Waste rock can contain high concentrations of deleterious trace elements, which upon oxidation can be released, having a significant impact on water quality. Therefore, knowledge about their occurrence and overall mobility is crucial to ensure suitable environmental protection measures. Sulfide-rich waste rock was characterized and quantified using automated mineralogy (QEMSCAN). Selected pyrite grains were analyzed for trace element occurrence using LA-ICP-MS before, during, and after leaching the waste rock in 10 L small-scale test cells for two years to assess trace element occurrence and mobility. Sequential extraction was used to estimate elemental sequestration during the experiment. The high abundance of pyrite (66%) and scarcity of buffering minerals resulted in low pH (<1.3) leachate with high concentrations of trace elements such as As (21 mg/L), Cu (20 mg/L), Hg (13 µg/L, Pb (856 µg/L), Sb (967 µg/L), Tl (317 µg/L ), and Zn (23 mg/L) in solution with limited retention in secondary minerals, primarily due to these elements’ association with pyrite either as inclusions or impurities showing an average abundance of 193 ppm As, 15 ppm Cu, 13 ppm Hg, 20 ppm Pb, 24 ppm Sb, 26 ppm Tl, and 74 ppm Zn in the waste rock. The occurrence of Cu and Zn as inclusions associated with the pyrite led to their extensive mobilization of 79% and 72%, respectively, despite their low abundance in the waste rock. Provided the overall leachability of S (11%) and limited formation of secondary minerals, the average oxidation rate suggests depletion of the pyrite within approximately 18 years. In conclusion, this study shows the importance of detailed mineralogical investigations and early preventive measures of waste rock to ensure sustainable mine waste and water management.

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.

During the operation of a mine, waste rock is often deposited in heaps and usually left under ambient conditions allowing sulfides to oxidize. To focus on waste rock management for preventing acid rock drainage (ARD) formation rather than ARD treatment could avoid its generation and reduce lime consumption, costs, and sludge treatment. Leachates from 10 L laboratory test cells containing sulfide-rich (> 60% pyrite) waste rock with and without the addition of lime kiln dust (LKD) (5 wt.%) were compared to each other to evaluate the LKD’s ability to maintain near neutral pH and reduce the sulfide oxidation. Leaching of solely waste rock generated an acidic leachate (pH < 1.3) with high concentrations of As (21 mg/L), Cu (20 mg/L), Fe (18 g/L), Mn (45 mg/L), Pb (856 μg/L), Sb (967 μg/L), S (17 g/L), and Zn (23 mg/L). Conversely, the addition of 5 wt.% LKD generated and maintained a near neutral pH along with decreasing of metal and metalloid concentrations by more than 99.9%. Decreased concentrations were most pronounced for As, Cu, Pb, and Zn while S was relatively high (100 mg/L) but decreasing throughout the time of leaching. The results from sequential extraction combined with element release, geochemical calculations, and Raman analysis suggest that S concentrations decreased due to decreasing sulfide oxidation rate, which led to gypsum dissolution. The result from this study shows that a limited amount of LKD, corresponding to 4% of the net neutralizing potential of the waste rock, can prevent the acceleration of sulfide oxidation and subsequent release of sulfate, metals, and metalloids but the quantity and long-term stability of secondary minerals formed needs to be evaluated and understood before this method can be applied at a larger scale.

This report is the outcome of the SIP STRIM project

StopOx-Utilization of industrial residuals for prevention of sulfide oxidation in mine waste implemented at Applied geochemistry, Luleå University of Technology running from 2015 to 2018. Boliden Mineral has been partner and co-funder of the project. Other partners in the project were Cementa, Dragon Mining, MEROX, Nordkalk, and SP Processum. The overall aim of the project was to develop prevention technologies to reduce the sulfide oxidation in mine waste, during and after operation, and thereby reduce the generation of acid mine drainage. The StopOx project has been focusing on sulfidic mine waste from the Boliden area which were disposed of and are causing acid mine drainage or have the potential. Industrial residues/products were supplied by BillerudKorsnäs, Cementa, MEROX, and Nordkalk. The report consists of chapters based on three subprojects.

Chapter 1. Introduction

Chapter 2. Inhibition technology with aim to minimize waste rock oxidation during operations by using residues from other industries (passivation of sulfidic surfaces by the formation of secondary minerals)

Chapter 3. The suitability of green liquor dregs as substitutes for or additives to till in a sealing layer as part of a cover system

Chapter 4. Weathering of waste rock under changing chemical conditions

The research described in chapters 2 and 3 was performed by Ph.D. students and will continue until 2021, while the subproject in chapter 2 ended in 2018.

Prevention and mitigation of acid rock drainage (ARD) from mine wastes are crucial for limiting environmental impact. However, preventive measures are often too expensive, potentially harmful to the environment or not applied early enough. This study aimed to test the potential of different secondary raw materials for maintaining a circumneutral pH (6–7) in a sulfide oxidation environment, allowing secondary minerals to form on reactive sulfide surfaces to prevent release of acid, metals and metalloids, and thereby ARD generation. Five materials (blast furnace slag, granulated blast furnace slag, cement kiln dust, bark ash, lime kiln dust) were selected based on their alkaline properties, availability and yearly yield. High sulfidic (>50 wt%, sulfide) waste rock from an active Cu–Zn–Au–Ag open pit mine in northern Sweden was leached in small-scale laboratory test cells under ambient condition for 4–8 weeks before adding secondary raw materials on the surface in an attempt to prevent ARD generation. During 52 subsequent weeks of leaching, the pH and electrical conductivity in the leachate from the waste rock varied between 1.7-4.6 and 2.1–22.8 mS/cm, respectively. All secondary raw materials were able to increase the pH to circumneutral. However, blast furnace slag, granulated blast furnace slag and cement kiln dust were not able to maintain a circumneutral pH for an extended time due to self-cementation or carbonation, whereas bark ash (1 wt%) and lime kiln dust (5 wt%) prevented acidity, metal and metalloid leaching. Materials such as cement kiln dust and bark ash contained elevated concentrations of, e.g., Cd and Zn, but the release of metals and metalloids was generally low for most elements, except for Cl, K and Na, most likely due to salt dissolution.

Pyrite contains varying content of trace elements. Upon oxidation, these elements can be released having a great impact on water quality. Therefore, it is crucial to identify trace elements occurrence in minerals, and their overall leachability to ensure that suitable measures are taken to protect the environment. Sulfide-rich waste rock was mineralogically investigated, screened and quantified using QEMSCAN and LA-ICP-MS. The water quality was determined by leaching the waste rock in small scale test cells for more than two years. Arsenic, Hg, Sb, and Tl were elevated and the dominating trace elements in the waste rock with a content of 217, 17, 38, and 26 ppm respectively. Results show that pyrite was the most abundant mineral (66%) with traces of other sulfides such as arsenopyrite, chalcopyrite, and sphalerite. The abundance of pyrite along with the scarcity of any buffering minerals resulted in high concentrations of Fe3+ which subsequently generated acidic leachate pH (<1.3) with extremely high concentrations of As (21 mg/L), Hg (13 µg/L), Sb (967 µg/L), and Tl (317 µg/L). The leachability of elements varied substantially. The highest leachability was observed for As (18%) due to the presence in pyrite preferentially to arsenopyrite or as a sulfosalt. Conversely, Sb was primarily identified in various sulfosalts such as Bournonite, found in cracks between pyrite grains, which can explain the lower leachability (5%). Results from LA-ICP-MS show that Hg was distributed in the more porous parts of the pyrite and displayed a partial correlation with Tl. However, Hg had low leachability compared to Tl implying at least two sources of Hg in the pyrite.

Additional leaching tests with pyritic waste rock treated with lime kiln dust (5wt.%) to inhibit the sulfide oxidation are ongoing. The treatment has limited oxidation and leaching of As and Tl but has not prevented the release of Hg and Sb indicating restricted ability to prevent the oxidation of sulfide minerals such as sulfosalts. The overall results from LA-ICPMS and leaching of the waste rock indicate that mineral association of trace elements profoundly influences the possibility to prevent their release during sulfide oxidation and the overall effectiveness of inhibition. Moreover, this suggests that the ability to prevent sulfide oxidation in more complex mine wastes could prove difficult.

One of the main and most challenging environmental problems related to mining is the generation of acid rock drainage (ARD), a leachate characterized by low pH and elevated concentrations of sulfate, metals, and metalloids formed when sulfide-bearing minerals are subjected to oxygen and water. During the operation of a mine, waste rock is often deposited in heaps and usually left under ambient conditions, enabling sulfides to oxidize. Generated ARD is commonly treated actively with alkaline material in an attempt to raise the pH and precipitate metals, with subsequent formation of sludge, which requires additional treatment. To focus on the treatment of waste rock rather than the ARD could prevent the generation of ARD; reduce the lime consumption, costs, and sludge treatment. This thesis aims to identify and evaluate the potential of different industrial residues to maintain circumneutral pH in a sulfide oxidation environment, allowing secondary minerals to form on the reactive sulfide surface to prevent sulfide oxidation and generation of ARD.

Five different industrial residues (blast furnace slag, granulated blast furnace slag, cement kiln dust, bark ash, and lime kiln dust) were selected in a feasibility study performed prior to this study. The selection was based primarily on their alkaline properties, availability, and early yield. The waste rock was selected due to its high content of sulfides (>50%) and potential to generate ARD. Initial characterization of the industrial residues included combining mineralogical and chemical composition with batch testing (L/S 10). Sulfide oxidation in the leaching of the waste rock accelerated after week 29 resulting in high concentrations of major elements such as Al, Fe and S but also extremely high concentrations of e.g. As, Cu, Mn, Pb, Sb and Zn despite their relatively low content in the waste rock. Leaching was conducted during 14-153 weeks. The initial characterization implied that all of the studied industrial residues has the potential to prevent ARD generation. However, the enrichment and leachability of Pb in the cement kiln dust, as well as Cr and Zn in the bark ash, suggested the presence of elements of potential concern that could limit the use of the materials. When the industrial residues were added to the waste rock surface in small-scale laboratory test cells, blast furnace slag, granulated blast furnace slag, and cement kiln dust self-cemented and failed to maintain circumneutral pH, whereas bark ash (1wt.%) prevented acidity, metal and metalloid leaching. However, the use of bark ash may prove problematic due to the release of Cl, K, and Na likely related to salt dissolution. Lime kiln dust (5wt.%), the most promising of the industrial residues, maintained a circumneutral pH throughout the time of leaching, with an overall decrease of metal and metalloid concentrations by more than 99.9%. Results from investigations of secondary minerals formed combined with element release during the leaching period suggest that the addition of LKD to the waste rock led to decreasing concentrations of S in the leachate due to decreased sulfide oxidation, which subsequently led to gypsum dissolution. Moreover, the addition of LKD to the waste rock generated a lower amount of secondary minerals compared to when no addition was made.

The results from these studies increase the understanding of advantages and limitations of using selected industrial residues in the treatment of mine waste. Moreover, it shows that a rather small amount of alkaline material, corresponding to 4% of the net neutralizing potential of waste rock, can prevent the acceleration of sulfide oxidation and subsequent release of sulfate, metals, and metalloids. However, the quantity and long-term stability of the formed secondary minerals need to be evaluated and understood before this method can be applied at larger scale.

Prevention and mitigation of acid rock drainage from mining are decisive for limiting environmental impact. Five by-products and industrial remnants (lime kiln dust, blast furnace slag, granulated blast furnace slag, cement kiln dust and fly ash) were investigated for their suitability to prevent acidity and metal(loid)s during leaching from highly sulfidic (50wt%, sulfide) waste rock in small scale laboratory test cells. Variations in pH and electrical conductivity in leachate allowed differentiation between the different materials. Lime kiln dust (5wt%) and fly ash (1 and 2.5wt%) were observed to be the most suitable materials to prevent acidity and metal(loid)s leaching.

The ability to reduce sulfide oxidation in waste rock after mine closure is a widely researched area, but to reduce and/or inhibit the oxidation during operation is less common. Sulfide-rich (ca 30 % sulfur) waste rock, partially oxidized, was leached during unsaturated laboratory condition. Trace elements such as As and Sb were relatively high in the waste rock while other sulfide-associated elements such as Cu, Pb and Zn were low compared to common sulfide-rich waste rock. Leaching of unsaturated waste rock lowered the pH, from around six down to two, resulting in continuously increasing element concentrations during the leaching period of 272 days. The concentrations of As (65 mg/L), Cu (6.9 mg/L), Sb (1.2 mg/L), Zn (149 mg/L) and S (43 g/L) were strongly elevated at the end of the leaching period. Different alkaline industrial residues such as slag, lime kiln dust and cement kiln dust were added as solid or as liquid to the waste rock in an attempt to inhibit sulfide oxidation through neo-formed phases on sulfide surfaces in order to decrease the mobility of metals and metalloids over longer time scale. This will result in a lower cost and efforts of measures after mine closure. Results from the experiments will be presented.