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.     

A QEM*SEM study of a suite of pressure leach products from a gold circuit

High lime consumption during the neutralisation of oxidised solids at the Lihir gold operation can be related to both ore mineralogy and conditions within the autoclaves. During the pressure oxidation process, basic iron sulphate and potassium jarosite precipitated and silicate clay minerals were dissolved. Illite in the plant feed was identified as a source of potassium for potassium jarosite precipitation — a greater extraction of which was obtained during a period of ‘low’ lime consumption. By contrast potassium feldspar was found to be largely un-reactive.The dissolution of basic iron sulphate under alkali conditions resulted in the consumption of lime. The difference between the conversion of 4% and 8% of the total solid, by weight, from basic iron sulphate to ferric hydroxide can account for either ‘low’ or ‘high’ lime consumption. Potassium jarosite was largely non-reactive during neutralisation.The quantity of basic iron sulphate present in the autoclave discharge is likely to be a function of both ore mineralogy and autoclave conditions. These mineralogical changes occur mostly in the ultrafine, −10 μm size fraction.In order to control the consumption of lime in the plant, it is clear that during pressure oxidation the formation of basic iron sulphates needs to be minimised and that of potassium jarosite promoted.     

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).     

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.     

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).     

Treatment of Acid Leachate from Coal Discard using Calcium Carbonate and Biological Sulphate Removal

Abstract.   An integrated approach is proposed for treating acidic coal discard leachate, consisting of CaCO₃ handling and dosing, CaCO₃-neutralization, and biological sulphate removal. It was found that: powdered CaCO₃ can be slurried to a constant density and used to neutralize the acid water, remove Fe (II), Fe (III), and Al, and partially remove the sulphate (to saturation level); biological sulphate removal can be used to lower the sulphate to less than 200 mg/L using ethanol as the carbon and energy source; CO₂ produced during calcium carbonate treatment can be used for H₂S-stripping and; H₂S gas recovered in the sulphate removal stage can be used for iron removal.     

铀堆浸废水中和沉渣减容与改性方法研究

在传统石灰中和处理法的基础上，提出了酸法难浸废水用“石灰石-石灰两步中和-沉渣循环”的流程进行处理。废水先与廉价的石灰石接触反应，使废水中的强酸中和并使铁、铝等金属离子在较低pH值下形成氢氧化物，再用石灰乳进一步中和到要求的pH。生成的沉淀物（沉渣）在过程中不断循环。该方法与一次石灰中和法相比，试剂费用节省1／3，沉渣生成量（以体积计）减少2／3，并且沉渣的过滤和沉降性能也有所改善。本工作还对沉渣减容与改性的机理进行了研究探讨。     

Buffering of Acidic Mine Lakes: The Relevance of Surface Exchange and Solid- Bound Sulphate

Abstract.  Buffering mechanisms in an acidic mine lake in Lusatia, Germany were investigated. The titration curve has four sections with different buffering mechanisms: (1) buffering by free hydrogen ions and hydrogen sulphate (pH = 2.55-2.9), (2) buffering by Fe with bound SO₄ (pH = 2.9-4.3), (3) buffering by Al with bound SO₄ (pH = 4.3-5.5), and (4) buffering by surface exchange of SO₄ and basic cations (pH > 5.5). Three different phase models were applied to simulate the titration curve: (1) an iron and aluminium hydroxide model; (2) an iron and aluminium hydroxysulphate model; and (3) an iron hydroxide model with surface exchange for SO₄, Ca, and Mg, coupled with an aluminium hydroxysulphate model. The uncertainty of model input parameters was accounted for in a sensitivity analysis. Only the third model, which considers surface exchange, was able to adequately reproduce the measured titration curve.     

Acid mine drainage treatment through a two-step neutralization ferrite-formation process in northern Japan: Physical and chemical characterization of the sludge

The acid mine drainage of a closed iron sulphide mine in northern Japan was treated on-site using a continuous flow bench scale plant. In the bench scale plant of the two-step neutralization ferrite-formation process, magnesium oxide or calcium carbonate was used during the first neutralization step to raise the pH to around 4.8 to produce aluminium hydroxide sludge. In the second neutralization step sodium hydroxide was applied to reach a pH of 8.5 to produce ferrite sludge. Initial settling rates of the produced aluminium and ferrite sludge were from 1.2 to 4.8 times higher than the initial settling rates of the sludge produced by the treatment plant currently operated at the mine. The suspended solids (SS) concentration in the sludge ranged from 1.0 to 11.8 times higher than the SS concentration in the sludge produced by the current facility. The sludge volume index (SVI) of aluminium sludge was 11 and 36 mL/g when produced with magnesium oxide or calcium carbonate, respectively. The SVI of ferrite sludge was 4 mL/g regardless of the type of neutralizer used in the first neutralization, while the same parameter (SVI) of the sludge produced by the current facility is 70 mL/g, which indicates that the two-step neutralization ferrite-formation process generates sludge with much higher density. In addition, the process effectively reduced the concentration of toxic heavy metals from above 800 ppb, 13 ppm, and 15 ppm to as low as 1.4 ppb, 0.02 ppm and 0.2 ppm for arsenic, copper, and zinc, respectively.     

Water Cover Over Mine Tailings and Sludge: Field Studies of Water Quality and Resuspension

Flooding is considered one of the best available technologies for long term storage of acid generating mine waste when suitable site-specific conditions exist. There is, however, a concern that oxidation may still occur. In cases where lime neutralization sludge and reactive sulfide tailings are co-disposed in the tailings pond, wind-induced waves could resuspend the waste and negatively impact the quality of the water cover. Studies were undertaken at the Noranda Inc. Heath Steele Lower Cell tailings impoundment, located in northeastern New Brunswick, Canada, 50 km northwest of city of Miramichi. The stored material in the cell consisted of unoxidized tailings with small amounts of sludge. The 90 ha impoundment acted as a polishing pond, prior to the discharge of final effluent. The pond was keptalkaline (pH of 8.5-10.5) in order to meet regulated discharge limits. On some windy days when the Lower Cell experienced turbulent water conditions, the final effluent exceeded the suspended solids water quality standard of 25 mg/L. The dry mass of suspended sediment measured in 1999 ranged from 1.5 to 434 mg with relatively more material (> 100 mg) being suspended under shallow water cover (= 1 m). Both x-ray diffraction and scanning electron microscopy analyses indicated that the suspended material was mostly lime neutralization sludge and other material composed primarily of calcite and brucite and coatings of aluminum, iron, zinc and manganese hydroxides. Solubility considerations of the carbonate system confirmed that the water cover was supersaturated with respect to calcite. The results suggest that sludge and tailings re-suspension and precipitation of solid phases in the water cover likely combined to produce the observed, occasionally high total suspended solids concentration.     

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.     

Limestone Neutralisation of Arsenic-Rich Effluent from a Gold Mine

Traditionally acid mine water is neutralised with lime. Limestone is a cheaper alternative for such applications. A case study showed that limestone can be used effectively to replace lime for the neutralization of arsenic rich acid water. The cost of limestone treatment is 45.8% less than that of lime. The acidity can be removed from 33.5 to 0.06 g/L (as CaCO₃). The study also showed no significant differences in the TCLP characteristics of the resultant sludge when water is treated with lime or with limestone. Sludge from the limestone treatment process can be disposed of on a non-hazardous landfill site. An erratum to this article can be found at     

Effects of a Barite Mine on Ground Water Quality in Andhra Pradesh,India

Abstract.  An investigation was undertaken to determine the effects of a large barite mining operation on local ground water quality near Mangampeta,Andhra Pradesh, India.Water samples were collected from drinking water wells in the mining and adjacent regions. The drinking water in the mining region had sulphate concentrations that ranged from 211 to 589 mg/L, compared to sulphate concentrations of 25 mg/L or less in the non-mined areas. The natural existence of barite and the widespread mine waste dumps at Mangampeta are believed to be responsible for the higher levels of sulphate in the ground water.     

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.     

Processing of Phosphate Mine Tailings by Coagulation Flocculation to Reduce Marine Pollution in Togo: Laboratory Tests

Abstract  About 2.5 million t of sedimentary phosphorite mine tailings, highly enriched with trace metals such as Cd, Cr, Cu, Ni, and Zn, are dumped annually in the coastal waters of Togo without any pre-treatment, causing serious pollution problems in the region. We conducted laboratory jar tests of a coagulation-flocculation procedure with coagulants RM45U and AN945MPM to clarify the sludge. The efficiency of the method depends particularly on two factors: the amount of coagulant and the solid concentration of the sludge to be treated. Thus, with a mud concentration of 47.7 g/L, the average optimal amount of the two coagulants was 25 mg/L. With both coagulants, water turbidity passed from 60 x 103 NTU to approximately 3 NTU after clarification with the optimal amount of the two coagulants. RM45U reduced concentrations of Pb by 40%, Zn by 98.8%, Fe by 80.6%, and Cd by 32.8%. AN945MPM reduced Pb by 20%, Zn by 98.5%, Fe by 48%, and Cd by 32.8%.     

Contaminated Alluvial Ground Water in the Butte Summit Valley

Abstract.  Ground water in alluvial sediments of upper Silver Bow Creek is chronically contaminated with heavy metals, including Cd, Cu, Fe, Mn, and Zn. Most of this contamination stems from slag, mill tailings, and waste rock from the Butte mining district that had been deposited along the ancestral Silver Bow Creek floodplain. Much of this mine waste is now buried by fill, topsoil, buildings, or parking lots. Although the pH values of most wells in the region are in the 5.5 to 7.0 range, a cluster of monitoring wells near the site of a former mill and smelter contain water that is strongly acidic (pH < 4.5), with extremely high dissolved metal concentrations (Cu up to 750 mg/L; Zn up to 490 mg/L). Ground water discharging from the area is currently collected by a subsurface French drain and conveyed to a treatment facility where lime is added to precipitate metals from solution.     

Removal of Metals and Acidity from Acid Mine Drainage Using Municipal Wastewater and Activated Sludge

Co-treatment of acid mine drainage (AMD) and municipal wastewater (MWW) using the activated sludge process is an innovative approach to AMD remediation that utilizes the alkalinity of MWW and the adsorptive properties of the wastewater particulates and activated sludge biomass to buffer acidity and remove metals. The capacity of these materials to treat AMD was investigated in batch mode metal removal tests using high-strength synthetic AMD (pH 2.8, Al 120–200 mg/L, Cu 18–30 mg/L, Fe 324–540 mg/L, Mn 18–30 mg/L, and Zn 36–60 mg/L). Using material from a range of MWW treatment plants, the performance of screened and settled MWW, activated sludges with mixed liquor suspended solids (MLSS) concentrations of 2.0 and 4.0 g/L, and return activated sludges with 6.0 and 7.4 g/L MLSS were compared. Similar trends were observed for the MWW and activated sludges, with removal efficiency generally decreasing in the order Al = Cu > Mn > Zn > Fe. Trends in Fe removal using settled MWW and activated sludges were highly variable, with removal <30 %. Using activated sludges, average removal efficiencies for Al, Cu, Mn, and Zn were 10–65 %, 20–60 %, 10–25 %, and 0–20 %, respectively. Sludge solids concentration was an important controlling factor in metal removal, with removal of Al, Cu, Mn, and Zn increasing significantly with solids concentration. Municipal wastewaters had greater neutralization capacities than activated sludges at high AMD loading ratios. Mixing AMD with screened MWW gave the highest removal efficiency for all metals, achieving average removal of 90–100 % for Al, Cu, and Fe, 65–100 % for Zn, and 60–75 % for Mn. These empirical findings are useful for developing process design parameters in co-treatment systems. Utilizing MWW and activated sludge to remediate AMD can potentially reduce materials and energy requirements and associated costs.     

Use of a Multiple Orifice Spray Reactor to Accelerate Ferrous Iron Oxidation in Acidic Mine Water

Abstract  The potential of a multiple orifice spray reactor to enhance aeration and oxidation of ferrous iron in acidic mine water was investigated in a bench-scale experiment. The reactor consists of two concentric cylinders, with a series of orifices that act like a venturi in the inner cylinder. Neutralization and aeration are combined in the reaction chamber where air is aspirated and alkaline agent dispensed. Ferrous iron oxidation rates were about four orders of magnitude faster than theoretical rates at a treatment pH of 6.5; at an influent ferrous iron concentration of around 150 mg/L, the orifice spray reactor oxidized about 30% of the ferrous to ferric iron within 1 second. Under certain conditions, the orifice spray reactor oxidizes more ferrous iron than can be simply attributed to the oxygen transfer capabilities of the system. It is suggested that the excess oxidation capability is due to, or initiated by, the effects of hydrodynamic cavitation.     

Production of a poly-alumino-iron sulphate coagulant by chemical precipitation of a coal mining acid drainage

The aim of this work was to produce a poly-alumino-iron sulphate coagulant from acidic coal mine drainage. Precipitating the iron and aluminium at pH 5.0, followed by dissolution in sulphuric acid, produced a coagulant consisting of 8.7% iron and 3.3% aluminium. Water treatment tests proved that this coagulant was as efficient as the coagulant chemicals conventionally used in water treatment plants. The process can be easily incorporated into conventional AMD treatment plants, thereby reducing sludge waste issues and producing a valuable chemical reagent.     

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.