Alkali extraction of valuable metals from spent Mo–Ni/Al2O3 catalyst

•Mo can be selectively extracted by alkali autoclaving in low NaOH/Mo ratio solution.•Molybdates and aluminates do not affect the selective extraction of Mo.•Residual Mo can be dissolved along with alumina in the Bayer process.•Mo dissolved by the Bayer process can be precipitated by Ba(OH)2.

Extraction of metal values from the spent Mo–Ni/Al2O3 catalyst containing complex oxides was investigated in this work. Aiming at studying selective extraction of metals, a sequential recycle process is conceived as follows: First, acidic oxide MoO3 is selectively extracted by alkali leaching. Then amphiprotic Al2O3 is dissolved by caustic soda autoclaving. Al(OH)3 is obtained by seeding precipitation and used to prepare the support of new catalysts. Besides, Ni(OH)2 remains in the digestion residue as “green mud” of the modified Bayer process. In the light of the idea, alkali leaching experiments were conducted, using several common basic leaching agents. The results show that caustic autoclaving at low NaOH/Mo ratio could selectively extract Mo without significant dissolution of the matrix. Under the optimum operating conditions of temperature 160 °C, NaOH dosage 1.2 times the stoichiometric amount, solid/liquid ratio 1:4, retention time 120 min and stirring rate 300 rpm, over 96% of Mo was recovered while less than 0.2% of Al was dissolved. After leaching of Mo, the Al2O3 matrix was dissolved completely by caustic soda autoclaving at 160 °C. Sodium aluminate solution was purified by CaO and Ba(OH)2. The overall Mo and Al extraction yields were up to 99%.