Towards environmentally compatible in situ leaching of mine waste using low concentration acids

Abstract

Conventional mining of metalliferous rock typically involves physical excavation followed by mineral processing. These are energy intensive and environmentally invasive processes which produce large amounts of wastes. In situ leaching of mine wastes with acidic lixiviants may provide an opportunity to recover additional metals and decontaminate residual material. However, the use of high concentration mineral acids (such as >1 M sulfuric acid) can be harmful to the environment and may necessitate post treatment to remove the acidity from the residual rock. The feasibility of using less aggressive lixiviants that may mitigate these risks was explored; 0.1 M citric and sulfuric acid were applied as lixiviants to determine the leaching behaviour of As and Cu from legacy sulfidic Cu/As mine waste. This was achieved via triplicate column upflow tests with a cumulative liquid-solid ratio of 31. Citric acid was more efficient than sulfuric acid for As recovery, with recovery (leaching efficiency) rates of 40.7 wt% and 35.2 wt%, respectively, but slightly less effective for Cu recovery: 33.8 wt% and 43.2 wt% of the total content, respectively. The comparable performance, despite citric acid being a weaker acid, is attributed to its affinity to form soluble As/Cu-citrate complexes. Numerical modelling of heap leaching of the mine waste at field scale was undertaken to allow exploration of scenarios for metal recovery versus time, lixiviant application rate and reagent consumption. This study highlights the suitability of applying lixiviants for the in situ leaching of metals from mine waste in low concentrations over a prolonged timescale. The work therefore provides a step towards unlocking a new paradigm of metal recovery and remediation for modern and legacy mine wastes with minimal environmental disturbance and energy input.