The Evolution of Cyanide Heap Leach Closure Methods

The last two decades have seen a significant evolution in the approaches used to close gold heap leach pad facilities in an environmentally responsible manner. With the promulgation of new cyanide regulations in the State of Nevada, USA in 1989, the mining industry was faced with the need to develop new methods to safely close cyanide heap leach pads in a manner that eliminated long-term risks associated with cyanide and other chemical constituents that could be discharged from the heap leach pads following closure.

Initially, rinsing of the heaps was thought to be the only approach to reduce concentrations of cyanide, and this was reflected in the new regulations as a requirement to rinse heaps, unless other methods could be demonstrated to adequately stabilise the spent ore and solutions. However, the regulations were unclear as to when closure of the heap began and if circulation of process solutions, after cyanide addition ceased, constituted rinsing.

Many closure plans developed during the early years immediately following promulgation of the new regulations assumed that rinsing would be done with fresh water in quantities of up to three times the total pore volume of the heap based solely on the results of a study published in 1996 (Cellan et al.). If implemented, this would result in the consumption of significant quantities of clean water in a region where water resources are limited, and the benefits of freshwater rinsing were not always apparent. Rinsing heaps during closure also generated large quantities of additional “process” solutions that required management through other methods, such as forced evaporation, or chemical or biological treatment.

By the late 1990s, the mining industry had closed enough heap leach pads to allow review of ideas and to use this to make changes in the regulations and approaches employed by the industry. A better definition of when heap closure begins was needed. The currently accepted criterion for closure was after economic gold recovery ceases. Continued research on this led to the demonstration for closed heaps that additional rinsing after the residual gold recovery period is often not needed to reduce cyanide risks from the heap drainage because the more toxic forms of cyanide volatilise and other deleterious elements become mobile as solutions are recirculated through the heap (Bowell et al., 2009).

To address the shortcomings of heap rinsing as the primary method of chemical stabilisation of spent ore, a number of approaches were tried, and many of the early approaches were abandoned in favour of more practical and economic methods which were equally effective. This paper documents the history of heap leach closure approaches since the late 1980s and discusses the evolution of understanding and consequently the methods proposed to stabilise spent heap ore during closure. Data from several closed heaps are presented as illustrations of the progression of approaches to heap rinsing. Current trends in the existing best practice and their probable impact on the focus of future of heap closure methods are also discussed.