Sciencemadness Discussion Board

DIY acid washed sand changed color unexpectedly

SuperOxide - 26-10-2020 at 12:01

A bit ago I cleaned some extra sand I got from Home Depot with the intention of being able to use it for chemistry (sand baths, prevent bumping, maybe filtering, etc), and to do so, I processed it like so:

  1. Froth wash (to get rid of anything that would easily float when agitated)
  2. demagnetized it pretty well using a series of strong magnets
  3. passed it through some screens to get a specific mesh size
  4. acid washed it multiple times (with concentrated HCl)
  5. dried it in the oven

And it worked pretty well, the sand seems nice and clean, not really dusty when I pour/scoop some out, it's all mostly a very consistent size, etc.

However, the other day I was using it as a sand bath to dehydrate some sodium acetate with this setup:
Sodium acetate dehydration - Imgur.jpg - 391kB
So that's a large porcelain dish with the sand, then a smaller porcelain dish sitting in the center, on top of the sand.
You can see the thermocouple is in the sodium acetate which is in the smaller dish, and that never got over 220C.

After the dehydration, when I took out the center porcelain dish, I could see the sand under it seemed to have changed color, darkening slightly, maybe even turning a little red:
Sand changing color - Imgur (1).jpg - 487kB
Another picture with flash enabled:
Sand changing color - Imgur.jpg - 656kB

My first thought was maybe there was some iron that got oxidized into iron oxide, but if that was the case, I would think the sand that was exposed to oxygen would have gotten oxidized while the sand under the porcelain dish wouldn't have gotten oxidized, right? (less oxygen available under the dish, so slightly anaerobic environment?) Not to mention I thought the acid wash would have gotten rid of any iron contaminants to begin with.
Also, when I dried the sand in the oven in the process listed above, it got way over 220C, and it never turned this color.

My goal is to make this sand as clean and inert as possible, so maybe I should have done some other washing steps? Maybe concentrated NaOH wash and/or a concentrated H2SO4 wash, possibly with H2O2? I've read a few documents on preparing sand for laboratory use, and one of the steps is washing it with piranha solution, which I didn't do, but may end up doing.
What compounds could have been created when it was heated like that without oxygen, and what could they be washed away with?

Thanks in advance for any input.

[Edited on 26-10-2020 by SuperOxide]

[Edited on 26-10-2020 by SuperOxide]

SuperOxide - 26-10-2020 at 12:47

P.S. - This isn't necessarily related to the above question, but shows I may have some work to do in terms of cleaning it up a little more.

I did a small scale test of washing small 5g samples with NaOH, conc. H2SO4, and piranha solution:
Sand washing wet - Imgur.jpg - 235kB
(I know the NaOH on the left says 1M solution, but I ended up adding more NaOH so it ended up being more concentrated than that)

Here's the picture of the results:
Sand washing - Imgur.jpg - 422kB
It looks like the NaOH had virtually no change in appearance, the H2SO4 seems to be lighter, and the H2SO4 with H2O2 maybe even cleaner than that (but not by much - though if you look at the picture of them in test tubes, it's evident that it did more just by the color of the solution).

I'm heating some sand up right now in an anaerobic environment to see if it gets darker - and if it does, I may run the same test I ran here on that sand to see what happens.

[Edited on 26-10-2020 by SuperOxide]

Ubya - 26-10-2020 at 13:20

NaOH will react with the sand to make sodium silicates (soluble in water), if there are metal impurities you'll precipitate insoluble metal silicates while dissolving the sand. You are cleaning the impurities from the sand XD

[Edited on 26-10-2020 by Ubya]

SuperOxide - 26-10-2020 at 13:59

Quote: Originally posted by Ubya  
NaOH will react with the sand to make sodium silicates (soluble in water), if there are metal impurities you'll precipitate insoluble metal silicates while dissolving the sand. You are cleaning the impurities from the sand XD

[Edited on 26-10-2020 by Ubya]


lol. I figured if there was alumina oxide or something like that in the sand, then the NaOH would react with it to make sodium aluminate which could be washed away. That was only a small scale test any ways (5g).

I know that highly concentrated NaOH at very high temperatures can etch/dissolve glass, but I don't think it would in the concentration or temperatures I'm using.

Also, the sand that changed colors in the first post wasn't treated with NaOH, only concentrated HCl.

[Edited on 26-10-2020 by SuperOxide]

MidLifeChemist - 26-10-2020 at 15:06

Yeah, dissolving sand in NaOH is not easy. Takes a long time in hot, concentrated NaOH.

Btw, I did a few minutes of quick research, and it looks like it is very hard to remove Iron impurities from sand. Good luck!
https://patents.google.com/patent/US4405588A/en


[Edited on 26-10-2020 by MidLifeChemist]

SuperOxide - 26-10-2020 at 16:35

Quote: Originally posted by MidLifeChemist  
Yeah, dissolving sand in NaOH is not easy. Takes a long time in hot, concentrated NaOH.

Right, that's definitely not what I was trying to do.
Quote: Originally posted by MidLifeChemist  
Btw, I did a few minutes of quick research, and it looks like it is very hard to remove Iron impurities from sand. Good luck!
https://patents.google.com/patent/US4405588A/en

Yeah, that's for sure. But do you think that's what the color change is? It's just such a clear color change that I doubt its entirely because of iron impurities. I did 3 washes with concentrated HCl (heating it quite a bit, then letting it sit for a few days, stirring and heating it intermittently), so I know I got a lot of iron out. On the last wash, the HCl wasn't as yellow as the first two washings (which is what happens with iron gets dissolved in it).
And again, if it was iron being oxidized under higher temperatures, then one would think that the sand that wasn't exposed to the oxygen wouldn't have been oxidized, but it was the opposite.

So I doubt its iron contaminants that's the culprit.

[Edited on 27-10-2020 by SuperOxide]

clearly_not_atara - 26-10-2020 at 17:13

Your sand looks greenish, that suggests iron or nickel chlorides. If there is some kind of alkali in the porcelain it could convert to the oxide and lose the green tint. Consistently the sand in the NaOH looks kind of reddish, like the sand under the dish. The red of course suggests iron.

SuperOxide - 27-10-2020 at 09:43

Quote: Originally posted by clearly_not_atara  
Your sand looks greenish, that suggests iron or nickel chlorides. If there is some kind of alkali in the porcelain it could convert to the oxide and lose the green tint. Consistently the sand in the NaOH looks kind of reddish, like the sand under the dish. The red of course suggests iron.

I agree that the sand treated with NaOH does seem a little red as well, but I don't see any green at all. I think maybe the lighting in the first picture might look greenish perhaps? But if you look at the one right after it, with flash enabled, you'll see there's no green.

Also, if the red was due to iron, wouldn't all of the heated sand turn red? Not just the sand immediately under the porcelain dish? I would think that the iron in the sand that was exposed to the oxygen would turn red as well, but it doesn't seem so.

I think I should do some more HCl washes anyways though.. I'm doing a test run on 400g of the sand and like 150mL of 31% HCl (but obviously added an appropriate amount of water, making sure it covered all the sand). I just started it and it's emitting some odd smell.. not exactly like chlorine, more like some poopy smelling sulfur compound, but can't be sure (I hesitate saying it smells like a poopy sulfur compound because I can't imagine how sulfur got in there, and I would think I would have smelled it before, but maybe wearing a mask masked the smell.)

[Edited on 27-10-2020 by SuperOxide]

[Edited on 27-10-2020 by SuperOxide]

macckone - 27-10-2020 at 14:50

You might try a nitric acid or aqua regia soak and rinse.
My plan for washing a bag of sand is:

Float rinse to remove light stuff.
Magnetic removal of iron and nickel.
Pan to leave behind heavy stuff (mostly black sand that isn't magnetic, trace amounts of gold and silver can show up).
Treat with acetic acid first to remove carbonates (most of the contaminants).
30% H2O2 + close to glacial acetic acid soak will dissolve almost anything, using a little (1%) concentrated nitric acid to activate it converts it to peracetic acid.
The exception is silver acetate is not super soluble and platinum group metals do not fully react.
Rinse well.
Then treat with nitric acid (will dissolve silver and mercury compounds).
If you suspect remaining platinum group metal contamination treat with aqua regia.
Then several long soaks with HCl to remove iron contamination (iron is the worst because it is high concentration).

Several other compounds that can show up red.
Mercury, Silver and Lead are not soluble in HCl.
All three can form complexes or compounds that are red.
Other platimun group metals will not dissolve in HCl under normal conditions.
Even metal oxides can be slow to dissolve. Particularly calcined oxides.

A good question is the actual source of the sand.
Sometimes it is just mining tailings that are deemed 'safe for resale'.

SuperOxide - 27-10-2020 at 16:50

Quote: Originally posted by macckone  
You might try a nitric acid or aqua regia soak and rinse.
My plan for washing a bag of sand is:

Float rinse to remove light stuff.
Magnetic removal of iron and nickel.
Pan to leave behind heavy stuff (mostly black sand that isn't magnetic, trace amounts of gold and silver can show up).
Treat with acetic acid first to remove carbonates (most of the contaminants).
30% H2O2 + close to glacial acetic acid soak will dissolve almost anything, using a little (1%) concentrated nitric acid to activate it converts it to peracetic acid.
The exception is silver acetate is not super soluble and platinum group metals do not fully react.
Rinse well.
Then treat with nitric acid (will dissolve silver and mercury compounds).
If you suspect remaining platinum group metal contamination treat with aqua regia.
Then several long soaks with HCl to remove iron contamination (iron is the worst because it is high concentration).

Several other compounds that can show up red.
Mercury, Silver and Lead are not soluble in HCl.
All three can form complexes or compounds that are red.
Other platimun group metals will not dissolve in HCl under normal conditions.
Even metal oxides can be slow to dissolve. Particularly calcined oxides.

A good question is the actual source of the sand.
Sometimes it is just mining tailings that are deemed 'safe for resale'.


Wow, thanks for such a detailed response!

I follow some of those steps (froth/float wash, magnetic removal, HCl, etc). The hematite gets left behind because it's not magnetic (which I didn't realize initially), so I hoped the concentrated HCl wash would help with that, but maybe it wasn't enough.

Do you by any chance have a picture of the before and after? Curious to see how pretty looking it is (I know... it's just sand, but still, lol).

I haven't tried nitric or acetic acid, or H2O2, but I think I will give that a shot. Thanks!

DraconicAcid - 27-10-2020 at 16:56

Washing with acid is only going to get iron off the surface. If your sand particles are contaminated with iron oxides all the way through, acid won't take it off, but heat can still change the colour.

MidLifeChemist - 27-10-2020 at 17:21

Quote: Originally posted by DraconicAcid  
Washing with acid is only going to get iron off the surface. If your sand particles are contaminated with iron oxides all the way through, acid won't take it off, but heat can still change the colour.


Yes, this is what I was alluding to in my previous post on this topic, I don't washing with acid is going to remove the Iron, it may be embedded in the structure of the minerals used to form the sand - e.g. part of the chemical formula.

macckone - 27-10-2020 at 20:23

Draconic acid is correct, you can only remove the surface contaminants without turning it into sodium silicate and then reconstituting the silica, which will be much finer than the original sand. Of course you can also crush glass and the only contaminants of significant amount will be sodium, potassium and calcium.

Processing sand is a pain in the butt.
However, it is cheaper than buying high grade silica sand of a specific grit which is surprisingly expensive.

Really pure silica can be obtained by first converting to sodium silicate and then reacting with HCl.
Followed by dissolving the cleaned hydrosilic acid precipitate with ammonia fluoride or bifluoride then precipitating silica with ammonia.

Patent US5266289 for the ammonium fluoride process

This gives fine powdered silica rather than sand though.

chornedsnorkack - 27-10-2020 at 23:44

Quote: Originally posted by macckone  
Draconic acid is correct, you can only remove the surface contaminants without turning it into sodium silicate and then reconstituting the silica, which will be much finer than the original sand. Of course you can also crush glass and the only contaminants of significant amount will be sodium, potassium and calcium.

Most window glasses that look clear when seen across a few mm thickness are green when looked edge on. Due to contamination of iron.
And crushing glass gives you sand of, well, glass, not of quartz.
Quote: Originally posted by macckone  

Processing sand is a pain in the butt.
However, it is cheaper than buying high grade silica sand of a specific grit which is surprisingly expensive.

Really pure silica can be obtained by first converting to sodium silicate and then reacting with HCl.
Followed by dissolving the cleaned hydrosilic acid precipitate with ammonia fluoride or bifluoride then precipitating silica with ammonia.

Patent US5266289 for the ammonium fluoride process

This gives fine powdered silica rather than sand though.


So how do you then grow your quartz into crystals of desired size?
Silica solubility increases in either alkaline solution or acid fluoride solution. How do you get fine silica to undergo rapid Ostwald ripening, like in agitated fluoride solution?

SuperOxide - 28-10-2020 at 09:27

Quote: Originally posted by DraconicAcid  
Washing with acid is only going to get iron off the surface. If your sand particles are contaminated with iron oxides all the way through, acid won't take it off, but heat can still change the colour.

I see, so even using something like nitric acid wouldn't be that much better, because the acid may not even reach the iron oxides...
Quote: Originally posted by macckone  
Draconic acid is correct, you can only remove the surface contaminants without turning it into sodium silicate and then reconstituting the silica, which will be much finer than the original sand. Of course you can also crush glass and the only contaminants of significant amount will be sodium, potassium and calcium.

Processing sand is a pain in the butt.
However, it is cheaper than buying high grade silica sand of a specific grit which is surprisingly expensive.

Yep, this is why I'm trying to do it myself (also just so I can learn the process and get my hands dirty with some chemistry..)
Quote: Originally posted by macckone  
Really pure silica can be obtained by first converting to sodium silicate and then reacting with HCl.
Followed by dissolving the cleaned hydrosilic acid precipitate with ammonia fluoride or bifluoride then precipitating silica with ammonia.

Patent US5266289 for the ammonium fluoride process

This gives fine powdered silica rather than sand though.

Yeah, I see the process for converting it to powdered silica, and I may do that as a means to get powdered silica, but I also need some actual sand grain as well.

Some examples of what I would like to use the sand for is:

  1. Sand bath for high temp experiments
  2. a layer at the top and bottom of small chromotography column
  3. prevent bumping in some high temp reactions
  4. layer of sand for filters (thus needs to be as inert as it can get)

So without converting sand all the way to silicone dioxide powder, is there a decent way to accomplish this?

[Edited on 28-10-2020 by SuperOxide]

MidLifeChemist - 28-10-2020 at 10:27

SuperOxide,

Based on what you what to use the sand for, I'll bet it is fine just the way after you processed it with a froth wash and HCl wash. But that is just my speculation, I'm not a sand expert.

SuperOxide - 28-10-2020 at 11:27

Quote: Originally posted by MidLifeChemist  
SuperOxide,

Based on what you what to use the sand for, I'll bet it is fine just the way after you processed it with a froth wash and HCl wash. But that is just my speculation, I'm not a sand expert.
Yeah I think it should be ok. Just the color changing caught me off guard I suppose.

I do wish it was a little lighter in color. Also, I see some black specs still in it, and they aren't magnetic, so they must be hematite.

SuperOxide - 28-10-2020 at 12:05

Heres a bit of a closer picture, so you can see there definitely is some extra contaminants. I get that HCl can't get to all of the iron oxides, but I would think some of these particles that are seemingly mostly not silicone dioxide would be easily processed.

I think it was macckone that suggested I try nitric acid, and I may give that a shot.

Sand - up close - Imgur (1).jpg - 273kB Sand - up close - Imgur.jpg - 426kB

phlogiston - 28-10-2020 at 14:22

If the impurities don't leach out after all of these different extreme treatments, it seems unlikely to me it's going to leach out and interfere when you use it for filtering, anti-bumping material, etc.
Maybe you're also doing this for the challenge though, which I can appreciate.

In my country, hardware stores sell an exceptionally clean variety of sand locally known as 'zilverzand', which literally translates as 'silver sand'. It looks perfectly white to the eye and it said to be quite pure SiO2. It is used for mortar and to cover the floor of bird cages.
Can you get something like that in your country? It seems to me that would be a much more suitable starting material.

SuperOxide - 28-10-2020 at 16:35

Quote: Originally posted by phlogiston  
If the impurities don't leach out after all of these different extreme treatments, it seems unlikely to me it's going to leach out and interfere when you use it for filtering, anti-bumping material, etc.
This may be true, but if I use the sand for whatever reason in some reaction, and the reaction has some weird failure, I'd like to be a little more confident that it wasn't due to some impurity introduced by the sand that was used as boiling stones (or filter, or whatever).
Quote: Originally posted by phlogiston  
Maybe you're also doing this for the challenge though, which I can appreciate.
Ok, yeah. That's definitely a factor, lol.

Quote: Originally posted by phlogiston  
In my country, hardware stores sell an exceptionally clean variety of sand locally known as 'zilverzand', which literally translates as 'silver sand'. It looks perfectly white to the eye and it said to be quite pure SiO2. It is used for mortar and to cover the floor of bird cages.
Can you get something like that in your country? It seems to me that would be a much more suitable starting material.

zilverzand.. I've never heard of that before. I'll look around to see if I can get my hand on some. From what I can tell it certainly looks pretty clean.


I re-read these two documents:

  1. Iron and Aluminium Removal from Algerian Silica Sand by Acid Leaching
  2. Impurity removal process for high-purity silica production by acid leaching

And saw some details that were suggested here, such as washing with HCl:HNO3 and even H2SO4:H2O2, so I may give that a try, and then I can be satisfied that it's as inert as I want it to be - and pretty... it has to look pretty.. lol

[Edited on 29-10-2020 by SuperOxide]

SuperOxide - 28-10-2020 at 20:08

Quick question - I don't have any HNO3 on hand so I plan on making some tomorrow, but not sure how much I will make (limited time and KNO3 on hand).
In some of the literature it suggests using just HNO3, some suggest using HCl:HNO3, and I think I will eventually do a some testing on different batches to see which one has a better result, but if I only have enough nitric acid for a small amount what do you guys think I should try? Just nitric acid, or aqua regia (which I admit I've never used before, but I will definitely read up on some SDS docs and other safety procedures first).

Is there any reason that using HNO3 would yield a better result than HNO3:HCl? Or should I just skip right to the HNO3:HCl? I get that aqua regia isn't some specific acid, but rather a mixture of acids, so my intuition says there isn't anything that HNO3 can do on its own that aqua regia can't, but I wanted to get some of your input.

Thanks

[Edited on 29-10-2020 by SuperOxide]

macckone - 29-10-2020 at 10:23

HNO3 is superior for silver, mercury and lead.
HNO3:HCl is superior for other platinum group metals.
HCl causes silver, mercury and lead to precipitate chlorides.

SuperOxide - 29-10-2020 at 11:08

Quote: Originally posted by macckone  
HNO3 is superior for silver, mercury and lead.
HNO3:HCl is superior for other platinum group metals.
HCl causes silver, mercury and lead to precipitate chlorides.


Right, I get that, and I'm not sure how many platinum group metals or silver/mercury/lead is in the sand (I doubt very much, if any at all), but I know it can be used to clean other metal contaminants.

In the PDF document in my previous post in section 2, it states:
Quote:
The silica used in this work is a natural one from the region of Siliana in Tunisia. In first time of purification, a silica was dipped for a few minutes in an acids mixture solution HCl:HNO3 with volume proportion of (2:1) to remove metallic impurities located at the surface of silica grains. Then it was abundantly rinsed with deionized water and dried under vacuum in an oven at a temperature of about 120°C.

And a similar procedure is listed in other documents too in addition to other acids such as sulfuric acid, phosphoric acid and oxalic acid. Some of them even suggest using HF, which seems odd as that would dissolve the silicone dioxide itself, as opposed to the impurities - wouldn't it? I'll probably do a small scale test to see what works best, which I plan on carrying out later today. I will update yall on my results :-)

macckone - 29-10-2020 at 12:01

The level of silver/mercury/lead contamination is probably low, but it is non-zero.
Just as other contaminants are non-zero. Beach sand is going to be lower in lead but could be higher in mercury if it is from the great lakes. If it is mine tailings they are supposed to be below a certain limit of mercury and lead to be resold. But that isn't necessarily true and a number of countries use gold amalgamation extraction and sell the crushed tailings as sand.

The hydrofluoric method does result in a loss of silica. But the impurities are removed to low levels.
The method I provided actually solubilizes the silica with ammonium fluoride/bifluoride and produces fine silica.

Someone mentioned that crushed glass is glass not quartz, for the most part there isn't a difference unless trace amounts of iron, sodium or boron are a problem. And by trace I mean much less than in the particles of play sand.

Obviously all of this depends on the end use. Mine is as a dispersion medium for making nitric acid via birkeland eyde method. The sand helps break up the bubbles and increases absorption. But it needs to be as pure as possible and have a certain grain size. Crushed glass works but it is hard to get a good grain size. It tends to form chuncks and powder. The chunks are fine for boiling stones since they are generally the same composition as the glass distilling container. Back to the sand, the biggest impurity in mine was calcium carbonate, the second was various forms of iron. The third was aluminum oxide which I did not separate and for most uses, it doesn't matter. Glass making would obviously be a different matter. If you were using it as a reagent then converting it to sodium silicate and then using the ammonium fluoride process makes more sense.

[Edited on 29-10-2020 by macckone]

SuperOxide - 29-10-2020 at 12:45

Quote: Originally posted by macckone  
The hydrofluoric method does result in a loss of silica. But the impurities are removed to low levels.
The method I provided actually solubilizes the silica with ammonium fluoride/bifluoride and produces fine silica.
Interesting.... I may try to use Whink Rust Stain Remover to see if that works. Jeri Ellsworth (one of my fave YouTubers ... and nerd crush) has a video on safe glass etching using this product, so it may suffice as a low HF source.

[Edited on 29-10-2020 by SuperOxide]

macckone - 29-10-2020 at 14:42

That is an interesting product.
Yeah, HF preferentially removes the areas of the sand grains with the most impurities.
It also increases the surface area and 'activates' the silica to some extent.
It is used as a pretreatment in making catalysts

SuperOxide - 29-10-2020 at 15:12

Quote: Originally posted by macckone  
That is an interesting product.
Yeah, HF preferentially removes the areas of the sand grains with the most impurities.
It also increases the surface area and 'activates' the silica to some extent.
It is used as a pretreatment in making catalysts
Awesome. Well that rust stain remover hopefully would suffice, if not, I'm not going to risk dealing with any other higher concentration of HF (I am still a chemical newbie, after all, and this is just sand, lol).

Will update you after I do the HNO3[:HCl] leaches. Thanks for the replies everybody! These forums are awesome :-)

SuperOxide - 29-11-2020 at 14:58

Just some updates - The aqua regia wash definitely whitened it up quite a bit, even more so than the H2SO4:H2O2 wash did.

Here's a photo of the difference:
sand-aqua-regia-washes.jpg - 56kB
(full size image here, or here for same image with flash enabled)

  1. The control sand (you can see how it's a little a little brownish or even reddish
  2. First wash with decently concentrated aqua regia (soaked for at least 4 or 5 days)
  3. Second wash with aqua regia for just as long, not much of a difference from the first wash
  4. Third wash with aqua regia, this time I even applied heat for some time (not quite boiling, but 80C or so for a while)


Here's a link to the Imgur image gallery (which contains the other acid/base washes as well for comparison).

The sand is clearly much cleaner looking than the control, so that's a big plus, but even after 3 long washes with concentrated aqua regia, it still has some black bits in it (presumably hematite), any idea what might take care of that? The only two acids I haven't tried yet are HF and glacial acetic acid, and I'm not positive that either of those would work for it, but I think I'll give it a shot on a small scale.

I'm going to wash the rest of the sand with aqua regia as well since it clearly makes it much cleaner, but id like to figure out a good solution for the black sand that persists through multiple HCl washes, conc. sulfuric acid, piranha solution, conc. nitric acid, aqua regia and sodium hydroxide. They're invincible!

[Edited on 29-11-2020 by SuperOxide]

CharlieA - 30-11-2020 at 14:49

Have you checked the black particles to see if they are magnetic? I'm thinking that magnetite may be a possibility.

SuperOxide - 30-11-2020 at 16:53

Quote: Originally posted by CharlieA  
Have you checked the black particles to see if they are magnetic? I'm thinking that magnetite may be a possibility.


I have, and I used a series of neodymium magnets to remove all of the magnetic particles in there (which I should have mentioned that was the first thing I did, I apologize).
Here's a picture of me holding the test tube with the acid washed sand in it next to a relatively large neodymium magnet, you can see that nothing is jumping over to it:
magnetite-test.jpg - 49kB
Full size image here

The hematite is usually either not magnetic or very weakly magnetic, and that seems to match what I'm seeing here, but if that's the case, I'm surprised that the repeated concentrated HCl washes and even repeated aqua regia hasn't gotten to them (though they still had an overall positive effect, just not the expected/intended result, lol).

B(a)P - 30-11-2020 at 17:38

Have you had a look with a hand lens. The dark minerals may also be biotite or amphibole, though the treatments you subjected the sand to would likely remove biotite.

SuperOxide - 17-12-2020 at 15:44

Quote: Originally posted by B(a)P  
Have you had a look with a hand lens. The dark minerals may also be biotite or amphibole, though the treatments you subjected the sand to would likely remove biotite.

I have, pretty sure thats not what they are.

However, quick update, I ran everything through my new (very fine) screen to separate out the silt from the sand, and the silt seems to contain much more black particulate than the larger sand particles (or perhaps it's just because they were heavier and made it to the screen easier).
Out of curiosity, I put some of the silt in a test tube with some more concentrated aqua regia. Every day I would shake the test tube up a bit, and that was about a week ago. And it appears that there's a red layer settling out above the sand, which I presume to be iron oxide from the black particulate...

Here are two photos, one w/o flash and one w/ flash.
silt-in-AR-1.jpg - 23kB silt-in-AR-2.jpg - 25kB
And here are some other photos.

So perhaps I just either need to re-wash the same sand, but in much more concentrated aqua regia for even longer (which is surprising, because I thought it was decently concentrated before, but I suppose I was just skimping out on using my precious HNO3).

RustyShackleford - 17-12-2020 at 15:52

I think its very likely to be iron contamination. If you are ever in need of fine extra pure sand, get some hamster sand (almost pure white) and heat that in HCl for a couple hours. probably the best purity you can get outside very complicated procedures or actual laboratory sand from a lab supplier.

Morgan - 17-12-2020 at 17:59

There's a fair amount of mostly quartz sand where I live. It's a fine sand and it squeaks or chirps when you walk on it. In spots it looks like snow almost. Like this example someone posted.
https://wandareddish.wordpress.com/2011/12/24/pensacola-beac...

SuperOxide - 17-12-2020 at 19:52

Quote: Originally posted by RustyShackleford  
I think its very likely to be iron contamination. If you are ever in need of fine extra pure sand, get some hamster sand (almost pure white) and heat that in HCl for a couple hours. probably the best purity you can get outside very complicated procedures or actual laboratory sand from a lab supplier.

hamster sand, I've never heard of that... I will check it out. Thanks!

And I think I agree about it being iron contaminants... I suppose I just skimped on the use of HNO3 since I only have like 400mL of azeotropic. I was mostly just doing this for the analytical experience and the challenge to see if I could get it very pure (without actually reacting it with NaOH then converting it back to pure silicone dioxide, since I do plan to use the granulated sand itself).
Quote: Originally posted by Morgan  
There's a fair amount of mostly quartz sand where I live. It's a fine sand and it squeaks or chirps when you walk on it. In spots it looks like snow almost. Like this example someone posted.
https://wandareddish.wordpress.com/2011/12/24/pensacola-beac...

Yeah I saw someone mention that. That's pretty awesome, I don't think I've ever seen anything that white in a natural setting such as a beach... Lucky you! :)

macckone - 18-12-2020 at 16:02

A lot of really white sand contains large amounts of calcium carbonate.
It is a major impurity in a lot of sands.
Some sands even contain titanium dioxide which gives a whiter color.
Titanium dioxide is very inert and would require real effort to remove.
The calcium carbonate comes out with vinegar.

As for the black impurity, it is likely something black in a silica matrix.
If it was pure hematite it would have dissolved in aqua regia.
You could try a round of gravity separation, ie. panning the sand.
Density separation is used extensively in the mining industry as well as the hazardous waste clean up industry.

The orange color is likely iron oxychloride of variable composition.
That would indicate the black impurity is hematite in a silica matrix.

One trick that might help is heating the sand to 200C then dropping it in ice water slowly to avoid too much boiling and steam. This will cause the impure particles to shatter while the pure silica will remain in one piece. If you are going for a particular particle size this has the added benefit that the smaller particles will be the impure stuff.

SuperOxide - 19-12-2020 at 10:40

Quote: Originally posted by macckone  
The calcium carbonate comes out with vinegar.

I did react some with some [nearly] glacial acetic acid, and that seemed to yield no difference at all (in weight or appearance). I figured that would happen considering it's already had a few acid washes, but since it was in the sand purification documents I looked at, I just wanted to rule it out.
Quote: Originally posted by macckone  
As for the black impurity, it is likely something black in a silica matrix.
If it was pure hematite it would have dissolved in aqua regia.
I think I agree with you on this. However maybe the aqua regia with sand in suspension should have been heated for longer - I did 3 AR washes, each time it set for at least 4 days, sometimes a week, but I only heated the last batch, and it didn't get to a boil, but rather just kept it at around 80C for like an hour. Maybe that wasn't enough.
Quote: Originally posted by macckone  
You could try a round of gravity separation, ie. panning the sand.
Density separation is used extensively in the mining industry as well as the hazardous waste clean up industry.
Yeah, I don't have a gold panning pan thingy, so I'll have to figure something out (I'm sure it's not too difficult). When I have the sand in a test tube with some acid on it, and I shake it around, I often notice that the black particulates will concentrate in some spot and form a band mostly at the same level. So perhaps I should have given this a shot before. I just assumed that the acid washes would take care of it better, but I guessed wrong.
Quote: Originally posted by macckone  
The orange color is likely iron oxychloride of variable composition.
That would indicate the black impurity is hematite in a silica matrix.
Iron oxychloride, interesting. The photos with the red/orange particles in suspension was a _very_ concentrated solution of AR, so I may neutralize it and see how that looks. If it has a noticeable difference, then I may use the same process on the rest.

Quote: Originally posted by macckone  
One trick that might help is heating the sand to 200C then dropping it in ice water slowly to avoid too much boiling and steam. This will cause the impure particles to shatter while the pure silica will remain in one piece. If you are going for a particular particle size this has the added benefit that the smaller particles will be the impure stuff.
Now that's a hell of an idea.

SuperOxide - 22-12-2020 at 14:55

Quote: Originally posted by macckone  
If it was pure hematite it would have dissolved in aqua regia.
You could try a round of gravity separation, ie. panning the sand.
Density separation is used extensively in the mining industry as well as the hazardous waste clean up industry.

I put some silt (that was just filtered off from the sand with a very fine screen) in a flask with some acid, shook it up, and look at the blatantly obvious line it formed...
silt-hematite-line.jpg - 69kB
So I think what this all boils down to is my shitty gravity separation techniques and trying to over-compensate for it with use of acids.
I don't have a panning thing that I can use, but I'm sure I can come up with something. Since it's decently pure sand all of similar grade, maybe it can pass through a sep funnel even... I'll test that out on a small scale. Or even just put it in a jar and shake it with some water, then put it on the side, and the lip will hold in the lower layers as I pour it out (think I'll try that first before clogging up my sep funnels).

[Edited on 22-12-2020 by SuperOxide]

MidLifeChemist - 22-12-2020 at 19:34

Your dedication to this project amazes me!

SuperOxide - 23-12-2020 at 11:01

Quote: Originally posted by MidLifeChemist  
Your dedication to this project amazes me!

Haha, thanks. I know it's a bit of a silly project to be so interested in, but I thought it would be a fun experimental exercise.... Get some sand, test various cleaning processes on it and see which ones yield the best results. Plus it's not like it's a full on days work in each step, some of them are just letting it sit in acid for a few days, then wash it and analyze it a bit.

VeritasC&E - 30-12-2020 at 02:47

Quote: Originally posted by SuperOxide  
Quote: Originally posted by MidLifeChemist  
Your dedication to this project amazes me!

Haha, thanks. I know it's a bit of a silly project to be so interested in, but I thought it would be a fun experimental exercise.... Get some sand, test various cleaning processes on it and see which ones yield the best results. Plus it's not like it's a full on days work in each step, some of them are just letting it sit in acid for a few days, then wash it and analyze it a bit.


Given the many applications of pure SiO2 I feel this is a super interesting project and I look forward to reading up on future updates.

Have you maybe thought of starting off with automated physical separation techniques before the leaching steps?

Along the way of:

Gravitational: Shaking the sand on a purely vertical axis inside of a metal cylinder ending up with a cone at the bottom (with help of a simple electric motor). The shaking amplitude and frequency should be such that all the sand inside the cylinder is freely suspended in air for some time at each cycle, without coming in contact with the cylinder's upper lid. You'd simply pour in some sand from the top (maybe 1kg at a time or more if you have a large enough cylinder with a powerful enough motor), put the power on, power it off after a while and emptying the cylinder from the bottom cone by fractions (with increasingly small fraction sizes at the beginning and end of the sand batch inside). When emptying the cylinder you could put it on a scale to know where you're at. Any contamination from the metal cylinder will dissolve before anything else in the later acid washes.

Magnetic: I'm not sure how the magnetic step was carried out but it could be easily automated as well by making a simple electric rotary tumbler a putting the sand inside by 5Kg batches or so together with the neodymium magnet. The magnet being cleared from particles at exponentially increasing intervals.

[Edited on 30-12-2020 by VeritasC&E]

SuperOxide - 30-12-2020 at 09:30

Quote: Originally posted by VeritasC&E  

Given the many applications of pure SiO2 I feel this is a super interesting project and I look forward to reading up on future updates.

I agree! However keep in mind that if it was just the pure SiO2 that I was after, then there's a method to go about getting that which involves doing some basic purification techniques such as separating out the magnetite/hematite from the sand, then heat the sand up with molten NaOH, then a few other steps.
There are a few good videos on YT about that as well (eg: videos by Vibzz lab and The Canadian Chemist), but the end results there are a pure SiO2 powder, which isn't the same as pure SiO2 sand. The sand granules are actually what make the substance quite useful to me (and I think most others).
If I can get it to where it's mostly SiO2 still contained in sand granules, then that would be a win, however I know thats not 100% possible, as the individual granules aren't just one substance (eg: SiO2 on the outside, but maybe some iron oxide on the inside, etc).

Quote: Originally posted by VeritasC&E  

Have you maybe thought of starting off with automated physical separation techniques before the leaching steps?

I've thought about it, and once I get the process down, then maybe that would be the way to go. I'm actually mostly a tech nerd, and chemistry is just a recently acquired passion of mine. So I think automating the process with an Arduino or RaspberryPi would be a fun thing to do when I know i have the underlying process down to pat. Or perhaps that would be overkill, not sure, lol.
Quote: Originally posted by VeritasC&E  

Gravitational: Shaking the sand on a purely vertical axis inside of a metal cylinder ending up with a cone at the bottom (with help of a simple electric motor). The shaking amplitude and frequency should be such that all the sand inside the cylinder is freely suspended in air for some time at each cycle, without coming in contact with the cylinder's upper lid. You'd simply pour in some sand from the top (maybe 1kg at a time or more if you have a large enough cylinder with a powerful enough motor), put the power on, power it off after a while and emptying the cylinder from the bottom cone by fractions (with increasingly small fraction sizes at the beginning and end of the sand batch inside). When emptying the cylinder you could put it on a scale to know where you're at. Any contamination from the metal cylinder will dissolve before anything else in the later acid washes.

So that's a yes and a no.... I thought I did some decent gravitational separation, but I'm now realizing that I just sucked at it. I didn't have a gold panning pan thing, so I just used a saucer with a lip on the end, but that clearly didn't work as well as I thought. I did get a lot of the heavy particles (magnetite and hematite) out, enough that I was convinced that I did a good job, but clearly I was wrong. This is definitely the step I need to go back and reassess (I'm just wasting my precious acids on this step when all it needs is some mechanical work done by yours truly).
Quote: Originally posted by VeritasC&E  

Magnetic: I'm not sure how the magnetic step was carried out but it could be easily automated as well by making a simple electric rotary tumbler a putting the sand inside by 5Kg batches or so together with the neodymium magnet. The magnet being cleared from particles at exponentially increasing intervals.

Yep! This step I'm 100% sure I got right. I have a series of very powerful neodymium magnets, and the sand I have now, I can throw the magnets in there, shake it around, and when I pull them out they have literally nothing on them. However the black particles are still in the sand, so they must be hematite (since hematite is weakly magnetic if magnetic at all). This is what I need to work on getting out by the gravity separation.

macckone - 30-12-2020 at 13:02

Based on the photo with the clear line, gravity separation is the way to go.
Something like this would make an excellent riffle mat.

https://www.amazon.com/Mini-Ice-Cube-Trays-Food-Grade/dp/B01...

They also sell them but they cost more:
https://www.amazon.com/Sluice-Fox-Rubberized-Pocket-Prospect...

SuperOxide - 31-12-2020 at 08:48

Quote: Originally posted by macckone  
Based on the photo with the clear line, gravity separation is the way to go.
Something like this would make an excellent riffle mat.

https://www.amazon.com/Mini-Ice-Cube-Trays-Food-Grade/dp/B01...

They also sell them but they cost more:
https://www.amazon.com/Sluice-Fox-Rubberized-Pocket-Prospect...


Using an ice cube tray as a riffle mat... genius. I just looked in my kitchen and I think the tray I have would work even better, actually.

raffle-mat.jpg - 50kB