Reducing cadmium emissions to water during removal of electrostatic precipitator ash : Investigating the effect of process parameters on the formation and precipitation of insoluble cadmium salts in water solutions

Sammanfattning: The removal of electrostatic precipitator (ESP) ash by solving the ash in water constitutes an environmental problem if heavy metals, such as cadmium, present in the ash remain in the outgoing effluent streams. The ESP ash solution is alkaline, meaning cadmium is present as the insoluble salt Cd(OH)2, cadmium hydroxide. The aim of this work is to reduce the amount of cadmium released to water recipients from pulp mills by investigating different precipitation mechanisms in ESP ash/ water solutions. Solutions with different concentrations, pH and added process chemicals were prepared and the Cd content in the supernatant measured with atomic absorption spectroscopy. The Cd removal through precipitation reached the highest values when ESP ash concentrations were >150 g/l, pH was above 11.5 and the ash solution was allowed to precipitate for 24h. The addition of polymers was found to reduce needed precipitation time. The effect of concentration on Cd removal could be caused by maximum solubility for the Cd2+ ion being reached at roughly 100 g/l ash concentration. The need for higher pH values could be caused by Cd forming complex ions with other solutions at pH<11 and theoretical maximum Cd2+ ion concentration decreasing from pH 10 to pH 13.