I have at least 500oz of sterling that I have melted into flakes and chunks. It does have small amounts of lead and tin in it from broken out knife handles. What is the most economical and/or easiest way to refine the silver to .999 purity of this quantity? What methods do the large refiners use? I have access to nitric acid and power supplies for a cell.
You have created a problem - you should have done a HCl leach on the silver to get rid of the tin/lead before melting
Then a GOOD washing to get rid of the vast majority of the HCl leach used to rid the tin/lead
Then incineration to rid the traces of chlorides
Then ether dissolve the sterling with nitric - cement with copper to up grade the 925 silver (sterling) to (plus/minus) 998 silver - then melt/pour anodes to run in the silver cell --- or - pour anodes as 925 (sterling) & run direct in cell --- when running 925 direct in the cell you ether have to make your electrolyte with MUCH more silver dissolved in it to prevent co-depositing of copper - or change out the electrolyte more often to prevent copper co-depositing
The third option is to dissolve the 925 in nitric (after removing tin/lead as above) then using the salt/lye/sugar process to get to your 999 silver
But you now have a problem because you did not get rid of the tin/lead BEFORE melting so you now have an alloy of silver/tin/lead
Tin & lead when alloyed with precious metals is what is known as "the refiners nightmare" - & that is because those metals like to follow along with the precious metals in the refining process
In other words - at least
traces of the tin & lead like to stay with the precious metals - somewhere - in the refining process & become a real pain in the butt trying to get rid of them - meaning - somewhere in the process - you need to take extra steps to get rid of the tin/lead to get to you end goal of pure precious metals
In your situation you have several options
1) dissolve the sterling in nitric acid (50/50 nitric/D-water if the nitric is 67 - 70%) this will dissolve the silver & the lead - the tin will end up as metastannic acid which is an ultra fine paste like substance in a state of tin oxides - because the tin paste particles are ultra fine they don't like to settle well & they also tend to go through filters (at least traces) so you will likely end up with traces of tin in your silver/lead nitrate
You can the drop the lead out of your silver/lead nitrate by adding some sulfuric acid which will drop the lead out as lead sulfate which can then be filtered out & it does not take much sulfuric acid to drop out the lead - don't overdo the sulfuric acid --- you can then filter out the lead sulfate
Once the lead sulfate is filtered out you can then cement the silver out with copper & then melt/pour the cemented to anodes to run in your silver cell
The anodes will likely have
traces of tin in them (because of tin problem as described above) but that is ok because when you run the silver anodes in the cell the tin will now stay behind in the anode slims
That is going to make processing the anode slims a real pain in the butt - but - the silver crystals that deposit on the cathode should be pure
2) run the sterling as sterling anodes direct in in the cell - this leaves all of the tin in the anode slims as well as most of the lead ---
some of the lead will go into the electrolyte along with copper as the cell runs - but should not be a problem of co-depositing with the silver on the cathode - unless you run the cell long enough to also start co-depositing copper at the cathode (read my last post about running sterling direct in a cell)
Once you are done running the cell - you can then drop whatever lead went into the electrolyte in the same way as dropping out the lead in option (1) & the silver can then be recovered from the electrolyte
the problem with this option is that the anode slimes are going to have MUCH more tin in them as well as some lead - so processing the slims is going to be even more of a pain in the butt then processing the slimes in option (1)
3) dissolve the sterling with nitric - again you are going to end up with the tin paste - which again you can filter settle most of that out - you just wont get all of it due to poor settling/filtering problem
You can then drop the lead as before (sulfuric) - or - add salt or HCl to your silver/lead chloride - this will drop both the silver & the lead as their chlorides
Lead chloride dissolves in HOT water (silver chloride does not) so you can wash MOST (if not all) of the lead chloride out of silver chloride with LOTS of (multiple) HOT water washes
You can then convert the silver chloride to silver with the lye/sugar part of the silver chloride process & then melt the silver
The silver will likely have traces of tin & possibly "traces" of lead depending on how well you do with getting rid of the lead as described above (sulfuric - or hot water washes)
Because traces of tin are likely to follow the silver chloride you will likely have to still run this silver in the silver cell
The point being - that because you alloyed the silver with the tin & lead - the tin & the lead are now going to be a pain in the butt to get rid of "somewhere in the refining process
Your other option is to sell it direct to an actual refiner - as is --- The refiner I sell my silver to pays me the same price for silver whether it is refined silver - or not refined --- the only reason I refine "some" of my silver is because I have a "few" buyers that buy "small" amounts of some of my refined silver for a bit better price then the refiner I send most of my silver to
Kurt
