Mixing silver grades for dissolution.

[mike6]

Gentlemen.
First, let me express my gratitude for this forum, and to the members, without whom, my understanding of precious metal refining would not be augmented on a daily basis.
As this is my first post, kindly bear with me if I seem laborious in my question. I wish to get a meaningful answer first time round (without getting anyone's back up), and as such, feel that, the more information I give, the more concise the answer will be.

I have duplicated the silver cell as presented by "Palladium", and am achieving excellent results.
I refine .925 silver (coin and jewelry), .800 silver (coin), and .500 silver (coin). Step 1 of the process requires the dissolution of the raw stock, with nitric acid and distilled water. I use 70% nitric in the following ratios:
1).925- 1,45ml/g nitric, and an equal volume of water.
2).800-1,53ml/g nitric, and an equal volume of water.
3).500- 2,00ml/g nitric, and an equal volume of water.
Presently, I process each purity grade individually, as in my ignorance, I am afraid of messing up a tried and tested method. Logistically, as my batches are getting larger, I find more and more plastic containers, of different volumes, breeding uncontrollably!

Is the following hypothetical solution possible?
1kg of raw .925 would require 2900ml of dissolution liquid.
1kg of raw .800 would require 3060ml of dissolution liquid.
1kg of raw .500 would require 4000ml of dissolution liquid.
This is a total of 9960ml of liquid, technically, in the correct weight/volume ratio.
My question is thus, will I cause down-stream process problems, specifically during the cementation phase if I mix 3x 1kg batches, of differing purities (as specified above) into a single reaction vessel, and dissolve with the hypothetical 9960ml of dissolution liquid?, will full dissolution of the raw material even take place?
Please help!

 

[Harold_V]

My personal opinion-----don't worry about the specific alloys, so long as you add your diluted nitric such that it is fully consumed before adding more. So long as it is fully consumed, it really makes no difference. My practice was to heat the vessel to the point of boiling (keep it covered with a watch glass), and observe the resulting discharge. When brown fumes cease to result, I'd add more water, diluting the now well concentrated solution, then pour off the solution and start with a new addition. I'd repeat this process until all had been dissolved.

Harold

 

[Palladium]

:arrow:

 

[mike6]

Gentlemen.
Now that I know that I have not missed the bus entirely, I will proceed with a small scale trial. It appears that I am overshooting the nitric requirement by nearly 0.5ml/g on the .500 and will rectify.Thank you for your reply.
Mike

 

[goldsilverpro]

I get bigger numbers. I am using 1.2ml/g of 70% for Ag and 1.2 x 3.4 = 4.08ml/g of 70% for copper. It always takes 3.4 times more for Cu than for Ag.

.925
(925 x 1.2) + (75 x 4.08) = 1110 + 306 = 1416 ml 70% HNO3. Add 1416ml pure water = 2832ml 50/50 HNO3

.800
(800 x 1.2) + (200 x 4.08) = 960 + 816 = 1776 ml 70% HNO3. Add 1776ml pure water = 3552ml 50/50 HNO3

.500
(500 x 1.2) + (500 x 4.08) = 600 + 2040 = 2640ml 70% HNO3. Add 2640ml pure water = 5280ml 50/50 HNO3

Total of 5832ml 70% HNO3 and 5832ml Distilled water = 11664ml 50/50 HNO3

This is maybe the maximum amount of 50/50 HNO3 that it would take and that would be in a completely open top container. If you had a more closed system, it could theoretically take 1/2 of that. If the container were a beaker with a watch glass on it, it would be somewhere in between. You just never know exactly what it will take. Ideally, you should use what is needed but not more than needed to dissolve everything.

 

[mike6]

Chris!
I do believe I have number envy!
I have come to the conclusion, that, the more I think I know, the less I know I know.The year I have spent on the tip of the iceberg has only served to illustrate a glimpse of what lies beneath.My introduction to the forum is going a long way in explaining the ''why' behind the ''how'. I have just purchased your book on line, and look forward to gleaning further information from you veterans!
Thank you for your suggestions.

 

[Lou]

GSP,

We should emphasize that a totally closed system without the presence of excess oxygen will rapidly and dangerously pressurize.

 

[FrugalRefiner]

I'd like to add one comment to this thread. I agree that with the three examples you've given (sterling silver, 800 coins, and 500 coins), you shouldn't have any problems combining them for dissolution.

However, if you have occasion to work with silver contacts, silver bearing brazing alloys, dental amalgam, or other similar materials, I would treat those as separate batches. With sterling and coins, copper is usually the predominant, if not the only, alloying metal used. With contacts, brazing alloys, amalgams, or other unknown alloys, there can be a host of other metals in the mix that can complicate recovery. Contacts may contain cadmium, tungsten, etc. Brazing alloys can contain tin, cadmium, and many other metals. Amalgams contain tin and mercury. Since some of these can complicate recovery, it's better not to contaminate a large batch of otherwise clean silver and copper scrap with these metals. You can treat your bulk, copper waste solution easily, and deal with small quantities of the more highly toxic wastes.

Dave

 

[mike6]

Gentlemen.
My dissolution takes place in 25l (6.6 gallon) polyethylene containers, and nitrogen dioxide fumes are vented through my scrubber.On occasion, the container would be two thirds full, and tiny droplets of solution would escape.I combat this by merely placing the lid of the container loosely on top.The lid now prevents the droplets from escaping (like a watch glass), however, significantly less NO2 is being vented, as it is ''contained'' in the vessel.Is it possible that I sometimes do not achieve full dissolution of my raw material because I am not effectively removing the NO2 as it is being produced?

Mike

 

[Palladium]

Do you use heat?
5 gallon stainless pots are fairly cheap and come with a lid.

 

[mike6]

Sir.
Where I live we have a lot of sun! Today's ambient in the shade was 104F.By placing my dissolution containers in a home-made solar oven, I am able to maintain a solution temperature of 140F. I have recently procured 2x submersible heaters, along with thermocouple and control unit. This new equipment is graded to maintain a steady 176F.As my adventure progresses, I am attempting to eliminate as many variables as I am able.Now that I am coming to understand dissolution formulas (thanks to you guys), my next research will be on what optimum dissolution temperatures should be.It should be noted, that electricity in the 3rd world, is not always a given.The South African electricity grid is in imminent danger of collapse, and as I type this, my diesel generator is supplying my power, and also running a 2,6kg batch of silver in one of your cells, Ralph

 

[Palladium]

Glad the cell design is working out for you sir. No need to worry about temperatures. Anything short of boiling works. With a stainless pot you can heat it with gas or over a camp fire of coals. all you need is nitric, heat, and stainless.

 

[mike6]

GSP,
I have just dissolved 2x batches using your numbers( .800 and .500), no additional heat other than solar, and presto!, total dissolution!
Thank you.

 

[goldsilverpro]

GSP,
I have just dissolved 2x batches using your numbers( .800 and .500), no additional heat other than solar, and presto!, total dissolution!
Thank you.

They would probably dissolve overnight with no added heat.

 

[mike6]

GSP,
Both were 2kg batches of silver coins, contained in titanium baskets, and suspended in the solution. Dissolution was complete in 3 hours! Imagine my surprise. Thanks once again.
Mike

 

[goldsilverpro]

My personal opinion-----don't worry about the specific alloys, so long as you add your diluted nitric such that it is fully consumed before adding more. So long as it is fully consumed, it really makes no difference. My practice was to heat the vessel to the point of boiling (keep it covered with a watch glass), and observe the resulting discharge. When brown fumes cease to result, I'd add more water, diluting the now well concentrated solution, then pour off the solution and start with a new addition. I'd repeat this process until all had been dissolved.

Harold

Using only a portion of the total acid needed, working it to completion, decanting the spent solution, and adding a fresh portion is a good thing to know. I'm glad you brought it up. I've mentioned it about once a year but never got a response.

I started doing it like that in most dissolving operations that involved a lot of solution. I found that, when the solution level is more than 2" above the metal sitting on the bottom, there is a noticeable decrease in the rate of dissolution, which gets worse as the acid is used up. You can really tell the difference at 3" or 4". At 8", it can slow to a crawl. After all, the better solution is up at the top, 1/2 or 3/4 spent solution is sitting on the metal, and the metal is way down on the bottom. Of course, you could overcome this somewhat with heat, which tends to mix the solution, or by stirring it a lot. However, when the acid nears the point where it is used up, it can take forever unless you start with fresh.

So, I started adding in portions of acid about 2" above the metal, at most. When the acid was spent, I diluted to cool it a bit, poured it off, and added fresh acid, until the metal was dissolved. Definitely went faster. 1" might be even better.

An interesting adjunct to this is that you can start with, say, 5 times more metal in the dissolving container and just keep using portions of the needed acid until complete. This is much faster than running 5 normal sized batches.

USE A PLASTIC CATCH TRAY WHEN DECANTING! Those drips are valuable.

 

[nickvc]

Ralph
While you seem to be working only with silver I'd bet that some traces of other values will report in your processes. Make sure you keep any un dissolved materials and the slimes from your cell for later recovery and processing. It all adds up and may make for a nice little bonus at some stage.

 

[Palladium]

Ralph
While you seem to be working only with silver I'd bet that some traces of other values will report in your processes. Make sure you keep any un dissolved materials and the slimes from your cell for later recovery and processing. It all adds up and may make for a nice little bonus at some stage.

While most of my production (90%) comes from silver oxide batteries and brazing silver you are absolutely correct. I love running old sterling silver flatware and jewelry. You would be surprised at how much gold comes from it. Seems that the older the silver is the more it's sprinkled with gold residue.

 

[4metals]

Seems that the older the silver is the more it's sprinkled with gold residue.

There are 2 very good reasons for that. One is that often sterling silverware was plated gold. Unless there was a very heavy nickel underplate the gold will eventually migrate down into the silver and you will not even notice it.

The other thing is that years ago when the older stuff was made, the price of gold was $35 an ounce and the price of silver was like 35 cents per ounce, The cost of recovering the fractional percentage of gold retained by silver when it was refined the first time (often from the ore at a mine) was more than the gold was worth.

Today when refining primarily sterling silver it is not uncommon to recover 0.3 ounces of gold per 1000 ounces of sterling material. In a silver cell it is much more feasible, but if you were digesting it in nitric you would need about 11.8 gallons of nitric and an equal amount of distilled water. Then you have to get the silver back out of solution and make it into saleable silver metal again.

It is possible for sterling to run higher but the long term running averages that I have seen from refiners producing volume, peg the gold at about .3 oz per 1000.

 

[Palladium]

That would explain a few things! You right i have what i call a dusting of silver from just the flatware, but when i run older jewelry i find small pieces of karat gold that is attached usually as decorations to the silver. That's where i tend to get the majority of the gold that comes from running old sterling. Since the gold was so cheap back then i guess small ornamental pieces attached really didn't drive the price of the product up that much considering.