how can i use thermal decomposition to prepare so3 from "caso4 gypsum "?

You will have to use rather high temperatures (sources state 700°C -
1100°C) to decompose CaSO4 gypsum.

"Thermal decomposition of gypsum into CaSO4 and H2O above approx: 140°C (413 K)
into CaO and SO3 above approx: 700°C (1273 K)"

from:
Transmix
( http://www.transmix.co.uk/safety.htm )

Other sources state even higher temperatures required:
"thermische Zersetzung ab ca. 1000°C in Calciumoxid und Schwefeltrioxid."
thermal decomposition above appr. 1000 °C into CaO and SO3
( http://www.chdb.de/anorganik/chemikalien/chemikalie_anorg_7778-18-9.htm )
(Translation by me)
Same temperture range (above 1000° C) is given in
( http://ceramic-materials.com/cermat/material/834.html )

I hope this helps to solve your question.

till-ga

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