99,99% Gold with one procedure

[goldandsilver123]

Hello,

I'm working as consultant on a company that is trying to change it's method of refining (because of waste regulations and waste of chemicals).

They refine from around 85% gold to 99,99% with one procedure, that it is:

1-Dissolve with AR overnight without any heating (with excess nitric)
2-Next day add urea to remove the excess nitric, add 2 times the original volume in water and make a filtration
3- Bubble SO2 in the solution until all gold is precipitated
4- filter the precipitate and wash with water (tap cold water)
5- wash the funel with ammonia, then nitric acid, then ammonia. Until the filtrate it's not blue anymore.
6- the gold is melted and sampled

With this method they achieve 99,995% minimum by atomic absorption with ocasionaly ICP-OES check in other laboratories.

I changed the procedure to (urea and ammonia not used anymore, change the SO2 to SMB because of risk management, change nitric acid to hydrochloric to make easier on the waste water treatment):

1-Dissolve with AR without excess nitric with heating
2-Next day add 3 times the original volume in water and make a filtration
3- Add Sodium Metabissulfite dissolved in water in the solution until all gold is precipitated (until ORP reading of +400 mV)
4- filter the precipitate and wash with water (tap cold water)
5- wash the funel with hot distilled water then cold hydrochloric acid, repeated 3 times
6- the gold is melted and sampled

With this method the maximum purity achieved was 99,98%.
Silver is the main contaminant in all cases.

Until now I only achieved 99,99% with one procedure without reducting all of the gold from the solution or refining it 2 times.

Anyone has any tips?

Thank you all

 

[FrugalRefiner]

You may find some insight in Lou's post on washing in Gold has been refined and melted, now has orange hue.

Dave

 

[UncleBenBen]

For your step #2. Try crushed ice for your diluent. The drop in temperature will greatly lower the solubility of the silver chloride. Which can then be filtered out before precipitation. That should save from having to deal with so much silver in the washing stages.

 

[mls26cwru]

if i recall correctly, an ammonia rinse can help filter out silver chloride... i think it can also help remove trace amounts of copper... it might be helpful to add that back in the rinse procedure.

you also mentioned rinsing/diluting with tap water... careful with that. Tap water has chlorides in addition to a bunch of crud.... you could be introducing contaminates if the water isn't pre-filtered and deionised.


****EDIT: just read the post from Lou linked above... very interesting read!***

 

[4metals]

You don't mention what the other 15% is in the feedstock. Or the quantity of material processed in each run. Both of these little tidbits of information are required to make the proper decision as to your best path forward.

Also, what are the regulations causing concern?

 

[goldsilverpro]

All of this is iffy at best. With AA, I can't see how it would be possible to directly analyze the gold, itself, with a gold lamp, to a accuracy of 4 nines. If you had an AA lamp for every possible impurity, it would take many hours to run an analysis and the accuracy still wouldn't be there. Enlighten me as to how this is done.

 

[g_axelsson]

5- wash the funel with hot distilled water then cold hydrochloric acid, repeated 3 times

Why washing with cold HCl? I think hot should be better at dissolving silver chloride.

Then washing in the funnel, I could see parts of the gold not being properly washed. Washing in a beaker with ample stirring would probably use less acid and remove more silver chloride. It could pose other problems depending on the amount of gold to wash each day, you never mentioned how much gold they process per day.

Göran

 

[4metals]

All of this is iffy at best. With AA, I can't see how it would be possible to directly analyze the gold, itself, with a gold lamp, to a accuracy of 4 nines.

Assuming they have lamps for all of the elements present in the original sample, then it is possible, but time consuming as Chris pointed out, but possible to get a good handle on purity to the 4th digit. For most secondary refiners today the expense of an ICP is tough to justify and I am very happy to see secondary refiners with a lab and a capable AA technician on staff. It still comes down to what was there to start with which has yet to be answered.

 

[goldandsilver123]

You don't mention what the other 15% is in the feedstock. Or the quantity of material processed in each run. Both of these little tidbits of information are required to make the proper decision as to your best path forward.

Also, what are the regulations causing concern?

I know it's placer mining gold ("ouro de garimpo"), the other 15% is majoritary copper (~8%) and silver (~7%).

The gold is from many locations and the lot processed is a mix. Usually 10 kilos is processed in each batch.

The problem with the waste treatment is the time consumed and cost, because it produces waste with ammonia, sulfates, nitrates in excess that makes the treatment harder, longer and more expensive.

All of this is iffy at best. With AA, I can't see how it would be possible to directly analyze the gold, itself, with a gold lamp, to a accuracy of 4 nines. If you had an AA lamp for every possible impurity, it would take many hours to run an analysis and the accuracy still wouldn't be there. Enlighten me as to how this is done.

They don't analyse gold directly. At each batch is made an AA analysis of 15 elements, those elements were chosen by the impurities given in the routine ICP-OES analysis. For what I know in 35 years of the company they never had problems with their purity in the national market or the international market.

What condemn is always silver, the lowest silver content in the test was 160 ppm (with the other impurities reach total 200 ppm).

They want a safe margin of 50 ppm of silver maximum.

5- wash the funel with hot distilled water then cold hydrochloric acid, repeated 3 times

Why washing with cold HCl? I think hot should be better at dissolving silver chloride.

Then washing in the funnel, I could see parts of the gold not being properly washed. Washing in a beaker with ample stirring would probably use less acid and remove more silver chloride. It could pose other problems depending on the amount of gold to wash each day, you never mentioned how much gold they process per day.

Göran

For what I know cold hydrochloric acid can dissolve more silver chloride then hot.

Thanks for all of your responses, I got something in mind to try. It's very good to have this forum, normally I don't have anybody to talk about this chemistry.

 

[Topher_osAUrus]

Lou suggests cold HCl because the acid is merely a dissolved gas, and has higher solubility cold than when hot. (Kind of like how soda loses its carbonation faster when its warm) The crummy thing about it though, is the reaction rate suffers when the temperature is cold compared to when its simmering.

I do hot HCl washes though, simmering hot to be precise (tisk tisk), simply because I dont have a fancy stir-bar hot plate, and the agitation from the boiling action helps to make sure the acid easily contacts all of the contaminated gold.

So, I would think either of these 3 things would work: boiling HCl, an ammonia wash, or highly agitated cold HCl washing.
Lou also mentioned (4metals did too), sulfuric sonification, which may be something that could be suited to your needs. But I am not sure of the solubility of silvet chloride in hot sulfuric acid (either strong or dilute), but I presume it removes it sufficiently in the ppm quantities you speak of.

 

[Lou]

1. Sulfamic to remove excess nitric. Use minimum nitric.
2. Add crushed ice to chill and dilute and use flocculant to remove AgCl.
3. Continue to use SO2 if possible. Preferably with ORP meter.
4. Wash gold in with c. HCl, rinse with hot water and use the filtrate for next gold digestion. The gold can then be treated or pulped with hot 60-70% sulfuric until HCl fume gone, filtered on a glass fiber filter (or polypropylene if acid below 55 C) and rinsed with hot water. A little sulfuric residue will not hurt gold or crucible.

Will be very pure.

Silver chloride, freshly precipitated will form soluble silver sulfate in concentrated, hot sulfuric solutions. HCl goes off as a gas.

 

[4metals]

The problem with the waste treatment is the time consumed and cost

So they are not up against any environmental regulations?

Just eliminating the ammonia was a big step as the ammonia re-dissolves copper after it is dropped in waste treatment.

What equipment do they have for waste treatment and how many liters per day do they treat? For 10 kg a day the waste generated should be under 100 liters per batch. That is a small treatment system.

Are you sure there are no PGM's in the feedstock?

 

[g_axelsson]

Lou suggests cold HCl because the acid is merely a dissolved gas, and has higher solubility cold than when hot. (Kind of like how soda loses its carbonation faster when its warm) The crummy thing about it though, is the reaction rate suffers when the temperature is cold compared to when its simmering.

HCl is a dissolved gas but it is protonated in water, ie split into a chloride ion and a hydrogen ion. The silver chloride salt is dissolved as a complex silver chloride salt and I can't think of any example when increased temperature would decrease solubility of a salt. Boiling off HCl would precipitate silver chloride again so hot but maybe not boiling would be a good point to aim for.

In Wet Analysis of Gold-Silver Alloys of High Gold Content, Caley, Earle R.; Shank, Lowell W., The Ohio Journal of Science. v68 n2 (March, 1968), 100-104, https://kb.osu.edu/dspace/handle/1811/5375 you find:

The solubility of silver chloride in aqueous solutions is increased by high
temperature, high chloride concentration, high hydrogen-ion concentration, and
by combinations of these conditions. This increase is largely due to the formation
of soluble complex ions such as AgCl₂^-, so the maximum effect occurs when the
active concentration of chloride ions is as high as possible. Results from experiments
on the simultaneous effect of high temperature and high alkali-chloride
concentration are shown in table 1. In these experiments a boiling salt solution
containing an excess of solid silver chloride was refluxed until no more dissolved.

Table 1 :

Solubility of silver chloride in solution of alkali chlorides at the boiling point
		Mg of AgCl dissolved in 50 ml of solutions of stated molarity
Salt		1.0 M	3.0 M	5.0 M
KCl    	11		123		614
NH4Cl	  12		111		472
NaCl  	 10		82		 292
LiCl   	9		 38		 100

Table 2:

Solubility of silver chloride at the boiling point in
hydrochloric acid and in hydrochloric acid 3.0 M
in potassium chloride
Concentration	Solubility in	Solubility in
of acid M 		acid alone		acid plus KCl
			mg/50 ml		mg/50 ml
1.0			11			222
2.0			33			433
3.0			66			698
4.0			112			--
5.0			175			--

If a concentrated solution of ammonium chloride is used to wash away silver chloride it would not add any contaminants even if there are some remnants of ammonium chloride left after washing. It would evaporate when heated before melting.

I haven't tested these procedures myself, I only read the scientific article along a few other while doing research.

Göran

 

[snoman701]

Hydrogen chloride is the gas. It is present, dissolved as a molecule, in the water. Hydrochloric acid is the aqueous
solution of hydrogen and
chlorine ions.

If hydrogen chloride were not present in hydrochloric acid, it would not create the white vapor as that is the interaction between water in the atmosphere and hydrogen chloride.

It is thus my guess that silver chloride has greater solubility in hydrogen chloride, with greater presence at lower temperatures, than it does in hydrochloric acid.


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[g_axelsson]

Yes, solubility of HCl in water is lower at higher temperature.

https://pubchem.ncbi.nlm.nih.gov/compound/hydrochloric_acid#section=Chemical-and-Physical-Properties

Solubility in water
82.3 g/100 g water at 0 deg C
67.3 g/100 g water at 30 deg C
63.3 g/100 g water at 40 deg C
59.6 g/100 g water at 50 deg C
56.1 g/100 g water at 60 deg C

up to the azeotrope at 20% wv at 108.6 °C

37% weigt/volume HCl that we use is 37g HCl in 100 ml (density = 1.19g/ml) = 37g HCl in 119g acid = 37g HCl dissolved in 82 g water = 45.2 g/100 g water. So we are below maximum solubility of HCl, it won't matter if you lower the temperature as there are already more water in our acid. The above table shows us that we can go at least to 60 °C without forcing out any HCl because of solubility issues.

HCl reacts with water to give H+(aq) and Cl−(aq), not dissolved HCl molecules, and its dissociation into ions results in a much higher solubility than expected for a neutral molecule.

But solubility of salts increases with higher temperature, so there is a sweet spot somewhere. The question is where? My guess is closer to boiling than freezing.

Adding some potassium chloride to the wash or just washing with strong potassium chloride while boiling first could be a real alternative to ammonia wash. Even a strong and hot ammonium chloride wash should be more effective at removing silver chloride than HCl.

Finally I found it, https://en.wikipedia.org/wiki/Hydrochloric_acid#Physical_properties (table is in weight/weight %)
The boiling point of hydrochloric acid is around 50-60 C depending on concentration... that's really low. Never thought it would be so low... I learned a new thing today.

Still, I think it would be better to wash with warm HCl rather than cold.

Göran