# Precipitation



## Platdigger (Mar 6, 2008)

Does any one know, or have a chart of the different PH levels that different metal ions will come out of solution at?

I saw a chart like this at one time, but not sure were to find it.

What I am talking about is like carefully adjusting the PH of a pregnant solution with say sodium hydroxide. And having the different metal ions in solution precipitate out as their hydroxide.

Iron, copper, gold.......and so forth.
Randy


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## lazersteve (Mar 6, 2008)

Randy,

I've got a copy in my files, give me some time to locate it and post it.

Steve


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## lazersteve (Mar 7, 2008)

Here's the chart I promised:

pH values at which various metal hydroxides form a 0.001 Molar Saturated solution:

Metal Hydroxide = 'Precipitation pH'
Fe(OH)3 = 2.5
Al(OH)3 = 4.1
Cr(OH)3 = 4.9
Cu(OH)2m = 5.9
Zn(OH)2 = 7.3
Co(OH)2 = 7.6
Pb(OH)2 = 7.7
Fe(OH)2 = 8.1
Mn(OH)2 = 8.4
Ni(OH)2 = 8.6
Mg(OH)2 = 9.9

At pH 12 precipitations most likely as complete as it can be. The metal hydroxides are usually non-crystalline and form gelatinous high volume precipitates that settle slowly and filter poorly. 

The sludges retain large amounts of water and will need further drying. 
The solution will still require neutralization back to pH7 with acid.

This material is an excerpt from the book by Roland Loewen, Small Scale Refining of Jewelers Waste pg. 212.

I hope this is what you are looking for.

Steve


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## Platdigger (Mar 8, 2008)

Thanks Steve, perfect!


Except......he didn't list any of the precious ones..... 
Randy


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## lazersteve (Mar 8, 2008)

Randy,

That's because this is meant for disposal pretreatment of your wastes and the PMs have all been removed at this point.

Steve


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## markqf1 (Mar 8, 2008)

Wouldn't it be safe to say that at a ph of 7, all of the pms would already have been precipitated?


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## Platdigger (Mar 8, 2008)

Not always, I believe some plats come out at about 8 or better.
Randy


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## Richard NL (Nov 30, 2015)

Platdigger said:


> Not always, I believe some plats come out at about 8 or better.
> Randy


Analytical separations by means of controlled hydrolytic precipitation by Raleigh Gilchrist https://goo.gl/HOYqYV + https://en.wikipedia.org/wiki/PH_indicator

And:
http://goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?f=37&t=18892


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## g_axelsson (Nov 30, 2015)

Here is a downloadable copy of the Gilchrist paper :
Analytical Separations by Means of Controlled Hydrolytic Precipitation by Raleigh Gilchrist

Göran


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## Grelko (Nov 30, 2015)

lazersteve said:


> Metal Hydroxide = 'Precipitation pH'
> Fe(OH)3 = 2.5
> Al(OH)3 = 4.1
> Cr(OH)3 = 4.9
> ...



I've been looking for this information, thanks very much.

I was also wondering if different solutions, will evaporate faster, the closer they get to PH 7 "neutral/water". Has anyone tried this?


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## lazersteve (Dec 1, 2015)

Evaporation has to do many factors like convection, temperature, solution concentration, solute (what compound is dissolved), solution the compound is dissolved in, etc. 

For example, strongly basic solutions of water and sodium hydroxide tend to adsorb water from the air as sodium hydroxide is hygroscopic, so the solution needs to be dehydrated by heating to distill off the water. Similarly, dilute sulfuric acid does not readily dehydrate for the same reason, yet the pH is the opposite end of the scale as a strong lye (NaOH) solution. On the flip side, many of the metal hydroxides either retain water in the compound structure (combined water), but do not 'dissolve' in water to any appreciable extent. The combined water has to be forced out, whereas water that is not combined more readily evaporates from the mix. 

High surface area is another factor in natural dehydration. Piping a solution over a large surface area or placing in a shallow dish exposes more of the solution for evaporation. Flowing solutions tend to evaporate the fastest generally.

To your question, neutral solutions of metal mineral acid salts (not hydroxides, but chlorides, sulfates, and nitrates), where the salt is soluble in water, tend to find an equilibrium of water content depending on external factors like temperature, pressure, and whether or not the container is covered/sealed and the affinity of the compound/acid for water vapor. 

Steve


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## kurtak (Dec 1, 2015)

Grelko said:


> lazersteve said:
> 
> 
> > Metal Hydroxide = 'Precipitation pH'
> ...



It should be noted that precipitating metals from solutions as hydroxides is not an "exact" science - the numbers provided by Steve are the "optimum" numbers for each metal but in reality hydroxides of each metal will "start to form (drop out) at lower Ph numbers then there listed "optimum" number - meaning hydroxide precipitation occurs within a "range" both above & below that of the listed "optimum" number - also some metals will re-dissolve when the go beyond the high end of the "range" with in which they will precipitate

Example - the listed "optimum" number for Zn is 7.3 however Zn will "start" to precipitate before then & will "start" to re-dissolve as you go over (higher)

The "range" is somewhat effected by how dilute &/or concentrated the solution is you are precipitating from - the range is greater with a dilute solution then with a concentrated solution

The point being that you can not do a "selective" precipitation of metals by PH adjustment (with hydroxides) (NaOH) because there is an over lap with in the range that metals will start to precipitate compared to the listed optimum number --- in other words as you hit the optimum number for one metal other metals with a higher optimum number are also going to start to precipitate as well

Here is an example of what I am talking about :arrow: http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=9058&p=226091&hilit=hydroxides#p226025 read from this post down the next 7 posts

you will see that the silver starts to precipitate long before (about 6.3) it optimum number of 8.3

Kurt


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## Grelko (Dec 1, 2015)

lazersteve said:


> Evaporation has to do many factors like convection, temperature, solution concentration, solute (what compound is dissolved), solution the compound is dissolved in, etc.
> Steve



This explains alot thank you. I wondered because I had some left over AP that was at PH 1, and some that I slowly raised to PH ~3.5. The 3.5 looks like it evaporated a little on its own, but the PH 1 didn't do much. They both contained the same used solution I had from dissolving pins a while back. It looks like I'll have to get out my crock pot. It's ALOT easier to evaporate most of a solution, before trying to neutralize it.



kurtak said:


> It should be noted that precipitating metals from solutions as hydroxides is not an "exact" science - the numbers provided by Steve are the "optimum" numbers for each metal but in reality hydroxides of each metal will "start to form (drop out) at lower Ph numbers then there listed "optimum" number - meaning hydroxide precipitation occurs within a "range" both above & below that of the listed "optimum" number - also some metals will re-dissolve when the go beyond the high end of the "range" with in which they will precipitate
> Kurt



Thank you, this answered a question I had about the disposal process when I first came here, and why we should raise the PH to 9, then lower it back to 7.

On a side note - This is the best, most fun, hardest, need to read everything, take safety precautions, etc, etc, hobby that I've ever had the pleasure of starting :mrgreen: 

Nothing in here really seems to be "concrete". It's all based on ranges, we can't get 100% of a certain material, it's always 99.9 or something. It's hard to be selective about pushing certain materials out of solution. Other elements come down with it, even if it's not much. You can "accidently dissolve" other metals if you add a bit too much of this or that to your solutions. Including the actual price of the material after recovering/refining it "which isn't much right now", but it's worth the effort learning all of this.


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