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Topic: Silver purification from silver chloride and silver nitrate  (Read 43836 times)

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Offline flightman233

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Silver purification from silver chloride and silver nitrate
« on: April 01, 2009, 11:03:23 PM »
Hey Guys,

I've got a rather interesting question.  I'm sure there is something that I am missing here.

What would be the simplest method of purification of silver from silver chloride?  I've thought about this in a number of ways, and my initial trials of reduction have shown little success.

I have tried to take some of the silver chloride, and heat it while it was mixed with carbon to see if it would reduce to silver metal and I am almost positive that got me no where.

After mixing some with sodium carbonate, I actually got a small amount of silver from this reaction.

Other thoughts included running an electroplating system on it (read somewhere else to use graphite anode, and stainless steel cathode).  This idea seems the most prudent now, because my other thoughts or methods didn't work out so well so far.

I have silver nitrate, and i added copper wire (grinding this down into powder was more difficult that it was worth), and the silver obtained here was somewhat littered with copper flakes and such.  To purify this product I am considering hitting it with hot concentrated sulfuric acid to dissolve the copper and would leave me my desired silver product (assuming it was filtered hot and such).  Any ideas on purification of this product?  Other methods perhaps?

Thanks guys, look forward to hearing your response.

Offline AWK

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Re: Silver purification from silver chloride and silver nitrate
« Reply #1 on: April 02, 2009, 01:28:22 AM »
AgCl is soluble in ammonia, even diluted.
Both Cu and Ag are soluble in concentrated H2SO4

Offline flightman233

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Re: Silver purification from silver chloride and silver nitrate
« Reply #2 on: April 02, 2009, 08:14:45 AM »

This presents an interesting problem... I knew that Cu was soluble in sulfuric, but this is only if it is hot, or near boiling... now if silver is too then I can't use this to purify the copper loaded product that I have collected.

If silver chloride is dissolved into ammonia, and then electroplated, then that appears it will be one of the only ways to get the purest silver back out of the compound..

Do you know of anything else that can be used for the purification?

Offline Fleaker

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Re: Silver purification from silver chloride and silver nitrate
« Reply #3 on: April 03, 2009, 09:35:38 PM »
Since you seem to have done your due diligence, I'll help you out.

Having refined more than quite a few ounces of silver, here are some things I've learned:

Avoid silver chloride like the plague. If it can't be avoided because you want to separate it from lead contamination, do not form the diammine silver (I) complex in concentrated ammonia. Instead, wash the lead (II) chloride out with several portions of very hot water and save your ammonia. To make sure all the lead is out, check the last was with either sodium chromate or KI solution, a yellow ppt indicates Pb (II) and you need to keep washing.

If there are other metals present, dissolving in ammonia is desirable. Acidification after filtration with nitric or hydrochloric acid will return the silver chloride. Recall that a blue colour indicates copper contamination. Assuming you just want pure silver from your AgCl, you're best off to add some more base (i.e. NaOH) and reduce with a suitable reducing sugar, namely dextrose. You may also stop at silver oxide, and avoid the sugar reduction by simply heating until the silver oxide disproportionates (350oC is a good hold temp, then proceed to melt the silver powder).

Silver chloride should be avoided simply because it is very difficult to manipulate--making Ag2O from it is exceptionally troublesome if the AgCl is allowed to dry because it will clump up and you will find that it does not react. If you want to get it to silver metal without using the aldose reduction method, I suggest you utilize sulfuric acid and zinc or iron strips with heavy stirring. This will take a few hours, and if using anything but the purest iron and zinc, it will stink due to phosphine and hydrogen sulfide formed from impurities in the reductant metal.

I should advise that you should never attempt to melt silver chloride, it will volatilize and end up in your lungs and all over the place. If you do attempt to melt it, make sure you melt it with a gross excess of sodium carbonate with a slight amount of borax, which will reduce it to silver metal. Unfortunately, sodium carbonate is extremely aggressive on most refractory mixes, so your crucibles will suffer.

While I do not know your feedstock that you are obtaining this silver from, I can advise that the easiest method to obtain decent 99% pure silver suitable for the Thum cell is to use copper as your reductant (cementation).

Make a concentrated silver nitrate solution, put in a large bar of copper metal and let it sit. Periodically scrap the silver off the copper with a car window scraper. The shiny silver crystals collect on the bottom of the tank/beaker/drum. Do NOT introduce aluminum as it will add an extra step because you'll need to boil your silver with concentrated NaOH to remove the hydrous alumina that will inevitably precipitate. That is hard on glassware and a safety risk to your eyes.

Once you have the crude cement silver, usually about 99% pure, you should rinse well and cast it into an anode and electrorefine it in a nigh saturated solution of silver nitrate acidified with 30 mL or so 16M HNO3 per liter. I forget what current density I use, but an offhand voltage is about 3-5V depending on electrode gap/temperature etc.

Your electrolytic silver should run 999 fine or better (usually 99,97) depending on if your cement silver was free of palladium and nickel.

Hope that helps.
Neither flask nor beaker.

Offline BluRay

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Re: Silver purification from silver chloride and silver nitrate
« Reply #4 on: April 06, 2009, 05:15:19 PM »
What would be the simplest method of purification of silver from silver chloride? 
Don't know if it can be useful for you, however in some precious metals firms they get silver from AgCl by heating it with a much greater amount of soda mixed:

2AgCl + Na2CO3 --> Ag2O + 2NaCl + CO2

at high temperature (but don't know the value), Ag2O decomposes into Ag:

2Ag2O + heat --> 4Ag + O2

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