Max nitric conc in scrubber solution and other question

[geedigity]

If you don't want to read the entire post, my questions are in bold.


I am hoping someone could shed some light on a few questions I have.

Quite a ways back, I tried to reclaim a little gold from gold plated copper items and other iron based metals that consisted of pins, fingers, CPUs, transistors, etc. I subjected the material to a severe case of smashing and crushing in an attempt to provide maximum surface area for the subsequent processes to act upon it.

Following the pulverization of the material, nitric acid was added and allowed to react until no further action was observed, when the liquid was decanted off and a bit more nitric was added to the solids. At that point, I added aqua regia (AR) to the solids and began dissolving the gold and other left over base metals. After it appeared that everything had dissolved, I tested the solution using my stannous chloride test solution, and it immediately turned a black color with the fringes of the test spot being purplish, indicating there was gold in solution. I noticed when I added a few more pins and fingers, the gold was still dissolving, so I added a whole bunch more material. Why? Not quite sure why I added a whole bunch more material.

That was a mistake, because I ran out of nitric acid in the solution and didn’t have anymore on my shelf. I ended up with a mess of half dissolved base metals, dark brown/green solution, thick sludge (like mud), plastic floaters, and on and on. When I tested the liquid, no gold was in solution anymore. I had just made the batch of testing solution earlier in the day, and although I didn’t have any standard solutions, per se, I had some auric chloride (unknown purity) that someone gave to me that I used to mess around with. So my testing solution was good. With my tail between my legs, I put the liquids and muck into a container labeled “screwed up AR batch” and forgot about it.

Later, I decided to reprocess the material and the first thing I did was decant off the liquids, since everything had settled nicely. Knowing I still had HCL in the solution, I figured I may as well dissolve the whole mess using AR, cement out the gold using copper and the refine the gold from there. Since I didn’t have any nitrate to make nitric, I used a some NaNO2 (sodium nitrite) to make NOx that I bubbled through two, 3+ feet long, 3-inch inner diameter PVC scrubbers I made. Each scrubber column contained just under 4 liters of water with some 35% peroxide added. Needless to say, my reactions are always done under pressure, so I can force the gases generated through the scrubbers at about 1 psi and a flow rate of 3 to 5 liters per minute.

Question: How much NOx can I dissolve into water at 1 psi at a temperature ranging from 0 C up to 20 C (32F – 68 F). I assume it is a linear relationship. I have been looking for this info, but can not seem to be able to figure out what maximum concentration nitric acid I could expect from using this method.

The reason I am asking this is that, apparently the resultant liquid in the scrubber, or as I call it, the “scrubbant” (a word made up by me), is much stronger than I thought it was.

So getting back to reprocessing, after I thought I had dissolved everything that would dissolve in the AR, I tested the solution and confirmed there was gold in it. I added some sulfuric acid to be able to drop-out the lead, filtered and decided to cement out the gold using copper. Upon slightly heating the solution with copper wire in it, the solution began to vigorously boil and fume and all of the copper wire dissolved. So I added more copper wire. Then suddenly, the reaction stopped and by now the solution was a dark green-blue with brown (not blue-green) color. When I say suddenly, I mean in under a minute. I let it sit with excess copper wire in it overnight. The next day, I had pulled out some of the copper wire and it was covered with light tan colored crystals. I put some of the crystal covered copper wire into a small container, added hot water, and shook it vigorously. The majority of the crystals fell off of the wire and settled at the bottom of the container and didn’t appear to dissolve. The decanted liquid did not show the presence of gold in it (using test solution).

I then added several drops of concentrated HCL, and heated it slightly and quite a bit of the light tan crystals dissolved, turning the solution a brown color. For the sake of completeness, I tested the resultant liquid; the spot went from brown to clear, but no gold.

A test of the main solution (dark green-blue solution) changed the green-blue spot to a light brown color that eventually dried to a yellow (with a slight hint of green) color. It did not appear to indicate the presence of gold.

Question: I thought that based on the reactivity series that copper would cement out any metal below it, Ag (although not in this case), Hg, Au, Pt, etc. With respect to the light tan crystals, what is it most likely? Tin?

Pouring off the main solution resulted in only light tan crystals at the bottom that I subsequently dissolved in HCL. Little, if any brown sediment remained. I assumed this was the gold, but have yet to do anything with the main solution, muck and sediment. Recently, I was thinking about throwing the whole mess, minus the very little brown sediment, back into one container and dropping everything with Al, and either trying again or recycling the copper et al dropped by the Al. Anyway, I thought I would tell an amusing story and see if anyone could answer my couple questions above.

I am sure many other questions will be asked of me, but the above is generally what I had in my notes that I kept.

 

[butcher]

If you form nitrate salts (even from the base metals), (example Cu(NO₃)₂) and then add an acid H2O4 you make, or reform a nitric in solution, adding HCl your making an aqua regia in solution.


The goal is getting rid of nitric acid that keeps the gold oxidized so it cannot be reduced back to metal, and not having nitrate salts in solution, that when acidified that will convert nitrate salts back into nitric acid in solution, or aqua regia (depending on the acid used) to dissolve gold, or keep the gold from being reduced.

With base metals you create many problems, nitrate salt of the base metal is only one problem.

Base metals also create many other problems in recovery, the type of base metals and how much are involved, in with a solution of dissolved gold, can only complicate a recovery process, or even make it almost impossible to get your gold back without going through extreme measures.

I see so many problems, with what you tried and are continuing to try, not removing the base metals properly, and the base metals forming nitrate salts, overuse of nitric acid...

I can see one main problem your having and that is not spending enough time learning before spending time in experimenting.

Your question should have ben where do I find Hoke's book.
Where do I find the Guide to the forum where I can begin study of how this stuff works, and how to do it properly?

instead of taking notes of your failed experiments, I suggest taking note of what you study from Hoke's book and the forum.
This way your not spending your time trying to find a way out of a mess and trying to figure out how to get your lost gold back.
You are spending your time learning to recover and refine precious metals.

 

[geedigity]

Butcher - I completely understand what you are saying, and with all due respect, I have read Hoke’s book multiple times and continue to read in the forum. After spending much time reading, I still have the first and revised my other question into questions. I did the above quite some time back. My questions were more from a chemistry standpoint, not so much from a reclamation standpoint although I did mention in the OP that I was contemplating returning to the mess. I understand that I could have increased the HCL concentration neutralized the nitric using urea and dropped the gold using the various methods available. I also realize I can drop the whole mess down with AL, rinse well, heat to redness, dissolve the base metals in nitric, discard, then dissolve remaining in AR and drop the gold. I first started reclaiming gold from e-scrap in 1985, since I was taking chemistry in college and my dad had a bunch of gold fingers that he wanted the gold from. I took on the challenge just using chemistry and talking to a few folks and eventually added a couple books that I purchased from a bookstore in OR. As to my notes, I always try to take good notes so I can remember things.

My first mistake as I openly admitted, was the addition of other gold plated items to my AR solution. I know I should have processed the dissolved the gold (with minimal base metals present) after neutralizing the nitric either by evaporation or urea and then using the numerous methods available and not adding more gold plated items at that point. I also realize following the addition of too much gold plated scrap that the gold in solution would have cemented out. That was the first time (and the last) I added a sizable amount of other gold plated items to a batch of AR that had effectively dissolved everything to try and use up the nitric acid. I also realize that I could have lowered the nitric acid to HCL ratio to be 0.3 or less, but that really wasn't the intent of my question. I guess the questions got clouded by the story, so next time I will just post a question and not include the whole story.

Also, realizing only after I had added my “scrubbant,” that my nitric acid was stronger than I thought it was, making the solution excess in nitric. Thus my first question concerning a way to calculate the max nitric acid concentration I could theoretically obtain in the scrubbers. I know I could get a rough idea using mass volume calculations, but was wondering using a calculator.

Onto the next question, given the excess nitric in solution, and dissolving copper metal in that solution, wouldn't the less reactive metal chlorides be reduced to the metal, since the chloride is limited in the solution?

I understand the tin concept and that is why I mentioned that upon adding additional hydrochloric acid the crystals dissolved. I also realize it could have been copper chloride or other metal chlorides, since I was careful to exclude tin from my gold plated scrap. This indicated to me that I was lacking HCL in the solution. I guess another question could be, if base metals consisting primarily of iron, nickel, copper, chromium, and possibly manganese, cobalt, lead, tin, mercury, arsenic, etc. and gold in AR where the ratio of nitric acid to HCL is greater than 1, I would expect that the formation of nitrates would be favored over the formation of chlorides? Also doesn’t excess copper metal generally reduce any of the metal ions (associated with soluble chlorides and nitrates and some insoluble in acid solutions) that are lower in the reactivity series? Again I would think yes, but I wanted to see if folks could shed some light on some of the exceptions or products that could be formed in the above scenario. Thanks

 

[sri steve]

I do more reading than posting because I've still got my L plates on but if theirs one thing I have learnt is to keep your waste/scrap separated into the same batches and not to mix them and use tried and tested processes.

I have noticed that some try and skip or alter processes and end up with dangerous solutions and wasted scrap, I'm not telling anybody how to do your business but I would advise to stick to methods tried and tested to work and have the patience needed to do what we do, Sri.

 

[necromancer]

i had to look up "L Plate" we don't have these in ontario

from wikipedia

An L-plate is a square plate bearing a sans-serif letter L, for learner, which must be affixed to the front and back of a vehicle in many countries if its driver is a learner under instruction, or a motorcycle rider with provisional entitlement to ride restricted motorcycles.

Canada[edit]
Motor vehicles are provincially regulated in Canada, and so regulations vary by province. In British Columbia, following the instructional phase and after passing a road test, new drivers are required to show a green N (for novice) plate until they have passed an additional road test, usually after 24 months.

In British Columbia, a red L (for learner) plate is required for 12 months before the N can be issued, and new drivers with this marking are not permitted to drive without a licensed driver over the age of 25 present in the vehicle.

EDIT:

changed Canada to Ontario in first line, added quote for British Columbia

 

[butcher]

geedigity,

If I form nitrate salts in solution (or in a mess of metals, and salts of those metals, say of nitrates and chlorides...), and I evaporate the solution, or do not have enough acid (or water in solution), evaporation would not do much of any good to try and drive off nitric acid, the nitric may just concentrate with evaporation in solution with the base metals and form more insoluble nitrate salt crystals, driving off water making these nitrate salts, chloride salts... less soluble, which may just form more of these insoluble salts, like nitrate salt.

Nitrate salts which if acidified can go back into solution to re-activate the solution again with nitric acid, or nitrate ions (keeping any gold in solution oxidized).

Without free acid in solution, I cannot expect evaporation to drive off the nitrate from the salt at the temperatures we are dealing with, if base metal nitrate salts were in solution, or insoluble as nitrate salt, I cannot expect sulfamic acid or urea to convert these nitrate salts, or to de-NOx the solution, or de-nitrate the mess, I cannot expect the copper to convert or cement these nitrate salts if it cannot contact every particle of the salt, cementation can reduce ions in solution, but has a hard time converting salts, and if the nitrate salt were of copper nitrate even then touching copper would not help any to convert it to another form.(several things can come into play here without the needed free acid, or water, the chemistry of the solution with sulfamic acid or even urea will not work the same).


But if I add acid to the nitrate salts, I can put them back into solution, or make them active again in solution as ions, or at least put them back into solution as nitrate ions, where I may be able to get rid of them with one of the above methods of de-NOxing of the solution, or conversion of the salt to another metal salt.

The acid can acidify the nitrate salt, (oxidize the base metal nitrate back to nitric in solution), where we at least have a chance of de-NOxing or converting the nitrate salt it to another salt, like the less volatile chloride...

You can have many different metals in solution at once as nitrates, chlorides, sulfates... in the same solution, you can even have several insoluble salts of these acids crystalized out of solution as precipitate, or insoluble metal salt crystals.

Say I have a solution like this, and I try to cement with copper, and all of my nitrates and chlorides are in solution, (or even sulfates if I had previously added some H2SO4) and I have an excess of acid so that the cementation process would proceed.

The copper bar I used to cement, would dissolve into solution adding copper to this mix of metal ionic salts, either consuming free acids, giving up electrons to the metals less reactive than copper in the reactivity series of metals, to attempt to cement them from solution as metal,. note the solution will still hold the soup of all of these other metals as salts of the acids used to put these metals in solution, you could still have an array of nitrates chlorides and sulfates all happy to be in solution with each other, if you kept the solution acidic with at least some free acid, you would be able to consume the oxidizer keeping the gold, or less reactive metal than copper in solution, and once consuming the oxidizer (nitric) you could convert the gold chloride ion, to gold metal atoms, by replacing its missing electron from one donated by the copper metal buss bar, as the copper bar dissolved to form more copper metal ions in solution.

Note: in the case above if nitrates or salts of nitrates are involved. adding acid can reform (nitric or aqua regia, depending on the acid), and try to dissolve the more noble metal like gold, or keep the solution in such a state of oxidation that the gold cannot cement from solution, until the copper metal dissolved and would consume the nitric acid (formed in solution), before we could cement the gold out of solution.

Note also as the acid dissolves the copper it is forming salts of those acids and your copper in solution, depleting the free acid of your solution, surely you can see here once the free acid is gone from solution we pretty much stop the process of ridding the solution of nitric acid or of nitrates by the cementing process, and no more copper dissolves or easily gives up its electrons.

It is hard to describe what happens in a mess of metals, as there can be so much going on in solution.
It is also so much harder to recover gold from the mess as so many reactions are working against you getting your gold back out of solution.

 

[Jado]

Question: How much NOx can I dissolve into water at 1 psi at a temperature ranging from 0 C up to 20 C (32F – 68 F). I assume it is a linear relationship. I have been looking for this info, but can not seem to be able to figure out what maximum concentration nitric acid I could expect from using this method.

The temperature relative solubility of each NO, NO2 would be fairly linear, but as long as the vapor temp comes down to 20C it shouldn’t be a notable issue.

The limiting factor is O. Standard reaction is 50% conversion to HNO3, 50% left as NO/NO2. This will never reach 100% in liquid state without added O3 or use of H2O2.

The maximum concentration should never exceed 35%, although that’s with industrial equipment. I’d aim for 25 and adjust to match your materials and engineering.