Regeneration of chromic

Regeneration of chromic-sulfuric acid mixtures from wax bleaching processes... 

Since 1927 chromic acid has been used for the oxidative bleaching of Montan waxes. Hoechst has been operating the electrochemical regeneration of chromic acid for more than 90 years. The newly developed electrochemical process (as enlargement and partial replacement for the existing plant) is based on the... 

The revival of spent chromic acid is closely connected with these oxidations, since it has been the practice, for some time, to regenerate the chromic acid after use by electrolysis6 —... 

The first regeneration of the strong base anion exchange resin with a near stoichiometric quantity of sodium hydroxide converts the loaded dichromate form of the resin to the chromate form whereafter it is able to efficiently take up chromic acid again. The anion column effiuent of sodium chromate may be cation exchanged across a strong acid resin in the hydrogen form to recover chromic acid ... 

Chromic acid solutions can also be produced from acid chromium(III) solutions anodically (regeneration of chromium(III) solutions obtained from chromic acid solutions employed in oxidation processes). 

H SO CrC (usually around 1 100) and an organic acid is used as catholyte, the small amounts of organic anion which do migrate across the membrane are oxidized by the chromic acid. An additional problem in the regeneration of chrome plating solutions is removal of low concentrations of cations such as Fe3+, Cu2+, Ni2" ". 

Maintenance of chromium plating baths causes ecological problems that can be solved by electrochemical methods, especially by using membrane electrolysers (i.e. electrodialysis) [39-41]. In these processes, chromium(III) is oxidized at the anode, usually made of lead, to chromium(VI), whereby chromic acid is regenerated. The chromic acid consumed in the plating process is then replaced by adding CrO in the solid form. 

These etchants for solder- and tin-plated boards were preferred for many years. More recently, their use has been completely eliminated due to Cr(VI) listing as a critical environmental hazard. Other problems with chromic-sulfuric etchants are difficulty in regeneration, inconsistent etch rate, the low limit of dissolved copper (4 to 6 oz/gal), and dangerous degradation of PVC and polypropylene equipment. Chromic add etchant is suitable for use with solder, tin/nickel, gold, screened vinyl lacquer, and dry or liquid film photoresists. Chromic-sulfuric mixtures etch copper slowly, and additives are needed to increase the etch rate. For example, sodium sulfate and iodine have been used for rate increase. Alkaline etchants have become so well controlled that there is no justification for the risks and costs of chromic acid formulations. 

If iodide is present and the molecular ratio iodide chloride exceeds 1 15, the chromyl chloride formation is prevented completely in favor of the liberation of chlorine. This result is due to the production of iodic acid from iodide and chromic acid. In the presence of concentrated sulfuric acid, and especially on warming, the iodic acid liberates chlorine from chlorides with regeneration of iodide. 

---