Immobilization of Heavy Metals in Waste Phosphate Coating Sludge Using Kiln Dust as Portland Cement Substitute

Abstract

In the present study, stabilization and solidification (S/S) of the waste phosphate sludge (WPS) using Portland cement (PC) and cement kiln dust (CKD) was investigated. In the first place, only PC was used to stabilize 5, 10 and 15% WPS. At a later stage, 10 and 15% CKD were used as PC substitute to stabilize 15%WPS. WPS contained initially 130.2 mg/L Zn and 22.6 mg/L Ni. U.S. Environmental Protection Agency (US-EPA) landfilling limits for Zn and Ni are 4.3 mg/L and 11 mg/L, respectively. Setting times and unconfined compressive strength (UCS) values were measured and permeability of selected samples was determined. TCLP (Toxicity Characteristics Leaching Procedure) and SPLP (Synthetic Precipitation Leaching Procedure) were applied to determine the concentrations of Zn and Ni leached from the mortar samples and leachate pH values were measured. Use of the CKD as a cement substitute delayed setting times, decreased UCS and increased leached heavy metal concentrations. Leached Zn and Ni concentrations of the WPS stabilized with PC and CKD following SPLP were lower than the EPA landfilling limits. However, leached Zn concentrations following TCLP were not compatible with the EPA limit. Leached Ni concentrations remained lower than the EPA limit. It was concluded that relatively low levels of alkalies, CaO and SiO2 content of CKD negatively affected the setting, compressive strength and leaching characteristics.