Metal Reclamation from Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 299

On the basis of the analysis of experimental data, thermodynamic results and observations by
other authors, three basic stages were distinguished:
(1) Formation of the chlorinating agent.
(2) Chlorination of molybdenum trioxide.
(3) Carbon oxidation.
Based on the kinetic treatment of the experimental data, a “nucleation and growth” type model
was proposed for the carbochlorination reaction of MoO 3 .
11.1.4 Metal Recovery by Carbothermic Reduction
Karr et al. [672] investigated the lime aided carbothermic reduction of spent catalyst. For this
purpose, they used stoichiometric amounts of carbon and different mole ratio of CaCO 3 . The
pelletized mixture was reduced in a carbolyte furnace at temperature range of 1000-1200 ◦ C
under reduced pressure. After cooling, the reduced mass was separated from slag (CaS) by
elutriation. The metal powder was again compacted and sintered for subsequent purification
step using a fused salt electrolysis. The efficiency of electro deposition was enhanced by
optimizing experimental parameters, such as temperature and cathode current density. The
schematic of electrolytic cell used for electrorefining is shown in Fig. 11.20 [672] .
11.1.5 Metal Recovery Using Electrolytic Cells
The Batelle process represents the first attempt to recover metals from spent hydroprocessing
catalysts using electrochemical dissolution [673,674] . In this process, the spent catalyst which
contained sulfides of Mo, V, and Ni with carbonaceous material and sulfur is added to the
solution containing a strongly oxidizing ion, such as Ce 4+ . The solution is added at the anode
side of the electrochemical cell where aqueous Ce 3+ is oxidized to Ce 4+ , which in turn oxidizes
coketoCO 2 and H 2 O, and metals to soluble oxides. Thus, the oxidation/solubilization of the
metal sulfides was achieved by this method. However, separation of the dissolved metals by
electrolysis was not studied in this work.
Recently, Lai et al. [675] used a combined acid leaching and electrolysis process to recover
valuable metals from spent HDS catalyst. The electrolytic cell was equipped with a glass bead
medium, an iridium oxide mesh anode, and a stainless steel plate cathode. An acid solution
consisting of concentrated HNO 3 /H 2 SO 4 /HCl with a volume ratio of 2:1:1 was found to be
better than the other tested solution (HNO 3 /H 2 SO 4 = 1:1) to leach the metals. For the
three-acid mixture, the best solid/liquid ratio and leaching time were 40 g/L and 1 h,
respectively, at 70 ◦ C. Under these conditions, the leaching yields of target metals, such as Mo,