garage chemist
Member
Hi Everyone,
I have refined dental gold alloys several times in the past, my method has been translated from German and posted here as "Stefans Dental Gold experiment". My username is "garage chemist" on the Sciencemadness board as well. I decided to post my experience here since there seem to be many members with knowledge and experience in PGM separations here.
In these separations I have always precipitated gold from the denoxed AR solution with SO2, then reoxidised the PGMs with chlorine, boiled to reduce Pd(IV) back to Pd(II), precipitated Pt with excess ammonium chloride and finally Pd with chlorine, always storing the final solutions due to their possible residual PGM content.
Many different alloys were refined this way, and I know for a fact that these generally contain not only Pt and Pd (both at 4-8%), but also Ir, Rh and Ru in amounts of up to 1% each, which is now causing problems in the final purification of the platinum.
Now I have combined all the ammonium hexachloroplatinate precipitates, which had varying shades of color ranging from muddy yellow over brown to greenish, calcined them in a quartz dish to crude platinum sponge, washed this well with hot aqueous HCl and, after drying and weighing, redissolved it in AR. There was a vigorous initial reaction with the first portion of AR, a much less vigorous reaction with the later portions, and finally there remained a small amount of black powder (perhaps 2-5% of the initial weight) that was entirely insoluble in boiling AR.
The solutions were denoxed by repeated evaporation with HCl (all of this done in a distillation setup with additional absorption of HCl gas to avoid corroding the fume hood fan more than necessary), filtered through a porosity 4 glass frit, and precipitated with an excess of NH4Cl solution (calculated such that the solution contains about 1 mol/L excess NH4Cl after precipitation). The muddy yellow precipitate was immediately filtered at the pump through a Por. 4 glass frit and washed with cold aqueous NH4Cl solution.
Now the unexpected thing is the deep brown coloration of the filtrate, which looks a lot like Pd(II) to me. I filtered immediately to avoid contaminating the Pt with the time-delayed precipitation of iridium, but now after 2 hours, absolutely nothing is precipitating in the deep brown filtrate. How could so much Pd get into my platinum precipitates?
I have attached pictures of the Pt precipitate on the glass filter, the black insoluble powder and the dark brown filtrate in the round-bottom flask.
The platinum precipitate is currently drying in an oven at 120 degrees centigrade.
I have a book that says that the dirty yellow coloration of the platinum salt indicates contamination with rhodium, which generally co-precipitates with platinum in the NH4Cl precipitation.
To remove rhodium, it is recommended to reduce the impure hexachloroplatinate in a stream of town gas and then repeatedly fuse the platinum sponge with fresh portions of potassium bisulfate at a dull red heat until the salt no longer takes on a yellow powder. If iridium has been previously removed by the lead fusion method, this procedure is said to give pure platinum.
Now, another book (Hoke) says that iridium doesn't even dissolve in AR when dissolving dental alloys, and stays as a black powder with the silver chloride. I find this statement doubtful at least (insoluble residue from my AR dissolution of crude Pt sponge and brown coloration of some hexachloroplatinate precipitates), and the types of dental aloys have surely changed a lot since Hoke's book was written.
Now I am pondering possible methods of proceeding with the purification of my platinum.
I am planning to remove rhodium from the Pt sponge with the bisulfate fusion. Does it make a difference in Rh extractability whether the Pt sponge was produced by calcining, calcining in a reducing atmosphere, or by zinc reduction?
Concerning Ir removal: Is there a difference in solubility of the Ir in AR when the Pt sponge is produced by calcination or reduction with zinc? How much of the Ir and how much of the Pt is left behind in the insoluble black powder when calcined Pt sponge is redissolved in AR?
My current plan is: calcine the hexachloroplatinate, extract Rh by bisulfate fusion, redissolve in AR (noting amount of insoluble residue), denox and reprecipitate with NH4Cl, noting color of precipitate and filtrate. Anything wrong with that?
Thanks for all educated answers and suggestions!
I have refined dental gold alloys several times in the past, my method has been translated from German and posted here as "Stefans Dental Gold experiment". My username is "garage chemist" on the Sciencemadness board as well. I decided to post my experience here since there seem to be many members with knowledge and experience in PGM separations here.
In these separations I have always precipitated gold from the denoxed AR solution with SO2, then reoxidised the PGMs with chlorine, boiled to reduce Pd(IV) back to Pd(II), precipitated Pt with excess ammonium chloride and finally Pd with chlorine, always storing the final solutions due to their possible residual PGM content.
Many different alloys were refined this way, and I know for a fact that these generally contain not only Pt and Pd (both at 4-8%), but also Ir, Rh and Ru in amounts of up to 1% each, which is now causing problems in the final purification of the platinum.
Now I have combined all the ammonium hexachloroplatinate precipitates, which had varying shades of color ranging from muddy yellow over brown to greenish, calcined them in a quartz dish to crude platinum sponge, washed this well with hot aqueous HCl and, after drying and weighing, redissolved it in AR. There was a vigorous initial reaction with the first portion of AR, a much less vigorous reaction with the later portions, and finally there remained a small amount of black powder (perhaps 2-5% of the initial weight) that was entirely insoluble in boiling AR.
The solutions were denoxed by repeated evaporation with HCl (all of this done in a distillation setup with additional absorption of HCl gas to avoid corroding the fume hood fan more than necessary), filtered through a porosity 4 glass frit, and precipitated with an excess of NH4Cl solution (calculated such that the solution contains about 1 mol/L excess NH4Cl after precipitation). The muddy yellow precipitate was immediately filtered at the pump through a Por. 4 glass frit and washed with cold aqueous NH4Cl solution.
Now the unexpected thing is the deep brown coloration of the filtrate, which looks a lot like Pd(II) to me. I filtered immediately to avoid contaminating the Pt with the time-delayed precipitation of iridium, but now after 2 hours, absolutely nothing is precipitating in the deep brown filtrate. How could so much Pd get into my platinum precipitates?
I have attached pictures of the Pt precipitate on the glass filter, the black insoluble powder and the dark brown filtrate in the round-bottom flask.
The platinum precipitate is currently drying in an oven at 120 degrees centigrade.
I have a book that says that the dirty yellow coloration of the platinum salt indicates contamination with rhodium, which generally co-precipitates with platinum in the NH4Cl precipitation.
To remove rhodium, it is recommended to reduce the impure hexachloroplatinate in a stream of town gas and then repeatedly fuse the platinum sponge with fresh portions of potassium bisulfate at a dull red heat until the salt no longer takes on a yellow powder. If iridium has been previously removed by the lead fusion method, this procedure is said to give pure platinum.
Now, another book (Hoke) says that iridium doesn't even dissolve in AR when dissolving dental alloys, and stays as a black powder with the silver chloride. I find this statement doubtful at least (insoluble residue from my AR dissolution of crude Pt sponge and brown coloration of some hexachloroplatinate precipitates), and the types of dental aloys have surely changed a lot since Hoke's book was written.
Now I am pondering possible methods of proceeding with the purification of my platinum.
I am planning to remove rhodium from the Pt sponge with the bisulfate fusion. Does it make a difference in Rh extractability whether the Pt sponge was produced by calcining, calcining in a reducing atmosphere, or by zinc reduction?
Concerning Ir removal: Is there a difference in solubility of the Ir in AR when the Pt sponge is produced by calcination or reduction with zinc? How much of the Ir and how much of the Pt is left behind in the insoluble black powder when calcined Pt sponge is redissolved in AR?
My current plan is: calcine the hexachloroplatinate, extract Rh by bisulfate fusion, redissolve in AR (noting amount of insoluble residue), denox and reprecipitate with NH4Cl, noting color of precipitate and filtrate. Anything wrong with that?
Thanks for all educated answers and suggestions!