Net Acid Production Potential Research Articles

Analysis of PAF and NAF Tests Using NAPP and ANC Methods at Hajak Site, North Barito, Central Borneo

Acid mine drainage (AMD) is a significant challenge for the global coal mining industry, necessitating specialized treatment to prevent its occurrence. A crucial step in AMD prevention is identifying rocks that contribute to its formation. These rocks are classified into Potential Acid-Forming (PAF) rocks and Non-Acid-Forming (NAF) rocks. PAF rocks have the potential to produce acid, while NAF rocks do not. Laboratory analyses, including Maximum Potential Acidity (MPA), Acid Neutralizing Capacity (ANC), Net Acid Producing Potential (NAPP), and Net Acid Generating (NAG) tests, identified 3 sample points with potential PAF characteristics out of 35 samples tested (see Table 3.1). Among these, 2 sample points were classified as Potential Acid Forming-Low Capacity, and 1 sample point was categorized as Potential Acid Forming-Medium Capacity (NAG pH 4.5, NAPP 10 kg H2SO4/ton). The acidity level of mine water resulting from landfilling varies significantly based on the mineral content and landfilling techniques used. Mined material with high carbonate mineral content tends to have lower acidity levels in leachate and can even neutralize formed acid. The stockpiling strategy implemented involves layering PAF material followed by a final cover of NAF material and a rooting zone to mitigate acid formation.

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.

Heavy metal release and acid generation during the oxidation of sulfide waste rock induced by ferric iron

As aby-product of waste rock from sulfide ore, Fe3+ is one of the important factors that promote acid production and heavy metals release from waste rock pile fields. In this paper, the dynamic leaching tests were conducted to investigate the behavior of acid generation and heavy metal release from the sulfide waste rock of Dexing copper mine under different Fe3+ concentrations. The Sulphur content in the waste rock samples was 3.56 % and the net acid production potential (NAPP) reached 47.95 kg·t−1 H2SO4. Under the oxidation of 600 mg·L-1 Fe3+, the pH value of the leaching system decreased to 5.53, which is 1.85 units lower than that of the control check group (CK). However, the quantity of SO42- in the leachate was reduced by 41.81 % compared to the CK group, as the jarosite precipitation was more likely to form under high Fe3+ conditions. The oxidation of Fe3+ effectively accelerated the release of heavy metals from the waste rock. In addition, the heavy metals release quantity gradually increased with the increase of the Fe3+ concentration, with the maximum release quantities of Cu, Pb, Zn, and As of 569.69, 333.75, 222.69, and 31.11 μg·kg−1, respectively, which increased by 27.66 %-129.60 % compared to the CK group. The release of SO42-, Cu, Pb, and Zn was controlled by the combined internal diffusion and the chemical reaction, while the release of As was best fitted to the interfacial chemical reaction-controlled model. The SEM analysis showed that the waste rock near the top of the column was strongly corroded, resulting in the increase of the specific surface area of the waste stone by 24.48 %. The re-precipitation reaction of heavy metals mainly occurred near the bottom of the leaching column. Leaching under Fe3+ oxidation caused a decrease in the proportion of residual heavy metals in the waste rock, which will lead to the easier release of heavy metals.

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.

Prediction Potential Acid Mine Drainage of Epithermal High Sulfidation Deposits using Static Test

Acid mine drainage can be predicted by testing the geochemical characteristics of rocks, to be knowing acid-forming rocks and acid neutralizing. Prevention and mitigation efforts from acid mine drainage can be predicted by knowing the characteristics of rocks and looking for the direct contact of acid rocks with air and water. The static test aims to determine the potential for acid formation that occurs in rocks as a first step to characterize rocks. Tests were carried out on argillic silica alteration rock samples from the Lanut site, PT J-Resources Bolaang Mongondow. The calculation of total sulfur shows a smaller percentage of the value of the advance argillic than the silisic sample. The results of advance argillic samples, NAG pH> 4.5 were obtained and the value of Net Acid Producing Potential (NAPP) in the range of 110-153 kg H2SO4 / ton rocks which means that the potential nature of rocks cannot be ascertained PAF or NAF. Whereas for silisic alteration rock samples the pH value of NAG <4.5 and NAPP values in the range of 146 - 355 kg H2SO4/ton rocks means silisic alteration has the potential acid forming.

Determination of acid forming potential of massive sulfide minerals and the tailings situated in lead/zinc mining district of Balya (NW Turkey)

Weathering of sulfide minerals is a major source of acid production in nature and especially in mining territories. Pyrite is not the only principal mineral that generates acid drainage: other sulfide minerals (sphalerite, galena, chalcopyrite, etc.) may also be responsible for acid production. In addition to massive sulfide minerals, sulfide-bearing mine tailings may also produce acid drainage due to oxidation and hydrolysis reactions in waste dumps. The lead/zinc (Pb/Zn) mining region in Balya and Balıkesir, in Turkey, has operated mines intensively since the 1860s; so that characterization of the sulfide minerals and tailings situated and formed around the mining site is of great importance to secure a sustainable environment. For this purpose, acid production and neutralization potentials of massive sulfide ores of the region, and in the Pb/Zn process facility mine tailings from ten different points of tailings dam, have been determined by applied conventional Acid-Base Accounting (ABA) and Net Acid Generation (NAG) static tests after chemical and mineralogical analysis. The NAG pH and net acid production potential (NAPP) values were compared on a chart in order to classify the samples as either acid generating or non-acid generating. According to the comparisons, the sulfide minerals were classified as potentially acid forming (PAF). Massive pyrite had the highest NAPP and NAG pH value of 1966.6 kg H2SO4/ton and 1.91, respectively and the galena had the lowest NAPP value of 558.9 kg H2SO4/ton. However, the sphalerite NAG leachate pH value of 4.30 was the highest in sulfide minerals so that the sphalerite plotted near the uncertainty reference border in the PAF zone. In the mine tailings, NAPP values of 105.9 kg H2SO4/ton on average and the NAG pH values of over 7.5 were determined. In addition to these tests, water leaching (agitation test) was carried out on tailings in order to generate more information. The tailings did not generate acidic leachates as they lie on limestone bed rock which neutralized the acidity.

Acid mine drainage analysis for the Reddale Coal Mine, Reefton, New Zealand

ABSTRACTThe Reddale Coal Mine is located within the Brunner Coal Measures, which are often assumed to be potentially acid-forming rocks that will release acidity when disturbed by mining. The pre-mining assessment of the site acid mine drainage (AMD) potential assumed overburden acid-neutralising capacity was negligible, similar to Brunner Coal Measures in other areas, and that AMD would occur almost immediately. Column trials using poorly stored and partially oxidised drill core indicated potentially acid-forming (PAF) overburden readily produced acidity with no lag period. Based on the pre-mining assessment, a Ca(OH)2 dosing plant was constructed for AMD management. Once mining commenced, tiphead samples collected during construction of the Ferndale engineered landform showed that the net acid-producing potential of the potentially acid-forming overburden was overestimated by about one-third when acid-neutralising capacity and non-sulphide forms of sulphur were accounted for. When coupled with material properties which resulted in good compaction of waste rock, construction of the engineered landform in 4 m lifts to minimise convective oxygen flux and aglime (<2.5 mm CaCO3) amendments to each lift, acidity loads from the Ferndale engineered landform were negligible. Compliance monitoring downstream of the mine site at Red4 shows that, to date, site discharge pH has remained circum-neutral. Pre-mining AMD management decisions, primarily construction of a Ca(OH)2 dosing plant that was never commissioned, therefore proved overly conservative. The timing and expense of AMD management has the potential to influence project feasibility and needs to be well understood.

Assessment of the alteration and acid-generating potential of deep boreholes for geological disposal

An integrated study, including mineralogy, geochemistry, lead isotopic composition, acid–base accounting, and factor analysis, was conducted on the rock core samples from two deep boreholes (AH1 and BH3) to assess the alteration and acid-generating potential of two representative lithologies of Korea (granite and geniss) for geological disposal. The alteration condition represents water circulation features in crystalline basement rocks, which is one of the key points to be assessed in repository sites. In addition, this study proposes that the acid-producing potential and acid-consuming minerals in the host rock are important for the long-term safety of a radioactive waste repository, given that sulfide oxidation may occur in artificial fractures created during construction. Fe3+ ions can then react with other sulfide minerals in reducing conditions and release H+ and SO4 2−, which can deteriorate cement-based grouting materials used for carven walls as well as the host rock. The results of the study show that the composition of Pb isotopes is a good indicator of the alteration of rock core samples compared to the chemical index of alteration and loss on ignition. The acid potential ratio and net acid production potential indicate that most of the core samples in both boreholes have a high alkali-generating potential. Factor analysis shows that the neutralizing capacity in AH1 is highly correlated with the alteration, while the neutralizing capacity in BH3 is correlated with secondary minerals. The alteration appears to be related to secondary minerals and the acid-producing capacity, while the changes in major elements are not related to the Pb isotopes in BH3 due to leaching along fractures.

Construction of Dry Cover System for Prevention of Acid Mine Drainage at Mine Waste Dump in Open Cast Coal Mines, Indonesia

Acid Mine Drainage (AMD) which occurs when sulfide minerals are exposed to water and oxygen with an excavation is one of the serious environmental problems in the world. A dry cover system is generally constructed in waste dump for the prevention of AMD in Indonesia by virtue of low cost and availability of waste rocks for a cover layer. However, the failure of the system caused by the lack of information related to the construction of cover system in mines, which leads to AMD, has been reported recently in Indonesia. In this study, the field investigation was conducted in pit and waste dump in open cast coal mine in Indonesia with the aim of obtaining the information on the construction of a cover layer and backfilling conditions of waste rocks in the waste dump. The rock samples taken in two areas of the mine were analyzed by geochemical analysis and sequential extraction with acids. The results indicated that Net Acid Producing Potential (NAPP) of the rocks in the waste dump down to 100 cm depth in both areas was from 10 to 30 kg H2SO4/ton, suggesting that Potentially Acid Forming (PAF) was backfilled in a cover layer. The backfill of PAF was contrary to the concept of cover system, which caused the failure of constructing a cover layer. The cause of the failure was likely attributed to the shortage of cover rocks which are classified as Non Acid Forming (NAF) or the failure of proper placement of them by an operational problem in the areas. Moreover, the results of the extraction with acids suggested that the form of iron and sulfur has to be taken into account to discuss the occurrence of AMD.

Inhibition of pyrite oxidation by surface coating: a long-term field study.

Pyrite and other iron sulfides are readily oxidized by dissolved oxygen in aqueous phase, producing acidity and Fe(2+), which causes significant environmental problems. Applications of surface coating agents (Na2SiO3 and KH2PO4) were conducted at Boeun (Chungbuk, South Korea) outcrop site, and their efficiencies to inhibit the oxidation of sulfide minerals were monitored for a long-term period (449days). The rock sample showed positive Net Acid Production Potential (NAPP=20.23) and low Net Acid Generation pH (NAGpH=2.42) values, suggesting that the rock sample was categorized in the potential acid-forming group. For the monitored time period (449days), field study results showed that the application of Na2SiO3 effectively inhibited the pyrite oxidation as compared to KH2PO4. Na2SiO3 as a surface coating agent maintained pH 5-6 and reduced oxidation of pyrite surface up to 99.95 and 97.70% indicated by Fe(2+) and SO4 (2-) release, respectively. The scanning electron microscope and energy-dispersive X-ray spectrometer analysis indicated that the morphology of rock surface was completely changed attributable to formation of iron silicate coating. The experimental results suggested that the treatment with Na2SiO3 was highly effective and it might be applicable on field for inhibition of iron sulfide oxidation.

Oil sands thickened froth treatment tailings exhibit acid rock drainage potential during evaporative drying

Bitumen extraction from oil sands ores after surface mining produces different tailings waste streams: ‘froth treatment tailings’ are enriched in pyrite relative to other streams. Tailings treatment can include addition of organic polymers to produce thickened tailings (TT). TT may be further de-watered by deposition into geotechnical cells for evaporative drying to increase shear strength prior to reclamation. To examine the acid rock drainage (ARD) potential of TT, we performed predictive analyses and laboratory experiments on material from field trials of two types of thickened froth treatment tailings (TT1 and TT2). Acid–base accounting (ABA) of initial samples showed that both TT1 and TT2 initially had net acid-producing potential, with ABA values of −141 and −230t CaCO3 equiv. 1000t−1 of TT, respectively. In long-term kinetic experiments, duplicate ~2-kg samples of TT were incubated in shallow trays and intermittently irrigated under air flow for 459days to simulate evaporative field drying. Leachates collected from both TT samples initially had pH~6.8 that began decreasing after ~50days (TT2) or ~250days (TT1), stabilizing at pH~2. Correspondingly, the redox potential of leachates increased from 100–200mV to 500–580mV and electrical conductivity increased from 2–5dSm−1 to 26dSm−1, indicating dissolution of minerals during ARD. The rapid onset and prolonged ARD observed with TT2 is attributed to its greater pyrite (13.4%) and lower carbonate (1.4%) contents versus the slower onset of ARD in TT1 (initially 6.0% pyrite and 2.5% carbonates). 16S rRNA gene pyrosequencing analysis revealed rapid shift in microbial community when conditions became strongly acidic (pH~2) favoring the enrichment of Acidithiobacillus and Sulfobacillus bacteria in TT. This is the first report showing ARD potential of TT and the results have significant implications for effective management of pyrite-enriched oil sands tailings streams/deposits.

Waste rock dump investigation at Roşia Montană gold mine (Romania): a geostatistical approach

Rosia Montană, the largest European gold mine, could be re-opened. Environmental problems led to severe pollution of the Rosia and Abrud Rivers. The two main potentially toxic element (PTE) sources in mine sites are, in general, the abandoned underground workings and the piles of waste rock. Since the composition of waste rocks is often heterogeneous, this study faces the problem of estimating their mineralogical and chemical features starting from a set of sampling point. Twenty-five samples were collected on the main waste dump of the Rosia Montană mine following a virtual squared grid (knots distance about 25 m). Grain size, color, bulk chemistry, mineralogy, acid mine drainage potential and a set of selected PTE (Ag, As, Cu, Ni, Pb and Zn) concentrations were determined on each sample. In a first approximation two main waste rock groups were identified: WR1 bulk composition is dacite-dominated, while WR2 is andesite-dominated. In both of them the concentrations of PTEs are below the regulatory limits for soils, with the exception of As. In terms of acid mine drainage, WR1 has a net acid-producing potential, while WR2 has a net neutralizing potential. The anisotropy of WR properties were analysed by means of semi-variograms and displayed with contour maps. Application of positive matrix factorization for the analysis of all the data relative to waste rocks allowed defining, in a semi-quantitative way, the factors controlling pollution and their spatial distribution. The processing of score matrix G factor associated with the geostatistical elaboration promises to be a powerful tool to discern the composition of mine dumps and support the exploitation and remediation phases.

Geochemical hazard evaluation of sulphide-rich iron mines: The Rio Marina district (Elba Island, Italy)

Rio Marina mining district (Elba Island) is characterised by hematite + pyrite ore association and was exploited for iron till 1981, leaving waste rock dumps of several millions m 3. The effect of open pit mining activity in this site is to produce acid mine drainage (AMD) processes leading to environmental pollution, testified by all the sampled waters (Giove stream, drainage channels, superficial pools and settling basin) which have pH values ranging from 2.08 to 3.35 and heavy metal concentrations that reach 903.16 mg/l for Fe, 45.02 mg/l for Mn, 10.08 mg/l for Zn and 1.75 mg/l for Cu. In the present work a space and time related approach to geochemical hazard evaluation was applied. The geochemical hazard is mainly related to high heavy metal concentration, acid mine drainage processes development and topographic setting. As all these parameters are related in space, hazard evaluation was performed by geostatistical methods. Fifty-four earth material samples (residual soils, waste rocks or debris materials) were collected in a central aligned 100 m mesh square grid. These were analysed for major elements by XRF, for Cu, Pb, Zn by ICP-AES and for AMD potential following the AMIRA procedure. The concentration of heavy metals was compared with Italian law limits. The overlap of Cu, Pb and Zn content maps show that at least one of these heavy metals exceed law limits in all the area. The AMD test results show that more than 50% of samples have a positive NAPP (Net Acid Producing Potential) that could reach 258.9 kg H 2SO 4/t. According to the obtained data, three main geochemical hazard classes were established and their distribution in the mining area was assessed. About 51% of the mining area surface belongs to the major hazard class, where AMD process occurs, about 49% belongs to an intermediate hazard class, where AMD process could occur only if certain conditions are met. Finally, the persistence of the AMD process in the Rio Marina area was evaluated on the basis of yearly rainfall, mining waters pH and NAPP values. A complete leaching of the first 0.25 m of the earth materials can retain the current environmental conditions for several centuries.

English

Research was carried out to improve acid rock drainage (ARD) prediction methodologies, focussing on resolving uncertainties in acid base accounting (ABA) methods. Advancements, their application, and interpretation are reported. Consideration of both net acid production potential (NAPP) and net acid generation (NAG) test results provide a more reliable routine screening technique than either test alone, increasing confidence in acid potential classification and identifying apparent conflicts requiring further investigation. Techniques such as sequential NAG, modified organic carbon NAG, modified ANC methods to account for siderite and acid buffering characteristic curve (ABCC) testing can be used to help resolve these conflicts in a relatively short time frame. Samples with abundant siderite require complex modified ANC methods, involving excess addition of H2O2 to ensure all dissolved ferrous ion is oxidised before completion of the back titration step. It is demonstrated that at siderite abundances of less than 15 wt %, standard ANC methods are sufficient, improving the efficiency of ANC assessment by reducing the number of modified ANC tests required. Comparison of ABCC profiles with the developed set of standard carbonate curves provides an indication of the relative reactivity of the ANC measured in waste rock samples, something not evident in ANC test results alone. A relationship was established between kinetic NAG and leach column test results, which provides an indication of lag times without the need to carry out leach columns on all samples. This has the great advantage of allowing kinetic prediction on a broad sample set in a short time frame. The test refinements and developments described in this paper improve the reliability of ARD prediction from short duration tests, provide information on relative reactivity of acid forming and neutralising phases, and improve predictions of acid forming potential.

Leaching of heavy metals in acid mine drainage

Acid mine drainage is one of the most serious environmental problems that the coal and metal mining industry is currently facing. The generation of low pH drainage enhances the dissolution of heavy metals in water. The samples used in this research originated from three pits at mine dumps. In a study reported in this paper, three types of tests; namely static test, kinetic test and column test were conducted to estimate acid generation and acid neutralization reaction rates, and to predict the solubility of metals and their release rates. Static test showed that all samples had a pH of net acid generation (NAG pH) <4, a net acid producing potential (NAPP) >10 kg H2SO4tonne(-1), and a S-content >3%, which can be classified as a high acid-forming capacity. Simulated runoff in the column tests was equivalent to 5-year average rainfall in Indonesia, the resultant leachates showed acidic behaviour (pH < 3.5). Based on the results, it was found that high mobilization of heavy metals (Cr, Cu, Zn, Cd and Pb) takes place under strong acidic conditions (pH approximately equal 2).

Geochemical effects of oxidation products and framboidal pyrite oxidation in acid mine drainage prediction techniques

Acid mine drainage predictive testwork associated with the Australian Mineral Industries Research Association (AMIRA) P387A Project: Prediction and Kinetic Control of Acid Mine Drainage (AMD) has critically examined static acid assessment and kinetic information from acid–base accounting techniques, including net acid production potential (NAPP), net acid generation (NAG) and column leach tests. This paper compares results on two waste rock samples that were obtained from the Kaltim Prima Coal mine (KPC) containing significant quantities of fine-grained framboidal pyrite. In agreement with other research, the authors' results indicated that framboidal pyrite is more reactive than euhedral forms due to the greater specific surface area of framboidal pyrite. This is evidenced by optical microscopy of reacted samples. Importantly, the results showed that NAPP testing is biased by the rapid acid generating oxidation of framboidal pyrite prior to, and during the acid neutralisation capacity (ANC) test. This can result in negative ANC values for samples containing significant framboidal pyrite (often “corrected” to zero kg H 2SO 4/t) when significant ANC is actually present in the sample. NAG testing using H 2O 2 indicated that samples containing a significant quantity of framboidal pyrite can result in the catalytic decomposition of the H 2O 2 prior to complete oxidation of the sulfide minerals present, requiring sequential addition of H 2O 2 for completion. A benefit of the NAG test, however, is that it assesses the net acid generation capacity of the sample without bias towards acid generation as is observed using NAPP methods. The kinetic NAG test also gives information on the reaction sequence of framboidal and euhedral pyrite. Periodic (kinetic) analysis of sub-samples from column leach tests indicated rapid oxidation of the framboidal pyrite compared to the euhedral pyrite, which was correlated with the greater framboidal pyrite surface area. Calculations to determine the sulfide/sulfate acidity derived from the oxidation of framboidal pyrite prior to; and during the ANC test have been developed to provide a better indication of the actual ANC (ANC Actual) of the sample. Paste pH values of <pH 4–5 may be one suitable trigger mechanism for the implementation of this new method. This has led to an improved NAPP estimation of total acid production. Together with NAG and column leach testing this improved methodology has resulted in accurate AMD characterisation of samples containing acidic oxidation products and framboidal pyrite.

Improved acid neutralisation capacity assessment of iron carbonates by titration and theoretical calculation

The acid neutralisation capacity (ANC) of a rock sample containing significant amounts of Fe carbonates, as conducted to determine net acid production potential (NAPP), can be a difficult parameter to determine. Various ANC tests are available to determine the ANC of carbonates. This work does not attempt to create another ANC test protocol; rather, it provides a refinement for existing tests. Results showed that a significant lag period may be needed (up to 432 h) after standard Sobek-type ANC tests for the complete hydrolysis of Fe associated with the ANC testing of siderite. This lag occurred even with standard industry modifications that include the addition of 2 drops of H 2O 2 at pH 4.5 during the back-titration. In this work the authors used a modification to the Sobek ANC test (the Modified Sobek ANC test) that included filtering and the addition of H 2O 2 at pH 4.5. This test was further modified by the continuance of this H 2O 2 addition (the H 2O 2 ANC test) until there was no subsequent pH drop (which is due to Fe hydrolysis reactions), thereupon the back titration was continued to pH 7.0. Results indicated that the ANC for siderite (after 0 h) using the H 2O 2 ANC test was similar to the ANC determined after 432 h lag by the Modified Sobek ANC test. This modification reduces the uncertainty related to static-test results for samples containing Fe carbonates. The test is simple to use, has industry application, and provides a better indication of the NAPP. The Modified Sobek ANC results for calcite and dolomite and the H 2O 2 ANC test for siderite were in good agreement with the mineralogical carbonate ANC (ANC carb). ANC carb was determined by calculation based on electron probe micro-analysis. Although lower than both the ANC carb and the ANC determined by titration, the chemical ANC calculated from the ions present in the ANC digestion liquor also provided a good indication of the overall acid neutralisation capacity of the sample.

Acidification of lead/zinc mine tailings and its effect on heavy metal mobility

The acid-forming potential of lead/zinc (Pb/Zn) mine tailings at Lechang City of Guangdong Province was studied using both net acid generation (NAG) and acid–base accounting (ABA) methods. The pyritic and total sulfur contents of the tailings were 12.6% and 18.7%, respectively. The mean acid neutralization capacity (ANC) was 63.5 kg H 2SO 4/t while three oxidized tailings samples had an ANC less than zero. The NAG and net acid production potential (NAPP) values were 220 and 326 kg H 2SO 4/t, and both the NAG and NAPP results indicated that the tailings had a high acid-forming potential. NAG was more accurate than NAPP in predicting acid-forming potential of the tailings due to uncompleted oxidization of pyritic sulfur. Analysis of samples from two profile tests indicated that acidification mainly occurred at the surface (0–20 cm) and had little effects at deep layer of the tailings. Total concentrations of Pb, Zn, Cu, and Cd were increased greatly with depth at the acidified tailings profile, while heavy metal concentrations at different depths of nonacidified tailings profile were similar. The results indicated that depletion of heavy metals at the acidified surface was due to acidification. The diethylenetetramine pentaacetic acid (DTPA)-extractable Pb, Zn, Cu, and Cd concentrations of acidified tailings surface (0–20 cm) were significantly higher than those of nonacidified tailings, which further revealed that acidification enhanced the mobility of heavy metals in the tailings.