3.2.3. Fe-Rich Material

Compared to the original material, Fe-phases have been depleted. Comparing these findings with generic labelling, some of the Fe-phases (generic) must contain phosphorus. Thus, they are not part of Fe-phases group (target phase). Target phase grouping thereby highlights that iron phosphates (generated in phosphate precipitation in wastewater treatment plants) are highly stable and not easily acid-accessible.

#### 3.2.4. Residues of Pre-Treated Digestions (Target Grouping)

Pre-treatment led to an interesting shift in material phases. While the calculated assay values for P increase from SSA to pre-treated SSA (relative increase, as CaSO4 is removed), the share of material components containing more than 10 wt% P decreases. At the same time, the amount of key spectra rises by about 10 area-%. Thus, the improved P-recovery after thermochemical pre-treatment is explained: thermochemical pre-treatment led to the formation of accessible material (see also [11]).
