Development of a Recovery System for Metals and Acids from Pickling Baths using Nanofiltration and Crystallisation

Within the project a possible combination of techniques for recovery of metals and acids from stainless steel pickling has been investigated, as there is a lack of cost and energy efficient methods that allow recycling of both metals and acids today. Crystallisation and nanofiltration were combined in order to separate the metals as metal fluorides and recycle nitric and hydrofluoric acid to the pickling bath. A total treatment concept will also include the thermal treatment of the crystals, in order to gain metal oxides that can be reused. The fluorides will be recovered as HF from the exhaust gas. This thermal treatment has not been investigated in this project, but it should be possible to adapt existing techniques for that. Experiments in laboratory scale were performed in order to increase understanding of the processes and for design of a pilot plant. A Pilot plant was built within the project. Process liquids from two stainless steel companies in Sweden were investigated with the pilot plant. The experiments were used for recommendations on full-scale solutions as well as rough estimations on energy and cost savings. Tests for a longer period of time would be needed in order to be able to assess the life-time of the equipment and performance stability better, but the results show that the techniques could be used for metal separation to allow recycling of acids and prepare for metal recovery. The results also indicate possible energy savings when installing the system. Investigations from a parallel project show promising results regarding the on-line measurement of concentrations, which will allow a more efficient control of the process