Cu instead of Al / Zn in cementing Pd ?

[9kuuby9]

Cu instead of Al / Zn in cementing Pd?

If you take a good look at the reactivity series you will notice that most of the base metals are between (Al / Zn) and Cu (the base metals in between);

Cr Cr3+ -0,74
Ga Ga3+ -0,560
Ga Ga2+ -0,45
Fe Fe2+ -0,441
Cd Cd2+ -0,404
In In3+ -0,3382
Tl Tl+ -0,338
Co Co2+ -0,28
In In+ -0,25
Ni Ni2+ -0,234
Mo Mo3+ -0,2
Sn Sn2+ -0,141
Pb Pb2+ -0,126
H2 H+ 0
W W3+ +0,11
Ge Ge4+ +0,124
Sb Sb3+ +0,240
Ge Ge2+ +0,24
Re Re3+ +0,300
Bi Bi3+ +0,317

So isn't it more efficient to use Cu instead of (Al / Zn)? [So that you don't have to digest the base metals in the next recovery process.]

Or doesn't Cu cement Pd as good as (Al / Zn) because of the higher reactivity?

Any helpful info is welcome!

 

[lazersteve]

In theory your proposal sounds good, but in reality, the reason we use the more reactive metals when cementing mixed PGMs is two fold:

1) The higher reactivity metals are easier to remove from the resulting mixed PGM blacks, resulting in cleaner blacks.
2) Copper, because of it's lower reactivity, has problems getting the last of the PGMs out of the solution in a timely manner.

One must remember that the reasons for using a particular reagent are not always evident from charts and graphs, even though this type of data gives an excellent starting point for setting up small experiments for testing our theories. What looks right on paper is not always what works best in the lab. It comes down to the difference between theory and practice.

Steve

 

[4metals]

Steve's advice is excellent, all things are not what they seem. Another reason zinc is better is the availability of fine zinc dust powder, the surface area of the zinc makes fast work of the drop. I have been using fine copper dust for dropping silver from silver electrolyte and it reacts so much quicker than slabs of metal but it is pricey and not easy to find but the reactivity rate is due to surface area.

You also have to consider the solubility of the metal you are using to drop the PGM 's and the solubility of the PGM's in different solvents. That is another plus in the zinc dust column. The excess copper will not come out without some of the PGM's in the same solution.

 

[9kuuby9]

Thank you for clearing that up steve; greatly apriciated.

I've also noticed that their is a big difference between the theoretical and the practical side.
In theory we are able to discuss things on a clear and proper view whereas in the practical side a lot of conditions have to be met before it can actually take place and the verity of factors is too great in comparison with theory.
In theory we're able to dismiss these conditions and factors because they don't exist unless you add it to the given theory.

 

[9kuuby9]

An observation from an experiment.

A clean solution only containing Ag nitrate and Pd nitrate. Addition of HCl to fully participate Ag as AgCl. This reaction creates HNO3.
after filtering Al is added to the filtrate. Al starts reacting with the leftovers of HCl and cements Pd at the same time. Pd is digested by the free nitric acid in the solution.
This keeps taking place until no nitric acid remains in the solution. Then the Pd cements successfully. after some hours the solution is decanted and Pd is left at the bottom.

I added some Al in the decanted filtrate and left it for around 4 days. after careful observation I noticed Pd blacks at the bottom of the vessel that holds the solution.

Does Al take that long to almost fully cement Pd from the solution? Since Al(-1,700) has a higher reactivity then Zn(-0,763) in theory Al should cement Pd twice as fast as Zn.

 

[FrugalRefiner]

Does Al take that long to almost fully cement Pd from the solution? Since Al(-1,700) has a higher reactivity then Zn(-0,763) in theory Al should cement Pd twice as fast as Zn.

A cementation (replacement) reaction only happens when the ion in solution comes in contact with the solid metal being used to replace it. Stirring or other agitation helps, but it can take a while for every ion in solution to "bump into" the aluminum, regardless of its potential reactivity.

Dave

 

[Marcel]

I dont like the highly reactive metals such as zinc or aluminum, because the reaction is quite strong and tends to foam over if your vessel is not large enough.
After precipitation it is easier to remove those metals though by simply adding HCl. Copper takes longer and theoretically even CuCl.
Btw. I use my old AP/CuCl to precipitate the last little bit out of the solution after dropping with copperbars or powders. I use a stirrer and let it work for several hours. I doubt that there are large amounts of Pd left, but one should simply add it to the stockpot and turn to more profitable activities.

 

[9kuuby9]

Thank you both for your explanations!

 

[solar_plasma]

I dont like the highly reactive metals such as zinc or aluminum, because the reaction is quite strong and tends to foam over if your vessel is not large enough.

Can the foaming be avoided for sure by adding the zinc in small portions?

 

[9kuuby9]

I don't like the highly reactive metals such as zinc or aluminum, because the reaction is quite strong and tends to foam over if your vessel is not large enough.

Can the foaming be avoided for sure by adding the zinc in small portions?

Well that sure will make it less foam.

But It really only reacts heavy when the solution is hot and because of the reaction it maintains it's high temperature and may introduce the given problem.
Because when I added plenty of Al to a cold solution; the reaction was rather slow and didn't react that heavy to introduce over foaming.

 

[solar_plasma]

But It really only reacts heavy when the solution is hot and because of the reaction it maintains it's high temperature and may introduce the given problem.
Because when I added plenty of Al to a cold solution; the reaction was rather slow and didn't react that heavy to introduce over foaming.

When I have come so far, I think I would cool it and ofcourse add Zn slowly. Last one is almost always a good idea when doing something new. :lol:

 

[9kuuby9]

But It really only reacts heavy when the solution is hot and because of the reaction it maintains it's high temperature and may introduce the given problem.
Because when I added plenty of Al to a cold solution; the reaction was rather slow and didn't react that heavy to introduce over foaming.

When I have come so far, I think I would cool it and ofcourse add Zn slowly. Last one is almost always a good idea when doing something new. :lol:

Always prepare for the worst :lol: ; for instance a large enough beaker so when it does happen it does not spill over :shock:

 

[9kuuby9]

Would Fe be a good alternative in cementing Pd from a nitrate based solution instead of Zn?

Al is almost completely inert in nitrate based solutions.

Any help would be appreciated! :mrgreen:

 

[9kuuby9]

New observation, Fe will not cement Pd & Ag from a nitrate based solution.

I've filed the surface to expose the bear metal, it Cemented some Pd. afterwards (some 10 seconds) it stopped cementing and formed an oxide layer to prevent further cementation.

Ph measured at 0.5 to 1. (ideally it should be 2-3 for cementation)

Copper however did cement the Ag & Pd from the nitrate solution; Ag cements as fluffy crystals and Pd cements as a black thin film.

Has this ever occurred to someone else? Maybe I've missed some knowledge concerning "cementing" ?