Sintered CaO tablets covered with platinum marker on the surface were exposed to simulated coal gas containing SO_2 at 1200 ℃ in a tube reactor. The SEM micrograph and reflex optical micrograph were used to observe morphology changes of the tablets. After the CaO tablet covered with platinum marker being reacted for 120 h, a thick layer of product CaSO_4 formed on the surface of the platinum marker. The layer of product CaSO_4 is compact. At high temperature, the conductivity of CaSO_4 reached 3.5 * 10~(-3) S·cm~(-1). This means that Ca~(2+) ions in CaSO_4 have higher mobility. A new reaction mechanism of sulfur retained by CaO is suggested as follows: In the diffusion stages of GaO desulphurization, Ca~(2+) ion in CaO diffuses outward through the sulfate product layer to the interface of CaSO_4 and gas to form GaSO_4 with O~(2-) , O_2 and SO_2, rather than that SO_2, O_2 diffuse through the sulfate product layer into the interface of CaSO_4 and CaO to react with CaO.

Key words: CALCIUM OXIDE, SULFUR, ionic diffusion, ELECTRICAL CONDUCTIVITY, REACTION MECHANISM, SEM, XRD