Popov K.I., Djokic S.S., Grgur B.N. Fundamental Aspects of Electrometallurgy
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172
Chapter 5
5.6 CONCLUSIONS
The beneficial effect of EPCR in the electrodeposition of metals are
clearly demonstrated and explained in a semiquantitative way. Therefore, for
the same effective current density EPCR requires a somewhat larger energy
consumption
7
, than in DC deposition as well as the more expensive current
generators. This should not be considered as an important drawback of such
electrolysis, bearing in mind all the considerable improvements of the
process of metal deposition that result.
It appears that in each particular case one should compare all the
characteristic of both deposition with constant rate and EPCR, and then
decide which one to apply, considering all the requirements in relation to the
quality of the deposits, the productivity of the electrolyser, and other
parameters of the electrolytic production operation.
5.7 FURTHER READINGS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Bibikov, Nikolai, Electrodeposition of Metals by AC (in Russian). Moscow, Leningrad:
Mashgiz, 1961.
Bakhvalov, Grigorii, New Technology of Metal Electrodeposition (in Russian). Moscow:
Metallurgiya, 1966).
Aleksandar; Popov, Konstantin. “Transport Controlled Deposition and
Dissolution of Metals.” In Modern Aspects of Electrochemistry, Vol. 7, Brian E.
Conway, John O’M. Bockris, eds. New York, NY: Plenum Press, 1972.
Ibl N., Puippe J. A., Angerer H. Electrocrystallization in pulse electrolysis. Surf. Technol
1978; 6: 287-30
Polukarov Yu. M., Grinina V. V., “Electrodeposition of Metals by Periodically Changing
Rate and Single Pulses” (in Russian) Itogi nauki i tekhniki, Seriya Elektrokhimiya Vol.
22, Yu. M. Polukarov, ed, Moscow: Acad. Sci. Moscow: 1985.
Theory and Practice of Pulse Plating, Jean-Claude Puippe, Frank Leaman, eds. American
Electroplate and Surface Finishing Society, Orlando, Florida: 1986.
Popov, Konstantin; Miodrag. “Theory of the Effect of Electrodeposition at
a Periodically Changing Rate on the Morphology of Metal deposits.” In Modern Aspects
of Electrochemistry, Vol. 19, Brian E. Conway, John O’M. Bockris, Ralph E. White, eds.
New York, NY: Plenum Press, 1989.
Pesco, Anthony; Cheh, Huk. “Theory and Application of Periodic Electrolysis.” In
Modern Aspects of Electrochemistry, Vol. 19, Brian E. Conway, John O’M. Bockris,
Ralph E. White, eds. New York, NY: Plenum Press, 1989.
Popov, Konstantin; Miomir.. “Electrodeposition of Metal Powders with
Controlled Grain Size and Morphology.” In Modern Aspects of Electrochemistry, Vol.
24, Ralph E. White, John O’M. Bockris, Brian E. Conway, eds. New York, NY: Plenum
Press, 1993.
M.D. The influence of the charge and discharge of the electrical double
layer in electrodeposition by periodically changing rate. J. Serb. Chem. Soc. 1965; 60:
449-58
5.
Electrodeposition at a Periodically Changing Rate
173
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
M.D., Popov K.I. Mass transfer during electrodeposition of metals at a
periodically changing rate. J. Serb. Chem. Soc. 1999; 64: 317-40
Viswanathan K., Cheh H.Y. Mass transfer aspect of electrolysis by periodic currents. J.
Electrochem. Soc. 1979; 126: 398-401
Chin D.T. Mass transfer and current-potential relation in pulse electrolysis. J.
Electrochem. Soc. 1983; 130: 1657-67
Popov K.I., M.D.,
M.S. The effect of periodic reverse current on the
surface rougneses of metal deposits and maximal deposition rate, Surf. Technol. 1982;
16: 209-18
Popov K.I., M.D., B. M., B.J. Fundamental aspects of
pulsating current metal electrodeposition I: The effect of the pulsating current on the
surface roughness and the porosity of metal deposits, Surf. Technol. 1980; 11: 99-109
M.D., Totovski A.P. The effect of AC superimposed on DC in
the electrodeposition of metals. Surf. Technol. 1983; 18: 233-241
Popov K.I., M.D., S.K., M.R. Surface roughening and
dendritic growth in pulsating overpotential copper electrodeposition. Surf. Coat. Technol.
1986; 27: 117-129
Popov K.I., Maksimovic M.D. Maximum deposition rate in metal electrodeposition by
reversing current in the second range J. Serb. Chem. Soc. 1991; 56: 25-31
Romanov V.V. The Zinc spongy electrodeposits formation during deposition from
zincate alkaline solution (in Russian). Zh. Prikl. Khim. 1963; 36: 1057-63
Popov K.I., D.N.,
M.D. Electrodeposition of zinc on copper from alkaline
zinkate solutions. J. Appl. Electrochem. 1978; 8: 19-23
Diggle J.W., A.R., Bockris J.O’M. The mechanism of the dendritic
electrocrystallization of zinc. J. Electrochem. Soc. 1969; 116:1503-14
Barton J.L., Bockris J.O’M. The electrolytic growth of dendrites from ionic solutions.
Proc. Roy.Soc. London 1962; A268: 485-505
Bek R.Yu., Kudryavtsev N.T. The effect of a periodically changing rate on the zinc
electrodeposition from zincate alkaline solutions (in Russian). Zh. Prikl. Khim. 1961; 34:
2020-27
Arouete S., Blurton K.F., Oswin H.G., Controlled current deposition of zinc from
alkaline solution, J. Electrochem. Soc. 1969; 116: 166-169
Popov K.I., E.R., V., M.G. Morphology of lead
dendrites electrodeposited by square-wave pulsating overpotential. Powder Technology
1997;
93:
55-61
M.G., M.D., Popov K.I., Kršul M.B. The effect of pulsating
overpotential on the morphology of electrodeposited silver powder particles. J. Appl.
Electrochem. 1978; 8: 61-65
Popov K.I., M.D., M.G., G.R. Formation of powdered
copper deposits by square-wave pulsating overpotential, J. Appl. Electrochem. 1977; 7:
331-37
M.G., Lj.J., E.R., V., Popov K.I. The effect of
particle structure on apparent density of electrolytic copper powder. J. Serb. Chem. Soc.
2001; 66:913-933
Popov K.I., Lj.J., E.R., V. The effect of reversing
current deposition on apparent density of electrolytic copper powder. J. Serb. Chem. Soc.
2002; 67:61-67
174
Chapter 5
30.
31.
32.
33.
34.
35.
36.
37.
Popov K.I., M.D., R.M. N.V. Fundamental aspect of
pulsating current metal electrodeposition. VIII: The effect pulse to pause ratio on
microthrowin
g
power of metal deposits. Surf. Technol. 1984; 22: 155-58
Popov K.I., M.G., Lj. J. M.I. G.Ž. Electrode
surface coarsening in pulsating overpotential copper electrodeposition. Surf. Coat.
Technol. 1988; 34: 355-63
Popov, Konstantin “The Effect of Periodically Changing Rate in Electrodeposition on
the Morphology of Metal Deposits.” (in Serbian) In Application of Pulsating Regimes in
Electroplating, Miomir Aleksandar Dekanski, eds. ICTM, Department of
Electrochemistry, Belgrade, 1992.
Popov K.I., M.D., Totovski Fundamental aspects of pulsating current
metal electrodeposition. VI: The comparison of electrode surface roughening in pulsating
current and periodic reverse current electrodeposition of metals. Surf. Technol. 1982; 17:
125-29
Popov K.I., Totovski M.D. Fundamental aspects of pulsating current
metal electrodeposition. VII: The comparison of current density distribution in pulsating
current and periodic reverse current electrodeposition of metals. Surf. Technol. 1983; 19:
191-85
Popov K.I., T.M., E.R., N.D., M.G. The
determination of the optimum current wave in reversing current metal electrodeposition.
J. Serb. Chem. Soc. 1998; 63: 537-44
B.M., T.M., The properties of chromium electrodeposited with
programed currents. Part I: Direct current. J. Serb. Chem. Soc. 2000; 65: 55-63
B.M., T.M., The properties of chromium electrodeposited with
programed currents. Part II: Reversing current. J. Serb. Chem. Soc. 2000; 65: 65-72
Chapter 6
ELECTROWINNING
6.1
THEORETICAL ASPECTS OF
ELECTROWINNING
Electrowinning of a metal is based on the electrolysis of aqueous
solutions or melts of metal salts with insoluble anode
1,2
.
The basic reactions during electrowinning from aqueous solutions are
2
:
- cathodic deposition of the desired metal:
and
- oxygen evolution on the anode:
Electrowinning of a metal by the electrolysis is usually performed in an
aqueous solution of the sulfates of a metal in dilute sulfuric acid, with the
aim to avoid problems related with the deposition of the hydroxide of the
metal. Sulfuric acid is chosen because of its relatively low price
2
.
Several processes of preparations and enrichment of the metal ore proceed
the electrolysis, which is the final phase in the electrowinning process.
Bearing in mind the fact that metals are found in nature usually in the
form of their sulfide and oxide ores, the preparation usually consists in
2
:
I Enrichment of the ore, serves for the removal of the wastes by flotation
which leads to a concentrate of metal sulfides and/or metal oxides.
175
176
Chapter 6
II Roasting – oxidation of the concentrate, if the dominate form of the
metal in the concentrate is the metal sulfide which is insoluble in sulfuric
acid, then the roasting is performed in the air with the aim of transforming
the sulfide into the oxide which is soluble in sulfuric acid.
III Leaching of the concentrate, is performed with dilute sulfuric acid,
impoverished electrolyte from the electrolysis process, which gives a
solution of the metal sulfate with some impurities.
IV Refining of the leach means the elimination of impurities with more
positive potentials, which could be deposited together with the metal, thus
decreasing its purity, or result in the hydrogen evolution reaction.
Elimination of the impurities is performed by increasing the pH of the
solution, by precipitation with hydrogen sulfide or by cementation.
V Electrolysis serves for the final extraction of the metal by cathodic
deposition in the electrochemical reactor. Since oxygen is evolved at the
anode according to Eq. 6.2, the concentration of metal ions decreases and the
acidity of the electrolyte increases, which can result in hydrogen evolution
together with metal deposition at the cathode. The compensation of the metal
ions concentration is performed in phase III.
Although practically all metals can be obtained by the electrochemical
procedure, the most wide spread technologies involve the electrowinning of
zinc from a solution of its sulfate
1
.
6.2 THEORETICAL ASPECTS OF ZINC
ELECTROWINNING
Apart from the conventional metallurgical procedure of zinc production,
today the most common procedure is the electrowinning of zinc by
electrolysis of an aqueous solution of zinc sulfate. This electrochemical
procedure gives zinc of 99.99 % purity and less loss of cadmium than the
classical pyrometallurgical treatment.
During the electrolysis of an aqueous zinc sulfate solution, the basic
electrode reactions are:
cathodic deposition of zinc
and oxygen evolution at the insoluble anode
6. Electrowining
177
Although the electrode potential of the reduction of zinc ions is much
more negative than the electrode potential of the reduction of protons (about
0.8 V), it is possible to deposit zinc from the acid solution with a high
current efficiency thanks to the extremely high value of the hydrogen
overpotential on zinc.
However, the presence of impurities (e.g metal ions with reduction
potentials more positive than that of protons) will cause these metals to be
deposited together with zinc. The results is the formation of cathodic
surfaces having a low value of the hydrogen overpotentials. Intensive
hydrogen evolution will occur on such surfaces together with the dissolution
(corrosion) of the already deposited zinc, following the mechanism of local
galvanic cells. Impurities such as copper, bismuth, germanium and antimony
do not just decrease the current efficiency, but completely prevent the
deposition of zinc. It can be concluded that the electrolyte for zinc
electrowinning has to be completely freed from such impurities before the
electrolysis. Hence, the basic aim of zinc ore treatment is the production of
pure zinc sulfate without impurities which would exhibit a bad influence on
the cathodic reaction
2
.
6.2.1
Technological scheme of zinc electrowinning
The composition of the zinc ore influences the manner of treatment of the
zinc sulfate solution. Zinc sulfate is extracted from an oxide ore after
removing the wastes, with the spent solution of zinc sulfate and sulfuric acid
from the electrolysis, according to the reaction
2
:
Extraction from a sulfide ore necessitates roasting in an air atmosphere,
during which the main reaction is the formation of zinc oxide according to:
Impurities in a zinc ore can be classified into three main groups
3
:
1.
2.
3.
Fe, Si, Al, Sb, Bi, Ge, Sn
Cu, Cd, Ni, Co
Pb, Ag,
During the dissolution of zinc sulfate, according to Eq. 6.4, the extent of
the free acid decreases, which leads to an increase of pH of the solution. This
results in the precipitation of Fe(III), Al and partly Cu in the form of
hydroxides. Owing to the high specific surface of Fe(III) hydroxide, Sb, Bi
178
Chapter 6
and Ge are adsorbed on its surface and in that manner removed from the
solution, while As forms the insoluble Fe(II) hydroxide
precipitates at pH 6.5, at which pH zinc(II) hydroxide precipitates too. For
this reason it is necessary to oxidize to This is achieved by the
addition of which reacts with according to the reaction:
and also react with at lower values of pH, giving the
corresponding three-valent ions which are deposited in the form of
hydroxides, and NiOOH, and so in this way partly removed.
During the electrolysis is formed by the anodic oxidation of
ions according to the reaction:
The impurities of the third group are insoluble in the leach.
The precipitate, containing impurities of the I, III, and partly of the II
group, are removed from the leach by filtration. The removal of group II
impurities is based on cementation of the metal ions from the solution with
zinc powder, according to the reaction:
After these treatments, the concentration of zinc ions varies between 120
and and the concentration of the free acid between 150 and
Typical contents of metal ions in the electrolyte for the
electrowinning of zinc is given in Table 6.1
3
.
It is common to add between 10 and of glue which serves to
enhance the quality of the deposit and to avoid the formation of dendrites
which could lead to short circuits between the anode and cathode during
electrolysis, and also to a decrease in the current efficiency
2
.
The described procedures are schematically given in Fig. 6.1.
The cathode material used in the electrochemical cell is usually
aluminium sheet 4 to 8 mm in thickness
3
. Aluminium is used owing to the
poor adhesion of the deposited zinc, which can later be easily removed from
the cathode
2
. Lead sheet 5 to 10 mm in thickness, and in average, having the
6. Electrowining
179
dimensions of 100 x 600 mm, serves as the anode
2,3
. The life time of these
lead anodes is 1.5 – 2 years, but if the lead anode is alloyed with 1 – 2 % of
silver, the life time is increased to 3 – 4 years
2
.
The average electrolysis time is between 10 and 24 h. during which time the
concentration of zinc should drop to After the electrolysis the
cathodes are taken from the cell and the zinc is mechanically removed.
Typical conditions for the electrowinning of zinc are as follows
2,3,4
:
the cathodic current density:
current efficiency:
voltage:
temperature of the electrolyte:
specific electrical energy consumption:
88–92%
3.3–3.6V
30–40°C
180
Chapter 6
For the electrowinning of 1 t of zinc in average about: 2.5 t of the
concentrate, 4 kg of 16 kg of Zn powder and 62 kg of sulfuric acid
are consumed
2
.
The zinc concentrate usually contains 0.5 – 1 wt. % of Cd, which is why
in most zinc electrowinning plants, Cd is obtained parallel by a similar
process
3,4
.
6.3 FURTHER READINGS
1.
2.
3.
4.
Pletcher, Derek, Industrial Electrochemistry, New York: Chapman and Hall, 1984.
Strahinja; Snežana; Branislav, Electrochemical Engineering
(In Serbian), Belgrade: Faculty of Technology and Metallurgy, 2001.
Spasoje, Electrometallurgy (In Serbian), Belgrade: Faculty of Technology and
Metallurgy, 1972.
Kubasov, Vladimir; Bannikov, Vladimir, Electrochemical Technology of Inorganic
Compounds (In Russian), Moscow: Khimia, 1989.
Chapter 7
ELECTROREFINING
7.1 THEORETICAL ASPECTS OF
ELECTROREFINING
Electrorefining of a metal by electrolysis is a way of purification of a
metal previously extracted by classical metallurgical or electrochemical
processes, with the aim of removing impurities, which could exhibit in
negative effects on the physico–chemical and mechanical properties of the
metal. In principle, almost all metals can be electrorefined, but judging by
the amount of 8 million tones per year, copper electrorefining outweighs all
others
1,2
.
The procedure of a metal electrorefining is schematically given in Fig.
7.1.
181