A review on management and recycling of spent selective catalytic reduction catalysts

Catalysts for selective catalytic reduction are widely used for the reduction of nitrogen oxides into gaseous nitrogen in many industrial applications. These catalysts usually contain titanium, tungsten, and vanadium. Due to the increasing demand, a new strategy for the management of end-of-life catalysts is urgent, in particular for the recovery of valuable metals and the reduction of the material disposed of. Titanium, tungsten, and vanadium shall be recovered and reused for the production of new catalysts or other products, to establish a circular economy approach. In this paper, a systematic review of the main strategies adopted for the management of spent catalysts was proposed and, in particular, an overview of the regeneration and rejuvenation techniques, the current processes for the recovery of such catalysts and the treatments patented. The present study found that among the recycling processes for the extraction of metals, those based on alkaline roasting and/or leaching are the most used: vanadium and tungsten are indeed water-soluble, so that can be easily separated from titania during the leaching stage. Instead, the acid leaching is not developed because of the downstream stages required to separate the three metals from each other. The recovery of the metals is currently negligible at full scale. For this reason, the recovery of titanium, tungsten, and vanadium with high extraction yields and grades shall be pursued in the future at a large scale, and the research shall be oriented towards this scope, whereas landfilling, that is still the main option in the management of this material, shall be penalized with a disposal fee.