# Waste Treatment, copper cementation.



## Gsracer (Feb 2, 2022)

Hello gents! Ive been keeping my copper waste solutions from my silver refining in 5 gallon buckets filled with iron for about a month at a time to ensure the copper and other metals are being cemented out before further processing. They have agitation (bubbler) going constantly. Recently ive gotten to a point where the waste solutions are stacking up on me and i wanted to see if i can revisit my process to make it more efficient.

Question is how long should it take for the copper and other metals to cement out on the iron, or is there a way for me to test the green iron solution for copper and such before further processing? 

Just trying to be thorough as i can be. 

Thanks in advance.


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## Stibnut (Feb 2, 2022)

I imagine you could take a sample and add ammonia to see if you get a blue color. Not sure if that's sensitive enough for proper wastewater disposal or not, but it would be good enough for me on a small scale.


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## Gsracer (Feb 2, 2022)

I had thought about ammonia. However as dark green as the solution is. I doubt I would be able to tell the color change. 

I’m hoping to cut down to a week per cementacion. I know 4 metals mentions in the thread on dealing with waste that it’s a few hours. But I’m unsure if that’s in a commercial setting or under certain conditions. Etc.


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## Stibnut (Feb 2, 2022)

I think the ammonia would drop both iron(II) and iron(III) as Fe(OH)2/3, which should allow you to see the blue color of tetraamminecopper. It would be a good idea to test that first on a ferrous sulfate solution though.


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## orvi (Feb 3, 2022)

Stibnut said:


> I think the ammonia would drop both iron(II) and iron(III) as Fe(OH)2/3, which should allow you to see the blue color of tetraamminecopper. It would be a good idea to test that first on a ferrous sulfate solution though.


It will go to some extent, but precipitate of iron hydroxides is very gooey and voluminous. Depends on how accurately you like to test it. If there is significant copper in solution, you will see it no matter how much iron you will have in solution. Then it only depend on your eyesight  

I once did rough test like this (ammonia "shortage" happened to me):
Cut and sand some small piece of steel sheet. No need to be perfect, but it should be fairly clean, rust-free, exposing fresh metal. Take like 100mL of the suspected "spent" solution and dip the steel sheet in for several hours. Personally, if I did not see any Cu precipitation/coating/mud forming, I conclude the solution is sufficently copper-free  I trusted my eye to recognize copper precipitate from other possible metals cementing.
If you are not sure, just collect bit of the coating/mud and add few drops of HCl/peroxide or easier - nitric. And than test with ammonia. 
It sound complicated and time consuming, but it could be done in a minute.


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## Ohiogoldfever (Feb 7, 2022)

Kinda unrelated but interesting.. I learned today that Ammonia has more hydrogen atoms than even hydrogen.. 100lbs of ammonia contains more hydrogen then 100lbs of hydrogen.....

Kind makes my head hurt...


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## orvi (Feb 8, 2022)

Ohiogoldfever said:


> Kinda unrelated but interesting.. I learned today that Ammonia has more hydrogen atoms than even hydrogen.. 100lbs of ammonia contains more hydrogen then 100lbs of hydrogen.....
> 
> Kind makes my head hurt...


I don´t get it. Nitrogen 14g/mol hydrogen 1g/mol. Ammonia is 82% nitrogen by weight...
Maybe measuring it by volume will do that magic difference, as liquid hydrogen is extremely light.


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## Yggdrasil (Feb 8, 2022)

I think it has to do with how you see it, by volume or weight.
One mol of Nitrogen as Ammonia stores 3 moles of Hydrogen.


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## Quiklearner (Feb 8, 2022)

This is why ammonia is being considered as a hydrogen storage medium for electric vehicles.


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## Yggdrasil (Feb 8, 2022)

Well ammonia can be combusted directly or converted to hydrogen for FC electric use. There are a couple of ships beeing built for Ammonia motors in Norway now.


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## Quiklearner (Feb 8, 2022)

Yes, I have heard that this is being applied to marine transport.


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## db87 (Feb 8, 2022)

Gsracer said:


> I had thought about ammonia. However as dark green as the solution is. I doubt I would be able to tell the color change.
> 
> I’m hoping to cut down to a week per cementacion. I know 4 metals mentions in the thread on dealing with waste that it’s a few hours. But I’m unsure if that’s in a commercial setting or under certain conditions. Etc.


You could dilute the solution down. If there is decent amount of metal you will still see it. Alternatively, Dilute solution by adding 50% water, warm to 40-50c and add a clean piece of iron to see if any copper cements out. you should see some copper plating out fairly quickly if there's any remaining.


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## Gsracer (Feb 8, 2022)

Well thank you for the replies.

I did the ammonia test after a week and a half in my waste processing rig.

What i did was take a sample from the solution in a test tube, i diluted it 50 percent with water. I then added some ammonia and i did get a browish black precipitate with the consistency of silver chloride that settled pretty quickly upon agitation. I assumed this was the iron as stibnut and orvi suggested. 

Most importantly the solution after the precipitate cleared up substantially and was relatively clear and not blue at all.

This helps me out tremendously and i can start processing my was quicker.

Thank you everyone again!


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## orvi (Feb 10, 2022)

Yggdrasil said:


> I think it has to do with how you see it, by volume or weight.
> One mol of Nitrogen as Ammonia stores 3 moles of Hydrogen.


Yes, it has more with what is called "volume energy density".
Liter of liquid hydrogen weight 71 grams approx.
Liter of liquid ammonia weight 730 grams approx.

So there is 10 times more mass per unit of volume. Additionally, when you burn hydrogen, you produce water. Burning the ammonia also produce nitrogen (and other stuff) alongside water. So you get the energy also from burning that "nitrogen" part.

deltaH(combustion) for ammonia is 319 kJ/mol
deltaH(combustion) for hydrogen is 286 kJ/mol

But you must consider that mole of hydrogen weight approx. 2grams. Mole of ammonia 17grams.
When you add density for the comparison of "volume performance", you get 35,5mol of H2 in 1L of liquid H2. That means it is capable of producing 10,2 MJ of heat energy.
Liter of liquid NH3 contains approx. 43 moles of NH3, thus combustion of liter of ammonia will produce 13,7 MJ of heat energy.
Aspects of the storage are also preferring ammonia, as it does not require that high pressure to store as liquid. 
On the other hand, you must manufacture the ammonia in the first place  and that is energy intensive process requiring hydrogen to start with. Nevertheless, Haber-Bosch and Ostwald processes are one of the most important chemical discoveries for humanity in past 150 years.


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## Yggdrasil (Feb 10, 2022)

I think they have discovered an alternative not too long ago, but it may not be available yet.
They are working hard at all these old processes now, to try to replace or refine them with less energy intensive processes.


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## Martijn (Feb 10, 2022)

The thing with testing for metals in solution by formation of hydroxides, is that iron comes out first at around pH 3. This is rusty brown. Copper will still stay in solution until you reach pH 5 / 6 or so. 


Putting clean iron in will indicate if there are still heavy metals above iron in the reactivity series present. 
Leaving the iron in too long will consume the iron completely. Once the color of the solution has become a pale green and after a few days, you can pretty much assume most copper is out and move on to raising pH. 
The solution will become clear as you said, but you need to measure pH and raise it too at least 9 to get all copper out and then to 11 to ensure all metals are out. 
Adding a pH + substance like soda ash or lye will indicate if any metals are left. At Ph11 you can discard the filtered solution. 


Martijn.


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## Gsracer (Feb 10, 2022)

Thank you for the explanation martin, after i cement out. I raise the pH in the steps as you stated with lye. I mix it thoroughly and the iron hydroxide usually settles in a day or two. Then i filter and dry the precipitate and dispose of the clear liquid. My main concern was speeding the iron cementation process as up till now i had no quantifiable way of knowing it was complete for sure. So in caution I just added an abundant excess of iron and lots of time.


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## Martijn (Feb 10, 2022)

I just tap the copper cement off the rebar every now and then. You'll see when no more cement is forming after a while. 
And depending on what you dissolved, there are other metals between iron and copper that will cement out on iron. 





Reactivity series - Wikipedia







en.wikipedia.org




I use a 10 liter see through bucket for the small amounts of what i process. It helps seeing whats going on.


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## orvi (Feb 11, 2022)

Martijn said:


> The thing with testing for metals in solution by formation of hydroxides, is that iron comes out first at around pH 3. This is rusty brown. Copper will still stay in solution until you reach pH 5 / 6 or so.
> View attachment 48263
> 
> Putting clean iron in will indicate if there are still heavy metals above iron in the reactivity series present.
> ...


Could you trace the source of the graph you added to your post ? I will like to read more about this, if it is a part of some article or so


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## Martijn (Feb 12, 2022)

orvi said:


> Could you trace the source of the graph you added to your post ? I will like to read more about this, if it is a part of some article or so


Many different sources and graphs are found when searching for solubility of metal hydroxides 


solubility of metal hydroxides - Google Zoeken



Liķe


https://www.researchgate.net/figure/Metals-hydroxide-solubility-as-a-function-of-their-concentration-and-pH-15_fig3_267326121


Have fun!


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## orvi (Feb 12, 2022)

Martijn said:


> Many different sources and graphs are found when searching for solubility of metal hydroxides
> 
> 
> solubility of metal hydroxides - Google Zoeken
> ...


Thanks, interesting reading. Possibility of treatment with bases is somewhat less implemented into general refining I guess. So any information is appreciated


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## Cap1 (May 31, 2022)

Gsracer said:


> Hello gents! Ive been keeping my copper waste solutions from my silver refining in 5 gallon buckets filled with iron for about a month at a time to ensure the copper and other metals are being cemented out before further processing. They have agitation (bubbler) going constantly. Recently ive gotten to a point where the waste solutions are stacking up on me and i wanted to see if i can revisit my process to make it more efficient.
> 
> Question is how long should it take for the copper and other metals to cement out on the iron, or is there a way for me to test the green iron solution for copper and such before further processing?
> 
> ...


In this context, instead of iron, could I use the ferrite rings found in ewaste? I've got about 5 kg of the stuff and would rather put it to use than just dumping it at the scrap yards.


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## olawlor (Jun 1, 2022)

Cap1 said:


> In this context, instead of iron, could I use the ferrite rings found in ewaste? I've got about 5 kg of the stuff and would rather put it to use than just dumping it at the scrap yards.


Most ferrites are not electrically conductive (to eliminate eddy currents), and contain iron oxides instead of iron metal, so I don't think they'd work as a direct scrap iron replacement. But they often do contain other metals, either nickel (which is too noble to cement copper) or manganese and zinc (which will cement almost everything).

I'd try a test tube sized sample just to see what happens. Please let us know!


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## dpgold (Jun 5, 2022)

I tried to do a copper cementation out of some second wash AP solution and this is what happened overnite. I put in a piece of steel and the next day I found one brown layer and a reddish layer above it. What would they be? I assume one layer should be copper one would be iron?


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## Martijn (Jun 5, 2022)

Could be copper with other base metals between iron and copper in the reactivity series.


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## dpgold (Jun 6, 2022)

Hello everyone, I tried to do a cementation of the AP second wash solution and I got a bunch of reddish powder on the top and brownish at the bottom. If you can tell me what are these. I do this so that I can get some experience and learn about the process not to recover the material. From what I understand the steel dissolves and goes into solution and the copper cements out (precipitates). The liquid was turquoise light colour from the beginning.


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## dpgold (Jun 6, 2022)

Sorry, I did not see the response for some reason. thank you.


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## 4metals (Sep 7, 2022)

Gsracer said:


> I’m hoping to cut down to a week per cementacion. I know 4 metals mentions in the thread on dealing with waste that it’s a few hours. But I’m unsure if that’s in a commercial setting or under certain conditions. Etc.


Just saw this thread. The method originally posted for waste was primarily for waste from aqua regia refining. What happens is the copper is replaced in solution by the iron going into solution as a chloride so the copper comes out as a metal and the iron goes in as a soluble salt. 

I prefer to treat the copper nitrate waste by adding sulfuric acid followed by distillation to generate copper sulfate which can be plated out of the solution on to a copper cathode and make very pure copper. One bonus of this admittedly tedious process is you do regenerate nitric acid so you cut down on nitric usage. And you do get back metallic copper to use for cementing Silver from contaminated electrolyte.


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## snoman701 (Sep 8, 2022)

4metals said:


> I prefer to treat the copper nitrate waste by adding sulfuric acid followed by distillation to generate copper sulfate which can be plated out of the solution on to a copper cathode and make very pure copper. One bonus of this admittedly tedious process is you do regenerate nitric acid so you cut down on nitric usage. And you do get back metallic copper to use for cementing Silver from contaminated electrolyte.


And if you had Cu + Ni + Zn + Pb and maybe even Cd in that nitrate soup?


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## 4metals (Sep 8, 2022)

snoman701 said:


> And if you had Cu + Ni + Zn + Pb and maybe even Cd in that nitrate soup?


They will be removed in the slimes from the copper cell, or better yet in the pre anode pouring smelt of the copper.


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## snoman701 (Sep 8, 2022)

4metals said:


> They will be removed in the slimes from the copper cell, or better yet in the pre anode pouring smelt of the copper.



No, I'm saying if you had more than just copper in your waste nitrate liquid. You add sulfuric and drive off the nitrates, so then you get a mixture of Copper Sulfate, Nickel Sulfate, Zinc Sulfate, etc. You can plate out the Copper pretty easy, but the others like to stay in solution....or, as I think you or Chris had referred to them in the past, they end up as floating turds. 

Nitrate liquids do not treat by neutralization nearly as nicely as chloride wastes. The hydroxides are slimy and icky whereas chloride wastes are relatively easily filtered.


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## 4metals (Sep 8, 2022)

If you do a smelt type of anode casting process, where you flux to oxidize those metals and do an air or O2 sparge you can minimize those metals before they make it to the anodes to begin with. 

Anything remaining in small quantities in your cast anodes will not mess up the anode corrosion and will stay in the slimes.


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## Alondro (Sep 8, 2022)

To make the copper cement faster, you can make a simple 'battery' by attaching the iron rod to a copper wire at the top, then bend the other end of copper wire into the beaker/bucket the iron is in. This will create a weak current and rapidly reduce the copper in the solution. Some will even crystalize onto the copper wire, as it acts as a cathode in the cell. 

The iron oxidizes, releasing electrons which pass through the wire to the copper cathode, creating another place for copper cations to reduce. 

For a big volume, I attach the wire to a copper rod. 

The copper in solution is usually all removed down to a trace amount within 4 days, unless there's so much copper ion that it totally dissolves the iron anode before finishing. Then a new iron rod is needed to complete the copper reduction.


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## 4metals (Sep 8, 2022)

Alondro said:


> To make the copper cement faster, you can make a simple 'battery' by attaching the iron rod to a copper wire at the top, then bend the other end of copper wire into the beaker/bucket the iron is in. This will create a weak current and rapidly reduce the copper in the solution. Some will even crystalize onto the copper wire, as it acts as a cathode in the cell.


I'm having a hard time picturing what you are describing here, perhaps you could make a sketch or take a photo of a "battery" like the one you describe. 

Thanks


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## snoman701 (Sep 8, 2022)

It's a short circuited battery with an iron anode and a copper cathode.


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## 4metals (Sep 8, 2022)

So a big slab or scrap iron with a wire lug on top and a length of insulated copper to get to the other side of a container and attach to a length of copper buss immersed in the solution will do? 

I've got a friend hounding me to get rid of drums of spent electrolyte and he doesn't want to do a distillation. I think I'll go downstairs and set up a 5 gallon pail of copper nitrate to try this. He will still have iron in solution to deal with at the end.


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## snoman701 (Sep 8, 2022)

Please report back....nitrate waste is my nemesis right now.


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## Alondro (Sep 8, 2022)

snoman701 said:


> Please report back....nitrate waste is my nemesis right now.


If you have the setup, you can also heat copper nitrate solution and bubble the gas released through 20% hydrogen peroxide and regenerate dilute nitric acid.

At a fast boil, copper nitrate will begin to decompose as the solution boils away and becomes more concentrated into copper oxide and nitrogen dioxide at the bottom of the container of solution, where it contacts the heat and passes 180C. This becomes very obvious as it's the same red-brown gas that evolves when nitric is eating at a metal.


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## Martijn (Sep 9, 2022)

4metals said:


> I'm having a hard time picturing what you are describing here, perhaps you could make a sketch or take a photo of a "battery" like the one you describe.
> 
> Thanks


It is basically a galvanic cell, like simple batteries work. Two different metals in an acidic solution electrically connected with copper wire outside the solution, creates a voltage. The voltage helps with displacing the copper. 
He posted this idea a couple of months ago. Cant find it now though.


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## orvi (Sep 9, 2022)

snoman701 said:


> Please report back....nitrate waste is my nemesis right now.


Same here. But I am wondering if aluminium does not do the trick. Precipitating all metals. I will see how the ppt will appear, and how the reduction will go - if there wouldn't be much nitrate deep reduction to ammonia. Somewhere I read that even HN3 could be produced in the process, what scares me quite a bit.

I wondered that adding NaCl to the juice in just about right ammount can ppt bulk of Pb as PbCl2. Or even better add H2SO4. Cementation with Fe would then give much cleaner copper. On the other hand, another filtration needed.

My waste is very diluted and contain nearly no Cu, lots of Cd and some SnPb traces. No benefit from recovery, and no possibility to scrubb Cd fumes at the moment. For this canister I will probably go with straight hydroxide, and we will see how things end up. I have regulated filtering unit which can be turned on for days... So if some gooey s..t form, I am prepared


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## Alondro (Sep 10, 2022)

orvi said:


> Same here. But I am wondering if aluminium does not do the trick. Precipitating all metals. I will see how the ppt will appear, and how the reduction will go - if there wouldn't be much nitrate deep reduction to ammonia. Somewhere I read that even HN3 could be produced in the process, what scares me quite a bit.
> 
> I wondered that adding NaCl to the juice in just about right ammount can ppt bulk of Pb as PbCl2. Or even better add H2SO4. Cementation with Fe would then give much cleaner copper. On the other hand, another filtration needed.
> 
> My waste is very diluted and contain nearly no Cu, lots of Cd and some SnPb traces. No benefit from recovery, and no possibility to scrubb Cd fumes at the moment. For this canister I will probably go with straight hydroxide, and we will see how things end up. I have regulated filtering unit which can be turned on for days... So if some gooey s..t form, I am prepared


Aluminum won't work with nitrates. It forms a layer of impermeable iron nitride on the surface. 

Besides, Al is much more expensive than iron. 

I also learned that the decomposition of both forms of iron nitrate (iron +2 and +3 oxidation states) takes place at a much lower temperature than copper nitrate, BELOW the boiling point of water. In fact, in hot aqueous solution over 80C, both iron nitrate forms decompose into iron hydroxide and nitric acid vapor. 

So, the BEST processing idea is displacing the copper with iron, then pouring the iron nitrate in a flask connected to bubbler set up and heating it to about 90C, with the vapor bubbling into 3% hydrogen peroxide, to ensure that all the nitrogen oxides released are converted back to nitric acid. What will remain in the flask will be aqueous iron hydroxide goop.


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## 4metals (Sep 10, 2022)

Alondro said:


> So, the BEST processing idea is displacing the copper with iron, then pouring the iron nitrate in a flask connected to bubbler set up and heating it to about 90C, with the vapor bubbling into 3% hydrogen peroxide, to ensure that all the nitrogen oxides released are converted back to nitric acid. What will remain in the flask will be aqueous iron hydroxide goop.


The battery idea works for someone running silver cells on sterling silver because the electrolyte is mostly copper nitrate (if the Palladium is removed with Dimethylglyoxime ). That insures the insolubles from the battery are mostly copper. 

The setup to distill the nitric for re-use benefits from the lower temperature but only marginally. If one has a proper distillation setup the extra few degrees isn't much different. However free dilute nitric can be concentrated in the same distillation apparatus. I think for some the removal of the copper as a metal is the big plus, and a classic treatment with caustic followed by filter pressing would be easier. 

The chemist in me loves the chemical apparatus, but the practical refiner in me often chooses other routes. None are wrong, just another means to an end.


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## Alondro (Sep 10, 2022)

4metals said:


> The battery idea works for someone running silver cells on sterling silver because the electrolyte is mostly copper nitrate (if the Palladium is removed with Dimethylglyoxime ). That insures the insolubles from the battery are mostly copper.
> 
> The setup to distill the nitric for re-use benefits from the lower temperature but only marginally. If one has a proper distillation setup the extra few degrees isn't much different. However free dilute nitric can be concentrated in the same distillation apparatus. I think for some the removal of the copper as a metal is the big plus, and a classic treatment with caustic followed by filter pressing would be easier.
> 
> The chemist in me loves the chemical apparatus, but the practical refiner in me often chooses other routes. None are wrong, just another means to an end.


Since I deal with small volumes, the little setup I mention is ideal. I'll only be dealing with a maximum of a few gallons of waste solution at time, even when I ramp up processing. Still in the experimental phase right now, testing all the different parts for what's recoverable in them, creating standardized and efficient protocols for each main group of parts and pieces, and so forth. 

Bigger refiners already would have the equipment to handle thermally decomposing copper nitrate directly.


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## orvi (Sep 10, 2022)

Alondro said:


> Aluminum won't work with nitrates. It forms a layer of impermeable iron nitride on the surface.
> 
> Besides, Al is much more expensive than iron.
> 
> ...


I did cementation with Al also in nitrate enviroment, and it went quite well.. Altough, pH was above 4 to be precise. Loť of nitrate get reduced to ammonia (clearly smelled), but overall, it works in described conditions.
I liked aluminium for it´s quick action... Took 20L bucket, fill 10L of AR copper waste, grab old piece of aluminium roof sheet, rolled it into the spiral and toss it in  it kickstarted the reaction instantly, and it was slowly chewing the plate downwards  thus moderating the insanely exothermic reaction. Fully saturated spent AR usually reduced in volume by 1/3 at the end of the process just by evaporation/boiling caused by cementation heat.

But iron seems to be much better idea in terms of efficiency and reusability. After removing lead with H2SO4, cemented copper would be fairly clean - nice benefit. 
Only thing to resolve is how to deal with cadmium... In a way we can recycle some nitric from the end juice. But... Cd(NO3)2 shouldn´t cause much problems as I think about it.


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## Alondro (Sep 10, 2022)

orvi said:


> I did cementation with Al also in nitrate enviroment, and it went quite well.. Altough, pH was above 4 to be precise. Loť of nitrate get reduced to ammonia (clearly smelled), but overall, it works in described conditions.
> I liked aluminium for it´s quick action... Took 20L bucket, fill 10L of AR copper waste, grab old piece of aluminium roof sheet, rolled it into the spiral and toss it in  it kickstarted the reaction instantly, and it was slowly chewing the plate downwards  thus moderating the insanely exothermic reaction. Fully saturated spent AR usually reduced in volume by 1/3 at the end of the process just by evaporation/boiling caused by cementation heat.
> 
> But iron seems to be much better idea in terms of efficiency and reusability. After removing lead with H2SO4, cemented copper would be fairly clean - nice benefit.
> Only thing to resolve is how to deal with cadmium... In a way we can recycle some nitric from the end juice. But... Cd(NO3)2 shouldn´t cause much problems as I think about it.


Oh, you're talking about aqua regia, not pure nitrate. The chloride ion in the AR will react strongly with aluminum.


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## orvi (Sep 11, 2022)

Alondro said:


> Oh, you're talking about aqua regia, not pure nitrate. The chloride ion in the AR will react strongly with aluminum.


I also did pure nitrate, but in very weakly acidic pH, then practically basic pH. Altough not as waste, but scavenging last bits of PGMs from the solution after hydrolysis, which refused to cement on iron, zinc, or drop as ammonium platinate  that was running nicely on Al. Altough much more ammonia was produced than PGMs cemented, it does the trick well for me


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## Alondro (Sep 11, 2022)

orvi said:


> I also did pure nitrate, but in very weakly acidic pH, then practically basic pH. Altough not as waste, but scavenging last bits of PGMs from the solution after hydrolysis, which refused to cement on iron, zinc, or drop as ammonium platinate  that was running nicely on Al. Altough much more ammonia was produced than PGMs cemented, it does the trick well for me


Hmm, strange. The few tests I did with Al in pure nitric solution didn't react in the least. I let the Al rod sit in there for days and it did nothing at all.


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## orvi (Sep 12, 2022)

Alondro said:


> Hmm, strange. The few tests I did with Al in pure nitric solution didn't react in the least. I let the Al rod sit in there for days and it did nothing at all.


Maybe your mixture had passivating properties. Hard to say. But I can say not all kinds of aluminium reacted the same. Sheet metal worked the best, but some aluminium spare parts which I had in the lab (broken) didn´t reacted that well... Just on few spots, and grinding with angle grinder to show fresh surface doesn´t help much


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## Alondro (Sep 12, 2022)

orvi said:


> Maybe your mixture had passivating properties. Hard to say. But I can say not all kinds of aluminium reacted the same. Sheet metal worked the best, but some aluminium spare parts which I had in the lab (broken) didn´t reacted that well... Just on few spots, and grinding with angle grinder to show fresh surface doesn´t help much


It might be Al-Mg alloys that react. The Mg would dissolve rapidly, constantly exposing fresh Al sponge with immense surface area that wouldn't be protected by nitride formaiton.

Alternatively, I was reading up on this and found that very dilute nitric acid will indeed react with aluminum. Concentrated will form the nitride layer and stop the reaction. So that's likely why the solution I tested didn't react, it was still too concentrated.


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