STONE REMOVAL QUESTION

[EMRE]

Hi All,
we do diamond removal with AR. we use three batched of AR, 1/3 of the necessary AR at a time. After each batch we pour off the acid and rinse the solids with prismatic rice to get rid of silver chloride,then apply the second and third batches. This speeds up the process a lot. Problem starts here, gold in the first batch AR partially drops out of the solution its self without us doing anything. why do you think this is happens? is it because Nitric is well spent so AR can not hold the gold in it? Any comments would be greatly appreciated...

 

[FrugalRefiner]

Are you heating the AR during the leach? It may be becoming super saturated, and when it cools it can no longer hold all the dissolved metals.

I had to look up prismatic rice to see it is sodium thiosulfate, so you taught me something new.

Dave

 

[snoman701]

Hi All,
we do diamond removal with AR. we use three batched of AR, 1/3 of the necessary AR at a time. After each batch we pour off the acid and rinse the solids with prismatic rice to get rid of silver chloride,then apply the second and third batches. This speeds up the process a lot. Problem starts here, gold in the first batch AR partially drops out of the solution its self without us doing anything. why do you think this is happens? is it because Nitric is well spent so AR can not hold the gold in it? Any comments would be greatly appreciated...

No, nitric only oxidizes gold to get it in solution, it is not necessary to keep it in solution.

Are you somehow getting the sodium thiosulphate residue in with the leach?

Hydrochloride acid + thiosulphate not only consumes free acid but also releases sulfur dioxide that can precipitate gold.


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[EMRE]

Hi,
No, there is no contamination from thiosulfate at all. I decant the wasted first batch AR to another beaker and let it sit as I work on the second batch. I check it some time later and I see silver chloride (gray), and gold dust(brown) laying at the bottom of the beaker..

Regards.

 

[snoman701]

If it's hot and concentrated you can get some precipitation just from the evaporation of acid.


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[EMRE]

Are you heating the AR during the leach? It may be becoming super saturated, and when it cools it can no longer hold all the dissolved metals.

I had to look up prismatic rice to see it is sodium thiosulfate, so you taught me something new.

Dave

Dave,

i do not heat it since the reaction is very vigorous and fast, it gets hot by its self anyways. I am not a chemist but my guess is same as yours.I think since I use only 1/3 of the needed AR, it gets super saturated at the end of the reaction and may be the copper in it pushes the gold out. I also tried adding some more nitric to it after I decanted the AR hoping the gold would redissolve back into the solution but it did not happen.
in my earlier technique, I used to put the full amount of hydrochloric acid and only 1/3 of the nitric and keep adding the other two as the prior depleted.But I did not take the metal out to get rid of silver choloride. Hence the digestion time was much longer and slower. I do not recall gold ever dropping out that way. Problem was processing time.

 

[FrugalRefiner]

I also tried adding some more nitric to it after I decanted the AR hoping the gold would redissolve back into the solution but it did not happen.

Did you try adding any additional HCl along with the nitric? If all the HCl was consumed in the original reaction, adding nitric alone would not cause any gold to dissolve as there are no free chloride ions available. When a reaction stops, it's always a good idea to add some more HCl to be sure it's not depleted. A little extra HCl is not a problem. Excess nitric is.

How do you know it's gold that precipitated?

Try adding a little fresh HCl to the nitric and the precipitate and let us know what happens.

Dave

 

[EMRE]

Dave

No I did not add hydrochloric acid when I added nitric acid. I thought there was plenty in the solution but you may be right about that. I will add them together and will let you know the results next time I do stone removal. The reason that I know that it is gold is when I do filteration to get silver chloride out, I see brown and gray color mud on top of the filter paper. I scanned it with xrf machine and I read high grade of silver and gold.
By the way I put that mud back into Beaker and I applied fresh aqua regia and it dissolved the gold. Like you said it may need it fresh hydrochloric acid.

 

[Platinum]

Are you running a 3:1 or a 4:1 ratio of Hcl in your ar process?
Can you give us the break down of the karat gold and the weights?

 

[EMRE]

Are you running a 3:1 or a 4:1 ratio of Hcl in your ar process?
Can you give us the break down of the karat gold and the weights?

Lots come in as mixed karats. We don't have time to go through one at the time to see what karat they are. However on average it is usually 70% 14 carat 20% 10 carat and 10% 18 karat. We use one to three ratio and we apply 500 mL Hydrochloric and 155 ml nitric acid per 100 pennyweights

 

[EMRE]

I also tried adding some more nitric to it after I decanted the AR hoping the gold would redissolve back into the solution but it did not happen.

Did you try adding any additional HCl along with the nitric? If all the HCl was consumed in the original reaction, adding nitric alone would not cause any gold to dissolve as there are no free chloride ions available. When a reaction stops, it's always a good idea to add some more HCl to be sure it's not depleted. A little extra HCl is not a problem. Excess nitric is.

How do you know it's gold that precipitated?

Try adding a little fresh HCl to the nitric and the precipitate and let us know what happens.

Dave

Dave,
We did another batch today and we added Hydrochloric + nitric acid to the depleted first batch of AR. Like you mentioned the cemented gold went back into the liquid. It was a success. However I think we added too much nitric acid because we had to neutralize a lot of active nitric acid at the end of the process. Let me throw you a curveball ball here; do you think the first batch stop working and depleted because it ran out of Nitric or because hydrochloric acid was so saturated so it could not take any more gold in? I guess the way to find that out is to add just hydrochloric acid to the first batch after it is depleted and see if cemented gold goes back in to the Lakewood or not? Correct?

 

[FrugalRefiner]

I think you ran out of HCl. Harold wrote about this often. If an AR reaction stalls, add more HCl before HNO₃.

Dave

 

[Platinum]

How large are these solutions volume wise in ml? It looks like your nitric consumption figures are based on the whole alloy being gold when in reality it's got a lot of silver. This will lead to excess nitric.
You said you are not heating them. If you don't heat it then you are going to have excess chemicals that are not consumed in the reaction. You said you tried it the other way, but it took to much processing time. I have never done straight karat without heat. Heat will not only assure you consume all the available chemicals, but that the ar can penetrate and undercut the silver chloride layer and do it job faster and more efficient.

Which do you add first to kick the process off, nitric or hcl?
I add about a 1/4 of the nitric i need first and heat it for a little while to help start eating away any silver that it can before you add the hcl and cause a chloride layer. It helps to undercut the gold and reduce the chloride layer. Once you add the hcl to the solution it will still form silver chlorides, but the chloride will be on the bottom of the beaker and not encrusting your gold from the start slowing down your process.

 

[EMRE]

I think you ran out of HCl. Harold wrote about this often. If an AR reaction stalls, add more HCl before HNO₃.

Dave

I agree. I should.probably start off with some excess HCL. Probably 1/4 ratio may work better too..
Thanks.

 

[EMRE]

How large are these solutions volume wise in ml? It looks like your nitric consumption figures are based on the whole alloy being gold when in reality it's got a lot of silver. This will lead to excess nitric.
You said you are not heating them. If you don't heat it then you are going to have excess chemicals that are not consumed in the reaction. You said you tried it the other way, but it took to much processing time. I have never done straight karat without heat. Heat will not only assure you consume all the available chemicals, but that the ar can penetrate and undercut the silver chloride layer and do it job faster and more efficient.

Which do you add first to kick the process off, nitric or hcl?
I add about a 1/4 of the nitric i need first and heat it for a little while to help start eating away any silver that it can before you add the hcl and cause a chloride layer. It helps to undercut the gold and reduce the chloride layer. Once you add the hcl to the solution it will still form silver chlorides, but the chloride will be on the bottom of the beaker and not encrusting your gold from the start slowing down your process.

Based on the calculation sheets I have, I should be using 2000 mL of hydrochloric acid and one third of that for nitric acid for 400 pennyweights of karat gold. I divide this volume two three batches and apply them one at the time. I also do Prismatic rice leaching in between Cycles to get rid of silver chloride and speed up the cycling times.
I have never applied the nitric acid first like you recommended. I know that is the proper way for refining. However, since we cannot do inquartation, I would guess the effect of nitric acid alone would not be all that much.. I digest about 90% of the 400 pennyweight of karat gold in about 90 minutes. At that point all the diamonds are loose so I don't really have to continue for the complete digestion. We pick out the insolubles and I melt and assay them and pay accordingly.

 

[g_axelsson]

You mentioned cemented gold and I think that is what it is.

When the acid is depleted some of the gold is cemented back and other base metals dissolve. The gold is in the form of a very fine powder and stay suspended in the solution for a long time. It's only when you filter it or allow it to stand for a while it settles to the bottom.

Is this a problem? The amount suspended is probably very small and wouldn't warrant the cost of treating the excess nitric acid or the time it takes to recover it. Let it ride with the silver chloride and recover it when you recover the silver.

Göran

 

[EMRE]

You mentioned cemented gold and I think that is what it is.

When the acid is depleted some of the gold is cemented back and other base metals dissolve. The gold is in the form of a very fine powder and stay suspended in the solution for a long time. It's only when you filter it or allow it to stand for a while it settles to the bottom.

Is this a problem? The amount suspended is probably very small and wouldn't warrant the cost of treating the excess nitric acid or the time it takes to recover it. Let it ride with the silver chloride and recover it when you recover the silver.


Göran

Goran,
Yes the problem is exactly like what you described. I think this happens because Hydrochloric is completely saturated like Dave mentioned. However, the amount drops out of the liquid is not very little like you mentioned. Once we did the recovery process for the gold dropped out onto the filter by applying AR and it was almost 20 pennyweights.

 

[g_axelsson]

That sounds like a huge amount, close to 30 grams. I had to look up pennyweights.
5% out of 400 pennyweights, it's hard to imagine that much staying suspended. Maybe it's sticking to the silver chloride.

Well, at least you understand what is happening and that's a long way towards finding a working solution or a work around.

Göran