Barium Carbonate Process for Sulphate and Metal Removal from Mine Water

Abstract.  The removal of sulphate and metals from mine water was assessed using the integrated barium carbonate process and the co-precipitation of barium sulphate with calcium carbonate. The rate of sulphate removal was influenced by the BaCO₃ concentration and the cation associated with sulphate, and increased with increasing BaCO₃-concentration. BaCO₃ can only be used for removal of sulphate that is associated with calcium, as calcium is needed to remove the added carbonate associated with the barium cation. Sulphate removal was only marginally influenced by alkalinity. Sulphide can be stripped with CO₂ from a BaS-solution. The (CO₂ dosed/sulphide removed) molar ratio was close to unity for the first 50% of sulphide in solution. The stripped H₂S-gas can be absorbed into zinc acetate. BaSO₄ and CaCO₃ can be converted simultaneously to BaS and CaO, respectively at an optimum temperature of 1050°C. The CaCO₃/BaSO₄ molar ratio has little influence on the yield of BaS. The running cost of the barium carbonate process for the removal of 2 g/L of sulphate totalled ZAR1.28/m³ (US$1.00 = ZAR7.0, Feb 2007), the capital redemption cost was R1.08/m³, and the value of the products (water and sulphur) totalled R2.76/m³.     

Treatment of Mine Water for Sulphate and Metal Removal Using Barium Sulphide

Abstract.   The integrated barium sulphide process consists of: preliminary treatment with lime, sulphate precipitation as barium sulphate, H₂S-stripping, crystallization of CaCO₃, and recovery of barium sulphide. Our tests showed that during lime pre-treatment, sulphate was lowered from 2 800 mg/L to 1 250 mg/L by gypsum crystallization; metals were precipitated as hydroxides. The BaS treatment then lowered sulphate to less than 200 mg/L. Sulphide was lowered from 333 to less than 10 mg/L (as S) in the stripping stage, using CO₂ gas for stripping. The stripped H₂S-gas was contacted with Fe (III)-solution and converted quantitatively to elemental sulphur. The alkalinity of the calcium bicarbonate-rich water was reduced from 1 000 to 110 mg/L (as CaCO₃) after CO₂-stripping with air due to CaCO₃ precipitation. Fe (II), after sulphur production, was re-oxidized to Fe (III) using an electrolytic step. The running cost of the BaS process is R2.12/m³ (US$1 = SAR6.5) for the removal of 2 g/L of sulphate.     

Removal of Sulphate, Metals, and Acidity from a Nickel and Copper Mine Effluent in a Laboratory Scale Bioreactor

Abstract  Mine effluents should be treated so that they can either be re-used (e. g. for mining activities or irrigation purposes) or discharged into a river system. The results of this study showed that applying laboratory scale biological sulphate removal technology to a nickel/copper mine effluent (BCL mine, Botswana) consistently lowered sulphate concentrations from an average of 2000 to 450 mg/L, and increased the pH from 5.8 to 6.5. During this period, the hydraulic retention time varied from 24 to 12 h. The Ni and Zn concentrations were reduced from a maximum of 5.86 to 0.15 mg/L and from a maximum of 38 mg/L to 0.03 mg/L, respectively, presumably precipitated as metal sulphides.     

Design Criteria for Limestone Neutralization at a Nickel Mine

Abstract.   Design criteria were developed for the construction of a full-scale limestone neutralization plant to treat leachate from the waste rock of a nickel mine, using data from laboratory studies, pilot-scale studies, and operation of a full-scale limestone handling and dosing facility. We learned that: limestone powder can be slurried to a constant slurry density of 60 g/L; Fe (II) can be oxidised at low pH (2.5) at a rate of 16.1 g/(L/d) using geotextile as a medium; and that the integrated Fe (II)-oxidation and limestone neutralization process allows neutralization, Fe (II)-oxidation, and gypsum crystallization to take place at the same time, provided that the solids concentration is high (greater than 30 g/L). A full-scale plant with a capacity of 50 m³/h was designed and constructed. The plant consists of the following stages: biological Fe (II)-oxidation, a fluidised-bed limestone neutralization reactor, a complete-mix gypsum crystallization reactor, and a clarifier.     

Neutralizing Coal Mine Effluent with Limestone to Decrease Metals and Sulphate Concentrations

Abstract.   This paper describes pilot scale tests of a novel process for the neutralisation of acidic mine water. Leachate from a waste coal dump was neutralised with limestone, and iron, aluminium, and sulphate were removed. Specific aspects studied were: the process configuration; the rates of iron oxidation, limestone neutralisation, and gypsum crystallisation; the chemical composition of the effluents before and after treatment; the efficiency of limestone utilisation; and the sludge solids content. The acidity was decreased from 12,000 to 300 mg/L (as CaCO₃), sulphate from 15,000 to 2,600 mg/L, iron from 5,000 to 10 mg/L, aluminium from 100 to 5 mg/L, while the pH increased from 2.2 to 7.0. Reaction times of 2.0 and 4.5 h were required under continuous and batch operations respectively for the removal of 4 g/L Fe (II). The iron oxidation rate was found to be a function of the Fe (II), hydroxide, oxygen, and suspended solids (SS) concentrations. The optimum SS concentration for iron oxidation in a fluidised-bed reactor was 190 g/L. Up-flow velocity had no influence on the rate of iron oxidation in the range 5 to 45 m/h. Sludge with a high solids content of 55% (m/v) was produced. This is high compared to the typical 20% achieved with the high density sludge process using lime. It was determined that neutralisation costs could be reduced significantly with an integrated iron oxidation and limestone neutralisation process because limestone is less expensive than lime, and a high-solids-content sludge is produced. Full scale implementation followed this study.     

Optimizing the Effluent Treatment at a Coal Mine by Process Modelling

Abstract:   The water network of a coal mine was audited and simulated by an interactive steady state model and the results were used to optimise the mines water management strategy. Simulation of the interactions showed that calcium carbonate powder could be used as an alternative to lime for neutralization of acid water at a reagent cost saving of 56%. Gypsum crystallization would reduce sulphate concentrations in the neutralization plant by 30% and in the coal processing plant by 60%. The capital cost for a neutralization/gypsum crystallization plant for separate treatment of coal discard leachate and less polluted streams would cost 3.0 million Rand (R), compared to R10.3 million for combined treatment. Only slightly less (8.9 t/d vs. 9.5 t/d) sulphate removal would be achieved during separate treatment. The over-saturation index (OSI) value can be controlled effectively by removing sulphate from the feed water for coal processing. Sulphate has to be lowered to 350 mg/L in a flow of 222 m³/h to obtain an OSI value less than 1. The capital cost of a 222 m³/h biological sulphate removal plant was estimated at R21.8 million (R4.1 million/(ML/d)); the running cost was estimated at R13.7 million/a (R4.10/m³). Pre-washing of the coal would reduce capital and running costs.     

Anaerobic Bioremediation of Acid Mine Drainage using Emulsified Soybean Oil

Abstract  Batch incubation and flow-through column experiments were conducted to evaluate the use of emulsified soybean oil for in situ treatment of acid mine drainage. Addition of soybean oil, soluble substrates, and a microbial inoculum to the batch incubations resulted in complete depletion of SO₄, 50% reduction in Fe, and an increase in pH to >6. A one time injection of emulsified soybean oil, lactate, yeast extract, and a microbial inoculum stimulated SO₄ and metal ion reduction for ≈300 days in laboratory columns packed with mine tailings receiving influent solutions with a pH≈3 and≈5. In all emulsion treated columns, SO₄ and Fe were reduced, pH increased to >6, and Al, Cu and Zn removal efficiency was 99% or greater. Cu, Fe, Mn and Zn were removed as metal sulfides and/or carbonates with removal efficiency decreasing with increasing metal sulfide solubility. The low pH and high heavy metals concentrations did not significantly inhibit biological activity. However, SO₄ removal with associated precipitation of metal sulfides may have been limited by the short hydraulic retention time (6-7 days) of the columns. There was a significant hydraulic conductivity loss in one of the four treated columns, indicating that hydraulic conductivity loss may be an issue under certain conditions.     

A Bench-scale Assessment of a Combined Passive System to Reduce Concentrations of Metals and Sulphate in Acid Mine Drainage

Abstract  Passive treatment of acid mine drainage (AMD) requires a combined strategy to minimize the effect of climatic variability on the treatment performance of the system. A vertical-flow combined passive treatment system was developed and evaluated in a bench-scale laboratory test for a 290-day period. The combined system consisted of four components with specific treatment functions: an oxidation/precipitation basin for excess iron removal; a peat biofilter for heavy metal sorption and the establishment of anoxic conditions; a bioreactor for alkalinity generation and sulphate reduction; and an anoxic limestone drain for alkalinity addition. The benchscale system was dosed with moderate strength synthetic AMD at a surface loading of 95 L/m²/d, and operated under continuous flow conditions. Removal efficiencies were 99.7%, 99.9%, 99.9%, 98.6%, 98.2%, and 99.9% for Fe, Al, Zn, Mn, Ni, and Cu, respectively, while Cd remained more mobile with a removal efficiency of 66.5%. Sulphate concentrations were reduced from 3030 mg/L to 814.9 mg/L and the acidic drainage was neutralized to an effluent pH of 7.2 and an alkalinity of 1353.6 mg/L (as CaCO₃).     

Improving the Accuracy of Geochemical Rock Modelling for Acid Rock Drainage Prevention in Coal Mine

Abstract.   The results of static tests are used to geochemically model the distribution of potentially acid and non-acid forming materials and plan mining excavation and overburden dumping to prevent or minimize the generation of acid rock drainage (ARD). The accuracy of the model depends very much on the amount and validity of the available pre-mine data and how the data is interpreted in both lateral and vertical directions. This results of such modelling was compared with subsequent overburden information provided by analysis of blast hole drill cuttings. We found that the model overestimated the amount of potentially acid forming material, but that it was still useful in ARD prevention.     

Pilot-scale Studies of Different Covers on Unoxidised Sulphide-rich Tailings in Northern Sweden: the Geochemistry of Leachate Waters

Abstract.  Leachate water quality from covered and uncovered unoxidised sulphide-rich tailings in six pilot-scale (5x5x3 m³) test cells was monitored during 2004 and 2005. The covers consisted of a layer of clayey till, sewage sludge, apatite or Trisoplast (a commercial mixture of tailings, bentonite, and a polymer). All layers were protected by an unspecified till except in one reference cell, where the tailings were left open. All leachate waters showed near-neutral pH as a result of neutralization by calcite in the tailings and by Ca(OH)₂ added prior to deposition. Average dissolved sulphur concentrations in the leachates were ≈ 600 mg L^-1, except in the cell with sewage sludge (300 mg L^-1). The source of sulphur was mainly pyrite oxidation, but residual sulphur probably remained from the enrichment process. The near-neutral pH favoured precipitation of metal-(oxy)hydroxides with subsequent removal of trace elements such as Cd, Cu and Pb (< 15 μg L^-1) from the solutions. High concentrations of Co, Mn, Ni, and Zn were found in leachates from the apatite, Trisoplast, and uncovered tailings cells. High As concentrations were found in the leachates in the sewage sludge and clayey till cells. The lowest metal concentrations, redox potential, and highest pH were found in the sewage sludge cell. Decreased elemental metal concentrations during 2004 suggest improved performance over time.     

Chemical and Biological Oxidation of Iron in Acid Mine Water

Abstract.   Prior to limestone neutralization of acid water, ferrous iron needs to be oxidized to prevent downstream oxidation and the formation of acid. This study assessed the effect of various parameters on the biological and chemical rate of iron oxidation, both chemically and biologically. In batch experiments, it was found that although the use of support media had no effect on the chemical iron oxidation rate, it was important when iron was oxidised biologically. Under continuous flow conditions, the highest oxidation rate occurred when the initial Fe (II) concentration was between 4.5 to 4.8 g/L, geotextile was used as the support medium, and when nutrients were added to the reactor. The optimal iron oxidation rate was achieved at a hydraulic retention time of 8 h. The chemical iron oxidation rate depends on the concentration of suspended Fe(OH)₃ and CaSO₄, which act as catalysts. The biological iron oxidation rate was dependent on the bacterial growth, which was influenced by several parameters (support media, nutrients, CO₂, and oxygen).     

Modelling the Long-term Release of Sulphate from Dump Sediments of an Abandoned Open Pit Lignite Mine

Abstract.  The generation of acid drainage from overburden spoil piles at open-pit lignite mines impacts water quality in large parts of the Lusatian mining area in Germany. The Lohsa Mine was exploited until the early 1990s and is to be flooded by 2005. It will then be used as a reservoir basin for the river Spree. Future acidity and sulphate concentrations in the surface water are of great interest because considerable amounts of the bank filtrate of the river are used to supply drinking water to communal water plants downstream. In our study, the input of sulphate from the unsaturated zone of the heap into the groundwater was calculated using the one dimensional reactive transport code SAPY. The SAPY program, which had been calibrated for effective diffusion and tortuosity using oxygen breakthrough curves of a column experiment with original heap sediments, was scaled up to field conditions and verified by measuring the oxygen and sulphate profile of the heap. Scenarios for a period of 80 years were simulated for different distances of the groundwater level to the subsurface, and the mass input of sulphate from the unsaturated zone into the groundwater was calculated in terms of specific fluxes for different times. Plans are to use the calculated source terms in a regional three-dimensional model to predict the evolution of the ground- and surface water in the area.     

Acid Mine Drainage Flowing from Abandoned Gas Wells

Abstract  In northwestern Pennsylvania (USA), numerous abandoned natural gas wells are producing artesian flows of Fe-contaminated water. The origin of the polluted water has been generally assumed to be brines from the gas-producing sands. We sampled 20 artesian discharges where iron staining was conspicuous. The waters were not brines, but were more characteristic of acid mine drainage (AMD). The dominant cations were Fe, Ca, and Mg, while the dominant anion was sulfate. The study area has a long history of coal mining in the lower Allegheny formation; however, the coal beds are generally at higher elevations than the discharges. We propose that AMD formed in the coal mines is infiltrating into lower aquifers, moving outside the lateral limits of mining, and using abandoned gas wells as conduits to the surface. While flowing through the underlying sandstones, the AMD chemistry is modified by contact with siderite, the dominant carbonate mineral in this stratigraphy. This would suggest that current remediation strategies that emphasize plugging the pollution-producing gas wells may be ill-advised because the source of the polluted water is more shallow than currently assumed.     

The Influence of Coal Quality Variation on Utilization of Water from Abandoned Coal Mines as a Municipal Water Source

Abstract.   As population increases and high quality water becomes more difficult to obtain, many communities will seek alternative water supply sources. Some municipalities have realized that they have a reservoir of unexploited water readily available in abandoned underground coal mines. Analysis of the mines history, the quality of the coal and water that reside within the mine, and knowledge of local hydrology, geology, and mine chemistry will provide communities with the information they need to determine the best mine sites to use.     

Performance of a Natural Wetland Treating Acid Mine Drainage in Arid Conditions

Abstract  A wetland naturally formed in the discharge from a copper mine tailing impoundment in Rajasthan, India. The wetland is abundantly vegetated. This study investigated changes that occurred in the seepage as it travelled 180 and 380 m (P₁ and P₂) through the wetland. The pH increased from 6.17 to 7.10 at P₁ and 7.34 at P₂ in the pre-monsoon season, 6.53 to 7.36 at P₁ and 7.77 at P₂ in the post-monsoon season, and from 6.20 to 6.63 at P₁ and 6.89 at P₂ in the winter. Contaminant removal at P₂ ranged from 40 to 95%.     

Waste from Biodiesel Manufacturing as an Inexpensive Carbon Source for Bioreactors Treating Acid Mine Drainage

Abstract.  Alcohol-fed, semi-passive bioreactors have been used to support the growth of sulfate-reducing bacteria (SRB) for treatment of acid drainage from mine sites. An alcohol source not previously examined for use in these reactors is the glycerol-methanol waste remaining after the production of biodiesel fuel. In the laboratory, rock-filled columns were used to investigate biodiesel waste (BDW) as a carbon source for SRB. Columns were provided with water containing 900 mg/L sulfate, and fed reagent-grade glycerol or BDW in sufficient quantity to reduce 50% of the sulfate. Addition of 246 mg/L of reagent-grade glycerol resulted in 50% sulfate reduction and production of up to 59 mg/L of soluble sulfide, while the equivalent of 246 mg/L of glycerol provided as BDW resulted in 55% sulfate reduction and the production of up to 92 mg/L of soluble sulfide. During the initial stages of acclimation, propionic, acetic, formic, and lactic acids were observed. Acid concentrations were reduced over time in the effluent, and organic carbon in the BDW was nearly completely converted to carbon dioxide.     

Using the DRASTIC System to Assess the Vulnerability of Ground Water to Pollution in Mined Areas of the Upper Silesian Coal Basin

Abstract  An attempt was made to use the U.S. EPA DRASTIC ranking system to assess the vulnerability of ground water in the Upper Silesian Coal Basin. Analysis of the various system components indicate that several DRASTIC factors would have to be modified to consider the effects of mining, subsidence, and ground water rebound.     

Prediction of Cadmium Removal Using an Artificial Neural Network and a Neuro-Fuzzy Technique

Abstract.  The prediction of adsorption of cadmium by hematite using an adapted neural fuzzy model and a back propagation artificial neural network was compared. Adsorption was found to depend on the Cd concentration, agitation rate, temperature, pH, and the particle size of the hematite. The adaptive neuro-fuzzy inference system proved to be more efficient in predicting Cd adsorption than a single layered feed forward artificial neural network.     

Removal of Sulfate,Zinc, and Lead from Alkaline Mine Wastewater Using Pilot-scale Surface-Flow Wetlands at Tara Mines,Ireland

Abstract.    Passive treatment systems have primarily been used at abandoned mines to increase pH and remove metals from the drainage water. Two pilot-scale treatment wetlands were constructed and monitored at an active lead/zinc mine (Tara Mines) in Ireland to treat alkaline mine water with elevated sulfate and metal levels. Each system comprised three in-series surface-flow cells that contained spent mushroom compost substrate. Typically, aqueous concentrations of 900 mg L^-1 sulfate, 0.15 mg L^-1 lead, and 2.0 mg L^-1 zinc flowed into the treatment wetlands at c. 1.5 L min^-1. During a two-year monitoring period, removal of sulfate (mean of 10.4 g m^-2 day ^-1 (31%), range of 0-42 g m^-2 day ^-1 (0-81%)), lead (mean of 1.9 mg m^-2 day ^-1 (32%), range of 0-6.6 mg m^-2 day ^-1 (0-64%)) and zinc (mean of 18.2 mg m^-2 day ^-1 (74%), range of 0-70 mg m^-2 day ^-1 (0-99%)) were achieved. These contaminants were somewhat associated with the vegetation roots but more significantly with the substrate. Communities of colonizing macroinvertebrates, macrophytes, algae, and microorganisms contributed to the development of a diverse ecosystem, which proved to be a successful alternative treatment process. The interacting processes within the wetland ecosystems responsible for wastewater decontamination are being further elucidated and quantified using a systems dynamic model.     

Long-term Performance of Passive Acid Mine Drainage Treatment Systems

Abstract.   State and federal reclamation programs, mining operators, and citizen-based watershed organizations have constructed hundreds of passive systems in the eastern U. S. over the past 20 years to provide reliable, low cost, low maintenance mine water treatment in remote locations. While performance has been reported for individual systems, there has not been a comprehensive evaluation of the performance of each treatment type for a wide variety of conditions. We evaluated 83 systems; five types in eight states. Each system was monitored for influent and effluent flow, ph, net acidity, and metal concentrations. Performance was normalized among types by calclating acid loading reductions and removals, and by converting construction cost, projected service life, and metric tonnes of acid load treated into cost per tonne of acid treated. Of the 83 systems, 82 reduced acid load. Average acid load reductions were 9.9 t/yr for open limestone channels (OLC), 10.1 t/yr for vertical flow wetland (VFW), 11.9 t/yr for anaerobic wetlands (AnW), 16.6 t/yr for limestone leach beds (LSB), and 22.2 t/yr for anoxic limestone drains (ALD). Average costs for acid removal varied from $83/t/yr for ALDs to $527 for AnWs. Average acid removals were 25 g/m²/day for AnWs, 62 g/m²/day for VFWs, 22 g/day/t for OLCs, 28 g/day/t for LSBs, and 56 g/day/t for ALDs. It appears that the majority of passive systems are effective but there was wide variation within each system type, so improved reliability and efficiency are needed. This report is an initial step in determining passive treatment system performance; additional work is needed to refine system designs and monitoring.