Physical and geochemical behavior of mine rock stockpiles in high rainfall environments

A full-scale trial dump was constructed at Grasberg Mine, Indonesia, to investigate the geochemical processes and flow behavior within a waste rock pile in a high rainfall environment. The effectiveness of using different limestone blending methods with acid rock and impervious covers to minimize acid rock drainage (ARD) was assessed. The chemistry, temperature, and oxygen data from the trial dump showed that oxidation occurred throughout the full profile of the dump; however, the oxidation rates at the trial dump were approximately 11% to 58% lower than those calculated for smaller scale experiments. The low-permeability cover that was constructed using the materials available at the Grasberg Mine was not effective in reducing infiltration into the trial dump and had little benefit in reducing the acid generation. Conversely, it was found that limestone blending with a crusher/stacker system was an effective method to mitigate ARD when the geochemical criteria are adequately fulfilled. Excavations were conducted in the test panels at the end of experiment revealing that inclined coarse and fine layers formed the internal structure of the waste rock pile. The presence of this coarse and fine layered structure within the profile tended to increase the potential for preferential flow and thus was found to adversely affect the performance of limestone blending and the limestone cover. In order to develop a fundamental understanding of water flow and geochemical mixing within the layered mine rock, a meso-scale laboratory experiment that contained the inclined coarse and fine layers of acid mine rock and limestone was constructed. The results of the meso-scale experiment demonstrated that the distribution of water flow in the coarse and fine layered pile was dependent on rainfall intensity. Furthermore, the flow path and residence time were found to control effluent quality from the layers. When rainfall intensity was increased, the flow of water was distributed more evenly and vertical flow became the dominant flow mechanism, promoting chemical mixing between the leachate from the different layers. In summary, this study indicates that limestone blending and limestone covers are most effective in mitigating ARD in waste rock in high rainfall environments.Applied Science, Faculty ofMining Engineering, Keevil Institute ofGraduat