Raffinate Neutralization Experiments at the McClean Lake Mill – Removal of Arsenic and Nickel John Mahoney Hydrologic Consultants, Inc., 143 Union Blvd., Suite 525, Lakewood, Colorado, USA Donald Langmuir Hydrochem Systems Corp., Denver, Colorado, USA Maynard Slaughter Crystal Research Laboratories, Greeley, Colorado, USA John Rowson COGEMA Resources, Saskatoon, Saskatchewan, Canada

Location Map

Overview Arsenic and Nickel Bearing Uranium Ores Arsenic and Nickel Bearing Uranium Ores Tailings disposal issues Tailings disposal issues Raffinate mg/L As, mg/L Ni, pH~1.0 Raffinate mg/L As, mg/L Ni, pH~1.0 Diffusion of As into Fox Lake required As and Ni in tailings pore waters be ~ 1 mg/L Diffusion of As into Fox Lake required As and Ni in tailings pore waters be ~ 1 mg/L

Mill and JEB TMF

Tailings Disposal System

Tailings Optimization and Validation License Application - Initial model based on mineral precipitation (scorodite) and surface complexation of As on Hydrous Ferric Oxide (Langmuir et al., 1999) License Application - Initial model based on mineral precipitation (scorodite) and surface complexation of As on Hydrous Ferric Oxide (Langmuir et al., 1999) Demonstrated that As < 1 mg/L was possible Demonstrated that As < 1 mg/L was possible Tailings Optimization and Validation Program (TOVP) ongoing studies to Tailings Optimization and Validation Program (TOVP) ongoing studies to Verify and improve process, Verify and improve process, Monitor tailings management facility, and Monitor tailings management facility, and Validate model assumptions Validate model assumptions

Tailings Neutralization Tailings neutralization circuit increases Fe:As > 3 Tailings neutralization circuit increases Fe:As > 3 Ferric sulfate is used to increase Fe:As Ferric sulfate is used to increase Fe:As Neutralization by slaked lime Neutralization by slaked lime Two stage process Two stage process pH 4 first tank pH 4 first tank pH second tank pH second tank

Tailings Neutralization Circuit

Raffinate Neutralization Experiments Plant raffinate spiked ~ 700 mg/L As, 500 mg/L Ni Plant raffinate spiked ~ 700 mg/L As, 500 mg/L Ni Ferric sulfate added Ferric sulfate added Dry Lime (CaO), slaked lime (Ca(OH) 2, or NaOH Dry Lime (CaO), slaked lime (Ca(OH) 2, or NaOH Some experiments included leach residue Some experiments included leach residue Short duration to simulate residence times in Neutralization Circuit Short duration to simulate residence times in Neutralization Circuit Filtered samples - solution analyses Filtered samples - solution analyses Solids air dried - mineralogical analysis Solids air dried - mineralogical analysis Raffinate 2 series, Redox measurements, single beaker experiments Raffinate 2 series, Redox measurements, single beaker experiments

Setup

Raffinate Compositions (mg/L) Parameter Raffinate 1 Unspiked Raffinate 1 Raffinate 2 Unspiked pH Total As (920) 693 (729) Sum of As (III+V) As(III) As (V) Fe Total6292,400 (2,700)808 1,854 (2,400) Fe (II) Al Ca Na Sulfate 14,100 21,430 Ni (550) (529) Eh (mv) 668 Fe/AsNA4.4NA 3.6

Neutralization Tests at pH Values of 2.2 to 7.4 (right to left) Beaker on Far Left is Slurried Lime)

Samples and 6

Filter Cake Wet

Mineralogy Studies X-ray diffraction SEM with Energy Dispersive Spectrometry Chemical analysis, mainly XRF, Fe(II) EXAFS- Canadian Light Source and Argonne APL Microprobe (electron beam, synchronous radiation) Quantitative mineralogy - XRD, analysis

Single Beaker Tests Raffinate 2 Separated mineral precipitates rather than accumulate all precipitates Enhanced X-ray diffraction determinations Initial volume of solution ~ 5 L Three solution samples collected Four solid samples

Single Beaker Experiment

Single Beaker, #1 First Filter Cake

Single Beaker #4 Wet Product

Arsenic Concentrations as Function of pH

Nickel Removal

Solution Concentrations

Molar Amounts Removed from Solution Raffinate 2

Saturation Indices (based upon As (V) and Fe(III) concentrations)

Ion Activity Products

Eh-pH diagram for the systems Fe – As – O – H based upon stabilities of scorodite and ferrihydrite using speciated As and Fe data from the Raffinate 2 neutralization experiments

Geochemical Model PHREEQC used PHREEQC used Ferric arsenate complexes included in database Ferric arsenate complexes included in database Disequilibrium steps included to explain removal of Ni at low pH Disequilibrium steps included to explain removal of Ni at low pH Force fit Force fit Simple mineral precipitation reactions Simple mineral precipitation reactions Gypsum – Sulfate [CaSO 4 2H 2 O] Gypsum – Sulfate [CaSO 4 2H 2 O] Scorodite (Ksp adjusted) – Fe and As to pH ~5 [FeAsO 4 2H 2 O] Scorodite (Ksp adjusted) – Fe and As to pH ~5 [FeAsO 4 2H 2 O] Ferrihydrite/Green Rust II- Iron Fe(OH) 3 / Fe 3 (OH) 8 Ferrihydrite/Green Rust II- Iron Fe(OH) 3 / Fe 3 (OH) 8 Theophrastite – Nickel [Ni(OH) 2 ] Theophrastite – Nickel [Ni(OH) 2 ] Annabergite – Nickel [Ni 3 (AsO 4 ) 2 8H 2 O] Annabergite – Nickel [Ni 3 (AsO 4 ) 2 8H 2 O] Basaluminite – Aluminum [Al 4 SO 4 (OH) 10 nH 2 O] Basaluminite – Aluminum [Al 4 SO 4 (OH) 10 nH 2 O]

Tailings Neutralization Process Initial Reaction Analyte(mg/L)(% prec.) Al As Fe Ni Si , Solution pH 1.5 McClean Lake Operation

Tailings Neutralization Process pH Gradient Analyte(mg/L)(% prec.) Al As Fe Ni Si , ~10 ~3 ~5 0.0 Solution pH 1.5 McClean Lake Operation

Tailings Neutralization Process Bulk Neutralization Analyte(mg/L)(% prec.) Al As Fe Ni Si , Solution pH 2.2 McClean Lake Operation

Tailings Neutralization Process Bulk Neutralization Analyte(mg/L)(% prec.) Al As Fe Ni Si Solution pH 4.0 McClean Lake Operation

Tailings Neutralization Process Bulk Neutralization Analyte(mg/L)(% prec.) Al As Fe Ni Si Solution pH 6.1 McClean Lake Operation

Tailings Neutralization Process Bulk Neutralization Analyte(mg/L)(% prec.) Al As Fe Ni Si Solution pH 7.4 McClean Lake Operation

Tailings Neutralization Process Aging Analyte(mg/L)(% prec.) Al As Fe Ni Si Solution pH 7.4 McClean Lake Operation

Flow Diagram RAFFINATE pH = 1.5 pe = As = 747 mg/L Ni = 571 mg/L Fe = 2,450 mg/L EQUILIBRATE pH = 7 Ni 3 (AsO 4 ) 2 8H 2 O FERRIHYDRITE SOLUTION 11 pH = 7 As = 291 mg/L Ni = 35.8 mg/L Fe = 554 mg/L MIX RAFFINATE SOLUTION 11 MODEL SOLUTION 6-1 pH = 2.18 pe = 11.7 As = 64.7 mg/L Ni = 518 mg/L Fe = 1,830 mg/L EQUILIBRATE pH = 2.18 SCORODITE GYPSUM 90% 10%

Comparison Data and Model

Conclusions Scorodite is dominant As - bearing phase Scorodite is dominant As - bearing phase Balance between relatively stable scorodite and less stable ferrihydrite Balance between relatively stable scorodite and less stable ferrihydrite Adjustments to Ksp produce best fit Adjustments to Ksp produce best fit Precipitation stops by pH ~ 5 Precipitation stops by pH ~ 5 As concentrations < 1 mg/L As concentrations < 1 mg/L Little or no evidence of As sorption process Little or no evidence of As sorption process Disequilibrium removes some nickel Disequilibrium removes some nickel Nickel mainly in theophrastite Nickel mainly in theophrastite Model concentrations higher than measured Model concentrations higher than measured Sorption may also play a role Sorption may also play a role