Net Acid Generation Test Research Articles

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.

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.

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.

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.

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.

Comparison of static and mineralogical ARD prediction methods in the Nordic environment

Acid rock drainage (ARD) is a major problem related to the management of mining wastes, especially concerning deposits containing sulphide minerals. Commonly used tests for ARD prediction include acid–base accounting (ABA) tests and the net acid generation (NAG) test. Since drainage quality largely depends on the ratio and quality of acid-producing and neutralising minerals, mineralogical calculations could also be used for ARD prediction. In this study, several Finnish waste rock sites were investigated and the performance of different static ARD test methods was evaluated and compared. At the target mine sites, pyrrhotite was the main mineral contributing to acid production (AP). Silicate minerals were the main contributors to the neutralisation potential (NP) at 60% of the investigated mine sites. Since silicate minerals appear to have a significant role in ARD generation at Finnish mine waste sites, the behaviour of these minerals should be more thoroughly investigated, especially in relation to the acid produced by pyrrhotite oxidation. In general, the NP of silicate minerals appears to be underestimated by laboratory measurements. For example, in the NAG test, the slower-reacting NP-contributing minerals might require a longer time to react than is specified in the currently used method. The results suggest that ARD prediction based on SEM mineralogical calculations is at least as accurate as the commonly used static laboratory methods.

Determination of the available acid-generating potential of waste rock, part I: Mineralogical approach

Significant volumes of waste rock are produced in mining operations, particularly in open pit operations. Waste rock is characterized by a high heterogeneity in their mineralogical, hydrogeological, and physical properties, especially in comparison to tailings. The purpose of this study was to develop an improved method to quantify the net acid-generating potential of waste rock by incorporating the degree of liberation of acid-generating and neutralizing minerals. Three lithologies were sampled immediately after waste rock blasting. Each lithology was separated into seven fractions: D < 0.053 mm, 0.053 mm < D < 0.3 mm, 0.3 mm < D < 0.85 mm, 0.85 mm < D < 2.4 mm, 2.4 mm < D < 5 mm, 5 mm < D < 15 mm, 15 mm < D < 50 mm; particles >5 cm were not considered. Mineralogical and chemical characterizations showed that sulphides, mainly pyrite, were enriched in the fine to mid-sized fractions (between 0.053 mm and 0.85 mm), and carbonates, mainly calcite, decreased as the particle size increased. Sulphides were more liberated within the fine fractions and their liberation was considered negligible at sizes >2.4 mm. For coarser fractions (>5 mm), sulphides were associated with non-sulphide gangue minerals which was confirmed by micro-computed tomography. The waste rock samples were submitted to both acid base accounting (ABA) and net acid generation (NAG) tests to evaluate their acid-generating potentials. Sixteen size fractions were non-acid generating and five were classified as uncertain. However, standard ABA and NAG tests are based on pulverized samples that don't consider the initial textures of samples. Therefore, ABA results (acid-generating potentials and neutralization potentials) were corrected using the degrees of sulphide and carbonate liberation. The corrected classifications reduced the acid-generating potential of most of the uncertain samples due to the high carbonates content in relation to sulphides. This study also defines a parameter, the diameter of physical locking of sulphides, that can be used to separate waste rock into a reactive and non-reactive fraction based on the relationship between sulphide liberation and particle-size fraction. The reactive fraction refers to waste rock likely to require active acid rock drainage (ARD) management, whereas non-reactive fraction refers to waste rock likely not require active management.

Reliability improvement for predicting acid-forming potential of rock samples using static tests.

In predicting the acid-forming potential of rock samples, a combination of acid-base accounting (ABA) and net acid generation (NAG) tests has been commonly used. While simple and economical, this method sometimes shows low reliability such as categorizing certain samples as uncertain (UC). ABA and NAG tests were modified to selectively recover valid minerals in nature and substituted for the original tests. ABA test overestimated acid-producing capacity (in the case of weathered samples) and acid-neutralizing capacity (in the case of plagioclase-including samples) compared to the modified ABA test. NAG test yielded lower NAG pH compared to modified NAG test for samples with high total C content and low total S content. By comparing the correlation coefficients between acid generation amounts by the two evaluation methods, it was confirmed that modified evaluation method (MEM) has a much higher reliability (R 2=0.9582) than existing evaluation method (EEM) (R 2=0.5873). It was also concluded that exploiting advantages of both EEM and MEM is recommended where EEM is initially applied for general classification and a supplemented static test of MEM is executed for the purpose of correcting the error of UC categorized samples.

Evaluation of net acid generation pH as a single indicator for acid forming potential of rocks using geochemical properties.

The main purpose of this research was to evaluate the geochemical properties of rocks for a single indicator of acid-forming potential. The indicators, such as net acid generation (NAG), NAG pH and total S, were applied to 312 rock samples of various geological characteristics. Additional indicators, such as a Modified NAG pH, paste pH and available acid neutralizing capacity (ANC), were applied to 22 selected samples. Among them, NAG pH was considered the most plausible single indicator in evaluating acid-forming potential, as it is simple to measure, widely applicable to various samples and can be used to estimate the NAG value. The acid-forming potential of 287 samples (92% of samples examined in this research) was classified as either non-acid forming (NAF) or potentially acid forming (PAF) by NAG pH, with an NAF criteria of <3.21 and PAF of >4.52. The NAG pH was also a good estimate of the risk of short-term acid release when combined with paste pH information. However, application of NAG pH to coal mine wastes, with high organic carbon contents, produced erroneous results due to the generation of organic acid during the NAG test. In this research, a Modified NAG pH was assessed as an alternative to NAG pH in such situations.

Physical chemical characterization of historical mining waste and ARD prediction tests

There are thousands of historical mine tailings and metallurgical residues present on inactive metal mining sites which have been abandoned. However, the potential release of dissolved metals, acidity, or suspended particulates from mine-waste dumps can be a serious and long-lasting problem. A variety of environmental impacts may occur at this abandoned mine sites and leading the list is acid generation, which is one of the main problems from metal mining. The objective of the present study was to characterize and evaluate the Romanian historical tailing of Sasar-Red Valley, near Baia Mare. This involved physical and chemical characterization of the materials and its acid generating potential. Sasar tailing impoundment contains 8.5 million m3of tailings and occupies an area of 32.5 hectares. Tailings samples were collected from different depths in three sampling points, and characterized by grain size, chemical elements concentration, elements distribution of elements in depth, paste pH, acid-base account and net acid generation tests. Acid base accounting (ABA) tests in conjunction with net acid generation (NAG) tests classified the samples into the category of ‘potentially acid generating’. This paper presents a synthesis of works performed in the Improve Resource Efficiency and Minimize Environmental Footprint (REMinE) project.

The Effect of Particle Size and Mineral Liberation on the Acid Generating Potential of Sulphidic Waste Rock

Acid Rock Drainage (ARD) prediction is complicated by the number and complexity of influencing parameters. The objective of this study was to determine the effect of particle size and mineral liberation on the production of ARD. This information was used to classify waste rocks from an Australian mine site for their potential use as cover material. Samples were crushed and sieved into different particle size fractions, and were subjected to varying static and kinetic tests including paste-pH, Acid-Neutralization-Capacity tests, kinetic Net Acid Generation tests and Humidity Cell tests. The results showed a strong influence of particle size, which can be attributed to the widely different mineral liberation characteristics of acid generating and neutralizing minerals. Whereas standard Acid Base Accounting (ABA) resulted in an uncertain or Non Acid Forming classification of open-pit waste rocks, the application of modified ABA test procedures using different particle size fractions resulted in an uncertain or Potentially Acid Forming classification of the very same material. The study demonstrated that the standard ABA procedure may lead to erroneous classification with potentially costly environmental consequences by overlooking particle size specific mineral liberation effects. Testing different particle size classes proved to be a practicable method to investigate the ARD characteristics of waste rocks and to gain a clearer insight into the consequences of mineral liberation.

Prediction of acid mine drainage generation potential of various lithologies using static tests: Etili coal mine (NW Turkey) as a case study.

The Etili neighborhood in Can County (northwestern Turkey) has large reserves of coal and has been the site of many small- to medium-scale mining operations since the 1980s. Some of these have ceased working while others continue to operate. Once activities cease, the mining facilities and fields are usually abandoned without rehabilitation. The most significant environmental problem is acid mine drainage (AMD). This study was carried out to determine the acid generation potential of various lithological units in the Etili coal mine using static test methods. Seventeen samples were selected from areas with high acidic water concentrations: from different alteration zones belonging to volcanic rocks, from sedimentary rocks, and from coals and mine wastes. Static tests (paste pH, standard acid-base accounting, and net acid generation tests) were performed on these samples. The consistency of the static test results showed that oxidation of sulfide minerals, especially pyrite-which is widely found not only in the alteration zones of volcanic rocks but also in the coals and mine wastes-is the main factor controlling the generation of AMD in this mine. Lack of carbonate minerals in the region also increases the occurrence of AMD.

A Study of Residual Cyanides and Potential Stabilities in Tailings Storage Facility of Gold Mining Operation

The study aimed to investigate residual cyanides and their potential stabilities in tailings from a gold processing plant deposited in the Tailings Storage Facility (TSF) of a gold mining site. For this purpose, several samples of tailings were taken from the recently abandoned TSF to a depth of 2.5 meters, representing 1 to 2 years old tailings. The samples were analyzed for pH, metals, residual cyanides, and Net Acid Generation Test (NAG test). The results showed that, in all samples, pH values were neutral to slightly alkaline. Fe concentrations were relatively high (22,800-35,000 mg kg-1) indicating that Fe can readily react with free cyanide (CN-) to form iron cyanide complexes (strong metal cyanide complexes). The residual cyanide concentration in tailings was found to be very low; free cyanide (CNFree), weak acid dissociable cyanide (CNWAD), strong acid dissociable cyanide (CNSAD) and total cyanide (CNT) were in the range of 0.16-0.64, 0.37-1.06, 1.96-3.89 and 2.33-4.95 mg kg-1, respectively. Cyanides in the tailings were predominantly strong metal cyanide complexes being more stable to dissociate. Total cyanide (CNT) concentration was quite low compared with Thailand’s environmental regulations for soil quality standards andwere environmentally insignificant. The NAG test for all samples revealed that NAG pH was in the range of 5.02-6.10 and NAG was in the range of 0.08-0.31 kg H2SO4t-1indicating all samples have low or non-acid forming potentials. The findings indicated a potential to form a high proportion of strong metal cyanide complexes in the tailings, which is difficult to dissociate and releasehigh toxic cyanide to the environment.

Geochemical characterization of coal and waste rocks from a high sulfur bearing coalfield, India: Implication for acid and metal generation

An integrated study on coal and mine wastes from the Jaintia Hills coalfield of Meghalaya, India involving mineralogy, acid base accounting (ABA), and net acid generation (NAG) potential and sequential leaching was undertaken to examine their potential in controlling the acid mine drainage (AMD). Mineralogical study revealed that pyrite is the major sulfide mineral in coal and mine waste, being more abundant in sandstone and carbonaceous shale; while, dolomite and calcite are abundant in a few shale and siltstones, and Fe and Al copiapites are enriched in sulfate salts. During the ABA test, all coals and >50% of mine waste showed paste pH<4, implying their acid generating nature. Further, the relations between net neutralization potential (NNP) and acid producing ratio (APR) revealed that blocky pyrite, pyritiferrous sandstone and efflorescent salts contribute acid much higher than coal, siltstone and carbonaceous shale. This inference is consistent with the NAG test. Partitioning of metals in mine waste indicates high proportions of them in the blocky pyrite being bioavailable than from other rocks. While Mn is highly bioavailable, major portions of Pb and Zn are in the reducible and oxidizable fractions. In efflorescent sulfate salts, >80% of metals are available in water soluble fraction; thus these minerals can be considered as the highest polluting residue in the mining environment. These minerals easily precipitate from AMD solution in dry periods and can re-dissolve under rain events because of their high solubility; therefore, they can play an important role in controlling the chemistry of mine drainage in regard to local climate change. In the case of coal, only small fractions of Mn, Ni, Zn, Cd, and Pb are released to the environment, though high proportions of them can become bioavailable under oxidizing conditions besides their other bioavailable forms.

Environmental geochemistry of the Hycroft Mine: a case study on the limitation of Sobek-style acid-generation predictions

The Hycroft Mine is an open pit gold mine situated along the eastern edge of the Black Rock Desert in NW Nevada, a semi-arid region where annual evaporation significantly exceeds precipitation. The Hycroft deposit is hosted in volcanic breccias and conglomerates of the Tertiary Kamma Mountain volcanic group that have been faulted by north-trending structures that acted as conduits for hydrothermal fluids. An acid-leaching event resulted in the development of vuggy, quartz alunite-altered zones produced by the removal of most minerals, including those capable of buffering acid. Accessory minerals associated with the acid leach zones are rare, except native sulphur which is locally abundant. The complex sulphur mineralogy of the Hycroft deposit, coupled with the deficiency of acid-neutralizing minerals, limits the application of traditional Sobek-style Acid-Base Accounting (ABA) methods that rely upon sulphur speciation data to predict acid-generation potential. A comparison of results from geochemical predictive tests including the modified Sobek ABA method, Net Acid Generation (NAG) test and kinetic humidity cell test (HCT) illustrates the complexity of interpreting ABA data for material containing native sulphur and low solubility sulphate minerals such as alunite. Comparing ABA data to HCT results shows the predictions of acid generation from the ABA test are not consistent with those from the HCT, which is considered a more definitive test for the prediction of acid and metals mobility. The results of this study indicate the ABA test does not provide a reliable prediction of acid generation for samples containing native sulphur and low solubility sulphate minerals. A reasonably good correlation is, however, observed between the HCT and NAG results, suggesting the NAG test is a more effective tool for predicting the geochemical behavior of the Hycroft deposit, and potentially also other deposits with complex sulphur mineralogy. The presence of native sulphur in the acid leach material presents additional challenges with respect to acid-generation prediction due to the unique properties of this mineral. Although acid generation is predicted from the NAG test for samples containing native sulphur, the actual amount of acid and metal release generated by this mechanism in the HCT is limited in comparison to that associated with natural pyrite oxidation, which is the main contributor to acid rock drainage. The potential for acid generation from this mechanism is further limited by the semi-arid site conditions that are unlikely to produce conditions similar to the humidity cell tests that are designed to accelerate the weathering process and enhance mineral-water reaction rates. This study demonstrates the importance of considering the site-specific mineralogical characteristics of the deposit in the selection of the most appropriate geochemical test methods to use in the prediction of potential acid generation.

전황함량과 순산발생능력의 상관관계를 통한 잠재특이산성토양 기준 설정

황철석 (Pyrite, FeS2)을 함유한 잠재특이산성토는 강하류 삼각지 토양, 간척지 등의 해성토뿐만 아니라, 영일만과 같은 융기해성토 지대, 내륙의 선상지하단 유기물이 많은 암흑색 토층이 있을 때에 존재하는 수가 있다. 또한 안산암 지역의 열수작용에 의해 생성되어 암맥을 따라 형성된 황철석이 광산개발이나 도로건설로 절취사면에서 노출되어 산화되면 매우 강한 산성을 띠는 특이산성토층을 형성하여 주변농경지에 피해를 주고 있다. 현재 잠재특이산성토양의 판정은 현장에서는 과산화수소로 반응 시 수증기발생 정도로 판단하거나 실내실험에서는 전황 (Total-S)성분의 함량으로 판단한다. 하지만 이들 방법은 시군농업기술센터 및 현장 진단 시 적용이 용이하지 않다. 산발생 능력평가 중 순산 발생능력실험 (Net Acid Generation, NAG pH)은 대상지역의 산성발생 가능성에 대한 예측을 정량적 계산으로 가능하다. 순산발생능력실험을 이용하여 전황함량과 NAG pH와의 상호관계를 통해 특이산성토양 판정을 제안하기 위해 화산기원의 잠재특이산성 토양과 사양질 토양을 일정비율로 혼합된 토양과 특이산성토양인 김해통과 해척통 토양에 대해 실험을 수행하였다. 전황의 함량이 0.75% 이상인 시료의 NAG pH가 2.5이며 0.75-0.50%의 중간 특이산성토양은 NAG pH 3.0으로 측정되었다. 그리고 전황 함량이 0.5-15% 약한 특이산성 토양은 NAG pH 3.8로 측정되었다. 따라서 순산발생량은 NAG pH를 이용하여 토양 내 황철석을 모두 산화시키고 pH를 측정하여 pH 3.8이하인 토양은 특이산성토양으로 구분하는 것이 타당할 것으로 판단되었다. Acid sulfate soil (ASS) and potential acid sulfate soil (PASS) are distribution in worldwide and originate from sedimentary process, volcanic activity, or metamorphism and are problematic in agriculture and environmental due to their present and potential acidity developed by the oxidation. The PASS was defined as soil materials that had sulfidic layer more than 20 cm thick within 4 m of the soil profile and contained more than 0.15% of total-sulfur (T-S). A tentative interpretative soil classification system was proposed weak potential acid sulfate (T-S, 0.15-0.5%), moderate potential acid sulfate (T-S, 0.5-0.75%) and strong potential acid sulfate (T-S, more than 0.75%). PASS due to excess of pyrite over soil neutralizing capacity are formed. It provides no information on the kinetic rates of acid generation or neutralization; therefore, the test procedures used in acid base account (ABA) are referred to as static procedures. The net acid generation (NAG) test is a direct method to measure the ability of the sample to produce acid through sulfide oxidation and also provides and indication. The NAG test can evaluated easily whether the soils is PASS. The samples are mixed sandy loam and the PAS from the hydrothermal altered andesite (1:3, 1:8, 1:16, 1:20, 1:40, 1:80 and 1:200 ratios) in this study. We could find out that the NAG pH of the soil containing 0.75% of T-S was 2.5, and that of the soil has 0.15% of T-S was 3.8. NAG pH test can be proposed as soil classification criteria for the potential acid sulfate soils. The strong type has NAG pH of 2.5, the moderate one has NAG pH of 3.0, and the weak one has NAG pH of 3.5.

Acid–base accounting assessment of mine wastes using the chromium reducible sulfur method

The acid base account (ABA), commonly used in assessment of mine waste materials, relies in part on calculation of potential acidity from total sulfur measurements. However, potential acidity is overestimated where organic sulfur, sulfate sulfur and some sulfide compounds make up a substantial portion of the sulfur content.The chromium reducible sulfur (CRS) method has been widely applied to assess reduced inorganic sulfur forms in sediments and acid sulfate soils, but not in ABA assessment of mine wastes. This paper reports the application of the CRS method to measuring forms of sulfur commonly found in mine waste materials. A number of individual sulfur containing minerals and real waste materials were analyzed using both CRS and total S and the potential acidity estimates were compared with actual acidity measured from net acid generation tests and column leach tests. The results of the CRS analysis made on individual minerals demonstrate good assessment of sulfur from a range of sulfides. No sulfur was measured using the CRS method in a number of sulfate salts, including jarosite and melanterite typically found in weathered waste rocks, or from dibenzothiophene characteristic of organic sulfur compounds common to coal wastes.Comparison of ABA values for a number of coal waste samples demonstrated much better agreement of acidity predicted from CRS analysis than total S analysis with actual acidity. It also resulted in reclassification of most samples tested from PAF to NAF. Similar comparisons on base metal sulfide wastes generally resulted in overestimation of the acid potential by total S and underestimation of the acid potential by CRS in comparison to acidity measured during NAG tests, but did not generally result in reclassification. In all the cases examined, the best estimate of potential acidity included acidity calculated from both CRS and jarositic S.

Effectiveness of amendments on re-acidification and heavy metal immobilization in an extremely acidic mine soil

Previous studies have shown that the application of soil amendments is efficient in reducing acidity and heavy metal bioavailability in mine soils. However, it remains a challenge for environmentalists to predict accurately and control economically the re-acidification in re-vegetated mine soils. In this study, net acid generation (NAG) test and bioassay technique were employed to assess the effectiveness of the amendments [including lime, N-P-K (nitrogen, phosphorous and potassium) fertilizer, phosphate and river sediment] on re-acidification and heavy metal immobilization in an extremely acid (pH < 3) mine soil. Our results suggested that NAG test was a rapid and accurate approach to assess the effectiveness of the amendments on re-acidification potential of the mine soil. Interestingly, it was found that phosphate and river sediment played quite specific roles in preventing the re-acidification in the mine soil. In addition, the results also indicated that the addition of 25 t ha(-1) lime combined with river sediment (30%) might be an economical method to successfully control the acidification and re-acidification in the extremely acid mine soil, allowing the re-establishment of the plants. Collectively, our results implied that the combined use of NAG test and bioassay assessment was effective in evaluating a reclamation strategy for extremely acidic mine soils.

Chemical characteristics and acid drainage assessment of mine tailings from Akara Gold mine in Thailand

Acid Mine Drainage (AMD) is a great concern in many abandoned mines because of its adverse effect on the environment. In mining processes, many kinds of wastes are produced. These wastes may become eventually sources of environmental degradation. The focus of this study is the geochemical characterization of the end-processed tailings generated by Akara Gold Mine, the biggest gold mine in Thailand. Tailing samples were systematically collected for analyses of chemical and mineralogical compositions. As a result, their quantitative chemical analyses are slightly different from place to place, but mineral components cannot be clearly differentiated. For instance, it may be assumed that the end-processed tailings, which were a mixture between high and low grade concentrates, would have similar mineral components. However, the little variation of chemical composition may be caused by the ore refining processes that are somehow varied in proportion to chemical additives, alkali cyanide and quick lime in particular. In addition, clay composition in ore-bearing layers may also influence alumina content of tailings, accordingly. Distribution of the tailings is not related to depth and distance of the tailing storage pond because the disposal has sped them over the pond during operation. Total heavy metals of the tailing samples were analyzed on the basis of the EPA 3052 method. Consequently, the most toxic elements (e.g., Co, Cu, Cd, Cr, Pb, Ni and Zn) were found falling within the standard of Thailand Soil Quality Standards for Habitat and Agriculture. Only Mn appears to have higher content than the standard. In addition, leaching tests proved that these tailings contain low metal concentrations. As a result, at pH 2, Mn can leach out exceeding the Thailand Surface Water Quality Standard for Agricultural (Mn <1 ppm) and the Thailand Industrial Effluent Standard (<5 ppm). Although leachate at pH 4 and neutral conditions contains lower Mn than the Industrial Effluent Standard it still exceeds the surface water quality standard. Interestingly, Pb can be leached out exceeding both standards (0.2 ppm for the industrial effluent standard and 0.05 ppm for the surface water quality standard). For Ni leaching, its concentration is lower than the Industrial Effluent Standard at all pH conditions but still exceeds the Surface Water Standard at pH 2 and 4. This information should be taken into consideration for further environmental monitoring. Acid generating potential of the tailings was estimated using acid–base accounting (ABA) and net acid generation (NAG) tests. The results of ABA and NAG tests show that the tailing samples contain a high amount of sulfur. However, they also contain high acid neutralization capacity. Consequently, these tailings may not have potential to generate acid drainage; in the other words, they can be classified as a non-acid forming (NAF) material. However, since these tailings contain some heavy metals (e.g., Ni, Mn and Pb) that are observed in leachates exceeding the standards at low pH, the AMD conditions may lead to heavy metal release. Therefore, prevention of oxidizing process and dissolution should be considered with great care. In addition, Mn and Pb can also be leached at neutral conditions. Barrier of air and water, clay layer for example, should be placed over the tailings pound before covering by topsoil for re-vegetation. Growing native grass is recommended for stabilization of the surface and reducing erosion rate. Monitoring of water quality should also be carried out annually.

Assessing the acidic potential of waste rock in the Akara gold mine, Thailand

Acid mine drainage (AMD) is the environmental issue that generates the greatest public concern regarding the mining industry. Thus, characterization of mine waste rock according to acid generation potential is necessary for mining operations to ensure proper waste rock storage and to avoid future adverse environmental effects. Therefore, this study was conducted to estimate the potential of AMD generation in the largest operating gold mine in Thailand by using acid base accounting and net acid generation tests. Representative samples of six types of waste rock classified by mining geologists for mineral processing and waste dumping were collected for this study: volcanic clastic, porphyritic andesite, andesite, silicified tuff, silicified lapilli tuff, and sheared tuff. Under various conditions, experimental results indicate that only silicified lapilli tuff and shear tuff are potentially acid-forming materials. The results indicate that AMD generation may possibly occur a long time after mine closure due to the lag time of the dissolution of acid-neutralizing sources. Acidic generation from some waste rocks may occur in the future based on environmental conditions, particularly the oxidation of sulphide minerals by the combination of oxygen and water. Therefore, a proper design for waste rock dumping and storage is necessary to reduce the risk of AMD generation in future. It is advisable to install a surface management system to control the overland flow direction away from the waste dump area and tailing storage facility and to install a second water storage pond next to the main storage pond to store the spilled water during storms and the rainy season. A water quality monitoring plan that focuses on disturbed areas such as water storage ponds and mine pits should be put in place.