lazersteve said:
Solid zinc objects have some merit, but it is not free of short comings. Firstly, it is slow to react compared to turnings or powder since it has a low surface area for a given mass.
True that it is slower to react then turning or shaving or powder - but only a "bit" slower --- at least as long as the leach solution is HCl based (AR or HCl/Cl) & not nitric --- zinc is VERY reactive with HCl & large chunks will react "vigorously" even in a HCl solution that has been diluted 2 - 3 times --- so vigorous it will become a rolling boil with the "chunks" moving & bouncing across the bottom of the reaction vessel --- dilute it 4 -5 time it well still be a relatively vigorous reaction with "lots" of bubbling - the chunks just wont move around on the bottom of the vessel
Be prepared to leave your solid zinc hanging for days or even weeks to effect recovery of the PGMs
Simply not true --- I have rarely had to wait more then 4 hours (or less) to do my cementing with chunks of zinc --- 4 - 6 hours "max" & that is in cases of "large" volumes of solution so it simply takes longer to use up the large volume of acid (HCl) that is dissolving the zinc - or if it is "really" diluted
Secondly, some cast zinc items often are not pure zinc and you end up introducing other metals into the reaction that are part of the 'zinc alloy'.
True - but - if you use the electrical conduit connecters I have pictured they are pure zinc --- you can get them "cheap" from most local scrap yards
Lastly, and perhaps the worst trait of cast zinc, it that it forms two types of precipitates. The first is a fluffy 'moss' that floats easily (due to hydrogen bubbles trapped in its structure), but breaks apart into super fine dust, similar to the precipitate from powdered zinc
The fluffy moss is a problem regardless of what zinc (powder, turnings, chunks) you use including (dare I say) the zinc turnings I purchased from you a couple/few years ago
The second form the precipitate takes is a crusty nodular substance on the surface of the cast zinc object.
I have NEVER had this problem - at least not as long as you don't use more zinc then the HCl will dissolve - cementing from a nitric solution (Pd that has been dissolved with nitric) is a different story - you will definitely have this problem cementing from a nitric solution regardless of the zinc you use (powder, turnings, chunks)
That is because although zinc is reactive with both HCl & nitric it is much less reactive with nitric so the slower reaction allows the PGMs to crust on the zinc --- zinc is so reactive with HCl (so much faster) the PGMs don't have time to adhere (crust) on the zinc
This is the problem precipitate as it requires lots of work to remove or you need to completely dissolve the cast zinc scrap to recover it. I have spent a lot of time trying to clean this type of precipitate from the surface of the solid zinc used for PGM precipitations, and it's no fun and never results in 100 percent recovery unless you fully dissolve the cast zinc object which requires more acid and creates more waste.
This is only a problem if you use more zinc then the (HCl) solution will dissolve & it is true regardless of the zinc you use - it just a matter of the size of the PGM incrusted zinc you end up with --- to much zinc powder & you end up with "small" PGM incrusted nodules --- to much zinc turnings = small pieces of PGM incrusted zinc --- to big of a zinc chunk = a chunk (or larger pieces) of PGM incrusted zinc --- in "any" case you will need more acid to rid the zinc
It takes the same "weight" of zinc to cement the PGMs from a solution regardless of the zinc you use (powder, turnings, chunks) so its a question of how much acid you have that determines how much "weight" in zinc you need to do the cementing --- so as an example - if you need a 100 grams of powder &/or turnings - a 100 grams solid chuck(s) will give you the exact same end result of cementing --- so choose your solid piece size accordingly (pic for size example)
Also here is a pic of my zinc barrel - this barrel used to be full to about 6 - 8 inches from the top - so as you can see I have used a lot of zinc chunks for cementing PGMs
It's a matter of personal preference which one you choose.
I totally agree --- I "personally" like using chunks of zinc because (1) it gives me the "same" end results as powder &/or turnings (2) I "never" have to worry about a boil over (3) chunks of zinc from the scrap yard are cheaper then powder or turnings & even "free" if you do any amount of "general" scraping (my barrel of zinc was totally free from back in the days when I did "a lot" more general scrapping)
Kurt