Thermally induced transformations of Fe oxide-stabilized residues from waste incineration

Air pollution control (APC) facilities at waste incinerator plants produce large quantities of solid residues rich in salts and heavy metals. Heavy me tals are readily released to water from the residues and it has, therefore, been found suitable to apply a rapid co-precipitation/adsorption process a s a means to immobilize the toxic elements. In the 'Ferrox process', this i mmobilization is based on co-precipitation with an Fe(III) oxide formed by oxidation of Fe(II) by air in an aqueous slurry with the APC residue at alk aline pH. In this work we have undertaken a Mossbauer spectroscopy study of the Fe oxide phase formed by precipitation at room temperature and of the oxides present after heating to 600 and 900 degreesC. The only Fe oxide obs erved in the Ferrox product at room temperature is a very poorly crystallin e ferrihydrite. Analytical transmission electron microscopy showed that the main elements associated with the ferrihydrite are Si and Ca. Following he ating to 600 degreesC the oxide is still characterized as an amorphous Fe o xide, and it is probable that Si associated with the ferrihydrite is decisi ve in preventing crystallization. After the 900 degreesC treatment a transf ormation into defect maghemite is observed. Reducing gases produced from ca rbon in the samples probably induces this transformation. It eases, thus, t he reduction of Fe(III) and the consequent formation of magnetite that even tually oxidizes to maghemite during cooling in air.