Electroplating solutions, purification

Other industrial applications of electrolysis include extraction/purification of metals from ores, electroplating, and the manufacture of certain chemicals such as sodium hydroxide. In the latter, sodium chloride solution when electrolysed is converted to sodium hydroxide to produce chlorine at the anode and hydrogen at the cathode. Both of these gaseous by-products are collected for industrial use chlorine is used in the production of bleach and PVC hydrogen is used as a fuel, to saturate fats, and to make ammonia. 

However, it can be assumed for most electrochemical applications of ionic liquids, especially for electroplating, that suitable regeneration procedures can be found. This is first, because transfer of several regeneration options that have been established for aqueous solutions should be possible, allowing regeneration and reuse of ionic liquid based electrolytes. Secondly, for purification of fiesh ionic liquids on the laboratory scale a number of methods, such as distillation, recrystallization, extraction, membrane filtration, batch adsorption and semi-continuous adsorption in a chromatography column, have already been tested. The recovery of ionic liquids from rinse or washing water, e.g. by nanofiltration, can also be an important issue. 

The purification of electroplating baths can be carried out using a batch process or a continuous process. In the batch process, the bath solution is treated with activated carbon for 25 to 50 min and then filtered. It is ensured that it is clear and does not contain any activated carbon. The clear filtrate to be used again is the bath. The batch process can also be carried out by filtering the bath solution through a fabric coated with a suitable activated carbon. The frequency of the purification depends upon the operation of the electroplating bath, and the amount of carbon dose depends upon the degree of contamination of the bath, but usually it is 0.5 to 1.3 kg of activated carbon for each 100 liter of the bath solution. 

After pretreatment by roasting to form metal oxides, the next general purification step for many metals is to chemically treat the metal oxides by using aqueous solutions to leach out the metal. The chemical solution selected for treatment depends on the metal oxide. As examples, zinc oxide will dissolve into an acidic solution, copper oxide forms a water-soluble complex ion when mixed with ammonia and ammonium chloride in water, and tin oxides can be dissolved in fluoroboric acid. The water solutions can then be reacted to obtain specific metal salts, or electroplating techniques can be used to plate out the desired metal. Plating solution waste can either be destroyed or reclaimed. 

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