In the past, wastewater treatment was mainly a process of removing the contaminants. Today, the focus is increasingly shifting to the recovery of raw materials and useful metals for reuse. The energy from waste plants, including the sludge final processors, also focus on reuse. This mainly concerns the recovery of phosphate as a finite raw material, but there are also opportunities for rare, scarce and heavy metals. By inventorying the contents of the metals in the different material streams, recovery technologies for different metals / metal groups are selected.
The process of recovering metals is based on an electrolysis process. In electrolysis, a liquid is put under a direct electrical voltage using inert electrodes and a voltage source. By applying this voltage to the waste water, electrons are exchanged between the electrodes and the liquid. The electrons can be used to convert metal ions into solid metal. This solid metal deposits on the cathode. The cleaned waste water then leaves the electrolysis cell and can in many cases be reused. The method is best applicable if only one metal contaminant is present in the liquid to be treated in a sufficiently high concentration. With complex mixtures, recovery of metal and water is more difficult.
The main application of electrolysis is in the metal industry for the purification of rinse waters from pickling baths or galvanic baths.
These applications include:
- Recovery of metals and spent electrolytes;
- Detoxification of nitrite and Cr (VI);
- The recovery of silver from fixing baths in the graphics sector.
If electrolysis is not possible with low concentrated solutions, conventional techniques are used to increase the concentration, such as ion exchange or membrane techniques followed by electrolysis of the regenerate.
An additional variation of electrolysis is membrane electrolysis. This technique finds its applications in, among other things, the production of chlorine gas, the surface treatment sector, extending the service life of pickling baths, metal recovery from rinsing baths, etc.
The change from chromium (VI) to chromium (III) is also a recurring application. Residual concentrations of < 0.1 mg/l of chromium (VI) are found here. In order to achieve a high current efficiency (80 to 90%), the pH value should remain less than 3 to 4. Environmental aspects are very important here. The deposit of the metal on the electrode forms the residual material that can possibly be reused. Chemical aids are required for the discharge of the used lye and acid to correct the pH values reached.
MEMOX – Ti/RuO2 electrodes are often used as anode for an electrolysis process.
The market for metal recovery will become increasingly intensive in the future due to the increasing attention to the environment, in order to minimize the waste discharge of metals.