Acid-forming capacity of lead–zinc mine tailings and its implications for mine rehabilitation

Acid mine drainage problems were experienced in a Pb–Zn mine operation at Lechang, in the northern part of Guangdong Province, People's Republic of China. Geochemical and acid generation evaluations were made on fresh tailings including tailings fine, tailings sand and high sulphide tailings, and oxidised tailings with the aim of providing information on mine rehabilitation. All fresh tailings had a pH higher than 7 while the oxidised tailings had a pH of 4.9 (range 1.6 to 7.4). Only samples with pH < 3 had an electrical conductivity (EC) > 4 dS m⊃-1. Total S contents of all tailings samples were very high with the high sulphide fraction having a mean S content of 38%. All fresh tailings had a high acid neutralisation capacity (ANC) while half of the oxidised tailings had an ANC less than zero. The results from the acid–base account and the net acid generation test indicated that all fresh and oxidised tailings samples were acid-forming except for the sand fraction samples. All tailings samples contained high total concentrations of Cd, Cu, Fe, Pb and Zn but low concentrations of total nitrogen and phosphorus. The preliminary results demonstrated that the tailings were all acid-forming especially the high sulphide fraction which should be kept permanently unexposed under impermeable cap or water.