Effect of sulfate and ferrous iron on Ca, Co, Cr, Pb, U and Zn leaching from green liquor dregs

Nanna Stahrea, M. Bäckströma, and L. Sartza

a Man-Technology-Environment Research Centre, Örebro University, Sweden

nanna.stahre@oru.se

Leaching tests for evaluation of materials for co-disposal, landfilling or use as remediation material is often performed using HCl. But when evaluating a material for acidic sulfide mine waste remediation leaching with HCl is not ideal since the typical ARD often contains high concentrations of sulfate and ferrous iron that have an impact on the mobility of trace elements. Green liquor dregs (GLD) is an alkaline by-product from the paper and pulp industry with a pH between 10 and 14. It mainly consists of non-process elements from the wood pulping, organic material and spent cooking chemicals as well as calcite. Samples were collected from two different mills in the southern part of Sweden (Aspa and Frövi). Frövi adds lime mud (mainly calcite) during the final dewatering of GLD while Aspa does not. Each GLD was leached with three acidic solutions: HCl, H2SO4 and H2SO4+Fe2+ (final Fe2+ concentration 100 mg/L). 11 subsamples of each GLD was added different amounts of leaching solution (including an untreated subsample). Leaching of calcium and lead is strongly affected by the composition of the leaching solution due to the formation of sulfate mineral precipitates. HCl leaches much higher concentrations than H2SO4 and H2SO4+Fe2+. For calcium leaching with H2SO4 regardless of addition of Fe results in calcium reaching a constant concentration of 437 mg/L for Aspa and 432 mg/L for Frövi at or above pH 8. For lead, when lime mud is present, leaching with H2SO4+Fe2+ yields lower leached concentrations than when only using H2SO4. This effect can also be seen for chromium but the HCl leached amount is not as much higher as for calcium and lead. For chromium it can also be seen that from approximately pH 7.75 the leached concentrations from the H2SO4 leached samples decrease linearly to about pH 6.75 where the leached concentrations are at their lowest (the effect is clearer when lime mud is present). This suggest that there is a presence of negatively charged chromium as Cr(VI) and its subsequent reduction to Cr(III). In the presence of lime mud uranium and cadmium leached less when leached with H2SO4 compared to when leached with HCl (5-10% for cadmium, 4-14% for uranium). Addition of Fe gives slightly lower concentrations for uranium and slightly higher concentrations for cadmium. However, these effects are not seen as clearly for uranium when lime mud is not present in the samples and not at all for cadmium. This is attributed to the speciation of uranium ($\mathrm{\text{UO}}_{\mathrm{2}}^{\mathrm{2 +}}$) forming negative complexes when carbonate and/or sulfate are present. For zinc and cobalt no significant difference was noted for different leaching solutions. pH is lowered by the addition of ferrous iron with around 0.55 to 0.14 pH units. Elements most susceptible to the presence of sulfate, carbonate and ferrous iron are lead and chromium whereas zinc and cobalt are not affected. In conclusion when evaluating an alkaline material through acid leaching for use in reclamation of acidic mine waste it is important to use a leaching solution similar to the ARD and not only HCl in order to not overestimate the leached concentrations of trace elements.

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