scotthensler5810
poorman5810
I’m building a fume hood for gold/silver refining. I would like to know if I’m using Nitric concentrate 67% to 70% or Sulfuric Acids, what cfm air flow would be needed for this?
Making my own Fume Hood!
- Thread starter scotthensler5810
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Martijn
Well-known member
Deoends on how much you want to process in one go, how fast, and if you scrub the fumes or not.
If you expect the worse case scenario: runaway reaction of HNO3, broken beaker. Fumes need to be taken away from you AND diluted enough to be expelled to the air in a safe concentration. So a tall exhaust would diffuse the red cloud more before it can hurt someone. A big air scrubber would help in that case.
Catch basin to catch any overflow in stead of pouring over your lab floor and reacting with whatever may be laying around.
A good airflow and circutaion in the fumehood should keep the fumes away from you. I tested mine with smoking incense sticks to see the flow good enough.
Prevention is key here. Keep reactions slow, small amounts, add reagents in small amounts, keep a loose lid on a beaker to condense and reuse the fumes. Scrub the fumes through a scrubber before releasing them in the fume hood.
If you expect the worse case scenario: runaway reaction of HNO3, broken beaker. Fumes need to be taken away from you AND diluted enough to be expelled to the air in a safe concentration. So a tall exhaust would diffuse the red cloud more before it can hurt someone. A big air scrubber would help in that case.
Catch basin to catch any overflow in stead of pouring over your lab floor and reacting with whatever may be laying around.
A good airflow and circutaion in the fumehood should keep the fumes away from you. I tested mine with smoking incense sticks to see the flow good enough.
Prevention is key here. Keep reactions slow, small amounts, add reagents in small amounts, keep a loose lid on a beaker to condense and reuse the fumes. Scrub the fumes through a scrubber before releasing them in the fume hood.
scotthensler5810
poorman5810
Thank you for the information!
Generally lab hoods are designed to evacuate 100 CFM for every square foot of hood opening. This gives decent airflow to remove the dangerous fumes from the work area. Fume scrubbing is a different matter. Build a hood with an appropriately sized blower and add a scrubber to remove emissions in need of scrubbing, like NOx, or the Sulfur Dioxide from precipitating the gold. That’s where sizing the scrubber comes in.
Making effective enough fumehood isn´t particularly difficult task. You choose good fan to be built-in and practically the biggest issue of just pumping the air through is solved.
On the other hand, as 4metals said, scrubbing is much more difficult to design. You need to adjust the dimensions of the scrubbing column, compute how many nozzles for spraying the liquid you will need, and most importantly adjust the size for retention time needed to obtain scrubbing efficiency needed.
With just SO2 or HCl/Cl2 gasses, scrubbing is relatively quick, but NOx gasses are harder to scrubb efficiently, and need longer retention times in the column.
On the other hand, as 4metals said, scrubbing is much more difficult to design. You need to adjust the dimensions of the scrubbing column, compute how many nozzles for spraying the liquid you will need, and most importantly adjust the size for retention time needed to obtain scrubbing efficiency needed.
With just SO2 or HCl/Cl2 gasses, scrubbing is relatively quick, but NOx gasses are harder to scrubb efficiently, and need longer retention times in the column.
Here in the US, hood exhaust is called fugitive emission because the only bad fumes the blower sees is what escapes the scrubber or is emitted when you pour acids or do general in hood tasks. Don't make the mistake of going too small, make a hood with an opening at least 4 feet wide and 3 feet tall. You will not regret the space that hood will afford you. For a hood with a 4 foot by 3 foot opening, you should exhaust 1200 CFM. A little trick which will allow a blower half this size is to make a "face shield" out of plexiglass that folds up and out of the way when you need full access. The face shield is the full width of the hood and when down it usually hangs about 18" down from the top. It allows plenty of room for tasks to be accomplished with your hands under the shield working in the hood with your face behind the plexiglass. In addition to protecting you from splash, it effectively halves the exhaust requirement because with the shield down, the opening is now 4 feet by 1.5 feet or only requiring 600 CFM.
There are 2 schools of thought about blowers. One is use a cheap plain steel blower that you spray a resistant spray paint through to crudely coat the inside and when it craps out you replace it. The other school says get a corrosion resistant blower, much more costly. A third option is a blower made for high humidity which is usually plastic. They are better than coated steel but not as good as a true corrosion resistant fan.
But keep in mind you will need to address the corrosive fumes because, unless you. have a corrosion resistant blower, they will eat the blower. That is where a scrubber comes into play.
There are 2 schools of thought about blowers. One is use a cheap plain steel blower that you spray a resistant spray paint through to crudely coat the inside and when it craps out you replace it. The other school says get a corrosion resistant blower, much more costly. A third option is a blower made for high humidity which is usually plastic. They are better than coated steel but not as good as a true corrosion resistant fan.
But keep in mind you will need to address the corrosive fumes because, unless you. have a corrosion resistant blower, they will eat the blower. That is where a scrubber comes into play.
The thing that one really has to consider on hoods is the cost of the makeup air. Might not seem like much, but when your winter is sub freezing, you also need to heat that air. My average monthly gas/electric bill for the house is $500. Without the fume hood running it's closer to $250. This winter it's all getting shut off and I'm moving any refining to an outbuilding.
The cost of refining in beakers as opposed to closed vessels is outrageous.
The cost of refining in beakers as opposed to closed vessels is outrageous.
Make up air always helps but refineries are always hot in summer and cold in the winter. Making a hood like this helps.
making a hood with a plain steel blower
Air intake from outside the refining space exhausts less CFM from the workspace.
making a hood with a plain steel blower
Air intake from outside the refining space exhausts less CFM from the workspace.
Yeah, I figured this out after the fact. Biggest reason I'm moving it is I need the space so I can set up an assay lab and jewelry studio. Assay hood will be tiny and blower will be set up with a VFD so I can seal it up tight and just keep a negative pressure on it.
When I started out I didn't realize how much I needed to invest in the assay side....no matter how many times I read it.
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Just curious to see if an effective fume scrubber could be built using course Limestone contained within a plastic pipe. Putting the blower on the vent (out) side to induce suction through the Limestone column. This would leave the blower at a greatly reduced exposure to corrosive elements. Engineering would dictate size needed for volume desired obviously. Just thinking out loud.
KalleMP
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I once saw an axial flow fan intended for aggressive extraction use at a second hand goods store, almost brought it for later use. It had heavy epoxy coating on all the exposed metal and a plastic impeller. I expect such professional fans are available for a price.
While hunting around for something cheaper I saw that there are various extractor fans that are sold as non corroding that are mostly made of plastic and even if they were not powerful they were cheap enough from Ali-express that one could mount multiples to gain the flow required.
Still planning what I have been wondering is how noisy or energy consuming would a venturi vacuum generator be for a fume hood. Everything that contacts the fume stream could be of a suitable plastic or composite. Picture a Dyson fan or hair-dryer arrangement powered by a suitable compressor remotely located in a sound proof enclosure.
I would prefer to not use a scrubber as it is a maintenance point, a water mist curtain might be an option but even there frost could cause issues.
While this is always true, the more remote your location is the necessity of fume scrubbing decreases. The US EPA will grant a “de minimus” status to a small generator using less than 10 pounds of Nitric Acid a day. For most here that is a lot of nitric. To be granted this status you have to be a commercial operation and apply for the exemption. There is a caveat in the rule. Even if you are under the 10 pounds per day limit, if a neighbor complains you will need to scrub the fume.
I live in a very rural area, can’t even see my neighbors, yet I refined as a sideline hobby kind of thing, (actually it was my own investment towards retirement) but I used a home made scrubber and it was effective. I felt I had to do it because if I can advise people here to do it, I should do it too. Note; I only processed about 50 oz of karat scrap at a time and refined roughly once a week.
Skeeter8154
Active member
What kind of sealant is use to seal the seams in a fume hood
That depends on what material the hood is made from.
PVC is welded
Fiberglass uses fiberglass resin
Plywood uses a good epoxy sealer and silicone. I wonder how flex seal would hold up to the acid fumes.
https://flexsealproducts.com
Any members have first hand experience with flex seal?
Skeeter8154
Active member
I’m using frp panels on luan so a chemical resistant silicone will work
