Palladium (Pd) refining can be a daunting task, similar to the other Platinum group metals (PGM's) refining process. This is mainly due to their similar (but different) physical and chemical properties. Separating Palladium from base metals and especially from sister PGM's, is rarely achieved completely in one refining cycle. Sounds kind of rough, but rest assured that this process is doable, even by the non-chemist in a garage lab. However, basic experience and acquaintance with lab techniques and chemicals is required.
In this step by step tutorial a process of refining and re-refining Palladium metal is described in detail (including a video tutorial, see bottom of page). It covers attaining a desirable purity of 99.9 and above metal sponge. Melting of the metal is only briefly overviewed mainly because most buyers of Palladium metal prefer it in powder form (or as salt) rather then a bullion bar or an unmarked button.
This is a refining process in which the initial feedstock is already of relatively high PGM content and has little to no presence of base metals. Recovered black PGM powder (AKA 'PGM black') can have a variety of sources; whether it is Catalytic Converters, Monolithic Ceramic Capacitors, Dental scrap or any other source, in which a recovery process is applied.
It is important to mention that almost every step in the following described process can be performed using an alternative method. Therefore, the writer does not claim this process to be the only or the best one. One should take the time to do more research on the alternatives, some of which are mentioned here.
IMPORTANT SAFETY NOTE: All procedures described here MUST only be performed outdoors and away from people and animals, or in a fume hood. The reader has sole responsibility not to hurt himself or others when following this tutorial.
Abstract of the process:
- Dissolution of black PGM's powder with Aqua Regia (AR). - Neutralization of AR solution. - The Ammonium Chloride (NH4Cl) wash solution. - Precipitating Platinum, Rhodium and Iridium (if present) as salts. - Precipitating Palladium as salt. - Re-refining of palladium salt (optional). - Conversion of Palladium salt to metal sponge.
Dissolution of black PGM's powder with Aqua Regia (AR)
- As mentioned at the end of the Palladium Recovery Tutorial out of MCC's, the black PGM powder is washed several times with water and then covered with Hydrochloric acid (HCl 32%). - Place the beaker on heat set to low and allow the solution to come to temperature. - Nitric acid is then added SLOWLY, with very small increments. Do not bother calculating how much nitric acid is needed as per some recipes. Your main goal is to use as little as possible to achieve a full dissolution of any metals that will dissolve in AR. - When no more reaction occurs upon addition of nitric acid, remove the beaker from heat and allow it to cool back to ambient temperature. Once cooled, place it in an ice bath or a fridge for the night. The next day you might notice solids at the bottom, these solid may very well be; base metals oxides, silver/lead chlorides and possibly some un-reacted PGM powder such as Rhodium, Iridium, Ruthenium and Platinum (assuming they were present with in the feed stock). Any of these combinations might show up, therefore one should keep and collect these solids for further testing and treatment. - Filter the solution, the receiving beaker should be of wide mouth and bottom, also, try and use minimal amounts of water when rinsing the filter and solids.
Neutralization of AR solution
Neutralizing the solution basically means to make sure there is no more free Nitric Acid as well as nitrate ions left in solution. This can be achieved by means of slow evaporation and repeated additions of HCl. That's why you were advised earlier to use only minimal amounts of rinse water. - Place the wide beaker on heating element, set on low heat and allow the solution to slowly evaporate (NOTE: Do not boil!).
- When the solution turns to syrup like consistency and crystallizations start to appear, add enough HCl to dissolve the crystals and shift the solution consistency back like water. Notice if brown fumes are emitted upon addition of HCl. If so, repeat the previous step again until no more brown fumes appear. The color of the solution is basically orange but the high concentration makes it look like blood, almost brown.
As you probably understand by now, when working with any of the Platinum group metals, the key to the process success and for achieving good separation, is to work with concentrated solutions. - Transfer the solution to a more graduated flask or cup if needed.
The Ammonium Chloride (NH4Cl) wash solution
Before going forward with this tutorial you should familiarize yourself with this wash solution. Since some of the salts you will be working with later in this tutorial are somewhat soluble in water (some are more soluble then others), you can't use plain water to wash these salts. Luckily, these salts do not dissolve in a cold dilute solution of Ammonium Chloride (Amazon Link). How to produce 1N Ammonium Chloride wash solution: - For every 100 ml of water, add 5.4 grams dry Ammonium Chloride salt and let it dissolve.
- Add this solution to a spray bottle or a wash bottle (Amazon Link) and place it in the fridge or ice bath. When called for, use this solution only when it is very cold to insure it will not dissolve any salts. When not in use, store it back in the fridge or ice bate. For convenience sake, we will refer to this wash solution as wash 'solution A' to distinguish it from water washes or other steps that call for the use of Ammonium Chloride.
Precipitating Platinum, Rhodium and Iridium if present as salts.
To precipitate the above metals as insoluble salts (IV) and separate them from the Palladium solution, Ammonium Chloride is added to the solution. - Prepare a saturated solution of ammonium chloride (NH4Cl). Simply dissolve Ammonium Chloride salt in water until no more salt will dissolve. (Keep track of how much Ammonium Chloride was dissolved.)
- Add just a little of the above solution to the PGM bearing solution and observe. If Platinum (Pt) is present in the solution, it will come down at once as bright yellow crystals of Ammonium Hexachloroplatinate (NH4)2PtCl6. Rhodium (Rh) will come out with the Platinum if present. Iridium (Ir) will also come out of solution upon addition of NH4Cl, but will take its time doing so. If Iridium is suspected to be in the solution, wait at least 1-2 hours before moving on to the next step. Remember, the more concentrated the PGM solution, the faster it will drop.
- If any colored salts form upon addition of Ammonium chloride, filter them out with a Vacuum System (Amazon Link) and wash it in the filter several times with 'solution A' until wash solution turns out clear of color. Using a vacuum filter system will reduce this step (and further steps) down to few minutes and minimize the use of wash solution. Try not to dilute the main liquor too much. The solution now contains mostly Palladium (and Ru / base metals if present). If too dilute, evaporate the solution down once again to a more concentrated volume.
Precipitating Palladium as salt.
The palladium in the solution is already bonded to the Ammonium salt in form of its tetrachloride (NH4)2PdCl4. To get it out, we actually further oxidize it with Chlorine (Cl) to form the brick red insoluble salt of Ammonium hexachloropalladate (NH4)2PdCl6. - You will need approximately 1.5-2 grams of Ammonium Chloride salt to precipitate 1 gram of Palladium metal that is in the solution. Try and estimate how much ammonium salt was already added in previous steps and add more if needed. The salt can be added dry at this point.
- There are several ways to introduce Chlorine to the solution, each with its own pro's and con's. In the Video Tutorial, a chlorine gas generator was used and the gas was bubbled through the solution.
An alternative, is to use Sodium/Potassium Chlorate, adding few crystals at a time and let tem react completely before next addition, the chlorates react with the acid to form Cl gas within the solution. Be VERY careful handling and storing chlorates, especially Sodium chlorate, those are strong oxidizers that can prove explosive if not handled correctly. Another alternative is to use dry pure Sodium/Calcium Hypochlorite pellets or granules, again adding slowly and let them react completely before adding more. Bleach (clorox 3% and higher) can also be used to provide the Chlorine.
- Keep adding Chlorine until no more red palladium salt is forms. -Notice the color of the solution. It should be much lighter in color as the Pd drops out of solution. Add a bit more ammonium salt and more chlorine to make sure no more Pd can be precipitated (due to low Pd ppm). - Using a Vacuum filter system, filter the brick red Pd powder. Wash the filter and powder several times with 'solution A' until washes come out clear of color.
At this point one can choose to re-refine the brick red palladium salt (recommended) or move on to the conversion step. Converting the palladium salt at this point will most likely produce a final product purity of above 99% (depending on starting material). To ensure purity of 99.9% and above, a re-refining step is necessary.