Abstract:

Reduction of long-term environmental liabilities at a mine site requires robust characterization of acid rock drainage (ARD) and metal leaching (ML) potential of mined/processed materials. Standard laboratory testing is useful for preliminary characterization of the ARD/ML potential and, in many instances, can provide sufficient understanding to guide selection and implementation of ARD/ML management strategies. However, due to major differences in laboratory testing and actual field conditions, standard laboratory testing may not always provide sufficient or even accurate prediction of future ARD/ML evolution. Field test pads constructed using larger amounts of materials provide a more representative demonstration of ARD/ML generation from mine stockpiles under field conditions. The main objective of the three-year field test pad study at the Bagdad copper mine in Arizona was to improve the laboratory prediction of development rock ARD/ML potential under field conditions. Due to the semi-arid site conditions, drip-line water irrigation was used to supplement natural precipitation and generate seepage for the estimation of solute loading and mineral reaction rates. The main study findings discussed in this paper include reclassification of ARD potential of development rock materials, seepage water quality evolution under field conditions, and effect of geochemical reactions on rock geotechnical strength. The major outcome of this study was reclassification of the test pad materials from ‘uncertain’ to ‘non-acid generating’. Post-test analyses indicated no significant degradation of geotechnical strength after the three-year study. We will also share lessons from construction, operation, and deconstruction of the test pads. Findings from the study will be used to support mine planning, permitting, and operational materials management at the Bagdad Mine.

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Dr. Michael Hay