Modification of the sewage sludge ash P-mineralogy by thermo-chemical treatment with LF slags

Phosphorous is one essential element and thereby an important nutrient for plant and animal growth. In the form as phosphate, it is one major constituent in fertilising materials for agriculture (Tulsidas et al., 2019). The three most important exporting nations worldwide are China and Morocco followed by the USA with an annual production of 90 Mt, 37 Mt and 24 Mt of phosphate rock, respectively (Jasinski, 2021). Jasinski (2020) summarized a world production of 223 Mt and a static reserve of approx. 71 Gt. The demand for P-fertilisers in the EU in the year 2010 was with a quantity of 4 Mt low in comparison to other regions as Asia whereas annually approx. 25 Mt Pfertiliser are used (Killiches, 2013). In Europe, only in Finland, a significant amount (1 Mt/a) of rock phosphate from magmatic type deposits is mined (Neukirchen & Ries, 2016) (Jasinski, 2021). Thereby, most of the used P-fertilisers, despite farmyard manures, must be imported from countries outside the EU which means an immense dependency on such imports. Because of the import dependency, in the German national regulation for sewage treatment (AbfKlärV, 2017) Precycling from SSA is mandatory for wastewater treatment facilities with a capacity ≥ 250,000 citizen units from 2029.
Sewage and finally sewage sludge produced in wastewater treatment facilities is the most important anthropogenic source for P. Nowadays, sewage sludge is treated by onsite drying and often external incineration to SSA. As reported by (Drissen & Rex, 2016) SSA can contain P2O5 in a range of 5-25 wt.%. The most dominant phosphate minerals found in the SSA are whitlockite and apatite (Drissen et al., 2016). Latter phase can also be found as major P-carrier in most phosphate rock. However, both phases show, in general, low plant availability and, consequently, low fertilizing efficiency in pot trials as well as on site experiments where these phosphate-minerals are applied directly (Drissen & Rex, 2016) (Rex et al., 2013). Hence, several authors suggest the thermo-chemical treatment of SSA to generate P-phases featuring higher plant availability
with different additives and methods. For enhanced plant available P (Herzel et al., 2021) developed a process at pilot scale which uses an eutectic mixture of K2SO4 and Na2SO4 as additive to SSA to produce K-Na containing P-phases of which good plant availability are attested. Another way of treatment was suggested by (Rex et al., 2013) by the injection of SSA and animal meat ash into molten Ca-rich converter slag. For the created products high plant availability of the Pphases were proven by chemical as well as by plant trials. As major P-carriers Ca-Si-P phases as nagelschmidtite and silicocarnotite were identified (Drissen et al., 2016).