Reaction sequences in the thermo-chemical treatment of sewage sludge ashes revealed by X-ray powder diffraction - A contribution to the European project SUSAN

The sequence of reactions accompanying the thermochemical treatment of an iron- and aluminium-bearing sewage sludge ash was ascertained by investigating two systematic series of samples. The ash was thermochemically treated in a lab-scale rotary furnace after mixing it with a chlorine-donor, either CaCl(2) or MgCl(2). Within each of these two sample series only a single process parameter, the reaction temperature, was varied, namely between 350 and 1050 degrees C. It was found, that among the numerous crystalline phases present in the raw ash only quartz and hematite continue to exist after thermochemical treatments carried out at 1050 degrees C, whereas all other components undergo at least one decomposition-recrystallization cycle. Some of the components re-crystallize even several times. It was proved that the restructuring of the calcium- and phosphorus-bearing mineral phases proceeds via the formation of chlorspodioside, Ca(2)PO(4)Cl. The influence of the type of chlorine-donor on the final product was elucidated in detail and - to the best of our knowledge - for the first time crystalline AlPO(4) was found in a sewage sludge ash and its decomposition was investigated, too.