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