a rotary drum, or a series of belt-transfer points. In agglomeration, large particles play a part as a natural nucleus for the attaching action of fines. Hence, a tumbling action as strong as for pelletizing is not required. Common methods to accomplish agglomeration include: • Cascade belt system with either all belts moving forward or some of them moving forward and at least one moving backward; • Combination of belt plus inclined vibrating chute; • Large concrete mixer drum; and • Belt discharging directly onto stockpiled ore. The proper agglomeration system will depend basically on the size distribution of the ore and the amount and type of clays present. In all of the agglomerating systems described, the tumbling action takes place at the transfer points either from belt to belt, or from belt to inclined chute ore belt to ore pile. Usually, two or three mixing bars at each transfer point are also included to help the mixing action. Plows to move the ore around on a moving belt are also used.