10K question.

[GotTheBug]

I started with a small 10K necklace. I got a bit ahead of myself, not remembering it has to be 6K for inquarting to work.
So, direct to nitric, not much reaction, as what should have been expected.
Filtered off nitric solution, positive for silver, go figure.
So the rest in hot AR, good results and have the gold powder sitting in my special jar.
Now the question, I have some interesting stuff left that will not react in boiling AR.
Thoughts?
The following pictures are the initial leftover, and then the same after a boil in hot AR.
Thanks as always,
Paul.

 

[Geo]

Silver chloride mostly. Roast the solids and grind in a mortar and pestle and run through AR to finish the gold dissolution.

 

[GotTheBug]

Thank You Geo, had a feeling that might be what it was. Since I've removed most of the gold already (looks like better than a gram of powder), could I roast and then straight nitric to dissolve the silver, leaving the gold behind?

 

[4metals]

The reason you inquart this material is so the nitric acid can penetrate the alloy completely and dissolve all of the silver. What you did was dissolve some of the silver and the rest remained in the alloy because it could not dissolve.

Nitric will dissolve any silver it comes into contact with, inquarting assures you it can contact most of it.

When you proceeded to the aqua regia, the remaining silver crusted over forming silver chloride and likely entrapping some gold in the chloride crust. Almost like rust, goes quickly on clean iron but the thicker it gets the more it tends to protect the surface from rusting further.

You can dissolve the silver chloride in ammonia, and expose any undissolved gold. Then add Hydrochloric to the ammonia solution to drop silver chloride and save it up until you have enough to process.

 

[GotTheBug]

4Metals, perfect! That is what I shall do first thing tomorrow.

 

[Harold_V]

The use of the term inquartation involves the melting and alloying of high karat gold alloy, lowering it to roughly 25% (6K). You implied that inquartation doesn't work when the alloy is 10k, which, in a sense, is true (it will work, but with difficulty, as I detail below), but in your case you did NOT inquart, so it isn't a part of the equation.

While it doesn't cooperate well, you can digest 10K gold without inquartation, but the requirement is to expose the alloy to a prolonged heated dilute nitric solution, and it must be relatively thin, as the gold that remains will isolate the remaining alloy from nitric, assuming the piece is thick enough. I would hesitate to process something like a gent's ring made of 10k gold for that reason.

To carry this further, even 14K can be processed without inquartation, assuming it is finely divided enough. I routinely processed 14K, even 18k filings from the jeweler's bench without inquartation. A prolonged boil in nitric would penetrate the alloy deeply enough for the AR to consume the remains, even though silver still remained. The pieces were so fine that before the silver chloride could isolate the alloy from the AR, the alloy was totally dissolved, liberating silver chloride. It goes without saying, the resulting gold solution was heavily contaminated with base metals, so the quality of the gold was suspect, but in my case all gold was twice refined, so that wasn't an issue.

If you'd like to avoid going through the same drill, once you've dissolved the silver chloride (ammonium hydroxide), incinerate the remains, then expose them to heated dilute nitric. I recommend you cover the vessel with a watch glass and drive the solution to a low boil. That should penetrate to the core, removing any remaining silver, which is the very thing that prevents dissolution of the values with AR, just as was stated by 4metals.

Harold

 

[nickvc]

To follow up on 4metals and Harold's excellent posts I regularly used to go straight to AR without inquartation especially with 18k + alloys as the silver content was low enough to get complete dissolution of the gold. Much low karat jewellery from Italy has little or no silver in its alloying so again it can go straight to AR the key been the silver content or lack of, the point made by Harold of high base metal contamination is especially valid as drag down will occur while precipitating your gold,good washes can solve that but if you have the time and want a guaranteed product simply re refine to achieve high purity.

 

[4metals]

When doing stone removal the option of inquartation simply does not exist, as a result of seeing countless lots of goods for stone removal I have witnessed first hand the menace of silver chloride and how it can "freeze" a reaction by coating over the material.

With stone removal, or with any refining when you forego the inquartation, the 10 and 14 karat goods will digest almost completely, but 18 karat with high silver and green gold (not seen much these days) poses a problem. The initial acid usually loosens the stones sufficiently to do a complete stone extraction but the remaining chunks are best processed by inquartation. Even if a lot appears to have digested completely, when the chlorides are reduced into a silver bar the assay is usually over 1% for gold so you are not getting all of the gold in the first shot without inquartation.

The benefit of inquartation for a small refiner is you keep all of your silver in one place. Your accumulated silver is constantly re-cemented after the process and the quantity of silver constantly increases as you collect the silver from each lot you process. When your accumulated silver grows to a certain point, a good problem to have, you simply keep what you need for inquartation and upgrade the rest to .999 using one of the many designs for small silver cells discussed on this forum.

 

[GotTheBug]

One of the many things I love about you guys is that I learn something very valuable every time I mess up.

Next time I will definitely inquart, seems it will be a much easier process. The ammonia did the trick this morning, cooling now in fact. The good news
is the amount of powder, presumably gold, is very little after dissolving the silver chloride.

Just wanted to say Thank You, again.

 

[Lou]

Just a reminder of some caveats on using ammonia to remove AgCl crusts:

1. Make absolutely certain that one rinse the gold solution off the silver chloride with dilute 1% HCl so that when one adds the ammonia, it will not inadvertently form dangerous "fulminating gold". Check your rinse in the filter with stannous or wait until it comes through colorless.

2. Any ammonium chloride solution obtained from the ammonia wash HCl neutralization should be kept separate from any base metals so it does not inadvertently chelate your nickel/copper when you do your waste treatment. What happens is that the ammonium chloride forms free ammonia when the base metals are neutralized with lime or lye; this free ammonia binds up the nickel/copper and holds it in solution. The best thing to do is to evaporate the ammonium chloride solution down and manage it as a silver containing RCRA waste until TCLP ( a type of test used to determine if the material can be landfill disposed) proves otherwise.


Useful solubility guide: http://www.saltlakemetals.com/Solubility_Of_Silver_Chloride.htm

Note, if you see that thiosulfate dissolves AgCl (does quite nicely and without the stink of aqueous NH3), also understand that it can dissolve some gold. You get this gold back after plating out the silver or cementing it out with iron. Reports in the anode slimes or as a powder in the nitric parted silver.

 

[Geo]

Since the gold is the objective and we are talking about a small quantity, what I suggested will work in this instance. Of course you should have done it the right way the first time but now you have a small quantity of gold left in what you have. The silver content would be no more than a few pennies, so it's not worth the effort, in my opinion, to attempt to reclaim. It is much simpler to dry and crush what you have and finish the dissolution. It might not have the finesse of a professional refiner but it will work.

 

[goldsilverpro]

Besides ammonia, sodium or potassium cyanide solution (with no oxidizer added) will dissolve AgCl and so will a sodium thiosulfate solution. Along with ammonia, they are about the only 3 things that will dissolve AgCl. With either cyanide or thiosulfate, the silver could be easily recovered. I would probably do electrolytic on the cyanide and sulfide precipitation on the thiosulfate.

With any acids in the mix, you will immediately get hydrogen cyanide gas off the cyanide solution and SO2 off the thiosulfate. For that reason, you must rinse the AgCl very thoroughly, preferably several hot water rinses, preferably in a filter, preferably vacuum, before using NaCN or Na2S2O3. No matter how well you think you rinsed it, you should still use a good fume hood. With cyanide, I would heat it to about 130F before using it. I've tried both and the cyanide is faster.

In saying all this, I most always use ammonia because it's easier but it definitely doesn't sit around long before I get it back to the acid side with HCl. Don't put it off and don't forget about it.

Geo,

Since the gold is the objective and we are talking about a small quantity, what I suggested will work in this instance. Of course you should have done it the right way the first time but now you have a small quantity of gold left in what you have. The silver content would be no more than a few pennies, so it's not worth the effort, in my opinion, to attempt to reclaim. It is much simpler to dry and crush what you have and finish the dissolution. It might not have the finesse of a professional refiner but it will work.

I always direct dissolve the karat gold in aqua and I always work in covered open top containers. During the dissolving, I very often crush off the AgCl with the flat end of about a 3/8" solid stiff plastic stir rod. Sometimes this doesn't work 100% but it often works well enough to expose some fresh gold and allow the acids to get back at it. In most cases, crushing it often while dissolving will allow you to get most all of the gold dissolved. You can feel undissolved gold with the stir rod. The small amount you don't get is with the AgCl and is pretty easy to get.

 

[4metals]

Tumbling the material to break off any silver chloride crust also helps, I have enjoyed great success doing direct Aqua regia dissolves on karat material by always melting and granulating the material before acid. By tumbling, the gold can be successfully processed with silver content approaching 10% directly in Aqua Regia. Melting makes the lot homogenous in silver content so any pieces higher than 10% are diluted by pieces of lower silver assay allowing this process to be more versatile.

This process is effective for larger scale refining where the customer is paid on assay so you own the gold you are refining already. Since you will not get 100% of the gold as a small percentage will be tied up in the chlorides, you have to get paid on the "hung up" metal after silver refining and processing of anode slimes.

Inquarting is still the way to go if you are "toll refining" lot by lot and paying the customer on out turn, but if your lots grow and you decide to skip the individual lot processing for bulk refining, think tumblers.

This type of tumbler is very effective, the larger one tumbles buckets from 5 to 12 gallons. A 12 gallon bucket safely processes 200 ounces of karat scrap. http://www.quikspray.com/QuikstirMixerBucket.aspx

 

[4metals]

I should add that these quickspray tumblers are great for nitric dissolves on gold filled material and the nitric dissolves of inquarted karat or sterling.

They are available with stainless steel buckets but the bucket attachment makes it pretty easy to use most any bucket or stainless pail.

 

[goldsilverpro]

I wish I had one of those bucket mixers when I was doing fuel manifolds from aircraft scrap.

At that time, I strapped a hopper vibrator on the outside of the bucket and that did speed things up. The mixer would have been better, though.