ammonium sulfate etching process

I am currently building a home PCB etching tank based on the ammonia/ammonium sulfate chemistry. The etching system consists of a etching tank, and an electrolytic copper recovery tank. The result is a infinitely replenishable etching system without requiring any addition of ingredients. Elo-Chem is the only commercial etching system I have found that works off this principle. Several patents describe some information about the etching process but nothing on the desing of the electrolytic cell. I'd be very interested in any information about this process. I'm particularly interested knowing what anode materials are suitable. My tests showed a solution composed of 60 g/l of dissolved copper (as copper(II) tetrammine sulfate), 0.5M ammonium sulfate and at pH 9.0 gave an etching rate of around 12 minutes for 35um copper at 28 deg.C in air bubbled solution. This etching rate is quite adequate for hobby work.

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