Questions about precipitating with Zinc, when is enough?

[Gs70350]

Ive leached some catalyst material that I had sitting in a bucket for a long while, it had a few crushed up cats inside. I performed the process of HCL/CL that lazersteve describes in his video outdoors in a rubermaid tote and so-on. All the colors looked spot on for the palladium in the first batch, more orange on second pass for platinum?, and so on. On the first pass I ran honeycomb only using a 1/8 inch mesh sieve. Second pass incorporated all the ground up fine mesh material that was not processed at first (heeding the whole filter nightmare scenario). After getting 4 grams of black pgm powder/sponge from the first batch, i was impressed. I did not measure the zinc but used zinc metal, not powder to precipitate. This worked out with less fine PGM powder in the end, easier to filter/cleanup. The zinc seemed to get used up fast. It took a lot of it. I figure a few ounces of zinc were added in all, to get the pgm powder that I have. How much zinc is really needed?

The second batch I added more zinc and also used a lower ph, more soda ash before precipitating. I did not use much the first time since i cut the acid with water. This time I processed about twice as much material, half extra being fine dust/crumbs. This plus the previously leached honeycomb. I ended up with about two to three times as much liquid solution in the end.
After a day and a half or so the solution turned clear, and all black powder in the bottom. This time I recovered 10.2 g of black pgm powder. Is this possible? Or is this bunch of black zinc? I added quite a few oz zinc as well this time..

 

[solar_plasma]

I have no experience with this process, just read. But I know zinc, and it will readily dissolve in HCl, producing hydrogene gas. So one drop HCl on a sample of the washed black powder would show.

Though I do not believe, there will be much if any zinc in it.

when is enough?

enough is, when your tests do not show any pgm's in the solutions anymore

 

[lazersteve]

Have you confirmed your zinc does not contain Cab-O-Sil?

Steve

 

[Marcel]

I have used both: Zinc in solid form and very fine zinc powder.
It is a foolproof precipitant, that knocks down anything in solution. But it can be quite tricky.
Image you could follow the process with bare eye. The fine zinc dust gets covered with PM atoms until it shields the inner core from the solution. That happens when the powder is very fine, but even with solid zinc bars that can happen.
So when all the precipitation has ended ( As you said you receive a clear or light purple solution ) There might be still zinc enclosed in the black powder. That is why a good wash with HCl is needed afterwards and will reduce the mass. If you used nitric you cannot do this because it will create new AR and redissolve your PMs. But with HCl+Cl a good HCl wash afterwards is possible.
I know what you mean by: When is the reaction over, do I have to put in more zinc or not?
The solution will still react even when all the PMs have been precipitated long ago, because zinc is so reactive!
So you will always add more zinc than needed to replace the PMs in solution. That is normal. But then again you will have to wash the result with HCl throughly.

Because of that I have switched to using copper and time as main precipitants. It is a slow reaction and will require a few days to complete, but it is very reliable and wont bring down any metals inbetween like zinc will. Agitation and heat will speed things up and allow you to control the reaction a bit.
With zinc again, it creates so much heat by itself, once the reaction has started you can only watch it until it is over.

Btw. I use zinc from toys (cars, toyguns etc.) that my children dont play with any more ( I hope at least).
It is a constant source for me... :lol:

 

[Gs70350]

Ok well that's reassuring I will purify with an hcl wash and rinse again and recheck the mass. Sounds logical.
In my case I've been using zinc that I melted down into large chunks, skimming off the copper slag on top. I'm sure you know what I am using with this information. I don't know if cab o sil is found in my zinc source or not, but as I said is is fairly pure as I'm only heating to the melting point of the zinc, copper and whatever else floats and is skimmed off. So wash in hcl to remove remaining zinc, rinse with distiller water, dry and remeasure the mass. Got it!

 

[kkmonte]

What I learned when you make your first HCl rinse, use diluted cold HCl or it could dissolve some palladium if you have any there, then do you 3x distilled rinses.