19 High-Temperature Electrochemical Synthesis of Nanopowders. . .
319
The dried tungsten monocarbide is analyzed to determine the content of tungsten,
bound carbon, and impurities, such as sodium, lithium, iron, nickel, copper,
molybdenum, etc.; the content of impurities and of tungsten carbide is found at
the level of several hundredths of a percent while that of free carbon 2–7% of the
target product. The recommended regime of washing tungsten monocarbide free of
the salt phase is as follows: leaching of the bulb of heated water (T = 70–80 ◦ C) at
a ratio W/S = 1:8 and total yield of hydrochloric acid (1:1) 0.3 l and of ammonium
hydroxide (1:1) 1.5 l per kilogram of cathode deposit. The yield of the target product
amounts to 8–9% of the mass of the bulb.
Purification of tungsten monocarbide from free carbon is an important step. Free
carbon may be removed from the carbides by annealing in a hydrogen medium [8, 9]
at a temperature in the range 800–1000 ◦ C. However, application of this annealing
regime to powders measuring 0.1–0.5 μm in dimension obtained by means of hightemperature electrochemical synthesis leads to removal of the carbon bound to the
carbide even at temperatures of 700–800 ◦ C. Positive results on purification of
tungsten monocarbide were obtained by the present authors in [10, 11] in the course
of annealing tungsten carbide in an activated hydrogen medium. The use of this
strategy made it possible to remove free carbon and to reduce the content of sorbed
oxygen down to 0.2–0.3%.
19.3.4 Recovery of Tungsten and Lithium from Spent
Electrolyte and Carbide-Salt Deposits
In a carbide-salt bulb, the total mass of Na 2 WO 4 , lithium tungstate, and lithium
carbonate significantly exceeds the mass of the target product. It is thus useful
to recover these components of the synthesis for subsequent reuse. Electrolyte
remaining in the electrolysis bath following conclusion of the synthesis process may
also be subjected to reprocessing. Lithium is recycled into lithium carbonate. For
this purpose the filtrate obtained following leaching of the cathode deposit or the
electrolyte, which contains Na 2 WO 4 and Li 2 WO 4 , is concentrated by evaporation,
and Li 2 CO 3 is deposited as sodium carbonate:
Li 2 WO 4 + Na 2 CO 3 → Li 2 CO 3 + Na 2 WO 4 .
(1)
As a consequence of its low solubility in the water (not more than 7.3 g/l at
T = 100 ◦ C), lithium carbonate separates out into the deposit and passes readily
through a filter. Following repeated washing in heated distilled water, Li 2 CO 3 is
dried and repeatedly reused. 54% of the lithium contained in the salt phase of
the cathode deposit is recovered. More complete deposition of Li 2 CO 3 requires
multistage concentration by evaporation of the filtrate and sedimentation, which is
not cost-effective.
319
The dried tungsten monocarbide is analyzed to determine the content of tungsten,
bound carbon, and impurities, such as sodium, lithium, iron, nickel, copper,
molybdenum, etc.; the content of impurities and of tungsten carbide is found at
the level of several hundredths of a percent while that of free carbon 2–7% of the
target product. The recommended regime of washing tungsten monocarbide free of
the salt phase is as follows: leaching of the bulb of heated water (T = 70–80 ◦ C) at
a ratio W/S = 1:8 and total yield of hydrochloric acid (1:1) 0.3 l and of ammonium
hydroxide (1:1) 1.5 l per kilogram of cathode deposit. The yield of the target product
amounts to 8–9% of the mass of the bulb.
Purification of tungsten monocarbide from free carbon is an important step. Free
carbon may be removed from the carbides by annealing in a hydrogen medium [8, 9]
at a temperature in the range 800–1000 ◦ C. However, application of this annealing
regime to powders measuring 0.1–0.5 μm in dimension obtained by means of hightemperature electrochemical synthesis leads to removal of the carbon bound to the
carbide even at temperatures of 700–800 ◦ C. Positive results on purification of
tungsten monocarbide were obtained by the present authors in [10, 11] in the course
of annealing tungsten carbide in an activated hydrogen medium. The use of this
strategy made it possible to remove free carbon and to reduce the content of sorbed
oxygen down to 0.2–0.3%.
19.3.4 Recovery of Tungsten and Lithium from Spent
Electrolyte and Carbide-Salt Deposits
In a carbide-salt bulb, the total mass of Na 2 WO 4 , lithium tungstate, and lithium
carbonate significantly exceeds the mass of the target product. It is thus useful
to recover these components of the synthesis for subsequent reuse. Electrolyte
remaining in the electrolysis bath following conclusion of the synthesis process may
also be subjected to reprocessing. Lithium is recycled into lithium carbonate. For
this purpose the filtrate obtained following leaching of the cathode deposit or the
electrolyte, which contains Na 2 WO 4 and Li 2 WO 4 , is concentrated by evaporation,
and Li 2 CO 3 is deposited as sodium carbonate:
Li 2 WO 4 + Na 2 CO 3 → Li 2 CO 3 + Na 2 WO 4 .
(1)
As a consequence of its low solubility in the water (not more than 7.3 g/l at
T = 100 ◦ C), lithium carbonate separates out into the deposit and passes readily
through a filter. Following repeated washing in heated distilled water, Li 2 CO 3 is
dried and repeatedly reused. 54% of the lithium contained in the salt phase of
the cathode deposit is recovered. More complete deposition of Li 2 CO 3 requires
multistage concentration by evaporation of the filtrate and sedimentation, which is
not cost-effective.