Metal anodes for electrolytic acid solutions containing fluorides or fluoroanionic complexes

Abstract

The present invention relates to metal anodes for oxygen evolution from solutions containing fluorides or artionic fluorocomplexes such as tetrafluoroborates and hexafluorosilicates, the anodes having a metal substrate or matrix selected in the group comprising nickel-copper alloys with a copper content in the range of 2.5 and 30% by weight, tungsten or tantalum, niobium or titanium, combinations thereof or alloys of the same with palladium, nickel or yttrium. The anodes further comprise electrocatalytic compounds for oxygen evolution dispersed in the metal matrix. In the case of nickel- copper alloys, useful electrocatalytic compounds are cerium or tin dioxides, with suitable additives, while for tungsten, cobalt added with nickel, iron, copper or palladium may be used. The same electrocatalytic compounds may be advantageously applied to said metal substrate or matrix in the form of a coating using the conventional technique of thermal decomposition of paints containing suitable precursors or by thermal deposition such as plasma-spray.

Description

Electrolytes containing anionic fiuorocomplexes are commonly used in conventional technologies for the electrolytic recovery of metals, such as lead, tin, chromium. In the specific case of lead recovery from batteries scraps, the scraps are leached with acid solutions containing tetrafluoroborates BF.sub.4 - and hexafluorosilicates SiF.sub.e .dbd.. The electrolysis of these solutions produces lead as a solid deposit; therefore, the electrolytic cells are diaphragmless and have a very simple design. However, this advantage has been so far counterbalanced by the scarce resistance of the substrates to the aggressive action of anionic fluorocomplexes on the anodes whereat oxygen is evolved. Further, a parasitic reaction may take place with formation of lead dioxide which subtracts lead to the galvanic deposition of the metal; thus, reducing the overall efficiency of the system.

Upon carefully considering the prior art teachings found for example in U.S. Pat. Nos. 3,985,630, 4,135,997, 4,230,545, 4,272,340, 4,460,442, 4,834,851 and in Italian patent application no. 67723A/82, it may be concluded that:

anodes made of carbon or graphite, as such or coated by lead dioxide, are known in the art but offer a rather limited active lifetime, in the range of a hundred hours due to the oxidizing action of oxygen evolution. Obviously, this brings forth higher maintenance costs for substituting the anodes and additional costs connected to the consequent production losses;

anodes made of titanium, coated by lead dioxide or platinum or oxides of the platinum group metals, still undergo corrosion, though to a far less extent with respect to carbon or graphite, in any case, insufficient for counterbalancing the higher construction costs;

anodes made of tantalum coated by platinum metal or metal oxides offer a much longer lifetime than titanium but the production costs are extremely high;

the parasitic reaction of lead dioxide deposition onto any type of anode may be prevented adding a suitable inhibitor to the leaching solution; for example phosphoric acid, antimony acid or arsenic acid. However, the quantities required may spoil the compactness of the lead metal deposit. This problem is overcome by resorting to an anode having a coating made of metals or oxides of the platinum group metals and at least one element comprised in the group of arsenic, antimony, bismuth, tin. In this case, a remarkably lower quantity of inhibitor to prevent the anodic deposition of lead dioxide is required, and the deterioration of the produced lead deposit is eliminated. It is, therefore, evident that the prior art does not provide for an anode offering both a long lifetime (higher than 1000 hours) and a limited cost, which are both necessary features for wide industrial application.

THE INVENTION

The present invention permits to overcome the disadvantages of the prior art by providing for an anode characterized by a reduced cost, high resistance to the aggressive conditions of oxygen evolution in solutions containing anionic fluorocomplexes and even free fluorides, and good catalytic properties for oxygen evolution; that is lower electrolysis potential with consequently reduced energy consumptions.

The anode of the present invention comprises a matrix made of one or more metals or metal alloys capable of passivating by forming a protective layer of oxides or oxyfluorides and one or more compounds of suitable elements capable of flavoring oxygen evolution; said elements being embedded into the matrix or alternatively applied to the same in the form of an external coating. Said anode is suitable for use in electrometallurgical processes for the deposition of lead, tin, chromium, from solutions containing fluorocomplex anions such as tetrafluoroborates and hexafluorosilicates or free fluorides.

The present invention also comprises the electrolytic process for recovering metals in cells equipped with anodes and cathodes and fed with acid solutions containing metal ions and anionic fluorocomplexes such as tetrafluoroborates and hexafluorosilicates, wherein said anodes are of above mentioned type.

The following description will take into consideration the particular case of electrolytic recovery of lead, for simplicity sake. In this process, the leaching solution to be electrolyzed has the following composition:

tetrafluoroboric acid, HBF.sub.4, or hexafluorosilic acid H.sub.2 SiF.sub.6 : 40-240 g/l; dissolved lead: 40-80 g/l; temperature: 15.degree.-35.degree. C.; current density (anodic and cathodic): 150-2000 A/m.sup.2.

Electrolysis occurs between the anode and the cathode, with the following reactions:

cathode: Pb.sup.++ (complex)+2e.sup.- .fwdarw.Pb (compact metal) anode: H.sub.20 O--2e.sup.- .fwdarw.2H.sup.+ +1/12O.sub.2 (main reaction) Pb.sup.++ (complex)+2H.sub.2 O-2e.sup.- .fwdarw.PbO.sub.2 +4H.sup.+ (parasitic reaction)

Suitable elements for the anode are : titanium, niobium, tantalum, tungsten or alloys thereof such as:

titanium-palladium (Pd 0.2%), titanium-nickel (Ni 0.5-1.5%); titanium-yttrium titanium-tantalum ( Ta 0.5-5.0%) titanium-niobium (Nb 0.5-5.0%) titanium-tungsten (W 0.5-5.0%) copper-tantalum (niobium); titanium-tantalum (niobium)

Further, it has been surprisingly found that alloys of nickel-copper, obtained either by sinterization of the powders of the elements or by melting and casting in suitable molds readily passivate when put in contact with the aforementioned solutions; that is they become coated by a protective layer of oxides or oxyfluorides or insoluble fluorides when the copper content is in the range of 2.5 to 30% and more preferably between 5 and 20%.

The poor conductivity of the protective film formed on the above metals gives rise to a high potential, and consequently, to high energy consumptions in the process of lead recovery.

It has been found that when using tungsten and nickel-copper alloys, if suitable elements are dispersed into the metal matrix, the oxygen evolution potential is remarkably reduced, bringing the energy consumption to quite acceptable levels for industrial applications for the production of lead.

Suitable compounds for anodes based on nickel-copper are cerium oxide, CeO.sub.2, added with Nb.sub.2 O.sub.5 (1-5%), NiO (0.5-2%), Pr.sub.6 O.sub.11 (0.5-2%), CuO (0.5-2%) and tin dioxide, SnO.sub.2, added with Sb.sub.2 O.sub.3 (0.5-4%) and CuO (0.5-2%); while for anodes based on tungsten, addition of cobalt (5-35%) optionally mixed with minor amounts of iron and nickel (1-2%), copper, palladium and cerium result more positive.

The same results are alternatively obtained by applying to the metal matrix a coating exhibiting electrocatalytic properties for oxygen evolution, chemical stability and possibly limited porosity to ensure an adequate protection to the metal matrix.

In the case of tungsten and nickel-copper alloys, suitable coatings are obtained by cerium and tin oxides as above described for the dispersion in the metal matrix. As for the other alloys, testing has shown that a suitable coating must comprise a matrix made of tungsten or other metal of the VIB group (70-99%), cobalt (1-30%) as the electrocatalyst for oxygen evolution to inhibit possible parasitic reactions and further comprising suitable additives selected from the group comprising nickel, palladium, cerium and copper, or optionally, a combination of the same, (0.5-2%).

The following examples describe various embodiments of the present invention without limiting the invention to the same.

EXAMPLE 1

Eight rods having a diameter of 20 mm, 100 mm long, made of nickel-copper alloys, having different compositions, have been prepared by monostatic lateral pressing (about 250 kg/cm.sup.2) starting from the powders of the elements (1-10 microns) and subjected to subsequent thermal treatment in inert environment at 950.degree.-1150.degree. C. for 6-12 hours (preferably between 980.degree.and 1080.degree. C. for 8-10 hours) followed by a second oxidizing treatment in air at 900.degree.-1300.degree. C. for 100-600 hours (preferably 970.degree.-1000.degree. C., 300-400 hours for copper contents higher than 10-15%).

At the same time, three reference samples have been prepared as follows:

two rods having a diameter of 20 mm, 100 mm long, based on commercial Monel.RTM., (nickel, copper alloy) one of the 400 type and the other of the K500 type oxidized at the conditions used for the samples obtained by sinterization

one sheet of 10.times.100.times.1 mm made of commercial graphite coated by a deposit of beta-PbO.sub.2 obtained by galvanic deposition from nitrate bath. The sintered rods and the reference samples have been tested as anodes in the electrolysis of a fluoroboric solution, which is the typical electrolyte used for metal lead recovery from batteries scraps.

The operating conditions and the results are reported in the following Table.

______________________________________OPERATING CONDITIONS______________________________________HBF4, tetra- 80 g/lfluoroboricacid:Temperature: AmbientCathode: LeadProcedure: Determination of the corrosion potential (PC) by electrochemical potentiostatic procedure and analysis of the solution and cathodic deposit; comparison with the oxygen evolution potential (PO) detected on a graphite electrode coated by beta-PbO.sub.2. The value Delta V = PC - PO defines the stability or instability degree of the various materials.______________________________________ TABLE 1.2__________________________________________________________________________ RESULTS Anodic Potential V (NHE) Delta V O.sub.2 Evolution PC-POSAM- PO Volts (Volts)PLES Corrosion 400 1000 400 1000No. PC Volts A/m.sup.2 A/m.sup.2 A/m.sup.2 A/m.sup.2__________________________________________________________________________1 Beta-Pb.sub.2 2.07 2.24 on graphite2 Monel 400 type +0.38 -1.69 1.86 corroded3 Monel K500 +0.39 -1.68 -1.86 1.86 corroded type4 Ni 99-Cu 1 -0.1 -2.17 -2.34 1.86 corroded5 Ni 98-Cu 2 +0.36 -1.71 -1.88 1.86 corroded6 Ni 97.5-Cu 2.5 +1.30 -0.77 -0.94 1.86 corroded7 Ni 95-Cu 5 >2.3 >0.23 >0.06 1.86 passivated8 Ni 90-Cu 10 >2.3 >0.23 >0.06 1.86 passivated9 Ni 80 Cu 20 >2.3 >0.23 >0.06 1.86 passivated10 Ni 70 Cu 30 +0.99 - 1.08 -1.25 1.86 corroded11 Ni 65-Cu 35 +0.43 -1.65 -1.81 1.86 corroded__________________________________________________________________________

The above results lead to the following considerations:

oxygen evolution on beta-PbO.sub.2 occurs at potentials (PO) comprised between 2.07 and 2.24 Volts at current densities between 400-1000 A/m.sup.2. It is evident that any material having a Corrosion Potential (PC) lower than these values is characterized by instability (tendency to dissolve). The various potentials refer to a reference normal hydrogen electrode (NHE);

the materials with a copper content between 5 and 20% are stable under oxygen evolution.

Similar materials obtained not by sinterization but by molding with casting wax showed the same behaviour.

EXAMPLE 2

Twelve rods having a diameter of 20 mm, 100 mm long, made of sintered nickel-copper alloys have been prepared as described in Example 1, the only difference being the addition of preformed powders (pigments) based on tin oxide and cerium oxide. The electrolysis conditions and the results expressed in terms of anodic potentials, V(NHE) for oxygen evolution at 1000 A/m.sup.2 after 300 h, cathode faradie efficiency % calculated on lead and stability/un-stability of the material under corrosion, are reported in Tables 2.1. and 2.2

TABLE 2.1______________________________________HBF.sub.4, tetrafluoroboric acid: 150 g/llead ion: 60 g/lH.sub.3 PO.sub.4, phosphoric acid: 2 g/ltemperature: Ambientcathode: Leadanodic current density: 1000 A/m.sup.2______________________________________ TABLE 2.2__________________________________________________________________________ RESULTSSAM- Composition O.sub.2 evolution FaradicPLES % PO V (NHE) efficiencyNo. Matrix Additives initial 300 h % REMARKS__________________________________________________________________________Ni--Cu SnO.sub.2 --Sb.sub.2 O.sub.3 90-101 95 5 = 6.8 = 100 corroded2 95 4.90 0.10 2.5 2.6 100 not corroded3 90 9.80 0.20 2.45 2.8 100 not corrodedNi--Cu SnO.sub.2 --Sb.sub.2 O.sub.3 80-204 95 5 = 6.8 = 100 corroded5 95 4.9 0.1 2.5 2.38 100 not corroded6 90 9.8 0.2 2.45 2.38 100 not corrodedNi--Cu CeO.sub.2 --Ta.sub.2 O.sub.5 --NiO--Pr.sub.6 O.sub.11 90-107 95 5 = = = 8.5 = 100 corroded8 95 4.9 0.1 = = 2.8 2.65 100 not corroded9 95 4.8 0.1 0.1 = 2.9 2.6 100 not corroded10 95 4.8 0.1 0.05 0.05 2.8 2.55 100 not corroded11 90 9.6 0.2 0.1 0.1 2.7 2.3 100 not corroded 80 2012 90 9.6 0.2 0.1 0.1 2.8 2.40 100__________________________________________________________________________ The results obtained on Ni-Cu alloys bring to the following conclusions:

Tin Dioxide

corrosion on SnO2 without additives

no visible corrosion under operation with O.sub.2 evolution on SnO.sub.2 added with Sb.sub.2 O.sub.3 after 300 hours of operation.

Cerium Dioxide

anodic corrosion on CeO.sub.2 without additives

no visible corrosion under operation with oxygen evolution after 300 hours of operation with CeO.sub.2 containing additives increasing electrocatalytic activity according to the following order:

CeO.sub.2 <CeO.sub.2 +Ta.sub.2 O.sub.5 <CeO.sub.2 +Ta.sub.2 O.sub.5 +NiO<CeO.sub.2 +Ta.sub.2 O.sub.5 +NiO+Pr.sub.6 O.sub.11

Similar results may be obtained with Ni-Cu structures coated by an electrocatalytic coating, having the same composition as the particles used for the dispersion embedded in the matrix, said coating being applied by thermal decomposition of a paint containing suitable precursors. It is also to be pointed out that the addition of only 2 g/l of phosphoric acid ensures 100% cathodic Faradic efficiency: this means that no lead dioxide is formed at the anode.

EXAMPLE 3

Four rods, with a diameter of 20 mm, 100 mm long, made of nickel-copper alloy, have been obtained by casting the component metals together with powders based on tin oxide and/or cerium oxide (diameter 40-60 microns). Said samples have been tested as anodes for the electrolysis of fluoroboric solutions according to the conditions and procedures described in Example 2. The results are reported in Table 3.1.

TABLE 3.1______________________________________ O.sub.2 EvolutionSAM- PO Volts FaradicPLE Composition % (NHE) Efficien-No. Matrix Additives Init. 300 h cy (%)______________________________________Ni--Cu SnO.sub.2 Sb.sub.2 O.sub.3 CuO 90-101 95 4.8 0.15 0.05 2.5 2.8 1003 95 4.8 0.15 0.05 2.5 2.38 100Ni--Cu CeO.sub.2 Ta.sub.2 O.sub.5 NiO Pr.sub.6 O.sub.11 80-202 95 4.8 0.1 0.05 0.05 2.9 2.7 1004 95 4.8 0.1 0.05 0.05 2.8 2.35 100______________________________________

The samples reported in Table 3.1 showed also that metal structures made of Cu.sub.20(10 -Ni.sub.80(90) after addition of SnO.sub.2 or CeO.sub.2 containing additives do not undergo any visible corrosion when used as anodes for oxygen evolution.

EXAMPLE 4

Fifteen commercial tungsten rods with different contents of cobalt, nickel and iron have been used as anodes for oxygen evolution in the electrolysis of fluoroboric solutions as illustrated in Example 2. The results are reported Table 4.1

TABLE 4.1______________________________________ RESULTS O.sub.2 Evolution FaradicSAMPLES PO VOLTS Effi-Composition (%) (NHE) ciency RE-No. W Co Ni Fe Initial 300 h % MARKS______________________________________1 100 >6.00 passivated2 90 10 2.6 2.4 100 slight Co corroded3 80 20 2.3 2.3 100 heavy Co leachng4 70 30 2.2 2.2 100 heavy Co leaching5 65 35 2.2 2.2 100 corroded6 95 = 5 = 3.2 = = close to passivation7 90 = 10 = 2.6 3.5 = close to passivation8 95 = = 5 3.8 = = close to passivation9 90 = = 10 2.3 4.1 = close to passivation10 90 8 1 1 2.2 2.3 100 not corroded11 80 15 2.5 2.5 2.1 2.2 100 not corroded12 63 35 1 1 2.1 2.1 100 not corroded13 60 38 1 1 2.1 2.1 = not corroded14 58 40 1 1 2.0 4.0 = corroded15 58 38 2 2 2.0 5.0 = passivated______________________________________

These results lead to the following conclusions:

tungsten is stable when used as anode in fluoroboric solutions (passivation)

elements like Co, Ni, Fe in minor amounts perform an electroatalytic activity for oxygen evolution

the following series show an electrocatalytic activity increasing as per the following order: Fe<Ni<Co<Co+Ni+Fe

a critical concentration threshold for each additive or combination of the same has been found beyond which passivation or corrosion phenomena occur.

Similar results may be obtained by applying to the tungsten structure an electrocatalytic coating as described in Example 2.

EXAMPLE 5

Six rods having a diameter of 20 mm, 100 mm long, labelled as follows:

sample 1 as in Example 2, no. 6 sample 2 as in Example 2, no. 12 sample 3 as in Example 3, no. 3 sample 4 as in Example 3, no. 4 sample 5 as in Example 4, no. 4 sample 6 as in Example 4, no. 11 have been used as anodes for electrolysis of fluorosilic solutions containing lead ions and phosphoric acid.

The electrolysis conditions are reported in Table 5.1.

TABLE 5.1______________________________________H.sub.2 SiF.sub.6, fluorosilicic acid: 100 g/lH.sub.3 PO.sub.4, phosphoric acid: 6 g/llead ions: 60 g/ltemperature: ambientanodic current density: 1000 A/m.sup.2cathode: lead______________________________________

The results are reported in Table 5.2.

TABLE 5.2______________________________________RESULTS O.sub.2 Evolution PO FaradicSAMPLES Volts (NHE) EfficiencyNo. Initial 300 h % REMARKS______________________________________1 2.45 2.38 100 not corroded2 2.8 2.45 100 not corroded3 2.5 2.38 100 not corroded4 2.8 2.35 100 not corroded5 2.2 2.22 100 not corroded6 2.1 2.2 100 not corroded______________________________________

EXAMPLE 6

Seven anodes having a passivatable metal matrix and a coating based on tungsten and cobalt were prepared; further four anodes were also tested as shown herebelow. The anodes, in the form of sheets, 100.times.10.times.1 mm, of commercial pure titanium, were sandblasted and samples 1 to 3 were further subjected to chemical pickling in boiling 20% HCl. All the samples were then coated by different kinds of coatings and tested at the same conditions illustrated in Example 2. The description of the anodes and the results of the tests are reported in Tables 6.1 and 6.2.

TABLE 6.1______________________________________SAM-PLE Coating Thickness Load ApplicationNo. Composition % (micron) g/m.sup.2 Procedure______________________________________1 RuO.sub.2 TiO.sub.2 11.2 20 painting + thermal (50) (50) (Ru) decomposition2 IrO.sub.2 Ta.sub.2 O.sub.5 10.5 20 painting + thermal (50) (50) (Ir) decomposition3 Pt Sb 2 21.5 galvanic deposition (>98) (<2) (Pt)4 beta PbO.sub.2 800 // galvanic deposition5 W 155 // plasma jet6 Co 130 // plasma jet7 Co 140 // thermo spray8 W + Co 145 // plasma jet (97.5) (2.5)9 W + Co 135 // plasma jet (90) (10)10 W + Co 130 // plasma jet (80) (20)11 W + Co 130 // plasma jet (70) (30)______________________________________ TABLE 6.2______________________________________RESULTS O.sub.2 Evolution FaradicSAMPLE PO Volts (NHE) Efficien-No. Initial 300 h cy % REMARKS______________________________________1 1.75 // // corroded after 125 h2 1.80 // // corroded after 140 h3 1.76 1.67 80 corroded4 1.93 1.63 70 corroded5 >3.0 // // passivated6 1.9 1.59 60 Co leaching, corroded7 1.93 1.68 50 complete Co leaching, corroded8 2.09 2.35 100 slight Co leaching, incipient passivation9 2.05 2.08 100 slight Co leaching incipient passivation10 2.00 1.75 60 heavy Co leaching, corroded11 2.00 1.63 30 heavy Co leaching, corroded______________________________________

Conventional coatings on titanium, such as noble metal oxides (e.g. RuO.sub.2 and IrO.sub.2) stabilized by valve metals, noble metals ((e.g. Pt) and lead dioxide (beta PbO.sub.2) are mechanically (PbO.sub.2) and/or chemically (Pt, IrO.sub.2, RuO.sub.2) unstable also after a few dozens of hours with the consequent corrosion of the substrate areas remained uncoated. The coatings based on tungsten passivated after a few minutes. The coatings based on cobalt corroded after a few hours while coatings based on tungsten-cobalt with cobalt contents around 10% show neither corrosion nor passivation. Lower cobalt contents do not prevent the passivating action of tungsten from prevailing with time while with higher cobalt contents dissolution is observed which causes mechanical unstability of the remaining coating.

EXAMPLE 7

Fifteen sheets, 10.times.10.times.1 mm, of commercial pure titanium, after sandblasting with corindone (pressure: 7 aim: distance of spraying pistol from substrate: 30-35 cm; abrasive grain: irregular shape, sharp edged, average diameter about 300 microns) were coated by plasma jet or thermospray technique with tungsten and cobalt coatings containing nickel, palladium and copper as doping elements. The samples thus obtained were used as anodes in the electrolysis of lead fiuoroborate solutions at the same conditions as illustrated in Example 2. The characteristics of the anodes are reported in Table 7.1 and the relevant results in Table 7.2.

TABLE 7.1______________________________________ COATINGSAM- Thick-PLE Composition ness ApplicationNo. Matrix % microns Procedure______________________________________1 Ti W + Co 140 plasma spray (90) .sub. (10)2 Ti W + Co + Ni 145 plasma spray (89) .sub. (10.5) (0.5)3 Ti W + Co + Ni 145 plasma spray (89) .sub. (10) (1.0)4 Ti W + Co + Ni 135 plasma spray (89) .sub. (9.5) (1.5)5 Ti W + Co + Ni 130 plasma spray (88) .sub. (10) (2.0)6 Ti W + Co + Pd 100 plasma spray (89) .sub. (10.5) (0.5)7 Ti W + Co + Pd 110 plasma spray (89) .sub. (10) (1.0)8 Ti W + Co + Cu 105 plasma spray (88) .sub. (11.5) (0.5)9 Ti W + Co + Cu 115 plasma spray (89) .sub. (10.5) (0.5)10 Ti W + Co + Cu 125 plasma spray (89) .sub. (10) (1.0)11 Ti W + Co + Cu 125 plasma spray (90) .sub. (8.5) (1.5)12 Ti W + Co + Ni + Pd 130 plasma spray (89) .sub. (10) (0.5).sub. (0.5)13 Ti W + Co + Ni + Pd 130 thermo spray (89) .sub. (10) (0.5).sub. (0.5)14 Ti W + Co + Ni + Cu 145 plasma spray (89) .sub. (10) (0.5).sub. (0.5)15 Ti W + Co + Ni + Cu 120 thermo spray (89) .sub. (10) (0.5).sub. (0.5)16 Ti W + CeO.sub.2 100 plasma spray (97.5) (2.5)17 Ti W + CeO.sub.2 + Co 110 plasma spray (92.5) (2.5) (5)18 Ti W + CeO.sub.2 + Co 100 plasma spray (87.5) (2.5) (10)______________________________________ TABLE 7.2______________________________________RESULTS O.sub.2 Evolution FaradicSAMPLE PO Volts (NHE) Efficien-No. Initial 300 h cy % REMARKS______________________________________1 2.04 2.17 100 slight Co leaching2 2.05 2.19 100 slight Co leaching3 2.04 2.08 100 no corrosion4 2.04 2.09 100 no corrosion5 2.02 2.18 100 slight Co leaching6 2.09 2.06 100 no corrosion7 2.07 2.10 100 Pd traces in solution8 2.09 2.22 100 no corrosion9 2.07 2.18 100 no corrosion10 2.07 2.14 100 no corrosion11 2.06 2.23 75 Cu traces in solution12 2.06 2.09 100 no corrosion13 2.07 2.07 100 no corrosion14 2.05 2.10 100 no corrosion15 2.08 2.08 100 no corrosion16 // // 100 passivated17 2.11 2.15 100 no corrosion18 2.05 2.10 100 no corrosion______________________________________

The results permit to state that minimum quantitites of nickel, palladium, copper (1-1.5%) in a possible combination improve the chemical and electrochemical stability of the coatings. For each additive an optimum concentration has been determined in the range of 1-1.5% corresponding to the best performances. The presence of nickel, copper and palladium in the above concentrations avoids or, in any case, remarkably reduces the anodic leaching of cobalt. The combined presence of the above elements, for example Ni+Pd or Ni+Cu to an amount of 1-15% stabilizes the operating potential. This effect is particularly enhanced when the coating is applied by thermospray.

EXAMPLE 8

Seventeen sheets made of commercial titanium and titanium alloys (100.times.10.times.1 mm) were prepared according to the procedures described in Example 7 and coated by plasma or thermospray technologies with deposits based on W+Co, W+Co+Ni, W+Co+Ni+Pd, W+Co+Ni+Cu. The samples were tested as anodes in the electrolysis conditions described in Example 2 but with a double anodic current density (2000 A/m.sup.2). The characteristics of the samples are reported in Table 8.1 while the results are reported in Table 8.2.

TABLE 8.1______________________________________ COATINGSAM- Thick-PLE Composition ness ApplicationNo. Matrix % microns Procedure______________________________________1 Ti W + Co 120 plasma (90) .sub. (10)2 Ti W + Co + Ni 140 plasma (89) .sub. (10) (1)3 Ti W + Co + Ni + Pd 135 plasma (89) .sub. (10) (0.5).sub. (0.5)4 Ti W + Co + Ni + Cu 135 plasma (89) .sub. (10) (0.5).sub. (0.5)5 Ti W + Co + Ni + Pd 120 thermo-spray (89) .sub. (10) (0.5).sub. (0.5)6 TiPd W + Co 110 plasma (90) .sub. (10)7 TiPd W + Co + Ni 105 plasma (89) .sub. (10) (1)8 TiPd W + Co + Ni + Pd 110 plasma (89) .sub. (10) (0.5).sub. (0.5)9 TiPd W + Co + Cu + Pd 115 thermo-spray (89) .sub. (10) (0.5).sub. (0.5)10 TiNi W + Co 120 plasma (90) .sub. (10)11 TiNi W + Co + Ni 105 plasma (89) .sub. (10) (1)12 TiNi W + Co + Ni + Pd 115 plasma (89) .sub. (10) (0.5).sub. (0.5)13 TiNi W + Co + Ni + Pd 115 thermo-spray (89) .sub. (10) (0.5).sub. (0.5)14 Ti--Y W + Co 120 plasma (90) .sub. (10)15 Ti--Y W + Co + Ni 125 plasma (89) .sub. (10) (1)16 Ti--Y W + Co + Ni + Pd 130 plasma (89) .sub. (10) (0.5).sub. (0.5)17 Ti--Y W + Co + Ni + Pd 130 thermo-spray (89) .sub. (10) (0.5).sub. (0.5)______________________________________ TABLE 8.2______________________________________RESULTS O.sub.2 Evolution FaradicSAMPLE PO Volts (NHE) Efficien-No. Initial 300 h cy % REMARKS______________________________________1 2.14 1.93 100 Co leaching, corroded2 2.18 2.05 100 slight Co leaching, corroded3 2.19 2.05 100 corroded4 2.19 2.08 100 corroded5 2.15 2.10 100 corroded6 2.18 2.13 100 not corroded7 2.18 2.13 100 not corroded8 2.18 2.13 100 not corroded9 2.18 2.15 100 not corroded10 2.19 2.15 100 not corroded11 2.17 2.15 100 not corroded12 2.17 2.15 100 not corroded13 2.17 2.16 100 not corroded14 2.20 2.01 100 Co leaching, corroded15 2.22 2.02 100 Co leaching, corroded16 2.18 2.08 100 Co leaching, corroded17 2.19 2.01 100 Co leaching, corroded______________________________________

The results obtained at 2000 A/m.sup.2 led to the following considerations:

titanium structures, accidentally contacting the electrolyte due to chemical or mechanical removal of the coating, undergo a remarkable corrosion; this negative behavior is less important with ternary or quaternary deposits, for these latter especially when obtained by thermo-spray, being more compact;

titanium-yttrium (Y 0.35% ) samples show a similar behavior compared with samples of commercial titanium, with the same coating;

titanium-palladium (Pd 0.20%) and titanium nickel (Ni 1.5%) samples show a higher stability. Corrosion is lower as it can be seen from the anodic potential values which are stable with time: in fact an increasing potential is a symptom of passivation of the coating, while a decreasing potential shows of the corrosion of the substrate.

EXAMPLE 9

Five sheets (100.times.10.times.1 mm) made of titanium, tantalum, niobium, tungsten and of a nickel (90%)-copper (10%) alloy, after a surface treatment as described in Example 7, have been coated by a coating of W (89)+Co(10)+Ni(0.5)+Pd(0.5) applied by plasma jet. The samples have been tested as anodes in the eleetrolysis of lead fluoroborates solutions at the same conditions as illustrated in Example 8. The results are reported in Table 9. The cathodic deposition efficiency of lead was 100%.

TABLE 9______________________________________ RESULTS Coat- ing O.sub.2 EvolutionSAM- Matrix Thick- PO VoltsPLES Composition ness (NHE)No. % micron Initial 500 h REMARKS______________________________________1 Ti 140 2.20 2.06 corroded2 Ta 135 2.17 2.17 no corrosion3 Nb 145 2.21 2.17 slightly corroded4 W 125 2.18 2.18 no corrosion5 Ni(90) + Cu(10) 130 2.20 2.20 no corrosion______________________________________

The results lead to the following considerations:

when the substrate is made of tantalum, tungsten or Ni(90)- Cu(10) alloy, a good stability and constant anodic potentials of the coatings applied to the same are experienced;

the substrate made of titanium is unstable and the anodic potential of the coating rapidly decreases with time;

an intermediate situation is experienced with the substrate made of niobium with anodic potentials slightly decreasing with time.

Claims

1. In a process for the electrocatalytic recovery of metals from aqueous solutions containing metal ions and fluoride ions or anionic fluorocomplexes, wherein the improvement comprises using as the anode

a) a passivatable metal matrix comprising a nickel-copper alloy containing 5 to 20% by weight of copper, b) an electrocatalytic compound for oxygen evolution and c) at least one other additive.

2. The process of claim 1 wherein the metal ions are lead.

3. The process of claim 1 wherein said electrocatalytic compound is present in said metal matrix as an alloy or as a dispersion.

4. The process of claim 1 wherein the electrocatalytic compound is in the form of a coating on said metal matrix.

5. The process of claim 1 wherein the electrocatalytic compound is selected from the group consisting of cobalt, cerium dioxide and tin oxide.

6. The process of claim 5 wherein the electrocatalytic compound comprises cobalt and the additive is at least one element selected from the group consisting of nickel, copper, iron, palladium and cerium.

7. The process of claim 5 wherein the electrocatalytic compound is cerium dioxide and the additive is at least one member selected from the group consisting of niobium oxide, nickel oxide, praseodymium oxide and copper oxide.

8. The process of claim 5 wherein the electrocatalytic compound is tin oxide and the additive is at least one oxide selected from the group consisting of antimony oxide and copper oxide.