Evaluation of acid mine drainage and metal release potential of cemented paste backfill of sulfide rich tailings

Sulphide-containing tailings can be stored in underground cavities as cemented paste backfill (CPB). Due to the sulphur (S wt.%) content, it is significant to predetermine the possible environmental impacts of CPB. This study investigates the potential for metals leachability and acid mine drainage (AMD) formation (SO4=, pH) from sulphide-rich tailings (TA) and granular- and monolithic forms of CPB produced using these tailings with static, modified acid-base accounting (ABA) tests and kinetic humidity cell tests (HCT). The results have shown that the pH of CPB leachates remained close to neutral levels (~pH 7) throughout the 40-week HCT period, whereas TA leachates became acidic (≤pH 7) after the 5th week. The average SO4= release from monolithic CPB (CPBm) was about 2.5-fold lower than granular CPB (CPBg) and, 10.8-fold lower than TA samples. The extent of metal leachability from TA was observed to depend on pH, increasing steadily with decreasing pH. The leachability of metals from CPBs remained comparatively at low levels, i.e. 60–1000 times lower than those from TA. The leaching of metals was observed to reach almost a plateau after the initial 33 weeks for all samples. These findings reveal that CPB generates effluents, containing heavy metals within safe levels as per regulatory limits for water quality standards (EPA, TS, and WPC). Additionally, CPB provides a higher neutralisation potential than TA. This study has shown that a well-designed CPB is highly effective in mitigating the formation of AMD and metals leaching compared to TA, providing an environmentally friendly storage for sulphide-rich tailings.