PARTIAL PRESSURE DIAGRAMS OF THE CA-O-C SYSTEM

Abstract

This paper presents the results of a thermodynamic analysis of the Ca–O–C system carried out using partial pressure diagrams in the context of calcium carbide production from phosphate–siliceous raw materials. The analysis focuses on chemical equilibria between condensed phases (Ca, CaO, CaC₂) and gaseous phases (CO and CO₂) within the temperature range of 1600–2100 °C. Both planar and volumetric partial pressure diagrams were constructed to reflect the conditions for calcium carbide formation, either directly from CaO or via the intermediate formation of metallic calcium. An invariant point and line were identified, describing the interaction of three condensed and two gaseous phases. It was established that the stability region of calcium carbide is influenced by temperature and the partial pressure ratio CO₂/CO. An increase in lgPCO reduces the lgPCO₂/lgPCO ratio, favoring CaO formation; conversely, a decrease in lgPCO increases the ratio, promoting CaC₂ formation. An equation was derived to describe the relationship between the partial pressure ratio and the temperature of invariant equilibrium. The obtained results allow for optimization of high-temperature calcium carbide synthesis and expand the theoretical foundation of thermal processing for calcium-containing (including phosphate) raw materials.