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A field cell and humidity cell study of metal attenuation in neutral rock drainage from the Antamina Mine, Peru Hirsche, Dustin Trevor
This work focuses on the attenuation of Mo and Zn in neutral pH drainage from waste rock at the Antamina mine in Peru. The study was designed to test the hypothesis that Mo or Zn containing leachate from one waste rock type can be attenuated when allowed to contact a different waste rock type. Mixed material stacked field cells and humidity cells connected in series, where leachate from a Mo or Zn - releasing waste rock type flowed through a second waste rock material type, were used to test this hypothesis. Both of these were new methods, which had not before been reported in the peer reviewed scientific literature related to the study of waste rock geochemistry. Results from both the humidity cells (laboratory conditions) and field cells (field conditions) showed the same general attenuation patterns. When drainage from Mo-releasing waste rock flowed through Pb-rich black marble waste rock, Mo was removed from solution. Mo attenuation was not observed when the order of the waste rock materials was reversed such that drainage from Pb-rich material flowed through Mo-releasing intrusive rock. As, also released from the same Mo-releasing intrusive rock, showed the same attenuation pattern as Mo. Geochemical modeling suggested that wulfenite precipitation was responsible for the observed attenuation of Mo. Zn was removed from leachate both by contact with Mo-releasing intrusive rock and by contact with calcite-rich grey hornfels material. Like Zn, Cd was removed from solution by contact with calcite-rich grey hornfels. Scanning Electron Microscopy (SEM) suggested that Zn may have been incorporated into the crystal structure of phyllosillicate clay minerals; however, further work is needed to confirm this mechanism. Insufficient data were available to develop a hypothesis as to the specific attenuation mechanisms responsible for removing Cd and As from solution. In addition to shedding light on the geochemical processes controlling Mo and Zn in neutral mine drainage, this research also demonstrated the effectiveness of stacked field cells and humidity cells connected in series for the study of metal attenuation by waste rock mixing.
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