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This article reviews electrochemical processes and devices that can contribute to a cleaner environment. Electrochemical processes for treatment of waste water.
Table of contents

Browse All Figures Return to Figure. Previous Figure Next Figure. Email or Customer ID. Forgot password? Old Password. New Password. Password Changed Successfully Your password has been changed. Returning user. C1- from the anode compartment figure 3a or 0, from the cathode one figure 3b b prevention of redox shuttles due to e.

C1- is excluded from the anode compartment to avoid anode corrosion or Cl2 evolution. Metal powder is prevented from contacting the anode where rapid redissolution would occur. Figure 3. Examples of divided cells for metal-ion removal showing the importance of an ion permeable separator. In all cases, the cathode process is metal deposition. In certain cases, the electrode materials and conditions may result in a spontaneous reaction i. Recently, however, controlled electrolyte flow and electrode dispersion conditions have resulted in significant improvements as in the case of the Actimag process which utilises a pulsed magnetic field [ Another example of a galvanic cell reaction is provided by open circuit corrosion of the metal deposit.

Freshly deposited and particularly finely- divided metals are more active than their bulk, compact counter parts. Corrosion of the mixed electrode deposit may ensue if the cathode surface is left under open circuit conditions; metal dissolution is balanced via reduction of species such as dissolved oxygen, protons o r higher oxidation states of transition metal ions.

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The possible role of electrochemical techniques has been outlined and illustrated by typical electrochemical reactions. In practice, the choice of reactor design and the reaction conditions are extremely important. In the present case, the following guidelines [27] apply: i Moderate costs: Capital costs may be minimised by choosing a simple cell design which utilises regular components and offers a modular approach to scaleup.

It must also facilitate maintenance as well as being safe and reliable. In many process environments, the reactor may have to operate for long periods without attention. The need for a high and uniform rate of mass transport will often necessitate careful control of the hydrodynamics. The active electrode area per unit reactor volume should be high, particularly when a compact design is needed, or when the desired reaction is restricted t o a low current density. In certain cases, an alternating anodic and cathodic electrode region may be useful in destabilising metal complexes.

These considerations are developed in the following section. Such an approach will minimise costs and will often produce hardware which is attractive to users and easy to maintain. Inevitably, the above considerations often result in conflicting requirements of a reactor design. For example in some cases, a more complex reactor design in terms of its construction may reward the design engineer with enhanced performance o r market advantages table 2.

Hence the electrochemical engineer must exercise considerable skill and experience in order to achieve suitable compromises in the process of cell design or selection. The cathode current density is defined as:. This is particularly true in dilute liquors, where the bulk reactant concentration, cB is low. The situation may be characterised by a mass transport coefficient, k,: where j, is the limiting current density.

The combined parameter kLA is a useful figure of merit and a central driving force in reactor design is the achievement of high values. Idealised current density versus cathode potential curves for copper deposition from uncomplexed acid solution.

The maximum rate of metal-ion removal corresponds t o operation at the limiting current density. Two variants of this approach are shown in figure 6. It is also possible for one of the stages particularly the polishing one to be non electrochemical i. Examples of two-stage reactor strategies for metal-ion removal.

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In the present case, the cathode may be static or moving with 2- or 3-dimensional character as a subdivision table 4 and typical cell designs suggested by this classification have been considered elsewhere [4,5,8, 17, Pump Cell - tumbled bed cell Rotating Cylinder e. Eco-Cell 4. This important classification is illustrated in figure 7 for selected reactors. A classification of electrochemical reactors.. Relatively few of these have survived the rigours of scaleup to a pilot scale and fewer still have proved successful in a competitive marketplace.

In this section, emphasis will be placed upon those reactors which are or have been in commercial use and which have, therefore, been applied to practical problems of metal-ion removal a t a reasonable scale or in moderately large numbers. Due to the size and scope of this subfield of electrochemical engineering, only a profile of certain designs will be given.

Information on suppliers and other organisations is provided in a n appendix. More detailed information is available elsewhere [l, 4, including data on reactor performance [24, Table 5 summarises the design concepts and means of performance enhancement for selected reactors, a subdivision being made according t o the nature of the metal product and its frequency of removal. Table 5 Examples of electrochemical reactors for continuous metal recovery Reactor Organisation Type of cathode Frequency and Cell Performance enhancement n ainly via method of product normally removal divided?

The concentric cylindrical geometry is preferred for some small scale operations. Porous, 3-dimensional electrodes are capable of treating particularly dilute liquors while maintaining a reasonably high fractional conversion and current efficiency. However, care must be taken to avoid potential and flow distribution problems, or 'plugging' by metal.

Electrode movement may be achieved by rotation of cylindrical electrodes the anode or the cathode , or by vibration, impaction, fluidisation or a pulsed magnetic field. Electrolyte movement may involve trickle or more usually flooded flow through a porous matrix o r past a solid surface, air sparging or fluidisation via inert glass beads. Some reactors may be classified into several categories, depending upon the nature of the electrode geometry which may facilitate a choice of metal removal strategy. For example, porous, 3-dimensional carbon electrodes may be leached to produce a metal-ion concentrate or the metal depositlcarbon matrix may be removed, followed by furnace refining in which the substrate is removed as carbon dioxide.

Rotating cylinder electrode reactors may operate in several different modes, depending upon the electrolysis conditions and the nature of the cathode surface.

Environmental Oriented Electrochemistry, Volume 59

For example, possibilities include: a intermittent recovery of smooth, compact metal deposits b intermittent recovery of roughened metal flake or powder c continuous recovery of metal foil or d continuous recovery of metal flake or powder. The majority of reactors involve the direct electrolytic deposition of metal. However, other possibilities include open-circuit cementation of metal or indirect, electrolytic precipitation of metal oxides and hydroxides. A diverse range of process liquors can be treated, in terms of their source and the concentration of metal-ion.

In the following section, the construction and performance of certain reactors developed in the one of the author's laboratories will be briefly considered. In addition, the reactors have shown a creditable performance during the electrolytic treatment of a wide range of metals and electrolytes. Mass transport may be enhanced by the use of tangential manifolds and the use of a reasonable flow rate. An example of an inner concentric cathode is provided by the 'Cyclone Cell' which is shown schematically in figure 8a; this device has been used to extract gold from electroplating rinse waters [31] and silver fom photographic processing liquors The performance of a typical cell is illustrated in figure 8b which shows data for the removal of silver from various photographic fixer solutions under constant current conditions.

The rate of concentration decay is seen to depend both upon the nature of the fixer and the batch processing time. At early times, the rate of concentration decay is fixed i. The results also show the relatively high rate of redissolution of a fresh silver deposit under open-circuit conditions, particularly in the case of the bleach-fix solutions. The current efficiency for silver removal was relatively constant during the early stages of metal removal but it decayed rapidly during the later, mass transport controlled period.

In the case of colour and black and white fixers, the rapid decrease in current efficiency may be partially prevented by potentiostatic operation as described elsewhere [ Despite the drawbacks associated with a rotating drive assembly and associated electrical power brushes, RCE reactors have exploited the following key characteristics i The rate of mass transport is high due to turbulent flow and surface enhancement of microturbulence occurs in the presence of roughened metal deposits.

Concentric cylindrical cathode cell [31, A 20 d m 3 batch of electrolyte is processed at 25OC using a cathode of The continuous extraction of cadmium powder from an acidic sulphate liquor containing adversely high concentrations of zinc has been examined on a pilot plant A scale using the reactor shown in figure 9a and the arrangement in figure 9b.

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As demonstrated in table 7 [, a high rate of cadmium removal was observed if the process conditions were suitably controlled. The results refer to single pass conversion measurements and the assumption of complete mass transport control.

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The cadmium deposits typically had a purity of Table 7 Summary of results for cadmium removal from a zinc sulphate liquor RCE diameter: A pilot scale rotating cylinder electrode reactor system for continuous extraction of cadmium powder from hydrometallurgical process streams A polypropylene cell body; B cell flange; C rotating cathode; D cation exchange membrane; E nickel mesh anode; F catholyte inlet manifold tube; G catholyte outlet manifold; H reciprocating scraper; I removable inspection cover; J reference electrode probe.

Earlier studies [ employed layers of hollow carbon cylinders known as Raschig rings see figure 10a. The relatively low active cathode area per unit reactor volume together with constructional difficulties associated with these packings led to the examination of alternative materials such a s felt, particles, foam and perforated plates A prototype commercial reactor, developed in collaboration with Wilson Process Systems Ltd, figure lob has been used t o remove gold from a n alkaline, cyanide-based electroplating dragout solution under the conditions stated in table 8.

The fluid contacting pattern and potential distribution in bipolar trickle tower reactors has been shown to offer a particularly effective reaction environment for the breakdown of cyanide complexes which facilitates metal-ion removal [42].

The Bipolar Trickle Tower Reactor c Concentration-time behaviour, showing the removal of gold, total cyanide via the Liebig method and free cyanide via an ion selective electrode. The important requirement of design simplicity often dictates the use of a static cathode geometry incorporating porous, 3-dimensional electrode materials One example is reticulated vitreous carbon which has been extensively investigated in our laboratories [ and elsewhere, e.

A reactor comprising of eight identical RVC cathodes figure l l a has been used for cupric ion removal from an acidic sulphate solution. Each of the cathode segments was controlled at a potential of Typical steady state results are shown in figure l l b.

The cupric ion level was systematically reduced from an inlet value of approximately 10 ppm to an outlet concentration of approximately 0. Figure Reticulated vitreous carbon cell [ Reticulated vitreous carbon cell. Catholyte: deaerated 0. The solid line shows the predicted performance based upon a cascade of eight identical PFRs, each having a kLA value of 9.