Patent Publication Number: US-6210558-B1

Title: Steel pickling process in which the oxidation of the ferrous ion formed is carried out electrolytically

Description:
This application is a 371 of PCT/EP97/02346 filed May 7, 1997. 
    
    
     FIELD OF THE INVENTION 
     Object of the present invention is the achievement of a steel pickling process and particularly of a stainless steel one, carried out in a bath containing as essential components HF and ferric ions, in which the oxidation of Fe 2+  formed during the pickling process to Fe 3+  necessary in order to maintain the Redox potential of the solution at the predetermined value, is carried out by an electrolytic oxidation method acting directly on the pickling solution exactly as it is, preferably in a continuous way. 
     The electrolytic oxidation method according to the present invention can be advantageously applied to all the known pickling processes, the electrolytic oxidation of Fe 2+  to Fe 3+  in the pickling solution can replace the traditional oxidation methods of Fe 2+  to Fe 3+  by chemical oxidizers such as for instance H 2 O 2 , peracids, persalts, chlorates, oxygen (air), HNO 3 . 
     TECHNOLOGICAL BACKGROUND 
     The electrolytic oxidation of the Fe 2+  ions in an exhausted pickling solution in order to restore the necessary concentration of Fe 3+  ions has been already disclosed by U.S. Pat. No. 3,622,478 for the treatment of a pickling solution based on H 2 SO 4  and Fe 3+  ions introduced in starting solution as Ferric sulphate. The treatment is carried out in an electrolytic cell without separation between the catholyte and the anolyte (cell without separating diaphragm). 
     In FR 2.341.669 it is disclosed the electrolytic oxidation of Fe 2+  ions in an exhausted pickling solution based on HCl and Fe chlorides, in order to restore the necessary concentration of Fe 3+  ions. The treatment is carried out in a cell provided by separating diaphragm. 
     The above mentioned process for electrolytic oxidation of Fe 2+  ions are relevant to pickling solutions based on H 2 SO 4  or HCl and not containing HF or F −  anions. 
     The presence of HF and F −  anions in the pickling solution involves the formation of fluorinated complexes of trivalent iron and consequently the properties and behaviour of the solution is not equivalent to those of pickling solutions not containing HF (or F −  anions). 
     Consequently the above prior art electrolytic oxidation methods cannot be considered obviously applicable to HF containing pickling solutions. 
     SUMMARY OF THE INVENTION 
     According to the present invention the conventional methods of chemical oxidation can be advantageously substituted, in order to restore the preestablished value of the ferric ions concentration defined by the sort of the pickling process and of the material to be treated by a method for the electrolytic oxidation of the pickling solution carried out batch-wise or continuously according to the requirements of the plant. 
     The solution to be treated can be cooled before entering in the electrolytic cell or can be treated at the same temperature of the pickling process. The electrolytic oxidation according to the present invention is carried out with two electrodes acting respectively as a cathode and as an anode in contact with the pickling solution to be oxidized, to which a continuous tension having a sufficient value for the oxidation of Fe 2+  to Fe 3+  to the anode and for the reduction of H +  to gaseous H 2  to the cathode is applied. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     In the diagrams of the FIGS. 1-4 it is reported the Fe 2+  content decrease with time, during the electrolytical oxidation process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The process can be carried out in a proper electrolytic cell in which the solution coming from the pickling bath is continuously or discontinuously sent, said electrolytic cell being preferably equipped with a diaphragm to separate the cathodic area from the rest of the electrolyte. From the electrolytic oxidation tests of the pickling solution carried out the result showed that it is also possible to carry out the operation in a “single build” electrolytic cell that is without separating diaphragm of the cathodic area from the anodic one provided that the current density on the cathodic surface is very high, in the range of 400±50 A/dm 2  that is up to 100 times the anodic on the anodic one current density which is kept at values in the range from 0.4 to 15 A/dm 2  and mostly of the order of 4±0.5 A/dm. 
     As far as the tension applied to the electrodes is concerned, this one is related among other things to the intensity of the current flow that one wants to keep in the electrolytic cell. It is generally comprised between 1 and 8 V preferably between 1 and 5 V and more preferably between 1 and 3V. 
     The schematic representation of the electrolytic reoxidation of the pickling solution is shown in the following for the case of “single build” cell and of the “double build” cell (namely cell having diaphragm):                    
     The double build cell affords the highest conversion allowing an easier control of the possible parasitic reactions such as the reduction of Fe 3+  to Fe 2+ . 
     In the double build cell the catholyte can be the same pickling solution provided that the volume of the catholyte is very limited in order to reduce at the most the amount of Fe 3+  therein contained (which is reduced to Fe 2+ ). 
     Of course it is preferred to utilize as a catholyte any aqueous solution, preferably acid, which does not contain metallic ions, in particular Fe 2+ , which can be reduced at the cathode. 
     The catholyte solution can be restored in situ owing to the spontaneous inflow of hydrogen cations from the outside solution through the diaphragm, or can occurs through addition of acid solution from outside by pump at controlled flow rate. A proper example is an aqueous solution of H 2 SO 4  owing to the low cost, the high electrolytic conductivity and limited corrosion effect on the building materials of the cell. 
     If the cathode is made of proper material the catholyte can be an aqueous solution of HF. 
     An embodiment economically advantageous consists in feeding in the cathodic compartment the exhausted pickling solution no more suitable for recycling in pickling bath which should be definitively discharged. Such a solution contains yet enough acidic components and acts well as catholyte having a good electrical conductivity. 
     In an industrial embodiment of the present invention double build cells in series arrangement can advantageously be of the type “bipolar electrode” wherein a face of the electrode acts as cathode in a cell and the opposite face of the same electrode acts as anode in the adjacent cell. 
     As far as the electrode material is concerned, platinum is certainly suitable thanks to its inalterability in the solution to be treated but it is obviously to be excluded in production plants for financial reasons. 
     As anode, any carbonaceous material or metallic material, possibly pretreated on the surface, can be used. In particular anodes made of graphite, glassy carbon, carbon felts and also metals for example lead after an activation surface treatment, come out to afford satisfactory results. Graphite can be also used as support for anodic materials consisting of particulate of graphite or of carbon felt. 
     The anode can be bidimensional in form of bar, plate and any other commercial form, or tridimensional in form of fixed or fluidized bed: particularly good results have been obtained with tridimensional anodes made of carbon felt, or made of graphite particulate in form of fixed or fluidized bed wherein the surface available for the electric change for a unitary volume of the anode results to be the maximum. 
     The cathode can be bidimensional or tridimensional and can be made of ferrous or carbonaceous materials or of a metal chosen amongst vanadium, tungsten, tantalium, niobium, yttrium, and in the case the process is carried out by excluding the presence of HF in the catholyte also amongst titanium and zirconium. Shape and size of the cathode are those required by the working conditions of the process. 
     In the case of a double build cell the separation between catholyte and anolyte is carried out by a porous diaphragm made of a material inert to the pickling solution or by a ions exchange membrane (cations or anions exchange). The diaphragm can be made of asbest or materials consisting of ceramic oxides or organic polymers suitable for the manufacture of fabrics, felts and microporous films. Such polymeric materials can be choosen among polyoxyphenylene, polyfluorovinyl, polyphenylensulfide, polyperfluoroalkoxyl, polytetrafluoroethylene. For the ion exchange membrane is suitable a matirial of the type perfluorosulphonic acid sold under the trade mark Nafion (Du Pont). 
     The process of electrolytic oxidation above disclosed can be carried out in a large range of temperature between ambient temperature and 100° C.: at high temperature the reaction speed is increased but the life of the electrodes is compromised. The preferred working temperature is comprised between 20° and 60° C. 
     The possibility of oxidizing in an electrolytic way the pickling solution within the scope of a stainless steel or common steel pickling process comes out of the laboratory tests carried out. From these tests the working conditions that can be applied within the scope of an industrial pickling process which uses a bath containing Fe 3+  ions and hydrofluoric acid or HF+H 2 SO 4 , are drawn. 
     The electrolytic oxidation method according to the invention is useful in the stainless steel pickling as well as in the pickling of other kind of steel where the Fe 2+  ion in the pickling solution is to be continuously oxidized to Fe 3+ , for instance in the pickling of nickel-steels or nickel-cobalt steels according to the Japanese Uyemura&#39;s patent n. 235 581. 
     The pickling solutions which can be advantageously reoxidized by the electrolytic method according to the present invention are of different type. 
     We can mention in this connection the solutions containing as acid component only HF, generally comprised in the following limits: 
     HF between 10 and 60 g/l (as free acid) 
     Fe 3+  between 15 and 70 g/l 
     anion F −  total, between 30 and 140 g/l 
     Fe 2+ +Fe 3+ =80 g/l 
     Fe 3+ /Fe 2+ =0.2+7 
     Of particular interest are the pickling solutions used in the process Cleanox  R  of the Applicant wherein the composition can range between the following wide limits according to the type of the material to be treated and of the upstream manufacturing steps: 
     HF between 5 and 60 g/l (as free acid) 
     H 2 SO 4  between 30 and 200 g/l (as free acid) 
     Fe 3+  between 5 and 80 g/l 
     Fe 2+  between 4 and 80 g/l 
     anion F −  total between 5 and 150 g/l anion SO 4   −−  total between 60 and 330 g/l 
     Fe 3+ /Fe 2+  between 0.05 and 20 g/l 
     In order to avoid troubles owing to saturation of the bath in iron salts generally a total Fe content of 120 g/l is not exceeded. The above solution can contain, for specific uses, small amount of Cl −  anions up to a maximum of 20 g/l. 
     It is possible also to treat pickling solutions containing: 
     HF between 5 and 60 g/l (as free acid) 
     HCl between 20 and 60 g/l (as free acid) 
     Fe 3+  between 5 and 80 g/l 
     anion F −  total between 5 and 150 g/l 
     A further application of the electrolytic oxidation method according to the invention consists in the reoxidation of solutions used in passivation treatments subsequent to the pickling process and having composition similar to those above considered for the pickling process. 
     The following tests and examples have illustrative purpose and do not limit the possible application of the invention process. 
     TEST 1 
     The “single build” cell was equipped with a 5.3×11 cm “screened” platinum anode having an actual total surface of 100 cm 2 . The cathode made of platinum too having an actual surface of about 1 cm 2 . The volume of the electrolytic solution was 100 ml. The solution in the test had the following composition: 
     HF=20.69 g/l 
     H 2 SO 4 =71.2 g/l 
     Fe 2+ =41 g/l 
     Fe 3+ =32 g/l 
     Cr 3+ =2.7 g/l 
     The solution had a room temperature. 
     The applied tension was comprised between 1 and 2 V and it was set in order to maintain a constant current intensity of 1 A. 
     The Redox potential of the solution was measured at regular intervals of 15 minutes and it is reported in the following table: 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Electrolysis time (min.) 
                 Measured Potential (Pt/SCE) 
               
               
                   
                   
               
             
            
               
                   
                 0 minutes 
                 0.213 Volt 
               
               
                   
                 15 minutes 
                 0.218 Volt 
               
               
                   
                 30 minutes 
                 0.226 Volt 
               
               
                   
                 45 minutes 
                 0.234 Volt 
               
               
                   
                 60 minutes 
                 0.243 Volt 
               
               
                   
                 75 minutes 
                 0.255 Volt 
               
               
                   
                 90 minutes 
                 0.263 Volt 
               
               
                   
                 105 minutes 
                 0.272 Volt 
               
               
                   
                 120 minutes 
                 0.290 Volt 
               
               
                   
                 135 minutes 
                 0.295 Volt 
               
               
                   
                 150 minutes 
                 0.304 Volt 
               
               
                   
                 165 minutes 
                 0.308 Volt 
               
               
                   
                 180 minutes 
                 0.315 Volt 
               
               
                   
                 195 minutes 
                 0.318 Volt 
               
               
                   
                 210 minutes 
                 0.320 Volt 
               
               
                   
                   
               
            
           
         
       
     
     Total growth of the potential=0.107 Volt. 
     TEST 2 
     The single “build” cell was equipped with a platinum anode as the one of test 1 and with an iron cathode having a cathodes surface/anodic surface of 1/100. The electrolytic solution volume was 1000 ml, the composition was the following one: 
     HF=46.6 g/l 
     H 2 SO 4 =122.4 g/l 
     Fe 2+ =38.1 g/l 
     Fe 3+ =11.7 g/l 
     The solution has a room temperature. The applied tension was included between 1 and 2 V and it was set in such a way to obtain a 4 A constant current intensity. 
     After 60 minutes of electrolysis the analytic data compared with the initial ones are as follows: 
     
       
         
           
               
               
             
               
                   
               
               
                 Content of Fe 2+  and Fe 3+  at 
                 Content of Fe 2+  and Fe 3+  at 
               
               
                 the beginning, t = 0 minutes 
                 t = 60 minutes 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Fe 2+  = 
                 38.1 g/lt 
                 Fe 2+  = 
                 31.8 g/l 
               
               
                 Fe 3+  = 
                 11.7 g/lt 
                 Fe 3+  = 
                 18 g/l 
               
               
                 Redox = 
                 120 mV/Ag, AgCl 
                 Redox = 
                 150 mV/Ag, AgCl 
               
               
                   
               
            
           
         
       
     
     The results show that a quantity of 6.3 g of Fe 2+  is thus oxidized to Fe 3+  with a potential Redox increase of about 30 mV. From these data a 75% current efficiency is reckoned. 
     TEST 3 
     The “single build” cell was equipped with the same electrodes as in test 2, the volume of the electrolyte being of 1000 ml and the temperature being a room temperature. 
     The initial solution was as follows: 
     HF=46.6 g/l 
     H 2 SO 4 =122.4 g/l 
     Fe 2+ =31.8 g/l 
     Fe 3+ =18 g/l 
     The applied tension was comprised between 1 and 2 V and it was set in such a way to have a 4 A constant intensity. After 60 minutes of electrolysis the analytic data compared with the initial ones are as follows: 
     
       
         
           
               
               
             
               
                   
               
               
                   
                 Content of Fe 2+   and Fe 3+   at 
               
               
                 Content of Fe 2+   and Fe 3+   at 
                 the end of the electrolysis 
               
               
                 the beginning, t =0 minutes 
                 t = 60 minutes 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Fe 2+  = 
                 31.8 g/lt 
                 Fe 2+  = 
                 26.4 g/l 
               
               
                 Fe 3+  = 
                 18 g/lt 
                 Fe 3+  = 
                 23.4 g/l 
               
               
                 Redox = 
                 150 mV/Ag, AgCl 
                 Redox = 
                 183 mV/Ag, AgCl 
               
               
                   
               
            
           
         
       
     
     The results show the oxidation of 5.4 of Fe 2+  to Fe 3+  and a potential Redox increase of about 33 mV. From these data a 64% current efficiency is reckoned. 
     TEST 4 
     The used cell was equipped with platinum cathode and anode which were also used in test 1. The cell contained 80 ml of electrolyte and was equipped with a NAFION separating membrane of the cathodic area. The tension was set between 1 and 2 V so as to have a constant intensity of 0.5 A. 
     The initial electrolyte composition was the same as in test 1. At regular intervals of 15 minutes the Redox potential of the solution which is reported in the following table was measured. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                   
                 Potential of the solution 
               
               
                   
                 Electrolysis time 
                 measured by Pt/SCE electrode 
               
               
                   
                   
               
             
            
               
                   
                 0 minutes 
                  0.22 Volt 
               
               
                   
                 15 minutes 
                 0.268 Volt 
               
               
                   
                 30 minutes 
                 0.308 Volt 
               
               
                   
                 45 minutes 
                 0.344 Volt 
               
               
                   
                 60 minutes 
                 0.364 Volt 
               
               
                   
                 75 minutes 
                 0.384 Volt 
               
               
                   
                 90 minutes 
                 0.398 Volt 
               
               
                   
                 105 minutes 
                 0.412 Volt 
               
               
                   
                 120 minutes 
                 0.414 Volt 
               
               
                   
                   
               
            
           
         
       
     
     Total growth of the potential of the solution=0.194 Volt. 
     A comparison of the test 1 data with the test 4 ones showed a higher oxidation speed of Fe 2+  in the solution together with a higher current efficiency in test 4 than the one obtained in test 1. This is substantially due to the fact that in test 4 a cell provided with a diaphram for the separation of the cathodic area from the remaining electrolytic solution is used. A practical application of the electrolytic oxidation process of the pickling solution is shown in the following examples. 
     EXAMPLE 1 
     A pickling solution containing: 
     HF=25 g/l (as free acid) 
     H 2 SO 4 =110 g/l (as free acid) 
     Fe 2+ =50.7 g/l 
     Fe 3+ =39.3 g/l 
     is placed in an electrolytic cell having capacity of 700 ml, provided with separating diaphragm formed by a Nafion membrane, with anode and cathode of rectangular shape, made of graphite, having each working surface of 23.48 cm 2 , and is subjected to electrolysis during 18 h. The tension applied to the cell was about 6 V (average value). 
     Working data and the results ascertained are reported in the following: 
     applied current: 0.92 A 
     anode current density: 392 A/m 2    
     volume of the electrolysed solution: 700 ml 
     initial Fe 2+  content in the treated solution: 30.1 g 
     amount of Fe 2+  oxidized in g calculated by Faraday law: 34.5 g 
     Faraday yield: 87.2% 
     bivalent iron oxidation rate, kg/m 3 /day=57.34 
     EXAMPLE 2 
     A pickling solution containing 40 g/l HF, Fe 3+  and Fe 2+  ions for a total of 40 g/l Fe, was subjected to electrolytic oxidation in a two-compartment cell provided with a separating diaphragm consisting of a Nafion ionic exchange membrane and with graphite electrodes. Two tests were carried out by varying some operating conditions. In both cases, a colloidal Fe(OH) 3  suspension was formed as a result of pH increase (due to protons migration toward the cathode compartment through the membrane). This phenomenon did not take place when treating pickling solutions also containing substantial quantities of H 2 SO 4 . 
     The operating conditions and the results of the first and second tests are reported hereinafter. 
     FIG.  1  and FIG. 2 show the Fe 2+  content variation with time, detected in the first and, respectively, in the second test. 
     Test 1 
     electrolysis total time: 8 h 
     immersed anode area: 21.73 cm2 
     cell applied voltage: about 7V 
     applied current, A: 0.9 
     anode current density, A/m2: 414 
     electrolysed solution volume: 700 ml 
     initial Fe 2+  content in the treated volume: 21.77 g/l 
     oxidized Fe 2+  quantity, g, referred to the treated solution volume: 17.76 g/l 
     oxidized Fe 2+  quantity, g, calculated by the Faraday law: 21.42 g/l 
     Faraday yield: 82.9% 
     bivalent iron oxidation rate, kg/m3/day: 54.55 
     Test 2 
     electrolysis total time: 4 h 
     immersed anode area: 21.73 cm2 
     applied current, A: 1.7 
     anode current density, A/m2: 782 
     electrolysed solution volume: 700 ml 
     initial Fe 2+  content in the treated volume: 19.48 g/l 
     oxidized Fe 2+  quantity, g, referred to the treated solution volume: 15.76 g/l 
     oxidized Fe 2+  quantity, g, calculated by the Faraday law: 20.2 g/l 
     Faraday yield: 77.9% 
     bivalent iron oxidation rate, kg/m3/day: 93.4 
     A moderate oxygen evolution was observed in test 2. which was due to the high anode current density (7.82 A/dm2 vs. 4.14 A/dm2 of test 1). 
     In both tests, the Fe 2+  oxidation rate decreased with decreasing the concentration, which indicates a diffusion control on the kinetics of the whole electrolytic process. 
     EXAMPLE 3 
     This example has been carried out in an electrolytic cell having separating diaphragm made of Nafion ion exchange membrane of 100 cm 2  of surface. This comparatively large surface has been chosen in order to avoid the too high local current densities detected in some preceding tests (cell geometry optimisation). The pickling solution to be treated was as utilized in the Applicant&#39;s Cleanox R  process and consisted of HF 40 g/l, H 2 SO 4  130 g/l, Fe 2+  47.75 g/l, Fe 3+  40 g/l. The catholyte consisted of a H 2 SO 4  aqueous solution (127 g/l). 
     Catholyte (5 l) and anolyte (5 l) were contained in two separate container and let to circulate continuously respectively in the cathodic compartement and in the anodic compartement each of work capacity of about 0.5 l. 
     The test data are as follows: 
     catholyte volume: 5 l; anolyte volume: 5 l 
     total immersed anode area: 168.68 cm2 
     total immersed cathode area: 84.34 cm2 
     applied current, A: 6.7 
     measured voltage across cell: 3.75 
     anode current density (theoretical), J: 398.8 A/m2 
     initial Fe 2+  content: 47.75 g/l 
     final Fe 2+  content: 11.75 g/l 
     total electrolysis time: 14 h 
     Cleanox solution temperature during electrolysis: 40° C. 
     Faraday yield: 89.2% 
     Fe ++  decrease with time is illustrated in the graph of FIG.  3 . 
     Processing of the experimental data by a linear regression procedure gave an oxidation rate of 61.54 kg/m3/day. 
     Remarks 
     Once electrolysis had been completed, the graphite cylinders used as anodes did not show chemico-mechanical corrosion phenomena. 
     EXAMPLE 4 
     This test was carried out under the same operating conditions as adopted in Example 3 (NAFION membrane area: 100 cm2), introducing graphite particulate prepared in the lab into the anode compartment. 
     The test data are as follows: 
     catholyte volume: 5 l 
     anolyte volume (Cleanox): 5 l 
     total immersed anode area: 168.68 cm2+graphite particulate area: 600 cm2, total 770 cm2 
     total immersed cathode area: 84.34 cm2 
     applied current, A: 6.7 
     measured voltage across cell (mean): 2.8 
     initial Fe 2+  content: 43.00 g/l 
     total electrolysis time: 12 h 
     Cleanox solution temperature during electrolysis: 40° C. 
     Faraday yield, π farad : 93.4% 
     Fe ++  decrease with time is illustrated in the graph of FIG.  4 . 
     Processing of the experimental data by a linear regression procedure gave an oxidation rate of 62.6 kg/m3/day. 
     Remarks 
     With this type of anode the energetic balance of the process improves with an average decrease of the cell voltage of 0.7 V. 
     EXAMPLE 5 
     A commercial-scale plant for the production of austenitic steel wire comprises a pickling stage consisting of a vat having a capacity of approx. 12,000 l and operating with a solution containing 
     sulphuric acid *: 100 g/l, 
     hydrofluoric acid *: 30 g/l, 
     Fe 3+ : 40 g/l, 
     Fe 2+ : 25 g/l; 
     operating T: 50° C. 
     * concentration values referred to free acids. 
     To secure the highest efficiency of the pickling reaction, the solution was fed with an air flow of approx. 360 m3/h. The solution also contained chromium and nickel in an overall amount of approx. 12 g/l, deriving from the pickling reaction. During the process, the Fe 3+ /Fe 2+  ratio had to be maintained at values ranging from 1.5 to 2.0. 
     Considering the plant productivity, to maintain said ratio at the predetermined values, approx. 300 kg of Fe 2+  (derived from the pickling reaction) was to be oxidized to Fe 3+  within a period of 12 h. Said operation had been formerly carried out by adding approx. 400 kg/day of Cleanox 352 Z (H 2 O 2  stabilized at 28% by wt.). 
     The use of hydrogen peroxide has been now replaced by electrolytic oxidation according to the invention. The solution is continuously sent to a multiple electrolytic cell (filter press type) consisting of electrolytic cells in series provided with bipolar electrodes and including 16 anode semicells alternating with 16 cathode semicells, each being 1 m×1 m in size, separated by a NAFION semipermeable cationic membrane. The pickling solution, fed from a common header by means of a variable delivery pump (up to 5,000 l/h), after filtration continuously enters each anode semicell from the bottom (semicell working volume: 15 l), outflows from the top and then returns to the pickling vat. 
     The catholyte consists of a ca. 100 g/l sulphuric acid solution coming from an approx. 500 l adjacent tank, in which it is continuously recirculated. The electrode of bipolar type consists of a stiff plate, thickness of 1 cm, made of graphite. 
     The total cathode surface, like the anode one, is 16 m 2 . Nafion membranes are placed between two polyethylene porous panels which serve as a reinforcement and prevent the membrane from being contaminated by suspended solids, if any. 
     A direct current flow corresponding to a current density on the electrode of about 4 A/dm 2  is caused to pass through each cell. The average voltage across the cell is 3 volts. The quantity of bivalent iron oxidized to trivalent iron averagely is comprised between 11 to 13 kg/h, with a Fe 3+ /Fe 2+  ratio being maintained within the predetermined range.