Simultaneous recovery of Pb and PbO2 from battery plant effluents. Part II

Results are reported of experiments and modelling of cathodic Pb and anodic PbO₂ electrodeposition, aimed at developing a process using both reactions simultaneously for treating aqueous effluents from lead–acid battery recycling plants. Pb(II) solubilities and equilibrium potentials were calculated as functions of pH and sulfate activities. Using kinetic parameters from the literature or determined experimentally, models were developed for current density–potential and current efficiency–potential relationships, together with the current density dependence of specific electrical energy consumptions for Pb(II) recovery. Experimental current density–potential and charge efficiency–potential relationships were in broad agreement with model predictions, with near unity current efficiencies for mass transport controlled PbO₂ deposition from electrolytes containing 1 mol Pb(II) m^–3 at pH 12. However, charge efficiencies for cathodic deposition of lead were typically 0.2 for 1 mol Pb(II) m^–3 + 1 mol O₂ (aq) m^–3; removal of dissolved oxygen was predicted and determined to increase current efficiencies to near unity. Pb(II) concentrations were depleted to <60 ppb in a batch recycle reactor system with graphite felt anodes and graphite or titanium felt cathodes. Simultaneous cathodic Pb and anodic PbO₂ electrodeposition resulted in more rapid Pb(II) depletion than for either reaction separately.     

Scale-up of the perforated bipole trickle-bed electrochemical reactor for the generation of alkaline peroxide

This paper reports work on the scale-up of a perforated bipole trickle-bed electrochemical reactor for the electro-synthesis of alkaline peroxide. The reactor uses a relatively simple cell configuration in which a single electrolyte flows with oxygen gas in a flow-by graphite felt cathode, sandwiched between a microporous polyolefin diaphragm and a nickel mesh/perforated Grafoil anode/bipole. Both one and two-cell reactors are scaled-up from cathode dimensions 120 mm high by 25 mm wide and 3.2 mm thick (reactor-A) to 630 mm high by 40 mm wide and 3.2 mm thick (reactor-B). The scale-up is achieved by the use of constrictions that prevent segregation of the 2-phase flow in the larger cell, combined with switching from a polypropylene to a polyethylene diaphragm with improved transport properties and raising the electrolyte feed concentration from 1 to 2 M NaOH.For the one-cell reactor-B with a polypropylene diaphragm, operating on a feed of 1 M NaOH and oxygen at 900 kPa(abs)/20 °C, the peroxide current efficiency at a superficial current density of 5 kA m⁻² increases from 27% (un-constricted cathode) to 57% with a constricted cathode. The corresponding current efficiencies at 3–5 kAm⁻² for reactor-A and the constricted reactor-B are respectively 69–64% and 66–57%. Under similar conditions at 3–5 kA m⁻² the one-cell constricted reactor-B with a polyethylene diaphragm gives current efficiencies of 88–64%, and changing to an electrolyte of 2 M NaOH raises this range to 90–80%. At 3–5 kA m⁻² the equivalent two-cell (bipolar) constricted reactor-B shows current efficiencies of 82–74% and at 5 kA m⁻² obtains 0.6 M peroxide in 2 M NaOH with specific energy 6.5 kWh per kg H₂O₂.     

Effect of leakage currents on the primary current distribution in bipolar electrochemical reactors

The primary current distribution in a bipolar electrochemical reactor with outside inlet and outlet electrolyte manifolds was investigated by numerical solution of the Laplace equation and by experimental measurement in simulated cells made from conductive paper and segmented electrodes. The geometric parameters determining the distribution were the interelectrode gap, electrode length, transverse section and length of the electrolyte manifold. The effect of the number of electrodes in the bipolar stack was also analysed. Values obtained numerically have been compared with those obtained experimentally and a good agreement is observed between them. These results are useful for estimating the performance of the bipolar stack.     

Electrochemical investigation of Al–Li/Li<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>FePO<Subscript>4</Subscript> cells in oligo(ethylene glycol) dimethyl ether/LiPF<Subscript>6</Subscript>

1 M LiPF₆ dissolved in oligo(ethylene glycol) dimethyl ether with a molecular weight, 500 g mol⁻¹ (OEGDME500, 1 M LiPF₆), was investigated as an electrolyte in experimental Al–Li/LiFePO₄ cells. More than 60 cycles were achieved using this electrolyte in a Li-ion cell with an Al–Li alloy as an anode sandwiched between two Li _x FePO₄ electrodes (cathodes). Charging efficiencies of 96–100% and energy efficiencies of 86–89% were maintained during 60 cycles at low current densities. A theoretical investigation revealed that the specific energy can be increased up to 15% if conventional LiC₆ anodes are replaced by Al–Li alloy electrodes. The specific energy and the energy density were calculated as a function of the active mass per electrode surface (charge density). The results reveal that for a charge density of 4 mAh cm⁻² about 160 mWh g⁻¹ can be reached with Al–Li/LiFePO₄ batteries. Power limiting diffusion processes are discussed, and the power capability of Al–Li/LiFePO₄ cells was experimentally evaluated using conventional electrolytes.     

Simplified model to predict the effect of the leakage current on primary and secondary current distributions in electrochemical reactors with a bipolar electrode

A simplified mathematical model to calculate the current distributions in bipolar electrochemical reactors is proposed. The current distributions are deduced from a combination of the voltage balance in the reactor with a voltage balance including the electrolyte inlet and outlet. Thus, equations to predict the effect of geometric and operational variables on the current distributions at the electrodes are reported. The parameters acting upon the current distributions were lumped into two dimensionless variables and their effects on the current distributions are discussed. The primary current distributions are obtained as a limiting case. Comparisons between calculated and experimental primary current distributions are reported.     

Modelling of three-dimensional bipolar electrodes with irreversible reactions

The behaviour of an electrochemical reactor with three-dimensional bipolar electrodes for irreversible reactions is analysed. Copper deposition at the cathodic side and oxygen evolution at the anodic one were adopted as test reactions at the bipolar electrode, from an electrolyte solution with a copper concentration lower than 1000 mg dm⁻³, pH 2 and 1 M Na₂SO₄ as supporting electrolyte. A mathematical model considering the leakage current is proposed, which can represent the tendency observed in the experimental data related to cathodic thickness and potential at both ends of the bipolar electrode. High values of leakage current were determined, which restricts the faradaic processes to small thicknesses at both ends of the bipolar electrode. Likewise, the performance of the bipolar electrochemical reactor for the treatment of effluents is experimentally and theoretically examined. In this case, the conversion for copper removal was 90.1% after 480 min of operation with one bipolar electrode and 94.8% after 300 min of operation with two bipolar electrodes at a total current of 3 A.     

Removal of Pb(II) from simulated wastewaters using a stainless-steel wool cathode in a flow-through cell

The investigation of an electrolytic process to remove Pb(II) from simulated wastewaters using a stainless-steel wool (SSW) cathode in a flow-through cell under potentiostatic condition is reported. Voltammetry under hydrodynamic conditions was used to estimate the diffusion coefficient, which was found to be 1.4× 10⁻⁵ cm² s⁻¹ in the supporting electrolyte (0.10 mol l⁻¹ NaNO₃ and 0.10 mol l⁻¹ H₃BO₃, pH 4.8). The performance of the flow-through cell was evaluated for three potentials: −0.70, −0.80 and −0.90 V vs. saturated calomel electrode (SCE). At −0.70 V, the reaction was found not to be completely controlled by mass transfer, while at −0.80 V and −0.90 V the Pb(II) concentration decayed exponentially. At −0.90 V, using a flow rate of 250 l h⁻¹, after a 90-min electrolysis, the Pb(II) concentration decayed from 50 ppm to only 1 ppm, corresponding to a 98% removal.     

Comparative study of chemical and electrochemical Fenton treatment of organic pollutants in wastewater

The technological and economic aspects of using the Fenton process to treat industrial wastewater containing morpholyne and diethylethanolamine, as well as sodium salts of naphthalene sulfonic acid and of ethylenediaminetetraacetic acid based on data obtained in pilot tests are discussed. Chemical Fenton technology was tested using commercial 30–35% solutions of H₂O₂ and iron (II) salts, which was followed by the additional electrochemical destruction of organic pollutants in an undivided reactor with catalytic stable anodes (CSA) and 1 g L⁻¹ NaCl as a supporting electrolyte and a source of active chlorine. An alternative electrochemical method involving the electrogeneration of hydrogen peroxide in polluted water at the gas -diffusion cathode was studied both with the addition of ferrous salt to the electrolyte prior to electrolysis (in-cell electro-Fenton) as well as with the post-electrolysis addition of Fe²⁺ in another reactor (ex-cell electro-Fenton). The accumulation of hydrogen peroxide in concentrations sufficient for the mineralization of organic pollutants was achieved in both cases with near 100% current efficiency. In comparison with wastewater treatment processes which use a purchased hydrogen peroxide reagent, the Fenton-like processes achieved an economic savings of as much as 64.5% in running costs due to the on-site electrochemical generation of H₂O₂. Preparative electrolysis in the membrane reactor showed higher current efficiencies and lower specific energy consumptions for H₂O₂ electrogeneration in comparison with the results of tests carried out in an undivided cell.     

Electrodeposition of CuNiW alloys: thin films,nanostructured multilayers and nanowires

Electrodeposition operating conditions were determined for the deposition of copper–nickel–tungsten alloys and compositionally, multilayered deposits. Multilayered alloys with one layer rich in Cu and the other layer rich in NiW were fabricated as both thin films and nanowires. The electrolyte contained 0.6 M Na₃C₆H₅O₇, 0.2 M Na₂WO₄, 0.3 M NiSO₄ and variable CuSO₄ concentration at a pH of 8 adjusted with ammonium hydroxide at 70 ± 2 °C. The deposit composition and current efficiency were characterized using rotating cylinder electrodes with and without a Hull configuration. Addition of Cu(II) to the electrolyte lowered the tungsten partial current density and hence the W wt% in the deposit. Thin film multilayered alloys, with a modulation in composition, were fabricated with pulse current deposition and conditions to selectively etch one layer was determined with a view towards fabricating nanotemplates. Nanowires with modulated composition were also demonstrated, electrodeposited into alumina nanoporous templates. However, the nanowire deposition was confounded by the formation of oxide during the modulation, and results herein recommend that the potential of the more noble step be more negative than −0.9 V versus SCE to avoid this situation, despite metallic alloy formation in unrecessed electrodes.     

Electrochemical treatment of effluents from adipic acid plants

Electrochemical measurements have been made on the system Cu²⁺-adipic acid-HNO₃ (which models the effluent from adipic acid plants) to investigate the reasons for the observed low current efficiency for copper deposition from such solutions. The most probable cause is a cathodic shift in the deposition potential of copper making the reduction of NO₃^? the preferred process. Depletion experiments have been carried out on real effluent in two three-dimensional cells, a bipolar trickle tower and a porous reticulated carbon bed. Each performs reasonably well and, while the current efficiencies are low (c 20%), the deposition is essentially mass-transfer controlled.     

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.     

Models of Hypochlorite production in electrochemical reactors with plate and porous anodes

Pseudo two-dimensional finite element models were developed to predict the hypochlorite (chloric(I)) (HOCl + OCl⁻) production by electrolysis of near-neutral aqueous sodium chloride solution, in reactors with (a) an anode and cathode in the form of plates, and (b) a lead dioxide-coated graphite felt anode and titanium plate cathode. The model was used to investigate the feasibility of using a porous anode to achieve high single pass conversions in oxidising chloride ions. For the model reactor with planar anode, the effects of diffusion, migration and convection on the mass transport of the reacting species were considered, whereas with the porous anode, a supporting electrolyte (Na₂SO₄) was notionally present to eliminate the migrational contribution to reactant transport. For an electrolyte flow rate of 10⁻⁶ m³ s⁻¹ (Re = 10 for plate electrodes, Re _porous = 0.76 for porous anode), a cell voltage of 3.0 V and an inlet NaCl of 100 mol m⁻³, the single-pass conversion of Cl⁻ was predicted to increase from 0.45 for the reactor with a planar anode to 0.81 for the reactor with a porous anode. For the same operating conditions, the overall current efficiency was also predicted to increase from 0.71 to 0.77 by replacing the plate with the porous anode.     

Electrochemical conversion of 2,3-butanediol to 2-butanone in undivided flow cells: a paired synthesis

A procedure has been developed for converting 2,3-butanediol inca. 10% aqueous solution to 2-butanone by passing it through a porous anode at which it is selectively oxidized to acetoin by electrogenerated NaBrO and then pumping to a porous cathode at which it is reduced to 2-butanone. The not fully optimized yields and current efficiencies are 75% and 60%, respectively. The procedure employs: Pb/Hg or Zn/Hg cathodes, graphite anodes, pHca. 7, ambient temperature, current density of 2 mA cm^–2, five minute residence time outside the cell, packed bed electrodes, and parallel electrolyte and current flow.     

Electrochemical removal of hydrogen sulfide from polluted brines using porous flow through electrodes

Carbon felt was used in porous electrodes to achieve electrochemical oxidation of sulfide ions from flowing chloride brines. Using X-ray photoelectron spectroscopy (XPS) and Energy Dispersive Spectroscopy (EDS), sulfur was identified as the final reaction product under various potentials and temperatures. While some of the resulting sulfur flows out with the electrolyte, the rest remains adsorbed on the graphite surface. The rate of the process and the removal efficiency increase with potential, temperature, flow rate and sulfide concentration. The measured limiting currents are substantially lower than those predicted from mass transfer correlations. This was attributed to the passivating effects of the sulfur deposited on the internal surface of the porous electrode. Potentiostatic current transients show that the carbon felt electrodes have higher capacity for removing sulfide ions than planar electrodes, which is attributed to the large internal surface area of the carbon felt.     

Reverse electrodialysis: evaluation of suitable electrode systems

Reverse electrodialysis (RED) is a method for directly extracting electrical energy from salinity gradients, especially from sea and river water. For the commercial implementation of RED, the electrode system is a key component. In this paper, novel electrode systems for RED were compared with existing systems on safety, health, environment, technical feasibility and economics. Systems with inert DSA-type electrodes and a NaCl–HCl supporting electrolyte with the reversible Fe²⁺/Fe³⁺ redox couple or the [Fe(CN)₆]^4–/Fe(CN)₆]^3– couple achieved the highest ranking. Improvements of the electrode system are also discussed like the use of special stable metal electrodes, graphite electrodes, other reversible redox couples, capacitive electrodes and electrolytes with carbon particles.     

Polythiophene, polyaniline and polypyrrole electrodes modified by electrodeposition of Pt and Pt+Pb for formic acid electrooxidation

The electrosynthesis of polythiophene (PTh), polyaniline (PANI) and polypyrrole (PPy) films modified by dispersion of Pt or Pt+Pb and its employment in the electrocatalytic oxidation of HCOOH are studied and compared. The influence of parameters such as polymer film thickness, the number of dispersed Pt particles, the amount of Pb deposited and the presence of Pb2+ in the electrolyte on the electrooxidation of HCOOH is investigated. Electrode systems including the polymer and a mixture of Pt and Pb particles show a better electrocatalytic activity than electrodes having a polymer–Pt combination or bulk Pt electrodes. Furthermore, during the electrooxidation of HCOOH using polymer–(Pt+Pb) electrodes the presence of fewer poisoning species is observed, indicating that the role of Pb in these electrode systems is in agreement with the Pb adatom effect observed when bulk Pt electrodes are used. However, the presence of Pb(ii) in the electrolyte is not required for the PTh–(Pt+Pb) electrode system and, in addition, a better electrocatalytic effect is obtained in this case. With application of an appropriate E/t program the activity is unchanged over a long time.     

Impedance spectroscopy on porous materials: A general model and application to graphite electrodes of lithium-ion batteries

Electrochemical impedance spectroscopy (EIS) was applied to porous negative graphite electrodes for lithium-ion batteries in the EC:DMC, 1 M LiPF₆ electrolyte. The effect of porosity on the electrode response time was studied and a theoretical model was developed, based on free path of the current lines between subsequent reaction sites. The effect of porosity on the electrode response is evidenced by the impedance spectra in which the high frequency capacitive semicircle is distorted. Fresh electrodes (before the formation of the solid electrolyte interphase, SEI) and cycled electrodes have different shapes of the impedance spectra indicating a change of processes at the surface. In particular, the shape of the spectrum for a fresh electrode can be related to an adsorption process. Impedance spectra of fresh electrodes were fitted using a simple model that considers porosity and the assumed electrochemical processes, giving good agreement between model and data. A correlation was found between adsorption sites and irreversible charge capacity in the first cycle.     

Electrochemical processing of porosity gradients for the production of functionally graded materials

The present paper lays the theoretical foundations of a new production process for functionally graded materials (FGMs). The process is based on the evolution of porosity gradients in porous electrodes which undergo electrochemical dissolution or deposition. The electrodes with graded porosity serve as preforms for the production of graded composites by infiltration processing. A one-dimensional macroscopic model of the porous electrode has been used for the prediction of the porosity gradients. A numerical approach allows utilization of experimentally determined current–potential curves for nonporous electrodes, with the incorporation of changes of the pore structure during the course of the electrode reaction, to predict the porosity gradients. For porous copper cathodes and anodes the results of this model are compared with experimentally observed polarization behavior and porosity distributions for different current densities and electrolyte conductivities.     

Assessment of the Wettability of Porous Electrodes for Lithium-Ion Batteries

The wettability of lithium cobalt oxide (LiCoO₂) and mesocarbon microbead electrodes in nonaqueous electrolyte is analyzed by a mathematical model of capillary liquid movement. Results show that wetting in the LiCoO₂ electrodes is difficult as compared with the MCMB electrodes at the same electrolyte composition. Wetting in the porous electrodes is controlled mainly by electrolyte penetration and spreading in pores. Electrolyte penetration is determined by viscosity. On the other hand, electrolyte spreading is controlled by surface tension. Organic solvent composition and lithium salt concentration may influence the wettability of porous electrodes due to changes in the viscosity and surface tension of the electrolyte. Increasing the amount of EC and/or lithium salts can cause poorer electrolyte spreading and penetration. Furthermore, careful pressure control has a positive effect on increasing the surface area of the solid–liquid interface. AC impedance data show that batteries with vacuuming prior to electrolyte filling may reach a maximum wetting in a few hours. If no vacuuming is applied, a few days are required to obtain sufficient wetting.     

Enhancing the electrical and thermal stability of metallic fiber-filled polymer composites by adding tin–lead alloy

This article presents a novel way of greatly enhancing the electrical and thermal stability of copper fiber (CuF)-filled acrylonitrile–butadiene–styrene (ABS) composites via the incorporation of small amount of tin–lead (Sn–Pb) alloy. It was observed that many fibers are soldered together by Sn–Pb, and a continuous CuF/Sn–Pb network is formed throughout the ABS matrix. As a result, the percolation concentration of ABS/CuF composite containing 1 vol% Sn–Pb is lower than for ABS/CuF composite, and the addition of Sn–Pb to the ABS composites containing 5 vol% CuF leads to a further decrease of electrical resistivity compared to ABS/CuF composites with corresponding filler contents. Furthermore, the electrical resistivity of ABS/CuF/Sn–Pb composite shows no temperature dependence, and remains constant during the thermal post-treatment.