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

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

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

14. Example Calculation
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 Released Arsenic in JEB Mill Tailings using 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