Sciencemadness Discussion Board

Aqua regia scrubber

a11051605 - 25-8-2017 at 19:19

Hey all, this is my first post and I don't claim to be a chemist but I do enjoy this forum and knowledge it provides so please bear with me. Also my question below is based on many hours of my time spent reading and studying on a safe way to do experiments and ore/metal extraction with aqua regia so if I am missing something please, please clue me in.

Now for some background to my idea.
Basically what I hope to do is perform my own wet assays and small scale extraction of various polymetallic ore's. These ores contain a soup of pretty much the entire spectrum of metals all in various forms of sulfides and oxides, (mostly sulfides) and range from precious metals to some what nasty arsenide's and pyrites to lead's and possibly mercury traces (ya I know kinda nasty). Now here is the kicker, I live in a sub division and am also environmentally conscious and I try and be a good neighbor, meaning try not to gas my neighbors out...who also happen to be all local law enforcement lol. My plan is to use aqua regia on -100 mesh crushed ore under a home made fume hood that is in a detached stand alone shed that will vent to atmosphere. However I would really like to clean up any fumes that would be generated first before I vent to atmosphere both from an environmental perspective and a smell/angry neighbor perspective.

Now for the equipment and means.
For my aqua regia I will produce this using 32% muriatic acid (HCl) and 67.2% concentrate nitric acid (HNO3) in the normal 3:1 ratio and diluted by 3 times that volume of distilled water.
The regia will be slow dripped into a erlenmeyer flask containing the ore to start the digestion. From this I digestion I anticipate Nitric Oxide gas (NO) and chlorine gas (Cl2) being generated from initial reaction and maybe traces of sulfur dioxide gas (SO2) later on in the reaction (being used as gold participant). All of these fumes would go down a gas line to a series of other flasks to be used as bubbler scrubbers. First flask would be a solution of Hydrogen peroxide (H2O2) at 35% concentrate to oxidizes the NO into Nitrogen Dioxide (NO2). Next the gas would bubble through the H2O2 and down a gas line to a flask filled with distilled water to pull the NO2 into solution and get free dilute HNO3 and also maybe pull alittle of the chlorine gas out, kinda doubt it will do much though. Finally any remaining gas would make its way to another flask which would contain a strong solution of aqueous Sodium hydroxide ( NaOH) this should neutralize any of the gases that make it this far (in theory). This final flask would vent to my fume hood and thus atmosphere.

So am I missing anything? Sound good to much wiser people then I? My main concerns is preventing any explosions or fires due to pressure or exothermic reactions and of course actually do a decent job of cleaning out noxious gases.
Any thoughts? Should I maybe keep the bubbler flasks in ice buckets to prevent heating? Do you foresee me having to swap out different bubblers with new solutions periodically? I.e the H2O2 every half hour or so? Would you expect strong odors even after the lye treatment? Should I add a 4th flask of activated carbon and water? I imagine these 3 or 4 flask bubblers would not lead to an over pressure but maybe I am wrong? Any way please give me your thoughts and concerns because I want to do this right and safely. :o



[Edited on 8-28-2017 by zts16]

Sulaiman - 26-8-2017 at 01:02

I guess that your choices of procedures and precautions will depend upon volumes,

e.g. if operating at test tube scale then you would not need scrubbers, just airflow/extraction, as the tiny quantities of NOx, Cl2, SO2, H2S etc. would easily be diluted by air to safe or un-noticeable levels, and loss of NOx etc. is negligible, cost wise.
Personal protection required would be gloves and goggles level.

If you are processing 10's kg per day then your safety/scrubbing/recycling requirements are beyond my experience, but I'm sure that you will need scrubbers, and would want to recycle the NOx, and a good quality respirator or gas mask plus face shield would definitely be a good investment, and a canary.

So, what volumes of reagents/gasses are expected to be involved ?

Also, why dilute the aqua regia ?
if not neccessary then you are just creating larger volumes to process, clean up, or store and dispose of.


P.S. adding the aqua regia drip-wise is good as it will limit the gas flow rate BUT sometimes reactions are very slow to start, then suddenly, as the reactants warm up, all hell breaks loose, e.g. sudden massive quantities of NOx could overwhelm your scrubbers (or you)

wg48 - 26-8-2017 at 03:04

General analysis of rock for elements usually involves the use of hydrofluoric acid in combination with other acids and as that combination dissolves most rocks and glass hence PTFE lab ware is required.

I guess that if you are analysing for specific metals, you know what they are combined with and you don’t mind inaccuracy (some ore particles will trapped in insoluble rock matrix) you could tailor your digestion for those specific metal ores. You will find lots of procedures on the net.

A word of caution 35% H2O2 can be catalytically decomposed in to boiling water and more than 100 times its volume of oxygen. Are you confident your gases will not be catalytic for H2O2.

One more point if your not scrubbing the H2S, it will require a lot of dilution for it not to be detected by your neighbours as most of their noses can detect it at a few parts per billion.






MeshPL - 26-8-2017 at 23:18

Scrubbing H2S shouldn't be a big deal. Considering that those sulfides will dissolve rather slowly and the solution will contain a lot of oxidisers, all of the hydrogen sulfide should be oxidised before it leaves the solution.

a11051605 - 27-8-2017 at 08:36

Was not awear of the h2s hazard but I would imagine it would be scrubbed out with those other gases from solution by the time it reached the vent or so diluted as to be a non-issue? Was also not awear of the h2o2 being a potential catalytic reaction? Maybe play it safe n go with 30% h2o2? Thank you all for the replys so far.

Melgar - 27-8-2017 at 08:44

You can use HDPE for hydrofluoric acid, no need for HDPE. You can't heat it in that, true, but you shouldn't be heating HF anyway.

a11051605 - 27-8-2017 at 14:04

I don't think I would use hf due to not knowing proper ways to selectively participate out various metals with it or its interactions. Pretty much all I know about hf is that it is toxic and eats most silicates n thus glass, as well as being midly corrosive towards most metals.

a11051605 - 27-8-2017 at 14:22

Also as a note* iv read more about niritc acids reactions with h2o2 and from what iv read it sounds like they use various organic compounds to stabilize the h2o2 and that's what can react violently with the nitric...so it's advised to always do a small test sample of your nitric with the h2o2.

MrHomeScientist - 28-8-2017 at 06:11

Obligatory HF warning: Forget about working with HF. It is one of the most dangerous chemicals there is; a surprisingly small amount spilled on the skin can be enough to kill you. It penetrates skin readily and deadens nerves on the way, so you may not even know anything is wrong until hours later (too late). Inhaling HF fumes probably isn't great for you either. DON'T work with it outside a professional lab and with maximum safety precautions.


Anyways,

There isn't a large difference between 35% and 30% H<sub>2</sub>O<sub>2</sub>. If something were to go wrong, I don't think there'd be a noticeable difference either way. Cooling your scrubbing flasks is a good idea, both to help prevent thermal runaway and because gases are more soluble in cold solutions (increasing your capture efficiency).

Think about what your aqua regia will produce as it reacts. Just the acid itself undergoes decomposition:
HNO<sub>3</sub> (aq) + 3 HCl (aq) → NOCl (g) + Cl<sub>2</sub> (g) + 2 H<sub>2</sub>O (l)
2 NOCl (g) → 2 NO (g) + Cl<sub>2</sub> (g)
2 NO (g) + O<sub>2</sub> (g) → 2 NO<sub>2</sub> (g)

Plus if you have sulfide ores:
MS + H<sup>+</sup> → M<sup>+</sup> + H<sub>2</sub>S (g)

I bolded all the gases produced. Let's look at your proposed scrubbing setup vs what they take care of:
(1) H<sub>2</sub>O<sub>2</sub> - NO, NOCl (I think)
(2) H<sub>2</sub>O - NO, NO<sub>2</sub>
(3) NaOH - any remaining acidic gases (NO<sub>2</sub>, Cl<sub>2</sub>, H<sub>2</sub>S)

Overall it seems pretty reasonable. When you're done, these flasks will need to be disposed of as well. (1) and (2) should be dilute nitric acid, and (3) is some complicated mix of stuff depending on how well the previous stages worked.

Remember: long absorption paths and small bubbles are what you want. So tall wash bottles and some aquarium air stones. Also be wary of suckback: if pressure in the reaction flask drops, it can suck your wash bottle contents into the reaction. I'd put an empty flask in between each stage of your scrubbing setup to be safe.


The big question, as Sulaiman asked, is what quantities are you looking at? If it's small-scale, don't worry about scrubbing too much (though it couldn't hurt). If it's kilograms at a time, you may want even more safety procedures in place.

The point I'm trying to make is that there's a lot to consider before starting an experiment if you want to do it safely and responsibly. Apparatus setup, gas scrubbing, and disposal of all waste solutions needs to be given a lot of thought before you get started.


[Edited on 8-28-2017 by MrHomeScientist]

Sulaiman - 28-8-2017 at 07:30

just a few idle thoughts;

. liquid suckback via fine sintered glass or ceramic (for bubblers) has a very slow flow rate / high resistance to flow

. sintered bubblers create a significant resistance to gas flow, causing 'back pressure'

. deeper = better for gas absorbtion, but adds back pressure

so if you have fine sintered bubblers at the bottom of deep scrubbers;

- gas absorbtion will be relatively good
but
- the pump will need to maintain a reasonable vacuum to achieve acceptable flow rates.
- IF THERE IS A SUDDEN INCREASE IN GAS PRODUCTION RATE - SOMETHING WILL BURST

Bubbling through a bed of sand, glass beads etc. sounds like a viable alternative to mechanical stirring and simpler,
and sudden surges in pressure can be handled more easily, I intend to try it soon.

P.S. a U-tube with water (or other liquids) forms an over-pressure release valve, to vent directly to an extractor, or outside, in case of sudden pressure rises.
The liquid in the U-tube will be expelled under these conditions.

[Edited on 28-8-2017 by Sulaiman]

XeonTheMGPony - 28-8-2017 at 07:35

for allot of my gas scrubber systems I've used 2 to 4 inch pvc pipe packed with marbles.

T on the top and bottom, at the ends I use threaded caps, the top cap has a shower nozzle the bottom a course filter of the cartridge type that feeds into a DC pump to circulate the solution, and a chiller if needed.

This allows for a great deal of surface contact with low back pressure, the stacks are usually 6 feet high or better for what I was doing.

but can be scaled up as needed easily.

So you have your resavour tank that you can test for the condition of your scrubber solution.
The filter (Shouldn't be fouled normally)
Scrubber stack with the marbles or other large surface area media
Pump to spray the solution down the stack.

Vent fan top or bottom, top you need some baffles to catch any mist, no matter what you need a coalescing stage on the exhaust I used 316ss mesh rolled tightish in a 4" pvc tube about 2 feet long.

Needles to say the whole system was big, heavy, and took a fair bit to make, so if you plan to do this allot that's what you'll be in for!

a11051605 - 28-8-2017 at 09:52

Alot of these systems sound like great deisgns. Wish their was a youtube video or a place with pics so I could design it right. The system I was talking about doing would just use the normal gas pressure of the gas being produced to kind of push it's self along as it seeks the lower pressure of atmosphere at the end of the scrubbers... also forgot to mention that yes I would gave an empty flask in line to catch any kind of back blow or in the event or pressure loss or something abnormal go on. I was thinking that the weight of the various scrubbing liquids would be present as back pressure but I imagine that it would still be less then the pressure it would take to cracked any of the glasses and thus avoid some kind of pressure explosion?

XeonTheMGPony - 28-8-2017 at 16:29

the back pressure is why I went with a wet film contact system, very little back pressure, a squirrel cage fan is what moved the gasses along the column.

Because the scrubbing solution was pumped my reservoir capacity was unlimited in theory (1K L at the time :D)

But with a pond pump and a 20l garbage pail with a bunch of ping pong balls would work, use a cold water loop to cool the absorbent media if required.

you could use 4" stacks with 2 inch connects and a good blower for a fairly low back pressure system

[Edited on 29-8-2017 by XeonTheMGPony]

Sulaiman - 28-8-2017 at 23:46

Quote: Originally posted by XeonTheMGPony  
...
But with a pond pump and a 20l garbage pail with a bunch of ping pong balls would work, use a cold water loop to cool the absorbent media if required.
...


Just what I need - 20 litres of celluloid (nitrocellulose + camphor) balls in my scrubber :o:P

XeonTheMGPony - 29-8-2017 at 03:48

well get the thermal plastic type! if they hold up in a rotating biomass treatment plants sure they'll hold up there!

Melgar - 30-8-2017 at 21:22

Quote: Originally posted by Sulaiman  
Just what I need - 20 litres of celluloid (nitrocellulose + camphor) balls in my scrubber :o:P

It's already nitrated, so what's the worst it can do? Nitrate it some more?

Sulaiman - 31-8-2017 at 04:34

Quote: Originally posted by Melgar  
Quote: Originally posted by Sulaiman  
Just what I need - 20 litres of celluloid (nitrocellulose + camphor) balls in my scrubber :o:P

It's already nitrated, so what's the worst it can do? Nitrate it some more?


I was more concerned about this, just 100 ping pong balls in a bucket

https://www.youtube.com/watch?v=RzYoyXwL_A0

100_ping_pong_balls.jpg - 30kB

XeonTheMGPony - 31-8-2017 at 07:44

I'm lazy so I go for simple solutions, just buy non nitro lacquer ones!

MeshPL - 2-9-2017 at 11:23

Quote: Originally posted by MrHomeScientist  
Obligatory HF warning: Forget about working with HF. It is one of the most dangerous chemicals there is; a surprisingly small amount spilled on the skin can be enough to kill you. It penetrates skin readily and deadens nerves on the way, so you may not even know anything is wrong until hours later (too late). Inhaling HF fumes probably isn't great for you either. DON'T work with it outside a professional lab and with maximum safety precautions.
[Edited on 8-28-2017 by MrHomeScientist]


That's why they sell it in grocery stores in Italy. At least I saw 15% HF in Italian grocery store.