Hoke (Refining Precious Metal Wastes) pg. 146-147

[BAGross85]

I’m at the stage with black powder, recovered by displacement with Zn. I’m thinking I need to continue to wash with hot H2O, but I’ve reached “igniting in a muffle furnace”

I believe that this powder contains a few percent rhodium. Hoke says this step is not absolutely necessary, but I would like to isolate rhodium if present: Hoke says that this converts the iridium to a powder that won’t easily dissolve, but is that also the case with rhodium here?

Or, is the ignition step mainly for iridium, as she stated? Basically, will the subsequent, dilute aqua regia treatment leave behind the rhodium as insoluble powder, if I don’t ignite it prior in a muffle furnace?

 

[Yggdrasil]

I’m at the stage with black powder, recovered by displacement with Zn. I’m thinking I need to continue to wash with hot H2O, but I’ve reached “igniting in a muffle furnace”

I believe that this powder contains a few percent rhodium. Hoke says this step is not absolutely necessary, but I would like to isolate rhodium if present: Hoke says that this converts the iridium to a powder that won’t easily dissolve, but is that also the case with rhodium here?

Or, is the ignition step mainly for iridium, as she stated? Basically, will the subsequent, dilute aqua regia treatment leave behind the rhodium as insoluble powder, if I don’t ignite it prior in a muffle furnace?

This is from converters, right?
Unless you tell us it is all a guess.
Zinc is only applicable if there are no base metals present.
Igniting is what we call calcining and it will drive off the Chlorine or other Gaseous components, if it is done slow enough with correct temperature.
It will be relatively lossless.
The fine powder left behind may render the Powders somewhat soluble even in HCl.
At least Pd.
Formate reduction might be a better option, the powders may be denser and less soluble.

 

[BAGross85]

This is from converters, right?
Unless you tell us it is all a guess.
Zinc is only applicable if there are no base metals present.
Igniting is what we call calcining and it will drive off the Chlorine or other Gaseous components, if it is done slow enough with correct temperature.
It will be relatively lossless.
The fine powder left behind may render the Powders somewhat soluble even in HCl.
At least Pd.
Formate reduction might be a better option, the powders may be denser and less soluble.

Yes, converters.

I’m confused, I thought zinc was one of the better choices for cementing in the presence of base metals. So, copper then?

 

[4metals]

Zinc was the go to metal for cementation when Hoke published but the trouble is it also pulls down other base metals that reside beneath it on the electromotive series. Copper, sits just above the precious metals so it only drops PM's (and mercury)

 

[BAGross85]

Zinc was the go to metal for cementation when Hoke published but the trouble is it also pulls down other base metals that reside beneath it on the electromotive series. Copper, sits just above the precious metals so it only drops PM's (and mercury)

So, what’s the best solution to wash the product of cementing with Cu? Hot H2O then dilute nitric?

Edit for spelling

 

[eaglekeeper]

So, what’s the best solution to wash the product of cementing with Cu? Hot H2O then dilute nitric?

Edit for spelling

Zinc is fine.... As long as your converters are clean of any metal shavings from the de-canning, there shouldn't be any base metals. I've always used zinc, because I don't want to deal with the PGM's anymore than I have too.

After cementing on zinc, make sure you rinse the blacks until the wash water is a neutral PH.

 

[BAGross85]

Zinc is fine.... As long as your converters are clean of any metal shavings from the de-canning, there shouldn't be any base metals. I've always used zinc, because I don't want to deal with the PGM's anymore than I have too.

After cementing on zinc, make sure you rinse the blacks until the wash water is a neutral PH.

On page 165, Hoke says “”First, wash the black powder with unusual care…”

What is the prefered wash solution when the cementation is from AR, and cemented with Zn?

What is the preferred wash solution, when the cementation is from AR, and cemented with Cu?

On pg. 168 Hoke says “wash with great care”



So, Hoke uses the phrases “unusual” and “great” care, when talking about washings. I’m not sure what that means literally: any suggestions on techniques? I read through the forum and didn’t quite land on a method yet. I like the concept of dilution and decantation, as it allows the powder to remain in one vessel.

Edit for clarification

 

[eaglekeeper]

What is the prefered wash solution when the cementation is from AR, and cemented with Zn?

I just rinse with water...let it settle, then pour off (or use a pipette) the rinse water. I'll wash the PGM blacks three or four times then check the PH of the rinse water before decanting off. Once the PH test strip stays close to neutral, it's safe to assume that all the zinc has been removed.

Others may have different ways of cleaning PGM blacks.

 

[Martijn]

What is the prefered wash solution when the cementation is from AR, and cemented with Zn?

I would say "wash" with AR, since cementing on zinc is recovery imo, depending on the source material.

What is the preferred wash solution, when the cementation is from AR, and cemented with Cu?

Hot HCl, wash until HCl comes out clear. Then wash until pH neutral.

So, Hoke uses the phrases “unusual” and “great” care, when talking about washings. I’m not sure what that means literally:

Extra care, give it some tender love and care, don't rush it, being meticulous, etc.

 

[Yggdrasil]

If the cemented PGMs are very fine, HCl might not be good.
It may dissolve some Pd.

 

[eaglekeeper]

Yes, converters.

I’m confused, I thought zinc was one of the better choices for cementing in the presence of base metals. So, copper then?

Maybe I should mention... I use a small chunk of zinc, that way when the solution tests negative for PGM's I simply remove the chunk and wash any cement back into the beaker.

Zinc powder works just as well, but I stopped using it because I always managed to get the powder everywhere somehow.

 

[kurtak]

Zinc is fine.... As long as your converters are clean of any metal shavings from the de-canning, there shouldn't be any base metals.

There are "some" foreign made CATs that contain "small" amounts of copper

Kurt

 

[eaglekeeper]

There are "some" foreign made CATs that contain "small" amounts of copper

Kurt

Interesting...... does anyone know what purpose the copper serves in the process?

 

[galenrog]

Interesting...... does anyone know what purpose the copper serves in the process?

My understanding, and my understanding could be wrong, is that alloying copper to the catalyst (platinum, palladium, rhodium) saves quite a bit of money in manufacturing, although it likely reduces useful life of the catalytic converter.

Time for more coffee.