Proper mixture of various precipitants

[Coldgarage]

Would looooooove a 'freebie:' list of various pH steps to cement a deNOx'ed AR with small increments of Sodium Hydroxide. Have not run accross it yet in Hoke's book or searching forum, have no doubt it has been mentioned here 100 times

 

[FrugalRefiner]

Cementing is a "replacement" reaction, where a solid piece of a metal is put into a solution that contains a less reactive metal that is already dissolved. The more reactive metal dissolves into solution, giving up electrons to the less reactive metal, which precipitates as a fine powder that can look like cement.

When you raise the pH of a solution that contains dissolved metals, the metals precipitate as oxides, hydroxides, etc. Instead of a powdered metal, the values precipitate as a compound which will require additional treatment to get them to a metallic state. It is better to use a selective precipitating agent like SMB, ferrous sulfate, etc. to reduce the oxidized metals and cause them to precipitate as a metal.

Dave

 

[lazersteve]

Would looooooove a 'freebie:' list of various pH steps to cement a deNOx'ed AR with small increments of Sodium Hydroxide. Have not run accross it yet in Hoke's book or searching forum, have no doubt it has been mentioned here 100 times

This is the base metals list I posted several years ago:

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

As for precious metals valence plays an important part in determining which metal hydroxide/oxide will precipitate at a particular pH (if at all). Here's a great document detailing many aspects of the use of pH to "selectively" precipitate various metals including precious metals.

Hydrolysis of Metals
Steve

 

[lazersteve]

As a additional note to the above post typically a carbonate base is used in the "selective" precipitations as these are not as strong as hydroxide bases. This aspect of the carbonates makes them the better choice when attempting to "fine tune" the pH of a solution.
You should also note that temperature greatly affects what form of precipitate you will get whether it will be fluffy and problematic to separate or dense and filterable.

Steve