Dispersion modelling and leachability of heavy metals from tailings dam material of a gold cyanidation plant in Zimbabwe

Abstract

Notable pollution impacts on tailings dams and surrounding areas at gold mines have been a major concern in Zimbabwe. This study investigated the major environmental pollutants propagating from tailings dams in gold operations. Soil and water samples were
collected from a gold mine’s active slimes dam and its surroundings. The samples were analysed for physico-chemical parameters and 13 elements which included; lead, chromium, iron, manganese, nickel, copper, arsenic, cobalt, cadmium, mercury, sodium, cyanide and sulphide. Atomic Absorption Spectroscopy (Varian Spectra AA 200 model) was used in the chemical analysis of heavy metals. In order to determine the relationship between parameters in the tailings dam and seepage (SP) water, a correlation matrix and a panel of regression equations were formulated. Single batch, 24-hour column leach tests were done on the tailings dam soil to ascertain the dissolution and mobility rate of the heavy metals, manganese, arsenic, iron, chromium and lead, under varying pH conditions. Highly positive correlations (> 0.90) were observed between the following pairs; Mn(SP) and Fe (0.99), Mn(SP) and Ammonia (0.96), Mn(SP) and Na (0.92), Fe(SP) and Fe(0.975), Fe(SP) and Ammonia (0.93), Ni(SP) and Fe (0.908), and Ni(SP) and Na (0.92). Relatively weak correlations (<0.50) were found between the following variables: Sulphate(SP) and TDS (0.293), TDS(SP) and Na (0.248), TDS(SP) and Ammonia (0.382), and TDS(SP) and TDS (0.316). The results also showed that heavy metals;
manganese, iron and nickel in the see-page water stream have a high correlation value (> 0.80). The findings indicate that theseheavy metals are being released from a common source; dissolution of sulphide minerals within the tailings dam soil.Manganese had the
highest extraction rate across the pH range investigated, having a maximum of 5.8% extraction at pH 3.It was concluded that a pH range of 6 to 9 proved to be the optimum in minimizing the heavy metals solubility.