Cell phone cyanide leaching

[kjavanb123]

Hi,

Today I prepared a cyanide solution to dissolve the gold plated on cell phone boards.

Prepared a hot solution consists of following;

16000 ml of hot tap water
240 grams of sodium cyanide

Mix them well, added boards into a basket and submerged it to solution. Added few ml of hydrogen peroxide and stirred the basket.

Within a mintue or cell all visible gold plating gone and only gray (Ni) layer remained. I continued this batch by batch until 30 kg boards finished.

In total 50 ml hydrogen proxide was added to solution for all the batches.

Solution had turned to yellow to tinted green by the time I was done.

Added three tablespoons of lye to make sure pH is 11-12, then while stirring I added 6 tablespoons of 325 mesh zinc powder.

Solution turned gray in color and some white foams formed on top surface.

After 10 mintues I transfered the main solution to another barrel, and there was small amount of residue left which after few washes with hot water, dissolved it in nitric acid, and only garbage from cell phone boards remained.

So I am going to weight till tommorow, filter paper the solution as it had a lot of floating gray stuff, and save that.

Also going to hang two stainless steel plates 24" by 24" and connect them to 6V 6A rectifier.

Then wash the residue in filter paper with plenty of hot water then add nitric under fume hood, see if I can recover the lost gold.

Strangely, this is my second cell phone boards batch that I have used cyanide with similar outcome.

Please advise

Regards
Kj

 

[nickvc]

Kevin I suspect the problem is that the cell phone boards are very low in gold so it may be hard to recover the gold from the solution.
I would suggest agitation such as a bubbler to keep the solution moving and exposed to the zinc, perhaps you could add a little heat as most chemical reactions are stronger when heated.

 

[anachronism]

With 240g NaCN in 16 litres of water, to do 30Kg of boards, you're using way way too much NaCN. You're going to be looking to find 5g -8g of gold in there and using 240g of cyanide to do it.

Not only are you aggressively attacking copper/other metals from the boards but what do you think all those free CN- ions are going to be doing whilst you precipitate the gold out using zinc? Think it through please and come back to us but imagine using 240g of Nitric to dissolve 6g of gold in AR and then try to drop it using SMB.

 

[kjavanb123]

Those are excellent points. I used a big stainless steel pot and a large plastic strainer that I filled it up with cell phone boards then dip them in solution.

Since I have been smelting I rarely used cyanjde leach, but we wanted to test if blast of air used in our smelting could cause some gold platijg losses or not.

So we decided to leach the cell phone boards with cyanide, after neturalizing the boards, smelt them.

Over the past few batches of mixed cell phone boards we consistently recovering 0.750 to 0.800 grams of gold per kg of mix boards.

With result of these leaching plus smelting tests we could test if any losses specially for gold plated on boards did occur.

Since the volume of cyanide solution is high compare to expected gold, you think using stainless steel plates and electricty would plate the dissolved gold?

Regards
Kj

 

[anachronism]

Kevin the question I would be asking is how often you get these boards or boards you can use in cyanide. Ive got a thread going about the electrowin cell. That's designed exactly for this kind of leach and has the added benefit of leaving the solution in a useable state for further processing.

Edit: You've always been really good at building things- this would probably be of benefit to you.

Double edit: No I am not promoting my version of the cell - I'm promoting the concept that Deano gave to the forum for nothing.

 

[kjavanb123]

Hi,

Thanks for noting the EW usage in cyanide leaching. I will give it a try.

I found the following post by the "Guru" of cyanide and any other refining procedure, GoldSilverPro, at the following,

http://www.goldrefiningforum.com/~goldrefi/phpBB3/viewtopic.php?t=20047

".. I then adjusted the pH of the solution to 12-12.5 with sodium hydroxide and added an extra 10g of NaCN per gallon. I then stirred vertically, slowly up and down, with a plunger stirrer I made from a 5" diameter plastic disk on the end of a 5' length of plastic pipe. Stir continuously. While stirring, I added zinc dust in small increments though a flour sifter. At first, you will see brown gold power being brought to the surface by the stirrer. Keep stirring. After 1 or more further additions, the powder will start to turn gray. When it's all gray, with no hint of brown, stop adding. Hang 2 zinc bars, near the top of the solution, and let it settle overnight..."

So based on above I did recover gold, but transfered the solution to another barrel too soon.

Will check the solution today.

Thanks again.

Kj

 

[autumnwillow]

It seems that you are recovering the gold by zinc dust method. What is the purpose of the stainless steel?

 

[kjavanb123]

Autumn,

I recall a post by GSP where he talked about two stainless steel plates connected to 6V 6A to plate the gold or silver from pregnant cyanide solution.

Continuing of the same electrolysis would destroy the cyanide.

 

[nickvc]

Kevin I feel Jon,s point about the amount of free cyanide is important, any gold that was converted will have been redissolved by the free cyanide and to get the zinc to dissolve fully and precipitate the gold needs plenty of agitation to make sure the whole solution has been in contact with the zinc, think about it this way GPC the plating salts is 68.4 percent Au by weight so just over an ounce of cyanide will absorb over two ounces of gold, you have used 240 grams to dissolve 8 grams....

 

[autumnwillow]

On my experience with regards to dissolving gold in cyanide.

Precipitated pure gold powder from sulfide was used. 1.33g was weighed out to be digested in the process.
Only about 4 grams of KCN was used in a 400ml solution. pH was checked before proceeding with H202 addition.
I added about 10ml of H202. Waited an hour. Not all gold dissolved. I kept adding H202 and still it did not dissolve the gold.
I decided to add more KCN, it dissolved some, then I added more and more along with H202 until it fully dissolved. I think I used about 20g of KCN and about 50ml of H202.
With these observations it seems that the amount of KCN to dissolve gold is not directly proportional to the gold weight. You will use more KCN in order to dissolve gold.

As for free cyanide, you do not need to destroy it. As long as the oxidizer (H202 in your case) is removed by heat or light exposure it will not re-dissolve the gold. You can check if the oxidizer is completely removed by hanging a pure gold piece, it should not bubble.

As for GSP's method of hanging a bar, I don't do that, instead I add excess amount of zinc powder.
I use 40grams per litre of cyanide for my digestions, as most of my digestions have lots of gold content. Low cyanide concentrations are ideal for low gold content such as ores. I think its a different story for what you are digesting. As cyanide will also digest other base metals. You used 240g thats 66gpl. A little too much I think, I would use 40gpl max.

For destroying the solution, it requires an ORP meter, a pH meter, pH increaser and bleach. I have posted the info somewhere in this forum.

 

[anachronism]

As for free cyanide, you do not need to destroy it. As long as the oxidizer (H202 in your case) is removed by heat or light exposure it will not re-dissolve the gold. You can check if the oxidizer is completely removed by hanging a pure gold piece, it should not bubble.

Yes the part about oxidiser is completely valid. This is why many "leach enhancers" cause problems with precipitation. That given I hope we can agree that there was still way too much cyanide used in this particular instance.

Edit: in fact my analogy with the Nitric would therefore need changing but I'm not going to edit the post retrospectively.

 

[autumnwillow]

kjavanb123,

Did you have wash barrels too? A caustic solution to wash the boards after removing them from the cyanide solution? I would use 2-3 wash barrels. Then combine all these with the pregnant solution before precipitating with zinc.

Also please remember to wash your zinc/gold precipitate well with a caustic solution before proceeding with Nitric.

I am still not sure what is the stainless electrolysis for. Is it to destroy the cyanide solution? If so, I do not think that is the proper process. Would you mind posting the link of that thread where GSP mentioned it? I think that stainless electrolysis is meant for gold recovery not cyanide destruction.

I think you need to stop and do a little more research about cyanide. You might hurt yourself or the people around you. Keep all the solutions at pH 11. Everything that went in that cyanide solution should also be stored in a drum filled with water at pH 11.

Detail out all the things that you did and I would gladly help so as the other members. Some pictures would also be nice.


anachronism,

Yes way too much cyanide, I would probably use 30gpl for cp boards.

I am not sure what happens with cyanide overnight. My experiment was done in four hours. Also, when I precipitate with zinc I do not leave the solution overnight like GSP does. So I'm not sure what free cyanide without an oxidizer does overnight. So your post about the nitric could be correct or wrong. I'm not sure. I'm no chemist.

I'm pretty sure Deano or Lou would know the answer.

 

[Deano]

Cyanide without any oxidiser present will do nothing to gold.

It is much more difficult to totally remove any oxidisers from a cyanide solution than people think.

Most operators who used cyanide in large scale would do a vacuum removal of air from the pregnant solution.

This would minimise the oxygen level in the liquor but not totally remove it all.

Such a removal was good enough for most processing requirements.

Totally removing all oxygen and metal complexes from the liquor becomes a very complex and expensive exercise.

If you leave a de-aerated cyanide solution just sitting overnight the solution will adsorb oxygen from the air and dissolve gold.

Many metal complexes in solution will also act as oxidants for cyanide.

Deano

 

[kjavanb123]

Autumn

Please read the following post by GoldSilverPro,

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=86&t=24627&p=260570&hilit=SS+cyanide#p260570

Here is another post by Master GSP, where he mentions stainless steel plating

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=86&t=19877&p=202517&hilit=Plate+cyanide#p202517

Regards
Kj

 

[autumnwillow]

kjavan,

After zinc precipitation do you proceed with the electrolysis (stainless anode/cathode)?
Or are you destroying the cyanide solution and discarding it using the same method?

If so you did not read his post right.

I used to electrolyze strong cyanide solutions, using 2 stainless sheet electrodes, to destroy the bulk of the cyanide. After a couple of days, at least 90 percent of the cyanide was gone, proven by analysis.

Let me tell you why. In the electrolysis the anode produces oxygen which is only a part of the entire process to destroy the cyanide. It is similar to using H202 for cyanide destruction.

Remember there is a proper process to completely destroy the cyanide solution.
You can simply google H202 cyanide destruction or calcium hypochlorite cyanide destruction.
Calcium hypochlorite is the cheapest way for me and its easy to find as it is just a bleach/chlorine used for swimming pools.

You can however re-use the solution if you decide to just plate out the gold without using zinc precipitation. Look into Deano's cell or the steel wool cell. Your losses here will be close to zero but the process takes time.

 

[anachronism]

If you're making a complete list don't forget activated carbon Autumn

 

[kjavanb123]

As for treatment of cyanide waste solution please read the following post by 4metals,

http://goldrefiningforum.com/phpBB3/viewtopic.php?f=40&t=7627&p=69862&hilit=Cyanide+waste#p69862

Regards
Kj

 

[autumnwillow]

kjavanb,

I made the same post in this thread about using an ORP and pH meter for complete cyanide waste destruction.
How are your results doing? Did you get more out of the cyanide process? Or did smelting provided a better result?

anachronism,

I'm not sure if electroplating and using activating carbon are the only ways to get the gold out of a cyanide solution while being able to re-use the solution. I'm no expert in this field. I'm just providing information of what I know.

 

[kjavanb123]

Autumn

I have filtered the cyanide solution and filtered the zinc powder.

I have not gone back to the shop to check on the situation yet.

Regards
Kj