Stabilization of Filter Cake and its Leaching Behaviour: A Case Study with Cementitious and Soluble Phosphate Additives

Filter cake is one of the main waste products of zinc processing industries and it contains a high amount of toxic heavy metals. In this research in order to reduce heavy metals leachability in filter cake, Portland cement, natural pozzolan, diammonium phosphate (DAP), triple superphosphate (TSP), lime, zinc oxide and ground granulated blast furnace slag (GGBFS) have been used. This research’s results suggest that even though leachability of lead in the filter cake is quite low, it should be noted that limestone doubles the solubility level of Pb due to the mineralogical nature of the waste, by converting its sulphate form (PbSO4) to its carbonate form (PbCO3), which is more soluble. TSP was the only additive capable of reducing the leachability of metal elements in the filter cake as well as stabilizing the amounts of Pb and Cd in TCLP extracts within the limit. Although all additives were able to reduce Cd leachability, TSP and ZnO had the most remarkable efficiency. According to XRD results, the main factor in lead stabilization using TSP was a reduction in the amount of PbSO4 (with high Ksp) and an increase in PbS (with low Ksp), which ultimately reduced lead leachability. The Pozzolan which was used for the first time as a stabilizer demonstrated a good performance in reducing the leachability of Cd, Ni and Cu. In addition, due to the pseudocementitious properties of Pozzolan, it can reduce leachability of metal elements, along with other additives such as cement, by increasing the solidification efficiency.