There are many guides for making copper chloride PCB etchant, but very few on reversing the process and recovering the chemicals. Some guides advocate disposal of the spent etchant by neutralizing with baking soda and filtering off the copper carbonate. While this does work, you don't get back the acid you put into the etchant originally.
In the above video we fully recycle copper chloride etchant back into the hydrochloric acid and copper that went into it.
First the spent copper chloride and hydrochloric acid etchant is filtered to remove any insoluble particles. If the solution contains greater than 20% by mass of hydrogen chloride then it's a good idea to dilute so that during distillation the hydrogen chloride is not lost to the atmosphere. (Usually most solutions contain less than 20% so this is not an issue).
The solution is distilled to first remove water and then to recover the free hydrochloric acid. The distillation continues until the residue is dry.
Now the residue is recovered and weighed. For every 1g of copper chloride residue 7 mL of water is added followed by 1 mL of sulfuric acid (98%). The solution is thoroughly mixed. What we're trying to do here is convert the copper chloride into copper sulfate and hydrochloric acid.
If there is insoluble white copper (I) chloride then air is bubbled in to oxidize it. This process can be accelerated by heating the solution to 60 degrees Celsius with stirring. This oxidizes the insoluble copper(I) chloride to soluble copper (II) chloride.
After everything has been dissolved. The solution is again distilled to recover the remaining hydrochloric acid and fully convert the copper chloride into copper sulfate. The distillation proceeds until dry.
Now for every 1 mL of sulfuric acid originally added 4 mL of water is added to the copper sulfate residue. The solution of copper sulfate is electrolyzed with a platinum anode and copper cathode. The current density used is 100 ma per square centimeter of platinum. At this point, the copper is plated into the cathode and the copper sulfate is slowly converted to sulfuric acid. The electrolysis is finished when the solution is clear. The sulfuric acid solution may be purified by distillation or save for use in future etchant recycling runs.
The hydrochloric acid and copper have now been recovered.
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