The calcium oxidecarbon dioxide equilibrium has been studied a t pressures of carbon dioxide up to 304 atm. by using a pressure vessel with an internal heating arrangement. For the reaction CaCO,(s) = CaO(s) + CO,(g), in the pressure range 1-39.5 atm., the results agree well with those of previous authors and can be expressed by the relation, AGO = 38,000 -32.4T. The eutectic point has been estimated to lie a t 1242" f 3' and 39.5 f 0.5 atm. Above the eutectic point, the pressure-temperature relations have been plotted for the binary melts in equilibrium with solid calcium carbonate and solid calcium oxide, severally. By means of a special quenching device, the compositions of these melts have been determined and a phase diagram has been derived for the eutectic portion of the system. The eutectic composition has been found to be CaCO, 88.3 moIe %, CaO 11.7 mole yo.THE equilibrium CaCO, + CaO + CO, has been well studied at pressures up to 1 atm. and many thermodynamic expressions have been derived for the pressure-temperature relation.lPa At higher pressures, however, the only systematic thermodynamic study has been that of Smyth and Adams who used thermal analysis to follow the dissociation up to a pressure of 1000 atm. These workers found that at above 1240" and 39.5 atm. dissociation was accompanied by fusion of the resultant oxide with unchanged carbonate to give a single liquid phase.Since the system involves both gas-solid and gas-liquid equilibria at high temperatures its study is particularly useful for establishing techniques of general application to moltensalt systems of this type. As a result of the present work on the equilibrium, special apparatus has been developed, for example, a device for quenching samples, and furnaces which are stable in high-pressure carbon dioxide. Knowledge of the equilibrium has been extended by determining the oxide and carbonate contents of the binary melts which are formed and from these results a phase diagram has been constructed for the eutectic portion of the system.

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