Understanding phosphorus phases in sewage sludge ashes: A wet-process investigation coupled with automated mineralogy analysis

Phosphorus (P) recovery from SSA (sewage sludge ashes) is highly relevant to address future P-shortages. Efficient recycling strategies require an in-depth knowledge of P-phases present. These may be subject to changes during treatment. Treatment was simulated, initial and treated material studied using automated mineralogy. Automated mineralogy, using the MLA system, is an established tool in ore processing evaluation and adapted here to examine P-phases in SSA. Coupling SSA treatment for P-recovery routines with MLA investigations allows for documenting phase compositions during chemical and thermochemical processes. Behaviour of P-phases during treatment and type of SSA govern P-recycling capacity. Findings lend additional insight to the identification of SSA types.Two different P-recovery routines have been investigated. Target phase groups (phosphates) have successfully been digested. Understanding phosphate setup in terms of composition, particle sizes and phase association is vital for an efficient P-extraction from SSA. P-recovery is not straightforward, but rather a series of steps in order to address different P-phase types. SSA is a complex secondary resource, which needs to be understood prior to P-extraction.