Phosphorus rich sewage sludge ash is a promising source to produce phosphorus recycling fertilizer. However, the low plant availability of phosphorus in these ashes makes a treatment necessary. A thermochemical treatment (800–1000 °C) with alkali additives transforms poorly plant available phosphorus phases to highly plant available calcium alkali phosphates (Ca,Mg)(Na,K)PO₄. In this study, we investigate the use of K₂SO₄ as additive to produce a phosphorus potassium fertilizer in laboratory-scale experiments (crucible). Pure K₂SO₄ is not suitable as high reaction temperatures are required due to the high melting point of K₂SO₄. To overcome this barrier, we carried out series of experiments with mixtures of K₂SO₄ and Na₂SO₄ resulting in a lower economically feasible reaction temperature (900–1000 °C). In this way, the produced phosphorus potassium fertilizers (8.4 wt.% K, 7.6 wt.% P) was highly plant available for phosphorus indicated by complete extractable phosphorus in neutral ammonium citrate solution. The added potassium is, in contrast to sodium, preferably incorporated into silicates instead of phosphorus phases. Thus, the highly extractable phase (Ca,Mg)(Na,K)PO₄ in the thermochemical products contain less potassium than expected. This preferred incorporation is confirmed by a pilot-scale trial (rotary kiln) and thermodynamic calculation.

富含磷的污水污泥灰是生产磷循环肥料的有前途的来源。然而，这些灰烬中磷的植物可用性低，因此需要进行处理。使用碱性添加剂进行热化学处理 (800–1000 °C) 将植物可利用性差的磷相转化为植物可利用性高的碱金属磷酸钙 (Ca,Mg)(Na,K)PO ₄。在本研究中，我们研究了使用 K ₂ SO ₄作为添加剂在实验室规模的实验（坩埚）中生产磷钾肥。纯 K ₂ SO ₄不适合，因为 K ₂ SO _{4 的}熔点高，因此需要高反应温度. 为了克服这一障碍，我们使用 K ₂ SO ₄和 Na ₂ SO _{4 的}混合物进行了一系列实验，从而降低了经济可行的反应温度（900-1000 °C）。通过这种方式，生产的磷钾肥（8.4 wt.% K, 7.6 wt.% P）对磷具有高植物利用率，这表明磷在中性柠檬酸铵溶液中完全可提取。与钠相反，添加的钾优选并入硅酸盐而不是磷相。因此，高度可萃取相 (Ca,Mg)(Na,K)PO ₄热化学产品中的钾含量低于预期。这种优选的掺入通过中试规模试验（回转窑）和热力学计算得到证实。