Precipitating carbonates

[Geo]

I am wondering if this can be used to selectively remove some base metals with careful PH adjustments using soda ash. Please let me know one way or another if it's possible. Using sodium carbonate on solutions with certain metals create carbonates of those metal salts that are insoluble. From wiki :

Sodium carbonate test[edit]
The sodium carbonate test (not to be confused with sodium carbonate extract test) is used to distinguish between some common metal ions, which are precipitated as their respective carbonates. The test can distinguish between Cu, Fe and Ca/Zn/Pb. Sodium carbonate solution is added to the salt of the metal. A blue precipitate indicates Cu2+ ion. A dirty green precipitate indicates Fe2+ ion. A yellow-brown precipitate indicates Fe3+ ion. A white precipitate indicates Ca2+, Zn2+ or Pb2+ ion. The compounds formed are, respectively, copper(II) carbonate, iron(II) carbonate, iron(III) oxide, calcium carbonate, zinc carbonate and lead(II) carbonate. This test is used to precipitate the ion present as almost all carbonates are insoluble. While this test is useful for telling these cations apart, it fails if other ions are present, because most metal carbonates are insoluble and will precipitate. In addition, calcium, zinc and lead ions all produce white precipitates with carbonate, making it difficult to distinguish between them. Instead of sodium carbonate, sodium hydroxide may be added, this gives nearly the same colours, except that lead and zinc hydroxides are soluble in excess alkali, and can hence be distinguished from calcium. For the complete sequence of tests used for qualitative cation analysis, see qualitative inorganic analysis.

My real question would be, will gold precipitate out as a carbonate before the base metals or will the base metal precipitate first?

 

[solar_plasma]

I have read some documents about that, regarding treatment and parting of blended metal waste solutions. Quite interesting, since carbonates do not clog the filter as much as hydroxides. But my conclusion from the data I saw was, that just like hydroxide precipitation, it is hard or impossible to get a selective precipitation. The precipitation of one mostly starts at a point, where another metal is still in full progress of precipitation. At least there is nothing won compared to other methods.

If anyone comes to another conclusion, let us hear it. This is a quite interesting field.

 

[Gratilla]

Yes. Do a google on "metal [carbonate/hydroxide] solubility curves" and choose "picture" (ie not the default "web" links) and you'll be shown a slew of graphs.

You'll note (as you mentioned) that pH/solubility behaviour varies with cation, anion, etc. Some compound solubilities will decrease with increasing pH, some decrease with increasing pH, many decrease then increase and some are not much affected. Note that the solubility scale is usually logarithmic (pH is also, of course, logarithmic), so there can be a very significant solubility change with a relatively small change in pH. AND solubility can also change fairly significantly with temperature. So careful choice of pH and temp can produce quite effective separations.

In fact, the mining industry uses calcium oxide/hydroxide and calcium carbonate on acid-side leaches for exactly this purpose, dropping (for example) As and Fe salts at around pH 2.5 and other hydroxides/carbonates (selectively) as they gradually increase pH.

 

[arthur kierski]

a few years ago,i extracted gold ,pd,pt ,ag using a na2co3(106 grams per liter) --i boiled this solution with my metals containing solution---this formed a precipitate containing carbonates ,oxides,hydroxides and free
metals(pt,au)----this precipitate was treated with concentrated hno3 diluted and formed a new precipitate,containing au,pt that was treated with hcl-h2o2------the nitric solution containing ag and pd was treated with hcl for agcl and then the liquid with dmg for pd-------
note:the first solution treated with na2co3,after filtration also contained pt,au with soluble carbonates as cr,mo,ti, etc--

note2--this method was used by ( Jimeno) in qualitative analysis ---he did qualitive analises using na2co3 instead of h2s----this method is used more in unknown solutions containing pms
hope that i helped,
Arthur Kierski

 

[Geo]

Thank you everyone. While processing CPU's, it occurred to me that there may be a way to clean the solution of bothersome metals first and leave the precious metals in solution. Just removing the iron would be an advantage so when I came across this and it was talking about the same thing but added lead and zinc. I'm so worried about arsenic from CPU's but apparently, it precipitates out as well. The more I learn about this, the more amazed I am at what can be done.

 

[g_axelsson]

You don't need to be afraid of the arsenic in CPU:s. It is used as a dopant and diffused into the silicon die. It only exists in trace level amounts and sits securely inside the silicon die. As long as you don't dissolve the die no arsenic should be released.

By the way, we need some arsenic in our food, too little arsenic could lead to increased cancer rates. http://www.ars.usda.gov/Main/docs.htm?docid=10906
But don't start eating CPU:s now! :mrgreen:

Göran