Attenuation of metals in tailings Lars Lövgren, Tomas Hedlund and Torbjörn Karlsson Department of Chemistry, Umeå University
Background - Acid Mine Drainage Weathering (oxidation) of sulphidic mine waste produces acidity and releases metal ions and sulphate. H 2 O O 2 O 2 PbS ZnS feldspars FeS quarts 2 FeS calcite CuFeS 2 2 FeAsS acidity arsenic iron(ii) lead(ii) copper(ii) sulfate zinc(ii) calcium sodium magnesium potassium FeS 2 + 3,5O 2 + H 2 O Fe 2+ + 2SO 2-4 + 2H + Pyrite + oxygen + water divalent iron + sulfate + hydrogen ions ZnS + 2O 2 Zn 2+ + 2SO 4 2- Sphalerite + oxygen zinc ions + sulfate
Oxidised tailings (weathered) silicates + iron(hydr)oxides Oxidation front Unweathered tailings The acidity produced by sulphur oxidation is, at least partly, neutralised by weathering of carbonate minerals and more soluble silicate minerals. Photo: LTU
Background: Previous studies (LTU/MiMi) Metal concentrations ti in pore water decrease with depth. Kristineberg, Impoundment 1 Arsenic Cadmium From: Holmström, Salmon, Carlsson, Petrov and Öhlander, 2001
Attenuation of metals by unweathered tailings may be significant In absence of oxygen (unoxidised tailings): Adsorption of metal ions to mineral particles: sulphides, silicates etc Precipitation of hydroxides, carbonates and sulphates, e.g. FeCO 3 (s) and ZnCO 3 (s) F S d F S i t ili f ti f FeS and FeS 2 in tailings formation of secondary sulphides, e.g. CuS, FeAsS
Goals of the present project The main goals are to: identify the minerals in mining waste able to contribute metals in short and long term, and to facilitate prediction of the quality of discharging ground water by quantifying the metal retardation capacity of tailings of different dff composition.
Methods Sampling - Core drilling Chemical characterisation of tailings and pore water Inductively Coupled Plasma-AES/MS, Scanning Electron Microscopy Physical characterisation of tailings - Porosity, Specific surface area Mineralogical characterisation - X-ray Powder Diffraction, Mineral Liberation Analysis Structural investigations - X-ray Absorption Spectroscopy (XAS/XANES) at MAX-lab, Lund, X-ray Photoelectron Spectroscopy Quantification of uptake capacity Column and batch experiments Construction of a spreadsheet model for metal retention in tailings Construction of a spreadsheet model for metal retention in tailings considering chemical and mineralogical composition.
Sampling sites Kristineberg: Cu-Zn-mine with a high content of FeS 2 (ca. 30%). Background information available in large amounts. Aitik: Cu-mine with low sulphide content (<1%), but net acid producing. Zinkgruvan (Askersund): Zn-mine, moderate sulphide content. Not net acid producing. Boliden: Fresh tailings from the concentrator.
Sampling of tailings late autumn 2008 Aitik Kristineberg Enemossen, Zinkgruvan
Results Copper is removed from water at ph > 5 Copper uptake by tailings in batch experiment % Cu remov ved from solution 100% aitik batch1 80% aitik batch2 60% Zinkgruvan 40% Boliden 20% solubility curve Cu(OH)2(s ) 0% 4,00 6,00 8,00 10,00 12,00 ph in pore water Zinkgruvan ph ph in pore water Aitik
Results Copper removal by tailings at ph 5 /g tailings) Cu adso orbed (µmol Copper 60.00 50.0 40.0 30.0 20.0 10.0 0.0 0.0 20.0 40.0 60.0 80.0 100.0 120.0 Cu added (μmol/g tailings) Kriberg 1 Kriberg 2 Zinkgruvan Aitik
Results Column experiments with Cu at ph 5 0,30 10,0 Column with tailings Cu umol out 025 0,25 0,20 015 0,15 0,10 005 0,05 0,00 0,0 0 20 40 60 80 9,0 8,0 7,0 6,0 50 5,0 ph 4,0 3,0 2,0 1,0 copper ph Cu umol in Outgoing water is collected and analysed A copper solution is pumped through the column
Results Column experiments with copper at ph 5: Uptake of Cu(II) ions by neutralized tailings from Aitik, ph=5 Uptake of Cu: ca. 0.2 g/kg Conc. of Cu in Aitik tailings: ca 0.2 g/kg Cu uptake µm mol/g tailing 3 2,5 2 1,5 1 0,5 100% removal fast reaction Slower process 0 0 2 4 6 8 10 Cu added µmol/g tailing Column experiment (flow rate 1ml/h = 0.013µmol /g h)
Results XANES: Added Cu does not bind to sulphide on mineral surfaces at least not initially CuS Boliden rent Copper sulphide pure substance Fresh tailings from Boliden First Derivative Abso orbance CuO CuOH B5, 400 µmol B4, 200 µmol Z3, 100 µmol Kri2 14, 50 µmol CuAq 8,965 8,975 8,985 8,995 9,005 Energy (kev) Copper oxide pure substance Copper hydroxide pure substance Fresh tailings from Boliden + Cu added Tailings from Zinkgruvan + Cu added Tailings from Kri-berg + Cu added Copper dissolved in water pure substance
Results Arsenic removal by tailings adsorb bed (µmol/g ta ailings) Arsenic 70.0 60.0 50.00 40.0 30.0 20.0 10.0 0.0 0.0 20.0 40.0 60.0 80.0 100.0 Cu added (µmol/g tailing) Kriberg 1 Kriberg 2 Zinkgruvan Aitik
Results Arsenic removal at ph 8 Boliden: S content in tailings 4.4 % Zinkgruvan: S content in tailings 0,9 %
Summary Metals are removed from water by tailings Copper is taken up at ph>5 Initial uptake is likely explained by precipitation of Cu(OH) 2. Supported by XANES analysis. A slower process may involve phase transformation, e.g. formation of secondary copper sulphide. To be shown. As long as ph can be maintained (at least) neutral Cu will be retained ion the tailings. Uptake of arsenic appears to be more strongly correlated to sulphide content of tailings.