Very early when I first started out in refining (like 13 years ago) is when I first encountered the metastannic acid problem - that was the result of putting some gold plated pins in nitric to get rid of the base metals to recover the gold foils - the pins still had solder on them - which resulted in the formation of the dreaded metastannic acid
At that time I had no idea what the white goo was so I asked about it here on the forum & that is when I first learned about metastanic acid & is when I was told by some of this forums very finest & most experienced refiners that tin is "a refiners nightmare" due to the formation of metastanic acid "anytime" nitric & tin are involved - & yes that included the nitric in AR --- so - I was told by those WAY smarter then me (at the time) that any time you have tin involved (mostly as solder) that you are better/best served to get rid of the tin with HCl before using nitric to dissolve base metals - &/or before dissolving gold in AR
Therefore - for the last 13 year - any time I know I have tin to deal with (mostly as solder) I ALWAYS use HCl to get rid of the tin first - whether the next step is to use nitric to dissolve base metals - or going to AR to dissolve gold (along with base metals)
So -------
Metastannic acid is not produced in AR,
I never had a problem regarding solder in AR. In AR, everything is nicely dissolved into solution without any residues of insoluble tin compounds.
After reading the above quotes I thought - Hmmm - ok -
maybe I am wrong - maybe the formation of metastanic acid by tin in AR is not a concern
So yesterday while I was waiting for my furnace to heat up to melt about 4 kilos of silver cement to make anodes for my silver cell I decided to do an experiment
So I got out my roll of 95% tin 5% antimony solder (used for making my stannous chloride) & cut off a 2 gram piece of it
I then got out a 50 ml beaker & put 6 ml HCl & 2 ml 67% nitric in the beaker ("standard" 3 to 1 AR)
I then dropped in the 2 gram piece of tin solder
There was an immediate reaction of the acid with the solder with no white cloud (metastanic acid) forming "in" the acid & no brown fumes coming off - this lasted for about a minute to about a minute & a half & at first I thought - ok maybe metastannic acid does not form in AR - BUT ----------
Then - after that first minute to minute & half the reaction took off like a rocket with LOTS of boiling/foaming/brown fumes & a very obvious white cloud forming "in" the solution - this reaction only lasted about 30 seconds before dying down - at which time I set the beaker aside & went to work melting my silver
This morning I went to check on the beaker --- there is so much metastanic acid in that beaker that there is no separation of metatastannic acid settled in the bottom of the beaker with solution on top of the metastanic acid
It is just a
"mass" of wet metastannic acid in the beaker
The conclusion of this experiment is that tin in AR can & most certainly
will cause the formation of metastannic acid
Now then - with that said - it "may" be true that if you are using A LOT of HCl & very little nitric - AND - there is NOT a lot of tin in the first place - you "may" get away with very little (& "maybe" even no) metastannic forming by the tin in such AR (LOTS of HCl - not a lot of tin - & not a lot of nitric)
But there is NO question that metastannic acid can
& will form in AR --- hence my original post (this thread) that tin is a refiners nightmare due to the fact that tin - which has a tendency/probability of forming - whether in just nitric - OR - AR - can
& will follow/contaminate your gold
You are always better/best served to get rid of as much tin as possible with just HCl - before moving on to any other recovery/refining step
This is the advice I was given
by the masters of refining 13 years ago - I have been following that advice ever since & it
has served me well
When there is tin (mostly as solder) I never go to recovery or refining without FIRST removing the tin with good old HCl
It is my insurance that my final PMS will have little
or NO tin contaminating my end results of recovery/refining efforts
For what it is worth
Kurt