Urea question

[Acidrain]

I read in a thread somewhere that it's generally advised not to use urea for PH adjustment in refining.
I'm new to refining and am reading Hokes book among other materials but haven't learned why yet.
Can someone explain please?

 

[Martijn]

Welcome to the forum.

Urea does not remove free nitric acid from solution, it is only used to remove NOx from solution. NOx is the gas formed by digesting with nitric. Urea could even at the end increase the nitric content of your solution.
Urea is also used in some types of combustion engines to destroy the NOx in exhaust gases. And as a fertilizer.

Some ways to not have excess HNO3 in an Aqua Regia solution:

• do not add to much in the first place (e.g. premixing AR at 3:1)
• add more metal to use up the free nitric, preferably gold
• add sulfamic adic to destroy the nitric acid.

Martijn.
edited to make sense

 

[Martijn]

You asked about pH adjustment. That is done with soda ash or lye as pH plus or HCl as pH minus.

edited to remove wrong comment

 

[Acidrain]

You asked about pH adjustment. That is done with soda ash or lye as pH plus or HCl as pH minus.

edited to remove wrong comment

Oh thank you! That really cleared things up for my understanding

 

[Deano]

Urea has two useful effects for someone who is looking to drop cleaner gold from acid chloride solution.
1. If you have divalent base metals, such as copper, in your solution these base metals will act as an oxidant to redissolve some of the precipitated gold.
The base metals will usually be present in the solution as a nitro complex such as copper nitrate.
Urea is a cheap and simple way to destroy these complexes before precipitating the gold.
2. The pH which gives you the highest purity gold from precipitation is 2.5, this equates to instrumental pH 1.5 as the chlorides in solution will cause an instrument error of around minus 1 pH unit.
Adding urea to a denoxxed solution from aqua regia digestion will raise the pH of that solution without you getting any of the base metal adherence to the precipitated gold you will get by either not raising the pH to 2.5 or by using strong caustic agents such as sodium or calcium hydroxides as pH adjusters.
If you have a high aqua regia to solids ratio during leaching then there may be so much acid present that requires pH adjustment that the urea method is not viable and it may be cheaper to redigest the precipitated gold and use an alternative precipitant in order to get a higher purity on the next precipitation.
Deano

 

[anachronism]

Urea has two useful effects for someone who is looking to drop cleaner gold from acid chloride solution.
1. If you have divalent base metals, such as copper, in your solution these base metals will act as an oxidant to redissolve some of the precipitated gold.
The base metals will usually be present in the solution as a nitro complex such as copper nitrate.

Deano

Hence why people redissolve gold during clean up from a denoxxed solution in a recovery situation. Many a night spent scratching my head over that problem until you explained this one to me Dean! A question if I may- this problem appears to occur less when using Ferrous Sulphate as the initial precipitant. Is there any chemical reason why this would be the case or is it blind luck?

Jon

 

[Shark]

Adding urea to a denoxxed solution from aqua regia

Will this affect denoxing with sulfamic in a detrimental way? Or would it be bettter to denox using one of the methods?

 

[anachronism]

Will this affect denoxing with sulfamic in a detrimental way? Or would it be bettter to denox using one of the methods?

I'd suggest denoxxing the usual way then adding Urea to bring the pH up whilst destroying the base metal complexes, and I'm sure Dean will correct me if I'm wrong.

 

[Yggdrasil]

I'd suggest denoxxing the usual way then adding Urea to bring the pH up whilst destroying the base metal complexes, and I'm sure Dean will correct me if I'm wrong.

I was of the impression that Sulfamic acid destroyed the Nitrates as well.
If so the Urea has only a pH adjustment effect here.

 

[Shark]

I was of the impression that Sulfamic acid destroyed the Nitrates as well.
If so the Urea has only a pH adjustment effect here.

This is my understanding as well. But used this way, it explains some of the issues with dropping gold and the reformation of NOx when running some ewaste material with a heavy copper base as a contaminants. These often need to be dropped and filtered immediately to stop the gold from resolving, given time, such as sitting over night, may completely redissolve small batch’s completely. I have ran into this in the past, but just recently started to understand why, now maybe how to stop it.

 

[anachronism]

This is my understanding as well. But used this way, it explains some of the issues with dropping gold and the reformation of NOx when running some ewaste material with a heavy copper base as a contaminants. These often need to be dropped and filtered immediately to stop the gold from resolving, given time, such as sitting over night, may completely redissolve small batch’s completely. I have ran into this in the past, but just recently started to understand why, now maybe how to stop it.

Hehe so true. When I learned this a few years ago I used to get funny looks from people when I told them that in effect the base metals were having the effect of oxidising the gold! They thought I was mad.

 

[Deano]

Sulfamic acid will not destroy nitrates, there is plenty of literature attesting to this. It is, however, an efficient way of removing free nitric acid from solution.
Urea will not destroy free nitric acid in solution but will destroy nitrates in solution. Once again there is plenty of literature attesting to this.
I have always used heat to remove free nitric acid from solution as in Australia it is cheaper than using chemical methods such as sulfamic acid.
Heat is also cheaper than the excess of metabisulfite needed to compensate for the presence of free nitric acid in solution if you have not performed any free acid removal steps.
The presence of nitrates in solution will also require an excess of metabisulfite if there is a divalent metal, usually copper, in solution. This is where urea, which is usually cheaper than metabisulfite, will not only give you a cheaper drop but a cleaner one when used before the metabisulfite addition.
It will also minimise the redissolution process which can still be caused by dissolved oxygen in the liquor raising the divalent metals to their higher valency and then these metals act as an oxidant for gold.
Deano

 

[Acidrain]

Sulfamic acid will not destroy nitrates, there is plenty of literature attesting to this. It is, however, an efficient way of removing free nitric acid from solution.
Urea will not destroy free nitric acid in solution but will destroy nitrates in solution. Once again there is plenty of literature attesting to this.
I have always used heat to remove free nitric acid from solution as in Australia it is cheaper than using chemical methods such as sulfamic acid.
Heat is also cheaper than the excess of metabisulfite needed to compensate for the presence of free nitric acid in solution if you have not performed any free acid removal steps.
The presence of nitrates in solution will also require an excess of metabisulfite if there is a divalent metal, usually copper, in solution. This is where urea, which is usually cheaper than metabisulfite, will not only give you a cheaper drop but a cleaner one when used before the metabisulfite addition.
It will also minimise the redissolution process which can still be caused by dissolved oxygen in the liquor raising the divalent metals to their higher valency and then these metals act as an oxidant for gold.
Deano

Excellent information! Thank you

 

[Shark]

I may break into the eWaste stash and try this out for the experience. This explains many early problems I had with recovery of from some materials.

 

[Badman]

I use Urea primarily in my Garden. 6 foot tall flowers is what it produces. An angry white Leander bit me and spit on me. It actually turned its flower head and struck out.

 

[minero74]

Interesting information about the use of Urea
that’s explain many question.
Thank You for sharing .

I will keep Urea near by.

In Thiosulfate leaching method,copper sulfate is use as the oxidizer.
and it work great.

Edited.

 

[shadybear]

Let me see if I have this right. sulfamic acid destroys the free nitric in solution or unused nitric,
But urea still has affect on nitrates.
So should you use sulfmic first then settle and filter, then use urea?

 

[Yggdrasil]

Let me see if I have this right. sulfamic acid destroys the free nitric in solution or unused nitric,
But urea still has affect on nitrates.
So should you use sulfmic first then settle and filter, then use urea?

It seem so.

 

[Deano]

The usual way I treat gold chloride solution from aqua regia digestion is to simmer the solution on a hotplate until the fumes change colour from brown to white.
Give the beaker a further few minutes on the hotplate to be sure that residual nitric acid has been removed.
Add urea to instrumental reading 1.5 pH.
Drop your gold with metabisulfate or any precipitant of your choice.
If you prefer to remove free nitric acid by chemistry then replace the simmering step with a sulfamic acid addition.
You then add urea to remove any nitrates present such as copper nitrate.
If you add enough urea to get to instrumental pH 1.5 you will get a cleaner drop but this step is not necessary if you are happy to have dirtier gold from just the nitrates removal step at a lower pH.
After this step you precipitate the gold.
Deano

 

[Acidrain]

Awesome! Thank you for sharing