Material solutions for a new process of nutrient recovery by thermochemical treatment of sewage sludge

The project aims to find material solutions for a recently developed process that allows the recovery of Phosphorus, an essential plant nutrient and therefore integral component of many fertilizers, from sewage sludge ashes. The process offers the possibility to overcome the previsible shortage of Phosphorus as a natural resource. It is distinguished by the use of highly chlorine-containing atmospheres at temperatures up to 1000°C. Unfortunately, there are currently no materials commercially available that can withstand such conditions over longer periods of time.

For the development of a sufficiently resistant material system, ceramic materials (SiC-ceramics and corundum) as well as nickel base alloys were chosen as a base material. Nickel base alloys feature outstanding high temperature corrosion resistance and strength. To further advance these qualities, protective diffusion coatings, applied by pack cementation, were developed. The thermodynamic assessment of different coating elements revealed that aluminum and silicon have the best prerequisites to form and maintain slow-growing, stable oxide layers with the highest potential for being protective against corrosive attacks.

The performance of all the materials is being examined in long-term tests under simulated field conditions at high temperatures and under atmospheres of up to 10% Cl₂ in air. A better knowledge about the corrosion mechanisms and the mechanical degradation allows a subsequent material optimization, which is an inevitable precondition for a large-scale realization of the process.