Factor effect of operating conditions on the leaching of fly ashes from a pulp and paper mill

The non-process-elements (NPE's) in the pulp and paper production, in particular chloride (Cl−) and potassium (K+), eventually accumulate in a sodium sulfate recovery cycle of a paper plant, especially on boiler fly ashes (flue gas cleaning). This accumulation often leads to fouling and corrosion. Thus, in order to reduce such problems, a leaching unit is typically integrated aiming to remove Cl− and K+ and recover sodium sulfate (Na2SO4) from boiler fly ashes. In this context, this research is focused on investigating the effects of temperature, ash concentration and pH on the separation efficiency (Cl− and K+ removal and Na2SO4 recovery) of recovery boiler fly ashes using design of experiments in a devised laboratory setup. As expected from the literature, the results obtained reveals that concentration has the most significant effect on the response variables followed by temperature; whereas pH, not mentioned in this application so far, also has a significant effect. The optimum operating condition, which yields 84 wt% of Na2SO4 recovery and 97 wt% of Cl− and K+ removal, is achieved at 60 °C, 50 wt% of ash and pH 9.0. For practical applications, this optimal condition reduces the accumulation of NPE‘s and mitigates the risks of fouling and corrosion.