Sulfidic Waste Research Articles

AFRA – Heuristic expert system to assess the atmospheric risk of sulphide waste dumps

In 2006, an unprecedented atmospheric confined space accident took place in a sampling shed at the Sullivan Mine in Kimberley, British Columbia. This accident suggests that a risk assessment should be carried out on a regular basis at mine reclamation sites for many years after closure. In this paper, an Atmospheric Fuzzy Risk Assessment (AFRA) tool is described that can assess atmospheric risk given heuristic and measured data at such sites. It can also serve to transfer knowledge about atmospheric hazards in an enclosed structure. The system uses fuzzy logic to input and output information and to perform weighted inferencing. The paper describes the developmental process as well as system verification and validation based on a number of known test and reference waste dumps. AFRA is a heuristic expert system based on fuzzy logic and the first tool that was developed to assess the atmospheric risk of mine waste dumps. The atmospheric risk is estimated by fuzzy Mamdani system given the values of four major elements of risk comprising of: gas generation, gas emission, gas confinement, and human exposure. The ability of AFRA to adapt its risk assessment to different climate conditions is explained. There are many physical, chemical, and environmental factors which fluctuate over time affecting oxygen-depletion in waste dumps. AFRA can help mining engineers and mine managers recognize this type of danger when conducting a confined space inventory at a reclamation site.

The Diavik waste rock project: Initial geochemical response from a low sulfide waste rock pile

Three large-scale experimental waste rock piles (test piles) were constructed and instrumented at the Diavik Diamond Mine in the Northwest Territories, Canada, as part of an integrated field and laboratory study to measure and compare physical and geochemical characteristics of experimental, low sulfide waste rock piles at various scales. This paper describes the geochemical response during the first season from a test pile containing 0.053wt.% S. Bulk drainage chemistry was measured at two sampling points for pH, Eh, alkalinity, dissolved cations and anions, and nutrients. The geochemical equilibrium model MINTEQA2 was used to interpret potential mineral solubility controls on water chemistry. The geochemical response characterizes the initial flushing response of blasting residues and oxidation products derived from sulfides in waste rock exposed to the atmosphere for less than 1year. Sulfate concentrations reached 2000mgL−1 when ambient temperatures were>10°C, and decreased as ambient temperatures declined to<0°C. The pH decreased to<5, concomitant with an alkalinity minimum of<1mgL−1 (as total CaCO3), suggesting all available alkalinity is consumed by acid-neutralizing reactions. Concentrations of Al and Fe were<0.36 and<0.11mgL−1, respectively. Trends of pH and alkalinity and the calculated saturation indices for Al and Fe (oxy)hydroxides suggest that dissolution of Al and Fe (oxy)hydroxide phases buffers the pH. The effluent water showed increased concentrations of dissolved Mn (<13mgL−1), Ni (<7.0mgL−1), Co (<1.5mgL−1), Zn (<0.5mgL−1), Cd (<0.008mgL−1) and Cu (<0.05mgL−1) as ambient temperatures increased. Manganese is released by aluminosilicate weathering, Ni and Co by pyrrhotite [Fe1−xS] oxidation, Zn and Cd by sphalerite oxidation, and Cu by chalcopyrite [CuFeS2] oxidation. No dissolved metals appear to have discrete secondary mineral controls. Changes in SO4, pH and metal concentrations indicate sulfide oxidation is occurring and effluent concentrations are influenced by ambient temperatures and, possibly, increasing flow path lengths that transport reaction products from previously unflushed waste rock.

Adaptive predictive control of the sulfur recovery process at Pemex Cadereyta refinery

SUMMARYThis paper describes the design and application of an optimized control system, based on adaptive predictive expert control methodology, to the sulfur recovery units (SRUs) of the Pemex Refinery in Cadereyta, and presents comparative results obtained in relation to those of the conventional control system operating the plant. The sulfur recovery process represents the final stage in the refinery production chain, and is responsible for removing the sulfur content, usually in the form of hydrogen sulfide (H2S) waste gas generated by other refinery processes. The main goal is minimizing the sulfur content in the gasses released into the atmosphere to comply with emission level requirements. This goal can be obtained by a precise control of the ratio between hydrogen sulfide and sulfur dioxide (SO2) in the tail gas, which determines, through a Claus reaction, the maximum sulfur recovery. The SRUs operating in Cadereyta were connected to a common amine acid gas collector and a common ammonia acid gas collector. Optimized control strategies were designed to control the pressure on both acid gas collectors and the flow rate of air entering each SRU. The results show that the H2S:SO2 ratio in the tail gas is considerably more stable under optimized control, particularly in the presence of abrupt changes in the acid gas load. The peaks observed in this ratio under conventional control disappear under optimized control showing a significant improvement in the process operation. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Electrical and Seismic Tomography Used to Image the Structure of a Tailings Pond at the Abandoned Kettara Mine, Morocco

The Kettara site (Morocco) is an abandoned pyrrhotite ore mine in a semi-arid environment. The site contains more than 3 million tons of mine waste that were deposited on the surface without concern for environmental consequences. Tailings were stockpiled in a pond, in a dyke, and in piles over an area of approximately 16 ha and have generated acid mine drainage (AMD) for more than 29 years. Geophysical methods have been used at the Kettara mine site to determine the nature of the geological substrate of the tailings pond, the internal structure of the mine wastes, and to investigate the pollution zones associated with sulphide waste dumps. Electrical resistivity tomography (ERT) and seismic refraction data were acquired, processed, and interpreted; the results from ERT and seismic refraction were complementary. A topographical survey of the tailings disposal area was also undertaken to estimate the volume of wastes and quantify the AMD process. Two-dimensional inverse models were used to investigate the geophysical data and indicated alteration zones at depth. It was determined that the material could be classified into three categories: tailings, with low resistivity (5–15 Ω m) and low velocity (500–1,800 m/s); altered, black shales, with intermediate resistivity (20–60 Ω m) and velocity (2,000–3,500 m/s), and; materials with high resistivity and velocity (>60 Ω m and >4,000 m/s, respectively), including unaltered shales associated with quartzite seams. The low-resistivity zone generates AMD, which migrates downward through fractures and micro-fractures. The substrate is composed of broken and altered shale, which facilitates AMD infiltration.

The Diavik Waste Rock Project: Design, construction, and instrumentation of field-scale experimental waste-rock piles

The physicochemical processes that affect acid mine drainage (AMD) in unsaturated waste rock piles and the capabilities of small-scale laboratory experiments to predict AMD from waste rock are not well understood. An integrated laboratory and field study to measure and compare low sulfide waste rock and drainage characteristics at various scales has been initiated. This paper describes the design, construction and instrumentation of three field-scale experimental waste rock piles (test piles), and six active zone lysimeters at the Diavik diamond mine in the Northwest Territories, Canada. The test piles are comprised of granitic and sulfide-bearing metasedimentary waste rock excavated during open pit mining operations. One test pile contains waste rock with a target S content of <0.04wt.% S; the second test pile contains waste rock with a target S content of >0.08wt.% S; and the third test pile contains the higher sulfide waste rock (>0.08wt.% S) and was re-sloped and capped with a low permeability till layer and a low sulfide waste rock cover. The first two test piles are approximately 15m high with bases of 50m by 60m, and the re-sloped test pile has a larger base of 80m by 125m. Instrumentation was selected to measure matrix flow, geochemistry of pore water and drainage, gas-phase O2 concentration, temperature evolution, microbiological populations, waste rock permeability to air, and thermal conductivity, as well as to resolve mass and flow balances. Instrument locations were selected to characterize coupled physicochemical processes at multiple scales and the evolution of those processes over time. Instruments were installed at a density such that the number of instruments that survived construction (40% to >80% by instrument type) was sufficient to allow adequate characterization of the physicochemical processes occurring at various scales in the test piles.

Traditional agricultural practices enable sustainable remediation of highly polluted soils in Southern Spain for cultivation of food crops

This study relates elemental content of a range of edible crops grown in soils severely polluted by metals and metalloids as affected by traditional smallholder management practices. Five agricultural plots close to a sulfidic waste dump were monitored. Soil analysis demonstrated elevated concentrations of As, Cu, Pb and Zn that were greatly in excess of maximum statutory limits for agricultural soils in the studied region. The main vegetables (lettuce, chard, onion, potatoes) and lemon, together with their associated soils, were measured for elemental content. Extractable soil element concentrations were very low. There were differences in elemental accumulation between crops, but none exceeded statutory concentrations in edible parts. Soil–plant transfer factors were uniformly low for all elements and crops. It is concluded that traditional soil management practices (annual liming and application of animal manures) have created conditions for sustainable long-term safety use, with potential for multiple end-use, of these highly polluted soils

Phosphate amendment of metalliferous waste rocks, Century Pb–Zn mine, Australia: Laboratory and field trials

The aim of the study was to determine whether the application of phosphate compounds (phosphorite rock, phosphate fertilizer) to polyminerallic waste rocks can inhibit sulfide oxidation and metal mobility (Cu, Pb, Zn, Cd, Ni, Mn, Mg). Waste rocks comprised sulfidic carbonaceous shales and were sourced from the Century Pb–Zn mine, NW Queensland, Australia. The acid producing, Pb–Zn rich rocks consisted of major quartz, muscovite/illite, dolomite, siderite and kaolinite as well as smaller amounts of sulfide minerals (e.g. galena, sphalerite, pyrite). Laboratory leach experiments were conducted on finely granulated phosphate-treated waste rocks (>2 to <30 mm) over 13 weeks, whereas phosphate amendment of coarsely granulated waste rocks (sand to boulder size) was investigated using heap leach piles at the mine site over an 11 months period. Results of the laboratory experiments demonstrate that the treatment of finely granulated waste rocks with phosphorite rock produced leachates with near-neutral pH values due to calcite dissolution. This in turn did not allow the leaching of apatite, formation of secondary phosphate phases and phosphate stabilization to occur. Metal mobility in these amended wastes was restricted by the dissolution of calcite and the resultant near-neutral pH conditions. By contrast, the application of the water-soluble phosphate fertilizer MKP (KH2PO4) to polyminerallic sulfidic waste rocks during the short-term laboratory experiments led to the formation of phosphate coatings and precipitates and inhibited acid and metal release (Cd, Mn, Ni, Pb, Zn). At least in the short term, the application of phosphate fertilizers proved to be an effective method. However, results of the long-term field trials demonstrate that coarsely granulated waste rocks were not coated by secondary phosphate phases and that amendment by phosphorite rock or superphosphate fertilizer did not improve leachate quality compared to the unamended waste. Thus, phosphate stabilization appears ineffective in suppressing oxidation of sulfides in coarsely granulated mine wastes.

Diversity of Iron Oxidizing Bacteria from Various Sulfidic Mine Waste Dumps

More than 100 cultures of acidophilic Fe(II)- and/or sulfur-oxidizing microorganisms from mine waste dumps in 10 different countries all over the world have been maintained in liquid media in the BGR-strain collection for many years. Our 16S rDNA analysis showed that most of the cultivated Fe(II)-oxidizers belong to four genera: Acidithiobacillus, Acidimicrobium, “Ferrimicrobium” and Leptospirillum. All analyzed Acidithiobacillus strains were identified as At. ferrooxidans. The Leptospirillum strains were affiliated with L. ferriphilum or L. ferrooxidans. The Gram-positive strains related to Acidimicrobium or ”Ferrimicrobium” were phylogenetically more diverse than the strains of the genera Acidithiobacillus and Leptospirillum and fell into three separate clusters. While several strains could be identified as syngeneic (16S rDNA) with “Ferrimicrobium acidiphilum”, two other 16S rDNA clusters were distantly related and might represent new species or even new genera. In addition, one new Sulfobacillus strain and one new Alicyclobacillus strain could be identified. Furthermore several strains related to Acidiphilium acidophilum have been detected and form one 16S rDNA cluster.

Process Decisions Focused on the Prevention of AMD Formation on Beneficiating Sulfide Minerals

Acid Mine Drainage (AMD) from voluminous sulfide-bearing wastes contributes to the environmental burden of mineral processing. To diminish burden and its time frame of risk, the disposal of sulfidic waste materials is re-examined through the valuable distinction between reactive or active gangue materials contributing to AMD generation and largely unreactive passive gangue. In an example process for disposal of tails from mineral sulfide beneficiation, a proof of concept is provided for inclusion of a non-selective sulfide float preceding or following selective separation of the desired mineral sulfide. The environmental benefit of subsequent reduction of sulfide in the tailings from 3.7 to 0.2% on AMD generation potential is quantified through a combination of methodologies including MPA, ANC, NAPP, NAG and microbial testing. Sulfide reduction from 4 to 1% reduces AMD generation. Further reduction below 1% S yields diminishing returns. The environmental benefit of separating ‘active’ and ‘passive’ gangue material prior to disposal is shown, and the reduced active gangue fraction determined through material balance.

Evaluation of heavy metal bio-availability from Almagrera pyrite-rich tailings dam (Iberian Pyrite Belt, SW Spain) based on a sequential extraction procedure

The beginning of polymetallic sulphide ore exploitation at Almagrera, Iberian Pyrite Belt, SW Spain, has been dated back to pre-Roman times; mining activity lasted until 2001 when it was definitely halted. The Almagrera sulphide tailings dam was built at the Sotiel–Coronada mine, where Cu, Zn and Pb concentrates were obtained by flotation processes, and sulphide (mainly pyrite) wastes formed. The dam was built as a water-reservoir, on a tectonic fault; its hazard is currently documented by the released wastes after a strong storm in the region, then, its stability is not monitored by the owner. The present study focuses on the pollutant potential of the Almagrera sulphides tailings, and estimates the environmental impact (volume affected) that would be caused by a hypothetical dam collapse. The chemical speciation of potentially toxic elements (As, Cd, Cr, Cu, Fe, Mn, Pb, S, Sb and Zn) in the sulphide tailings was performed by modified BCR sequential extraction tests. Results show that the most pollutant fraction in these wastes consists of metals bounded to sulphides and released under oxidizing conditions in acid mine drainage (AMD) production processes. Part of this fraction is temporally retained in solid phases by precipitation of soluble secondary minerals in warm periods, but such minerals are easily dissolved in rainy periods. Taking into account the amount of pollutants in the bio-available fraction (capability of these polluting elements to be absorbed or become available for the human body), and GIS modelling, the mining wastes from Almagrera could release to the environment up to e.g. 6960 tons (t) of Fe, 5857 t of S, 2756 t of Zn, 2463 t of Pb, 741 t of Cu, 633 t of Mn, and 589 t of As. Although these estimates would be affected by seasonal variations, high amounts should be expected every year by the continuous oxidation of the sulphide fraction. The supposed toxic spill would have a dramatic impact, like the one that occurred in 1998 at Aznalcóllar, an abandoned mining site showing strong similarities with Almagrera.

Field scale remediation of mine wastes at an abandoned gold mine, Australia II: Effects on plant growth and groundwater

This second paper reports the results of plant growth, plant mortality, plant leaf tissue metal and salt concentrations and leachate quality monitoring from lysimeters in four large field trial treatments established on sulfidic waste rock/soil that was used for haul road construction at a closed gold mine in Australia. The TerraB™, lime and clay treatments allowed good tree growth of four Eucalypt species, compared to the control. There was no statistical difference in tree growth between the TerraB™, lime or clay treatments over the 2 years of monitoring in this paper. However, the growth of one tree species was poor in the TerraB™ treatment. Leaf tissue metal and major ion data are also presented. Leachate pH in the control became increasingly acidic (pH 4.57–3.95). The addition of Ca(OH)2 and biosolids led to an initial increase in leachate pH, compared to the control; however, this has decreased over the duration of the study (pH 5.37–4.89) and may affect the sustainable growth of plants in the future. In the TerraB™ and biosolids treatment leachate pH increased to 6.92 after the first rainfall event and continued to increase over the duration of the study to pH 7.4 after 24 months. After 24 months average heavy metal leachate concentrations (mg/L) in the lysimeters for Al, Cd, Cu, Mn and Zn were, control: 32.55, 5.67, 12.71, 39.29, 121.80, TerraB™: 0.07, 0.02, 0.07, 0.57, 0.23, and lime: 2.19, 1.19, 2.33, 3.6, 28.4. No leachate was available for collection from the clay treatment indicating that this technique was functioning in terms of minimizing the infiltration of water into the mine soil.

Quantification of Microorganisms Involved in Cemented Layer Formation in Sulfidic Mine Waste Tailings (Freiberg, Saxony, Germany)

Cemented layers predominantly consisting of gels/poorly crystalline mineral phases have been formed as a consequence of mineral weathering in sulfidic tailings near Freiberg, Saxony, Germany. These layers function as natural attenuation barrier for toxic compounds and reduce oxidation and erosion processes of tailings surfaces. Quantitative molecular biological and cultivation methods were applied to investigate the role of microorganisms for mineral weathering and cemented layer formation. High resolution depth profiles of numbers of microorganisms showed maximal cell numbers in the oxidation zone where cemented layers had been formed. Highest total cell numbers of &gt;109 cells g-1 dry weight (dw) were detected by SybrGreen direct counting. Using quantitative real-time PCR (Q-PCR) between 107 and 109 Bacteria g-1 dw and up to 108 Archaea g-1 dw were determined. As well high numbers of cultivable and living Bacteria could be detected by MPN (most probable number) for Fe(II)- and S-oxidizers and CARD-FISH (catalyzed reporter deposition - fluorescence in situ hybridization). Overall, the high numbers of microorganisms determined with various quantification techniques argue for a significant role of microorganisms in cemented layer formation due to microbial mineral weathering. It is hypothesized that EPS (extracellular polymeric substances) mediate the formation of secondary mineral phases.

Immobilization of toxic elements in mine residues derived from mining activities in the Iberian Pyrite Belt (SW Spain): Laboratory experiments

In the mining environments of the Iberian Pyrite Belt (IPB), the oxidation of sulphide wastes generates acid drainage with high concentrations of SO 4, metals and metalloids (Acid Mine Drainage, AMD). These acid and extremely contaminated discharges are drained by the fluvial courses of the Huelva province (SW Spain) which deliver high concentrations of potentially toxic elements into the Gulf of Cádiz. In this work, the oxidation process of mine tailings in the IPB, the generation of AMD and the potential use of coal combustion fly ash as a possible alkaline treatment for neutralization of and metal removal from AMD, was studied in non-saturated column experiments. The laboratory column tests were conducted on a mine residue (71.6 wt% pyrite) with artificial rainfall or irrigation. A non-saturated column filled solely with the pyrite residue leached solutions with an acid pH (approx. 2) and high concentrations of SO 4 and metals. These leachates have the same composition as typical AMD, and the oxidation process can be compared with the natural oxidation of mine tailings in the IPB. However, the application of fly ash to the same amount of mine residue in another two non-saturated columns significantly increased the pH and decreased the SO 4 and metal concentrations in the leaching solutions. The improvement in the quality of leachates by fly ash addition in the laboratory was so effective that the leachate reached the pre-potability requirements of water for human consumption under EU regulations. The extrapolation of these experiments to the field is a promising solution for the decontamination of the fluvial courses of the IPB, and therefore, the decrease of pollutant loads discharging to the Gulf of Cádiz.

English

Source control of acid rock drainage (ARD) requires consideration of both biological and abiotic mechanisms of metal sulfide oxidation. A promising approach is to couple a biocide with phosphate for application to sulfidic waste materials. This approach aims to 1) inhibit or kill with thiocyanate (SCN) the iron-oxidizing acidophilic microorganisms that accelerate sulfide oxidation and 2) precipitate FePO4 and AlPO4, thereby removing ferric ion oxidant and Lewis acidity, and, in the process, armoring the surface of pyrite to retard its abiotic oxidation. Thiocyanate effectively reduces sulfide biooxidation if it is applied efficiently and is not washed out of the system by rainfall or adsorbed to the rock. Several sources of phosphate including phosphate rock, waste material and agricultural products were characterized and tested in combination with SCN for their ability to retard oxidation of sulfidic waste rock. Waste rock sources include samples from base metal and precious metal mines and a coal mine. The acid neutralizing capacities (ANC) of different sources of phosphate were compared and evaluated using an artificial ARD solution which included iron and aluminum sulfates. Thiocyanate alone in laboratory tests sharply reduced ARD production, approaching the abiotic rate of sulfide oxidation. Whether thiocyanate plus phosphate further reduced ARD production depended on the mineral sulfide composition of the waste rock and the parameter measured. The abiotic sulfide oxidation rate of sphalerite-containing waste rock was reduced with phosphate treatment, most likely by removal of residual ferric iron from leach solutions. Phosphate did not further reduce abiotic sulfide oxidation rates with a pyritic waste rock, but did significantly reduce the soluble iron and other metal content of leachates. The combined thiocyanate plus phosphate treatment minimized biooxidation, removed ferric ion oxidant, and restricted formation of Lewis acidity but has not shown evidence of armoring pyrite. The laboratory test work guided the set up and operation of a 3000 t field test. ARD production was reduced over 50% in the first season with combined thiocyanate and phosphate treatment. However, washout of thiocyanate and its adsorption to rock reduced its concentrations in leachates to near zero in the second year of the test, greatly reducing the effectiveness of chemical treatment. Water soluble forms of thiocyanate (NaSCN) are best used to reduce ARD in situations where rainfall infiltration is low. The development of slowor controlled-release thiocyanate products combined with phosphates would be beneficial. Additional key words: ARD prevention, phosphate, thiocyanate ______________________ 1 Paper presented at the 7 th International Conference on Acid Rock Drainage (ICARD), March 26-30, 2006, St. Louis MO. R.I. Barnhisel (ed.) Published by the American Society of Mining and Reclamation (ASMR), 3134 Montavesta Road, Lexington, KY 40502 2 Gregory J. Olson, Vice President, Thomas R. Clark, Senior Scientist, Little Bear Laboratories, Inc., Golden, CO 80403; Terry I. Mudder, Managing Director, TIMES, Ltd., Sheridan, WY 82801; Mark Logsdon, Principal Geochemist, Geochimica Inc., Aptos, CA 95003. 7 th International Conference on Acid Rock Drainage, 2006 pp 1435-1452 DOI: 10.21000/JASMR06021435

English

Acidic and Metalliferous Drainage (AMD) from sulfidic waste rock piles is a major issue facing the mining industry worldwide. Existing dry cover systems are designed to minimize the acid load discharged from waste rock piles by lowering infiltration rates. The strategic blending of alkaline amendments, such as limestone, within waste rock piles is also used to minimize the acid load discharged. The benefit of this blending is limited in most cases due to armoring (passivation) of limestone by precipitates. Armoring can be overcome by placing limestone on top of the waste rock piles, where it only interacts with rain water. Alkalinity released from the limestone-bearing covers can react with acid and metallic salts along preferential flow pathways within a waste rock pile to create inert, precipitate-coated channels that inhibit further reaction (ie. acid release). Due to the low solubility and slow dissolution rates of limestone, acid generation is not completely prevented and may still occur within the waste rock pile. Nevertheless, an overall reduction in the physical interaction between water and acid producing materials can sometimes be achieved using limestone-bearing covers. This mechanism has proven successful at the Freeport mine in Indonesia, which is characterized by very high rainfall. However, the effectiveness of limestone-bearing covers is limited under most climatic conditions, as a result of the low solubility and slow dissolution rates of limestone in near-neutral rainwater. New magnesium-based materials with superior solubility and dissolution rate characteristics to limestone have been developed for use in waste rock covers. These materials will permit the controlled release of alkalinity to infiltrating rainwater over a wide range of climatic conditions. The required amount of the magnesium-based alkalinity producing material in a waste rock cover will be insignificant relative to the amount of acid producing material in the waste rock. As a result, the magnesium-based cover materials represent an innovative and cost-effective solution to minimization, rather than treatment, of acid drainage from waste rock piles. The new materials being developed are based on calcium-enriched caustic magnesia (MgO) with controlled calcination grades and grain sizes to maximize both solubility and dissolution kinetics. Alkalinity concentrations of up to 510 mg/L CaCO3 equivalent in pure water can be achieved with the new materials (c.f. only 10-15 mg/L for limestone), with a typical saturation pH of 9.0-9.5 (c.f. only 8.0-8.5 for limestone). Compared to limestone it is envisaged that relatively small amounts of these new magnesium- based materials can be deployed within existing and new cover systems to minimize short, medium and long term acid discharges, thereby providing a significant advance in the control of AMD (acid and metalliferous drainage) from waste rock piles.

Variably Saturated Reactive Transport of Arsenic in Heap‐Leach Facilities

A variably saturated reactive transport software package, UNSATCHEM, has been modified for the purposes of simulating As oxyanion transport in gold mine heap‐leach facilities. Gold‐ore deposits in the Carlin Trend, located in northeastern Nevada and western Utah, are characterized in part by the occurrence of As‐bearing minerals. The large‐scale mining methodology used on the Carlin Trend includes the use of large‐scale heap‐leach facilities in which an extractive solution is used to dissolve the gold from the ore. This solution also is capable of dissolving other minerals, including those that contain As. While these facilities are generally environmentally benign compared with sulfide waste rock dumps associated with other mining provinces around the world, controlling the flux of As from the heap‐leach facility after mining has ceased is of interest to the mining and regulatory communities. We present three examples that use the As transport capable version of UNSATCHEM to simulate the effect of three commonly applied heap‐leach facility closure methodologies. The results demonstrate the importance of including geochemistry in transport simulations and illustrate the highly nonlinear behavior that is produced from a coupled flow and reactive geochemistry numerical solver.

Hydrochemistry, mineralogy and sulfur isotope geochemistry of acid mine drainage at the Mt. Morgan mine environment, Queensland, Australia

Mineralogical, hydrochemical and S isotope data were used to constrain hydrogeochemical processes that produce acid mine drainage from sulfidic waste at the historic Mount Morgan Au–Cu mine, and the factors controlling the concentration of SO 4 and environmentally hazardous metals in the nearby Dee River in Queensland, Australia. Some highly contaminated acid waters, with metal contents up to hundreds of orders of magnitude greater than the Australia–New Zealand environmental standards, by-pass the water management system at the site and drain into the adjacent Dee River. Mine drainage precipitates at Mt. Morgan were classified into 4 major groups and were identified as hydrous sulfates and hydroxides of Fe and Al with various contents of other metals. These minerals contain adsorbed or mineralogically bound metals that are released into the water system after rainfall events. Sulfate in open pit water and collection sumps generally has a narrow range of S isotope compositions ( δ 34S = 1.8–3.7‰) that is comparable to the orebody sulfides and makes S isotopes useful for tracing SO 4 back to its source. The higher δ 34S values for No. 2 Mill Diesel sump may be attributed to a difference in the source. Dissolved SO 4 in the river above the mine influence and 20 km downstream show distinctive heavier isotope compositions ( δ 34S = 5.4–6.8‰). The Dee River downstream of the mine is enriched in 34S ( δ 34S = 2.8–5.4‰) compared with mine drainage possibly as a result of bacterial SO 4 reduction in the weir pools, and in the water bodies within the river channel. The SO 4 and metals attenuate downstream by a combination of dilution with the receiving waters, SO 4 reduction, and the precipitation of Fe and Al sulfates and hydroxides. It is suggested here that in subtropical Queensland, with distinct wet and dry seasons, temporary reducing environments in the river play an important role in S isotope systematics.

Effectiveness of various cover scenarios on the rate of sulfide oxidation of mine tailings

Long term environmentally sound disposal of the millions of tons of mining residue is a serious challenge to the international mining industry. This paper evaluates, through a numerical investigation, the potential performance of desulfurized tailings as a cover material for the reduction of acidic drainage from sulfidic tailings. This evaluation is facilitated through a comparison of various cover types as decommissioning options. The cover types considered consist of a desulfurized tailings material cover exposed to ambient climate conditions, a water cover (flooded tailings), and a combination cover type. As part of the evaluation of cover performances, the effect of climatic variability on the potential rate of sulfide oxidation in tailings with an open ground surface, was also assessed. The numerical analysis was conducted using the model PYROX, which was modified to allow for variably-saturated conditions, time varying moisture contents, and to account for the temperature dependence of Henry's law and gas diffusion. In the case study presented here, the benign cover material consists of a low sulfide waste stream (cassiterite float tails, CFT), a by-product of the production of tin concentrate (cassiterite, SnO 2). Modelling results after a simulation period of 100 years indicate that a water cover alone or an exposed CFT cover alone are both less effective options than the combined cover type. A water cover alone leads to a reduction of approximately 99.1%, in the oxidation rate relative to uncovered tailings while the combined cover type results in the lowest potential extent of sulfide oxidation after mine closure-an approximately 99.8% reduction. Importantly, a CFT cover exposed to ambient environmental conditions can still substantially reduce the sulfide oxidation rate, by approximately 75–82% over a 100-year time period, relative to uncovered tailings. Variability in precipitation (and hence percent saturation of the surface layer) had less of an effect on the potential sulfide oxidation rate than did the cover type. The performance of the exposed CFT cover varied by less than 10%, within the range of climatic conditions expected at the Renison Bell mine site in southwest Tasmania, Australia. Although the modelling results indicate that the combined water and CFT cover is the best option, this approach achieves only a minor improvement over the water cover alone.

Large-scale experiments for microbiological evaluation of measures for safeguarding sulfidic mine waste

Large-scale experiments for microbiological evaluation of measures for safeguarding sulfidic mine waste

Geochemical trends in metal-contaminated fiord sediments near a former lead–zinc mine in West Greenland

Disposal of sulfidic waste in marine environments implies an environmental risk due to potential release and spreading of heavy metals to sediments and biota on a regional scale. However, tailings disposal in marine systems is practised in several places. Fiord sediments near Black Angel Mine in West Greenland are contaminated by Pb and Zn as a result of mining activity and marine disposal in the period 1973–1990. Chemical analyses were performed on 6 fiord sediment cores collected up to 10 km away from the disposal area and included heavy metal analysis, high-resolution stable Pb isotope stratigraphy and radiochemical dating ( 210Pb). The results were used for evaluating spatial and temporal distribution of anthropogenic heavy metals from the marine disposal. A significant accumulation of Pb and Zn on a regional scale is still observed 9 years after mine closure. Stable Pb-isotope data provide a unique fingerprint of the mining-related Pb sources in the area. Today spreading of mining-related Pb up to 10 km away from the mining area accounts for more than 80% of total Pb in sediment deposited within the last 100 years.