Sustainable phosphate mining: Enhancing efficiency in mining and pre-beneficiation processes.

Phosphate holds a critical role as a vital, limited, strategic, and irreplaceable resource. Throughout its production chain, residual phosphate can be found in waste streams. This study aims to enhance production efficiency by exploring methods to limit residual phosphate presence in waste stocks. It investigates the presence of residual phosphate in a phosphate mining site. The presence of residual phosphate throughout the production chain is investigated. Through meticulous analyses of extraction, destoning, and screening processes, the study identifies three primary stages where residual phosphate exists, the study simulates different scenarios of residual phosphate recovery and prevention. The principal data sources are data from mining site, recent literature, and information from a lithological log, the study meticulously analyzes the extraction, crushing, and sieving processes to assess the persistence of residual phosphate. The production chain diagnostic revealed that 76% of resource present is recovered (either integrated into the value chain or stored in the mine for future use), from which 8% goes to the destoning waste rocks (75% of which is residual phosphate) and the screening waste rocks (72% of which is residual phosphate), with an average grade that reaches 25% P2O5. Approximately, 24% of the initial phosphate rock (with an average grade of 22% P2O5) remains as residual phosphate which is retained in the spoil piles. To recover and prevent the presence of residual phosphate, the study proposes four new scenarios for improvement, including an integrated scenario where all the solutions are combined for a comprehensive approach. Both quantity and grade of recovered residual phosphate are assessed in each scenario. To evaluate these enhancements, the study utilizes the AnyLogic software to simulate existing process configuration and the maximal recovery of each scenario. The current flowsheet indicates that extracted phosphate can be directed either to pre-beneficiation and expedition or stored for future use. By prioritizing the extraction of phosphate over the final product, the simulation results suggest that implementing these novel scenarios could potentially save 25% of the total phosphate resource and increase storage by twofold, preserving phosphate that would otherwise be unused. This recovered phosphate can then be destined to various uses, meeting the company's present or future needs. Considering this, the study opts to keep stocks separated based on their grades and avoid mixing new phosphate streams with the final product. The implications of this research extend to sustainable mining practices, with direct ramifications for environmental impact mitigation and the conservation of valuable resources.

Multi-year in situ hydrogeochemical monitoring of hard rock lithium mine tailings in a large-scale experimental pile.

Spodumene, a lithian mineral found in granitic pegmatites, is a major source of lithium. In situ, multi-year and large-scale (>10s of tons) predictive hydrogeochemical studies can be of great value for informing mine waste rock and tailings management, not least because the material is exposed to the actual climate where it is meant to be stored. However, such studies are rare given their cost and size, and previous ones have focused mainly on sulfide-bearing materials. An experimental field cell filled with spodumene beneficiation tailings was built at the Whabouchi lithium mine site in northern Québec (Canada) and monitored during four consecutive years. In parallel, column laboratory kinetic testing on the same material was conducted for water quality parameters to compare the effect of testing scales on the geochemical behavior. Geochemical results and release rates were overall similar between laboratory and field, and consistent with previously published laboratory results for materials from the same site. The leachates were neutral to slightly alkaline, with ions from feldspars (Ca > Na > K) and residual spodumene (Li) being notable solutes. Concentrations for most solutes surveyed remained higher in the field after four years than in columns at the end of the experiment. One major difference between laboratory and field was a brief (less than 2 years) increase in iron concentrations from the field cell, resulting in Fe-oxyhydroxides precipitation, that was not observed in laboratory. The source of this iron is arguably residual ferrosilicon (used for dense medium separation) and this difference is attributed to the different testing conditions and configurations in the laboratory versus in situ. Field hydrogeological results highlight strong seasonal patterns and the rapid response of this sand-like, sulfide-poor material to ambient temperature changes and wetting-drying events. The tailings both wet and drain rapidly and easily given their water retention curve and the range of matrix suctions recorded during frost-free months. Net infiltration through the tailings was estimated to represent 55% of total precipitation in summer (June-October) 2021. This study provides a comprehensive assessment of the environmental behavior of hard rock lithium mine tailings under real, sub-arctic climatic conditions and outlines similarities and differences between laboratory- and field-generated geochemical results.

Environmental assessment of phosphogypsum: A comprehensive geochemical modeling and leaching behavior study.

Understanding the variations in the geochemical composition of phosphogypsum (PG) destined for storage or valorization is crucial for assessing the safety and operational efficacy of waste management. The present study aimed to investigate the environmental behavior of PG using different leaching tests and to evaluate its geochemical behavior using geochemical modeling. Regarding the chemical characterization, the PG samples were predominantly composed of Ca (23.03-23.35 wt%), S (17.65-17.71 wt%), and Si (0.75-0.82 wt%). Mineralogically, the PG samples were primarily composed of gypsum (94.2-95.9 wt%) and quartz (1.67-1.76 wt%). Moreover, the automated mineralogy revealed the presence of apatite, fluorine and malladrite phases. The overall findings of the leaching tests showed that PG could be considered as non-hazardous material according to US Environmental Protection Agency limitations. However, a high leachability of elements at a L/S of 2 under acidic conditions ([Ca] = 166.52-199.87 mg/L, [S] = 207.9-233.59 mg/L, [F] = 248.62-286.65 mg/L) is observed. The weathering cell test revealed a considerable cumulative concentration over 90 days indicating potential adverse effects on the nearby environment (S: 8000 mg/kg, F: 3000 mg/kg, P: 700 mg/kg). Based on these results, it could be estimated that the surface storage of PG could have a serious impact on the environment. In this context, a simulation model was developed based on weathering cell results showed encouraging results for treating PG leachate using CaO before its disposal. Additionally, PHREEQC was used to analyze the speciation of major elements and calculate mineral phase saturation indices in PG leaching solutions. The findings revealed pH-dependent speciation for Ca, S, P, and F. The study identified gypsum, anhydrite, and bassanite as the key phases governing the dissolution of these elements.

Performances of stabilization/solidification process of acid mine drainage passive treatment residues: Assessment of the environmental and mechanical behaviors.

Residues from passive treatment of acid mine drainage (AMD) have variable chemical stability and could regenerate contaminated drainage. Stabilization/solidification (S/S) can prevent contaminant leaching. Residues were collected from a tri-step AMD field passive treatment system, operated for 6 years at the reclaimed Lorraine mine site, Quebec, Canada. General Use Portland cement (GU), blended binders based on GU with pozzolanic additives (ground-granulated blast-furnace slag; GGBFS and fly ash type C; FAC) were used as hydraulic binders, in proportions (w/w %) of 100GU, 20GU/80GGBFS, and 50GU/50FAC, respectively. Residues were mixed with wood ash (35%) and sand (25%), while reference samples (100% sand) were also prepared. Prior to S/S, raw materials were characterized. The S/S effectiveness was assessed mineralogically and mechanically (unconfined compressive strength; UCS). Environmental behavior assessment (static vs semi-dynamic leaching tests) was also performed. UCS results showed that strength increase with age. At 56 days, GU- (1.3 MPa) and GU/GGBFS (0.7 MPa) satisfied Quebec's strength requirements for landfill disposal (0.7 MPa), but not GU/FAC (0.6 MPa), while all samples satisfied USEPA criteria (0.35 MPa). The semi-dynamic test showed that all elements can be immobilized successfully in GU- and GU/GGBFS. The GU binder had the best stabilizing performance. Based on USEPA requirements, S/S using GU, GGBFS, and FAC can be also considered for contaminant immobilization in AMD passive treatment residues. Finally, the comparison between replicates using Student's t-test indicated good reproducibility of S/S treatment.

Leaching and geochemical behavior of fired bricks containing coal wastes.

High amounts of mine wastes are continuously produced by the mining industry all over the world. Recycling possibility of some wastes in fired brick making has been investigated and showed promising results. However, little attention is given to the leaching behavior of mine wastes based fired bricks. The objective of this paper is to evaluate the geochemical behavior of fired bricks containing different types of coal wastes. The leachates were analyzed for their concentration of As, Ba, Cd, Co, Cr, Cu, Mo, Ni, Pb, Zn and sulfates using different leaching tests; namely Tank Leaching tests (NEN 7375), Toxicity Characteristic Leaching Procedure (TCLP) and pH dependence test (EPA, 1313). The results showed that the release of constituents of potential interest was highly reduced after thermal treatment and were immobilized within the glassy matrix of the fired bricks. Moreover, it was also highlighted that the final pH of all fired samples changed and stabilized around 8-8.5 when the initial pH of leaching solution was in the range 2.5-11.5. The release of heavy metals and metalloids (As) tended to decrease with the increase of pH from acidic to alkaline solutions while Mo displayed a different trend.

Iron removal in highly contaminated acid mine drainage using passive biochemical reactors.

Passive biochemical reactors (PBRs) are a viable alternative to neutralization plants for the treatment of acid mine drainage (AMD) because they require lower investment costs and use residual materials. However, high iron (Fe) concentrations (≥0.5 g/L) in AMD are challenging for their long-term efficiency. Sorption and precipitation are the main Fe removal mechanisms, but the relative importance of each is mostly unknown. In this study, locally available natural materials (organic and inorganic) were characterized and tested for their performance in Fe removal from highly contaminated AMD (pH 3.5, 4 g/L of Fe, and 9 g/L of sulfate). Iron retention capacity of the materials was then evaluated and the efficiency of eight mixtures of materials was compared through 40-day laboratory batch tests. All batch-type PBRs increased the pH up to 6.5 and decreased dissolved metals concentrations, including Fe, up to 99%. Results showed that organic residual materials (manures, municipal wastewater sludge, and compost) were the best substrates for Fe removal.These findings allowed for the selection of three reactive mixtures with distinct characteristics (mixture #1 - 30% organic wastes; mixture #4 - 50% calcite; and mixture #7 - 50% sand) to be further evaluated in column type PBRs.

Removal of copper in leachate from mining residues using electrochemical technology.

This research is related to a laboratory study on the performance of a successive mining residues leaching and electrochemical copper recovery process. To clearly define the experimental region for response surface methodology (RSM), a preliminary study was performed by applying a current intensity varying from 0.5 A to 4.0 A for 60 min. By decreasing the current intensity from 4.0 A to 0.5 A, a good adhesion and a very smooth and continuous interface of copper was formed and deposited on the cathode electrode. However, the removal rate of Cu decreased from 83.7% to 37.9% when the current intensity passed from 4.0 A to 0.5 A, respectively. Subsequently, the factorial design and central composite design methodologies were successively employed to define the optimal operating conditions for copper removal in the mining residues leachate. Using a 2(3) factorial matrix, the best performance for copper removal (97.7%) was obtained at a current intensity of 2.0 A during 100 min. The current intensity and electrolysis time were found to be the most influent parameters. The contribution of current intensity and electrolysis time was around 65.8% and 33.9%, respectively. The treatment using copper electrode and current intensity of 1.3 A during 80 min was found to be the optimal conditions in terms of cost/effectiveness. Under these conditions, 86% of copper can be recovered for a total cost of 0.56 $ per cubic meter of treated mining residues leachate.

A review of organic and inorganic amendments to treat saline-sodic soils: Emphasis on waste valorization for a circular economy approach.

Soil salinization poses a significant challenge to the sustainable advancement of agriculture on a global scale. This environmental issue not only hampers plant growth and soil fertility but also hinders the advancement of the national economy due to restrictions on plant development. The utilization of organic and/or inorganic amendments has demonstrated the ability to mitigate the detrimental impacts of salt stress on plant life. At the outset, this review, in addition to summarizing current knowledge about soil amendments for saline-sodic soils, also aims to identify knowledge gaps requiring further research. The organic or inorganic amendments modify soil conditions and impact plant development. For instance, organic amendments have the potential to improve the structure of the soil, augment its capacity to retain water, and stimulate microbial activity. As this occurs, salts gradually leach through the porous structure of the soil. Conversely, inorganic amendments, such as gypsum and phosphogypsum, displace sodium from soil-negative sorption sites reducing the salinity, they also increase base saturation, altogether positively impacting plant growth conditions. This review emphasizes that, under adequate rates, the combination of organic and inorganic amendment has a high potential to enhance the poor physicochemical properties of saline-sodic soils, thereby reducing their salinity. Consequently, an in-depth examination of the mineral composition, texture, and chemical composition of the soil is required to choose the most effective amendment to implement. Future research necessitates a thorough investigation of techno-economic and life cycle assessment, with active involvement from stakeholders, to enhance the decision-making process of the amendments in specific localities.

Native plant diversity for ecological reclamation in Moroccan open-pit phosphate mines.

Mining activities have significant impacts on the environment, particularly in terms of the destruction of natural habitats andbiodiversity loss. With the increasing awareness of the importance of ecological restoration and conservation, there is a growing need to study and understand the flora that thrives in mining sites in order to facilitate successful reclamation efforts. This study aimed to investigate the floristic composition and plant diversity of four phosphate mine sites (PMSs) in Morocco, namely Bou Craa mine (BCM), Ben Guerir mine (BGM), Youssoufia mine (YSM), and Khouribga mine (KHM). The study found a total of 215 vascular plant species from 166 genera and 49 taxonomic families across the four sites. BGM was the most diverse site with 120 plant species, followed by KHM with 75, YSM with 57, and BCM with 54. Compositae family species were the most common at BGM and KHM, while Amaranthaceae species were dominant at BCM, and Poaceae and Compositae at YSM. Therophytes (annual species) were the most common functional group (45.0%), followed by chamaephytes (19.6%) and hemicryptophytes (15.9%). Atriplexnummularia and Chenopodiumalbum were the most common species found at all four sites, while Atriplexsemibaccata, Bassiamuricata, Haloxylonscoparium, and 12 other species were common at three sites. However, 156 plant species were found at only one site. The findings of this study highlight the significant abundance of plant species in Moroccan PMSs and provide a basis for successful ecological engineering rehabilitation plans. The study emphasizes the importance of studying the indigenous plant species that naturally populate these marginal lands to ensure successful reclamation efforts.

Advancing phosphate ore minerals separation with sustainable flotation reagents: An investigation into highly selective biobased depressants.

Froth flotation has been a commonly employed technique to enrich natural ores by removing impurities based on the surface properties of minerals. This process involves the use of various reagents, including collectors, depressants, frothers, and activators, which are often chemically synthesized and may represent environmental risks. Therefore, there is a growing need to develop biobased reagents that offer more sustainable alternatives. The aim of this review is to provide a comprehensive assessment of the potential of biobased depressants as a sustainable alternative to traditional reagents in selective flotation process for phosphate ore minerals. To achieve this objective, the review investigates the extraction and the purification methods of different biobased depressants, analyzes the specific conditions for reagent interaction with minerals, and assess the biobased depressants' performance through a range of fundamental studies. These studies aim to (i) provide a better understanding of the adsorption behavior of some biobased depressants onto the surfaces of apatite, calcite, dolomite, and quartz comprised in different mineral systems by measuring their zeta potential and analyzing their Fourier transform infrared spectra before and after contact with these reagents, (ii) determine the depressants' adsorption amounts, (iii) evaluate their effect on the contact angle of bare minerals, and (iv) assess their ability to inhibit the flotation of the studied minerals. The outcomes revealed the potential use and the promising applicability of these unconventional reagents since their performance is comparable to that of conventional reagents. In addition to their good effectiveness, these biobased depressants have the added advantages of being cost effective, biodegradable, non-toxic, and ecofriendly. Nevertheless, further research and investigations are required to improve the selectivity and, consequently, the effectiveness of biobased depressants.

Coal slime as a good modifier for the restoration of copper tailings with improved soil properties and microbial function.

In recent years, the solid wastes from the coal industry have been widely used as soil amendments. Nevertheless, the impact of utilizing coal slime for copper tailing restoration in terms of plant growth, physicochemical characteristics of the tailing soil, and microbial succession remains uncertain.Herein, the coal slime was employed as a modifier into copper tailings. Their effect on the growth and physiological response of Ryegrass, and the soil physicochemical properties as well as the bacterial community structure were investigated. The results indicated that after a 30-day of restoration, the addition of coal slime at a ratio of 40% enhanced plant growth, with a 21.69% rise in chlorophyll content, and a 62.44% increase in peroxidase activity. The addition of 40% coal slime also increased the content of nutrient elements in copper tailings. Following a 20-day period of restoration, the concentrations of available copper and available zinc in the modified tailings decreased by 39.6% and 48.51%, respectively, with 40% of coal slime added. In the meantime, there was an observed augmentation in the species diversity of the bacterial community in the modified tailings. The alterations in both community structure and function were primarily influenced by variations in pH value, available nitrogen, phosphorus, potassium, and available copper. The addition of 40% coal slime makes the physicochemical properties and microbial community evolution of copper tailings reach a balance point. The utilization of coal slime has the potential to enhance the physicochemical characteristics of tailings and promote the proliferation of microbial communities, hence facilitating the soil evolution of two distinct solid waste materials. Consequently, the application of coal slime in the restoration of heavy metal tailings is a viable approach, offering both cost-effectiveness and efficacy as an enhancer.

Assessment of arsenic immobilization in synthetically prepared cemented paste backfill specimens.

Mine tailings coming from the exploitation of sulphide and/or gold deposits can contain significant amounts of arsenic (As), highly soluble in conditions of weathering. Open mine voids backfilling techniques are now widely practiced by modern mining companies to manage the tailings. The most common one is called cemented paste backfill (CPB), and consists of tailings mixed with low amounts of hydraulic binders (3-5%) and a high proportion of water (typically 25%). The CPB is transported through a pipe network, to be placed in the mine openings. CPB provides storage benefits and underground support during mining operations. Moreover, this technique could also enhance contaminant stabilization, by fixing the contaminants in the binder matrix. CPB composites artificially spiked with As were synthesized in laboratory, using two types of hydraulic binders: a Portland cement, and a mix of fly ash and Portland cement. After curing duration of 66 days, the CPB samples were subjected to several leaching tests in various experimental conditions in order to better understand and then predict the As geochemical behaviour within CPBs. The assessment of the As release indicates that this element is better stabilized in Portland cement-based matrices rather than fly ash-based matrices. The As mobility differs in these two matrices, mainly because of the different As-bearing minerals formed during hydration processes. However, the total As depletion does not exceed 5% at the end of the most aggressive leaching test, indicating that As is well immobilized in the two types of CPB.

Improvement of water recovery from phosphate sludge at the M'Dhilla Mine, Tunisia.

In Tunisia, phosphate beneficiation from ores by the Gafsa Phosphate Company (GPC) is a water-intensive process that generates large amounts of sludge. Responsible mining minimizes water use and prioritizes its recycling and reuse to limit the impact on water resources. Recovery of water from the phosphate sludge (PS) using the densification with adapted flocculants-a low-energy consuming process-is plausible for efficient management of water resources. The objective of this study was to improve low-cost water recovery from PS produced at the M'Dhilla Mine plant operated by GPC in Tunisia. Representative samples of PS were first collected and characterized for physicochemical and mineralogical properties. To maximize water recovery, densification based on flocculation was then performed using two different flocculants (Slim Floc vs Flomin 905) with different doses (0.1 g/L vs 0.3 g/L), consumption (100-1200 g per ton of dry sludge g/tds), sludge concentration (50 g/L vs 60 g/L), and settling time (15-1200 s). Results showed that PS particles were fine-grained and contained carbonates, silicates, and significant residual fluorapatite (59%) that could be valorized. Up to 91% of water was recovered using anionic flocculant Flomin 905. These findings show an improvement of 24% relative to the current water recovery at M'Dhilla plant (66%) while using a flocculant dose three times lower than the conventional flocculent Sim Floc (0.1 g/L vs 0.3 g/L). The best sludge settlement conditions were obtained with 0.1 g/L Flomin 905 at 600 g/tds and 10 min of settling time. The densification process using Flomin 905 proved efficient in maximizing water recovery (91%) with a consumption of flocculant that could be decreased by up to 70% annually in comparison with Slim Floc, thus decreasing treatment costs by 63%. Results will help to prevent exhaustion of groundwater resources and limit land exploitation while decreasing the volume of settling ponds.

Passive treatment of acid mine drainage from the Sidi-Kamber mine wastes (Mediterranean coastline, Algeria) using neighbouring phosphate material from the Djebel Onk mine.

Passive abiotic treatment of acid mine drainage (AMD) was investigated using phosphate mining residuals (raw low-grade phosphate ore, phosphatic limestone wastes, and phosphate mine tailings) from the Djebel Onk mine, Algeria. Laboratory batch tests were performed using the main expected lithologies of phosphate materials in contact with synthetic AMD, which had a low pH (3.08) and contained high concentrations of Fe (600 mg/L), Mn (40 mg/L), Mg (10 mg/L), Zn (20 mg/L), Cu (25 mg/L), As (50 mg/L), and sulfate (3700 mg/L). Phosphate materials were used as an oxic limestone drain to evaluate the increase in the pH of the AMD and metal removal by sorption and precipitation mechanisms. The results showed that all phosphatic lithologies were efficient in the passive treatment of AMD. The pH rapidly increased from 3.08 to 8.47 during water-rock interactions. The neutralization potential comparisons also showed that the phosphatic limestone wastes neutralized more acid than other lithologies. In addition, metals were efficiently removed (95.5% to 99.9%) by all materials. The results of batch sorption tests showed that the concentrations of metals in residual leachates did not exceed the Algerian criteria for industrial liquid effluents. Overall, these findings indicate that passive systems using phosphatic materials from the Djebel Onk mine can be effective for AMD treatment. The use of these mine wastes for passive treatment of AMD would allow the development of integrated management strategies for these residual materials in the context of sustainable development of phosphate mining.

Role of secondary minerals in the acid generating potential of weathered mine tailings: Crystal-chemistry characterization and closed mine site management involvement.

Mine tailings exposed to water and oxygen generate acid mine drainage (AMD) when the neutralizing minerals are insufficient to buffer the acid produced by sulfide oxidation. Mineral reactivity, such as sulfide oxidation and carbonate dissolution, leads to several changes within mine tailings in terms of their physical, mineralogical, and geochemical properties, which may lead to the release of metal(oid)s (e.g., As, Cu, Zn, Fe, S) into the environment. Fresh and oxidized tailings were sampled at two vertical profiles in a tailings storage facility (TSF). The TSF contains tailings from gold ore processing at a mine that has been closed for more than 25 years. Oxidized tailings have formed by in-situ oxidation of fresh tailings over more than 20 years. The collected samples were analyzed for: i) chemical composition by inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray fluorescence (XRF), and total S/C; and ii) mineralogical composition by X-ray diffraction (XRD), Mineral Liberation Analyzer (MLA), Mossbauer spectroscopy, and Fe L-edge X-ray absorption near-edge spectroscopy (XANES). Mineralogically, the fresh tailings included more than 22 wt% carbonates and more than 10 wt% sulfides. In contrast, the oxidized tailings were composed mainly of secondary minerals such as iron oxy-hydroxides and gypsum. Geochemically, the fresh tailings exhibited a circumneutral behavior during weathering cell experiments and contaminants such as As were negligibly released (<0.3 mg/L). The latter is explained by formation of secondary iron oxy-hydroxides, which are known for the capacity to uptake several contaminants from the leachate. Long term oxidation of fresh tailings will lead to highly oxidized tailings similar to those collected in situ. The oxidized tailings exhibited an acidic behavior despite sulfide depletion due to latent acidity. The geochemical behavior was strongly controlled by the reactivity of secondary minerals (e.g., dissolution of gypsum and iron oxy-hydroxides). Quantitatively, the oxidized tailings released 163 mg/kg Fe, around 12,000 mg/kg S, and around 6 mg/kg Zn.

Environmental behavior of metal-rich residues from the passive treatment of acid mine drainage.

In closed or abandoned mine sites, passive systems are often used for acid mine drainage (AMD) treatment. They generate metal-rich residues with variable chemical stability, which is rarely reported. The objective of the present study was to evaluate the potential mobility of contaminants (metals and sulfates) from AMD post-treatment residues to better anticipate their fate and enable their proper management. Sampling of a field tri-step passive system, consisting of two passive biochemical reactors (PBR1 and PBR2), separated by a wood ash reactor (WA), implemented in the reclaimed Lorraine mine site, QC, Canada, was carried out. Samples were collected from the inlet (In) and the outlet (Out) of each treatment unit. Physicochemical and mineralogical characterization was performed. The potential mobility of the metals was then assessed via static and kinetic leaching tests. Results showed that all residues had high metal contents (e.g. Fe content >29 g/kg in PBR1-In, > 76 g/kg in WA-In and > 80 g/kg in PBR2-Out). A high residual neutralizing potential was also found in the WA residues (inorganic carbon 6.5%). Native and organic sulfur were found in the PBR2 residues, while Fe-oxyhydroxide (hematite, goethite and magnetite), carbonate and sulfate minerals were present in all residues. According to USEPA regulations, all residues were considered non-hazardous, but Quebec's provincial regulation relative on mining effluents classifies these residues as leachable for some metals, such as Fe, Al, Ni, Zn and Mn. A potential generation of contaminated neutral mine drainage (Al, Ni, Mn and Zn concentrations exceeding criteria) could occur from PBR1 (In & Out) and WA (In & Out) residues. Moreover, the PBR2 residues (In & Out) regenerated AMD rich in Fe and sulfates, especially for PBR2-Out (1 g/L Fe and 6 g/L sulfates). Therefore, all residues were proven to require stabilization prior to their landfill (co-)disposal with municipal waste.

Treatment efficiency of iron-rich acid mine drainage in a tri-unit pilot system.

Treatment efficiency of iron-rich acid mine drainage (AMD; pH 3, and 2 and 4 g/L Fe) was tested in a laboratory tri-unit pilot-scale reactor (2.65 m3) for 1 year. The first unit consisted of a passive biochemical reactor (PBR1), filled with reactive mixture (50% of manure, sawdust, maple chips, compost, urea, sediment, and sand; 50% of calcite), with the aim to neutralize acidity and to partially remove metals. The second unit contained wood ash and acted as neutralizer and iron retention filter (by sorption and precipitation). The last unit was a second polishing PBR2, filled with reactive mixture (98% of manure, sawdust, maple chips, compost, urea, sediment, and sand; 2% of calcite), which aim was to remove the residual metals. The results showed that pH increased to about 6 and redox potential decreased significantly (from 550 mV to -100 mV). Iron, the most challenging metal in the AMD, decreased from 4 g/L (the highest tested concentration) to approximately 100 mg/L. The performance of the multistep treatment system was controlled by the capacity of the wood ash to immobilize iron.

Stability of metal-rich residues from laboratory multi-step treatment system for ferriferous acid mine drainage.

Passive systems are often used for the treatment of acid mine drainage (AMD) on closed and abandoned mine sites. Metal-rich residues (solid precipitates) with variable chemical composition and physical properties can be generated. Their characterization is required to better anticipate the potential fate, including stability for disposal, potential recovery, or reuse. The present study evaluated the leaching potential of solids from a laboratory passive multi-step treatment for Fe-rich AMD (2350 ± 330 mg/L Fetot, 0.7 ± 0.4 mg/L Ni, 0.2 ± 3.0 mg/L Zn, and 5073 ± 407 mg/L SO42-, at pH 3.04 ± 0.45). To do so, post-treatment solids from three units (Fe-pretreatment reactor (50% wood chips and 50% wood ash, WA50), passive biochemical reactor, PBR for SO42- treatment (30% inorganic materials, 70% organic substrate), and polishing reactor (50% calcite and 50% wood chips, C50)) of a pilot laboratory treatment system were sampled. Physicochemical and mineralogical characterization, as well as static leaching tests were then performed. Results showed that all solids had high neutralizing potential, while high inorganic carbon was found in C50. Moreover, high metal concentrations were found in WA50. Metals and sulfates in all solids precipitated in the form of oxyhydroxides, oxy-hydroxy-sulfates, carbonates, sulfides, sulfate, and native sulfur. The Fe was not found as problematic contaminant in solids, but it was in AMD. However, a probable generation of contaminated neutral drainage by Ni and Zn could occur from WA50. The C50 had the highest acid neutralizing capacity and could better resist to acid aggression relative to solids from PBR and WA50. The PBR and C50 solids were considered as non-hazardous towards regulation's limits and a potential co-disposal with municipal wastes could be a storage option. Further studies should be undertaken by testing other leaching and kinetic tests to assess long-term metal stability.

Geochemistry of rare earth elements within waste rocks from the Montviel carbonatite deposit, Québec, Canada.

Several rare earth element (REE) mine projects around the world are currently at the feasibility stage. Unfortunately, few studies have evaluated the contamination potential of REE and their effects on the environment. In this project, the waste rocks from the carbonatites within the Montviel proterozoic alkaline intrusion (near Lebel-sur-Quévillon, Quebec, Canada) are assessed in this research. The mineralization is mainly constituted by light REE (LREE) fluorocarbonates (qaqarssukite-Ce, kukharenkoite-Ce), LREE carbonates (burbankite, Sr-Ba-Ca-REE, barytocalcite, strontianite, Ba-REE-carbonates), and phosphates (apatite, monazite). The gangue minerals are biotites, chlorite, albite, ankerite, siderite, and calcite. The SEM-EDS analyses show that (i) the majority of REE are associated with the fine fraction (< 106 µm), (ii) REE are mainly associated with carbonates, (iii) all analyzed minerals preferably contain LREE (La, Ce, Pr, Nd, Sm, Eu), (iv) the sum of LREE in each analyzed mineral varies between ~ 3 and 10 wt%, (v) the heavy REE (HREE) identified are Gd and Yb at < 0.4 wt%, and (vi) three groups of carbonate minerals were observed containing variable concentrations of Ca, Na, and F. Furthermore, the mineralogical composition of REE-bearing minerals, REE mobility, and REE speciation was investigated. The leachability and geochemical behavior of these REE-bearing mine wastes were tested using normalized kinetic testing (humidity cells). Leachate results displayed higher LREE concentrations, with decreasing shale-normalized patterns. Thermodynamical equilibrium calculations suggest that the precipitation of secondary REE minerals may control the REE mobility.

Characterization of how contaminants arise in a dredged marine sediment and analysis of the effect of natural weathering.

Millions of tons of contaminated sediments are dredged each year from the main harbors in France. When removed from water, these sediments are very reactive, therefore their geochemical behavior must be understood in order to avoid dispersion of contaminated lixiviates in the surrounding soils. In this objective, it is necessary to evaluate the principal physicochemical parameters, and also achieve advanced mineralogical characterization. These studied sediments are highly contaminated by metals, notably copper (1445 and 835mg/kg, in the unweathered and naturally-weathered sediments, respectively), lead (760 and 1260mg/kg, respectively), zinc (2085 and 2550mg/kg, respectively), as well as by organic contaminants (PAH, PCB) and organometallics (organotins). A high concentration of sulfide minerals was also observed both in the unweathered sediment preserved under water (3.4wt% of pyrite especially), and in the naturally weathered sediment (2wt% pyrite), and in particular framboïdal pyrite was observed in the two materials. The presence of reactive mineral species in the naturally-weathered sediment can be explained by the deposit of a protective layer, composed of sulfide and their oxidation products (sulfate and iron oxides), thus preventing oxygen from diffusing through to the sulfide minerals. Additionally, the presence of aluminosilicates aggregates coating the sulfide minerals could also explain their presence in the naturally-weathered sediment. As organic matter is one of the principal constituents of the sediments (5.8 and 6.3wt% total organic carbon in the unweathered and weathered sediment, respectively), the aggregates are probably partially constituted of refractory humic material. It therefore appears that the natural weathering has led to a significant decrease in PAHs and organotins, but not in PCBs. The evolution of the granulometric structure and the distribution of the metallic contaminants could therefore lead us to consider a treatment by size separation, and a possible valorization of the dredged sediments in civil engineering.