Development of SLL equilibrium speciation and data fitting tool and its application to P recovery process from sludge

A solid-liquid-liquid equilibrium (SLLE) speciation tool has been developed in MATLAB in order simulate species that would exist in the different phases as a function of pH. The SLLE modelling tool may be applied to processes such as leaching, precipitation and reactive extraction. This work is a part of research carried out under the Phos4You project to develop a new process for recovery of Phosphorus (P) from dried sewage sludge. The work carried out at the University of Liège is funded by the Interreg North West Europe Program and SPW (Région wallonne). The process being developed is the PULSE (Phosphorus ULiège Sludge Extraction). The concept of PULSE is adopted from the PASCH process that was previous developed by RWTH Aachen for extraction of P from ashes of incinerated sewage sludge. The PULSE process involves drying of sludge followed by acidic leaching to dissolve P. After leaching and separation of the solids, reactive extraction is used to separate the metals from the P-rich aqueous fraction. Finally P is precipitated as calcium phosphate. The knowledge of speciation, complex formation, and phase equilibria are essential for design and optimization of unit operations that are employed in PULSE process to gain deeper understanding and reduce the load of experimental work. The SLLE tool being developed can simulate the speciation and complex formation for both aqueous and organic phases as well the precipitation of solid phases. Due to non-ideality of the system and ionic interactions, activities are used for the computation instead of concentrations. The user can choose the activity model from a list of models. The input parameters required for the SLLE tool are the equilibrium constants, stoichiometric coefficients, and valence of the species along with the total input concentration of the components and the phase ratios of the different phases involved in the system under consideration.
As the PULSE process deals with sewage sludge, which contains organics and bio solids, experimental results would significantly vary from modelling results that considers pure substances or phases. Also the thermodynamic data for reactive extraction using organic solvents is highly system specific and not available in many cases. In order to overcome these challenges, a data fitting algorithm is coupled with the SLLE speciation tool. The data fitting algorithm enables determination of the speciation modelling parameters based on experimental data such as equilibrium concentrations and degrees of extraction. Arbitrary complexes can be accounted for in the reactive extraction, which also allows describing synergistic effects, for which stoichiometry and equilibrium constants also will need to be fitted to experimental data.
An example of the speciation and solid-liquid phase equilibria computed using the developed tool is shown below in figure 1. With regards to the PULSE process, the speciation information from the figure below helps us to identify the pH that should be reached during leaching in order to completely dissolve P from sludge or the pH at which calcium phosphate may be precipitated. The predominant specifies of various metals that are existing in different pH can identified from the speciation diagram as shown in the figure below, which is very critical in case of reactive extraction as depending on the type of solvent used only certain specific species or complexes are extracted. The SLLE tool is being continually improved and developed to include more features such as temperature dependency of the equilibrium constants. It will also be extended to interlink the unit operations- leaching, reactive extraction, and precipitation for application to PULSE process.