Reverse electroplating for environment-friendly gold recovery / refining

A. Recycling gold plated circuit board scrap is not a simple process that can be set up using only e-mail/bulletin board responses.

Hire a reputable consultant who can spend the time showing you processes that will:
1. recover the metals
2. be economical to operate
3. be safe for the operators
4. not violate environmental regulations

You will find that aqua regia or chloride-based electrowinning/refining are not going to work on "raw" scrap.

A. I have a simplicity refiner from Shor at home and use it to refine karat gold. The alloy is dissolved into a saltwater solution by using either a rectifier or battery charger ⇦this on eBay or Amazon [affil links] (12 v 10 amp+) then the gold is precipitated by adding a powdered chemical (I believe is sodium metabisulfite). The "mud" is collected rinsed and dried and melted back to 999.5+ purity. I know of a guy in the USA who uses one to refine the amalgam he gets from stripping the gold plating off circuit boards.

hope this helps.

Regards
Jeremy

A. YOU NEED A HIGH CURRENT, LOW VOLTAGE UNIT FOR ELECTRODEPOSITION, A SOLUTION OF GOLD CHLORIDE, A FIBERGLASS OR CERAMIC SINK (BLANK ALL METAL), AND TWO WEEKS OF THEORY, SAFETY IN HANDLING, AND PROTECTIVE CLOTHING. I AM SENDING YOU A SCHEMATIC FOR THE UNIT. GOOD LUCK. HAVE FUN !

2.5 V, 20 A. CIRCUIT FOR ELECTROPLATING

http://www.maxim-ic.com/appnotes.cfm/appnote_number/2040/ln/en

PRODUCTS SOLUTIONS DESIGN APPNOTES SUPPORT BUY COMPANY 50W Current-Mode Forward Converter Design with the MAX8540 This application note details the design of a 50-watt, isolated, forward converter, using the MAX8540 synchronizable, high-frequency, current-mode PWM controller. Design procedures for both the power stage and controller are presented, along with actual performance measurements. The converter delivers 20 A of Load current at an output voltage of 2.5 V. It employs synchronous rectifiers for secondary rectification. The input voltage range for the converter is 36-75 VDC. This design is available as an evaluation board. The evaluation board demonstrates how easy it is to implement the features network and telecom applications require. The design methods can easily be adapted to the design of high performance, full featured off-line power supplies. Key features: 300 kHz switching frequency, Programmable input UV/OV Protection Programmable hiccup current limit or latch mode overcurrent protection, Programmable maximum duty cycle clamp with feedforward, Programmable slope compensation with single resistor, Synchronization to external clock, Adjustable current limit threshold, Active low-enable feature for easy turn on/off of converter, Internal leading edge blanking on the current sense pin, Output overvoltage protection Space-saving 16 pin QSOP. Description of Application Circuit Operation Figure 1 shows the circuit diagram of a 2.5 V, 20 A isolated forward converter that uses the MAX8540 current mode controller (U1). At startup, the total capacitance at the Vcc pin is charged through MOSFET Q7 and the parallel combination of resistors R30 and R22 from the dc input voltage VIN. When Vcc exceeds the undervoltage lockout threshold of the MAX8540, it goes through the soft-start mode and pulses of gradually increasing duty cycle are applied to the gate drive IC, U8. The MOSFET Q1 therefore starts to switch the input DC voltage across the power transformer T1, used to provide isolation and to step down the input DC voltage to the required level. (Selection of power transformer turns ratio is dealt with in the following section.) Since the energy for driving Q1 comes from the capacitance at Vcc, the Vcc voltage falls. The hysteresis of the MAX8540 undervoltage lockout feature allows this to happen. Pulses on the bias winding of transformer T1 are rectified by D1, regulated, and applied to the Vcc pin. The rectified and regulated bias circuit voltage builds up and prevents the Vcc pin from falling below the undervoltage lockout threshold. The primary side control continues to operate from the bias winding. The MAX8515 (U2) is configured to sense the Vcc pin and turn off Q7 at a voltage slightly higher than the worst-case startup voltage for U1. This avoids unnecessary power dissipation in Q7, R30, and R22. Figure 1. Circuit diagram of 2.5 V, 20 Amp, Isolated Current mode Forward Converter using the MAX8540.

Cheaper to build, and, Oh, might learn something new.

A. A site called "SREETIPS" on you tube has many videos that are incredibly informative on the subject. He shows values recovered and post processing too. Very well respected.

A. All you need is a "passive" cathode. Stainless steel is probably adequate. Copper foil is routinely fabricated by electroplating copper onto a stainless steel belt and then separating them; the same idea should work for precious metals.

A. Hi Eddie,

The book I'm reading uses sodium sulfite for the precipitation of Gold. At the end of there is an article describing Platinum recovery. If you'll share, where are you finding Platinum?

Extracted from a link:

Platinum-

If you had platinum in your gold, it will not dissolve, to any appreciable degree, in the room temperature aqua regia. It will be left behind when you pour off the aqua regia, prior to precipitation. To insure high purity of the platinum, you will need to re-refine this material. Put this material in a fresh aqua regia bath. This time, however, heat the acid to simmering. Continue heating until all the platinum is dissolved (that may take 1-2 hours). When completely dissolved add 1 ounce of ammonium chloride for every ounce of dissolved platinum. The platinum will precipitate as a red mud. If you want to leave the iridium in the platinum, then wait for it to precipitate before recovering the platinum. Iridium will precipitate as a blue-black mud after the platinum precipitates. Platinum group metals will also show up on the stannous chloride test. Platinum turns red, palladium turns orange and iridium turn blue-black.

Good Luck,

A. Hello Eddie, as a chemist working in the recovery of precious metals from electronics scraps I advise you to keep away from the aqua regia process. It's very dangerous because of it's toxic fumes and time expensive. Furthermore, the aqua regia process attacks the resins of the boards compounds leaving you with an insoluble mud. In fact, most of the operators in this sector prefer to use a basic process composed of cyanide (2% solution) and other components. This process is safer because there are no toxic gas involved, as long you take care not to originate some hydrocyanic gas. Search for literature on this matter.

I can only tell you that the process I have developed, based on cyanide, recover all precious metals from any board in about 15 minutes time. Unfortunately I can't give you other indications, it's a company secret. Good luck

A. Hi Eddy

If you live in the USA and you want to use the (process composed of cyanide (2% solution) and other components) that the guy from Spain recommended. You will find that there are tons of environmental laws concerning the use of cyanide leach. I have chatted with several people on a few forums that are using AR and they are recovering the gold. They work outside so the fumes won't cause problems. Do your home work on it before messing with it because it is dangerous if you don't know what you are doing with it. Shor International has a page that describes the process and they have a list on the site of tools and safety tools.

A. I agree that the Cyanide treatment is a lot more hazardous than the AR method. Cyanide will kill you in 15 minutes or less, it takes less than 1 teaspoon of 2% cyanide solution to cause death. Also you will have toxic waste compared to the Aluminum Hydroxide that you can turn your AR into which has relatively no impact on the environment.

If you aren't careful working with acids they can kill you also but, with a little common sense you will be safer (do it outside). I developed a fume abatement apparatus for my lab out of PVC piping, some sealed plastic buckets and an air compressor [adv: air compressors on eBay or Amazon [affil links] . The fumes that are lighter than air rise out on their own and if the condense on the way out to the exhaust system (about 8 ft) then they fall into an acid collection container. If they are heavier then when I use the pump to push the fumes off they run through the activated carbon filter and fall into the acid condensing tank. I don't remove this tank for about a week to allow the acids to turn into a liquid then I neutralize them and dispose of them properly.

With a little time you can figure out how I made this as I sell the plans in my online stores and sometimes on auction sites. I am not going to attempt to advertise here as this is an informative site which I have a lot of respect for!

THE POINT IS DO NOT UNDER ANY CIRCUMSTANCES USE CYANIDE IN THE US. MINES MAKE ENOUGH POLLUTION WITH IT ALREADY!

A. Shawn,
Cyanide can be destroyed using sodium hypochlorite (bleach). As long as you neutralize the cyanide before you dispose of the waste water (into the nearby crystal clear mountain stream crammed full of salmon) then I don't see what the fuss is all about.

See this page on how to do that: www.shorinternational.com/cyanodestruct.htm

Alan Jacobson
- Eugene, Oregon
2006

A. You're certainly welcome to voice your opinion, Alan, and we thank you for it. But if you don't understand the fuss, please listen to the other side :-)

Neutralization of cyanide waste is NOT trivial. A man in a local retirement village here died doing so in the last couple of years. Sewer workers have died as a result of such disposal. 6 employees in a plating shop in Indiana (Bastion Plating) died some years ago, and numerous individual workers since. First responders have refused to treat heart attack victims, believing they ingested cyanide.

Without careful pH control, not only will the salmon be dead from cyanogen chloride, but perhaps some people as well. And once the cyanide has complexed with nickel or iron, you can't destroy it with alkaline chlorination anyway. It amazes me that the government demands that any employee who works with this must receive annual professional safety training; but hobbyists, dealing with larger amounts under far more dangerous circumstances -- no co-workers, no one with a Scott air-pak, no MSDS available to first responders in a mailbox outside the building, no registration with the hospital so antidotes are on hand -- feel that a paragraph of internet advise from a stranger (whose name is often fictitious) is the only education they need when working with an instantaneous-acting, airborne poison like hydrogen cyanide.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Butyl Diglyme, available from FERRO and others, takes a bit of getting used to, but delivers 99.9 Gold as a final product.
Route is not too difficult.
Dissolve gold with straight HCl or HC + Chlorine (use fume hood or stay upwind outside...serious fumes)
In a separatory funnel, pour the strained HCl- Gold Chloride.
Add 1/4 measure or volume of Butyl Diglyme. Agitate until all the gold has transferred to the BD.
Separate out the stripped water/HCL portion.
Rinse and repeat until aqueous portion is clean.
Pour the Butyl Digyme portion into a heatproof beaker [beakers on eBay or Amazon [affil links] , place over warm heat,on a sand-filled pan. Add a small amount of Oxalic acid, dissolved in DI water, gently heating to simmer.
The gold will form clean flakes and fall out as it precipitates.
Alternate procedure once you have separated the Butyl Diglyme portion is to add H2O2 to precipitate.
Results may be melted with a bit of borax flux in a crucible by torch or furnace to solidify to a 99.5 % Gold button.
Butyl Dyglyme can be strained thru a coffee filter to clean it, and used endlessly, over and over.