Popov K.I., Djokic S.S., Grgur B.N. Fundamental Aspects of Electrometallurgy

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182

Chapter 7

7.1.1

Selectivity of metal dissolution and deposition

The main principle of electrorefining is based on the anodic dissolution

of a metal containing impurities and its cathodic deposition with an as much

as possible reduced amount of impurities. The selectivity principle of metal

dissolution and deposition can be explained starting from a metal M placed

in a suitable solution containing its ions which leads to the

establishment of the equilibrium electrode potential Under some

value of current density, j, the base metal is anodically dissolved

having an anodic potential and cathodically deposited

having a cathodic potential as shown in Fig. 7.2

All metal-impurities having an equilibrium potential more positive than

(such is metal ) will not be anodically dissolved, but owing to the

dissolution of the base metal (erosion) they will be transferred into the

6. Electrowining

183

electrolyte and fall down to the bottom of the reactor forming the anodic

slime.

Metal-impurities which have an equilibrium potential more negative than

(such as: and will be dissolved together with the base metal

M, and transferred into the electrolyte in the form of metal ions. Exceptions

are those metals whose cations can react with the anions in the electrolyte

forming sparingly soluble salts which sediment as anodic slime

During cathodic deposition of a metal, all the metals whose equilibrium

potentials are more positive than the potential of the cathode (metals

and could be deposited, depending on their concentrations and

deposition overpotentials. In the example given in Fig. 7.2. metal will be

anodically dissolved, but not cathodically deposited, while metal will not

be anodically dissolved, but transformed into the anodic slime

It can be concluded that the selectivity principle of electrorefining is

based on such electrolysis conditions which ensure that metal impurities that

could be cathodically deposited will not be anodically dissolved (anodic

slime) and impurities that could be anodically dissolved will not be

cathodically deposited, but remain in the electrolyte.

7.2

THEORETICAL ASPECT OF COPPER

ELECTROREFINING

Copper produced by metallurgical processes contains significant amount

of impurities, between 0.5 and 2 %, which have a bed influence on the

mechanical and electrical properties of copper

. Amounts of 0.15 % of

phosphorous or 0.5 % of arsenic, dramatically decrease the electrical

conductivity of copper, as shown in Table 7.1.

Hence, the improvement of

the electrical properties of copper is the main reason for electrorefining.

The second reason for electrorefining is the extraction of some noble

impurities, normally present in non-refined copper (anodic metal), such as:

gold, silver, platinum and palladium etc

As stated previously, anodic copper contains between 0.5 and 2 % of

admixtures that can be divided into four groups

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Chapter 7

Ni, Co, Fe, Sn, Zn, Pb

Au, Ag, Pt, Pd, Se, Te

As, Sb, Bi

The main reactions occurring at the electrodes, which determines the

potentials of anode and cathode, during electrorefining of copper are as

follows:

while the impurities can react in the following manners

2,3

Metals of the first group, whose equilibrium potentials are more

negative (less noble) than copper (see Table 2.1) will during anode

dissolution be transferred together with copper into solution. Due to the

presence of sulfate ions in the electrolyte, lead ions will form lead sulfate

and fall into the anodic slime. Other metals, such as Ni, Co, Sn and Zn, will

be concentrated in the electrolyte, since their deposition potential is more

negative than the potential of the cathode, whose value is around 0.3 V. The

presence of these metals in the electrolyte has no bad influence, but it

necessitates periodical purification of the electrolyte. The presence of iron

ions is completely undesirable. Iron can be anodically dissolved in the form

but can not be deposited since its equilibrium potential is much more

negative than the potential of the cathode. ions can be oxidized to

at the anode, but the value of the standard electrode potential of this reaction

(+0.77 V) is more positive than the potential of the anode

Bearing in mind the Nerst relation (Eq. 2.2) for the equilibrium electrode

potential:

it could be concluded that the potential is dependent on the ratio of the

and concentration. If the concentration of is much lower than that of

the equilibrium potential will become more negative than the actual

potential of the anode which could result in the oxidation of to

This would decrease the concentration of and increase the concentration

of which leads to a possibility for the reduction of to at the

cathode. This unwanted closed circle would consume a part of the current

during the electrorefining process.

6. Electrowining

185

All the metals with which the electrolyte is enriched lead to a

consumption of free acid.

Insoluble impurities from the second group, such as and

can not be anodically dissolved, but sediment and form anodic slime.

Only can be dissolved, according to:

Hence, half of the copper is dissolved into the electrolyte in the form of

ions, while the other half forms a fine powder which partly reacts with

the air oxygen and sulfuric acid in the electrolyte, and partly sediments to

form anodic slime.

Metals of the third group as Au, Ag, Pt, Se and Te also sediment as the

anodic slime. They are more noble than copper and according the rule that

reactions with lower electrode potentials occur first (see section 2.3.1) at the

anode, these metals act as insoluble inert inclusions and fall down as the

anodic slime.

The most undesirable impurities are the metals of the fourth group: As,

Sb, and Bi. Their presence drastically decreases the quality of the cathode

copper deposit, as shown in Table 7.1. These metals are the most difficult to

be removed becouse of the values of their reduction potential which are near

the potential of copper, precisely between the equilibrium potential of copper

and hydrogen. However, their concentration in the electrolyte is usually very

small, so their deposition potential is more negative compared with the

potential of the cathode, and under normal conditions they are not deposited.

If for some reason the concentration of copper ions decreases, their

deposition could be expected. On the other hand, Sb and Bi could react with

As forming sparingly soluble arsenates, which floats in the form of fine foam

on the electrolyte and make electrolysis process difficult.

The effects of electrorefining could best be illustrated by Table 7.2,

which gives the distribution of impurities in the anodic slime, electrolyte and

cathode metal

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Chapter 7

7.2.1

Technological scheme of copper electrorefining

The process of copper electrorefining is schematically illustrated in Fig

7.3.

Anodes of 99 – 99.5 % pure copper obtained, by pyrometallurgical

treatment, are prepared by casting after melting in the anode furnace. The

surface area of the anode depending on the capacity of the electrolytic cell, is

between 1 and their width is between 40 and 50 mm, while their

weight is between 300 and 350 kg

2,4

. A part of the anodes is prepared to

cathodes by rolling into 1 mm thick copper sheets 5 kg in weight

2,4

. During

the manufacture of the anodes and cathodes, hooks for hanging the

electrodes onto electrode tracks are also produced.

After manufacture, the electrodes are placed into electrochemical cells,

the number of which can, depending on the capacity of the plant, be a few

tens connected in series or parallel.

Electrorefining of copper takes place in an electrolyte, a typical

composition of which is given in Table 7.3

Thiourea and gelatine (10-100 g per 1 t of copper) are used as additives

for enhancing the quality of the deposit and preventing the formation of

dendrites which can produce short circuits during the electrolysis process

The conditions of the copper electrorefining process are

2,4

cathodic current density:

current efficiency

cell voltage:

temperature of the electrolyte:

specific electrical energy consumption:

94 – 96

0.25–0.35V

55–60°C

6. Electrowining

187

After the electrorefining process, cathodic copper of 99.97 – 99.99 %

purity is dried, and further processed by melting, rolling into thin metal

sheets, wires or similar commercial products.

During electrolysis process the electrolyte is enriched in Ni, Sb, and As,

which are removed by regeneration, while the anodic slime is removed from

the electrochemical cell and transported for further processing. It is

important to mention that the process of the anode slime can bring in such

high incomes that the entire cost of electrorefining is covered.

As previously stated, copper electrorefming is an electrolysis process

with a soluble anode and theoretically the composition of the electrolyte

should not change during the process. However, in a real system some

composition changes do occur. First by the acid electrolyte dissolves some

amount of anodic copper in the presence of air oxygen, which results in an

increase in the concentration of copper ions, and a concurrent decrease in the

free acid concentration. Secondly, impurities from the anodic copper, which

did not sediment into the anodic slime, remain in the electrolyte and

additionally decrease the concentration of free acid.

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Chapter 7

7.2.1.1

Refining of the electrolyte

In practice, two methods of correcting the electrolyte composition and

refining are in common use

The first way consists of removing copper, As and Sb by cathodic

deposition in separate electrochemical cells with insoluble anodes. This

procedure increases the concentration of the acid after which the solution is

concentrated by vaporization whereby crystallization of nickel, iron and

copper sulfates occurs.

The second procedure consists of the neutralization of the free sulfuric

acid by dissolving copper. Copper sulfate crystallizes from the saturated

solution. The rest of the electrolyte is freed from Cu, As and Sb by

electrolysis in a separate cell and from Ni, Zn and Fe by evaporation.

After this refining, the electrolyte, practically contains only sulfuric acid,

which is further, corrected by dissolving copper.

7.2.1.2

Processing of the anodic slime

The anodic slime is a precious crude from which noble and rare metals

and metalloids such as the platinum group metals, gold, silver, selenium,

tellurium etc are extracted

1,2

Copper is removed from the anodic slime by dissolution in hot dilute

sulfuric acid with air circulation, after which selenium and tellurium are

removed. Subsequently, by melting after the addition of quartz sand, sodium

carbonate and sodium nitrate, the anodic slime is transformed into an alloy

composed of 93 % silver, 3% gold, 1% copper, 0.05% palladium, 0.03%

platinum, and traces of other metals

Silver of 99.99 % purity is extracted from the alloy by the process of

anodic dissolution in a solution of silver nitrate. Owing to the high value of

the exchange current density which leads to the poor adhesion of the silver

deposit, some specific constructions of the refining cell are required which

enable the separation of the anodic slime collected in polypropylene bags

containing the anodes from the crystals of silver that drop from the cathode

to the bottom of the cell

After the process of silver electrorefining, the anodic slime is composed

of 95% gold, 5% silver and about 1% copper, palladium, platinum and some

other metals. After melting and casting, the anodes are subjected to a

electrorefination in the electrolyte containing and free hydrochloric

acid. Owing to the high value of the electrode potential of gold deposition of

about 1.4 V, all of impurities, except silver which forms insoluble silver(I)

chloride, are dissolved and remain in the electrolyte

When the amount of platinum and palladium reaches a value of about

the electrolyte is replaced. These metals are subsequently recovered

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189

in the form of ammonium salts: and By the

reduction of these salts, palladium and platinum are obtained

It is important to mention that the price of noble metals salts are a few

times higher than the price of the noble metals themselves.

7.3 FURTHER READINGS

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, Electrometallrgy (In Serbian), Belgrade: Faculty of Technology and

Metallurgy, 1972.

Kubasov, Vladimir; Bannikov, Vladimir, Electrochemical Technology of Inorganic

Compounds (In Russian), Moscow: Khimia, 1989.

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Chapter 8

OPTIMUM CONDITIONS FOR

ELECTROPLATING

Metal coating represents a metal electrodeposit which changes the surface

properties from those of the basic metal to those of electrodeposited one. It

should be adherent, nonporous and without internal stresses. A good adhesion

depends mainly on the preparation of the substrate for electrodeposition, resul-

ting in a clean surface amenable to accepting a satisfactory deposit. Poor surface

preparation can cause peeling of the coating making it useless. Crashing and

peeling of a deposit can also be caused by internal stresses, which arise mainly

from the incorporation of foreign materials in the lattice. If lattice of an electro-

deposited metal is free of such inclusions the mechanical properties of the

electrodeposit are practically the same as those of thermally prepared metal.

Finally, the surface properties of the basic metal can not be completely trans-

formed into the properties of the electrodeposited one if the deposit is porous.

The preparation of a substrate for electrodeposition is not connected with

metal electrodeposition and is not treated in this book. It is treated in details

elsewhere

1,2

. The appearance of stresses, which is partially connected with the

electrodeposition process, but has not yet been clarified is also not treated here.

On the other hand the effect of cementation on the adhesion of a coating

and the effect of deposition condition on the porosity of the metal deposits

can be treated in a semi-quantitative way.

8.1

CEMENTATION AND DEPOSITION FROM THE

COMPLEX SALT SOLUTIONS

If zinc is immersed in a copper sulfate solution the reaction

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