Electrochemistry Lead Cathode

[jmdlcar]

I want to make a lead cathode for a sulfuric cell. My question is will lead melt aluminum if not will lead stick to aluminum? I want to use aluminum to make a mold to make my lead cathode. I know that a dumb question. Thanks Jack

I forgot to say it is thin aluminum same kind as is on a house.

 

[Geo]

sorry Jack, it wont work. you need a piece of steel.

 

[Geraldo]

Hi Jack!

Your questions:

1. Will lead melt aluminum?

Lead melting temp about 320C (about 620 F)
Aluminum melting temp about 660 C (about 1220 F)

These are for the pure materials - alloys will have lower melting points.

If you make sure that your lead is near its melting point (e.g. minimize the molten superheat) then no, it won't melt the aluminum. It may soften it though, which is a concern if you have thin sheet as mold material rather than a plate. The other thing going for aluminum is it has high heat conduction, so it cools quickly, making it harder to melt or soften the aluminum.

2. Will lead stick to aluminum?
If the aluminum is smoothly polished, and well passivated (treated with a strong oxidizer like hydrogen peroxide), then generally lead doesn't stick to aluminum well. A lot of lead molds are made of aluminum - bullet molds, fishing lure molds and similar.

3. Will lead corrode aluminum? (you didn't ask this)
If the aluminum is well passivated, and the lead is cold (cast maybe 50 C above melting or less) then the aluminum can last for years. If you cast very hot, then the aluminum can be attacked. Lead can dissolve a little aluminum, but generally it isn't that big a deal at lower temps.

Industrially (operations that cast lead anodes for use in commercial electrowinning, such as for zinc or for lead electrorefining) they tend to use steel molds. People also make molds for casting lead from wood and plaster with decent short term results (those molds eventually die, but hold up surprisingly well for a few casts). Lead has such a low melting point it is pretty easy to cast.

I will ruin your day now and suggest that pure lead anodes won't survive as long as you would like in sulfuric acid. Commecially, people use lead-silver, lead-tin-silver, and lead-calcium alloys.

Have fun casting! And make sure you have great ventilation and ideally wear a respirator.

Best Regards, Gerald

 

[Geo]

i used molding clay i bought at the craft store for a few bucks that i pressed into a baking dish. i used a small piece of wood to make the pattern and baked it dry. it made four cathodes before it fell apart.

when you say aluminum like goes on a house, what comes to mind is siding, flashing or window or door frame. if you try to follow lazersteves videos for making a cathode, none of these will work. you may melt and pour into a window or door frame made of extruded aluminum. but for safety reasons, i will not recommend that you use any of these. trust me, molten lead pouring down your shoe will cause you to have a bad day.

 

[jmdlcar]

The lead I was going to use was from auto wheel weight. I don't know if it is pure or not.

 

[Geo]

take each one and scratch a spot, then put a drop of vinegar on it. if it bubbles, its made of zinc or tin and zinc, if it doesnt bubble then its lead.

 

[goldsilverpro]

Geraldo,

Unlike most binary alloys, the melting points of the alloys of Pb/Al are higher, and not lower than that of the pure metals. Check the phase diagram. Note that the melting point of a 30/70, Pb/Al is about 1700K (about 2600F). Does that change your thinking?
http://resource.npl.co.uk/mtdata/phdiagrams/alpb.htm

I think it will stick, at the least. At the worst, I think it will eat a hole through the aluminum.

 

[jmdlcar]

I guest when I get the money I'll buy one from lazersteve. If it will fit my dish the size of it is 9 x 5 x 3 inch made by PYREX.

Geo, I already melt them down into a pan of water.

Just like the Lye I'm just going to buy it. I was just trying to make both to save me money.

 

[Geo]

ill see how much lead i have. im sure i have enough to make you one if you want to wait on the other package, i can send them both by the end of the week. if you cant wait for the other, ill still send it monday and try to get you a cathode sent out in a couple of weeks. i only mail packages twice a month so i wont send anything out for another two weeks. since your the only one im mailing to, i could wait till monday after next. its up to you.

man, i just read what i wrote and its all over the place. :shock: ill just send the package out to you tomorrow and see if i can send you a cathode next monday. ill try to anyway.

 

[jmdlcar]

I can wait for both. Thanks Geo for your help.

 

[samuel-a]

Geraldo,

Unlike most binary alloys, the melting points of the alloys of Pb/Al are higher, and not lower than that of the pure metals. Check the phase diagram. Note that the melting point of a 30/70, Pb/Al is about 1700K (about 2600F). Does that change your thinking?
http://resource.npl.co.uk/mtdata/phdiagrams/alpb.htm

I think it will stick, at the least. At the worst, I think it will eat a hole through the aluminum.

Hi Chris.

These two metals (Al/Pb) are Immiscible at low temp', just looking at the giant hump there you can tell.
I'm not sure what you say is completely wrong then again i'm not sure it is completely right.

It seems as if Geraldo is talking from experiance, and it does make sense while looking at the diagram.

The Zone which is relevant to this discussion is the encircled one in the attached photo. (as 100% molen Lead meets the surface of solid Aluminum)
I have renovated the diagram a bit (btw, that's a wonderful resource, thanks), according to the diagram, molten Lead poured over (unpassivated) Aluminum at below 700K (800F/427C) give or take, should not dissolve it (and as a result stick to it). Well that's in theory, in practice, i'm sure there will be some sort of corrosion (due to minute dissolution) to the Al without a thick oxide layer in between the two metals.



Perhaps i get it all wrong. Please correct me if that's the case. I'm no metallurgist and this subject can be extremely confusing.

 

[Geo]

Sam, if that pertains to pure metals it wouldn't apply to the discussion from the OP. wheel weights may contain other metals than lead and zinc. i saw one of those "how is it made" videos, and since its a non-critical component, they use recycled material. the only time purity is an issue is in states with a law prohibiting lead in certain applications, some lead still makes it through so theres a minimum percentage level they can accept.

 

[etack]

lead ingot molds are Al http://www.ebay.com/itm/Lee-Ingot-mold-Handle-Bullet-Casting-Lead-melting-/270980837612?_trksid=p3284.m263&_trkparms=algo%3DSIC%26its%3DI%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D21%26pmod%3D390414907159%26ps%3D54#ht_500wt_1413

I want to make a lead cathode for a sulfuric cell. My question is will lead melt aluminum if not will lead stick to aluminum? I want to use aluminum to make a mold to make my lead cathode. I know that a dumb question. Thanks Jack

I forgot to say it is thin aluminum same kind as is on a house.

it should work as a mold but not like Steve did in his video.

Couldn't he just hang the wheel weight on the cell?

Eric

 

[Geo]

the most typical wheel weights have a piece of steel in them. lead is used because its neutral in sulfuric acid. steel will foul the electrolyte.

 

[etack]

GSP said that his drum cell was made from mild steal http://goldrefiningforum.com/phpBB3/viewtopic.php?f=86&t=13948&hilit=plastic


Eric

 

[Geo]

yep, but he also used very specific voltage, amperage within a certain temperature. http://goldrefiningforum.com/phpBB3/viewtopic.php?f=86&t=13948&hilit=plastic#p140721

when i use my cell with my battery charger, it may be 12v but amps go up and down and im sure the temps run over 110 degrees.ferrous sulfate forms from steel being dissolved in sulfuric acid.

 

[Geraldo]

Interesting discussion.

Just some added info here:

1. The classical wheel weight alloy is lead-antimony (antimony is a hardener). These alloys are less corrosive than pure lead - like any solvent, its solvating power goes down as the amount of solutes in it increases. Lead antimony alloys have a lower melting point than pure lead, although since the alloy is typically <1% antimony, the freezing point depression is only a few degrees. Of course, wheel weights contain lots of junk, so your mileage may vary...

2. Aluminum is only sparingly soluble in lead. The reverse is also generally true - lead is only sparingly soluble in aluminum. Lead is added to aluminum alloys to make "free machining" alloys. It works because the lead doesn't dissolve, but rather forms little islands that are soft and allow chips to break. Think of oil in vinegar (a dispersion of droplets) rather than salt in vinegar (true solution). The free machining alloys are actually aluminum-lead-bismuth alloys, with the bismuth helping to disperse the lead.

It takes exceptional efforts to "dissolve" lead in aluminum - typically very special aluminum-tin alloys with extensive processing. In the molten state, aluminum and lead don't even mix effectively.

3. If you have ever cast bullets, you will find that a lot of molds are cast aluminum. Some are also brass (which you would imagine would "stick" to lead, but again, at the typical casting temp it doesn't). Some (usually older ones) are cast iron. For modern consumer molds, the vast majority are aluminum.

4. I have cast lead alloy plates on a modified cookie sheet, which is also aluminum. This was mainly because I wanted something quickly and didn't have the materials to make a proper mold. It works. I don't know about aluminum siding, however - it seems too thin to me. An old cookie sheet is quite thick in comparison. You should also, ALWAYS, have a secondary containment system in case your mold fails. This is true of all molds. Anything can fail at any time - this is why we always have to think about failure and have a secondary system to contain the spill. There is never any excuse to have "molten lead in your shoes."

5. Release isn't much of an issue if the mold is smooth. If you have concerns or complex profiles (deep ridges and such) then boron nitride works well but may be overkill. I have literally greased molds with coconut oil and gotten very good results, although the mold is preheated to a couple hundred degrees, and is EXTREMELY well ventilated, as it will burn or smoke. Classically, in old lead smelters people used to coat runners and cold pots with tar to assist in lead flow and avoid sticking for cold runners (hot runners, where the lead might be 800-1000 celsius, are often water cooled and don't have sticking issues since the lead has very low viscosity at those temps).

As I suggest, commercially people use steel or iron molds for casting lead anodes , such as for a Betts Lead Electrorefining plant, or for a zinc electrowinning facility. Of course, these plants cast hundreds of large anodes a day. It is unlikely the OP will be casting hundreds of anodes in his lifetime. So, if you control your melt temperature, I suggest aluminum alloys can make acceptable mold materials. However, I would personally never use something as flimsy as house siding or flashing.

Best Regards, Gerald

 

[qst42know]

I want to make a lead cathode for a sulfuric cell. My question is will lead melt aluminum if not will lead stick to aluminum? I want to use aluminum to make a mold to make my lead cathode. I know that a dumb question. Thanks Jack

I forgot to say it is thin aluminum same kind as is on a house.

Many lead molds are made of aluminum but are heavy walled cast aluminum. Heated once should provide a no-stick surface. However house siding has no mass to draw heat from the lead to freeze it quickly and aluminum becomes weak at elevated temperatures and fails without warning. I suspect siding would simply collapse before the lead solidified. Quarter inch plate would be about my limit for about a one pound pour.

A sooted iron skillet should cast all the lead anodes you might need but they will be thick from the surface tension of molten lead.

 

[Oz]

A sooted iron skillet should cast all the lead anodes you might need but they will be thick from the surface tension of molten lead.

Fine idea qst.

Lead is actually rather fluid when at just above its melting point. I used to reclaim lead used as the seal over oakum from cast iron pipes to cast fishing weights with the kids. I enjoyed showing them all the junk that will float on molten lead, there were often stones or iron nails included in the seals. I used a cast iron sauce pan to do this, with a propane weed burner as the heat source. I would wager that if you used a cast iron skillet that was heated by propane and leveled properly you could get a half inch thick slab of lead without surface tension interfering. Even melting in cast iron I never observed any soldering properties to the iron.

It would not be hard to cut your cathode from a round lead “puck”. You have to be sure you actually have lead though.

 

[Jason1]

take each one and scratch a spot, then put a drop of vinegar on it. if it bubbles, its made of zinc or tin and zinc, if it doesnt bubble then its lead.

Good information