AR didn't dissolve karat gold completely and lots of black precipitate

[HarryHJ]

I'm trying to refine gold with AR from karat scraps recently. My first try with 18k gold was successful . I dissolved the piece in hot AR, got dark green solution and white precipitate (supposed to be silver chloride), then filtered the solution, settled with SMB (a little more than 1g SMB for 1g gold), filtered and wash and finally I got a little less than 75% of the original weight of gold.

But my second try with 14k got more trouble. I had mixed 14k white and yellow gold, I melted them and poured into water to form small nuggets. Then I put the metal in AR and keep heating. The solution turned dark green, but there were lots of dark grey or black precipitate , they covered some small pieces (about 10% left) and prevented the reaction. Then I took the undissolved ones out, removed the surrounding precipitate, cut the metal into 1-2mm grains, and put in new AR. Again, black precipitates formed dense shell surrounding the metal, so still small amount of metal couldnot get dissolved. There's metal in the middle of the grains shown in the picture.

Could someone help me on these questions?
1. Are the black precipitates silver chloride? AgCl should be white like I got from first time. Could other base metal in the 14k affect the color, such as rhodium, nickel?
2. I heard that silver chloride is not stable, could it turn black when exposed to long time heat or light? (I did the refining day time outside under sun)
3. Any tips to improve the process to completely dissolve gold? Since the precipitate is dense and tight on the metal.
4. After this 2 rounds of refining, my gold weight is 5% less than it should be (58.5% x original weight). Is this normal loss? and how to test if gold remains in the waste solution? and how to get more gold from the solution? (I think I added SMB more than enough)
I really appreciate your help.

 

[FrugalRefiner]

Harry, welcome to the forum. From your questions, we can see you have much studying to do. That's OK. We all started like that.

1. The undissolved bits (not precipitates, because they weren't first dissolved, then precipitated) could well be silver chloride. That's much more likely than rhodium or nickel.

2. Yes, silver chloride turns black when exposed to the sun or other strong light.

3. It is usual to "inquart" alloyed gold, meaning that you add silver or other base metal to bring the gold content to about 25% (one quarter - inquart), then dissolve the base metals with nitric acid before moving on to dissolving the gold in AR.

4. The gold content of jewelry in the U.S. is rarely as high as the karat markings would indicate. Older regulations allowed jewelry manufacturers to be up to a half karat below the karat stamped. If the piece included solder, the regulations allowed up to another half karat. So an item could be up to one full karat below what is stamped. Although the regulations have been tightened over the years, it's hard to know when a piece was made, and some small makers still take the chance of underkarating their goods.

Dave

 

[Yggdrasil]

I'm trying to refine gold with AR from karat scraps recently. My first try with 18k gold was successful . I dissolved the piece in hot AR, got dark green solution and white precipitate (supposed to be silver chloride), then filtered the solution, settled with SMB (a little more than 1g SMB for 1g gold), filtered and wash and finally I got a little less than 75% of the original weight of gold.

But my second try with 14k got more trouble. I had mixed 14k white and yellow gold, I melted them and poured into water to form small nuggets. Then I put the metal in AR and keep heating. The solution turned dark green, but there were lots of dark grey or black precipitate , they covered some small pieces (about 10% left) and prevented the reaction. Then I took the undissolved ones out, removed the surrounding precipitate, cut the metal into 1-2mm grains, and put in new AR. Again, black precipitates formed dense shell surrounding the metal, so still small amount of metal couldnot get dissolved. There's metal in the middle of the grains shown in the picture.

Could someone help me on these questions?
1. Are the black precipitates silver chloride? AgCl should be white like I got from first time. Could other base metal in the 14k affect the color, such as rhodium, nickel?
2. I heard that silver chloride is not stable, could it turn black when exposed to long time heat or light? (I did the refining day time outside under sun)
3. Any tips to improve the process to completely dissolve gold? Since the precipitate is dense and tight on the metal.
4. After this 2 rounds of refining, my gold weight is 5% less than it should be (58.5% x original weight). Is this normal loss? and how to test if gold remains in the waste solution? and how to get more gold from the solution? (I think I added SMB more than enough)
I really appreciate your help.
View attachment 61958View attachment 61959

When refining karat Gold one need to perform something called inquarting, which is to dilute the Gold to appr 6 karat ie 25%.
Then part it, ie dissolve the base metals and Silver in Nitric.
If the Gold content is above 25% it will protect the Silver and base metals and the dissolving will be incomplete.
If the Gold is high enough in karat (ie the Silver content is sufficient low) it can be dissolved directly.

There is not enough Rhodium to cause this.
It looks like slag from the s/melting, did you use Borax?
Yes Silver Chloride will turn purple then black in light, but it takes time.

Your first post did not indicate you planned to process so I did not give you your study material.

Here is what you need to study.

We ask our new members to do 3 things.
1. Read C.M. Hokes book on refining jewelers scrap, it gives an easy introduction to the most important chemistry regarding refining.
It is free here on the forum: https://goldrefiningforum.com/phpBB3/viewtopic.php?f=54&t=19798
2. Then read the safety section of the forum: https://goldrefiningforum.com/forums/safety.47/
3. And then read about "Dealing with waste" in the forum: https://goldrefiningforum.com/threads/dealing-with-waste.10539/

Suggested reading:
https://goldrefiningforum.com/forums/the-library.101/
https://goldrefiningforum.com/threads/when-in-doubt-cement-it-out.30236/
https://goldrefiningforum.com/threa...le-read-this-before-you-post-about-ore.33333/


Forum rules is here.
https://goldrefiningforum.com/threads/gold-refining-forum-rules.31182/


Edit to add Dave beat me to it

 

[HarryHJ]

Thanks a lot