This case history of a new heap leach operation is courtesy of E&MJ, May 2008. They describe a new heap leach pad at the Talvivaara Mine in Finland thus:

The process flow at Talvivaara will consist of four main steps: mining, crushing, bioheap-leaching, and metals recovery. Open pit mining is planned to be at a scale of approximately 15 million mt/year, making it the largest open-pit mine in Finland. Three-stage crushing will be followed by agglomeration with sulphuric acid in order to consolidate the fines with coarser ore particles. After agglomeration, the ore will be conveyed and stacked 8 m high on the primary heap pad for 18 months of bioheap-leaching. After the primary leaching interval, leached ore will be reclaimed, conveyed and restacked onto the secondary heap pad, where it will be leached further in order to recover additional metals. After secondary leaching, the barren ore will remain permanently on the secondary heaps. In metal recovery, nickel, copper, zinc, and cobalt are precipitated from the pregnant leach solution and filtered to produce saleable metal products.

The issue about this planned heap leach pad is the impact of freezing temperature. The mine proponents note that heap leaching is successful in Chile in cold regions and that tests on site have indicated that the heap leach material does not loose its “heat” to the point that leaching is not possible. The following is from the Talvivaara website on their field testing:

During summer 2005, a 17 000 tonne demonstration plant was constructedat the mine site. A representative ore sample was mined, crushed to 80 % -8 mm, agglomerated and built to an 8 m high heap. Irrigation of the heap started in August 2005. The pilot heap was inoculated with indigenous bacteria collected from the site. The amount of bacteria in the pregnant leaching solution has been in the range of 106 – 108 cells/ml. The bacteria involved are mesophilic and thermophilic. The start-up of the solution flow resulted soon in elevated temperatures of over 50 0C in the pregnant leach solution. The rise is due to the oxidation of the large quantity of pyrrhotite and pyrite in the ore. The elevated temperatures have also been maintained over the boreal winter conditions. Metal recovery was started in November 2005. In winter 2007 the heap was reclaimed and the secondary leaching phase begun.

For more information on the mine and their field heap leach testing see these links:

Talvivaara Mine Project General Presentation