Inorganic phosphorus and nitrogen–phosphorus–potassium (NPK) fertilizers based on phosphates from thermochemically treated sewage sludge ash were analyzed using mid-infrared (mid-IR) and far-infrared (FIR) spectroscopy. The different compounds present in the fertilizers were qualitatively determined with the help of recorded reference spectra of pure substances. Differentiation between various phosphates and other compounds such as sulfates, nitrates, and oxides was possible using combined interpretation of the mid-IR and FIR spectra. The results are in agreement with previous X-ray diffraction (XRD) measurements of the same samples. The main phosphate phases detected were NH<sub>4</sub>H<sub>2</sub>PO<sub>4</sub>, MgHPO<sub>4</sub>·3H<sub>2</sub>O, Mg<sub>3</sub>(PO<sub>4</sub>)<sub>2</sub>, Ca<sub>5</sub>(PO<sub>4</sub>)<sub>5</sub>Cl, CaHPO<sub>4</sub>·2H<sub>2</sub>O, Ca(H<sub>2−</sub>PO<sub>4</sub>)<sub>2</sub>·H<sub>2</sub>O, and AlPO<sub>4</sub>. Furthermore, K<sub>2</sub>SO<sub>4</sub>, NH<sub>4</sub>NO<sub>3</sub>, Fe<sub>2</sub>O<sub>3</sub>, and SiO<sub>2</sub> were identified in the IR spectra. However, ammonium and sulfate compounds were only identified in the mid-IR region but were not detectable in the FIR region.

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