My Refined Silver is not Coming Out Pure. Help!

[achab86]

Hi All,

First time poster, long time lurker.

Here's my process:

I'm refining 366 40% Kennedy half dollars. I work for a PM refinery, so the costs for processing these are next to nothing. About 10 cents per coin.

I am using a formula I found on this forum. 1.22ml/g of silver and 3.6ml/g of copper. Equal amounts of nitric and distilled water.

After the initial reaction calms down, I turn the heat on until all the metal is dissolved.

Once dissolved, I filter out any contaminants, then add my copper. I use a solid quarter inch bar of copper that I suspend in the solution using a titanium wire.

Everything seems to go ok and the silver cements as expected. Once it cements, I rinse everything with boiling water - multiple times until the rinse water stays clear.

Here's my problem. Once everything dried out, I xrayed the silver powder and it came up 80% silver and 20% copper.

I have no idea why there is so much copper. Any help would be greatly appreciated.

Thanks in advance!
Alex

 

[GotTheBug]

Take some time and watch Palladium's videos, WELL worth it. Think of it this way, first you extract, then you refine. I'm a noob for sure compared to many on this forum, but on a similar path to yours, hence the recommendation of watching those videos.

Start here:
http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=50&t=17233&p=202931#p202931

 

[Lou]

Have you melted it yet? Melt it and I bet your assay is 99%+

I hope your refinery doesn't rely on XRF. It's useful but really, I think everyone just considers it magic and believes everything it says.

You're doing a simple displacement reaction--don't complicate it!

Lou

 

[achab86]

Have you melted it yet? Melt it and I bet your assay is 99%+

I hope you're refinery doesn't rely on XRF. It's useful but really, I think everyone just considers it magic and believes everything it says.

You're doing a simple displacement reaction--don't complicate it!

Lou

We don't rely on it. Just for preliminary testing. We have an ICP machine for real assays.

I haven't melted it yet since I'll be doing the melting at home. I was expecting the powder to be almost white in color when it dried, but it's more of a dark gray.

Thanks for the input. I can try a small test melt tomorrow.

Here's what I'm using to make molds. Can't wait to get it done!

 

[Lou]

NIce!!


Cement silver should be grey in color, hence the cement nomenclature.

 

[achab86]

NIce!!


Cement silver should be grey in color, hence the cement nomenclature.

Thanks again for the help.

Its more dark gray than cement colored gray.

I just did a 2oz test melt and ran an assay.
86% Ag
14%Cu

Any suggestions? I have no idea what I could have done wrong...

 

[Lou]

You're starting from copper and silver nitrate and adding in pure copper to reduce out the silver.

Done correctly, cement silver should be relatively high grade silver. Sounds to me that 1.) your copper is falling apart and contaminating the silver, 2.) you're not rinsing it well enough.

You should take your cement silver and run it through the Thum cell.

Lou

 

[achab86]

You're starting from copper and silver nitrate and adding in pure copper to reduce out the silver.

Done correctly, cement silver should be relatively high grade silver. Sounds to me that 1.) your copper is falling apart and contaminating the silver, 2.) you're not rinsing it well enough.

You should take your cement silver and run it through the Thum cell.

Lou

Again, thank you for the help.

I guess I'll re-digest it and try again. The copper i am using is thick, heavy, and solid... so I didn't think anything would fall off of it. I can't see anything copper in the cement silver.

I'll also rinse like there is no tomorrow!

I haven't looked much into silver cells. When I do, I get confused and my head starts to hurt...

 

[goldsilverpro]

About the only ways for Cu contamination that I can think of are:
(1) The copper bearing solution is not thoroughly removed by rinsing. I prefer hot distilled water rinses.
(2) Slivers of copper are falling off of the copper rod as it dissolves and are intermingling with the silver. This seems to occur more often when using copper wire or rods. The best source of copper for this is clean, unplated, scrap copper buss bar.
(3) Copper nitrate is crystallizing because the solution is saturated with copper. This can be alleviated by diluting the solution with a equal amount of distilled water and then filtering before cementing.
(4) In an open container, it takes a little more nitric to dissolve copper than what you're using - about 4.15ml/g. Maybe you're leaving undissolved metal in the solution. Dilute and filter before cementing.

 

[Palladium]

I bet he used excess nitric. The only time i have high cu contaminates is when the bar lies in the bottom of the bucket in thick silver sediment or i use to much nitric. When you first added the cu bar to drop the silver did you notice any fizzing or red gas? How much silver are you dealing with? Did you use an air bubbler to keep the solution stirred? Was the bar suspended in the solution by the wire or did it lay on the bottom?

 

[achab86]

I bet he used excess nitric. The only time i have high cu contaminates is when the bar lies in the bottom of the bucket in thick silver sediment or i use to much nitric. When you first added the cu bar to drop the silver did you notice any fizzing or red gas? How much silver are you dealing with? Did you use an air bubbler to keep the solution stirred? Was the bar suspended in the solution by the wire or did it lay on the bottom?

I think you might be right. I noticed that the copper was being dissolved briefly when I put the bar in (silver was cementing then being re-digested), but I thought that once an equilibrium was found, the copper would be dissolved the the silver pushed out. I know I've read things about that on here. Perhaps i just had toooo much nitric?

I used 1.22ml per gram of silver, and 3.6ml per gram of copper.

4200g of total weight
1680g Ag
2520g Cu
------------
2050mL for Ag
9072mL for Cu
------------
11122mL for the entire batch. (11122mL of nitric and 11122mL of distilled)

I didn't have an air bubbler to use, but constantly stirred it. The bar was suspended with the titanium wire.

 

[Palladium]

Your first problem is relying on numbers. Numbers are only meant as a rough guide. Experience is the key and you can only get that with practice. You should have heated the lot while adding you nitric in steps while observing the red gas given off. If you just stop short of dissolving everything and have some small pieces of the coins left over then you know that all you nitric was used and you want have a problem. Better to have some small pieces left over than to over shot the nitric and cause a problem. Excess nitric during the drop causes major copper contaminants. As a lesson dissolve it all again if you want and watch the difference. In the end though if you want to pour some pretty bars you need to go on and send it through the cell. Melting that cemented copper/silver will make them bars look rough!
I have 7 plus hours of video on this subject on Youtube that explains it all A-Z.

 

[achab86]

Your first problem is relying on numbers. Numbers are only meant as a rough guide. Experience is the key and you can only get that with practice. You should have heated the lot while adding you nitric in steps while observing the red gas given off. If you just stop short of dissolving everything and have some small pieces of the coins left over then you know that all you nitric was used and you want have a problem. Better to have some small pieces left over than to over shot the nitric and cause a problem. Excess nitric during the drop causes major copper contaminants. As a lesson dissolve it all again if you want and watch the difference. In the end though if you want to pour some pretty bars you need to go on and send it through the cell. Melting that cemented copper/silver will make them bars look rough!
I have 7 plus hours of video on this subject on Youtube that explains it all A-Z.

I'm actually re-digesting it now, and only adding acid in small amounts. Since it was my first attempt, I thought I'd try it "by the book" using the numbers i found.

I'll check out your videos. The cell would be a nice thing to do.

Thanks for the advice!!!

 

[etack]

If you want pure Ag and not use a cell. Convert It to AgCl and reduce it. There is a learning curve with it too, but the Cu contamination is gone.

eric

 

[achab86]

Thanks all for the great suggestions.

I have something new...

I re-digested the cemented silver last night. When I looked at the solution this morning, it had a hard, almost plastic-like film on the top. I put the solution back on heat to see if it would dissolve, but it has not.

Does anyone know what this stuff is? Is it silver?

Thanks again!

 

[kurtak]

another thing that will cause copper contamination is not having the copper "entirely" submerged in the silver nitrate solution - if the copper is sticking out above the surface of the solution it will cause copper above the solution to oxidize (blue/green copper corrosion) some of this then falls down into the solution & once it gets into the solution it somehow cause's more of the copper down in the solution to oxidize as well (during the cementing process)

I discovered this in my early days of refining - if my bus bar was not entirely submerged I would end up with a layer of copper oxide between the bus bar & the silver cement when I pulled the bus bar out & peeled the silver cement away from the bus bar

at least that's all I could figure because it doesn't seem to happen if the bus bar is entirely submerged

Kurt

 

[achab86]

another thing that will cause copper contamination is not having the copper "entirely" submerged in the silver nitrate solution - if the copper is sticking out above the surface of the solution it will cause copper above the solution to oxidize (blue/green copper corrosion) some of this then falls down into the solution & once it gets into the solution it somehow cause's more of the copper down in the solution to oxidize as well (during the cementing process)

I discovered this in my early days of refining - if my bus bar was not entirely submerged I would end up with a layer of copper oxide between the bus bar & the silver cement when I pulled the bus bar out & peeled the silver cement away from the bus bar

at least that's all I could figure because it doesn't seem to happen if the bus bar is entirely submerged

Kurt

I think you may have hit the nail on the head. The bars I was using were only about half submerged because I didn't want the bars to touch what was cementing.

I've decided to go the AgCl route since I think it will be faster than dropping with copper. But I will not forget this for the future!

Thanks!!!!

 

[Palladium]

Two other important tricks i noticed. You can have a solution that you have driven to the edge and consumed all the nitric and when you put the bar in the process won't kick off. You can add some nitric to it and the process will start. Also if you take the bar out of the solution and expose it to the air say for storage, then when you place it back in the solution it will have formed a layer on the surface that will interfere with the cementation. I solve both problems by holding the bar over the bucket and pouring a little nitric down the bar and into the bucket. Lower the bar into the bucket and you're off to the races!

 

[goldsilverpro]

I've decided to go the AgCl route since I think it will be faster than dropping with copper. But I will not forget this for the future!

Copper cementation is much simpler than going the AgCl route, assuming you do the dissolving/cementation properly (which you obviously haven't done). It's easy to drop the AgCl but the steps afterward are not that simple. It's not easy to get all the AgCl converted and I doubt if the purity will be any better than cementation, mainly because the AgCl is more difficult to rinse.

 

[Palladium]

I hate AgCl ! I used to salt my gold filled nitric acid and recover my silver that way because i though it was fast and simple. It may be fast for the start of recovery, but it's a pita for rest unless your 1. Set up for that specific process or 2. doing small amounts. I go with cementation on every process now because i don't want the hassle. Only thing worse than washing AgCl to me is washing my silver nitrate from tin when i run brazing alloys.