AR removing nitric acid

[crazyboy]

I was reading Hoke's book in preparation for my first gold recovery (I plan to use AR) and they stress the importance of removing nitric acid by boiling the mix down and adding sulfuric acid and adding water and boiling it down again.


Can't you just add sodium bicarbonate soln. or ammonia to neutralize the acid?

 

[lazersteve]

You missed the part about adding HCl.

The sulfuric is only added in a very small amount to precipitate any lead as lead sulfate.

The purpose of the boiling down and adding HCl is to remove the NOx gas and convert the precious metal nitrates to there chlorides. The free NOx gas in the solution will cause the precipitated metals to be reabsorbed and also will allow a mixture of PGMs to precipitate with gold when SMB is added. It can also interfere with some reducing agents. If you convert all the nitrates to chlorides they can be cleanly separated with SMB (gold), ammonium chloride (platinum), and sodium chlorate(palladium).

If you neutralize with sodium carbonate or hydroxide (or any other strong base for that matter). All or some of your precious metals will precipitate together and you will have not separated and refined anything.

Steve

 

[arthur kierski]

steve you gave me an idea in the end of your reply to crazy boy
question:if i neutralize with a strong base(naoh) ar containing the 3pgm(pt,pd and rh) and then add smb to the neutralised solution will all the pgms prepicipitate?
this would solve my problem with ar solutions,not having to boil down and add hcl or having to use urea--------------------------------------------
i did this once with zinc powder and it worked----
thanks
arthur

 

[lazersteve]

Arthur,

I've never tried it but it may work with the Pd. I know nitrate solutions of Pd will drop with SMB. Pt and Rh are another story. I'll try to find you some more details to save you from wasting any chemicals and causing yourself more headaches than necessary.

Steve

 

[Blacktoadd]

I always boil down my AR solutions a couple of times Replacing solution with HCl % water to remove the bulk of the nitric. Then into the warm solution I add a few pellets of Urea until there is no more reaction, then a few more just in case. Then I feel the solution should be ready for SMB precipitation. Did I miss anything. Before this I have used several HCl diguestions to remove all that is soluable with HCl and rinsed until solution is clear. I am processing finely crushed ore. The ore was previously headed to drive off sulfer and other unwanted's.
Does this sound reasonable?

 

[Blacktoadd]

A line in one of the articles said Dont use SMB on gold chloride solutions. Thats what I thought it was for. Who is mistaken.

 

[lazersteve]

The article may have been refering to the the addition of SMB to gold chloride solutions PRIOR to removing the nitric acid and NOx. As you have already pointed out the nitric and NOx must be removed brfore adding SMB. This is because the free nitric and NOx can redissolve the gold powder plus due to the fact that any dissolved Pd (and perhaps others) in a nitrate solution will preciptate with SMB (via SO2).

If you specify the referenced article perhaps the context of the comment can be discerned.

Steve

 

[Rag and Bone]

Kjerski wrote: "if i neutralize with a strong base(naoh) ar containing the 3pgm(pt,pd and rh) and then add smb to the neutralised solution will all the pgms prepicipitate?"