Scrubbing of NOx fumes

Letter 30044

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Dear Sir,

I'm working for a company refining 200 kg pure gold from 350 kg scrap per day. I have a problem with NOx abatement.Our scrubbing system is not sufficient. When I open the lid of the process bottle(has volume about 350 lt being heated and involving aqua regia and scrap) no NOx evolving occurs. Does it make any problem with dissolving gold or dissolving time. If yes how can NOx can be removed. I will appreciate any suggestion.

Thank u.

Ahmed Has
chemical engineer - Istanbul, Turkey

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Hi Ahmed ! Oh dear ! You sure do have a problem, eh.

I've known a very competent chemical engineer with extensive mass transfer knowledge fail when dealing with nitric emissions (faucet Company) ... and yet, on very minor but mixed acid emissions, a brown so-called NOX cloud emerged which, to my utter surprise, was knocked out by using a simple 12 micron blade type mist eliminator.

Your application is the worst. Pure NOX, I believe.

One method is to SLOW the airflow down CONSIDERABLY to achieve a languid retention time. Then to add to it minor amounts of air (ie. oxygen). In theory pure oxygen but that is not practical. Then and then only allow the fumes to enter the scrubber. As a rider I'd suggest you at least install a good eliminator after that scrubber ... but the pretreatment of a very slow air movement with the addition of small amounts of 'fresh' air is a prerequisite.

I believe that an Italian Company (A.J.M. Vieri ???) has this knowledge. From the hot glass retorts the low cfm airflow went into a 3" diameter header and THEN into a 12" dia x 10 feet long (or longer, I forget)vertical tube which had mini valves allowing fresh air to be sucked in ... THEN this 12" dia tube was joined to 8, yes, EIGHT other identical tubes to achieve a long but SLOW air movement in order for some of the NOX compounds to have time enough to reconvert from NO to N02 ... which is scrubbable.

I hope that this will help you! Let me know if it does.

Freeman Newton
- White Rock, B.C. Canada ... 40 odd years in plastics for chem. plants but also a scrubber designer

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Dear Ahmed,

You need to take a good look at your whole system, some modifications in the digestion of the gold scrap may help limit the amount NOX emissions as follows;

*Add the measured amount of HCL to your vessel with the scrap gold and let it heat up.
*Meter in the measured amount of HNO3 slowly over a 4 hour period as this will help control the reaction rate and minimize the peak emissions, avoid using excess HNO3.
*Use a condenser on the digestion vessel exhaust to capture acid vapors and return them to the vessel. You will often find metal values in the returned liquid.
*Use a minimum amount of speed on your scrubbing system to create the longest possible residence time in your fume scrubber. If you are venting your whole refinery with one high speed scrubber, consider getting a separate scrubber for the concentrated fume and run it at the slowest speed possible.
*Better control of reaction times will help reduce NOX emissions.

You may want to check the sponsors of this website for advice and help with your problem. It is often wise to hire a consultant rather than trying to reinvent the wheel,it can be cheaper in the long run.

Good luck & Best regards,

Jim Sivertsen
Refining & Alloys - Alden, New York

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Dear Mr Ahmed,

ya. you have a problem of nox emission. the brownish fumes can be controlled by the following method.

u calculate the nox generation during your process.
u connect the duct to a scrubber, which diameter should have less than 2 mtr/sec velocity of your air flow.
try to add atmospheric air thru openings.
add filter media not less than 3/4th of scrubber height.
and finally use mist eliminator.
this will definitely solve ur problem.

Ramesh Miyyapur
- Hyderabad, Andhra Pradesh, lndia