About 6 hours I let it cement out in copper until no reaction is seen happening. I’m finding even cementing with copper and leaving it over night I still can get silver chloride out of the solution with HCL.
Cementation on copper can be complete, that is for sure. You just need two things:
1. good surface area of copper - eg some copper plate is the best, or thick spirals from heavy duty breakers. Wire isn´t very good.
2. MIXING. If the solution don´t mix, still pregnant silver solution is locked up in the silver cement fallen to the bottom of the beaker/vessel. Frequent disturbance of the silver deposit and good mixing is essential to drive this to completion.
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I never had any problems regarding completeness of cementation. Small batches (under 200 g) can be done in bigger beaker with thick magnetic stirbar on magnetic stirplate. However, more silver then cause problems as stirbar become overwhelmed and stop working. Overhead stirrer, or modified drill can serve nicely, with stirring paddle from some plastic. It is beneficial to drop heavy copper plate to the bottom and stir the solution above it with mechanical stirrer. This way, you can be done in less than one hour of operation. Some guys done the opposite - modified the cast copper heavy plate that way it can be attached to the drill, and then stirred the solution with this copper paddle
Altough I knew about this, I most of the times went with plastic bucket, hanging thick copper plate, glass stirrod and patience
Stirring the deposit thoroughly every 15 or so minutes (I was doing my work alongside this in a lab), and about 5 hours later, it was done. I always tested directly the solution in the bucket with one drop of dilute NaCl solution to see if there is any silver left, when I did not see more silver cementing on the copper plate. Fish the copper out and spray some water onto it to see if something fall from it - and if the dripping water is greyish - indicating something detached from it.
