# Why fear aluminum?



## qst42know (Nov 6, 2008)

According to Hoke you use zinc to clean your stock pot but alternately she also states you can use iron, copper, and aluminum which in her day was the priciest option, So I can see why she used other metals. 

And yet many have warned to avoid any aluminum in your solutions.

As a point of discussion why is aluminum so dreaded today? 

It is cheap and plentiful and easier to come by than zinc dust.

Is it the alloying elements? Or something I'm missing?


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## lazersteve (Nov 6, 2008)

Gst,

I've tried aluminum foil, aluminum rod, zinc dust, zinc turnings (just today), and solid iron.

The Aluminum was way too reactive for my taste, it foams up a lot.

The solid iron was way too slow and the PGMs seemed to bond to the iron.

Zinc dust was nice, but contained what Lou identified as Cab-O-Sil, a stabilizing compound (SiO2) used by zinc dust shippers to prevent fire. I tried several suppliers and all contained the mysterious ultra fine 'sand'.

Today I made my own zinc turnings from a 20# zinc block and they work only slightly slower than powdered zinc and produced a better precipitate. The material clumped together better than the dust precipitate. Filtering was also much easier because the precipitate particles where not as fine as with the zinc dust. I also used less of the turnings than the dust for the same volume of solution.

Overall I'm happy with zinc. If you use too much it cleans out of the precipitate with a short soak in diluted HCl.

I'm planning on making 50+ pounds of the turnings this weekend (last weekend was ate up finalizing my new PT DVD). I will be selling the zinc turnings on my website. I'll have a price per pound once I see how long it takes me to turn the 20# blocks.

Steve


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## Lou (Nov 6, 2008)

I'll give you the short answer:

In this case, Hoke is wrong. Aluminum is inconvenient to use because it is tough contaminant to remove because it forms sticky gels (like what tin does with conc. nitric, metastannic acid goo!).

To understand why it is such a royal pain, you must know something of its aqueous chemistry.

When you go to do a cementation reaction, you are basically using a reactive metal to displace a less reactive metal. That much you know. When the aluminum or zinc goes into an acidic solution, it produces hydrogen gas. It is both the nascent (new-born) hydrogen that does the actual reduction of these metals and the chemical potential caused by the flow of electrons. Gross simplification here.

The problem with aluminum is that it has problematic aqueous chemistry and can do several things. When you put aluminum metal into this hydrochloric acid solution, you get "aluminum chloride hexahydrate" (not really, actually it is a mixture of many things: [Al(H2O)6]3+ + H2O --><-- [Al(OH)(H2O)5]2+ + H3O+, and also the benign complex AlCl4-). In short, this is hydrolysis. This is driven by the low Ksp of hydrous aluminum oxide, Al(OH)3. Now you may ask, why am I getting a seemingly basic precipitate (note the OH function in it) when I am doing a reduction at low pH in hydrochloric acid? Well, alumina is called an acidic oxide for this reason, it precipitates at low pH! Recall that water autoionizes into both hydronium (H3O+) and hydroxyl (OH-) ions. This is because water is amphoteric and acts as both an acid and a base. Well, as luck would have it, so do many other things, in particular aluminum hydroxide. The hydroxyls from water's autoionization will hook up with any AlCl4- and any hexaaqualuminum (III) floating about (which is what happens when aluminum cation is present with excess water from HCl).

So what does this mean in practical talk? It means that even at low pH of around 1 (which is equivalent to 1M HCl) you will have some aluminum hydroxide beginning to drop out because of hydrolysis. I think the actual math (which gets nasty if solved exactly, 8 or 9 unknowns) says it starts at pH 1.4. Anyhow, this gets entrained and mixed up with your cemented values and becomes hell to remove. It requires a boil in base (to complex the aluminum hydroxide as soluble aluminate, Al(OH)4-) or in a very conc. solution of HCl. Aluminum hydroxide forms alumina upon heating, a very stable entity--it is used as refractory for lining steel ladles or as an abrasive, or in sapphires in rings.

Did I also mention that this aluminum hydroxide is very flocculent and slimy and will clog filters? It is a pain to remove from your metals. Pure Al(OH)3 made by an analytical chemist with good technique and know how won't do this, but I can guarantee you, it will when you try it. Not trying to insult, but there is a lot of math, technique, and art to it. It ties in with selective precipitation techniques, also a good tool for refining! I'd be willing to discuss this all in more detail with math and examples if there is enough interest...


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## lazersteve (Nov 6, 2008)

WOW Lou, 

Great answer...

That explains the slimy stuff I saw in the aluminum reaction. I assumed it was a coating on the Aluminum foil I was using.

Are you saying that Aluminum hydroxide (pure) would be a useful non-selective refining precipitant?

Steve


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## Lou (Nov 6, 2008)

Thank you Steve.


I did not mean that Al(OH)3 could be used as a precipitant, I meant only that it is a good example of pH controlled selective precipitation. Aluminum is a bear to deal with in any refining process, even in the gas phase! (recall that anhydrous AlCl3 is a strong Lewis acid and a very water reactive compound that also vapor transports, this is the problem with catalytic converters extraction). 


Pure aluminum hydroxide is made by manipulating thermodynamics and solution equilibria. You likely know that there are active forms of alumina typically finding use in filtration setups (think water treatment), chromatography, and, in my particular case, as a support for the PGMs for catalysis. There are many cool things to do with it and one can even change its properties based off of the way you precipitate and calcine it. Did I ever mention that I love chemistry?

It's very difficult to digest the stuff, so keeping aluminum out of the refining process is imperative. Zinc doesn't cause nearly the problems that aluminum does. It too will hydrolyze, but at a much higher pH than alumina, so much so that you can go ahead and cement at pH of 4.



Lou


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## qst42know (Nov 7, 2008)

Lou said:


> I'll give you the short answer:



I get from the SHORT answer :wink: the chemistry of aluminum in solution adds far too many complications. 

I had though it might be the sources of aluminum generally considered pure such as foil (1100 and 3003 series) contain 1% alloying elements. However from your thorough enough for me answer, I see it's the complex nature of aluminum itself.

Thanks for the explanation and clarification.


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## DNIndustry (Nov 14, 2008)

I have bit different take.

I droped my gold as gold hydroxide. Au(OH)3. Perfect for Aluminum.
I decanted as much water as I could. Then placed it in these 4x6" solid aluminum containers. It started reduce an dehydrate the Yellow mud to an olive green paste over night. Just by sitting in it. I flame tested the gas and it was hydrogen. No mess what so ever.

Any comments?


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## Harold_V (Nov 14, 2008)

DNIndustry said:


> I have bit different take.
> 
> I droped my gold as gold hydroxide. Au(OH)3. Perfect for Aluminum.
> I decanted as much water as I could. Then placed it in these 4x6" solid aluminum containers. It started reduce an dehydrate the Yellow mud to an olive green paste over night. Just by sitting in it. I flame tested the gas and it was hydrogen. No mess what so ever.
> ...



Yes, but from a person that has no chemical education. 

At some point you will have to dissolve your gold. When you do, you will come to terms with the problems that occur because you included aluminum in your processing. Aluminum borders on the impossible to filter----so unless you can work around the filtration problem, you'll have regrets for having used the aluminum in any way. 


For the record, I used aluminum for converting silver chloride to elemental silver----but I also had the option to rinse the resulting cement silver numerous time in order to eliminate the dissolved aluminum. Filtering, even with a Buchner, was not an option until the silver had been rinsed many times. 

Wise people avoid aluminum when refining. 

Harold


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## DNIndustry (Nov 14, 2008)

I will agree... 
Powdered Al is not the way to go.

To tell the truth it was an accident that I put the gold in the Al container.


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