On the Reaction between CaO-bearing Compound and Cr2O3

• Published in: Journal of the Ceramic Association, Japan Vol. 75; no. 865; pp. 259 - 264
• Main Authors: NISHINO, Tadashi, SAMURAI, Tadashi
• Format: Journal Article
• Language: English; Japanese
• Published: The Ceramic Society of Japan 1967
• ISSN: 0009-0255; 1884-2127
• DOI: 10.2109/jcersj1950.75.865_259

In the reaction of several binary or ternary compounds containing CaO with Cr2O3 in oxidizing atmosphere, the variation of Cr3+-Cr6+ ebuilibrium depends on the acidity of the compounds which has been used as indicator for examining the acidity of glasses. This paper deals with the thermal reaction between CaO-bearing compound and Cr2O3 in air or O2 stream. Synthesized compounds for the study are as follows; CaO⋅Fe2O3, 2CaO⋅Fe2O3, 3CaO⋅Al2O3, 5CaO⋅3Al2O3, CaO⋅Al2O3, 3CaO⋅5Al2O3, CaO⋅TiO2, 3CaO⋅2TiO2, 3CaO⋅SiO2, 2CaO⋅SiO2, CaO⋅SiO2, CaO⋅ZrO2, 3CaO⋅MgO⋅2SiO2, 2CaO⋅MgO⋅2SiO2, CaO⋅MgO⋅2SiO2, and CaO⋅MgO⋅SiO2. Reactivity, which is indicated by the amount of CaO to be converted to CaCrO4, is determined by means of thermal, X-ray diffractometric and chemical analyses. Taking into account of the reactivity for the binary compounds in view of ionic potential Z/r (Z; number of charge, r; radius of ion) of constituent metal ion excepting Ca2+ ion, compound constituted of metal ion with higher Z/r, such as CaO⋅SiO2 or CaO⋅TiO2, is regarded acidic for its higher effect of polarizing power to O2- ion, and it does not react with Cr2O3. On the contrary, such compoumd as CaO⋅Fe2O3 or 2CaO⋅Fe2O3 is involved in basic compound for its lower ionic potential of Fe3+ ion, and as the result of it the compound reacts with Cr2O3 thoroughly to produce CaCrO4 and Fe2O3. Concerning the other binary compounds such as 3CaO⋅SiO2, 2CaO⋅SiO2, 3CaO⋅2TiO2, CaO⋅Al2O3, 3CaO⋅5Al2O3 and CaO⋅ZrO2, a part of CaO is consumed for producing CaCrO4 until remaining CaO⋅SiO2, CaO⋅TiO2, CaO⋅2Al2O3, and cubic ZrO2 respectively. 3CaO⋅MgO⋅2SiO2 reacts with Cr2O3 to yielding CaCrO4 and CaO⋅MgO⋅2SiO2 up to 800°C, but both of which decrease successively to forming 2CaO⋅MgO⋅2SiO2, uvarovite (3CaO⋅Cr2O3⋅3SiO2) and other compounds, and which is illustrated by determination of the amounts of dissolved CaO and MgO into diluted HCl solution during the reaction process and is shown in DTA and TGA curves on the mixture of CaCrO4 and CaO⋅MgO⋅2SiO2. Reaction can not be detected between the other ternary compounds and Cr2O3. Classifying the compounds into acidic or basic roughly with regard to the occurrence of reaction with Cr2O3, CaO⋅SiO2, CaO⋅TiO2, CaO⋅2Al2O3, cubic ZrO2, 2CaO⋅MgO⋅2SiO2, CaO⋅MgO⋅2SiO2 and CaO⋅MgO⋅SiO2 are involved in the former and the other compounds the latter.