Method diagram
Method and installation description
Electrolysis is a process which, under the influence of an electrical current, involves effective electron transfer taking place in an electrolyte, whereby chemical changes are induced. During the process, a liquid is subjected to electrical current using inert electrodes and an electrical source. By subjecting wastewater to this current, electrons are exchanged between the electrodes and the liquid. The electrons can be used to convert metal ions in solid metal. This solid metal leaves deposits on the cathode. The purified wastewater then leaves the electrolysis cell and can, in many cases, be re-used. The method is most appropriate when only one metal pollutant is present in a sufficiently high concentration in the to-be-treated liquid.
The technique is also used for chemical oxidation/reduction and precipitation applications (see electro-coagulation file).
The efficiency of the technique can be improved by separating the cathode and anode areas using a membrane. The membrane consists of polymer with a high charge density. The anion or cation can be made selective by altering the membrane’s charge. This separation allows the conditions surrounding the electrodes to be individually optimised (e.g. pH correction), whereby after-effects are avoided and yields are increased.
Specific advantages and disadvantages
The advantage of this technique is that purified water as well as deposited metals can be re-used. In addition to metals, organic components can also be degraded using oxidation and reduction to realise an exchange of electrons. When re-using metals, only 1 metal pollutant must be present in a sufficiently high concentration. Recuperation of metal and water is difficult in complex mixes.
Because the used 'inert’ anodes partially dissolve, this reduces the quality of recuperated metals. This can be overcome by using a graphite anode. Energy use is high when low end concentrations are required. Hydrogen gas and chlorine gas may form on the electrodes in the presence of (hydrochloric) acid.
Application
The main application of electrolysis is found in the metal industry for the purification of rinse waters originating from pickling tanks or galvanic baths.
The applications include:
- Recuperation of metals and used electrolytes;
- Disintoxication of nitrite and Cr (VI) (also see disintoxication file);
- Silver recuperation from fixing baths in the graphics sector;
If heavy metals are leached in leachate waters, this technique is also suitable to purify such waters.
If electrolysis is not possible for solvents with low concentrations, then conventional techniques are implemented to increase the concentration – such as ion exchange or membrane techniques followed by electrolysation of the recuperated material.
Membrane electrolysis is a variation to electrolysis. This technique is used in the creation of chlorine gas, in the surface treatment sector, to extend the redress life of pickling tanks, in recuperating EDTA, etc.
Boundary conditions
The ideal pH is determined by the application. Side-reactions such as gas-forming or forming of HCN in cyanide must be prevented using pH correction.
The retention period in the electrolysis cell is between 5 and 30 minutes.
Effectiveness
The purification yield is normally very high and, for main purification, end concentrations less than 0.1 mg/l are possible. However, in order to realise these concentrations, special reactors must be implemented that have a large specific surface and/or improved mass transfer.
When recuperating Pd and Ni from palladium-nickel-alloy baths, the concentration of both metals is reduced to 20 mg/l in both cases. The remaining concentration of 20 mg/l can be recuperated using other techniques, such as ion exchange.
Another example is the disintoxication of cyanides from galvanic baths. During this application, residual concentrations of CN of < 1 mg/l can be achieved in starting concentrations of, for example, 5 g/l. The remaining concentration can then be further reduced using ion exchange.
Another important application is disintoxication of electrochemical chrome (VI). This involves using an electrolysis cell where a reduction of chrome(VI) to chrome(III) takes place. In this application, residual concentrations of < 0.1 mg/l of chrome (VI) can be realised. In this case, the pH conditions are of great importance. In order to realise a high current efficiency (80 to 90 %), the pH value must remain less than 3 or 4.
Support products
Leach solution is used to prevent hydrogen gas forming from H+-ions. Acid can be used for pH modification to prevent metals precipitating as hydroxide before they deposit on the electrodes.
Environmental issues
Residual substances are formed when a metal deposits on the electrode. This deposit can possibly be re-used. Leach solution and acid are chemical products for which pH correction may be necessary before they are discharged.
Costs
The cost of an electrolysis installation is greatly determined by the type of electrodes that are used. A specific case has been provided to demonstrate this:
In a small plant for the recuperation of palladium from drag-out rinse tanks, 26 g/u Pd is recuperated.. The costs for required electricity amount to 0.75 € per kg/Pd. Investment costs amount to 93 € per anode and 13 € per cathode, whereby 8 anodes and 7 cathodes are needed for each electrolysis cell. The anode is a Ti/RuO2 electrode and the cathode is a three dimensional electrode. The investment cost for the cell with electrodes is €3000 and a rectifier for the voltage amounts to approximately €1000.
Comments
none
Complexity
The process is fairly complex. For each metal or organic component that needs to be recuperated, one must be aware of the possible oxidation/reduction reactions. The type of electrode is also important.
Level of automation
The level of automation for electrolysis is primarily determined by the need for factors such as pH-modification and in-line measurement equipment for analysing metal ions and anions.
References
- Baeyens J., Hosten L. and Van Vaerenbergh E., Wastewater purification, Environment Foundation - Kluwer Editorial, 1995
- Frenzel I. et al., Journal of Membrane Science, 261, 49-57, 2005
- Gylien O. et al., Journal of Hazardous Materials, B116, 119–124, 2004
- Tsai, T. H., Separation and Purification Technology, 68, 24-29, 2009
- VITO-SCT, revision of technical files WASS, 2009
Version February 2010