I need to dry a sample of allumina. What is the minimum temperature for drying allumina without vacuum? Is it enough 250°C for 4 hours? Is it important the allotropic form (alpha or corundum)?
Thank you in advance for every reply!
There is a difference between simply drying Al2O3 ceramics and converting gamma Al2O3 into alfa Al2O3 (by the way alfa Al2O3 and corundum are one and the same). Gamma Al2O3 powder is just a raw material that must be calcined into alfa Al2O3 powder prior to further processing into a ceramic. This calcination happens at temperatures of around 1000°C. After calcination, Al2O3 remains in alpha form and can be further processed. Drying of this powder or green compacts from it can be carried out at any temperature.
As Dr. Vadim Verlotski mentioned, alpha-phase of alumina is also called corundum, and this is the generally used phase of alumina. Alpha phase is the stable phase, that means you don't have to worry about its transformation to other phases at low temperatures. Alumina is an oxide, so vacuum is not needed if you are not concerned about environmental contamination. The drying temperature and time depend on what solvent you use in case it's mixed in a solvent, and the quantity of the suspension. You will be drying in a beaker most probably, so you can use a magnetic stirrer, and keep the temperature above boiling point of the solvent. The higher the temperature you give above the boiling point, the less time it will take to finish the drying.
Thank you for your replies.
I only need to remove water from Alumina, I am not really concerned on the allotropic change. The alumina is not mixed with any solvent, it contains only some humidity, perhaps as water of crystallization. I "googled" and I usually found it is dried up to 350°C under nitrogen stream. I only have an oven and I can reach 250°C. I was thinking to keep it, as you say, in a beaker, at this temperature for, i.e. 4h. Can I be confident water is removed?
I have an application which requires it to be 100% water-free.
If you have alpha Al2O3, then it does not contain any crystallization water and completely dries already at a temperature exceeding the dew point (usually 40-50°C). A higher drying temperature will not make the material drier, but will only break down possible organic impurities. Of course, if alumina after drying is stored in air at a temperature below the dew point, then it can again absorb a small amount of water from the air.
Dear Giacomo Del Bianco
The partial release of crystallization water of alumina Al2O3.nH2O is start at 200oC, followed by a major release between 200oC and 300oC. At higher temperatures in range 275-475oC, you get alumina full free of crystallization water. So, furnace temperature of 250oC is just about releasing most of crystallization water but not all of it. In this respect, I remind you that loss of alumina crystallization water is temperature dependent process in first place.
Best regards
Thank you Adim and Mohammed.
Mohammed Hussein j. H. Al'Atia do you have any literary reference (also minimal) of these figures you mention?
Dear Giacomo Del Bianco
Yes, you can stand on the experimental results as seen in my article [1] under paragraph titled "Furnace thermal analysis" on page 7. Also you can stand on the conclusion in article [2] page 9.
Best regards
[1]Conference Paper Experimental Stability and Thermal Characteristics Enhanceme...
[2]Article Thermal decomposition of bauxite minerals: Infrared emission...
The diffraction line is altered at a heat treatment temperature (HTT) of 400 ° C.
This change in the diffraction line corresponded to the phase transition from the beam to γ-Al2O3.
The diffraction line of the HTT 1000 ℃ sample matched the diffraction line of θ-Al2O3.
Furthermore, the diffraction line of the HTT 1200 ° C sample matched the diffraction line of α-Al2O3.
The drying temperature should be 250 ° C. Please adjust the drying time according to the thickness of the work.
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