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Topic: HF Acid Substitute  (Read 9372 times)

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Offline SteveE

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HF Acid Substitute
« on: January 16, 2018, 09:50:43 AM »
Hello Everyone

I was a gold prospector years ago. Occasionally I would find gold in quartz specimens. I'd like to dissolve some of the quartz off these specimens to expose more gold. Before 9-11, HF acid was pretty much easily obtainable. Since 9-11, it's a little more difficult. I've tried Armour Etch with some piss poor results and never expected it would work in the first place. Is anyone aware of an HF acid substitute that can dissolve quartz?
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Offline SteveE

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Re: HF Acid Substitute
« Reply #1 on: January 16, 2018, 10:07:06 AM »
Also, would nylon fishing string dissolve or degrade in HF acid or a substitute?
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Offline wildfyr

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Re: HF Acid Substitute
« Reply #2 on: January 16, 2018, 10:30:52 AM »
Nylon will be vulnerable to any strongly acidic or basic solutions.

You can dissolve quartz (or any silicon/silica based material) in hot concentrated NaOH. I don't know how long it will take.
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Offline SteveE

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Re: HF Acid Substitute
« Reply #3 on: January 16, 2018, 10:57:52 AM »
Quote from: wildfyr on January 16, 2018, 10:30:52 AM
Nylon will be vulnerable to any strongly acidic or basic solutions.

I'd like to find some type of flexible string, wire or something I can wrap around the specimens to hang them in the solution without touching the bottom of the beaker. Is there something cheap that's impervious to acids or bases?
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Online Corribus

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Re: HF Acid Substitute
« Reply #4 on: January 16, 2018, 11:05:15 AM »
When I've digested clay minerals (which contain a lot of silicates), I've found success using nitric / fluoroboric acid systems at very high temperature in a microwave digester. But this was small amounts (<200 mg), so YMMV.

Can you find Teflon string? Teflon is impervious to just about everything.
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Offline SteveE

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Re: HF Acid Substitute
« Reply #5 on: January 16, 2018, 11:54:01 AM »
Quote from: Corribus on January 16, 2018, 11:05:15 AM
When I've digested clay minerals (which contain a lot of silicates), I've found success using nitric / fluoroboric acid systems at very high temperature in a microwave digester. But this was small amounts (<200 mg), so YMMV.

Can you find Teflon string? Teflon is impervious to just about everything.

I'm looking for something a little less complex then nitric / fluoroiboric acid at high temps.

The Teflon string sounds like it might work but couldn't find it listed as such. Is PTFE the same thing?
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Online Corribus

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Re: HF Acid Substitute
« Reply #6 on: January 16, 2018, 11:59:46 AM »
Yes, PTFE is basically the same thing.

There's really not a whole lot that will completely dissolve silica. HF is mostly the only game in town. NaOH or KOH will etch it, but the dissolution is really slow.
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Offline SteveE

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Re: HF Acid Substitute
« Reply #7 on: January 16, 2018, 01:39:10 PM »
Quote from: Corribus on January 16, 2018, 11:59:46 AM
Yes, PTFE is basically the same thing.

There's really not a whole lot that will completely dissolve silica. HF is mostly the only game in town. NaOH or KOH will etch it, but the dissolution is really slow.

Do you really think sodium hydroxide would dissolve quartz even if it does work slowly? I'm in no hurry and as a matter of fact, slower might be better since I wouldn't have to watch it as much as I did using HF acid.
« Last Edit: January 16, 2018, 03:41:20 PM by Arkcon »
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Online Corribus

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Re: HF Acid Substitute
« Reply #8 on: January 16, 2018, 02:04:39 PM »
No, I don't it's a very viable option.
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Offline wildfyr

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Re: HF Acid Substitute
« Reply #9 on: January 16, 2018, 03:17:55 PM »
I ran the math for KOH etching. This source (https://cleanroom.byu.edu/KOH) shows etching rates of 1-2 microns microns/hour in hot, 20% KOH. This would result in noticeable dissolution over months.

My math is taking their 20% KOH at 90°C:

(1500 nm/hour)(24 hour/day)(30 day/month)(1nm/1E7cm)=0.072 cm/month. Not a lot, but its something.

What about buying sodium or potassium fluoride and acidifying the solution with HCl?
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Online Corribus

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Re: HF Acid Substitute
« Reply #10 on: January 16, 2018, 04:17:01 PM »
The reaction would be limited by the solubility of silica, which is very low in water.
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Offline SteveE

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Re: HF Acid Substitute
« Reply #11 on: January 16, 2018, 06:12:15 PM »
Thanks guys. I'm getting a lot of helpful information here. Some things I never thought of as well.

I'm not planning on removing pounds of quartz. I have four specimens. I figure between all of them, I need to remove 10 to 12 grams of quartz. One alone will need 4 or 5 grams removed with the rest split between the other 3.
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Offline SteveE

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Re: HF Acid Substitute
« Reply #12 on: January 17, 2018, 08:49:48 AM »
Quote from: Corribus on January 16, 2018, 04:17:01 PM
The reaction would be limited by the solubility of silica, which is very low in water.

Can someone give me an general idea how much KOH it would take to dissolve 10 to 12 grams of quartz?
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Offline SteveE

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Re: HF Acid Substitute
« Reply #13 on: January 17, 2018, 08:56:04 AM »
Quote from: wildfyr on January 16, 2018, 03:17:55 PM


What about buying sodium or potassium fluoride and acidifying the solution with HCl?

I'm assuming this is a procedure to make HFL.. Is this a dangerous process or can it be done safely? I have a chemistry background from when I was younger. As a hobby and worked in a chemical factory.
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Offline Enthalpy

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Re: HF Acid Substitute
« Reply #14 on: January 17, 2018, 01:44:06 PM »
Polypropylene and polyethylene are cheap and they resist many acids and bases, but not heat. +80°C would already be a lot.

Stainless steel should resist NaOH and KOH but not HF. Some cooking sieves are made of it and they resist heat.

The answer to slow etching is crushing to a fine powder. Ball mill?
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