Fume scrubbing system at Gold Refinery

[4metals]

This thread is being broken off from the consolidation of gold loss thread to make it easier to compartmentalize by topic.

My original question to Gold Refinery was;

"There are a few technical specifications we need to know about the fume scrubber to evaluate it properly. We need to know the air flow rate through the scrubber, probably expressed in cubic meters per hour. Then we need to know the cross sectional area of the packed section of the scrubber. The packed section is the area that contains the tower packing where the chemical reactions take place. To effectively scrub NOx fumes they have to remain in the "scrubbing zone" inside the scrubber for about 8 seconds. If we know the actual area of the packing and the flow rate we can see if the scrubber has the necessary retention time for effective scrubbing of NOx. Finally, is there a pH controller and dosing pump to maintain the pH of the scrubber liquid?"

His response was;

Yes,We have a dosing pump that regulates the pH of the water inside the scrubber. Of course, for the first pair of scrubbers, the second pair of scrubbers do not have pH control with the dosing pump.

In the tank of the first pair of scrubbers, ammonia is added to control the pH of 7. In the tank of the second pair of scrubbers, there is a caustic soda.

The white smoke I mentioned earlier comes out of the chimney (at the peak of the reaction), after the addition of ammonia. The smell of chlorine gives very little.

The electric motor has an inverter to regulate the air flow.

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[Yggdrasil]

I became a bit confused here, it did not really answer any of the questions 4metals asked.
It did how ever outline the overall design some what.

 

[4metals]

The airflow rate through the system is an important number to know as retention time in the scrubbing zone is critical. Also a photo of the back side of the scrubber so we can see the air path is helpful. Because the 220 cm height is not filled with packing material so the actual area is less. Scrubber internals include things like mist eliminators and redistribution plates. (the plumbing will tell us if there are redistribution plates)

In any event, more photo's and the blower airflow rating will help.

 

[4metals]

I question the chemistry they have you using for the scrubber. Specifically ammonia.

The fume coming off the aqua regia digestion produces NOx as well as Nitrosyl chloride. The addition of ammonia to the nitrosyl chloride will generate an amide plus ammonium chloride. Ammonium chloride is the white fume you see. Typically sodium hydroxide is used in a fume scrubber for aqua regia work. By maintaining the pH at 9.5 to 10 the nitrosyl chloride (the yellow gas component of a NOx cloud) will break down into water, salt, and sodium nitrate in the scrubber.

For most applications caustic scrubbing is all that is needed. For exceptional and heavy NOx there is an excellent product called appropriately BioNOxSolver which is added with an O.R.P. controller. If it came to it, the second sump could be dosed with BioNOxSolver and you would be in good shape. I used caustic and BioNOxSolver in a scrubber in midtown Manhattan NY, legally permitted, and passed all air tests with no complaints. You cannot operate any closer to your neighbor than in Manhattan!
BIONOxSOLVER™ NOx Scrubbing Solution - Bionomic Ind..
For starters I would be using the first pair of towers and the pH controller to dose liquid caustic and the second pair with liquid caustic which is pH controlled manually.

The electric motor has an inverter to regulate the air flow.

In the US we call these inverters variable frequency drives, they work by controlling the frequency of the motor to lower the speed of a blower. If they gave you the ability to lower the flow rate, chances are it produced the required Cubic meters per hour required. Still knowing the rating would be helpful.

 

[Gold.refinery]

The airflow rate through the system is an important number to know as retention time in the scrubbing zone is critical

Unfortunately I do not know the exact amount of air flow. The frequency driver has a tuner from 1 to 10, the manufacturer said always set between 4 to 5, it is suitable for the best gas washing. He said 300 m3/h to 800 m3/h is appropriate.

Because the 220 cm height is not filled with packing material so the actual area is less.

I guess up to a height of 70 cm the tower is filled with Rasching ring.

 

[Gold.refinery]

The fume coming off the aqua regia digestion produces NOx as well as Nitrosyl chloride. The addition of ammonia to the nitrosyl chloride will generate an amide plus ammonium chloride. Ammonium chloride is the white fume you see. Typically sodium hydroxide is used in a fume scrubber for aqua regia work. By maintaining the pH at 9.5 to 10 the nitrosyl chloride (the yellow gas component of a NOx cloud) will break down into water, salt, and sodium nitrate in the scrubber.

As I mentioned, the first pair of scrubbers has a dosing pump and the automatic pH control and the second pair of scrubbers do not have a dosing pump.
Since we do not have access to bioNoxSolver, would you recommend using just caustic soda in both scrubbers? Do you agree to use ammonia or not?

How much should the pH in both scrubbers be set to?
Doesn't the use of caustic soda reduce the life of the pumps?

If we maintain the liquid pH inside the scrubber tanks 9.5 to 10 with caustic soda, will the white fume be removed? What about with ammonia?

 

[Lou]

Why would anyone use ammonia in a NOx scrubber?? The white fume is ammonium chloride fume and that is extremely difficult to scrub. That is so fine that it requires a candle filter not a packed bed scrubber. Two different technologies.

Caustic will work fine. Typically 10.5-12.5 pH range is acceptable. Too much higher and the pH probes start shortening their lifespan. If additional reducing power is needed, sodium sulfite, sodium dithionite, and sodium thiosulfate can all be added to ensure better/faster reduction.

 

[Gold.refinery]

Why would anyone use ammonia in a NOx scrubber?? The white fume is ammonium chloride fume and that is extremely difficult to scrub. That is so fine that it requires a candle filter not a packed bed scrubber. Two different technologies.

Caustic will work fine. Typically 10.5-12.5 pH range is acceptable. Too much higher and the pH probes start shortening their lifespan. If additional reducing power is needed, sodium sulfite, sodium dithionite, and sodium thiosulfate can all be added to ensure better/faster reduction.

So ,you are against using ammonia.
I can set the pH 10.5 to 12.5 with by dosing pump in the first pair of scrubbers.
I do not have a doring pump for the second pair of scrubbers. Manual setting is enough?

If I use SMB, can caustic soda break the so2 gases in the scrubber?

Should sodium sulfite, sodium dithionite and sodium thiosulfate be added as a solution by dosing pump ? Are these all together or separately is necessary?

 

[silver1]

Wow! Love the pictures and your setup sir. That's a lot of equipment to scrub a lot of air.
When i first started to look for the best system for all my needs i chose a different route myself. Everything i do is seal reactors. I used flooded column bubble scrubbers. Packed with Bio-balls. First two stages are H2O2. Next 4 stages are NaOH, and the last two stages after that are H2O or can be filled with another solution according to demand. 8 column total. Because i use sealed reactors the amount of air needed for scrubbing is way lower and surface contact efficiency is improved in the scrubber without all that added air and space. Doing that i can reduce the surface footprint of floor area need and reduce the size of my equipment considerably. I also eliminate any pumps from having contact with the solution. It also eliminates a chemical resistant fan system. Not clogged heads filters or any thing. Very cheap and efficient for me both cost and maintenance wise. I monitor the ph in column5 and 6. Without probes. I just pull a sample from the bottom of the drain cock on the column. When the ph starts to change i will dump 1-4 and refill. I can do a change over in a matter of a few minutes. 5-6 are secondary backup for any sudden spikes and to serve as my control base for measurements.

All based on a design found in this PDF. Yawl should really like this PDF.

 

[4metals]

Unfortunately I do not know the exact amount of air flow. The frequency driver has a tuner from 1 to 10, the manufacturer said always set between 4 to 5, it is suitable for the best gas washing. He said 300 m3/h to 800 m3/h is appropriate.

I guess up to a height of 70 cm the tower is filled with Rasching ring.

At a diameter of 60 cm and a packed height of 70 cm each tower has an area of 0.1978 cubic meters and all 4 together have an area of 0.79 cubic meters. To give you a retention time in the packed columns of 8 seconds for effective NOx scrubbing, the effective flow rate through the scrubber should be about 5.9 cubic meters per minute. 300 cubic meters per hour is 5 cubic meters per second. The blower is capable of delivering the flow rate you need but it is also capable of providing a flow rate so fast the scrubber will be ineffective.

Ask your manufacturers rep if a setting between 4 & 5 will provide the 5.9 m3 per minute you need. Better yet ask him to bring in a velometer and show you how he can measure it in situ so he proves it to you. If he tells you he cannot do this for you, tell him his shoelace is untied, and when he bends over to tie it, kick him in the ass!

If you maintain the pH at 10 with a dosing pump using sodium hydroxide (Lou says 10.5-12.5, either will work) and manually maintain a pH of 10 in the second pair of stacks, you should be fine.

If additional reducing power is needed, sodium sulfite, sodium dithionite, and sodium thiosulfate can all be added to ensure better/faster reduction.

For now don't try to fix something until you are sure you need it, add the nitric slowly, preferably by dosing pump, and the scrubber will work fine IF THE FLOW RATE THROUGH THE SCRUBBER IS 5.9 M3/M/MIN.

 

[Gold.refinery]

Now we want to change the additive of the first pair of scrubbers from ammonia to caustic soda and set the pH of both tanks to 11.

We also definitely find airflow of 5.9 m3 / min.

Do you think this will be enough?
Will Nox gases break completely?
Will SO2 gases be absorbed?

(Some recommend using a more effective and different additive in the first pair of scrubbers than the second pair of scrubbers:
H2O2, sodium sulfide and derivatives)

 

[4metals]

Now we want to change the additive of the first pair of scrubbers from ammonia to caustic soda and set the pH of both tanks to 11.

Yes pH of 11 will work

We also definitely find airflow of 5.9 m3 / min.

This is based on the number you gave us of a 70cm depth of the tower packing

I guess up to a height of 70 cm the tower is filled with Rasching ring.

Actually 70 cm seems a bit low for a tower 220 cm tall but that is what I based the calculation on. If the packing is deeper the towers will support a higher flow rate and still maintain the retention time you need.

Do you think this will be enough?
Will Nox gases break completely?
Will SO2 gases be absorbed?

Caustic scrubbing with the proper retention time will eliminate the red fume.
You will need additional chemistry to completely break the NOx. Most refiners get by if they eliminate the red fume.
As we said earlier the Sulfur Dioxide is less of an issue so it will be fine.

Some recommend using a more effective and different additive in the first pair of scrubbers than the second pair of scrubbers:
H2O2, sodium sulfide and derivatives

It’s becoming apparent that you purchased this equipment with practically no knowledge of refining. Start with caustic as we have suggested and you will be much better off than you were with ammonia and caustic where you were refining your first lots. If you need additional scrubbing the forum will still be here to answer your questions.

 

[orvi]

Now we want to change the additive of the first pair of scrubbers from ammonia to caustic soda and set the pH of both tanks to 11.

We also definitely find airflow of 5.9 m3 / min.

Do you think this will be enough?
Will Nox gases break completely?
Will SO2 gases be absorbed?

(Some recommend using a more effective and different additive in the first pair of scrubbers than the second pair of scrubbers:
H2O2, sodium sulfide and derivatives)

Ammonia is not a good option here as Lou and others said. HN4Cl is usually fine, and it stains everything white. Any leak, untight joint etc in the scrubber or in the facility - you inevitably mix HCL and NH3 vapors and everything will be covered with white corrosive stuff. Ammonia itself is quite unpleasant on its own, so if you were to do it completely - you need to scrubb also the ammonia. Double work. Unnecessary.

Sodium hydroxide or carbonate do not evaporate. That is the advantage. They are cheap and available in any form. NaOH is much stronger base than ammonia - so it will outperform ammonia in SO2 scrubbing. Sodium carbonate with also outperform ammonia in SO2 scrubbing, because sodium sulfite or hydrosulfite are much more stable compounds than ammonium sulfite/hydrosulfite. Stronger base (or acid) usually form more stable salts. If you are competing with weak acid as SO2, you will better use strong base to tame it, than weak base like ammonia.

It was said here numerous times - SO2 is easier to scrubb than NOx.

 

[Gold.refinery]

I removed ammonia from the first pair of scrubbers.
I added a caustic soda tank for injection with a dosing pump and set the pH of the first pair of scrubbers to 10.5.
I also set the pH to 11 in the second scrubber pair with the caustic.
We will do a new operation tomorrow.

But I have concerns that when NOx gas combines in the first scrubber, I predict the dosing pump will not be able to maintain the pH at 10.5 at the proper speed and will lag behind.
I also predict that excessive injection of caustic in scrubbers will reduce their lifespan and cause crystal formation.

 

[4metals]

I have never seen a dosing pump pumping a concentrated caustic solution on a pH controller not be able to keep up with the fume being scrubbed from an aqua regia operation.

I also predict that excessive injection of caustic in scrubbers will reduce their lifespan and cause crystal formation.

Since you are using a pH controller, and you have calibrated your pH meter, you are not making excessive injections of caustic, but required injections of caustic in order to maintain the desired pH.

Any scrubbing method you choose will convert the fume to a soluble salt. And that soluble salt, in this case sodium nitrate, will become an insoluble salt when the solution is over saturated. That is, you will experience crystal formation and in extreme cases, blockage of air passages and plumbing without periodic maintenance of your scrubber water.

 

[EMRE]

At a diameter of 60 cm and a packed height of 70 cm each tower has an area of 0.1978 cubic meters and all 4 together have an area of 0.79 cubic meters. To give you a retention time in the packed columns of 8 seconds for effective NOx scrubbing, the effective flow rate through the scrubber should be about 5.9 cubic meters per minute. 300 cubic meters per hour is 5 cubic meters per second. The blower is capable of delivering the flow rate you need but it is also capable of providing a flow rate so fast the scrubber will be ineffective.

Ask your manufacturers rep if a setting between 4 & 5 will provide the 5.9 m3 per minute you need. Better yet ask him to bring in a velometer and show you how he can measure it in situ so he proves it to you. If he tells you he cannot do this for you, tell him his shoelace is untied, and when he bends over to tie it, kick him in the ass!

If you maintain the pH at 10 with a dosing pump using sodium hydroxide (Lou says 10.5-12.5, either will work) and manually maintain a pH of 10 in the second pair of stacks, you should be fine.

For now don't try to fix something until you are sure you need it, add the nitric slowly, preferably by dosing pump, and the scrubber will work fine IF THE FLOW RATE THROUGH THE SCRUBBER IS 5.9 M3/M/MIN.

i would love to see the full setup picture of his equipment. I think he has a big fume hood about 6 feet wide that why the manufacturer installed a big blower to suck all that air in. he needs to change his setup to close reactors that are attached to with much smaller size blower so needed retention times can be achieved,,

 

[4metals]

i would love to see the full setup picture of his equipment

I agree, a series of photo's to show the path or a video of the space would be helpful.