Stabilization/solidification of industrial sludge and fly ash using cement based technology

Abstract

The immobilization of chromium, copper and nickel in industrial sludge and fly ash waste was investigated using a 2: 1 waste to cement ratio with varying amounts of sodium silicate for a stabilisation/solidification (S/S) process. Maltese sand (Lower Globigerina limestone - 'franka') spiked with 10mg/g Cr, Cu and Ni was also used in this study as control waste samples. A sample of sludge, fly ash and sand was pre-treated at 550°C for six hours. The effectiveness for immobilizing the metals was measured by a leaching test procedure performed at different slurry pH's. The addition of cement and silicate showed a reduction in the amount of metals leached, with values far below the leaching limits. The leaching limits of chromium, copper and nickel according to Directive 2003133/EC are 70, 100, and 40 mg/kg dry weight respectively. Leaching is dependent of the pH of the solution since at an acidic slurry pH the amount of Cr, Cu and Ni leached increased when compared to values obtained at the sample's natural pH and alkaline pH. Thermally treated samples showed a decrease in the leaching of copper and nickel. Chromium was observed to become more soluble after thermal treatment, with values reaching 982.13 mg/kg dry weight of sand after two days. As a result of this increase in chromium solubility, thermally treated waste was submitted to the SIS process. The addition of cement and silicate slightly decreased the amount of Cr leached.