Reusing NO2 from nitric acid

[4metals]

It's better to be a pain asking questions than to be in pain from a reaction thats gone bad! I'll answer anything I can.

Glass is good for the reaction except for the fact that it can break, glass lined steel reactors are the best but incredibly expensive. I've heard of refineries using titanium vessels but I have no experience with them, I know that the titanium has to be passivated first but that's all I've heard of it. For the fumes a vinyl ester fiberglass holds up well, I've installed them over 20 years ago and they're still holding up to aqua regia which is caustic scrubbed. The fumes coming off an aqua regia reaction have some chlorides in them and will condense as a dilute aqua regia, more nitric than hydrochloric. When I say they will condense it is when they are passed through a condenser to cool them and collect the condensate. Usually when doing stone removal I like to condense the fumes to make the nitric perform double duty. If working in a condensed reactor I like to use the aqua regia made up 5:1 (so 1 part nitric) because the recycling of the condensate saves on acid. So to sum it up, nitric dissolves do not produce the same fumes, they may look the same because of the dominant player is the nitric, but aqua regia passes off chlorides which will condense if cooled.

The cooling in the packed tower is just so the tower stays cooled to help in the condensing of the fumes, it needs to be cooler than the reaction producing the fumes, the cooler it is the more efficient it condenses the fume.

Plastic flowers, that's right up there with plastic hair curlers! The thing you want to avoid with packing is voids or depressions where either unreacted fumes or falling solutions will collect, we want a film on the packing caused by surface tension of the liquids, no puddles. So if flowers work, go for it!

Anhydrous nitric acid is pure acid and the best you'll get is 50% by condensing it from fumes. Don't get too carried away with cooling, cold running tap water in a teflon coated coil will work fine.

 

[Lou]

Anhydrous nitric acid has no purpose in refining and is a serious hazard--it is stored cold because it is constantly decomposing. It is a radically different animal than the regular 16M 67-70% that everyone here is familiar with and will visibly attack and destroy plastics which are otherwise inert at lower concentrations.

As for column packing, one can find small marbles. Those are better than using saddles or raschig rings because there is less risk of holdup and accidental overpressure.


Glass reaction vessels are just fine if one is careful. I use jacketed glass reactors that have a 5" flange at the top and use overhead stirring and recirculating heaters. 5L, 12L and 50L but that would depend upon how much material.

Glass lined reaction kettles are indeed expensive, and will rust all to hell, even if they're epoxy painted. Pfaulder makes these kettles. Every now and then you'll see one up on ebay for 500 or so. Which is a deal, because a 50 gal glass-lined reactor costs about 20 times that new (or more!).

 

[goldsilverpro]

Glass lined reaction kettles are indeed expensive, and will rust all to hell, even if they're epoxy painted. Pfaulder makes these kettles. Every now and then you'll see one up on ebay for 500 or so. Which is a deal, because a 50 gal glass-lined reactor costs about 20 times that new (or more!).

Several places I worked had glass lined Pfaudler kettles. They were all jacketed and steam was used to heat them. Instead of a reactor top, they were all covered with a round sheet of fiberglass with a fiberglass fume vent coming up out of it. The lid was hinged for easy access and cleaning. As far as I'm concerned, they are the ultimate dissolving machine. Far better than those huge awkward round bottom glass flasks sitting on a heating mantle.

 

[4metals]

No doubt that glass lined phaudler vessels are top of the line, but a 72 liter glass reactor with a mantle on a tipping base is cheaper and tough to beat. You never have to pick up the vessel, just invert it to empty and hose it clean. Condensers and mixers can all be added to the 4" conical head with a simple clamping device.

 

[Lou]

Much like my setup, but I do not using a heating mantle...mine is jacketed.

Glas-col heating mantles are still around 2k for that size. Do you use overhead stirring?

 

[4metals]

Funny you should ask about stirring, I use these reactors for stone removal, once years ago I did have an expensive gasketed stir motor so the reaction stayed sealed and boy did it look scientific! Running the stirrer caused the diamonds to swirl around and score the glass which needless to say resulted in the acid draining out of the wrong end of the reactor. After replacing the mantle (you're right they are expensive) and the 72 liter flask (again pricey) the stirrer earned a corner of distinction from which it was never removed. Glass does have its drawbacks, although my glassblower doesn't see it that way!

 

[Harold_V]

The gold can be re-boiled in nitric and rinsed and dried to yield a product that may serve your purposes.

I'm very outspoken about not using nitric for the wash process. From experience, I know that the chlorides are not easily eliminated from precipitated gold, so the average guy is likely to encounter some re-dissolving of the recovered gold. My process recommends HCl for the wash, which appears to do an adequate job without any of the risks or complexities of using nitric. It in no way implies that nitric is not suited, just that it presents problems that may or may not be easily overcome by those with limited equipment.

Harold

 

[oldtimmer]

There is one for sale on ebay: 370153970905..

 

[goldsilverpro]

The one on Ebay doesn't look like a kettle. I think it's columns. It's not what I was thinking of.

 

[4metals]

The e-bay posting is only parts of a complete column, I think that most would be surprised at the amount of money you would have to spend to complete the column; clamps, caps, and elbows cost big bucks when purchased new and most times, when you buy parts at a "great" price, the only way to get the missing pieces is to buy new. In the long run you'll spend more than you intended to. It is also critical when buying used glass lined equipment to closely inspect the glass to assure there are no small cracks which will allow the acids we use to attack the steel. Valves are the hardest things to inspect when buying used.

 

[4metals]

The gold can be re-boiled in nitric and rinsed and dried to yield a product that may serve your purposes.

[I'm very outspoken about not using nitric for the wash process. From experience, I know that the chlorides are not easily eliminated from precipitated gold, so the average guy is likely to encounter some re-dissolving of the recovered gold. My process recommends HCl for the wash, which appears to do an adequate job without any of the risks or complexities of using nitric. It in no way implies that nitric is not suited, just that it presents problems that may or may not be easily overcome by those with limited equipment.

Harold
For aqua regia refining I completely agree with your advice against using nitric acid to remove silver chlorides. The process I was talking about does not use aqua regia, only nitric as a parting acid. The resultant gold sponge, which has never dissolved, is slightly contaminated with silver but it is metallic silver, not the chloride. For this reason nitric will clean it up nicely. What this process does is essentially the same as the parting acids do in a huge fire assay. This process will not produce .9995 gold but may approach .999. We have produced gold for sale directly to Johnson Matthey Canada this way because their acceptance criterion was only .995.

 

[Harold_V]

Harold
For aqua regia refining I completely agree with your advice against using nitric acid to remove silver chlorides. The process I was talking about does not use aqua regia, only nitric as a parting acid.

Thanks for the clarification, 4metals. I failed to consider that you were referencing nitric processed material, so it would make perfect sense in that situation, and I'd certainly endorse the process.

The HCl wash I referenced isn't intended to remove silver chloride, as you likely may understand. It's intended to help remove drag down of other contaminants, and appears to be very effective. The contaminants that are removed are generally very visible in the way of discolored solutions.

Because silver chloride isn't removed by this process, I used a wash in ammonium hydroxide to remove any traces of silver chloride. That seemed to be effective, for in the end, my quality was very acceptable.

Harold

 

[semi-lucid]

Metals

I've spent quite a bit of time studying column's, and working on a CAD model.
I'm wondering if the 14 feet number is the overall height, water tank + column. Or if it is the column alone. I'm trying to calculate the volume of the packed area, so I can look at it on a percentage basis. Do you think a volume of 33% to 50% would be overkill?

Better to uber-kill it, than to under-kill it.

John

 

[4metals]

Semi,

The tower I referenced was 14' tall by itself. Now despite how cool it sounds to do all of the chemical and mathematical calculations what it comes down to is this;

How high is your ceiling?
What size column can you get at the right price?

Refineries and tall ceilings are nice, as a rule of thumb with tower packing the fume should be redistributed every time it moves up by twice the diameter of the column. To redistribute the fume you need a short unpacked section of column and more packing support. Packing support is the grid the packing sits on to keep it from falling to the bottom. So in a 18" diameter column I would redistribute the fume every 36 inches. I would leave a 12" void before the next set of packing supports. In the void I like to add spray nozzles so whatever chemistry you are recirculating can enter the column. In our 14 foot column there are 3 sections of packing plus the necessary voids where the solutions are sprayed and the fume redistributes. If you are building a scrubber to scrub and neutralize NOx the air will be pulled through the scrubber with a blower, because of this a tightly woven packing, as compared to the relative density of your primary packing, needs to be above the top spray nozzles to act as a mist eliminator. This keeps large drops of your chemistry from spraying out of the column.

So to summarize, overkill never hurts when scrubbing, if you design to produce 1000 ounces a day, it never fails you'll need to process 1500! So I would use the fattest, tallest scrubber I could find at a good price, and still fit in the doors and under the ceiling.

 

[semi-lucid]

Yes. You don't study column's very long before you start looking at the ceiling. :?

J

 

[dick b]

Its quite easy to break a 15' High column into three short 5' columns with the discharge of the first column being piped down to the bottom of the second column with the water sprays in the falling section of pipe, etc.
That way you can achieve any length of column necessary to scrub the fumes plus you can change the chemistry in each set of sprays to scrub for different fumes and or neutralize them.

 

[4metals]

Absolutely true, all you need is a bigger exhaust blower to make up for the resistance.

Good point!

 

[semi-lucid]

I was thinking about that also. You could have a separate water reservoir for each column, and route the fumes to the column with the strongest acid first. Then when you changed out the acid and installed fresh water, change the order of the series. Why keep it simple when you can make it complicated. :roll:

John

 

[4metals]

I'm a huge believer in KISS technology.

KEEP IT SIMPLE STUPID!

 

[semi-lucid]

When I first started thinking about this, I planned to bubble the fumes through a series of reservoirs, and as the concentration got high enough in the first reservoir, I would remove it, and move the second reservoir to the first position, and the third reservoir to the second position. Then add a new reservoir with fresh water to the third position.

That line of thinking naturally led me to thinking about a series of column's, because I wonder if the fresh water would be more effective at reacting with the fumes then a 50% acid concentration. With a series, you would always have a low acid concentration at the end of the scrub.

Not sure if it's worth the complication though. I have heard a quote attributed to Albert Einstein; Keep it as simple as possible, but no simpler.

John