How much copper to use before dissolving precious metal in aqua regia?

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anthonyulo8

Member
Joined
Oct 2, 2023
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8
Location
Adelaide
Hi guys, I have a question and have been thinking about it for a while. I’ve tried researching it online and in the books but I can’t seem to find the answer. I know that we add copper (or melt the precious metal with copper) in order for the aqua regia to fully dissolve the metals. If there is a high concentration of silver, which does not dissolve in aqua regia, it forms a sludge blocking the aqua regia from further reaction. The copper helps the precious metals such as gold, platinum and palladium to react fully with aqua regia. However, I am not sure how much copper to use. What happens when we add too much copper? Anyone who can answer my question or direct me to the answer would be helpful.
 
anarxi said:
the most important thing is that there is no more than 25% silver
quarter
hence the name of the quartering process
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I think you meant that there are no more than 25% Gold, right?
If there are more than 25% Gold the Gold will protect the Silver (Silver/Copper/what not) from the Nitric.
In the other end, too high Silver content will form Silver Chloride which will passify it as well.
 
Yggdrasil said:
I think you meant that there are no more than 25% Gold, right?
If there are more than 25% Gold the Gold will protect the Silver (Silver/Copper/what not) from the Nitric.
In the other end, too high Silver content will form Silver Chloride which will passify it as well.
Click to expand...
yes, you are right
 
anthonyulo8 said:
Hi guys, I have a question and have been thinking about it for a while. I’ve tried researching it online and in the books but I can’t seem to find the answer. I know that we add copper (or melt the precious metal with copper) in order for the aqua regia to fully dissolve the metals. If there is a high concentration of silver, which does not dissolve in aqua regia, it forms a sludge blocking the aqua regia from further reaction. The copper helps the precious metals such as gold, platinum and palladium to react fully with aqua regia. However, I am not sure how much copper to use. What happens when we add too much copper? Anyone who can answer my question or direct me to the answer would be helpful.
Click to expand...
The best is to use Silver as Silver needs approximate 1 ml Nitric per gram to dissolve, while Copper needs closer to 4 ml per gram.
If you have ample access to Nitric this may not be an issue for you,
but it still adds to the cost as the Silver will be reclaimed and used again in the next lot, not quite that easy with Copper.

By tumbling during the dissolution the Silver Chloride may be mechanically removed during the process and this can be skipped.
 
but by the way, I think the same scheme applies to silver.
In any case, when I etch holders the silver-paladium contact ,in the relay (80/20 alloy) in aqua regia there are practically no losses.
if the alloy is 70/30 some palladium goes into solution.
 
I just found out that the gold we produced recently had a mix of orange colour in it. I wish I could post photos. Too much copper perhaps?
 
anthonyulo8 said:
Hi guys, I have a question and have been thinking about it for a while. I’ve tried researching it online and in the books but I can’t seem to find the answer. I know that we add copper (or melt the precious metal with copper) in order for the aqua regia to fully dissolve the metals. If there is a high concentration of silver, which does not dissolve in aqua regia, it forms a sludge blocking the aqua regia from further reaction. The copper helps the precious metals such as gold, platinum and palladium to react fully with aqua regia. However, I am not sure how much copper to use. What happens when we add too much copper? Anyone who can answer my question or direct me to the answer would be helpful.
Click to expand...
You want to make an alloy of about 6k (25%) gold. Silver will require far less nitric acid than copper when parting.
 
anthonyulo8 said:
Hi guys, I have a question and have been thinking about it for a while. I’ve tried researching it online and in the books but I can’t seem to find the answer. I know that we add copper (or melt the precious metal with copper) in order for the aqua regia to fully dissolve the metals. If there is a high concentration of silver, which does not dissolve in aqua regia, it forms a sludge blocking the aqua regia from further reaction. The copper helps the precious metals such as gold, platinum and palladium to react fully with aqua regia. However, I am not sure how much copper to use. What happens when we add too much copper? Anyone who can answer my question or direct me to the answer would be helpful.
Click to expand...
Silver has a very high affinity to Chlorine, more than most other metals. Ideal more than 10% silver in a Gold alloy will make it difficult to dissolve gold efficiently. Adding copper to bring the Silver content down to 10% should help. But this adds to your cost of refining and you need to recover copper for recycling for lots having high silver content. Hope, this may help you to some extent.
 
Anil said:
Silver has a very high affinity to Chlorine, more than most other metals. Ideal more than 10% silver in a Gold alloy will make it difficult to dissolve gold efficiently. Adding copper to bring the Silver content down to 10% should help. But this adds to your cost of refining and you need to recover copper for recycling for lots having high silver content. Hope, this may help you to some extent.
Click to expand...
The issue is not the affinity, but the fact that it forms insoluble Silver Chloride.
If the amount of Silver Chloride gets too high it blocks the acids access to the other metals and thus stops the reaction.
This can be solved by pushing the Silver content low enough or tumbling during the processing mechanically removing the Silver Chloride layer.
The other approach is to alloy it and get the Gold content to 25% so Nitric can part the alloy.
 

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