Application of High Pressure Grinding Roll Technology in Silver Heap Leaching

High Pressure Grinding Roll (HPGR) technology has been evaluated to replace tertiary cone crushing at Coeur Rochester operations (Coeur Rochester, Inc.) as an opportunity to improve silver recovery. In 2016 and 2017, onsite and third-party pilot programs were initiated to evaluate the impact of metallurgical recovery and heap leach performance of crushed product generated by an HPGR. In 2018, Forte Dynamics evaluated the HPGR laboratory metallurgical data from Coeur Rochester and all third parties to develop field-scale leach curves based on new crushing scenarios. The laboratory recovery test work data was consolidated and scaled to field conditions to determine projected heap leach performance. Based on the metallurgical testing conducted, the replacement of current tertiary cone crushers with HPGR technology has the potential to increase Coeur Rochester’s silver recovery rate by 8%, from 61% to 69%, and shorten the overall timing of silver recoveries from 20 years to just over two years. INTRODUCTION The Rochester mine, and associated valley fill heap leach facilities is an open-pit silver and gold mine located in Pershing County, Nevada, approximately 26 miles northeast of the city of Lovelock. In 2017, Rochester produced approximately 4.7M troy oz silver (Ag) and 51k troy oz gold (Au). Ore mined at Rochester is either fed directly to the primary X-Pit crusher, crushed at an in-pit crusher system, or placed directly onto a heap leach pad as run-of-mine (ROM). Both crushing circuits are comprised of three stages of crushing to produce a nominal 3/8” ore. Primary crushing consists of jaw crushing while secondary and tertiary crushing are composed of varying cone crushers. Historical recovery rates of crushed Rochester ore average 61.4% Ag and 95.9% Au over a period of 20 years [1]. These rates were derived from third party verification of in-house metallurgical test results, crusher production and heap leach production results from 1986 through 2004 in a private report for Rochester produced by KD Engineering Co., Inc. Recovery rates are regularly verified by compositing daily samples of ore delivered to each leach pad from the X-pit crusher, in-pit crusher, and/or ROM. A proportionate split of each sample is used to generate monthly composites for metallurgical analysis utilizing column leaching and bottle roll leaching. These tests are run in a manner analogous with production conditions and deliver test results that provide expected heap leach performance. As the Rochester oxide deposit has been mineralogically and metallurgically consistent, the gold and silver recovery trends have also been consistent over the life of the project. In 2016 and 2017, onsite and third-party pilot programs were initiated to evaluate the impact of HPGR technology to metallurgical recovery and heap leach performance of crushed product in an open circuit configuration, as a potential replacement for the currently in place tertiary cone crushers. During these years, HPGR was evaluated in three phases of test work.