Photo courtesy of Allied Nevada
Hycroft Mine’s gyratory crusher.
Hycroft ... Continued from page 113 to management of the heap leach pads. Pumping capacity and access to water on a daily basis have increased so that flows to the heap leach pads now average 22,000 gallons per minute, an increase of 36 percent from the beginning of the third quarter. However, remediation on the Lewis leach pads is ongoing. Allied Nevada has indicated all permits are in place and solution is currently being introduced to the leach pad at a rate of 1,500 gallons per minute. The company expects to begin recovering gold and silver from the pad in 2014. Another important turning point for the mine was the new Merrill-Crowe plant capable of processing 21,500 gallons-per-minute, which went online in October. The Merrill-Crowe process is used for removing gold and silver from cyanide once the solution is recovered from the heap leach pads. The old process, which included a 5,000 gallon per minute Merrill-Crowe plant and 6,500 gallon per minute carbon column could not process at the rate needed. In addition, the carbon columns were not efficient at recovering silver from solution, which was not lost, but continued to recirculate in solution until it was run through the Merrill-Crowe process. The carbon columns have since been discontinued at Hycroft. Thom explained silver is the first metal captured by the carbon columns, but is “chased off� as gold is captured later in the process. Allied Nevada anticipates increase capture of silver with the new plant should increase to approximately 6 ounces of gold for every ounce of silver, as compared to the 1 ounce of gold for each ounce of silver captured in the carbon process. In addition the the new Merrill-Crowe plant, the company is building a crushing system that will help to increase the gold and silver recoveries in the heap leach operation. It is expected that the crushing system will be completed and commissioned by the end of 2013 and fully operational in 2014.
114 MINING QUARTERLY, Elko, Nevada WINTER 2013