I was going through some old papers from my first refinery and came across some sketches I thought some may find useful. This was the first hood I ever made and, all things considered, it wasn't bad. There are a few options I would add today, having had the experience of refining a few ounces under my belt, and maybe we can get some discussion going about what others have experienced and would do to change this design.
Because the blower is basically causing the exhaust by powering an eductor, it only sees room air, not fume laden air so any blower will do. The duct from the blower should be PVC along its entire length but especially from where it enters the hood and goes up into the stack. All elbows should be the biggest sweeps you can find so the air bends around the corner not bangs into a 90 degree wall like it will with a more compact elbow. One alternative is to use 2, 45 degree elbows with a gap equal to the diameter of the duct between them if you can't find an affordable sweep.
One benefit of the blower was the ability to choose where the input air came from. So in summer I used air from the shop to feed the blower but in winter I used outside air. This made heating the shop easier as the only air I had to re-heat was the air sucked out by the draw of the eductor effect.
The window folded up so I could work in the hood and when down I could still stick my hands in and do things. Back then I was a relatively small operation and the hood was capable of handling my production which rarely exceeded I kilo of fine gold a day before I needed to upsize. I controlled the rate I generated NOx by controlling the nitric additions so I did not use a scrubber with this setup. When I got larger I did, and the hoods and scrubber feeds changed but that's another story.
The bottom of the hood was sealed and came up about 4" all around to act as a catch for all spills. (If you say you never have spills then you are one of two things, either a bold faced liar or new at this!)
One of the things I would add to this system would be a waste drainage port. Sounds complicated but it is simply a bulkhead fitting permanently glassed into the bottom to feed into a drum for waste. The translucent drum (so you can see how full it is easily!) is placed under the hood so the legs of the hood must be tall enough. Then a 2" threaded pipe is threaded into the drum bung which is centered beneath the bulkhead so that you can thread the fitting on by looking down through the bulkhead fitting inside the hood and thread in the pipe. The pipe should extend a few inches above the bulkhead and you can cap it with an easily removable cap. The bulkhead should extend above the base of the hood enough to maintain your spill catching capability. (no sense making a 4" deep reservoir and adding a 2" bulkhead) Offsetting the drain port so it is not in the premium front workspace is a good idea.
Now when you need to pour waste into a drum it is no longer a funnel full of stinky acid poured while holding your breath, it is simply uncapping the pipe, inserting the funnel, and pouring inside the vented hood.
When the time comes to empty the drum, the pipe can be removed from inside the hood and the drum slid out for transfer and processing. I quickly realized that if it settled long enough a little more gold always came down so I modified my setup to have an overflow to a second drum by adding a bulkhead overflow a few inches from the top of the drum under the hood and feeding a shorter drum alongside it. That gave me a long retention time for settling and a yearly draining of the tank to recover the settled values on the bottom. I always used Viton gaskets for the bulkhead fittings and never had a leak with spent aqua regia waste.
I also added lights by cutting a round 10" hole in the top of the hood (one on each side of the exit duct) and glassing in a 10" pyrex glass pie plate. Then a "chicken brooding lamp" can be fitted with a bright bulb and placed on top of the hood, out of the fumes so it will not be eaten up, and you have plenty of light to work with. (The older i get the more I appreciate good light!)
When I first joined the forum I posted about building a hood and I think this information can add on to that post to help out a few members looking to make a lab for themselves.
Any other suggestions, or add ons to make this work better are welcome.
Because the blower is basically causing the exhaust by powering an eductor, it only sees room air, not fume laden air so any blower will do. The duct from the blower should be PVC along its entire length but especially from where it enters the hood and goes up into the stack. All elbows should be the biggest sweeps you can find so the air bends around the corner not bangs into a 90 degree wall like it will with a more compact elbow. One alternative is to use 2, 45 degree elbows with a gap equal to the diameter of the duct between them if you can't find an affordable sweep.
One benefit of the blower was the ability to choose where the input air came from. So in summer I used air from the shop to feed the blower but in winter I used outside air. This made heating the shop easier as the only air I had to re-heat was the air sucked out by the draw of the eductor effect.
The window folded up so I could work in the hood and when down I could still stick my hands in and do things. Back then I was a relatively small operation and the hood was capable of handling my production which rarely exceeded I kilo of fine gold a day before I needed to upsize. I controlled the rate I generated NOx by controlling the nitric additions so I did not use a scrubber with this setup. When I got larger I did, and the hoods and scrubber feeds changed but that's another story.
The bottom of the hood was sealed and came up about 4" all around to act as a catch for all spills. (If you say you never have spills then you are one of two things, either a bold faced liar or new at this!)
One of the things I would add to this system would be a waste drainage port. Sounds complicated but it is simply a bulkhead fitting permanently glassed into the bottom to feed into a drum for waste. The translucent drum (so you can see how full it is easily!) is placed under the hood so the legs of the hood must be tall enough. Then a 2" threaded pipe is threaded into the drum bung which is centered beneath the bulkhead so that you can thread the fitting on by looking down through the bulkhead fitting inside the hood and thread in the pipe. The pipe should extend a few inches above the bulkhead and you can cap it with an easily removable cap. The bulkhead should extend above the base of the hood enough to maintain your spill catching capability. (no sense making a 4" deep reservoir and adding a 2" bulkhead) Offsetting the drain port so it is not in the premium front workspace is a good idea.
Now when you need to pour waste into a drum it is no longer a funnel full of stinky acid poured while holding your breath, it is simply uncapping the pipe, inserting the funnel, and pouring inside the vented hood.
When the time comes to empty the drum, the pipe can be removed from inside the hood and the drum slid out for transfer and processing. I quickly realized that if it settled long enough a little more gold always came down so I modified my setup to have an overflow to a second drum by adding a bulkhead overflow a few inches from the top of the drum under the hood and feeding a shorter drum alongside it. That gave me a long retention time for settling and a yearly draining of the tank to recover the settled values on the bottom. I always used Viton gaskets for the bulkhead fittings and never had a leak with spent aqua regia waste.
I also added lights by cutting a round 10" hole in the top of the hood (one on each side of the exit duct) and glassing in a 10" pyrex glass pie plate. Then a "chicken brooding lamp" can be fitted with a bright bulb and placed on top of the hood, out of the fumes so it will not be eaten up, and you have plenty of light to work with. (The older i get the more I appreciate good light!)
When I first joined the forum I posted about building a hood and I think this information can add on to that post to help out a few members looking to make a lab for themselves.
Any other suggestions, or add ons to make this work better are welcome.
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