The hygroscopic nature of MgO 2

[heaterguy]

Hello,

These Eng-Tips forums are very helpful. Hopefully this group can he me with the hygroscopic nature of MgO. There is a website that shows the molecular structure of granular MgO. The website is:

http://www.webelements.com/webelements/compounds/text/Mg/Mg1O1-1309484.html.

We use MgO inside of heater elements and we have to bake the heater elements prior to sealing the ends. This prevents future moisture problems. However, we don't fully understand how H2O attaches to MgO.

For instance, does the H2O attach the outside plains of the granular cubes of the MgO or does H2O flow inside of the granular cubes?

When we bake out the heater element does the process of difusion take place so that the partial pressure of H2O inside the heater element match the partial pressure of H2O inside the oven?

We check resistance with a Megger device. Why is the resistance of MgO highest at 220-280'F? Why is it lower at higher temperatures? Of course the resistance is lower at temperatures less than 212'F because, we think, H2O is liquid.

Any insites into the hygroscopic nature of MgO will be helpful.

Regards,

Craig (aka heaterguy)

 

[bewdley]

Well, MgO hydrates at ordinary temperatures to produce Mg(OH)2, probably evolving heat, so it should not be a problem of crystalline form.
Sorry, no idea on other topics.

 

[moltenmetal]

heaterguy: the process of diffusion to give you an equilibrium between the MgO inside your element tubes and the furnace environment will take time. The problem is the path length along which diffusion must take place- the water vapour must diffuse down a long, narrow tube to escape. It'll take a long time for the water at the middle of your element to diffuse to the open end to escape, and if you don't wait long enough you may have dry MgO at the ends and still very hydrated MgO at the middle.

You can improve the rate of drying by increasing the bake-out temperature, hence increasing the driving force, but diffusion will still take a while. And given your geometry there's probably no way to switch from diffusion alone to convection plus diffusion (i.e. by sweeping the water vapour out of the MgO inside the heater elements with a bone dry carrier gas). You should of course ensure that your furnace contains dry gas.

You should of course be doing what you can to keep the water out in the first place. Ensure that you start with properly calcined MgO, and that you get this MgO where it's going as quickly as possible to minimize the amount of time it has to pick up water from the atmosphere. This will minimize your problems with bake-out later.

The assumption that the water in your MgO is "liquid" below 212 C isn't correct. This water is crystal hydration water rather than liquid water. I doubt sincerely that you have any "liquid" water in your MgO if you're handling it sensibly during manufacture.

As to the electrical conductivity of the material versus temperature, above a certain temperature you're dealing with increasing mobility of the ions within the MgO itself I would imagine.

 

[heaterguy]

Thank you for your responses. We have a 100% successful method of baking out our heater elements. I am trying to understand the hygroscopic nature of MgO for other reasons.

Imagine a 0.475" diameter x 10 foot long, infinite resistant, unsealed heater element exposed to a humid environment at room temperature. It's megger reading will quickly drop to around 5 Mohm. After 24 hours it will drop to less than 1 Mohm. After a week it will drop to less than .1 Mohm. It will return to infinite when baked out at 280'F in about 3 hours.

Help me, if you can, understand the chemistry of hygroscopic MgO.

Does MgH2O2 form inside the heater element?
Can the H2O diffuse deep into the heater element?
Does H2O penetrate into the MgO cube (see diagram on link above)? Does H2O stick to the side of the cube?
How does the electricity pass through the hydrated MgO?

MoltenMetal, I like your idea of the electrical conductivity of the MgO changing with temperature. This explains why an infinite resistant element will drop to 1500 Mohm at 800'F, for instance.

 

[25362]

These are excerpts from my Googleing around on the subject:

1. For MgO particles (when prepared by firing carbonates), hydration is apparently affected by surface availability. Meaning that the higher the porosity (ie, the higher the internal or "specific surface", m²/g), the more effective the hydration process.

2. The hydration reaction: MgO(s) + H₂O(g) => Mg(OH)₂(s) is exothermic (H^o=-81 kJ/mol). The activation energy has been calculated at 62.3 kJ/mol.

3. It is generally recognized that reactions with activation energies above 60 kJ/mol have rates that depend strongly on temperature. Assuming Arrhenius behaviour, an increase of 10 deg C from 300 K to 310 K would more than double the rate of hydration. However, it has been found that after a certain hydration level, the rate stabilizes.

4. Successive hydration and drying processes may activate pore surfaces making them more adsorbent. On the other hand, it appears that impurities, such as iron oxides, may increase hydration resistance.