Electrochemistry good amp draw poor deplate

[Blissman22]

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Now on to my problem. I have been using this setup to deplate my gold military style pins with great success until today. Here is my setup 6x8 pyrex dish sulphuric acid drain cleaner from lowes lead cathode aluminum mesh anode 12v 6-2 amp manual charger with a 1.5 Amp auto trickle charger. I always leave manual charger on the 12v 6amp setting and use auto charger also until it says battery full. To me this meant that deplate was complete. I have had great success until today. On my first batch for day I placed about 1/4 pound of pins into my aluminum mesh anode and watched it deplate with great success. This dunk only drew 3 amps total 1.5 from auto and 1.5 From manual charger. Now on to second batch I used same setup as I had been using. This time when I was rinsing I noticed about 1/3 of my pins didn't deplate. The amp draw on this batch was a little higher than my first about 3 amps on manual plus 1.5 From trickle. Now my third batch went crazy. The manual charger got pegged at 8amps (HIGH AS METER GOES) Plus the 1.5 From trickle. I thought wow there is a boat load of gold on these ones. But very little bubbling was happening. So I waited for trickle to register battery full (about 15 minutes). When i pulled pins only a few were done. I put them back in jiggled basket and no bubbling. I made sure it wasn't touching bottom. But no more reaction. Any suggestions?
I know copper mesh is better for anode but I have read on here that almost any metal works because it conducts electricity just copper is better. The aluminum has worked for me flawlessly for 5lbs of these pins until today.

 

[g_axelsson]

Maybe your acid is getting too diluted and starting to attach the base metals instead of the gold.
Try to change the acid to fresh one. Be careful that the basket is absolutely dry if you wash it in water between the runs, store the acid under cover to minimize absorption of moisture from the atmosphere.

And please report back when you find out what the problem was so people can learn from your experience.

Göran

 

[Blissman22]

Acid has been covered when not in use. My acid level seems to be dropping i would say fast. When pulling basket from cell I have been waiting for most of acid to drain back into cell. Maybe rinse water has been diluting cell since iI haven't let mesh fully dry between dunks. I do not use same basket consecutively. I have 3 that I rotate. Could I add more acid or is it recommended that I change all acid? It has been very cold here in N.Y. this week so my acid started out thick today by time I got to third dunk acid had gotten considerably warmer and thinned out. Does this help get a better clue for anyone?

 

[g_axelsson]

If you add fresh acid to the old one, you still have the water in it but it will be less water per volume of acid.
For example, if you have 90% acid in your cell now and add an equal amount of 96% acid you end up with 93%. Hotter acid is also increasing the base metal attack, cool it down and add some fresh acid if you like that.
I would rather pour off the old acid and add the same volume of fresh acid to the cell, It is always possible to add the old acid back to the cell afterwards, but then you have already tested with the fresh acid.

The old acid can be brought back to strength by boiling the acid. But before doing that read all the safety threads about boiling sulfuric acid because that is really DANGEROUS!!! if done the wrong way.
Search for "Concentrating battery acid" and you will find a ton of information.

Göran

 

[butcher]

Aluminum is so reactive with acid, being above hydrogen in the reactivity series of metals, it will displace hydrogen from acid easily, I would not use it in a cell.

High current means you have low or no resistance in the cell, either a short in the cell, or very high metal content in the electrolyte lowering the electrolyte solutions resistance to current, pointing to a possibility the solution is loaded with base metal ions.

 

[Blissman22]

Thank you both for the insight. Guess its time to change my tactics. Now its time for me to tweek my cell and do things properly. I guess I must have done so much reading I short circuited my medulla oblongata. Thank you for keying me in on the chemistry of my aluminum in acid, so there is probably my issue. Any who I am going to invest in some copper mesh and change my acid completely out. Oh yeah I read a great post on how to dry my mesh between runs. Until my copper arrives I shall be practicing in my head and of course reading and hopefully learning.

 

[bmgold2]

I was very curious about your aluminum mesh basket instead of the usual copper just because that would be a lot easier to get. I never would have guessed that aluminum would handle the sulfuric at all but then I found this information from:

Handbook of Corrosion Data edited by Bruce D. Craig, David S. Anderson found at this site:

http://tinyurl.com/mtv7z3u

In that book it states:

Because the rate of attack is very low, aluminum alloys may be used to handle both dilute (below 10%) and concentrated (above 98%) sulfuric acid solutions. However, rapid attack occurs in the concentration range of 40 to 95%, with the maximum corrosion occurring at 80% concentration.

You probably got away with the aluminum until the sulfuric acid got diluted with either rinse water or just absorbed from the air. Maybe the aluminum dissolved (slowly) but eventually built up in the acid enough to cause the problem.

I don't think I'll attempt using aluminum since (so far) my cell is very small and doesn't take much copper mesh to make a basket but this was the first time I ever heard of anyone using aluminum mesh (window screen?) for the basket.

 

[FrugalRefiner]

Nice research bmgold!

Dave

 

[Harold_V]

Someone should discuss the fact that the electrolyte will attack base metals with higher temperatures. If the cell is run repeatedly without being allowed to cool, the problem can be temperature related.

Harold

 

[goldsilverpro]

Someone should discuss the fact that the electrolyte will attack base metals with higher temperatures. If the cell is run repeatedly without being allowed to cool, the problem can be temperature related.

Harold

I totally agree with Harold and that was my 1st thought with Blissman's problems, which progressively got worse. Most problems are caused by using too much amperage which results in getting the solution too hot. It can essentially ruin the solution. Unless you have a way of cooling the solution, I would limit the current to about 5A/gallon.

 

[bmgold2]

Let's say the problem was with the increased temperature of the acid. What other problems would be caused from using aluminum instead of copper for the basket?

If kept at a low enough temperature and the acid concentration is kept high the only other problem I can see is that any aluminum that did get dissolved could drop not only the gold but also the copper and basically any other base metal that might be in the pins and got dissolved. This would cause more contamination to be cleaned up later but could this be a viable option for use in a stripping cell if you don't want to spend the money on copper mesh or if you just can't get any? This isn't a problem for me since I still have a big enough piece of copper mesh left to build at least a couple more baskets for my micro stripping cell but aluminum window screens can often be found for free in junk screen doors or from people replacing old windows (not electronic junk). If nothing else hardware stores probably carry it and may even sell it by the foot.

Just a thought. I'm always looking for the cheapest way to do things (not that they often work).

By the way, on my tiny 1/2 cup micro stripping cell, I limit my current to 1 amp and only run it for short periods of time. Last run was a bit less than 10 grams of pins spread over around a hour (maybe less) and the temperature did raise but not enough to tell without the thermometer stuck of the outside. Maybe 80 degrees F. but didn't really pay that much attention to the number. Since his first batch was 1/4 pound, it probably did heat up.

EDIT:After thinking about it for a few minutes, I think (In my case) a piece of stainless steel screen ripped out of a cheap, dollar store strainer would be a better idea if I didn't have copper mesh.

 

[Blissman22]

I'm not really sure how hot my cell got during my runs. It was an average of 20°f here in upstate NY last week.I was working outside in my cold garage. I frequently touched the side of my cell while running it. It was Luke warm at best. I cannot give a more accurate guesstimate.

Now after letting my cell sit for a couple of days it looks like there is some aluminum floating on top of cell. When the cell was running there was a metallic foam being created now I believe this foam was the aluminum being attacked. It no longer looks like foam but crinkled foil.

It also looks like some gold is floating and stuck to sides. Is this possible? It is not black but golden in color.

 

[Harold_V]

It also looks like some gold is floating and stuck to sides. Is this possible? It is not black but golden in color.

Sulfuric stripping cells work in a strange manner. Persulfuric acid is created at the anode, which readily dissolves gold. However, as the gold migrates away from the anode, persulfuric ceases to exist, so the gold is precipitated as the black sludge that is harvested.

Assuming that there's a state of being whereby gold is in solution, it stands to reason that the presence of aluminum, some distance away from the anode, may cement some of the gold. It could well be that the gold you're seeing is the result of that cementation.

Mind you, I'm just thinking out loud. I have no evidence to support my thoughts.

Easy enough to eliminate the unwanted aluminum, which can be very troublesome in solution. Gather all of the solids, which should be well rinsed (use tap water), then give the solids a wash in heated HCl. That should dissolve any traces of aluminum that may have made the trip with the values.

If you'd like to guild the lily, incinerate the solids before the HCl wash. That should help eliminate other unwanted substances. Before dissolving the values, rinse well (with tap water) after the dissolution process to eliminate the base metals.

Harold

 

[sharkhook]

Please do not count me as an expert, but I had a strange thing happen similar to this. My mini cell has worked great so far, I have used it 5 times now with the same acid. I had some pieces from HD cables for the newer T.V.s and all of them went well. The last piece looked identical but I knew it came from something else, some part of a satellite receiver box, a very old one. When I ran it, doing one piece at a time, it held steady amps at 1.1, but never deplated. I tried it for 3-4 seconds three times, usually more than long enough for the other materials to deplate. This one piece never came clean. I tried some decent pins, I would call them mid grade, afterwards just for comparison and they went well, very quick, like the others I had ran. I am thinking that one piece may not even be gold in my case, but have no idea what it could be. I haven't tested it yet, I just moved on to other materials when I saw that they processed as usual.