Non-Chemical Au precipitate re-disolving in HCL boil

[elfixx]

Is it normal for the gold precipitate from AR to start redisolving when proceding to the HCL wash?? When I ppt the gold from AR I first decant and wash with distilled water until washing water come clear and then I boil the powder in 30% HCL for a few minute. The acid become yellowish and test positive for Au with stanous. Is that usual??How can I avoid this whitout having to eliminate the HCL wash which would affect the purity....

 

[lazersteve]

Two things can help:

1) Wash thoroughly with hot water before boiling in acid.

2) Incinerate to red heat the gold powder before the acid wash.

Steve

 

[elfixx]

You mean to bring the gold powder to a red glow but not a melt and then wash it in HCL, then water and then HCL and water again?

 

[lazersteve]

Yes, but do a water hot water rinse before incineration.

 

[Harold_V]

One thing to consider is that you may be dragging down some gold chloride that isn't getting precipitated. I had that happen on many occasions when I'd precipitate large volumes. Stirring the lot while bubbling SO2 gas through it would precipitate what little that remained. That is only a concern for appreciable volume, where you end up with a thick layer of gold at the bottom of your vessel.

It is possible to precipitate gold when there are traces of nitric present. It doesn't take much to dissolve a respectable amount of gold. I'm not going to suggest that's what has been happening, but it's a distinct possibility. In such a case, boil for a prolonged time, until you're sure you have eliminated the nitric (by dissolving the gold). Make sure you have some free HCl so it can do its work. When you are comfortable that the nitric is gone, add some ice to cool the solution, then re-precipitate. That will avoid incinerating the pure gold.

I like incineration between operations when it's required, but pure gold is too easily contaminated by the handling and heating, so it's best if you can avoid doing so. You're better served to try what I recommended. It works, and very well.

Harold

 

[Oz]

Harold,

That makes a lot of sense when you have an inch of precipitate in the bottom of the beaker. You are sure to have gold chloride hidden in the mass. It is similar to your typical suggestion of adding a sacrificial button of pure gold to eliminate all remaining nitric but in this case it is the (hopefully) light brown precipitate.

It is funny to hear you recommend against incineration but in this case such a finely divided powder would be easy to have losses doing so.

I would like to hear more about your thoughts on the icing of the solution.

 

[Harold_V]

I would like to hear more about your thoughts on the icing of the solution.

My comments may be controversial, but bear with me. I have years of experience in precipitating from concentrated solutions, and achieved consistent results.

When I re-refined, I have mentioned, frequently, that I processed in amount in the 75 ounce range. The gold was processed in four vessels, each 4 liters in size. I started with roughly a liter of gold chloride, which would contain anywhere from 17 to 20 troy ounces. To the containers I would add three liters of ice (I had, and currently still have an ice machine).

Few have given any thought to the process of precipitation in the way of what happens with the energy contained within. If you precipitate from dilute solutions, especially if you add SMB or you use ferrous sulfate, you add more solution to what is already likely quite dilute. You see little, if any, change in the solution temperature.

That is not the case when you precipitate from concentrated solutions, using SO2, which was my normal process. I started with what was almost four liters of iced solution, and ended up with a solution that was often too hot to touch.

Gold precipitation is obviously an exothermic process, so you must deal with the heat. How I discovered heat was an issue was getting near the end, having a solution that was exceedingly hot, yet the last bit of gold wouldn't precipitate, in spite of a constant stream of SO2. In desperation, I finally tried adding more ice, and got instant results.

So then, I recommend cooling the solution after boiling, so the resulting heat of precipitation can be absorbed. Perhaps precipitating with oxalic acid would eliminate the need to do so. I tried it only once, and was not impressed. I should also note that I did so cold, not hot. I should have had my copy of Hoke at the ready, but I did not. As a result, I don't have an opinion on oxalic acid.

Harold

 

[Oz]

That is so funny “my comments may be controversial”, it is your experience I seek when I ask you a direct question on forum Harold. Anyone’s input will be welcome, but I put your name at the top of my post because I am looking for a reply from your personal experience that you had referenced. If someone else does not believe it, oh well.

Having a liter of gold chloride then adding ice of 3 liters reminds me of the dilution of gold chloride with H2O to precipitate any silver that is in your chloride (yes I know that silver should not be in a chloride solution, it does happen). I am guessing here that you had already precipitated the silver then concentrated your gold chloride.

You are very correct in pointing out the energy (heat) given by the reaction. That is the cause of most foam overs, as the reaction produces heat it speeds the reaction. Silver refiners know this all too well.

Not being a chemist I would still think that oxalic acid would generate heat in precipitation as well. Don’t hold me to it, but I think oxalic acid reactions should be done a fuzz (yes, very scientific I know) above room temperature. I haven’t tried it myself but have some in case I run into a situation where I can’t get my gold down clean. It could then be worth trying a different precipitant to get that stray contaminant.

I should probably take a known mass of .999 gold into solution and precipitate with oxalic acid to see the reaction before trying it as a remedy.

Unfortunately I am not running 20 Troy Au at a shot, so that may be why I have not had the difficulty you did with heat. Thanks to you, if I find myself in that situation I will have the answer.

 

[jsargent]

That is so funny “my comments may be controversial”, it is your experience I seek when I ask you a direct question on forum Harold. Anyone’s input will be welcome, but I put your name at the top of my post because I am looking for a reply from your personal experience that you had referenced. If someone else does not believe it, oh well.

Having a liter of gold chloride then adding ice of 3 liters reminds me of the dilution of gold chloride with H2O to precipitate any silver that is in your chloride (yes I know that silver should not be in a chloride solution, it does happen). I am guessing here that you had already precipitated the silver then concentrated your gold chloride.

You are very correct in pointing out the energy (heat) given by the reaction. That is the cause of most foam overs, as the reaction produces heat it speeds the reaction. Silver refiners know this all too well.

Not being a chemist I would still think that oxalic acid would generate heat in precipitation as well. Don’t hold me to it, but I think oxalic acid reactions should be done a fuzz (yes, very scientific I know) above room temperature. I haven’t tried it myself but have some in case I run into a situation where I can’t get my gold down clean. It could then be worth trying a different precipitant to get that stray contaminant.

I should probably take a known mass of .999 gold into solution and precipitate with oxalic acid to see the reaction before trying it as a remedy.

Unfortunately I am not running 20 Troy Au at a shot, so that may be why I have not had the difficulty you did with heat. Thanks to you, if I find myself in that situation I will have the answer.

Something to watch for with oxalic acid: Been experimenting with oxalic acid as an agent to selectively remove iron from non-selective precipitations. Works well except for one thing... it apparently precipitates calcium out of the well water I am using, as calcium oxalate crystals, which of course are insoluble. Incineration of the resulting precip burns the calcium oxalate to oxide which then can be easily removed with mild acetic acid. A note here, I am assuming the white precip is calcium oxalate because I can do this same procedure with citric acid and I get no white precip, but the reaction takes much longer and the dissolution of the iron is not as complete. Oxalic acid is a VERY strong "weak" acid. Safety first.

 

[elfixx]

Could you pro's teach me how to proced to incineration the right way
I have never incinerated anything so i'm not shure how to do it correctly.
In fact I have no clue of the purpose or the chemical reaction involved.
I'd realy appreciate a step by step procedure as well as the kind of equipment required or those which can acheive the best result.

Thx

 

[Anonymous]

Dr. Glazo out here in the Mojave Desert...

Say Harold, would you be so kind as to describe the color of a 1 liter solution that contains 15 Troy ounces of Au before precipitation... I presume that is AuCl III ...

***
Greetings Oz...

Straying from the subject... I achieved resolution with Verizon by posting a detailed complaint on the Verizon Guideline Employee website that originate from the Office of the Board Of Directors in the Big Apple...

I'll fill you in later... Good questions and comments between Harold and Oz... Thanks

 

[Oz]

Jsargent,

Thanks for the input and caution on oxalic acid. I should not have a problem with calcium as I use distilled water from a dehumidifier (can’t beat free) in all my procedures. Am I reading your post correctly that you are precipitating Fe out of a Au containing solution instead of the other way around?


Dr Glazo,

I am glad to hear your situation is resolved with Verizon. It is too bad you were not reimbursed by the hour for all your wasted time, not to mention lack of service.

I have taken gold chloride to a red but do not know how many grams per liter it contained. It might be worth knowing as a general guideline at what concentration you start seeing the shift to red. It would be dependant of course on the contaminates in solution especially in a first precipitation. We will talk soon.

 

[Harold_V]

I am guessing here that you had already precipitated the silver then concentrated your gold chloride.

All depends on what I was processing. I was never concerned about a little silver chloride hitching a ride up to the point of filtration. Otherwise it would have driven me crazy, and I'd have spent considerable time trying to eliminate traces that would get eliminated in the end by filtration. That, of course, does not take into account the traces that remain in solution that are precipitated upon dilution. I relied on the ammonium hydroxide wash to limit the amount of silver carried over to the final product. It may not have been the best scenario, but from all indications, my quality was superb, so I stayed the course.

Because I inquarted my gold, the vast majority of silver was removed in the preliminary operation, in preparation for dissolving the values. Those that process e scrap may well never encounter silver, so for them it may be a moot point.

As you likely know, it borders on the impossible to wash these substances adequately to insure that everything has been removed. You are far better served to live with the traces, allowing them to follow the gold chloride solution to filtration. They will be removed in that operation, which would be true even if you went far out of your way attempting to eliminate them by continual rinsing. Note, also, that I am referring to gold from the jeweler's bench, which is guaranteed to contain silver.

Note that I am not suggesting my method is the best way to go. For me it was, because of my work load. I had to keep all batches flowing nicely, otherwise I got jammed up and ran late on deliveries. One of my selling points was fast and honest refining.

As I've stated previously, once I understood the need for a second refining, all of my gold received the treatment. That may well have been a few years after I started refining. Remember-----I had no one with whom I could discuss the issues ---very unlike the readers of this forum. Fact is, it took me many years to discover the secret of pouring shot instead of cornflakes, and that was only because of a fellow Greek that worked at the refinery in Sparks, Nevada (now defunct) that advised me of the proper procedure. I may have never deducted it from reading or personal experience. There are times when we simply must rely on those that have gone before us.

Anyway, back to the second refining----by now, what little silver may have been following the gold would still remain, and be eliminated in filtration, or when washing with ammonium hydroxide. I got two shots at removing traces of silver, and feel it was effective enough that I'd recommend the procedure to anyone that was interested in turning out comparable quality.

All I can say is if anyone can come up with a method that yielded gold of better quality than that shown below----power to them!

Harold

 

[Harold_V]

Say Harold, would you be so kind as to describe the color of a 1 liter solution that contains 15 Troy ounces of Au before precipitation... I presume that is AuCl III ...

Sigh!
I can only assume it is AuCl III, for I honestly do not know. What ever the product of dissolution with AR is, that's what I dealt with. Sorry to appear to be a moron, but I am not educated, and do not understand chemistry well. I understood the mechanics of refining, in spite of not having a great chemical understanding.

So then, in answer to the color question. It all depends on traces included. If you are processing gold of high purity, the color will be a deep orange-red. You have likely seen pictures of dilute solutions, often the color of beer.

The level of concentration dictates, as do traces of other elements. A hint of palladium, for example, would deepen the color considerably, as would a trace of platinum. Platinum is inclined to turn the solution a more red-orange color, while palladium turns it more towards a brown-orange color. Again, all depends on percentages. It takes very little palladium to turn the solution brown.

It was not unusual to see a hint of blue in the remaining solution after precipitation in the second refining. It had little effect on the color of the solution.

The presence of the blue tint was evidence that I was on the right track. In spite of careful washing, I knew that gold from a first precipitation, at least by my methods, was generally not clean. While washing improved the quality considerably, it was not a cure all. The particles agglomerate and trap residues within the clumps, and removing them is not accomplished easily. Be processing a second time, the impurities are liberated, resulting in a solution that is very dilute with unwanted substances. Little is dragged down on the second precipitation, so gold quality benefits greatly.

Sorry I can not display a photo. In all my years of refining, I never photographed any of the processes. Didn't think anyone would care to see them.

Harold

 

[Harold_V]

Could you pro's teach me how to proced to incineration the right way
I have never incinerated anything so i'm not shure how to do it correctly.
In fact I have no clue of the purpose or the chemical reaction involved.
I'd realy appreciate a step by step procedure as well as the kind of equipment required or those which can acheive the best result.

The subject has been discussed often. Please do a search for incineration, which will help avoid my spending time on a subject that has been well covered.

I'll give a couple guidelines here, however.

To incinerate, I like a stainless fry pan. They are thin and don't require a lot of added heat to raise the temperature of the contents. One should avoid pans made of aluminum (they'll melt), and pans made of steel, which oxidize readily, yielding a very short useful life, and adding contaminants to the lot being incinerated. Stainless adds virtually nothing. I purposely avoided cast iron, not only because of it's thickness, but it is also thermally sensitive. If you play a hot torch on the surface, you risk cracking the pan. Others see it totally differently. For me, they were wrong.

I incinerated on a hot plate, gas fired. I had adjusted the burner to maximum performance, so it provided excessive heat.

I did all my incineration in a fume hood (made totally of asbestos, so it could not burn), which was filtered to catch particles that were carried off in the gasses (smoke). Depending on what is incinerated, losses can be excessive. There is little to worry about in the way of losses if you are incinerating solid objects. My concern was in incinerating polishing wastes from the jeweler's bench. After incineration, the end product is very fine, and easily carried off.

Do not incinerate on a wooden surface. Prolonged heating reflects a large amount of heat downward, which can ignite the substrate. When I moved my operation to the castle, I found an area roughly six inches in diameter that had been burned almost all the way through the 3/4" plywood top. There was a layer of 3/8" asbestos on top of the wood surface.

A large air gap between a non-combustible work surface and any wood below will serve to eliminate any dangers. I built my next hood and counter accordingly, spacing the heated surface, which was now 1¼" thick, six inches from anything combustible.

Hope this helps. If you have a specific question, please ask. Mean time, study Hoke-----she discusses incineration. Do avoid her recommendation to toss on gasoline liberally. That is absolutely not acceptable, and can be the cause of burning down one's building. Gasoline has no place in a refining lab.

Harold