Reaction Between CaO and Cr2O3

As a part of the study of binary system, CaO-Cr2O3 the reactions which occur when mixtures of CaCO3, Ca(OH)2 and Cr2O3, CaO⋅Cr2O3 and CaCO3 fired in air were investigated using differential thermal analysis, thermal balance, and X-ray diffraction analysis at room and high temperatures.The results obtained are summarized as follows:(1) When the mixtures of Ca(OH)2 and Cr2O3 were heated in air an oxidation reaction has occured at about 720° with the formation of CaCrO4 showing an exothermic peak in the thermal curve. In the case of mixtures CaCO3-Cr2O3 decomposition and oxidation occured simultaneously at the temperatures between 700° and 900°C giving the thermal curves with two or three peaks.(2) From the relations of weight loss against the ratio CaCO3/Cr2O3 and from X-ray data the compounds formed during the heating may be estimated.By heating in air up to 1000°C CaCrO4 was formed from the mixtures of CaCO3/Cr2O3=0.33-2, and a new compound “X” from those of 2-4, and only “X” when the ratio was larger than 4.“X” was not the same as 9 CaO⋅4CrO3⋅Cr2O3, a compound given by Ford and Ree (Trans. Brit. Ceram. Soc. 41, 207, 1948), but a higher oxidized compound. From the weight loss curves of mixtures CaCO3/Cr2O3=4-6 the composition of “X” was calculated to 18CaO⋅10CrO3⋅Cr2O3.The mixtures of CaCO3/Cr2O3=2-4 showed a secondary loss in weight at a temperature between 920° and 1000°C corresponding to an endothermic peak at 950°C in the diffrential thermal curve. Also high temperature X-ray analysis has revealed that the change was a reversible phase transformation. This may be regarded as a decomposition of 18CaO⋅10CrO3⋅Cr2O3 with O2-loss at the presence of CaCrO4. The stable form at high temperatures should have the lattice belonging to simple crystal system, since the number of lines in the diffraction pattern were few.(3) CaO⋅Cr2O3, which appeared as green needle, anisotropic crystals showing straight extinction could be obtained easily by acid treatment of the mass produced by keeping molten for 3 hours the mixture 2CaCO3+Cr2O3 at 1300°C. At 1000°C CaO⋅Cr2O3 was oxidized partially giving CaCrO4, and the mixture, CaCO3+CaO⋅Cr2O3 showed a phase transformation at 960°C.