Patent Publication Number: US-6210550-B1

Title: Anode with improved coating for oxygen evolution in electrolytes containing manganese

Description:
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 &gt;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 (&lt;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&lt;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.&lt;65° C.) and current density (e.g. &lt;5000 ANm 2 ). Under higher current densities (e.g. &gt;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; 
     high electrical resistivity of the deposited MnO 2  layer. 
     It has been proposed to modify the coating based on iridium and tantalum oxides by the addition of ruthenium oxide, to decrease the potential for oxygen evolution to values below those of the parasitic reaction, and of titanium oxide in order to achieve the structural stabilisation of ruthenium. 
     The following compositions have been suggested: Ta—Ir—Ru, 20-75-5% by weight respectively and Ta—Ir—Ru—Ti, 17,5-32,5-32,5-17,5% by weight respectively. 
     The above described anodes, provided with the protective interlayer and the electrocatalytic coating containing ruthenium and titanium, have found only experimental and not yet satisfactory applications so far. These applications are summarised in table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Classification of industrial processes using experimental 
               
               
                 activated titanium anodes 
               
            
           
           
               
               
            
               
                   
                 ACTIVATED TITANIUM 
               
               
                   
                 ANODE DESCRIPTION 
               
            
           
           
               
               
               
            
               
                 PROCESS 
                   
                 Surface 
               
            
           
           
               
               
               
               
            
               
                 Definition 
                 Operating conditions 
                 Interlayer 
                 coating 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Electrolytic 
                 Temperature 
                 ≅45° C. 
                 Pt Ir 
                 TaIrOx 
               
               
                 production 
                 Anodic current 
                 150-200 
                 or 
                 or 
               
               
                 of copper 
                 density 
                   
                 TiTaOx 
                 TaTiIrRuOx 
               
               
                 (primary) 
                 A/m 2   
               
               
                   
                 Cu 
                 40-55 g/l 
               
               
                   
                 H 2 SO 4   
                 150-200 g/l 
               
               
                   
                 Mn 
                 30-300 
               
               
                   
                 ppm 
               
               
                   
                 F 
                 100-200 
               
               
                   
                 ppm 
               
               
                 Copper 
                 Temperature 
                 30-34° C. 
                 Ti − TaOx + 
                 TaIrOx 
               
               
                 refining 
                 Anodic current 
                 150-200 
                 IrOx 
                 or 
               
               
                 (secondary 
                 density 
                   
                 or 
               
               
                 copper 
                 A/m 2   
                   
                 Pt − Ir 
               
               
                 exhaustion 
                 H 2 SO 4   
                 10-50 g/l 
               
               
                 cells) 
                   
                 ≅170 g/l 
               
               
                 Chromium 
                 Temperature 
                 55-65° C. 
                 TiTaOx + 
                 TaIrOx 
               
               
                 deposition 
                 Anodic current 
                 2500-6000 
                 IrOx 
               
               
                 from 
                 density 
               
               
                 sulphate + 
                 A/m 2   
               
               
                 fluoride 
                 CrO 3   
                 250-300 g/l 
               
               
                   
                 H 2 SO 4   
                 1,0-1,5 g/l 
               
               
                   
                 H 2 SiF 6   
                 1,0-1,5 g/l 
               
               
                 Chromium 
                 Temperature 
                 55-65° C. 
                 TiTaOx + 
                 TaIrOx 
               
               
                 deposition 
                 Anodic current 
                 2500-6000 
                 IrOx 
               
               
                 from 
                 density 
               
               
                 sulphate + 
                 A/m 2   
               
               
                 organics 
                 CrO 3   
                 250-300 g/l 
               
               
                   
                 H 2 SO 4   
                 1,5-2,5 g/l 
               
               
                   
                 C 2 H 5 SO 3 H 
                 100-1000 
               
               
                   
                 ppm 
               
               
                   
               
            
           
         
       
     
     The present invention is directed to overcoming the drawbacks still affecting the experimental anodes previously described which mainly consist in the deposition of manganese dioxide and/or the corrosion of the titanium substrate, even if remarkably delayed in time. 
     In particular, the present invention is directed to an anode for oxygen evolution in electrochemical processes carried out with electrolytes containing sulphuric acid or sulphate, metals to be deposited at the cathode, high quantities of manganese and, in some cases, limited concentrations of fluorides (&lt;5 ppm). The anode of the invention comprises a titanium substrate provided with an electrocatalytic and selective layer for oxygen evolution and is unaffected by the parasitic reaction of electrochemical precipitation of non-conductive manganese dioxide. The main components of the electrocatalytic layer are iridium oxide, which acts as electrical conductor and catalyst for oxygen evolution, and bismuth oxide, electrically non-conductive and directed to stabilise iridium. The coating may comprise doping agents selected from the groups IVA (e.g. Sn), VA (e.g. Sb), VB (e.g. Nb and Ta), as promoters of both the electronic conductivity and compactness of the coating. In a different embodiment of the invention, the anode may comprise one or more protective interlayers applied between the titanium substrate and the coating. The interlayer, the components of which are selected in the groups IV B (e.g. Ti), V B (e.g. Ta), VIII2 (e.g. Ir), VIII3 (e.g. Pt), acts as a protective barrier for the titanium substrate against corrosion. 
     The anode exhibits the following operating characteristics: 
     anodic potentials for oxygen evolution close to the reversible value also under high current density (e.g. 1.65 V (NHE) at 3000 A/m 2 ); 
     high overvoltage for the deposition of MnO 2 ; this reaction is practically inhibited also with high concentrations of manganese (e.g. Mn&gt;5 g/l) and temperatures up to 60° C.; 
     chemical and mechanical stability of the coating under operating conditions; 
     Faradic efficiencies of the cathodic process of metal deposition higher than those of the prior art anodes (lead anodes and anodes of titanium provided with a coating made of iridium and tantalum oxides). 
     The invention will be now described making reference to some examples. which are not intended to be a limitation thereof. The samples were made of titanium grade 2 with dimensions of 10 mm×50 mm×2 mm, subjected to mechanical sandblasting with corindone (grain dimensions 0.25-0.35 mm average), at a pressure of 5-7 atm, with a distance between the sample and the nozzle of 20-30 cm. The paint comprised hydro-soluble chlorides as precursor salts. In particular, the following salts or solutions have been used, suitably mixed as explained hereinafter: 
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 H 2 Ir Cl 6   
                 20-23% solution as Ir 
               
               
                 TaCl 5   
                 hydrochloric solution 50 g/l as Ta 
               
               
                 BiCl 3   
                 salt or slightly hydrochloric solution at 50 g/l as Bi 
               
               
                 SnCl 2 2H 2 O 
                 salt or hydrochloric solution at 10 g/l as Sn 
               
               
                 SbCl 3   
                 salt or hydrochloric solution 10 g/l as Sb 
               
               
                 NbCl 5   
                 salt or hydrochloric solution 10 g/l as Nb 
               
               
                   
               
            
           
         
       
     
     The following painting procedure was used: 
     application of the aqueous solution containing the precursor salts of the various components in the defined ratio, by brushing or equivalent technique (e.g. rolling, electrostatic spraying); 
     drying at 105° C., thermal decomposition for 15 minutes at 490° C. in oven under forced air ventilation; 
     repeating of the painting and thermal cycle until the pre-defined amount of noble metal in the final coating is obtained; 
     annealing at 510° C. 
     The samples thus obtained have been subjected to electrolysis as anodes in the solutions reported in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Anodic Electrochemical Characterisation 
               
            
           
           
               
               
               
            
               
                   
                 Type of solution 
                   
               
               
                   
                 and operating 
                 Relevant Industrial Applications 
               
            
           
           
               
               
               
               
            
               
                 Reference 
                 conditions of the test 
                   
                 Industrial operating 
               
            
           
           
               
               
               
               
               
            
               
                 process 
                 Code 
                 Description 
                 Specific process 
                 conditions 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Electrolysis of 
                 A 
                 H 2 SO 4   
                 170 g/l 
                 electrolytic 
                 pH 
                 1.2-1.8 
               
               
                 sulphuric 
                   
                 Mn 
                 4 g/l 
                 production of 
                 Co 
                 50-80 g/l 
               
               
                 solutions 
                   
                 temp. 
                 40° C. 
                 cobalt 
                 Mn 
                 15 g/l 
               
               
                 containing 
                   
                 current 
                 500 A/m 2   
                   
                 temp. 
                 60° C. 
               
               
                 manganese 
                   
                 density 
                   
                   
                 current 
                 200 A/m 2   
               
               
                   
                   
                   
                   
                   
                 density 
               
               
                   
                   
                   
                   
                 electrolytic 
                 H 2 SO 4   
                 180 g/l 
               
               
                   
                   
                   
                   
                 production of 
                 Cu 
                 ≅50 g/l 
               
               
                   
                   
                   
                   
                 copper (primary 
                 Mn 
                 &lt;300 ppm 
               
               
                   
                   
                   
                   
                 copper) 
                 temp. 
                 ≅50° C. 
               
               
                   
                   
                   
                   
                   
                 current 
                 ≅200 A/m 2   
               
               
                   
                   
                   
                   
                   
                 density 
               
               
                   
                   
                   
                   
                 electrolytic 
                 H 2 SO 4   
                 180 g/l 
               
               
                   
                   
                   
                   
                 production of 
                 Zn 
                 70 g/l 
               
               
                   
                   
                   
                   
                 zinc (&lt;90% of 
                 Mn 
                 4 g/l 
               
               
                   
                   
                   
                   
                 the world-wide 
                 temp. 
                 &lt;40° C. 
               
               
                   
                   
                   
                   
                 electrolytic 
                 current 
                 500 A/m 2   
               
               
                   
                   
                   
                   
                 production) 
                 density 
               
               
                 Electrolysis of 
                 B 
                 as above + 
               
               
                 sulphuric solutions 
                   
                 ZnSO 4   
                 (Zn 70 g/l) 
               
               
                 containing 
                   
                 Fluorides 
                 &lt;5 ppm 
               
               
                 manganese 
               
               
                   
               
            
           
         
       
     
    
    
     EXAMPLE 1 
     8 samples of titanium, pre-treated as described above, have been activated by different coatings selected among the most representative of the prior art, according to the above described procedure. 
     The final compositions of the prepared samples and the corresponding code numbers are specified in table 1.1. The percentages are expressed by weight and refer to the components in the elemental state. 
     
       
         
           
               
             
               
                 TABLE 1.1 
               
             
            
               
                   
               
               
                 Description of the reference samples 
               
            
           
           
               
               
               
            
               
                 Code 
                 Protective interlayer 
                 Electrocatalytic coating 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 (° ) 
                 Ti 
                 Ta 
                 Ir 
                 Ir 
                 Ta 
                 Ir 
                 Ti 
                 Ru 
                 Ir + Ru 
               
            
           
           
               
               
               
               
               
            
               
                 No. 
                 % molar 
                 g/m 2   
                 % by weight 
                 g/m 2   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 5.1.1 
                 80 
                 20 
                 // 
                 // 
                 35 
                 65 
                 // 
                 // 
                 10 
               
               
                 5.1.2 
                 80 
                 20 
                 // 
                 // 
                 17,5 
                 32,5 
                 17,5 
                 32,5 
                 10 
               
               
                 5.1.3 
                 75 
                 20 
                 5 
                 1 
                 35 
                 65 
                 // 
                 // 
                 10 
               
               
                 5.1.4 
                 75 
                 20 
                 5 
                 1 
                 17,5 
                 32,5 
                 17,5 
                 32,5 
                 10 
               
               
                   
               
               
                 (° ) Each code number corresponds to two samples having the same formulation.  
               
            
           
         
       
     
     EXAMPLE 2 
     This example concerns anodic materials of titanium activated with the coating of the invention based on bismuth and iridium oxides with and without doping agents. 
     8 samples of titanium, pretreated as described above, have been activated with different coatings whose code numbers and final compositions expressed in percentages by weight with respect to the components in the elemental state are reported in table 2.1. 
     
       
         
           
               
             
               
                 TABLE 2.1 
               
             
            
               
                   
               
               
                 Description of the samples of the invention 
               
            
           
           
               
               
            
               
                   
                 Coating components 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 Code 
                 Ir 
                 Bi 
                 Sn 
                 Sb 
                 Ta 
                 Nb 
               
               
                   
                 N. 
                 % 
                 % 
                 % 
                 % 
                 % 
                 % 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 5.2.1 
                 65 
                 — 
                 35 
                   
                   
                   
                   
               
               
                   
                 5 2.2 
                 65 
                   
                 30 
                   
                   
                 5 
               
               
                   
                 5.2.3 
                 65 
                   
                 17,5 
                   
                   
                 17,5 
               
               
                   
                 5.2.4 
                 65 
                 — 
                 30 
                 5 
               
               
                   
                 5.2.5 
                 65 
                   
                 25 
                 10 
               
               
                   
                 5.2.6 
                 65 
                   
                 25 
                 5 
                 5 
               
               
                   
                 5.2.7 
                 65 
                   
                 30 
                   
                 5 
               
               
                   
                 5.2.8 
                 65 
                 — 
                 30 
                   
                   
                   
                 5 
               
               
                   
                   
               
            
           
         
       
     
     For all the samples the iridium content was 10 g/m 2 . The samples were tested as anodes in sulphuric electrolyte containing manganese, as an impurity, under the operating conditions described in table 4 for the electrolyte code A. The anodic potential with time and visual observations of the morphological state of the coatings at the end of the test are reported in table 2 and compared with the data obtained with the prior art samples prepared by procedure described in example 1. 
     
       
         
           
               
             
               
                 TABLE 2.2 
               
             
            
               
                   
               
               
                 Electrochemical behaviour of the tested samples 
               
               
                 (Electrolyte code: A) 
               
            
           
           
               
               
               
            
               
                 Code 
                 Anodic Potential: V (NHE) 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 N. 
                 Initial 
                 1000 h 
                 2000 h 
                 3000 h 
                 FINAL MORPHOLOGICAL STATE 
               
               
                   
               
            
           
           
               
               
               
            
               
                   
                 ANODES OF THE INVENTION 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 5.2.1 
                 1,68 
                 1,72 
                 1,75 
                 1,77 
                 MnO 2  deposit in a highly 
               
               
                   
                   
                   
                   
                   
                 distributed form, undetermined 
               
               
                 5.2.2 
                 1,68 
                 1,72 
                 1,83 
                 1,94 
                 Thin and porous MnO 2  deposit 
               
               
                 5.2.3 
                 1,68 
                 1,78 
                 1,87 
                 1,95 
                 Thin and porous MnO 2  deposit 
               
               
                 5.2.4 
                 1,68 
                 1,75 
                 1,77 
                 1,85 
                 Extremely thin MnO 2  deposit 
               
               
                 5.2.5 
                 1,67 
                 1,78 
                 1,87 
                 1,92 
                 MnO 2  deposit unevenly distributed 
               
               
                   
                   
                   
                   
                   
                 (zones) 
               
               
                 5.2.6 
                 1,68 
                 1,75 
                 1,78 
                 1,95 
                 Thin and porous MnO 2  deposit 
               
               
                 5.2.7 
                 1,68 
                 1,79 
                 1,80 
                 1,94 
                 MnO 2  deposit unevenly distributed 
               
               
                   
                   
                   
                   
                   
                 (zones) 
               
               
                 5.2.8 
                 1,68 
                 1,74 
                 1,85 
                 1,97 
                 Thin and porous MnO 2  deposit 
               
            
           
           
               
               
               
            
               
                   
                 PRIOR ART ANODES 
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 5.1.1 
                 1,62 
                 1,98 
                 2,08 
                 2,15 
                 Compact MnO 2  deposit 
               
               
                 5.1.2 
                 1,65 
                 1,76 
                 2,00 
                 2,05 
                 MnO 2  deposit in scales 
               
               
                 5.1.3 
                 1,64 
                 2,00 
                 2,07 
                 2,12 
                 Compact MnO 2  deposit 
               
               
                 5.1.4 
                 1,65 
                 1,76 
                 1,97 
                 2,06 
                 MnO 2  deposit in scales 
               
               
                   
               
            
           
         
       
     
     The experimental results of Table 2.2 show that: 
     all the prior art samples are passivated when manganese is present in the solution: in particular, passivation is quick for the coatings without ruthenium (a few hundred hours); passivation is less quick but nevertheless significant and irreversible for the coatings containing ruthenium (a thousand hours as a maximum). 
     None of the samples of the invention exhibits any passivation after more than 3000 hours of operation in solutions containing manganese. In particular, coatings containing tantalum or niobium are covered with a thin and porous, mechanically inconsistent layer, which is removed under operation. The coatings without tantalum or niobium did not give rise to macroscopic precipitates of MnO 2  for the whole electrolysis period. 
     EXAMPLE 3 
     This example concerns the use of anodes, provided with a protective interlayer and an electrocatalytic coating used in industrial sulphuric electrolytes for the production of zinc containing fluorides and manganese. N. 16 samples of titanium pre-treated as described above have been activated with different coatings based on bismuth, iridium with and without doping agents. In particular, a first series of samples identified by code no. 5.3 was without the interlayer; a second series of samples identified by code no. X 5.3 comprised a protective interlayer made of noble metals only in the elemental state; a third series of samples, identified by code no. Y 5.3 comprised a protective interlayer made of valve metal oxides containing small quantities of noble metals. The code numbers and the final compositions of the coatings, expressed as percentages by weight relative to all the components in the elemental state are reported in table 3.1. For all the samples the iridium loading was 10 g/m 2 . 
     
       
         
           
               
             
               
                 TABLE 3.1 
               
             
            
               
                   
               
               
                 Description of the coatings and relevant codes 
               
            
           
           
               
               
            
               
                   
                 Components of the coatings 
               
               
                   
                 Protective Interlayer 
               
               
                   
                 Electrocatalytic Coating 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Code 
                 Ti 
                 Ta 
                 Ir 
                 Pt 
                 Bi 
                 Sn 
                 Sb 
                 Ta 
                 Nb 
                 Ir 
               
               
                 No. 
                 % 
                 % 
                 % 
                 % 
                 % 
                 % 
                 % 
                 % 
                 % 
                 % 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 5.3.1 
                   
                   
                   
                   
                 35 
                 // 
                 // 
                 // 
                 // 
                 65 
               
               
                 5.3.2 
                   
                   
                   
                   
                 30 
                   
                   
                 5 
                   
                 65 
               
               
                 5.3.3 
                   
                   
                   
                   
                 17,5 
                   
                   
                 17,5 
                   
                 65 
               
               
                 5.3.4 
                   
                   
                   
                   
                 30 
                   
                   
                   
                 5 
                 65 
               
               
                 5.3.5 
                   
                   
                   
                   
                 30 
                 5 
                   
                   
                   
                 65 
               
               
                 5.3.6 
                   
                   
                   
                   
                 25 
                 10 
                   
                   
                   
                 65 
               
               
                 5.3.7 
                   
                   
                   
                   
                 30 
                   
                 5 
                   
                   
                 65 
               
               
                 5.3.8 
                   
                   
                   
                   
                 25 
                 5 
                 5 
                   
                   
                 65 
               
               
                 X5.3.1 
                   
                   
                 30 
                 70 
                 35 
                   
                   
                   
                   
                 65 
               
               
                 X5.3.2 
                   
                   
                 30 
                 70 
                 30 
                   
                   
                 5 
                   
                 65 
               
               
                 X5.3.5 
                   
                   
                 30 
                 70 
                 30 
                 5 
                   
                   
                   
                 65 
               
               
                 X5.3.8 
                   
                   
                 30 
                 70 
                 25 
                 5 
                 5 
                   
                   
                 65 
               
               
                 Y5.3.1 
                 75 
                 20 
                 5 
                   
                 35 
                   
                   
                   
                   
                 65 
               
               
                 Y5.3.2 
                 75 
                 20 
                 5 
                   
                 30 
                   
                   
                 5 
                   
                 65 
               
               
                 Y5.3.5 
                 75 
                 20 
                 5 
                   
                 30 
                 5 
                   
                   
                   
                 65 
               
               
                 Y5.3.8 
                 75 
                 20 
                 5 
                   
                 25 
                 5 
                 5 
                   
                   
                 65 
               
               
                   
               
            
           
         
       
     
     The samples have been tested as anodes in an electrolyte for the production of zinc, under the electrolytic and operating conditions of Table 4, electrolyte code C. The test comprised the use of transparent plastic lab cells, each one comprising: 
     an anode as previously described; 
     a counter-electrode with dimensions of 10 mm×50 mm×2 mm; 
     a dosing pump for the circulation of the solution; 
     The electrolyte was partially renewed every 24 hours. 
     The results obtained with the anodes of the invention, that is anodic potential with time, zinc yield (determined by removal of cathode every 48 hours and relevant weighing) and visual observations of the morphological state of the coating at the end of the test are reported in table 3.2. 
     These data are compared with the data obtained with the prior art anodes, prepared according to the procedure described in Example 1. 
     
       
         
           
               
             
               
                 TABLE 3.2 
               
             
            
               
                   
               
               
                 Electrochemical Behaviour 
               
               
                 (Electrolyte code: B) 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Zinc deposition 
                   
               
               
                   
                   
                 faradic Yield 
               
               
                 Code 
                 Anodic Potential: V (NHE) 
                 (average 
                 Final morphological 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 No. 
                 Initial 
                 1000 h 
                 2000 h 
                 3000 h 
                 values) % 
                 observations 
               
               
                   
               
               
                 5.3.1 
                 1,67 
                 1,72 
                 1,83 
                 1,87 
                 90-92 
                 MnO 2  deposit, 
               
               
                   
                   
                   
                   
                   
                   
                 undetermined 
               
               
                 5.3.2 
                 1,67 
                 1,73 
                 1,85 
                 1,87 
                 90-92 
                 MnO 2  deposit, 
               
               
                   
                   
                   
                   
                   
                   
                 undetermined 
               
               
                 5.3.3 
                 1,68 
                 1,73 
                 1,84 
                 1,88 
                 90-92 
                 MnO 2  deposit, 
               
               
                   
                   
                   
                   
                   
                   
                 undetermined 
               
               
                 5.3.4 
                 1,68 
                 1,73 
                 1,86 
                 1,88 
                 90-92 
                 MnO 2  deposit, 
               
               
                   
                   
                   
                   
                   
                   
                 undetermined 
               
               
                 5.3.5 
                 1,68 
                 1,73 
                 1,85 
                 1,88 
                 90-92 
                 MnO 2  deposit, 
               
               
                   
                   
                   
                   
                   
                   
                 undetermined 
               
               
                 5.3.6 
                 1,68 
                 1,73 
                 1,86 
                 1,9  
                 90-92 
                 Thin and unevenly 
               
               
                   
                   
                   
                   
                   
                   
                 distributed MnO 2   
               
               
                   
                   
                   
                   
                   
                   
                 deposit (in zones) 
               
               
                 5.3.7 
                 1,69 
                 1,73 
                 1,87 
                 1,9  
                 80-83 
                 Thin and unevenly 
               
               
                   
                   
                   
                   
                   
                   
                 distributed MnO 2   
               
               
                   
                   
                   
                   
                   
                   
                 deposit (in zones) 
               
               
                 5.3.8 
                 1,68 
                 1,75 
                 1,87 
                 1,9  
                 80-82 
                 Thin and unevenly 
               
               
                   
                   
                   
                   
                   
                   
                 distributed MnO 2   
               
               
                   
                   
                   
                   
                   
                   
                 deposit (in zones) 
               
               
                 X5.3.1 
                 1,68 
                 1,76 
                 1,81 
                 1,87 
                 90-92 
                 MnO 2  deposit, 
               
               
                   
                   
                   
                   
                   
                   
                 undetermined 
               
               
                 X5.3.2 
                 1,68 
                 1,80 
                 1,81 
                 1,87 
                 90-92 
                 MnO 2  deposit, 
               
               
                   
                   
                   
                   
                   
                   
                 undetermined 
               
               
                 X5.3.5 
                 1,68 
                 1,8  
                 1,81 
                 1,9  
                 90-92 
                 Thin and unevenly 
               
               
                   
                   
                   
                   
                   
                   
                 distributed MnO 2   
               
               
                   
                   
                   
                   
                   
                   
                 deposit (in zones) 
               
               
                 X5.3.8 
                 1,68 
                 1,81 
                 1,87 
                 1,9  
                 90-92 
                 Thin and unevenly 
               
               
                   
                   
                   
                   
                   
                   
                 distributed MnO 2   
               
               
                   
                   
                   
                   
                   
                   
                 deposit (in zones) 
               
               
                 Y5.3.1 
                 1,68 
                 1,77 
                 1,81 
                 1,87 
                 90-92 
                 MnO 2  deposit, 
               
               
                   
                   
                   
                   
                   
                   
                 undetermined 
               
               
                 Y5.3.2 
                 1,68 
                 1,78 
                 1,81 
                 1,99 
                 90-92 
                 Thin and unevenly 
               
               
                   
                   
                   
                   
                   
                   
                 distributed MnO 2   
               
               
                   
                   
                   
                   
                   
                   
                 deposit (in zones) 
               
               
                 Y5.3.5 
                 1,68 
                 1,78 
                 1,88 
                 1,94 
                 80-82 
                 Thin and unevenly 
               
               
                   
                   
                   
                   
                   
                   
                 distributed MnO 2   
               
               
                   
                   
                   
                   
                   
                   
                 deposit (in zones) 
               
               
                 Y5.3.8 
                 1,68 
                 1,77 
                 1,82 
                 1,84 
                 81-83 
                 Thin and unevenly 
               
               
                   
                   
                   
                   
                   
                   
                 distributed MnO 2   
               
               
                   
                   
                   
                   
                   
                   
                 deposit (in zones) 
               
               
                 5.1.1 
                 1,65 
                 2,05 
                   
                   
                 −90 
                 Thick and compact 
               
               
                   
                   
                   
                   
                   
                   
                 MnO 2  deposit 
               
               
                 5.1.2 
                 1,65 
                 1,73 
                 1,84 
                   
                 −82 
                 Thick and compact 
               
               
                   
                   
                   
                   
                   
                   
                 MnO 2  deposit 
               
               
                 5.1.3 
                 1,65 
                 2,0  
                   
                   
                  90 
                 Thick and compact 
               
               
                   
                   
                   
                   
                   
                   
                 MnO 2  deposit 
               
               
                 5.1.4 
                 1,64 
                 1,74 
                 1,87 
                   
                  79 
                 Thick and compact 
               
               
                   
                   
                   
                   
                   
                   
                 MnO 2  deposit 
               
               
                   
               
            
           
         
       
     
     The results reported in Table 3.2 permit to state that: 
     All prior art anodes passivated in sulphuric solutions containing at the same time fluorides, manganese and precursor salts of zinc. The average faradic yield of zinc deposition with the prior art anodes is lower than 90% as an average. 
     The samples of the invention do not exhibit any passivation phenomena after 3000 hours of electrolysis in industrial solutions containing at the same time fluorides, manganese and zinc precursor salt. The faradic yield in the average is higher than 90%.