Attenuation of metals in tailings Lars Lövgren, Tomas Hedlund and Torbjörn Karlsson Department of Chemistry, Umeå University

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1 Attenuation of metals in tailings Lars Lövgren, Tomas Hedlund and Torbjörn Karlsson Department of Chemistry, Umeå University Background Oxidation of sulphide minerals releases metals and acidity Weathering (oxidation) of sulphidic mine waste produces acidity and releases metal ions and sulphate. Limited access to O 2 (within tailings) (di)sulphide is oxidised FeS 2 + 3,5O 2 + H 2 O Fe SO H + (In contact with air: Fe(II) is oxidised) Fe 2+ + ¼ O H 2 O Fe(OH) 3 (s) + 2H + ZnS + 2O 2 Zn SO 4 2-1

acidity and releases metal ions and sulphate.

2 The acidity is partly or completely neutralised and metal ions may be removed from water in weathered tailings in unweathered tailings in the underlying ground in the discharge area in downstream surface waters all depending on geochemical conditions Several geochemical processes removing metals from water are possible e.g. FeS and FeS 2 in tailings adsorption of Me 2+ /As(III, V) to mineral surfaces formation of secondary sulfides (e.g. CuS, FeAsS) FeS(s) + Cu 2+ CuS(s) + Fe 2+ Precipitation of sulphates (e.g. CaSO 4 and PbSO 4 ) Precipitation of carbonates and hydroxides (e.g. ZnCO 3 (s), CuCO 3 (s) and Cu(OH) 2 (s)) 2

are possible e.g. FeS and FeS 2 in tailings adsorption of Me 2+ /As(III, V) to mineral surfaces formation of secondary sulfides (e.g. CuS, FeAsS) FeS(s) + Cu 2+ CuS(s) + Fe 2+ Precipitation of sulphates (e.

3 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 composition. Methods Sampling - Core drilling Chemical analysis of tailings and pore water Physical characterisation of tailings Mineralogical characterisation Structural investigations Quantification of uptake capacity Column and batch experiments Literature study of relevant geochemical processes Construction of a spreadsheet model for metal retention in tailings considering chemical and mineralogical composition. 3

Methods Sampling - Core drilling Chemical analysis of tailings and pore water Physical characterisation of tailings Mineralogical characterisation Structural investigations

4 Sampling sites Kristineberg: High content of FeS 2 (ca. 30%). Zinkgruvan: Low sulphide content. Not net acid producing. Aitik: Low sulphide content (<1%), but net acid producing. Boliden: Fresh tailings from the concentrator; high in sulphide. Copper uptake by tailings in batch experiment % Cu removed from solution 100% 80% 60% 40% 20% 0% aitik batch1 aitik batch2 Zinkgruvan Boliden solubility curve Cu(OH)2(s) 4,00 6,00 8,00 10,00 12,00 ph ph in pore water ph in pore water Aitik Zinkgruvan 4

Copper uptake by tailings in batch experiment % Cu removed from solution 100% 80% 60% 40% 20% 0% aitik batch1 aitik batch2

5 XANES measurements indicate Cu(OH) 2 (s) CuS Boliden rent Copper sulphide pure substance Fresh tailings from Boliden First Derivative Absorbance 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 Arsenic removal at ph 8 Boliden: S content in tailings 4.4 % Zinkgruvan: S content in tailings 0,9 % 5

substance Copper hydroxide pure substance Fresh tailings from Boliden + Cu added Tailings from Zinkgruvan + Cu added Tailings from Kri-berg +

6 Results from column experiments Cadmium removal in tailings at ph Cd adsorption mmol/g mmol/g Aitik Zinkgruvan Only a small fraction (c.a. 5-6%) of the injected Cd is taken up by the tailings. But Cd in /mmol Uptake of Cd(II) to tailings exceeds adsorption at monolayer level Tailings from Aitik ph 6.5 Specific surface area of tailings 2.5 m 2 /g Uptake normalised to surface area: 0,020 mmol/g ca. 8 µmol/m 2 which exceeds theoretical surface site concentrations (ca 5 µmol/m 2 ) 6

2 0.25 0.3 0.35 Cd in /mmol Uptake of Cd(II) to tailings exceeds adsorption at monolayer level Tailings from Aitik ph 6.

7 Capacity of arsenic(iii) uptake to tailings from Aitik is lower than uptake of Cu(II) umol As adsorb/g tailing Aitik As (batch) 3,0 27 umol As added 2,5 22 umolas added 2,0 16 umolas added 11 umol As added 1,5 6 umol As added 1,0 0,5 0,0 ph 6,000 6,500 7,000 7,500 8,000 0,5 Specific surface area of tailings 2.5 m 2 /g Uptake normalised to surface area: 0.8 µmol/g ca. 0.3 µmol/m 2 which is lower than theoretical surface site concentrations (ca 5 µmol/m 2 ) Sulphide minerals seems to play a greater role for As uptake. An estimate of adsorption capacity of tailings minerals in Kristineberg Assumptions: Silicates active in metal adsorption (65 %), sulphides (25 %) and other minerals (10 %). Density of wet tailings: kg/m 3 Porosity: 40 % (pores are saturated with H 2 O: 400 kg) Mineral phases constitute kg/m 3 Silicates (model substance: kaolinite): Amount of silicates in 1.0 m 3 of tailings: 0.65 * kg = kg Specific surface area: 3 m 2 /g Density of metal bindning surface sites: 5.5 µmol/m 2 (Angove et al., 1997) Amount of surface sites: 21 mol/m 3 7

An estimate of adsorption capacity of tailings minerals in Kristineberg Assumptions: Silicates active in metal adsorption (65 %), sulphides (25 %) and other minerals (10 %).

8 An estimate of adsorption capacity of tailings minerals in Kristineberg Sulphides (pyrite): Fraction of minerals composed by pyrite: 25% Amount of pyrite in 1.0 m 3 of tailings: 0.25*2 000kg = 500 kg Specific surface area: 0.90 m 2 /g (Axelsson, 2002) Density of metal binding surface sites: 4.3 µmol/m 2 (Kornicker and Morse, 1991) Amount of surface sites: 1.9 mol/m 3 Amount of metals in tailings from Impoundment 1 (Holmström et al., 2001) Zn: 0.9 % of 2000 kg/m 3 = 18 kg/m mol/m 3 Cu: 0.1 % of 2000 kg/m 3 = 2.0 kg/m 3 31 mol/m 3 Solubility of carbonates of Cu(II), Cd(II) and Zn(II) atmospheric and enhanced of CO 2 pressure ph log conc (mol/l) Zn ppm CO2) Zn 2+ (1.7 ppm CO2) Cd 2+ (0.4 ppm CO2) Cd 2+ (1.7 ppm CO2) Cu 2+ (0.4 ppm CO2) Cu 2+ (1.7 ppm CO2)

9 mol/m 3 Amount of metals in tailings from Impoundment 1 (Holmström et al., 2001) Zn: 0.9 % of 2000 kg/m 3 = 18 kg/m 3 275 mol/m 3 Cu: 0.1 % of 2000 kg/m 3 = 2.

9 High bicarbonate concs. in tailings groundwater - Saxberget Alkalinity (HCO 3- ) calculated from charge balance Anions: [OH] + [HCO 3- ] + [COO - ] + 2[SO 4 2- ] Cations: [H + ] + [Na + ] + [K + ] + 2[Mg 2+ ] + 2[Ca 2+ ] + 2[Fe 2+ ] + 2[Mn 2+ ] + 2[Ni 2+ ] ph 6.45 at point of discharge ph ca 6.8 at a point 15 m downstream Calculated a alkalinity ca 3.5 meq./l Pressure of CO 2 (g)> atmospheric HCO 3- + H + CO 2 (g) + H 2 O High potential of forming carbonate minerals with Cu(II), Cd(II), Zn(II) and Pb(II) Conclusions The uptake of metal cations (Cu(II), Cd(II) (and Zn(II)) to tailings at neutral ph exceeds adsorption at monolayer level. XANES spectroscopy indicate that sorbed Cu(II) is bound in a form resembling Cu(OH) 2 (s). The uptake capacity with respect to As relates to the tailings content of sulphide minerals. Several of the tailings sampled in the field exhibit a high ph and all samples show large acid neutralising capacities. Relatively large concentrations of bicarbonate ions in tailings pore water makes formation of carbonate phases of e.g. Cu(II), Cd(II) and Zn(II) likely. 9

2[Fe 2+ ] + 2[Mn 2+ ] + 2[Ni 2+ ] ph 6.45 at point of discharge ph ca 6.8 at a point 15 m downstream Calculated a alkalinity ca 3.5 meq.

10 Axknowledgements Linda Malm and Malgorzata Sikora for help with some of the experiments. 10

Hardness - Multivalent metal ions which will form precipitates with soaps. e.g. Ca 2+ + (soap) Ca(soap) 2 (s)

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