Dear Mr. Adekitan,

under atmospheric pressure (calcining) kaolinite is dehydrated and converted into amorphous metakaolinite (Al2Si2O7) in the T range 550 to 600°C. Upon rising temperature to 925 to 950°C a silica-deficient spinel (Si3Al4O12) comes into being, also called gamma-Al oxide. At 1050° is the point of conversion into mullite (Si2Al6O13) and cristobalite (SiO2).

This technical process is different from the metamorphic reactions at lithostatic pressure, e.g., 2 kbar. Kaolinite converts in a silica-enriched environment into pyrophyllite at about 400°C, into andalusite just underneath 500°C and into sillimanite at ca. 650°C . The next stage is the anatexis. The lower phase transformations are not very much affected by the pressure in nature.

Best regards

H.G.Dill

Hello dear Sir;

I fully agree with Harald.

Regards,

Hamed

Dear Mr. Adekitan,

Conditions of kaolin calcination depends on the use of the product.  What properties should the product possess? You can not establish the same calcination conditions for all kaolins because as a natural product kaolins are different depending on the origin of the kaolin deposit. At the beginning it is proposed to perform thermal analysis testing by DTA / DSC-TG method, dillatometry or hot stage microscopy. Having results of these tests it will be possible to set the  conditions of kaolin calcination.

Best regards.

Pawel

the temp. depend , which use of the calcened kaolin should be.

350 degree , for porcaline 1st stage , after glassing the tempreture should be 850  c , for ceramic industry & isolaters tempreture should be 1100 C , also the tempreture depends on the iron oxide content in kaoline clay

Dear Mr. Adekitan;

It seems somebody were disagree with my confirmation to comments of Harald; however I still agree with his comments because it is absolutely true and there is not necessity have my confirmation on his comments!

It is not clear for me that what you want to do after calcination of Kaolin? Normally Thermogravimetric analysis (https://en.wikipedia.org/wiki/Thermogravimetric_analysis) such as DTA/ DSC/ TG is good approach for better recognition of clay behaviors (sometimes could be applied for better clay type recognition and better description for researches).

The T-O clay mineral kaolinite does not contain inter-layer cations or inter-layer water. The temperature of dehydroxylation depends on the structural layer stacking order. Disordered kaolinite dehydroxylates between 530 and 570 °C, ordered kaolinite between 570 and 630 °C. Dehydroxylated disordered kaolinite shows higher pozzolanic activity than ordered. The dehydroxylation of kaolin to metakaolin is an endothermic process due to the large amount of energy required to remove the chemically bonded hydroxyl ions. Above the temperature range of dehydroxylation, kaolinite transforms into metakaolin, a complex amorphous structure which retains some long-range order due to layer stacking. Much of the aluminum of the octahedral layer becomes tetrahedrally and pentahedrally coordinated. In order to produce a pozzolan (supplementary cementitious material) nearly complete dehydroxylation must be reached without overheating, i.e., thoroughly roasted but not burnt. This produces an amorphous, highly pozzolanic state, whereas overheating can cause sintering, to form a dead burnt, non-reactive refractory, containing mullite and a defect Al-Si spinel. Reported optimum activation temperatures vary between 550 and 850 °C for varying durations, however the range 650-750 °C is most commonly quoted. In comparison with other clay minerals kaolinite shows a broad temperature interval between dehydroxylation and recrystallization, much favoring the formation of metakaolin and the use of thermally activated kaolin clays as pozzolans. Also, because the octahedral layer is directly exposed to the interlayer (in comparison to for instance T-O-T clay minerals such as smectites), structural disorder is attained more easily upon heating.

Nevertheless what is your purpose? It seems you are civil engineer? Therefore are you going to use Metakaolin for concrete application? As you know MetaKaolin is considered to have twice the reactivity of most other pozzolans, metakaolin is a valuable admixture for concrete/cement applications. Replacing portland cement with 8–20% (by weight) metakaolin produces a concrete mix, which exhibits favorable engineering properties, including: the filler effect, the acceleration of OPC hydration, and the pozzolanic reaction. The filler effect is immediate, while the effect of pozzolanic reaction occurs between 3 to 14 days. However I am not professional expert in civil engineer and concrete industry but I advise you to use mixture design of statistical software's such as DX7.0 if your approach is for concrete industry; in this case you can design experiments with variables of calcinations temperature, duration and so on and your responses could be your objective in civil industry.

Hereafter I have attached 2 papers I have found in my database and some links which it could be impressive.

https://www.researchgate.net/publication/232405592_Effects_of_calcination_temperature_of_kaolinite_clays_on_the_properties_of_geopolymer_cements

http://www.sciencedirect.com/science/article/pii/S2187076414001225

http://www.imerys-perfmins.com/calcined-kaolin/eu/calcined-kaolin.htm

http://www.imerys-perfmins.com/calcined-kaolin/eu/calcined-kaolin-products.htm

http://www.degruyter.com/view/j/gse.2012.58.issue-4/v10205-011-0022-2/v10205-011-0022-2.xml

Thanks and best regards,

Hamed

Article Effects of calcination temperature of kaolinite clays on the...

The behavior of your sample of kaolinite could be different in comparision with other samples, it depend of the nature of the sample, I recommend you to do a TGA/DSC experiment to have information about the thermal behavior of your sample of clay. If you take the TGA/DSC, you can upload the result here, and will discuss. 

what is the effect of weight gain on sintering of kaolin sample on same temperature (1600 degree C) but with different hodlding time (2-8 hours) ?