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.
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.
