Investigation of sodium trapped by the filter cake during separation of iron from sewage sludge ash

Abstract

The company EasyMining has developed a process, Ash2Phos, to recover P from incinerated sewage sludge ash. The purpose of this report was to investigate the cause of Na losses to the filter cake during separation of Fe in one of the process steps. The project also aimed at exploring the influence of washing on the Na losses. The investigation was done by conducting a process reproduction experiment on a semi-pilot scale, including cycles with different washing conditions. Since the focus of the project was the Fe separation step, the experimental setup was simplified compared to the large scale process and did only include upstream process steps. The evaluation of Na losses to the filter cake and the influence of washing were done by constructing a mass balance, based on analyses with ICP-AES and ICP-MS, over the process step for the cycles with different washing conditions. The evaluation of the filter cake composition and elemental distribution was conducted by SEM-EDS analysis on filter cakes with different washing conditions. Analysis with XRD was also performed to investigate possible crystalline structures containing Na. Additional experiments on lab scale were performed on one of the known filter cake components, Al-silicates, to study its adsorption potential, washing behaviour and influence on the Na losses. The studies identified three main Na containing particles in the filter cake: Fe-precipitates, Al-silicates and Ca-precipitates. Extensive washing was found to be efficient for Na removal from the filter cake, however, only to a limited extent. Further, it was not possible to identify any crystalline structures containing Na. Based on the lab scale experiment, the losses of Na to the Al-silicates were not due to adsorption, instead it was related to trapped liquid in the cake. Finally, Fe-precipitates were identified as the main contributors to the Na losses but further studies are required to identify the mechanism and reduce the losses. It would also be of great interest to optimise the washing by including a cost perspective and large scale process conditions.