Metal Reclamation from Spent Hydroprocessing Catalysts - Spent Hydroprocessing Catalysts - page 298

both types of spent catalysts were compared. Chlorination of the unroasted sample at 500 ◦ C
for a reaction time of 30min, with a gas mixture containing Cl 2 /N 2 or Cl 2 /O 2 equal to one lead
to the extraction of more than 98% of molybdenum and 80% of vanadium, respectively. The
volatilization of the cobalt and nickel chlorides was negligible. In addition, the chlorination of
the alumina was limited to 3 and 9%. The carbochlorination of the roasted sample, that was
free from carbon and sulfur, in the same conditions with a gas mixture having a Cl 2 /CO ratio
equal to one, lead to the extraction of 97, 82 and 3 of the Mo, V, and Al, respectively. Less

than 5% of the cobalt and nickel compounds were transported by the vapor phase. The Mo and
V chlorides were recovered separately from the condensates, while chlorides of cobalt and
nickel were recovered from the chlorination residues by leaching with acidified water. The
final residue composed essentially of alumina and is environmentally safe. Clearly, relatively
good selectivity is obtained with a reasonable recovery rate of the valuable elements.

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Ojeda et al. [668] studied the effect of different chlorination procedures on the recovery of
metals from a spent CoMo/Al 2 O 3 catalyst. The following three procedures were used in their
study:

(1) Mo extraction by chlorination at low temperatures and subsequent chlorination at elevated
temperatures to recover Co, employing a Cl 2 -CO mixture.

(2) Extraction of the three metals by chlorination with Cl 2 -CO gaseous mixture at elevated
temperatures and subsequent separation by precipitation of Co at controlled pH.

(3) Mo extraction by chlorination of the catalyst with CCl 4 vapors in air current and
subsequent purification of the recovered Mo solutions by precipitation of impurities (Fe
and Al) at controlled pH.

In every case, almost 100%Mo was recovered, while Co extraction was 90% in procedure I and
95% in II. In all the assayed methods, hydrated aluminum oxide was obtained as by-product.

Recently, Ojeda et al. [669-671] investigated the chlorination of molybdenum trioxide in the
presence of carbon with the aim of determining the experimental conditions at which
chlorination occurs and to propose a mechanism for this reaction. The effects of several
variables, such as carbon content, mixing time, flow rate, temperature, chlorine mole fraction,
and reaction time on the chlorination of MoO 3 in the presence of carbon, were studied. The
results showed that carbochlorination starts at 543 K, significantly increasing with temperature
and reaction time, and slightly affected by chlorine molar fraction and flow rate, while direct
chlorination starts at 803 K. The following global reaction was obtained by identification of
the reaction products:

2MoO 3 (s) + 2Cl 2 (g) + C(s) ⇒ 2MoO 2 Cl 2 (g)+CO 2 (g).