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A Method to Measure Arsenic Readily Released to Pore Waters from Uranium Mill Tailings John Mahoney Hydrologic Consultants, Inc., 143 Union Blvd., Suite.

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Presentation on theme: "A Method to Measure Arsenic Readily Released to Pore Waters from Uranium Mill Tailings John Mahoney Hydrologic Consultants, Inc., 143 Union Blvd., Suite."— Presentation transcript:

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 Polymetallic U ores with As and Ni - in Raffinate Raffinate 300-700 mg/L As, 200-500 mg/L Ni, pH~1.0 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 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 Tailings neutralization circuit increases Fe:As >3 Neutralization with slaked lime to pH ~ 7 Neutralization with slaked lime to pH ~ 7

4 Tailings Neutralization Circuit

5 Mill and JEB TMF

6 Final Decommissioned TMF AthabascaSandstone Tailings Mass Waste Rock Glacial Till

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) 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

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

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

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

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

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

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

14 Example Calculation With Sorption

15 EPICS Plot Dilution Kd = 0/1 = 0

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

17 Initial Modeling to Verify Procedure Simulated models using PHREEQC Simulated models using PHREEQC Defined Pore water and Surface - Diffuse Layer Model Defined 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 steps Arsenic free leaching solution added in steps Changing proportions to mixtures Changing 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 conditions 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 PHREEQC models verified that overall approach and calculational method was correct Laboratory forms checked with PHREEQC results Laboratory forms checked with PHREEQC results

22 Measurements on Tailings Models required constant major ion composition Models required constant major ion composition Surface charge effects may be important Surface charge effects may be important Maintain major ion concentration and pH Maintain 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 solution Leach solution made from neutralized solution Very Small Sample Aliquots < 25 ml Very Small Sample Aliquots < 25 ml Dilutions ~ 1L arsenic free pore water/step Dilutions ~ 1L arsenic free pore water/step 3 Steps - V4/V1 > 4 3 Steps - V4/V1 > 4 1 hour (12 hours) between extractions 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 PHREEQC used to verify approach Simulation of experiments Simulation of experiments Assist in experimental design Assist in experimental design Check spreadsheets setup Check spreadsheets setup

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

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


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