Sciencemadness Discussion Board

Mantle weak magnet issue fix?

Fyndium - 10-2-2021 at 13:15

My mantle might not actually be the culprit, but a chinese 2L flask that has been made slightly too large, leaving about 10-20mm space to the bottom, causing stirbar engagement to be very weak to the extent I cannot use stirring at all. Schott and another 2L chinese flask fit well, but they're only 1-necked. I got many disengagements when doing vacuum distillation with the 2L too, though, so it ain't too strong to begin with.

Could the magnet be upgraded somehow? The magnets are 19.5x9.5x9.5mm blocks placed in an aluminum casing. I cut sheets of cardboard and placed under the magnets to raise them about 2mm, but it doesn't seem to have any apparent improvement. Could a stronger stirbar be used? The longer the stirbar, the easier it disengages, and the largest one doesn't actually engage at all, or keeps on only at very low, like 60rpm. I eyed at neodymium magnet blocks, if there'd be fitting sizes available, but nope.

Final option is to order more 2L flasks and hope they fit better.

paulll - 10-2-2021 at 13:44

How about eBay item# 265030302979?

Fyndium - 10-2-2021 at 14:28

Thanks, put one coming.

zed - 11-2-2021 at 05:43

Ummm. I've posted about this elsewhere. But, here goes.

Those Neodymium Stir bars are excellent. But, they demagnetize before water boils. They entirely lose their mojo in heated solutions.

I just got some Samarium Cobalt Magnets. In the size that I got them, they are god-awful strong.
They need to be coated or covered with something. They aren't plated, and they chip easily. Stack 'em, !-2-3-4

HDPE might be a good idea, boiling does not trouble it... Samarium Cobalt does not demagnetize at normal lab temperatures.

[Edited on 11-2-2021 by zed]

[Edited on 11-2-2021 by zed]

Fery - 11-2-2021 at 09:32

SmCo magnet can usually operate upto 350 C without loosing / weakening magnetic properties

Fyndium - 11-2-2021 at 11:10

HDPE melts at 110-130C so for boiling water it'd do, but for any other use it will give up. Also, HDPE is resistant to almost anything at room temp, but is surprisingly easily attacked at 100C.

Sealing in teflon could work, if the plastic is weldable at it's melting temp around 330C. In this instance it would be trivial to put the magnets inside a teflon tube and seal it with soldering iron.

Btw, how glass stir bars are made? First sealed in glass, then magnetized?

zed - 12-2-2021 at 03:28

Well, nothing seems to be easy. A stainless steel capsule would be nice, and not impossible.

Even Teflon has problems for some reactions. I've noted that NurdRage's Sodium synthesis, eats teflon stir-bars.

I have Neodymium, inside HDPE for strong coupling at low temps. I have regular Teflon stir-bars. I have Samarium Cobalt for higher temps and strong coupling. I suppose I could make a stainless capsule, or utilize stainless steel foil.

Sigh... I really hate overhead stirring. It brings with it, a whole new spectrum of things that can screw-up.

Expensive too. Good for stirring sludges and Peanut-butters though.

Fyndium - 12-2-2021 at 10:44

What complications overhead stirring can cause, except for having difficult time doing vacuum jobs?

zed - 13-2-2021 at 21:12

Alignment, breaking joints, mounting problems, sparking, insufficient torque.

Had a buddy that used it a lot. There were a lot of expenses, special motors, controllers.

Might used it myself.... Sometime.

I'll shop for an air-motor. For the nitty-gritty, it is helpful to use an air-motor, controlled by a remote compressor. Expensive, but lots of torque, and.... Explosion proof.

Fyndium - 14-2-2021 at 01:25

Breaking joints is one thing I'm worried about because of the teflon plug that expands when heated, and I've discussed about this on another thread.

Now that I think of it, I might try putting a glass joint adapter to the joint, so if it expands, it will crack the $4 adapter, not the $400 flask.

Other than that, I've liked my overhead a lot. It just churns through large reaction volumes and viscous liquids like nothing. Last time I was betrayed by a magstir was when I did a reaction where the product precipitated as it formed from the solution, and off I went for few hours, and when I came back, it was all settled to the bottom as a clean, white mass, the stirrer in a standstill. Changed to overhead, and no problem.

Alignment is not an issue, but you'll need a sturdy stand. I've mounted my hood with 16mm stand and machined bosses from delrin stock, and the stirrer is just solid like rock. I also got a 3d-printer axial coupling adapter to the shaft so it can bend a little. With little adjusting, overhead stirrer can be made run very smooth.

Sparking issue is another matter, but it should also be be addressed on what one is working with. If you're refluxing ether, extra safety measures could be smart, but not so when doing non-flammable stuff. Brushless DC motors are rather safe on sparking, though.

[Edited on 14-2-2021 by Fyndium]

macckone - 16-2-2021 at 22:41

if the magent in the stirrer is 19.5mm then the magnet in the stir bar needs to be 19.5mm for maximum coupling.
You can get glass coated alnico magnets.
They are suitable for temperatures up to the melting point of glass.
They can also be remagnetized.

Mine is curiously about 19mm.

You can get samrium cobalt magnet stir bars in the teflon variety but they are not suitable for hot sodium hydroxide.
For that matter glass doesn't like it too much either if there is even the slightest water present.

I got mine specifically for the sodium production reaction.
That and stirring boiling sulfuric acid to help prevent bumping.

Fyndium - 7-4-2021 at 11:06

I got my neodymium magnets, and installed them. I also added 1mm of al plate pieces under the magnets to bring them just a little closer. Ran it dry, and it seems to be stronger, it can now spin the same stirbars without decoupling, before it ran off almost every time. True test to be done when I next time distill solvents.

Fyndium - 30-4-2021 at 10:52

Forget it. Apparently the heat is enough to kill off any neodymium magnets. If it was weak for the start, now you need to summon spells to keep it coupled over 120rpm, and much less if the liquid happens to have any sort of viscosity higher than water, or acetone, or sometimes air. I think I just crap the whole mag and use overhead instead.

BromicAcid - 30-4-2021 at 20:27

I have used the rare earth stir bars with the black dot on the Teflon. The issue is that when they decouple and spontaneously re-couple sometimes do a flip. We had one instane at work where this shattered the bottom out of a flask.

Fyndium - 1-5-2021 at 03:44

Yeah, ain't taking that chance with vacuum distillations. I don't know if your equipment scale was larger than fel L though, as someone posted here an 8-incher.

Rare-earths also seem to command a bit higher price. For 60€ I got my entire overhead, which has worked 100% flawless for few hundred hours now.