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1、The Basics ofAcid Mine Drainage,ByAndy Robertson and Shannon Shaw,Disclaimer,These slides have been selected from a set used as the basis of a series of lectures on Acid Mine Drainage presented in 2006 at the University of British Columbia,Vancouver,BC.No attempt is made here to provide linking text
2、 or other verbal explanations.If you know about Acid Mine Drainage,these slides may be of interest or fill in a gap or twogoing back to basics never hurts the expert.If you know nothing of Acid Mine Drainage,these slide may be incomprehensible,but on the other hand they may be an easy way to ease in
3、to a tough topicgood luck.,Overview of ARD,Metal Sulphide+Water+Oxygen=Acid+MetalMS+H2O+O2=H2SO4+M(OH)x(not stoichiometrically balanced)Acid+Alkali=“Salt”+Carbon DioxideH2SO4+CaCO3=CaSO4+CO2Environmental Impact from:AcidityMetals in solution(in acid or alkaline environments)SalinitySludge precipitat
4、es,Bacterial Catalization of Oxidation,Temperature Effects on Oxidation,Buffering of ARD during Oxidation of a Mineral Assemblage,Buffering of ARD during Oxidation of a Mineral Assemblage,pH,Time,Buffering of Mineral A(e.g.calcite,dolomite),Buffering of Mineral B(e.g.ankerite,siderite),Buffering of
5、Mineral C(e.g.Al(OH)3),Buffering of Mineral D(e.g.feldspars),Mechanisms Controlling ARD in Tailings,Mechanisms Controlling ARD in Waste Rock,Advective Air Transport,Oxygen Diffusion,Mechanisms Controlling ARD in Open Pits,Mechanisms Controlling ARD in Underground Workings,Sulphide Minerals,Pyrite(Fe
6、S2)Pyrrhotite(Fe(1-x)Sx)Marcasite(FeS2)Chalcopyrite(CuFeS2)Galena(PbS)Sphalerite(ZnS)Arsenopyrite(FeAsS)Bornite(Cu5FeS4),Alkali Minerals,TypesCarbonates Calcite(CaCO3)Dolomite(Ca,Mg(CO3)2)HydroxidesFe(OH)3Al(OH)3Silicates Clays,Development of ARD,Water chemistry depends on:Rate and extent of oxidati
7、onRate and extent of metal releaseQuantity of materialContained metalsSite hydrology and climateAccumulation of oxidation productspH/solubility controls,flowpath reactionsControl technology,Site Characterization,DesignField investigation&SamplingLab testing,New Mines vs.Existing Mines,New MinesARD p
8、robably not evidentObjective is to determine ARD potentialFresh samples used for testing and predictionLong term behavior based on kinetic testing,modeling and predictionExisting and Abandoned MinesARD may be evident/matureField reconnaissance used to define ARDHistoric data(time trends)extremely us
9、efulLimited laboratory testing requiredField instrumentation and monitoring possibleBackground altered,requires simulation,Design,Review existing data,e.g:Geology&mine planDrill core logsWater quality monitoring resultsAssays on ore/waste rock and tailingsWaste type volumesWaste placement historyDev
10、elop reconnaissance&sampling plan,Field Investigations,ObjectivesDetect early signs of ARDDetermine potential for ARD Assess factors that control ARDEvaluate control measuresDetermine environmental impactAssess compliance with regulatory standards,Field Investigations,What to bring:Eyes that know wh
11、at to look forpH and conductivity metersAcid bottle,hydrogen peroxide,sulfate kitGeological pick,hand lens,sampling bags,camera,GPS unitSite map,history,data,2.2,Field Investigations,Things to look for:Visible pyrite or other sulfides(oxidation)&calciteRed,orange,yellow,white,blue staining(precipita
12、tes,water)Dead vegetation or bare groundMelting snow or steaming vents on wasteDead fish or other biotaLow pH in seeps,groundwater,decants&streams,Field Investigations,Things to log in the field:Paste pHPaste conductivityColourLithologySulfide contentSecondary mineralogyDegree of fizzMoisture conten
13、tGrain size,Field Investigations,General MethodologyVisual observation of site Paste pH and water quality dataField extraction testingClassify types of wastesSolids sampling(for lab testing),Field Investigations,Geochemistry:Low paste pH of mine wastesHigh conductivity of waste pasteContaminants in
14、leach extraction testsStatic(ABA)testsProducts from Reconnaissance:Physical disturbance and drainage mapWaste deposit map and characterizationExposed rock map and characterizationPaste pH and conductivity surveyObservations and sampling mapARD site assessment report,TDS vs pH,Sample Selection(New Mi
15、nes),Step 1:On geological sections:Define rock typesDefine sulfide and alkali mineral distributionPreliminary rock units classificationStep 2:Sample each rock unit class allowing for:Area distribution of classVariability of rockStep 3:Perform static lab tests and use results to refine rock unit clas
16、sificationStep 4:Sample each new rock class and repeat Step 3 until satisfied.Step 5:Sample each rock class for appropriate kinetic testing and use results to refine rock classificationStep 6:Repeat Step 5 until satisfied with classifications and characterization.,Sampling(existing mines),Steps:Defi
17、ne geology,mineralization,waste types etc.Define objectives(i.e.sampling for reveg,cover,water quality evaluations etc.may have different focus)Consider mine plan and waste placement history Identify sources of samplesInitial sampling and testing programFurther sampling if necessary,Sampling(Existin
18、g Mines),A Becker hammer-type drill rig can be used in order to minimize sample crushing and the geochemical disturbance of the samplesSamples typically collected at specified intervals(e.g.every 10 ft)&paste pH and EC measured,A sub-set of samples can then be selected using observations and field m
19、easurements as a guide for more detailed laboratory testing,Test Methods,Static ARD Testsbalance between potentially acid generating and consumingtool for waste managementincludes geological/mineralogical characterizationindividual samplesShort-term Leaching Testsreadily soluble componentKinetic Tes
20、tsoxidation and metal leaching rateswater chemistry prediction,Geochemical Static Tests,Objective:Potentially Acid Generating MineralsvsAcid Neutralizing MineralsCautions for ARD assessment:pH of alkalinity(NP)determinationAssumes instant availability of NPAssumes all sulphur/sulphide minerals react
21、iveIgnores reaction rates(kinetics)Extrapolation to field,Geochemical Static Tests,ProceduresPaste pH and conductivity on the as received fines Acid-Base Accounting TestsNet Acid Generation(NAG)-also an accelerated kinetic testB.C.Research Initial TestLapakko Neutralization Potential TestH2O2 Oxidat
22、ion(modified for siderite correction)Net Carbonate Value(NCV)for ABA TestsLeach extraction analysesForward acid titration testsMulti-element ICP analysesDetailed procedures can be found on:and in prediction course on,Geochemical Static Tests,Definitions:AP=acid potential=%S x 31.25NP=neutralization
23、potentialNNP=net neutralization potential=NP-APNP:AP ratio=NP/APAll expressed as:kg CaCO3 equivalent/tonne,or CaCO3 eq./1000 tonnes,Example:S=2%AP=62.5 kgCaCO3/tNP=90 kgCaCO3/tNNP=27.5 kgCaCO3/tNP/AP=1.4:1,Note:units and acronyms used are different in Australiasia,local references should be sought f
24、or correct usage,terminology,guidelines etc.,Interpretation,Start with guidelines”or general criteria for classification,then develop site-specific criteria,Typically criteria are based on a set of tests,not just one type of test e.g.ABA&NAG results,NAG Test,Developed in Australia as an alternative
25、and/or compliment to ABA test,Developed as a“one-off”test that can assess the net acid generation potential both acid generation and acid neutralization in one test.NAG test varies among users,typically:Adding 250 mL of 15%H2O2 at room temp to 2.5 g of sample pulverized to pass 200 mesh.React for 12
26、 h then boiled until visible reaction ceases(or Cu catalyst added)or initial reaction period is extended to 24 hMeasure pH of the reacted solution(NAGpH)Titrate reacted solution with NaOH to a specified pH end-point(pH 4.5 and/or pH 7)to determine the NAG value of the sample.,Interpretation,There ar
27、e numbers of modifications to the test for different scenarios,including:Sequential addition NAG test(multiple additions of H2O2)Kinetic NAG test(track pH,temperature and EC during test)Modifications to account for organic matter effects(analyze for organic acids and sulphuric acid in reacted soluti
28、on,extended boiling step).Modifications to leach carbonates prior to NAG test(i.e.measure of acidity not net acidity).NAG results are generally interpreted as such:If the final NAGpH is 4.5,sample said to be non-acid formingIf the final NAGpH is 4.5,the sample is said to be potentially acid formingT
29、he NAG value then provides a quantitative assessment of potential acid formation in units of kg CaCO3/t equivalent(or kg H2SO4/t equivalent),Applications of the NAG test,In conjunction with ABA tests etc to reduce the risk of mis-classificationAs an operational scale management tool(e.g.for segregat
30、ion of different material types)For identifying material for prioritization(e.g.AML ranking)As an indicator test that can be run on greater number of samples than if using other methods due to the fact it is quick,simple and inexpensiveUsed very widely in Australasia,Some potential pitfalls,Organic
31、matter,Cu,Pb and MnO2 can catalyze decomposition of H2O2.Samples high in these parameters can have unpredictable results(OShay et al.,1990)Samples with a lot of Zn can be buffered between pH of 4 to 5 by the formation of Zn(OH)2(Jennings et al.,1999)NAG test can underestimate potential acidity if sa
32、mples have(Amira,2002):Sulphide content 1%High carbonate contentHigh organic contentNot as conservative as ABA testing,Rumble et al.2003 ICARD proceedings,Example Ok Tedi,Example Ok Tedi,Single addition NAG test showed the dredged material was NAF but river bars showed elevated SO4 and metals and sl
33、ightly depressed pHSequential NAG test consistently showed a drop in the NAGpH of the material below 4.5 after additional H2O2 additions,Pile et al.2003 ICARD proceedings,perhaps due to presence of Cu or higher S content,Short-term Extraction Tests,ObjectiveDetermine readily soluble loadDetermine ac
34、id soluble loadProcedureUncrushed sample including finesAgitate in deionised water or mild acidFilter and analyse filtrate*Always account for dilution in concentration assessments,Kinetic Testing,ObjectivesValidation of static test results and boundariesDetermination of leaching behaviourSimulation
35、of site conditionsEvaluation of extent of oxidation Evaluation of stored productsPrediction of drainage water qualityProduces raw data for modelingInvestigate factors controlling ARDSelection of control measures,Kinetic Testing,Humidity Cells,ObjectivePredict lag to,and rate of,acid generationSemi-q
36、ualitative water quality prediction*AdvantagesWidely used in North America in the pastSimple to operateAppropriate for fine samples,disseminated mineralizationDisadvantagesCrushed sample-does not address surface area,mineralogyNot representative of waste rockHigh flushing rate,saturation,pH&solute m
37、odification*Always account for dilution in concentration assessments,Columns,ObjectiveEvaluate kinetics of oxidation&leaching for waste rockData to predict drainage water qualityAdvantagesRepresentative of rock pile size distributionDevelopment of local pH environmentsEvaluate storage/flushingEvalua
38、te control optionsEstimate production rates DisadvantagesSize of sample requiredInterpretation of dataEdge effectsHigh flush ratesLaboratory conditions of temp and oxygen availability,Kinetic Testing Data,Field Test Plots,ObjectiveEvaluate leach kinetics&drainage water quality in field conditionsAdv
39、antagesRepresentative of site conditionsCalibration of water quality predictionTest control options on a realistic scaleAlready exist?DisadvantagesLimited control of test conditionsTime requiredExpensive for new installationsMaintenance and damage Interpretation of results,Field Test Plots,Field Tes
40、t Plots,Field Test Plots,Field Barrel Tests,ARD Model,ARD Model(pore-water),Acid Neutralization,Sulphide Oxidation,Secondary Mineral Precipitation,Mineral Dissolution,Ion Exchange,Metal Attenuation,Scale-up to Field Conditions,Dynamic Systems,Sulfate Generation Over Time,Water Quality,Examples:Highl
41、y Acid Generating Rock SeepagepH 4000 mg/LHigh Cu(5 mg/L),Zn(3 mg/L),Fe(10s mg/L),Al(10s mg/L)Moderately Acid Generating Rock SeepagepH 3.5-5.5,SO4 2000-4000 mg/LModerate Cu(0.5-5 mg/L),Zn(0.3-3 mg/L),Fe(0.3-10mg/L),Al(0.1-10mg/L)Neutral pH/Metal Leaching Rock SeepagepH 5.5-7.5,SO4 2000 mg/LModerate
42、 Zn(0.3 mg/L),+/-As,Cd,NiLow Cu(0.5 mg/L),Fe(0.3 mg/L),Al(0.1 mg/L)Buffered/Low Metal Leaching Rock SeepagepH 7-8,SO4 2000 mg/LNegligible Cu,Zn,Fe,Al etcNote:in arid climates evapo-concentration can drastically change any of these water types,salinity can become an issue in particular for revegetati
43、on purposes,Chemical-Physical Interactions,The time dependant change in geotechnical characteristics of a rock results from:Physical Weathering-e.g.sheeting due to unloading;thermal expansion and contraction,abrasion,salt and ice crystal growth;slaking due to clay mineral expansion and contraction d
44、uring wetting and dryingChemical Weathering-e.g.oxidation;hydrolysis;dissolution;diffusion;precipitationThese weathering processes may result in an increase or a decrease in rock strength,and an increase or decrease in permeability.Most commonly a decrease in shear strength and permeability occur.,P
45、re-mining Alteration,The natural geothermal processes that are associated with sulphide ore genesis alter alumino-silicate minerals in the rock mass.Sericite-clay and chlorite-epidote altered zones surrounding such ore bodies often exhibit reduced strength properties and an increased propensity to s
46、lake when exposed to air and water.Additional alteration occurs as a consequence of exposure of the mineral deposits to air and water and the resulting oxidation of pyrite and further hydrolysis of the alumino-silicates.,Mineral Alteration,Under non-acidic conditions,primary minerals like feldspars
47、weather to form clay and amorphous hydroxide minerals,such as kaolinite and gibbsite Under acidic and sulphate-rich conditions,produced by pyrite oxidation,alumino-silicates weather far more rapidly.Aluminum is highly soluble under these conditions.Acid leaching is concentrated on weak zones such as
48、 fractures in rock particles and mineral cleavages causing a breakdown of the rock fabric.When this occurs over natural sulphide bodies it results in the production of gossan or oxide zones,often with high percentages of clays,including smectite clays.,Consequence of Mining Pyritic Rock,Mining of al
49、tered and acid-generating sulphide containing waste rock increases,by several orders of magnitude,the surface area of rock surface exposed to air and water resulting in hugely increased rates of slaking(physical weathering)as well as geochemical weathering.Hydrolysis,fragmentation and breakdown of t
50、he rock fabric,results in an increase in the percentage of fines,including clays.This in turn results in changes in both the permeability and shear strength of the mine rock,Oxidation Products Mass Balance,1%by weight of sulfide sulfur can produce:3.2%by weight of sulfuric acid and this can hydrolyz
