# How does SMB work?



## rtl326

I have been using Campden tables that I got for a wine making supply store.
Each tablet is 7/10 of a gram of Sodium Metabisulfite. I grind these up with a motar & pestle and dump the powder into AR solution to precipitate gold powder. 

Can someone tell me what is the actual reaction taking place between the SMB and the Auric Chloride?

I am trying to get a handle on how much SMB is too much or not enough.

How does residual HNO3 interfere with the SMB?

Sometimes the SMB makes the solution turn brown like coffee with too much cream and it is very slow to settle out. Is this a function of partice size? how can I adjust things to get faster settling?

I often get gold plating out on the sides of the beaker and bits of gold floating on the top of the liquid even before adding SMB. It really isn't a problem since I just wash the gold off the the glass with freah AR. But can someone tell me why the gold seems so eager to come out of solution? Have I used too little AR?

I know some will ask 'Why AR? Why not HCL/Chlorox?' 
It's because I am trying to follow C.M. Hokes directions as close as possible
while processing gold foils from fingers and PC boards and I am assuming all manner of base metal contaminations.


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## Palladium

Potassium metabisulfite has a molar mass of 222.32 g/mole
Sodium metabisulfite is 190.1 g/mole

So you'll need to use about 17% more (by weight) of the potassium. I usually add about 1.2 grams of smb per gram of gold you expect to recover. Then use stannous chloride to gauge how much more is needed. Residual nitric will eat up your smb by precipitating the gold and then the excess nitric will re dissolve it. Some people eliminate the nitric by evaporation like hoke teaches. Some people use So2. Some people use urea. Or you can keep adding smb until the nitric is gone. If you buy smb off ebay this can become expensive quick. I buy it in 50 lb bags so sometimes it's easier to just use smb and when needed in a process sometimes excess nitric. Excess nitric will also affect a stannous chloride test. Their are many ways to learn the tricks of refining. On a small scale it is more prudent for a new person to learn to control the additions of acid. By doing this it will take less chemicals later (smb) to drop the gold. In the end it equals up when your buying nitric acid and your chemicals on a small scale. On a larger scale sometimes time is more important. On the point of particle size and settling the more diluted your solution is the finer the particulate size will be. Temperature will effect this also as well as other things. Lou is going to do a thread on this i think.


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## NobleMetalWorks

rtl326 said:


> I have been using Campden tables that I got for a wine making supply store.
> Each tablet is 7/10 of a gram of Sodium Metabisulfite. I grind these up with a motar & pestle and dump the powder into AR solution to precipitate gold powder.





> Can someone tell me what is the actual reaction taking place between the SMB and the Auric Chloride?



When you add SMB to an acidic solution, it dissolves and releases SO2 gas, it's the gas that actually precipitates the Au out of solution. If you bubbled SO2 into your AuCl it would precipitate Au.



> I am trying to get a handle on how much SMB is too much or not enough.



General rule of thumb is to use as much SMB as you are expecting gold, if you add SMB and get no brown gas, then add what you expect to get in Au, then test after precipitation to ensure you have no more Au in solution. If you do, add small amounts until you have no reaction with Stannous Chloride. If you put too much SMB in, you may get a brown color when you test with Stannous.



> How does residual HNO3 interfere with the SMB?



If you have HNO3 in solution, when you attempt to precipitate Au, it immediately is re-dissolved right back into solution, which gives off NOx as it is dissolved. If you make small additions of HNO3 when added to HCl to make your AR, you can observe and not add too much which will not create the problem precipitating in the presence of too much HNO3 



> Sometimes the SMB makes the solution turn brown like coffee with too much cream and it is very slow to settle out. Is this a function of partice size? how can I adjust things to get faster settling



It takes as long as it takes, you don't really want to filter it, the equipment to settle it faster will cost you a lot. Just plan on it taking a long time to settle. Adding water helps to thin your solution and allows for Au to settle faster, also ice cold solution helps because it condenses.



> I often get gold plating out on the sides of the beaker and bits of gold floating on the top of the liquid even before adding SMB. It really isn't a problem since I just wash the gold off the the glass with freah AR. But can someone tell me why the gold seems so eager to come out of solution? Have I used too little AR?



Two things, first, the gold that you see floating on the top is floating on surface tension created by oils probably from your skin, incineration will remove oils before you process so they do not become a problem, if you do have this issues you can knock it down by using a spray bottle. Second, if you do not have enough solution to hold Au in solution, when more Au is dissolved it may cement gold out of solution at the same time. It is being displaced out of solution because your solution is super saturated.

Scott


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## rtl326

Thank for your input.

So the nitric just fights the SMB until it is driven off.
What becomes of the sodium from the SMB? Simply forms NaCl?

Yes, I read where Hoke says the floaters in the process of lifting the foils from the PC boards is caused by the oils on our skin. I have had good luck using the smallest posssible drop of Dawn detergent in the 5 gallon HCL bucket when the floaters get too numerous. I assumed it worked by lowering the surface tension, but maybe it is also acting as a surfactant on the oils from my skin.

The floating gold In was wondering about forms after the AR dissolves everything, has been filtered and is diluted by all the rinsing water cleaning the debris & the filter. It shows up after standing overnight. I added the AR in portions as Hoke says to and put in portions of foil until the AR was killed. I add a bit more AR at the end to be certain I am not missing any gold left in the debris left behind. Perhaps the solution is just too saturated and the gold appears upon cooling overnight. Or is it the dilution that triggers the appearance of metalic gold?

The next day I evaporate it down w/o boiling until it starts to thicken and there is no more nitric smell. Do I lose Auric Chloride during this process?

I periodically test a drop of the AR with drop of stannous in a watch glass. I often get black particles and sometimes gold particles. I wash the test drops into a small beaker and now I have quite a bit of black powder from these tests - too much to simply throw away. Can this be added to the SMB precipitants? Or should in be dissolved in the next batch of AR?

The process goes fairly well and I have nearly 30 grams of recovered gold powder. But I have been simply following the directions and, for the most part, flying blind. I am trying to better understand the process from a chemical standpoint. 

Thanks again for your insights.

(edited for typos)


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## kadriver

rtl326 said:


> How does residual HNO3 interfere with the SMB?



Hello:

Another important aspect that must be considered; free nitric will allow other metals (if present) to precipitate with the pure gold when using sodium metabisulfite.

The result is the pure gold will be contaminated with these other metals.

This can be minimized or even eliminated by ensuring that all the free nitric has been removed before precipitating the gold with sodium metabisulfite.

Evaporation seems to do the best job of removing excess nitric - but it takes a long time and you risk spattering the gold bearing solution.

Urea can be used to neutralize excess nitric, but it does not seem to work completely - plus contamination can be introduced if the urea is dirty.

The best way to guard against excess nitrc (in my experience) is to add small amounts of nitrc acid - just enough to dissolve the gold. 

I add the HCl to the gold - no nitric acid yet, and place on low heat.

Then I begin to add small amounts of nitric acid with a disposable pipette to form the aqua regia, 2 or 3 ml at a time, and then wait for the reaction to slow then add more until all the gold is dissolved.

By adding nitric acid in this manner, instead of pre-mixing the aqua regia, nearly all the free nitric gets used up.

Excess HCl is no problem, but excess nitric creates many problems.

kadriver


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## Lino1406

SO3-- takes O--, gives 2 electrons becomes SO4--
Au+++ takes 3 electrons becomes Au
Na2S2O5 = 2NaHSO3 - H2O
NaHSO3 becomes NaHSO4


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## rtl326

Thank you kadiver. 

I have 19 grams of gold powder that I assume is contaminated with at least some tin, lead and copper. I plan to heat it red hot to oxidize any tin then repurify. I will use your method to dissolve it with AR and use a few drops of H2SO4 to knock out any lead. I am trying for best purity.

And thank you Lino1406 

That was the original answer I was looking for with this question. I used SMB for the first precipitaion. But Hoke, page 172, tells me I should use a different precipitant for repurificantion. I have not seen that mentioned in this forum and everyone seems to use only SMB. Any thoughts?


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## samuel-a

rtl326 said:


> That was the original answer I was looking for with this question. I used SMB for the first precipitaion. But Hoke, page 172, tells me I should use a different precipitant for repurificantion. I have not seen that mentioned in this forum and everyone seems to use only SMB. Any thoughts?



rtl326
This fact was mentioned dozens of times before as far as i have seen... but anyway, here's the complete SMB reaction expressed in a balanced equation:
Na2S2O5 + H2O = 2 NaHSO3
3 NaHSO3 + 3 H2O + 2 HAuCl4 = 3 NaHSO4 + 8 HCl + 2 Au

You can try use copperas, oxalic or even solvant extraction with BDG.
But all in all, you can get good gold even with twice SMB and proper acid and water washes.


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## farhad69

i dissolved gold plated pins in AR . i did not add Urea before adding SMB and directly add much more SMB than i needed. 2 days later i flitered the solution. i did not get much gold and there was insoluble powder at the bottom of the bucket. i washed it off and add again to my solution. what shall i do now? neaturaliz the AR and then add enough SMB??


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## Topher_osAUrus

farhad69 said:


> i dissolved gold plated pins in AR . i did not add Urea before adding SMB and directly add much more SMB than i needed. 2 days later i flitered the solution. i did not get much gold and there was insoluble powder at the bottom of the bucket. i washed it off and added again to my solution. what shall i do now? neaturaliz the AR and then add enough SMB??



Urea is unnecessary. Use sulfamic acid to denox the ar solution, do not neutralize the acid either, sodium metabisulfite needs acidic conditions to release SO2 to reduce the gold back to metallic gold.

Lets start from the beginning.

How many kilos of pins did you dissolve? Why ar and not ap or nitric? Was there any sediment left after the ar on the pins? If so, your gold is there. As the gold cements back out on the baser metals, via the principles of the reactivity series of metals.
What did the stannous test say? Before and after?
What does the insoluble powder look like?

Gold plating is incredibly thin, 30 microinches or less usually, so it takes a great deal of pins to get any decent sized button of gold. If you can state how you started, what exact processes you have taken and what you encountered along the way, we could all help you much much better.

Regards,
Toph


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## farhad69

thank you for the response. i usually use dissolving gold plated pins in only nitric acid . then collect gold foils and dissolve them in HCL plus Bleach. then precipitate it by SMB. but since my nitric acid is not pure (it contains CL and forms weak AR) i decided to use AR this time. so i added 1.5 kgs of big sized pins into a bucket. then i added 1 litre water then 2 liters HCL and around 600 mls of HNO3 in 3 steps. all pins dissolved after 4 hours.and i got a thick dark green solution. i filtered it using a sieve. there was no other base metals in it. i almost tripled the solution to dilute it and added 110 mgs of SMB .immideatly i felt the gas and the solution turned brown to black. 2 days later i filtered it and there was white powder at the bottom of the bucket and some black powder.i seperated them by washing off . but the white powder immediately settles down after mixing it in water. why? what is this powder? and the original solution is still dark green n the stannous test is positive. what do i do now? do i still hv nitric in the solution and need to be removed by either evaporating or adding urea? but the solution volume is high . i can not evaporate it down


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## Topher_osAUrus

farhad69 said:


> thank you for the response. i usually use dissolving gold plated pins in only nitric acid . then collect gold foils and dissolve them in HCL plus Bleach. then precipitate it by SMB. but since my nitric acid is not pure (it contains CL and forms weak AR) i decided to use AR this time. so i added 1.5 kgs of big sized pins into a bucket. then i added 1 litre water then 2 liters HCL and around 600 mls of HNO3 in 3 steps. all pins dissolved after 4 hours.and i got a thick dark green solution. i filtered it using a sieve.
> What was caught in the filter? Any brown to black sludge?
> there was no other base metals in it. i almost tripled the solution to dilute it and added 110 mgs of SMB .immideatly i felt the gas and the solution turned brown to black. 2 days later i filtered it and there was white powder at the bottom of the bucket and some black powder.i seperated them by washing off . but the white powder immediately settles down after mixing it in water. why? what is this powder?
> Probably lead chloride maybe lead sulfate or other junk. See if it dissolves in hot water, if not, try HCl ..the smb releases SO2, which in a chain of complex reactions will form a wee bit of sulfuric and convert the lead chloride to lead sulfate. (Something like Cl2 +SO2 +H2O=H2SO4+2HCl, it helps to add a bit of sulfuric acid before you filter the solution, so the lead sulfate is caught in the filter, not your gold)
> and the original solution is still dark green n the stannous test is positive. what do i do now?
> Is it a purple stain, or brown? Brown is a false positive from the smb.
> do i still hv nitric in the solution and need to be removed by either evaporating or adding urea? but the solution volume is high . i can not evaporate it down


Forget the urea, it just causes more problems down the line. Can you get sulfamic acid? It is used as groute cleaner,
sold in most hardware stores. (In the US anyways, no idea what its common uses are elsewhere 

After two days, I would bet that all the gold has precipitated out and its the remaining SO2 in solution thats causing the positive. I would just add a slab of copper in the solution and cement out any other traces of precious metals, then take that black powder(thats cemented on the copper) and the powder that precipitated out with smb and redissolve it in HCl/Bleach, filter crystal clear, and precipitate again (after heating it up to remove excess bleach), remember the bleach additions will begin to make the solution basic, so add small additions of HCl too, to keep it acidic and working at dissolving your gold.

Hope this helps


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## farhad69

so you say
1. add sulfamic acid to solution
2. put a piece of copper to collect other precious metal
3.dissolving powders in HCL plus Bleach or HCL and little by little HNO3
4. precipitation by SMB

by the way. may i hv your number to talk more on whatsapp? 
i hv large piles of Telecommunication boards and i want to recover them. i hv not studied chemistry but i hv some knowledge.i want an expert to work together based on profit share.

00989335070951


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## farhad69

i forgot to ask how much sulfamic acid is needed? it is sold in powder?i hv 18 liters solution


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## Topher_osAUrus

farhad69 said:


> so you say
> 1. add sulfamic acid to solution
> 2. put a piece of copper to collect other precious metal
> 3.dissolving powders in HCL plus Bleach or HCL and little by little HNO3
> 4. precipitation by SMB
> 
> by the way. may i hv your number to talk more on whatsapp?
> i hv large piles of Telecommunication boards and i want to recover them. i hv not studied chemistry but i hv some knowledge.i want an expert to work together based on profit share.
> 
> 00989335070951



Well, not exactly.
I was saying to use sulfamic the next go around to remove nitric acid from the solution.

The copper is used to cement out the remaining gold and other precious metals. Eventually the solution will become saturated with copper and all metals below it in the reactivity series will be displaced out.

The nitric is not necessary if you oxidize the gold with bleach instead.

Filtering is a must, the solution needs to be crystal clear, with no signs of turbidity at all before you precipitate. I even dissolve the smb in water, and filter it before i add it to my gold solution.


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## farhad69

dear. can you plz tell me what to do step by step like below
1.
2.
3.
...


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## farhad69

i think u were saying sulfamic acid is added to AR before adding SMB to eliminate excess nitric acid

but this time i should put a piece of copper in the solution to pick up gold and other precious metals. 
right?


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## Topher_osAUrus

Sure, why not.
1. Add a bunch of copper to the solution that gold has been precipitated from.
2.add an air bubbler to help speed up the process, agitation of any kind works
3.after a couple days, remove the air bubbler and let it settle a day
4. Siphon off barren solution
5. Take the solids out of that container and add them to the gold powder that was reduced with smb
6, add enough HCl to cover the powder and warm it up a little bit
7.add a pipette of bleach(a few mL at most)
8. Stir with vigor, be careful not to breathe the chlorine gas
9. After 20 minutes or so, when dissolving action has ceased, add another pipette of bleach.
10. Continue stirring until action stops again
11. If there is still powder to dissolve, add 50 mL of HCl then another pipette of bleach
12. After all the gold is dissolved, turn the heat up a little more. You want it to steam, but not boil! That is important, boiling is not good
13. Woft a bit toward you, if it smells like Cl, keep steaming, if it is not strong. Then turn off the heat and let it cool.
14. Let it settle overnight, or filter until crystal clear. (Also important)
15. Take your clear solution and add some smb that has been dissolved in water and filtered clear also - your gold should immediately begin to precipitate, do not over add the smb, do small additions of the aqueous sodium metabisulfite. Ive noticed that adding too much can give a dark precipitant, but just the right amount will keep it a beautiful tan coloured powder.
16. Test the solution to see if there is still gold in it. If positive, add a little more smb, if negative, siphon it off to your stock pot (a bucket with a LOT of copper in it to catch traces of PM's)
17. Wash the gold powder with HCl, no heat, just stirring.
18. Wash with water until neutral pH
19. Repeat 17/18 until the washes are clear
20. After washing is clear, and neutral pH is attained, slowly dry the gold in the beaker (it should never leave the vessel it was precipitated in until it is dried and going in the crucible to melt)
21. After gold is dried, well, melt it up!

I think I covered everything in there, oh, nope, I didn't.
After the gold is dissolved, before you let it settle, you need to add sulfuric acid to remove any lead chloride, which will turn to insoluble lead sulfate, which will be filtered off at step 14. This is very important, as lead will make the gold brittle.

-to your recent reply, that is correct! Spot on!


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## farhad69

thx a bunch for complete explanation. this time i made a mistake and i hv to use your procedure. and also using sulfamic acid TOTALLY to remove excess nitric acid in AR. BUT as you know electroic scrap hs a wide range of devices . i hv also 500 kgs of chips. fingers. mlcc. tantalum capacitors and .....
i'd be grateful to work together. i hv good access to catalyst too. what do you think? 
is there any email or phone number to get in touch more easily?


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## Topher_osAUrus

Well, I live in the middle of the usa, but, yes, id gladly help you when and where I can.
I sent you a private message with mt phone number

Or my email address is
[email protected]


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## Topher_osAUrus

I must add to that.

I will gladly help you when and where I can, but, BUT, I also want to help you with the safety side of it too.
I want to know that it's being done in a practical and safe manner, where you aren't endangering your life or others.

Everything from proper personal safety gear, down to waste treatment. I want you to set a precedent for other refiners in your country. It's not hard to do at all, and doing it correctly will even give you a bonus in the end, by recovering traces of metals that would otherwise be lost down the drain.


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## farhad69

i sent you an email.thx


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## Chirou

Well guys, i had beautiful green solution after Poor's man AR with some PCB's without solder i added SMB (not much like a little baby spoon) and everything turned black and nothing changes it's been like an hour.


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## FrugalRefiner

Did you test the solution with stannous before you added the SMB?

Dave


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## bwalyack

I want to know if smb is selective....I tried to use it and I got a brown precipitate but when I assayed it there was not gold...


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## butcher

bwalyack,

Do not confuse recovery of gold with refining gold.
SMB is a great tool in refining but is not a good tool for recovery of gold.

You were trying to recover gold, SMB, in this case, was a poor choice, as you found out SMB will reduce other metals.



SMB is selective for gold when used properly in refining gold, note here we are discussing refining gold.
where most all of the base metals have been removed from the gold in earlier recovery processes, and the solution is fairly clean of base metals.

SMB is used for refining gold, and would not be a good choice in using for recovering gold from dirty solutions.


SMB can precipitate or change the oxidation state of other metals like copper tin... 

In a dirty complex of solutions of metals like in a recovery process, SMB is not selective and is not the best to recover gold from these complex metal solutions, copper cementation would be a better choice to recover gold from these base metal solutions.

Recovery of gold and refining gold are two different processes, SMB works great for refining gold but is not much help in recovery of gold from a mess of metals, the more metals in solution the more complicated the mess, and less likely to precipitate gold, if the mess is not too bad, with just some other base metals like copper the gold will precipitate dirty (Black) with copper chloride and other metals, if the solution is mostly base metals and little gold and tin in solution the gold can get locked up in solution as colloids SMB overuse trying to precipitate the colloids can increase the problem, overuse of SMB will reduce copper in solution, and even give a false positive (Brown) reaction to a stannous chloride test of the dirty solution.


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## nickton

I think I got that false positive stannous test in my solution, and was wondering what it was. Definitely a brown color, and after letting it all settle overnight, got some precipitate in the form of black powder but still got that weird brown test. I wonder if I should boil it down or what? How does one fix this?


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## anachronism

Brown isn't a false positive because the positive for gold is purple/black. 

Brown can indicate other metals molybdenum for one. I'm sure one of the pro chemists can add meat to the bones of my post here but suffice to say don't worry about a bit of brown there, take a good look at the raw product you used and work out what other metals could be present.

Jon


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## nickton

I don't expect any thing exotic because I kept it simple with this one and just processed gold fingers.


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## kuli

Hello.

One dilemma. So if I drop SMB into a solution and previously did not miss the necessary NHO3 gas, can I lose gold with brown steam? Yes or no please


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## anachronism

kuli said:


> Hello.
> 
> One dilemma. So if I drop SMB into a solution and previously did not miss the necessary NHO3 gas, can I lose gold with brown steam? Yes or no please



No


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## nickvc

anachronism said:


> kuli said:
> 
> 
> 
> Hello.
> 
> One dilemma. So if I drop SMB into a solution and previously did not miss the necessary NHO3 gas, can I lose gold with brown steam? Yes or no please
> 
> 
> 
> 
> No
Click to expand...


Agreed but bear in mind those fumes are toxic :shock:


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