I tested solutions made from silver plated parts processed in 2 different ways to deal with base metal:
Batch #1: about 10 grams of plated parts were soaked in roughly half the volume of sulfuric acid required to dissolve that mass (assuming it was 100% copper to calculate the moles required) and warmed slowly for several hours to 100C, then cooled and allowed to react for a week.
Batch #2: same mass of parts was submerged in an excess of HCl with a little copper chloride added, and allowed to sit for a week.
I then washed both with distilled water, making sure to settle and save any powders (silver could be present). I then separated the powders and remaining solid metal/ceramic from both trials and dissolved each of the 4 resulting masses with 50% nitric acid in separate tubes.
After all metal dissolved as completely as possible, I then settled down the solutions, poured the clear solutions into new tubes, and added salt water to drop silver chloride.
Results: The powder resulting from the sulfuric acid step contained all detectable silver from the parts. The remaining base metal fraction had no detectable silver whatsoever. The HCl method did not achieve a separation. Some flakes of silver ended up in the powder, but much was still left on the base metal that remained, and some had converted to silver chloride initially in the HCl, as determined by a some of the white residue turning grey upon exposure to sunlight. The sulfuric acid method cleanly split silver from the remaining base metal, and left it entirely to be found in the powders with not a trace left in the base metal. The HCl method did not accomplish this, and left the silver mixed with other metals, and turned some into a silver chloride initially mixed in with waste gunk and ceramic powder from certain parts. Thus, to recover silver with the HCl method, it is necessary to use far more acid to completely dissolve all base metals. But, some silver is lost as silver chloride mixed into insoluble waste (ceramics, other insoluble metal chlorides, etc). Recovery of this silver would require chemically transforming the silver chloride into silver oxide, and then smelting out the silver from the waste.
Conclusion: sulfuric acid stripping is the superior 1st step method for plated silver. Much less acid is required, and since the silver reforms tiny crystals which fall off remaining base metal in the initial dissolution when allowed to sit, the silver powder can be mechanically separated successfully from the remaining base metal parts by stirring vigorously to dislodge all the silver crystal powder, then simply washing it through a sieve which catches all the larger base metal and ceramic pieces. This powder, while still containing a small amount of copper, is much more highly concentrated in silver, and therefore requires a much smaller amount of nitric acid to dissolve the silver for purification, as compared to the HCl method, and uses only a tiny fraction of the nitric acid which would be required to dissolve all the parts completely. It also avoids the metastannic issue.
The sulfuric acid step must be done outside to clear SO2 gas, combined with a gas capture setup, or performed in a ventilation hood. However, since concentrated HCl also fumes heavily and its vapors are corrosive and damaging to respiration and objects, that process must also be conducted in similar fashion. Therefore, there is no added disadvantage to sulfuric acid in that regard. As a plus to the waste stream, the sulfate salts left over are far less hygroscopic than the chlorides and can be easily dried to powder for disposal. Copper displaces iron just as well in copper sulfate solution as in copper chloride solution.
Batch #1: about 10 grams of plated parts were soaked in roughly half the volume of sulfuric acid required to dissolve that mass (assuming it was 100% copper to calculate the moles required) and warmed slowly for several hours to 100C, then cooled and allowed to react for a week.
Batch #2: same mass of parts was submerged in an excess of HCl with a little copper chloride added, and allowed to sit for a week.
I then washed both with distilled water, making sure to settle and save any powders (silver could be present). I then separated the powders and remaining solid metal/ceramic from both trials and dissolved each of the 4 resulting masses with 50% nitric acid in separate tubes.
After all metal dissolved as completely as possible, I then settled down the solutions, poured the clear solutions into new tubes, and added salt water to drop silver chloride.
Results: The powder resulting from the sulfuric acid step contained all detectable silver from the parts. The remaining base metal fraction had no detectable silver whatsoever. The HCl method did not achieve a separation. Some flakes of silver ended up in the powder, but much was still left on the base metal that remained, and some had converted to silver chloride initially in the HCl, as determined by a some of the white residue turning grey upon exposure to sunlight. The sulfuric acid method cleanly split silver from the remaining base metal, and left it entirely to be found in the powders with not a trace left in the base metal. The HCl method did not accomplish this, and left the silver mixed with other metals, and turned some into a silver chloride initially mixed in with waste gunk and ceramic powder from certain parts. Thus, to recover silver with the HCl method, it is necessary to use far more acid to completely dissolve all base metals. But, some silver is lost as silver chloride mixed into insoluble waste (ceramics, other insoluble metal chlorides, etc). Recovery of this silver would require chemically transforming the silver chloride into silver oxide, and then smelting out the silver from the waste.
Conclusion: sulfuric acid stripping is the superior 1st step method for plated silver. Much less acid is required, and since the silver reforms tiny crystals which fall off remaining base metal in the initial dissolution when allowed to sit, the silver powder can be mechanically separated successfully from the remaining base metal parts by stirring vigorously to dislodge all the silver crystal powder, then simply washing it through a sieve which catches all the larger base metal and ceramic pieces. This powder, while still containing a small amount of copper, is much more highly concentrated in silver, and therefore requires a much smaller amount of nitric acid to dissolve the silver for purification, as compared to the HCl method, and uses only a tiny fraction of the nitric acid which would be required to dissolve all the parts completely. It also avoids the metastannic issue.
The sulfuric acid step must be done outside to clear SO2 gas, combined with a gas capture setup, or performed in a ventilation hood. However, since concentrated HCl also fumes heavily and its vapors are corrosive and damaging to respiration and objects, that process must also be conducted in similar fashion. Therefore, there is no added disadvantage to sulfuric acid in that regard. As a plus to the waste stream, the sulfate salts left over are far less hygroscopic than the chlorides and can be easily dried to powder for disposal. Copper displaces iron just as well in copper sulfate solution as in copper chloride solution.
When I will do this, I let you guys know and post some pictures.
