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Mine Water and the Environment

, Volume 34, Issue 4, pp 364–374 | Cite as

Jarosite Solid Solution Associated with Arsenic-Rich Mine Waters, Macraes Mine, New Zealand

  • G. Kerr
  • J. Druzbicka
  • K. Lilly
  • D. CrawEmail author
Technical Article

Abstract

Jarosite is one of the principal precipitates from process waters of the pressure-oxidation autoclave system at the Macraes gold mine, and this jarosite is discharged with the process waters to a tailings impoundment for long-term storage. Dissolved sulfate and arsenate concentrations in the autoclave, inferred from mineralogy, both exceed 0.1 mol/L (≈10,000 mg/L). Coarse-grained (≈1 mm) jarosite crystals, precipitated as mineral scales, provide insight into the mineralogical nature of the jarosite. The jarosite crystals are zoned, with some zones having up to 12 wt% Al, as part of the jarosite-alunite solid solution series. Substitution of up to 1.7 wt% Na for K also occurs. Both the monovalent and trivalent sites in the jarosites have apparent deficiencies (≈10 atm.%) because of protonation of the hydroxyl groups. There is <0.3 wt% As substitution for S in the jarosite anionic site, despite the abundant dissolved As in the autoclave solutions. Instead, jarosite is intergrown with ferric arsenate, and most jarosite growth zones contain numerous small (<20 μm) inclusions of this ferric arsenate. Ferric arsenate precipitation with jarosite occurs as dissolved As decreases by several orders of magnitude during the latter stages of oxidation in the autoclave slurry. The lack of structural As and only minor Na substitution limits the destabilization effects of solid solution within the jarosite in contact with the tailings waters, and jarosite dissolution is expected to be slow. Dissolution is further limited by high (up to 8,000 mg/L) dissolved sulfate in the tailings impoundment. Both jarosite and associated ferric arsenate dissolve incongruently in the surficial environment, yielding ferric oxides/hydroxides, which adsorb As, so that dissolved As is generally <10 mg/L in the tailings water.

Keywords

Ferric arsenate Scorodite Alunite Mine tailings Autoclave 

Jarosit-Mischkristalle assoziiert mit arsenreichen Bergbauwässern, Macraes Mine, Neuseeland

Zusammenfassung

Jarosit ist eines der Hauptpräzipitate aus Prozesswässern des Oxydationsautoklaven der Macraes Goldmine. Dieser Jarosit wird zur Langfristlagerung zusammen mit den Prozesswässern in ein Absetzbecken eingebracht. Die Konzentration gelöster Sulfate und Arsenate im Autoklaven, aus der Mineralogie hergeleitet, übertrifft 0,1 Mol/L (≈10.000 mg/L). Grobkörnige (≈1 mm) Jarositkristalle, welche als Krusten an den Wänden ausfallen, erlauben Einblicke in die mineralogische Natur der Jarosite. Die Kristalle sind zoniert, wobei manche Zonen bis zu 12 Gew. % Al erreichen, als Teil der Jarosit-Alunit Mischkristallserie. Substitution von K durch bis zu 1,7 Gew. % Na kommt vor. Auf Grund der Protonation von Hydroxylgruppen haben monovalente und trivalente Gitterorte der Jarosite scheinbare Lücken (≈10 Atom %). An anionischen Positionen der Jarosite wird S durch <0,3 Gew. % As substituiert, obwohl in den Autoklavenfluiden reichlich As gelöst ist. Dagegen ist Jarosit mit Ferri-Arsenat verwachsen und die meisten Anwachszonen in Jarosit enthalten unzählige kleine (<20 μm) Einschlüsse von Ferri-Arsenat. Während der letzten Oxydationsstadien der Suspension im Autoklaven nimmt gleichzeitig mit der Ausfällung von Ferri-Arsenat und Jarosit das gelöste As um mehrere Potenzen ab. Der geringe Anteil von strukturellem As und die geringe Na-Substitution begrenzen die destabilisierende Wirkung der Jarositmischkristalle in Kontakt mit dem Wasser im Absetzbecken. Es wird erwartet, daß die Lösung der Jarosite sehr langsam sein wird. Begrenzt wird die Lösung ferner durch hohe (bis zu 8000 mg/L) Sulfatkonzentration in dem Absetzbecken. Unter Oberflächenbedingungen zeigen Jarosit und das begleitende Ferri-Arsenat inkongruentes Lösungsverhalten, indem Ferrioxyde und Hydroxyde entstehen, welche As adsorbieren. Deshalb bleibt die Konzentration von gelöstem As im Wasser des Absetzbeckens generell <10 mg/L.

Solución sólida de jarosita asociada con aguas de minas ricas en arsénico, mina Macraes, Nueva Zelanda

Resumen

Jarosita es uno de los principales precipitados en los procesos de oxidación en autocalves de la mina de oro Macraes y dicha jarosita es descargada con las aguas del proceso a los embalses de colas para un largo almacenamiento. Las concentraciones de sulfato y arseniato en el autoclave, inferidas a partir de la mineralogía, exceden 0,1 moles/l (≈10,000 mg/L). El tamaño de grano (≈1 mm) de los cristales de jarosita, precipitada a escala mineral, proporciona cierta información sobre la naturaleza mineralógica de la jaorista. Los cristales de jarosita están divididos en zonas, algunas de las cuales contienen hasta 12 % p/p Al, como parte de la serie de soluciones sólidas jarosita-alunita. También se observa la sustitución de hasta 1,7 % p/p Na por K. Los sitios monovalentes y trivalentes de la jarosita tienen aparentes deficiencias (≈10 % en átomos) debido a la protonación de los grupos oxhidrilos. Hay <0,3 % p/p de sustitución de As por S en el sitio aniónico de la jarosita, independientemente de la abundancia de As disuelto en las soluciones del autoclave. Jarosita está intercrecida con arseniato férrico y la mayoría de las zonas de la jarosita contienen numerosas y pequeñas inclusiones (<20 μm) del arseniato férrico. La precipitación de arseniato férrico con jarosita ocurre cuando el As disuelto decrece por varios órdenes de magnitud durante los últimos estadíos de la oxidación en el autocalve. La ausencia de As estructural y mínimas sustituciones por Na limita los efectos de desestabilización de la solución sólida de la jarosita en contacto con aguas de cola, por lo que se espera que la disolución de la jarosita sea baja. La disolución está, además, limitada por la alta cantidad (hasta 8000 mg/L) de sulfato disuelto en los embalses de colas. Tanto jarosita como el arseniato férrico asociado se disuelven de modo incongruente en el ambiente superficial, dando óxidos e hidróxidos férricos que adsorben As de modo que el As disuelto es generalmente <10 mg/L en el agua de cola.

Notes

Acknowledgments

This research was financed by the University of Otago and NZ Ministry of Business, Innovation and Employment. This project would have been impossible without the ongoing enthusiasm and logistical support for research on the Macraes mine site by Oceana Gold Ltd, which is much appreciated. In particular, discussions with Geua Mola and Quenton Johnston were very helpful. Constructive comments by two anonymous referees substantially improved the presentation of the paper.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2014

Authors and Affiliations

  1. 1.Department of GeologyUniversity of OtagoDunedinNew Zealand

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