A Method to Measure Arsenic Readily Released to Pore Waters from Uranium Mill Tailings

1. A Method to Measure Arsenic Readily Released to Pore Waters from Uranium Mill Tailings • John Mahoney Hydrologic Consultants, Inc., 143 Union Blvd., Suite 525, Lakewood, Colorado, USA • Donald Langmuir Hydrochem Systems Corp., Denver, Colorado, USA • John Rowson COGEMA Resources, Saskatoon, Saskatchewan, Canada

2. Location Map

3. Introduction • Polymetallic U ores with As and Ni - in Raffinate • Raffinate 300-700 mg/L As, 200-500 mg/L Ni, pH~1.0 • Diffusion of As into Fox Lake required that As and Ni concentrations in tailings be around 1 mg/L • Tailings neutralization circuit increases Fe:As >3 • Neutralization with slaked lime to pH ~ 7

4. Raffinate Fe (SO ) 2 4 3 CaO (pH 8) Tailings CaO (pH 4) BaCl 2 Flocculant pH pH NEUTRALIZATION TANKS MIXING TANK THICKENER Process Air to TMF 24 hour metallurgical composite sample Tailings Neutralization Circuit

5. Mill and JEB TMF

6. Final Decommissioned TMF Glacial Till Waste Rock Tailings Mass Athabasca Sandstone

7. 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) • Demonstrated that As < 1 mg/L was possible • Tailings Optimization and Validation Program (TOVP) ongoing studies to • Verify and improve process, • Monitor tailings management facility, and • Validate model assumptions

8. Readily Released Arsenic • Regulatory Agencies required - quantify amount of arsenic released from tailings via “desorption” process • Surface Complexation • Complicating factors • Amorphous Arsenic bearing phases - Scorodite, Annabergite, Coprecipitates • Need procedure that only targets “surface complexed” arsenic

9. Issues • Discriminate As released through desorption while not releasing As in phases (Scorodite) • Minimize time for dissolution • Use a gentle extraction procedure • Amorphous phases preclude extraction agents • hydroxylamine hydrochloride, oxalic acid • Spiking methods may change solution composition and require model calculations • adding phosphate or molybdate

10. Methodology • Developed to Measure Chlorinated Solvents in Soils - Portable Gas Chromatograph using Headspace Analysis • Changing liquid/solid ratio changes concentration in solution • Basically a Matrix Correction • Serial Dilution Method • Similar to Method of Standard Additions (MSA) • Crawling Down the Isotherm

11. EPICS Procedure • EPICS = Equilibrium Partitioning In Closed System • Found Alternative Method in Gossett (1987) • Calculate Henry’s Law constants

12. Pure water Pure water EPICS Method, No SorptionEs gibt nichts praktischeres als die Theorie - Boltzmann 3L 0.33 mg/L 2L 0.5 mg/L 1L 1 mg/L

13. Pure water Pure water As As EPICS Method, With Sorption 3L 0.45 mg/L 2L 0.655 mg/L 1L 1 mg/L

14. ExampleCalculation With Sorption

15. EPICS Plot Dilution Kd = 0/1 = 0

16. Features of Method • Everything referenced to solution changes Requires solution concentrations only • Matrix effect for soil water system • Produces dimensionless Kd • Suited for other hydrologic studies

17. Initial Modeling to Verify Procedure • Simulated models using PHREEQC • Defined Pore water and Surface - Diffuse Layer Model • Defined Arsenic Free Solution • Use MIX keyword to re-equilibrate with Surface • Arsenic free leaching solution added in steps • Changing proportions to mixtures • Surface re-equilibrates with new solid/solution ratio • Various simulation to cover range of conditions

18. PHREEQC Modeling

19. PHREEQC Based EPICS

20. Verification of the EPICS Procedure by Comparison with PHREEQC Modeling Results

21. Verification of Method • PHREEQC models verified that overall approach and calculational method was correct • Laboratory forms checked with PHREEQC results

22. Measurements on Tailings • Models required constant major ion composition • Surface charge effects may be important • Maintain major ion concentration and pH • Experimental Design • 1Kg Slurry (tailings and pore water) • Leach solution made from neutralized solution • Very Small Sample Aliquots < 25 ml • Dilutions ~ 1L arsenic free pore water/step • 3 Steps - V4/V1 > 4 • 1 hour (12 hours) between extractions

23. Initial Experiments

24. Summary of Arsenic Desorption Measurements, One Hour Extractions V4/V1 is the overall dilution for the tests.

25. Long Duration Experiments

26. Summary of Arsenic Desorption Measurement Results - 12 Hours Extractions V4/V1 is the overall dilution for the tests.

27. Calculation of Readily ReleasedArsenic in JEB Mill Tailingsusing the EPICS Method

28. Percentage of Readily Released Arsenic in Mill Tailings

29. Conclusions - Modeling • PHREEQC used to verify approach • Simulation of experiments • Assist in experimental design • Check spreadsheets setup

30. Conclusions - Methodology • Method is simple application of accepted technique • Various approaches are essentially same method • Straightforward method to measure distribution coefficient • Should be applied to other systems

31. Conclusions – Arsenic in Tailings • Amount of readily released arsenic in JEB TMF is very small ~ 0.2 percent of total • Consistent with observation - adsorption of arsenic on ferrihydrite is not reversible • Most Arsenic in Scorodite • Suggests that adsorption of As is not occurring on other surfaces