Pyrolysis of (NH4)2ZrF6: some practical problems

Today I made my first attempt at pyrolysing ammonium hexafluorozirconate under argon, acc.:

(NH₄₂ZrF₆(s) → ZrF₄(s) + 2 NH₄F(g)

Below is a photo of the apparatus (dress rehearsal):



Left: Argon bottle and connecting silicone tube to reactor
Middle: reactor with copper boat. The glass tube is a thick walled borosilicate combustion tube, 17 x 300 mm
Right: cold trap and exit to a simple gazometer (not shown)

Slightly more up close:



The copper boat, fashioned from 14 mm gas pipe, designed to fit quite snugly in the combustion tube:



It's mounted on a small cardboard stand to allow loading and weighing.

About 2.75 g of finely ground ammonium hexafluorozirconate was loaded into the copper boat, then inserted into the combustion tube and the front seal and tube reinstated. The apparatus was then flushed with argon at high flow rate for a couple of minutes and flow rate was then adjusted to about 10 ml/minute.

Heating was then provided from a max air opening propane Bunsen burner. White fumes started coming off the boat immediately, some settling just behind the boat and a bit further down, some floating into the cold trap, where they settled (at the end of the experiment a white powder was found at the bottom of the 200 ml Erlenmeyer).

After about 15 minutes of heating something started going wrong: at the point where the first part of the copper boat touched the glass, a ‘bubble’ (protrusion) started to form in the glass, which eventually started leaking argon gas. The experiment was stopped for that reason.

After cooling the boat was extracted from the tube and the weight loss of its content was calculated to be 37 w%. The reaction predicts 30.7 %, which seems to suggest the conversion was already complete after that short time. Perhaps a bit of ZrF4 had also volatilised.

Then another surprise popped up. The product in the boat had become hollow and the part sticking to the boat stuck to it… like mad. Unremovable, as if it had been fused on to it. Subsequent soaking in simmering water caused the product to hydrolyse and come off. This may be due to too high temperature of the pyrolysis.

New attempts will (or may) include:

1. Lower temperature and more gradual increase of heating
2. Possibly use of a copper instead of borosil tube
3. Possibly use of a nickel boat.



[Edited on 13-4-2014 by blogfast25]

Quote: Originally posted by deltaH

A suggestion may be to heat the tube (so effectively just the argon gas) slightly upstream of the boat and not have the flame directly below the boat, thereby having hot Ar heating the boat and avoiding a hot spot on the boat surface underneath that fuses the product locally. [Edited on 13-4-2014 by deltaH]

When I was working in catalysis research, we routinely used calcination tubes to calcine small amounts of catalyst that had a counter current gas flow for heat exchange.

These were made from vertical tubes and I thought you could make your own from a large fat test tube and a smaller open tube with some stoppers and heat with your bunsen flame from the bottom.

Ideally, if you can and have the means for some basic glass work, attaching a smaller side arm tube for your feed would be good.

Importantly, the powder would fluidise in the upflowing gas so you got very uniform heating in the powder bed and good mixing, but don't run your gas flow too high or it will become entrained in the upflow. I've drawn the diagram here with a second piece of glass wool plug to prevent crystals of NH4F falling onto you powder from the top (presumably they will grow from the cooler wall at the top of the apparatus).

Don't make you glass wool ball very dense, obviously, or everything will lift to the top. A loose ball does the job!

You can also control your temperature to some degree by varying the Ar gas flow speed... slower for hotter. Having a thermocouple in there to see the temperature in the middle of the bed would be excellent!

Uniformity of heating was the main advantage with such a setup.

The ones I used to use were a bit more sophisticated in that they were heated by electric heating coils that ran about half the length of the tube and had a thermocouple that we would insert in the centre of the fluidising powder from the top so that we could monitor and control the temperature in the powder bed accurately. Also they had ground glass joints and not stoppers, but those are expensive!

Enjoy the drawing

Note: there's some bug here in that the drawing background appears black in the preview image, but it loads correctly as white when you click on it. I've notified a moderator, so I think it will be resolved soon...



[Edited on 14-4-2014 by deltaH]