RECYCLING OF SPENT DENOX CATALYSTS

Denox catalysts, used to eliminate NO, from gaseous streams, consist o f MoO3 or WO3 on a TiO2 matrix. When they are contaminated by heavy me tals, such as As (>2000 ppm) and Hg (8.8 ppm), they have to be recycle d. A process has been developed to recover the refractory metals and t o enable the TiO2 to be recycled in titanium production installations. The latter objective requires that the As content of the leached TiO2 be as low as possible. A one-step process, which consisted in direct NaOH-leaching of the spent catalyst to extract 93% of both molybdenum and arsenic, was considered first. The final As content ranged between 57 and 145 ppm depending on the particle size (d(80) -36 mu m to ungr ound). To lower these values a pretreatment using dilute sulphuric aci d solution in the presence of aluminium scrap was found to be necessar y. The purpose of this was to transform the arsenic into a compound th at would dissolve readily in the subsequent alkaline leach so as to le ave a TiO2 residue analysing around 30 ppm As. Treatment of the soluti on consists in purification by neutralization to remove the leached im purities (As, Al, etc.) and adsorption of molybdenum on the ion-exchan ge resin Amberlite IRA 94 S followed by elution to produce a purified solution of molybdenum.