# Trouble filtering black powder



## Uljunky (Aug 10, 2013)

I recently dissolved high purity platinum into AR. Since the platinum was nearly pure i decided to skip the ammonium chloride step and cemented it out with zinc powder. I had an inky black solution that would not settle. I tried to filter but it clogged within a few seconds. after many hours of trying different techniques I decided to just boil away the acid and water. After it boiled for about an hour i noticed that i had clear liquid on top of the pot!
In the future I will cement the blacks, boil, then filter. I spent many hours fiddling with the inky solution. No more. Dropping boiling water in as suggested by Hoke didn't work it just gave me more solution to filter. 

Cheers and have fun.


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## NobleMetalWorks (Aug 10, 2013)

Uljunky said:


> I recently dissolved high purity platinum into AR.



I can understand dissolving high purity Pt if your intent was to make Chloroplatinic Acid (H2PtCl6), or possibly a standard solution, but why would you do so only to precipitate later? Depending on how you precipitate, reduce, cement and then melt, you can have a melt loss of up to about 8% with only a small amount recoverable. In other words, dissolving high purity Pt guarantees you are going to loose some Pt.



Uljunky said:


> Since the platinum was nearly pure i decided to skip the ammonium chloride step and cemented it out with zinc powder. I had an inky black solution that would not settle.



I personally feel that Pt melts better after calcination, and prefer ammonium chloride to cementing with zinc.

By using zinc turnings you may get a better result, the platinum would be cemented on the bottom rather than somewhere in the solution. Depending on the purity of your Zn powder, you might actually introduce impurities which would be cemented with the Pt. These impurities would of course have to be removed.



Uljunky said:


> I tried to filter but it clogged within a few seconds. after many hours of trying different techniques I decided to just boil away the acid and water. After it boiled for about an hour i noticed that i had clear liquid on top of the pot!



When you say


> just boil away the acid and water


did you add HCl and then observe the reaction as the solution was boiling, to make sure NOx gas was not evolving? So long as you observe NOx gas evolving, you have to continue evaporating, and adding HCl when the solution is cool, then boiling again and observing, in order to ensure you have used up all the HNO3. 



Uljunky said:


> In the future I will cement the blacks, boil, then filter. I spent many hours fiddling with the inky solution. No more. Dropping boiling water in as suggested by Hoke didn't work it just gave me more solution to filter.
> 
> Cheers and have fun.



Perhaps if you state your entire procedure from start to finish, exactly what was done, someone might be able to help you, or point out what you might have done differently so that you don't experience these problems in the future?

Scott


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## butcher (Aug 10, 2013)

I am not sure what your problem was with this batch.

I normally do not filter when I can, or where I can process without it, I normally let things settle and decant liquid (filtering the clear liquid when and if needed).

There are times when things will not settle, or when filtering is needed and the solution just will not filter usually if the solution is dirty with organics or tin, or very high in base metals, hydroxides are another thing that give problems, add a couple of these together and you really have trouble, like tin in solution with the solution tending toward the hydroxide or less acidic side of the pH scale, make the solution even harder to filter.

(depending on the problem sometimes acidifying and strong heating, or concentrating in an acid will help).

If the solution is bad enough in contamination (usually where tin is involved) there are time's where it may be necessary to evaporate the solution down, to remove most of the liquids, if chlorides I normally neutralize it with NaOH and adjust PH,as chlorides of gold and silver can be volatile when roasted or melted, and rinse with water to remove most of the chlorides as salt water, dry to powder, raise the heat keeping the powders crushed, sometimes they will fuse back to a syrupy solution with more heat if so slow continued low heat and they will dry back out after acid or salts vapor off and can then be re-crushed, then incinerate the powders to red hot stirring them to give them good exposure to air or oxygen,cool and wash them in a boil of HCl, and some hot water washes, this will help to remove organics, oxidize base metals to make them easier to dissolve in acids, and put tin in a oxidized state making it easier to deal with, as Harold states this can help to eliminate many of the problems with filtering.

Eliminating these problems before hand can save you all of this trouble.


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## Lou (Aug 10, 2013)

Well, as you can guess, it wasn't a good idea to hit it with zinc. If it was really pretty good platinum, you should have just chilled the solution, then saturated it with ammonium chloride solution and filtered. 

When you get fine blacks with zinc, it's because there are other elements present that interfere with the reduction process and/or your solution is too dilute or your zinc is too high a surface area. 

Zinc should be the weapon of last resort for PGMs!

Boiling a solution of platinum black in HCl does help it come together and make a more filterable precipitate--you are ripening the platinum particles by making them stick together. That and the HCl is probably dissolving impurities and clumps.


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## Uljunky (Aug 11, 2013)

Thank you very much gentlemen. I did drive off the nitric and i was at the point where i would normally add the ammonium chloride, skipped that and went straight to zinc. The reason for dissolving the high purity platinum in AR was to remove it from the AR immune base material.


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