While I was typing with my one finger, Pantherlikher, beat me to the answer, it took me so long to type this I will post it anyway.
Copper would not cement back out onto the pins.
Read up on the document of this leach to get a good understanding of how it works (Laser Steve has a copy of the document on his website).
Cementing, is a reduction / oxidation process, a chemical replacement reaction, where we use a metal lower in the reactivity series to replace a metal in solution.
Oxidation is:
Gain of oxygen
Loss of hydrogen
Loss of electrons
Reduction is:
Loss of oxygen
Gain of hydrogen
Gain of electrons
There is no oxidation without reduction, for something to be oxidized something else has to be reduced, and like-wise we cannot reduce something without the oxidation of something else.
so we cannot reduce copper ions in solution back to metal unless something else is oxidized in the reaction.
When we dissolve a metal into solution we are making metal ions (or metal salts dissolved in solution).
Metal + acid = salt of that metal and acid
HCl alone will not dissolve copper, but it will attack copper oxide, so by adding air, or oxygen, in our case H2O2 to the acidic solution we can oxidize the copper to a copper oxide that the acid can attack.
copper is oxidized, H2O2 is reduced.
Cu + H2O2 --> CuO + H2O + 1/2 O2
(oops)
oxidized copper and acid forms copper II Chloride and water (copper ions in solution)
CuO + 2HCl --> CuCl2 + H2O
overall reaction
2HCl + Cu + H2O2 --> CuCl2 + 2H2O
The above reaction put copper into solution as copper II Chloride.
Adding elemental copper metal to this CuCl2 solution will dissolve more copper (oxidize more copper into solution as the CuCl2 is reduced to Copper I Chloride (CuCl). Here the elemental copper gives up electrons and copper II chloride gains electrons.
CuCl2 + Cu --> 2CuCl
We see above adding copper metal to the CuCl2 solution will not reduce the copper from solution back into copper metal. But adding the copper metal will reduce the copper II chloride (green solution) to copper I chloride (brown solution with a precipitant of white copper I chloride powders).
Adding copper metal to copper I chloride will produce no reaction, we cannot oxidize the copper metal, and cannot reduce the copper I chloride in the reaction.
But we can regenerate the copper I chloride solution by adding HCl.
CuCl + HCl --> CuCl2 + H
putting the white powder of copper I chloride back into solution ready to dissolve more copper.
So we see here the copper ions from solution will not plate back out of the solution onto our copper pins, although we can form a copper I chloride.
To be able reduce the copper ions in solution back to copper metal we would need some other electro-chemical reaction, for example a metal lower in the reactivity series of metals.
Do a study on the reactivity series of metals this study is very helpful in recovery and refining of metals.
Here we can use elemental Iron (or another metal higher in the reactivity series than copper to give the copper ions back its missing electrons, to reduce the copper from solution (cement copper from solution), back into copper metal, here the copper is reduced to metal, as the iron is oxidized (dissolved into solution as metal ions), with iron giving up it electrons to the copper, copper gaining these electrons goes from being a copper ion missing electrons to copper metal with all of the electrons in the atoms, these copper atoms join together to form copper metal powder falling to the bottom of the vessel, while some of the copper atoms will plate out onto the undissolved iron until it is completely consumed in the reaction. Now we are back to our elemental copper with all of its electrons in its atoms, and Iron ions in solution missing electrons in its atoms.
We could also reduce iron from solution with a metal higher in the reactivity series than iron...
CuCl2 + Fe --> FeCl2 + Cu
(Reactions simplified for discussion).
Heating the solution can help it react a little faster, but you do not want to heat it too much if you do heat it too much you just waste the oxygen and acid needed in solution.
The reaction is slow cold, and faster if warmed.
Too much heat drives off the needed oxygen, gases in solutions are driven out with heat, while cold solutions retain gases much better.
Heating can concentrate solutions, once the free acid is concentrated to its azeotrope you will drive off your free acid as HCl gas changing the hydrochloric acid into hydrogen chloride gas, wasting your acids.
Keep studying, read the document on Steve's web site and study the reactivity series of metals, keep reading the forum, all of this will become more clear and you will gain a better understanding the more you study.
