how to build an acid resistant hood & scrubber

Building an acid resistant hood.

A good sturdy acid resistant hood is the backbone of any refining and or assaying lab. They do get expensive if you buy them from a scientific supply house. To build one yourself you need to use 3/4" plywood and build the equivalent of a doghouse with a 4 foot length and 30" depth. It should look the attachment when done.

Next google spray on bed liners on your computer. I've used Line X brand and Rhino and both have worked well. Find a local guy doing the work and take the hood to have the inside of the hood sprayed. Make sure he gets the corners and the bottom extra heavy. If you're flush with cash he can do the outside as well, for me I used good epoxy paint on the outside. If you live in an area where alot of fiberglass work is done, that is also an option, both work well, it's all about price.

If you make the bottom like a "boat" with a lip in front it will also serve as a spill containment because something always spills, especially when you don't have containment.

Now all you need is legs or a table to put this on, keep in mind you will be processing in 5 gallon buckets so plan your height accordingly.

If you add a "U" channel vertically on both sides of the opening, you can add a sheet of 1/4" plexyglass as a sliding door. Make the channels long enough to contain the plastic sheet from the bottom in the closed position to the top in the open position. Place a stop so the door always stays 3 or 4 inches up from the bottom, and drill a pin so it can be pinned in the open position.

Beautiful!....

You can also have a coating, in a variety of plastics, "chopped" on. The last time I had a hood coated, I found a local guy with the equipment that made tornado shelters. He "chopped" on a coating of PVC fairly inexpensively. I also used the same PVC coating on wooden silver cells with great success.

I don't know exactly how these "choppers" work. I think it is molten plastic, chopped into small increments by a "chopper" gun and sprayed on. No solvents are involved. Maybe, bed liners are applied the same way.

Once, I had a 12' long hood made from bare plywood and I used it for about 5 years with no noticeable deterioration. I used two fair sized belt-driven squirrel cage blowers, in tandem, run off the same motor. The motor sat on top of the hood. I never, never incinerated in that hood and never had any problems.

Because of the incineration factor I have been gathering slate from old pool tables when I find it free. I have no difficulty machining it to size however for containment I would be interested in suggestions as to sealants for the bottom meeting the sides that will hold up over time.

4Metal, thank you so much. My main fear about refining is about toxic fumes. Now with this hood design of yours I would reconsider about quitting because the money from gold recovery is simply irresistable.
Would it be necessary to attach an exhaust fan to the duct in order to suck out the fume easily? Will these fumes be harmful to your neighbor and the environment I mean when it gets outside over the roof? I hope it just neutralise in the air otherwise it (the hood) is still not the solution that I want. Thank you again in advance.

Would it be necessary to attach an exhaust fan to the duct in order to suck out the fume easily?

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Yes. This particular hood has an opening of 3' x 4' = 12 sq.ft. When it's wide open, for excellent exhaust you'll need an exhaust fan of 100 cfm/sq.ft., or a total of 1200 cfm. You must also consider what the exhaust fan is made from if your design pulls the fumes through the fan. Steel will rust out quickly.

Will these fumes be harmful to your neighbor and the environment I mean when it gets outside over the roof?

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Yes. The fumes are heavy and, under calm weather conditions, they can tend to hug the ground. They will corrode metals - especially cars and metal buildings. Also, any usage of nitric acid will produce very visible red-brown fumes which can readily be seen by the authorities.

I hope it just neutralise in the air otherwise it (the hood) is still not the solution that I want.

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The fumes won't necessarily be neutralized but they will tend to disperse into the air. How fast depends on the weather.

A fume scrubber to handle 1200 cfm would be quite large, due to the great amount of air dilution of the fumes. If you want a wide open hood, you should consider building a multi-layer fume scrubber like the one described by 4metals in another thread. A sealed dissolving system, with little or no air dilution, would require a much smaller scrubbing system.

The hood described in the earlier post is only the hood to contain the fumes, it is part of a system which removes the fumes from the hood and passes them through a scrubber. The scrubber is a device made of a large diameter PVC pipe which is filled with tower packing.

Tower packing is basically obstructions to the air path which provide surface area for the chemicals we are pumping down through the scrubber to react with the fumes we are sucking up through the scrubber. They can get expensive if you purchase packing from a manufacturer so there are options to improvise. Wiffle balls work great, but they probably cost too much as well. Ladies hair curlers work too. Marbles work but that’s what chemists use in smaller columns, I’d venture a guess that a 12” diameter by 6 foot deep bed of marbles would be too heavy to be practical. One option is to cut slices of plastic soda bottles into rings about half to three quarters of an inch thick. Don’t use the ends because we do not want any pockets that will trap air or liquids. Anything you can think of to provide a corrosion resistant surface, that doesn’t trap liquids or gasses, that isn’t too heavy, that will allow the air to flow through the spaces will work.

We will be able to use a considerably smaller scrubber than required for a 1200 cfm opening of the hood I described earlier because scrubbers work when there is some contact time available. So if you’re trying to scrub 1200 cubic feet of air and you want it to stay in contact with the packing for a few seconds, you need a large scrubber. We will only be scrubbing the fume generated from an enclosed reaction so we can do a good job with a smaller sized scrubber.

The way we get to split the fume and do double duty with one corrosion resistant blower is by building a PVC manifold to exhaust the fume to the scrubber. Using a 3” PVC pipe, and 3 tee’s and one 90 degree elbow make a manifold for 4 equidistant inlets. Bush down the openings to inch and a half. Our reaction vessel is nothing more than a spackle bucket, our contained reaction vessel is a spackle bucket with a lid. In the lid we drill a 2” hole off center and we connect a section of flexible tubing from the hole in the lid to one of the holes in the manifold. The piping of the manifold is directed to the bottom of the scrubber, below the packing support and above the liquid level.

The blower we need is an 8” intake blower, usually either PVC or FRP (fiberglass reinforced plastic) Now from the blower we go into a reducing wye, it has to either be or be bushed down to 8” in the top, 6” out the bottom, and 4” out the wye. The throat of the wye has to be a full 8” so an 8” wye and a 8 by 6 reducer on one leg and a 8 by 4 reducer on the other leg will work. The 6 inch drop goes into the top of the hood and the 4” drop runs to the top of the scrubber. In the 6” line, just above the hood where you can reach it, add a blast gate damper. This will allow fine tuning between the 6” exhaust and the scrubber.

The reason I make a 4 port manifold is that this scrubber will handle 1 if not 2 ports working together. Cap off the unused ports. The damper on the 6” line is for adjusting the exhaust so the suction going into one of the manifold ports is just enough to suck the fume so it doesn’t overflow the container, and not hard enough to suck out all of the fume. The goal is to allow the fume to be in the scrubber for as long as possible and this is achieved by sucking in as little red fume as necessary.

The liquid in the scrubber should flow at a rare of 3 to 5 gpm from the pump up to the spray nozzles. We will use liquid caustic to keep the pH over 10. A corrosion resistant pump is needed here.

Some thoughts about spray nozzles.

The water and caustic that performs the scrubbing of the fume is delivered through a pipe to the top of the scrubber where it sprays on the packing, wetting the surfaces so they can interact with the NOx. To assure that the most surface is wetted as possible a manifold should be made forming an “X” over the top of the packing. Into this manifold you can either insert plastic spray nozzles to disperse the liquid or my personal favorite is to drill 1/8th inch holes.

Whether you use nozzles or holes I can assure you they will clog eventually, either from caustic buildup or from generic floating crap clogging the holes. Because of this the scrubber has to be built in such a way as to facilitate periodic cleaning of the nozzles.

There are many way to skin a cat but the two I have used work like this.

Option 1
Do not glue at least 1 coupling leading to the blower in the 4” PVC duct. Also do not glue the cap which serves as the top of the scrubber. This will allow you to remove the top of the scrubber as needed to clean the nozzles. Another benefit of this method is it allows you to witness the flow coming from the manifold you have made and judge if there should be more or less holes. If you use this method, enter the side of the scrubber with a PVC bulkhead fitting followed immediately by a PVC union, followed by a cross tee at the center and the necessary 3 legs and end caps. This allows you to open the union and take the manifold to the sink and clean it in hot water. The down side to this method is the crusty growth you will get as caustic drips find their way out of the unglued fittings. Not hard to clean if you keep up with it.

Option 2
Make the “X” of the manifold using 2 straight pipes install 2 bulkhead fittings which allow you to thread in a bushing and pass a pipe through into the scrubber. Mount the bulkheads at 90 degrees so the pipes cross, and mount one 1” higher than the other. Cap the pipes and drill the holes, add unions outside so you can disconnect to unthread and clean the nozzles. Connect the piping together and run it to the pump. Mark the outside of the assembly so you know which way the nozzles spray, this way when you re-assemble they are spraying at the packing not up into the stack. If you use spray nozzles instead of holes, make sure you have clearance to get through the bulkhead, because the nozzles project downward about 1”. This method is a little more difficult to construct.

One helpful precaution is to add an in-line strainer to trap the floating crud before it clogs the nozzles, remember to clean it regularly.

This scrubber, because of height considerations, does not have a mist eliminator. What that does is it allows the mist to condense on its surface and drip back into the scrubber. If you choose to use spray nozzles, the finer mist may require you to add a mist eliminator. If you are running your scrubber, and your kids come in and say “Gee Daddy it’s raining and the sun is out” you need a mist eliminator.

So nice. Thank you 4metals.

I have been doing research on the subject of
how to build an acid resistant hood & scrubber

The reading material posted by 4metals is good
Some of it I dont understand but i have to read it a few times

Anyway If I look at this picture hood_and_scrubber_001_162
I see a pipe over a pot with smoke coming out of it
and that smoke goes into a container filled with water in the bottom
the air passes through that water and i guess the toxic fumes are now clean.

Isnt it enough to do what i just explained above.

Can someone explain to me what the other things are in this post
for example what is 4metals saying here

Wiffle balls work great, but they probably cost too much as well. Ladies hair curlers work too.
Marbles work but that’s what chemists use in smaller columns, I’d venture a guess that a 12”
diameter by 6 foot deep bed of marbles would be too heavy to be practical.

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The smoke you refer to is wrong. That is the packing in the column. The water and ph reducer spray down on the packing, absorbing the fumes and rendering the exhaust harmless. That is what your quote is refering to. Hope this helps you.

Frank

What you refer to as smoke coming out of a pot is actually what I referred to in the text as a length of corrugated flexible pipe. The exhaust will never be strong enough to suck a fume into the manifold without the aid of ducting. I can't say I ever passed an art class in college, I guess my drawing reflects that.

The quote you pasted is an example of what can be used as tower packing. The column is referred to as a packed tower, what you pack it with depends on what you can get at the right price. These were just examples of things that work.

Hi 4metals & dick b
Thanks for your help
This is begining to make more sense.
I still have to read this post a few times to get all the details
Since I have your attention
I have a few Questons

Question 1
I was thinking about buying this pipe to put in a CHEMICAL FUME HOOD
Like what you have in this post
http://shorinternational.com/refining.htm
Shor ARVT Pumping Table System..................$4,495
Is this a good idea

Question 2
would it not be better to make 2 holes for gloves and do all the work by sticking your hands in the gloves

The Shor system is not cheap, either for the hardware, instructions etc. or for the chemicals to operate the system. From a business standpoint, the one really profiting from using their system is them! With the money you are talking about spending for it you should be able to set up a chemical refining setup which can produce gold cheaper than the Shor system. The reason it will be cheaper is no one will be gouging you for chemical prices.

It all depends on your needs and how much commitment you want to make to learning a process. I'd opt for classical chemical refining.

A glove box will work but the restrictions they place on your maneuverability is a big price to pay.

Hi 4metals
Thanks for your help
I would like to summarize what I have understood from this project
1) the bucket is in the hood
2) your pour acid on the gold
3) reaction takes place
4) hole no 1 sucks up the toxic fumes I guess it needs a motor
5) what happens to the fumes that escape through the pipe no 2
6) Wiffle balls work great or One option is to cut slices of plastic soda bottles into rings about half to
three quarters of an inch thick. Is this here I marked no 3

I will definitly have more questions tommorow for today that is all
Thanks

sorry i forgot to send the picture

The fumes which enter pipe 1 pass through the scrubber, the fumes that pass up pipe 2 bypass the scrubber. One blower supplies the suction for both pipes.

The fumes going out pipe 2 are not the noxious fumes, they are diluted by the total volume of air passing through the hood. The noxious fumes pass through the scrubber where they contact the solution which is pumping down through the scrubber and they are neutralized.

The point you labeled 3 is where the tower packing goes.

I am sorry I cannot explain this any more clearly, possibly because I wrote this and understand it completely I've missed some details which I'm leaving out. Maybe someone else reading this can chime in here and explain what seems to be missing.

I have edited my original schematic, maybe it is clearer now.

4 Metals:
You didn't mention that to get the air to flow thru the scrubber, that the blast gate needs to be closed.

So far I like all that you and Harold have posted about this subject.

I would rather see all of the air flow to go thru the scrubber. Even if there is low toxic fumes or smoke leaving the hood there are some. By scrubbing all of the air and by adding paper air filter elements to the hood discharge into the ductwork you are improving the air quality emissions that are emitted from your lab.

This is just a personal choice that I think is beneficial.

Frankk12

Try this one:

http://goldrefiningforum.com/phpBB3/viewtopic.php?t=4284&highlight=scrubber

John

Dick B

The blast gate should not be closed completely. Based on the diameters of the legs of the wye an 8" blower should supply exhaust to both the scrubber and the hood. If your packing provides enough air space for flow. The damper is only in there to increase flow in the column if it is too low from the resistance. If you went out and spent big bucks on tower packing you probably wouldn't need the damper. If you close the damper the flow through the scrubber will be too high and thus ineffective.

The goal is just enough flow through the scrubber to capture the fumes coming off the top of the reaction, not strong enough to suck out all of the fume.