Is there a way to drop gold faster?

[Wonka]

Sometimes it takes days for my gold to settle. I wonder if a little heat will speed up the process?

 

[FrugalRefiner]

Heat can help. Dropping from a clean, concentrated solution helps a lot.

Dave

 

[Wonka]

What temp is best?

Edit: I have it set at 50C right now.

 

[4metals]

Sometimes it takes days for my gold to settle. I wonder if a little heat will speed up the process?

Are you dropping from aqua regia? What are you dropping with? What have you done to consume any excess nitric? How concentrated is the gold in solution?

Good details precipitate better answers…….. faster too!

 

[Ultrax]

I wrote about the non-linearity of chemical reactions in this thread Oxidize silver chloride using urea in October 2023.

The higher temperature will help to increase drop of the first 95%, but will not significantly change the overall proportion and involve much more impurities into precipitate.

 

[Yggdrasil]

And this is why we settle for good enough and divert the rest to the stock pot.

Edited to correct the language for precision.

 

[nickvc]

With really dirty solutions or low concentrations of gold it might pay to cement on copper , collect the powder and dissolve using minimal reagents which should speed up the precipitation time.

 

[Thomas M Joseph]

The formation of a precipitant is a two step process.
1. nucleation
2. particle growth

To obtain large particles that settle and filter easily, nucleation is inhibited and particle growth is enhanced by using a homogeneous precipitation technique. This includes heating, using dilute precipitating solutions, and adding the precipitant slowly with constant stirring. Using a homogeneous technique also provides a cleaner percipitant.

Hope this helps.

 

[Martijn]

Sometimes it takes days for my gold to settle. I wonder if a little heat will speed up the process?

Giving the precipitated gold solution a gentle boil with a coulpe ml of sulfuric acid can help the solution clear up fast.
It cleans the small gold particles and makes them clump together so they fall to the bottom. The gentle bubble action also provides agitation.
This will not work if an oxidizer like free nitric is present. This will redissolve some gold.

 

[Ayham Hafez]

The formation of a precipitant is a two step process.
1. nucleation
2. particle growth

To obtain large particles that settle and filter easily, nucleation is inhibited and particle growth is enhanced by using a homogeneous precipitation technique. This includes heating, using dilute precipitating solutions, and adding the precipitant slowly with constant stirring. Using a homogeneous technique also provides a cleaner percipitant.

Hope this helps.

So SMB dissolved in water precipitating bigger gold particles than dry SMB?

 

[Thomas M Joseph]

According to gravimetric chemistry, the more nucleii, the finer the precipitate. The object is to promote particle growth rather than the formation of more nucleii. Fast reactions favor the formation of more nucleii and less particle growth. Introduce SMB(precipitant) slowly in the form of your choice so the reaction will favor particle growth...mixing everything at once is not promoting a homogeneous technique, regardless of concentration. Fine particles carry more contaminants as the smaller the particle, the more surface area for contaminants to attach to. Post reaction see Martijn's good advice.

 

[Ayham Hafez]

Giving the precipitated gold solution a gentle boil with a coulpe ml of sulfuric acid can help the solution clear up fast.
It cleans the small gold particles and makes them clump together so they fall to the bottom. The gentle bubble action also provides agitation.
This will not work if an oxidizer like free nitric is present. This will redissolve some gold.

Do you mean after adding SMB add some sulphuric and heat the solution?

 

[orvi]

With really dirty solutions or low concentrations of gold it might pay to cement on copper , collect the powder and dissolve using minimal reagents which should speed up the precipitation time.

According to gravimetric chemistry, the more nucleii, the finer the precipitate. The object is to promote particle growth rather than the formation of more nucleii. Fast reactions favor the formation of more nucleii and less particle growth. Introduce SMB(precipitant) slowly in the form of your choice so the reaction will favor particle growth...mixing everything at once is not promoting a homogeneous technique, regardless of concentration. Fine particles carry more contaminants as the smaller the particle, the more surface area for contaminants to attach to. Post reaction see Martijn's good advice.

I can personally confirm both of these are beneficial and correct, not even from theory, but also from my observation - as I used both techniques A LOT in my refining "career".

When I explored the possibility to de-NOx + simultaneously drop gold on copper+addition of sulfamic acid, I never treated crude AR low gold solution differently (speaking of solutions arising from metallic material <5g/kg Au dissolved in AR).

Also, when I saw, how much differently the gold precipitate using SO2 gas than using dry/slurried SMB, I always went with SO2, when there was possibility of creating small particles during precipitation.

Most significant factors that can alter the speed of the whole "dropping" operation (from my observation) are:

1. Concentration of solution - this is probably one of most influencing factors. More concentrated the Au is, bigger, heavier and nicer precipitate you get - and also you get it much more quickly than from dilute Au solutions.

2. Particulate contamination of the solution - crystal clear solution will give you better precipitate than cloudy/merky solution with lots of nucleation sites (as correctly mentioned above).

3. Presence of an acid - acidic enviroment change the surface behaviour and charges, and apparently help particles to coagulate better. Overshooting SMB in already acid-deprived solutions can lead to finer precipitates, even if the gold content in the solution is OK.

4. Mixing. Contraintuitively, AFTER the first microparticles of gold form, stirring of the solution can indeed help to coagulate them faster than letting it sitting still. If all of the gold drop from solution, aka the solution goes to translucent and gold appear in solution, I found it beneficial to let it on it´s own for 5 or so minutes, and then stir occasionally in the first hour. Mixing help particles to bump to one another, and you reach equilibrium much faster.

5. Temperature. "Packing" precipitates by boiling is well known technique, and it really works also with gold. Not miraculosly, but it helps if your concern is to speed up the process.
6. Careful choice of precipitant "formulation". In other words, it is very different if you use powdered SMB or powdered sodium sulfite, if you slurry these reagents and use them as liquid slurry, or if you use them as clear solutions. If you stir the mixture during gold drop or just spoon in the SMB/sulfite. In my experience, using powdered reagents in dry form usually lead to finer precipitates. I have some hypotheses why this can be the case, but they are just my theories, which I cannot back properly by any experiments or literature. So I leave this open. But I definitely observed the difference there. Best precipitant in my opinion is gaseous SO2, if you can choose from dry SMB, slurried SMB, clear SMB solution or directly bubbling SO2 gas. It probably has something to do with temporary pH drop (SMB and sulfites are essentially basic), nucleation sites etc, but here I cannot be sure.

7. Precipitant itself and it´s action. There is so much going on in terms of surface chemistry, physical forces of "in statu nascendi" isolated gold nanoparticles, that I wouldn´t be able to explain it with my poor knowledge in this field. But it is true, that cementing on copper from hot solution will give you relatively nice and settling precipitate usually in order of max few hours. Hydrazine produce very nice gold precipitate, despite the fact that it is relatively fast acting. SMB is relatively fast acting precipitant too, but colloidal suspensions or microsuspensions are much more likely to form with it... My choice from common precipitants, if I would need to treat dilute Au solution (like from dissolution of scrap that is below 1-2g/kg in AR) is copper cementing+heating.

8. Chelating impurities or surfactants present. In scientific community, we are usually interested in studying the behaviour of nanoparticles, and thus there is quite a big field of how to make stable solutions of nanoparticles (of whatever metal or compound). Most of the times, surfactants or chelating/binding chemicals are added to prevent particle agglomeration - substances which very strongly bond to the nanoparticles of gold in the very moment they are created, and thus preventing further agglomeration. When we refine stuff, we are dealing with contamined solutions, various dissolved junk from polymers (plastic in acid gets attacked and decomposed etc.) or impurities in our reagents. All of this can have negative effect on later gold coagulation.

I do not want to get this too long. We can talk hours about what could influence behaviour of gold drop, and things listed above are certainly not all of the factors.

Concentration is one of the biggest contributors to nice and easy gold drop. Cementation on copper+heating works wonderful, if you need to treat dilute contamined solutions. There is also a possibility to perform extraction of gold with organic solvent, but this have it´s drawbacks too (mainly you need to know what is in the solution, how much free acid is left, is there any free oxidizer etc...).

 

[Martijn]

Do you mean after adding SMB add some sulphuric and heat the solution?

Yes.

 

[Wonka]

The formation of a precipitant is a two step process.
1. nucleation
2. particle growth

To obtain large particles that settle and filter easily, nucleation is inhibited and particle growth is enhanced by using a homogeneous precipitation technique. This includes heating, using dilute precipitating solutions, and adding the precipitant slowly with constant stirring. Using a homogeneous technique also provides a cleaner percipitant.

Hope this helps.

Thanks! You guys always say heating but never say what the best temp is?

 

[Wonka]

I do AR with urea and SMB. I do stir it but never tried heating until now.

 

[Wonka]

I do AR with urea and SMB. I do stir it but never tried heating until now.

Edit: the entire bucket will turn black from a clear solution, but it takes days to drop. Idk if it’s because I’m in MO and it’s already down to below freezing at night or what.

 

[Thomas M Joseph]

50 degrees C/122 degrees F is enough to promote a homogeneous technique. It helps to remove contaminant ions from the adsortion layer of small precipitant particles reducing any net charges of the particles and it provides kinetic energy. So it helps the particles overcome repulsive action of being charged and coagulate into larger particles. Hope I made sense as this is pretty watered down for some complex physical chemistry.

 

[Ultrax]

50 degrees C/122 degrees F is enough to promote a homogeneous technique. It helps to remove contaminant ions from the adsortion layer of small precipitant particles reducing any net charges of the particles and it provides kinetic energy. So it helps the particles overcome repulsive action of being charged and coagulate into larger particles. Hope I made sense as this is pretty watered down for some complex physical chemistry.

The physical chemistry of disperse systems is a rather complex topic for the forum.
To explain, for example, the difference in the size of gold particles when precipitated by hydrazine and sulfur dioxide would require rather long lectures.

It may make sense to use scientific terminology in a slightly different way on the forum since there are many newbies here.

For example, instead of using the term “homogeneous method” (which will generally be difficult to understand outside the context of chemistry, since the question arises “homogeneous method of what?”) use human-readable "slow uniform mixing"; instead of "impurity ions from an adsorption layer of fine precipitant particles, reducing any net charges on the particles and providing kinetic energy", you can simply use the term "coagulation (clumping) of particles using an electrolyte (sulfuric acid).

I may be wrong, but it seems to me that the subtleties that you outline are more likely to be needed in analysis and research than in gold refining. No offence.

 

[Wonka]

50 degrees C/122 degrees F is enough to promote a homogeneous technique. It helps to remove contaminant ions from the adsortion layer of small precipitant particles reducing any net charges of the particles and it provides kinetic energy. So it helps the particles overcome repulsive action of being charged and coagulate into larger particles. Hope I made sense as this is pretty watered down for some complex physical chemistry.

Thanks! Completely understood!