Contaminated Neutral Drainage Research Articles

Mineralurgical and Environmental Characterization of the Mine Tailings of the IOCG Mine of Guelb Moghrein, Akjoujt, Mauritania

Since 2004, the processing of the iron oxide–copper–gold (IOCG) ore of Guelb Moghrein, Akjoujt, Mauritania, has resulted in the generation of approximately 40 million tons of mine tailings. The storage of these tailings poses significant environmental challenges particularly to surface and underground water resources. To address this issue, we propose an approach involving both mineralurgical and environmental characterization. Our mineralogical analysis reveals that the TSF tailings consist of sulfides and iron oxides associated with a silico-carbonated matrix. This mineralogical analysis also shows that the TSF tailings consist of secondary minerals, resulting from sulfides oxidation. Furthermore, our findings indicate that the chemical analysis of the TSF tailings contains potential toxic elements (PTEs) such as Cu, As, Co, Ni, Sb, and Se. Regarding the environmental characterization of the tailings, conducted through acid-based accounting (ABA) static tests, we demonstrate that leaching from the tailings generates a contaminated neutral drainage (CND).

Feasibility of reprocessing gold tailings: Integrated management approach for the control of contaminated neutral mine drainage

Sulfide ore processing generates finely ground mine tailings that often contain sulfide minerals such as pyrite, pyrrhotite, arsenopyrite, and gersdorffite. These minerals can generate acid mine drainage or contaminated neutral drainage (CND) due to weathering (aqueous oxidation by oxygen, and bacterial activity). Contaminated neutral mine drainage affects the surrounding environment by contaminant release (e.g., As, Ni, Fe, Cu, and Zn), which is one of the most challenging environmental issues for the mining industry. This paper examines the feasibility of desulfurization by flotation at the laboratory scale as a sustainable tailings management approach to prevent CND generation from tailings produced by the Amaruq mine (Nunavut, Canada). The flotation tests were conducted using a five-level statistical experimental design (DOE) using five parameters that include: collector dosage (PAX), activator dosage (CuSO4), frother dosage (MIBC), pH, and solid content (Cs). The response surface methodology (RSM) was used to predict sulfur recovery and to choose the optimal conditions of flotation. The mineralogical properties of the samples before and after reprocessing were investigated by combining optical microscopy, X-ray diffraction spectroscopy (XRD), and the quantitative evaluation of materials by scanning electron microscopy (QEMSCAN®). The trace element (As, Co, Ni, etc.) microanalysis of the identified minerals was quantified using electron probe micro-analyser (EPMA), and the bulk chemical composition was analyzed using induction furnace, X-ray fluorescence, and ICP-AES/MS. Finally, the geochemical behavior of the initial (WTT) and desulfurized (DWT) tailings was evaluated using kinetic weathering cells. The main results demonstrated that sulfur recovery can reach up to 97 % using the following optimal conditions: pH of 11.5, PAX dosage of 158 g/t, CuSO4 dosage of 300 g/t, MIBC dosage of 55 g/t, and Cs of 29.3 %. Moreover, the residual sulfides within the desulfurized tailings were completely locked within the non-sulfide gangue minerals as analyzed by QEMSCAN® and optical microscopy. Kinetic testing was performed on WTT and DWT to assess their contaminant release rates (especially As, Ni, Cu, Fe, and Zn).The desulfurized material did not cause any significant risk of contaminated mine drainage generation. All released contaminants were below the limits of Quebec provincial legislation.

Lead Mobilization and Speciation in Mining Waste: Experiments and Modeling

Mining produces significant amounts of solid mineral waste. Mine waste storage facilities are often challenging to manage and may cause environmental problems. Mining waste is often linked to contaminated mine drainage, including acidic waters with more or less elevated concentrations of trace metals such as lead. This work presents a study on the mobilization of lead from waste from two typical mining sites: Zeida and Mibladen, two now-closed former Pb–Zn mines in the Moulouya region of Morocco. Our research investigates the mobilization potential of Pb from the waste of these mines. The study involved acid–base neutralization capacity tests (ANC–BNC) combined with geochemical modeling. Experimental data allowed for the quantification of the buffering capacity of the samples and the mobilization rates of lead as a function of pH. The geochemical model was fitted to experimental results with thermodynamic considerations. The geochemical model allowed for the identification of the mineral phases involved in providing the buffering capacity of carbonated mining waste (Mibladen) and the meager buffering capacity of the silicate mining waste (Zeida). These cases are representative of contaminated neutral drainage (CND) and acid mine drainage (AMD), respectively. The results highlight the consistency between the ANC–BNC experimental data and the associated modeling in terms of geochemical behavior, validating the approach and identifying the main mechanisms involved. The modeling approach identifies the dissolution of the main solid phases, which impact the pH and the speciation of lead as a function of the pH. This innovative approach, combining ANC–BNC experiments and geochemical modeling, allowed for the accurate identification of mineral phases and surface complexation phenomena, which control the release of lead and its speciation in drainage solutions, as well as within solid phases, as a function of pH.

ANC–BNC Titrations and Geochemical Modeling for Characterizing Calcareous and Siliceous Mining Waste

Pyrite and calcite are mineral phases that play a major role in acid and neutral mine drainage processes. However, the prediction of acid mine drainage (AMD) or contaminated neutral drainage (CND) requires knowledge of the mineral composition of mining waste and the related potential for element release. This paper studies the combination of acid–base neutralizing capacity (ANC–BNC) with geochemical modeling for the characterization of mining waste and prediction of AMD and CND. The proposed approach is validated with three synthetic mineral assemblages: (1) siliceous sand with pyrite only, representing mining waste responsible for AMD, (2) siliceous sand with calcite and pyrite, representing calcareous waste responsible for CND, and (3) siliceous sand with calcite only, simulating calcareous matrices without any pyrite. The geochemical modeling approach using PHREEQC software was used to model pH evolution and main element release as a function of the added amount of acid or base over the entire pH range: 1 < pH < 13. For calcareous matrices (sand with calcite), the results are typical of a carbonated environment, the geochemistry of which is well known. For matrices containing pyrite, the results identify different pH values favoring the dissolution of pyrite: pH = 2 in a pyrite-only environment and pH = 6 where pyrite coexists with calcite. The neutral conditions can be explained by the buffering capacity of calcite, which allows iron oxyhydroxide precipitation. Major element release is then related to the dissolution and precipitation of the mineral assemblages. The geochemical modeling allows the prediction of element speciation in the solid and liquid phases. Our findings clearly prove the potential of combined ANC–BNC experiments along with geochemical modeling for the characterization of mining waste and the assessment of risk of AMD and CND.

Treatment of manganese, in the presence or absence of iron, in acid and neutral mine drainage using raw vs half-calcined dolomite

Efficient removal of Mn from mine drainage is complex because of its high solubility minerals, especially in the presence of Fe. This study evaluates the comparative efficiency of raw and half-calcined dolomite for Mn removal in synthetic and real contaminated neutral drainage (CND) as well as in real acid mine drainage (AMD). Batch isotherms were used to evaluate the removal of Mn by raw and half-calcined dolomite, in the presence or the absence of Fe, in synthetic CND and in synthetic AMD. Batch kinetics tests were also performed, only for synthetic CND, in the absence of Fe. Raw dolomite showed limited efficiency in batch tests. Hence, only the half-calcined dolomite was used in column tests. Removal of Mn from synthetic CND (pH 6.84; 46.1 mg/L Mn) was effective for 106 days. For the following 119 days of monitoring, Mn was treated ≥95%. For Mn removal in real AMD (pH 2.40; 22.8 mg/L Mn and 679 mg/L Fe), half-calcined dolomite proved less effective. After 1 week, Mn, Ni and Zn were no longer treated, whereas after 3 weeks, the pH and the concentration of Cu in the final effluent were also no longer conform to the Canadian discharge criteria. In the same time, Co, Cr and Fe were still removed even after 8 weeks. For the real CND, Mn concentration decreased by >90% (0.65 mg/L at the inlet and 0.05 mg/L at the exit) with half-calcined dolomite. Therefore, half-calcined dolomite seems highly promising for the treatment of Mn in CND.

Comparison of organic materials for the passive treatment of synthetic neutral mine drainage contaminated by nickel: Short- and medium-term batch experiments

This paper addresses the comparison of various low-cost organic materials for the passive treatment of circum-neutral pH contaminated mine water. First, the effectiveness of five organic materials to remove Ni from a contaminated neutral drainage (CND) was compared in short-term batch experiments (24 h) at various pH values. Second, based on results of the short-term experiments, two substrates (brown algae and sawdust) were eliminated and three (horticultural peat, compost, wood ash) were compared along with a new substrate (field-collected surface peat) in medium-term (56 days) batch experiments to treat CND. In these experiments, calcite was added to peat samples and all substrates performed equally well, sequestering over 97% of the Ni. Chemical extractions revealed that Ni was more strongly bound to the horticultural peat-calcite (HD-peat-calcite) residue than to the field-collected peat-calcite (LT-peat-calcite) residue. Compost, because of its higher density, was identified as the most promising candidate for sorption-based fixed-bed column experiments. Nevertheless, wood ash should not be discarded as its alkaline properties favor nickel removal.

Removal and Recovery of Ni and Zn from Contaminated Neutral Drainage by Thermally Activated Dolomite and Hydrothermally Activated Wood Ash

Raw and activated dolomite or wood ash were comparatively evaluated in column reactors for the simultaneous removal of Ni and Zn in synthetic contaminated neutral drainage (CND) over 119 days. Dolomite was thermally activated by charring, while wood ash was subject to alkaline fusion followed by hydrothermal treatment. Column testing was performed using CND with 51.5 mg/L Ni and 46.5 mg/L Zn, at pH 6. Testing was considered complete when metal concentrations in the final effluent reached 0.5 mg/L to meet Canadian regulations on discharge. Results showed that thermally activated wood ash was the most efficient among all tested materials as it removed 64 mg/g Ni and 61.6 mg/g Zn. Based on these results, thermally activated wood ash could be considered a promising option for Ni and Zn treatment in CND, although final pH correction of treated effluent might be necessary. At the end of the testing period, metals could be recovered by dissolution with concentrated acid and by hydrometallurgical processes from spent activated dolomite and wood ash, respectively. As spent material could be used for recovery of treated metals, sludge management issues could be alleviated.

Kinetic Testing for Oxidation Acceleration and Passivation of Sulfides in Waste Rock Piles to Reduce Contaminated Neutral Drainage Generation Potential

Contaminated neutral drainage (CND), generated when metals are leached and enough neutralizing minerals are present to keep a near-neutral pH, is a growing environmental concern related to mine waste management. There is a need to find ways to reduce CND generation from mine waste to avoid perpetual effluent treatment. Nickel concentrations at the Lac Tio mine effluent have been sporadically higher than provincial environmental standards. In this project, three different treatment solutions were applied to Lac Tio mine waste rock samples in 70 kg waste rock columns in an attempt to reduce nickel CND generation potential by forcing sulfide oxidation and/or passivation. Following the applications, kinetic tests were used to determine any reduction in sulfide oxidation and nickel leaching. Hydrogen peroxide and sodium silicate were the main products tested, alone and in combination with a sodium bicarbonate buffer. Buffered hydrogen peroxide showed the best potential to reduce CND generation, with nickel release reduced from 1.4 × 10–3 mg/kg/day (untreated) to 4.0 × 10–5 mg/kg/day (treated) and near-neutral pH values. This passivation treatment also reduced the sulfur release rate by more than 50%. Optical and scanning electron microscopy observations supported the laboratory results. To conclude, this research project proposes a new way to reduce the CND generation potential of low sulfide waste rock.

Efficiency of eight modified materials for As(V) removal from synthetic and real mine effluents

Arsenic (As) contamination is a major problem especially for active gold mine operations. In the present study, eight low-cost materials including biochar (B), Fe-loaded biochar (BF), activated biochar (BC), Fe-loaded activated biochar (BCF and BFC), thermally modified dolomite (MD), wood ash (WA), and modified wood ash (MWA) were comparatively used for the efficiency in As(V) removal from synthetic and real mine effluents, through batch and column testing. Batch adsorption tests were conducted in beakers with a ratio adsorbent material and As(V) synthetic and real solutions of 0.1 g: 10 mL at concentrations of 850 and 300 µg/L As, respectively. Column adsorption tests were performed in 3 reactors with As(V) concentration of up to 900 µg/L in contaminated neutral drainage (CND) collected from a local gold mine. Results from batch testing with synthetic effluents showed the best performance for As(V) removal in the following order: MD > WA > BCF > BF > BFC > MWA > BC > B. Consistent findings were obtained in batch and column testing with the real mine effluent. Although iron grafted biochars are good adsorbents, their performance for As(V) removal was limited probably because of the very low As concentration in this study. In the same time, MD was found to be the most efficient material for As(V) removal but the final pH must be monitored and eventually adjusted. As(V) was completely removed by MD in batch testing (99.9%) and column testing (99.6%) after>112 days to bellow the authorized monthly mean allowed by Canadian discharge criteria. Thus, MD seems to be the most efficient material among the tested ones for the removal of As(V) in batch and column testing from synthetic and mine effluents.

Environmental Impact of Mine Exploitation: An Early Predictive Methodology Based on Ore Mineralogy and Contaminant Speciation

Mining wastes containing sulfide minerals can generate contaminated waters as acid mine drainage (AMD) and contaminated neutral drainage (CND). This occurs when such minerals are exposed to oxygen and water. Nowadays, mineralogical work—when it is done—is independently and differentially done according to the needs of the exploration, geotechnics, metallurgy or environment department, at different stages in the mine development process. Moreover, environmental impact assessments (EIA) are realized late in the process and rarely contain pertinent mineralogical characterization on ores and wastes, depending on countries’ regulations. Contaminant-bearing minerals are often not detected at an early stage of the mine life cycle and environmental problems could occur during production or once the mine has come to the end of its productive life. This work puts forward a more reliable methodology, based on mineralogical characterization of the ore at the exploration stages, which, in turn, will be useful for each stage of the mining project and limit the unforeseen environmental or metallurgical issues. Three polymetallic sulfide ores and seven gold deposits from various origins around the world were studied. Crushed ore samples representing feed ore of advanced projects and of production mines were used to validate the methodology with realistic cases. The mineralogical methodology consisted in chemical assays and XRD, optical microscopy, SEM and EPMA were done. Five of the ores were also submitted to geochemical tests to compare mineralogical prediction results with their experimental leaching behavior. Major, minor, and trace minerals were identified, quantified, and the bearing minerals were examined for the polluting elements (and valuables). The main conclusion is that detailed mineralogical work can avert redundant work, save time and money, and allow detection of the problems at the beginning of the mine development phase, improving waste management and closure planning.

Removal efficiency of As(V) and Sb(III) in contaminated neutral drainage by Fe-loaded biochar.

Performance of raw and two Fe-loaded biochars, produced either by evaporation (E-product, 26.9% Fe) or precipitation (P-product, 12.6% Fe), was evaluated in batch and column testing for As(V) and Sb(III) removal from contaminated neutral drainage (CND). Batch testing results showed that sorption capacity of the E-product tripled for As(V) and quintupled for Sb(III), whereas for the P-product, it doubled for both contaminants, relative to the raw biochar. Moreover, As(V) removal by the E-product reached 90% in less than 8h, for initial concentrations up to 50mg/L. In column testing, the E-product efficiently treated the influent [pH6; 1mg/L As(V)] for more than 286days. The pH of the final effluent was within the legally allowed limits (6-9.5) while less than 0.3mg/L Fe leached out. Based on these findings, Fe-loaded biochar by evaporation (E-product) seems promising for As(V) treatment in CND.

Development of a modified kinetic test using EDTA and citric acid for the prediction of contaminated neutral drainage

Predicting the quality of mine drainage is a very important step in the management of mine wastes. A variety of useful tools are available for acid mine drainage (AMD), but new tests must be developed to improve the accuracy of predictions for contaminated neutral drainage (CND), or metal leaching. This study was performed using 0.01M EDTA and citric acid in the leaching solutions of kinetic tests on waste rocks and low-grade ore from the BlackRock project. Results showed an increase in metal (Co, Cu, Mn, Ni, and Zn) concentrations in the leachates, except for Zn with citric acid. Several major cations (Al, Ca and Mg) were also chelated by EDTA, but did not appear to be in competition with metal chelation because the chelating agent was present in excess in the kinetic test leachates. Ca and Mg were also chelated by citric acid, but Al was not. The release rate of iron was compared to the release rate of total sulphur in the leachates. For the waste rock samples, these release rates corresponded to the dissolution of sulphides. For the low-grade ore sample, the release rate of iron was higher and likely associated with the dissolution of iron oxides. Further studies will be needed to explore how the selection and concentration of specific chelating agents for use in modified kinetic tests must be adjusted depending on the mineralogy of the samples.

Removal of Ni and Zn in contaminated neutral drainage by raw and modified wood ash

ABSTRACTIn the present study, wood ash was modified by alkaline fusion, prior to hydrothermal synthesis, for potential application in the treatment of mine drainage impacted water. With this objective, two types of wood ash (both raw and modified) were evaluated for the treatment of Ni and Zn in contaminated neutral drainage (CND). Batch adsorption experiments were initially conducted on synthetic CND, and then on two real CND, sampled on two active mine sites, contaminated by either Ni (3.7 mg/L) or Zn (9.1 mg/L). Leaching of Zn was observed during the kinetic tests for the raw wood ash, whereas its modification suppressed the leaching. The cation exchange capacity acquired by modification of the two samples of wood ash exceeded 300 meq/100 g (which is two to fourfold higher than those of the raw ash), while sorption capacity for Ni and Zn tripled relative to the raw material. The Langmuir model best described the sorption process for all materials, while potential mechanisms of metal removal include adsorption, precipitation and ion exchange, following pseudo second-order kinetics. Results also showed that within 2 h of contact of mine effluents with one modified wood ash, Ni and Zn concentrations decreased below the maximum authorized monthly mean concentration allowed by the Canadian law (0.5 mg/L), whereas the other modified wood ash allowed reaching the regulatory conformity after 2 h for Ni but 7 days for Zn (although 93% removed after 2 h). Nonetheless, the pH was raised (10.9–11.8) above the legally allowed limits (6–9.5). Based on these findings, modified wood ash could be considered as a promising option for the treatment of Ni and Zn in CND, but the pH correction of final effluent might be necessary.

Performance of thermally activated dolomite for the treatment of Ni and Zn in contaminated neutral drainage

Intensive research is ongoing for developing low-cost and highly efficient materials in metal removal from contaminated effluents. The present study evaluated dolomite [CaMg(CO3)2], both raw and modified by thermal activation (charring), for Ni and Zn treatment in contaminated neutral drainage (CND). Batch adsorption testing (equilibrium and kinetics) were conducted at pH 6, to evaluate the performance of initial vs. modified dolomite, and to assess potential mechanisms of metal removal. Charring of dolomite led to a rigid and porous material, mainly consisting of CaCO3 and MgO, which showed a sorption capacity increased sevenfold for Zn and doubled for Ni, relative to the raw material. In addition, Freundlich model best described the sorption of the both metals by dolomite, whereas the Langmuir model best described their sorption on charred dolomite. Plausible mechanisms of metal removal include cation exchange, surface precipitation and sorption processes, with carbonate ions and magnesium oxides acting as active centers. Based on these results, charred dolomite seems a promising option for the efficient treatment of Ni and Zn in CND.

Use of EDTA in modified kinetic testing for contaminated drainage prediction from waste rocks: case of the Lac Tio mine.

The tools developed for acid mine drainage (AMD) prediction were proven unsuccessful to predict the geochemical behavior of mine waste rocks having a significant chemical sorption capacity, which delays the onset of contaminated neutral drainage (CND). The present work was performed in order to test a new approach of water quality prediction, by using a chelating agent solution (0.03M EDTA, or ethylenediaminetetraacetic acid) in kinetic testing used for the prediction of the geochemical behavior of geologic material. The hypothesis underlying the proposed approach is that the EDTA solution should chelate the metals as soon as they are released by sulfide oxidation, inhibiting their sorption or secondary precipitation, and therefore reproduce a worst-case scenario where very low metal attenuation mechanisms are present in the drainage waters. Fresh and weathered waste rocks from the Lac Tio mine (Rio tinto, Iron and Titanium), which are known to generate Ni-CND at the field scale, were submitted to small-scale humidity cells in control tests (using deionized water) and using an EDTA solution. Results show that EDTA effectively prevents the metals to be sorbed or to precipitate as secondary minerals, therefore enabling to bypass the delay associated with metal sorption in the prediction of water quality from these materials. This work shows that the use of a chelating agent solution is a promising novel approach of water quality prediction and provides general guidelines to be used in further studies, which will help both practitioners and regulators to plan more efficient management and disposal strategies of mine wastes.

Numerical modeling of contaminated neutral drainage from a waste-rock field test cell

Contaminated drainage related to the leaching of soluble metals under near-neutral conditions, known as contaminated neutral drainage (CND), may arise when metal species are soluble at neutral pH. Such a phenomenon has been sporadically observed in effluent fromthe Tiomine waste-rock pile in Quebec, Canada, particularly from older sections of the pile, where Ni concentrations are increasing with time. It has been postulated that Ni is retained within the fresh waste rock as sorbed species, but as the rock ages, sorption sites become saturated and more Ni is released to the effluent. A field test program was initiated to evaluate the geochemical behavior of the waste rock. This paper presents a numerical analysis of CND generation from waste-rock field test cells including water flow and multi-component transport with geochemical reactions (e.g. sulfide oxidation, pH neutralization, and Ni sorption), using the code MIN3P. The model was able to represent the delay before Ni is seen in the effluent, as caused by sorption of Ni onto Fe-oxide particles. Once the sorption sites are saturated, the model allows Ni release into the effluent by millerite dissolution, expressed by the shrinking core model. A sensitivity analysis indicated that sorption parameters significantly affected the simulated results, so their selection should be based on sound independent field or experimental data.

Kinetic Testing and Sorption Studies by Modified Weathering Cells to Characterize the Potential to Generate Contaminated Neutral Drainage

Techniques developed for acid mine drainage (AMD) prediction might not be suitable for contaminated neutral drainage (CND) generating sites. The Tio mine waste is known to generate Ni contaminated neutral drainage in some of the piles, but humidity cell tests fail to generate the Ni concentrations observed in the field. Weathering cell tests (small-scale humidity cell tests) were performed on fresh and weathered (produced 25 years ago) waste rock samples from the Tio mine containing various levels of hemo-ilmenite ore, and results were compared to humidity cell results on similar samples. The main constituents of the waste rock are the hemo-ilmenite ore and the plagioclase gangue; these minerals were purified from the waste rocks and the purified fractions were also submitted to weathering cell tests. The fresh waste rock samples were also submitted to sorption cells (modified weathering cells), which showed that the waste rocks have an important Ni sorption potential and that the sorbed phases are stable under weathering cell conditions. Even though the Ni concentrations obtained from the laboratory tests remain significantly lower than those obtained in field conditions (from field test pads and from waste rock piles), the results from the present study give important insight into the geochemical processes implicated in CND generation.