Recovering precious metals from polishing sweeps

[silveras]

Hi Thanks for the info. I mixed the 10 kg pulverized, burnt polishing waste with 17 liters of water plus 3 liters of 70% nitric acid. I had it leached for 4 days. I strained the solution and rinsed the slurry with water several times. I actually have total of 80 liters of solution. How do I go about cementing the silver? Do I add more water? If so how much more? What is the ideal ph of the solution for cementing? Can I use zinc dust instead of copper to cement? Thanks.

 

[Harold_V]

How do I go about cementing the silver?

If the solution is relatively free of solids, all you need to do is put in pieces of copper. Don't use wire unless you're sure you will consume all of it. Otherwise as it gets smaller, it breaks in to small lengths, which are virtually impossible to remove because they're hard to find. That sort of defeats the purpose of cementing the silver, which will yield quite good quality, assuming you've done things right.

Do I add more water? If so how much more?

If you've added rinse water to the original solution, no, don't add more water. It should already be quite dilute. I used to cement from relatively concentrated solutions. It can be troublesome in that on occasion the silver will plate out as a solid surface, adhering to the copper doggedly. It's best when it cements as fine particles, which yield the general appearance of (Portland) cement, which is the source of the name we use, at least as I understand it.

What is the ideal ph of the solution for cementing?

Don't know that I can answer that. I was never too concerned, and on occasion dissolved some copper because there was free nitric present. It does no harm aside from consuming copper. Eventually it achieves equilibrium where no copper is lost to dissolution from acid, just from cementation.

Can I use zinc dust instead of copper to cement? Thanks.

Yes, you can---but that's a mistake. Zinc will cement copper, too. At first the copper may be consumed, cementing silver, but eventually you'll end up with copper and silver mixed, which defeats the purpose. You'd be far better served to recover the silver as silver chloride than to use zinc----with the sole exception being if you intend to re-use the recovered silver for inquartation, it doesn't make too much difference. The one negative (if you use the recovered material for inquartation) is the reason you use silver instead of copper, however, in that copper requires almost three times as much nitric to digest as an equal amount of silver.

Harold

 

[4metals]

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