Reactions between Vapors from Molten Mixed Alkali Salts and Refractory Materials

As a fundamental study on corrosion of refractories by alkali-containing vapor, the present study has been made for the purpose of researching the reactions occurred in the refractory materials such as corundum, mullite and forsterite when they were exposed to vapors from the melt of mixed salt, K2CO3-Na2CO3, at 1200°C and from the melt of mixed salt, K2SO4-Na2SO4, at 1300°C. In the tests with carbonate vapor, the reactions took place through two stages under the condition held in a steady vapor composition and at a constant temperature. At the early stage the mole ratio of K2O to Na2O found in the reaction products was almost the same as that in the vapor, whereas at the later stage the products were altered progressively into those richer in Na2O by replacing K in the products by Na in the vapor. In the respective cases of corundum, mullite and forsterite, the solid solutions of K- and Nacompounds as shown by the following formulas were formed, namely, (K, Na) AlO2, (K, Na)3Al3SiO8 and (K, Na)2xMgxSi10-xO20 (4≤x≤5). During cooling, however, the solid solutions were separated into two phases of K- and Na-compounds. Consequently, the amount of Na-compounds in the reaction products increased with increasing time of reaction. In comparison to the tests with carbonate vapor, the reaction velocity observed when tested with sulphate vapor were very sluggish, nevertheless, at the early stage of reaction the mole ratio of K2O to Na2O in the product was almost the same as that vapor also in this case.