Hydrolysis Thread.

[HAuCl4]

Taking off from this thread:
http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=6200&p=54384&hilit=bromate+hydrolysis#p54384

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.

Some of the things you say fourmetals, some would be displeased to hear. It's all in the literature though.

Some additional points to mention:
- use only sodium bromate, not potassium bromate (supposedly you can lose platinum to a potassium platinum double salt, plus it is less reactive than NaBrO3).
- use sodium carbonate, not bicarbonate--more acid neutralized per amount of mass
- if adding bulk Na2CO3, use overhead stirring to break the foam.


Also, the sulfite drops any Ir as its dioxide if I remember correctly. EDIT: Also forgot, sometimes it is necessary (especially after the sulfite, well known for reducing Pt (IV) to Pt (II) to sparge with chlorine gas or add extra HCl and sodium chlorate/bromate (either work, but chlorate is cheaper). This ensures that all platinum is "H2PtCl6".

From the above posts and in the hopes on furthering my own knowledge and that of others, I'd like to delve deeper into the hydrolysis process, which I have recently experimented with to obtain commercial platinum 9995+, and also used it as an intermediate process to obtain spectroscopically pure gold 999999+ expeditiously (measured by an ICP by someone else).

If anyone can post how the sodium bromate mechanism exactly works to prevent platinum and gold forming aurates or platinates above pH 7, I'd like to read about it.

My use of it (in gold chloride solutions) was to rid of the solution from persistent traces of silver chloride and copper chloride contamination, and it worked beautifully where other methods like cooling and diluting the solution did not work as good (silver chloride was the worst contaminant). I raised the pH at 7.5 at the most but wonder what happens at higher pH levels. All silver, palladium, and base metal traces were removed succesfully from gold.

Notice that if the Pt solution is contaminated with small quantities of Ru or Os, an additional step of distillation must be added to remove these contaminants in platinum solutions.(See flow sheet posted by Lou in this other thread: http://goldrefiningforum.com/phpBB3/viewtopic.php?f=51&t=11139&start=20)

I'm sure our readers from Republic, Matthey and INCO will love this thread. :shock: :lol:

 

[garage chemist]

I am also considering trying this method to purify my platinum, though I am looking for simpler methods first.
One thing that I am wondering: how does one measure the pH of such a concentrated and highly colored platinum solution?
The solution also contains abundant free bromine due to the acid and bromate, which would immediately bleach indicator solutions or pH test strips. Is there a kind of pH sensor / glass electrode that is unaffected by such a corrosive environment?

Another potentially highly useful method for purification of platinum is the lead fusion. When crude platinum is dissolved in ten times its own weight of molten lead at 1000°C and left to cool, the Pt and Ir are now present as separate particles.
Dilute HNO3 removes the lead, AR dissolves the Pt, Ir is left behind. Quantitative and sharp separation of Ir from Pt is achieved, though Rh partially stays with the Pt. This deserves its own thread when someone here decides to try it.

 

[freechemist]

As far as I know, the bromate-hydrolysis after Gilchrist is a combined bromide/bromate driven process. Bromide and bromate react together with acid to form elemental bromine, Br2, according to the following stoichiometry:

BrO3(-) + 5 Br (-) + 6 H(+) => 3 Br2 + 3 H2O.

This reaction uses up dissolved acid. Elemental bromine formed can be driven off by strong heating. The reaction comes to an end, when practically no more acid is available. Therefor it can be used, to use up protons (acid), formed in a different process, like the slow precipitation of PGM-hydroxides, formed by slow hydrolysis of their chloro-complexes in aqueous solution, holding the pH of the reacting mixture at a constant low value. In practice, platinum solutions containing significant amounts of Rh, Ir, Pd as impurities are first brought up to a pH of 3-4 with NaHCO3, before addition of the stoichiometrically calculated amounts of NaBr and NaBrO3, and then heated to nearly boiling. When no more bromine is evolved, NaHCO3 is added in small portions (if necessary) to the mixture until the pH reaches a constant value of 6-7, always being heated and agitated. The precipitated hydroxides are separated by filtration, and the purified Pt-solution is processed further by known procedures.

freechemist

 

[HAuCl4]

I am also considering trying this method to purify my platinum, though I am looking for simpler methods first.
One thing that I am wondering: how does one measure the pH of such a concentrated and highly colored platinum solution?
The solution also contains abundant free bromine due to the acid and bromate, which would immediately bleach indicator solutions or pH test strips. Is there a kind of pH sensor / glass electrode that is unaffected by such a corrosive environment?

Another potentially highly useful method for purification of platinum is the lead fusion. When crude platinum is dissolved in ten times its own weight of molten lead at 1000°C and left to cool, the Pt and Ir are now present as separate particles.
Dilute HNO3 removes the lead, AR dissolves the Pt, Ir is left behind. Quantitative and sharp separation of Ir from Pt is achieved, though Rh partially stays with the Pt. This deserves its own thread when someone here decides to try it.

Indeed garagechemist. Lead fusion works very well for separating binary alloys of Pt-Ir. Most Pt alloys are binary, so an idea would be to start one thread for each of the 5 possible binary alloys of platinum. You can do this for the Pt-Ir thread if you so wish, and post the details of the procedure you follow there. 8)

Also garagechemist, I have had no problem using pH test strips with the procedure as posted by 4metals as the Pt solution is not very concentrated. Also all bromine had been boiled out of the solution at the time of the pH adjustment and measurement. I can also see that would be the case for the modifications as suggested by freechemist. There are several electronic pH meters available in the market, but I do not have one. Cheers.

Thanks for your explanation freechemist. I can see how a modified procedure, using your observations, can save some costs on sodium bromate and bromide. Cheers. 8)

 

[Lino1406]

Bromine will oxidize Pd, Rh because its oxidation potential
1.066v. Iridium (about 1.2v)? I wonder

 

[Lou]

They make Pt electrodes that stand up well.

You absolutely do not need to employ the Gilchrist if 999 fine platinum is your goal.

In fact, some Pt I purchased from a member here had been "refined" 3 times and was melted many, many times. By WD-XRF it was 9997 +/- 0.02. Suffice it to say, all he did was use ammonium chloride (or potassium chloride, not sure which) and repeated precipitations and melting. It's not tenable to refine platinum in the fashion this member did, as it simply does not make economic sense. High purity is possible with care and noting visual cues.

Attached is Gilchrist's original method, for y'all to peruse.
Pt is relatively routine as far as refining is concerned. Iridium and rhodium are the dogs-- I don't even mess with them unless I have a kg or more to do at a time.

HAuCl4, did you know that a method for 3N iridium is to alloy it with manganese? Or that iridium is not quantitatively reduced by zinc, but instead requires magnesium?

 

[HAuCl4]

HAuCl4, did you know that a method for 3N iridium is to alloy it with manganese?

No. I didn't know that method. But I have used nickel iteratively instead with some success. Awkwardly I must admit. I sure would like to read about the manganese-iridium method, melting and shotting must be awkward too. Thanks for your post and clues. 8)

Also Lou: Do you prefer the sequence as described by 4metals, bromate first, carbonate later, or do you prefer as described by freechemist, bicarbonate first, bromate/bromide later?.

In the same line of thought, is it possible to use sodium hydroxide instead of carbonate/bicarbonate, and what are the advantages and/or shortcomings?. 8)

 

[Lou]

If the dilution is right, you can use carbonate. I often use carbonate, although bicarb is preferred as it is slower and dilutes the solution more.

And no, don't use hydroxide, it is too strong a base and easy to overshoot.


As for your other question, how I do the Gilchrist is however I do it. I have some modifications made to it but I will not discuss it here on the forum. 4metals is not principally in the platinum group metal refining business, and freechemist I think is retired from it. Between them and myself, I think you have sufficient information. As I am now actively in this business, unlike almost every refiner who claims to refine these metals but brokers them on to those who do, I hope you'll understand my reticence in discussing active protocols. My best customers are refineries and some of them read this board. Academic discussion is fine, but commercially relevant information is another. I'm still not convinced of your motives anyway

 

[HAuCl4]

I was asking about the manganese method to purify iridium...not the Gilchrist...which works great on gold chloride with very little bromate needed...I'm sure Republic will be thrilled to read it here...

My motives are purely educational self enlightment and revenge against Matthey, Inco, establishment, etc... A strong motive, the police would say...:lol:

Did you see Pt under gold by more than $100 per ounce?. Amazing. Happens rarely. I have no more physical gold now!. :lol:

 

[goldenchild]

From the above posts and in the hopes on furthering my own knowledge and that of others, I'd like to delve deeper into the hydrolysis process, which I have recently experimented with to obtain commercial platinum 9995+, and also used it as an intermediate process to obtain spectroscopically pure gold 999999+ expeditiously (measured by an ICP by someone else).

Six nines you say.

 

[HAuCl4]

Goldenchild: One AR leach of yellow 18K gold, or 90% miners gold, one filtering of cold solution (iced), one hydrolysis, another filtering, and one precipitation with oxalic acid (ammonium oxalate in slightly acid solution of oxalic to be exact) was sufficient to produce spectroscopically pure gold. Either that or the ICP used was broken. :shock: Solvent extraction was used first, but later removed from the process as deemed unnecessary.:lol: 8) :shock:

Maybe I got lucky.

 

[HAuCl4]

Lou: Thanks for posting that paper and all the other tips and bits, here and there, about PGM processing and chemistry. Your posts have certainly contributed much to both improve and simplify my understanding of that pet topic of mine. Cheers. 8)

I had the paper already somewhere, but read it again in .pdf form, and I smiled at the separation of Rh and Ir. I have never attempted that process of titanous chloride and cupferron. Never had any quantities worth processing. Looks very interesting and cumbersome at the same time. The best I had done with Iridium, to rid it of Rh and any other metal still clinging to it (no Au, Pt, Os, Ru at this stage) was to boil the blacks in concentrated sulfuric acid, remelt with Nickel, dissolve the Nickel with AR and do it over and over 3 times. Awkward and cumbersome, but simple.

 

[Lou]

So happy that it is appreciated.