ELECTROCHEMICAL REGENERATION OF CHROMATE SOLUTION.
Chromate solutions used for the treatment of metals and alloys such as Zn, Cd, Cu, Al, brass, bronze, etc. can be regenerated by means of the membrane electrolysis. In the course of the process metal ions (Zn, etc.) are removed from the solution, initial pH value is restored and trivalent chromium is reoxidized back to chromate. Due to the regeneration periodic dumping of chromate solution is eliminated. For example at a big job-shop in Frankfurt, Germany ( « Assmus Metallveredelung GMBH») 500 gal (2000 l) of silver-containing black zinc chromate solution is used for already 15 years with continuous regeneration ( before it had been dumped and made fresh portions every 5 weeks). Similar solution is continuously regenerated for 12 years at «Gatto Industrial Platers Inc.» plating shop (Chicago, USA).
The process has following advantages:
- Consumption of chromic acid (dichromates) and other components of the chromate solution is considerably reduced.
- Quality of chromate conversion coatings is maintained constant due to continuous regeneration.
- There is no need to increase continuously the concentration of chromate which is inavoidable in normal operation without regeneration.
- Drag-out from the chromating tank brings much less chromium, zinc, etc. into rising tanks due to lower concentrations of these ions in the chromationg solution.
- The latter fact and elimination of dumping drastically reduces the amount of solid wastes and the consumption of chemicals used for waste treatment.
- Regeneration runs round-the-clock and needs very little care.
It takes only few days to build a regeneration module from PP or PVC by welding. This module is immersed into any suitable tank with the chromate solution to form in this way a regeneration cell connected by a circulation system to the chromating tank ( see Fig.1)
The module can also be placed directly into the chromating tank close to one of its walls since it is only 5 to 8 in (15-20 cm) thick (see Fig.2). Both systems are successfully used in industry. Relatively low currents (usually 50 to 200 A) and voltages (below 12 V) are needed for the regeneration.
Cell design, material and the size of the electrodes and membranes, current value and other parameters are determined in the course of tests made in a portable pilot-scale regeneration cell with real solutions at the customer’s plating shop. It takes usually 5 days for a single type of chromate solution to complete regeneration pilot-scale tests, calculations and preparation of drawings of the module.
If you are interesting in further information and in the implementation of the process at your company, contact please Dr. Sergey S. Kruglikov.
Tel: +7-916-616-9699,
Fax: +7 (495) 609-2964,
E-mail: skruglikov@mail.ru
Fig. 1. Separate regeneration unit.
1, process tank with chromate-based solution;
2, circulation pump;
3, regenerated solution returning into process tank;
4, anodic compartment of the regeneration cell;
5, anode;
6, cation – exchange membrane;
7, cathode;
8, cathodic compartment;
9, catholyte (auxiliary solution);
10, catholyte container;
11, regeneration cell,
12, circulation pump.
Fig. 2. Regeneration directly in the process tank.
1, immersed electrochemical module (cathodic half-cell);
2, cathode;
3, anode;
4, cation-exchange membrane;
5, process tank with chromate-based solution.
