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A Method to Measure Arsenic A Method to Measure Arsenic Readily Released to Pore Readily Released to Pore Waters from Uranium Mill Waters from Uranium Mill
TailingsTailings
John Mahoney Hydrologic Consultants, Inc.,
143 Union Blvd., Suite 525, Lakewood, Colorado, USA
Donald Langmuir Hydrochem Systems Corp., Denver, Colorado, USA
John RowsonCOGEMA Resources, Saskatoon, Saskatchewan, Canada
Location MapLocation Map
IntroductionIntroduction
Polymetallic U ores with As and Ni - in RaffinatePolymetallic U ores with As and Ni - in Raffinate
Raffinate 300-700 mg/L As, 200-500 mg/L Ni, pH~1.0Raffinate 300-700 mg/L As, 200-500 mg/L Ni, pH~1.0
Diffusion of As into Fox Lake required that As and Ni Diffusion of As into Fox Lake required that As and Ni concentrations in tailings be around 1 mg/Lconcentrations in tailings be around 1 mg/L
Tailings neutralization circuit increases Fe:As >3Tailings neutralization circuit increases Fe:As >3
Neutralization with slaked lime to pH ~ 7 Neutralization with slaked lime to pH ~ 7
Tailings Neutralization CircuitTailings Neutralization Circuit
to TMF
24 hour metallurgical composite sample
Raffinate Tailings BaCl 2
Fe 2 (SO 4 ) 3
CaO (pH 4)
pH pH
THICKENER
NEUTRALIZATION TANKS
Flocculant
CaO (pH 8)
MIXING TANK
Process Air
Mill and JEB TMFMill and JEB TMF
Final Decommissioned TMFFinal Decommissioned TMF
AthabascaAthabascaSandstoneSandstone
Tailings MassTailings Mass
Waste Rock
Glacial Till
Tailings Optimization and Tailings Optimization and ValidationValidation
License Application - Initial model based on License Application - Initial model based on mineral precipitation (scorodite) and surface mineral precipitation (scorodite) and surface complexation of As on Hydrous Ferric Oxide complexation of As on Hydrous Ferric Oxide (Langmuir et al., 1999)(Langmuir et al., 1999)
Demonstrated that As < 1 mg/L was possibleDemonstrated that As < 1 mg/L was possible
Tailings Optimization and Validation Program Tailings Optimization and Validation Program (TOVP) ongoing studies to (TOVP) ongoing studies to Verify and improve process, Verify and improve process, Monitor tailings management facility, andMonitor tailings management facility, and Validate model assumptionsValidate model assumptions
Readily Released ArsenicReadily Released Arsenic Regulatory Agencies required - quantify Regulatory Agencies required - quantify
amount of arsenic released from tailings via amount of arsenic released from tailings via
“desorption” process“desorption” process Surface ComplexationSurface Complexation
Complicating factorsComplicating factors Amorphous Arsenic bearing phases - Amorphous Arsenic bearing phases -
Scorodite, Annabergite, CoprecipitatesScorodite, Annabergite, Coprecipitates Need procedure that only targets “surface Need procedure that only targets “surface
complexed” arseniccomplexed” arsenic
IssuesIssues Discriminate As released through desorption Discriminate As released through desorption
while not releasing As in phases (Scorodite)while not releasing As in phases (Scorodite) Minimize time for dissolutionMinimize time for dissolution Use a gentle extraction procedureUse a gentle extraction procedure
Amorphous phases preclude extraction agentsAmorphous phases preclude extraction agents hydroxylamine hydrochloride, oxalic acidhydroxylamine hydrochloride, oxalic acid
Spiking methods may change solution Spiking methods may change solution composition and require model calculationscomposition and require model calculations
adding phosphate or molybdateadding phosphate or molybdate
MethodologyMethodology Developed to Measure Chlorinated Solvents in Developed to Measure Chlorinated Solvents in
Soils - Portable Gas Chromatograph using Soils - Portable Gas Chromatograph using
Headspace Analysis Headspace Analysis
Changing liquid/solid ratio changes Changing liquid/solid ratio changes
concentration in solutionconcentration in solution
Basically a Matrix CorrectionBasically a Matrix Correction Serial Dilution MethodSerial Dilution Method
Similar to Method of Standard Additions (MSA)Similar to Method of Standard Additions (MSA)
Crawling Down the IsothermCrawling Down the Isotherm
EPICSEPICS Procedure Procedure
EPICS = EPICS = EEquilibrium quilibrium PPartitioning artitioning IIn n
CClosed losed SSystemystem Found Alternative Method in Gossett Found Alternative Method in Gossett
(1987)(1987) Calculate Henry’s Law constantsCalculate Henry’s Law constants
EPICSEPICS Method, No Sorption Method, No Sorption Es gibt nichts praktischeres als die Theorie - Es gibt nichts praktischeres als die Theorie -
BoltzmannBoltzmann
Pure waterPure water
2L0.5 mg/L
3L0.33 mg/L
1L1 mg/L
EPICSEPICS Method, With Sorption Method, With Sorption
Pure waterPure water
2L0.655 mg/L
3L0.45 mg/L
As As
1L1 mg/L
ExampleExample CalculationCalculation
With Sorption
EPICSEPICS Plot Plot
Dilution Kd = 0/1 = 0
Features of MethodFeatures of Method
Everything referenced to solution Everything referenced to solution changes changes
Requires solution concentrations onlyRequires solution concentrations only
Matrix effect for soil water systemMatrix effect for soil water system Produces dimensionless KdProduces dimensionless Kd Suited for other hydrologic studiesSuited for other hydrologic studies
Initial Modeling to Verify Initial Modeling to Verify ProcedureProcedure
Simulated models using Simulated models using PHREEQCPHREEQC Defined Pore water and Surface - Diffuse Layer ModelDefined Pore water and Surface - Diffuse Layer Model Defined Arsenic Free Solution Defined Arsenic Free Solution Use MIX keyword to re-equilibrate with Surface Use MIX keyword to re-equilibrate with Surface
Arsenic free leaching solution added in stepsArsenic free leaching solution added in steps Changing proportions to mixturesChanging proportions to mixtures Surface re-equilibrates with new solid/solution ratio Surface re-equilibrates with new solid/solution ratio
Various simulation to cover range of conditionsVarious simulation to cover range of conditions
PHREEQCPHREEQC Modeling Modeling
PHREEQCPHREEQC Based Based EPICSEPICS
Verification of the Verification of the EPICSEPICS Procedure by Comparison with Procedure by Comparison with PHREEQCPHREEQC Modeling Results Modeling Results
Verification of MethodVerification of Method
PHREEQCPHREEQC models verified that models verified that
overall approach and overall approach and
calculational method was correctcalculational method was correct
Laboratory forms checked with Laboratory forms checked with
PHREEQCPHREEQC results results
MeasurementsMeasurements on Tailings on Tailings Models required constant major ion compositionModels required constant major ion composition
Surface charge effects may be importantSurface charge effects may be important Maintain major ion concentration and pHMaintain major ion concentration and pH
Experimental Design Experimental Design 1Kg Slurry (tailings and pore water)1Kg Slurry (tailings and pore water) Leach solution made from neutralized solutionLeach solution made from neutralized solution Very Small Sample Aliquots < 25 mlVery Small Sample Aliquots < 25 ml Dilutions ~ 1L arsenic free pore water/step Dilutions ~ 1L arsenic free pore water/step 3 Steps - V4/V1 > 43 Steps - V4/V1 > 4 1 hour (12 hours) between extractions1 hour (12 hours) between extractions
Initial ExperimentsInitial Experiments
Summary of Arsenic Desorption Summary of Arsenic Desorption Measurements, One HourMeasurements, One Hour
Extractions Extractions
Test Number
Initial pH Final pHInitial As(aq) (mg/L) (C1)
Final As(aq) (mg/L) (C4)
V4/V1Inverse slope
Inverse Intercept R2 Kd
rr
1.1 7.65 7.68 0.62 0.14 5.8 2.01 0.89 0.64 0.44
1.2 7.7 7.67 0.32 0.12 4.4 1.83 1.1 0.95 0.6
1.3 8.1 8.04 0.81 0.31 4.5 0.63 0.79 0.96 1.25
1.4 8.03 7.94 0.95 0.49 4.2 0.31 0.86 0.92 2.77
1.5 7.57 7.7 0.98 0.32 5.1 0.68 0.6 0.99 0.88
V4/V1 is the overall dilution for the tests.
Long Long DurationDuration Experiments Experiments
Summary of Arsenic Desorption Summary of Arsenic Desorption Measurement Results - 12 HoursMeasurement Results - 12 Hours
Extractions Extractions
Test Number
Initial pH
Final pH
Initial Concentration
of As (C1) (mg/L)
Final Concentration
of As (C4) (mg/L)
V4/V1Inverse slope
Inverse Intercept R2 Kd
exch
2.1 - 1 hr 7.82 7.76 0.93 0.38 5.0 0.49 0.79 0.98 1.6
2.2 -12 hr 7.89 7.88 0.93 0.39 4.4 0.49 0.78 0.95 1.6
2.3 -12 hr 7.89 7.88 0.93 0.4 4.3 0.47 0.76 0.97 1.6
V4/V1 is the overall dilution for the tests.
Calculation of Readily ReleasedCalculation of Readily ReleasedArsenic in JEB Mill TailingsArsenic in JEB Mill Tailingsusing the using the EPICSEPICS Method Method
Test Kdrr F(desorb)
Initial As(aq) (mg/L)
As desorbed (mg/L)
Readily Released As (mg/L)
1.2 0.6 0.38 0.32 0.19 0.51
1.3 1.25 0.56 0.81 1.01 1.82
2.1 - 1 hr 1.6 0.62 0.93 1.49 2.42
2.2 -12 hr 1.6 0.62 0.93 1.49 2.42
Percentage of Readily Percentage of Readily Released Arsenic in Mill Released Arsenic in Mill
TailingsTailings
Test
Initial Volume of Solution
(L)
Readily Released As
(moles)
Mass solids (g)
% As in Solids
Total As in Solids (g)
Readily Released As
%
1.2 0.88 6.01E-06 298.2 0.126 0.376 0.120
1.3 0.85 2.07E-05 277.9 0.149 0.414 0.374
2.1 - 1 hr 0.78 2.52E-05 575.4 0.177 1.018 0.185
2.2 -12 hr 0.87 2.81E-05 575.1 0.167 0.960 0.219
ConclusionsConclusions - Modeling - Modeling
PHREEQCPHREEQC used to verify approach used to verify approach
Simulation of experimentsSimulation of experiments
Assist in experimental designAssist in experimental design
Check spreadsheets setupCheck spreadsheets setup
Conclusions - MethodologyConclusions - Methodology
Method is simple application of Method is simple application of
accepted techniqueaccepted technique
Various approaches are essentially Various approaches are essentially
same methodsame method
Straightforward method to measure Straightforward method to measure
distribution coefficient distribution coefficient
Should be applied to other systemsShould be applied to other systems
ConclusConclusiions – Arsenic in ons – Arsenic in TailingsTailings
Amount of readily released arsenic in JEB Amount of readily released arsenic in JEB TMF is very small ~ 0.2 percent of totalTMF is very small ~ 0.2 percent of total
Consistent with observation - adsorption Consistent with observation - adsorption of arsenic on ferrihydrite is not reversible of arsenic on ferrihydrite is not reversible
Most Arsenic in ScoroditeMost Arsenic in Scorodite
Suggests that adsorption of As is not Suggests that adsorption of As is not occurring on other surfacesoccurring on other surfaces
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