Cementing Silver

[viperD]

Hello everyone, let me just start by saying this forum is amazing! It has been a great place to learn various techniques and I wouldn't be anywhere near where I am today without it. I am relatively new to the world of PM refining. I have been collecting various silver pieces for quite some time now and just recently started to refine some of it with nitric acid. I've done about 4 different runs now and it always seems like I don't get as much silver cement at the end of the process that I should be. I will give you my last run's numbers as an example. I started with 60.4 grams of coin silver flatware pieces. Using about 1.2 ml of nitric per gram of silver, I added 75 ml of nitric acid to the beaker. I then boiled distilled water and added that until reaching about a 50/50 mix. The magic started happening and after letting everything dissolve and settle I was left with about 5 grams of undissolved silver flatware pieces left on the bottom. I strained these out and added another 2 parts of distilled water to the silver nitrate. So at this point about 55 grams of (.90) silver flatware had been dissolved into the solution. I added my copper and started the cementing process. After cementing was complete (or I think it was complete) I filtered and rinsed the cemented silver with boiling distilled water many times to clean it. After drying the cemented silver I came up with a final weight of 39 grams. Here's where the questions come in. Having 55 grams of silver dissolved into the silver nitrate, shouldn't I be recovering closer to 49-50 grams of refined silver cement? Am I losing about 10 grams of silver in the solution? Again, I want to say thanks to everyone on this forum. It has been a great learning tool so far. Hopefully someone can give me some insight as to what I'm doing wrong here!

 

[martyn111]

After cementing was complete (or I think it was complete)

A quick test for you to determine if all the silver has been cemented out.
Take a small sample of the solution you think has no more silver left in it and add a few grains of table salt or a couple of drops of HCl, if you get a white precipitate (silver chloride) then you haven't removed all the silver from the solution.

 

[viperD]

Thanks for the reply! I just took a small sample, added a few grains of table salt and did get a white precipitate. I guess the solution wasn't done cementing. From here should I just suspend more copper in the solution and try to cement the rest out? Also how long do you recommend I let it continue cementing? Thanks again, I appreciate the response.

 

[Geo]

dont be impatient. if you have time leave the copper in the solution for several days. if you need it in a hurry, you can heat the solution. it will speed the process. dont overheat, less than 100°F should work fine.

 

[viperD]

I'm in no hurry at all. I will put the copper back in and let it sit for a few days. Thanks again for the responses. I'll keep you updated on how this turns out!

 

[butcher]

viperD,
It sounds to me like you had free nitric in your solution, I would bet it probably could have dissolved more silver from your original solution with more heat and time, before you tried to cement the silver on copper.

 

[Mojeho]

Hello,
When i'm doing cementation with remaining nitric in solution, i use large bars of copper, and add urea to remove nitrous acid and NO2 gas(Also recommended when cementing cadmium with zic in nitrate solutions). Note that when nitric is present, than silver cementation can take up to week when not heated. Also make sure to stir the solution as often as you can, it helps too(no need to stand there and stir all the time though).
Have a nice day
Moje

 

[viperD]

Hey Butcher, thanks for the comment. Free nitric didn't occur to me at first. Would the remaining silver be unable to be cemented if that is the case? I left the solution sit over night with copper suspended and it appears that no more silver has cemented. I have not added any sort of heat source so I will continue to let it sit and see what happens. Thanks for the input!

 

[philddreamer]

Hi Viper!

Would the remaining silver be unable to be cemented if that is the case?

As I understand, if there's still free nitric and you stop adding copper, some of the cemented silver will re-dissolve until all the free nitric is used.

But, if you continue to add copper and all the silver cements, and there's still free nitric, the nitric will digest the added copper until all the nitric is used up. At this point, your silver will not re-dissolve, and neither will any undissolved copper.

I have not added any sort of heat source so I will continue to let it sit and see what happens.

But, like Butcher stated, if you add heat, you might start dissolving some of the metals again.

Butcher, please correct me if I'm wrong.

Thanks!

Phil

 

[mitchd]

Hi all, don't know if this will help but I had the same problem, had the copper in for days with nothing happping all I did was add about a quarter of the volume of distilled water and it started to work again.

 

[Oz]

You have been given good advice, but I will add a piece. Given your feedstock you may reach a point while cementing with copper that you think you have reclaimed all of the silver from solution. If you test that by adding sodium chloride or HCl you may still get a white precipitate that is not silver. Depending on the sterling silver you have or other silver scrap there may be lead present that will form a very similar white precipitate with the addition of chlorides. If you have this happen, place the precipitate in sunlight for a few hours. If it does not turn purple or black, it is indeed lead chloride. If it does change color, get back to us with a picture, and we will tell you how to get your full recovery.

 

[Lobby]

You added hot distilled water to your nitric?

:shock:

 

[Harold_V]

Thanks for the reply! I just took a small sample, added a few grains of table salt and did get a white precipitate. I guess the solution wasn't done cementing. From here should I just suspend more copper in the solution and try to cement the rest out? Also how long do you recommend I let it continue cementing? Thanks again, I appreciate the response.

The rate at which you will cement the silver is directly related to the amount of copper you provide. If you provide a large surface area, and there's enough copper present, you can recover everything in minutes. Use plenty if you're in a hurry.

One thing to consider. What silver cements, it is dropped in a pregnant solution. Some of the dissolved silver will be dragged down with the cemented silver. If you hope to make a full recovery, when the solution tests barren, before removing the copper, stir the cemented copper well, using a gloved hand. That will release the dragged down silver nitrate so it can be recovered. Do this until stirring makes no difference, although it likely will be needed only once. Test as you have been directed---by removing a drop or two and testing with HCl or salt.

If you suspect that your silver may contain dissolved platinum or palladium, do not remove the copper until it has been cemented. You can test the solution for values with stannous chloride. Both platinum and palladium will cement after the silver is down, and may do so slowly, although you can expect a full recovery.

Assuming you do happen to have these values in solution, the color should be green. When they are fully recovered, the solution should be a nice even blue color, lacking any green tint.

Harold

 

[odif740]

If having "free nitric" is a problem, could or should the excess be neutralized with something that leaves the Ag in solution?

 

[Harold_V]

If having "free nitric" is a problem, could or should the excess be neutralized with something that leaves the Ag in solution?

On occasion I'd have too much nitric present. It doesn't prevent cementation, but it does dissolve more copper, and will yield the familiar brown fumes. When the appropriate level of free acid is achieved (by dissolving copper), silver will then cement and remain. Some free nitric is desirable.

Harold

 

[butcher]

I see no good reason to neutralize the acid, or add some other chemical to the mix, if free nitric acid you could consume more silver, or just let it dissolve more copper while cementing the silver, if you did not wish to try dissolve more silver, this way you do not add any other chemical and your silver would be more pure.

Some excess acid is beneficial when cementing, if you have a lot of free nitric it will consume more copper, but will do no harm to the purity of the silver (of coarse the copper is a large bus bar, and not thin sheets or wire that may let pieces of copper metal fall off into your silver cement powders).

Here is where just a little figuring before hand can help you save nitric acid, copper and other troubles, spending a little time learning to calculate the approximate amount of acids needed, can save a lot of frustrations later, using too much acids in some processes can be very troublesome, and people new to this field usually dump in way too much of everything.

 

[Dirtburner]

What I do to Before I hang copper in my solution to cement silver out, is dry all the liquid out on a hot plate under a fume hood. I do this until it is almost completely dry. Then I put the silver nitrate crystals on a ceramic plate to air dry stirring occasionally until I smell no more nitric acid. Next, I mix my nitrate crystals in a generous amount of warm distilled water use enough to dissolve what you have, filling a jar or beaker just large enough to give you enough room for the reaction and copper surface area. An exact amount of water is not important. I use copper wire bent in a coil. Now hang it and cement! You should get very nice crystals of silver hanging from the wire very quickly!! Filter and use. Watch your solution colors for palladium. I get 99.99 fine like this. The larger crystals grow when your PH is correct! These absorb fewer impurities, and are easy to handle and clean. You can double check purity after you pour by taking drill cuttings and parting them, then use a streak plate for the bar too. When you initially digest your silver, any gold or platinum groups other than Palladium, should be left behind as a black filtrate. Upon heating to redness, the gold will regain its yellow color. Use white ceramic parting dish for this.

 

[butcher]

Dirtburner,
Interesting but you are not cementing silver you are growing crystals these are two different processes.

I have questions about that process,and comments of what I see, some of the questions that come to mind.

Evaporating off the nitrate will concentrate acid and metals as this will drive off excess acid which can be a plus if you over use acid in the first place, but as you do this you will also form crystals of other metals nitrates that would not normally form when using the normal process, these you will have to dissolve again in the water you are using later, some metals salts once dried or hardened are harder to redissolve after they form, also when you dry some of the more reactive base metals, and expose them to air they can change states, especially if you drive off nitrate acids in the salts, copper even copper which is fairly unreactive being low in the reactivity series, can oxidize or worse form copper carbonates which are fairly insoluble in water.

Then you take copper and grow crystals on it, this sounds to me you are collecting these crystals on undissolved copper wire (how would these crystals be pure when plated to copper?), it does not sound like you are replacing the silver from solution with copper as we normally do when we cement silver.

I do not see how your copper wire coated with silver crystals would not contaminate your silver, you are growing silver crystals and granted the crystals would be pure as long as solution was not too contaminated in which they grew, but you have them stuck to copper metal and where copper and silver joined they would not be pure, I would not call this a cementation process which is using a displacement reaction, replacing one metal in solution with a another metal lower in the reactivity series, what you are doing is growing silver crystals on copper which is using a totally different chemical process.

If you wish to grow silver crystals using this method why use copper wire which will contaminate your silver, why not just use pure silver wire or just, a sting ,or a string with some silver salt as a seed to grow the crystals onto, this way your silver would not be contaminated with copper, also the more pure your solution the more pure the crystals would form, although the crystals will try to reject impurity some metals would also reduce onto the matrix electrolytically or mechanically, especially when grown onto copper which will have a greater potential voltage difference than several of the metals in your solution.

 

[g_axelsson]

Dirtburner,
Interesting but you are not cementing silver you are growing crystals these are two different processes.

It's the same process, cementing, both when going from silver nitrate directly from nitric digestion or from dissolved silver nitrate crystals. Just as when running silver in a cell you can get a lot different crystal forms, mostly a function of what other content there is in solution, temperature and concentration. It sounds like Dirtburner creates larger more massive crystals than I'm used to at least, but the size of the crystals doesn't decide the name of the process.

I have questions about that process,and comments of what I see, some of the questions that come to mind.

Evaporating off the nitrate will concentrate acid and metals as this will drive off excess acid which can be a plus if you over use acid in the first place, but as you do this you will also form crystals of other metals nitrates that would not normally form when using the normal process, these you will have to dissolve again in the water you are using later, some metals salts once dried or hardened are harder to redissolve after they form, also when you dry some of the more reactive base metals, and expose them to air they can change states, especially if you drive off nitrate acids in the salts, copper even copper which is fairly unreactive being low in the reactivity series, can oxidize or worse form copper carbonates which are fairly insoluble in water.

Dirtburner said "Filter and use.", although not exactly where I would use it in the process. If I would repeat this process, then I would filter off anything not dissolved before I would start with the cementation. If any solids were left with the silver it would contaminate it.

Then you take copper and grow crystals on it, this sounds to me you are collecting these crystals on undissolved copper wire (how would these crystals be pure when plated to copper?), it does not sound like you are replacing the silver from solution with copper as we normally do when we cement silver.

The copper will be replaced with silver just as any normal cementation. What would else turn the silver nitrate into silver unless we dissolve copper and replace the silver in solution? Dirtburner never adds any other chemicals or electricity to reduce the silver.

I do not see how your copper wire coated with silver crystals would not contaminate your silver, you are growing silver crystals and granted the crystals would be pure as long as solution was not too contaminated in which they grew, but you have them stuck to copper metal and where copper and silver joined they would not be pure, I would not call this a cementation process which is using a displacement reaction, replacing one metal in solution with a another metal lower in the reactivity series, what you are doing is growing silver crystals on copper which is using a totally different chemical process.

You must have misread what Dirtburner wrote. It's the standard replacement process and as the copper goes into solution the silver that plated to the surface is dropping off as the copper goes into solution.

If you wish to grow silver crystals using this method why use copper wire which will contaminate your silver, why not just use pure silver wire or just, a sting ,or a string with some silver salt as a seed to grow the crystals onto, this way your silver would not be contaminated with copper, also the more pure your solution the more pure the crystals would form, although the crystals will try to reject impurity some metals would also reduce onto the matrix electrolytically or mechanically, especially when grown onto copper which will have a greater potential voltage difference than several of the metals in your solution.

If you just read what Dirtburner wrote you see that he doesn't add anything else than copper to the silver nitrate. Silver wire wouldn't work, no replacement process. Silver salts on a string is... what? Growing a large silver nitrate crystal from a concentrated solution? I don't understand what you are thinking here.
Next, yes "the more pure your solution the more pure the crystals would form" is as usual, a cleaner solution always gives a better result.

Watch your solution colors for palladium. I get 99.99 fine like this.

Dirtburner, exactly what will happen if you start with a solution heavy in palladium? Wouldn't that give you a result with less quality than 99.99%?

Göran

 

[butcher]

I thought he was growing crystals, not cementing silver, otherwise why go to the trouble of drying silver nitrate salts and then rehydrating them, I guess I do not understand what he is doing.