Selective removal of heavy metals from incinerated ash of sewage sludge

Abstract

There is an urgent need to promote the utilization of sewage sludge incineration ash as a phosphorus resource, because it contains P₂O₅ equivalent to phosphorus ore. Numerous tests have been conducted to utilize sewage sludge incineration ash as a phosphorus ore substitute in the current wet phosphoric acid refining processes. However, sewage sludge incineration ash contains more heavy metals (Cu, Zn, Pb, etc.) than phosphorus ore. The amount of sewage sludge incinerated ash that can be mixed is limited, because current wet phosphoric acid refining processes do not have a process to remove these impurities. The authors have proposed a method to selectively change certain oxide components into easily soluble or low-boiling salts and separate them by applying NH₄Cl to steelmaking slag and electric furnace dust generated in steel smelting. Based on these findings, we devised a method to reduce impurities (Cu, Zn, Pb, etc.) in sewage sludge incineration ash. In this presentation, the effects of ash/NH₄Cl ratio, temperature, and time were investigated according to the devised process to verify the possibility.