Sciencemadness Discussion Board

gold from E-waste

ahlok2002 - 9-2-2006 at 23:53

maybe this in not new for those who is in the line of electronic circuit board manufacture. the e-board contains trace amount of gold inorder to give certain performance.

and we know, the electronic waste become one of the issue as most of the part of it are hard to be recycled. however, we know that raw materials are getting more expensive due to the chain effect of the increase of the petroleum. and now people try to recover the materials from the waste and the targeted materials in the the e-waste are gold, silver, copper and many more.

from the information i get from online, i found out few way to recover the metals(gold) from the circuit board. Mainly two effective ways, one is acid treatment (aqua regia) another is cyanide leaching.

i have limited info in and very brief info on the procedures how to carry out the recovery process.

for the aqua regia process which involved the nitric acid and HCl to dissolve the gold. then the precipitate the gold with the sodium disulfite upon neutralized with urea.

and for cyanide leaching, the waste are dump in the alkaline solution of pH 9.5 -11.5 and wait for 48 hours for the metals to leach out. then precipitate the dissolve gold with zink powder.

so i may ask a question whether the e-waste need to be burned and grid into power inoder to start the process? the heating for the cyanide leaching solution will help to cut off the waiting time? what temperature is good if it does help?
what is the best reagent to precipitate the gold from the solution?

thanks for help.

garage chemist - 10-2-2006 at 03:12

The cyanide process requires good aeration since oxygen is needed for the gold dissolving reaction. Since air also contains carbon dioxide, some NaOH should be added to the cyanide solution in order to prevent release of HCN (desired pH = 9,5 to 11,5, you already said it).
The cyanide process is far superior to the acid process since the cyanide solution needs to be only 0,1-0,2% concentration and the cost for the chemicals is very low.
However, dissolution is a very slow process. The time for dissolution should be more like a week or even longer.

With cyanide, the gold from the connectors is removed, but not the gold from the inside of processors and the like, which would represent a loss. Therefore the circuit boards and especially processors should be crushed before leaching with cyanide.

Gold is precipitated by stirring the solution with zinc powder and then dissolving the excess zinc with HCl.
The gold needs to be purified by dissolution in aqua regia and reprecipitation from the diluted solution with FeSO4, FeCl2 or sodium disulfite.

densest - 10-2-2006 at 15:33

Also note that last time I looked, recovery operators paid US$1/lb = EU2/kilo for electronic scrap. Gold and platinum have gone up 50% and 150% respectively since then, so scrap may be worth a bit more.

Professional recovery operations get platinum, palladium, rhodium, mercury, silver, tantalum, and other exotics out of the input scrap. Separating platinum group metals increases the profit considerably but I'm not sure of the chemistry.

Isn't there a reducing compound like Na2S2O4 sodium dithionite/hydrosulfite which will work as well or better than Zn for getting Au+ to Au metal?

[Edited on 10-2-2006 by densest]

kryss - 11-2-2006 at 08:00

You ned to be careful if heating - in places like china they sort of stir fry them and all the volatiles like antimony and arsenic come off. They maybe amalgamate them also?

unionised - 12-2-2006 at 11:22

I have a nasty feeling that the gold won't disolve until all the copper is in solution so you would need a lot of cyanide/ acid before you got anywhere.

[Edited on 12-2-2006 by unionised]

ahlok2002 - 14-2-2006 at 18:15

thanks all for the info, lately i found addition info on the cyanide leaching process which the solution need to be aerated so that the oxidation reaction can catalyst the process.

besides sodium cyanide, the thiourea, thiocyanide salt have been tried but i dun have details on the futhur development. it is nice if we can use the alternative salts instead of cyanide salt due to the cyanide is easily decompose in alkaline environment and oxygenated solution.

it think the zink and sodium bisulfite are not so effective in precipitate the gold from solution. i had encounter commercial product which work better and yet to find what is the chemical .

i'm trying in lab, hope will update what i found later..


[Edited on 15-2-2006 by ahlok2002]

ahlok2002 - 15-2-2006 at 19:15

i saw the yellow fume emmitted from the reaction of aqua redia. the yellow fume is NO or cl2 gas?

i hope that is NO rather than cl2..that latter gas cause more harm to the enviroment if we conducted in the big scale.

thanks for help.

Ephoton - 26-2-2006 at 16:55

no one reads my posts :( ive answered this in a nice safe way.
whats it to be spelling or answers.
ill answer anyway.

ammonium persulfate + HCl for disolve.
drop with Al foil or hydroquinone.
If dropping with Al foil wash three times with hot acids HCl, HNO3 then H2SO4
make sure to wash with water before any acid wash to stop oxidation
of gold from happening.
no cyanides no nitrates nothing nasty in disolve so no horrific fumes.
I have and can get more Au than most of you I know what im talking about.

[Edited on 27-2-2006 by Ephoton]