Element leaching from green liquor dregs from 16 Swedish pulp and paper mills between 2017 and 2019

Several studies have found that green liquor dregs (GLD) can be a possible remediant for remediation of oxidised sulfidic mine sites. GLD is a heterogeneous alkaline waste from the pulp and paper industry, and as such, broad scale utilization requires environmental properties to be characterized in accordance with Swedish law. Among these properties, water soluble leaching properties of GLD must be examined. In this study samples were collected from 16 mills at five different occasions, approximately 6 months apart, totalling 71 samples. Leaching was performed in sequential steps at a L/S ratio 2 and 8 on wet samples corresponding to 25 g d.w, resulting in a total L/S ratio of 10. The liquid phase was analysed for electrical conductivity, pH, alkalinity, anions and element concentrations, after which correlation calculations and geochemical calculations were performed. Leaching of sodium and potassium was as expected very high while leaching of calcium, magnesium and iron was low. Trace elements leach generally less than ≤ 5%. Na, K, As, Ba, Na, K, Rb and U are present mainly as easily soluble salts that are washed out over time while Ag, Co, Cr, Cu, Ni, Pb, Se, Sr, V and Zn are mainly solubility controlled. Hydroxides are indicated by geochemical calculations to be the solubility controlling phases for Cd, Co, Ni, Zn and in some cases also for lead, while oxides are the suggested controlling phase for cobalt. For copper the results suggested both oxides and hydroxides as controlling phases. In general, most elements of concern had low enough leaching rates that their impact will be moderate as leaching in the study was under optimal conditions and in situ application is expected to lower leching rates further. When comparing the chemistry of leachates from GLD with the chemistry of leachates from acidic mining waste there are environmental benefits to be had from co-disposal even if GLD is not inert. This combined with the fact that both short term and long term buffering capacity is excellent indicates that GLD is suitable for remediation of acidic mining waste.