
GEOCHEMISTRY OF ARSENIC IN URANIUM MILL TAILINGS. SASKATCHEWAN. CANADA A Thesis Submitted to the College of Graduate Studies and Research in Partial Fulfillment of the Requirements for the Degree of Doctor of PhiIosophy in the Department of Geological Science University of Saskatchewan Saskatoon BY Robert Donahue Fall 2000 Q Copyrigh~Robert Donahue. 2000. AII rights reserved. Acquisitions and Acquisitions et Bibliographic Services services bibriraphiques 395 WoUingbn Street 385, me Weltingtcf~ 0th~~ON KIA OM OttawaON KlAW CaMda Canada The ador has granted a non- L'auteur a accorde une Licence non exclusive licence allowing the exclusive pennettant a la National Library of Canada to BibIioth&pe nationale du Canada de reproduce, loan, distriiute or sell reproduke, preter, distnibuer ou copies of this thesis in microform, vendre des copies de cette these sous paper or electronic formats. Ia forme de rnicrofiche/film, de reprocluction sur papier on sur format dectronique. The author retains ownership of the L'auteur conserve la proprikte du copyright in this thesis. Neither the droit d'auteur qui prdge cette these. thesis nor substantial extracts hmit Ni la these ni des extraits mbstantiels mybe printed or otherwise de celle-ci ne doivent %re imprim& reproduced without the author's ou autrement reproduits sans son permission, autorisation. h pfesenting this thesis in partial llfillment of the requirements for a Postgraduate degm from the University of Saskatchewan. I agree that the Libraries of this University may make it freely available for inspection. I fiuther agree that permission for copying of this thesis in any manner. in whole or in part, for scholarly purposes may be granted by the professor or professors who supervised my thesis work or. in their absence. by the Head of the Department or the Dean of the College in which my thesis work was done. It is understood hat any copying or publication or use of this thesis or parts thereof for financial gain shall not be allowed without my written permission. It is also understood that due recognition shall be given to me and to the University of Saskatchewan in any scholarly use which may be made of any material in my thesis. Requests for permission to copy or to make other use of material in this thesis in whole or part should be addressed to: Head of the Department of Geological Science University of Saskatchewan Saskatoon Saskatchewan (S7N 5E2) ABSTRACT The Rabbit Lake uranium mine &pit tailings body consists of alternating layers of ice. frozen tdings and untiozen tailings. The tailings sotids are predominately composed of quartz (I6 to 36 @A). calcium sulphate (03 to 54 wt%). iIlite (3 and 14 wt%)As-Ni sulphide 2 wt% and iron 2 wt%. Arsenic and Ni concentrations in the tailings showed similar panems with depth, which were strongly related to historical changes in As and Ni concen~onsin the mill feed, Mineralogy of the ore bodies indicated that As and Ni in the mill feed occurred primarily as 1:1 molar ratio arsenides such as niccolite and gersdorffite. EMP analysis suggested that solubilid arsenic is precipitated as Ca Fe. and Ni arsenates during the neutralization process. Dissolved As concentrations in five monitoring wells installed within the tailing body ranged hm 9.6 to 71 mgL. Pore fluids in the wells had a pH between 9.3 and 10.3 and measured Eh between +58 and +213 mV. Sequential extraction analyses of tailings samples showed that As above 34 m depth was primarily associated with amorphous iron and metal hydroxides whiie the As below 34 m depth was associated with Ca. Iikely as amorphous calcium arsenate precipitates. The change in the dominant As solid phases at this depth was attributed to the differences in the molar ratio of Fe to As in the mill feed. Below 34 m it was <2 whereas above 34 m it was >4. The high CdAs ratio during tahgs neutralization would Iikely pkferentially precipitate Ci14(OH)z(As04)~:4H~O.Geochemical modeling. using PHREEQC. suggested that if the pore fluids were brought to equilibrium with hydrated calcium arsenate. the long-term dissolved As concenuations would range between 13 and 81 ma. The predicted pH and speciation of arsenic in the filter sand was dependent on the redox conditions (oxidizing or reducing) assigned to the regional groundwater. Reducing conditions in the regional gmundwter caw HA SO^^ the dominant species in the railings. to be reduced to ~2~~03'-as difhses hmthe railings into the sand Under dphate reducing conditions, imn as ~e*in the filter and is oxidhed to Fe(m) species as the sulphate (S(VI)) present in the tailings diffuses into the filter sand and is reduced to sulphide (S(-Q). The pH in the tailings will decxwe as the high concentrations of protons in the ater sand dBkinto the tailings. As the soIubiIity of calcium arsenate minerals present in the tailings are pH dependent the decrease in pH in the tahgcauses an increase in solubiIiq of the dcium arsenate minerals resulting m the dissolution of caIcium armate minetals. DEDICATION This Thesis is dedicated to Wanda She provided me with the oppo&ty to pursue my goal as a scholar She watched my back when I was alone She provide funding when there was none And most important She believed in me Without you this work would never have been completed Thank you ACKNOWLEDGEMENTS Numerous people played a part in the research required for this thesis. Cameco Corporation NSERC. the Cameco/NSERC chair and Wanda Goulden provided the financial support without which this thesis could not have been completed Thank you to Dr. Don Langmuir. Dr. CAJ, Appeio. and Dr. David L. Parbust for teaching me geochemistry. A special thanks to Dr. Malcolm Reeves for the been, the consulting work and for listening, As well. a special thanks to Dr. Garth van der Kamp for being a pan of my thesis committee. Thanks to the Crew of the S.S. Minnow: The very capable drillers Darryl, Mike and Karl. and the frrst mates Tyler. Ray. Gideon. and Sheldon. Thank you to the staffof the Saskatchewan Research Council Analytical Services. particularly Brenda Stanek for the excellent job they did on the anaIysis of the solids and fluids samples, My thanks to the Rabbit Lake Environment Department. Mark. Rob. Cam and Tim for the help on site and Darryl McClarty and Michael Courtin of Envista Technoio@esfor setting up the database. Thanks to P.M. Huang in Soil Sciences for his help with the sequential extraction analysis and A. Mermout and A. Hossain for their help with the clay mineralogy. Thanks to my supervisors Dr S. Lee Barbow and Dr. M. Jim Hendry. I acknowledge the support ofthe staff and muate students in Geology and especially Tom Bonli for his help with the electron micropbe work A special thanks to Wayne Clifton, Phg. for aIl his support and encouragement over the years. A finally a big pet for my dog gromit for heburs of one sided discussions on reactive transportaaalysis. TABLE OF CONTENTS PERMISSIONTO USE ..................................... .. .................................................................... n ABSTRACT ..... ....................................... .......... .. .. .. ..... .. ......... 111 DEDICATION.......................... ACKNOWLEDGEMENTS ............................................................................................. V TABLE OF CONTENTS............................................................................................................ VI LIST OF TABLES ....................................................................................................................... IX LIST OF FIGURES ..................................................................................................................... XI CHAPTER I.0 INTRODUCTION ..................................... ... ............................................ I 1.1 URANiUM MILL TAILINGS MANAGEh4ENT ...................................................... I 1.2 OBJECTIVES ................................................... ........................*.*..*.......... *...*..* ........... 3 13 METHODOLOGY ..................................................................................................... 4 1.4 REFERENCES ...............................................................................................................5 CHAPm 2.0 DISTRIBUTION OF ARSENIC AND NICKEL IN tRANKhI MILL TAILINGS. RABBIT LAKE, SASKATCHEWAN.CANADA ............................,., .... 7 ABSTRACT ......................................................................................................................... 7 2.1 INTRODUCTTON .................. **..O* ...O..* ..................................................................... 7 22BACKGROUND ........................................ .............................................................. 10 2.2. I Climatic Conditions I 222 Uranium Ore Bodies Milied 1 2.23 MiKing Processes 1 22.4 Tailings Management 1 23 MATERIALS AND METHODS ....................................................... ,................. ,............... 12 2.4 RESULTS AND DISCUSSION .......................-- ........................................................... IS 2.4. t STRATIGRAPHY OF THE TMF ............................................................ 15 2.42 ELEMENTAL ANALYSIS ............................................ ................................... .... 17 2.43 MMERALOGY................................................ - .......... .......................................... 20 2.5 SUMMARY AND CONCLUSIONS.........................................-....-.......................
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