Bromate Hydrolysis
Impure platinum sponge which has been reduced from de-noxed aqua regia solutions treated with ammonium chloride are collected and processed in 100 ounce lots by a process known as bromate hydrolysis. The objective of this technique is to separate the platinum from other platinum group metals and base metals by the formation of their hydroxides. Platinum, not forming a hydroxide remains in solution for further refining.
Digest the impure sponge and destroy the nitrates by classic evaporation to syrup to drive off the excess nitric. Dilute the syrup with HCl to a concentration of 1.75 troy ounces of Platinum per liter of solution for hydrolysis. This is usually done in a glass lined reactor capable of stirring and either boiling via steam jacketing or heating mantle. Vessel should be sized to allow it to operate at ½ capacity. (for 100 ounces that is approx. 50 liters of solution)
Heat solution to boiling and add 5% of the precious metal weight in zinc chloride. This acts as a coagulating agent. Stir for a few minutes and begin adding a saturated solution of sodium bromate. Solution should be added slowly as it will boil up. Solution should be added below the solution level in the reactor by delivering through a glass tube which extends below the solution level in the reactor. Add a total of ½ ounce of sodium bromate (in a saturated solution) for each ounce of precious metal in the reactor. Go slowly as it will foam up, take your time and add water to maintain the original level of the boiling solution.
Siphon a saturated solution of sodium bi-carbonate into the vessel to neutralize the solution. Adjust the pH to exactly 7.5. Shut off the heat and adjust the volume with water to the original starting level.
Add 1 liter of a coagulant (magna floc) and allow solution to cool to 40 C.
Allow the solution to settle for at least 2 hours then decant the clear the liquid followed by siphoning the settled hydroxides which are collected on a filter and washed. Washes are added to the decanted solution.
The decanted solution is now acidified with 2 liters of HCl for every 100 ounces of metal and boiled for 1 hour. Add 5 ounces of oxalic acid for every 100 ounces of metal to precipitate any gold in the solution. If iridium is present, add 1 oz of sodium sulfite for every 100 ounces of metal. The solution is now filtered and transferred to precipitation tanks for conversion to ammonium chloroplatinate.
Heat the solution to 70 C and drop the ammonium chloroplatinate with ammonium chloride. Decant the solution and wash the collected salts in a room temperature solution of 1/3 saturated ammonium chloride solution, 1/3 HCl, and 1/3 H2O.
Either transfer the salts to a glass reactor and reduce with hydrazine or calcine in a silica tray at 800 C for 20 minutes.
If you started with relatively pure salts (99%) this will produce high purity sponge.
This procedure is completely scalable depending on your reaction equipment. The first time you do it will be the hardest, once you get the required addition volumes down, routine reproduction of results is predictable.
