Dissolved oxygen concentration in an undivided rotating cylinder electrode reactor

The steady state mass balance for dissolved oxygen in an undivided rotating cylinder electrode reactor was established. Cases involving the absence or presence of diffusion controlled oxygen reduction reaction were treated. The results show that in the absence of the oxygen reduction reaction and under low electrolyte velocities employed industrially, the rotating cylinder electrode reactor, exhibiting a continuous stirred tank reactor behaviour, yields lower dissolved gas concentrations than a plug flow reactor. Therefore, the rotating cylinder electrode reactor leads to lower redissolution rates of the detached metallic particles by oxygen corrosion. The effect of the dissolved gas concentration in the inlet electrolyte also demonstrates the benefits of a prior degassing step in the flow sheet. The simpler equation derived in the absence of oxygen reduction may be used in its presence for inlet dissolved oxygen concentrations below saturation and industrially relevant electrolyte velocities ranging from approx. 0 to 0.3 m s⁻¹ if metal deposition current efficiency is higher than 87% or, conversely, if the dissolved metal concentration to dissolved oxygen concentration ratio is greater than 27. An even simpler equation based on the added hypothesis of total desorption of the gas produced can also be used with electrolyte velocities below 0.025 m s⁻¹. Finally, the derived equations allow optimization of the rotating cylinder electrode reactor.     

Electrochemical removal of nitrate ions in waste solutions after regeneration of ion exchange columns

Electrochemical reduction of nitrate ions in synthetic regenerating solutions after ion exchange column regeneration was studied. The influence of current density on the current efficiency was determined in the range 2.8–7.6 mA cm–2 in a diaphragmless flow-through electrolyser in a batch recirculation mode. A Cu cathode and a Ti/Pt anode was used, the temperature being maintained at 25 C. Highest integral current efficiency occurred at 2.8 mA cm–2. The presence of about 6 mg dm–3 Cu ions in treated solution was found to prevent a decrease in cathode activity and, consequently, in electrolysis efficiency. The catalytic influence of Cu ions was verified by potentiodynamic polarisation experiments on a copper rotating disc electrode and by chronopotentiometry performed during the course of electrolysis.     

Silver Electrodeposition from Photographic Processing Solutions

This paper describes the parameters that are important in the industrial practice of silver removal from photographic fixers. The experiments were performed under potentiostatic control using synthetic solutions. The current efficiency was analysed as a function of the cathodic potential taking into account the deposit quality. A cathodic potential of ?0.5 V against a saturated calomel electrode is recommended. The conditions to prevent the darkening of the electrodeposit were investigated. The determination of silver concentration in the solution was made by direct potentiometry. The results obtained with synthetic fixers were corroborated by making the silver deposition from commercial fixers in an electrochemical reactor with rotating cylinder electrode intercalated in an electrolytic flow circuit in order to simulate practical conditions.     

Electrochemical removal of cadmium using a batch undivided reactor with a rotating cylinder electrode

The performance of an undivided electrochemical batch reactor with a rotating cylinder electrode under potentiostatic control is examined for the removal of cadmium from a sodium sulfate solution containing 500 ppm Cd(II) at pH ? 7. The effect of hydrogen evolution as a side cathodic reaction on the figures of merit of the reactor is analysed. The best results were obtained for a cathode potential of ?0.9 V against the saturated calomel electrode. With an angular velocity of 1500 rpm the space time yield and the normalized space velocity were 0.66 mol m^?3 s^?1 and 3.9 h^?1 respectively, while the fractional conversion was 67.3% with a current efficiency of 66.7%. The surface morphology of the deposits as a function of the applied potential is also reported. © 2001 Society of Chemical Industry     

Theoretical and experimental studies of the effect of side reactions in copper deposition from dilute solutions on packed-bed electrodes

This paper analyses the effect of side reactions, such as the reduction of oxygen or Fe(III), on copper deposition from dilute acid sulfate solutions. Fundamental studies with a rotating disc electrode were performed to determine the potential regions where the reactions take place and also to calculate kinetic parameters under the conditions of the experiments. Applied studies carried out in a through-flow electrochemical reactor with a packed bed cathode formed by a stack of nets are reported. The experimental data are compared with theoretical results, according to the continuous model, to corroborate its reliability for the case of simultaneous electrochemical reactions. It was found that the agreement between the mathematical treatment and the experimental data is similar to the case of a single reaction in the electrode. Taking into account the side reactions, criteria for the selection of bed thickness parallel to the current flow are discussed.     

The effect of magnetic field on the oxygen reduction reaction and its application in polymer electrolyte fuel cells

The effect of magnetic field gradients on the electrochemical oxygen reduction was studied with relevance to the cathode gas reactions in polymer electrolyte fuel cells. When a permanent magnet was set behind a cathode, i.e. platinum foil or Pt-dispersed carbon paper for both electrochemical and rotating electrode experiments and oxygen was supplied to the uphill direction of the magnetic field, electrochemical flux was enhanced and the current increased with increasing the absolute value of magnetic field. This magnetic effect can be explained by the magnetic attractive force toward O₂ gas. When magnet particles were included in the catalyst layer of the cathode and the cathode was magnetized, the current of oxygen reduction was higher than that of nonmagnetized cathode. A new design of the cathode catalyst layer incorporating the magnet particles was tested, demonstrating a new method to improve the fuel cell performance.     

Removal of cadmium and production of cadmium powder using a continuous undivided electrochemical reactor with a rotating cylinder electrode

The behaviour of a continuous undivided electrochemical reactor with a rotating cylinder electrode under potentiostatic control is examined for the abatement of cadmium from synthetic sodium sulfate solutions with Cd(II) concentrations lower than 500 mg dm^?3 at a reactor inlet pH ? 7. The process was designed to convert the metal ions in solution to metal powder, which settles to the conical of the reactor and may be removed at intervals as a sludge by opening a drop valve. The effect of applied potential, inlet cadmium concentration, rotation speed and hydrogen evolution as side cathodic reaction on the ‘figures of merit’ of the reactor are analysed. The best results were obtained for cathode potentials in the range from ?0.9 V to ?1.0 V against the saturated calomel electrode. Therefore, when the rotation speed was 1000 rpm the space time yield and the normalized space velocity were 0.64 ×10^?2 mol m^?3 s^?1 and 0.89 h^?1 respectively, while the fractional conversion per pass was 35% with a current efficiency higher than 74%. The surface morphology of the deposits as a function of the process variables is also reported. © 2002 Society of Chemical Industry     

A mass transfer study of a new electrochemical reactor stirred by gases evolved at the counter electrode

Mass transfer coefficients were measured for the deposition of copper from acidified copper sulphate solution at a horizontal cylinder cathode stirred by oxygen evolved at a horizontal lead anode placed below the cylinder. Variables studied were: oxygen discharge rate, electrolyte concentration and cylinder diameter. Oxygen discharge was found to increase the rate of mass transfer by a factor ranging from 1.5 to 5 over the natural convection value depending on the rate of oxygen discharge at the lead anode. The data were correlated by the equation: J = 25.0 Re^?0.53 Mass transfer coefficients measured at an array of horizontal cylinders agreed with the results for single cylinders. A new electrochemical reactor built of a set of parallel arrays of horizontal cylinders and stirred by the counter electrode gases is proposed as offering an efficient way of stirring with no external stirring power requirement.     

Electrochemical Removal of Tin from Dilute Aqueous Sulfate Solutions using a Rotating Cylinder Electrode of Expanded Metal

The performance of a batch undivided electrochemical reactor with a rotating cylinder electrode of expanded metal sheets for the removal of tin from synthetic sulfate solution is studied. The effect of the cathode potential, initial tin concentration, number of sheets forming the cathode and cathodic side reactions on the figures of merit of the reactor is analysed. For a cathode potential of –0.65 V vs SCE at 500 rpm, the tin concentration decreased from 393 to 94 mg l^–1 after 30 min of electrolysis with a specific energy consumption of 3.93 kWh kg^–1 and a normalized space velocity of 1.27 h^–1. The change in concentration was higher when the potential was more negative because of the turbulence-promoting action of the hydrogen evolution. The results suggest that the applied potential must represent a compromise between the increase in space time yield or normalized space velocity and the increase in the specific energy consumption.     

Prediction and measurement of multi-metal electrodeposition rates and efficiencies in aqueous acidic chloride media

A novel process is being developed for metal recovery from waste electrical and electronic equipment (WEEE) and involving a leach reactor coupled to an electrochemical reactor. Metals such as Ag, Au, Cu, Pb, Pd, Sn, etc. are dissolved from shredded WEEE in an acidic aqueous chloride electrolyte by oxidizing them with aqueous dissolved chlorine species. In the electrochemical reactor: (i) chlorine is generated at the anode for use as the oxidant in the leach reactor, and, simultaneously, (ii) at the cathode, the dissolved metals are electrodeposited from the leach solution. The Butler-Volmer equation was used to provide predictions of the electrode potential dependences of partial current densities, and hence total current densities, current efficiencies and alloy compositions, for acidic aqueous chloride electrolytes containing Ag(I), Au(III), Cu(II), Fe(III), Pb(II), Pd(IV) and Sn(IV) species, together with dissolved chlorine. With judicious choice of kinetic parameters, the predicted total current density—electrode potential behaviour of such solutions was in good agreement with experimental data for a rotating Pt disc electrode. Reduction of dissolved chlorine at a Pt rotating disc electrode exhibited mass transport controlled behaviour, in agreement with Levich's equation over the potential range 0.3-0.9 V (SHE). This could form the basis of a linear sensor, possibly using a microelectrode for measurement and control of the dissolved chlorine concentration in the efflux of leach reactors and inlet to cathodes of the electrowinning reactors, in the envisaged process.     

Electrodeposition of Ni-W-B amorphous alloys

Partial polarization curves at the glassy carbon rotating disc electrode have been used to study the electrodeposition of Ni and Ni-W alloy from citrate-containing solution. For deposition of Ni-W alloys, the partial polarization curves indicate diffusion control for nickel reduction and stoichiometric limitation for tungsten deposition by the composition of the alloy. Plating experiments show that current efficiency of the electrodeposition and composition of the resulting alloy depend on the parameters of the electrolysis. The best conditions for electrodeposition of the alloy Ni-W-B are current density of 45–50 mA cm^–2, temperature of 60–70 °C, Ni(II) concentration of 20–25 mm, and pH 8.5. Pulsed galvanostatic plating at 1 Hz increased slightly the current efficiency. The concentration of Ni(II) in the solution can be self-regulated by using a nickel bipolar electrode in the cathode compartment.     

Experimental investigation of cell design for the electrolysis of iron oxide suspensions in alkaline electrolyte

Following feasibility studies of iron production by electrolytic reduction of hematite particles suspended in a strong alkaline medium, this article concerns the use of engineering methods to investigate the performance of various cell configurations, in view to designing larger processes: a horizontal flow cell with parallel electrodes and two rotating cylindrical electrodes were used for this purpose. The performance was analyzed in terms of current efficiency at 0.1 A cm⁻² and deposit morphology. The results reveal a negligible role of the mass transfer of Fe (+III) ions from the bulk electrolyte on the process efficiency, as formerly suggested in reaction mechanism studies. Conventional ion-mass transfer theory is therefore not applicable and another approach is proposed. Dispersed phase transport processes, more precisely the mechanical forces acting on both the 10 μm ore particles and the evolved oxygen bubbles, can quantitatively and qualitatively explain the cells performance. The configuration with the external rotating cathode allows both efficient contacts of the particles with the cathode and rapid removal of the produced gas phase; however the two rotating electrode devices are subject to appreciable ohmic losses due to the current lead system. The parallel plate configuration, with the cathode at the bottom appears as the best configuration for the deposition which can be achieved with low energy consumption.     

Electrochemical removal of arsenic from technical grade phosphoric acid

The electrolytic de-arsenication of technical grade phosphoric acid 85 wt % by cathodic deposition in order to produce chemically pure phosphoric acid was examined. Polarization curves using Cu, Pb, 316L stainless steel and graphite as cathodic rotating discs were determined. The best results were achieved for copper where the arsenic deposition takes place in a range of potentials without hydrogen evolution. Long term experiments in a laboratory batch reactor with a rotating cylinder cathode of copper showed that the electrodeposited arsenic was a non-conductor, poorly adherent and can be detached from the electrode by means of an ultrasonic cleaner. Experimental results with a pilot plant reactor with a three-dimensional cathode of copper nets showed a high degree of scatter. However, typical values of the figures of merit are: fractional conversion per pass 24%, specific energy consumption 250 kWh kg⁻¹ As for a volumetric flow rate of 1.5 dm³ min⁻¹ at 60 A and 3.4 V of cell voltage.     

硝基苯电解合成对氨基苯酚的工业化试验

采用1000L阴极转动分隔式电解槽,研究了硝基苯电解合成对氨基苯酚(PAP)的工业化放大过程。研究表明,转动阴极电解槽的工作特性与阴极转速、电流强度、温度、隔膜材料等因素有关。当电解电流为3000A时,硝基苯的平均转化率为91.0%,PAP平均收率为67.6%,直流电单耗为7.34 kWh(kg PAP)-1。电解合成的PAP纯度在97%以上,熔点为187.2~188.4C。     

Novel electrochemical measurements on direct electro-deoxidation of solid TiO<Subscript>2</Subscript> and ZrO<Subscript>2</Subscript> in molten calcium chloride medium

A solid metal oxide cathode undergoes significant chemical changes during the molten salt electro-deoxidation process. The changes in the chemical composition lead to changes in the electrical resistivity and potential of the electrode. Two novel electrochemical techniques, based on these two parameters, have been employed to study the electro-deoxidation of solid TiO₂ and ZrO₂ in molten calcium chloride at 900 °C. The in situ resistance measurements carried out by the IR drop method conclusively proved that TiO₂ electrode remains highly conducting throughout the electro-deoxidation process and hence is amenable for reduction. The ZrO₂ electrode, on the other hand, developed very high resistance midway in the electro-deoxidation, and could not be reduced completely. The resistance measurements give strong indication that the electron-transfer reactions taking place at the cathode determine the rate and efficiency of the electro-deoxidation process to a great extent. The low-current galvanostatic electro-deoxidation of TiO₂ electrodes, in conjunction with a graphite pseudo reference electrode to monitor the half cell potentials, showed that the metal oxide passes through two stages during the electrolysis; a high current, low resistant stage 1, where Ca²⁺ ions are inserted to the metal oxide cathode to produce different intermediate compounds and stage 2 where electro-deoxidation of the cathode take place continuously. Removal of oxygen, from the cathode, in stage 1 of the electro-deoxidation is considered to be insignificant. The anodic and cathodic voltages in this stage remained more or less stable at ~1.4 V and ~−1 V, respectively. When the oxygen ions in the melt were depleted at the end of this stage, both the anode and cathode potentials were increased in the anodic direction and this behaviour suggested that the graphite pseudo reference electrode was changed from a C/CO electrode in stage 1 to a Ca²⁺/Ca electrode in stage 2.     

An experimental study of a fixed bed chlor-alkali reactor

A 100 A continuous ‘flow-by’ chlor-alkali membrane reactor was constructed with both anode and cathode consisting of fixed beds of 0.6 to 1 mm diameter graphite particles. The reactor was operated over a range of conditions with and without co-current flow of air or oxygen to the cathode. With an anolyte of 5 M NaCl and catholyte 1.4–3 M NaOH the reactor produced sodium hydroxide and chlorine with ≥80% efficiency at temperatures 28–100°C, absolute pressure 270–970 kPa and superficial current density up to 3.3 kA m^?2. For operation at 100°C and an average pressure of 870 kPa with no gas delivered to the cathode, the cell voltage increased linearly from 2.5V at 0.3 kA m^?2 (10 A) to 4.0 V at 3.3 kA m^?2 (100 A). When oxygen was delivered to the cathode at 1 litre min^?1 under 870 kPa average pressure, the corresonding cell voltages were 1.6 V at 0.3 kA m^?2 to 3.4 V at 3.3 kA m^?2. In operation with air under the same conditions the cell voltage rose from 1.6 V at 0.3 kA m^?2 to 3.1 V at 1.6 kA m^?2. The performance of the oxygen cathode deteriorated with lower pressure and temperature due to mass transfer constraints on the oxygen reaction in the fixed bed electrode.     

Kinetic Studies of the Electrochemical Treatment of Nitrate and Nitrite Ions on Iridium‐Modified Carbon Fiber Electrodes

Electrochemical reductions of nitrate and nitrite ions in neutral solution were studied using an Ir‐deposited carbon fiber electrode. The objective of the study was to set up a process to remove nitrate and nitrite ions from industrial and municipal wastes as well as from the nitrate and nitrite ion enriched water supplies. Since nitrate and nitrite ion reduction kinetics is very slow on plain carbon, the iridium‐modified fibers showed a significant reduction of nitrate and nitrite ions. Nitrogen and ammonia were identified as the two major gaseous products of the reduction process. A first‐order inhibition kinetics model is proposed and was used to analyze the nitrate and nitrite ion reduction kinetics. The reduction rates of nitrate and nitrite ions were favored at higher cathode potential (cf. –950 mV and –850 mV) in a solution of pH = 7.0 with hydrogen evolution efficiency increasing with cathodic potential. A simple model for the batch re‐circulation process was developed for the reduction of nitrate ion at surface‐modified fiber electrode. There was a good agreement between the model predictions and experimental results.     

Multi-cathode cell with flow-through electrodes for the production of iron(ii)-triethanolamine complexes

Nonregenerable reducing agents used in dye houses for the application of indigo, vat dyes and sulfur dyes can be substituted by indirect cathodic reduction techniques. An electrochemical cell for the indirect cathodic reduction of dispersed indigo dyestuff using an alkaline solution of the Fe(iii/ii)-triethanolamine complex as redox mediator was constructed and tested. The cell is built up as divided cell (cathode area 5–10m2, catholyte volume 12L, anolyte volume 1.5L) with several three dimensional cathode units (up to 10) in the same cathode compartment. The cathodes were connected to a common anode and to separately adjustable power supplies. The catholyte was circulated through the porous cathode units parallel to the current direction. Two different electrode materials (copper and stainless steel) and cathode constructions were tested, resulting in an optimized cell construction. The electrochemical cell was characterized by a series of batch electrolysis experiments. Results are given dealing with the cell voltage drop in the cathode, the product yield and the current efficiency at different current densities and cell current. After an optimization step the current efficiency reaches 70–80% at 2 A m–2 current density and 7.8×10–3moldm–3 Fe(ii)-complex. The cell current is 10A.     

Modelling potentials, concentrations and current densities in porous electrodes for metal recovery from dilute aqueous effluents

One-dimensional steady-state models have been developed for the recovery of Pb(II) ions from lead–acid battery recycling plant effluent by simultaneous lead and lead dioxide deposition, including oxygen evolution/reduction and hydrogen evolution as loss reactions. Both monopolar and bipolar reactor with porous graphite electrodes were modelled, as a design aid for predicting spatial distributions of potentials, concentrations, current densities and efficiencies, as well as specific electrical energy consumptions and by-pass currents. Since the industrial effluent contains a large excess of supporting electrolyte (Na₂SO₄), the electrical migrational contribution to reactant transport rates was neglected and the current density–potential relationship was described by the Butler–Volmer equation, allowing for both kinetic and mass transport control. The models were implemented and the governing equations solved using commercial finite element software (FEMLAB). The effects were investigated of electrolyte velocity, applied cathode potential, dissolved oxygen concentration and inlet Pb(II) ion concentration on single-pass conversion, current efficiency and specific electrical energy consumptions. According to model predictions, de-oxygenation of the inlet process stream was found to be crucial to achieving acceptable (i.e. >0.8) current efficiencies. Bipolar porous electrodes were also determined to be inappropriate for the recovery of Pb(II) from effluents, as the low concentration involved resulted in the predicted fraction of current lost as by-pass current, i.e. current not flowing in and out of the bipolar electrode, to be greater than 90% for the ranges of the variables studied.     

Anodic depolarization by aqueous pyrite slurries in the production of hydrogen

Electrolysis of pyrite slurries in aqueous acidic electrolytes at potentials of 0.6 V versus a saturated calomel electrode resulted in the formation of sulphate and ferric ions in the anolyte, while hydrogen gas was formed in the cathode compartment. Thermodynamic considerations suggest that oxidation of pyrite coupled to hydrogen production can be accomplished at an overall cell potential of 0.48 V. The Faradaic current efficiency for hydrogen production was found to be ≈95%. The rates of reaction were observed to increase with electrolysis time, suggesting that pyrite electro-oxidation reactions are autocatalytic in nature. The rates of reaction were found to increase monotonically with electrolysis temperature. In experiments with pyrite particles of various sizes, high reaction rates were generally found with smaller size particles. Other experiments indicated that higher reaction rates can be achieved by increasing the concentration of pyrite slurries.