Determination of SMBS addition into the Aqua regia solution

[Chemlab]

Dear Sir/Madam,
In the process of gold recovery using the aqua regia method, after leaching gold from e-waste, we proceed to neutralize the solution using urea. Following neutralization, we add sodium metabisulfite (SMBS) to precipitate gold dust.
However, I have encountered an issue: if an excess amount of SMBS is added to the solution, white precipitates form. Could you kindly advise on how to address this issue?
Additionally, how can the appropriate quantity of SMBS required for the precipitation process be determined?
Lastly, which reducing agent yields better results for gold precipitation: sodium metabisulfite (SMBS) or Ferrous sulfite (FeSO4)?
Looking forward to your guidance.

 

[Yggdrasil]

Dear Sir/Madam,
In the process of gold recovery using the aqua regia method, after leaching gold from e-waste, we proceed to neutralize the solution using urea. Following neutralization, we add sodium metabisulfite (SMBS) to precipitate gold dust.
However, I have encountered an issue: if an excess amount of SMBS is added to the solution, white precipitates form. Could you kindly advise on how to address this issue?
Additionally, how can the appropriate quantity of SMBS required for the precipitation process be determined?
Lastly, which reducing agent yields better results for gold precipitation: sodium metabisulfite (SMBS) or Ferrous sulfite (FeSO4)?
Looking forward to your guidance.

First of all we do not recommend using Urea.
And not neutralizing but decomposing NOx.
It works to a certain extent but has a few quirks.
It can under certain conditions form an explosive compound.
It has a relatively high pH and can if used too much neutralize the solution out of the range for Gold precipitation.
What we recommend is Sulfamic acid in a heated solution.

Regarding SMB we use what is "needed" that may be at ideal conditions as little as 1:1 gram or many times more in less favourable conditions.

Ferrous Sulfate needs a lot more but has the advantage of not giving the smell of SulfurDiOxide.
Ascorbic acid, Oxalic acid anfmd even Vitamine B can be used.

Sometimes if the solution is dirty, vementing on Copper and then dissolving without the use of Nitric may prove to be a better way.

 

[Yggdrasil]

Dear Sir/Madam,
In the process of gold recovery using the aqua regia method, after leaching gold from e-waste, we proceed to neutralize the solution using urea. Following neutralization, we add sodium metabisulfite (SMBS) to precipitate gold dust.
However, I have encountered an issue: if an excess amount of SMBS is added to the solution, white precipitates form. Could you kindly advise on how to address this issue?
Additionally, how can the appropriate quantity of SMBS required for the precipitation process be determined?
Lastly, which reducing agent yields better results for gold precipitation: sodium metabisulfite (SMBS) or Ferrous sulfite (FeSO4)?
Looking forward to your guidance.

I just noticed.
AR is not a good way of treating e-waste.
Can you elaborate how you treat it what it is and how you prepare your AR.
For e-waste Cupric Chloride etch and either HCl/bleach, HCl/Peroxide or HCl/pool Chlorine is a better solution than AR.
How much material do you treat at the time?

 

[Chemlab]

I just noticed.
AR is not a good way of treating e-waste.
Can you elaborate how you treat it what it is and how you prepare your AR.
For e-waste Cupric Chloride etch and either HCl/bleach, HCl/Peroxide or HCl/pool Chlorine is a better solution than AR.
How much material do you treat at the time?

Our e-waste raw materials are RAM, processors, and IC chips.
We need complete guidance on processing these materials to recover precious metals. If anyone can assist us, please let us know whom to contact for full support.
Currently, we are trialing CPU processors. We prepare Aqua Regia using 3L HCl, 1L HNO₃, and 500mL water for 5kg of processors. After the reaction, we filter the solution, neutralize it with urea, and add SMBS for gold precipitation.
Issues:

1. The green board releases white resin, causing complications.
2. We don't know the gold content in the processors to calculate the SMBS required (1:1 ratio). Excess SMBS settles at the bottom, and we’re unsure if gold is trapped in the residue or how to recover it.

Questions:

1. How can we accurately predict gold content in Aqua Regia?
2. Can you suggest better acid ratios or methods for processing e-waste?

 

[Yggdrasil]

Our e-waste raw materials are RAM, processors, and IC chips.
We need complete guidance on processing these materials to recover precious metals. If anyone can assist us, please let us know whom to contact for full support.
Currently, we are trialing CPU processors. We prepare Aqua Regia using 3L HCl, 1L HNO₃, and 500mL water for 5kg of processors. After the reaction, we filter the solution, neutralize it with urea, and add SMBS for gold precipitation.
Issues:

1. The green board releases white resin, causing complications.
2. We don't know the gold content in the processors to calculate the SMBS required (1:1 ratio). Excess SMBS settles at the bottom, and we’re unsure if gold is trapped in the residue or how to recover it.

Questions:

1. How can we accurately predict gold content in Aqua Regia?
2. Can you suggest better acid ratios or methods for processing e-waste?

Simply.
You DO NOT refine boards in AR or other leaching.
Boards are shredded and smelted.
For small scale the fingers and pins may be subjected to Nitric or Cupric Chloride to release the Gold and then the Gold can be refined.

All the information is in this forum.
You can follow the links here go to the library or search the forum.

We ask our new members to do 3 things.
1. Read C.M. Hokes book on refining jewelers scrap, it gives an easy introduction to the most important chemistry regarding refining.
It is free here on the forum: https://goldrefiningforum.com/phpBB3/viewtopic.php?f=54&t=19798
2. Then read the safety section of the forum: https://goldrefiningforum.com/forums/safety.47/
3. And then read about "Dealing with waste" in the forum: https://goldrefiningforum.com/threads/dealing-with-waste.10539/

Suggested reading:
https://goldrefiningforum.com/forums/the-library.101/
https://goldrefiningforum.com/threads/when-in-doubt-cement-it-out.30236/
https://goldrefiningforum.com/threa...le-read-this-before-you-post-about-ore.33333/


Forum rules is here.
https://goldrefiningforum.com/threads/gold-refining-forum-rules.31182/

 

[Martijn]

we proceed to neutralize the solution using urea

We advise against using urea.
You do not neutralize the solution, the SMB needs a low pH to work.

Additionally, how can the appropriate quantity of SMBS required for the precipitation process be determined?

for premixed AR on boards, you can't. You need to look and test.

Lastly, which reducing agent yields better results for gold precipitation: sodium metabisulfite (SMBS) or Ferrous sulfite (FeSO4)?

both are great and can drag down other metals.

Could you kindly advise on how to address this issue?

Don't add excess nitric to your AR, only enough to dissolve the gold. no more.
as for your solution, add up to one kilogram of gold to consume the nitric.

We prepare Aqua Regia using 3L HCl, 1L HNO₃, and 500mL water for 5kg of processors.

That is enough AR to dissolve one kilo of gold.. do you have processors containing 20% gold by weight?

I see you're from India, where HNO3 is cheap as water, i get the feeling you are not using the most environmentally friendly method.. can you tell us how you deal with chemical waste?

As Yggdrasil said, you need to study a lot.

 

[Chemlab]

We advise against using urea.
You do not neutralize the solution, the SMB needs a low pH to work.

for premixed AR on boards, you can't. You need to look and test.

both are great and can drag down other metals.

Don't add excess nitric to your AR, only enough to dissolve the gold. no more.
as for your solution, add up to one kilogram of gold to consume the nitric.

That is enough AR to dissolve one kilo of gold.. do you have processors containing 20% gold by weight?

I see you're from India, where HNO3 is cheap as water, i get the feeling you are not using the most environmentally friendly method.. can you tell us how you deal with chemical waste?

As Yggdrasil said, you need to study a lot.

We will finally neutralize acid waste into caustic soda lye treatment upto pH 7, then we transfer into ETP where metal residue and other solutions we will collect it, From Metal residue, we will once again do with Filter press to recover metals into it. Actually, we are finding the best way to recover precious metals from electronic waste, Can you give me best way other than Aqua Regia method?

 

[Yggdrasil]

We will finally neutralize acid waste into caustic soda lye treatment upto pH 7, then we transfer into ETP where metal residue and other solutions we will collect it, From Metal residue, we will once again do with Filter press to recover metals into it. Actually, we are finding the best way to recover precious metals from electronic waste, Can you give me best way other than Aqua Regia method?

This way if treating the waste may be insufficient and you will loose some PMs in the waste.
Treat it according to this link.
https://goldrefiningforum.com/threads/dealing-with-waste.10539/post-101993

And as said if you treat whole boards. Shredding and smelting is the way to go.

 

[Martijn]

We will finally neutralize acid waste into caustic soda lye treatment upto pH 7, then we transfer into ETP where metal residue and other solutions we will collect it, From Metal residue, we will once again do with Filter press to recover metals into it. Actually, we are finding the best way to recover precious metals from electronic waste, Can you give me best way other than Aqua Regia method?

AR is fine if you don't premix it. Add in bits until there is no more reaction.
Then precipitation will be much easier.
Rule of thumb is about one ml HNO3 for 1 gram of gold in AR and about one gram of SMB to precipitate one gram of gold.
Add SMB slowly and test for values in solution to know if you're done.

With free nitric left in solution, any gold that is precipitated will dissolve again. So there is no way to calculate the needed amount of SMB in that situation.

 

[Martijn]

AR is fine if you don't premix it.

But not on boards as said. On the recovered gold from e waste in mean.

 

[anarxi]

Dear Sir/Madam,
In the process of gold recovery using the aqua regia method, after leaching gold from e-waste, we proceed to neutralize the solution using urea. Following neutralization, we add sodium metabisulfite (SMBS) to precipitate gold dust.
However, I have encountered an issue: if an excess amount of SMBS is added to the solution, white precipitates form. Could you kindly advise on how to address this issue?
Additionally, how can the appropriate quantity of SMBS required for the precipitation process be determined?
Lastly, which reducing agent yields better results for gold precipitation: sodium metabisulfite (SMBS) or Ferrous sulfite (FeSO4)?
Looking forward to your guidance.

It is more practical for you to use iron sulfate for several reasons.
First: even if you pour a lot, it will not prevent the gold from settling out of the solution.
Second, it is more accessible
The disadvantages are slow settling and not very pure gold in the sediment.