Cementation of cyanidation tailings – effects on the release of As, Cu, Ni and Zn

Sammanfattning: Knowledge about mineralogy and chemical composition in sulfidic tailings is essential to predict how tailings management may affect the future leachate quality. At a gold mine in the north of Sweden, gold was extracted from inclusions in arsenopyrite and pyrrhotite by the use of cyanide. Sulfides in the ore dissolved to a large extent during the cyanide leaching process causing sulfide-related elements such as As, Cu, Ni and Zn to be mobilized to a various extent. In a subsequent water treatment process, a significant proportion of As and Cu was captured in secondary formed Fe-precipitates. Large proportions of water-soluble Ni- and Zn-species in tailings suggested that this treatment was insufficient to reduce the mobility of Ni and Zn. Maintaining oxidized, neutral conditions is of major importance for the immobility of As, Cu, Ni and Zn during further management of the cyanidation tailings (CT).Part of the CT were planned to be managed in underground cavities by the use of a cemented paste backfill (CPB) -application. In CPB, a monolithic mass is formed as tailings are mixed with small proportions (4-7 weight %) of pozzolanic materials and backfilled into underground excavated areas. Using a CPB-application may decrease the sulphide oxidation rate, reducing exposure of mineral surfaces to oxygen and increasing water saturation levels within the material. In this study, CT was mixed with binders (1-3 wt. %) for the formation of a low-strength (0.2 Mpa) CT-CPB-mass. These mixtures were stored at moisturized conditions and subsequently subjected to oxidized and flooded conditions in a laboratory-based study. During short-term storing, high water saturation levels were preserved in the CT-CPB-mixtures, but, sulfide oxidation still progressed, and the release of Zn, Cu, and Ni was still lower compared to that in CT. The opposite was true for As, probably due to a desorption from Fe-precipitates. The desorbed As was subsequently incorporated into less acid-tolerant species (i.e. Ca-arsenates and As bonded to cementitious phases) in the CT-CPB:s, that readily dissolved and released more As compared to that in CT.A complete flooding of CPB-filled workings may take a long time to be reached. During this transition period, zones with low levels of water saturation forms in the CPB-monoliths, which could increase the sulphide oxidation rate, lower pH and dissolve the cementitious binders. In this study, strength decreased along with the water saturation levels in the CPB-mixtures, due to a more extensive pyrrhotite oxidation. A minimal proportion (1 wt. %) of binders did not suppress Cu and As leaching during flooding, but Ni and Zn-leaching were still lower than from CT. In the CT-CPB:s, proportions of As, Cu, Ni and Zn associated with cementitious phases increased in tandem with the fraction of binders. Using higher binder proportions in the CPB, as water saturation levels were lowered, substantially increased the Zn-release while there was an insignificant change in the As-release, and substantially lower Cu- and Ni-release. Pyrrhotite oxidation proceeded in the CT-CPB-mixtures independent of water saturation level. So, increasing binder proportion in a CPB does not necessary mean that trace metals are more stabilized, due to the formation of acid-intolerant fractions. Results from this study, pinpoints the importance of having knowledge about trace element distribution and mineral assemblage in tailings before management methods are chosen and implemented.