USE THE H2SO4 CELL TO STRIP GF!

[MountainMike]

I know the main writers, whom I admire and respect, have given excellent reasons for NOT doing this. However, I believe we all would benefit from a united effort to develop a work-around for this method of stripping for some of the following reasons:

Reusable solution
Low fumes
Limited availability of HNO3
Low potency of alternatives

The main issue in stripping GF is that it is an alloy bonded to a substrate, apparently making it difficult to oxidize, enabling the persulphuric acid to work on the alloy.

Some ideas that may aid in this project are:

Adding 5% by volume H2O (suggested by GSP), perhaps 10% would work better for GF (?).

Vibration, maybe attached to a metal tank used as the cathode. Would it help move the copper out of the alloy?

Temperature, it has been suggested keeping under 100 degrees Fahrenheit. Maybe increasing the temperature to aid in removing the copper from the alloy?

Time, understand it would take more time, but the advantages listed above would more then off-set that.

Pat. #2,185,858 suggests using chromium trioxide as an accelerant. Has anyone tried that? GSP has advised NOT using the suggested nickel sulphate, as it is a "bad actor" leading to death of the cell.

So, there it is. Does anyone have any thoughts or interest in developing a work-around for GF stripping? MountainMike

 

[Noxx]

I stripped some gold filled material recently with good success. But probably because the plating had become thin over the age...

 

[lazersteve]

Mike,

I have stripped many pounds of Gold Filled scrap using the sulfuric cell, without any additives. I try to avoid using the cell for gold filled, for sevral reasons:

1) It takes soooo long to strip each piece of GF with the cell.
2) The waste from the cell is not as easy to recycle as the nitrate solutions.
3) You don't recover the silver content of the scrap.

As for the hazards and availability of nitric, I have modified the poor man's AR technique to overcome the bulk of the fumes.

The key is to add the nitrate to the hot HCl and scrap. Add the nitrate 2 teaspoons at a time every twenty to thirty minutes as the solution stops fizzing. If addition of nitrate does not produce a fizzing reaction, add a 1/2 cup of HCl. The fumes are easily controlled with a watch glass on top of the beaker and temperature.

Keep an eye on the reaction to determine when the base metals are all removed. When all the base metal is in solution the reaction is over. Pour off the solution and test it with stannous. It's usually void of gold. If a salt forms add some water.

The silver chloride and gold shells, foils, and gold powder remain as solids. thoroughly rinse them and then use the same technique above to dissolve the foils, shells, and gold powder from the silver chloride. Pick the stones and other solids out of the silver chloride.

Process the silver chloride with iron and 10% sulfuric acid.

Steve

 

[MountainMike]

Steve: Thank you for your detailed thoughts. I really am interested in and am looking forward to trying your new AR procedure. It seems ideally suited for "rough & ready" assay"s or Au in the hand quickly, on small amounts.

I have some thoughts and questions about your objections to using the H2SO4 Cell for stripping GF.

1. It takes soooo long to strip each piece of GF with the cell.

Quoting GSP, "After running this thing for years I have come to the conclusion that if, you have a controllable power supply, it makes no difference how large it is. The only difference is that a smaller power supply will be proportionally slower. Instead of 15 minutes for a 200 amp supply, a 20 amp supply might take 150 minutes. They are both 3,000 amp-minutes."

My personal answer to this is a 50 amp controllable supply

A system that can run by itself with little personal attention has a lot going for it.

2. The waste from the cell is not as easy to recycle as the nitrate solutions.

I do not understand that statement. One of the major advantages of the H2SO4 Cell is the recycling of solution. Again, I use GSP (because he has done this in a grand way) who stated he used the same solution for two (2) years.

3. You don't recover the silver content of the scrap.

The patent referenced stated it also stripped silver or palladium coated zinciferours articles (whatever that means).

Now, back to a work-around for the H2SO4 Cell. I have a mild steel tank that holds 7+ gallons. I purpose using 5 gallons of H2SO4 (96% Rooto) plus 5% (by volume, about 32 ounces) of H2O. The Tank will be the cathode. Holding the GF (or any Au plated material) will be a plastic perforated small parts plating basket (this eleminates the Faraday effect of a wire basket). The anode will be a solid copper rod, used for contact and stirring the material.

Are there any thoughts or suggestions to this plan?

Mike

 

[lazersteve]

Mike,

The pros you mentioned for the cell are all true when the scale of your operation is increased. My comments were geared around the small 1 quart cell that I operate and demonstrated.

For the small hobbyists cell, the time required to manually load each individual piece is tiresome to say the least. With large setups, the real key becomes intimate contact with the items to be stripped, even then GF items (being more thickly plated) take more time. It's really hard to beat the speed of 50/50 nitric in most cases.

The cleanup/recycling reference I made is related to the filtering, dilution, and inherent hazards associated with working with large (or small) volumes of hot concentrated sulfuric acid solutions, as well as the extra steps involved in processing the black powder after it is separated from the electrolyte. The bulk of the solution is poured off before diluting, and is therefore not the portion that will be recycled. It is the diluted portion that I was referring to with respect to recycling. Yes the diluted solution can also be recycled with additional effort and the hazards of working with boiling concentrated sulfuric acid.

The fact that silver is stripped is given. Some remains in the electrolyte and some precipitates with the black powder. Separating the stripped silver is the real point I was making. With AR, in one step the silver forms a nice solid (silver chloride) and the gold is filtered out in the liquid. With the cell you will have to process both the electrolyte and the solids for silver.

Perhaps GSP will chime I and let us know how he recovered silver from his cell electrolytes and black powder.

I do agree that on a large scale the cell can produce very good results, but unfortunately I don't operate on that scale. I use my small cell for larger gold plated and filled items.

Steve

 

[goldsilverpro]

I never used the sulfuric stripper for silver. When I first started this, silver was $1.29/oz.

After settling, I actually siphoned off the acid. I filled a small siphon hose with water and started siphoning. Of course, I used long gloves and a face shield. When the sulfuric meets the water in the hose, it immediately gets hot, but it cools rapidly as the acid flows. Remember, though, that I had a 50 gallon tank.

I always thought that an all mild steel spigot near the bottom would be good. It would be located up the side of the steel tank enough to give room for the gold to settle.

Probably the best arrangement would be a steel tank with the bottom sloping down from the 4 sides towards the bottom center. Coming down from the center would be a double valve arrangement with about an 8" long pipe between the valves. The pipe and valves should be at least 2". The top valve is right at the bottom center of the tank, at the top of the pipe. The bottom valve is at the bottom of the pipe. The tank would be on a stand in order to elevate the bottom valve enough to get a bucket under it.

(1) The bottom valve is closed, the top valve is opened, and the gold is allowed to settle into the pipe.
(2) A container (bucket) is placed under the bottom valve.
(3) The top valve is closed and the bottom valve is opened. The sludge drains into the bucket.

The main solution can be stirred and the above repeated to get more out. Also, an arrangement should be made to flush the gold out of the pipe.

The advantage of this is obvious; the solution remains in the tank. I have never done this with sulfuric. We did have about a 100 gal plastic tank like this that was used for cyanide stripping. We zinced out the gold and removed the precipitant with a 4" double valve arrangement. Worked great.

 

[MountainMike]

Steve: I absolutely agree, nothing beats 50/50 real nitric for GF. My purpose is not to prove the Cell is faster then nitric, it clearly is NOT. The two (2) main advantages of the Cell has to be its almost "closed loop" effect, and not dissolving pounds of metals. Ultimately the black sludge I generate with its filters will go directly into AR for the refining steps.

So, I am back requesting any thoughts, ideas, or suggestion to better operate the Cell. Mike



Hi GSP. Very good idea about the two (2) valve seperation system. I will incorporate that idea in the next tank I build, this tank (a 30# propane tank with the top cut off) is too thin for that.
Silver is not an issue for me. Mike