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

¹ Environment and Resources DTU, Technical University of Denmark, Build. 115, 2800 Kgs. Lyngby, Denmark
² Chemistry Department, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, 1871 Frederiksberg C, Denmark

^* E-mail: mas{at}imt.dtu.dk

Air pollution control (APC) facilities at waste incinerator plants produce large quantities of solid residues rich in salts and heavy metals. Heavy metals are readily released to water from the residues and it has, therefore, been found suitable to apply a rapid co-precipitation/adsorption process as a means to immobilize the toxic elements. In the ‘Ferrox process’, this immobilization 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 alkaline pH. In this work we have undertaken a Mössbauer spectroscopy study of the Fe oxide phase formed by precipitation at room temperature and of the oxides present after heating to 600 and 900°C. The only Fe oxide observed in the Ferrox product at room temperature is a very poorly crystalline ferrihydrite. Analytical transmission electron microscopy showed that the main elements associated with the ferrihydrite are Si and Ca. Following heating to 600°C the oxide is still characterized as an amorphous Fe oxide, and it is probable that Si associated with the ferrihydrite is decisive in preventing crystallization. After the 900°C treatment a transformation into defect maghemite is observed. Reducing gases produced from carbon in the samples probably induces this transformation. It eases, thus, the reduction of Fe(III) and the consequent formation of magnetite that eventually oxidizes to maghemite during cooling in air.

KEYWORDS: APC residues, stabilization, heavy metals, Fe oxides, transformation, thermal treatment, Mössbauer spectroscopy, analytical transmission electron microscopy

This article has been cited by other articles:

				M. E. Hodson and A. Bloodworth The mineralogy of waste and waste disposal Mineralogical Magazine, October 1, 2001; 65(5): 561 - 562. [Full Text] [PDF]