Purifying Lead with Caustic Soda

[B.C.Jay]

I had an interesting chat with a retired employee of our local smelter this last week. He worked in the melting room for a while where they do the final casting into pigs before shipping out the metals.
An interesting point he made, was that when they were going for five 9's pure lead, they would dump a 45 gallon drum of caustic soda into the melt pot as the final purification. Actually, they would keep doing it until they got the analysed purity they were after.

This got me wondering as to the caustic soda being a simple fluxing process as we are used to, or if this is more of a chemical reaction to remove the trace metals left. In other words, more like the copper sulfate / zinc displacement reaction.

Searching for an answer, I found that caustic soda (sodium hydroxide) should bind Tin, zinc and cadmium but no mention of antimony, copper, silver, calcium or the many other trace metals that are common in the lead refining process.


Any one with more metalurgical or chemistry knowledge want to chime in on this?

[triggerhappy243]

First off, is this caustic soda available to the gen. Public and if soo............. Where?

[farmerjim]

First off, is this caustic soda available to the gen. Public and if soo............. Where?

NaOH drain cleaner almost anywhere. Also used to make soap.

[triggerhappy243]

PLEASE FOREGIVE MY IGNORANCE, WHAT IS NaOH, and does it identify as caustic acid or is there another name for it?

[country gent]

Caustic Soda is an acid. On our steel cast iron or stainless equipment it may be hard on it. Hot acid is even more volatile than when at room temperature. We had a caustic soda cleaning tank at one shop I worked, aluminum brass zinc or pot metal parts were destroyed in a very short soak time. This tank was caustic soda and water at 180*. Another issue at lead melting temps would be the fumes that are given off and ventilation. Most industrial smelting have extremely strong ventilation pulling the smoke and fumes out. These acidic fumes might cause chemical burns in nasal passages lungs and on skin.

[salpal48]

Caustic Soda, is the Is the Fancy name for LYE. A good source for Caustic soda is ZEP industrial purple degreaser . It has a Very High Content. @ 90 %. Straight will Remove Paint . But the Vapor are not good to Inhale

[B.C.Jay]

Caustic Soda is either Sodium Hydroxide (NaOH) or Potassium Hydroxide (KOH), Yes, also known as Lye.
Sodium hydroxide is a whole lot less expensive than potassium hydroxide so it is used more often in the industrial world due to cost, but they pretty much do the same thing.

Na OH is available in 99%+ pure form as dry granule and is pretty much the main ingredient in powdered draino.

I buy it dry in 50 lb bags and use it for cleaning, therefore I have some.
That is what has perked my interest, I may just have to stir some into some roofing lead and see if it gets rid of some of that 'crud' that seems to stay with the roofing and cable lead. I've always found the roofing lead tends to have something in it that makes it a bit 'sticky' when you compare it to the really pure lead that is more purpleish when it's cast.

[triggerhappy243]

B.C.Jay, I am curious now. If you have a sample of say range lead that has 1.33% antimony, .90% tin and .90 copper, can you do an experiment on a 20 pound sample? test before and test after? This may be a venture worth pursuing.

[William Yanda]

"Caustic Soda is an acid." C. Gent
Please let me quibble. Lye, caustic soda, NaOH, KOH, whatever you call it, is a base, i.e. forms an OH ion when dissolved. That being said, it is a strong base, and will burn if splashed on bare skin because of its desire to dilute itself, pulling water from whatever is close. Acids do the same thing, but with an H ion. The big difference is that the H ion is positive while the OH ion is negative, IIRC. If I am wrong, please forgive me, HS Chemistry was over half a century ago.

[bassnbuck]

We get liquid caustic (a base) at work. One interesting thing is it will freeze at 56 degrees f. I have seen it get too thick to flow at 56 f. Verified with a temp. Gun several times.

[MAGA]

We use NaOH at work, you don't want to get it on your skin

[Jniedbalski]

All the machine shops in town had a caustic hot tank years ago to clean up cast iron engine blocks and heads. They had to get a different tank for the aluminum parts coming in the late 80's. It would eat up aluminum ant pot metal real quick.

[B.C.Jay]

Ok, I found it.
It's called the Harris process. It's one of the main refining processes of lead.
It uses non liquid NaOH and it looks like about 10% of that of Sodium Nitrate (NaNO3) depending on what your trying to remove.

The sodium nitrate salt causes various metals to oxidize and combine with the caustic soda. the big three are tin, antimony, arsenic and also Tellurium.

I guess they use this process both before and after electrolysis refining. I found a real interesting paper describing all of the processes in the big lead refinery in Germany. It's also interesting that they use sulphur for removing copper, but first they do a low temperature skim of copper (the oatmeal dross we're all used to, especially when playing with babbitts) and they add zinc to bind with any silver and use a vaccum distillation to remove the Zn/Ag. If there is any bismuth, gold, cadmium, calcium apparently that gets lost in the electrolysis slurry. That's a basic summary.

It was also interesting that one of the visual tests for pure lead is that when casting it, it has to remain shiny until frozen.

Yah, I'll probably try this on some of the roofing lead I've got and see if it cleans up.

[MaryB]

I worked at a feed testing lab for a few years after high school. We got in pure caustic soda in 55 gallon drums and I had to pump it up 1 floor to the lab holding tank(it was used as part of protein analysis, I do not remember the chemical combination...). I had to put on a rubber suit, full face mask, rubber gloves... and we used a stainless drum pump and had to replace it once a year because it ate it. That was one of my least liked parts of that job... there was a safety shower right out side the store room where it was kept and one of the guys had to use it one day when I was tied up. he went to pump it up and didn't bother putting on protective gear. Line going right over his head ruptured... I heard him scream and got him in the water(with a neutralizer). Burned my hands some touching him, to this day I have no fingerprints on my right thumb from it. He spent 3 weeks in the hospital and was fired for not following procedure.

So use extreme caution handling the stuff! They boiled it out of the feed samples and the stucco wall outside he vent hood was eaten away. I got the fun job of demoing and patching it with a special epoxy concrete that was supposed to resist it better.

[runfiverun]

borax will trap and hold heavy metals too.
it will also remove tin [but hey you can't have everything]

[hutch18414]

Removing tin, antimony and arsenic isn't what I want to do with lead. I add tin and antimony, and the arsenic is essential for heat treating. Seems like risking hazardous chemical exposure for not much gain in the casting arena. And I use pure food grade lye for soap making, purchased from Amazon. Not afraid of the lye, just doubt the need for lead of that purity in casting or alloying for casting. Now if it removed zinc.....

[runfiverun]

sulpher will pull zinc out.
so will copper sulphate, but you end up exchanging copper in for the zinc [and some tin] out

[Mytmousemalibu]

As mentioned Caustic Soda (lye) is a base, not an acid. Personally, strong bases scare me more than acids do. Acids are more easily neutralized and usually considered less dangerous than a base of similar strength and heating acids and bases tends to drastically increase reactivity. Sounds like unnecessary risk unless you have some serious contamination to deal with and proper means on implementing use of a caustic.

Molten sodium hydroxide will even dissolve GLASS! Think of how unreactive glass is to most chemicals and yet molten lye will dissolve it!
It will dissolve flesh and bone! Getting it on you would be horribly painful!

[Traffer]

I tried to get zinc out of a small batch of lead once. I used sulfur. It turned almost all of the alloy to useless black/brown chunky slag. I concluded that although I may have removed the zinc, I added sulfur to the lead. While doing a bit of research on the subject I came across the process of smelting raw galena. It has a crazy high sulfur content. The big smelters use a process called "roasting" to remove the sulfur. Basically they heat the lead in big batches to a very high temp and force air or oxygen into the bottom of the batch causing the galena to literally burn, or the sulfur in it to burn. This way they burn off the sulfur. So, if you were using sulfur to remove zinc, how do you keep the lead from absorbing (for lack of proper term) into the lead? How would caustic soda or lye, or sodium hydroxide react to the sulfur? So much chemistry for my non educated mind. Anyone care to respond about the roasting process?

[William Yanda]

Google "stewmaker" to find accounts of an individual who disposed of 300 bodies for Mexican cartels by putting the bodies in a barrel of lye, improperly identified by media as an acid.