Aqua Regia method & Gray Ag powder

[Scrapper-aw-]

Hi Everybody,

I'm new to these forums, and have been refining for two years now. The scrap we deal with on a regular basis contains Au, Ag, Pt, Pd and other metals. Because the Ag level of this alloy is usualy around 10% we go directly into Aqua Regia with the shot. After the shot is digested in aqua regia and filtered and rinsed we are left with a gray colored powdery precipitate.

This precip we have been drying, mixing with borax and soda ash and smelting to yeild Ag usualy contaminated with anyware from 4% to 16% Au. On rare occasion it is less than 1% Au. I have in turn taken the contaminated Ag and used the inquartation method to separate the Au from it.

I would like to be able to skip the smelting and shotting steps of this process if there is a way to purify the gray powdery precip chemically. I have not been able to digest it in Nitric acid, with/without heat.

Two questions:

1. Does anybody know what happens to Ag in Aqua Regia that makes it insoluble in nitric acid; what does it become?

2. Does all silver chloride need to be smelted with flux to return it to a lusterous solid metal state?

Thanks,

Scrapper-aw-

 

[Palladium]

Read Hoke's book in my sig line.
Welcome to the forum.

 

[butcher]

my thoughts on this, you have made a chloride salt, silver chloride and gold insoluble in nitric, incenerate to drive off chlorine gas, before nitric dissolve.

by adding nitric to this silver and gold chloride salt you are making some aqua regia, wich can dissolove back the gold, this is usually undisireable, inceneration is a great tool we all need to learn to use.

I would not try to melt silver chloride, I would convert to metal silver with NaOh and karo syrup before trying to melt,

there is also a method to convert silver chloride, using a metal such as aluminum and sulfuric, others can give details I have not used this process

 

[Harold_V]

1. Does anybody know what happens to Ag in Aqua Regia that makes it insoluble in nitric acid; what does it become?

You appear to know the answer in that you are already discussing the conversion of silver chloride to elemental silver via furnace.

2. Does all silver chloride need to be smelted with flux to return it to a lusterous solid metal state?

No. There's nothing preventing you from making the conversion using scrap steel or aluminum. Even zinc. You recover not only any silver that is present, but also all of the greater values that are included. The problem is base metals that are present will get reduced, too. Plus, you must rinse and dry the resulting cemented material, which must then be melted. You avoid the smelting process, but you can't avoid the melting process.

Harold

 

[Scrapper-aw-]

Thanks for the quick responses!

I will check out Hoke's book, Thanks!

I wasn't sure if it was silver chloride or some other form of silver.

8)

 

[4metals]

Is the hood with the glass vessels what you're working out of? If it is you may get more of the gold to digest if you heat the vessels. As you are working in aqua regia, the insolubles are a dealing with are already in the form of silver chloride or silver chloride encrusted gold. When these chlorides are sitting on your filter it would be easiest to slurry them in water, add some caustic to convert it to AgO and add Karo syrup to reduce the silver. Now it can be melted, the bar will look nice and shiny and the gold is evenly distributed in the silver. Easy to assay, you don't even need to cupel it, just part the drillings. Good for lot accountability.

You haven't said how much Pt and Pd your material contains. Different concentrations of PGM's call for different methods. But if PGM's are low, the method above is viable.

The photo in your avitar is also drawing the nitric out of the vessels pretty quickly, you would be better off to condense the fumes and you can get away with less nitric and less values in your scrubber.

 

[Juan Manuel Arcos Frank]

Scaper-aw-:

Let me put my two cents.

First of all,read Mrs. Hoke´s book.

Second,silver chloride is easy to convert to metalic silver,you can use cementation process,Karo syrup process or you can also melt it with sodium carbonate,the way to do this was posted by GSP,here in the Forum.

There is a process that uses AR with a hig concentrated alkali salt to dissolve the silver chloride formed.In this moment I am not at home,but I promess to post it tomorrow morning.

Best regards

Manuel

 

[Scrapper-aw-]

Is the hood with the glass vessels what you're working out of?

Yes, it is. Those are 22 liter flasks. I normaly staart out with about 20 Oz. T. to about 3 liters of AR. The shot that we make is very fine in size, I would estimate no bigger than 1mm. The reaction is very violent and highly exothermic. Initally I only put about half of the burden into the flask at a time. The remainder is added once the reaction has calmed slightly. The solution rises in temperature to about 135F to 145F after the final addition of shot. The shot is actually more like sand or grain.

The photo in your avitar is also drawing the nitric out of the vessels pretty quickly, you would be better off to condense the fumes and you can get away with less nitric and less values in your scrubber.

The exhaust flow of each of the hoses on my bench are controllable via a ball valve at the top of the hose. I meter the exhaust to apply just a slight negative pressure in the flask.

You haven't said how much Pt and Pd your material contains.

Most of the material I work with has a Pt content of between 1% and 4%. The Pd content can be anywhere between 0% and 40%, but usualy somewhere in the 20's.

When these chlorides are sitting on your filter it would be easiest to slurry them in water, add some caustic to convert it to AgO and add Karo syrup to reduce the silver

I'm guessing, hoping, there is a step by step on the forums for this technique. I have a bunch of this stuff that I would much rather melt than smelt.

You know, after using the acccess to Hoke's book via Palladium's sig I realize this is the book that I have as well. I just haven't read it cover to cover. I will now.

 

[butcher]

are you recovering the Pd and PGM ? and if you deal in lots of material a silver cell may be worth the trouble. I think testing your silver or silver chloride for PGM should also be considered.

 

[4metals]

Search using Karo syrup, I believe Lazersteve has a video using karo syrup to reduce silver chloride on his web site.

If the particle size you are starting with is that small a good aggressive reaction is assured without heat. With a good reaction and residual gold as high as you say, your alloy must be 10% or better silver to start. You may be better served to inquart with silver and digest in nitric. Your silver will dissolve along with the Pd leaving gold and platinum undissolved. (Actually if Pt is low enough it will dissolve along with the silver and Pd in nitric)

Your silver can be cemented out of the acid along with the PGM's by using copper and melted into anodes to purify in a silver cell or the silver can be re-used until the PGM's concentrate reaches a level where they can be dropped cost effectively. If you run the bars directly in a cell, the Pt will be in the anode slimes and the Pd will concentrate in the electrolyte.

Your gold powder can be digested in Aqua regia to purify it and the Pt remaining in the acid can be dropped by cementation with copper. If the Pt content is high enough, and the particle size low enough you can digest it in warm HCl and sodium chlorate. By not using nitric the solution can be filtered and the platinum dropped as ammonium chloroplatinate before you drop the gold. It takes a small particle size to dissolve the gold with HCl chlorate but it's worth trying.

Is your goal to produce 99.99. fine gold and PGM's?

 

[Scrapper-aw-]

are you recovering the Pd and PGM ? ......I think testing your silver or silver chloride for PGM should also be considered.

We routinely test all of our precipitate. Our silver chloride is Pd & PGM free. It is always well rinsed. And, yes, we are recovering the Pd and PGM.

With a good reaction and residual gold as high as you say, your alloy must be 10% or better silver to start.

I know that our Ag is less than 10% in the alloy.....is it possible I have overburdened the AR or possibly been slightly impatient and rushed the process? I'm a technician at heart, very methodical, but I have been known to get excited from time to time.

Is your goal to produce 99.99. fine gold and PGM's?

Well, our goal os to produce 99.9 of all precious metals. I'm not sure that the additional refining to 99.99 is cost effective or necessairy for our market. We want to market our products to the dental and manufacturing industries. We have been producing 99.9 Au and Ag. We are just learning how to separate Pt from Pd and having some nice successes. But there's so much that I don't have experience with.

8)

 

[butcher]

I agree with 4 metals, if your silver content was this low, then how is it collecting so much gold?
either you have so much Ag that the acid is not penetrating to extract the gold out completely, or the AgCL is collecting gold as it forms salts, surely you are diluting the aqua regia afterwards with water to precipitate silver.

when are you getting platinum before or after silver?
silver would need well diluted,
gold best diluted,
platinum from concentrated solution better.
some more details as to when and how you are getting these are helpful

and I gather you are using not using nitric prestep? and recovering palladium and silver there?
but you stated you were going directly to aqua regia? when are you geting the palladium last?

as you can see I get confused easily.
your silver chloride should be whiteish , if gray I would think other metals are with it.but of coarse you mentioned drying so light will change some chloride to silver and change it's color, silver chloride should be kept wet and from light if possible.

maybe more details of your process helpful,
you have two well informed refiners Harold and 4Metals assisting you, as I am just a backyard hobbiest. just learning.

 

[4metals]

When you prepare a refining lot are all of the bars going into the lot melted together? How are you getting the material you are digesting to its small size, by melting and shotting or mechanically?

A few years back a client was having issues with chlorides coating over on some lots and not on others. The cumulative assays were added up so the silver remained under 7%. Turns out the melt shop never melted all of the bars together to let the silver be the same concentration in all of the shot they poured. They were shotting some of the bars separately and occasionally those bars had higher silver so they crusted over with silver. Is it possible that the same thing is going on in your shop?

 

[Scrapper-aw-]

Metals,
Yes, when we prep a lot for refining we melt all bars in the lot together at the same time. Once molten we pour it through a water spraying aray of four nozzles each spraying a sheet of water at 750 psi each. This essentially turns the metal into a fine powder to sand like material. (I actually filter all the water that we pour the shot into as some of the particles are so small they remain suspended in the water.) [We first tried it with normal houshold water pressure levels and that yeilded cornflake type shot. That works fine for mostly Au scrap in AR. But I wanted to inquart in order to purify my silver and recover the Au in it. Everything I had read about the inquartation method said it was most sucessful when the particle size of the alloy was 3mm or less. So we put in the new pump and nozzles and we got Au-Ag, 1:3, powder basically. It doesn't take long to digest when it's that small!]

I do have a heater that the large flasks fit in. I will try heating my solution to see if I can get a complete digestion of all gold.

Normally,(going straight into AR with our shot), after our AR solution is finished digesting we filter and capture the silver chloride on the paper filter as a grey sludg or sponge. This is a different consistancy entirely from the white silver chloride you would precipitate out of a nitric pre-step. The white stuff is like a mix of toothpaste and glue--the grey stuff is more like sand or powder, and rinses off of tools and gloves easily. And now I understand that they are the same material chemically. (Up till now we just set this material aside untill we have enough to make the oven time devoted to smelting worth our while. Then we mix with borax and soda ash and smelt. I would like to eliminate the smelting process. Melting, I don't mind. Smelting is messy, relatively speaking.)

The liquid containing Au, Pd & Pt is then treated with urea to bring the Ph to 1.
Then we dilute to a 1:1 ratio with tap water and preciitate the Au with SMB.

We test with stannus chloride to detect that all gold is out of solution then filter.(Gold sponge is then, subsequently, redigested in AR , filtered, and re-dropped to bring it to 99.9%.)

The remaining solution containing Pt & Pd is heated to 170*F and remove from heat. Then we add HCl and amonium chloride and allow Pt to precipitate.

Filter again, and remaining liquid is treated with sodium chlorate, small amounts.... until all Pd is dropped and liquid is a pale green or yellow. Another check with the stannus chloride shows that all precious metal, (of any quantity) is out of solution.

Filter one last time to capture palladium amonium chloride. We roast this to a sponge state then ignite to bring to metal state. We use the same technique with our platinum amonium chloride.

Our waste liquid is stored for the time being. We used to send it out for further recovery but are in the process of seting up our own waste water treatment system.

Question: When using the Kayro Syrup method on silver chloride, what kind of fumes are given off?

 

[Palladium]

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=53&t=120&hilit=Karo+syrup

 

[4metals]

Karo syrup fumes are mostly steam and oxygen as the AgCl passes from chloride to silver oxide and finally to silver metal. Small cement mixers with removable polypropylene tubs are great for doing this. In a hood of course.

Adding urea to your acid after the gold is dropped makes your ammonium chloride precipitation incomplete. Platinum doesn't get come down completely in the presence of nitrates, that's why classic methods require the 3X boiling down to remove nitric before dropping the Pt with ammonium chloride. Urea is pure nitrates so you're really not helping yourself with urea. Gassing with SO2 is better because it drives off the nitric before the gold drops so you don't need urea. But the best is the 3X boil down and dropping the platinum first.

Your description of the shotting process is interesting, you are not far away from atomizing your alloy. A powerful water jet hits a stream of molten metal just as the metal hits the water surface causing the metal to fracture into micro small particles. This gives you a tremendous advantage because the alloy now can be digested using warm HCl and sodium chlorate. The metal dissolves and after filtration the Pt can be dropped with Ammonium chloride because there was no nitric introduced. Plus it all comes down (except a few ppm) Then you go for the gold as usual. Considering you are so close to atomizing your alloy already, perfecting this process can streamline your process.

 

[Palladium]

I'm no expert by far, but even i can see the genuis it that system. The atomization is a big factor.

Please make checks payable to 4metals
c/o The gold refining forum.com.

On the poor mans side could you use warm Hcl and H2O2 to avoid the nitric ?

 

[goldsilverpro]

A couple of ideas:

(1) To get complete conversion of silver chloride to silver metal, it takes at least 20 grams of sodium hydroxide and 13.3 ml of light Karo syrup per troy ounce of silver (31.1 grams) as silver chloride (41.32 grams). It is best to make sure all the silver chloride has been converted to silver oxide, by the sodium hydroxide, before adding the syrup. The silver chloride should never be allowed to dry before reacting it with the sodium hydroxide. If dry, a layer of silver oxide can more easily form on the surface of the silver chloride and prevent further conversion. When reacting the sodium hydroxide with the silver chloride, vigorous mechanical stirring is required to break up the lumps. A blender action would be ideal, but the seal might not stand up to the sodium hydroxide. A stainless potter's slip stirrer should work well.

How to determine if you have complete conversion of silver chloride to silver oxide. Rinse a small sample several times with hot distilled water to remove all traces of sodium chloride, a by-product of the reaction. Any traces of sodium chloride will give a false reading. You can test the rinse water with silver nitrate to make sure all the chlorides have been removed. When fully rinsed, dissolve the silver oxide in a little 50/50 nitric acid, diluted with distilled water. Heat if necessary. If any silver chloride remains, the reaction is incomplete , stir about 15 minutes more and test it again.

The conversion to silver oxide works better when hot. If you freshly make up about a 25% sodium hydroxide solution, it will be quite hot from the dissolving. Then, add the silver chloride, as a slurry, immediately after the sodium hydroxide has dissolved.

I might mention that the moist silver oxide will decompose at a temperature above 575F to form a silver powder. If you go this route, the syrup is not necessary.

Also, you could just dissolve all the silver oxide in nitric. The nitric fumes would be far less than when dissolving silver metal.

(2) I don't know if your setup would allow it but, when dissolving the gold "shot", you can control the reaction and keep the nitric at a minimum by first covering the the gold with a calculated excess of HCl (about 140 ml/troy ounce of shot) and heat. Then, add the nitric in increments. When each increment has fully reacted, add more nitric. At some point, an addition of nitric will produce no further reaction. Stop adding nitric at this point. If you do this right, no urea will be needed. I would add about 1/4 of the nitric each time. After about 75% of the nitric has been added and reacted, start adding the nitric in smaller increments.

 

[butcher]

scrapper, I think collecting the silver as you are from AR, is why your gray filtrate of silver contains so much gold, and I also wonder if it is completely converted completely to silver chloride, which would be white not gray?
I dilute my aqua regia and white silver chloride forms as white cottage cheese,some times more HCL needed,
with this method I believe less other metals are dragged down with the silver chloride.

I understand you are refining a little differently, Not inquarting your gold, and with a nice shotting system, I can not give any advise on how to adapt your operation, but I feel by reading Hoke, Harold's previous posts, Gsp posts, and 4Metals posts, I think you will find many good Ideas for adapting your processing operations. and by reading gain much knowledge into how and why's.

 

[Scrapper-aw-]

Thanks Butcher,

I have quickly found that there is an abundance of knowledge and experience among the people here in this forum. I have been reding, and reading, and reading...... you guys talk alot! But that's a good thing. 8)

I experimented with the Kayro method and the sulfuric method yesterday. We'll see how they turn out today.

You say that you dilute your AR.......how much of a diilution?

And this cottage cheese silver chloride that forms, is that coming from a 4 metals+ alloy refining lot? I have on a few occasions had similar type material form, but I couldn't tell you why. I only noticed it was out of the norm....I didn't do anything differently during those batches. The only variables were the % make-up of the alloy and perhaps time in soultion.

I'll write more about this and other questions you fellows posted above.....right now....got to go to work!

:lol: