Chemistry Reference
In-Depth Information
Table 19.3
Electrochemical synthesis processes
Interestingly the use of sulfur dioxide (possibly as a
waste gas) to produce dithionite—widely used in the
pulp and paper industry—is now commercialised
(Olin). The process is based on a three-dimensional,
carbon electrode (graphite felt) and achieves current
efficiencies of near 100% [13].
Al, Na, Mg, Li
Molten salt electrowinning
Cu, Zn, Cu, Ni, Cr, Pb
Hydrometallurgy
Cd, Mn, Ti, Ga, In, Ag, Au
Electrowinning or refining
Chlorine/caustic
Noble metal oxide anode, brine
electrolyte
Chlorate
Nobel metal oxide anode, brine
electrolyte
5.1 Metal salt preparation
Perchlorate
Pt/ Ti, PbO
2
anodes, chlorate
electrolyte
There are several small-scale processes in operation
for the manufacture of metal salts by anodisation.
The electrochemical method offers the feature of
controlled purity and is based on the overall 'simple'
formation of a soluble metal ion. The following are
examples of metal salt production [14,15], which are
typically formed by anodic dissolution of the base
metal in the appropriate acid or alkali solution:
Persulfate
Pt/Ti anode, conc. H
2
SO
4
DSA
®
, aqueous NaCl
Hypochlorite
Permanganate
Ni, monel anode, KMnO
4
electrolyte
Fluorine
Carbon anode, KF/2HF eutectic
Manganese oxide
C, Pb, Ti anodes, MnSO
4
Water electrolysis (H
2
, O
2
)
Ni on steel, KOH
Hydrogen peroxide
Carbon cathodes, NaOH
Ozone
Vitreous carbon anode, conc. aq.
HBF
4
(1)
Potassium gold cyanide solutions from gold.
(2)
Silver nitrate liquors by the anodic dissolution of
Ag in nitric acid.
(3)
Titanium(III) chloride.
(4)
Nickel acetate, carbonate, chloride, etc.
(5)
Potassium and sodium stannate (from tin/lead
solder).
(6)
Cuppra-ammonium nitrate from copper scrap
dissolution in ammonium hydroxide.
(7)
Copper acetate and pyrophosphate.
Bromate
C, Pt/ Ti, PbO
2
aq. NaBr
Chromic acid
Lead anode, Cr(III) in H
2
SO
4
Cuprous oxide
Copper, aq. NaCl
Potassium stannate
Anodic dissolution
Chlorine dioxide
DSA
®
, carbon cathode, sodium
chlorate and HCl
• Avoidance of aggressive and hazardous reagents
• Use of alternative feedstocks
• Decrease processing costs
The current efficiencies for these salt productions are
typically greater than 98%. Cathodic reduction also
can be use for the production of metal salts, e.g.
vanadium(II) formate by the cathodic reduction of
vanadium.
It is important to stress that electrochemistry is not
an expensive process in terms of energy use and,
with appropriate cell design, when operating at high
current densities the processes are very competitive.
Electrochemical synthesis covers the production of
inorganic and organic chemicals, metals and alloys,
semiconductors, conductive polymers and compos-
ites. Many electrochemical syntheses involve a
change of phase in forming the product, thus sim-
plifying product recovery and separation and reduc-
ing the tendency to generate waste streams in
downstream processing. Good examples are the
production, by reduction, of solid metals from
electrolytes, the production of gases from aqueous
solutions or eutectics, e.g. oxidative generation of
chlorine and fluorine, and the production of metal
salts by anodic oxidation. In addition, several of
these species are non-chlorinated bleaching, sterilis-
ing or oxidising solutions (peroxide, ozone, persul-
fate) used to a small extent as alternatives to
chlorine, which itself is produced electrochemically.
5.2 In situ generation of reagents
The production of chemical reagents that are haz-
ardous to transport and store, both on site and on
demand, is becoming an increasingly important area
for electrochemistry. The production of hydrogen by
electrolysis using, for example, solid polymer elec-
trolyte (SPE) cells for use in chromatography analy-
sis is well established. Processes also exist for the
production of hypochlorous acid, chlorine, hydrogen
peroxide, ozone and arsine. Notably the production
of ozone essentially uses SPE cell technology (Fig.
19.11), generating the gas (O
3
) at the anode (with
oxygen) from pure water. The process requires high
anode potentials (
E
o
=-1.5 V) to produce an approx-
imate 30% by volume ozone concentration.
