Sciencemadness Discussion Board

Filtering

Terrence - 25-5-2012 at 03:26

We Have a problem. We make silverbars. The problem is that we disolve impure silver in a nitric solution.When we filter the solution to remove the impurities even using a .25 micron paper some of the impurities still go through. Is there a floculant or another method to extract these impurities

barley81 - 25-5-2012 at 03:49

What kind of impurities? Soluble ones like copper will not be stopped by filtration. You can remove these chemically. Frequently it is more convenient to remove the silver from the solution rather than the impurities.
A short method that some people have used here:
Filter to remove insoluble impurities. Add NaCl to precipitate AgCl. Filter the AgCl. Boil with dextrose (or formaldehyde) and NaOH to reduce the AgCl to silver metal. Melt down silver metal.
(This was from memory, I might be missing something)

You can search the forum for silver purification methods to get more info.

99chemicals - 25-5-2012 at 14:55

Use two layers of filters.

Or make a setup where you can decant the silver solution from insolubles without disturbing the layer. Then you could filter the remaing solution and precipitate siver chloride to use in your next batch.

CHRIS25 - 25-5-2012 at 15:11

If the impurity is copper, then you can remove that by inserting a copper wire into the solution after the reaction appears consumed. The silver will attach to the copper piece and you will be left with copper nitrate in solution, but also some silver. The sludge that you can scrape off the copper can be set on a burnt piece of wood and heated to form a piece of silver, This can obviously be further refined. But what impurities do you think you have?

barley81 - 25-5-2012 at 17:40

If you use copper wire, then frequently copper adheres to the silver crystals and you don't get pure silver.

It really does sound like your impurities are soluble if they pass through a 0.25 micron filter paper, so any amount of filtration won't remove the impurities.

CHRIS25 - 26-5-2012 at 03:24

Hi, Yes you are right Barley - absolutely - but I have come to understand that this amount of copper is negligable in as much that after this reaction your silver is increased from 92.5% say to 97 or 98% purity. Ok, but maybe he wants 100% purity which is in fact 99% in actual practise, which is by far a very pure product, so 97 or 98% should be adequate?

barley81 - 26-5-2012 at 06:26

For some purposes it is more than adequate, but silver bars are usually 99.9+% in purity. If the silver made by displacement with copper were to be used to make sterling silver, it would be suitable if the % copper could be determined. There's enough copper which adheres to the silver to make a solution of the silver in HNO3 blue, and that would interfere with many experiments.

Terrence - 27-5-2012 at 22:06

Thanks guys for all the help. We put the silver into solution using heated HN03. after all is disolved we filter it. Sometimes it can take up to 5 or 6 filtering before it is clear. We have tried even 3 filter papers but we still get a very fine residue going through which we suspect is Iron and Zinc. we are trying to get +999 grade silver

peach - 28-5-2012 at 10:48

Hi Terrence,

You could achieve submicronic filtering with membrane filter elements. Link to example from Gore.

Entegris provide filters down to the single nanometer range.

A more affordable option might be use standard micronic papers / sinters but run it through again (key being, without emptying the filter or changing the element). The filter will gradually clog it's self down towards that range. Not sure if that's what you mean when you say you run it through 5-6 times.

Entegris 3 and 5 nanometer UHDPE filter element:


{edit}What's the silver for by the way, is it audio related?

[Edited on 28-5-2012 by peach]

unionised - 28-5-2012 at 12:07

The traditional approach would be to use a filter aid like celite but that would probably add too many impurities here.
You could try adding a little HCl or NaCl solution to form a ppt of AgCl then filtering that off and hoping the impurities are trapped in it.
Obviously, you would need to reclaim the silver chloride but that's not a big problem.

If the solution is near neutral then you could possibly use hydroxide or carbonate to ppt some of the silver and filtering that of. If it traps the impurities that's good and it's easy to reclaim the silver by dissolution in nitric acid.

bahamuth - 28-5-2012 at 13:11

Quote: Originally posted by Terrence  
Thanks guys for all the help. We put the silver into solution using heated HN03. after all is disolved we filter it. Sometimes it can take up to 5 or 6 filtering before it is clear. We have tried even 3 filter papers but we still get a very fine residue going through which we suspect is Iron and Zinc. we are trying to get +999 grade silver


The only viable solution I see here is what is already mentioned.

Having tried a lot of methods of cleaning silver from a nitric solution, the precipitation with sodium chloride then reduction with NaOH/sucrose (IIRC achievable purity is above 98% when alot of other metals are present) is the best way by far. If you have particles they are not of metallic origin (iron and zinc are both very soluble in nitric acid) and if they are organic you may want to combust the silver before dissolution in nitric acid sol.

Terrence - 29-5-2012 at 00:45

Thanks

I will give these options a try