could you use NaOH to help get rid of AgCl

[ericrm]

i know that it is not a recommended procedure ,but could i use sodium hydroxide to remove silver from silver chloride/gold powder mix.
my opinion is yes but i wish to have your opinion.

i start with wet silver chloride and gold powder that i wash with water to dilute as much possible the hcl content (it will still have some for sure)
than i add sodium hydroxide witch will turn the silver to silver oxide and watever little hcl left to salt and wash again.
i will add nitric acid to dissolve the silver oxide than decant and wash gold powder

my main concern is: will the salt left even in this minimum amount will be enuf to make a ssn and dissolve gold or make again silver chloride? or will it simply do nothing because of something that i dont know?
***The SSN leach will convert gold to sodiumaurochlorate NaAuCl4***

yes i could use amonia to dissolve silver but i wont.
yes if i had filtered my solution that would not be a probleme
yes i could simply redissolve the gold back and filter the silver Chloride
but that not the point...

 

[NeMonstr]

For example. Stir the dry powder with sodium hydroxide or carbonate and fusion. Silver recovers. Alloy dissolve in nitric acid.
Alloy does not dissolve if a lot of gold. Perhaps this is a difficult path. Dissolve silver chloride easier.

Melt alkali or soda can dissolve crucible.

 

[butcher]

My thoughts are you could convert the silver chloride to silver oxide with the NaOH, (or by one of the other silver chloride conversion methods), and rinse out most of the chloride salts that formed in the process with several water washes, but my concern would be you may have a hard time trying to wash all of the chlorides out, there could also be some silver chloride left that did not quite get completely converted, so before adding nitric acid to dissolve silver oxide, I would heat it strongly and then incinerate to help remove any trace chloride, so as not to dissolve the gold when nitric is used.

Another thought is to melt it as silver oxide, or a fusion as suggested above by NeMonstr, and use nitric to remove silver from the gold.
Or if not much too much gold in the silver you could recover the gold in the slimes of the silver cell.

Another thought is you already have silver chloride, just use HCl/NaClO and dissolve the gold from the silver chloride powders, rinse the silver chloride well, and then convert the AgCl to silver.

 

[ericrm]

Butcher
if i dissolve the gold from the sample ,will i by rincing the silver chloride in hcl manage to get enuf of the gold chloride out to make an accurate yield test on both the silver and the gold?

 

[NeMonstr]

Boiled in sulfuric acid. Silver sulfate dissolved in water.
2AgCl+H2SO4=Ag2SO4↓+2HCl↑
2Ag+2H2SO4=AgSO4↓+SO2↑+2H2O

 

[ericrm]

Boiled in sulfuric acid. Silver sulfate dissolved in water.
2AgCl+H2SO4=Ag2SO4↓+2HCl↑
2Ag+2H2SO4=AgSO4↓+SO2↑+2H2O

i tought that silver sulfate was insoluble ,but i was mistaken AgSO4 with Ag2SO4, thanks NeMonstr

how would you recover the silver? cementation with copper as with AgNO3? or can I precipitate it with some simple chemical?

 

[butcher]

Boiled in sulfuric acid. Silver sulfate dissolved in water.
2AgCl+H2SO4=Ag2SO4↓+2HCl↑
2Ag+2H2SO4=AgSO4↓+SO2↑+2H2O

I do not think this would be the best approach, boiling hot concentrated sulfuric acid with a pot full of silver powder bubbling and splattering hot solution, is just not too safe.

To answer the question you could dilute the silver sulfate and add NaCl, to convert it back to silver chloride.
Ag2SO4 +2NaCl --> 2AgCl + Na2SO4
http://books.google.com/books?id=9HnPAAAAMAAJ&pg=PA506&lpg=PA506&dq=Ag2SO4+%2B+2NaCl+--%3E&source=bl&ots=lngBfeF7Z5&sig=3Ey6ZmWMTbURgKn-XOORF4F1hfg&hl=en&sa=X&ei=c6nEUpLkDI74oATAwIHgCg&ved=0CHQQ6AEwCA#v=onepage&q=Ag2SO4%20%2B%202NaCl%20--%3E&f=false

You can dissolve the gold from the silver chloride solution, you may have to stir the solution well, the silver is already a silver chloride, so the solution to dissolve the gold will not do much to the silver chloride, with good stirring and warming to get the fine gold in contact with solution, you should be able to dissolve the gold leaving the silver chloride,decant the gold solution, rinse to wash the silver chloride, and convert back to silver, Heat the gold solution remove excess chlorine, and let it sit overnight for any AgCl to precipitate, decant and filter the solution and them precipitate gold.

 

[NeMonstr]

can also try not to boil, and just strongly warming.

 

[ericrm]

aww , i tough that dilute hot sulfuric would work... why would the hot concentrated sulfuric acid work and not the hot 30% sulfuric acid?

 

[butcher]

2AgCl+H2SO4=Ag2SO4↓+2HCl↑

In the above reaction we can have a dilute sulfuric acid in with our silver chloride, basically not much would happen without heat, we would need heat to drive the reaction to the right, notice on the right side of the reaction we are driving off HCl as a gas (↑) or (g),the (g) or the arrow means gas.
The HCl forms acid in solution (but will reform AgCl if left in solution), heat is use to decompose the HCl acid to force it to leave the solution as HCl gas. The H2SO4 has a much higher boiling point and will not leave solution or begin to decompose until almost all of the HCl has been removed, this leave the silver to form a silver sulfate.

2Ag+2H2SO4=AgSO4↓+SO2↑+2H2O↑
In the reaction above silver metal is dissolved in Hot concentrated sulfuric acid, hot enough to decompose the sulfuric acid to SO2 gas and 2H2O vapors, again forming silver sulfate.
this reaction is dangerous to perform...

 

[NeMonstr]

search for other complexing silver and silver chloride. does not dissolve the gold in which.

 

[solar_plasma]

Isn't the ammonium complex method safe, as long as you are converting the silver back to silver chloride within some minutes (and as long as the solution tests negative for gold)?

Btw.: Making silver mirrors is a common school experiment, though there you probably are handling minor amounts of silver, which may be the difference.

 

[ericrm]

the reson why i dont want to use amonia ,its because i try to remove it from my stock pot... when i deal with my waste i cement copper and turn the rest to hydroxide, but im scared that amonium chloride would find a way to get back as hydroxide and form some weird explosive compond ... and if i dont put it in my stock pot i would have to pay to discard it wich im not interested to do

 

[ericrm]

Butcher. only to the purpose of turning silver chloride in soluble silver sulfate will the hot dilute 30% sulfuric acid work?

Edit: ok i understand your post ,hot dilute sulfuric acid will work but i have to wait that all the hcl is evaporate... but wont the hcl form an azeotrop mixture with water refraining total evaporation of the hcl?

 

[solar_plasma]

because i try to remove it from my stock pot

now I see! I wouldn't put it in my stock pot either. I treat acidified solutions containing ammonium separate at work.

 

[butcher]

If you use 30% H2SO4 what happens as you heat the solution (with stirring) is the H+ hydrogen (from H2SO4, and H3O hydronium ions) takes on some of the Chlorides and forms HCl in solution with the dilute H2SO4 and AgCl, water evaporate off this acid mix until the HCl concentrates to its azeotrope the HCl gas vapors off, this continues until almost all of the HCl is removed, as this goes on the H2SO4 concentrates and taking the chloride from the silver.

H2SO4 has a high azeotrope around 98%, and as we see with the process basically drive out the water (free water at fist), the water with the 32% HCl gases, so basically before we drive out all of the HCl we can concentrate the H2SO4 in the heated reaction to become dangerous to be around especially without proper safety gear.

To help keep it safer before all of the HCl has vaporized off we can add some more 30% H2SO4, this will add some more acid, and with the water involved in the dilute acid, will help to keep the solution of hot sulfuric at a safer level of concentration, more liquid left with the silver powders can also help these powder s from bubbling and popping so much splashing acid everywhere, HCl has a low azeotrope around 32%, so our H2SO4 concentration would be somewhere around this.

Having an excess of H2SO4 will do no harm.

Note insoluble powders in volume do not like to be heated in acids (without constant stirring), what happens is they settle to the bottom of the dish, they push most of the acid out of them, they sit on the bottom of the dish insulating the cooler acid above them from the heat, making the bottom of the dish hotter, too high of a heat can burn some powders or salts to the bottom of the hot dish, the little acid at the bottom of the dish decomposes to gases, the powder layer traps and hold down these gases until they build enough pressure to lift the heavy silver powder, they the shove there way to the surface with all of that power and explode in a bubble of acids spitting and carrying acids and salts of metals with them.

Constant stirring to keep the powders suspended, and keeping the heat down to the just enough heat to drive off the HCl will help, but standing over a pot of hot fuming acid stirring all day is no fun, and if you do not have a fume hood even doing this outside is difficult, I suppose if you had a good fume hood, and a heated stirring hot plate and done just a few powders at a time it would be a bit easier.

Even dilute H2SO4 is some nasty stuff, get it hot and it really becomes nasty, I do not know how many blue jeans I have washed later to find them full of holes and the cotton rotten from working as a mechanic around car battery's, and many times I did not know I had that acid all over my clothes until I washed them.

If you do heat sulfuric acid you should dress for the occasion, full face shield, long rubber gloves made for dealing with sulfuric acid, fold cuffs so acid does not run up the glove when you lift your hands and run back down into the glove, a full apron made for acids, boots with no open tops where acid could fill your boot and burn your foot off your leg before you could get the boot off, and a good system to keep from breathing the fumes, lab techniques are just as important in safety, proper equipment to heat the acids in, knowledge of thermal shock of vessels, catch basins... you do not want a broken vessel of boiling hot acids dumping all over you, or to be holding a hot vessel of acids and have the slip right out of your hands...

Even with the best safety precautions accidents happen. preparing for an accident can help, when an accident happens, but it will not always save you from harm, so if possible not getting yourself into that dangerous situation is better, than even the best safety gear you could use.

 

[ericrm]

thanks Butcher, as soon you said that i would have to deal with 98% sulfuric acid ,i was already gone lol. i hate that stuff im too clumbsy for it...

but if i go back with the beginning idea how much gold can chloride trace could realy dissolve, were talking trace maybe les than 1/1000 of a gram(i dont know what trace realy is as a ppm number) but if it is this low i guess i wont mater much

edit found it...In analytical chemistry, a trace element is an element in a sample that has an average concentration of less than 100 parts per million

lets say that i have 1 g of salt ,and dilute it in 100g of water, after that i decant my solution i will be left with maybe 10g of salt/water at 1% salt.than i add 90g of water i will have 0.1g in 100g of water,than decant and add water ... so after 3 wash i will be at 0.001g ... how much gold can realy be made with this little salt quantity ...

1g in 100
0.1 in 100
0.001 in 100g

 

[freechemist]

According to my experience, the best method would be, to stir the mixed gold/AgCl-powders, after thorough washing with water, and still wet, in excessive sodium- or ammoniumthiosulfate solution. Metallic gold will remain untouched, whereas silver chloride will be dissolved slowly as a complex thiosulfate compound, easily soluble in water.

AgCl + 2 S₂O₃^2- ==> [Ag(S₂O₃)2]^- + Cl^-

The gold can be recovered by filtration, thorough washing with water and further purification with AR, if needed. The silver can be recovered by reduction (Zn-powder, Fe-filings, steel-wool, electrolytic) or by 1.) precipitating as black, insoluble silver sulfide, Ag₂S, followed 2.) by roasting at about 600^oC in an oxygen-rich atmosphere, to obtain spongy metallic silver.

2 [Ag(S₂O₃)2]- + S^2- ==> Ag₂S + 2 S₂O₃^2-
Ag₂S + O₂ ==> 2 Ag + SO₂.

 

[ericrm]

i guess the sodium thiosulfate ,will become silver thiosulfate than zinc thiosulfate? but what will i do with the zinc thiosulfate pay to discard? or does it have a simple and cheap way to be back in the stock pot for copper/metal recovery

 

[freechemist]

I am very sorry, to have given wrong reaction equations for dissolution of silver chloride with thiosulfate, so I correct them here.

Corrected version for dissolution of AgCl:
AgCl + 2 S₂O₃^2- ==> [Ag(S₂O₃)₂]^3- + Cl^-

Correct equation for precipitation of silver sulfide:
2 [Ag(S₂O₃)₂]^3- + S^2- ==> Ag₂S + 4 S₂O₃^2-

Of course, the complex anion formed from one silver cation, Ag⁺ and two doubly negatively charged thiosulfate anions has over all to be triply negatively charged.

@ericrm: The silver cation contained in the triply negative-charged anion [Ag(S₂O₃)₂]^3- is reduced to metallic silver, and metallic zinc is oxidized to the doubly charged cation Zn²⁺, about whose further fate I don't know much. However, I personally would not add such a zinc-thiosulfate solution to the stock pot, containing an acidic solution usually, for two reasons:
1.) There's no more metal to be recovered in it, and 2.) A lot of finely divided, elemental sulfur precipitates on mixing acid with thiosulfate-solutions, possibly leading to a rather messy mixture.