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Cleaner Technology Projects in Denmark 1996

Recovery of Process Bath of Chromic Acid

Recovery of process bath with chromic acid is an important element by introduction of cleaner technology in the electroplating industry and related industries. By using this recycling technique process chemicals will be saved and the amount of chemical waste will be reduced. The preliminary project indicates that this method in some cases also will save money for the industry. At the same time the solution will create possibilities to establish partly closed systems without emission of wastewater and waste.

Chromic acid is the key component in chromatin baths. When metal items of zinc and aluminium are treated in the bath a chemical reaction is taking place on the metal surface where a corrosion resistant layer of chromate is formed. Metal is removed from the surface and this metal - typically zinc and aluminium - will accumulate as metal ions in the process bath as a pollution . At the same time some chromic acid is reduced to chromium(+3). Therefore new chromic acid must be added to the bath. When the concentration of metal ions and chromium(+3) has reached an unacceptable level the bath is discarded. By treating the bath in a membrane electrolysis cell the bath can be reused.

The pollution of a chromium bath and a chromic acid pickling bath is nearly the same. Metals are accumulated in the baths, where also some chromium(+3) is formed. The pickling bath is used for etching of aluminium surfaces. The chromium bath is used for electrolytic depositing of bright chromium layers on items of steel, copper and brass.

Lab-tests have been carried out on the cleaning and regeneration of chromatin baths and chromic acid pickling baths, because no practical experiences were available. The test has produced valuable results and knowledge about the possibilities of this recycling method.

There have also been made investigations of 2 full-scale plants - one for recycling of a bright chromium bath and one for recycling of a yellow chromatin bath for zinc. These investigations show that the method does work, but considerable amounts of chromium are lost by the treatment dependent of the degree of pollution.

Yellow chromatin baths for zinc normally contain a lot of chloride which is oxidised by the anode in the electrolysis cell. Hereby a heavy chlorine emission is taking place, and this problem must be solved before the method can be used in practice. Chromatin baths with chloride should be treated batchwise, and effective ventilation from the cell is needed. The bath should not be kept in the production area during treatment, because this will cause chlorine gas in the production hall

Chlorine problems can be solved by using chloride free chromatin chemicals with nitric acid instead of hydrochloric acid. After all there is a long tradition for using hydrochloric acid to obtain a special yellow appearance and a good corrosive resistance of the treated surface. Tests have shown that this can be done using nitric acid, too.

Even if the project has exposed some problems by using membrane electrolysis for regeneration of process baths based on chromic acid we have seen that the equipment can function. It should after all be possible to optimise the equipment and system. Alternative catholytes should be tested and oxidation of chromium(+3) should be improved. Furthermore the operation routines must be optimised and systematised to obtain a complete and quicker removal of metals from the bath.

The preliminary project disclosed that foreign suppliers only have experience of membrane electrolysis for treatment of bright chromium baths. Nobody has tried to regenerate chromatin baths and chromic acid pickling baths for zinc and aluminium. This project has clearly demonstrated that these baths also can be regenerated. The first full-scale plant has already been established by Astral Galvano for regeneration of a yellow chromatin bath for zinc. With this cell 2/3 of the chromatin chemicals are recovered instead of sending the discarded baths to Kommunekemi.

By chromatin most of the process chemicals are lost to the wastewater, and the plan for the future must be to recover these chemicals too. This may be possible by a combination of counter current rinse and nano-filtration in a closed loop. Hereby we obtain a concentrate, which can be treated by membrane electrolysis and sent back into the process bath. It could be a future cleaner technology solution, which is more complete than just treating the discarded baths.

Author/ institution

Flemming Dahl og Peter Hardis, MILJØ-KEMI, Dansk Miljøcenter A/S

This report is subsidised by the National Council for Recycling and Cleaner Production

ISSN no. 0908-9195
ISBN no. 87-7810-562-5

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