Heavy Metal Removal From Simulated Acidic Mining Wastewater By Chemical Precipitants And Sludge Characterization

Chemical precipitation is a widely used and proven technology for the removal of heavy metals from wastewater. Generally speaking, precipitation is a method of causing contaminants that are either dissolved or suspended in solution to settle out of solution as a solid precipitate, which can then be filtered, centrifuged, or otherwise separated from the liquid portion. In this thesis I examined the technical applicability of chemical precipitation for the removal of heavy metals such as Zn ion, Cu ion and Pb ion from acidic mining wastewater. A particular focus was given to three chemical precipitating agents:Lime, Soda ash and Sodium sulfide. The operation conditions such as pH, dose required and removal efficiency were studied by the jar test experiments which were performed using a series of five beakers (1L each). The prepared acidic contaminated water was placed in each beaker and various dosages of lime, soda ash and sodium sulfide powders were added to precipitate the heavy metal as metal hydroxide, carbonate and sulfide respectively. Lime precipitation with an addition of 170,117mg/l of calcium hydroxide has achieved a complete removal of copper and Zinc metals respectively by which removal can be up to～100%. The results can be comparable to that of carbonate precipitation (99.9%,99.8%) of zinc and copper removal respectively, where as sulfide precipitation with an addition of 174mg/l of sodium sulfide has been found as one of the most effective method to treat synthetic wastewater containing lead metal (98% removal was achieved). It was found that the sludge derived from zinc and copper precipitation had generally better dewatering characteristics (turbidity, sludge volume and zeta-potential). Lead formed ultra-fine precipitates (small size flocs) that are difficult to separate by settling. Generally, the results show that with lime and Sodium sulfide, the treated synthetic wastewater was more easily dewatered. As may be expected, the XRD results show that the most significant metal composition in the sludge obtained from the three precipitants (lime, Sodium carbonate and Sodium sulfide) was the metal hydroxide, metal carbonate and metal sulfide respectively. However, there was a spontaneous dehydration of metal hydroxide and carbonate forming metal oxide; this is due to their low solubility.