Scrubbing

SCRUBBING NOx FUMES

To be legal, you all must find some way to scrub the brown fumes from aqua regia and nitric digestions, at the very least. Besides being toxic, they are very visible. Scrubbers are used to neutralize the acid and solubilize the NOx. They can be wet or dry. The air coming out the end should be clean

There are several situations where scrubbers are needed, in terms of air dilution:

(1) Maximum air dilution. A fume hood or other opening, such as a flexible duct or an overhead hood. They require a very big scrubber. At one place I worked, we had a 8' diameter scrubber, 25 or 30 feet tall and could only run about 4, 2'x3' open fume hoods at a time, assuming 100 cfm/sq.ft. of open area. The blower at the top was about 2500 cfm.

(2) Medium to low air dilution. This is where you limit the air being mixed with the fumes. We had a glass lined, steam jacketed, cast iron kettle about 4' in diameter that we used for big AR jobs. On top was a hinged lid with a 4" duct coming off it. Also, we had 10, 4 liter pyrex beaker AR stations for karat gold. Each had a loose-fitting clear plastic cover with flexible tubing coming out of the top. Each tubing had a valve and they all were connected to a manifold, which was connected to the scrubber. This required a scrubber, but it was much smaller - about 5' x 15'. The blower was 250 cfm.

(3) No air dilution. Most all self-contained, do-it-in-your-livingroom, karat gold machines on the market use this approach. Everything is sealed up, as much as possible. The only thing going through the scrubber is pure fumes. I worked with one of these a long time ago. It was made by Shor. It had about a 4 gallon flask and could run about 100 oz at a time - no inquartation. The scrubber was interesting. The fumes went through 2 inline glass cylinders, each about 4" x 15". One was filled with 1" chunks of lime. The other with 1" chunks of charcoal. I don't remember which came first. The fumes went through the system very slowly.

A Standard NaOH Scrubber

A standard scrubber is a big vertical plastic or fiberglass cylinder. At the bottom is a reservoir of NaOH solution. The solution is circulated through a large amount of spray nozzles at the top, which are directed downward. Above the NaOH is a shelf with holes in it. On it is stacked a foot or more of pieces of plastic shaped to have a maximum surface area. In Hong Kong, we used reject plastic flowers from a neighboring plastic flower maker. The top is sealed except for the exhaust hole. The blower is mounted on top. A drum of 50/50 Caustic Soda (tech grade NaOH) is located on the ground next to the scrubber. As the NaOH is neutralized by the acid fumes, a pH meter senses it and meters in some liquid caustic soda.

The fumes enter into the NaOH where they are partially neutralized. The are slowed by the pieces of plastic, where they travel a tortuous path. The plastic also acts as a condenser. When they leave, they enter the spray. By the time they exit, they should be nothing but air.

If I remember right, the fumes have to be contained in the scrubber for a total of 8 seconds. The size of the blower has to be matched with the volume of the scrubber in order to keep the 8 second confinement. It's easy to calculate. Also, in the large scrubber mentioned above, The blower had a variable speed. We balanced it to the area of the openings that we were using.

Another type of scrubber I saw was actually 3 small scrubbers, about 2.5' x 8', in a chain. Each scrubber contained a different solution - H2O2, NaOH, and water. The man wouldn't tell me in what order. I figured it out once but can't remember which was 1, 2, and 3. I think the H2O2 oxidized the NOx to one form. It may have converted it all to HNO3. Maybe, Lou can work it out. It was supposed to work very well.

Very informative GSP, thank you! It is good to see more information on waste streams especially systems that can be adapted to small users. I try to be as environmentally responsible as I can, but even readers that are not concerned with that will find it is far cheaper to be clean than fined.

I look forward to hearing Lou’s commentary on it as well.

How small scale do you want? This marble scrubber works well for me.

http://goldrefiningforum.com/phpBB3/viewtopic.php?t=3269&start=0&postdays=0&postorder=asc&highlight=genie

Pawnbroker Bob diagrams his scrubber system at the end of this thread as well.

Thanks.

I will implement a scrubber in my shack lab.

The following patent is also very interesting.

GSP, this info will be very usefull, Thanks a Ton

Hey Chris, I think I figured the order of the scrubbers correctly.

First, your NOx go through NaOH. NO2 will dissolve in the aqueous solution and make nitric acid:

2 NO2 + H2O => 2 NO + HNO3

The nitric acid is then neutralized by the NaOH but the NO does not dissolve in the solution.

Then it goes through the H2O2 solution which oxidizes the NO into NO2.

Final scubbing is in water. The release of NO at this point should be very minimal.

If your fumes are Cl and NO of any flavor you can use a AgNO3 tower first to capture the Cl and then a Quartz tower with circulating HNO3 to capture all the NO and NO2. Then a NaHO tower to clean up the rest. In this fashion your waste can be controlled more and you can recycle your HNO3. I did one of these setups once and have some drawings of the quartz tower somewhere. The idea came from the actual production of HNO3. While running it the reactions are not open but sealed and vented directly to the scrubbers. The AgNO3 tower is swapped out periodically to remove the AgCl and to recharge with AgNO3. Usually made from the product of the Quratz tower. Without the Cl gas the AgNO3 tower is removed as it would be transparent to the NO2 gases.

Interesting method of removing the chloride, if you can cut down on carryover of spray from the scrubbing it could scrub out the Cl and allow the NOx to be converted to nitric. I assume the quartz tower was quartz glass? I don't think pumping nitric acid in this tower would capture the NOx and convert it to nitric. Injecting O2 and Distilled water will convert the NOx. Do you know flow rates for the fumes? Scrubbing tower details? (dimensions)

I used a column that was 150mm dia. and 600 mm tall. The fumes entered about 100 from the bottom. 125 mm from the bottom was a perforated disk to hold the quartz sand. 4.5 Kg quartz sand size about 5mm pieces. Also entering the same level as the fumes is a O2 inlet with a check valve. It was manual but I adjusted the O2 rate to about match the fumes. In the bottom was a drain that pulled from the 60 mm deep pool of HNO3 and it was pumped with a Teflon pump and sprayed over the top of the quartz. The flow rate was set to leave a 10mm layer standing on the shelf plate. I used this setup to run about at most a 1 CFM fume rate and never had an issue. The NaOH tower was 400mm tall 80mm diameter and didn't have to be changed even after 30 Kg of anode slime was processed. I also had pH probs in and monitored each tower. I had to pull HNO3 out every so often and replace with pure water because it would get pretty strong. It worked best I found running the acid strong and just adding about 10 ml of water at a time.

Metals

I've been thinking about the chloride question. When we discussed the nitric column before, using water and O2, I came away thinking I needed a separate column system for use with AR, to avoid chloride contamination of the clean nitric. Then I read a post by LaserSteve, suggesting the introduction of a little AgNO3 to the nitric, to drop AgCl, and thereby clean up the nitric of any chlorides.

Would the chlorides coming off of AR be too much to clean up in this manner? Would it be better to have a separate scrubber for AR?

And then what about the chloride fumes in general? If your scrubbing the nitric fumes with the aim of recovery, it seems like you would need a separate system to deal with HCl fumes. Is it possible to regenerate the HCl, or is it necessary to just neutralize?

J

Semi,

The fume coming off the aqua regia is nitrosyl chloride, it is higher in nitric than chloride. If the chloride is removed in silver nitrate, which is an excellent chloride scavenger, the scrubber we discussed earlier with the O2 could be second in line.

By using some creative ductwork you could digest in aqua regia and scrub in a scrubber with silver nitrate followed by an O2 nitric scrubber. The nitric scrubber could have 2 feeds one from the aqua regia side, one from a straight nitric side.

At the 1 cfm flow rate madderoftime mentioned there should be no carryover of silver nitrate from the first scrubber. I have never heard of using quartz sand as a tower packing before but this unit has a flow rate set up by the flow of the O2 input so there is pressure behind it.

If you have a reservoir at the bottom to collect the chlorides, you could use a cold trap to drop them and filter the acid to recirculate it. Pumps for this type of system won't be cheap, teflon pumps and 316 stainless pumps are pricey.

Very cool approach. Worth some consideration.

A quick look at Wiki turned up this:

"NOCl reacts with water to release HCl."

http://en.wikipedia.org/wiki/Nitrosyl_chloride

It must also release NO, but if there was no O2 available, (or not much), the NO could possibly exit the first column, to be mixed with O2 in the second column.

But what would push it? Perhaps you could introduce an inert gas such as N2 to push the flow through the first column, and then O2 at the second and third columns. (I already picked up eight flow meters to introduce gas at each column independently.)
Then you could recover hydrochloric at the first column, instead of having AgCl to deal with. (But the hydrochloric would no doubt be contaminated with nitric, unless the water in the first column was thoroughly depleted of O2.)

Or, as madderoftime and LaserSteve have suggested, scavenging the chlorides with AgNO3 may be the better approach.

J

Here is a couple of scan from some old books on the manufacture of HNO3 and a rough drawing of where I started for the system I built.

To stop spray over from one tower to the next cyclone seperators like what is used to take moisture from airlines for painting should also be employed. Only a chemically resistant one of course.

I also had the idea to ionize the O2 input to O3 but had not the time to try it.

Madderoftime,
I like your setup, although this is only the NO scrubbing portion detail.
In your scan of the apparatus, after removing the confusion of daisy-chained condenser tubing, I have a few questions. The reactor on the left I assume is for the digestion, why is the condenser not vertical, causing the condensate to return to the reaction and only allowing fumes into the scrubber. This would allow immediate nitric recycling for the condensed portion. The round impeller shaped circles, are they pumps? If so the one on the left is pumping gas, what type of pump was used? The condensed vapor entry into the scrubber is confusing, does it employ a check valve to keep the O2 bubbling into the column? This setup appears to have been designed from off the shelf QVF glass pipe and fittings, considering you said they’re 6” in diameter. Who was your glass supplier?

4metals said:

Madderoftime,
I like your setup, although this is only the NO scrubbing portion detail.
In your scan of the apparatus, after removing the confusion of daisy-chained condenser tubing, I have a few questions. The reactor on the left I assume is for the digestion, why is the condenser not vertical, causing the condensate to return to the reaction and only allowing fumes into the scrubber. This would allow immediate nitric recycling for the condensed portion. The round impeller shaped circles, are they pumps? If so the one on the left is pumping gas, what type of pump was used? The condensed vapor entry into the scrubber is confusing, does it employ a check valve to keep the O2 bubbling into the column? This setup appears to have been designed from off the shelf QVF glass pipe and fittings, considering you said they’re 6” in diameter. Who was your glass supplier?

Click to expand...

I had all the same questions.

J

Like I said that was a starting point. Took me a while and a bunch of testing to get it right. Some of the items were omitted entirely. Like the bottom valve was replaced by a teflon distribution block. The fume input was actually teflon tubing. The pumps were teflon diaphragm pumps from Cole-Parmer. Glass was all Ace fittings (most all custom made) and reactor tops were all solid teflon. O2 line entered directy into the column and yes it had a check valve. When it was complete it looked and worked awesome. Wish I would have gotten a picture of it. Should dust it off and do that at least for the picture!