Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
1999-09-14
2001-04-03
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S290140
Reexamination Certificate
active
06210550
ABSTRACT:
DESCRIPTION OF THE INVENTION
The evolution of oxygen from solutions containing sulphuric acid or sulphates is a well-known reaction. In fact, all electrometallurgical processes based on sulphuric acid or sulphates presently under operation were developed at the beginning of the century. In these processes the anodic counter-reaction to the cathodic deposition or production of metals from the respective salts is represented in fact by the evolution of oxygen.
The industrial processes known so far, where oxygen is evolved at the anode, consist in:
the electrometallurgy of primary and secondary copper, zinc, cobalt, nickel from sulphuric electrolytes;
the high speed galvanic deposition of copper and zinc (tapes and wires) and the traditional deposition of chromium, nickel, tin and minor elements.
The most commonly used commercial anode is made of lead or, more precisely, lead alloys (e.g. Pb—Sb; Pb—Ag; Pb—Sn etc.). It consists of a semi-permanent system wherein the lead base undergoes spontaneous modification under anodic polarisation to lead sulphate, PbSO
4
, (intermediate protective layer with low electrical conductivity) and lead dioxide, PbO
2
, (semiconducting surface layer relatively electrocatalytic for the oxygen evolution with an electrode potential of >2.0 V (NHE) at 500 A/m
2
). This system under operation is, on the one hand, immune from progressive or irreversible passivation (spontaneous renewal of the electrodic surface), but, on the other hand, it is subject to the corrosive action of the electrolytic medium, which leads to its increasing dissolution (non-permanent system).
Industrial lead anodes are based on alloys containing, as alloying agents, elements selected from the groups I B, IV A and V A of the periodic table.
Examples of anodic compositions are given in Table 1.
TABLE 1
Anodic material
Electrometallurgical process
Pb—Ag (0.2-0.8%)
Zinc electrometallurgy
Pb—Sb (2.6%)
Electrometallurgy of cobalt, nickel,
Pb—Ag (0.2-0.8%)
primary and secondary copper
Pb—Sn (5-10%)
These materials are characterised by:
high anodic potentials, above 2.0 V (NHE) even at low current densities (e.g. 150 -200 A/m
2
);
lifetimes varying from 1 to 3 years;
High electrical resistivity and high electrical disuniformity (formation under operation of thick and solid layers of PbSO
4
(intermediate passivating layer) and PbO
2
(electrocatalytic surface layer for oxygen evolution).
This situation negatively affects the cathodic products, which undergo:
loss of faradic efficiency, never exceeding 90% for the zinc metallurgy and 95%for the cobalt electrometallurgy;
uneven and dendritic aspect of the deposit, especially for zinc and copper contamination by lead, in the range of 20-40 ppm Pb/ton Zn and 10-30 ppm Pb/ton Co.
As an alternative to lead anodes, cobalt anodes are used for a very limited part of the cobalt electrometallurgy. Three alloys are substantially utilised, corresponding to the following compositions:
Co—Si (5-20%)
Co—Si (5-20%)—Mn (1.0-5.0%)
Co—Si (5-20%)—Cu (0.5-2.5%)
The materials based on cobalt-silicon, as compared to lead, are characterised by a longer lifetime, but at the same time have a lower electrical conductivity and are brittle. The materials based on Co, Si and Cu exhibit values of electrical resistivity similar to those of lead but have a shorter lifetime and in any case are more fragile.
Table 2 summarises the general operating conditions of the prior art materials based on lead and cobalt alloys under the most common electrolytic conditions.
TABLE 2
Prior art anodic materials based on lead and cobalt alloys
Current
Anodic material and lifetime (years)
density
Pb—Sn
Co—Si,
Co—Si—
Process
Electrolyte or bath
A/m
2
Pb—Sb
Pb—Ag
Co—Si—Mn
Cu
Zinc
Zn
2+
(40-90 g/l)
300-500
//
2-4
//
//
H
2
SO
4
(150-200 g/l)
Fluorides (50 ppm)
Manganese (2-5 g/l)
Zn
2+
(40-90 g/l)
300-500
1-3
2-4
//
//
H
2
SO
4
(150-200 g/l)
Fluorides (<5 ppm)
Manganese (2-8 g/l)
Cobalt
Co
2+
(50-80 g/l)
150-250
2-3
4-5
3-4
2-3
H
2
SO
4
(pH 1.2-1.8)
Manganese (10-30 g/l)
Primary
Cu
2+
≅ (40-55 g/l)
150-200
3-4
—
//
//
Copper
H
2
SO
4
(150-200 g/l)
Fluorides 100-200
ppm
Manganese 300 ppm
Secondary
Cu
2+
(10-50 g/l)
150-200
3-4
—
//
//
copper
H
2
SO
4
≅ (170 g/l)
Fluorides ≅ 2-5 ppm
Nickel
Ni
2+
(60-70 g/l)
150-200
3-4
H
2
SO
4
(pH 2.3-3.0)
More recently the use of activated titanium anodes has been proposed, comprising a permanent titanium substrate provided with an intermediate protective coating made of oxides and/or noble metals and a surface electrocatalytic coating for oxygen evolution based on tantalum and iridium oxide, more active than lead (electrode potential 1.7 (NHE) at 500 ANm
2
) and suitable for reactivation ex-situ of the substrate.
This anode is suitable for operation in electrolytes containing sulphuric acid or sulphates free of or scarcely contaminated by impurities, as is the case for some galvanic processes of limited commercial interest. Conversely, at least on the basis of the experience gathered so far, this anode is not suitable for use with electrolytes containing a significant amount of manganese (zinc and cobalt electrometallurgies and some galvanic processes) due to:
i. progressive and irreversible passivation due to the manganese dioxide deposit;
ii. mechanical and chemical attack of the active layer;
iii. loss of noble metal and
iv. corresponding loss of faradic efficiency for the cathodic process.
The use of tantalum and iridium oxide, described for the first time in U.S. Pat. No. 3,878,083, arises from the following three reasons:
electrocatalytic activity of iridium and its oxides for the evolution of oxygen with a Tafel slope b<15 mV/decade;
stabilisation of iridium in the oxide state due to the action of tantalum;
structural compatibility between the tantalum and the iridium oxides.
This system is suitable also for concentrated sulphuric electrolytes (e.g. H
2
SO
4
150 g/l), provided they are free from impurities and subject to mild conditions in terms of temperature (e.g.<65° C.) and current density (e.g. <5000 ANm
2
). Under higher current densities (e.g. >5000 ANm
2
: zinc, copper, chromium electrometallurgies) and/or with electrolytes containing corrosive impurities (fluorides or their derivates and organic compounds in the zinc, copper, chromium electrometallurgies), an interlayer has been added to provide a protective barrier of the titanium substrate against corrosion.
Examples of known compositions of protective interlayers are:
a ) Titanium—Tantalum as oxides, 80-20% on atomic basis respectively. The oxide is formed by thermal decomposition of paints containing suitable precursors, as described in U.S. Pat. No. 4,484,999.
b) Platinum—Iridium in the metal state, 70-30% by weight respectively. Also in this case the layer is obtained by thermal decomposition of paints containing suitable precursor salts, as described in Italian patent application no. MI97A908, filed by the applicant on Apr. 18, 1997.
c) Titanium, tantalum and iridium, and particularly the first two as oxides, the third as metal and/or oxide, 75-20-5% on atomic basis respectively.
As previously said, the tantalum and iridium electrocatalytic coating for oxygen evolution, progressively loses its active properties in sulphuric solutions containing manganese, as is the case with primary copper zinc and cobalt electrometallurgies. In fact, the presence of manganese in the solution involves, in addition to the oxygen evolution reaction, also the electrodeposition of manganese dioxide according to Mn
2+
+2H
2
O=MnO
2
+4H
+
+2e at the anode in a scarcely conducting compact layer. This causes a masking of the original electrocatalytic coating and a gradual passivation whose rate is a function both of the manganese content in the electrolyte and of the temperature.
This ageing mechanism illustrates three main concepts:
concurrence of two reactions, the desired and the parasitic one, whose anodic potentials are very close;
mechanical stability of the MnO
2
, compact and adhering deposit;
hig
Nevosi Ulderico
Nidola Antonio
Bell Bruce F.
Bierman, Muserlian and Lucas
De Nora S.p.A.
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