Preventing Acid Rock Drainage Formation from Sulfidic Waste Rock Using Secondary Raw Materials

Abstract: One of the central and most challenging environmental problems related to mining is acid rock drainage (ARD) formation. The drainage is characterized by low pH and elevated concentrations of sulfate, metals, and metalloids formed when sulfide-bearing minerals are subjected to oxygen and water. Current remediation solutions, including active and passive techniques, have been developed to reduce ARD's negative impact. However, these treatments require continuous maintenance with an incessant addon of chemicals, energy consumption, not to mention long-term monitoring, until sulfide oxidation has ceased. Once it has been initiated, ARD formation could last for hundreds to thousands of years, making these approaches costly and unsustainable. A more strategic and environmentally sustainable approach would focus on preventing sulfide oxidation rather than treating its symptoms. This thesis explores five different secondary raw materials (SRM) (denoted below) for their use as amendments to prevent pyrite oxidation during storage. A combination of several mineralogical and geochemical methods was used to assess the materials' ability to maintain circumneutral pH when leaching pyritic waste rock (> 60% pyrite) in small-scale test cells (10L) to promote HFO precipitation on the pyrite surfaces.  The oxidation of waste rock resulted in a drainage characterized by low pH (<2) and extensive element mobilization of up to 80% of the original content during the first two years. The results highlight the importance of trace element characterization and the need for early preventive measures to hinder or reduce the risk of acid drainage formation that requires active and costly long-term treatment. Conversely, adding 1-5 wt.% SRM to the waste rock created drainages with circumneutral pH and substantially lower sulfate and metal concentrations. However, not all materials could maintain circumneutral pH for an extended time, such as blast furnace slag (air-cooled and granulated) and cement kiln dust. These materials either require larger volumes of water to dissolve or contain minerals that allowed the material to harden upon water contact, inhibiting its neutralization capacity. Biomass bark ash showed a similar but less extensive, hardening effect resulting in a better ability to maintain circumneutral pH for more than two years despite its small addition (1-2.5wt.%). A similar ability was observed for lime kiln dust (5 wt.%). Conversely to lime kiln dust, the ash contained high soluble elements of potential concern, and its usage should be questioned despite only a temporary increase of elements through wash-out. However, the correlation between the amount of bark ash added and the timespan of circumneutral pH was not linear, resulting in the risk of prematurely declining pH if too little is added. Conversely, adding too much bark ash increases the risk of material hardening. One major concern with this treatment method is that it can inflate secondary minerals formation, leading to latent acidity and element release through their dissolution in changing geochemical conditions, such as wet or dry coverage measures. However, the addition of small amounts of SRM (1-4% of the waste rock's net neutralizing potential) to the waste rock dramatically improves the overall drainage quality without increasing the total amount of secondary minerals formed compared to no addition. In general, the type of secondary minerals formed on the waste rock without SRM treatment was considered less stable in an oxidizing environment than those formed through SRM treatment, suggesting that not treating the waste rock is inferior to SRM treatment both before and after covering measures.In conclusion, this thesis's results show that using small amounts of SRM can prevent oxidation during at least two years, likely due to HFO formation on the reactive surfaces. Consequently, it can substantially limit the need for treatment measures, both before and after remediation, decreasing the overall need and cost for chemicals, energy, and long-term monitoring, stressing the need for applying preventive measures during the storage time from mining to remediation. However, secondary minerals' long-term stability needs further evaluation and understanding before this method can be applied on a larger scale.

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