Net Acid Generation Research Articles

5-year leaching experiments to evaluate a modified bauxite residue: remediation of sulfidic mine tailings.

The ALTEO company produces approximately 300,000 tons per year of bauxite residue after alumina extraction, which is washed and dried in a press filter to produce Bauxaline®. In this study, different ways for recovering and reusing this residue were explored, namely transformation into a vegetated soil, use in acid mine drainage depollution, and application in sulfide-mine tailings remediation. The Bauxaline® was therefore transformed into modified bauxite residue (MBR), resulting in reduced alkalinity, salinity, and sodicity. To counterbalance the net acid generation potential of two sulfidic mine tailings with 1 mol H+ kg-1 (1.5% sulfide) and 3.3 mol H+ kg-1 (5.3% sulfide), respectively, various treatments were applied. These treatments included the addition of 10% MBR or 10% MBR plus limestone, or by limestone only, within 40-l lysimeters. Six lysimeters were monitored over a 5-year period to assess the long-term emissions from treated materials. Vegetation was tested under various conditions, and its impact on emission was evaluated. The emissions of mine tailings treated with MBR and limestone were very low. The mine tailings with limestone showed intermittent peaks of emission, probably due to the coating of calcite grain by ferric oxide, hindering contact with percolating water. Vegetation successfully grew in the treated tailings. This study demonstrated that the alkalinity of limestone can temporarily immobilize elements in sulfidic mine tailings, with a reduction factor of emissions of 300 and 40 for the two mine tailings, respectively. For long-term immobilization, the alkalinity provided by both limestone and MBR and the Al and Fe oxides of MBR are more effective and necessary for long-term immobilization, with a reduction factor of 300 and 900, respectively.

The comparison of 7.5 and 15% hydrogen peroxide as oxidizing agent in static tests of acid mine drainage potential in Indonesia

Static tests of acid mine drainage potential are an important part of mining water management. Net acid generation (NAG) test is widely used in Indonesian coal mines because of its convenience. This test uses H2O2 to oxidize sulfide minerals within rock samples to determine their net acid-forming capacity. This study aimed to determine the difference between H2O2 at concentrations of 7.5% (the standard in several Indonesian coal mines) and 15% (the standard of the Acid Rock Drainage Test Handbook and Indonesian National Standard) in categorizing rocks as potentially acid-forming and non-acid-forming and in terms of NAG solution characteristics. A total of 564 rock samples collected from two Indonesian coal mining sites were analyzed using pH, NAG, total sulfur, and acid-base-accounting tests. The results of the study showed that there was no significant difference in rock classification or the behavior of contaminants in the NAG solution between 7.5% and 15% H2O2. The characteristics of sulfide minerals in Indonesian coal mines were the main factors influencing the results of the NAG test and behavior of contaminants in the NAG solution. Therefore, H2O2 at a concentration of 7.5% can be used in Indonesian coal mines with relatively low total sulfur concentrations (<5%) and minerals in framboidal form.

Soil Contaminated with Hazardous Waste Materials at Rio Tinto Mine (Spain) Is a Persistent Secondary Source of Acid and Heavy Metals to the Environment

Mineralogical analysis and laboratory-based leaching tests coupled with speciation modeling were undertaken to quantify the potential for short-term acid generation and the release of trace elements from soils heavily contaminated with mine waste at Rio Tinto. Three different waste materials were considered as case studies: roasted pyrite, copper slags, and leached sulfide ores. The results showed elevated values of net acid generation (up to 663 mmol H+/kg), the major pools being potential sulfidic acidity and acidity retained in jarosite. Remarkable contents of As and toxic heavy metals were found especially in the slag-contaminated soil. Copper, Zn, and Pb were the most abundant metals in the acid leach solutions resulting from mine soil-water interaction, with peak values of 55.6 mg L−1, 2.77 mg L−1, and 2.62 mg L−1, respectively. Despite the high total contents of trace elements occurring in soil, the mobile fraction was limited to maximum release values of 12.60% for Cd and 10.27% for Cu, according to the test leaching. Speciation calculations indicated that free metal ions (M2+) and sulfate species (MSO40) accounted for most of the dissolved load. Acid soil drainage is a secondary source of acid and heavy metals in the mine site and, therefore, an effective land reclamation program should ensure that acidity and metal mobility are reduced to environmentally sustainable levels.

Assessment of potential contamination and acid drainage generation in uranium mining zones of Peña Blanca, Chihuahua, Mexico.

Potential pollution of mining environmental liabilities' locations can be preliminarily and efficiently assessed by the potential generation of acid mine drainage and indices of contamination. This research evaluates the potential pollution by potentially toxic elements at locations with uranium mining liability evidence, using the net acid generation test and determining the background values to estimate acid mine drainage and indices of contamination. Sixty soil samples were collected, and the mineralogy and potentially toxic elements' total contents were determined by x-ray diffraction and optical spectrometry. The findings suggest that the soils related to a specific lithology might not present potential acid mine drainage generation but potential soil and sediment contamination. Future research is recommended on applying leaching tests to identify which potentially toxic elements are effectively being solubilized. Finally, it can be concluded that the study area's potential contamination is relatively low overall.

Effect of Carbide Slag Combined with Biochar on Improving Acidic Soil of Copper Sulfide Mines

Heavy metal-rich acidic soils of copper sulfide mines can easily cause harm to the surrounding environment and ecosystem safety if not treated properly. Currently, the most common method of mine ecological remediation is to improve acidic soils of copper sulfide mines by adding different types of amendments. In this paper, different dosages of biochar and carbide slag applied singly and in combination were designed to investigate the changes of physicochemical properties and ryegrass growth in the acidic soil of copper sulfide mines. Through soil incubation experiments and potting tests, different treatments explored the improvement effect of carbide slag combined with biochar on the acidic soil of copper sulfide mines. Then, it was found that 2% (w/w) carbide slag in combination with 10% (w/w) biochar had the best effect on the improvement of copper-sulfide mine acidic soil. Among them, carbide slag significantly increased the pH, cation exchange capacity (CEC) and acid neutralization capacity (ANC), and reduced the electrical conductivity (EC), net acid generation (NAG) and sulfate ion (SO42−) concentrations in the acidic soil of copper sulfide mines; biochar obviously reduced the bio-effectiveness of heavy metals Cu, Pb and Zn, and DPTA-Cu, Pb and Zn decreased by 8~80%, 7~76% and 17~79%, respectively. Apart from that, there was a positive cooperativity between carbide slag and biochar, and their application effectively controlled the acidification process and heavy metal toxicity in the acidic soil of copper sulfide mines. In summary, the results will provide a theoretical basis for the large-scale reclamation of carbide slag and biochar in improving acidic soils of copper sulfide mines, filling the gap of calcium carbide slag and biochar in soil remediation.

Static tests to assess acid mine drainage potential of copper sulfide flotation tailings

Most copper flotation tailings contain a significant amount of sulfide minerals that can generate effluent resulting from the oxidation of sulfides when they are exposed to oxygen and water. This study aimed to characterize and evaluate the acid drainage potential of five different copper flotation tailings, named samples I, II, III, IV, and V. Sample I was taken from an industrial operation, while Samples II, III, IV, and V were produced in pilot plant tests. All samples were characterized and submitted to static acid drainage tests, such as paste pH, Net Acid Generation (NAG) pH, and Modified Acid-Base Accounting (MABA). The sulfur as sulfide/total sulfur mass ratios varies from 0.40 up to 0.89 showing the presence of a high amount of minerals rich in pyrite and alunite, being higher than 13 wt% of pyrite in samples IV and V and around 19 wt% of alunite in sample III. Samples I and II presented a high number of alkaline elements, totalizing more than 8 wt% of K2 O+CaO+MgO, while samples III, IV, and V show the lowest amount of these elements (<4 wt%). Only samples III, IV, and V show Paste pH results lower than 5.0, indicating superficial oxidation on their surfaces and/or the presence of water-soluble acidity minerals. Due to the high pyrite content in samples III and IV, it was not possible to measure their NAG pH due to the high heat released with the addition of hydrogen peroxide. The low NAG pH obtained with sample III (5.30) was due to its high pyrite content (0.30 wt%) in comparison with the results obtained with sample I (NAG pH of 8.15 and 0.10 wt% of pyrite), and sample II (NAG pH of 8.18 and 0.10 wt% of pyrite). The MABA results show that samples III, IV and V presented acid drainage potential due to the neutralization potential (NP)/acidity potential (AP) ratio being lower than 1. On the other hand, samples I and II presented NP/AP ratio higher than 4, not showing potential to generate acid drainage. Due to these results, it is recommended to carry out acid drainage kinetic tests with samples III, IV, and V to determine the long-term weathering rates, evaluate lag time to acid generation and provide reaction rates for geochemical modeling.

Prediction of acid production potential of self-combusted coal mining wastes from Douro Coalfield (Portugal) with integration of mineralogical and chemical data

The prediction of acid production potential (APP) of coal waste piles that underwent self-combustion was investigated by integrating mineralogical and chemical data to secure a sustainable environment. Burned and unburned material from three waste piles in Douro Coalfield (Portugal) were studied to understand the effect of self-combustion on APP. For this purpose, mineralogical analysis, paste pH, infrared spectroscopy, acid ammonium oxalate leaching, and nitric acid leaching were performed to determine the most reliable APP scores, while British Columbia Research Initial test was carried out for the neutralization potential (NP) determination. The major acid producers were pyrite and jarosite, while the neutralizers were calcite, aragonite, azurite, siderite and hydrotalcite as rare accessory minerals. Hence, APP scores were determined in terms of the calculation factors of pyrite and jarosite. A thermal transformation from jarosite to hematite was detected due to self-combustion. The presence of hematite and mullite minerals in extremely burned wastes is in the foreground. After obtaining APP and NP, net acid production potential (NAPP) scores were calculated and subsequently interpreted with net acid generation (NAG) test results. The NAPP scores between 0.7 and 24.1 kg CaCO3/t were obtained, while the negligible level of NP scores was between 0.1 and 1.6 kg CaCO3/t. According to six different acid mine drainage (AMD) screening criterion, most of samples were detected as “potentially acid forming”. The interpretation of “non-acid forming” was not concluded for any wastes. Consequently, the piles are moderately prone to form AMD, and the NAPP in extremely burned zone of the piles is lower than the zones of partially burned and unburned due to the loss of sulfide and scarcity of acid sulfates.

Environmental desulfurization of mine tailings using froth flotation: The case of Amaruq Mine (Nunavut, Canada)

This research examined the the desulfurization of tailings from the Amaruq mine at a laboratory scale using froth flotation. Optimization of the flotation process was carried out by testing the conventional potassium amyl-xanthate collector (PAX) and three alternative collectors from the mercaptobenzothiazole (MBT) and dithiophosphate (DTP) families. The main goal was to assess the efficiency of the desulfurization process as a sustainable tailings management approach to prevent contaminated mine drainage generation. Mineralogical characterization of the Amaruq tailings using automated scanning microscopy (QEMSCAN®) revealed that the total sulfide and carbonate contents were about 8 wt% and 3 wt%, respectively; 63.39 % of the sulfur occurred as pyrrhotite, 30.17 % as pyrite, 3.63 % as gersdorffite, and 2.61 % as arsenopyrite. To compare the collector flotation yields, response surface methodology (RSM) was used for statistical design, analysis, and modelling of froth flotation experiments using three key-parameters: pH, activator (CuSO4) dosage, and collector dosage. Four quadratic mathematical models were developed for each collector to calculate sulfide recovery. Variance analysis (ANOVA) revealed that the factors that most affected sulfide recovery were collector type/dosage and pH. Process optimization yielded a maximum sulfide recovery of 96.73 % using 300 g/t PAX and 300 g/t CuSO4 at pH 11. A similar recovery of 95.83 % was achieved using an MBT-MTP based collector under the same conditions. The maximum recovery obtained using MTP and MTP-DTP based collectors while operating at optimal conditions (alkaline pH and maximum collector dosage at 350 g/t) was 88.65 %. The desulfurization process succeeded in decreasing the sulfur content in the tailings to<0.11 wt%. The final tailings were not acid-forming, as confirmed by the acid-base accounting (ABA) and net acid generation (NAG) static tests, compared to the highly acid-forming feed samples. Kinetic testing on the benign desulfurized tailings was also performed to predict and validate their acid generating potential, as well as their contaminant release rates (particularly As, Ni, Cu, Fe, and Zn). The Amaruq desulfurized material did not present any significant risk of contaminated mine drainage generation, and the leached contaminants did not exceed the criteria set by Quebec provincial legislation.

Environmental Geology of Barite Deposits in Nevada

Abstract Barite is an important industrial mineral used as a weighing agent in fluids during the drilling of oil and natural gas wells, with the majority of United States production originating from Nevada. This paper considers three barite properties at varying stages of development—Rossi, Heavy Spar, and Ann—all located within the northern Nevada barite belt. At all three sites, barite is hosted in the Roberts Mountains allochthon consisting of siliceous sedimentary rocks. Geochemical characterization of waste rock, ore, and process products has been undertaken as part of project development at each site, including 212 acid-base accounting (ABA) analysis and 19 humidity-cell tests operated for up to 114 weeks. This study demonstrates that the presence of barite greatly limits the application of using traditional Sobek-style ABA methods. Barite will undergo incomplete dissolution and extraction in the ABA tests that will result in an overprediction of acid generation for materials that contain barite (i.e., ore and jig by-products). Results indicate use of the net acid generation test to inform waste rock management represents a more reliable indicator of acid generation for these three deposits and potentially also other barite deposits. Potential barite mine sites should continue to be evaluated using the same geochemical testing tools as base metal mines but using those tools in a selective way to achieve balance between regulatory requirements and costs.

Sulfide Variation in the Coeur Rochester Silver Deposit: Use of Geologic Block Modeling in the Prediction and Management of Mine Waste

Abstract A critical issue in mine waste management is the management and mitigation of acid rock drainage and metal leaching particularly from waste rock. This requires good understanding and characterization of materials most likely to generate acid pH waters and generate leachable metals and metalloids, as well as the presence of minerals most likely to neutralize acidic pH such as carbonates and certain silicates. For low-sulfidation epithermal deposits in silica-rich volcanic rocks, neutralization minerals are rare. Consequently, determination of acid generation is almost entirely a function of sulfide content of the rocks. By understanding the distribution of sulfide minerals within such a deposit, an initial mine waste management plan can be developed. The Rochester mine currently exploits a low-sulfidation epithermal gold-silver deposit and is located on the southern flank of the Humboldt Range in west-central Nevada. The Rochester mine is a conventional open-pit mining operation with cyanide heap leaching and Merrill Crowe processing. Silver and lesser gold mineralization occur associated with sulfide minerals hosted in Permian-Triassic Koipato Group rhyolitic units with extensive quartz-sericite-pyrite alteration. Gangue mineralogy of the deposit is dominated by slow or unreactive silicate minerals with little or no carbonate. Silver-bearing mineralization was controlled by primary hypogene sulfides and secondary supergene oxidation processes, and most of the mineralized material mined to date has been fully or partially oxidized. Oxidation mobilizes and depletes all forms of sulfur from primary sulfide minerals. Due to the low sulfate content and low content of neutralizing gangue phases, the waste rock classification can be based on total sulfur content. Based on the analysis of pulp samples collected from within the mine area, a sulfur block model was developed and compared to the results of standard acid-base accounting and net acid generation tests. This comparison demonstrates the benefit of the block model in identifying zones of potentially acid-generating material in the mined waste. Application of this information to mine planning has allowed more efficient management of acid-generating mine waste by allowing the operator to develop areas of the waste rock facility to receive the waste in a timely fashion. Such an approach is based on good mine geologic and mineralogical characterization and the use of a representative and comprehensive geologic block model.

The potential of using alkaline gold mine tailing as a cover material to mitigate AMD formation from acidic gold mine tailing pile at Sabie-Pilgrim's Rest Goldfields, South Africa.

Acid mine drainage (AMD) caused by the oxidation of sulphide minerals found in mine waste is a global environmental concern, especially in water-restricted countries with heavy mining industries. Implementing AMD treatment and prevention programs can be extremely expensive, hence the need to identify environmentally sustainable treatment and preventative techniques to mitigate the potential of AMD formation. Soil covers and blends have been identified as an attractive approach. However, prior studies on the characteristics of the soils concerned and the acid-neutralisation rate should be carried out before considering the implementation of a soil cover or blending system to mitigate AMD formation. This study evaluated the acid generation capabilities of acidic gold mine tailings (AG), alkaline gold mine tailings (AN) and blends (MIX25, MIX50). Acid-base accounting (ABA), net acid generation (NAG) and acid-buffering characteristic curve (ABCC) test methods were used to evaluate the acid-generating and acid-neutralising capabilities of AG, AN, MIX25 and MIX50 samples. Leach column tests were conducted using alkaline gold mine tailings (AN) as the top pH neutralising cover (COV25) to determine the potential of the alkaline gold mine tailing to serve as a pH neutralising cover material to prevent and treat AMD generated by the acidic gold mine tailings. The ABA, NAG and ABCC results showed that AN has a high acid-neutralising capacity while AG has the potential to generate acid. The results further indicated that the AN to AG blend ratio of 1:3 by weight (MIX25) would neutralise the acid generated by AG. Leach column experiment (COV25) found that using AN as a pH neutralising cover would be a feasible option.

Improved mine waste dump planning through integration of geochemical and mineralogical data and mixed integer programming: Reducing acid rock generation from mine waste

Although the environmental significance of acid rock drainage (ARD) generated from mining wastes is well known, selecting the appropriate ARD management strategy can prove a complicated task. Chemical methods are favored for initial mine waste characterization but using these exclusively can overlook key factors, e.g., mineralogy, which controls the formation and elution of ARD. This paper first presents an ARD waste rock classification developed on Triple Characterization Criteria (TCC) which considers three input parameters: neutralizing potential ratio (NPR), net acid generation (NAG pH), and modal mineralogy weathering index (MMWI) values. Second, a new mixed-integer programming (MIP) model to guide waste dump construction with the dual aim of preventing ARD across the life-of-mine (LOM) and reducing waste rock re-handling, is introduced. Last, the spatial distribution of TCC in a planned waste dump is simulated via geo-statistical techniques to evaluate the MIP model. The proposed waste rock classification and dump planning model has been tested at an iron mine. The results of the MIP modeling and simulation of TCC showed the successful prevention of ARD by achieving large values of TCC (NPR ≥2, NAG pH ≥ 4.5, and MMWI ≥4.7) for dump cells, with the planned mine production maintained. The integrated TCC approach introduced in this study is intended to enable mine operators, at the start of the LOM, to effectively forecast ARD from future waste rock. Further, the MIP model will facilitate development of a mine schedule that optimizes the use of the waste materials based on TCC values. If used correctly, the TCC and MIP model have the potential to enable mine operators to reduce their environmental footprint across the entire LOM.

A Test of Two Methods for Waste Rock Drainage Quality Prediction: Aqua Regia Extraction and Single-addition Net-acid Generation Test Leachate Analysis

The mobility of contaminants from mine waste can be assessed using different extraction methods. Aqua regia (AR) extraction is the most commonly used method in Finland. Another method is the analysis of leachate from net acid generation (NAG) tests, which is primarily designed for acid production potential assessment. We investigated the performance of single-addition NAG test leachate analysis and AR extraction in drainage quality prediction, using waste rock and drainage water samples from several Finnish waste rock sites. Our objective was to improve interpretation of the AR and single-addition NAG test leachate analysis results in drainage quality prediction. The AR extraction effectively reflected elements that occurred in elevated concentrations in drainage water, though it over-predicted Al, As, Cd, Co, Cu, and Ni in some circumneutral drainages, and Cr in general. The single-addition NAG test leachate analysis also performed well in assessing the mobility of contaminants including Al and Cr at acid mine drainage sites. As the contaminants tend to precipitate in neutral NAG test solution, the usability of the method in neutral mine drainage cases should be further investigated. Furthermore, the conclusions presented in this study are limited to waste rock samples collected from the surface of piles; future work will examine waste rock history, dump cores, drainage quality changes, etc. in more detail.

Influence of Extremophiles on the Generation of Acid Mine Drainage at the Abandoned Pan de Azúcar Mine (Argentina).

The risk of generation of acid drainages in the tailings of the Pan de Azúcar mine that closed its activities more than three decades ago, was evaluated through biooxidation studies using iron- and sulfur-oxidizing extremophilic leaching consortia. Most of tailings showed a high potential for generating acid drainage, in agreement with the results from net acid generation (NAG) assays. In addition, molecular analysis of the microbial consortia obtained by enrichment of the samples, demonstrated that native leaching microorganisms are ubiquitous in the area and they seemed to be more efficient in the biooxidation of the tailings than the collection microorganisms. The acid drainages detected at the site and those formed by oxidation of the tailings, produced a significant ecotoxicological effect demonstrated by a bioassay. These drainages, even at high dilutions, could seriously affect a nearby Ramsar site (Laguna de Pozuelos) that is connected to the Pan de Azúcar mine through a hydrological route (Cincel River).

Reprocessing feasibility of polymetallic waste rock for cleaner and sustainable mining

Mining operation produces significant volumes of waste rocks (WR) characterised by high heterogeneity in their physical, mineralogical and hydrogeological properties. The purposes of this study were to assess the acid-generating potential, evaluate reprocessing feasibility of polymetallic WR using desulphurisation process (i.e., gravity separation and flotation) and study the geochemical behaviour of desulphurised materials. The fresh WR was collected in the waste dump from Langlois mine at Lebel-sur-Quévillon (Quebec, Canada). The WR sample was divided into six fractions using wet screening: D < 0.053 mm, 0.053 < D < 0.3 mm, 0.3 < D < 0.85 mm, 0.85 < D < 2.4 mm and 2.4 < D < 5 mm. Chemical and mineralogical characterisations showed that fine fractions (D < 2.4 mm) were enriched in sulphide minerals (i.e., 4.6 wt% pyrite, 3.5 wt% sphalerite, etc.) and carbonates (i.e., 3.7 wt% calcite and 5.1 wt% ankerite) compared to coarse fractions (1.2 wt% pyrite, 1.1 wt% sphalerite, 3.2 wt% calcite, 5.2 wt% ankerite). The acid-generating potentials of different fractions were evaluated using acid-base accounting (ABA) and net acid generation (NAG) tests. Only one size fraction (F2) was acid generating and others were classified as uncertain. The degree of liberation of sulphides in the size fractions greater than 2.4 mm was negligible (<16%). Consequently, for this WR sample, 2.4 mm was defined as the diameter of physical locking of sulphide (DPLS). Based on DPLS, the WR were separated into two fractions; reactive (<2.4 mm), non-reactive (<2.4 mm) and then submitted to kinetic tests. Geochemical analyses of leachates of the coarse fraction (2.4 < D < 10 mm), the total sample (D < 5 mm) and –DPLS fractions (D < 2.4 mm) and the benign desulphurised materials (D < 2.4 mm) showed that pH values remained between 6.62 and 8.11 and the instantaneous concentrations of metals such as iron (Fe), copper (Cu) and lead (Pb) were below environmental limits (D019, Quebec, Canada), except for zinc (Zn) leached from the total sample which indicates a risk of contaminated neutral drainage. The pyrite oxidation rate within the coarse fraction was low (16.6 μmol/kg/day) compared to fine fraction (35.8 μmol/kg/day). A combined gravity–flotation process gave better results to decontaminate the waste rock (D < 2.4 mm), with a benign desulphurised material reaching a low sulphur content (0.23 wt%) and high sulphur recovery (89 wt%). The final concentrates from the combined gravity–flotation process showed high base metals (Zn, Cu and Pb) recovery (more than 87 wt%).

Upstream environmental desulphurisation and valorisation of waste rocks as a sustainable AMD management approach

Open-pit mining generates a large volume of waste rocks (WR) that are deposited as unsaturated piles on the surface. WR are mainly characterised by the high heterogeneity of their hydrogeological, mineralogical and physical proprieties. The main objectives of this study consist of: i) a detailed characterisation of WR from Canadian Malartic mine in Malartic city (Quebec, Canada), ii) the use of environmental desulphurisation of the three classified waste rocks main lithologies to reduce their acid generation potential and iii) the recovery of the residual precious metals (e.g. Au, Ag). The studied materials were sampled after blasting while ensuring their representativity. Each lithology was first screened based on the DPLS (diameter of physical locking of sulphides) to remove the inert fraction higher than 2.4 mm. Then, the fraction (<2.4 mm) was separated into two fractions to optimize their integrated management: 0–1 mm referenced as fine fraction and 1–2.4 mm referenced as coarse fraction. Chemical and mineralogical determinations showed that the three lithologies are more enriched in carbonates (mainly as calcite) than sulphides (mainly as pyrite). The acid base accounting (ABA) testing and the net acid generation (NAG) tests both showed that these lithologies (<2.4 mm) and their corresponding fractions are non-acid generating except three fractions that are uncertain. In order to reduce the sulphur content, desulphurisation applied on the fraction <2.4 mm using laboratory gravity separation (Knelson separator and Mozley table) was more efficient than flotation. All desulphurised materials from gravity and flotation methods were in the non-acid generating zone. The gravity method offered a sulphur recovery of about 84 wt%. The pyroanalysis of gold (Au) and silver (Ag) on the concentrate of desulphurisation with gravity methods display a gold content of 3,78 g/t and of 4,9 g/t for silver. The precious metals recovery could offset partially the costs related to the waste handling and reprocessing.

Natural oxidation of coal tailings from Middelburg area (South Africa), and impact on acid-generation potential

SYNOPSIS The aim of this study was to determine the acid-generating potential of coal mine tailings located in the Middelburg area, South Africa, using conventional assessment techniques. Four coal-tailing samples were collected from different layers of a coal-tailing pile. The acid-base accounting (ABA), acid-buffering characteristic curve (ABCC), and net acid generation (NAG) methods were used in conjunction with a mineralogical investigation to assess the potential of acid mine drainage (AMD) formation from the tailings. The results showed that the top layer of the pile, which was exposed to the atmosphere, was most likely to form AMD, exhibiting the lowest paste pH (2.5) and ABA of zero kg H2SO4 per ton. The ABA results were compared to ABCC results and mineralogical calculations to confirm the findings. The results from the ABCC test and calculations based on mineralogy indicated that the ABA method overestimates the effective or readily available acid neutralization capacity (ANC) and the true MPA of each sample; only three of the four samples could therefore be classified as acid-forming, but with low acid-generating capacity. Although the findings show that the top layer of the coal tailings in the Middelburg area is most likely to form AMD, it is important for future studies to further investigate the kinetics of acid formation. Keywords: acid mine drainage, acid-base accounting, mineralogy, net acid generation, coal mine tailings.

Environmental Sustainability of Open-Pit Coal Mining Practices at Baganuur, Mongolia

In this paper, we studied one of the largest coal mines in Mongolia, the Baganuur Coal Mine, in terms of environmental sustainability related to mining practices, with a focus on discharged water and waste sediments. The present quality and potential for future pollution were assessed. Based on World Health Organization and Mongolian guidelines, groundwater pumped from the mining operations could be used for drinking and domestic purposes. In addition, based on the Na absorption ratio, groundwater samples from GW-2 and GW-3 could be used as agriculture water supplies with salinity reduction, or used to grow halophytes as a measure for desertification control and pasture production. All waste soil samples appeared to have a desertification potential. Dust particles smaller than 150 μm comprised more than 80% of soil samples, which had arsenic levels higher than the Mongolian soil pollution standards. In addition, soil collected between coal seams (S-5) showed high sulphur content based on X-ray fluorescence (XRF) and scanning electron microscopy–energy dispersive X-ray (SEM-EDX) spectroscopy analyses, strong potential for producing acid mine drainage in the analysis of pH of net acid generation and net acid production potential, and potential for leaching of metals, such as Co. Therefore, the Baganuur Coal Mine requires soil pollution control measures to mitigate the risks of dust and desertification. In this perspective, mine groundwater could be used to reduce environmental stresses by supporting pasture crops such as halophytes on waste disposal sites, thereby preventing dust issues and desertification. Continuous efforts, including monitoring and enacting environmental management measures, are needed from both the mining company and the government to ensure sustainable mine development.

Implications of Sulfur Speciation on the Assessment of Acid Rock Drainage Generating Potential: A Study of South African Coal Processing Wastes

The presence of sulfur in coal processing wastes can lead to environmental impacts, such as acid rock drainage (ARD). However, not all sulfur species are acid-forming, and the implications of sulfur speciation when assessing acid rock drainage potential by means of static chemical tests are not well understood. This study set out to evaluate the implications of different sulfur forms on the assessment of acid rock drainage potential using static laboratory-scale tests and to investigate the reliability of methods for the analysis of such forms for the case of three South African coal processing wastes. Both the International Organization for Standardization (ISO) 157:1996 and Australian Coal Association Research Program (ACARP) C15034 protocols were found to be suitable tools for analyzing the different forms of sulfur. Acid-generating sulfur forms constituted between 53% and 64% of the total sulfur in the wastes evaluated, with the maximum potential acidity (MPA) and net acid-producing potential (NAPP) values calculated on the basis of acid-forming sulfur being significantly lower than those calculated on the basis of total sulfur content. Results also showed that the partial conversion of sulfur species under the relatively aggressive conditions of the acid-neutralizing capacity (ANC) and net acid generation (NAG) tests may overestimate the potential acid generating potential in the case of coal. These findings highlight the uncertainties associated with standard ARD static tests and the importance of taking sulfur speciation into account when calculating the MPA for coal processing wastes.

Prediksi Laju Pembentukan Air Asam Tambang dengan Metode Column Leaching Test

Prediction of Acid Mine Drainage Forming using Method of Column Leaching Test. One of the environmental problems in coal mining activities is the formation of acid mine drainage. Prediction of the formation of acid mine drainage is important as an effort to control environmental impacts. Acid mine water occurs with interactions between potentially acid-forming material with oxygen, bacteria and water. Objective of study is to analyze the potential for acid mine drainage forming based on material characteristics. The research method was carried out using static and kinetic tests. The static test method classifies materials according to the ability to produce clean acids with observed parameters such as paste pH, total sulfur, Acid Neutralizing Capacity (ANC), Net Acid Generation (NAG), Maximum Potential Acid (MPA), and Net Acid Producing Potential (NAPP). The Kinetic test method predicts the rate of acid-forming of a material. The kinetic test uses the Column Leaching Test Method by using material with composition designed to represent field condition. The kinetic method parameters are pH, electrical conductivity, acidity, alkalinity, sulfate content, and dissolved metal content (Fe, Mn, and Cd). Results of the static test classified the material into NAF Non-Acid Forming (NAF), Potential Acid Forming (PAF) and Uncertain (UC) material categories. The results of the Column Leaching Method classified the material into categories of potential and no potential to form acid mine water. The columns that have the potential to form acid mine drainage occur in columns with large amounts of tonnage of PAF material or those in the upper layer so that it reacts with oxygen. The columns that have no potential to produce acid mine drainage in columns with PAF material are in the middle layer or mixed with NAF material.