The electrowinning cell electrode question.

[SapunovDmitry]

Hello everyone. Time for another brainstorming....
If i had a silver nitrate/copper nitrate solution with silver (not a nitrate) content of 50 gr/l, and copper content of 60 gr/l and i wanted to make a silver cell what electrodes would i normally use? I am particulary interested in anode material. Stainless steel anode? Or it would dissolve?
Thanks in advance guys.

 

[nickvc]

Most stainless steel alloys if not all are fine in nitric acid.When I ran a silver cell I used a stainless drum as the anode, be aware it will dissolve if you have any hydrochloric in the mix.

 

[SapunovDmitry]

Yep, it really works with stainless at voltages below 2-2,3 volts. The anode is stainless bolt and the cathode is stainless 10 cm x 10 cm plate. I used it for about an hour but despite the current being at about 1,5 Amps there is no sight of silver crystals forming. Could it be because of the redissolving of silver? There still is some excessive nitric in the solution.

 

[nickvc]

My bad as usual I get things backwards with cathode and anode,it was the stainless drum as the cathode and a silver bar as the anode
The solution your using wil cause co deposition of metals as the silver content is too low and the copper too high. My advice would be to cement the silver out with copper, wash and clean it up, use some of the cemented silver to make your electrolyte afresh and the rest to use as the anode. A little copper in the solution helps the silver crystal form and make harvesting easier but too much copper and it will co deposit.
Hope that helps and my apologies for giving you the wrong information initially..

 

[SapunovDmitry]

We had experience with cementing out with copper. It gives about75-90% clean silver with other being Cu- 5-20% and Tin the rest which is not what i wanted. Usually i make silver chloride from the source solution and then melt it with soda, borax and the filter. There are several problems with that way. First of all i have a 300 ml crucible (2,5 kg of silver) and you wouldn't be able to melt much of chloride with furnace of that size because of the bublbing. The second one is that i use graphite crucibles which are rather expensive to use since in that process salt is formed and it really kills the graphite at elevated temperatures. That's why i think it is either buying a bigger induction furnace or making the silver cell. Cell is faster and cheaper.

 

[nickvc]

If your using clean copper bars you shouldn't get any tin and cement silver can be 95% + which is good for the electrolyte. You need to clean the cemented silver well use a vacuum pump and a decent funnel and filter, the copper in your cement is either from the copper used to cement, did you use small pieces, or left in the cement from the solution. I'm not sure you can selectively electrowinn from a mixed metal solution without very accurate voltage and ampage control and even then you won't get pure metals.
Personally I wouldn't try to form silver chloride on purpose it is a difficult material to treat, some material you can't help it but I avoid it if at all possible.
There is plenty of information here on the forum concerning cells, Ralph, Palladium did a really smart set up if you can find the thread which was good and small.

 

[solar_plasma]

I could be wrong, but I in my opinion an electrowinning cell is only recovering, not refining.

Both -
cementing on a massive piece of copper followed by intense washing or
precipitation as chloride followed by intense hot HCl washes and NaOH/sugar method reduction or
precipitation as chloride followed by intense washing and reduction by zinc/H2SO4 method

- will all work fine and will yield a purer silver from a dirty solution than any electrowinning cell will.

 

[nickvc]

Bjorn
You missed Lou's method for conversion from chloride, a few iron nails and sulphuric tumbled to ensure full conversion to metallic silver, personally I'd prefer to avoid large volumes of chloride as it is a pain to convert efficiently and completely.

 

[FrugalRefiner]

precipitation as chloride followed by intense hot HCl washes and NaOH/sugar method reduction

Washing with concentrated HCl will dissolve a bit of the silver chloride. Lou uses it as part of his gold washing procedure to remove small amounts of silver chloride.

Dave

 

[goldsilverpro]

Yep, it really works with stainless at voltages below 2-2,3 volts. The anode is stainless bolt and the cathode is stainless 10 cm x 10 cm plate. I used it for about an hour but despite the current being at about 1,5 Amps there is no sight of silver crystals forming. Could it be because of the redissolving of silver? There still is some excessive nitric in the solution.

At 50 g/l Ag, you should definitely be depositing silver crystal - 60 g/l Cu is not that excessive. However, if there is too much free nitric, the silver will redissolve (or, won't deposit). As the Ag redissolves, the free HNO3 decreases. I don't know exactly how much free nitric will prevent crystal formation, but I would guess about 5%, or less, by volume.

I could be wrong, but I in my opinion an electrowinning cell is only recovering, not refining.

I have never subscribed to this definition and never will. Technically, the word "refining" does pertain to the final purification of the Au to a minimum purity of 999.5 Fine. However, industry-wide, the word denotes any and all steps leading to this purification. Many e-scrap refiners are actually semi-refiners. They process the scrap until everything is in a form that can be sampled and assayed and then they ship it out to a copper smelter (usually) who finalizes the refining. These semi-refiners never call themselves "recoverers". They always call themselves "refiners". To me, an industry wide definition always takes precedence over a dictionary definition. I have worked for, consulted for, or owned 34 refineries in my life. Everything done in all of these always fell into the category of "refining". If we "recovered" anything, it was part of the "refining", not separate from it.

 

[solar_plasma]

Bjorn
You missed Lou's method for conversion from chloride, a few iron nails and sulphuric tumbled to ensure full conversion to metallic silver, personally I'd prefer to avoid large volumes of chloride as it is a pain to convert efficiently and completely.

My fault, it was Lou's method I meant. I confused it with cementation on zinc, which I read somewhere and tried, before I came to the forum.

Dave, I sometimes used water with some HCl in it. I did not mention concentrated HCl. But you are right, thinking about it, there seems not to be a reason for using HCl in this case. Thank you for correction.

Chris, I weigh your opinion about chemistry and metallurgy higher than anything I ever would read anywhere in books and texts from authors I do not know as much as I know about you, so thank you for clarification. Wouldn't you agree, that electrowinning in this case and in this setup would hardly yield a purity compared to chloride precipitation or cementation - or at least would cause higher efford doing so?

 

[g_axelsson]

As the discussion is dealing with electrowinning and not a silver refining cell you have to take the anode reaction in consideration too. Electrowinning from a nitrate solution would to a certain extent also create new nitric acid from the nitrate ions. (Other anode reactions might happen, depending on the composition of electrolyte and anode.)
I would have expected some silver at least on the cathode where it should be protected from the acid by the electrical potential, but if it falls off it would be redissolved, especially if you already have an excess of nitric acid.

This is just my view of what should happen. I haven't run any electrowinning of silver. The last time I played around with electrolysis and nitrates I definitely felt the smell of nitric acid when the cell was running. Sorry, no details as it was 15 years ago.

Göran

 

[SapunovDmitry]

Looks like everything is okay now. I got rid of some excessive nitric (as many of us have suspected) with urea and it did good for the process. Silver started to deposit. We'll see when it will stop and what the final weight will be.

 

[goldsilverpro]

You will never be able to plate out 100% of the silver. There will always be at least a little left in the solution that will have to be recovered by other means.

 

[SapunovDmitry]

Yes, i think that it will be good until silver content of about 0,5- 1,5 gr/l since there is a considerable copper content and at given copper concentration silver and copper eh-ph diagrams will have the same eh of precipitation. So copper will start to co-precipitate.

 

[solar_plasma]

So copper will start to co-precipitate.

This is was what I meant. Please remember to tell us your experiences with this experiment and how it worked out. I would love to hear about it and to learn in what cases it is useful.

 

[abolfazlsadeghi34]

hi friends
I had an ore containig /2gram au and some copper ,iron, ag per kilo. I dissoled 5 kilos of this ore by 10 leters of aqau regia. after heating for 6hour and agitating I filtered the solution . I want to electrolisis the solution to get gold. I want to know the density of solution the density of acids(nitric and hcl) and......
please help

 

[nickvc]

With all electowinning it’s almost impossible to remove just one metal unless you are using very high purity solutions and high purity feed stock so I doubt you can remove just the gold from your solution using this method.

 

[butcher]

dissolving the ore in acid sounds like a dangerous way to waste acids and put yourself in harm.

silver chloride could be collected in the filter with your gangue.
You could use copper to recover gold.
Iron to recover copper.
and treat the toxic waste for proper disposal.