Environmental Engineering Reference
In-Depth Information
process was carried out by EURECAT's partner, a large mining company specialized in nickel
metallurgy
[7]
. The process consisted of a fusion of the solid in an arc furnace to separate
cobalt and nickel in a matte form from the aluminium and residual impurities, which are
eliminated in the slag. The nickel/cobalt matte was then treated conventionally by solvent
extraction to separate nickel and cobalt before electrolysis of the metals. The alumina
containing slag was completely inert and it could be disposed safely or could be used as a road
construction material. The valuable metals extracted from the spent catalysts were
electrolytical nickel and cobalt metals, molybdenum oxide, ammonium molybdate (or
tungstate), and vanadium sulfate. The yield recovery for molybdenum, vanadium, and tungsten
was over 90%, and for cobalt and nickel as high as 97%. This process is now abandoned and a
complete pyrometallurgical route is used
[7]
. The pyrometallurgical route starts with dry
catalysts that are melted in a furnace at temperatures around 1200-1500
◦
C. Heavy metals sink
to the bottom as alloys and are separated from slag, containing the alumina or silica support. It
is claimed that the pyrometallurgical route has the advantage of a complete recycling scheme:
metals for manufacturing in special alloys, the inert slag for manufacturing insulation material,
such as rock wool.
11.3.4 Taiyo Koko Company Process
In Taiyo Koko Company's metal recovery plant, soda ash roasting process is used to recover
Mo and V from spent desulfurization catalysts
[692]
. A simplified schematic diagram of the
process is shown in
Fig. 11.23
. The spent catalyst is mixed with soda ash and roasted in a kiln
at 850
◦
C for about 2-5 h in an oxidizing atmosphere to convert the Mo and V to their sodium
salts. The roasted material is wet ground in a ball mill, leached with water and filtered. The
filtered leach solution containing sodium molybdate and sodium vanadate is treated with
magnesium chloride and ammonium chloride for the removal of aluminum and phosphorous.
The purified solution is mixed with ammonium chloride to precipitate AMV. The AMV is
calcined to remove ammonia and the resulting V
2
O
5
is fused and flaked. The vanadium
product contains over 98% V
2
O
5
with iron and molybdenum as minor impurities. After the
removal of vanadium, the solution containing Mo is acidified with HCl to precipitate molybdic
acid which is calcined to produce MoO
3
.
11.3.5 Full Yield Industry Process
Full Yield Industry in Taiwan is reprocessing spent hydrotreating catalysts to produce
molybdenum salts and oxide, vanadium salts and oxide, cobalt salts, nickel salts, and
ferrovanadium alloy. The process
[569]
involves soda ash roasting followed by leaching and
separation of individual metals by precipitation, solvent extraction/ion exchange
methods.
