Sustainable extraction of valuable metals from petroleum spent hydroprocessing catalysts using a biodegradable lixiviant

The petroleum spent hydroprocessing catalysts contain high concentrations of toxic and valuable metals (Ni, V, Mo, and Al). Herein, a sustainable leaching process using a biodegradable green chelating agent (nitrilotriacetic acid- NTA) has been developed for leaching valuable metals from spent hydroprocessing catalysts in a single stage of operation. Our findings indicate that NTA can selectively extract all valuable metals with high efficiency (Ni- 93.3 %, Mo- 93.4 %, and V- 80.6 %), and minimal Al2O3 dissolution. The leaching kinetic was explained using a semiempirical Avrami equation (R2 > 0.9). After leaching, the dechlation of the NTA-metal complex led to an almost complete recovery of NTA (ca. 94 %). The leach residue (Al2O3) was subjected to 'Toxicity Characteristic Leaching Procedure' (TCLP) analysis and it was found that it poses a low ecological risk. Moreover, due to the leaching of deposited metals, there was a remarkable improvement in the textural properties of the leach residue (BET surface area- 193.5 m(2)/g and pore volume- 0.59 ml/g) thus making it a potential reuse candidate for preparing fresh hydrotreating catalyst. Therefore, this green method not only successfully permits the sustainable leaching of valuable metals, but allows the potential reuse of residue to prepare fresh hydroprocessing catalysts.