Sciencemadness Discussion Board

perlite for high temperature purposes

Cloner - 21-3-2006 at 15:07

The material perlite as available here easily is the raw mineral vulcanic stone, containing some water. When heated, the so-called expanded perlite is formed. This material I only encountered in insulation slabs and so on.

What has been used by the members of this forum who used perlite, normal or expanded? Will things crack at high temperatures when non-expanded perlite is used?

12AX7 - 21-3-2006 at 15:59

Uh...probably...

It's pretty easy to imagine what will happen to your furnace if you build it with non-expanded perlite then put some heat to it.

Where are you, NZ? I've heard difficulty there finding the fluffy pellet-like expanded stuff used in gardening.

Tim

Cloner - 22-3-2006 at 02:33

No, netherlands. This is the growshop capital of the world and perlite is sold there. They use it for indoor marijuana growing, apparently. The problem is the guy in the store probably didn't know jack about insulation. I can't tell for sure now if his perlite is truly 'unexpanded'. He said so. I doubt it from what I read elsewhere. But EP 'for insulation' is a consumer product only in slabs and such, the bags of powder are really something industrial, I think.

I can get 100 kilos of the expanded stuff for cheap at a factory outlet, so it's no big deal other than I can't lift that without getting a hernia.

DrP - 22-3-2006 at 02:52

LOL.. yea you want to take it easy and lift it 25 kg at a time (probably comes in 25kg bags anyway).

I was looking for pearlite about a year ago but it seems it is harder to get than it used to be (so I was told??) I found that mullite was very good, or even Zirkonium Mullite if you can get it. (I've actually got a couple of bags of Zirconia as well, but this is quite expensive and very heavy - a 25 kg bag is quite small - s.g. about 2)

12AX7 - 22-3-2006 at 07:16

Mullite is great for very high temperatures (cast steel for instance) but f-all on insulation. Perlite melts pretty low (you are limited to brass / bronze and maybe slow cone 10 ceramic firing) but all those air pockets work nicely to insulate.

One easy way to find out, take a handful and 1. look at it, 2. heat it. Expanded is pure white, while unexpanded is, well, a generic piece of rock! It fluffs like popcorn below red heat so a small torch will suffice.

Tim

DrP - 22-3-2006 at 07:49

Quote:

Mullite is great for very high temperatures (cast steel for instance) but f-all on insulation


Mullite (Allumina Silicate) is an excellant thermal insulator - better than alumina. (admittedly perlite is better but couldn't find a value for it off hand).
Thermal conductivity - W/mK
Mullite = 3.5
Zirconia = 2.7
Alumina = 24
Silicon = 125

I think these values vary according to temperature, I can't remember.

12AX7 - 22-3-2006 at 10:48

Probably varies a little. My experience with refractory is that conductivity increases most with airy materials (e.g., kaowool).

In contrast, castable refractory places 1W/m-K, or less, and that's for the hard assed stuff. Insulating castable is in the 0.5 range or less, IIRC. (I'd give an example but I can't find my refractory data sheets ATM.)

Tim

DrP - 23-3-2006 at 01:46

Quote:

In contrast, castable refractory places 1W/m-K, or less


OK then your right Tim - perlite is well below 1 W/m-K from what I could find. I want to get some too!

Cloner - 23-3-2006 at 03:19

Some searching the net comes up with more info. As I started to suspect, the growshop owner has been smoking too much! The stuff for sale on ebay and in growshops is probably expanded perlite after all, though it won't hurt to ask the vendor and even try out by yourself.

Horticultural Perlite

White perlite granules are the result of the expansion of combined water vaporizing in the crude mineral and forming enclosed air pockets. Because of the great quantity of air in these pockets, the product is very light and has excellent physical qualities for use in plant propagation. The physical properties of the product also further enhance its use in horticulture. It is inorganic, sterile, safe to handle and has no odor.


Now, to test your perlite. I thought of using the microwave. After all, perlite mineral contains water pockets. On vaporizing, the water should pop the perlite like popcorn, right? Or should one look at density? I can imagine that a float test would suffice. Perhaps EP will float on ethanol where mineral perlite won't?

DrP - 23-3-2006 at 03:34

Quote:

White perlite granules are the result of the expansion of combined water vaporizing in the crude mineral and forming enclosed air pockets


Thus its enhanced insulative properties I expect.
Quote:

Now, to test your perlite. I thought of using the microwave. After all, perlite mineral contains water pockets. On vaporizing, the water should pop the perlite like popcorn, right? Or should one look at density? I can imagine that a float test would suffice. Perhaps EP will float on ethanol where mineral perlite won't?


Would hitting it with a blow torch do the job? Unexpanded will expand and expanded wil not do much I expect.

Cloner - 25-3-2006 at 03:30

Aight, got some agricultural perlite. It's expanded allright, and comes in bags of 100 liters which can't be much more than 10 kgs of weight.

Love this kind of saturday morning shopping in a backward village 20 km out of town. After calling lots of garden shops, the first one that had perlite also had bags of 13-0-46 and calcium nitrate for over the counter sales. That's getting pretty rare here nowadays.