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hydrofluoric acid

Posted by Albert Russ  
avatar Re: hydrofluoric acid
August 18, 2010 01:07PM
I have a copper crystal group with a crust of what appears to be prehnite covering it. I previously requested information about the interaction of HF with copper and prehnite - will it remove the prehnite? Will it etch the copper? I received numerous warnings about the dangers of HF, but no information of its interactions with the minerals. While I appreciate the warnings, I work in a facility where we use HF as a de-scaler in an automated anodizing line, so safe handling and use of the chemical is standard operating procedure. I would not be using the chemical myself; I would leave that to the guys who do it every day. The questions remain: Will HF remove the prehnite? Will it etch the copper? Thanks in advance for any information you might offer.
avatar Re: hydrofluoric acid
August 18, 2010 02:33PM
From some of the literature it appears that there will be some solution of the copper (and possible redeposition forming masks) that is also influenced by how oxidizing the solution is. I would guess that you would end up with a pretty "bright" copper, not unlike that when you etch a sample with HCl.
avatar Re: hydrofluoric acid
August 18, 2010 02:42PM

Thank you for the reference!

avatar Re: hydrofluoric acid
August 18, 2010 04:30PM
The most vulnerable portions of the body to acids are the eyes. Even to dilute acids! ALWAYS wear safety glasses (preferably splash proof goggles) when working with acids!. If any acid splashes into the eyes, rinse with running cool water for at least 10 minutes (You will need help, as you will be unable to hold your eye open.) and then get to a doctor - - fast.
avatar Re: hydrofluoric acid
August 18, 2010 05:04PM
The only advice we give about HF on is DO NOT, EVER, USE IT.

If you are trained well and are carrying out work with the appropriate safety gear and equipment, even then we wouldn't advise you go near the stuff!

avatar Re: hydrofluoric acid
August 19, 2010 03:14AM
I have used HF over a period of time to remove quartz from Michigan copper and silver specimens. It may etch them a bit, but I am not sure about that. At any event, the results that I got I felt were well worth the effort, but again, unless you know how to work with acid, my advice is also not to do it.

Rock Currier
Crystals not pistols.
avatar Re: hydrofluoric acid
August 19, 2010 02:45PM
During my time working in Mount Isa, I was trained in the use of HF. There, it is used to prepare rocks for the detection of what we called tuffaceous marker beds, continuous k-feldspar rich layers within the siltstone containing the orebodies, which could be used to define the rocks' stratigraphic position. The issue with these TMBs is that they are hard to visually identify. Thus, HF is used to prepare the suspect rock before a stain is applied. If the stain sticks, then you are likely looking at a TMB. This was a reasonably routine procedure during core logging and pit mapping. The safety gear required to handle the HF safely deserves no less description than being the equivalent of a space suit. It really isn't worth the trouble unless you need to use it for commercial mineral preparation, or are doing something worthwhile as far as the science goes. In the latter case, it's probably better to give the work to someone who has dealt with HF before.
Jeffrey Rank
Re: hydrofluoric acid
August 31, 2010 05:38AM
I agree with rock currior. gotta know your shit! and it really only comes with trial and error. gotta learn what will do what with acids, personally I work with fine chisels, a dremal and diamond and carbide tips to achieve its natural condition. but certain acids at times to achieve a result. either way, its an art!
avatar Re: hydrofluoric acid
August 31, 2010 10:41AM
Well, the only problem with trial and error experience is that with something like HF, it can leave you very dead.
Re: hydrofluoric acid
August 31, 2010 05:12PM
I have successfully used hydrofluoric acid diluted to 3% and find that 4 hours at that strength softens most feldspar, dissolves pocket clay, and just starts to attack quartz. I have also used ammonium bifluorite for similar problems.

The smoky quartz from the Smoky Bear Claim, New Mexico were all treated with HF/ammonium bifluorite in sequence to remove the outer quartz crust and maintain luster. Novices tend to oversoak these in the HF, resulting in telltale white cracks where the HF penetrated "burning" the quartz.

John Betts
Re: hydrofluoric acid
August 31, 2010 05:46PM
Most wolframites from Bolivia have been cleaned with HF; nothing else seems to do the job. I use it as a last resort, with fear and trepidation, and great care, outdoors.
avatar Re: hydrofluoric acid
August 31, 2010 08:31PM
Before getting close to HF make sure to have a neutrilising-gel tube next to you! If an accident happens it might save your life. Unfortunatly I don't remember brands or names, but it was always next to the rescue-shower in the lab at university when I studdied...

Personaly, I'll never bring HF to my home, NEVER!
Re: hydrofluoric acid
August 31, 2010 09:05PM in California sells the calcium gluconate gel. They mail it worldwide.
Tim Schafer
Re: hydrofluoric acid
April 06, 2012 10:17PM
I was searching for a method of dissolving quartz sands from super-fine gold concentrates, when HF popped up as the only real solution. Then I read the MSDS. Back to the drawing board. I came across the Igoli merecury-free extraction. Using pool acid and bleach 1:1 to dissolve, and sodium metabisulphate to precipitate the gold after filtering. When done correctly, this should make for a very eco-friendly, and productive reclamation technique for the small scale mine operator. Being that I am not a chemist, and would like very much to maintain still alive status, not offend my neighbors, and stay out of court, I would like to ask for some direction from people who know.
How difficult is it to set up a scrubber for the chlorine gas in a home lab, and what would I need? I have seen some pictures, and a couple in person, but these are large scale scrubbers at chip plants. Any advice, and or direction WILL be greatly appreciated.
I have stumbled upon a very sweet find, but the gold is so fine that mercury amalgamation is just not possible efficiently. The make-up appears to be quartz, alot of calcium (numerous caverns in the area), ridiculous amounts of mica, gold, visible copper and silver, and magnetics. I hope I have not violated your sacred forum with such queeries. Have a fantastic day!
Re: hydrofluoric acid
April 07, 2012 03:28AM
Mercury is much safer and easier than what you are proposing. Use a retort to recover the mercury and all will be well. smileys with beer
Re: hydrofluoric acid
April 07, 2012 10:58PM
The semiconductor foundry at school uses HF.
They're quite fortunate to be across the street from the hospital!
Tim Schafer
Re: hydrofluoric acid
April 11, 2012 05:47AM
Your response is appreciated Mr. Petrov, and your concerrns are noted, but mercury is ineffective as the gold is just so fine and flat tthat Hg simply rolls right over it. I used mercury first, and recovered some, but only a very small percentage. A scrubber just does'nt seem too extreme. Please indulge me.
Re: hydrofluoric acid
April 11, 2012 09:22AM
Albert, I have used both HFl and ammonium bifluoride on quartz and would recommend the latter as the reaction is slower and easier to control. HFl can frost quartz rather quickly. It is good for removing quartz from fluorite or etching gold out of quartz, but is harder to judge when removing silica scale or secondary growth off quartz crystals. It is also dangerous because you don't feel it burn like other acids as it is very penetrating and will damage nerve endings without pain. My wife was wearing rubber gloves using HFl while processing some Bessemer pseudomorphs 25 years ago and didn't notice the pin hole in one glove. She ended up with a very nasty ulcer on her hand. The Ammonium bifluoride is the same radical in solution, but especially when used cold is excellect for removing opaline scale or secondary growth off quartz crystals. You can monitor the progress and decide when to stop. It will slowly attack feldspars and other associations so as usual work with test pieces first, pull samples and let them dry periodically to see dry results and be patient.

Edited 1 time(s). Last edit at 04/11/2012 09:24AM by John Lindell.
Re: hydrofluoric acid
April 11, 2012 01:04PM
Tim, another option for your consideration: Instead of dissolving the quartz in HF, how about dissolving the gold in aqua regia? Since the quantity of gold is, I imagine, less than the quantity of quartz, you'd need less acid. Quartz will not dissolve in aqua regia.
avatar Re: hydrofluoric acid
April 11, 2012 01:05PM
We have a policy here on mindat not to tell people how to use HF because it is so dangerous. So unless you know how to work with this dangerous reagents we strongly discourage its use. Also it is pretty expensive. To buy a gallon of the stuff you will probably have to pay ninety dollars or more. Smaller quantities are proportionally more expensive per ml. If you really feel you have to work with this reagent you should find an old chemist or someone that has used it before and get them to help you and make sure you have all the safety equipment you need to work with it. If your gold is really fine and there is not much of it, it is possible that the acid might cost you more than the value of the gold you recover.

Rock Currier
Crystals not pistols.


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