Smb

[Binhaze]

I’ve done the whole process completely my stannous confirms my solution is pregnant but when I dropped the smb nothing really comes from it and I even made some lead free silver tin Sauter stannous chloride to just test again and just like before goes black instantly with my solution just as before but the smb is bringing nothing

 

[4metals]

How much material was digested and how much Gold do you expect? How much aqua regia did you use? How much nitric was added? Without this type of information it is difficult to give you much useful information.

 

[Yggdrasil]

I’ve done the whole process completely my stannous confirms my solution is pregnant but when I dropped the smb nothing really comes from it and I even made some lead free silver tin Sauter stannous chloride to just test again and just like before goes black instantly with my solution just as before but the smb is bringing nothing

Welcome.
Looking forward to your answers to 4metals.

 

[Marcel]

@4metals I am not the SMB dropping expert since I rarely had problems with SMB, but how about adjusting PH, adding ice to cool down the solution and get a close-to-complete precipation ? We rarely talk about this here in recent times.

@Binhaze Maybe using copperas could help if nothing else works out.
Worst case: Drop your PMs with zinc and start over again.

 

[100tific]

When gold is dissolved in aqua regia , the main reaction is:
Au+4Cl−+NO3−+4H+→AuCl4−+NO+2H2O
When sodium metabisulfite is added, it hydrolyzes in aqueous solution and releases sulfur dioxide (SO₂), which acts as a reducing agent:
SO2+2H2O⇌HSO3−+H3O+
SO₂ reduces the Au³⁺ present in AuCl4−, promoting the formation of metallic gold:
AuCl4−+3SO2+3H2O→Au(s)+3H2SO4+4Cl−
However, if there are too many Cl⁻ ions in solution, that is, if the solution is very concentrated, undiluted, the equilibrium shifts towards the formation of more stable complexes, preventing the reduction and precipitation of gold. This last reaction would be shifted to the left.

At high Cl⁻ concentrations, the stability constant of the AuCl⁻ complex increases, making it more difficult for metabisulfite to reduce it.
This is corrected by diluting the solution before adding the MBS.

 

[Yggdrasil]

When gold is dissolved in aqua regia , the main reaction is:
Au+4Cl−+NO3−+4H+→AuCl4−+NO+2H2O
When sodium metabisulfite is added, it hydrolyzes in aqueous solution and releases sulfur dioxide (SO₂), which acts as a reducing agent:
SO2+2H2O⇌HSO3−+H3O+
SO₂ reduces the Au³⁺ present in AuCl4−, promoting the formation of metallic gold:
AuCl4−+3SO2+3H2O→Au(s)+3H2SO4+4Cl−
However, if there are too many Cl⁻ ions in solution, that is, if the solution is very concentrated, undiluted, the equilibrium shifts towards the formation of more stable complexes, preventing the reduction and precipitation of gold. This last reaction would be shifted to the left.

At high Cl⁻ concentrations, the stability constant of the AuCl⁻ complex increases, making it more difficult for metabisulfite to reduce it.
This is corrected by diluting the solution before adding the MBS.

The overwhelming cases of trouble dropping the Gold here on the forum seem to be in two categories.

1. Dirty solutions containing too much base metals, which if you dilute it too much will give a very hard to drop precipitate.
2. Or way too much Nitric/NOx/Nitrates.

And of course too high pH which will prevent the formation of SO₂.

The best precipitation is often achieved with concentrated clean solution which is very quick and complete.

The only times I dilute, is when I expect Silver in the solution, in which case I dilute and filter it Cold prior to dropping the Gold.

 

[haveagojoe]

Sometimes you just have to wait. If you leave it overnight you might find it has dropped by morning. This seems to happen when it's quite dilute, and the gold comes out as very fine powder.

 

[Yggdrasil]

One can also have a gradual drop.
It seems to drop all by the addition of SMB and you siphon the solution to a new beaker. The next day you will find it has dropped even more.
Of course if you have a dirty solution you may never see this.

 

[4metals]

One thing that troubles me about threads like this is their potential to pollute the forum. Or put another way, fill the forum with threads full of assumptions because we do not know enough from the original poster's question. That is why I asked about how much gold he expected and how much acid he used. without those answers we cannot really be helpful.

Maybe in the future we can wait for details before we offer options.

That said,

I am not the SMB dropping expert since I rarely had problems with SMB, but how about adjusting PH, adding ice to cool down the solution and get a close-to-complete precipation ? We rarely talk about this here in recent times.

I never adjusted pH before dropping the gold, I used both metabisulfite and SO₂ gas. My aqua regia lots were either from karat gold scrap or slimes from copper electrolytic cells so they always had at least Gold and Silver. For this reason I always added ice before filtering. This accomplished 2 things, first it decreased the solubility of Silver Chloride in the acid allowing more complete filtration and second temperature effects the solubility of gasses in solution and cooler acid will hold more SO₂ in solution than warm acid. Third, even though I only said two, it also melts and dilutes the acid with water which @100tific correctly pointed out as beneficial.

At high Cl⁻ concentrations, the stability constant of the AuCl⁻ complex increases, making it more difficult for metabisulfite to reduce it.
This is corrected by diluting the solution before adding the MBS.

My fear on this thread is the OP put 100ish grams of plated pins into acid and is looking for maybe 200 mg of gold while he used too much nitric acid. Without any details, all we can say is conjecture.

 

[SRM]

At the risk of 4metals ire.....

I assume you are using Bonide Stump Out. Bonide makes two similar products, Stump Out which is granular SMB and Stump Out Stump and Vine which is a liquid organic. Just checking to be sure you are using the correct product.

 

[4metals]

At the risk of 4metals ire.....

Pointing out that the OP possibly used the wrong product in error is actually helpful.