The extraction of potassium from a tablet mixture of K-feldspar ore and CaSO

4

by roasting was studied with a focus on the effects of the decomposition behavior of CaSO

4

on the potassium extraction process. The roasted slags were characterized by X-ray diffraction (XRD)

scanning electron microscopy (SEM)

energy-dispersive X-ray spectroscopy

and thermogravimetric (TG) analysis. The XRD analysis revealed that hydrosoluble mischcrystal K

2

Ca

2

(SO

4

)

3

was obtained by ion exchange of Ca

2+

in CaSO

4

and K

+

in KAlSi

3

O

8

. Meanwhile

the intermediate product

SiO

2

separated from KAlSi

3

O

8

and reacted with CaSO

4

to decompose CaSO

4

. The SEM results showed that some blowholes emerged on the surface of the CaSO

4

particles when they reacted with SiO

2

at 1200℃

which indicates that SO

2

and O

2

gases were released from CaSO

4

. The TG curves displayed that pure CaSO

4

could not be decomposed below 1200℃

while t

he mixture of K-feldspar ore and CaSO

4

began to lose weight at 1000℃. The extraction rate of potassium and decomposition rate of CaSO

4

were 62% and 44%

respectively

at a mass ratio of CaSO

4

to K-feldspar ore of 3:1

temperature of 1200℃

tablet-forming pressure of 6 MPa

and roasting time of 2 h. The decomposition of CaSO

4

reduced the potassium extraction rate; therefore

the required amount of CaSO

4

was more than the theoretical amount. However

excess CaSO

4

was also undesirable for the potassium extraction reaction because a massive amount of SO

2

and O

2

gas were derived from the decomposition of CaSO

4

which provided poor contact between the reactants. The SO

2

released from CaSO

4

decomposition can be effectively recycled.