Re-refining of palladium salt (optional)
- While red Palladium salt is still in the filter (Buchner Funnel as shown above), cover the powder with concentrated ammonia solution (i.e. Ammonium Hydroxide 28% - Amazon Link). Be very careful in adding ammonia solution to the red Pd salt as it reacts quite violently as it dissolves and releases ammonia gas. Cover with watch glass if needed. - Let the solution react completely with the powder. When done reacting, suck it down the filter into a new and clean receiving flask. Wait at least 20 minutes before draining the solution. - Keep doing so until all of the red Pd powder dissolves. Sister PGM salts (if present) will partially dissolve in ammonia solution with the Palladium and some of it will stay as colored salts on the filter. Most base metals (beside Silver and copper) will form solid hydroxides in this basic solution and will be left behind on the filter. The resulting Palladium solution should now be clear and almost colorless with only a hint of orange/yellow hue. - Once done, slowly add concentrated HCl (29-32%) to re-acidify the Palladium solution and mix. A bright yellow Palladosammine Chloride (NH3)2PdCl2 precipitate will appear and slowly settle on the bottom. There's no need for much HCl. Keep adding small HCl increments until no more yellow precipitates appear upon addition and mixing. - Vacuum Filter the yellow Pd salt and wash several times with wash 'solution A' as you did before. Test left over solution with Stannous Chloride for the presence of any precious metals. Discard solution if results are negative.
Re-refining demonstration
Conversion of Palladium salt to metal sponge.
Colored Palladium salt is now ready for the conversion step.
Conversion is achieved by calcining the colored Pd salt to its decomposition temperature, where Ammonium chloride is gassed off to leave behind metallic powder of Palladium (AKA- Pd sponge).
Calcining of small quantities (up to few ounces of salt) can be performed in a fused quartz dish or in a high end (German made) borosilicate crystallization dish. It is important that the dish can take the heat of the process which will exceed 400C (752F). Larger batches are usually done in specialized quartz vessels that fit into a tube furnace.
The calcining process is shared by other PGM salts and is done pretty much the same way.
An alternative method to calcining is a reduction of the Pd salt with Formic acid in aqueous conditions.
- Let the colored Pd salt dry on the filter for few days (or force dry with gentle warm air flow). Once dried, break the caked powder and transfer as much of it as possible to the crystallization dish.
Soak the filter paper with the left over powder in warm water do dissolve traces of Pd Salt and combine it with former left over dilute Palladium solutions to be recovered later with Zinc/Aluminum.
- Place the dish on a hotplate and set it on low heat.
As in the picture, use sand to evenly spread the heat across the dish, thus, preventing thermal shock to the glass.
Soon, white smoke will appear and the powder will start changing colors as it converts. Temperature must be raised slowly and gradually. A sudden spike in temperature will cause values to escape with the smoke. The smoke needs to be only white, any other color to it, is a sure sign that some values are evaporating. You can better test the color of the smoke by holding a flask filled with ice water above the smoke and notice the color of the condensation.
Raise temperature gradually and slowly (every few minutes take it up a notch) to max heat. Most hot plates max out at around 350-400C. The process is pretty much done when all that's left is gray/black powder and no more white smoke is emitted. If a thick layer of powder is calcined, it is recommended to mix the powder with a glass rod to fully expose all of the salt to the heat. Yellow Palladosammine Chloride salt will convert to metal sponge at much lower temperature then brick red Ammonium Hexachloropalladate salt. It only takes about 150C (302F). One more good reason (out of many) why one should re-refine Palladium salt. The entire calcining process should take about 1-2 hours, depending on batch size. The bigger the batch, the longer it takes. - When the calcining process is done and the Palladium sponge has cooled down to ambient temperature, wash is once or twice with warm water to remove traces of Pd salt that had not converted to metal, join this wash with previous dilute Pd solutions to be recovered later. - Dry and collect the black Palladium powder. Store it in an air tight bag or vial. The reason for an air tight storing of the powder is because Palladium is a catalyst. It will slowly react with Carbon monoxide in the atmosphere to form carbonates on the surface of the metal. The phenomena can affect metal purity later, while melting it.
The Palladium powder/sponge is ready to be sold or stored as is. If one wishes to melt it, it is highly recommended to melt it in a high back fused silica (Amazon Link) or alumina melting dish under Oxy/hydrogen flame or alternatively, in an induction furnace. Melting Palladium with a Fuel/Oxy based torch (e.g. propane, acetylene) may prove troublesome for the same reason mentioned above. The metal may absorb carbon, resulting in a dark blackish metal rather than silvery white.
