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     

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     

Electrochemical removal of cadmium from dilute aqueous solutions using a rotating cylinder electrode of wedge wire screens

Rates of mass transfer at rotating cylinder electrodes of wedge wire screens were studied by measuring the limiting current for the cathodic reduction of ferricyanide as test reaction. The experimental data are well correlated by an empirical expression between the Sherwood number and the Reynolds number, both in terms of the internal slot opening as characteristic length, and including two additional dimensionless parameters in order to characterize the geometry of the screens. The performance of an undivided electrochemical batch reactor with a rotating cylinder cathode of wedge wire screens was tested analyzing the cadmium removal from dilute solutions. The effect of cathodic applied potential and size of the screen is studied. Taking into account the residual cadmium concentration the best results were obtained for a cathode potential of −1.1 V vs. SCE at 700 rpm, where the cadmium concentration decreased from 54 to 0.9 mg l⁻¹ after 30 min of electrolysis with a specific energy consumption of 10.7 kWh kg⁻¹ and a normalized space velocity of 3.54 h⁻¹.     

Mass transfer studies at rotating cylinder electrodes of expanded metal

Mass transfer has been studied at rotating cylinder electrodes of expanded metal using the reduction of ferricyanide as test reaction. The experimental data are well correlated by an empirical expression between the Sherwood number and the Reynolds number, both in terms of the hydraulic diameter as characteristic length, and including two additional dimensionless parameters in order to characterize the geometry of the expanded metal. Comparisons of the mass-transfer performance of the expanded metal electrodes with other three-dimensional structures are made.     

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.     

Mass transport studies at rotating cylinder electrode during zinc removal from dilute solutions

The mass transport in the rotating cylinder electrode (RCE) for the Zn(II) recovery from dilute solutions was investigated. Global mass transport data were obtained by monitoring the (first order) concentration decay of dissolved zinc in sulfate media at pH 2. The electrolyses were performed at holding potential of −1.7 V vs. sat. MSE at Reynolds numbers comprised between 15 470 ≤ Re ≤ 123 680. Based on the analysis of Sh = aRe^bSc^0.356 correlation, the value of the constant a, associated with shape and cell dimensions, was 0.65; while the constant b, associated with hydrodynamic regime, exhibits a value of 0.48, which obeys to smooth zinc deposits on RCE interface. The mass transport in the Zn(II)/Zn cathode interface process differs with other deposition process, which usually gives roughness deposits.     

Construction and characterization of a rotating cylinder electrode for different technological applications

A rotating cylinder electrode (RCE) for corrosion studies has been constructed and hydrodynamically characterized. Empirical hydrodynamic parameters were determined from limiting current density and weight loss measurements for four different electrochemical processes. Statistical data analysis established the confidence limits of these parameters.     

Effluent treatment using a bipolar electrochemical reactor with rotating cylinder electrodes of woven wire meshes

BACKGROUND: The behaviour of a bipolar electrochemical reactor consisting of one or more rotating cylinder electrodes of woven wire meshes is reported using copper and cadmium deposition from dilute solutions as test reactions. RESULTS: The best performance related to electrode number was determined for copper deposition and was achieved by an arrangement with two bipolar electrodes, for which the conversion in a single pass was approximately 47%. The specific energy consumption was 3.27 kWh kg^?1 with a normalised space velocity of 23.05 h^?1. The copper powder obtained showed a nodular and dendritic surface morphology. This reactor configuration was also analysed for cadmium deposition, in which hydrogen evolution takes place simultaneously as a side cathodic reaction, considering the effect of flow rate and total current. The maximum conversion per pass for cadmium removal was 38.91%. In this case the reactor with two bipolar electrodes showed a performance similar to that of a monopolar reactor operated at a rotation speed three times higher. CONCLUSION: A continuous electrochemical reactor with two rotating bipolar electrodes of woven wire meshes presents a good performance for copper or cadmium removal from dilute solutions. Copyright © 2009 Society of Chemical Industry     

Cadmium removal from aqueous sulphate solutions by treatment with iron felts

The possibility of removing cadmium from effluents byelectrochemical treatment using iron felts as three-dimensional electrodes was investigated. It was found that iron felts remove cadmium by three paths: (i) adsorption of cadmium ions by hydrous ferric oxide, (ii) cathodic electrodeposition and (iii) precipitation of cadmium hydroxide due to the localized alkalinity produced by the hydrogen evolution. The adsorption isotherm, at 30°C, of cadmium ions from an aqueous sulphate solution on hydrous ferric oxide is given. The influence of cathodic potential, volumetric flow rate and interelectrode gap on the current, current efficiency and fractional conversion is discussed. The iron felts proved to be efficient in removing cadmium. The highest fractional conversion, ≌25% for single pass operation, with 100% current efficiency was obtained for a volumetric flow rate of 9·57×10⁻⁶ m³ s⁻¹ using iron felt cathodes of 4×10⁻³ m thickness potentiostated at potentials lower than −1·5 V against a saturated calomel electrode. © 1998 Society of Chemical Industry     

Removal of tin from dilute solutions

The fluidised bed cell of inert glass beads is an electrolytic reactor which is designed to provide higher ion‐transfer conditions during electrolysis, thus enabling metals to be removed efficiently from dilute solutions. The effectiveness of the method as a means of removing metals from effluent to meet discharge consent levels is studied for the in situ removal of tin from dilute solution (concentration range 0.25–1.00 gdm⁻³). The results show that the combination of high mass transport conditions and a moderately high electrode surface area per unit electrode volume provides a system for continuous removal of metal from dilute solutions. The effects of acid concentration, tin concentration, current density, fluidised bed agitation, electrode spacing, type of electrode and lead impurities on the removal of tin are reported and expressed in terms of the percentage removal of tin (α_Sn), the efficiency of tin deposition (ϕ_Sn), and the energy consumption (W_Sn) for 1 kg of tin deposited. The results show that tin can, under optimised conditions, be removed from dilute solutions to a residual concentration of 0.001 gdm⁻³. © 2001 Society of Chemical Industry     

Mass transfer studies at packed bed rotating cylinder electrodes of woven-wire meshes

Mass transfer has been studied at rotating cylinder electrodes of woven-wire meshes using the reduction of ferricyanide as test reaction. The experimental data are well correlated by an empirical expression between the Sherwood number and the Reynolds number, both in terms of the hydraulic diameter as characteristic length, and including two additional dimensionless parameters in order to characterize the geometry of the meshes. The mass-transfer enhancement factor for three-dimensional electrodes is analyzed.     

Removal of cupric ions from acidic sulfate solution using reticulated vitreous carbon rotating cylinder electrodes

The potentiostatic deposition of copper from acid sulfate solutions (0.50 mol dm^?3 Na₂SO₄ at pH 2 and 298 K) was studied at four porosity grades (10, 30, 60 and 100 pores per linear inch, ppi) of reticulated vitreous carbon (RVC) rotating cylinder electrode (RCE). The rate of removal of cupric ions from a 200 cm³ volume of electrolyte was examined as a function of the grade of RVC foam, the electrode potential and the initial cupric ion concentration. For the 100 ppi material, the product of the mass transport coefficient and the electroactive area per unit volume of electrode (k_mA_e) was equal to 0.28 s^?1 at a potential of ?500 mV vs SCE for an initial cupric ion concentration of 0.85 mmol dm^?3 and a constant rotation speed of 1500 rev min^?1. Under the experimental conditions, an initial dissolved copper concentration of 63.5 ppm could be reduced to <0.1 ppm in approximately 60 min using a 100 ppi RVC RCE. SEM studies showed some non‐uniform deposition of metal due to heterogeneous nucleation of copper together with the development of rough deposits. Copyright © 2004 Society of Chemical Industry     

Study of mass transfer in a vertically moving particle bed electrode

A study was made of the influence of process parameters on the mass-transfer coefficient in a flow-through cell with a cascade of rotating drums partially filled with conductive particles (called the vertically moving particle bed). Copper deposition from an acidic sodium sulphate solution was used as the model reaction. To evaluate the experimental data a macrohomogeneous mathematical model of potential and current density distribution inside the cell was developed. The electrolyte flow distribution between the empty space above the particle bed and through the bed was evaluated. On the basis of these results the following correlation is proposed:

The application of reticulated vitreous carbon rotating cylinder electrodes to the removal of cadmium and copper ions from solution

The removal of cadmium and cupric ions from 0.50 mol dm^?3 Na₂SO₄ at pH 2 and 298 K was studied using a reticulated vitreous carbon (RVC) rotating cylinder electrode (RCE). The cathode was a 100 pores per linear inch porosity grade with a radius of 0.5 cm, a height of 1.2 cm and a volume of 0.94 cm 3 . The cathode was rotated a constant speed of 1500 rev min^?1. A rate enhancement of approximately three times is reported for the removal of cupric ions from a chloride solution (0.05 mol dm^?3 cupric ions in 0.1 mol dm^?3 NaCl at pH 7) when compared with the analogous reaction in acid sulfate solutions (0.50 mol dm^?3 Na₂SO₄ at pH 2). SEM images of the metal deposit morphology allow the morphology of the metal deposits to be characterised. The deposits showed incomplete coverage of the RVC surface and appreciable roughness developed with time due to dendritic growths. Copyright © 2004 Society of Chemical Industry     

Removal of zinc from dilute aqueous solutions by galvanochemical treatment

The possibility of zinc removal, a common toxic metal, from simulated liquid effluents by the application of a novel treatment method, termed galvanochemical, was investigated. The galvanochemical process is considered as a simple, economic, friendly to the environment, method, which does not create harmful end‐products. Synthetic wastewaters were examined, these contained concentrations of zinc commonly found in real wastewaters from small‐to‐medium size industrial units, such as metal‐plating or metal‐treatment plants. These liquid wastes pose an important environmental problem, due to the content of heavy metals. The optimization of the main operating parameters was the objective of the study. The galvanic pair scrap ratio iron:coke was used for these investigations. It was found that the use of the galvanic pair mixture of 80 g dm^?3 (weight per solution volume ratio), agitated at 160 rpm for 5 h, removed more than 90% of zinc from the initial solution, containing initially 50 mg dm^?3 of zinc. The scanning electron microscopy/energy disperse spectrometry (SEM/EDS) analysis performed on the products showed that zinc was adsorbed onto the coke to a small extent and that the zinc removal was mainly based on the adsorption of zinc onto the surface of active iron oxides formed as a result of galvanochemical reactions. Copyright © 2005 Society of Chemical Industry     

Removal of cadmium from wastewater by enhanced ultrafiltration using surfactants

In order to recover cadmium ion from wastewater, a process based on enhanced ultrafiltration was finalised. The originality of the solution here proposed lies on the coupled use of ceramic membranes and of a surfactant added to the water to be treated. This paper deals with the study of the influence of experimental conditions upon permeation flux and selectivity of the separation. The process is efficient and flexible since a single stage treatment allows to reduce the concentration of cadmium in the effluent by a factor of about 750; this high performance in selectivity is on a par with high fluxes of permeate (roughly 170 L.h.^?1.m^?2 at 25°C and 3 × 10⁵ Pa).     

Effect of tubes bundle electrode on removal of cadmium from simulated wastewaters by electrodeposition

BACKGROUND: The performance of a novel pilot scale, fixed bed flow‐through cell, consisting of a cathode formed by a bundle of stainless steel tubes having two tube arrangements (920 tubes of 0.6 cm outer diameter and 360 tubes of 1.0 cm outer diameter) was investigated for the removal of cadmium in a batch re‐circulation mode. The electrochemical reaction studied was the cathodic reduction of Cd⁺² using 0.5 mol L^?1 sodium sulphate as supporting electrolyte. The parameters analyzed were initial Cd⁺² concentrations and electrolyte flow rates. RESULTS: Experimental results for the two different tube arrangements showed that the mass transfer coefficients were higher for the 0.6 cm diameter tubes than for the 1.0 cm diameter tubes. The former gave a fractional conversion of 80.4% with a current efficiency of 70.75% compared with a fractional conversion of 58.5% with a current efficiency of 45.7%, for the 1.0 cm diameter tubes, for a Cd⁺² concentration of 200 ppm and flow rate of 250 L h^?1. These results were correlated to: for 5 < Re < 28 and Sc = 649, where Sh is the Sherwood number, Re the Reynolds number and Sc the Schmidt number. CONCLUSION: Experimental results, analysis, correlations and figures of merit showed better performance of the cell for the removal of Cd⁺² with 0.6 cm diameter tubes than with 1.0 cm diameter tubes. Copyright © 2010 Society of Chemical Industry