In 1989, the first modern gold heap leach closure regulations were introduced in Nevada, USA. These regulations were based on limited information and assumptions, offering several ways to demonstrate geochemical stability, the first of which was the option to “rinse” a heap. This led some mine operators and regulators to believe that “rinsing” was essential to remove cyanide and to stabilize geochemically a heap. Although the recirculation of process solutions commonly occurred after cyanide addition ceased, this was not done to reduce cyanide, but rather to recover residual gold, a phase known as residual gold recovery1. After residual gold recovery, additional recirculation of process solutions continued, not to reduce weak acid dissociable (WAD) cyanide, but to reduce the quantity of process fluids from the circuit rather than cyanide, referred to as the “inventory reduction” phase of heap closure2.
As more heap leach facilities were closed in the 1990s and early 2000s, data continued to show that rinsing was unnecessary to reduce WAD and free cyanide concentrations in heap drainage. In fact, some research studies of heap closure during this period revealed that not only was rinsing not required to reduce WAD and free cyanide but could even release stable constituents from the heap3.
The belief that “rinsing” was required was largely influenced by a 1992 study by Cellan et al.4. That study was based on data from a small diameter laboratory column used for metallurgical testing, with solutions confined to a closed circuit. Using fresh water, the setup lacked the exposure to air and ultraviolet radiation that occurs during the recirculation in residual gold recovery and inventory reduction. Consequently, the study results misrepresented the necessity of rinsing to reduce WAD cyanide in heap solutions during closure.
Although data from all closed heaps in Nevada and other western U.S. states — along with the removal of rinsing from regulations — suggest otherwise, the belief that rinsing is required to reduce WAD and free cyanide still persists in some jurisdictions. Although there has never been a heap leach pad closed in North America using rinsing.
1. Parshley et al. (2012), Mine Closure 2012.
2. Ibid.
3. Bowell et al. (2009), Minerals Engineering, 22(4), 477–489.
4. Cellan et al. (1996), Comparison of Laboratory and Commercial Neutralization Rinse Data.
