Urea question

[anachronism]

You're not wrong Kurt. Each method has its pros and cons, and different methods are preferred by different people. Over use of Nitric has often been the root cause of many issues too. I remember the resistance to Sulphamic when first mentioned, because it wasn't used by Hoke, and it took time for people to accept something that wasn't in the established way of doing things.

Any process of destroying the Nitric is equally as good as the other provided it's one that genuinely works.

If a method kills two birds (or even three) with one stone then all the better!

 

[Shark]

I agree Kurt, and as well with Jon. It often seems I may be the only one looking for more than one way to do things. But I still want a way that works.

In this discussion, I recall several times when I was doing ewaste that no matter what it seemed to never stop the AR from coming back. Always in a heavy copper laden process. Looking back I recall discussions on the matter that didn’t make sense to me then. For some reason this time it sank into my tired old head I guess. When I read what Jon and Deano posted it “clicked” for me. Now I have another new “trick in the trunk” that may be useful again one day.

 

[kurtak]

Each method has its pros and cons, and different methods are preferred by different people.

I totally agree --- or as the old saying goes - "there is more then one way to skin a cat"

The only reason I posted what I did concerning using metal (whether gold or copper) to de-NOx AR is that it is "another" way of doing it - a way rarely talked about &/or often over looked

Kurt

 

[4metals]

Not to throw a fly into the ointment but here goes.........

There are some rules that refiners who learned here follow, hopefully, and some that a production refining environment would prove impractical. One of them is urea.

Case and point, the setup the refiner from Egypt is using is a setup made by a refining equipment manufacturer and using it requires different techniques than a refiner using a heated reactor. The thread about the Egyptian refinery is here.
The tumbler plant does not allow the reaction to be heated and does not make it easy to be exact with the nitric acid additions. Also production stone removal lots do not allow babysitting each beaker and adding nitric dropwise until the reaction is complete. These types of reactions work off a chart that says for X ounces of scrap add Y milliliters of Hydrochloric Acid and Z milliliters of Nitric Acid.

So it is easy to see that there is likely free Nitric Acid at the end of the reaction.
For the production refiner with excess nitric and time is of the essence refining schedules, urea is the route taken. Some equipment, like the tumbler, the operator can determine the best acid ratio to use the least amount of Nitric Acid efficiently but there is still excess.

For beaker stone lots, sulfamic can work well, as can heating, if time allows.

Some years back in a thread on the moderators section of the forum, Chris Owen (Goldsilverpro, RIP) was processing some aircraft stators basically using some reverse aqua regia. High nitric and low Hydrochloric Acid. He was having trouble getting rid of the free nitric so I suggested sulfamic acid, which he loved. Truth be told there was so much nitric in the reverse aqua regia he was using that the answer that worked was a combination of sulfamic and a Rotovap, which I believe Lou suggested.

After that, Chris started recommending sulfamic acid on the open forum. I personally use both but having taught so many refiners how to refine, mostly karat gold scrap, I never choose one over the other.

The production refiners of concentrated gold solutions, my strong suit, never leave solutions sitting long enough for chemical evolution of nitric to re-emerge and have never been an issue.

For my own processing, which I do less and less of these days, I believe what Kurt suggests which is less is better. Don't over use nitric.

 

[kurtak]

Awesome post 4metals with some very good points

And the first point I would like to address is -------

Also production stone removal lots do not allow babysitting each beaker and adding nitric dropwise until the reaction is complete.

The "fact" is that this advise of adding nitric a drop at a time so as not to end up with free nitric in the first place really only applies in one situation & that is when you are dissolving only a few grams of gold such as gold foils recovered from a pound or 2 of fingers &/or a second refine on first precipitated gold powders

So this advice is really only good advice in helping "new" - "hobby" refiners to understand the roll nitric plays in dissolving gold with AR in their very small (few grams) first attempts at "hobby" gold recovery/refining

Beyond that this "dropwise" advice just is not practical because once you start working with larger then a few grams of gold the real object is to get that gold into solution as quickly efficiently & effectively as possible so that you can get your gold back from solution as soon as possible so you can move on to your next batch as soon as possible

This requires the use of excess nitric simply in the fact that as gold is dissolved in the AR the nitric becomes less effective in dissolving the gold & because it (the nitric) become less effective - the reaction of dissolving the gold slows way down so you just simply can not sit around waiting for that reaction to come to "a stop" make a "small" addition - wait for the reaction to stop - make another "small" addition - etc. etc. - until the gold is dissolved as a method to insure I have no free nitric in my AR when done --- it just is not practical

Once you understand the roll nitric plays in dissolving gold with AR with the "small" batch "dropwise" method (as a teaching tool) you are then better served to calculate "about" how much nitric you are going to need - then over shoot that by "a bit" - &/or under shoot it by "a bit" - then make additions when the reaction has slowed but not stopped - knowing you will have excess nitric which needs to be de-NOxed - by the method of your choice

Then there are the situations where you actually want to overshoot the nitric by a lot

Examples - batches of mixed ceramic IC chips &/or mixed CPUs or epoxy chip ash bonding wire concentrates (just to name a few)

In those examples you have no real idea how much metal (both base metals & PM metals) you are needing to dissolve in the first place & as well you can not even see when all your metals (especially the PMs) have been dissolved so to insure you are getting all your metals into solution you don't really have much choice other then to overshoot your nitric (as well as HCl) by "a lot"

Then there is the case of reverse AR wherein the resulting AR is like 90% (plus) "free" nitric

My point being that other then as a teaching tool for "new" hobby refiners the "dropwise" advice is never really applied in refining --- in the best of situation we can keep the "free" nitric to a minimum - but there are also situations where you are best served to overshoot the nitric by a lot

And any method to de-NOx your AR will work - whether Hokes evaporation - simply heating - sulfamic - yes even urea - or my personal favorite of using up the nitric by loading it with metal

Kurt

 

[anachronism]

Then there are the situations where you actually want to overshoot the nitric by a lot

Examples - batches of mixed ceramic IC chips &/or mixed CPUs or epoxy chip ash bonding wire concentrates (just to name a few)

In those examples you have no real idea how much metal (both base metals & PM metals) you are needing to dissolve in the first place & as well you can not even see when all your metals (especially the PMs) have been dissolved so to insure you are getting all your metals into solution you don't really have much choice other then to overshoot your nitric (as well as HCl) by "a lot"

Definitely. If you're processing 50Kg of Ceramic processers in smaller parcels using the same liquor you're certainly not going to be adding Nitric by the drop. Unless of course you're wanting to spend a year doing them. The drop by drop process is absolutely fine for some circumstances and not for others.

 

[orvi]

Sulfamic acid will not destroy nitrates, there is plenty of literature attesting to this. It is, however, an efficient way of removing free nitric acid from solution.
Urea will not destroy free nitric acid in solution but will destroy nitrates in solution. Once again there is plenty of literature attesting to this.

I am looking a bit differently onto this.

You are certainly right in first case, as sulfamic reacts with free acid. If we talk about AR solutions (these we usually de-NOx), we are in acidic pH all the time. Also, coordination of chlorides to metal cations is much stronger than of nitrates. If you will throw some copper nitrate into acidic HCl/metal chlorides solution, equilibrium in the solution shifts towards formation of free nitric and respective chloride. Meaning that in typical case - acidic AR heavy metal chloride solution containing nitric acid residues - speaking about presence of some nitrate salt is OK, but presence of these species is severly supressed. And as all the ions are interchangeable in solution (nitrates and chlorides quite easily), free nitric acid can be indeed completely consumed by sulfamic acid.

In case of classic evaporative de-NOx-ing, we rely on decomposition of nitric acid by HCl, forming gaseous chlorine and NOCl, which are then easily and continously expelled from hot solution, as they form. Similarly, urea can facilitate de-NOx-ing in this way at higher temperatures - nitrogen (III) residues (forming by the same way as in evaporative deNOxing) and NO/NO2 complexed to the metal cations can be then "killed" by urea.
It is indeed quite nice chemistry, when one looks up the mechanism of that "fizzing" and what is actually going on (urea is diazotized by N(III) species, eliminates nitrogen and reacts further = that is the true explanation why it does not react with nitric acid - there is no such reaction with N(V) )

What I observed all the time is that first scoops of either urea or sulfamic react vigorously, and then it basically stop reacting, despite there is still plenty of oxidizer in solution. From literature I read, both urea and sulfamic acid reacts very quickly with nitrogen residues in oxidation state III (reaction rates are relatively high). But only sulfamic acid is capable of reacting with nitrogen in ox. state V.

This lead me more to the hypothesis that the first reaction is indeed reaction of complexed NO/NO2 residues or N(III) species in solution (which are in limited amount due to their lower stability). This also correlates with my observations that dissolutions of ferrous scrap in AR are harder to de-NOx than dissolutions of classic copper alloys.

But I am just hypothesizing based on my knowledge about the subject.

 

[4metals]

Metallic values vs e-scrap

Another unique feature of refining in aqua regia is the simple fact that once the oxidizer is consumed, that often convenient but potentially costly process called cementation comes into play.

With metallic scrap like karat gold, bullion bars, and scrap made entirely out of metal, no ceramics or plastics, it is easier to determine how much acid is required for a complete dissolution. (Especially true with karat scrap). Because of this determining how much Nitric Acid to add to minimize your excess is easier.

With e-scrap, it can be costly to just dissolve everything and then sort out the insolubles (ceramics, plastic, teflon, and of course Silver Chloride). But often a refiner will add an insufficient quantity of oxidizer, enough to dissolve some of the gold, but not all of it. Then that pesky little process called cementation starts to work. Any undissolved copper will begin dropping values back into the pile of insolubles on the bottom. The solution still tests positive for gold in solution so they think they got all of it. If the insolubles don't show signs of gold plating, they think they're done.

Whenever I take a solution sample from a large aqua regia digestion, I always heat it to see if it has any red fuming left in it. This tells me there is still free nitric in the solution and the cementation has not started yet. If this is the case, and the residues do not show any visible gold, the dissolution of the values is complete and the solution can be filtered and processed for values.

In my refining career I was always willing to accept refining residues because many refiners make this mistake. Thinking the reaction is done so the next step is recovery from solution. Totally forgetting the potential for the cementation to begin and drop metals before it is expected. I made a lot of money processing refining residues for this very reason. Especially from a refiner that was willing to let the "completed" reaction to sit over night.

So, especially with e-scrap where the acid additions are "blind" because you do not know the metal content, a little free nitric is a good thing in the solution at the end of your digestion. Making a little urea or sulfamic required if the lot is small enough. If it is a large lot, I always added copper and cemented the values rather than use chemicals and large filtrations. In this case, as many have pointed out, the de-NOxing is done by consuming the copper with the acid excesses.

I guess the moral of the story is cementation can be your friend.....until it's not!

 

[orvi]

Definitely. If you're processing 50Kg of Ceramic processers in smaller parcels using the same liquor you're certainly not going to be adding Nitric by the drop. Unless of course you're wanting to spend a year doing them. The drop by drop process is absolutely fine for some circumstances and not for others.

You're not wrong Kurt. Each method has its pros and cons, and different methods are preferred by different people. Over use of Nitric has often been the root cause of many issues too. I remember the resistance to Sulphamic when first mentioned, because it wasn't used by Hoke, and it took time for people to accept something that wasn't in the established way of doing things.

Any process of destroying the Nitric is equally as good as the other provided it's one that genuinely works.

If a method kills two birds (or even three) with one stone then all the better!

I strongly agree with both posts.

As research chemist, I know first hand that you want as many methods and reactions to synthesize something as you can. Many times people won´t understand and laugh why I am doing this and that when we know how to do this easily by other method. Circumstances and other factors can lead to complete failure of established protocols, that is why we need as much knowledge as we can about the subject to figure out alternative way.
I tweaked sulfamic de-NOxing by pairing it also with copper cementation. This way it can be accomplished in very short time, consumption of sulfamic is reduced and you get raw gold precipitate very quickly, filterable, finite (if no PGM´s are in the juice = no stockpot needed as you already cemented the solution) and straightforward... This worked for me, and I know that it can sound controversial for many as it is not standard technique

Situation about 50 kg ceramics in AR is still in my mind from good ´ol days. And yeah, you will pour in as much nitric as you sanely can to speed up the work, as impatient customer wait for the pay And killing that excess by brute-forcing the de-NOx with 1L pitcher and sodium sulfite slurry from bucket... Yeah, there are some things which can look very controversial for many But if they work and you know how to operate them consistently without harming yourself, then why not

 

[orvi]

So, especially with e-scrap where the acid additions are "blind" because you do not know the metal content, a little free nitric is a good thing in the solution at the end of your digestion. Making a little urea or sulfamic required if the lot is small enough. If it is a large lot, I always added copper and cemented the values rather than use chemicals and large filtrations. In this case, as many have pointed out, the de-NOxing is done by consuming the copper with the acid excesses.

I guess the moral of the story is cementation can be your friend.....until it's not!

I can say myself that I used this technique to my advantage. But I did it in "opposite" manner

I added intentially more material, and in the end less oxidizer (nitric), to intentially left exposed undissolved parts of metal in solution (most of the times pins+similar e-scrap). When reaction died down, I heated and stirred the pot, which caused all of the nitric to properly react and all of the gold cement on the remains of metal. Cooled overnight, gold cemented and settled completely to the bottom of the vessel. Barren liquid was then siphoned directly into waste (if there were no PGM´s) and "concentrated" residue treated. This allowed one to perform big dissolutions in smaller vessels - you divided the material into 2-3 portions and enriched the solid residue each time with gold, leading to very nice and heavy gold drop in the end. Another added benefit is that you can sequentially process material from ugliest/dirtiest/lower yielding to best yielding in one linear sequence.

I totally agree, cementation is your friend... until it´s not

 

[4metals]

I can say myself that I used this technique to my advantage. But I did it in "opposite" manner

Back in the days of refining large lots of hybrid ceramics when the yields were quite high, I used both methods. When the tech overestimated the Nitric Acid I realized that all of the solution would need processing. When done properly the mass of metal to dissolve is greater than the quantity of acid to dissolve it and the gold (albeit mixed with a lot of crap) was on the bottom. The next, smaller and easy to handle lot, was where the actual refining took place. Much less to handle and much less chemistry. And the bulk of the acid from the first dissolve went straight to waste treatment.

The trick to learn is the power of cementation and when it works in your favor and when it doesn't.

And Orvi, don't take this personally but I don't want your residues!

 

[anachronism]

I like this thread. Orvi, yes to all you've said! I use too much Nitric on a decent batch run because you need it done and fast! If you're running hot the last thing you need is to run out of Nitric and then have to start adding it because the risk of a violent boilover is high. Ive also purposefully let a batch cement out simply to get rid of the excess copper filled liquid so I can add fresh HCl and Nitric to keep the solution more concentrated with gold.

As 4metals says, you're often better having an excess to prevent that pesky cementation, especially when you're letting a reaction cool overnight and coming back to it the next day. We're all agreeing here that used wisely cementation is a good tool, but a cruel master.

 

[Acidrain]

I like this thread. Orvi, yes to all you've said! I use too much Nitric on a decent batch run because you need it done and fast! If you're running hot the last thing you need is to run out of Nitric and then have to start adding it because the risk of a violent boilover is high. Ive also purposefully let a batch cement out simply to get rid of the excess copper filled liquid so I can add fresh HCl and Nitric to keep the solution more concentrated with gold.

As 4metals says, you're often better having an excess to prevent that pesky cementation, especially when you're letting a reaction cool overnight and coming back to it the next day. We're all agreeing here that used wisely cementation is a good tool, but a cruel master.

I like this thread too! Talk about a crash course is refining chemistry! I'm really learning a lot from everyone's comments

 

[orvi]

I like this thread. Orvi, yes to all you've said! I use too much Nitric on a decent batch run because you need it done and fast! If you're running hot the last thing you need is to run out of Nitric and then have to start adding it because the risk of a violent boilover is high. Ive also purposefully let a batch cement out simply to get rid of the excess copper filled liquid so I can add fresh HCl and Nitric to keep the solution more concentrated with gold.

As 4metals says, you're often better having an excess to prevent that pesky cementation, especially when you're letting a reaction cool overnight and coming back to it the next day. We're all agreeing here that used wisely cementation is a good tool, but a cruel master.

I think this is very important for novice refiners to know about - if they intend to do some AR dissolution. You are always taught to do incremental nitric additions - but here it can be more complicated And this is also a thing that can causer the AR to have "bad reputation" among hobby refiners. Violent foamover in 50L barrel is the last thing you want

AR is complex in chemistry and behaviour, and often it´s power is underestimated.

 

[Yggdrasil]

I think this is very important for novice refiners to know about - if they intend to do some AR dissolution. You are always taught to do incremental nitric additions - but here it can be more complicated And this is also a thing that can causer the AR to have "bad reputation" among hobby refiners. Violent foamover in 50L barrel is the last thing you want

AR is complex in chemistry and behaviour, and often it´s power is underestimated.

AR in a 50L barrel is hardly hobby size is it, but you are right.

 

[anachronism]

AR in a 50L barrel is hardly hobby size is it, but you are right.

True I tend to use 3 x 5l at the same time.
Edit: You end up moving towards incremental additions of product rather than reagents. 3 x 5l operated "hot" and correctly can quite easily treat between 1.5Kg and 2Kg per hour of ceramic processors.

 

[orvi]

True I tend to use 3 x 5l at the same time.
Edit: You end up moving towards incremental additions of product rather than reagents. 3 x 5l operated "hot" and correctly can quite easily treat between 1.5Kg and 2Kg per hour of ceramic processors.

Yeah, bad example.

I found out that key to the prevention of these violent eruptions of gasses from hot AR is to never let the reaction "die down". If it is constantly rolling and reacting, it won´t happen (at least it did not happen to me). But if you have 70+°C AR and you let it "die down" in the middle of the dissolution, you are in for a tedious and messy time to kickstart it again without shooting it out of the beaker

Top hat transistors are similar in this fashion, as the legs inside the glass bottoms need some time and fresh oxidizing acid to be eaten up inside the glass portion. If you neglect this, you will lose the gold due to inevitable cementation in this place.

My favourite "modus operandi" was bigger plastic barrel, where I added the "dirty" portion of material (cut connectors with PCB´s, junk pins, material containing pieces of plastic etc...), pre-heat like 3-4 L of HCl on a hotplate in 5L beaker (to around 60-70 °C), pour it in, followed by immediate nitric to kickstart the AR reaction and alternately dosing in HCL and nitric to keep the reaction "alive". Paired with nice insulation on the barrel, you can cleverly use heat of the reaction to sustain it until completeion - of course this won´t be possible without excess of nitric. As the reaction ended and material was thoroughly eaten up, I decanted hot "active" liquid into second barrel, directly onto the "clean portion" of material (pins, cerams, CPU´s...) paying attention that there would be enough exposed base metals for subsequent cementation. Second barrel was tumbled for a while, leftover nitric happily reacted with new material, nitric was properly depleted and everything nicely cemented on the new batch of material. Tumbler was stopped overnight, everything settled down, and barren waste liquid was siphoned. ... And this was processed in common fashion.

 

[Yggdrasil]

This thread has so far been quite informative.
I like it, so keep it coming.
It might end up as Library material

 

[Yggdrasil]

I have always been trying to avoid or circumvent the "Cementing ghost" so to speak.
But when you see how others use it as a tool, it is of course completely obvious that it can be used this way,
I just didn't see/realize it.
Thanks guys!

Edit to rearrange the text a bit.

 

[kurtak]

WOW - some really awesome reading this morning - or as the moto of this forum says - "Gold Refiners Helping One Another"

Don't really have time to reply to all that has been said this morning - but likely will when more time allows

I have always been trying to avoid or circumvent the "Cementing ghost" so to speak.
But when you see how others use it as a tool,
it is of course completely obvious that it can be used this way, I just didn't see/realize it.
Thanks guys!

I have been "trying" to teach the value of cementation for years now - I first learned it's true "value" from 4metals back in 2014 in this thread -------

https://goldrefiningforum.com/threads/fuzz-button-interconnects-need-some-advice.22203/#p232030

And I have been using it ever since

That does NOT mean it is the only method I use

I guess the moral of the story is cementation can be your friend.....until it's not!

I totally agree, cementation is your friend... until it´s not

I agree

And Orvi, don't take this personally but I don't want your residues!

You probably wouldn't want mine then ether

Kurt