Silver still in solution and precipitating with gold?

Hi all,

This problem is killing me.. first some words on the method i use:

Source Au: 18k to 20k jewelry
Method: 1. Melt with KNO3 & borax (Na2B4O7.10H2O), create ingot
2. Remelt for granulation in water barrel
3. Put granulate in rotating drum
4. Add 1 part HNO3 (60%), 3 parts HCL (34%). (Part: 0.7 liters per 1 kg of granulate)
5. After about 4 hours add another part of HCL
6. After 3 to 4 hours when reaction is finished pull all the AR with a vacuum through a double filter cloth.
7. Neutralize AR with about 1 to 2 times hot water.
8. Precipitate with food grade Na2S2O5. About 2 to 2.3 parts per kg of Au in solution is needed. (very rarely red fumes)
9. Pull through another filter cloth.
10. Rinse 1 time with hot water (75°C). Rinse 2 times with cold distilled water.
11. Melt the gold.

My problem: Au is never 9999, always 9988 to 9995 with silver as the main problem

What i tried: - Boiling the AR to neutralize, then precipitating it -> no change (About 0.9 to 1.3 parts per kg of Au in solution is needed for precipitation)
- Adding H2SO4 to neutralize, then precipitating it -> no change (About 0.9 to 1.3 parts per kg of Au in solution is needed for precipitation)
- Refine the precipitated Au again -> no change
- Used about every filter cloth available, from cotton to paper to synthetic -> no change
- Rinsing precipitated Au with fresh _cold_ HNO3, water, water, water, HCL, water, water, water, NH3, water,water, water -> no change

What i didn't try: - Boiling in NH3 (24.5%), HCL, HNO3. This seems like ultimate last resort, the fumes from this are so extreme...

What i read here: Some AgCl stays in solution and mimics AuCl4 and as a result precipitates with it.

If so, what could be another possible solution, i'm no chemist and running out of ideas here.. All suggestions welcome.

Hoping for some clever insights & thanks in advance.

Inquartation is the route to go.
Use the forum search, in the top right of the page, keyword inquartation.

I don't know where you got your method of refining from but a study of the results from the above search and reading hokes book, available as a free download from many members signature lines, will point you in the correct direction.

Welcome to the forum.

I'm at a loss to understand why you'd use hot water to "neutralize" the acid. Hot water does no such thing (although I do admit, it does dilute the acid), and, because precipitation of gold is an exothermic reaction, it may actually hinder the precipitation of gold (not true if using oxalic acid). Likely not a problem, assuming you're precipitating from diluted solutions, however. The chief reason I'd worry is that cold, alone, gives silver cause to precipitate (as silver chloride), silver that is likely part of the problem with the purity of your end product.

I have no qualms with rinsing with water, but the idea that you can effectively wash gold with just water simply makes no sense. If you don't use a solvent of sorts, there are substances that will refuse to be removed. An example may be using ferrous sulfate to precipitate----which leaves behind iron in solution. If you alter pH by rinsing, that which has not been removed (there is always some) gets oxidized, leaving behind rust. Washing with HCl eliminates that possibility.

Silver, as chloride, is soluble in ammonium hydroxide. Does that leave you with a clue about what to do about residual silver? It won't be there as the element, but as the chloride.

I had a regimen I used to wash my gold. The end result was never disappointing.

As has been noted by martyn111, inquartation is advised when refining karat gold. It's an additonal step, but eliminates many of the issues that accompany dissolution of karat gold.

Harold

I don't think you're doing enough to remove the small amount of silver chloride that is dissolved in the strong aqua regia. It doesn't take much to lower the gold purity.The solubility of the dissolved silver chloride can be reduced in one of 2 ways: by dilution or by chilling. I usually diluted the AR with 3 volumes of water (not hot water). Many on the forum add ice to the AR - this has been discussed several times. In either case, the solution must be filtered to remove the silver chloride that precipitates, before precipitating the gold. In your procedure, this would be between steps 7 and 8. I see no filtration step at that point.

No matter what you do, producing 999.9 Fine gold with this method is pretty much a dream (occasionally, you might get lucky, though). However, with care, you can consistently produce 999.5 Fine or better.

BTW, what analytical method was used to determine the purity of your refined gold?

Inquartation:
I'm familiar with inquartation but this is no option for me, it takes too much time, time i don't have. I must be able to produce an ingot within 24 hours of receiving the juwelry.
But i dont think the solutions lies here, because if i refine the gold powder again i get a very clear/yellow AR and still have some Ag contamination after precipitation.

Hoke:
Hoke's book is very, very interesting, i've been reading it during my free time for the past 2 weeks, i guess i'm about midway now. It's like a bible (for me as a newbie it is)

Warm<->hot:
At certain intervals i test all my barrels of used AR for remaining Au by trying to precipitating it again with Na2S2O5. I have found that if the AR is below 50°C no reaction happens and no precipitation is occurring. If i heat the AR to just above 50°C i get a visible reaction (no red fumes), and very rarely some Au falls down which wasn't precipitated the first time.
If i don't dilute (thank you for the correct description) the AR with water or neutralize it in one way or the other precipitation is very hard and a lot of red fumes appear. Which is normal i guess since the acids are too strong (as described multiple times in Hoke's book).

Rinsing with water:
I do this, as you said, just to make sure no more acids are present before melting.

Gold washing:
I know washing the gold with boiling NH3 and HNO3 will resolve my problem since it would remove any Ag and AgCl. I'm just trying to understand why I have silver contamination even after refining twice. I don't see the logic of it. If i would, i could minimize the problem.
I want to determine if the residual Ag is in Cl form or not so i've ordered some materials enabling me to test washing the Au with boiling solvents, but i've been waiting for 3 weeks now for everything to be shipped. But still, the fumes produced by boiling solvents are quite of a hassle to deal with on a "production scale".

If it turns out the Ag is in Cl form, could this problem still be created by precipitation for Au or would the Ag fall down in pure metal form then?

Cooling down AR before filtration:
I haven't tried this, i could add ice to it before filtration, this shouldn't be much of a problem. I will tried it next time. Thanks!

Analytic method:
I always send my 9999 test samples to 3 recognized laboratory's. One to the official one of my country, one to Chimet (Italy) & a firm located in the Netherlands. They all use cupellation and ICP spectrometer. The results differ, but rarely one come's out 9999, Ag is always the main impurity.

Warm<->Cold:
So strange you guys speak of precipitating in cold AR since according to my tests, it should be at least 50°C before precipitation occurs with Na2S2O5.
I use BASF food grade minimum 99.9% pure fine powder, i guess that one should be good enough.
I will test this again... So very strange, can't really wrap my head around it.

goldsilverpro said:

In either case, the solution must be filtered to remove the silver chloride that precipitates, before precipitating the gold. In your procedure, this would be between steps 7 and 8. I see no filtration step at that point.

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This is described in step 6: "After 3 to 4 hours when reaction is finished pull all the AR with a vacuum through a double filter cloth."

goldsilverpro said:

No matter what you do, producing 999.9 Fine gold with this method is pretty much a dream (occasionally, you might get lucky, though). However, with care, you can consistently produce 999.5 Fine or better.

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I have read this over and over again. But, if you boil the Au in solvents afterwards, isn't this a viable method to produce 9999 with certainty?

So strange you guys speak of precipitating in cold AR since according to my tests, it should be at least 50°C before precipitation occurs with Na2S2O5.

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I said nothing about precipitating the gold from a cold solution. I was talking about chilling to precipitate out the silver chloride. Also, I most always precipitated the gold at room temperature. Always worked for me. However, my solutions contained little or no excess free nitric acid. I controlled this by only using enough nitric to dissolve the gold.

goldsilverpro said:

I said nothing about precipitating the gold from a cold solution. I was talking about chilling to precipitate out the silver chloride. Also, I most always precipitated the gold at room temperature. Always worked for me.

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Sorry for the misinterpretation.

goldsilverpro said:

However, my solutions contained little or no excess free nitric acid. I controlled this by only using enough nitric to dissolve the gold.

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How do you handle this? By, when no more reaction is occurring, visually checking and adding more if needed?

Na2S2O5:

I was looking over the wikipedia page:

wikipedia said:

Sodium metabisulfite releases sulfur dioxide in contact with strong acids:

Na2S2O5 + 2 HCl → 2 NaCl + H2O + 2 SO2

on heating it releases sulfur dioxide, leaving sodium oxide behind:

Na2S2O5 → Na2O + 2 SO2

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Anyone any idea what would be the theoretical minimum temperature required?
And what would be the main difference in result/effect between the NaCl & Na2O?

Thanks all for your input.

Notorious said:

Na2S2O5:

I was looking over the wikipedia page:

wikipedia said:

Sodium metabisulfite releases sulfur dioxide in contact with strong acids:

Na2S2O5 + 2 HCl → 2 NaCl + H2O + 2 SO2

on heating it releases sulfur dioxide, leaving sodium oxide behind:

Na2S2O5 → Na2O + 2 SO2

Click to expand...

Click to expand...

Anyone any idea what would be the theoretical minimum temperature required?
And what would be the main difference in result/effect between the NaCl & Na2O?

Thanks all for your input.

Click to expand...

The first part is right. The 2nd part makes no practical sense. If you heat it, it will drive the SO2, a gas, out of the solution and it will be wasted.

When you get ready to do your next batch, before you filter it put your container inside anther container and put crushed ice around it with some water to cool your solution, then when you filter your solution put some crushed ice in your filter. If you are using coffee filter's use 3-4 of them. Also do a search on the forum for "charmin plug" to use in your filter neck.

It is a well documented phenomenon that a trace of silver behaves like gold in a chloride solution. Whether the cause has been properly addressed, or not, I don't know---but I do know that the condition is well known. You can limit the problem by NOT using hot water, and by filtering after dilution.

I am the one that spoke of not precipitating gold from hot solutions. It was my routine to precipitate from concentrated solutions, often as much as 18 ounces in four liters, which was, primarily, ice. I'd concentrate the solution until it was about a liter in volume, filter, then add three liters of ice. My common precipitant was SO2, from a cylinder, which is no different from that which you have elected to use. What I discovered through repetitive operations was that as my solution warmed, precipitation slowed, in spite of a continuous flow of gas. At some point, precipitation ceased entirely, in spite of an excess of SO2 being introduced. The solution, at this point, was quite hot--too hot to handle the vessel comfortably by hand. All it took was the addition of a little more ice for the precipitation to resume. It is clear that hot solutions do not respond well to precipitation when using SO2.

It is equally clear from your comments that you are not eliminating all of the free nitric acid. That's the only reason you see brown fumes evolve from your solutions, which, if I recall, was upon heating. It is for that reason you achieve further precipitation. If you evaporate properly, all of your gold will precipitate in the first attempt, assuming the solution is not overheated, and assuming you have introduced enough SMB. There is NO need to precipitate a second time.

I talked about proper washing your product. You ARE NOT DOING THAT. You can not eliminate all contaminants by rinsing with water, and you will never eliminate any of the silver by rinsing with water, nor with nitric. There is no free silver mixed in your gold---what little silver that is present is in the way of silver chloride. That can be eliminated by washing the gold, first with HCl, followed by a prolonged boil with water. A second wash with ammonium hydroxide will dissolve and remove silver chloride. Follow that operation with another water rinse, then a third wash using HCl once again. Rinse a final time, then dry the gold. It is not a requirement that you eliminate all acid from the gold prior to melting, so multiple rinses perform no useful purpose, especially if you ignore the use of acid or ammonia to dissolve contaminants.

You can safely lose the idea that you're going to produce 4n's gold by the chemical process, as you've been advised. It can be done, but it requires far more attention to detail than you are offering. It is for that reason that the industry standard for gold has been established at 9995, and the reason that treatment by Wohlwill cells is applied if greater purity is desired.

Harold

Even though you filtered the AR in step 6, you precipitated more silver chloride when you added water in step 7, since the solubility of the silver in the dilute solution is less than in the strong solution. You must filter after dilution, rather than before.

To get the best purity from this process, you must properly leach and rinse the gold powder. The processes that Harold and I used for this are somewhat different but both of us used ammonia as one step of this process. The ammonia dissolves silver chloride, as Harold mentioned.

By not heating the AR when dissolving the gold, it's very difficult to control the amount of nitric acid being used. You will invariably use an excess and this must be eliminated in some manner before precipitating the gold.

Neutralizing AR:
Forgot to mention: I have noticed that if I use H2SO4 there is a lot more drag down occurring while precipitating.
Are there any other solutions for neutralizing the HNO3 except for boiling?

goldsilverpro said:

Even though you filtered the AR in step 6, you precipitated more silver chloride when you added water in step 7, since the solubility of the silver in the dilute solution is less than in the strong solution. You must filter after dilution, rather than before.

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I have ordered some materials to test dilution before filtration, should be arriving within the next 2 weeks. But i can never dilute with 3 parts of water. The amount is just to much.
First i will add ice until the temperature drops below 20°C. Then pull it through a filter, afterwards i will dilute it with 1 to 2 parts of cold water. Then pull through another filter. Afterwards i will try to precipitate the Au. Can't wait to try it out.

I have noticed that if I use H2SO4 there is a lot more drag down occurring while precipitating.

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Drag down of what? The only purpose of H2SO4 is to precipitate any dissolved lead and it takes very little to do this. It only takes 1 ml of concentrated H2SO4 or 3 ml of battery acid to precipitate 3.7 g of lead, which is quite a bit. The lead sulfate and silver chloride can be filtered out at the same time.

Are there any other solutions for neutralizing the HNO3 except for boiling?

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Like I said before, the simplest way is to not use way too much nitric to start with. That way, you don't have to evaporate (not boil!) the solution. The SMB will handle a small excess of nitric. It takes about 25 - 30 ml of 70% nitric, combined with about 120 ml of hydrochloric acid, to dissolve an oz of gold. If you are exceeding about 30 ml, you are using too much nitric, in my opinion.

Can you give more detail as to how you are tumbling the gold in the AR and with what type of tumbler?

Notorious said:

Neutralizing AR:
Forgot to mention: I have noticed that if I use H2SO4 there is a lot more drag down occurring while precipitating.
Are there any other solutions for neutralizing the HNO3 except for boiling?

goldsilverpro said:

Even though you filtered the AR in step 6, you precipitated more silver chloride when you added water in step 7, since the solubility of the silver in the dilute solution is less than in the strong solution. You must filter after dilution, rather than before.

Click to expand...

I have ordered some materials to test dilution before filtration, should be arriving within the next 2 weeks. But i can never dilute with 3 parts of water. The amount is just to much.
First i will add ice until the temperature drops below 20°C. Then pull it through a filter, afterwards i will dilute it with 1 to 2 parts of cold water. Then pull through another filter. Afterwards i will try to precipitate the Au. Can't wait to try it out.

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If you ignore the admonition to wash with ammonium hydroxide, you're spinning your wheels. You may see an improvement, but I expect you still won't achieve 9995 routinely, and likely never achieve 4 9's.

I see a reluctance on your behalf to make any changes in how you process----but what you do now is limited and will always yield marginal results. I get the impression you hope to have a reader tell you how to continue what you're doing, but achieving the hoped for results. That's not going to happen. You must make some changes in what you're doing if you expect to improve your quality.

Please DO NOT MAKE MENTION OF EVAPORATION BY DISCUSSING BOILING. One does NOT boil solutions when the intended purpose is to evaporate. That is not a proper procedure and leads to the loss of values. Readers that come to this forum looking for guidance take what you say literally, so the ignorance is repeated. We have to break that cycle.

Harold

Boiling!=Evaporating:
Sorry, my bad.

goldsilverpro said:

Drag down of what?

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All around dirt which is in solution, keep in mind i don't do a HNO3 attack before the AR treatment. As a result the AR is very dirty.
Normally i wait 24 hours, after precipitation and final filtering, before cementing (for other pm's) or neutralizing/floc so all the "black stuff"/dirt falls down. Then i pump over all the acid except for the sediment to a bigger barrel. This way i keep most of the sediment in one place .

goldsilverpro said:

The only purpose of H2SO4 is to precipitate any dissolved lead and it takes very little to do this. It only takes 1 ml of concentrated H2SO4 or 3 ml of battery acid to precipitate 3.7 g of lead, which is quite a bit. The lead sulfate and silver chloride can be filtered out at the same time.

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If i remember correctly, in Hoke's book, he describes alternate methods to neutralize HNO3, one of which is adding H2SO4. If i use this method, a lot of the dirt precipitates with the Au. (I add the H2SO4 a half hour before filtering out the AgCl)

goldsilverpro said:

It takes about 25 - 30 ml of 70% nitric, combined with about 120 ml of hydrochloric acid, to dissolve an oz of gold.

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Currently i use about 21.8 ml of 60% HNO3 and 87.1 ml of 34% HCL. I add the last part of HCL after the reaction slows down significantly.

goldsilverpro said:

Can you give more detail as to how you are tumbling the gold in the AR and with what type of tumbler?

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It's a simple cylinder manufactured out of PE rotating on a 45° angle. It has fins in it, you can compare it a little bit with a concrete mixer except its not open. Of course i connected it to a refluxer and a scrubber. It's quite handy, except you can't visually see the reaction unless you open it. But you get a good idea by looking at the refluxer.



@Harold_V:

I am very aware that, to make sure i always get 9999, i must boil the Au in solvents until no more traces are left behind in the solvents after filtration. I just want to make sure i maximize my result/time (efficiency) balance in every step of the way. The less i have to boil in solvents, the better.

Please understand, I am trying to adjust my process, that's why i'm reading and asking questions.

Notorious said:

Boiling!=Evaporating:
Sorry, my bad....

I am very aware that, to make sure i always get 9999, i must boil the Au in solvents until no more traces are left behind in the solvents after filtration. I just want to make sure i maximize my result/time (efficiency) balance in every step of the way. The less i have to boil in solvents, the better.

Click to expand...

If i remember correctly, in Hoke's book, he describes alternate methods to neutralize HNO3, one of which is adding H2SO4. If i use this method, a lot of the dirt precipitates with the Au. (I add the H2SO4 a half hour before filtering out the AgCl)

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In the Hoke book, I find no mention where it says to use H2SO4 as you are using it. SHE uses before the evaporation (about 1 part per 32 parts of AR - page 44) and only in conjunction with the evaporation (and lead removal). On page 45, she explains that its purpose is to help expel the nitric at the very end of the evaporation when it gets syrupy. At that point, it also prevents a crust from forming and provides sulfate ions for the precipitation of any lead that is present. Unless you evaporate the AR, H2SO4 would have no effect on the elimination of nitric.

The reason you have more "dirt" with the H2SO4 may be that some lead sulfate has dropped.

I am very aware that, to make sure i always get 9999, i must boil the Au in solvents until no more traces are left behind in the solvents after filtration. I just want to make sure i maximize my result/time (efficiency) balance in every step of the way. The less i have to boil in solvents, the better.

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Since you didn't inquart to eliminate the base metals before using the AR, it may be impossible to get even 999.5 Fine gold consistently by leaching in solvents. You will most probably have to redissolve the gold powder in AR and re-precipitate it, here again using all the ways you've learned to remove silver chloride and lead sulfate. Since the gold powder dissolves very quickly in AR, it's much easier to keep the nitric at a minimum. Here's how I do it:
(1) In a beaker placed inside of a catch container (I use a 5 liter Pyroceram Corning Ware dish - it's about the only thing that won't crack on the direct heat of a hotplate), cover the Au powder with at least 120 ml of HCl/oz of gold (I use 150 ml/oz). You should always use a little more HCl than you'll actually need. Some excess doesn't hurt. Use a beaker that's deep enough to allow for a lot of foaming.
(2) Heat at about med/low until you see a little fumes coming off. It shouldn't be extremely hot.
(3) Continue heating at a low level during the entire dissolving process - the dissolving reaction will also generate heat. Estimate (remember, though, that it's only an estimate - you may need a little more or a little less) how much nitric it will take and add about 1/4 of that. It should start reacting immediately (fizzing). If it doesn't, get the solution a little hotter and it will start reacting. When the reaction dies down or stops, give it a gentle stir and add more nitric. At the end, use smaller additions of nitric. Be patient and wait until there is little or no reaction between nitric additions. At some point, the gold is all dissolved. Stop adding nitric. Note: if the dissolving gets very slow towards the end, you can allow the nitric that's already in there to react, pour off the solution, and add a little fresh HCl and HNO3, with a little heat, to quickly dissolve that last little bit of gold.
(4) Either dilute with 3 volumes of water or treat with ice. Filter. Precipitate.

Once you do this a couple of times, you'll be able to get good control on the amount of nitric that is used. I would experiment on small amounts to start with. If you don't control the nitric and use too much, you will have to go through the standard evaporation process.

It's a simple cylinder manufactured out of PE rotating on a 45° angle. It has fins in it, you can compare it a little bit with a concrete mixer except its not open. Of course i connected it to a refluxer and a scrubber. It's quite handy, except you can't visually see the reaction unless you open it. But you get a good idea by looking at the refluxer.

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Not to change the subject but, have you ever had your cold processed silver chloride assayed for trapped gold content?