Valorisation of Partially Oxidized Tailings in a Cover System to Reclaim an Old Acid Generating Mine Site

Gabrielle Dublet-Adli,Andreas Smebye,Mari Tvedten,Gijs Breedveld,Anna Vårheim,Thomas Pabst,Marianne Kvennås,Christian Sætre,Gudny Okkenhaug,Samuel Gelena

doi:10.3390/min11090987

Abstract

The reclamation of acid-generating mine tailings typically involves building cover systems to limit interactions with water or oxygen. The choice of cover materials is critical to ensure long-term performance, and partly determines the environmental footprint of the reclamation strategy. The objective of this research was to evaluate if tailings pre-oxidized on-site could be used in cover systems. Column experiments were performed to assess the effectiveness of a cover with capillary barrier effects (CCBE), where the moisture retention layer (MRL) was made of pre-oxidized tailings with little to no remaining sulfides (LS tailings). The columns were submitted to regular wetting and drying cycles, and their hydrological and geochemical behaviour was monitored for 510 days. The LS tailings showed satisfying hydrological properties as an MRL and remained saturated throughout the test. The concentrations of Cu in the drainage decreased by more than two orders of magnitude compared to non-covered tailings. In addition, the pH increased by nearly one unit compared to the control column, and Fe and S concentrations decreased by around 50%. Despite these improvements, the leachate water remained acidic and contaminated, indicating that acid drainage continued to be generated despite a hydrologically efficient CCBE.

Highlights

Mining operations generate large quantities of solid wastes, which are usually disposed of in surface storage facilities [1,2]
Water covers [11,12], covers with capillary barrier effects (CCBE; [13,14,15]), and monolayer covers coupled with an elevated water table [16,17,18,19] are efficient reclamation techniques, which all rely on water to control oxygen diffusion
For columns 1 and 2, the degree of saturation in both the top and bottom coarse sand layers was between 10 and 25% around 8 days after the beginning of the wetting-drying cycle. This indicated that the coarse sand layers rapidly desaturated and, formed a capillary barrier effect with the fine-grained moisture retention layer (MRL)

Summary

Introduction

Mining operations generate large quantities of solid wastes, which are usually disposed of in surface storage facilities [1,2] These mine wastes and, in particular, mine tailings often contain residual sulfides (such as pyrite, pyrrhotite, or chalcopyrite), which can oxidize upon interaction with oxygen and water and generate acid mine drainage (AMD), following the general reaction: FeS2(s) + 15/4 O2(aq) + 7/2 H2O(l) -> Fe(OH)3(s) + 2 H2SO4(aq) (1). Because of the large volumes of waste, AMD could continue to be generated for several tenths or hundreds of years [8] It is more efficient and sustainable to prevent AMD generation at the source by controlling access of either water or oxygen to the reactive tailings by using engineered cover systems [9,10]. Oxygen concentration in water is around 30 times lower than in air, and the diffusion coefficient of oxygen in water is approximately 10,000 times smaller than in air [20,21], which makes water (or water-saturated granular materials) an efficient barrier to oxygen diffusion

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